METHOD FOR ACHIEVING IDEAL IMPURITY BASE PROFILE IN A TRANSISTOR

Abstract: A method for making a transistor having base, collector, and emitter regions, where the impurity doping profile of the intrinsic (32) and extrinsic (22) base regions can closely approximate that which is ideal for the transistor. The extrinsic (22) and intrinsic (32) base regions are formed in separate steps, where the extrinsic base region (22) is formed first, followed by formation of the intrinsic base region (32). The portion of the extrinsic base region located over the area where the intrinsic base region is to be formed is removed, leaving an opening (26) through which both the emitter (30) and the intrinsic base regions (32) are formed. Thus, the effect of the step in which the extrinsic base region is formed is removed prior to formation of the intrinsic base region. Furthermore, the extrinsic base region is protected during formation of the intrinsic base region. This technique can be applied to processes using either ion implantation or diffusion to form the emitter and base regions of the transistor.

Description: METHOD FOR ACHIEVING IDEAL IMPURITY BASE PROFILE IN A TRANSISTOR DESCRIPTION Technical Field This invention relates to a method for fabricating a bipolar transistor to obtain a desired ideal base impurity concentration profile, and more particularly to the use of separate and independent diffusion or ion implantation steps to form the extrinsic and intrinsic base regions of the transistor. Background Art Bipolar transistors have been fabricated using both diffusion and ion implantation steps in order to provide the desired impurity profile in the transistor. in particular,, ion implantation is a technique which provides a wide range of impurity concentrations with a high degree of control of both the impurity con¬ centration and its distribution. For this reason, the prior art has attempted to fabricate high frequency transistors using ion implantation, either in a double implant process wherein both the emitter and base regions are implanted in the semiconductor substrate, or in hybrid processes wherein the base is implanted and the emitter diffused. In the prior art, bipolar transistors have been fabricated using two separate processing steps to form the base region. For example, U.S. 3,856,578 describes a process having a first step comprising diffusion or ion implantation to determine the sheet resistivity at the surface of a semiconductor, followed by a second step in which impurities are implanted within the same area of the semiconductor material, but to a greater depth than the impurities introduced in the first step. In this manner, the impurity profile is dominated at the surface by im¬ purities determining sheet resistivity, and is dominated in the bulk of the semiconductor material by the impurities introduced in the second step. The emitter region is later formed by a chemical diffusion step, or by ion implantation together with thermal diffusion of the implanted impurities. Another method for obtaining a bipolar transistor having a specified concentration profile is that described in IBM Technical Disclosure Bulletin Vol. 20, No. 11A, Page 4452, April 1978. In that technique, the impurity profile of the collector-base junction is adjusted to obtain a desired impurity profile by subsequent boron implantation. In a bipolar transistor, it is desirable that the extrinsic base portion be doped more heavily than the intrinsic base portion and that the emitter be doped more heavily than either of the two base portions. A high peak impurity concentration in the extrinsic base region reduces the extrinsic base resistance and minimizes emitter sidewall injection. The in¬ trinsic base concentration profile is determined only by the desired intrinsic base characteristics, OMPI W1P0 regardless of the impurity profile of the extrinsic base. In ion implantation of the intrinsic base region, it is desired to minimize the amount of post ion implantation heat treatment that is required, in order to maintain the profile produced by the implan¬ tation. While the prior art describes two-step processes for forming the intrinsic and extrinsic base regions, these two steps follow one another, so that the two steps are additive. Thus, the steps wherein the intrinsic and extrinsic base regions are formed are not truly separate from one another, and processing tolerance is therefore more tight. Accordingly, it is a primary object of the present invention to provide an improved method for obtaining a desired ideal impurity profile in a bipolar tran¬ sistor. It is another object of the present invention to provide a process for forming a bipolar transistor, in which truly separate steps are used to provide the intrinsic and extrinsic base regions of the transistor. It is another object of this invention to provide an ion implantation technique for producing a bipolar transistor, in which first and second ion implantation steps are used, wherein the effects of the first implantation step are removed from the area which is to be subject to the second ion implantation step. It is another object of the present invention to provide a process for making a bipolar transistor having self-aligned emitter and base contacts, and wherein the extrinsic and intrinsic base regions are formed by separate steps. Disclosure of Invention This invention relates to a process for making a bipolar transistor, and particularly to a process in which the intrinsic base region and extrinsic base region are formed separately, in order to yield a transistor having an ideal implanted base impurity concentration profile. This technique can be applied in a process in which the emitter and base contacts are self-aligned with one another and can be imple¬ mented by either ion implantation or diffusion. The process is characterized by a first implantation or diffusion step, in order to form the extrinsic base region- The emitter, window is then opened by a mask- ing step, followed by an etching step in which the implanted or diffused region is selectively removed from this window area. The emitter is then formed, followed by formation of the intrinsic base using the same window. When the emitter is formed first, ion implantation is used to form the intrinsic base. If the intrinsic base is formed prior to formation of the emitter, either diffusion or ion implantation can be used to form the intrinsic base and the emitter. Since the extrinsic base region was protected by a mask during formation of the intrinsic base region, and since the effects of the first implantation were removed from the region in which the emitter and in¬ trinsic base were to be formed, the intrinsic and extrinsic base regions are separately provided in order to yield the most ideal impurity profile. The use of a polycrystalline layer for contacting the extrinsic base region allows self-alignment of the emitter and base contacts. Separate formation of the extrinsic and intrinsic base regions can also be applied to this self-alignment process. Whether or not self-aligned contacts are provided, the etching step in which the effect of the first ion implantation or diffusion is removed in the region of the intrinsic base does not contaminate the underlying semiconductor surface and therefore the emitter and intrinsic base • region are reliably produced. Furthermore, the etched region is subsequently heavily doped and consequently does not have any exposed or depleted surface region. For these reasons, there is no adverse effect due to the etching step, as might be anticipated by those having skill in this art. In this process, a single masking step provides the emitter and intrinsic base, which ensures that proper alignment of these regions with respect to each other and with respect to the extrinsic base region. Also, the width of the window through which the emitter is formed is less than the width of the first implant or diffusion so that a doped, low resistance region will be left between the intrinsic and extrinsic base regions. These and other objects, features, and advantages will be more apparent from the following more parti¬ cular description of the preferred embodiments. Brief Description of the Drawings FIG. 1.1 shows a cross section of a bipolar transistor (for example, an NPN transistor), while FIGS. 1.2 and 1.3 show the preferred impurity concentration profiles of the intrinsic base region (FIG. 1.2) and extrinsic base region (FIG. 1.3) of this transistor. FIGS. 2-8 illustrate a process for making a bipolar transistor in which separate steps are used to pro- vide the extrinsic and intrinsic base regions. In this process, the emitter and base contacts are not self-aligned. FIGS. 9-19 illustrate a process for making a bipolar transistor, in which the emitter and base contacts are self-aligned and in which the extrinsic and in¬ trinsic base regions are formed by separate steps. Best Mode for Carrying Out the Invention FIG. 1.1 shows a cross section of a bipolar transistor 10, having an N region 12 (emitter), a P-type region 14 (base) , and an N-type region 16 (collector) . The region of base layer 14 below the emitter 12 is the intrinsic base region (B.) while the regions of layer 14 surrounding emitter 12 are the extrinsic base regions (B ) . FIG. 1.2 shows the preferred impurity concentration profile taken along the line 1.2-1.2 of FIG. 1.1. This is the impurity profile through the emitter 12, intrinsic base region B. , and collector 16. A typical doping concentration in the emitter 12 is approximately 10 20 carriers/cm3, while a typical doping in the intrinsic base region is approximately 10 18 carriers/cm3. The intrinsic base doping profile is determined only by the desired intrinsic base characteristics, regardless of the extrinsic base profile. Usually, the intrinsic base doping level is less than that of the extrinsic base. The intrinsic base is formed by ion implantation, with minimum post O ion implantation heat treatment in order to have a well controlled doping profile. FIG. 1.3 shows the doping profile of the extrinsic base region B . A typical doping concentration is approximately e 1019 carriers/cm3. There is a high peak concentration in the extrinsic base region close to the surface of transistor 10 in order to reduce the extrinsic base resistance and to minimize emitter sidewall injection (i.e., to minimize injection of carriers from emitter region 12 into the extrinsic base region B ) . In the prior art, the intrinsic and extrinsic base regions have been formed by separate steps, but the steps have been additive so that the ideal profiles shown in FIGS. 1.2 and 1.3 have not been obtainable. The present invention describes a technique for closely approximating the profiles shown in FIGS. 1.2 and 1.3, without an adverse effect upon formation of the emitter region 12. FIGS. 2-8 In these figures, a process will be illustrated for forming a bipolar transistor in which the intrinsic and extrinsic base regions are separately formed. Although an NPN structure will be shown, it will be appreciated by those of skill in the art that a PNP structure could be formed equally well, by using different materials for the diffusion or implantation steps used to form the various regions of the tran¬ sistor. For example, boron is a material which can be ion implanted to form P-type regions, while arsenic is a material which can be implanted to form N-type regions. Thus, by choosing different dopings, PNP or NPN structures can be formed. In FIG. 2, an N-type wafer 18 of semiconductor material, such as silicon, has recessed oxide regions 20 therein. For simplicity, no subcollector is shown, although an N type layer in contact with N wafer 18 is well known in the art. In FIG. 3, a thin P layer 22 is formed by boron ion implantation or by boron diffusion. Portions of layer 22 will be used as the extrinsic base region while another portion will provide a low resistance bridge between the. intrinsic and extrinsic bases. An in¬ sulating layer 24, such as SiO^, is then deposited, as by chemical vapor deposition. Layer 24 is de¬ lineated and etched to open a window 26 therein (FIG. 4) . Window 26 is the emitter window through which an emitter (and the intrinsic base) will be formed. The P layer 22 is then preferentially etched through window 26 using, for example, the mixture HF:HN0_: CH 3 COOH=l:3:8. The results of this etching step are shown in FIG. 5. An important feature of this pro¬ cess is that selective etching occurs; that is, doped layer 22 is etched, but the etching stops at the interface between layer 22 and N wafer 18. This means that control is provided and the thickness of wafer 18 in the emitter region is not adversely ' affected by the etching step. In FIG. 6, a thin SiO- layer 28 is grown on the emitter window 26. Layer 28 is the screening oxide for later formation of the emitter and the intrinsic base region. The structure of FIG. 5 is also annealed to drive in the extrinsic base doping, as illustrated by the now deeper P regions 22. With boron implan¬ tation, a pre-anneal may be required in an inert ambient to preclude formation of "tertiary" defects, such as dislocations, in the P regions 22. OMP In FIG. 7, arsenic (As) is implanted and driven in to form the N emitter region 30. The P-type in¬ trinsic base region 32 is then formed by implantation with boron and an annealing step. If desired, the intrinsic base could be formed prior to the formation + of the N emitter region 30. FIG. 8 shows the structure after formation of the emitter (e) and base (b) contacts. Contact holes for the base are delineated through insulating layer 24 and a dip-etching step removes SiO_ layer 28 on the emitter. Metal contacts for the base, emitter, and collector (not shown) are then deposited. In the process illustrated by FIGS. 2-8, the extrinsic base region was formed prior to formation of the. in- trinsic base region, and the effects of the extrinsic base region were removed prior to formation of the emitter and the intrinsic base region. This etching step does not adversely affect the emitter region and the heavy doping of this region means that no exposed or depleted surface regions will be formed. Further, a portion of P-type layer 22 remains to ensure a low resistance path between the intrinsic and extrinsic base regions. FIGS. 9-19 (Self-Alignment) These figures illustrate a process in which the in¬ trinsic and extrinsic base regions are provided by separate steps and in which the emitter and base contacts are self-aligned. In FIG. 9, a wafer of semiconductor 34, such as silicon, has recessed oxide regions 36 therein. Regions 36 provide isola¬ tion between devices and serve the same function as the regions 20 illustrated in FIG. 2. For ease of illustration, no subcollector is shown, although the provision of such is well known in the art. In FIG. 10, a boron-doped P+ layer 38 of polysilicon is deposited, followed by deposition of a first Si0 2 layer 40, a layer 42 of Sig ., and a second layer 44 of Si0 2 . Alternatively, the Si_ 4 layer 42 and the Si0 2 layer 44 can be omitted if the Si0 2 layer 40 is sufficiently thick. Layer 38 is heavily doped to a concentration of about 10 20/cm3. FIG. 11 shows the structure after the emitter window 46 has been delineated and formed through a mask. A suitable mask is a layer of resist (not shown) and the window 46 can be made by reactive ion etching of the layers 40, 42, and 44 in a mixture CF. and H 2 . After the reactive ion etching, the resist layer is removed to leave the structure of FIG. 11. Alter- natively, the layer 38 of polysilicon can be reactive ion etched partially in CF . prior to removing the resist layer. In FIG. 12, P layer 38 has been preferentially etched in a solution of HF:HN0 3 :CH_COOH=l:3:8. This etching occurs through window 46, and leaves an overhang in the emitter region. A thin P layer 48 is then produced in the top sur¬ face of N-type wafer 34 by boron diffusion. After this, a layer 50 of Si0 2 is grown on the exposed surface of silicon wafer 34 and on the sidewall of the polysilicon layer 38. Alternatively, the layer 50 of Si0 2 can be a combination of thermally grown Si0 2 and chemically vapor deposited Si0 2 . Some drive-in of the P boron dopant in polysilicon layer 38 will occur during this step and, for this reason, the P portions of layer 48 under polysilicon layer 38 are shown to a greater depth than the portion of layer 48 under Si0 2 layer 50. The resulting structure is shown in FIG. 13. OMPI A reactive ion etching step in CF . and H_ is then used to remove Si0 2 layer 50 in the emitter region. This step will also etch the Si_N. layer 42 and the overlying Si0 2 layer 44. It will also remove a portion of the Si0 2 layer 40. The resulting structure is shown in FIG. 14. Preferential etching of the P layer 48- in the emitter region is then done, again using the mixture HF:HN0 3 :CH 3 COOH=l:3:8. This leaves the P + regions 52 (FIG. 15). As explained with respect to FIGS. 2-8, this etching step does not attack the surface of wafer 34. A thin layer 54 of SiO„ is then grown on the emitter region. Layer 54 serves as a screening oxide layer for later ion implantation in the emitter region. An annealing step then occurs to drive in the ex¬ trinsic base, which is the P regions 52. Thus, regions 52 have a greater depth in FIG. 16 than in FIG. 15. In FIG. 17, arsenic is ion implanted and driven in + to form the N region 56. Region 56 is the emitter of the device. Alternatively, the drive-in step may be postponed until the intrinsic base has been im¬ plemented. In FIG. 18, the intrinsic base region is formed by ion implantation of boron, which is driven in to provide P-type region 58. Alternatively, the sequence for forming the emitter and the intrinsic base may be reversed so that the intrinsic base region may be formed prior to formation of the emitter. In FIG. 19, contact locations are delineated, and contact holes- are opened to the polysilicon layer 38 After this, dip-etching occurs to remove Si0 2 layer 54 on the emitter. A metal layer is then deposited and delineated to form the base contacts b and the emitter contact e. In the process illustrated by FIGS. 9-19, the emitter- and base regions are self-aligned with one another, since a polysilicon layer 38 is used to provide base contact and as a mask when forming the emitter. As described previously, selective etching of the P silicon in the emitter region does not contaminate the silicon surface and no exposed or depleted surface regions result. What has been described is a process wherein an ideal doping profile can be approximated without adverse effects on other portions of the transistor. In addition to providing two steps for forming the extrinsic and intrinsic regions, the process com- pletely separates the fabrication of these two regions so that the desired characteristics of each can be enhanced. This is done in a manner which leaves a low resistance bridge between the intrinsic and ex¬ trinsic base regions, and in a manner which provides good process control. That is, the thickness of the semiconductor wafer can be established beforehand without having to allow extra thickness for etching, etc. The selective etching step does not attack the underlying semiconductor, and therefore its thickness can be determined with regard to only the desired electrical characteristics of the device. This is the key to a practical process and is a step which was not appreciated in the art. Still further, there is a self-alignment between the emitter and extrinsic/in¬ trinsic base regions since the same mask is used to define the emitter and intrinsic base region, after formation of the extrinsic base. It will be understood by those of skill- in the art that diffusion or ion implantation can be used to provide the extrinsic base regions, the emitter regions, and also the intrinsic base regions. Also, both NPN and PNP transistors can be fabricated in different semiconductor materials, including silicon, germanium, and gallium arsenide.

Claims: CLAIMS Having thus described my invention, what I claim as new, and desire to secure by Letters Patent is: 1. A method for forming a transistor having an emitter, base, and collector in a semiconductor wafer comprising a collector region having a first conductivity type, including the steps of: forming a region of opposite conductivity type in a surface of said wafer, portions of which are to serve as the extrinsic base region of said transistor, masking said region of opposite conductivity type to expose an area thereof, etching said exposed area of said region of opposite conductivity type to remove said opposite conductivity type region in said exposed area thereby exposing a portion of said surface of said wafer where said emitter is to be formed, OM />, WIP forming an emitter region having the same conductivity type as said collector region in said semiconductor wafer, said emitter being formed in said semiconductor wafer by incor- poration of a dopant having the same conduc¬ tivity type as said collector region, forming an intrinsic base region of opposite conductivity type to said emitter and collector regions, said intrinsic base region being located beneath said emitter region and con¬ tacting said extrinsic base region, said intrinsic base region having a doping level different than that of said extrinsic base region, forming electrical contacts to said extrinsic base region and to said emitter. 2. The method of Claim 1, where said extrinsic base region is formed by ion implantation. 3. The method of Claim 1, wherein said extrinsic base region is formed by diffusion. 4. The method of Claim 1, wherein said semi¬ conductor wafer is silicon, and including the further step of providing a layer of doped polycrystalline silicon over said extrinsic base region for electrical contact thereto. 5. The method of Claim 1, where said semiconductor wafer is comprised of a material selected from the group consisting of silicon, germanium, and gallium arsenide. OMPI ,- WWIIPPOO -v 6. The method of Claim 1, where said collector and emitter regions are N-type, and said in¬ trinsic and extrinsic base regions are P-type. 7. The method of Claim 1, where said collector and emitter regions are P-type, and said in¬ trinsic and extrinsic base region ' s are N-type. 8. The method of Claim 1, where said etching step selectively etches said region of opposite conductivity type and not said underlying se i- conductor wafer. 9. The method of Claim 1, where said intrinsic base region is formed prior to formation of said emitter region. 10. The method of Claim 1, where said emitter region is formed prior to formation of said intrinsic base region. OM 11. A method for forming a transistor in a wafer of semiconductor material having a collector region therein of a first conductivity type, including the steps of: forming a doped polycrystalline layer of said se iconductive material of the opposite con¬ ductivity type to said collector region over a surface of said semiconductor wafer, forming a base region of opposite conductivity type to said first conductivity type in said wafer of semiconductor material, portions of said base region being extrinsic base regions of said transistor, masking a portion of said base region to expose an area thereof to be removed by an etching step, selectively etching said base region in said exposed area to remove said base region in said exposed area, creating a window therein, without etching of said underlying semiconductor wafer, there being a portion of said base region left which is said extrinsic base region, forming an emitter region and an intrinsic base region through said window in said base region, said emitter having the same conductivity type as said collector, -said intrinsic base region having the same conductivity type as said ex¬ trinsic base region, opening contact holes through said layer of polycrystalline semiconductor material, and contacting said extrinsic base region by metal contacts which contact said layer of polycrystalline material, and depositing a metal layer which electrically contacts said emitter region. 12. The method of Claim 11, where said semiconductor material is silicon and said layer of doped semiconductor material is a polycrystalline layer of silicon. 13. The method of Claim 11, where said collector and emitter regions are N-type and said base is P-type. 14. The method of Claim 11, where said collector and emitter regions are P-type and said base regions are N-type. 15. The method of Claim 11, where said emitter is formed prior to formation of said intrinsic base region. 16. The method of Claim 11, where said intrinsic base region is formed prior to formation of said emitter region. 17. The method of Claim 11, where said intrinsic and extrinsic regions are formed by ion implantation. 18. The method of Claim 11, where said extrinsic base region is protected during formation of said intrinsic base region and said emitter region. O Λ. WI

Inventors: CROWDER B, ISAAC R, MING T
Applicants: CROWDER B, IBM, ISAAC R, MING T, IBM CORP
Application year: 1979
Main classification: H01L21
IPC codes: H01L21, H01L23, H01L29