# Patent

## Title
Structure and fabrication of insulated-gate field-effect transistor with hypoabrupt change in body dopant concentration below source/drain zone

## Abstract

An insulated-gate field-effect transistor (100, 100V, 140, 150, 150V, 160, 170, 170V, 180, 180V, 190, 210, 210W, 500, 510, or 530; or 220, 220W, or 540) is provided with a hypoabrupt vertical dopant profile below one (104; or 264 or 564) of its source/drain zones for reducing the parasitic capacitance along the pn junction between that source/drain zone and adjoining body material (108; or 268 or 568). In particular, the concentration of semiconductor dopant which defines the conductivity type of the body material increases by at least a factor of 10 in moving from that source/drain zone down to an underlying body-material location no more than 10 times deeper below the upper semiconductor surface than that source/drain zone.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This is a division of U.S. patent application Ser. No. 11/977,213, filed Oct. 23, 2007, now U.S. Pat. No. 7,838,930B1, which is a division of U.S. patent application Ser. No. 11/215,537, filed Aug. 29, 2005, now U.S. Pat No. 7,419,863 B1.


FIELD OF USE
This invention relates to semiconductor technology and in particular, to field-effect transistor (“FETS”) of the insulated-gate type. All of the insulated-gate FETs (“IGFETs”) described below are surface-channel enhancement-mode IGFETs except as otherwise indicated.
BACKGROUND
An IGFET is a semiconductor device in which a gate dielectric layer electrically insulates a gate electrode from a channel zone extending between a source zone and a drain zone. The channel zone in an enhancement-mode IGFET is part of a body region, often termed the substrate or substrate region, that forms respective pn junctions with the source and drain. In an enhancement-mode IGFET, the channel zone consists of all the semiconductor material between the source and drain. During IGFET operation, charge carriers move from the source to the drain through a channel induced in the channel zone along the upper semiconductor surface. The threshold voltage is the value of the gate-to-source voltage at which the IGFET switches between its on and off states for given definitions of the on and off states. The channel length is the distance between the source and drain along the upper semiconductor surface.
IGFETs are employed in integrated circuits (“ICs”) to perform various digital and analog functions. As IC operational capabilities have advanced over the years, IGFETs have become progressively smaller, leading to a progressive decrease in minimum channel length. An IGFET that operates in the way prescribed by the classical model for an IGFET is often characterized as a “long-channel” device. An IGFET is described as a “short-channel” device when the channel length is reduced to such an extent that the IGFET's behavior deviates significantly from the classical IGFET model. Although both short-channel and long-channel IGFETs are employed in ICs, the great majority of ICs utilized for digital functions in very large scale integration applications are laid out to have the smallest channel length reliably producible with available lithographic technology.
A depletion region extends along the junction between the source and body region. Another depletion region extends along the junction between the drain and body region. A high electric field is present in each depletion region. Under certain conditions, especially when the channel length is small, the drain depletion region can laterally extend to the source depletion region and merge with it below the upper semiconductor surface. This phenomenon is termed (bulk) punchthrough. When punchthrough occurs, the operation of the IGFET cannot be controlled with its gate electrode. Punchthrough needs to be avoided.
Various techniques have been employed to improve the performance of IGFETs, including those operating in the short-channel regime, as IGFET dimensions have decreased. One performance improvement technique involves providing an IGFET with a two-part drain for reducing hot-carrier injection. The IGFET is also commonly provided with a similarly configured two-part source.
FIG. 1 illustrates such a conventional long n-channel IGFET 20 as described in U.S. Pat. No. 6,548,842B1 (Bulucea et al.). The upper surface of IGFET 20 is provided with recessed electrically insulating field-insulating region 22 that laterally surrounds active semiconductor island 24 having n-type source/drain zones 26 and 28. Each source/drain zone 26 or 28 consists of very heavily doped main portion 26M or 28M and more lightly doped, but still heavily doped, lateral extension 26E or 28E.
Source/drain zones 26 and 28 are separated from each other by channel zone 30 of p-type body material 32 consisting of lightly doped lower portion 34, heavily doped intermediate well portion 36, and upper portion 38. Although most of upper body-material portion 38 is moderately doped, portion 38 includes ion-implanted heavily doped halo pocket portions 40 and 42 that respectively extend along source/drain zones 26 and 28. IGFET 20 further includes gate dielectric layer 44, overlying gate electrode 46, electrically insulating gate sidewall spacers 48 and 50, and metal silicide layers 52, 54, and 56.
Source/drain zones 26 and 28 are largely mirror images of each other. Halo pocket portions 40 and 42 are also largely mirror images of each other so that channel zone 30 is symmetrically longitudinally graded with respect to channel dopant concentration. As a result, IGFET 20 is a symmetrical device. Either source/drain zone 26 or 28 can act as source during IGFET operation while the other source/drain zone 28 or 26 acts as drain. This is especially suitable for digital situations where source/drain zones 26 and 28 respectively function as source and drain during certain time periods and respectively as drain and source during other time periods.
FIG. 2 illustrates how net dopant concentration NN varies as a function of longitudinal distance x for IGFET 20. Since IGFET 20 is a symmetrical device, FIG. 2 presents only a half profile starting from the channel center. Curve segments 26M*, 26E*, 28M*, 28E*, 30*, 40*, and 42* in FIG. 2 respectively represent the net dopant concentrations of regions 26M, 26E, 28M, 28E, 30, 40, and 42. Dotted curve segment 40′′ or 42′′ indicates the total concentration of the p-type dopant that forms halo pocket 40 or 42, including the p-type dopant introduced into the location for source/drain zone 26 or 28 in the course of forming pocket 40 or 42.
In addition to helping alleviate undesired roll off of the threshold voltage at short channel length, the presence of halo pockets 40 and 42 in IGFET 20 causes the net p-type dopant concentration in channel zone 30 to be increased along each source/drain zone 26 or 28, specifically along each lateral extension 26E or 28E. The onset of punchthrough is thereby alleviated because the thickness of the channel-zone portion of the depletion region extending along the junction of source-acting source/drain zone 26 or 28 is reduced.
Body material 30 is provided with an additional doping characteristic to further alleviate punchthrough. Based on the information presented in U.S. Pat. No. 6,548,842 B1, FIG. 3a roughly depicts how absolute concentrations NT of the p-type and n-type dopants vary as a function of depth y along a vertical line extending through main source/drain portion 26M or 28M as a result of the additional doping characteristic. Curve segment 26M′′ or 28M′′ in FIG. 3a represent the total concentration of the n-type dopant that defines main source/drain portion 26M or 28M. Curve segments 34′′, 36′′, 38′′, 40′′, and 42′′ together represent the total concentration of the p-type dopant that defines respective regions 34, 36, 38, 40, and 42.
The additional doping characteristic is achieved by ion implanting p-type upper portion 38 with p-type anti-punchthrough (“APT”) dopant that reaches a maximum concentration at a depth more than 0.1 μm below the upper semiconductor surface but no more than 0.4 μm below the upper surface. For the situation represented in FIG. 3a where main source/drain portions 26M and 28M extend approximately 0.2 μm below the upper surface, the p-type APT dopant reaches a maximum concentration at a depth of approximately 0.2 μm. By locating the p-type APT dopant in this manner, the thickness of the channel-zone portion of the depletion region extending along the pn junction of source-acting source/drain zone 26 or 28 is further reduced so as to further alleviate punchthrough.
Well region 36 is defined by ion implanting IGFET 20 with p-type well dopant that reaches a maximum concentration at a depth below that of the maximum concentration of the p-type APT dopant. Although, the maximum concentration of the p-type well dopant is somewhat greater than the maximum concentration of the p-type APT dopant, the vertical profile of the total p-type dopant is relatively flat from the location of the maximum well-dopant concentration up to main source/drain portion 26M or 28M. In particular, the concentration of the p-type dopant decreases by considerably less than a factor of 5 in going from the location of the maximum well-dopant concentration up to portion 26M or 28M.
U.S. Pat. No. 6,548,842 B1 discloses that the p-type dopant profile along the above-mentioned vertical line through main source/drain portion 26M or 28M can be further flattened by implanting an additional p-type dopant that reaches a maximum concentration at a depth between the depths of the maximum concentrations of APT and well dopants. This situation is illustrated in FIG. 3b for such a variation of IGFET 20 where curve segment 58′′ indicates the variation caused by the further p-type dopant. In FIG. 3b, the maximum concentration of the further dopant lies between the maximum concentrations of the APT and well dopants. Accordingly, the concentration of the total p-type dopant again decreases by considerably less than a factor of 5 in moving from the location of the maximum well-dopant concentration to portion 26M or 28M.
A symmetrical IGFET structure is not needed in situations, especially many analog applications, where current flows in only one direction through an IGFET during device operation. As further discussed in U.S. Pat. No. 6,548,842 B1, the halo pocket portion can be deleted from the drain side. IGFET 20 thereby becomes long n-channel IGFET 60 as shown in FIG. 4a. IGFET 60 is an asymmetrical device because channel zone 30 is asymmetrically longitudinally dopant graded. Source/drain zones 26 and 28 in IGFET 60 respectively function as source and drain.
FIG. 4b illustrates asymmetrical short n-channel IGFET 70 corresponding to long-channel IGFET 60. In IGFET 70, source-side halo pocket 40 closely approaches drain 28. Net dopant concentration NN as a function of longitudinal distance x along the upper semiconductor surface is shown in FIGS. 5a and 5b respectively for IGFETs 60 and 70.
Asymmetrical IGFETs 60 and 70 receive the same APT and well implants as symmetrical IGFET 20. Along vertical lines extending through source 26 and drain 28, IGFETs 60 and 70 thus have the dopant distributions shown in FIG. 3a except that dashed-line curve segment 62′′ represents the vertical dopant distribution through drain 28 due to the absence of halo pocket 42. When the IGFET structure is provided with the additional well implant to further flatten the vertical dopant profile, FIG. 3b presents the consequent vertical dopant distributions again subject to curve segment 62′′ representing the dopant distribution through drain 28.
U.S. Pat. Nos. 6,078,082 and 6,127,700 (both Bulucea) describe IGFETs having asymmetrical channel zones but different vertical dopant characteristics than those employed in the inventive IGFETs of U.S. Pat. No. 6,548,842 B1. IGFETs having asymmetrical channel zones are also examined in other prior art documents such as (a) Buti, et al., “Asymmetrical Halo Source GOLD drain (HS-GOLD) Deep Sub-half Micron n-MOSFET Design for Reliability and Performance”, IEDM Tech. Dig., 3-6 Dec. 1989, pp. 26.2.1-26.2.4, (b) Chai et al., “A Cost-Effective 0.25 μm Leff BICMOS Technology Featuring Graded-Channel CMOS (GCMOS) and a Quasi-Self-Aligned (QSA) NPN for RF Wireless Applications”, Procs. 2000 Bipolar/BiCMOS Circs. and Tech. Meeting, 24-26 Sep. 2000, pp. 110-113, (c) Cheng et al., “Channel Engineering for High Speed Sub-1.0 V Power Supply Deep Submicron CMOS”, 1999 Symp. VLSI Tech., Dig. Tech. Paps., 14-16 Jun. 1999, pp. 69 and 70, (d) Deshpande et al., “Channel Engineering for Analog Device Design in Deep Submicron CMOS Technology for System on Chip Applications”, IEEE Trans. Elec. Devs., September 2002, pp. 1558-1565, (e) Hiroki et al., “A High Performance 0.1 μm MOSFET with Asymmetric Channel Profile”, IEDM Tech. Dig., Dec. 1995pp. 17.7.1 - 17.7.4, (f) Lamey et al., “Improving Manufacturability of an RF Graded Channel CMOS Process for Wireless Applications”, SPIE Conf. Microelec. Dev. Tech. II, Sep. 1998, pp. 147-155, (g) Ma et al., “Graded-Channel MOSFET (GCMOSFET) for High Performance, Low Voltage DSP Applications”, IEEE Trans. VLSI Systs. Dig., December 1997, pp. 352-358, (h) Matsuki et al., “Laterally-Doped Channel (LDC) Structure for Sub-Quarter Micron MOSFETs”, 1991 Symp. VLSI Tech., Dig. Tech. Paps., 28-30 May 1991, pp. 113 and 114, and (i) Su et al., “A High-Performance Scalable Submicron MOSFET for Mixed Analog/Digital Applications”, IEDM Tech. Dig., December 1991, pp. 367-370.
The term “mixed signal” refers to ICs containing both digital and analog circuitry blocks. The digital circuitry typically employs the most aggressively scaled n-channel and p-channel IGFETs for obtaining the maximum potential digital speed at given current leakage specifications. The analog circuitry utilizes IGFETs and/or bipolar transistors subjected to different performance requirements than the digital IGFETs. Requirements for the analog IGFETs commonly include high linear voltage gain, good small-signal and large-signal frequency response at high frequency, good parameter matching, low input noise, well controlled electrical parameters for active and passive components, and reduced parasitics, especially reduced parasitic capacitances. Although it would be economically attractive to utilize the same transistors for the analog and digital blocks, doing so would typically lead to weakened analog performance. Many of the requirements imposed on analog IGFET performance conflict with the results of digital scaling.
More particularly, the electrical parameters of analog IGFETs are subjected to more rigorous specifications than the IGFETs in digital blocks. In an analog IGFET used as an amplifier, the output resistance of the IGFET needs to be maximized in order to maximize its intrinsic gain. The output resistance is also important in setting the high-frequency performance of an analog IGFET. In contrast, the output resistance is considerably less importance in digital circuitry. Reduced values of the output resistance in digital circuitry can be tolerated in exchange for higher current drive and consequent higher digital switching speed as long as the digital circuitry can distinguish its logic states, e.g., logical “0” and logical “1”.
The shapes of the electrical signals passing through analog transistors are critical to circuit performance and normally have to be maintained as free of harmonic distortions and noise as reasonably possible. Harmonic distortions are caused primarily by non-linearity of transistor gain and transistor capacitances. Hence, linearity demands on analog transistors are very high. The parasitic capacitances at pn junctions have inherent voltage non-linearities that need to be alleviated in analog blocks. Conversely, signal linearity is normally of secondary importance in digital circuitry.
The small-signal analog speed performance of IGFETs used in analog amplifiers is determined at the small-signal frequency limit and involves the small-signal gain and the parasitic capacitances along the pn junctions for the source and drain. The large-signal analog speed performance of analog amplifier IGFETS is similarly determined at the large-signal frequency limit and involves the non-linearities of the IGFET characteristics.
The digital speed of logic gates is defined in terms of the large-signal switching time of the transistor/load combination, thereby involving the drive current and output capacitance. Hence, analog speed performance is determined differently than digital speed performance. Optimizations for analog and digital speeds can be different, leading to different transistor parameter requirements.
Digital circuitry blocks predominantly use the smallest IGFETs that can be fabricated. Because the resultant dimensional spreads are inherently large, parameter matching in digital circuitry is often relatively poor. In contrast, good parameter matching is usually needed in analog circuitry to achieve the requisite performance. This typically requires that analog transistors be fabricated at greater dimensions than digital IGFETs subject to making analog IGFETS as short as possible in order to have source-to-drain propagation delay as low as possible.
In view of the preceding considerations, it is desirable to have a semiconductor architecture that provides IGFETs with good analog characteristics. The analog IGFETs should have high intrinsic gain, high output resistance, high small-signal speed with reduced parasitic capacitances, especially reduced parasitic capacitances along the source and drain junctions. It is also desirable that the architecture be capable of providing high-performance digital IGFETs.
GENERAL DISCLOSURE OF THE INVENTION
The present invention provides such an architecture. In accordance with the invention, a semiconductor structure contains a principal IGFET having comparatively low parasitic capacitance along at least one of the pn junctions that form source/drain boundaries. Although usable in digital applications, the principal IGFET is particularly suitable for analog applications and can achieve excellent analog performance.
The semiconductor structure of the invention may include an additional IGFET configured similar to, but of opposite polarity to, the principal IGFET. The two IGFETs thereby form a complementary-IGFET architecture especially useful for analog circuitry. The present semiconductor structure may also contain a further IGFET, or two further opposite-polarity IGFETs, particularly suitable for digital circuitry. The overall architecture can then be employed in mixed-signal ICs.
Returning to the principal IGFET, it contains a channel zone, a pair of source/drain zones, a gate dielectric layer overlying the channel zone, and a gate electrode overlying the gate dielectric layer above the channel zone. The principal IGFET is created from a semiconductor body having body material doped with semiconductor dopant of a first conductivity type so as to of the first conductivity type. The channel zone is part of the body material and thus is of the first conductivity type. The source/drain zones are situated in the semiconductor body along its upper surface and are laterally separated by the channel zone. Each source/drain zone is of a second conductivity type opposite to the first conductivity type so as to form a pn junction with the body material.
Importantly, the dopant of the first conductivity type in the body material has a concentration that decreases by at least a factor of 10, preferably at least a factor of 20, in moving upward to a specified one of the source/drain zones from an underlying body-material location no more than 10 times deeper, preferably no more than 5 times deeper, below the upper semiconductor surface than the specified source/drain zone. Alternatively stated, the concentration of the dopant of the first conductivity type in the body material increases at least 10 times, preferably at least 20 times, in moving from the specified source/drain zone downward to a body-material location no more than 10 times deeper, preferably no more than 5 times deeper, below the upper semiconductor surface than that source/drain zone. This body-material location normally lies below largely all of each of the channel and source/drain zones. By providing the body material with this “hypoabrupt” dopant distribution, the parasitic capacitance along the pn junction between the body material and the specified source/drain zone is comparatively low. The principal IGFET can thus achieve high analog performance.
The principal IGFET is normally an asymmetrical device in that the channel zone is asymmetrically longitudinally dopant graded. Specifically, the concentration of the dopant of the first conductivity type in the body material is lower where the channel zone meets the specified source/drain zone along the upper semiconductor surface than where the channel zone meets the remaining one of the source/drain zones along the upper surface. The specified source/drain zone then normally constitutes the drain during IGFET operation while the remaining source/drain zone constitutes the source. The concentration of the dopant of the first conductivity type in the body material is normally at least a factor of 10 lower, preferably at least a factor of 20 lower, where the channel zone meets the drain along the upper surface than where the channel zone meets the source along the upper surface. Alternatively stated, the concentration of the dopant of the first conductivity type in the body material is normally at least 10 times higher, preferably at least 20 times higher, where the channel zone meets the source along the upper surface than where the channel zone meets the drain along the upper surface.
The high dopant concentration along the source side of the channel zone shields the source from the comparatively high electric field in the drain because the electric field lines from the drain terminate on ionized dopant atoms which are situated in the channel zone near the source and which provide the higher channel-zone dopant concentration near the source rather than terminating on ionized dopant atoms in the depletion region along the source and detrimentally lowering the absolute value of the potential barrier for majority charge carriers coming from the source. This alleviates punchthrough. The combination of the above-mentioned hypoabrupt vertical dopant profile below the specified source/drain zone, i.e., the drain here, and the increased channel-zone dopant concentration at the source side can thereby achieve high analog performance without punchthrough failure.
The hypoabrupt vertical dopant profile below the specified source/drain zone can be implemented in various ways. In one implementation, the concentration of the dopant of the first conductivity type in the body material reaches a local maximum at the aforementioned body-material location that underlies the specified source/drain zone. The concentration of the dopant of the first conductivity type in the body material then typically progressively decreases in moving upward from this body-material location to the specified source/drain zone. Process-wise, this implementation is typically achieved by ion implanting suitable well dopant into the body material so that the well dopant reaches its maximum concentration at the aforementioned body-material location.
In another implementation, the concentration of the dopant of the first conductivity type in the body material substantially undergoes a step decrease in moving upward from the aforementioned body-material location to the specified source/drain zone. For example, the body material may contain a subsurface (buried) body-material portion and a directly overlying surface-adjoining body-material portion which extends to the upper semiconductor surface and which contains the source/drain zones. The subsurface body-material portion underlies the source/drain zones and, at its closest, is no more than 10 times deeper, preferably no more than 5 times deeper, below the upper semiconductor surface than the source/drain zones. The subsurface body-material portion can, for instance, be doped largely uniformly. The concentration of the dopant of the first conductivity type in the body material then substantially undergoes a step decrease, normally by at least a factor of 10, in crossing from the subsurface body-material portion to the surface-adjoining body-material portion and, in moving further upward through the surface-adjoining body-material portion to the specified source/drain zone, remains at least a factor of 10 lower than in the subsurface body-material portion.
To summarize, the present invention furnishes a semiconductor architecture having an IGFET, or a pair of opposite-polarity IGFETs, especially suitable for analog circuitry. The present architecture may include a further IGFET, or a pair of opposite-polarity further IGFETs, especially suitable for digital circuitry. The resultant architecture can handle mixed-signal applications very well. In short, the invention provides a substantial advance over the prior art.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a front cross-sectional view of a prior art symmetrical long n-channel IGFET.
FIG. 2 is a graph of net dopant concentration along the upper semiconductor surface as a function of longitudinal distance from the channel center for the IGFET of FIG. 1.
FIGS. 3a and 3b are graphs of absolute dopant concentration as a function of depth along vertical lines through the source/drain zones at two respective different well-doping conditions for the IGFETs of FIGS. 1, 4a, and 4b. 
FIGS. 4a and 4b are front cross-sectional views of respective prior art asymmetrical long and short n-channel IGFETs.
FIGS. 5a and 5b are graphs net dopant concentration along the upper semiconductor surface as a function of longitudinal distance from the channel center for the respective IGFETs of FIGS. 4a and 4b. 
FIG. 6 is a front cross-sectional view of an asymmetrical long n-channel IGFET configured according to the invention so as to have a semiconductor well portion of the same conductivity type as directly underlying semiconductor material.
FIGS. 7a-7c are respective graphs of individual, absolute, and net dopant concentrations as a function of longitudinal distance along the upper semiconductor surface for the IGFET of FIG. 6, 18a, 61a, or 61b. 
FIGS. 8a-8c are respective graphs of individual, absolute, and net dopant concentrations as a function of depth along a vertical line through the source of the IGFET of FIG. 6, 11, or 13.
FIGS. 9a-9c are respective graphs of individual, absolute, and net dopant concentrations as a function of depth along a pair of vertical lines through the channel zone of the IGFET of FIG. 6, 11, 13, or 15.
FIGS. 10a-10c are respective graphs of individual, absolute, and net dopant concentrations as a function of depth along a vertical line through the drain of the IGFET of FIG. 6, 11, 13, 18a, or 18b. 
FIG. 11 is a front cross-sectional view of an asymmetrical short n-channel IGFET configured according to the invention so as to have a semiconductor well portion of the same conductivity type as directly underlying semiconductor material.
FIGS. 12a-12c are respective graphs of individual, absolute, and net dopant concentrations as a function of longitudinal distance along the upper semiconductor surface for the IGFET of FIG. 11.
FIG. 13 is a front cross-sectional view of another asymmetrical long n-channel IGFET configured according to the invention so as to have a semiconductor well portion of the same conductivity type as directly underlying semiconductor material.
FIGS. 14a-14c are respective graphs of individual, absolute, and net dopant concentrations as a function of longitudinal distance along the upper semiconductor surface for the IGFET of FIG. 13, 15, 18b, or 18c. 
FIG. 15 is front cross-sectional view of a further asymmetrical long n-channel IGFET configured according to the invention so as to have a semiconductor well portion of the same conductivity type as directly underlying semiconductor material.
FIGS. 16a-16c are respective graphs of individual, absolute, and net dopant concentrations as a function of depth along a vertical line extending through the source of the IGFET of FIG. 15.
FIGS. 17a-17c are respective graphs of individual, absolute, and net dopant concentration as a function of depth along a vertical line extending through the drain of the IGFET of FIG. 15 or 18c. 
FIGS. 18a-18c are front cross-sectional views of three respective long n-channel IGFETs configured according to the invention so as to each have a semiconductor well portion of the same conductivity type as directly underlying semiconductor material.
FIGS. 19a-19c are respective graphs of individual, absolute, and net dopant concentrations as a function of depth along a vertical line extending through the source of the IGFET of FIG. 18a or 18b. 
FIGS. 20a-20c are respective graphs of individual, absolute, and net dopant concentrations as a function of depth along a vertical line extending through the source of the IGFET of FIG. 18c. 
FIG. 21 is a front cross-sectional view of an asymmetrical long n-channel IGFET configured according to the invention so as to have a semiconductor well portion of opposite conductivity type to directly underlying semiconductor material.
FIGS. 22a-22c are respective graphs of individual, absolute, and net dopant concentrations as a function of longitudinal distance along the upper semiconductor surface for the IGFET of FIG. 21 or 27a. 
FIGS. 23a-23c are respective graphs of individual, absolute, and net dopant concentrations as a function of depth along a vertical line through the source of the IGFET of FIG. 21 or 25
FIGS. 24a-24c are respective graphs of individual, absolute, and net dopant concentrations as a function of depth along a vertical line through the drain of the IGFET of FIG. 21, 25, 27a, or 27b. 
FIG. 25 is a front cross-sectional view of another asymmetrical long n-channel IGFET configured according to the invention so as to have a semiconductor well portion of opposite conductivity type to directly underlying semiconductor material.
FIGS. 26a-26c are respective graphs of individual, absolute, and net dopant concentrations as a function of longitudinal distance along the upper semiconductor surface for the IGFET of FIG. 25 or 27b. 
FIGS. 27a and 27b are front cross-sectional views of two respective long n-channel IGFETs configured according to the invention so as to each have a semiconductor well portion of opposite conductivity type to directly underlying semiconductor material.
FIGS. 28a-28c are respective graphs of individual, absolute, and net dopant concentrations as a function of depth along a vertical line extending through the source of the IGFET of FIG. 27a or 27b. 
FIGS. 29.1 and 29.2 are front cross-sectional views of two portions of a complementary-IGFET structure configured according to the invention.
FIGS. 30.1 and 30.2 are front cross-sectional views of two portions of another complementary-IGFET structure configured according to the invention.
FIGS. 31a-31o, 31p.1-31r.1, and 31p.2-31r2 are front cross-sectional views representing steps in manufacturing the complementary-IGFET structure of FIGS. 29.1 and 29.2. The steps of FIGS. 31a-31o apply to the structural portions illustrated in both of FIGS. 29.1 and 29.2. FIGS. 31p1-31r1 present further steps leading to the structural portion of FIG. 29.1. FIGS. 31p2-31r.2 present further steps leading to the structural portion of FIG. 29.2.
FIGS. 32 and 33 are three-dimensional graphs of net dopant concentration as a function of depth and longitudinal distance for respective computer simulations of (i) an asymmetrical short re-channel IGFET configured according to the invention and (ii) a reference symmetrical short re-channel IGFET.
FIGS. 34 and 35 are graphs presenting dopant contours as a function of depth and longitudinal distance from a source location for the respective computer-simulated IGFETs of FIGS. 32 and 33.
FIG. 36 is a graph of net dopant concentration as a function of longitudinal distance from a source location for the computer-simulated IGFETs of FIGS. 32 and 33.
FIGS. 37a and 37b are respective graphs of absolute and net dopant concentrations as a function of depth along vertical lines through the source and drain for the computer-simulated IGFETs of FIGS. 32 and 33.
FIGS. 38a and 38b are graphs of lineal transconductance and lineal drain current as a function of gate-to-source voltage respectively at threshold and saturation conditions for the computer-simulated IGFETs of FIGS. 32 and 33.
FIGS. 39a and 39b are graphs of lineal transconductance and lineal drain current as a function of gate-to-source voltage respectively at threshold and saturation conditions for computer simulations of (i) an inventive asymmetrical long n-channel IGFET generally corresponding to the inventive short-channel IGFET of FIG. 32 and (ii) a reference symmetrical long n-channel IGFET generally corresponding to the reference short-channel IGFET of FIG. 33.
FIG. 40 is a graph of lineal drain current density as a function of gate-to-source voltage for computer simulations of (i) the inventive IGFET of FIG. 32, (ii) the reference IGFET of FIG. 33, and (iii) a further reference symmetrical short n-channel IGFET lacking an anti-punchthrough implant.
FIG. 41 is a graph of lineal drain current as a function of drain-to-source voltage for the computer-simulated IGFETs of FIGS. 32 and 33.
FIG. 42 is a circuit diagram of an n-channel IGFET and associated parasitic capacitances.
FIG. 43 is a circuit diagram of a small-signal model of the n-channel IGFET and associated parasitic capacitances of FIG. 42.
FIGS. 44a-44c are circuit diagrams of single-IGFET amplifiers arranged respectively in common-source, common-gate, and common-drain configurations.
FIG. 45 is a circuit diagram of a single-IGFET amplifier arranged in a common-source shorted-output configuration.
FIG. 46 is a circuit diagram of a small-signal model of the amplifier of FIG. 45.
FIG. 47 is a graph of net dopant concentration as a function of distance from a pn junction for models of three different p-type dopant distributions.
FIG. 48 is a graph of depletion-layer capacitance as a function of reverse voltage for the models of the three dopant distributions of FIG. 47.
FIG. 49 is a graph of net body dopant concentration as a function of distance from a pn junction for a model of a junction capacitor whose more lightly doped side has a dopant profile that undergoes a step change in dopant concentration.
FIG. 50 is graph of areal junction capacitance as a function of reverse voltage for the junction capacitor modeled in FIG. 49.
FIGS. 51a and 51b are composite graphs/front cross-sectional views of dopant contours as a function of depth and longitudinal distance from the channel center for computer simulations of respective asymmetrical short and long n-channel IGFETs configured according to the invention.
FIG. 52 is a graph of lineal drain-to-body capacitance as a function of drain-to-body voltage for the computer-simulated IGFETs of FIGS. 32 and 33.
FIG. 53 is a graph of lineal source-to-body capacitance as a function of source-to-body voltage for the computer-simulated IGFETs of FIGS. 32 and 33.
FIG. 54 is a graph of cut-off frequency as a function of lineal drain current for the computer-simulated IGFETs of FIGS. 32 and 33 and the further inventive IGFET of FIG. 56.
FIG. 55 is a graph of cut-off frequency as a function of lineal drain current for computer simulations of (i) an inventive asymmetrical long n-channel IGFET corresponding to the inventive short-channel IGFET of FIG. 32, (ii) a reference symmetrical long n-channel IGFET corresponding to the reference short-channel IGFET of FIG. 33, and (iii) a further inventive asymmetrical long n-channel IGFET corresponding to the further inventive short-channel IGFET of FIG. 56.
FIG. 56 is a front cross-sectional view of another computer-simulated asymmetrical short n-channel IGFET configured according to the invention.
FIG. 57 is a graph of net dopant concentration as a function of longitudinal distance from a source location for the computer-simulated IGFET of FIGS. 32 and 56.
FIG. 58 is a graph of threshold voltage as a function of channel length for (i) asymmetrical n-channel IGFETs configured according to the invention, (ii) reference symmetrical re-channel IGFETs having halo pocket portions along each source/drain zone, and (iii) reference symmetrical n-channel IGFETs lacking a halo pocket along each source/drain zone.
FIG. 59 is a front cross-sectional view of an additional complementary-IGFET structure configured according to the invention.
FIG. 60 is a graph of absolute dopant concentration as a function of depth for (i) two asymmetrical n-channel IGFETs configured according to the invention and (ii) a reference symmetrical n-channel IGFET.
FIGS. 61a and 61b are front cross-sectional views of two respective further asymmetrical long n-channel IGFETs configured according to the invention.
FIGS. 62a-62c are respective graphs of individual, absolute, and net dopant concentrations as a function of depth along a vertical line through the source of the IGFET of FIG. 61a or 61b. 
FIGS. 63a-63c are respective graphs of individual, absolute, and net dopant concentrations as a function of depth along a pair of vertical lines through the channel zone of the IGFET of FIG. 61a or 61b. 
FIGS. 64a-64c are respective graphs of individual, absolute, and net dopant concentrations as a function of depth along a vertical line through the drain of the IGFET of FIG. 61a or 61b. 
FIGS. 65a-65d are front cross-sectional views of four additional respective complementary-IGFET structures configured according to the invention.



Like reference symbols are employed in the drawings and in the description of the preferred embodiment to represent the same, or very similar, item or items. The numerical portions of reference symbols having single prime (′), double prime (′′), asterisk (*), and pound (#) signs in drawings containing graphs respectively indicate like-numbered regions or zones in other drawings. “Xs” in a cross-sectional view of an IGFET provided with a well dopant indicate the location of the maximum concentration of the well dopant. Electrically insulating spacers (not shown) may be situated along the sidewalls of the gate electrodes of the IGFETs of FIGS. 13, 15, 18b, 18c, 25, and 27b depending on how those IGFETs are fabricated.
In the dopant-distribution graphs, “individual” dopant concentrations mean the individual concentrations of each separately introduced n-type dopant and each separately introduced p-type dopant while “absolute” dopant concentrations mean the total n-type dopant concentration and the total p-type dopant concentration. The “net” dopant concentration in the dopant-distribution graphs is the difference between the absolute (or total) n-type dopant concentration and the absolute (or total) p-type dopant concentration. The net dopant concentration is indicated as net “n-type” where the absolute n-type dopant concentration exceeds the absolute p-type dopant concentration, and as net “p-type” where the absolute p-type dopant concentration exceeds the absolute n-type dopant concentration.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Reference Notation and Other Conventions
The reference symbols employed below and in the drawings have the following meanings where the adjective “lineal” means per unit IGFET width and where the adjective “areal” means per unit lateral area:













AI ≡
current gain


Cda ≡
areal depletion-region capacitance


Cd0a ≡
value of areal depletion-region capacitance at zero reverse 



voltage


CDB ≡
drain-to-body capacitance


CDBw ≡
lineal drain-to-body capacitance


CGB ≡
gate-to-body capacitance


CGD ≡
gate-to-drain capacitance


CGIa ≡
areal gate dielectric capacitance


CGS ≡
gate-to-source capacitance


CL ≡
load capacitance


CSB ≡
source-to-body capacitance


CSBw ≡
lineal source-to-body capacitance


f ≡
frequency


fT ≡
cut-off frequency


fTpeak ≡
peak value of cut-off frequency


gm ≡
intrinsic transconductance of IGFET


gmw ≡
lineal transconductance of IGFET


gmb ≡
transconductance of body electrode


gmeff ≡
effective transconductance of IGFET in presence of source 



resistance


gmsatw ≡
lineal transconductance of IGFET at saturation


HA ≡
amplifier transfer function


ID ≡
drain current


IDw ≡
lineal drain current


ID0w ≡
leakage value of lineal drain current at zero gate-to-source 



voltage


ii ≡
small-signal input current


io ≡
small-signal output current


KS ≡
relative permittivity of semiconductor material


k ≡
Boltzmann's constant


L ≡
length of channel


LG ≡
length of gate electrode


LGDoverlap ≡
longitudinal distance that gate electrode overlaps drain


LGSoverlap ≡
longitudinal distance that gate electrode overlaps source


NA ≡
acceptor dopant concentration


NB ≡
net dopant concentration in body material


NB0, NB0′ ≡
values of net dopant concentration in junction-adjoining 



constant-concentration portion of material on more lightly 



doped side of pn junction


NB1, NB1′ ≡
values of net dopant concentration in junction-remote 



constant-concentration portion of material on more lightly 



doped side of pn junction


ND ≡
donor dopant concentration


ND0 ≡
value of net dopant concentration in constant-concentration 



material on more heavily doped side of pn junction


NI ≡
individual dopant concentration


NN ≡
net dopant concentration


NT ≡
absolute dopant concentration


ni ≡
intrinsic carrier concentration


q ≡
electronic charge


RD ≡
series resistance at drain of IGFET


RG ≡
series resistance at gate electrode of IGFET


Ron ≡
linear-region on-resistance of IGFET


RS ≡
series resistance at source of IGFET


s ≡
transform variable


T ≡
temperature


td ≡
depletion-region thickness


td0 ≡
value of depletion-region thickness at zero reverse voltage


tGI ≡
gate dielectric thickness


VBI ≡
built-in voltage


VBS ≡
DC body-to-source voltage


VDB ≡
DC drain-to-body voltage


VDD ≡
high supply voltage


VDS ≡
DC drain-to-source voltage


VGS ≡
DC gate-to-source voltage


Vg ≡
gate voltage amplitude


Vin ≡
input voltage amplitude


Vout ≡
output voltage amplitude


VR ≡
DC reverse voltage


VRmax ≡
maximum value of DC reverse voltage


VSB ≡
DC source-to-body voltage


VSS ≡
low supply voltage


VT ≡
threshold voltage


vgs ≡
small-signal gate-to-source voltage


vnsat ≡
electron saturation velocity


W ≡
channel width


x ≡
longitudinal distance


y ≡
depth, vertical distance, or distance from pn junction


yd ≡
value of distance from pn junction to remote boundary of 



depletion region in body material


yd0 ≡
value of distance from pn junction to remote boundary of 



junction-adjoining constant-concentration portion of body 



material having step change in uniform net dopant 



concentration


ydmax ≡
value of distance from pn junction to remote boundary of 



junction-remote constant-concentration portion of body 



material having step change in uniform net dopant 



concentration


yD ≡
value of depth at bottom of drain


yS ≡
value of depth at bottom of source


yST ≡
value of depth at remote boundary of junction-adjoining 



upper body-material portion


yW ≡
value of depth at location of maximum concentration of 



well dopant


ε0 ≡
permittivity of free space (vacuum)


μn ≡
electron mobility


ω ≡
angular frequency


ωin ≡
value of angular frequency at input pole


ωout ≡
value of angular frequency at output pole


ωz ≡
value of angular frequency at zero


ωp ≡
value of angular frequency at pole









Long-channel and short-channel n-channel IGFETs are respectively referred to here, i.e., both below and above, as long and short n-channel IGFETs. Similarly, long-channel and short-channel p-channel IGFETs are respectively referred to here as long and short p-channel IGFETs. As used below, the term “surface-adjoining” means adjoining (or extending to) the upper semiconductor surface, i.e., the upper surface of a semiconductor body consisting of monocrystalline, or largely monocrystalline, semiconductor material.
No particular channel-length value generally separates the short-channel and long-channel regimes of IGFET operation or generally distinguishes a short-channel IGFET from a long-channel IGFET. A short-channel IGFET, or an IGFET operating in the short-channel regime, is simply an IGFET whose characteristics are significantly affected by short-channel effects. A long-channel IGFET, or an IGFET operating in the long-channel regime, is the converse of a short-channel IGFET. While the channel length value of approximately 0.4 mm roughly constitutes the boundary between the short-channel and long-channel regimes for the background art in U.S. Pat. No. 6,548,842B1, the long-channel/short-channel boundary can occur at a higher or lower value of channel length depending on various factors such as gate dielectric thickness, minimum printable feature size, channel zone dopant concentration, and source/drain-body junction depth.
IGFETs in which Vertical Body-Material Dopant Profile Below Drain is Hypoabrupt Due to Subsurface Maximum in Well Dopant Concentration
FIG. 6 illustrates an asymmetrical long n-channel IGFET 100 configured in accordance with the invention so as to be especially suitable for high-speed analog applications. Long-channel IGFET 100 is created from a monocrystalline silicon (“monosilicon”) semiconductor body in which a pair of very heavily doped n-type source/drain zones 102 and 104 are situated along the upper semiconductor surface. Source/drain zones 102 and 104 are generally referred to below respectively as source 102 and drain 104 because they normally, though not necessarily, respectively function as source and drain.
Drain 104 is normally doped slightly more heavily than source 102. The maximum value of net dopant concentration NN in source 102 along the upper semiconductor surface is normally at least 1×1020 atoms/cm3, typically 4×1020 atoms/cm3. The maximum value of concentration NN in drain 104 along the upper surface is normally at least 1×1020 atoms/cm3, typically slightly greater than 4×1020 atoms/cm3 so as to slightly exceed the maximum upper-surface NN concentration in source 102. However, as discussed below in connection with the inventive IGFET of FIG. 56, drain 104 is sometimes doped more lightly than source 102. For example, the maximum value of concentration NN in drain 104 along the upper surface can be 5×1019 atoms/cm3 and can go down to at least as little as 1×1019 atoms/cm3 when maximum upper-surface NN concentration in source 102 is at least 1×1020 atoms/cm3.
Source 102 extends to a distance yS below the upper semiconductor surface. Drain 104 extends to a depth yD below the upper semiconductor surface. Source depth yS is normally 0.1-0.2 μm, typically 0.15 μm. Drain depth yD is normally 0.15-0.3 μm, typically 0.2 μm. Drain depth yD thus normally exceeds source depth yS, typically by 0.05-0.1 μm.
Source/drain zones 102 and 104 are laterally separated by an asymmetrical channel zone 106 of p-type body material 108 that forms (a) a source-body pn junction 110 with source 102 and (b) a drain-body pn junction 112 with drain 104. P-type body material 108 consists of a lightly doped lower portion 114, a heavily doped intermediate well portion 116, and an upper portion 118 that typically extends deeper below the upper semiconductor surface than source 102 and drain 104. Upper body-material portion 118 thereby typically contains all of channel zone 106.
P+ well portion 116 is defined by p-type semiconductor well dopant distributed vertically in an approximately Gaussian manner so as to reach a maximum subsurface concentration at depth yw below the upper semiconductor surface. The “Xs” in FIG. 6 generally indicate the location of the maximum subsurface concentration of the p-type well dopant. The concentration of the p-type well dopant at depth yW is normally 1×1018-1×1019 atoms/cm3, typically 5×1018 atoms/cm3. Maximum well-concentration depth yW, which exceeds source depth yS and drain depth yD, is normally 0.5-1.0 μm, typically 0.7 μm. Additionally, depth yW is normally no more than 10 times, preferably no more than 5 times, drain depth yD. That is, the location of the maximum concentration of the p-type well dopant is no more than 10 times, preferably no more than 5 times, deeper below the upper surface than drain 104.
The upper and lower boundaries of heavily doped well portion 116 are somewhat imprecise because well 116 is situated in doped semiconductor material of the same conductivity type (p type) as well 116. The semiconductor material that bounds well 116 does, as indicated below, have a low p-type background dopant concentration which is normally relatively uniform. The upper and lower boundaries of well 116 are typically defined as the locations where the concentration of the p-type well dopant equals the p-type background dopant concentration. Aside from any location where well 116 extends into other p-type material doped more heavily than well 116, the concentration of the total p-type dopant along the upper and lower boundaries of well 116 is then twice the p-type background dopant concentration. Under these boundary definitions, the upper boundary of well 116 is normally 0.2-0.5 μm, typically 0.3 μm, below the upper semiconductor surface. The lower boundary of well 116 is normally 0.9-1.3 μm, typically 1.1 μm, below the upper surface.
A depletion region (not shown) extends along the upper semiconductor surface from source-body pn junction 110 across channel zone 106 to drain-body pn junction 112. The average thickness of the surface depletion region is normally less than 0.1 um, typically in the vicinity of 0.05 um. Although the upper and lower boundaries of well portion 116 are somewhat imprecise, the concentration of the p-type well dopant normally drops to an electrically insignificant level at a depth less than 0.1 μm below the upper surface. Accordingly, well 116 is substantially located below the surface depletion region.
P-type upper body-material portion 118 includes a heavily doped pocket portion 120 that extends along source 102 up to the upper semiconductor surface and terminates at a location between source 102 and drain 104. FIG. 6 illustrates the example in which p+ pocket portion 120 extends deeper below the upper surface than source 102 and drain 104. In particular, FIG. 6 depicts the example in which pocket portion 120 extends laterally below source 102 and largely reaches p+ well portion 116. As discussed below in connection with FIGS. 18a-18c, pocket portion 120 can extend to a lesser depth below the upper surface than shown in FIG. 6. The remainder of p-type upper body-material portion 118, i.e., the part outside pocket portion 120, is indicated as item 124 in FIG. 6. Upper body-material remainder 124 is lightly doped and extends along drain 104. Channel zone 106, which consists of all the p-type semiconductor material between source 102 and drain 104, is thereby formed by part of source-side p+ pocket portion 120 and part of drain-side p− upper body-material remainder 124.
A gate dielectric layer 126 is situated on the upper semiconductor surface and extends over channel zone 106. A gate electrode 128 is situated on gate dielectric layer 126 above channel zone 106. Gate electrode 128 extends partially over source 102 and drain 104. In the example of FIG. 6, gate electrode 128 consists of polycrystalline silicon (“polysilicon”) doped very heavily n type. Electrode 128 can be formed with other electrically conductive material such as metal or polysilicon doped sufficiently p type as to be electrically conductive.
The upper surfaces of source 102, drain 104, and gate electrode 128 are typically respectively provided with thin layers (not shown in FIG. 6) of electrically conductive metal silicide to facilitate making electrical contact to regions 102, 104, and 128. In that case, gate electrode 128 and the overlying metal silicide layer form a composite gate electrode. Source 102, drain 104, and channel zone 106 are typically laterally surrounded by an electrical insulating field region (likewise not shown in FIG. 6) recessed into the upper semiconductor surface to define an active semiconductor island that contains regions 102, 104, and 106. Examples of the metal silicide layers and the field-insulating region are presented below in connection with FIGS. 29.1 and 29.2.
The presence of p+ pocket portion 120 along source 102 causes channel zone 106 to be graded longitudinally, i.e., in the direction of the channel length, with respect to channel dopant concentration. Because a substantial mirror image of source-side pocket portion 120 is not situated along drain 104, channel zone 106 is asymmetrically dopant graded in the longitudinal direction. P+ well portion 116 is situated below p− upper body-material remainder 124 that extends along drain 104. This configuration of p+ well 116 and p− upper body-material remainder 124 causes the vertical dopant profile in the portion of body material 108 underlying drain 104 to be hypoabrupt. That is, the concentration of the p-type dopant increases greatly, normally by at least a factor of 10, in going from drain-body junction 112 downward through p− upper body-material remainder 124 and into p+ well 116. The combination of the longitudinally asymmetrical dopant grading in channel zone 106 and the hypoabrupt vertical dopant profile through drain 104 in the portion of body material 108 underlying drain 104 enables IGFET 100 to have very good analog characteristics while avoiding punchthrough.
An understanding of the longitudinally asymmetrical dopant grading in channel zone 106 and the hypoabrupt vertical dopant profile in the portion of body material 108 underlying drain 104 is facilitated with the assistance of FIGS. 7a-7c, 8a-8c, 9a-9c, and 10a-10c. FIGS. 7a-7c present exemplary dopant concentrations along the upper semiconductor surface as a function of longitudinal distance x. Exemplary dopant concentrations as a function of depth y along a vertical line 130 through source 102 are presented in FIGS. 8a-8c. FIGS. 9a-9c present exemplary dopant concentrations as a function of depth y along a pair of vertical lines 132 and 134 through channel zone 106. Vertical line 132 passes through source-side pocket portion 120. Vertical line 134 passes through a vertical location between pocket portion 120 and drain 104. Exemplary dopant concentrations as a function of depth y along a vertical line 136 through drain 104 are presented in FIGS. 10a-10c. 
FIG. 7a specifically illustrates concentrations NI, along the upper semiconductor surface, of the individual semiconductor dopants that largely define regions 102, 104, 120, and 124 and thus establish the longitudinal dopant grading of channel zone 106. FIGS. 8a, 9a, and 10a specifically illustrate concentration NI, along vertical lines 130, 132, 134, and 136, of the individual semiconductor dopants that vertically define regions 102, 104, 114, 116, 120, and 124 and thus establish the hypoabrupt vertical dopant profile in the portion of body material 108 underlying drain 104. Curves 102′ and 104′ represent concentrations NI (surface and vertical) of the n-type dopant used to respectively form source 102 and drain 104. Curves 114′, 116′, 120′, and 124′ represent concentrations NI (surface and/or vertical) of the p-type dopants used to respectively form regions 114, 116, 120, and 124. Items 110# and 112# indicate where net dopant concentration NN goes to zero and thus respectively indicate the locations of pn junctions 110 and 112.
Concentrations NT of the total p-type and total n-type dopants in regions 102, 104, 120, and 124 along the upper semiconductor surface are shown in FIG. 7b. FIGS. 8b, 9b, and 10b depict, along vertical lines 130, 132, 134, and 136, concentrations NT of the total p-type and total n-type dopants in regions 102, 104, 114, 116, 120, and 124. Curve segments 114′′, 116′′, 120′′, and 124′′ respectively corresponding to regions 114, 116, 120, and 124 represent total concentrations NT of the p-type dopant. Item 106′′ in FIG. 7b corresponds to channel zone 106 and represents the channel-zone portions of curve segments 120′′ and 124′′. Total concentrations NT of the n-type dopant are represented by curves 102′′ and 104′′ respectively corresponding to source 102 and drain 104. Curves 102′′ and 104′′ in FIG. 7b are respectively identical to curves 102′ and 104′ in FIG. 7a. Curves 102′′ and 104′′ in FIGS. 8b and 10b are respectively identical to curves 102′ and 104′ in FIGS. 8a and 10a. 
FIG. 7c illustrates net dopant concentration NN along the upper semiconductor surface. Net dopant concentration NN along vertical lines 130, 132, 134, and 136 is presented in FIGS. 8c, 9c, and 10c. Curve segments 114*, 116*, 120*, and 124* represent net concentrations NN of the p-type dopant in respective regions 114, 116, 120, and 124. Item 106* in FIG. 7c represents the combination of channel-zone curve segments 120* and 124* and thus presents concentration NN of the net p-type dopant in channel zone 106. Concentrations NN of the net n-type dopant in source 102 and drain 104 are respectively represented by curves 102* and 104*.
With the foregoing general comments about FIGS. 7a-7c, 8a-8c, 9a-9c, and 10a-10c in mind, FIG. 7a indicates that the p-type dopant in source-side pocket portion 120 has two primary components, i.e., components provided in two separate doping operations, along the upper semiconductor surface. One of the primary components of the p-type dopant in pocket portion 120 along the upper surface is the p-type background dopant represented by curve 124′ in FIG. 7a. The p-type background dopant is normally present at a low, largely uniform, concentration throughout all of the monosilicon material including regions 102, 104, 114, 116, and 120. Below pocket 120 and upper body-material remainder 124, the p-type background dopant is represented by curve segment 114′ as indicated in FIGS. 8a, 9a, and 10a. The concentration of the p-type background dopant is normally 1×1015-1×1016 atoms/cm3, typically 5×1015 atoms/cm3.
The other primary component of the p-type dopant in source-side pocket portion 120 is the p-type pocket (or channel-grading) dopant indicated by curve 120′ in FIG. 7a. The p-type pocket dopant is provided at a high upper-surface concentration, normally 5×1017-2×1018 atoms/cm3, typically 1×1018 atoms/cm3, to define the shape of pocket portion 120. The specific value of the upper-surface concentration of the p-type pocket dopant is critically adjusted, typically within 10% accuracy, to set the threshold voltage of IGFET 100.
The boundary of source-side pocket portion 120 consists of (a) a section of the upper semiconductor surface, (b) a pn junction section formed by source-body junction 110, and (c) a p-type section of body material 108. Although the p-type section of the boundary of pocket 120 is somewhat imprecise, the p-type pocket section is typically defined as the location where the concentration of the p-type pocket dopant equals the concentration of the p-type background dopant. To the extent that pocket 120 does not intrude into well portion 116, the p-type dopant concentration along the p-type section of the boundary of pocket 120 is then twice the background dopant concentration, including where the p-type pocket-portion boundary section meets the upper semiconductor surface.
The p-type pocket dopant is also present in source 102 as indicated by curve 120′ in FIG. 7a. Concentration NI of the p-type pocket dopant in source 102 is substantially constant along its upper surface. In moving from source 102 longitudinally along the upper semiconductor surface into channel zone 106, concentration NI of the p-type pocket dopant is at the substantially constant upper-surface source level partway into zone 106 and then drops from that level essentially to zero at a location between source 102 and drain 104.
With the total p-type dopant in channel zone 106 along the upper semiconductor surface being the sum of the p-type background and pocket dopants along the upper surface, the total p-type channel-zone dopant along the upper surface is represented by curve segment 106′′ in FIG. 7b. The variation in curve segment 106′′ shows that, in moving longitudinally across channel zone 106 from source 102 to drain 104, concentration NT of the total p-type dopant in zone 106 along the upper surface is at a substantially constant high level partway into zone 106, decreases from the high level to the low p-type background level at a location between source 102 and drain 104, and then remains at the low background level for the rest of the distance to drain 104.
Concentration NI of the p-type pocket dopant in source 102 may, in some embodiments, be at the substantially constant source level along only part of the upper surface of source 102 and may then decrease in moving longitudinally along the upper semiconductor surface from a location within the upper surface of source 102 to source-body junction 110. In that case, concentration NI of the p-type pocket dopant in channel zone 106 starts decreasing immediately after crossing source-body junction 110 in moving longitudinally across zone 106 toward drain 104. Accordingly, concentration NT of the total p-type dopant in zone 106 along the upper surface similarly starts to decrease immediately after crossing junction 110 in moving longitudinally across zone 106 from source 102 to drain 104 rather than being at the substantially constant source level partway into zone 106.
Regardless of whether concentration NI of the p-type pocket dopant in channel zone 106 along the upper semiconductor surface is, or is not, at the substantially source level for a non-zero distance from source-body junction 110 longitudinally into zone 106, concentration NT of the total p-type dopant in zone 106 along the upper surface is lower where zone 106 meets drain 104 than where zone 106 meets source 102. In particular, concentration NT of the total p-type dopant in zone 106 is normally at least a factor of 10 lower, preferably at least a factor of 20 lower, more preferably at least a factor of 50 lower, typically a factor of 100 or more lower, at drain-body junction 112 along the upper surface than at source-body junction 110 along the upper surface.
FIG. 7c shows that, as represented by curve 106*, concentration NN of the net p-type dopant in channel zone 106 along the upper semiconductor surface varies in a similar manner to concentration NT of the total p-type dopant in zone 106 along the upper surface except that net dopant concentration NN of the net p-type dopant in zone 106 along the upper surface drops to zero at junctions 110 and 112. The source side of channel zone 106 thus has a high net amount of p-type dopant compared to the drain side. The high source-side amount of p-type dopant in channel zone 106 causes the thickness of the channel-side portion of the depletion region along the source-body junction 110 to be reduced.
Also, the high p-type dopant concentration along the source side of channel zone 106 shields source 102 from the comparatively high electric field in drain 104. This occurs because the electric field lines from drain 104 terminate on ionized p-type dopant atoms in pocket portion 120 instead of terminating on ionized dopant atoms in the depletion region along source 102 and detrimentally lowering the potential barrier for electrons. The depletion region along junction 110 is thereby inhibited from punching through to the depletion region along drain-body junction 112. By appropriately choosing the amount of the high source-side p-type dopant in channel zone 106, punchthrough is avoided in IGFET 100.
The p-type dopant in the portion of body material 108 below source 102, channel zone 106, and drain 104 has three primary components as indicated in FIGS. 8a, 9a, and 10a. One of the primary components of the p-type dopant in the body-material portion below regions 102, 104, and 106 is the p-type background dopant represented by curve segment 124′ or 114′ in FIGS. 8a, 9a, and 10a. The second primary component is the p-type well dopant that defines well portion 116 as indicated by curve 116′ in FIGS. 8a, 9a, and 10a. 
The final primary component of the p-type dopant in the body-material portion below regions 102, 104, and 106 is the p-type pocket dopant indicated by curve 120′ in FIGS. 8a and 9a. The p-type pocket dopant is substantially present only in the portion of body material 108 below source 102 and an adjoining part of channel zone 106. The amount of p-type pocket dopant present in the portion of body material 108 below drain 104 is either essentially zero or is so low as to be substantially electrically insignificant. Hence, the total p-type pocket dopant in the portion of body material 108 below drain 104 consists substantially only of the p-type well and background dopants as indicated respectively by curves 116′ and 124′ or 114′ taken along vertical line 136 in FIG. 10a. 
The total p-type dopant in the portion of body material 108 below drain 104 is indicated by curve segment 116′′ and its extensions 124′′ (upward) and 114′′ (downward) in FIG. 10b. Curve segment 116′′ represents the sum of the p-type well and background dopants in well portion 116. Curve segments 114′′ and 124′′ respectively correspond to p− lower body-material portion 114 and p− upper body-material remainder 124. Because concentration NI of the p-type background dopant is relatively uniform, concentration NT of the total p-type dopant in the portion of body material 108 below drain 104 reaches a maximum value at a subsurface location substantially equal to depth yW, i.e., substantially where the p-type well dopant reaches its maximum concentration.
As shown by the variation in combined curve segment 116′′/124′′ in FIG. 10b, concentration NT of the total p-type dopant in the portion of body material 108 below drain 104 decreases hypoabruptly by at least a factor of 10 in moving upward along vertical line 136 to drain 104 from the subsurface location of the maximum concentration of the p-type dopant in well portion 116. Concentration NT of the total p-type dopant in the portion of body material 108 below drain 104 preferably decreases by at least a factor of 20, more preferably by at least a factor of 40, even more preferably by at least a factor of 80, typically by a factor in the vicinity of 100 or more, in moving from the location of the maximum p-type well concentration up to drain 104. Additionally, concentration NT of the total p-type dopant in the portion of body material 108 below drain 104 normally decreases progressively in moving from the location of the maximum p-type well concentration up to drain 104 as indicated by combined curve segment 116′′/124′′.
The net dopant in the portion of body material 108 below drain 104 is p-type dopant.
FIG. 10c shows that, as represented by the combination of curve segments 116* and 124*, concentration NN of the net dopant in the portion of body material 108 below drain 104 varies vertically in a similar manner to concentration NT of the total p-type dopant in the portion of body material 108 below drain 104 except that concentration NN in the portion of body material 108 below drain 104 drops to zero at drain-body junction 112. The hypoabrupt vertical dopant profile in the portion of body material 108 below drain 104 causes the parasitic capacitance associated with junction 112 to decrease for the reasons discussed further below. This enables IGFET 100 to have increased analog speed.
Moving to the dopant distribution along vertical line 130 through source 102, the total p-type dopant in the portion of body material 108 below source 102 consists of the p-type well, background, and pocket dopants as indicated respectively by curves 116′, 124′, and 120′ in FIG. 8a. For the example of FIG. 6 in which source-side pocket portion 120 meets well 116 below source 102 so that concentrations NI of the p-type well and pocket dopants both exceed the p-type background concentration at the meeting location, the total p-type dopant in the portion of body material 108 below source 102 is indicated by the combination of curve segments 116′′ and 120′′ in FIG. 8b. In an embodiment where pocket 120 extends below source 102 but does not meet well portion 116, the total p-type dopant in the portion of body material 108 below source 102 would be indicated by the combination of curve segments 116′′ and 120′′ and a curve segment corresponding to p− upper body-material remainder 124.
As illustrated by combined curve segment 116′′/120′′ in FIG. 8b, concentration NT of the total p-type dopant in the portion of body material 108 below source 102 initially decreases by a factor of 10, typically by a factor in the vicinity 30, in moving upward through body material 108 from the subsurface location of the maximum concentration of the p-type dopant in well portion 116. Upon reaching a local minimum below source 102, concentration NT of the total p-type dopant in the portion of body material 108 below source 102 then rises before reaching source 102.
The net dopant in the portion of body material 108 below source 102 is p-type dopant.
FIG. 8c shows that, as represented by the combination of curve segments 116* and 120* for the example of FIG. 6 in which source-side pocket portion 120 meets well portion 116, concentration NN of the net dopant in the portion of body material 108 below source 102 varies vertically in a similar manner to concentration NT of the total p-type dopant in the portion of body material 108 below source 102 except that net dopant concentration NN in the portion of body material 108 below source 102 drops to zero at source-body junction 110. Depending on various factors such as the amount by which the depth of pocket 120 exceeds the depth of source 102, this vertical dopant profile below source 102 sometimes causes the parasitic capacitance along source-body junction 110 to be reduced, albeit normally less than the reduction in the parasitic capacitance along drain-body junction 112.
Turning briefly to FIGS. 9a-9c dealing with concentrations along vertical lines 132 and 134 through channel zone 106, the parenthetical entry “132” after reference symbols 120′, 120′′, and 120* indicates dopant concentrations along vertical line 132. Parenthetical entry “134” after reference symbols 124′, 124′′, and 124* indicates concentrations along vertical line 134.
FIG. 11 depicts an asymmetrical short n-channel IGFET 140 configured in accordance with the invention so as to be particularly suitable for high-speed analog applications. Short-channel IGFET 140 is a variation of long-channel IGFET 100 in which the channel length is shortened to such an extent that IGFET operation occurs in the short-channel regime. The channel length is shortened by appropriately reducing the length of gate electrode 128. In the example of FIG. 11, p-type pocket portion 120 extends sufficiently far across channel zone 106 to just meet drain 104.
FIGS. 12a-12c present exemplary dopant concentrations along the upper semiconductor surface of IGFET 140 as a function of longitudinal distance x in order to facilitate understanding the asymmetrical dopant grading in channel zone 106 of IGFET 140. As in FIGS. 7a-7c, FIGS. 12a-12c respectively depict individual dopant concentrations NI, absolute dopant concentrations NT, and net dopant concentrations NN along the upper semiconductor surface. All of the analysis presented above for IGFET 100 in connection with FIGS. 7a-7c applies to IGFET 140 except that the shortened length of channel zone 106 in IGFET 140 causes the dopant distribution in channel zone 106 of IGFET 140 to differ from that in channel zone 106 of IGFET 100 as explained below in connection with FIGS. 12a-12c. 
As in FIG. 7a, concentration NI of the p-type pocket dopant along the upper semiconductor surface is represented by curve 120′ in FIG. 12a. Unlike what occurs in channel zone 106 of IGFET 100, concentration NI of the p-type pocket dopant along the upper surface of channel zone 106 of IGFET 140 does not drop essentially to zero at a location between source 102 and drain 104. Instead, as illustrated by curve 120′ in FIG. 12a, concentration NI of the p-type pocket dopant along the upper surface of channel zone 106 of IGFET 140 is greater than zero along substantially all of the channel zone's upper surface and thus is at a small finite value where channel zone 106/pocket portion 120 meets drain 104. Concentration NI of the pocket dopant in IGFET 140 is at a much greater value where zone 106/pocket 120 meets source 102 along the channel zone's upper surface.
The preceding change in concentration NI of the p-type pocket dopant along the upper surface of channel zone 106 in IGFET 140 is reflected in absolute dopant concentration NT and net dopant concentration NN along the upper semiconductor surface of IGFET 140. As in FIG. 7b, curve segment 106′′ in FIG. 12b represents concentration NT of the total p-type dopant in channel zone 106 along the upper semiconductor surface for which the total p-type dopant in zone 106 along the upper surface is the sum of the p-type background and pocket dopants along the upper surface. The variation of curve segment 106′′ in FIG. 12b shows that, in moving from source 102 through channel zone 106 to drain 104 along the upper semiconductor surface, concentration NT of the total p-type dopant in zone 106 along the upper semiconductor surface of IGFET 140 is substantially constant at the high level in source 102 partway into zone 106 and then decreases from the high level to a low level slightly greater than the background concentration upon reaching drain 104. More particularly, concentration NT of the total p-type dopant in channel zone 106 of IGFET 140 increases progressively in going along the upper semiconductor surface from where zone 106 meets drain 104 partway to where zone 106 meets source 102.
Similar to what was said above about IGFET 100, concentration NI of the p-type pocket dopant along the upper semiconductor surface in IGFET 140 may start decreasing immediately after passing source-body pn junction 110 in moving longitudinally along the upper surface from source 102 across channel zone 106 to drain 104. As viewed in moving from drain 104 to source 102 rather than from source 102 to drain 104, concentration NT of the total p-type dopant in channel zone 106 in IGFET 140 then increases progressively in going along the upper semiconductor surface from where zone 106 meets drain 104 all the way to where zone 106 meets source 102. In any event, concentration NT of the total p-type dopant in zone 106 along the upper surface of IGFET 140 satisfies the specifications presented above for IGFET 100 of being lower where zone 106 meets drain 104 than where zone 106 meets source 102.
FIG. 12c shows that, as represented by curve 106*, concentration NN of the net p-type dopant along the upper surface of channel zone 106 of IGFET 140 varies in a similar manner to concentration NT of the total p-type dopant in zone 106 of IGFET 140 along the upper semiconductor surface except that concentration NN of the net p-type dopant in zone 106 of IGFET 140 along the upper semiconductor surface goes to zero at pn junctions 110 and 112. As in channel zone 106 of IGFET 100, the source side of channel zone 106 in IGFET 140 has a high net amount of p-type dopant compared to the drain side of IGFET 140. The high source-side p-type doping in channel zone 106 of IGFET 140 causes the thickness of the channel-side portion of the depletion region extending along source-body junction 110 to be reduced.
Source 102 and drain 104 are closer to each other in IGFET 140 than in IGFET 100. Accordingly, it is more likely that the depletion region extending along source 102 will punch through to the depletion region extending along drain 104 in IGFET 140 than in IGFET 100. However, the high amount of source-side p-type dopant in channel zone 106 of IGFET 140 reduces the likelihood of punchthrough occurring in IGFET 140 relative to an otherwise equivalent short re-channel IGFET lacking pocket portion 120.
The dopant concentrations along vertical lines 130, 132, and 136 respectively through source 102, channel zone 106, and drain 104 in IGFET 140 are substantially the same as in IGFET 100. Concentrations NI, NT, and NN shown in FIGS. 8a-8c, 9a-9c, and 10a-10c along vertical lines 130, 132, and 136 thus apply to IGFET 140. Accordingly, IGFET 140 has a hypoabrupt vertical dopant profile in the portion of body material 108 below drain 104. This causes the parasitic capacitance associated with drain-body pn junction 112 to decrease as further described below so that IGFET 140 has increased analog speed.
Source 102 can be longitudinally dopant graded to reduce its source (series) resistance RS. As discussed below, reducing source resistance RS is particularly advantageous in analog IGFET applications. This longitudinal dopant grading in source 102 typically involves configuring it as a main portion and a more lightly doped lateral extension that terminates channel zone 106 along the upper semiconductor surface. Drain 104 can be provided with a similar longitudinal dopant grading to reduce hot-carrier injection. Providing both of source/drain zones 102 and 104 with longitudinal dopant grading is therefore advantageous regardless of whether zones 102 and 104 respectively act as source and drain, the normal case, or respectively as drain and source.
FIG. 13 illustrates an asymmetrical long n-channel IGFET 150 configured in accordance with the invention to be especially suitable for high-speed analog applications and, in particular, to have longitudinal source/drain dopant grading for reducing source resistance RS and drain-side hot-carrier injection. IGFET 150 is arranged the same as IGFET 100 except that (a) n-type source 102 consists of a very heavily doped main portion 102M and a more lightly doped lateral extension 102E and (b) n-type drain 104 consists of a very heavily doped main portion 104M and a more lightly doped lateral extension 104E. Although more lightly doped than n++ main source portion 102M and n++ main drain portion 104M, lateral extensions 102E and 104E are still heavily doped in sub-μm complementary IGFET applications such as the present one. N+ lateral extensions 102E and 104E terminate channel zone 106 along the upper semiconductor surface. Gate electrode 128 extends partially over each of lateral extensions 102E and 104E but typically not over main source portion 102M or main drain portion 104M.
N++ main source portion 102M and n++ main drain portion 104M normally extend deeper below the upper semiconductor surface respectively than p+ lateral extensions 102E and 104E. Consequently, source depth yS and drain depth yD in IGFET 150 respectively are the depths of main source portion 102M and main drain portion 104M. Pocket portion 120 extends under, and partially alongside, main source portion 102M so that drain depth yD again normally exceeds source depth yS. Also, pocket 120 extends under, and alongside, source extension 102E. As a result, drain extension 104E normally extends deeper below the upper surface than source extension 102E.
Maximum net dopant concentration NN in n++ main source portion 102M along the upper semiconductor surface is normally at least 1×1020 atoms/cm3, typically 4×1020 atoms/cm3. Maximum net dopant concentration NN in n++ main drain portion 104M along the upper surface is normally at least 1×1020 atoms/cm3, typically slightly greater than 4×1020 atoms/cm3 so as to slightly exceed the maximum upper-surface NN concentration in main source portion 102M. Maximum net dopant concentration NN in n+ source extension 102E along the upper surface is normally 1×1018-1×1019 atoms/cm3, typically 3×1018 atoms/cm3. Maximum net dopant concentration NN in n+ drain extension 104E along the upper surface is typically slightly greater than 3×1018 atoms/cm3 so as to slightly exceed the maximum upper-surface NN concentration in source extension 102E.
Subject to the longitudinal dopant grading in source 102 and drain 104, channel zone 106 is asymmetrically longitudinally dopant graded substantially the same in IGFET 150 as in IGFET 100. FIGS. 14a-14c present exemplary dopant concentrations along the upper semiconductor surface of IGFET 150 as a function of longitudinal distance x for use in examining the longitudinal dopant grading in source 102 and drain 104. FIG. 14a depicts concentrations NI, along the upper semiconductor surface, of the individual semiconductor dopants that largely define regions 102M, 102E, 104M, 104E, 120, and 124. Concentrations NT of the total p-type and total n-type dopants of regions 102M, 102E, 104M, 104E, 120, and 124 along the upper semiconductor surface are depicted in FIG. 14b. FIG. 14c illustrates net dopant concentration NN along the upper semiconductor surface.
FIG. 14a is analogous to FIG. 7a except that curves 102M′, 102E′, 104M′, and 104E′ represent concentrations NI, along the upper semiconductor surface, of the n-type dopant used to respectively form regions 102M, 102E, 104M, and 104E. FIG. 14b is similarly analogous to FIG. 7b subject to total concentrations NT of the n-type dopant along the upper surface being represented by (a) curve 102′′ consisting of segments 102M′′ and 102E′′ respectively corresponding to main source portion 102M and source extension 102E and (b) curve 104′′ consisting of segments 104M′′ and 104E′′ respectively corresponding to main drain portion 104M and drain extension 104E. FIG. 14c is analogous to FIG. 7c subject to (a) curve 102* that represents net dopant concentration NN in source 102 along the upper surface being formed with segments 102M* and 102E* respectively corresponding to main source portion 102M and source extension 102E and (b) curve 104* that represents net dopant concentration NN in drain 104 along the upper surface being formed with segments 104M* and 104E* respectively corresponding to main drain portion 104M and drain extension 104E.
The longitudinal dopant grading in source 102 and drain 104 of IGFET 150 reduces source resistance RS and alleviates drain-side hot-carrier injection but does not have any significant effect on the asymmetrical dopant grading in channel zone 106. Accordingly, the asymmetrical channel-zone dopant grading in IGFET 150 avoids punchthrough in largely the same way as in IGFET 100.
The configuration of well portion 116 and upper body-material remainder 124 in IGFET 150 causes the vertical dopant profile through drain 104 and into underlying body material 108 to be hypoabrupt substantially the same as in IGFET 100. With vertical lines 130 and 136 respectively going through n++ main source portion 102M and n++ main drain portion 104M, the vertical dopant concentration graphs of FIGS. 8a-8c, 9a-9c, and 10a-10c substantially apply to IGFET 150. The resulting reduced parasitic capacitance associated with drain-body pn junction 112 enables IGFET 150 to have increased analog speed. The reduction in source resistance RS further enhances the analog performance in the manner discussed below.
Drain 104 can be vertically dopant graded to further reduce the parasitic capacitance associated with drain-body junction 112. Source 102 can similarly be vertically dopant graded for reducing the parasitic capacitance associated with source-body junction 110. The vertical dopant grading typically involves configuring each source/drain zone 102 or 104 as a main portion and a more lightly doped lower portion. The vertical source/drain dopant grading can be combined with the above-mentioned longitudinal dopant grading of source/drain zones 102 and 104.
In the foregoing regard, FIG. 15 illustrates an asymmetrical long n-channel IGFET 160 configured in accordance with the invention to be especially suitable for high-speed analog applications. IGFET 160 is provided both with source/drain longitudinal dopant grading for reducing source resistance RS and drain-side hot-carrier injection and with source/drain vertical grading for reducing source/drain parasitic capacitances. IGFET 160 is arranged the same as IGFET 150 except that (a) source 102 further includes a lower portion 102L more lightly doped than main source portion 102M and (b) drain 104 further includes a lower portion 104L more lightly doped than main drain portion 104M. Lower source portion 102L and lower drain portion 104L are heavily doped n-type.
Source depth yS and drain depth yD in IGFET 160 are respectively the depths of n+ lower source portion 102L and n+ lower drain portion 104L, since they respectively underlie n++ main source portion 102M and n++ main drain portion 104M. Pocket portion 120 extends below lower source portion 102L. Consequently, drain depth yD once again normally exceeds source depth yS.
Source 102 and drain 104 of IGFET 160 respectively include n+ lateral source extension 102E and n+ lateral drain extension 104E for achieving longitudinal source-drain dopant grading. See FIG. 15. The longitudinal source/drain dopant grading in IGFET 160 has substantially the same characteristics as in IGFET 150. Accordingly, the longitudinal upper-surface dopant concentration graphs of FIGS. 14a-14c and the associated description of FIGS. 14a-14c in connection with IGFET 150 apply to IGFET 160. Since the longitudinal source/drain dopant grading in IGFET 150, and thus in IGFET 160, does not have any significant effect on the asymmetrical longitudinal dopant grading in channel zone 106, the asymmetrical channel-zone dopant grading in IGFET 160 avoids punchthrough in IGFET 160 in substantially the same way as in IGFET 100.
An understanding of the vertical dopant grading in IGFET 160 is facilitated with the assistance of FIGS. 16a-16c and 17a-17c which present exemplary dopant concentrations as a function of depth y along vertical lines 130 and 136 respectively through source 102 and drain 104 including respectively through lower source portion 102L and lower drain portion 104L. FIGS. 16a and 17a specifically illustrate concentrations NI, respectively along lines 130 and 136, of the individual semiconductor dopants that vertically define regions 102M, 102L, 104M, 104L, 114, 116, 120, and 124. Concentrations NT of the total p-type and total n-type dopants in regions 102M, 102L, 104M, 104L, 114, 116, 120, and 124 along lines 130 and 136 are respectively depicted in FIGS. 16b and 17b. FIGS. 16c and 17c respectively illustrate net dopant concentration NN along lines 130 and 136.
FIGS. 16a and 17a are respectively analogous to FIGS. 8a and 10a except that (a) curves 102M′ and 102L′ represent concentrations NI along vertical line 130 of the n-type dopant used to respectively form main source portion 102M and lower source portion 102L and (b) curves 104M′ and 104L′ represent concentrations NI along vertical line 136 of the n-type dopant used to respectively form main drain portion 104M and lower drain portion 104L. Similarly, FIGS. 16b and 17b are respectively analogous to FIGS. 8b and 10b subject to (a) concentration NT of the total n-type dopant along line 130 being represented by curve 102′′ consisting here of segments 102M′′ and 102L′′ respectively cooresponding to main source portion 102M and lower source portion 102L and (b) concentration NT of the total n-type dopant along line 136 being represented by curve 104′′ consisting here of segments 104M′′ and 104L′′respectively corresponding to main drain portion 104M and lower drain portion 104L. FIGS. 16c and 17c are respectively analogous to FIGS. 8c and 10c subject to (a) curve 102* that represents net dopant concentration NN in source 102 along line 130 being formed here with segments 102M* and 102L* respectively cooresponding to main source portion 102M and lower source portion 102L and (b) curve 104* that represents net dopant concentration NN in drain 104 along line 136 being formed here with segments 104M* and 104L* respectively corresponding to main drain portion 104M and lower drain portion 104L. Additionally, concentrations NI, NT, and NN along vertical lines 132 and 134 through channel zone 106 of IGFET 160 are respectively shown in FIGS. 9a-9c. 
Subject to the preceding comments about the vertical dopant grading in source 102 and drain 104 of IGFET 160, the configuration of well portion 116 and pocket portion 120 in IGFET 160 is substantially the same as in IGFET 100. Accordingly, the vertical dopant profile below drain 104 is substantially the same in IGFET 160 as in IGFET 100. For this reason, the parasitic capacitance associated with drain-body junction 112 is reduced in IGFET 160, thereby enabling it to have increased analog speed. The vertical dopant gradings in source 102 and drain 104 enable IGFET 160 to have even greater analog speed by reducing (or further reducing) the parasitic capacitance along source-body junction 110 and by further reducing the parasitic capacitance along drain-body junction 112. The longitudinal dopant gradings in source 102 and drain 104 of IGFET 160 reduce source resistance RS while simultaneously alleviating drain-side hot-carrier injection.
FIGS. 18a-18c illustrate versions 170, 180, and 190 of respective asymmetrical long re-channel IGFETs 100, 150, and 160 in which pocket portion 120 extends to a lesser depth below the upper semiconductor surface than source 102 and drain 104. For long n-channel IGFET 180 or 190 whose source 102 and drain 104 respectively include source extension 102E and drain extension 104E, pocket portion 120 extends to a greater depth below the upper semiconductor surface than extensions 102E and 104E.
The p type section of the boundary of pocket portion 120 in each IGFET 170, 180, or 190 is, as explained above in connection with IGFET 100, defined as the location where the concentration of the p-type pocket dopant equals the concentration of the p-type background dopant. The total p-type dopant concentration along the p-type section of the boundary of pocket portion 120 is then twice the background dopant concentration in IGFET 170, 180, or 190. Hence, some of the p-type pocket dopant is present in source 102 of IGFET 170, 180, or 190 at a depth below that illustrated for pocket portion 120 in FIGS. 18a-18c. This additional p-type pocket dopant in source 102 cancels (compensates for) some of the n-type dopant that defines source 102 along its lower surface. Accordingly, drain depth yD in IGFET 170, 180, or 190 exceeds source depth yS, albeit by a lesser amount than in IGFET 100.
Channel zone 106 of each of IGFETs 170, 180, and 190 is asymmetrically longitudinally dopant graded substantially as described above respectively for IGFETs 100, 150, and 160. In this regard, dopant concentrations NI, NT, and NN along the upper semiconductor surface for IGFET 170 are also substantially respectively represented in FIGS. 7a-7c. FIGS. 14a-14c substantially respectively present concentrations NI, NT, and NN along the upper surface for IGFETs 180 and 190. Punchthrough is thus avoided in IGFETs 170, 180, and 190 in the manner described above for IGFET 100.
Each of IGFETs 170, 180, and 190 has a hypoabrupt vertical dopant profile below drain 104 substantially as described above for IGFETs 100, 150, and 160. FIGS. 10a-10c respectively also substantially present concentrations NI, NT, and NN along vertical line 136 through drain 104 for each of IGFETs 170 and 180. Concentrations NI, NT, and NN along vertical line 136 through drain 104 for IGFET 190 are substantially respectively represented in FIGS. 17a-17c. The parasitic capacitance along drain-body junction 112 in each IGFET 170, 180, or 190 is thereby reduced, as described above for IGFET 100, to enable each IGFET 170, 180, or 190 to have increased analog speed.
FIGS. 19a-19c and 20a-20c present exemplary dopant concentrations as a function of depth y along vertical line 130 through source 102 for IGFETs 170, 180, and 190. FIGS. 19a-19c apply to IGFETs 170 and 180. FIGS. 20a-20c apply to IGFET 190. FIGS. 19a and 20a specifically illustrate concentrations NI of the individual semiconductor dopants that vertically define regions 102, 114, 116, and 120 along line 130. Concentrations NT of the total p-type and total n-type dopants in regions 102, 114, 116, and 120 along line 130 are depicted in FIGS. 19b and 20b. FIGS. 19c and 20c respectively illustrate net dopant concentration NN along line 130.
As indicated by the variation of curve segments 116′′ and 124′′ in FIGS. 19b and 20b, concentration NT of the total p-type dopant in the portion of body material 108 below source 102 decreases hypoabruptly by at least a factor of 10 in moving upward along vertical line 130 to source 102 from the subsurface location of the maximum concentration of the p-type dopant in well portion 116. Configuring pocket portion 120 to be shallower than source 102 and drain 104 thus results in a hypoabrupt vertical profile for the total p-type dopant in the portion of body material 108 below source 102. This arises because much less of the p-type dopant is situated below source 102 in IGFETs 170, 180, and 190 than in IGFETs 100, 150, and 160. Curve segment 120′′ which represents concentration NT of the total p-type pocket dopant along line 130 through source 102 in FIGS. 8b, 16b, 19b, and 20b is largely located above source depth yS in FIGS. 19b and 20b but extends considerably below depth yS in FIGS. 8b and 16b. 
The hypoabrupt vertical dopant profile in the portion of body material 108 below source 102 of IGFETs 170, 180, or 190 is quite similar to the hypoabrupt vertical dopant profile in the portion of body material 108 below drain 104 of IGFET 170, 180, or 190 and thus below drain 104 of IGFET 100, 150, or 160. Compare combined curve segment 116′′/120′′ along vertical line 130 through source 102 in FIGS. 19b and 20b with combined curve segment 116′′/120′′ along vertical line 136 through drain 104 in FIGS. 10b and 17b. Similar to what occurs in the portion of body material 108 below drain 104, concentration NT of the total p-type dopant in the portion of body material 108 below source 102 in each IGFET 170, 180, or 190 preferably decreases by at least a factor of 20, more preferably by at least a factor of 40, even more preferably by at least a factor of 80, typically by a factor in the vicinity of 100, in moving from the subsurface location of the maximum p-type well concentration up to the very heavily doped material of source 102. In IGFET 170, 180, or 190, the hypoabrupt vertical dopant profile in the portion of body material 108 below the very heavily doped material of source 102 causes the parasitic capacitance associated with source-body junction 110 to decrease. The analog speed of IGFET 170, 180, or 190 is further increased.
Dopant concentrations NI, NT, and NN along vertical line 134 through channel zone 106 of IGFET 170, 180, or 190 appear substantially as respectively shown in FIGS. 9a-9c. Dopant concentrations NI, NT, and NN along vertical line 132 through channel zone 106, including pocket portion 120, of IGFET 170, 180, or 190 are similar to what is respectively illustrated in FIGS. 9a-9c except that curve 120′′ for concentration NI of the p-type pocket dopant along line 132 for IGFET 170, 180, or 190 is similar to curve 120′ along line 130 in FIG. 19a or 20a. 
For the purpose of simplicity in describing IGFETs 100, 140, 150, 160, 170, 180, and 190, it was assumed above that the concentration of the p-type background dopant is substantially constant throughout the semiconductor material containing any of IGFETs 100, 140, 150, 160, 170, 180, and 190. However, the concentration of the p-type background dopant can vary as long as the peak value of the p-type background dopant is relatively low compared to the concentration of the other p-type dopant.
Well portion 116 of body material 108 in each of IGFETs 100, 140, 150, 160, 170, 180, and 190 is of the same conductivity type as the directly underlying lightly doped semiconductor material (lower body-material portion 114). As indicated below in connection with the fabrication process of FIGS. 31a-31o and 31p.1-31r2, this situation normally arises when well portion 116 and pocket portion 120 are created in a starting region of lightly doped p-type semiconductor material. The dopant concentration in the bulk of upper body-material remainder 124 thereby largely equals the low background dopant concentration of the p− starting region.
Alternatively, the semiconductor material directly underlying well portion 116 can be of opposite conductivity type to well 116. Since well portion 116 is p type, the semiconductor material directly underlying well 116 is n type. This alternative typically arises when well 116 and pocket 120 are created in a starting region of n-type semiconductor material, normally lightly doped at a relatively uniform net background concentration. The portion of the starting n-type region intended to become upper body-material portion 118, i.e., the portion of the starting n-type region situated above well portion 116 (or above the intended location for well 116) is doped with p-type compensating dopant to an absolute concentration greater than the n-type background dopant concentration of the starting n-type semiconductor region so as to cause all of upper body-material portion 118 to be p type.
The minimum value of net concentration NN of the p-type compensating dopant in upper body-material portion 118 can be in the vicinity of n-type background dopant concentration. However, in order to ensure that all of body-material portion 118 is p type, the minimum value of concentration NN of the p-type compensating dopant in portion 118 is normally a significant amount greater than, e.g., at least twice as great as, the n-type background dopant concentration. The minimum value of concentration NN of the p-type compensating dopant in the bulk of body-material portion 118 outside pocket portion 120 is thus normally significantly greater than the n-type background dopant concentration.
FIG. 21 illustrates, in accordance with the invention and analogous to FIG. 6, a variation 100V of asymmetrical long n-channel IGFET 100 in which p− lower body-material portion 114 is replaced with a lightly doped n-type lower region 192 that forms a lower pn junction 194 with p+ well portion 116. Since lower region 192 is not of p-type conductivity, p-type body material 108 in IGFET 100V consists of well portion 116 and an upper portion 196 that replaces upper body-material portion 118 of IGFET 100. Part of upper body-material portion 196 of IGFET 100V is formed by p+ pocket portion 120. The remainder of upper body-material portion 196, i.e., the part outside pocket portion 120, is indicated as item 198 in FIG. 21. Upper body-material remainder 198 is lightly doped p type at a somewhat higher net concentration than n− lower portion 192. Aside from the preceding differences, IGFET 100V is configured and constituted substantially the same as IGFET 100.
IGFET 100V has the following features similar to those of IGFET 100: (a) a longitudinally asymmetrical dopant grading in channel 106 and (b) a hypoabrupt vertical dopant profile in the portion of body material 108 underlying drain 104. An understanding of these features of IGFET 100V, including how they can respectively differ somewhat from those of IGFET 100, is facilitated with the assistance of FIGS. 22a-22c, 23a-23c, and 24a-24c. FIGS. 22a-22c present exemplary dopant concentrations along the upper semiconductor surface of IGFET 100V as a function of longitudinal distance x. Exemplary dopant concentrations along vertical line 130 through source 102 of IGFET 100V are presented in FIGS. 23a-23c. FIGS. 24a-24c present exemplary dopant concentrations along vertical line 136 through drain 104 of IGFET 100V.
FIGS. 22a, 23a, and 24a illustrate concentrations NI of the individual semiconductor dopants that define regions 102, 104, 116, 120, 192, 196, and 198. Curves 192′ and 198′ specifically respectively represent concentrations NI of the n-type background dopant and the p-type compensating dopant that respectively define n− lower region 192 and p− upper body-material remainder 198. Item 194# indicates where net dopant concentration NN goes to zero below well portion 116 and thus indicates the location of lower pn junction 194.
Concentrations NT of the total p-type and total n-type dopants in regions 102, 104, 116, 120, 192, 196, and 198 are depicted in FIGS. 22b, 23b, and 24b. Curve segments 192′′ and 198′′ in FIGS. 22b, 23b, and 24b respectively correspond to n− lower region 192 and p− upper body-material remainder 198. FIGS. 22c, 23c, and 24c illustrate concentrations NN of the net p-type dopant and net n-type dopant variously in regions 102, 104, 116, 120, 192, 196, and 198. Curve segments 192* and 198* in FIGS. 22c, 23c, and 24c respectively correspond to n− lower region 192 and p− upper body-material remainder 198.
FIGS. 22a-22c, 23a-23c, and 24a-24c represent an example in which (a) concentration NI of the n-type background dopant in IGFET 100V approximately equals concentration NI of the p-type background dopant in IGFET 100, (b) concentration NI of the p-type compensating dopant along the upper semiconductor surface of IGFET 100V is 2-3 times concentration NI of the n-type background dopant in IGFET 100V, and (c) the maximum value of concentration NI of the p-type compensating dopant is 2-3 times concentration NI of the p-type compensating dopant along the upper semiconductor surface and thus is 4-9 times concentration NI of the n-type background dopant. Aside from these differences, concentrations NI of the other dopants in IGFET 100V are respectively largely the same as in IGFET 100.
More particularly, concentration NI of the p-type pocket dopant in IGFET 100V varies longitudinally in substantially the same way as in IGFET 100. The variation of curve segment 120′ in FIG. 22a shows that, in moving longitudinally from source 102 along the upper semiconductor surface of IGFET 100V into channel zone 106, concentration NI of the p-type pocket dopant is at an approximately constant upper-surface level partway into zone 106 and then drops from that level essentially to zero at a location between source 102 and drain 104.
The total p-type dopant in channel zone 106 along the upper semiconductor surface of IGFET 100V is the sum of the pocket and compensating dopants. This differs from IGFET 100 where the total p-type dopant in channel zone 106 along the upper semiconductor surface is the sum of the pocket and background dopants. Since concentration NI of the p-type compensating dopant in the illustrated example is 2-3 times concentration NI of the n-type background dopant and thus is 2-3 times concentration NI of the p-type background dopant in IGFET 100, the minimum value of concentration NT of the total p-type dopant along the upper surface of IGFET 100V in the illustrated example is 2-3 times the minimum value of concentration NT of the total p-type dopant along the upper surface of IGFET 100.
Item 106′′ in FIG. 22b represents the channel-zone portions of curve segments 120′′ and 198′′. Similar to what occurs in IGFET 100, the variation in curve 106′′ here shows that concentration NT of the total p-type dopant in channel zone 106 along the upper semiconductor surface of IGFET 100V is lower where zone 106 meets drain 104 than where zone 106 meets source 102. Concentration NT of the total p-type dopant in channel zone 106 of IGFET 100V is normally at least a factor of 10 lower, preferably at least a factor of 20 lower, more preferably at least a factor of 50 lower, typically a factor in the vicinity of 100 lower, at drain-body junction 112 along the upper surface than at source-body junction 110 along the upper surface. The reason why the typical value for this concentration difference is a factor in the vicinity of 100 in IGFET 100V, rather than a factor of more than 100 as can arise in IGFET 100, is that the minimum value of concentration NT of the total p-type dopant along the upper surface is 2-3 times higher in the illustrated example of IGFET 100V than in IGFET 100.
Referring to FIG. 22c, item 106* represents the combination of channel-zone curve segments 120* and 198* here. Subject to segment 198* of curve 106* for IGFET 100V being slightly higher than segment 124* of curve 106* for IGFET 100 in FIG. 7a, curves 106* in FIGS. 7a and 22a are quite similar. Accordingly, the source side of channel zone 106 in IGFET 100V has a high net amount of p-type dopant compared to the drain side. The thickness of the channel-side portion of the depletion region along source-body junction 110 is thereby reduced in IGFET 100V. In addition, the high p-type dopant concentration along the source side of channel zone 106 in IGFET 100V shields source 102 from the comparatively high electric field in drain 104 for the (field-line termination) reasons described above in connection with IGFET 100. Punchthrough is avoided in IGFET 100V.
The p-type dopant in the portions of body material 108 below source 102 and drain 104 of IGFET 100V consists primarily of the well and compensating dopants as indicated respectively by curves 116′ and 198′ in FIGS. 23a and 24a and, for source 102, also the pocket dopant as indicated by curve 120′ in FIG. 23a. The variation of curve 198′ in FIGS. 23a and FIG. 24 a shows that concentration NI of the p-type compensating dopant reaches a maximum value close to the bottoms of source 102 and drain 104. This maximum value is 4-9 times concentration NI of the n-type background dopant in the particular example of FIGS. 23a and 24a. Concentration NI of the p-type compensating dopant drops essentially to zero at a depth less than depth yW of the maximum p-type dopant concentration in well portion 116 in the example of FIGS. 23a and 24a. 
The amount of p-type pocket dopant present in the portion of body material 108 below drain 104 of IGFET 100V is either essentially zero or is so low as to be electrically insignificant. The total p-type dopant in the body-material portion below drain 104 of IGFET 100V thus consists substantially only of the well and compensating dopants.
The total p-type dopant in the portion of body material 108 below drain 104 of IGFET 100V is indicated by curve segment 116′′ and its (upward) extension 198′′ in FIG. 24b. Because concentration NI of the p-type compensating dopant drops essentially to zero at a depth less than depth yW, concentration NT of the total p-type dopant in the body-material portion below drain 104 reaches a maximum value at a subsurface location substantially equal to yW. As occurs in IGFET 100, the variation in combined curve segment 116′′/198′′ in FIG. 24b shows that concentration NT of the total p-type dopant in the portion of body material 108 below drain 104 of IGFET 100V decreases hypoabruptly by at least a factor of 10 in moving upward to drain 104 from the subsurface location of the maximum concentration of the p-type dopant in well portion 116.
Concentration NT of the total p-type dopant in the portion of body material 108 below drain 104 of IGFET 100V in the particular example of FIG. 24b typically decreases by a factor in the vicinity of 15 in moving from the location of the maximum p-type well concentration up to drain 104. The reason why the typical value for this hypoabrupt concentration decrease is in the vicinity of a factor of 15 in IGFET 100V rather than in the vicinity of a factor 100 as arises in IGFET 100 is that concentration NI of the p-type compensating dopant at the bottom of drain 104 in IGFET 100V in the example of FIG. 24a is 4-9 times concentration NI of the p-type background dopant at the bottom of drain 104 in IGFET 100. However, the vertical dopant profile for the p-type compensating dopant can be lowered to reduce the value of concentration NI of the p-type compensating dopant at the bottom of drain 104 while still ensuring that all of upper body-material portion 196 is p type. Concentration NT of the total p-type dopant in the body material portion below drain 104 in IGFET 100V can readily decrease by at least a factor of 20, and typically by at least a factor of 40, in moving upward to drain 104 from the location of the maximum concentration of the total p-type dopant in well portion 116.
As represented by the combination of curve segments 116* and 198*, FIG. 24c shows that concentration NN of the net p-type dopant in the portion of body material 108 below drain 104 in IGFET 100V varies vertically in a similar manner to concentration NT of the total p-type dopant in the body-material portion below drain 104 except that concentration NN in the body-material portion below drain 104 drops to zero at pn junctions 112 and 194. Due to the hypoabrupt vertical dopant profile in the body-material portion below drain 104 of IGFET 100V, the parasitic capacitance associated with drain-body junction 112 is again decreased for the reasons discussed further below. Although the decrease in the parasitic capacitance along junction 104 may be less in IGFET 100V than in IGFET 100, IGFET 100V still has increased analog speed.
The presence of the p-type compensating dopant at a concentration greater than that of the p-type background dopant in IGFET 100 has considerably lesser effect on the vertical dopant profile through source 102 of IGFET 100V than along the vertical dopant profile through drain 104 of IGFET 100V because the p-type pocket dopant is also present below source 102 in IGFET 100V. As is evident from comparing FIGS. 23a-23c respectively to FIGS. 24a-24c, the comments made above about the vertical dopant profile through source 102 of IGFET 100 apply generally to the vertical dopant profile through source 102 of IGFET 100V.
FIG. 25 illustrates, in accordance with the invention and analogous to FIG. 13, a variation 150V of asymmetrical long n-channel IGFET 150 in which n− lower portion 192 again replaces p− lower body-material portion 114. IGFET 150V likewise contains p-type upper body-material portion 196 that replaces upper body-material portion 118 of IGFET 150. P- remainder 198 of upper body-material portion 196 is again at a slightly higher net dopant concentration than n− lower portion 192. Subject to the presence of n− lower portion 192 and p− upper remainder 198, IGFET 150V is configured substantially the same as IGFET 150 so as to have longitudinal source/drain dopant grading for reducing source resistance RS and drain-side hot-carrier injection.
FIGS. 26a-26c present exemplary dopant concentrations along the upper semiconductor surface of IGFET 150V for use in examining the longitudinal dopant grading in source 102 and drain 104. Concentrations NI, along the upper surface, of the individual semiconductor dopants that largely define regions 102M, 102E, 104M, 104E, 120, 192, and 198 are depicted in FIG. 26a. FIG. 26b illustrates concentrations NT of the total p-type and n-type dopants of regions 102M, 102E, 104M, 104E, 120, 192, and 198 along the upper surface. Net concentration NN along the upper surface is illustrated in FIG. 26c. 
FIGS. 26a-26c respectively repeat FIGS. 14a-14c subject to respective modifications of FIGS. 22a-22c to account for n− lower portion 192 and p− upper body-material remainder 198. The longitudinal dopant grading in source 102 and drain 104 of IGFET 150V does not have any significant effect on the asymmetrical dopant grading in channel zone 106. The asymmetrical channel-zone dopant grading in IGFET 150V avoids punchthrough in largely the same way as in IGFET 150 and thus in largely the same way as in IGFET 100.
The configuration of p+ well portion 116 and p− upper body-material remainder 198 in IGFET 150V causes the vertical dopant profile through drain 104 and into underlying body material 108 to be hypoabrupt substantially the same as in IGFET 150. The vertical concentration graphs of FIGS. 23a-23c and 24a-24c along vertical lines 130 and 136 substantially apply to IGFET 150V. Drain-body junction 112 in IGFET 150V has reduced parasitic capacitance which, although typically not reduced as much as in IGFET 150, enables IGFET 150V to have increased analog switching speed.
FIGS. 27a and 27b illustrate, in accordance with the invention and respectively analogous to FIGS. 18a and 18b, variations 170V and 180V of asymmetrical long n-channel IGFETs 170 and 180 in which n− lower portion 192 replaces p− lower body-material portion 114. Each IGFET 170V or 180V likewise contains p-type upper body-material portion 196 that replaces upper body-material portion 118 of IGFET 170 or 180. P- remainder 198 of upper body-material portion 196 is once again at a slightly higher net dopant concentration than n− lower portion 192. Subject to the presence of n− lower portion 192 and p− upper remainder 198, pocket portion 120 of each IGFET 170V or 180V extends to a lesser depth below the upper semiconductor surface than source 102 or drain 104. IGFET 180V also has the longitudinal source/drain dopant grading of IGFET 150V for reducing source resistance RS and drain-side hot-carrier injection.
Channel zone 106 of each of IGFETs 170V and 180V is asymmetrically longitudinally dopant graded substantially as described above respectively for IGFETs 100V and 150V. FIGS. 22a-22c substantially respectively present concentrations NI, NT, and NN along the upper semiconductor surface for IGFET 170V. Punchthrough is thus avoided in IGFET 170V for the reasons described above in connection with IGFET 100V. Concentrations NI, NT, and NN along the upper semiconductor surface of IGFET 180V are substantially respectively represented in FIGS. 26a-26c. IGFET 180V avoids punchthrough as described above for IGFET 170V and thus as described above for IGFET 100V.
Each of IGFETs 170V and 180V has a hypoabrupt vertical dopant profile through drain 104 as described above for IGFET 100V. FIGS. 24a-24c also respectively present concentrations NI, NT, and NN along vertical line 136 through drain 104 for each IGFET 170V or 180V. As a result, the parasitic capacitance along drain-body junction 112 in each IGFET 170V or 180V is reduced for the reasons described above in connection with IGFET 100V. IGFETs 170V and 180V thereby have increased analog speed.
FIGS. 28a-28c respectively present concentrations NI, NT, and NN along vertical line 130 through source 102 for each IGFET 170V or 180V. As indicated by the variation of curve segments 116′′ and 198′′ in FIG. 28b, concentration NT of the total p-type dopant in the portion of body material 108 below source 102 decreases hypoabruptly by at least a factor of 10 in moving upward along line 130 to source 102 from the subsurface location of the maximum concentration of the p-type dopant in well portion 116. As occurs in IGFETs 170 and 180, configuring pocket portion 120 to be shallower than source 102 and drain 104 in IGFETs 170V and 180V results in a hypoabrupt vertical profile for the p-type dopant in the body-material portion below source 102.
The hypoabrupt vertical dopant profile in the portion of body material 108 below source 102 for each IGFET 170V or 180V is quite similar to the hypoabrupt vertical dopant profile in the portion of body material 108 below drain 104. In the particular example of FIGS. 28a and 28b, concentration NT of the total p-type dopant in the body-material portion below source 102 of IGFET 170V or 180V decreases by a typical factor in the vicinity of 15 in moving from the location of the maximum p-type well concentration up to source 102. Although this typical factor of 15 is considerably less than the corresponding typical factor of 100 which occurs in IGFET 170 or 180, the vertical dopant profile for the p-type compensating dopant can be lowered. Similar to what was said above about the vertical dopant profile through drain 104 of IGFET 100V, concentration NT of the total p-type dopant in the body-material portion below source 102 of IGFET 170V or 180V can readily decrease by at least a factor of 20, typically by at least a factor of 40, in moving upward to source 102 from the location of the maximum concentration of the p-type dopant in well portion 116.
The hypoabrupt vertical dopant profile in the portion of body material 108 below source 102 of IGFET 170V or 180V causes the parasitic capacitance associated with source-body junction 110 to decrease, albeit by typically a smaller amount than in IGFET 170 or 180. Consequently, the analog speed of each IGFET 170V or 180V is further increased.
Variations of IGFETs 140, 160, and 190 can be provided with n− lower region 192 and p− upper body-material remainder 198 in the same way that regions 192 and 198 are provided in IGFETs 100V, 150V, 170V, and 180V. These asymmetrical long n-channel variations of IGFETs 140, 160, and 190 are respectively referred to below as IGFETS 140V, 160V, and 190V.
Complementary IGFET Structures Suitable for Mixed-Signal Applications
Short-channel versions of long-channel IGFETs 150, 160, 170, 180, 190, 100V, 150V, 160V, 170V, 180V, and 190V can be produced in accordance with the invention by appropriately reducing the channel length. P-channel IGFETs can likewise be produced in accordance with the invention by reversing the conductivity types of the semiconductor regions of IGFETs 100, 140, 150, 160, 170, 180, 190, 100V, 140V, 150V, 160V, 170V, 180V, and 190V, including the short-channel variations of IGFETs 150, 160, 170, 180, 190, 150V, 160V, 170V, 180V, and 190V.
N-channel IGFETs 100, 140, 150, 160, 170, 180, 190, 100V, 140V, 150V, 160V, 170V, 180V, and 190V, including the short-channel variations of IGFETs 150, 160, 170, 180, 190, 150V, 160V, 170V, 180V, and 190V, and the p-channel IGFETs can be variously provided in the same semiconductor structure to produce a complementary-IGFET architecture especially suitable for high-speed analog applications. For instance, one or more of n-channel IGFETs 100, 140, 150, 160, 170, 180, and 190 can be combined with one or more p-channel variations of IGFETs 100V, 140V, 150V, 160V 170V, 180V, and 190V. The complementary-IGFET structure is then created from lightly doped p-type semiconductor material using p− lower body-material portion 114 as the p-type equivalent of n− lower portion 192 for each p-channel variation of IGFET 100V, 140V, 150V, 160V 170V, 180V, or 190V. Alternatively, one or more of n-channel IGFETs 100V, 140V, 150V, 160V 170V, 180V, and 190V can be combined with one or more p-channel variations of IGFETs 100, 140, 150, 160, 170, 180, and 190 fabricated from lightly doped n-type semiconductor material using p− lower portion 192 as the n-type equivalent of p− lower body-material portion 114 for each p-channel variation of IGFET 100, 140, 150, 160, 170, 180, or 190.
IGFETs, both n-channel and p-channel, particularly suitable for digital circuitry can also be provided in the semiconductor structure. Bipolar transistors, npn and/or pnp, can be variously provided in the semiconductor structure. The resulting semiconductor architecture is thus suitable for mixed signal applications.
FIGS. 29.1 and 29.2 (collectively “FIG. 29”) depict two portions of a complementary-IGFET structure configured according to the invention so as to be especially suitable for mixed-signal applications. The complementary-IGFET structure of FIG. 29 is created from a doped monosilicon semiconductor body having lower p− body-material portion 114. A patterned field region 200 of electrically insulating material, typically silicon oxide, is recessed into the upper surface of the semiconductor body to define a group of laterally separated active semiconductor islands. Four such islands 202, 204, 206, and 208 appear in FIG. 29.
Four long-channel IGFETs 210, 220, 230, and 240 are formed along the upper semiconductor surface respectively at the locations of islands 202, 204, 206, and 208. IGFETs 210 and 220 in FIG. 29.1 are asymmetrical devices primarily intended for high-speed analog applications. IGFETs 230 and 240 in FIG. 29.2 are symmetrical devices primarily intended for digital applications. IGFETs 210 and 230 are n-channel devices. IGFETs 220 and 240 are p-channel devices.
Asymmetrical n-channel IGFET 210 is an implementation of long n-channel IGFET 180 of FIG. 18b and contains all the regions of IGFET 180. Source 102, drain 104, and channel zone 106 of n-channel IGFET 210 are situated in island 202. In addition to the regions depicted in FIG. 18b, IGFET 210 contains a pair of electrically insulating sidewall spacers 250 and 252 situated along the opposite transverse sidewalls of gate electrode 128. Metal silicide layers 254, 256, and 258 are respectively situated along the tops of source 102, drain 104, and gate electrode 128.
Asymmetrical p-channel IGFET 220 is an implementation of a p-channel version of long n-channel IGFET 180V in which n− lower portion 192 is replaced with p− lower body-material portion 114. IGFET 220 has a p-type source 262 and a p-type drain 264 separated by an n-type channel zone 266 of n-type body material 268 consisting of a heavily doped well portion 276 and an upper portion 278. Source 262, drain 264, and channel zone 266 are situated in island 204. Each source/drain zone 262 or 264 consists of a very heavily doped main portion 262M or 264M and a more lightly doped, but still heavily doped, lateral extension 262E or 264E for reducing source resistance RS and drain-side hot-carrier injection. Lateral extensions 262E and 264E terminate channel zone 266 along the upper semiconductor surface.
A heavily doped pocket portion 280 of n-type upper body-material portion 278 extends along source 262, primarily source extension 262E. As with pocket portion 120 in IGFET 210, n+ pocket portion 280 extends deeper below the upper semiconductor surface than p+ source extension 262E but not as deep as p++ main source portion 262M. The remainder 284 of n-type upper body-material portion 278 is lightly doped and extends along drain 264. N+ well portion 276, n+ pocket portion 280, and n− upper body-material remainder 284 in IGFET 220 have largely the same longitudinal and vertical doping characteristics respectively as p+ well portion 116, p+ pocket portion 120, and p− upper body-material remainder 198 in IGFET 180V with the conductivity types reversed. IGFET 220 thereby avoids punchthrough and has reduced parasitic capacitances along the source-body and drain-body pn junctions.
A gate dielectric layer 286 overlies channel zone 266 of IGFET 220. A gate electrode 288 is situated on gate dielectric layer 286 above channel zone 266. Gate electrode 288 extends partially over each source/drain extension 262E or 264E. In the example of FIG. 29, electrode 288 consists of very heavily doped p-type polysilicon. A pair of electrically insulating sidewalls spacers 290 and 292 are situated along the opposite transverse sidewalls of electrode 288. Metal silicide layers 294, 296, and 298 are respectively situated along the tops of source 262, drain 264, and gate electrode 288.
Symmetrical n-channel IGFET 230 has a pair of n-type source/drain zones 302 and 304 separated by a p-type channel zone 306 of p-type body material 308 consisting of lower p- portion 114, a heavily doped immediate well portion 316, and an upper portion 318. Source/drain zones 302 and 304 and channel zone 306 are situated in island 206. Each source/drain zone 302 or 304 consists of a very heavily doped main portion 302M or 304M and a heavily doped, and thus more lightly doped, lateral extension 302E or 304E for reducing drain-side hot-carrier injection. Lateral extensions 302E and 304E terminate channel zone 306 along the upper semiconductor surface.
A pair of heavily doped halo pocket portions 320 and 322 of p-type upper body-material portion 318 respectively extend along source/drain zones 302 and 304 in a symmetrical manner. Halo pocket portions 320 and 322 extend primarily along source/drain extensions 302E and 304E. In the example of FIG. 29, p+ pocket portions 320 and 322 extend deeper below the upper semiconductor surface than n+ extensions 302E and 304E but not as deep as n++ main drain portions 302M and 304M. Item 324 is the moderately doped p-type remainder of upper body-material portion 318.
A gate dielectric layer 326 overlies channel zone 306. A gate electrode 328 is situated on gate dielectric layer 326 above channel zone 306. Gate electrode 328 extends partially over each source/drain extension 302E or 304E. Gate electrode 328 consists of very heavily doped n-type polysilicon in the example of FIG. 29. A pair of electrically insulating sidewalls spacers 330 and 332 are situated along the opposite transverse sidewalls of electrode 328. Metal silicide layers 334, 336, and 338 are respectively situated along the tops of source/drain zones 302 and 304 and electrode 328.
Subject to being formed over p- lower body-material portion 214, symmetrical p-channel IGFET 240 is a long-channel device configured substantially the same as IGFET 230 with the conductivity types reversed. IGFET 240 thus has a pair of p-type source/drain zones 342 and 344 separated by an n-type channel zone 346 of n-type body-material 348 consisting of a heavily doped well portion 356 and an upper portion 358. Source/drain zones 342 and 344 and channel zone 346 are situated in island 208. Each source/drain zone 342 or 344 consists of a very heavily doped main portion 342M or 344M and a more lightly doped, but still heavily doped, lateral extension 342E or 344E for reducing drain-side hot-carrier injection. Lateral extensions 342E and 344E terminate channel zone 346 along the upper semiconductor surface.
A pair of heavily doped halo pocket portions 360 and 362 of n-type upper body-material portion 358 respectively extend along source/drain zones 342 and 344 in a symmetrical manner. Halo pocket portions 360 and 362 respectively extend primarily along source/drain extensions 342E and 344E. In the example of FIG. 29, n+ pocket portions 360 and 362 extend deeper below the upper semiconductor surface than n+ extensions 342E and 344E but not as deep as n++ main source/drain portions 342M and 344M. Item 364 is the moderately doped n-type remainder of upper body-material portion 358.
A gate dielectric layer 366 overlies channel zone 346. A gate electrode 368 is situated on gate dielectric layer 366 above channel zone 346. Gate electrode 368 extends partially over each source/drain extension 342E or 344E. In the example of FIG. 29, electrode 368 consists of very heavily doped p-type polysilicon. A pair of electrically insulating sidewall spacers 370 and 372 are situated along the opposite transverse sidewalls of electrode 368. Metal silicide layers 374, 376, and 378 are respectively situated along the tops of source/drain zones 342 and 344 and electrode 368.
Gate dielectric layers 126, 286, 326, and 366 of IGFETs 210, 220, 230, and 240 typically consist primarily of silicon oxide but may consist of silicon oxynitride or/and other high permittivity dielectric material. The thickness of dielectric layers 126, 286, 326, and 366 is normally 2-8 nm, preferably 3-5 nm, typically 3.5 nm for operation across a 1.8-V range. The dielectric layer thickness is suitably increased for operation across a higher voltage range or suitably decreased for operation across a lower voltage range. Sidewall spacers 250, 252, 290, 292, 330, 332, 370, and 372 are illustrated in FIG. 29 as shaped roughly like right triangles with convex hypotenuses but can have other shapes. Silicide layers 254, 256, 258, 294, 296, 298, 334, 336, 338, 374, 376, and 378 typically consist of cobalt silicide.
Channel zones 306 and 346 of IGFETs 230 and 240 have symmetrical longitudinal dopant profiles similar to that illustrated in FIG. 2 for symmetrical IGFET 20 of FIG. 1. The presence of p-type halo pocket portions 320 and 322 in channel zone 306 alleviates threshold voltage roll-off and helps avoid punchthrough in IGFET 230. The presence of n-type halo pocket portions 360 and 362 in channel zone 346 similarly alleviates threshold voltage roll-off and helps avoid punchthrough in IGFET 240.
The vertical dopant profile through each main source/drain portion 302M or 304M of IGFET 230 and into underlying p-type body material 308 is similar to that shown in FIG. 3a for IGFET 20 and also to the computer-simulated reference dopant profiles shown in FIGS. 33, 37a, and 37b discussed below. The same applies to the vertical dopant profile through each main source/drain portion 342M or 344M of IGFET 240 and into underlying n-type body material 348 subject to body material 348 forming a pn junction with p− lower portion 114 instead of merging into underlying lightly doped n-type monosilicon. The moderate, but elevated, concentration of the p-type dopant in upper body-material portion 324 of IGFET 230 cooperates with the heavy p-type dopant concentration provided by halo pocket portions 320 and 322 to inhibit punchthrough from occurring in IGFET 230. The corresponding dopings in upper body-material portion 364 and halo pocket portions 360 and 362 of IGFET 240 similarly enable it to avoid punchthrough.
The p-type well dopant that defines well portion 316 of IGFET 230 typically reaches a maximum concentration at approximately the same depth below the upper semiconductor surface as the maximum concentration of the p-type well dopant that defines well portion 116 of IGFET 210. Because concentration NI of the p-type background dopant is relatively uniform, the maximum concentration of the total p-type dopant in well portion 316 of IGFET 230 typically occurs at approximately the same depth below the upper surface as the maximum concentration of the total p-type dopant in well portion 116 of IGFET 210. Upper body-material portion 318 of IGFET 230 is provided with p-type anti-punchthrough (again “APT”) dopant to raise upper portion 318 to a moderate p-type doping level. The p-type APT dopant in upper portion 318 reaches a maximum concentration at a lesser depth below the upper semiconductor surface than the maximum concentration of the p-type well dopant of well portion 316.
The combination of the total p-type dopant, i.e., the p-type well, APT, and background dopants, in the portion of body material 308 below n++ main source/drain portion 302M or 304M causes concentration NT of the total p-type dopant in that body-material portion to be relatively flat along a vertical line extending from the subsurface location of the maximum p-type dopant concentration in well 316 up to main source/drain portion 302M or 304M. In particular, concentration NT of the total p-type dopant in the portion of body material 308 below main portion 302M or 304M normally changes (decreases) by less than a factor of 10, typically by less than a factor of 5, in moving from the location of the maximum p-type dopant concentration in well 316 up to portion 302M or 304M.
The same arises with IGFET 240. The n-type well dopant that defines well portion 356 of IGFET 240 typically reaches a maximum concentration at approximately the same depth below the upper semiconductor surface as the maximum concentration of the n-type well dopant that defines well portion 276 of IGFET 220. The maximum concentration of the total n-type dopant in well portion 356 of IGFET 240 thus typically occurs at approximately the same depth below the upper surface as the maximum concentration of the total n-type dopant in well portion 276 of IGFET 220. Upper body-material portion 358 of IGFET 240 is provided with n-type APT dopant to raise upper portion 358 to a moderate n-type doping level. The n-type APT dopant in upper portion 358 reaches a maximum concentration at a lesser depth below the upper semiconductor surface than the maximum concentration of the n-type well dopant of well portion 356.
The combination of the total n-type dopant, i.e., primarily the n-type well and APT dopants, in the portion of body material 348 below source/drain zone 342M or 344M causes concentration NT of the total n-type dopant in that body-material portion to be relatively flat along a vertical line extending from the subsurface location of the maximum n-type dopant concentration in well portion 356 up to main source/drain portion 342M or 344M. Specifically, concentration NT of the total n-type dopant in the portion of body material 348 below main portion 342M or 344M normally changes by less than a factor of 10, typically by less than a factor of 5, in moving from the location of the maximum n-type dopant concentration in well 356 up to portion 342M or 344M.
Pocket portions 120 and 280 of respective IGFETs 210 and 220 may, of course, alternatively extend deeper below the upper semiconductor surface than respective sources 102 and 262. IGFET 210 then implements IGFET 150 of FIG. 13 while IGFET 220 implements a p-channel version of IGFET 150V. Halo pocket portions 320 and 322 of IGFET 230 may extend deeper below the upper semiconductor surface than source/drain zones 302 and 304 as occurs below in computer-simulated reference short-channel structure B of FIG. 33. Halo pocket portions 360 and 362 of IGFET 240 may likewise extend deeper below the upper semiconductor surface than source/drain zones 340 and 342.
FIGS. 30.1 and 30.2 (collectively “FIG. 30”) depict two portions of another complementary-IGFET structure configured according to the invention so as to be particularly suitable for mixed-signal applications. The complementary-IGFET structure of FIG. 30 contains asymmetrical p-channel IGFET 220 and an asymmetrical long n-channel IGFET 380 configured the same as asymmetrical n-channel IGFET 210 except that a heavily doped n-type subsurface layer 382 lies between p+ well portion 116 and p- lower portion 114 for isolating p+ well portion 116 and p-type upper body-material portion 118 from p- portion 114. Consequently, p-type body-material 108 for IGFET 380 does not include p- lower portion 114 but instead consist of only p+ well portion 116 and p-type upper body-material portion 118. The complementary-IGFET structure of FIG. 30 further includes symmetrical p-channel IGFET 240 and a symmetrical long n-channel IGFET 390 configured the same as symmetrical n-channel IGFET 230 except that a heavily doped n-type subsurface layer 392 lies between p+ well portion 316 and p- lower portion 114 for isolating p+ well portion 316 and p-type upper body-material portion 318 from p- portion 114. P-type body material 308 IGFET 390 thereby does not include p- lower portion 114 but instead consist only of p+ well portion and p-type upper body-material portion 318. Aside from n+ subsurface layers 382 and 392, n-channel IGFETs 380 and 390 operate respectively the same as n-channel IGFETs 210 and 230.
Circuit elements other than IGFETs 210, 220, 230, 240, 380, and 390 may be provided in other parts (not shown) of the complementary-IGFET structure of FIG. 29 or 30. For instance, short-channel versions of IGFETs 210, 220, 230, 240, 380, and 390 may be present in either complementary-IGFET structure. Bipolar transistors along with various types of resistors, capacitors, and inductors may be provided in the complementary-IGFET structure of FIG. 29 or 30. Depending on the characteristics of the additional circuit elements, suitable isolation is also provided in either complementary-IGFET structure for the additional elements. IGFETs 240 and 230 or 390 can, of course, be deleted in some purely analog complementary-IGFET applications.
Fabrication of Complementary-IGFET Structure Suitable for Mixed-Signal Applications
FIGS. 31a-31o, 31p.1-31r.1, and 31p.2-31r.2 (collectively “FIG. 31”) illustrate a semiconductor process in accordance with the invention for manufacturing a complementary-IGFET structure containing long-channel IGFETs 210, 220, 230, and 240 as generally shown in FIG. 29. The steps involved in the fabrication of IGFETs 210, 220, 230, and 240 up through the stage just before the creation of gate sidewall spacers 250, 252, 290, 292, 330, 332, 370, and 372 are shown in FIGS. 31a-31o. FIGS. 31p.1-31r.1 illustrate the fabrication of spacers 250, 252, 290, and 292 and later steps leading to IGFETs 210 and 220 as depicted in FIG. 29.1. FIGS. 31p2-31r.2 illustrate the fabrication of spacers 330, 332, 370, and 372 and later steps leading to IGFETs 230 and 240 as shown in FIG. 29.2.
Short-channel versions of IGFETs 210, 220, 230, and 240 may be manufactured simultaneously according to the fabrication steps employed in manufacturing long-channel IGFETs 210, 220, 230, and 240. The short-channel IGFETs are of lesser channel length than long-channel IGFETs 210, 220, 230, and 240 but otherwise are of generally the same intermediate IGFET appearances as shown in FIG. 31. The simultaneous fabrication of long-channel IGFETs 210, 220, 230, and 240 and their short-channel versions is implemented with masking plates (reticles) having patterns for both the long-channel and short-channel IGFETs.
Aside from the pocket (including halo pocket) ion implantation steps and the source/drain extension ion implantation steps, all of the ion implantation steps in the present fabrication process are performed roughly perpendicular to the lower semiconductor surface and thus roughly perpendicular to the upper semiconductor surface. More particularly, all of the implantation steps except the pocket and source/drain extension ion implantation steps are performed at a small angle, typically 7°, to the vertical. This small deviation from perpendicularity is used to avoid undesirable ion channeling effects. For simplicity, the small deviation from perpendicularity is not indicated in FIG. 31.
Unless otherwise indicated, the species of n-type dopant utilized in each of the n-type ion implantations in the fabrication process of FIG. 31 consists of the specified n-type dopant in elemental form. That is, each n-type ion implantation is performed with ions of the specified n-type dopant element rather than with ions of a chemical compound that contains the n-type dopant. The species of p-type dopant employed in each of the p-type ion implantations variously consists of the p-type dopant, normally boron, in elemental or compound form. Hence, each p-type ion implantation is normally performed with boron ions or with ions of a boron-containing compound.
In some of the fabrication steps in FIG. 31, openings extend (substantially) through a photoresist mask above the active semiconductor regions for two IGFETs. When the two IGFETs are formed laterally adjacent to each other in the exemplary cross sections of FIG. 31, the two photoresist openings are illustrated as a single opening in FIG. 31 even through they may be described below as separate openings.
The letter “P” at the end of a reference symbol appearing in the drawings of FIG. 31 indicates a precursor to a region which is shown in FIG. 29 and which is identified there by the portion of the reference symbol preceding “P”. The letter “P” is dropped from the reference symbol in the drawings of FIG. 31 when the precursor has evolved sufficiently to largely constitute the corresponding region in FIG. 29.
The starting point for the fabrication process of FIG. 31 is a monosilicon semiconductor body typically consisting of a heavily doped p-type substrate 400 and an overlying lightly doped p-type epitaxial layer 114P. See FIG. 31a. P+ substrate 400 is a semiconductor wafer formed with <100> monosilicon doped with boron to a concentration of approximately 5×1018 atoms/cm3 for achieving a typical resistivity of 0.015 ohm-cm. For simplicity, substrate 400 is not shown in the remainder of FIG. 31. Alternatively, the starting point can simply be a p-type substrate lightly doped substantially the same as p− epitaxial layer 114P.
Epitaxial layer 114P consists of epitaxially grown <100> monosilicon doped p type with boron to a concentration of approximately 5×1015 atoms/cm3 for achieving a typical resistivity of 5 ohm-cm. The thickness of epitaxial layer 114P is typically 5.5 μm. When the starting point for the fabrication process of FIG. 31 is a lightly doped p-type substrate, item 114P is the p− substrate.
Field-insulating region 200 is provided along the upper surface of p− epitaxial layer (or p− substrate) 114P as shown in FIG. 31b so as to define active semiconductor islands 202, 204, 206, and 208 respectively for IGFETs 210, 220, 230, and 240 going from left to right in FIG. 31b. Field insulation 200 is preferably created according to a trench-oxide technique but can be created according to a local-oxidation technique. In providing field insulation 200, a thin screen insulating layer 402 of silicon oxide is thermally grown along the upper surface of epitaxial layer 114P.
A photoresist mask 404 having openings above islands 202 and 206 is formed on screen oxide layer 402 as shown in FIG. 31c. P-type well dopant consisting of a boron species is ion implanted at a high dosage and a high energy through the uncovered sections of screen oxide 402 and into the underlying monosilicon to define (a) p+ well portion 116 for IGFET 210 and (b) a p+ precursor well portion 316P for IGFET 230. The portion of epitaxial layer 114P above well 116 constitutes a p− precursor upper body-material portion 118P for IGFET 210. Photoresist 404 is removed.
A photoresist mask 406 having an opening above island 206 is formed on screen oxide 402. See FIG. 31d. P-type APT dopant consisting of a boron species is ion implanted at a moderate dosage through the uncovered section of screen oxide 402 and into the underlying monosilicon to define a p precursor upper body-material portion 324P for IGFET 230. Photoresist 406 is removed.
A photoresist mask 408 having openings above islands 204 and 208 is formed on screen oxide 402 as shown in FIG. 31e. N-type well dopant consisting of phosphorus or arsenic is ion implanted at a high dosage and a high energy through the uncovered sections of screen oxide 402 and into the underlying monosilicon to define (a) n+ well portion 276 for IGFET 220 and (b) an n+ precursor well portion 356P for IGFET 240.
With photoresist mask 408 in place, n-type compensating dopant likewise consisting of phosphorus or arsenic is ion implanted at a low dosage and a moderate energy through the uncovered section of oxide 402 above island 204 and into the underlying monosilicon to define an n− precursor upper body-material portion 278P for IGFET 220. The n-type compensating dopant also passes through the uncovered section of oxide 402 above island 208 and into the underlying monosilicon for IGFET 240. Either of the two n-type doping operations that utilize photoresist 408 can be performed first. Photoresist 408 is removed. If it is desired that the monosilicon for IGFET 240 not receive any of the n-type compensating dopant, the n-type compensating doping operation can be performed with an additional photoresist mask having an opening above island 204 but not above island 208 (and also not above island 202 or 206) after which the additional photoresist is removed.
A photoresist mask 410 having an opening above island 208 is formed on screen oxide 402. See FIG. 31f. N-type APT dopant consisting of phosphorus or arsenic is ion implanted at a moderate dosage through the uncovered section of screen oxide 402 and into the underlying monosilicon to define an n precursor upper body-material portion 358P for IGFET 240. Photoresist 410 is removed.
A thermal anneal, such as a rapid thermal anneal (“RTA”), may now be performed on the resultant semiconductor structure to repair lattice damage and place the implanted p-type and n-type dopants in energetically more stable states so as to minimize diffusion of these dopants during subsequent thermal operations. See FIG. 31g. The upper semiconductor surface is cleaned. A gate-dielectric-containing dielectric layer 412 is provided along the upper semiconductor surface as shown in FIG. 31h. Dielectric layer 412 is created by a thermal growth technique.
Precursor gate electrodes 128P, 288P, 328P, and 368P are formed on gate-dielectric-containing dielectric layer 412 respectively above segments of upper body-material portions 118P, 278P, 318P, and 358P. See FIG. 31i. Precursor electrodes 128P, 288P, 328P, and 368P are created by depositing a layer of largely undoped (intrinsic) polysilicon on dielectric layer 412 and then patterning the polysilicon layer. The portions of dielectric layer 412 underlying precursor electrodes 128P, 288P, 328P, and 368P respectively constitute gate dielectric layers 126, 286, 326, and 366. A dielectric sealing layer 414 is thermally grown along the exposed surfaces of precursor electrodes 128P, 288P, 328P, and 368P. In the course of forming dielectric sealing layer 416, the portions of dielectric layer 412 situated to the sides of gate dielectric layers 126, 286, 326, and 366 thicken somewhat to become a composite surface dielectric layer 416.
A photoresist mask 418 having an opening generally above the intended location for p+ pocket portion 120 of IGFET 210 is formed on dielectric layers 414 and 416. See FIG. 31j. Photoresist mask 418 is critically aligned to precursor gate electrode 128P. P-type pocket dopant consisting of a boron species is ion implanted at a moderate dosage in an angled manner through the uncovered portion of surface dielectric layer 416 and into the underlying monosilicon to define a p+ precursor pocket portion 120P for IGFET 210. The p-type pocket implantation is normally performed at two opposite tilt angles to the vertical. Alternatively, the p-type pocket implantation can be performed at a single tilt angle. Photoresist 418 is removed.
A photoresist mask 420 having an opening generally above the intended location for n+ pocket portion 280 of IGFET 220 is formed on dielectric layers 414 and 416. See FIG. 31k. Photoresist mask 420 is critically aligned to precursor gate electrode 288P. N-type pocket dopant consisting of phosphorus or arsenic is ion implanted at a high dosage in an angled manner through the uncovered portion of surface dielectric 416 and into the underlying monosilicon to define an n+ precursor pocket portion 280P for IGFET 220. The n-type pocket implantation is normally performed at two opposite tilt angles but can be done at a single tilt angle. Photoresist 420 is removed.
A photoresist mask 422 having openings above islands 202 and 206 is formed on dielectric layers 414 and 416 as shown in FIG. 31l. N-type source/drain extension dopant consisting of arsenic or phosphorus is ion implanted at a high dosage through the uncovered section of surface dielectric 416 and into the underlying monosilicon to define (a) an n+ precursor source extension 102EP for IGFET 210, (b) a separate n+ precursor drain extension 104EP for IGFET 210, and (c) a pair of laterally separated n+ precursor source/drain extensions 302EP and 304EP for IGFET 230. Photoresist 422 is removed.
A photoresist mask 424 having an opening above island 206 is formed on dielectric layers 414 and 416. See FIG. 31m. P-type halo dopant consisting of a boron species is ion implanted at a high dosage in an angled manner through the uncovered section of surface dielectric 416 and into the underlying monosilicon to define a pair of laterally separated p-type precursor halo pocket portions 320P and 322P for IGFET 230. Photoresist 424 is removed.
A photoresist mask 426 having openings above islands 204 and 208 is formed on dielectric layers 414 and 416 as shown in FIG. 31n. P-type source/drain extension dopant consisting of a boron species is ion implanted at a high dosage through the uncovered section of surface dielectric 416 and into the underlying monosilicon to define (a) a p+ precursor source extension 262EP for IGFET 220, (b) a separate p+ precursor drain extension 264EP for IGFET 220, and (c) a pair of laterally separated p+ source/drain extensions 342EP and 344EP for IGFET 240. Photoresist 426 is removed.
A photoresist mask 428 having an opening above island 208 is formed on dielectric layers 414 and 416. See FIG. 31o. N+ halo dopant consisting of phosphorus or arsenic is ion implanted at a high dosage in an angled manner through the uncovered section of surface dielectric 416 and into the underlying monosilicon to define a pair of laterally separated n+ precursor halo pocket portions 360P and 362P for IGFET 240. Photoresist 428 is removed.
A low-temperature furnace anneal may be performed at this point to remove defects caused by the heavy dosages of the source/drain extension implants.
In the remainder of the process of FIG. 31, the complementary-IGFET structure at each processing stage is illustrated with a pair of FIGS. “31z.1” and “31z.2” where “z” is a letter varying from “p” to “r”. Each FIG. 31z.1 illustrates the processing done to create asymmetrical IGFETs 210 and 220 while each FIG. 31z.2 illustrates the processing simultaneously done to create symmetrical IGFETs 230 and 240. Each pair of FIGS. 31z1 and 31z.2 is, for convenience, collectively referred to below as “FIG. 31z”. For instance, FIGS. 31p.1 and 31p.2 are collectively referred to as “FIG. 31p”. 
Gate sidewall spacers 250, 252, 290, 292, 330, 332, 370, and 372 are formed along the transverse sidewalls of precursor gate electrodes 128P, 288P, 328P, and 368P as shown in FIG. 31p. The formation of sidewall spacers 250, 252, 290, 292, 330, 332, 370, and 372 is performed by depositing dielectric material on top of the structure and then removing, primarily by anisotropic etching conducted generally perpendicular to the upper semiconductor surface, the dielectric material not intended to constitute spacers 250, 252, 290, 292, 330, 332, 370, and 372. Portions of dielectric layers 414 and 416 are also partially, but not totally, removed. Items 430 and 432 in FIG. 31p respectively indicate the remainders of dielectric layers 414 and 416 not covered by spacers 250, 252, 290, 292, 330, 332, 370, and 372.
A photoresist mask 434 having openings above islands 202 and 206 is formed on dielectric layers 430 and 432 and spacers 290, 292, 370, and 372. See FIG. 31q. N-type main source/drain dopant consisting of arsenic or antimony is ion implanted at a very high dosage through the uncovered sections of surface dielectric layer 432 and into the underlying monosilicon to define (a) n++ main source portion 102M and n++ main drain portion 104M for IGFET 210 and (b) n++ main source/drain portions 302M and 304M for IGFET 230. The n-type main source/drain dopant also enters precursor electrodes 128P and 328P and converts them respectively into n++ gate electrodes 128 and 328. Photoresist 434 is removed.
The portions of regions 102EP, 104EP, and 120P outside main source portion 102M and main drain portion 104M now respectively constitute n+ source extension 102E, n+ drain extension 104E, and p+ pocket portion 120 for IGFET 210. P− upper body-material remainder 124 is the remaining lightly doped material of precursor upper body-material portion 118P, now p-type upper body-material portion 118. The portions of regions 302EP, 304EP, 320P, and 322P outside main source/drain portions 302M and 304M now respectively constitute n+ source/drain extensions 302E and 304E and p+ halo pocket portions 320 and 322 for IGFET 230. P− upper body-material remainder 324 is the remaining lightly doped p-type material of precursor upper body-material portion 318P, now p-type upper body-material portion 318.
When the main source/drain dopant consists of arsenic, a thermal anneal may now be performed to repair lattice damage, activate the main n-type source/drain dopant, and cause it to diffuse outward. This anneal, normally an RTA, also activates the pocket and source/drain extension dopants.
A photoresist mask 436 having openings above islands 204 and 208 is formed on dielectric layers 430 and 432 and spacers 250, 252, 330, and 332 as shown in FIG. 31r. P-type main source/drain dopant consisting of a boron species is ion implanted at a very high dosage through the uncovered section of surface dielectric 432 and into the underlying monosilicon to define (a) p++ main source portion 262M and p++ main drain portion 264M for IGFET 220 and (b) p++ main source/drain portions 342M and 344M for IGFET 240. The p-type main source/drain dopant also enters precursor electrodes 288P and 368P and converts them respectively into p++ gate electrodes 288 and 368. Photoresist 436 is removed.
The portions of regions 262EP, 264EP, and 280P outside main source portion 262M and main drain portion 264M now respectively constitute p+ source extension 262E, p+ drain extension 264E, and n+ pocket portion 280 for IGFET 220. N- upper body-material remainder 284 is the remaining lightly doped n-type material of n− upper body-material portion 278P, now n-type upper body-material portion 278. The portions of regions 342EP, 344EP, 360P, and 362P outside main drain portions 342M and 344M now respectively constitute p+ source/drain extensions 342E and 344E and n+ halo pocket portions 360 and 362 for IGFET 240. N− upper body-material remainder 364 is the remaining lightly doped n-type material of n− precursor upper body-material portion 358P, now n-type upper body-material portion 358.
A capping layer (not shown) of dielectric material, typically silicon oxide, is formed on top of the structure. The semiconductor structure is now thermally annealed to repair lattice damage and activate the implanted main p-type source/drain dopant. If the earlier anneal for activating the main n-type source/drain dopant is not performed, this final anneal activates the pocket dopants and all the source/drain dopants. The final anneal is typically an RTA.
The thin layers of dielectric material, including dielectric layers 430 and 432, are removed along the upper semiconductor surface and along the top surfaces of gate electrodes 128, 288, 328, and 368. Metal silicide layers 254, 256, 258, 294, 296, 298, 334, 336, 338, 374, 376, and 378 are respectively formed along the upper surfaces of regions 102M, 104M, 128, 262M, 264M, 288, 302M, 304M, 328, 342M, 344M, and 368. This typically entails depositing a thin layer of suitable metal, typically cobalt, on the upper surface of the structure and performing a low-temperature step to react the metal with underlying silicon. The unreacted metal is removed. A second low-temperature step is performed to complete the reaction of the metal with the underlying silicon and thereby form silicide layers 254, 256, 258, 294, 296, 298, 334, 336, 338, 374, 376, and 378. The metal silicide formation completes the basic fabrication of IGFETs 210, 220, 230, and 240. The resultant complementary-IGFET structure appears as shown in FIG. 29.
The p-type well, p-type APT, n-type well, n-type compensating, and n-type APT implantations of FIGS. 31c-31f can generally be performed in any order. The p-type pocket, n-type pocket, n-type source/drain extension, p-type halo, p-type source/drain extension, and n-type halo implantations of FIGS. 31j-31o can generally be performed in any order. The n-type main source/drain implantation of FIG. 31q is normally performed before the p-type main source/drain implantation of FIG. 31r, particularly when the main source/drain dopant consists of arsenic. However, the p-type main source/drain implantation can sometimes be performed before the n-type main source/drain implantation.
The tilt angles for the p-type pocket, n-type pocket, p-type halo, and n-type halo implantations of FIGS. 31j, 31k, 31m, and 31o are normally at least 15°. Although typically varying from one of the angled implantations to another, the tilt angle for each angled implantation is typically 25-45°.
The complementary-IGFET structure of FIG. 30 is, except for n+ isolation layers 382 and 392 that convert IGFETs 210 and 230 respectively into IGFETs 380 and 390, typically fabricated according to substantially the same steps as the complementary-IGFET structure of FIG. 29. Isolation layers 382 and 392 are normally formed between the stages of FIGS. 31b and 31c using an additional photoresist mask having openings above islands 202 and 206. The additional photoresist mask also has openings for use in creating heavily doped n-type regions that connect isolation layers 382 and 392 to the upper semiconductor surface for receiving a suitable isolation voltage. Isolation dopant consisting of arsenic or phosphorus is ion implanted at a high dosage through the uncovered sections of screen oxide 402 and into the underlying monosilicon to define (a) n+ isolation layers 382 and 392 respectively for IGFETs 380 and 390 and (b) the n+ isolation-layer connection regions.
General Computer Simulations
Computer simulations were conducted to check the device characteristics and performance advantages, especially for analog applications, of IGFETs configured according to the invention. The simulations were performed with (a) the MicroTec two-dimensional device simulator furnished by Siborg Systems and (b) the Medici two-dimensional device simulator furnished by Avant! Corp. The MicroTec simulator was utilized primarily for large-signal (DC) simulations. The Medici simulator was employed primarily for small-signal simulations.
Two types of n-channel IGFETs were computer simulated at the device level: (a) asymmetrical n-channel IGFETs configured according to the invention and (b) symmetrical reference n-channel IGFETs generally corresponding to (but different inventively from) the computer-simulated inventive n-channel IGFETs. The inventive computer-simulated asymmetrical IGFETs are generally indicated below as being of structure “A”. The computer-simulated inventive asymmetrical IGFETs of structure A generally correspond to long n-channel IGFET 150 of FIG. 13 or to a short-channel version of IGFET 150. The reference computer-simulated symmetrical IGFETs are generally identified below as being of structure “B”. The computer-simulated reference IGFETs of structure B generally correspond to long n-channel IGFET 230 of FIG. 29 or to a short-channel version of IGFET 230 for the situation in which halo pocket portions 320 and 322 of IGFET 230 extend respectively below source/drain zones 302 and 304.
Structures A and B were based on analytical-profile models generally using Gaussian dopant profiles. It was assumed that each set of structures A and B under comparison was manufactured according to the same process flow such as that of FIG. 31. Aside from variations that produce the body-material doping features of the invention and except as otherwise indicated below, structures A and B in each set under comparison had substantially the same dopant distributions. Structures A and B in each set under comparison also had basically the same geometrical dimensions, e.g., gate length, gate stack height, and source/drain length. The computer simulations on structures A and B represented devices manufactured at the 0.18-μm technology node, i.e., IGFETs manufactured with design rules for which the minimum printable feature size was 0.18 μm.
The computer simulations were generally directed toward enhancing analog performance. Structure B was thus computer simulated at parameter values expected to yield enhanced analog performance for structure A. Inasmuch as the basic architecture of structure B is for digital applications, the values for certain of the parameters used in computer simulating structure B to achieve enhanced analog performance differed from the parameter values that would yield enhanced digital performance.
It was assumed that the structures A and B were arranged in multiple-IGFET structures and that insulation-filled trenches were used to implement a field-insulating region, such as field-insulating region 200 in FIG. 29, for laterally isolating the IGFETs in the multiple-IGFET structures. The trenches, which had the same dimensions for structures A and B, were up to 50% deeper and up to 20% wider than that needed to optimize digital performance for reference structure B in the multiple-IGFET structures. The IGFET wells, corresponding to p+ well portions 116 and 316 respectively for asymmetrical n-channel IGFET 150 in FIG. 13 and symmetrical n-channel IGFET 230 in FIG. 29, were up to 20% deeper than that needed to optimize the digital performance of structure B. It appears that inventive structure A may need the wider/deeper trenches in order to maintain good inter-IGFET isolation in the presence of a well whose average dopant concentration is lower than that of structure B optimized for digital performance.
Structures A and B both had the same threshold voltage VT and the same background p-type dopant concentration, namely 0.4 V and 5×1015 atoms/cm3. For a given value of the p-type background dopant concentration, the p-type implants which define the pocket portions along the upper semiconductor surfaces of structures A and B control their threshold voltages VT. In light of this, the peak upper-surface concentration of the p-type pocket dopant in the single pocket portion of inventive structure A was appropriately adjusted to achieve the same threshold voltage that arose from the peak upper-surface concentrations of the p-type halo dopant in the two halo pocket portions of reference structure B. More specifically, inventive asymmetrical structure A received a higher dosage of the pocket dopant than reference symmetrical structure B to accommodate the fact that reference structure B had twice as many pocket portions as inventive structure A.
FIG. 32 presents a three-dimensional dopant profile for a short-channel version of inventive asymmetrical structure A. A three-dimensional dopant profile for a corresponding short-channel version of reference symmetrical structure B is presented in FIG. 33. FIGS. 32 and 33 specifically illustrate net dopant concentration NN as a function of longitudinal distance x and depth y. Only the monosilicon portions of structures A and B are presented in FIGS. 32 and 33. Although both of structures A and B were assumed to be fabricated at the 0.18-μm technology node, structures A and B in FIGS. 32 and 33 of the computer simulations both had a gate length LG of 0.2 μm leading to a channel length L of 0.12-0.14 μm due to lithography alignment.
Inventive structure A in FIG. 32 basically corresponds to a short-channel version of IGFET 150. Reference structure B in FIG. 33 basically corresponds to a short-channel version of IGFET 230 subject to structure B in FIG. 33 being simulated generally at the parameter values which enhance the analog performance of structure A and again subject to halo pocket portions 320 and 322 of IGFET 230 extending respectively below source/drain zones 302 and 304. For convenience, short-channel structures A and B in FIGS. 32 and 33 are labeled with the reference symbols employed to identify the monosilicon regions of IGFETs 150 and 230. Since structure B was simulated for analog applications, source/drain zones 302 and 304 are respectively labeled as source 302 and drain 304 in FIG. 33. Except as indicated below, all references to the short-channel versions of structures A and B, or to short-channel structures A and B, mean the versions respectively shown in FIGS. 32 and 33.
Each source/drain zone of each structure A or B consisted of a very heavily doped main portion and a more lightly doped, but still heavily doped, lateral extension. Drain 104 of the short-channel version of structure A thus consisted of main portion 104M and lateral extension 104E. However, regions 104M and 104E of drain 104 are difficult to distinguish in FIG. 32 and are therefore not separately labeled in FIG. 32.
FIGS. 34 and 35 present two-dimensional dopant contours for the respective short-channel versions of structures A and B. The dopant contours are taken along vertical planes through short-channel structures A and B. Regions 104M and 104E of short-channel structure A can be clearly distinguished in FIG. 34 and are so labeled in FIG. 34. As shown by the locations of pn junctions 110 and 112 in FIG. 34, main drain portion 104M of short-channel structure A extends deeper below the upper semiconductor surface than main source portion 102M. That is, drain depth yD is greater than source depth yS for inventive asymmetrical structure A. In contrast, an examination of FIG. 35 shows that main source portion 302M and main drain portion 304M in reference symmetrical structure B extend to virtually the same depth below the upper surface.
FIG. 36 presents net dopant concentration NN along the upper semiconductor surface as a function of longitudinal distance x from a source/drain zone location for the short-channel versions of structures A and B. The source/drain zone location is approximately 3.5 μm from the channel-zone center. In FIG. 36 and in the later graphs which present computer-simulated data for both of structures A and B, curves representing data for inventive structure A are marked with small empty circles to distinguish that data from the data for reference structure B whose data curves are not additionally marked. At locations where structures A and B have substantially identical data, the curve segments for structures A and B are not visually distinguishable from each other.
Analogous to the curve segments in FIGS. 12c and 14c, curve segments 102M*, 104M*, 102E*, and 104E* in FIG. 36 represent concentrations NN of the net n-type dopant respectively in regions 102M, 104M, 102E, and 104E along the upper semiconductor surface of inventive short-channel structure A. Curve segments 106* and 120* in FIG. 36 represent concentrations NN of the net p-type dopant respectively in regions 106 and 120 along the upper semiconductor surface of short-channel structure A. Curve segments 302M*, 304M*, 302E*, and 304E* represent concentrations NN of the net p-type dopant respectively in regions 302M, 304M, 302E, and 304E along the upper semiconductor surface of reference short-channel structure B. Curve segments 306*, 320*, and 322* represent concentrations of the net p-type dopant respectively in regions 306, 320, and 322 along the upper semiconductor surface of short-channel structure B.
Curve segment 106* in FIG. 36 illustrates the asymmetrical dopant grading in channel zone 106 of inventive short-channel structure A. In particular, curve segment 106* shows that net dopant concentration NN along the upper semiconductor surface of short-channel structure A reaches a high value at approximately 1×1018 atoms/cm3 near source 102 and then decreases progressively in moving from the location of that high value across channel zone 106 toward drain 104. Although not shown in the computer simulations, concentration NT of the total p-type dopant in channel zone 106 of short-channel structure A is at least a factor of 10 lower, typically a factor of more than 100 lower, where zone 106 meets source 102 along the upper surface than where zone 106 meets drain 104 along the upper surface. In contrast, the symmetrical dopant grading in channel zone 316 of short-channel structure B is illustrated by curve segment 306* which shows that concentration NN along the upper surface of short-channel structure B is at approximately equal peak values close to source 102 and drain 104 and is at a slightly lower value in the middle of channel zone 306.
FIG. 37a presents absolute (total p-type and total n-type) vertical dopant profiles through source/drain locations for the short-channel versions of structures A and B. With reference to longitudinal distance x in FIG. 36, absolute dopant concentrations NT for inventive short-channel structure A are illustrated in FIG. 37a along a vertical line (or plane) through main source portion 102M at distance x equal to 0.0 μm and along a vertical line through main drain portion 104M at distance x equal to 0.7 μm. FIG. 37a similarly depicts concentrations NT for reference short-channel structure B along a vertical line extending through main source portion 302M at distance x equal to 0.0 μm and along a vertical line extending through main drain portion 304M at distance x equal to 0.7 μm.
Analogous to the curve segments in FIGS. 8band 10b, curve segments 102′′ and 104′′ in FIG. 37a respectively correspond to source 102 and drain 104 of inventive short-channel structure A and thus represent concentrations NT of the total n-type dopant along the vertical lines respectively at distances x of 0.0 and 0.7 μm through source 102 and drain 104, specifically main source portion 102M and main drain portion 104M. Curve segments 114′′, 116′′, 118′′, 120′′, and 124′′ in FIG. 37a respectively generally correspond to regions 114, 116, 118, 120, and 124 of short-channel structure A and thus variously represent concentrations NT of the total p-type dopant along the vertical lines at distances x of 0.0 and 0.7 μm through source 102 and drain 104. Items 110# and 112# respectively indicate pn junctions 110 and 112 for short-channel structure A.
The maximum concentration of the p-type well dopant in well portion 116 of inventive short-channel structure A occurs at depth yw equal to 0.7 μm as indicated by curve segment 116′′ in FIG. 37a. The combination of curve segments 116′′ and 124′′ illustrates the hypoabrupt nature of the vertical dopant profile below drain 104 along the vertical line at distance x equal to 0.7 μm in short-channel structure A. In particular, combined curve segment 116′′/124′′ shows that concentration NT of the total p-type dopant in the body material portion directly below drain 104 decreases by a factor of approximately 100, and thus by a factor considerably in excess of 10, in moving from the location of the maximum p-type well dopant concentration up to pn junction 112 at the bottom of drain 104.
Curve segments 302′′ and 304′′ in FIG. 37a respectively correspond to source 302 and drain 304 of reference short-channel structure B and represent concentrations NT of the total n-type dopant along the vertical lines at distances x of 0.0 and 0.7 μm through source 302 and drain 304. Curve segments 114′′, 316′′, 318′′, 320′′, and 322′′ respectively correspond to regions 114, 316, 318, 320, and 322 of short-channel structure B and thus represent concentrations NT of the total p-type dopant along the vertical lines at distances x of 0.0 and 0.7 μm through source 302 and drain 304. In this regard, curve segment 114′′ is utilized for both of short-channel structures A and B.
As is the situation with well portion 116 of inventive short-channel structure A, the maximum concentration of the p-type well dopant in well portion 316 of reference short-channel structure B occurs at depth yw equal to 0.7 μm as indicated by curve segment 316′′ in FIG. 37a. However, the combination of curve segments 316′′, 318′′, and 322′′ (or 320′′) is relatively flat in the p-type body-material portion directly below drain 304 (or source 302) in short-channel structure B. Combined curve segments 316′′/318′′/322′′ shows that concentration NT of the total p-type dopant in the body-material portion directly below drain 304 changes considerably less than a factor of 5, and thus considerably less than a factor of 10, in moving from the location of the maximum p-type well dopant concentration up to the pn junction at the bottom of drain 304. That is, the vertical dopant profile in the body-material portion directly below drain 304 in short-channel structure B is not hypoabrupt.
The net vertical dopant profiles corresponding to the absolute vertical dopant profiles of FIG. 37a are presented in FIG. 37b for the short-channel versions of structures A and B. Curve segments 102* and 104* in FIG. 37b represent concentrations NN of the net n-type dopant respectively in source 102 and drain 104 of inventive short-channel structure A along the vertical lines respectively at distances x of 0.0 and 0.7 μm through source 102 and drain 104, specifically through main source portion 102M and main drain portion 104M. Curve segments 114*, 116*, 120*, and 124* represent concentrations NN of the net p-type dopant respectively in regions 114, 116, 120, and 124 of short-channel structure A variously along the vertical lines at distances x of 0.0 and 0.7 μm through source 102 and drain 104.
Curve segments 302* and 304* in FIG. 37b represent concentrations NN of the net n-type dopant respectively in source 302 and drain 304 of reference short-channel structure B along the vertical lines respectively at distances x of 0.0 and 0.7 μm through source 302 and drain 304, specifically through main source portion 302M and main drain portion 304M. Curve segments 114*, 316*, and 318* represent concentrations NN of the net p-type dopant respectively in regions 114, 316, and 318 of short-channel structure B along the vertical lines at distances x of 0.0 and 0.7 μm through source 302 and drain 304. Curve segment 114* is used for both of short-channel structures A and B.
FIGS. 38a and 38b respectively illustrate lineal linear-range transconductance gmw and lineal saturation transconductance gmsatw as a function of gate-to-source voltage VGS for the short-channel versions of structures A and B for which gate length LG was 0.2 μm. FIGS. 39a and 39b respectively illustrate lineal linear-range transconductance gmw and lineal saturation transconductance gmsatw as a function of gate-to-source voltage VGS for long-channel versions of structures A and B configured basically the same as short-channel structures A and B except that gate length LG was 0.5 μm. Drain-to-source voltage VDS was 0.1 V for the linear-range gmw graphs of FIGS. 38a and 39a. Voltage VDS was 2.0 V for the saturation gmsatw graphs of FIGS. 38b and 39b. FIGS. 38a, 38b, 39a, and 39b also indicate the variation of lineal drain current IDw from which linear-range transconductance gmw and saturation transconductance gmsatw were determined.
As FIGS. 39a and 39b show, the long-channel version of inventive structure A exhibited considerably higher transconductance, both linear-range transconductance gmw and saturation transconductance gmsatw, than the long-channel version of reference structure B. The short-channel version of structure A exhibited slightly higher, roughly 10% higher, linear-range transconductance gmw than the short-channel version of structure B as indicated in FIG. 38a. FIG. 38b shows that the short-channel version of structures A and B had nearly the same gmsatw characteristics. The generally higher values of transconductances gmw and gmsatw for inventive structure A enable it to have higher voltage gain and thus improve its analog performance.
FIG. 40 illustrates the current-voltage transfer characteristics, i.e., the variation of lineal drain current IDw with gate-to-source voltage VGS, for the short-channel versions of structures A and B at a drain-to-source voltage VDS of 2.0 V. As FIG. 40 shows, short-channel structures A and B had almost identical current-voltage characteristics. It is also expected that long-channel versions of structures A and B would have largely the same current-voltage characteristics.
Taking note of the fact that upper body-material portion 318 of reference short-channel structure B is provided with a high concentration of the p-type APT dopant for helping to avoid punchthrough in short-channel structure B, the nearly identical current-voltage characteristics for short-channel structures A and B show that the absence, in inventive structure A, of the p-type APT dopant at the location generally similar to that of the p-type APT dopant in reference structure B does not lead to punchthrough in inventive structure A. The qualitative physical explanation for this result is that the p-type pocket dopant performs the anti-punchthrough function in structure A. More particularly, the p-type pocket implant in pocket portion 120 of inventive structure A is provided at a greater doping than the p-type halo implant in either of pocket portions 320 and 322 of reference structure B in order for structure A to have the same threshold voltage VT as structure B. This difference can be seen by comparing curve segment 120* in FIG. 36 to curve segments 320* and 322*. The higher doping of the p-type pocket implant in inventive structure A enables it to avoid punchthrough while simultaneously operating at low current leakage comparable to that of reference structure B.
The preceding conclusion is further supported by computer-simulated data obtained on a further reference short-channel IGFET structure C lacking the APT implant of reference short-channel structure B but otherwise identical to short-channel structure B. The current-voltage transfer characteristics for reference structure C at the VDS value of 2.0 V are indicated by the curve tagged C in FIG. 40. Leakage current ID0w is the value of drain current IDw at a zero value of gate-to-source voltage VGS. As FIG. 40 shows, drain leakage current ID0w is approximately 50 times higher for reference structure C than for inventive short-channel structure A. This indicates that punchthrough occurs in structure C.
FIG. 41 depicts lineal drain current IDw as a function of drain-to-source voltage VDS for the short-channel versions of structures A and B at values of gate-to-source voltage VGS ranging from 0.5 V to 2.0 V. As indicated in FIG. 41, inventive short-channel structure A generally achieves slightly higher values of drain current IDw than reference short-channel structure B at each of the indicated VGS values. Inventive structure A thus has lower channel resistance than reference structure B. Also, drain current IDw increases less with increasing drain-to-source voltage VDS at high voltage VDS in inventive structure A than in reference structure B. This indicates that less avalanche multiplication or/and less channel-width modulation occurs in inventive structure A than in reference structure B.
Analytical Analysis and Performance Advantages of Inventive Asymmetrical IGFETs
For good analog performance, the source of an IGFET should be as shallow as reasonably possible in order to avoid roll-off of threshold voltage VT at short-channel length. The source should also be as heavily doped as possible in order to maximize the IGFET's effective transconductance gmeff in the presence of the source resistance RS. Effective transconductance gmeff is determined from the IGFET's intrinsic transconductance gm as:




  
    
      
        
          g
          meff
        
        =
        
          
            g
            m
          
          
            1
            +
            
              
                R
                S
              
              ⁢
              
                g
                m
              
            
          
        
      
    
    
      
        (
        1
        )
      
    
  




As Eq. 1 indicates, reducing source resistance RS causes effective transconductance gmeff to increase. The voltage drop across source resistance RS also subtracts from the intrinsic gate-to-source voltage so that actual gate-to-source voltage VGS is at a lower value. This debiases the IGFET at its gate electrode. In short, source resistance RS should be as low as reasonably possible.

The need to minimize source resistance RS in order to maximize effective transconductance gmeff is in addition to the need to have a lower series resistance at the source and drain of an IGFET in order to achieve a lower of value of the IGFET's on-resistance Ron. More particularly, the voltage drop across source resistance RS adds to the total source-to-drain voltage drop. This causes on-resistance Ron to increase.
For achieving high-voltage capability and reducing hot-carrier injection, the drain of an IGFET should be as deep and lightly doped as reasonably possible. These needs should be met without causing on-resistance Ron to increase significantly and without causing short-channel threshold voltage roll-off.
The parasitic capacitances of an IGFET play an important role in setting the speed performance of the circuit containing the IGFET, particularly in small-signal high-frequency operations. FIG. 42 illustrates parasitic capacitances CDB, CSB, CGB, CGD, and CGS variously associated with drain electrode D, source electrode S, gate electrode E, and body-region electrode B of an n-channel IGFET Q where CDB represents the drain-to-body capacitance, CSB represents the source-to-body capacitance, CGB represents the gate-to-body capacitance, CGD represents the gate-to-drain capacitance, and CGS represents the gate-to-source capacitance. A small-signal equivalent model of IGFET Q is presented in FIG. 43 where VBS is the body-to-source voltage, gmb is the transconductance of the body electrode, and items 440 and 442 are current sources.
The bandwidth of an amplifier is defined as the value of the frequency at which the amplifier's gain drops to 1/√{square root over (2)} (approximately 0.707) of its low-frequency value. It is generally desirable that the amplifier's bandwidth be as great as possible.
IGFET Q of FIG. 42 can be arranged in three primary amplifier configurations for providing amplified output voltage Vout as a function of input voltage Vin according to the relationship:

Vout=HAVin  (2)

where HA is the IGFET's complex transfer function. These three configurations are the common-source, common-gate, and common-drain configurations respectively shown in FIGS. 44a-44c where CL is the load capacitance, VDD is the high supply voltage, and VSS is the low supply voltage. Amplifier input voltage Vin is supplied from a voltage source 444. Element 446 in FIGS. 44b and 44c is a current source, while signal VG in FIG. 44b is the gate voltage. An examination of transfer functions HA for the three configurations of FIGS. 44a-44c shows that reducing parasitic drain-to-body capacitance CDB and/or parasitic source-to-body capacitance CSB improves IGFET performance in each of these configurations.

Transfer function HA for the common-source amplifier configuration of FIG. 44a is the input-pole/output-pole function:




  
    
      
        
          H
          A
        
        =
        
          
            
              -
              
                g
                m
              
            
            ⁢
            
              R
              D
            
          
          
            
              (
              
                1
                +
                
                  s
                  /
                  
                    ω
                    in
                  
                
              
              )
            
            ⁢
            
              (
              
                1
                +
                
                  s
                  /
                  
                    ω
                    out
                  
                
              
              )
            
          
        
      
    
    
      
        (
        3
        )
      
    
  




where RD is the drain (series) resistance, ωin is the angular frequency at the input pole, ωout is the angular frequency at the output pole, and s is the complex frequency operator equal to jω for which ω is the angular frequency. The parasitic capacitances of IGFET Q in the common-source configuration enter Eq. 3 by way of pole frequencies ωin and ωout given respectively as:





  
    
      
        ω
        in
      
    
    
      =
    
    
      
        1
        
          
            R
            S
          
          ⁡
          
            [
            
              
                C
                GS
              
              +
              
                
                  (
                  
                    1
                    +
                    
                      
                        g
                        m
                      
                      ⁢
                      
                        R
                        D
                      
                    
                  
                  )
                
                ⁢
                
                  C
                  GD
                
              
            
            ]
          
        
      
    
    
      
        (
        4
        )
      
    
  
  
    
      
        ω
        out
      
    
    
      =
    
    
      
        1
        
          
            R
            D
          
          ⁡
          
            (
            
              
                C
                DB
              
              +
              
                C
                GD
              
              +
              
                C
                L
              
            
            )
          
        
      
    
    
      
        (
        5
        )
      
    
  




Parasitic drain-to-body capacitance CDB appears in output pole frequency ωout of Eq. 5. For the situation in which source resistance RS is zero in the common-source configuration, input pole frequency ωin is infinite in accordance with Eq. 4. The bandwidth of IGFET Q in FIG. 44a then equals ωout as given by Eq. 5. Output pole frequency ωout decreasing with increasing drain-to-body capacitance CDB for given values of drain resistance RD and parasitic gate-to-drain capacitance CGD. Reducing parasitic drain-to-body capacitance CDB thus desirably increases the bandwidth for the common-source configuration of IGFET Q in FIG. 44a. 
Also, parasitic drain-to-body capacitance CDB in the common-source configuration is in parallel with load capacitance CL as shown in FIG. 44a. Reducing drain-to-body capacitance CDB thus advantageously reduces its output loading effect.
Transfer function HA for the common-gate amplifier configuration of FIG. 44b is the input-pole/output-pole function:




  
    
      
        
          H
          A
        
        =
        
          
            (
            
              
                g
                m
              
              +
              
                g
                mb
              
            
            )
          
          
            
              [
              
                1
                +
                
                  
                    (
                    
                      
                        g
                        m
                      
                      +
                      
                        g
                        mb
                      
                    
                    )
                  
                  ⁢
                  
                    R
                    S
                  
                
              
              ]
            
            ⁢
            
              (
              
                1
                +
                
                  s
                  /
                  
                    ω
                    in
                  
                
              
              )
            
            ⁢
            
              (
              
                1
                +
                
                  s
                  /
                  
                    ω
                    out
                  
                
              
              )
            
          
        
      
    
    
      
        (
        6
        )
      
    
  




where input pole frequency ωin for the common-gate configuration is given as:





  
    
      
        
          ω
          in
        
        =
        
          
            1
            +
            
              
                (
                
                  
                    g
                    m
                  
                  +
                  
                    g
                    mb
                  
                
                )
              
              ⁢
              
                R
                S
              
            
          
          
            
              R
              S
            
            ⁡
            
              (
              
                
                  C
                  GS
                
                +
                
                  C
                  SB
                
              
              )
            
          
        
      
    
    
      
        (
        7
        )
      
    
  




Reducing source-to-body capacitance CSB causes input pole frequency ωin to increase. This enables the performance of IGFET Q to improve in the common-gate configuration of FIG. 44b. 

Output pole frequency ωout is given by Eq. 5 for the common-gate amplifier configuration of FIG. 44b. Reducing parasitic drain-to-body capacitance CDB therefore increases the bandwidth of the common-gate configuration of FIG. 44b. 
Transfer function HA for the common-drain amplifier configuration of FIG. 44c is the single-zero/single-pole function:




  
    
      
        
          H
          A
        
        =
        
          
            (
            
              1
              +
              
                s
                /
                
                  ω
                  z
                
              
            
            )
          
          
            (
            
              1
              +
              
                s
                /
                
                  ω
                  p
                
              
            
            )
          
        
      
    
    
      
        (
        8
        )
      
    
  




where ωz is the angular frequency at the zero, and ωp is the angular frequency at the pole. The parasitic capacitances enter Eq. 8 by way of zero frequency ωz and pole frequency ωp given respectively as:





  
    
      
        ω
        z
      
    
    
      =
    
    
      
        
          g
          m
        
        
          C
          GS
        
      
    
    
      
        (
        9
        )
      
    
  
  
    
      
        ω
        p
      
    
    
      =
    
    
      
        
          g
          m
        
        
          
            
              g
              m
            
            ⁢
            
              R
              S
            
            ⁢
            
              C
              GD
            
          
          +
          
            C
            SB
          
          +
          
            C
            GS
          
          +
          
            C
            L
          
        
      
    
    
      
        (
        10
        )
      
    
  




Parasitic source-to-body capacitance CSB appears in pole frequency ωp of Eq. 10. By reducing capacitance CSB, pole frequency ωp increases. This improves the frequency characteristics of IGFET Q in the common-drain configuration of FIG. 44c. 

Similar to what occurs with drain-to-body capacitance CDB in the common-source configuration of FIG. 44a, parasitic source-to-body capacitance CSB is in parallel with load capacitance CL in the common-drain configuration as shown in FIG. 44c. Reducing source-to-body capacitance CSB thus beneficially reduces its loading effect in the common-drain configuration.
FIG. 45 illustrates a small-signal model of the shorted-output version of the common-source amplifier configuration of FIG. 44a. In the small-signal model of FIG. 45, drain electrode D of IGFET Q is electrically shorted to source electrode S. FIG. 46 presents a small-signal equivalent circuit of the model of IGFET Q of FIG. 45. Element 448 in FIG. 46 is a voltage-controlled current source. Items vgs, ii, and io, in FIGS. 45 and 46 respectively are the small-signal gate-to-source (input) voltage, the small-signal input current, and the small-signal output current.
Cut-off frequency fT of an IGFET is defined as the frequency at which the absolute value of current gain AI of the IGFET's shorted-output common-source configuration falls to 1. That is,




  
    
      
        
          
          
            
              A
              I
            
            ⁡
            
              (
              f
              )
            
          
          
        
        =
        
          
            
              
                
                
                  
                    I
                    out
                  
                  ⁡
                  
                    (
                    f
                    )
                  
                
                
              
              
                I
                in
              
            
            ⁢
            
              ❘
              
                
                  V
                  out
                
                =
                0
              
            
          
          =
          1
        
      
    
    
      
        (
        11
        )
      
    
  




Cut-off frequency fT is derived from the small-signal equivalent circuit of FIG. 46 as:





  
    
      
        
          f
          T
        
        =
        
          
            g
            m
          
          
            2
            ⁢
            
              π
              ⁡
              
                (
                
                  
                    C
                    GS
                  
                  +
                  
                    C
                    GD
                  
                  +
                  
                    C
                    GB
                  
                
                )
              
            
          
        
      
    
    
      
        (
        12
        )
      
    
  




Capacitance CGB in Eq. 12 is the parasitic capacitance between gate electrode G and the IGFET's body region outside the active area occupied by IGFET Q.

Increasing transconductance gm of an amplifying IGFET generally improves its performance capability because its voltage gain generally increases. Since cut-off frequency fT increases with increasing transconductance gm according to Eq. 12, increased frequency fT is an indicator of improved IGFET performance.
In the classical long-channel model of IGFET Q for which source resistance RS is zero, transconductance gm is:




  
    
      
        
          g
          m
        
        =
        
          
            (
            
              W
              L
            
            )
          
          ⁢
          
            μ
            n
          
          ⁢
          
            
              C
              GIa
            
            ⁡
            
              (
              
                
                  V
                  GS
                
                -
                
                  V
                  T
                
              
              )
            
          
        
      
    
    
      
        (
        13
        )
      
    
  




where W is the IGFET width, μn is the electron mobility, and CGIa is the gate dielectric capacitance per unit area. In the short-channel velocity-saturation model of IGFET Q, transconductance gm is:

gm=WvnsatCGIa  (14)

where vnsat is the electron saturation velocity since IGFET Q is an n-channel device. Examination of Eqs. 13 and 14 shows that transconductance gm is proportional to areal gate dielectric capacitance CGIa in both the long-channel and short-channel models.

For the classical long-channel model of IGFET Q in saturation, capacitances CGS, CGD, and CGB are:




  
    
      
        
          C
          GS
        
        =
        
        ⁢
        
          
            
              (
              
                2
                3
              
              )
            
            ⁢
            
              WLC
              GIa
            
          
          +
          
            
              WL
              GSoverlap
            
            ⁢
            
              C
              GIa
            
          
        
      
    
    
      
        
        ⁢
        
          (
          15
          )
        
      
    
  
  
    
      
        
          C
          GD
        
        =
        
        ⁢
        
          
            WL
            GDoverlap
          
          ⁢
          
            C
            GIa
          
        
      
    
    
      
        
        ⁢
        
          (
          16
          )
        
      
    
  
  
    
      
        
          C
          GB
        
        =
        
        ⁢
        
          WLC
          GIa
        
      
    
    
      
        
        ⁢
        
          (
          17
          )
        
      
    
  




where LGSoverlap and LGDoverlap are the longitudinal distances that the gate electrode respectively overlaps the source and drain of IGFET Q. The term WLGSoverlapCGIa is the parasitic capacitance arising from the gate electrode overlapping the source. The term WLGDoverlapCGIa is the parasitic capacitance arising from the gate electrode overlapping the drain. Inserting Eqs. 15-17 into Eq. 12 yields cut-off frequency fT for an ideal long-channel IGFET in saturation.

Eqs. 15 and 16 are not expected to be accurate for the asymmetrical IGFETs of the invention due to the asymmetrical longitudinal dopant grading in their channel zones. However, Eqs. 15 and 16 can be used as trend indicators in calculating parasitic capacitances CGS and CGD for assessing cut-off frequencies fT of the present asymmetrical IGFETs. More accurate values of capacitances CGS and CGD can be determined by computer simulation.
Cut-off frequency fT involves, by definition, the short-circuit condition at the output of the common-source configuration. As a result, frequency fT essentially obliterates the effects of parasitic drain-to-body capacitance CDB. Also, frequency fT does not reflect the effect of parasitic source-to-body capacitance CSB since it utilizes the common-source configuration.
Cut-off frequency fT has a peak cut-off value fTpeak dependent on the operating current, i.e., drain current ID. Although peak cut-off frequency fTpeak is useful in assessing high-frequency IGFET performance, circuits typically operate at frequencies one to two factors of 10 lower than peak value fTpeak. In addition to peak value fTpeak being high for an IGFET, it is generally desirable that there be reduced variation of frequency fT with decreasing operating current below operating current levels corresponding to peak value fTpeak.
Source-body and drain-body junctions, such as pn junctions 110 and 112 in inventive IGFETs 100, 140, 150, 160, 170, 180, 190, and 230, are normally reverse biased. When a pn junction is in reverse bias, the depletion region along the junction exhibits an areal small-signal capacitance Cda given as:




  
    
      
        
          C
          da
        
        =
        
          
            
              K
              S
            
            ⁢
            
              ɛ
              0
            
          
          
            t
            d
          
        
      
    
    
      
        (
        18
        )
      
    
  




where ∈0 is the absolute permittivity, KS is the relative permittivity of the semiconductor material, and td is the voltage-dependent thickness of the depletion region.

For a pn junction formed along uniformly doped substrate, thickness td for the depletion region of such an ideal pn junction is:




  
    
      
        
          t
          d
        
        =
        
          
            
              2
              ⁢
              
                K
                S
              
              ⁢
              
                
                  ɛ
                  0
                
                ⁡
                
                  (
                  
                    
                      V
                      R
                    
                    +
                    
                      V
                      BI
                    
                  
                  )
                
              
            
            
              qN
              
                B
                ⁢
                
                  
                
                ⁢
                0
              
            
          
        
      
    
    
      
        (
        19
        )
      
    
  




where VR is the applied reverse voltage, VBI is the built-in voltage of the junction, q is the electronic charge, and NB0 is the uniform background dopant concentration in the substrate. Built-in voltage VBI varies with background dopant concentration NB0 according to the relationship:





  
    
      
        
          V
          BI
        
        =
        
          
            (
            
              
                2
                ⁢
                kT
              
              q
            
            )
          
          ⁢
          
            ln
            ⁡
            
              (
              
                
                  N
                  
                    B
                    ⁢
                    
                      
                    
                    ⁢
                    0
                  
                
                
                  n
                  i
                
              
              )
            
          
        
      
    
    
      
        (
        20
        )
      
    
  




where k is Boltzmann's constant, T is the temperature, and ni is the intrinsic carrier concentration.

FIG. 47 illustrates how net dopant concentration ND−NA varies with distance y into p-type substrate material of a model of a pn junction that can have any of three basic types of dopant profiles in the p-type substrate, where ND and NA respectively are the absolute donor and acceptor dopant concentrations. The junction model is also shown in FIG. 47. As indicated by the illustrated junction model, the p-type material is thicker, and thus more lightly doped, than the junction's n-type material. Curves 450, 452, and 454 in FIG. 47 respectively indicate examples of hypoabrupt, abrupt, and hyperabrupt profiles in the p-type material. Distance yd indicates the thickness of the p-type portion of the depletion region along the junction.
Hypoabrupt profile curve 450 approximately represents the vertical dopant profile below drain-body junction 112 in each of inventive n-channel IGFETs 100, 140, 150, 160, 170, 180, 190, 210, 100V, 140V, 150V, 160V, 170V, 180V, and 190V. In IGFETs 150, 160, 180, 190, 210, 150V, 160V, 180V, and 190V where drain 104 includes main drain portion 104M and lateral drain extension 104E, curve 450 specifically represents the vertical dopant profile below the portion of drain-body junction 112 along the bottom of main portion 104M. For IGFETs 170, 180, 190, 170V, 180V, and 190V in which source 102 extends deeper below the upper semiconductor surface than pocket portion 120, curve 450 also represents the vertical dopant profile below source-body junction 110, specifically below the junction portion along the bottom of main source portion 102M for each of IGFETs 180, 190, 180V, and 190V. Subject to the conductivity types being reversed, curve 450 further represents the vertical dopant profile below the bottom of main portion 264M of drain 264 in IGFET 220. Flat curve 452 represents the p-type material of the ideal abrupt pn junction covered by Eqs. 18-20.
FIG. 48 depicts how parasitic areal capacitance Cda of the depletion region varies with reverse voltage VR across the pn junction modeled in FIG. 47. Curves 460, 462, and 464 in FIG. 48 indicate the Cda variations respectively for curves 450, 452, and 454 in FIG. 47. In particular, curve 462 qualitatively indicates the power-law variation of capacitance Cda for the ideal pn junction as determined from Eq. 18 using the td data from Eq. 19 (and the VBI data from Eq. 20).
Parasitic drain-to-body capacitance CDB along junction 112 in inventive n-channel IGFET 100, 140, 150, 160, 170, 180, 190, 210, 100V, 140V, 150V, 160V, 170V, 180V, or 190V is approximately proportional to areal depletion capacitance Cda as represented in FIG. 48 by curve 460 corresponding to hypoabrupt-junction-profile curve 450 in FIG. 47. As FIG. 48 shows, depletion capacitance Cda is lower for curve 460 than (a) for curve 462 corresponding to flat-profile curve 452 in FIG. 47 or (b) for curve 464 corresponding to hyperabrupt-junction-profile curve 454 in FIG. 47. Hence, the hypoabrupt vertical dopant profile below drain-body junction 112 in each IGFET 100, 140, 150, 160, 170, 180, 190, 210, 100V, 140V, 150V, 160V, 170V, 180V, or 190V causes its drain-to-body capacitance CDB to be reduced. The same applies to capacitance CDB along the bottom of drain 264 in inventive p−channel IGFET 220. Parasitic source-to-body capacitance CSB is also normally reduced, especially in n-channel IGFETs 170, 180, 190, 170V, 180V, and 190V where source 102 extends deeper below the upper semiconductor surface than pocket portion 120.
Additionally, areal depletion capacitance Cda for curve 460 corresponding to hypoabrupt-junction-profile curve 450 in FIG. 47 varies more slowly with reverse voltage VR than curve 462 or 464 as indicated by comparing curve 460 in FIG. 48 to curves 462 and 464. Parasitic drain-to-body capacitance CDB in each inventive IGFET 100, 140, 150, 160, 170, 180, 190, 210, 220, 100V, 140V, 150V, 160V, 170V, 180V, or 190V thus has reduced variance with reverse voltage VR. This is advantageous because less compensation is needed to account for variations in capacitance CDB. The same comments apply to parasitic source-to-body capacitance CSB, particularly in IGFETs 170, 180, 190, 170V, 180V, and 190V.
In further examining the hypoabrupt dopant profile below drains 104 and 264, consider the extreme example of a hypoabrupt junction profile in which net dopant concentration NB of the semiconductor material along the more lightly doped side of the pn junction makes a step change from a first dopant-concentration value to a higher second dopant-concentration value at a selected distance sufficiently close to the junction to affect the parasitic capacitance along the junction. This example is modeled in FIG. 49 which illustrates how net dopant concentration NB varies with distance y from the junction.
The two-step pn junction model of FIG. 49 is configured in the following way. The more lightly doped side of the pn junction is formed with p-type material in which net dopant concentration NB is at first value NB0 for a distance extending from the junction out to a distance yd0 at which concentration NB makes a step increase to a second value NB1. Distance yd0 is the location that would constitute the p-type boundary of the depletion region at zero reverse voltage VR if concentration NB in the p-type material were at low value NB0 beyond distance yd0 out at least to a location beyond which changes in concentration NB of the p-type material would not significantly affect the parasitic capacitance along the junction.
The depletion region along the junction in the model of FIG. 49 expands from distance yd0 to a maximum distance ydmax as reverse voltage VR increases from zero to some maximum value VRmax. Beyond distance ydmax, net dopant concentration NB can have an arbitrary profile in the p-type material as indicated in FIG. 49. As also indicated in FIG. 49, concentration NB of the more heavily doped n-type material along the junction is at a uniform value ND0 much greater than NB1.
FIG. 50 depicts the dopant profile in the p-type material as high concentration value NB1 ranges from NB0 (at which the step change in concentration at distance yd0 disappears) up to 20 NB0 at a typical yd0 value of 0.2 μm and a typical NB0 value of 3×1016 atoms/cm3. If the modeled pn junction is the drain-body junction at depth yD below the upper semiconductor surface, the step increase in concentration NB from NB0 to NB1 occurs at a depth yD+yd0 below the upper surface.
Areal depletion capacitance Cda of the two-step pn junction of FIG. 49 is governed by the differential equation:




  
    
      
        
          
            d
            
              V
              R
            
          
          
            d
            
              (
              
                1
                /
                
                  C
                  da
                  2
                
              
              )
            
          
        
        =
        
          
            
              qK
              S
            
            ⁢
            
              ɛ
              0
            
            ⁢
            
              N
              B
            
            ⁢
            
              
                t
                d
              
              ⁡
              
                (
                
                  V
                  R
                
                )
              
            
          
          2
        
      
    
    
      
        (
        21
        )
      
    
  




Integrating Eq. 21 subject to the condition that the depletion region extend to distance yd0 when reverse voltage VR is zero yields the following value for distance yd0:





  
    
      
        
          y
          
            d
            ⁢
            
              
            
            ⁢
            0
          
        
        =
        
          
            
              2
              ⁢
              
                K
                S
              
              ⁢
              
                ɛ
                0
              
              ⁢
              
                V
                BI
              
            
            
              qN
              
                B
                ⁢
                
                  
                
                ⁢
                0
              
            
          
        
      
    
    
      
        (
        22
        )
      
    
  




Combining Eqs. 18 and 22 produces the following result for depletion capacitance Cda:





  
    
      
        
          C
          da
        
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FIG. 50 illustrates how areal depletion capacitance Cda varies with reverse voltage VR for values of high concentration value NB1 ranging from NB0 (again, beyond the modeled junction) up to 20 NB0 as determined from Eq. 23. FIG. 50 shows that increasing concentration ratio NB1/NB0 causes capacitance Cda to vary more slowly with voltage VR. For this reason, it is desirable that concentration ratio NB1/NB0 be as high as reasonably possible in order to have parasitic capacitances CDB and CSD vary slowly in the asymmetrical IGFETs of the invention.
Areal depletion capacitance is at an initial value Cd0a when reverse voltage VR is zero. Setting voltage VR to zero in Eq. 23 yields:




  
    
      
        
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Initial depletion capacitance value Cd0a is, as expected, the classical value for an ideal pn junction at zero reverse voltage. In accordance with Eq. 24, capacitance value Cd0a decreases with decreasing low concentration value NB0 according to the square root of low value NB0. In combination with choosing a high value of concentration ratio NB1/NB0 so as to have slow variations in parasitic capacitances CDB and CSB, low concentration value NB0 should be low in order for capacitances CDB and CSB to be low at zero reverse voltage VR.

Computer Simulations Relating to Capacitance and Frequency Parameters

With the foregoing information about capacitance and frequency parameters in mind, small-signal simulations were performed with the Medici simulator to characterize the junction capacitances of inventive structure A. FIGS. 51a and 51b respectively depict short-channel and long-channel versions of structure A created by the Medici simulator for the junction capacitance characterization. Items 470 and 472 respectively indicate the source and drain contacts (or source and drain electrodes) in FIGS. 51a and 51b. Metal silicide layers 254 and 256 are respectively subsumed in contacts 470 and 472. The doping contour of p-type body material (or region) 108 in each of FIGS. 51a and 51b illustrates the graded nature of the doping in body material 108. The short-channel IGFET of FIG. 51a had a gate length LG of 0.15 μm. Gate length LG for the long-channel IGFET of FIG. 51b was 1.0 μm.
FIG. 52 illustrates parasitic lineal drain-to-body capacitance CDBw as a function of drain-to-body voltage VDB for a short-channel implementation of inventive structure A quite similar to the short-channel implementation of structure A in FIG. 51a and for a short-channel implementation of structure B substantially corresponding size-wise and, except for the doping features of the invention, dopant-wise to the short-channel implementation of structure A in FIG. 52. Gate length LG is 0.2 μm in FIG. 52 rather than 0.15 μm as in FIG. 51a. Gate-to-source voltage VGS was 0.9 V for the CDBw simulations of FIG. 52. As FIG. 52 shows, drain-to-body capacitance CDBw is considerably lower for this short-channel version of inventive structure A than for the corresponding short-channel version of reference structure B. In particular, capacitance CDBw for the examined short-channel version of inventive structure A was approximately 50% of capacitance CDBw for the examined short-channel version of reference structure B in the VDS range of 0 to 2 V.
FIG. 53 depicts parasitic lineal source-to-body capacitance CSBw as a function of source-to-body voltage VSB for the short-channel implementations of structures A and B examined in FIG. 52. Gate-to-source voltage VGS was again 0.9 V. As shown in FIG. 53, source-to-body capacitance CSB was considerably lower for the examined short-channel version of inventive structure A than for the corresponding short-channel version of reference structure B. Although the reduction in the CSBw value was not as great as the reduction in the CDBw value, the examined short-channel version of structure A had approximately a 35-40% lower CSBw value than the examined short-channel version of structure B at the VSB value of 2.0 V, and a 25-35% lower CSBw value than the examined short-channel version of reference structure B at the VSB value of 0 V.
The somewhat lesser improvement in source-to-body capacitance CSBw for the short-channel version of structure A examined in FIG. 52 is expected because the total p-type dopant in source 102 is increased by the p-type pocket implant. Also, source-to-body capacitance CSB is less important than drain-to-body capacitance CDB in many applications because source 102 is shorted to body material 108. As desired, further reduction in source-to-body capacitance CSB for short-channel versions of structure A can be achieved by making well portion 116 deeper.
FIG. 54 illustrates cut-off frequency fT as a function of lineal drain current IDw for the short-channel implementations of structures A and B examined in FIG. 52. FIG. 54 also illustrates the variation of cut-off frequency fT with lineal drain current IDw for a variation A′ of inventive short-channel structure A. In FIG. 54 and in later graphs that present computer-simulated data for inventive structures A and A′, curves representing data for structure A′ are marked with solid circles to distinguish that data from the data marked with empty circles for structure A. The particular characteristics of further inventive structure A′ are described below in connection with FIGS. 56 and 57. As FIG. 54 indicates, cut-off frequency fT was largely the same for simulated short-channel structures A, A′, and B.
The variation of cut-off frequency fT with lineal drain current IDw is illustrated in FIG. 55 for long-channel versions of the implementations of structures A, A′, and B in FIG. 54. As FIG. 55 shows, cut-off frequency fT was largely the same for the long-channel versions of inventive structures A and A′. Importantly, frequency fT for the long-channel versions of inventive structures A and A′ was considerably greater than for the long-channel version of reference structure B. Accordingly, the long-channel versions of inventive structures A and A′ had better performance capability than the long-channel version of reference structure B.
Additional IGFET in which Vertical Body-Material Dopant Profile Below Drain is Hypoabrupt Due to Subsurface Maximum in Well Dopant Concentration
FIG. 56 illustrates a short n-channel implementation 480 of asymmetrical IGFET structure A′ according to the invention. In addition to generally showing structural details, doping contours for IGFET 480 are depicted in FIG. 56 as a function of depth y and longitudinal distance x from a source location. The source location for measuring distance x is approximately 0.35 μm from the channel-zone center.
IGFET 480 is configured generally similar to short-channel IGFET 140 of FIG. 11 except that source 102 in IGFET 480 consists of n++ main source portion 102M and n+ more lightly doped lateral extension 102E as in long-channel IGFET 150 of FIG. 13. This enables source resistance RS to be reduced in IGFET 480 and thereby improves its analog performance. As in IGFETs 140 and 150, p-type pocket portion 120 extends deeper below the upper semiconductor surface than source 102. Drain depth yD for IGFET 480 is approximately 50% greater than source depth yS.
FIG. 57 presents net dopant concentration NN along the upper semiconductor surface as a function of distance from the preceding source location for inventive structure A of FIG. 32 and inventive structure A′, i.e., IGFET 480, of FIG. 56. As in FIGS. 12c and 14c, curve segments 106* and 120* here represent concentrations NN of the net p-type dopant in respective regions 106 and 120 while curve segments 102M*, 102E*, 104M*, 104E*, and 104* represent concentrations NN of the net n-type dopant in respective regions 102M, 102E, 104M, 104E, and 104. Although only marked with open circles, curve segments 102M*, 102E, 106*, and 120* apply to both structures A and A ′.
As curve segments 104* and 102M* in FIG. 57 indicate, short-channel IGFET 480 of inventive structure A′ reaches a somewhat lower maximum net dopant concentration in drain 104 along the upper surface than in main source portion 102M. More particularly, the maximum value of net dopant concentration NN in drain 104 along the upper surface of IGFET 480 is normally 20-50%, typically 30-40%, of the maximum upper-surface value of concentration NN in main source portion 102M. Although FIG. 57 illustrates an example in which the maximum upper-surface value of concentration NN in drain 104 slightly exceeds 1×1020 atoms/cm2, the maximum upper-surface NN concentration in drain 104 of short-channel IGFET 480 can readily be considerably lower, e.g., 5×1019 atoms/cm2 down to 1×1019 atoms/cm2 or less, dependent on the maximum upper-surface NN concentration in main source portion 102M. Also, drain 104 in IGFET 480 extends somewhat deeper below the upper surface than main source portion 102M. In essence, the two-part drain formed with a main portion and a more lightly doped lateral extension in an IGFET, such as IGFET 150, whose source consists of a main portion and a more lightly doped lateral extension is replaced in IGFET 480 with a deeper more lightly doped drain. The reduced doping in drain 104 of IGFET 480 results in a lower electric field in drain 104 and enables IGFET 480 to operate away from electric-field magnitudes at which undesirable drain impact ionization occurs.
FIG. 58 illustrates threshold voltage VT as a function of gate length LG for computer simulations of the IGFETs of inventive structure A′, reference structure B, and a further symmetrical reference structure D lacking both halo pocket portions of structure B but otherwise substantially identical size-wise and dopant-wise to structure B. Gate dielectric thickness tGI was 4.0 nm in the simulations of FIG. 58.
As FIG. 58 shows, threshold voltage roll-off was shifted to a lower value of threshold voltage VT in inventive structure A′ than in reference structure B or D. FIG. 58 also shows that inventive structure A′ incurred less undesired reverse short-channel effect than reference structure B or D. That is, inventive structure A underwent less change, normally less decrease, in threshold voltage VT with increasing gate length LG in the long-channel domain than reference structure B or D. Structure A thus had better short-channel and long-channel characteristics than structure B or D.
Additional Complementary-IGFET Structure Suitable for Mixed-Signal Applications
FIG. 59 illustrates a variation, in accordance with the invention, of the complementary-IGFET structure of FIG. 29.1. The complementary-IGFET structure of FIG. 59 is especially suitable for mixed-signal applications. The principal configurational difference between the complementary-IGFET structures of FIGS. 29.1 and 59 is that the complementary-IGFET structure of FIG. 59 is created from a starting structure such as a bonded wafer.
In the complementary-IGFET structure of FIG. 59, a subsurface electrically insulating layer 482 typically consisting primarily of silicon oxide separates a lower semiconductor layer 484 from an upper semiconductor layer having islands 202 and 204 laterally separted by field-insulation region 200 along the upper semiconductor surface. Lower semiconductor layer 484 normally consists of monosilicon, either p type or n type. FIG. 59 presents an example in which lower semiconductor layer 484 is lightly doped p type. An electrically insulating extension 486, typically of the trench type and likewise typically consisting primarily of silicon oxide, extends from field-insulation region 200 to subsurface insulating layer 482. Field insulation 200 and insulating extension 486 together laterally surround islands 202 and 204 so that they are fully dielectrically isolated from each other.
Islands 202 and 204 normally consist of doped <100> monosilicon. Island 202 has a low substantially uniform n-type background dopant concentration on which is imposed a low, but slightly higher, substantially uniform p-type background concentration typically provided by the p-type semiconductor dopant aluminum. Consequently, portions of island 202 that do not receive any other dopant (p type or n type) are lightly doped p type. Island 204 simply has a low substantially uniform n-type background dopant concentration.
Island 202 provides the monosilicon for a variation 210W of long n-channel IGFET 210. Source 102 and drain 104 long of n-channel IGFET 210W are separated by a channel portion of p-type body material 108 consisting of a lightly doped lower portion 488, p+ well portion 116, and an upper portion 490. P− lower body-material portion 488 and p-type upper body-material portion 490 respectively correspond to p− lower body-material portion 114 and p-type upper body-material portion 118 of IGFET 210. Upper body-material portion 490 of IGFET 210W consists of source-contacting p+ pocket portion 120 and lightly doped p-type remainder 492 that corresponds to p− upper body-material remainder 124 of IGFET 210. Due to the imposition of the low p-type background dopant concentration on the lower n-type background concentration in island 202 of IGFET 210W, net dopant concentration NN in the bulk of each region 488 or 492 is largely the difference between the p-type and n-type background dopant concentrations.
Aside from the above-mentioned configurational differences and the presence of the two background dopant concentrations in island 202, n-channel IGFET 210W is configured and constituted substantially the same as n-channel IGFET 210. P− lower body-material portion 488 can be deleted so that p+ well portion 116 extends down to subsurface insulating layer 482.
Island 204 provides the monosilicon for a variation 220W of long p-channel IGFET 220. Source 262 and drain 264 of long p-channel IGFET 220W are separated by a channel portion of n-type body material 268 consisting of a lightly doped lower portion 494, n+ well portion 276, and an upper portion 496 corresponding to n-type upper body-material portion 278 of IGFET 220. Upper body-material portion 496 of IGFET 220W consists of source-contacting n+ pocket portion 280 and lightly doped n-type remainder 498 that corresponds to n− upper body-material remainder 284 of IGFET 220. Unlike IGFET 220, IGFET 220W does not have a low p-type background dopant concentration and does not utilize an n-type compensating dopant to ensure that all of upper body-material portion 496 is of n-type conductivity. Net dopant concentration NN in the bulk of each region 494 or 498 is simply the n-type background dopant concentration.
Aside from the above-mentioned configurational differences and the absence of an n-type compensating dopant to ensure that all of upper body-material portion 496 is n type, p-channel IGFET 220W is configured and constituted substantially the same as p-channel IGFET 220. N− lower body-material portion 494 can be deleted so that n+ well portion 276 extends down to subsurface insulating layer 482.
Fabrication of Additional Complementary-IGFET Structure
The complementary-IGFET structure of FIG. 59 is manufactured in the following manner according to the invention. A structure is first provided in which subsurface insulating layer 482 is sandwiched between lower semiconductor layer 484 and an upper semiconductor region consisting of <100> n-type monosilicon at a low uniform dopant concentration. This initial structure can be created, for example, by bonding two semiconductor wafers together through electrically insulating material that forms subsurface insulating layer 482. One of the wafers provides the <100> n-type monosilicon for the upper semiconductor region. The other wafer provides lower semiconductor layer 484 again normally consisting of monosilicon, either p type as in the illustrated example or n type.
Insulating extension 486 is formed in the n− upper semiconductor region according to a deep trench-isolation technique. Field-insulating region 200 is then formed along the outside (upper) surface of the n− upper semiconductor region according to a shallow trench-isolation technique to define islands 202 and 204. Using a photoresist mask having an opening above for island 202, p-type semiconductor dopant normally consisting of aluminum is introduced into island 202 at a light dosage that is sufficiently high to convert all the material of island 202 to p-type conductivity at a low net concentration. When aluminum is used to perform the p-type doping of island 202, the aluminum diffuses relatively fast throughout island 202 so that it becomes substantially uniformly doped p type in a relatively short time.
P+ well portion 116 and n+ well portion 276 are formed respectively in islands 202 and 204 in the manner described above in connection with fabrication of IGFETs 210 and 220. Part of island 202 underlies well 116 and constitutes p− body-material portion 488. Part of island 204 similarly underlies well 276 and constitutes n-lower body-material portion 494. Regions 102, 104, 120, 126, 128, 250, 252, 254, 256, and 258 for IGFET 210W and regions 262, 264, 280, 286, 288, 290, 292, 294, 296, and 298 for IGFET 220W are then formed as described above for IGFETs 210 and 220. The p-type monosilicon above well portion 116 constitutes p-type upper body-material portion 490 of which the part outside p+ halo pocket portion 120 constitutes p− upper body-material remainder 492. The n-type monosilicon above well portion 276 constitutes p-type upper body-material portion 496 of which the part outside n+ halo pocket portion 280 constitutes n− upper body-material remainder 498.
IGFETs in which Vertical Body-Material Dopant Profile Below Drain is Hypoabrupt Due to Step Change in Body-Material Dopant Concentration
The vertical dopant profiles below the drains in the asymmetrical IGFETs configured according to the invention can be made hyperabrupt in ways other than having concentration NT of the conductivity-type-defining dopant in the body material decrease by at least a factor of 10 in going from the location of the maximum well dopant concentration up to the drain. In particular, the vertical dopant profile below the drain can be made hypoabrupt by arranging for the body material below the drain to include (a) a drain-adjoining portion in which the conductivity-type-defining dopant is at a largely uniform first concentration and (b) a directly underlying drain-remote portion in which the conductivity-type-defining dopant is at a largely uniform second concentration significantly greater than, normally at least 10 times greater than, the concentration of the conductivity-type-defining dopant in the drain-adjoining portion.
The concentration of the conductivity-type-defining dopant then undergoes a step decrease, normally by at least a factor of 10, in going from the drain-remote body-material portion up through the drain-adjoining body-material portion to the drain. An n-channel IGFET provided with this second type of drain-underlying hypoabrupt dopant profile, along with the asymmetrical channel-zone doping characteristics of inventive structure A or A′, is generally referred to here as inventive structure E.
FIG. 60 generally illustrates the vertical dopant profiles through the drains and into the underlying body regions for n-channel IGFETs configured as structures A/A′, B, and E. More particularly, the variation of absolute dopant concentration NT as a function of depth y along a vertical line through the drain is shown in FIG. 60 for an n-channel IGFET of each of structures A/A′, B, and E. Similar to what is shown in FIG. 49, concentration NT of the total p-type dopant along the vertical line through the drain of the n-channel IGFET of structure E is at uniform concentration value NB0 from the upper semiconductor surface to a depth yST equal to drain depth yD plus distance yd0. Concentration NT of the p-type dopant in the drain-adjoining body-material portion extending distance yd0 from depth yD to depth yST is thus NB0. Distance yd0 is normally 0.05-1.0 μm, typically 0.1 μm.
At depth yST, absolute dopant concentration NT makes a step change from NB0 up to value NB1 greater than, normally at least a factor of 10 greater than, NB0. Concentration NT of the p-type dopant in the drain-remote body-material portion extending downward from depth yST is at value NB1 out to some depth beyond which the concentration of the p-type dopant in the body material does not have any significant effect on the characteristics, especially drain-to-body capacitance CDB, at the drain-body junction. Accordingly, concentration NT of the p-type dopant in the body material makes a step decrease, normally by at least a factor of 10, in crossing from the drain-remote body-material portion where concentration NT of the p-type dopant equals NB1 up into the drain-adjoining body-material portion where concentration NT of the p-type dopant equals NB0 and then remains at NB0 up to the drain-body junction.
FIG. 61a illustrates an asymmetrical long n-channel IGFET 500 configured in accordance with the invention to implement structure E so as to be particularly suitable for high-speed analog applications. IGFET 500 is arranged substantially the same as IGFET 170 of FIG. 18a except that p-type body material 108 consists of a heavily doped lower subsurface portion 502 and an upper surface-adjoining portion 504 which extends to the upper semiconductor surface. P+ subsurface body-material portion 502 underlies source 102, drain 104, and channel zone 106. The upper boundary (top) of subsurface body-material portion 502 is at depth yST below the upper semiconductor surface. Depth yST is normally no more than 10 times, preferably no more than 5 times, drain depth yD. At its closest to source 102 and drain 104, subsurface portion 502 is thus normally no more than 10 times deeper, preferably no more than 5 times deeper, below the upper semiconductor surface than source 102 and drain 104.
P-type surface-adjoining body-material portion 504 overlies and meets p+ subsurface body-material portion 502. Channel zone 106 is part of surface-adjoining body-material 504. P+pocket portion 120, which is shallower than source 102 here, is also part of surface-adjoining body-material portion 504. Item 124 in FIG. 61a is now the lightly doped material of surface-adjoining body-material portion 504, i.e., the segment of portion 504 outside pocket portion 120.
The p-type dopant in the segment of surface-adjoining body-material portion 504 below drain 104 is present at a largely uniform concentration equal to NB0. A typical value for concentration NB0 is 5×1015 atoms/cm3. The p-type dopant in the segment of subsurface body-material portion 502 below the preceding segment of surface-adjoining body-material portion 504 and thus below drain 104 is present there at a largely uniform higher concentration equal to NB1. Value NB1 is normally at least 10 times NB0, preferably at least 20 times NB0, more preferably at least 40 times NB0, typically in the vicinity of 100 times NB0.
FIG. 61b illustrates another asymmetrical long n-channel IGFET 510 configured in accordance with the invention to implement structure E so as to be particularly suitable for high-speed analog applications. IGFET 510 is arranged the same as IGFET 500 except that a subsurface electrically insulating layer 512 typically consisting largely of silicon oxide contacts subsurface body-material portion 502 along its bottom surface. In IGFET 510, the p-type dopant in the segment of subsurface body-material portion 502 underlying drain 104 from depth yST down to subsurface insulating layer 512 is largely uniformly doped at concentration NB1.
An understanding of the hypoabrupt vertical dopant profile below drain 104 in underlying body material 108 of IGFETs 500 and 510 is facilitated with the assistance of FIGS. 62a-62c, 63a-63c, and 64a-64c. FIGS. 62a-62c, which are generally analogous to FIGS. 8a-8c, present exemplary dopant concentrations along vertical line 130 through source 102 of IGFET 500 or 510. Exemplary dopant concentrations along vertical lines 132 and 134 through channel zone 106 of IGFET 500 or 510 are presented in FIGS. 63a-63c which are generally analogous to FIGS. 9a-9c. FIGS. 64a-64c, which are generally analogous to FIGS. 10a-10c, present exemplary dopant concentrations along vertical line 136 through drain 104 of IGFET 500 or 510.
FIGS. 62a, 63a, and 64a illustrate concentrations NI, along vertical lines 130, 132, 134, and 136, of the individual semiconductor dopants that form source 102, drain 104, subsurface body-material portion 502, pocket portion 120 of surface-adjoining body-material portion 504, and remainder 124 of portion 504. Concentrations NT of the total p-type dopant and total n-type dopant in regions 102, 104, 502, 120, and 124 along lines 130, 132, 134, and 136 are depicted in FIGS. 62b, 63b, and 64b. FIGS. 62c, 63c, and 64c depict net dopant concentrations NN along lines 130, 132, 134, and 136.
Curves/curve segments 102′, 102′′, 102*, 104′, 104′′, 104*, 120′, 120′′, 120*, 124′, 124′′, and 124* in FIGS. 62a-62c, 63a-63c, and 64a-64c have the meanings presented above in connection with respectively analogous FIGS. 8a-8c, 9a-9c, and 10a-10c. Curves 502′ in FIGS. 62a, 63a, and 64a indicate concentration NI, along vertical lines 130, 132, 134, and 136, of the n-type dopant used to form subsurface body-material portion 502. Curve segments 502′′ in FIGS. 62b, 63b, and 64b represent concentration NT, along lines 130, 132, 134, and 136, of the total n-type dopant in subsurface portion 502. Curve segments 502* in FIGS. 62c, 63c, and 64c indicate concentrations NN along lines 130, 132, 134, and 136 of the net n-type dopant in portion 502.
Referring to FIG. 64a, the p-type dopant in the portion of body material 108 below drain 104 of IGFET 500 or 510 has two primary components referred to here as the “lower” p-type dopant and the “upper” p-type dopant. The lower p-type dopant is at high fixed concentration NB1 in subsurface body-material portion 502 as indicated by curve segment 502′. The upper p-type dopant is at low fixed concentration NB0 in remainder 124 of surface-adjoining body-material portion 504 as indicated by curve 124′. The upper p-type dopant is also present in drain 104 as indicated by the extension of curve 124′ into the area encompassed by curve 104′.
The total p-type dopant in the portion of body material 108 below drain 104 of IGFET 500 or 510 is indicated by the combination of curve segments 502′′ and 124′′ in FIG. 64b. As shown by the variation in combined curve 502′′/124′′, concentration NT of the total p-type dopant in the portion of body material 108 below drain 104 substantially undergoes a step decrease in crossing from subsurface body-material portion 502 at concentration NB1 into upper body-material remainder 124 at concentration NB0 and then remains at concentration NB0 in moving further upward to drain 104. Inasmuch as high concentration NB1 is normally at least 10 times NB0, concentration NT of the total p-type dopant in the portion of body material 108 below drain 104 thus decreases hypoabruptly by at least a factor of 10 in moving upward from subsurface body-material portion 502 through upper body-material remainder 124 to drain 104.
High concentration value NB1 is as mentioned above, preferably at least 20 times NB0, more preferably at least 40 times NB0. Accordingly, the hypoabrupt decrease in concentration NT of the total p-type dopant in the portion of body material 108 below drain 104 is preferably by at least a factor of 20, more preferably by at least a factor of 40.
FIG. 64c shows that, as represented by the combination of curve segments 502* and 124*, concentration NN of the net p-type dopant in the portion of body material 108 below drain 104 in IGFET 500 or 510, varies vertically in a similar manner to concentration NT of the total p-type dopant in the portion of body material 108 below drain 104 except that concentration NN of the net p-type dopant in the portion of body material 108 below drain 104 drops to zero at drain depth yD, drain-body junction 112. As in previously described IGFETs of the invention, the hypoabrupt dopant profile in the portion of body material 108 below drain 104 reduces parasitic capacitance along drain-body junction 112 of IGFET 500 or 510. Increased analog speed is thereby achieved for IGFETs 500 and 510.
Turning to the vertical dopant distribution below source 102 of IGFET 500 or 510, curve segments 502′ and 124′ in FIG. 62a have substantially the same shapes as in FIG. 64a. Although curve 120′ appears in FIG. 62a, the total p-type dopant in the portion of body material 108 below source 102 consists of the lower and upper p-type dopants at respective concentrations NB0 and NB1 because p-type pocket portion 120 is shallower than source 102 in the example of FIGS. 61a and 61b. The portion of combined curve segments 502′′ and 124′′ at depth greater than source depth yS in FIG. 62b is shaped substantially the same as the portion of combined curve segments 502′′/124′′ at depth greater than drain depth yD in FIG. 64b. Accordingly, concentration NT of the total p-type dopant in the portion of body material 108 below source 102 changes hypoabruptly in largely the same way as concentration NT of the total p-type dopant in the portion of body material 108 below drain 104. Consequently, the parasitic capacitance along source-to-body junction 110 is also reduced so as to further enhance the analog performance of IGFET 500 or 510.
Similar to what occurs in IGFET 100 of FIG. 6, p-type pocket portion 120 in IGFET 500 or 510 can be modified to extend deeper below the upper semiconductor surface than source 102 and drain 104. In that case, the p-type pocket dopant in pocket portion 120 causes concentration NT of the total p-type dopant in the portion of body material 108 below source 102 to rise somewhat immediately below source-body junction 110 and thus be somewhat greater than NB0 just below the bottom of source 102. The parasitic capacitance along source-body junction 110 is higher than in the example of FIGS. 61a and 61b but, with suitable choice for the doping and depth of pocket 120, is still reduced. This again enhances the analog performance of IGFETs 500 and 510. Modifying pocket portion 120 to extend deeper below the upper semiconductor surface than source 102 and drain 104 does not have any significant effect on the drain characteristics of IGFET 500 or 510 because substantially none of the p-type pocket dopant is then located in drain 104.
Channel zone 106 of IGFET 500 or 510 is asymmetrically longitudinally doped in largely the same way as channel zone 106 of IGFET 170 in FIG. 18a. Since the dopant distributions of FIGS. 7a-7c and the associated information presented above about FIGS. 7a-7c apply to IGFET 170, this information generally applies to IGFETs 500 and 510. Punchthrough is thereby avoided in IGFETs 500 and 510. The channel length of IGFET 500 or 510 can be reduced sufficiently to convert it into a short-channel device. In that case, the surface dopant distributions of FIGS. 12a-12c and the associated information presented above about FIGS. 12a-12c generally apply to IGFETs 500 and 510.
Each source/drain zone 102 or 104 of IGFET 500 or 510 can be modified to consist of main portion 102M or 104M and more lightly doped lateral extension 102E or 104E. Alternatively or additionally, each source/drain zone 102 or 104 of IGFET 500 or 510 may include more lightly doped lower portion 102L or 104L. In such cases, the dopant distributions presented in FIGS. 14a-14c, 16a-16c, and 17a-17c and the associated information about those dopant distributions generally apply to IGFETs 500 and 510 subject, in the case of FIGS. 16a-16c and 17a-17c, to replacing curves/curve segments 116′ and 114′, 116′′ and 114′′, and 116* and 114* respectively with curves/curve segments 502′, 502′′, and 502*.
Further Complementary-IGFET Structures Suitable for Mixed-Signal Applications and Having Step Change in Body-Material Dopant Concentration
FIG. 65a illustrates another complementary-IGFET structure configured according to the invention so as to be especially suitable for mixed-signal applications. The complementary-IGFET structure of FIG. 65a is created from doped silicon material such as that of a bonded wafer. A patterned field region 520 of electrically insulating material extends along the upper surface of the silicon material to define a group of laterally separated semiconductor islands, including islands 522 and 524. Two asymmetrical long-channel IGFETs 530 and 540 are formed along the upper semiconductor surface respectively at the locations of islands 522 and 524.
IGFET 530 is an n-channel device which implements IGFET 510 of FIG. 61b. Source 102, drain 104, and channel zone 106 are situated in island 522. Body material 108 of IGFET 530 is <100> p-type doped monosilicon. A lower semiconductor layer 550 consisting of lightly doped <100> p-type monosilicon underlies and contacts insulating layer 512 so that it is a subsurface layer. Field-insulating region 520, typically of the trench type, is vertically separated from subsurface insulating layer 512.
IGFET 540 is a p-channel device configured substantially the same as n-channel IGFET 500 of FIG. 61a with the conductivity types reversed. IGFET 540 thus has a heavily doped p-type source 562 and a heavily doped p-type drain 564 separated by a channel zone 566 of n-type body material 568 consisting of a heavily doped lower subsurface portion 572 and an upper surface-adjoining portion 574 that extends to the upper semiconductor surface. Source 562, drain 564, and channel zone 566 are situated in island 524.
Body material 568 is formed with <100> n-type monosilicon. Subsurface portion 572 of n-type body material 568 extends over p- lower semiconductor layer 550 and through an opening in subsurface insulating layer 512 to form a lateral pn junction 576 with semiconductor layer 550. Subsurface body-material portion 572 also forms a vertical pn junction 578 with p+ subsurface body-material portion 502 of IGFET 530. A reverse bias is applied across pn junction 578 to isolate IGFETs 530 and 540 from each other.
A heavily doped pocket portion 580 of n-type surface-adjoining body-material portion 574 extends along source 562 of IGFET 540. N+ pocket portion 580 causes channel zone 566 to be asymmetrically graded in a similar manner to the asymmetrical longitudinal dopant grading of channel zone 106 in IGFET 530. Item 584 is the lightly doped n-type remainder of surface-adjoining body-material portion 574. A gate dielectric layer 586, typically consisting primarily of silicon oxide, overlies channel zone 566. A gate electrode 588 is situated on gate dielectric layer 586 above channel zone 566. Gate electrode 588 extends partially over source 562 and drain 564. In the example of FIG. 65a, gate electrode 588 consists of very heavily doped p-type polysilicon.
The n-type dopant in subsurface body-material portion 572 is present at a largely uniform concentration NB0′. The n-type dopant in the segment of surface-adjoining body-material portion 574 below drain 564 is present in that segment at a largely uniform concentration NB1′ greater than NB0′. Analogous to concentrations NB1 and NB0, concentration NB1′ is normally at least 10 times NB0′, preferably at least 20 times NB0′, more preferably at least 40 times NB0′, typically in the vicinity of 100 times NB0′. In the portion of body material 568 below drain 564, IGFET 540 thus has a hypoabrupt dopant profile of generally the same nature that IGFET 530 has in the portion of body material 108 below drain 104. The vertical dopant profile in the portion of body material 568 below source 562 of IGFET 540 is likewise quite similar to the vertical dopant profile in the portion of body material 108 below source 102 of IGFET 530. Accordingly, IGFET 540 has reduced parasitic capacitance along its drain-body and source-body junctions.
FIG. 65b illustrates a variation of the complementary-IGFET structure of FIG. 65a. In the variation of FIG. 65b, field-insulation region 520 is provided with an electrically insulating extension 590, typically of the trench type, that reaches subsurface insulating layer 512. The combination of field-insulating region 520 and insulating extension 590 laterally surrounds subsurface body-material portion 502 of IGFET 530. This dielectrically laterally isolates IGFETs 530 and 540 from each other.
The conductivity types can be reversed in the complementary-IGFET structures of FIGS. 65a and 65b. The resultant n-type body material and n− lower semiconductor layer respectively corresponding to p-type body material 108 and p− lower semiconductor layer 550 are then both <110> n-type monosilicon. The p-type body material corresponding to n-type body material 568 is <110> p-type monosilicon.
FIG. 65c depicts a further variation of the complementary-IGFET structure of FIG. 65a. FIG. 65d depicts a corresponding variation of the complementary-IGFET structure of FIG. 65b. In the variations of FIGS. 65c and 65d, a lower semiconductor layer 592 consisting of lightly doped <110> n-type monosilicon replaces p− lower semiconductor layer 550. N-type body material 568 for p-channel IGFET 540 is formed with <110> n-type monosilicon, rather than <100> n-type monosilicon, in the complementary-IGFET structures of FIGS. 65c and 65d. P-type body material 108 for n-channel IGFET 530 continues to be <100> p-type monosilicon in the complementary-IGFET structures of FIGS. 65c and 65d. 
The conductivity types can be reversed in the complementary-IGFET structures of FIGS. 65c and 65d. In that case, the resultant p-type body material and p− lower semiconductor layer respectively corresponding to n-type body material 568 and n− lower semiconductor layer 592 are both <100> p-type monosilicon. The n-type body material corresponding to p-type body material 108 is <110> n-type monosilicon.
Manufacture of Further Complementary-IGFET Structures Having Step Change in Body-Material Dopant Concentration
The complementary-IGFET structure of FIG. 65a is fabricated in the following manner according to the invention. A structure is first provided in which (a) a subsurface semiconductor region consisting of heavily doped <100> p-type monosilicon at high uniform concentration NB1 adjoins a subsurface electrically insulating layer, (b) a surface-adjoining semiconductor region consisting of lightly doped <100> p-type monosilicon at low uniform concentration NB0 adjoins and overlies the subsurface semiconductor region, and (c) a lower semiconductor layer consisting of lightly doped <100> p-type monosilicon adjoins and underlies the subsurface insulating layer. The lightly doped lower semiconductor layer constitutes p− lower semiconductor layer 550.
The initial structure can be created, for example, by bonding two semiconductor wafers together through electrically insulating material that forms the subsurface insulating layer. One of the wafers has a lightly doped <100> p-type monosilicon substrate that forms lower semiconductor layer 550. The other wafer has a heavily doped <100> p-type monosilicon substrate and an overlying lightly doped <100> p-type monosilicon epitaxial layer respectively doped substantially uniformly at concentrations NB1 and NB0 to respectively form the subsurface semiconductor region and the surface-adjoining semiconductor region.
Field-insulating region 520 is formed along the outside (upper) surface of the p− surface-adjoining semiconductor region to define island 522 for IGFET 530 and to define the location of island 524 for IGFET 540. Field insulation 520 may extend partially through the p- surface-adjoining semiconductor region so that field insulation 520 extends deep into, but not fully through, p-type surface-adjoining body-material portion 504 in the completed complementary-IGFET structure as shown in FIG. 65a. Alternatively, field insulation 520 may extend fully through the p− surface-adjoining semiconductor region and partially into the underlying p+ subsurface semiconductor region. The portion of the p− surface-adjoining semiconductor region in island 522 constitutes a precursor to p-type surface-adjoining body-material portion 504. The underlying portion of the p+ subsurface semiconductor region substantially constitutes p+ subsurface body-material portion 502.
At the location for island 524, a cavity is formed through the p− surface-adjoining semiconductor region, through the underlying section of the p+ subsurface semiconductor region, and through the further underlying section of the subsurface insulating layer down to p− lower semiconductor layer 550. The remaining portion of the subsurface insulating layer constitutes subsurface insulating layer 512. Heavily doped <100> n-type monosilicon is epitaxially grown at uniform concentration NB1′ on the so-exposed section of lower semiconductor layer 550 to substantially form n+ subsurface body-material portion 572. Lightly doped <100> n-type monosilicon is epitaxially grown at uniform concentration NB0′ in the cavity on subsurface portion 572 to form a precursor to n-type surface-adjoining body-material portion 574. Body-material portion 572 and the precursor to body-material portion 574 form island 524.
Gate dielectric layers 126 and 586 are respectively formed along the exposed (upper) surfaces of the precursors to p-type surface-adjoining body-material portion 504 for IGFET 530 and n-type surface-adjoining body-material portion 574 for IGFET 540. Gate electrodes 128 and 588 are respectively formed on gate dielectric layers 126 and 586. N++ source 102, n++ drain 104, and p+ pocket portion 120 are formed in the precursor to surface-adjoining body-material portion 504. The remaining part of the p-type precursor to body-material portion 504 then substantially constitutes portion 504 for IGFET 530. P++ source 562, p++ drain 564, and n+ and pocket portion 580 are similarly formed in the precursor to n-type surface-adjoining body-material portion 574. The remaining part of the n-type precursor to surface-adjoining body-material portion 574 then similarly substantially constitutes portion 574 for IGFET 540. The operations involved in forming gate electrodes 128 and 588, n++ source/drain zones 102 and 104, p+ pocket portion 120, p++ source/drain zones 562 and 564, and n+ pocket portion 580 can be performed in various orders.
The complementary-IGFET structure of FIG. 65b is fabricated according to the invention in the same way as the complementary-IGFET structure of FIG. 65a except that insulating extension 590 to field-insulating region 520 is formed in the p+ subsurface semiconductor region in the course of forming field insulation 520.
The complementary-IGFET structures of FIGS. 65c and 65d are fabricated according to the invention in the same respective ways as the complementary-IGFET structures of FIGS. 65a and 65b except that a lower semiconductor layer consisting of lightly doped <110> n-type monosilicon replaces p− lower semiconductor layer 550. The lightly doped lower semiconductor layer constitutes n− lower semiconductor layer 592. The initial structure used for creating the complementary-IGFET structure of FIG. 65c or 65d can be created in the same way as the initial structure used to create the complementary-IGFET structure of FIG. 65a or 65b except that the first-mentioned wafer has a lightly doped <110> n-type monosilicon substrate rather than a lightly doped <100> p-type monosilicon substrate.
Also, after forming the cavity through the p− surface-adjoining semiconductor region, through the underlying section of the p+ subsurface semiconductor region, and through the further underlying section of the subsurface insulating layer down to n− lower semiconductor layer 592, n+ subsurface body-material portion 572 is epitaxially grown on lower semiconductor layer 592 in the cavity as heavily doped <110> n-type monosilicon at concentration NB1′. The precursor to re-surface-adjoining body-material portion 574 is then epitaxially grown on n+ subsurface portion 572 in the cavity as lightly doped <110> n-type monosilicon at concentration NB0′.
Variations
While the invention has been described with reference to particular embodiments, this description is solely for the purpose of illustration and is not to be construed as limiting the scope of the invention claimed below. For instance, silicon in the semiconductor body or/and in gate electrodes 128, 288, 328, 368, and 588 can be replaced with other semiconductor materials. Replacement candidates include germanium, a silicon-germanium alloy, and Group 3a-Group 5a alloys such as germanium arsenide.
Metal silicide layers can be provided along the upper surfaces of sources 102 and 562, drains 104 and 564, and gate electrodes 128 and 588 in IGFETs 530 and 540 of the complementary-IGFET structures of FIGS. 65a-65d. Composite gate electrodes 128/258, 288/298, 328/338, and 368/378 in IGFETs 210, 220, 230, 240, 380, and 390 of the complementary-IGFET structures of FIGS. 29 and 30 or/and gate electrodes 128 and 588 in IGFETs 530 and 540 of the complementary-IGFET structures of FIGS. 65a-65d can be replaced with gate electrodes consisting substantially fully of metal or substantially fully of metal silicide, e.g., cobalt silicide or nickel silicide with dopant provided in the silicide gate electrodes to control their work functions, Various modifications may thus be made by those skilled in the art without departing from the true scope of the invention as defined in the appended claims.

## Claims

1. A structure comprising a principal field-effect transistor (“FET”) which comprises:
a principal channel zone of principal body material of a semiconductor body having an upper semiconductor surface, the body material being sufficiently doped with semiconductor dopant of a first conductivity type as to be of the first conductivity type;
a pair of principal source/drain zones situated in a semiconductor body along the upper semiconductor surface, laterally separated by the channel zone, and being of a second conductivity type opposite to the first conductivity type, the body material extending laterally below the S/D zones, the dopant of the first conductivity type in the body material having a concentration that decreases by at least a factor of 10 in moving upward to a specified one of the source/drain zones from a principal underlying subsurface body-material location which is no more than 10 times deeper below the upper semiconductor surface than the specified source/drain zone;
a principal gate dielectric layer overlying the channel zone; and
a principal gate electrode overlying the gate dielectric layer above the channel zone.

2. A structure as in claim 1 wherein the aforementioned subsurface body-material location is no more than 5 times deeper below the upper semiconductor surface than the specified source/drain zone.

3. A structure as in claim 1 wherein the aforementioned subsurface body-material location lies below largely all of each of the channel and source/drain zones.

4. A structure as in claim 1 wherein the concentration of the dopant of the first conductivity type in the body material decreases by at least a factor of 20 in moving upward to the specified source/drain zone from the aforementioned subsurface body-material location.

5. A structure as in claim 4 wherein the concentration of the dopant of the first conductivity type in the body material decreases by at least a factor of 40 in moving upward to the specified source/drain zone from the aforementioned subsurface body-material location.

6. A structure as in claim 1 wherein the concentration of the dopant of the first conductivity type in the body material is lower where the channel zone meets the specified source/drain zone along the upper semiconductor surface than where the channel zone meets the remaining one of the source/drain zones along the upper semiconductor surface.

7. A structure as in claim 6 wherein the concentration of the dopant of the first conductivity type in the body material is at least a factor of 10 lower where the channel zone meets the specified source/drain zone along the upper semiconductor surface than where the channel zone meets the remaining source/drain zone along the upper semiconductor surface.

8. A structure as in claim 6 wherein the concentration of the dopant of the first conductivity type in the body material is at least a factor of 20 lower where the channel zone meets the specified source/drain zone along the upper semiconductor surface than where the channel zone meets the remaining source/drain zone along the upper semiconductor surface.

9. A structure as in claim 6 wherein the concentration of the dopant of the first conductivity type in the body material is at least a factor of 50 lower where the channel zone meets the specified source/drain zone along the upper semiconductor surface than where the channel zone meets the remaining source/drain zone along the upper semiconductor surface.

10. A structure as in claim 6 wherein the concentration of the dopant of the first conductivity type in the body material increases substantially progressively in going along the upper semiconductor surface from where the channel zone meets the specified source/drain zone along the upper semiconductor surface at least partially to where the channel zone meets the remaining source/drain zone along the upper semiconductor surface.

11. A structure as in claim 6 wherein the specified source/drain zone extends deeper below the upper semiconductor surface than does the remaining source/drain zone.

12. A structure as in claim 1 wherein the concentration of the dopant of the first conductivity type in the body material substantially undergoes a step decrease in moving upward to the specified source/drain zone from the aforementioned subsurface body-material location.

13. A structure as in claim 1 wherein each source/drain zone comprises a main source/drain portion and a more lightly doped lateral extension continuous with the main source/drain portion, the channel zone being terminated by the lateral extensions along the upper semiconductor surface.

14. A structure as in claim 1 wherein the remaining one of the source/drain zones comprises a main source/drain portion and a more lightly doped lateral extension continuous with the main source/drain portion, the channel zone being terminated by the specified source/drain zone and the lateral extension of the remaining source/drain zone along the upper semiconductor surface.

15. A structure as in claim 14 wherein each source/drain zone is defined with semiconductor dopant that, along the upper semiconductor surface, reaches a maximum net concentration which is lower in the specified source/drain zone than in the remaining source/drain zone.

16. A structure as in claim 15 wherein the maximum net concentration of the dopant in the source/drain zones is at least 20% lower in the specified source/drain zone than in the remaining source/drain zone.

17. A structure as in claim 1 wherein each source/drain zone comprises a main source/drain portion and a more lightly doped vertical extension that underlies, and is continuous with, the main source/drain portion.

18. A structure as in claim 1 further including a further FET of like polarity to the principal FET, the further FET comprising:
a further channel zone of the body material;
a pair of further source/drain zones situated in the semiconductor body along the upper semiconductor surface, laterally separated by the further channel zone, and being of the second conductivity type, the concentration of the dopant of the first conductivity type in the body material either being approximately constant or changing by less than a factor of 10 in moving upward to each further source/drain zone from a further underlying subsurface body-material location approximately as deep below the upper semiconductor surface as the aforementioned principal subsurface body-material location;
a further gate dielectric layer overlying the further channel zone; and
a further gate electrode overlying the further gate dielectric layer above the further channel zone.

19. A structure as in claim 18 wherein:
the concentration of the dopant of the first conductivity type in the body material is lower where the principal channel zone meets the specified principal source/drain zone along the upper semiconductor surface than where the principal channel zone meets the remaining one of the principal source/drain zones along the upper semiconductor surface; and
the concentration of the dopant of the first conductivity type is approximately the same where the further channel zone meets one of the further source/drain zones along the upper semiconductor surface as where the further channel zone meets the other of the further source/drain zones along the upper semiconductor surface.

20. A structure as in claim 1 further including an additional FET of opposite polarity to the principal FET, the additional FET comprising:
an additional channel zone of additional body material of the semiconductor body, the additional body material being sufficiently doped with semiconductor dopant of the second conductivity type as to be of the second conductivity type;
a pair of additional source/drain zones situated in the semiconductor body along the upper semiconductor surface, laterally separated by the additional channel zone, and being of the first conductivity type, the dopant of the second conductivity type in the additional body material having a concentration that decreases by at least a factor of 10 in moving upward to a specified one of the additional source/drain zones from an additional underlying subsurface body-material location no more than 10 times deeper below the upper semiconductor surface than the specified additional source/drain zone;
an additional gate dielectric layer overlying the additional channel zone; and
an additional gate electrode overlying the additional gate dielectric layer above the additional channel zone.

21. A structure as in claim 20 wherein:
the aforementioned principal subsurface body-material location is no more than 5 times deeper below the upper semiconductor surface than the specified principal source/drain zone; and
the aforementioned additional subsurface body-material location is no more than 5 times deeper below the upper semiconductor surface than the specified additional source/drain zone.

22. A structure as in claim 20 wherein:
the concentration of the dopant of the first conductivity type in the principal body material decreases by at least a factor of 20 in moving upward to the specified principal source/drain zone from the aforementioned principal subsurface body-material location; and
the concentration of the dopant of the second conductivity type in the additional body material decreases by at least a factor of 20 in moving upward to the specified additional source/drain zone from the aforementioned additional subsurface body-material location.

23. A structure as in claim 20 wherein:
the concentration of the dopant of the first conductivity type in the principal body material is lower where the principal channel zone meets the specified principal source/drain zone along the upper semiconductor surface than where the principal channel zone meets the remaining one of the principal source/drain zones along the upper semiconductor surface; and
the concentration of the dopant of the second conductivity type in the additional body material is lower where the additional channel zone meets the specified additional source/drain zone along the upper semiconductor surface than where the additional channel zone meets the remaining one of the additional source/drain zones along the upper semiconductor surface.

24. A structure as in claim 20 wherein:
the concentration of the dopant of the first conductivity type in the principal body material continuously decreases in moving upward to the specified principal source/drain zone from the aforementioned principal subsurface body-material location; and
the concentration of the dopant of the second conductivity type in the additional body material continuously decreases in moving upward to the specified additional source/drain zone from the aforementioned additional subsurface body-material location.

25. A structure as in claim 20 wherein:
the concentration of the dopant of the first conductivity type in the principal body material substantially undergoes a step decrease in moving upward to the specified principal source/drain zone from the aforementioned principal subsurface body-material location; and
the concentration of the dopant of the second conductivity type in the additional body material substantially undergoes a step decrease in moving upward to the specified additional source/drain zone from the aforementioned additional subsurface body-material location.

26. A structure as in claim 20 wherein:
the semiconductor body includes a substrate region of the first conductivity type and an isolation portion of the second conductivity type overlying the substrate region;
the principal body material comprises a principal well portion overlying the isolation portion above the substrate region such that the principal well portion is spaced apart from the substrate region, the concentration of the dopant of the first conductivity type in the principal body material reaching a local subsurface maximum in the principal well portion to enable the concentration of the dopant of the first conductivity type in the principal body material to decrease by at least a factor of 10 in moving upward from the location of the principal local subsurface maximum to the specified principal source/drain zone; and
the additional body material comprises an additional well portion which overlies the substrate region, the concentration of the dopant of the second conductivity type in the additional body material reaching an additional local subsurface maximum in the additional well portion to enable the concentration of the dopant of the second conductivity type in the additional body material to decrease by at least a factor of 10 in moving upward from the location of the additional local subsurface maximum to the specified additional source/drain zone.

27. A structure as in claim 20 further including a third FET of like polarity to the principal FET, the third FET comprising:
a third channel zone of the principal body material;
a pair of third source/drain zones situated in the semiconductor body along the upper semiconductor surface, laterally separated by the third channel zone, and being of the second conductivity type, the concentration of the dopant of the first conductivity type in the principal body material either being approximately constant or, changing by less than a factor of 10 in moving upward to each third source/drain zone from a third underlying subsurface body-material location approximately as deep below the upper semiconductor surface as the aforementioned principal subsurface body-material location;
a third gate dielectric layer overlying the third channel zone; and
a third gate electrode overlying the third gate dielectric layer above the third channel zone.

28. A structure as in claim 27 wherein:
the concentration of the dopant of the first conductivity type in the principal body material is lower where the principal channel zone meets the specified principal source/drain zone along the upper semiconductor surface than where the principal channel zone meets the remaining one of the principal source/drain zones along the upper semiconductor surface;
the concentration of the dopant of the second conductivity type in the additional body material is lower where the additional channel zone meets the specified additional source/drain zone along the upper semiconductor surface than where the additional channel zone meets the remaining one of the additional source/drain zones along the upper semiconductor surface; and
the concentration of the dopant of the first conductivity type in the principal body material is approximately the same where the third channel zone meets one of the third source/drain zones along the upper semiconductor surface as where the third channel zone meets the other of the third source/drain zones along the upper semiconductor surface.

29. A structure as in claim 27 further including a fourth FET of the same polarity as the additional FET and thus of opposite polarity to the principal FET, the fourth FET comprising:
a fourth channel zone of the additional body material;
a pair of fourth source/drain zones situated in the semiconductor body along the upper semiconductor surface, laterally separated by the fourth channel zone, and being of the first conductivity type, the concentration of the dopant of the second conductivity type in the additional body material either being approximately constant or changing by less than a factor of 10 in moving upward to each fourth source/drain zone from a fourth underlying body-material location approximately as deep below the upper semiconductor surface as the aforementioned additional subsurface body-material location;
a fourth gate dielectric layer overlying the fourth channel zone; and
a fourth gate electrode overlying the fourth gate dielectric layer above the fourth channel zone.

30. A structure as in claim 29 wherein:
the concentration of the dopant of the first conductivity type in the principal body material is lower where the principal channel zone meets the specified principal source/drain zone along the upper semiconductor surface than where the principal channel zone meets the remaining one of the principal source/drain zones along the upper semiconductor surface;
the concentration of the dopant of the second conductivity type in the additional body material is lower where the additional channel zone meets the specified additional source/drain zone along the upper semiconductor surface than where the additional channel zone meets the remaining one of the additional source/drain zones along the upper semiconductor surface;
the concentration of the dopant of the first conductivity type in the principal body material is approximately the same where the third channel zone meets one of the third source/drain zones along the upper semiconductor surface as where the third channel zone meets the other of the third source/drain zones along the upper semiconductor surface; and
the concentration of the dopant of the second conductivity type is approximately the same where the fourth channel zone meets one of the fourth source/drain zones along the upper semiconductor surface as where the fourth channel zone meets the other of the fourth source/drain zones along the upper semiconductor surface.

31. A method of fabricating a semiconductor structure from a semiconductor body, the method comprising:
defining, for a principal field-effect transistor (“FET”), a principal gate electrode above, and vertically separated by a principal gate dielectric layer from, a segment of the semiconductor body intended to be a principal channel zone of a first conductivity type;
introducing semiconductor dopant of a second conductivity type opposite to the first conductivity type into the semiconductor body to define, for the FET, a pair of principal source/drain zones of a second conductivity type opposite to the first conductivity type laterally separated by the channel zone such that, subsequent to completion of fabrication of the structure, (a) the semiconductor body has an upper semiconductor surface, (b) the channel zone is part of principal body material which is of the first conductivity type and which extends laterally below the source/drain zones, and (c) all semiconductor dopant of the first conductivity type in the semiconductor body has a concentration that decreases by at least a factor of 10 in moving upward to a specified one of the source/drain zones from a principal underlying subsurface body-material location which is no more than 10 times deeper below the upper semiconductor surface than the specified source/drain zone.

32. A method as in claim 31 further including, prior to the act of introducing the dopant of the second conductivity type, introducing semiconductor dopant of the first conductivity type into at least material of the semiconductor body underlying material of the semiconductor body intended for the source/drain zones.

33. A method as in claim 32 wherein the act of introducing the dopant of the first conductivity type is performed so that, subsequent to completion of fabrication of the structure, the dopant of the first conductivity type largely reaches maximum concentration along the aforementioned subsurface body-material location.

34. A method as in claim 31 wherein, subsequent to completion of fabrication of the structure, the aforementioned subsurface body-material location is no more than 5 times deeper below the upper semiconductor surface than the specified source/drain zone.

35. A method as in claim 31 wherein, subsequent to completion of fabrication of the structure, the aforementioned subsurface body-material location lies below largely all of each of the channel and source/drain zones.

36. A method as in claim 31 wherein, subsequent to completion of fabrication of the structure, the concentration of all dopant of the first conductivity type decreases by at least a factor of 20 in moving upward to the specified source/drain zone from the aforementioned subsurface body-material location.

37. A method as in claim 31 further including introducing semiconductor dopant of the first conductivity type into the semiconductor body to define a pocket portion extending along the remaining one of the source/drain zones up to its then-existent upper surface such that, subsequent to completion of fabrication of the structure, (a) the pocket portion constitutes a portion of the body material and is more heavily doped than adjacent material of the body material and (b) the concentration of all dopant of the first conductivity type in the body material is materially lower where the channel zone meets the specified source/drain zone along the upper semiconductor surface than where the channel zone meets the remaining one of the source/drain zones along the upper semiconductor surface.

38. A method as in claim 37 wherein, subsequent to completion of fabrication of the structure, the concentration of all dopant of the first conductivity type in the body material is at least a factor of 10 lower where the channel zone meets the specified source/drain zone along the upper semiconductor surface than where the channel zone meets the remaining source/drain zone along the upper semiconductor surface.

39. A method as in claim 38 wherein the act of introducing the dopant of the second conductivity type comprises:
introducing lateral-extension semiconductor dopant of the second conductivity type through an opening in a lateral-extension mask, through the then-existent upper surface of the semiconductor body, and into a pair of laterally separated primary portions of the semiconductor body using the lateral-extension mask, the gate electrode, and any material along the gate electrode as a dopant-blocking shield;
providing spacer material to the transverse sides of the gate electrode; and
introducing main-portion semiconductor dopant of the second conductivity type through an opening in a main-portion mask, through the then-existent upper surface of the semiconductor body, and into a pair of laterally separated further portions of the semiconductor body using the main-portion mask, the gate electrode, and the spacer material as a dopant-blocking shield.

40. A method as in claim 31 wherein the act of introducing the dopant of the second conductivity type comprises:
performing one of (a) introducing main-portion semiconductor dopant of the second conductivity type into the semiconductor body at a main-portion dosage and to a main-portion average depth below the then-existent upper surface of the semiconductor body and (b) introducing lower-portion semiconductor dopant of the second conductivity type into the semiconductor body at a lower-portion dosage and to a lower-portion average depth below the then-existent upper surface of the semiconductor body, the main-portion dosage being greater than the lower-portion dosage, the lower-portion average depth being greater than the main-portion average depth; and
performing the other of introducing the main-portion and lower-portion dopants such that the main-portion dopant defines a pair of laterally separated main source/drain portions respectively of the source/drain zones, such that the lower-portion dopant defines a pair of laterally separated lower source/drain portions respectively of the source/drain zones, and such that the lower source/drain portions are more lightly doped than, respectively underlie, and are respectively vertically continuous with the main source/drain portions.

41. A method as in claim 31 wherein:
the gate-electrode-defining act includes defining, for a further FET of like polarity to the principal FET, a further gate electrode above, and vertically separated by a further gate dielectric layer from, a segment of the semiconductor body intended to be a further channel zone of the first conductivity type; and
the act of introducing the dopant of the second conductivity type includes introducing further semiconductor dopant of the second conductivity type into the semiconductor body to define, for the further FET, a pair of further source/drain zones of the second conductivity type laterally separated by the further channel zone such that, subsequent to completion of fabrication of the structure, (a) the further channel zone is part of the body material which also extends below the further source/drain zones and (b) the concentration of all dopant of the first conductivity type either is approximately constant or changes by less than a factor of 10 in moving upward to each further source/drain zone from a further underlying subsurface body-material location approximately as deep below the upper semiconductor surface as the aforementioned principal subsurface body-material location.

42. A method as in 31 wherein:
the gate-electrode-defining act includes defining, for an additional FET of opposite polarity to the principal FET, an additional gate electrode above, and vertically separated by an additional gate dielectric layer from, a segment of the semiconductor body intended to be an additional channel zone of the second conductivity type; and
the method further includes introducing semiconductor dopant of the first conductivity type into the semiconductor body to define, for the additional FET, a pair of additional source/drain zones of the first conductivity type laterally separated by the additional channel zone such that, subsequent to completion of fabrication of the structure, (a) the additional channel zone is part of additional body material which is of the second conductivity type and which extends below the additional source/drain zones and (b) all semiconductor dopant of the second conductivity type in the semiconductor body has a concentration that decreases by at least a factor of 10 in moving upward to a specified one of the additional source/drain zones from an additional underlying subsurface body-material location which is no more than 10 times deeper below the upper semiconductor surface than the specified additional source/drain zone.

43. A method as in claim 42 further including, prior to the act of introducing the dopant of the first conductivity type, introducing additional semiconductor dopant of the second conductivity type into at least material of the semiconductor body underlying material of the semiconductor body intended for the additional source/drain zones.

44. A method as in claim 43 wherein the act of introducing the additional dopant of the second conductivity type is performed so that, subsequent to completion of fabrication of the structure, the additional dopant of the second conductivity type largely reaches maximum concentration along the aforementioned additional subsurface body-material location.

45. A method as in claim 42 wherein:
the gate-electrode-defining act includes defining, for a third FET of like polarity to the principal FET, a third gate electrode above, and vertically separated by a third gate dielectric layer from, a segment of the semiconductor body intended to be a third channel zone of the first conductivity type; and
the act of introducing the dopant of the second conductivity type includes introducing further semiconductor dopant of the second conductivity type into the semiconductor body to define, for the third FET, a pair of third source/drain zones of the second conductivity type laterally separated by the third channel zone such that, subsequent to completion of fabrication of the structure, (a) the third channel zone is part of the principal body material which also extends below the third source/drain zones and (b) the concentration of all dopant of the first conductivity type either is approximately constant or changes by less than a factor of 10 in moving upward to each third source/drain zone from a third underlying subsurface body-material location approximately as deep below the upper semiconductor surface as the aforementioned principal subsurface body-material location.

46. A method as in claim 45 wherein:
the gate-electrode-defining act includes defining, for a fourth FET of the same polarity as the additional FET and thus of opposite polarity to the principal FET, a fourth gate electrode above, and vertically separated by a fourth gate dielectric layer from, a segment of the semiconductor body intended to be a fourth channel zone of the second conductivity type; and
the act of introducing the dopant of the first conductivity type includes introducing further semiconductor dopant of the first conductivity type into the semiconductor body to define, for the fourth FET, a pair of fourth source/drain zones of the first conductivity type laterally separated by the fourth channel zone such that, subsequent to completion of fabrication of the structure, (a) the fourth channel zone is part of the additional body material which also extends below the fourth source/drain zones and (b) the concentration of all dopant of the second conductivity type either is approximately constant or changes by less than a factor of 10 in moving upward to each fourth source/drain zone from a fourth underlying subsurface body-material location approximately as deep below the upper semiconductor surface as the aforementioned additional subsurface body-material location.