Fully depleted lateral transistor

Abstract: The breakdown characteristics of a lateral transistor integrated in an epitaxial layer of a first type of conductivity grown on a substrate of an opposite type of conductivity and comprising a drain region formed in said epitaxial layer, are markedly improved without recurring to critical adjustments of physical parameters of the integrated structure by forming a buried region having the same type of conductivity of the substrate and a slightly higher level of doping at the interface between the epitaxial layer and the substrate in a zone laying beneath the drain region of the transistor.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to a lateral, double-diffused transistor (e.g. an LDMOS) having improved breakdown characteristics, particularly suited for high voltage integrated circuits (HVIC's). 2. Description of the prior art Typically, high-voltage, integrated circuits (HVIC's) contain one or more high-voltage power transistors together with a low voltage signal processing circuitry on the same chip. The use of this type of integrated circuits is becoming more and more viable alternative to the use of a plurality of discrete circuits, in a wide variety of applications. In these integrated circuits, lateral, double-diffused, MOS transistors (LDMOS) are widely used as active power devices. One way to improve the voltage handling capability of a lateral transistor is intrinsic to a so-called RESURF (an acronym for REduced SURface Field) technique. This particular technique is described in an article of J. A. Appels et al., appeared on: "Philips J. Res. 35 , 1-13, 1980". The physical structure of a RESURF LDMOS transistor, as depicted in Figures 1 and 2, is substantially identical to the structure of a conventional LDMOS transistor. The main difference between the two devices consists in the fact that the structure of a RESURF LDMOS is generally formed in a much thinner epitaxial layer than a conventional high voltage device. For this reason, the bottom-side depletion region pertaining to the junction between the epitaxial layer, for example of an n - conductivity, and a substrate layer, for example of a p - conductivity, has a significant effect on high-voltage withstanding capability in the case of a RESURF type LDMOS structure. The document US-4,300,150 discloses a lateral double diffused MOS transistor, the integrated structure of which includes field-shaping semiconductor layers. The field shaping layer may be a buried layer formed beneath the channel region of the transistor and having the same type of conductivity and a higher doping level than the substrate. Alternatively the field-shaping layer may be diffused in the epitaxial layer alongside the drain region of the transistor or the integrated structure may include both layers. The transistor is reportedly capable of having a breakdown voltage of 370 Volts. The document EP-A-69429, describes an insulated gate, field effect transistor, the structure of which includes a buried layer of the same conductivity type as the epitaxial layer, formed beneath the channel region of the transistor and a field plate connected to the source electrode or to the gate electrode which is separated from the semiconductor by an insulating layer having a thickness increasing in the direction of the drain area. For better illustrating the breakdown mechanism in a RESURF LDMOS structure, Figures 3, 4 and 5 show in a qualitative and schematic way the progress of the depletion region into the drift region (the region where electric charge carriers move under the influence of an electric field). The situations that develops in the drift region with an increase of the voltage applied to a drain terminal (D), in a grounded-source configured transistor, are schematically depicted in Figures 3, 4 and 5, wherein the depleted region is identified with a cross-hatching. The operating condition characterized by a relatively low voltage applied to the drain terminal of the device, i.e. a voltage lower than the "pinch-off" voltage (V d <V PO ), is depicted in Fig. 3. As may be observed, in such a low drain voltage condition, practically no interaction exists between the surface depletion region that develops under a gate structure (G) and the bottom-side depletion region (or more briefly bottom depletion region) that develops across the junction between the substrate and the epitaxial layer. In these conditions, the electric fields pertaining to the superficial regions of the structure will have values similar to those that occur in a conventional type LDMOS structure (i.e. in a similar transistor structure formed in a relatively thicker epitaxial layer). Upon an increase of the voltage applied to the drain (D) of the transistor, and when such a voltage reaches a "pinch-off" value (V D =V PO ), the two depleted regions (surface and bottom regions) merge. This "pinch-off" condition is schematically depicted in Fig. 4. Because of an expansion of the depletion region in the drift region, the increase of the electric field intensity under the edge of the gate electrode tends to be less than in the case of a conventional LDMOS structure. When the voltage applied to the drain (D) of the device rises above the pinch-off voltage (V D >V PO ) , the surface depletion region tends to extend laterally toward the drain region (the n + region in the case shown in the Figures), and eventually the whole drift region becomes completely depleted. This may occur as long as the electric field that develops under the edge of the gate electrode during such a lateral extension of the surface depletion region, remains lower than the critical electric field. Under these conditions, as schematically shown in Fig. 5, the drift region under the edge of the gate electrode becomes practically isolated from the drain region and therefore the local electric field itensity remains approximately constant even if the drain voltage is increased further. According to these hypotheses, in a grounded-source configuration, the breakdown mechanism of the device, is determined solely by the presence of intense electric fields near the drain diffusion (n + ) or at the junction between the substrate and the epitaxial layer. On the other hand, the main objectives in designing a power transistor, is that of reducing its internal resistance (ON-resistance) and of achieving the highest breakdown voltage as possible. These two objectives could be reached if the drift region was completely depleted just before electric fields of critical intensity would develop under the edge of the gate electrode. This would ensure that the device be in a working condition as the one depicted in Fig. 5; a condition that determines the best avalanche breakdown voltage that can be obtained for a certain charge density in the drift region. In other words, an optimal RESURF structure should operate under conditions of substantially complete depletion when the voltage that is applied to a drain terminal reaches or slightly rises above the pinch-off voltage (V D =V PO ). According to the known art, these objectives may be achieved or approached by accurately trimming common design parameters, such as for example the doping level of the epitaxial layer, the doping level of the substrate layer, the thickness of the field oxide and in particular the thickness and resistivity of the epitaxial layer. Optimization of the structure thus becomes a very critical process because while from one side a complete depletion region of the drift region must be favored, on the other side, the structure should retain the ability to withstand voltage breakdown between, for example, the source region p + and the substrate p - , under punch-through conditions. OBJECTIVES AND SUMMARY OF THE INVENTION Vis-à-vis this state of the art and of its present limitations, the instant invention provides a way to optimize an integrated structure of a RESURF transistor in a noncritical way. In practice, by providing an additional degree of freedom in designing the structure, the present invention permits to achieve a complete depletion of the drift region, irrespectively of breakdown withstanding considerations pertaining to a punch-through mechanism between a source region and the substrate of the integrated structure (breakdown conditions that notably occur when the transistor is functioning in a source-follower configuration). According to the present invention, this is obtained by a lateral RESURF LDMOS transistor according to claim 1. A buried region having a doping level higher than the doping level of the substrate, between the substrate and the epitaxial layer and projectively underneath the drain region. This buried region is kept at a sufficiently large distance from a source region so that a punch-through between the source region and the buried region (because of the curvature effect that the buried region may induce, though in an extremely limited fashion) does not become a limiting parameter in the functioning of the device at the design voltages. This buried region may be formed by ion implanting the substrate, within a defined area, before going through the normal steps of a standard fabrication process of these devices that bring about the formation of buried layers in general and the growth of the epitaxial layer. This buried region extends for a major part of its "thickness" into the substrate and there is not any ohmic contact path with other regions or conducting layers. In practice, this buried region permits to "modulate" the depletion along the junction between the epitaxial layer and the substrate in an important zone, underlying the drain region of the device, differently from other zones and in particular from the zone underlying the source region of the structure. In this way, a complete depletion of the drift region at the drain end from the horizontal junction up to the surface, is favored without necessarily modifying the values of other physical parameters of the integrated structure, such as for example without further decreasing the thickness of the epitaxial layer, or increasing the doping level of the epitaxial layer, both of which would be detrimental in terms of punch-through. BRIEF DESCRIPTION OF THE DRAWINGS The different aspects and advantages of the invention will become more evident through the following description of several important embodiments and by referring to the attached drawings, wherein: Figure 1 is a schematic partial cross section of an integrated LDMOS structure of a known type; Figure 2 is an enlarged view of the channel region of the LDMOS structure; Figures 3, 4 and 5 , as described above, show in a schematic way the development of a depletion zone in the drift region between a source region and a drain region of the integrated LDMOS structure, with the rising of the voltage applied to the drain; Figure 6 is a partial schematic cross section view of an integrated LDMOS structure made in accordance with the present invention; Figures 7 and 8 , are layout views of a RESURF LDMOS transistor made in accordance with the present invention. DESCRIPTION OF PREFERRED EMBODIMENTS With reference to Fig. 6, an LDMOS structure made in accordance with the present invention is characterized by the presence of a buried region 5 which extends for a major portion thereof into the substrate 1, in a zone laying under the drain region of the device that is represented by the n + region 4 and by the n DDD region (the notation being an acronym of Double Diffused Drain) 4a. The buried region 5 has a conductivity of the same type of the substrate 1 and a concentration of dopant that is slightly greater than the dopant concentration of the substrate. For example, in a typical case of a p - substrate having a bulk resistivity comprised between 100 and 150 Ω cm, the buried region 5 may be realized by implementing the substrate with Boron at 80 KeV for a dose of 6·10 11 atoms (boron)/cm 2 . Generally, the buried region 5 may be formed by implanting the surface of the susbtrate 1 with atoms of the desired dopant, within the areas defined by a mask, before proceeding to the growth of the epitaxial layer 2. The diffusion profile of the region 5 so created is such as to extend into the growing epitaxial layer 2 above the substrate 1 in a relatively lesser extent than into the substrate 1 itself. In practice, diffusion of the implanted dopant is considerably more pronounced in the substrate 1 than in the growing epitaxial layer 2. This fact determines a nontrivial and advantageous effect in minimizing the curvature of the junction that is created between the buried region 5 and the epitaxial layer. Therefore, the creation of the buried region 5 according to the present invention does not negatively effect the breakdown characteristics of the resulting structure because of the introduction of substantially negligible curvature effects. On the other hand, the presence of a buried region 5, has the remarkable effect of causing a shift of the bottom depletion region (identified in Fig. 6 by the cross-hatched area, D bot ), that is of the depletion region pertaining to the junction n-epi/p-substrate. The bottom depletion region is shifted toward the epitaxial layer side of the junction. In this way, in correspondence of the buried region 5, the "thickness" of the depletion region in the epitaxial layer (D epi ) is virtually increased while the "thickness" of the depletion region in the substrate (D sub ) is proportionally decreased. As a consequence of this local "lifting" of the bottom depletion region toward an overlaying drain region 4 (n + ), a complete depletion of any residual, not yet depleted, drain-end portion of the drift region between the source region and the drain region of the device is greatly enhanced. This may occur when the voltage applied to the drain D of the transistor reaches the level of the pinch-off voltage (V D ≥V PO ) in the drift region, that is when an interaction (merging) of the surface depletion region D surf. and of the bottom depletion region D bot. occurs. The improved RESURF LDMOS transistor structure according to the invention, has the intrinsic advantage of permitting that, in correspondence of the source region that is indicated as a whole with A in the figure, the bottom depletion region D bot. maintains a relatively pronounced extension for a consistent depth into the bulk of the substrate 1, so as to retain a high punch-through breakdown voltage. Viceversa, in the drain region that is indicated as a whole with B in the figure, the bottom depletion region D bot. is, as a whole, shifted proportionally more into the epitaxial layer 2, so as to favor and produce a complete depletion of the drift region at its drain-end. In this way, because the curvature effect created by the junction that is formed between the buried region 5 and the epitaxial layer is substantially negligeable, the breakdown of the integrated structure, is determined solely by the following factors: the electric field intensity near the drain region (n + ), in a grounded-source configuration; the punch-through voltage between the source region 3a (p + ) and the substrate 1, in a souce-follower configuration. It is evident that the novel RESURF structure is much less critical from the point of view of possible breakdown mechanisms as compared with a conventional structure, without a buried region 5 that characterize the structure of the present invention. Moreover, it has been found that the buried region 5 reduces also the probability of the occurrence of a so-called premature breakdown due to three-dimensional effects at the source/drain terminations of the fingers of an interdigitated integrated structure typical of power devices, besides substantially eliminating all instabilities of the breakdown voltage pertaining to intense surface fields. The arrangement of the buried region 5 underneath the drain regions in a typical interdigitated layout of a power LDMOS transistor is depicted in the simplified layout of a ten-finger transistor shown in Fig. 7. The buried region 5 may also be extended along the entire perimeter of the integrated LDMOS transistor structure, by extending it radially as far as the internal "wall" of an isolation diffusion that completely surrounds the area occupied by the integrated transistor structure and which extend throughout the entire thickness of the epitaxial layer, according to known techniques. This alternative embodiment is schematically shown in the simplified layout view of Fig. 8. According to a typical embodiment of the invention, the different regions that characterize an integrated RESURF LDMOS structure made in accordance with the present invention may have the following characteristics: REGION ID. IN IN FIG. 6 DOPANT RESISTIVITY AND IMPLANT PARAMETERS THICKNESS OR DEPTHS OF JUNCTION p substrate 1 B 100-150 Ω cm 375 µm epitaxial layer 2 P 6 Ω cm 18 µm gate oxide - 850 Å field oxide - 1.1 µm source contact region p + 3a B 1·10 15 cm -2 /80KeV 3.8 µm p-body 3b B 5·10 13 cm -2 /70KeV 3.0 µm source region n + 3 As 5·10 15 cm -2 /50KeV 0.5 µm drain region n + 4 As 5·10 15 cm -2 /50KeV 0.5 µm n DDD region 4a As 8·10 13 cm -2 /50KeV 1.2 µm buried region 5 B 6·10 11 cm -2 /80KeV 5 µm (in substrate) Electrical connection of the integrated high voltage structure with the external world may be arranged by employing specially devised techniques for this type of power devices. In particular, it is possible to employ a segmented capacitance-chain technique for source connection and a simple capacitance-chain for drain connection, as disclosed in a prior European patent application (EP-A-565 808) of the same Applicant, application No. 92830190.2, filed on April 17, 1992. A combination of the beneficial effects, as produced by the structure of the present invention, with the beneficial effects that may be obtained by arranging the electrical connections of the integrated transistor according to the teachings disclosed in said prior patent application, will permit to integrate power transistors capable of withstanding extremely high voltages.

Claims: A lateral RESURF LDMOS transistor integrated in an epitaxial layer (2) of a first type of conductivity grown on a semiconducting substrate (1) having a second type of conductivity, comprising a drain region (4) in said epitaxial layer (2) and further comprising a buried region (5) having the same type of conductivity as that of said substrate (1) and a doping level higher than said semiconducting substrate (1), at the interface between said substrate (1) and said epitaxial layer (2) in a zone laying beneath said drain region (4) of the transistor characterized in that said buried region (5) is laterally spaced from a source region (3) of the transistor by a distance being sufficient to be ininfluent as far as a punch-through breakdown voltage across said epitaxial layer (2) is concerned. A lateral RESURF LDMOS transistor as defined in claim 1, wherein said buried region (5) at the interface between said epitaxial layer (2) and said substrate (1), extends into said substrate (1) by a greater proportion of its thickness than in said epitaxial layer. A lateral RESURF LDMOS transistor according to claim 1, wherein said substrate (1) has a p - type conductivity, said epitaxial layer (2) has an n - type conductivity and said buried region (5) has a p type conductivity. A lateral RESURF LDMOS transistor according to claim 1, wherein the transistor has an interdigitated structure comprising at least said buried region (5) having a comb-like structure that extends beneath a plurality of fingers of a drain region interdigitated with a plurality of fingers of a source region (3). A lateral RESURF LDMOS transistor according to claim 4, wherein said comb-shaped buried region (5) extends in the form of a ring along the entire perimeter of the integrated structure of the transistor and said ring portion extends radially as far as a perimetral isolation region that extends vertically through the entire thickness of said epitaxial layer (2). A method of manufacturing a lateral RESURF LDMOS transistor for improving the breakdown characteristics, according to one of claims 1 to 5, said lateral RESURF LDMOS transistor being integrated in an epitaxial layer (2) of a first type of conductivity grown on a semiconducting substrate (1) of a second type of conductivity and comprising a drain region (4) formed in said epitaxial layer (2) which is contacted through a drain contact (D), further comprising: forming a buried region (5) having the same type of conductivity as that of the substrate (1) and a doping level higher than the doping level of the substrate (1), between the substrate (1) and said epitaxial layer (2) in a zone laying beneath the drain region (4) of the transistor and characterized in that said buried region (5) is laterally spaced from a source region (3) of the transistor by a distance being sufficient to be ininfluent as far as a punch-through breakdown voltage across said epitaxial layer is concerned.

Inventors: RAVANELLI ENRICO MARIA ALFONSO, VILLA FLAVIO, RAVANELLI,ENRICO MARIA ALFONSO, VILLA,FLAVIO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1993
Main classification: H01L29
IPC codes: H01L29, H01L21