Patent Publication Number: US-2013248935-A1

Title: Sige heterojunction bipolar transistor with a shallow out-diffused p+ emitter region

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a SiGe heterojunction bipolar transistor (HBT) and, more particularly, to a SiGe HBT with a shallow out-diffused p+ emitter region. 
     2. Description of the Related Art 
     A bipolar transistor is a well-known structure that has an emitter, a base connected to the emitter, and a collector connected to the base. The emitter has a first conductivity type, the base has a second conductivity type, and the collector has the first conductivity type. For example, an npn bipolar transistor has an n-type emitter, a p-type base, and an n-type collector, while a pnp bipolar transistor has a p-type emitter, an n-type base, and a p-type collector. 
     When the emitter and base are formed from different semiconductor materials, such as silicon and germanium, respectively, the interface is known as a heterojunction. The heterojunction limits the number of holes that can be injected into the emitter from the base. Limiting the number of injected holes allows the dopant concentration of the base to be increased which, in turn, reduces the base resistance and increases the maximum frequency of the transistor. 
       FIG. 1  shows a cross-sectional view that illustrates an example of a prior-art SiGe heterojunction bipolar structure  100 . As shown in  FIG. 1 , bipolar structure  100  includes a silicon-on-oxide (SOI) wafer  110 , which has a silicon handle wafer  112 , a buried insulation layer  114  that touches silicon handle wafer  112 , and a single-crystal silicon substrate  116  that touches buried insulation layer  114 . Silicon substrate  116 , in turn, has a heavily-doped, p conductivity type (p+) buried region  120  and a heavily-doped, n conductivity type (n+) buried region  122 . 
     As further shown in  FIG. 1 , bipolar structure  100  includes a single-crystal silicon epitaxial structure  130  that touches the top surface of silicon substrate  116 . Epitaxial structure  130  has a very low dopant concentration, except for regions of out diffusion. For example, a number of p-type atoms out diffuse from p+ buried layer  120  into epitaxial structure  130 , and a number of n-type atoms out diffuse from n+ buried layer  122  into epitaxial structure  130 . In the present example, epitaxial structure  130  is a very lightly doped, n conductivity type (n---) region, excluding the regions of out diffusion. 
     Bipolar structure  100  also includes a number of shallow trench isolation structures  132  that touch epitaxial structure  130 , and a deep trench isolation structure  134  that touches and extends through epitaxial structure  130  as well as silicon substrate  116  to touch buried insulation layer  114 . Buried insulation layer  114  and deep trench isolation structure  132  form an electrically-isolated, single-crystal silicon region  136  and a laterally-adjacent, electrically-isolated, single-crystal silicon region  138 . In addition, bipolar structure  100  includes a lightly-doped, p conductivity type (p−) region  140  that extends from the top surface of silicon epitaxial structure  130  down through epitaxial structure  130  to touch p+ buried region  120 , and a lightly-doped, n conductivity type (n−) region  142  that extends from the top surface of silicon epitaxial structure  130  down through epitaxial structure  130  to touch n+ buried region  122 . 
     Bipolar structure  100  also includes a p conductivity type sinker region  144  that extends from the top surface of silicon epitaxial structure  130  down through epitaxial structure  130  to p+ buried region  120 , and an n conductivity type sinker region  146  that extends from the top surface of silicon epitaxial structure  130  down through epitaxial structure  130  to n+ buried region  122 . 
     Sinker region  144  includes a heavily-doped, p conductivity type (p+) surface region and a moderately-doped, p conductivity type (p) lower region, while sinker region  146  includes a heavily-doped, n conductivity type (n+) surface region and a moderately-doped, n conductivity type (n) lower region. 
     Further, bipolar structure  100  includes a SiGe epitaxial structure  150  that touches and lies over silicon epitaxial structure  130 , a shallow trench isolation structure  132 , and p− region  140 . SiGe epitaxial structure  150  has a number of layers including a top layer  151  and a lower layer  152  that touches and lies below top layer  151 . 
     Top layer  151  includes a single-crystal silicon region and a polycrystalline silicon region. Top layer  151  also has an out-diffused emitter region  153 , and an outer region  154  that touches out-diffused emitter region  153 . Out-diffused emitter region  153 , which lies in the single-crystal silicon region, has a heavy dopant concentration and a p conductivity type (p+). 
     Outer region  154 , which horizontally surrounds out-diffused emitter region  153 , has a very low dopant concentration and, in the present example, an n conductivity type (n---). Lower layer  152 , in turn, includes a single-crystal germanium region that touches the single-crystal silicon region of top layer  151 , and a polycrystalline germanium region that touches the polycrystalline silicon region of top layer  151 . Lower layer  152  also has a heavy dopant concentration and an n conductivity type (n+). Thus, the single-crystal germanium region has an n+ dopant concentration. 
     Bipolar structure additionally includes a SiGe epitaxial structure  155  that touches and lies over silicon epitaxial structure  130 , a shallow trench isolation structure  132 , and n− region  142 . SiGe epitaxial structure  155  has a number of layers including a top layer  156  and a lower layer  157  that touches and lies below top layer  156 . 
     Top layer  156  includes a single-crystal silicon region and a polycrystalline silicon region. Top layer  156  also has an out-diffused emitter region  158 , and an outer region  159  that touches out-diffused emitter region  158 . Out-diffused emitter region  158 , which lies in the single-crystal silicon region of top layer  156 , has a heavy dopant concentration and an n conductivity type (n+). 
     Outer region  159 , which horizontally surrounds out-diffused emitter region  158 , has a very low dopant concentration and, in the present example, an n conductivity type (n---). Lower layer  157 , in turn, includes a single-crystal germanium region that touches the single-crystal silicon region of top layer  156 , and a polycrystalline germanium region that touches the polycrystalline silicon region of top layer  156 . Lower layer  157  also has a heavy dopant concentration and a p conductivity type (p+). 
     Bipolar structure  100  additionally includes an isolation structure  160  that touches SiGe epitaxial structure  150 , and an isolation structure  162  that touches SiGe epitaxial structure  155 . Isolation structures  160  and  162  are electrically non-conductive. Isolation structure  160  has an emitter opening  164  that exposes the single-crystal silicon region of top layer  151  of SiGe epitaxial structure  150 , and a contact opening  166  that exposes the polycrystalline silicon region of top layer  151  of SiGe epitaxial structure  150 . Similarly, isolation structure  162  has an emitter opening  170  that exposes the single-crystal silicon region of top layer  156  of SiGe epitaxial structure  155 , and a contact opening  172  that exposes the polycrystalline silicon region of top layer  156  of SiGe epitaxial structure  155 . 
     Bipolar structure  100  further includes a heavily-doped, p conductivity type (p+) polysilicon structure  180  that touches isolation structure  160  and extends through emitter opening  164  to touch the p+ out-diffused emitter region  153  of SiGe epitaxial structure  150 . Bipolar structure  100  also includes a heavily-doped, n conductivity type (n+) polysilicon structure  182  that touches isolation structure  162  and extends through emitter opening  170  to touch the n+ out-diffused emitter region  158  of SiGe epitaxial structure  155 . 
     P+ polysilicon structure  180  and p+ out-diffused emitter region  153  form the emitter, the remaining portion of SiGe epitaxial structure  150  forms the n-type base, and the combination of p+ buried region  120 , p− region  140 , and p-type sinker region  144  form the collector of a pnp SiGe heterojunction bipolar transistor (HBT)  190 . 
     In addition, n+ polysilicon structure  182  and n+ out-diffused emitter region  158  form the emitter, the remaining p-type portion of SiGe epitaxial structure  155  forms the p-type base, and the combination of n+ buried region  122 , n− region  142 , and n-type sinker region  146  form the collector of an npn SiGe HBT  192 . 
     During an anneal in the fabrication of HBT  190  and HBT  192 , p-type atoms in p+ polysilicon structure  180  out diffuse into top layer  151  of SiGe epitaxial structure  150  to form p+ emitter region  153 , and n-type atoms in n+ polysilicon structure  182  out diffuse into top layer  156  of SiGe epitaxial structure  155  to form n+ emitter region  158 . 
     One of the drawbacks of HBT  190  and HBT  192  is that p+ out-diffused emitter region  153  is significantly larger and deeper than n+ out-diffused emitter region  158  due to the higher diffusion rate of p-type atoms, such as boron, when compared to the lower diffusion rate of n-type atoms, such as phosphorous. 
     In applications where the pnp and npn parameters are to be matched as closely as possible, the significantly deeper depth of p+ out-diffused emitter region  153  when compared to the depth of n+ out-diffused emitter region  158  poses a problem. One approach to reducing the variation in the depths is to form a thin oxide layer on the portion of the single-crystal silicon region of top layer  151  of SiGe epitaxial structure  150  that is exposed by emitter opening  164 . 
       FIG. 2  shows a cross-sectional view that illustrates an example of a prior-art SiGe heterojunction bipolar structure  200 . SiGe heterojunction bipolar structure  200  is similar to SiGe heterojunction bipolar structure  100  and, as a result, utilizes the same reference numerals to designate the elements that are common to both structures. 
     As shown in  FIG. 2 , SiGe heterojunction bipolar structure  200  differs from SiGe heterojunction bipolar structure  100  in that SiGe heterojunction bipolar structure  200  utilizes a p+ out-diffused emitter region  210  in lieu of p+ out-diffused emitter region  153 . P+ out-diffused emitter region  210  is similar to p+ out-diffused emitter region  153 , except that p+ out-diffused emitter region  210  is smaller and shallower than p+ out-diffused emitter region  153 . 
     SiGe heterojunction bipolar structure  200  also differs from SiGe heterojunction bipolar structure  100  in that SiGe heterojunction bipolar structure  200  includes an oxide layer  212  that lies between and touches p+ out-diffused emitter region  210  of SiGe epitaxial structure  150  and p+ polysilicon structure  180 . 
     P+ polysilicon structure  180  and p+ out-diffused emitter region  210  form the emitter, the remaining portion of SiGe epitaxial structure  150  forms the n-type base, and the combination of p+ buried region  120 , p− region  140 , and p-type sinker region  144  form the collector of a pnp SiGe heterojunction bipolar transistor (HBT)  214 . 
     During the anneal that causes the atoms to out diffuse, oxide layer  212  is thin enough to allow p-type atoms to diffuse through from p+ polysilicon structure  180  into the top layer  151  of SiGe epitaxial structure  150  to form p+ emitter region  210 , but thick enough to slow down the rate at which the atoms diffuse into the top layer  151  of SiGe epitaxial structure  150 . As a result, the depth of p+ out-diffused emitter region  210  can be formed to be approximately the same as the depth of n+ out-diffused emitter region  158 . 
     One of the drawbacks of SiGe heterojunction bipolar structure  200  is that SiGe heterojunction bipolar structure  200  has a significantly larger 1/f noise than SiGe heterojunction bipolar structure  100  due to the presence of oxide layer  212 . In addition, next generation HBTs commonly use epitaxially-grown single-crystal silicon structures to form the emitters in lieu of polysilicon structures like polysilicon structure  180 . However, an oxide layer like oxide layer  212  cannot be used with epitaxially-grown single-crystal silicon emitters to reduce the depth of the p+ out-diffused emitter region because single-crystal silicon cannot be epitaxially grown on oxide. 
     Thus, there is a need for a SiGe HBT with a shallow p+ out-diffused emitter region which is approximately equal to the depth of the n+ out-diffused emitter region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating an example of a prior-art SiGe heterojunction bipolar structure  100 . 
         FIG. 2  is a cross-sectional view illustrating an example of a prior-art SiGe heterojunction bipolar structure  200 . 
         FIG. 3  is a cross-sectional view illustrating an example of a SiGe heterojunction bipolar structure  300  in accordance with the present invention. 
         FIGS. 4A-4G  are cross-sectional views illustrating a method  400  of forming a SiGe heterojunction bipolar structure in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 3  shows a cross-sectional view that illustrates an example of a SiGe heterojunction bipolar structure  300  in accordance with the present invention. SiGe heterojunction bipolar structure  300  is similar to SiGe heterojunction bipolar structure  100  and, as a result, utilizes the same reference numerals to designate the elements that are common to both structures. 
     As shown in  FIG. 3 , SiGe heterojunction bipolar structure  300  differs from SiGe heterojunction bipolar structure  100  in that SiGe heterojunction bipolar structure  300  utilizes a p+ out-diffused emitter region  310  in lieu of p+ out-diffused emitter region  153 . P+ out-diffused emitter region  310  is similar to p+ out-diffused emitter region  153 , except that p+ out-diffused emitter region  310  is smaller and shallower than p+ out-diffused emitter region  153 . Thus, outer region  154  touches and horizontally surrounds a smaller p+ out-diffused emitter region  310 . 
     SiGe heterojunction bipolar structure  300  also differs from SiGe heterojunction bipolar structure  100  in that SiGe heterojunction bipolar structure  300  utilizes an n+ out-diffused emitter region  312  in lieu of n+ out-diffused emitter region  158 . N+ out-diffused emitter region  312  is similar to n+ out-diffused emitter region  158 . Thus, outer region  159  touches and horizontally surrounds n+ out-diffused emitter region  312 . P+ out-diffused emitter region  310  has a depth that is approximately the same as the depth of n+ out-diffused emitter region  312 . 
     In addition, SiGe heterojunction bipolar structure  300  differs from SiGe heterojunction bipolar structure  100  in that SiGe heterojunction bipolar structure  300  replaces p+ polysilicon structure  180  with a p+ epitaxial structure  314 . P+ epitaxial structure  314 , in turn, has a number of layers including a bottom layer  316  and an upper layer  318  that touches and lies above bottom layer  316 . 
     Bottom layer  316 , which lies over isolation structure  160 , includes a single-crystal region that touches the single-crystal p+ out-diffused emitter region  310 . In addition, bottom layer  316  includes a single-crystal germanium region and a polycrystalline germanium region. Bottom layer  316  also has a heavy dopant concentration and a p conductivity type (p+). Thus, the single-crystal germanium region has a p+ dopant concentration. 
     Upper layer  318 , in turn, includes a single-crystal silicon region that touches and lies over the single-crystal germanium region of bottom layer  316 , and a polycrystalline silicon region that touches and lies over the polycrystalline germanium region of bottom layer  316 . Further, upper layer  318  has a heavy dopant concentration and a p conductivity type (p+). Thus, the single-crystal silicon region has a p+ dopant concentration. 
     SiGe heterojunction bipolar structure  300  also differs from SiGe heterojunction bipolar structure  100  in that SiGe heterojunction bipolar structure  300  replaces n+ polysilicon structure  182  with an n+ epitaxial structure  320 . N+ epitaxial structure  320 , in turn, includes a single-crystal silicon region and a polycrystalline silicon region. The single-crystal silicon region of n+ epitaxial structure  320  touches the single-crystal n+ out-diffused emitter region  312  of SiGe epitaxial structure  155 . 
     Thus, p+ epitaxial structure  314  and p+ out-diffused emitter region  310  form the emitter, the remaining portion of SiGe epitaxial structure  150  forms the n-type base, and the combination of p+ buried region  120 , p− region  140 , and p-type sinker region  144  form the collector of a pnp SiGe heterojunction bipolar transistor (HBT)  322 . 
     Further, n+ epitaxial structure  320  and n+ out-diffused emitter region  312  form the emitter, the remaining p-type portion of SiGe epitaxial structure  155  forms the p-type base, and the combination of n+ buried region  122 , n− region  142 , and n-type sinker region  146  form the collector of a npn SiGe heterojunction bipolar transistor (HBT)  324 . 
     In operation, during the anneal that causes the p-type atoms to out diffuse, the germanium in bottom layer  316  is thin enough to allow p-type atoms to diffuse from upper layer  318  into the top layer  151  of SiGe epitaxial structure  150  to form p+ out-diffused emitter region  310 , but thick enough to slow down the rate at which the atoms diffuse into the top layer  151  of SiGe epitaxial structure  150 . As a result, the depth of p+ out-diffused emitter region  310  can be formed to be approximately the same as the depth of n+ out-diffused emitter region  312 . 
       FIGS. 4A-4G  show cross-sectional views that illustrate a method  400  of forming a SiGe heterojunction bipolar structure in accordance with the present invention. As shown in  FIG. 4A , the method utilizes a conventionally-formed intermediate structure  408  that includes a silicon-on-oxide (SOI) wafer  410 , which has a silicon handle wafer  412 , a buried insulation layer  414  that touches silicon handle wafer  412 , and a single-crystal silicon substrate  416  that touches buried insulation layer  414 . Silicon substrate  416 , in turn, has a p+ buried region  420  and an n+ buried region  422 . 
     In addition, base structure  408  includes a single-crystal silicon epitaxial structure  430  that touches the top surface of silicon substrate  416 . In the present example, epitaxial structure  430  has a very low dopant concentration and an n conductivity type (n---), except for regions of out diffusion. For example, a number of p-type atoms out diffuse from p+ buried layer  420  into epitaxial structure  430 , and a number of n-type atoms out diffuse from n+ buried layer  422  into epitaxial structure  430 . As a result, substantially all of epitaxial structure  430  has a very low dopant concentration. 
     Intermediate structure  408  also includes a number of shallow trench isolation structures  432  that touch epitaxial structure  430 , and a deep trench isolation structure  434  that touches and extends through epitaxial structure  430  as well as silicon substrate  416  to touch buried insulation layer  414 . Deep trench isolation structure  434  forms an electrically-isolated, single-crystal silicon region  436 , and a laterally-adjacent, electrically-isolated, single-crystal silicon region  438 . 
     In addition, intermediate structure  408  includes a lightly-doped, p conductivity type (p−) region  440  that extends from the top surface of silicon epitaxial structure  430  down through epitaxial structure  430  to touch p+ buried region  420 , and a lightly-doped, n conductivity type (n−) region  442  that extends from the top surface of silicon epitaxial structure  430  down through epitaxial structure  430  to touch n+ buried region  422 . 
     Intermediate structure  408  also includes a p conductivity type sinker region  444  that extends from the top surface of silicon epitaxial structure  430  down through epitaxial structure  430  to p+ buried region  420 , and an n conductivity type sinker region  446  that extends from the top surface of silicon epitaxial structure  430  down through epitaxial structure  430  to n+ buried region  422 . 
     Sinker region  444  includes a heavily-doped, p conductivity type (p+) surface region and a moderately-doped, p conductivity type (p) lower region, while sinker region  446  includes a heavily-doped, n conductivity type (n+) surface region and a moderately-doped, n conductivity type (n) lower region. 
     Further, intermediate structure  408  includes a SiGe epitaxial structure  450  that touches and lies over silicon epitaxial structure  430 , a shallow trench isolation structure  432 , and p−region  440 . Intermediate structure  408  also includes a SiGe epitaxial structure  452  that touches and lies over silicon epitaxial structure  430 , a shallow trench isolation structure  432 , and n− region  442 . 
     SiGe epitaxial structure  450  has a number of layers including a top layer  454  and a lower layer  455  that touches and lies below top layer  454 . Top layer  454  includes a single-crystal silicon region and a polycrystalline silicon region. In addition, top layer  454  has a very low dopant concentration and, in the present example, an n conductivity type (n---). 
     Lower layer  455 , in turn, includes a single-crystal germanium region that touches the single-crystal silicon region of top layer  454 , and a polycrystalline germanium region that touches the polycrystalline silicon region of top layer  454 . Lower layer  455  also has a heavy dopant concentration and an n conductivity type (n+). 
     Similarly, SiGe epitaxial structure  452  has a number of layers including a top layer  456  and a lower layer  457  that touches and lies below top layer  456 . Top layer  456  includes a single-crystal silicon region and a polycrystalline silicon region. In addition, top layer  456  has a very low dopant concentration and, in the present example, an n conductivity type (n---). 
     Lower layer  457 , in turn, includes a single-crystal germanium region that touches the single-crystal silicon region of top layer  456 , and a polycrystalline germanium region that touches the polycrystalline silicon region of top layer  456 . Lower layer  457  also has a heavy dopant concentration and a p conductivity type (p+). 
     Intermediate structure  408  additionally includes an isolation structure  460  that touches SiGe epitaxial structure  450 , and an isolation structure  462  that touches SiGe epitaxial structure  452 . The isolation structures  460  and  462  are electrically non-conductive. Isolation structure  460  has an emitter opening  464  that exposes the single-crystal silicon region of top layer  454  of SiGe epitaxial structure  450 , and a contact opening  466  that exposes the polycrystalline silicon region of top layer  454  of SiGe epitaxial structure  450 . Similarly, isolation structure  462  has an emitter opening  470  that exposes the single-crystal silicon region of top layer  456  of SiGe epitaxial structure  452 , and a contact opening  472  that exposes the polycrystalline silicon region of top layer  456  of SiGe epitaxial structure  452 . 
     As further shown in  FIG. 4A , method  400  begins by epitaxially growing a lower layer  474  in a conventional manner on the exposed single-crystal silicon regions and the polycrystalline silicon regions of the SiGe epitaxial structures  450  and  452 . Lower layer  474  is also grown on p sinker region  444  and n sinker region  446 . Lower layer  474  is further grown on the isolation structures  460  and  462  as well as on the shallow trench isolation structures  432  and deep trench isolation structure  434 . 
     Lower layer  474  has a single-crystal region that touches and lies over the single-crystal silicon region of top layer  454  of SiGe epitaxial structure  450 , and a single-crystal region that touches and lies over the single-crystal silicon region of top layer  456 . Lower layer  474  also has a single-crystal region that touches and lies over single-crystal p sinker region  444 , and a single-crystal region that touches and lies over the single-crystal n sinker region. Lower layer  474  has a polycrystalline region that touches and lies over the isolation structures  432 ,  434 ,  460  and  462 . 
     In addition, lower layer  474  includes a single-crystal germanium region and a polycrystalline germanium region, and can optionally include single-crystal silicon and polycrystalline silicon that lie below and/or above the germanium. After lower layer  474  has been grown, a patterned photoresist layer  476  is formed on lower layer  474  in a conventional manner. 
     Following the formation of patterned photoresist layer  476 , as shown in  FIG. 4B , the exposed regions of lower layer  474  are etched to form a lower structure  480 . Lower structure  480  touches the single-crystal silicon region of top layer  454  of SiGe epitaxial structure  450  which is exposed by emitter opening  470 , and the top surface of isolation structure  460 . As a result, lower structure  480  has a single-crystal region that touches and lies over top layer  454  of SiGe epitaxial structure  450 , and a polycrystalline structure that touches and lies over isolation structure  460 . After lower structure  480  has been formed, patterned photoresist layer  476  is removed in a conventional manner. 
     As shown in  FIG. 4C , after patterned photoresist layer  476  has been removed, an upper layer  482  is epitaxially grown in a conventional manner on lower structure  480  and the single-crystal silicon region of top layer  456  of SiGe epitaxial structure  452 . Upper layer  482  is also grown on the polycrystalline silicon regions of the SiGe epitaxial structures  450  and  452 . In addition, upper layer  482  is grown on p sinker region  444  and n sinker region  446 . Upper layer  482  is further grown on the isolation structures  460  and  462  as well as on the shallow trench isolation structures  432  and deep trench isolation structure  434 . 
     Upper layer  482  has a single-crystal region that touches and lies over lower structure  480 , and a single-crystal region that touches and lies over the single-crystal silicon region of top layer  456  of SiGe epitaxial structure  452  exposed by emitter opening  470 . 
     Further, upper layer  482  has a single-crystal region that touches and lies over the single-crystal p sinker region  444 , and a single-crystal region that touches and lies over the single-crystal n sinker region  446 . Upper layer  482  has a polycrystalline region that touches and lies over the isolation structures  432 ,  434 ,  460  and  462 . In addition, upper layer  482  includes silicon. After upper layer  482  has been grown, a patterned photoresist layer  484  is formed on upper layer  482  in a conventional manner. 
     Following the formation of patterned photoresist layer  484 , as shown in  FIG. 4D , the exposed regions of upper layer  482  are etched to form a first upper structure  486  that touches lower structure  480 , and a second upper structure  488  that touches the single-crystal silicon region of top layer  456  of SiGe epitaxial structures  452  which is exposed by emitter opening  470 . After the upper structures  486  and  488  have been formed, patterned photoresist layer  484  is removed in a conventional manner. 
     As shown in  FIG. 4E , after patterned photoresist layer  484  has been removed, a patterned photoresist layer  490  is formed in a conventional manner. Following the formation of patterned photoresist layer  490 , a p-type dopant, such as boron, is implanted through patterned photoresist layer  490  to heavily dope (p+) upper structure  486 . After upper structure  486  has been doped, patterned photoresist layer  490  is removed in a conventional manner. 
     As shown in  FIG. 4F , after patterned photoresist layer  490  has been removed, a patterned photoresist layer  492  is formed in a conventional manner. Following the formation of patterned photoresist layer  492 , an n-type dopant, such as phosphorous, is implanted through patterned photoresist layer  492  to heavily dope (n+) upper structure  488 . After upper structure  488  has been doped, patterned photoresist layer  492  is removed in a conventional manner. 
     As shown in  FIG. 4G , after patterned photoresist layer  492  has been removed, the doped structure is annealed in a conventional manner. During the anneal, the germanium in lower structure  480  is thin enough to allow p-type atoms to out diffuse from upper structure  486  into SiGe epitaxial structure  450  to form a p+ out-diffused emitter region  494 , but thick enough to slow down the rate at which the atoms out diffuse into SiGe epitaxial structure  450 . 
     At the same time, n-type atoms out diffuse from upper structure  488  into SiGe epitaxial structure  452  to form an n+ out-diffused emitter region  496 . Thus, as a result of the slowing effect provided by the germanium, the depth of p+ out-diffused emitter region  494  can be formed to be approximately the same as the depth of n+ out-diffused emitter region  496 . Method  400  then continues with conventional steps. 
     It should be understood that the above descriptions are examples of the present invention, and that various alternatives of the invention described herein may be employed in practicing the invention. Thus, it is intended that the following claims define the scope of the invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.