A diffused channel insulated gate field effect transistor

Abstract: A diffused channel insulated gate field effect transistor, comprising: a gate isolation layer (14) and a gate electrode (16) positioned on an upper surface of a semiconductor substrate (10, 12) of a first conductivity type; a body region (24) of a second conductivity type present in the semiconductor substrate lying below a part of the gate electrode, on at least one side thereof, and extending downwards to a first depth; a source region (30) of said first conductivity type present in the body region, spaced away from the first end of the gate electrode, at the upper surface and extending downwards therefrom to a second depth, shallower than the first depth; and a lightly doped region (26) of the first conductivity type present in the body region, at least partly between the source region and the gate electrode, extending downwards to a substantially shallower depth than the second depth.

Description: The present invention relates to the manufacture of semiconductor integrated circuits, in particular to the manufacture of integrated circuits including diffused channel insulated gate field effect (DMOS) transistors. As is well known in the art, the terms DMOS and CMOS are used to refer to any type of planar insulated gate field effect transistor, not necessarily constructed with a metal-oxide-semiconductor structure. Figures 1A to 1D show steps in the manufacture of one cell of a vertical DMOS transistor according to the prior art. As shown in figure 1A, a first step begins with a highly n doped semiconductor layer 10 covered with a lightly doped n type layer 12 on its upper surface. Layer 12 is generally an epitaxied layer. The highly n doped region 10 may be a substrate or a buried layer within a p type substrate. As shown in figure 1B, a gate insulator region 14 and a gate electrode 16 are formed on the upper surface of the epitaxied layer 12. As shown in figure 1C, a p type body region 18 is introduced on both sides of the gate electrode 16. Typically, this region is formed by photolithographic masking followed by a p type implantation and a first thermal anneal step, at a very high temperature and over a long period of time. Subsequent to the thermal anneal step, each body region 18 extends under the gate electrode by a distance d. As shown in figure 1D, highly n doped source regions 20 and highly p doped body contact regions 22 are introduced into the body regions 18, adjacent to each end of the gate electrode 16. Typically, these regions 20, 22 are introduced by photolithographic masking followed by ion implantation and a second thermal anneal step. This second anneal step is performed for a shorter time and at a lower temperature than the first anneal step. The length L of the body region remaining at the upper surface of the epitaxied layer 12 is the channel length of the DMOS transistor. The highly doped layer 10 corresponds to the drain region of the DMOS transistor. The process continues in a known manner, with the formation of interconnect layers, and contacts to gate, source, body and drain. Electrical contact to the drain region 10 is made either by a metallisation of the lower surface of drain region 10 when the drain region is a substrate, or at the upper surface of an n type well contacting the drain region 10, when this is a buried layer. The formation of the p body region is typically done by implanting boron ions at an energy of 25 to 35keV and an intensity of 2x10 13 to 4x10 13 atoms.cm -2 . A severe thermal anneal step is then required to achieve a sufficient distance d of diffusion of this p body implant under the gate electrode. For example, for boron in silicon, which has a high diffusion coefficient, such a thermal anneal step may typically involve heating the circuit to 1160°C for a period of 1 to 2 hours. Use of a dopant with a lower diffusion coefficient would require even more severe diffusion conditions, or may not even be possible. Although shown as a vertical DMOS structure, a horizontal DMOS structure may be formed where the p body dopant is introduced on only one side of the gate electrode 16. A highly n doped drain contact region is then introduced on the other side of the gate electrode. It is often required to integrate DMOS, CMOS and bipolar transistors in a single integrated circuit. This occurs in "BCD" (Bipolar - CMOS - DMOS) processes, which currently have CMOS and DMOS transistors with gate electrodes over 2µm long. In this way, single integrated circuits may be used to control inductive devices, such as electric motors, or where high voltage holding capabilities are required. A typical application would be a computer disc controller, where bipolar circuitry is required for treating analogue signals, CMOS transistors for control logic circuitry, and DMOS transistors for a high power interface to control the disc drive motors. However, conventional techniques for producing DMOS transistors are incompatible with known processes for producing CMOS integrated circuits having transistor gate lengths of approximately 1,5µm or less. In such integrated circuits, a threshold adjust region must be included under the gates of CMOS transistors to achieve a desired threshold voltage. Subjecting such a transistor to a severe anneal step as described above would suppress the threshold adjust region by diffusion effects, and the threshold voltages would no longer be controllable. Furthermore, the severe first thermal anneal step described above would weaken the gate insulator layer, and reduce its breakdown voltage to an unacceptable level. The severe thermal anneal step cannot simply be eliminated, as the high doping density of the source region 20 compared to the p body region 18, would preclude the choice of a pair of dopants allowing an optimised channel region to be formed. Other methods of manufacturing DMOS transistors are disclosed in EP-A-0336393, US-A-5187117 and EP-A-0658940, but in both methods a high temperature, long duration anneal step is provided. An object of the present invention is to provide a method of fabricating DMOS transistors which are suitable for inclusion in manufacturing processes for CMOS integrated circuits with a transistor gate length of 1,5µm and less. A second object of the present invention is to provide such a process which does not require a high temperature, long duration anneal step. A third object of the present invention is to provide such a method which introduces as few new steps as possible into the manufacturing process for CMOS integrated circuits. The invention also provides a method of manufacture of a diffused channel insulated gate field effect transistor, as claimed in claim 1. Specific embodiments of the invention are claimed in the sub-claims. These and other objects, characteristics and advantages of the present invention, will be described in detail with respect to certain particular embodiments of the present invention, given by way of non-limiting examples with reference to the accompanying drawings, wherein: figures 1A to 1D show cross sectional views of a DMOS transistor at various stages in a manufacturing process according to the prior art; and figures 2A to 2D show cross sections of a DMOS transistor at various stages in a manufacturing process according to a preferred embodiment of the present invention. As is customary in the domain of semiconductor devices, these figures are not drawn to scale. In particular, the vertical dimensions of certain features has been exaggerated for clarity. Figures 2A to 2D illustrate a preferred process for the manufacture of DMOS transistors in integrated circuits containing both DMOS and CMOS transistors, according to the present invention. Features common with those shown in figures 1A to 1D carry identical reference labels. In figure 2A, a highly n doped semiconductor layer 10 is provided with a lightly n doped epitaxial layer 12, a gate insulator layer 14 and gate electrode 16 as described with reference to figures 1A and 1B. Generally, the device shown in figure 2A is created through the performance of a number of manufacturing steps. In a first step the semiconductor layer 10 is covered with the lightly doped n type layer on its upper surface. Preferably, layer 12 is an epitaxied layer. In a subsequent series of steps, the gate insulator layer 14 and the gate electrode 16 are formed on the upper surface of layer 12. P type body implants 24 are formed adjacent to both ends of the gate electrode 16. Significantly, no thermal anneal step is applied. Accordingly, the p type dopant does not diffuse into the semiconductor, neither vertically nor under the gate electrode. Figure 2B shows the structure at a later step in the process of the present invention. A light n type implant 26 has been formed in the same regions as the p body implants 24. No anneal is performed at this stage, so this implant also does not diffuse into the semiconductor. During the step illustrated in figure 2C, spacers 28 are produced adjacent to each end of the gate electrode. Spacers 28 may correspond to those currently used in the manufacturing of CMOS transistors. In CMOS transistors spacers such as spacers 28 allow the formation of lightly doped drain (LDD) regions between highly doped source and drain regions and the channel region. Without these LDD regions, the peak of the electric field would be at the edge of the drain region, just under the gate electrode 16. Such high electric fields could cause hot carrier injection into the gate insulation layer 14 and gate electrode 16. The LDD regions displace the point of maximum field strength to a location under the spacers 28, so that no damage to the gate insulation layer 14 can occur. Vertical DMOS transistors do not experience the problem of hot carrier injection. The presence of spacers does not appear to bring similar advantages to these structures. Accordingly, the step for forming spacers 28 is not performed in conventional DMOS manufacturing processes. After formation of the spacers 28, the highly n doped source regions 30 are formed, along with highly p doped body contact regions 32. Only now, as represented in figure 2D, is a single thermal anneal step applied to the structure. It will be understood that, while the above described implantations are performed, so also are the implantations necessary for the formation of other components of the integrated circuit. In particular, CMOS and maybe also bipolar transistors are formed. The anneal step must therefore be compatible with the other structures of the integrated circuit. Referring to figure 2D, upon completion of the anneal step and according to the present invention, the DMOS channel region is formed. Significantly, the DMOS channel region is not defined by the difference between a p type diffusion 18 and a highly doped n type diffusion 20 as in conventional devices. Instead, the DMOS channel region is defined by the difference between a p type diffusion 24 and an intermediately doped n diffusion 26. It is this arrangement which makes possible the realisation of the DMOS channel in a single thermal anneal step. It will, however, be clear for one skilled in the art that the implementation of the invention requires a suitable choice of different dopants, their implantation conditions, and the conditions of the single thermal anneal. Therefore, a certain specific combination of steps performed in accordance with the present invention will be described in detail below. Example The dopant which forms the p body region 24 is boron, implanted at an energy of 40keV, at a dose of 4x10 13 atoms.cm -2 . The dopant which forms the lightly doped n regions 26 is preferably arsenic implanted at an energy of 40keV, at a dose of 3x10 13 atoms.cm -2 . This light n implant is preferably the same as that used to produce lightly doped drain (LDD) regions in n channel MOS transistors of the integrated circuit. The dopant which forms the highly n doped source regions 30 is preferably arsenic, implanted at an energy of 40keV, at a dose of 4x10 15 atoms.cm -2 . This n type implant is preferably the same as that used to produce drain and source regions in n channel MOS transistors of the integrated circuit. The p body contact regions 32 are also doped with boron. However, to ensure that they have a depth after the thermal anneal step less than that of the p body 24, an implant species such as BF 2 is preferably used, which penetrates less deeply into the semiconductor than boron, and so produces a shallower region after anneal. This same dopant implant may be used to produce source and drain regions for p channel MOS transistors. For the anneal step, the structure may be heated to 980°C for 30 minutes, which corresponds to normal conditions for CMOS circuit manufacture. This allows the production of a 0.25µm channel length, and the following diffusion profiles: Cs (atoms.cm -3 ) xj (µm) p body 24 2x10 18 0.5 lightly doped n 26 4x10 18 0.15 n source 30 3x10 20 0.25 p body contact 32 1x10 20 0.5 where Cs represents a surface concentration, and xj a corresponding junction depth of a particular region. In an example of controlling a 12V motor, the dimensions of the DMOS transistor should be fixed to withstand a voltage of 20V, both to withstand voltage peaks caused by the inductance of the motor, and to allow for variation of the breakdown voltage with temperature. This may be achieved with the vertical DMOS structure of figure 2D, having a 2.5µm long gate electrode 16, a 3µm thick epitaxial layer 12, with a resistivity of 1Ω.cm. Such a structure could have a breakdown voltage as high as 35V. Lower resistivity and greater thickness of the epitaxial layer 12 would give a higher breakdown voltage. Thus, a DMOS transistor is provided that may be incorporated into a standard CMOS logic or bipolar circuit manufacturing process, with only the addition of a single p body region implant. Although described with reference to certain specific embodiments, numerous variations of the invention are possible, and will be apparent to those skilled in the art. For example, in addition to the vertical DMOS device produced according to the invention, with body and source regions on either side of the gate electrode, a horizontal DMOS structure could be produced as described with reference to the prior art.

Claims: A method of manufacture of a diffused channel insulated gate field effect transistor, comprising the following steps: providing a semiconductor substrate (10, 12) of a first conductivity type, having an upper surface; defining a gate isolation layer (14) and a gate electrode (16) on the upper surface of the substrate; implanting a first dopant (24) of a second conductivity type into a first region of the substrate, adjacent to a first end of the gate electrode, and not following this implant with an anneal; implanting a second dopant (26) of first conductivity type at a low density into the first region of the substrate; forming spacers (28) adjacent to at least the first end of the gate electrode; implanting a third, source, dopant (30) of first conductivity type at a high doping density into a part of the first region adjacent to the spacer (28); performing a thermal anneal, wherein the first dopant is chosen to diffuse more quickly than the second and third dopants, during the anneal step. A method according to claim 1, wherein the thermal anneal is performed at a temperature of less than 1000°C. A method according to claim 2, characterised in that the anneal is performed for a time of the order of 30 minutes. A method according to claim 3, characterised in that it further comprises the step of: implanting a fourth dopant (32) of second conductivity type into second regions of the substrate, where body contacts are to be formed, and in that this implantation is performed such that, after anneal, the diffusion depth of the fourth dopant is less than the diffusion depth of the first dopant. A method according to claim 4, characterised in that the first dopant is boron, the second and third dopants are arsenic and the fourth dopant is boron, implanted in the form of boron fluoride.

Inventors: JIMENEZ JEAN, JIMENEZ, JEAN
Applicants: ST MICROELECTRONICS SA, STMICROELECTRONICS S.A.
Application year: 1996
Main classification: H01L29
IPC codes: H01L29, H01L21