SELF-ALIGNED METAL-SEMICONDUCTOR FIELD EFFECT TRANSISTOR

Abstract: Disclosed is a self-aligned GaAs, lightly doped drain metal-semiconductor field effect transistor. The device (70) consists of a shallow n⁻ active channel region (53A) formed on a GaAs substrate (50), a Schottky gate (54) overlying the n⁻ region and highly doped and deep n⁺ source (64) and drain (65) regions formed on either side of the gate. In the channel regions between the gate edges and the source/drain regions are position­ ed n-type source/drain extensions (57A, 58A) which have an intermediate depth and doping concentration to mini­ mize the device series resistance, suppress short channel effects and permit channel length reduction to submicron levels. In addition, p-type pockets (59A, 60A) are provided under the source/drain extensions (57A, 58A) to better control the device threshold voltage and further reduce the channel length.

Description: The invention relates generally to semiconductor integrated circuit technology, and more specifically, to a metal-semiconductor field effect transistor (MESFET) comprising on a semiconductor substrate a pair of relatively deep and highly doped source and drain re­ gions, a relatively shallow and highly doped active channel region arranged between said source and drain regions, and source and drain extensions arranged between said channel region and said source and drain regions, respectively, and forming a contiguous arrangement of said source, source extension, channel, drain extension and drain regions, these regions all being of a first conductivity type. The subject MESFET structure is particularly well suited for application in gallium- arsenide (GaAs) technology. The simplest and most attractive transistor for high speed and low power GaAs digital integrated circuits is the normally-off or enhancement mode MESFET (E-MESFET). In very large scale integration (VLSI) applications, the E-MESFET is desirable since, as discussed in the article by K. Suyama et al, entitled "Design and Performance of GaAs Normally-Off MESFET Integrated Circuits", IEEE Transactions on Electron Devices, Vol. Ed-27, No. 6, pp. 1092-1097 (June 1980), it enables direct coupling of logic functions and can operate with a single positive power supply. The conventional E-MESFET structure is illustrated in a cross-sectional representation in Fig. 1. It consists of a semi-insulating GaAs substrate 10 have a n⁻ layer 11 which is formed by either im­ planting n-type ions or by epitaxial growth. A gate electrode 12 is supported directly on the n⁻ layer 11 between the heavily doped source 13 and drain 14 and forms a metal semi-conductor (Schottky) contact. The n⁻ layer 11 is made sufficiently thin, typically as thin as 0,1 µm, and it is lightly doped, typically to a doping density of about 1 × 10¹⁷ atoms/cm³ to enable the nor­ mally-off operations of the device by enabling the depletion region under the Schottky gate 12 to pinch off the channel at zero gate voltage. The source 13 and drain 14 are heavily doped to insure good ohmic contact with their respective alloy contacts 15 and 16 which in turn make electrical contact with metal interconnects 17 and 18, respectively. Due to its light doping and shallow depth the channel between the source and drain exhibits a high sheet resistivity. Consequently, this prior art MESFET suffers from a large source-drain series resistance which degrades the device performance. Also, the heavy doping provided in the source and drain regions, imposes re­ strictions on device operation. First, its places a limitation on the maximum supply voltage due to the low drain break-down voltage. Second, it requires the source and drain be spaced sufficiently far away from the gate edges to insure enhancement mode operation. This require­ ment further compounds the series resistance problem discussed above by increasing the length of the high resistivity channel regions. Another disadvantage of this prior art MESFET structure is that it is not amendable to a high level of circuit integration because of the rather large device size. An improved GaAs MESFET structure is disclosed in an article "GaAs LDD E-MESFET for ultra-high speed logic" (IBM Technical Disclosure Bulletin, Vol. 26, No. 4, September 1983, pp. 1988/89). A partial sectional view thereof is shown in Fig. 2. GaAs substrate 20 includes a self-aligned gate n-channel MESFET 40 having a relatively shallow and low dopant concentration n⁻ channel region 21A. Positioned adjacent to the channel region 21A on either side thereof are lightly doped n-type source extension 25A and drain extension 26A. The n⁺ doped source 29 and drain 30 are positioned adjacent extensions 25A and 26A, respectively. Overlying the channel region 21A, in a self-aligned relationship with the source 29 and drain 30 is a metal Schottky gate 22. Structure 40 further includes sidewall insulator spacers 31-31 abutt­ ing the gate 22, ohmic contacts 32 and 33 to the source 29 and drain 30, respectively, and their associated interconnecting metallization 34 and 35, respectively. It is a principal object of the present invention to provide a high performance self-aligned GaAs MESFET device having a reduced series resistance and short channel length. Another object is to provide a MESFET structure in which device punch-through is prevented and parasitic substrate currents are reduced which otherwise tend to degrade the device operation at submicron channel length. The invention as claimed is intended to meet these objectives and to provide for a further improved MESFET device in which hitherto encountered drawbacks are eliminated or reduced. The advantages offered by the invention are mainly that, with very short channel devices the short channel effects are reduced by prevent­ ing the drain electric field from extending into the active channel region underneath the gate. The diminished short channel effects translate into improved threshold voltage control. One way of carrying out the invention is described in detail below with reference to drawings in which Fig. 1 and 2 are cross-sectional views of prior art MESFET structures. Fig. 3 is a cross-sectional representation of an embodiment of a self-aligned MESFET in accordance with the invention. Figs. 3A-3F are sequential cross-sectional represen­ tations of the process steps of the present invention culminating in the structure shown in Fig. 3. An embodiment of the invention is illustrated in Fig. 3 which shows a partial sectional view of an en­ hancement mode E-MESFET. Structurally, the device con­ sists of a shallow n⁻ channel region formed on a semi- insulating GaAs substrate, a Schottky metal gate over­ lying the n⁻ channel region and relatively highly doped and deep n⁺ source and drain regions formed in the substrate in a spaced-apart relationship with and on either side of the gate. In the channel regions between the gate edges and source/drain are positioned n-type source/drain extensions of an intermediate depth and doping concentration. The source/drain extensions consti­ tute the lightly doped drain regions of the MESFET and serve to minimize the device series resistance, suppress short channel effects without affecting the enhancement mode operation and permit the device channel length reduction to submicron levels. The structure additionally comprises deep p-type pockets under the lightly doped n-extensions abuing the source/drain to enable better device threshold voltage control and thereby enable further reduction in the channel length. In the embodiment of Fig. 3, the n-channel E-MESFET 70 consists of a shallow (typically less than 1 µm depth) and very lightly doped (dopant concentration in the range of 1 × 10¹⁶ - 5 × 10¹⁶ atoms per cm³) n⁻ regions 53A formed on a semi-insulating GaAs substrate 50 having a sheet resistance of typically about 1 Meg Ohm-cm or more. Abutting the channel region 53A, in a contiguous relationship therewith are n-type lightly doped (typical­ ly to a doping concentration of 1 × 10¹⁷ - 1 × 10¹⁸ atoms/cm³) source extension 57A and drain extension 58A of intermediate depth (typically in the range 0.10 - 0.20 µm) and highly doped (typically to a doping concentration exceeding 1 × 10¹⁸ atoms/cm³) and deep (typically 0.20 - 0.30 µm) source 64 and drain 65 regions. Provided under the source/drain extensions 57A and 58A are deep p-type pockets 59 and 60, respectively. Overlying the channel region 53A is a conductive Schottky gate 54 which is self-aligned with the source 64 and drain 65. Reference is now made to Figs. 3A-3F wherein the successive steps of the fabrication process of the above described embodiment (Fig. 3) of the present invention are illustrated in detail. These successive steps are listed in Table I indicating the correspondence between the steps and Figs. 3A-3F. TABLE 1 While the following description is primarily direct­ ed to the fabrication of an E-MESFET, this description is exemplary of the fabrication of a class of devices which embody the principles of the present invention. In addition, it should be noted that the thickness and other dimensions shown herein are selected for clarity of illustration and not to be interpreted in a limiting sense. The fabrication process of the E-MESFET 70 starts, as illustrated in Fig. 3A with forming a passivation surface layer of silicon dioxide SiO₂ or silicon nitride Si₃N₄ over a semi-insulating GaAs substrate 50 and defining by photolithographic and etching techniques an active area 52 of the substrate 50 (Step 1). The passiva­ tion surface layer that remains at the completion of this step is designated by numeral 51. The remnant layer 51 is left in tact to prevent subsequently formed metallization (Step 9) from being formed in the isolation regions thereunder which would occur if layer 51 were removed. Further, non-removal of the layer 51 saves a mask step which would be necessary if layer 51 were removed since the device isolation regions would be required to be appropriately protected during metallization. Next, referring to Fig. 3A, the shallow and low conductivity n⁻ layer 53 is formed by implanting n-type ions such as silicon or germanium directly into the exposed substrate. The energy and dose of ions implanted are chosen such that the semiconducting n⁻ GaAs layer 53 resulting therefrom is shallow having a depth of less than 0.1 µm and has a relatively low dopant concentra­ tion in the range of about 1 × 10¹⁶ - 1 × 10¹⁷ atoms/cm³. The n⁻ layer 53 may also be formed by other techniques such as molecular beam epitaxy, liquid phase epitaxy, metal organic vapor epitaxy or chemical vapor deposition. Next, referring to Fig. 3B, the Schottky gate 54 is formed at a selected location directly on the active channel layer 53 by blanket depositing a conductive material such as aluminium, titanium or titanium-tungsten and defining the gate therefrom by conventional photo­ lithographic and etching techniques (Step 2). Alterna­ tively, the gate 54 can be formed by the lift-off tech­ nique consisting of forming a photoresist layer over the layer 53, removing the photoresist from the region where the gate electrode is to be formed, blanket depositing a layer of conductive material over the exposed layer 53 and the remaining photoresist layer and removing the photoresist and the conductive layer thereover. The gate electrode 54 has a substantially horizontal upper surface and substantially vertical side surfaces. The succeeding step in the process, which is illus­ trated in Fig. 3B is the formation of a conformal insula­ tor layer 55 such as silicon dioxide or silicon nitride over the gate electrode 54 and the layer 53 not covered by gate 54 (Step 3). If the layer 55 is silicon dioxide, one example of forming the layer 55 is by chemical vapor deposition using silane at a low pressure and tempera­ ture. As shown in Fig. 3B, the conformal insulator layer 55 consists of substantially horizontal and vertical surfaces. The thickness of layer 55 is typically in the range 0.2 - 0.5 µm. Next, the structure is subjected to anisotropic reactive ion etching (RIE) to substantially remove the insulator layer 55 from the horizontal surfaces and to form the sidewall spacers 56-56 abutting the vertical surfaces of the gate 54 (Step 4). The resulting structure is shown in Fig. 3C. The sidewall spacers 56-56 are preferred to have a thickness about 0.1 µm or less. The thickness dimension is constrained by manufacturing reproducibility and by other considerations which in­ clude, on the one hand, the desirability of having thin sidewall spacers so as to reduce both the overall MESFET device dimensions and the undesirable channel series resistance as well as the accompanying parasitic capacit­ ance, both of which tend to limit the MESFET performance; and on the other hand, the need to have thick sidewalls 56-56 to ensure that the gate 54 (Fig. 3C) does not sub­ stantially reach the (subsequently formed) source and drain extensions (designated by numerals 57 and 58 in Fig. 3C, for example) extending beneath the sidewall spacer 56-56 as a result of later diffusion. The latter is important to premature break-down of the MESFET device. The succeeding process step which is also illustrat­ ed in Fig. 3C is a double implantation step (Step 5) consisting of a relatively low energy and dose n-type source and drain extensions 57 and 58, respectively, and a relatively high energy and dose p-type ion implantation to form the p-type pockets 59 and 60 extending under the source and drain extensions 57 and 58 respectively. Typical n-type implantation species include Si and Ge having a dose in the range 1 × 10¹² - 1 × 10¹³ atoms/cm² and energy of about 60 Kev. Typical p-type implantation species to accomplish step 5 include zinc and cadmium having a dose of at least about 1 × 10¹² atoms/cm² and energy in the range 100-150 Kev. In this process step (Step 5), it is immaterial in what order the n-type and p-type implantations are accomplished. It should be noted that the n-type source and drain extensions 57 and 58, respectively, are of a depth and dopant concentration exceeding those of the n⁻ channel region 53A. In progressing toward the structure of Fig. 3D, the next process step is the formation of a second conformal insulator layer 61 not only over the gate 54 and its abutting aidewall spacers 56-56, but also over the source/drain extensions 57 and 58, respectively (Step 6). This step (Step 6) is typically accomplished in the same fashion as the first conformal insulator layer formation (Step 3). In Step 7 which is illustrated in Fig. 3E, the conformal layer 61 is subjected to a RIE process step along the lines of Step 4 to form a second sidewall spacer 62-62 abutting the previous sidewall spacer 56-56. The thickness of the sidewall spacer 62-62 is governed by the same considerations discussed above in connection with the spacer 56-56. The enlarged sidewall spacer which is a combination of spacers 56 and 62 is designated by numeral 63-63 (Fig. 3F). Once spacer 63-63 is formed masking a portion 57A of the source extension 57 and a portion 58A of the drain extension 58 adjacent the n⁻ layer 53A, the structure is subjected to a high energy and dose n-type ion implanta­ tion step (Step 8) to form the deep and heavily doped source and drain regions 64 and 65, respectively. The species for forming the source 64 and drain 65 include ions of silicon and germanium. The energy and dose of the ions used to form the source 64 and drain 65 are selected such that the depth and doping concentration of the source 64 and drain 65 exceed those of the source and drain extensions 57A and 58A, respectively. During this process step, the sidewall spacers protect portions 57A and 58A of the n-type source and drain extensions 57 and 58, respectively, and portions 59A and 60A of the p-type pockets 59 and 60, respective­ ly, from receiving further ion implantation. The dose of the n-type ions (typically, silicon of germanium) used in this step is adjusted such that at the completion of the step, the previously p-type doped pocket regions not masked by the sidewall spacers are not only compensated for, but also the resultant impurity type of the implant­ ed regions 64 and 65 is rendered overwhelmingly n-type. The relative depths of the n-type doped regions 57A, 59A and 64 (Fig. 3E) etc., are dictated by the particular device application and the desired device characteris­ tics. For example, if parasitic substrate current is a particular concern, making 59A deeper than 64 will help reduce it. After implanting the source 64 and drain 65, the structure is subjected to a low temperature anneal step to activate the impurities implanted not only in the source 64 and drain 65, but also those in the course extension 57A and drain extension 58A. This thermal anneal step may be accomplished at a temperature of about 800°C either by capping the entire structure with a layer of silicon nitride or capless under an arsenic gas over pressure. Referring to Fig. 3F, the next sequence of process steps is establishing self-aligned platinum diarsenide (PtAs₂) contacts with the source 64 and drain 65 (Steps 9 and 10). In Step 9, platinum is blanket deposited over the entire structure forming a thin conductive layer 66 thereon. The structure is then subjected to a thermal step at a temperature of about 300 to 350° C to react the platinum layer 66 formed over the source and drain regions 64 and 65, respectively, with the arsenic atoms in these regions forming a thin layer of platinum di­ arsenide. The unreacted platinum is then removed (Step 10) by etching it away in aqua regia which is a solution of water (H₂O), nitric acid (HNO₃) and hydrochloric acid (HC1) in the proportion of H₂O:HNO₃:HC1 of 1:1:2. The resulting structure which is illustrated in Fig. 3 consists of highly conductive, self-aligned PtAs₂ source and drain contacts 67 and 68, respectively. In summary, the E-MESFET fabricated by the above novel process involving sidewall spacer technology, due to its self-aligned feature and relatively shallow, short (possibly less than 0.1 µm) and lightly doped channel and its adjacent deeper and relatively highly doped source and drain extensions, has a greatly reduced series resistance making the devide particularly suitable for high speed logic applications. Another beneficial feature is that the double implanted source/drain reduces the peak electric field at the drain by spreading out the depletion regions through the n-type drain extension. Consequently, the breakdown voltage is increased permitt­ ing higher drain voltage and better performance. The smaller peak electric field at the drain also results in high mobility. The self-aligned E-MESFET structure also occupies less chip area thereby enabling a higher level of integration than hitherto possible. The deep p-type pockets underneath the source and drain extension further reduce the above short channel effects by further preventing the drain electric field from extending into the active channel (i.e. region under the gate). The diminished short channel effects means better threshold voltage control, etc. Although the invention has been described in connec­ tion with an enhancement mode MESFET, the basic structure disclosed in Fig. 3 can be conveniently modified to form depletion mode or normally-on D-MESFETs. The depletion mode device can be formed by making the shallow n⁻ channel region 53A in Fig. 3 more highly doped and/or deep. This may require an additional mask step. The resulting D-MESFET will have all the beneficial features discussed above in connection with the counterpart enhancement mode device and will be suitable for high speed and low power enhancement/depletion circuit appli­ cations.

Claims: 1. A self-aligned metal-semiconductor field effect transistor (MESFET) comprising on a semiconductor sub­ strate (50) a pair of relatively deep and highly doped source and drain regions (64, 65), a relatively shallow and lightly doped active channel regions (53A) arranged between said source and drain regions, and source (57A) and drain (58A) extensions arranged between said channel region and said source and drain regions, respectively, and forming a contiguous arrangement of said source (64), source extension (57A), channel (53A), drain extension (58A) and drain (65) regions, these regions all being of a first conductivity type, characterized in that the MESFET furthermore comprises highly doped pockets (59A, 60A) of a second conductivity type arranged underneath each of said source (57A) and drain (58A) extensions in a contiguous relationship with said source and drain regions, respectively. 2. A MESFET as claimed in claim 1, wherein said semiconductor substrate (50) comprises gallium arsenide. 3. A MESFET as claimed in claim 1, wherein said first conductivity type is n-type. 4. A MESFET as claimed in claim 1, wherein the dopant concentration in said channel region (53A) is in the range of 1 × 10¹⁶ to 5 × 10¹⁶ atoms/cm³. 5. A MESFET as claimed in claim 1, wherein the dopant concentration in said source and drain extensions (57A, 58A) is in the range of 1 × 10¹⁷ to 1 × 10¹⁸ atoms/ cm³. 6. A MESFET as claimed in claim 1, wherein the dopant concentration in said source (64) and drain (65) regions is greater than 1 × 10¹⁸ atoms/cm³. 7. A MESFET as claimed in claim 1, wherein the dopant concentration in said pockets (59, 60) is greater than 1 × 10¹⁸ atoms/cm³. 8. A process for forming a MESFET on a semi-insu­ lating gallium arsenide substrate, said process compris­ ing the steps of (a) forming a relatively shallow and lightly doped layer (53) of a first conductivity type on said substrate (50), (b) forming a conductive gate electrode (54) having substantially vertical surfaces in direct contact with and over a portion of said active layer (53); (c) forming a first conformal dielectric layer (55) over the resulting structure; (d) etching directionally using reactive ion etching to remove substantially said first dielectric layer (55), forming narrow dimensioned first dielectric sidewalls (56-56) on said vertical surfaces of the gate (54); (e) forming source (57) and drain (58) extensions by introducing dopant of the first conductivity type into the portion of the active layer (53) not masked by said gate (54) and its associated first dielec­ tric sidewalls (56-56), said extensions being of a depth and doping concentration exceeding those of the active layer; (f) forming pockets (59, 60) of a second conductivity type underneath the source (57) and drain (58) extensions by introducing dopant of a second con­ ductivity type into said semiconductor body (50) via said source and drain extensions, said pockets having a doping concentration exceeding that of the source and drain extensions; (g) forming a second conformal dielectric layer (61) over the resulting structure; (h) etching directionally using reactive ion etching to remove substantially said second dielectric, forming second dielectric sidewalls (62-62) abutt­ ing said first dielectric sidewalls (56-56); and (i) forming source (64) and drain (65) regions of said first conductivity type by introducing dopant of the first conductivity type into the portions of the source (57) and drain (58) extensions and the underlying pockets (59, 60) of the second conduc­ tivity type not masked by the second dielectric sidewalls (62-62), said source and drain regions being of a depth and dopant concentration ex­ ceeding those of the source and drain extensions. 9. A process as claimed in claim 8, further comprising the steps of (j) forming a layer (66) of platinum over the resulting structure; (k) subjecting said structure to a low temperature heat­ ing step to react the bottom portion of the platinum layer in contact with said source (64) and drain (65) regions thereby forming self-aligned platinum diarsenide ohmic contacts with said source and drain regions; and (l)removing the unreacted platinum.

Inventors: CODELLA, CHRISTOPHER FRANK, OGURA, SEIKI
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1985
Main classification: H01L29
IPC codes: H01L29, H01L21