Abstract:
A bipolar transistor structure is described incorporating an emitter, base, and collector having a fully depleted region on an insulator of a Silicon-On-Insulator (SOI) substrate without the need for a highly doped subcollector to permit the fabrication of vertical bipolar transistors on semiconductor material having a thickness of 300 nm or less and to permit the fabrication of SOI BiCMOS. The invention overcomes the problem of requiring a thick semiconductor layer in SOI to fabricate vertical bipolar transistors with low collector resistance.

Description:
RELATED APPLICATIONS 
   This application is a divisional of U.S. application Ser. No. 09/757,965, filed Jan. 10, 2001 now U.S. Pat. No. 6,949,871, which claims benefit of U.S. Provisional Application Ser. No. 60/242,339, filed Oct. 20, 2000. 

   FIELD OF THE INVENTION 
   This invention relates to bipolar transistors and more particularly to a bipolar transistor structure suitable for implementation on silicon on insulator (SOI) alone or with Complementary Metal Oxide Semiconductor (CMOS) devices to provide SOI BiCMOS essential for most RF and communication products or digital products that have some critical analog components. 
   BACKGROUND OF THE INVENTION 
   High-performance bipolar transistors are vertical bipolar transistors, as opposed to lateral bipolar transistors. A vertical bipolar transistor, for example an npn bipolar transistor  2  schematically shown in  FIG. 1 , comprises an n+ type emitter region  3 , a p type base region  4 , and an n type collector region  5  stacked one on top of the other. In order to reduce collector series resistance, there is usually an n+ type subcollector layer  6  beneath the collector region  5  and an n+ type reachthrough region  7  is used to bring the collector contact to the surface. 
   In normal operation, the emitter-base diode is forward biased, and the base-collector diode is reverse biased. The entire n type collector layer is usually thick enough to accommodate the space-charge region (also called the depletion region since it is normally depleted of mobile carriers) and a quasi-neutral region. The thickness, or width, of the space-charge region is determined by the collector doping concentration and the base-collector bias voltage. The quasi-neutral collector region can be very thin, usually just thick enough to prevent the space-charge region from reaching the n+ type subcollector layer. If the base-collector space-charge region touches the n+ type subcollector, it will cause the base-collector junction capacitance to increase and the base-collector junction breakdown voltage to decrease. The n+ type subcollector layer is usually rather thick, typically thicker than 1000 nm, in order to achieve an adequately small collector series resistance. 
   In normal operation, electrons are injected from the emitter E and collected at the collector C. The dotted arrow shown in  FIG. 2  indicates the electron path in normal operation, starting from the emitter contact.  FIG. 3  is the energy-band diagram along the electron path. In  FIGS. 2 and 3 , A indicates the location of the emitter contact, A′ indicates the boundary between the depleted part and the quasi-neutral part of the n type collector, and A″ indicates the top of the n+ subcollector layer. In  FIG. 3 , the ordinate represents electron and hole energy. 
   A more detailed description of the basic structure and operation of a bipolar transistor can be found in the book by Yuan Taur and Tak H. Ning entitled Fundamentals of Modern VLSI Devices, Chapter 6, Bipolar Devices, Cambridge University Press, 1998, pp 292-347 which is incorporated herein by reference. 
   Vertical bipolar transistors have been built in the silicon layer of SOI.  FIG. 4  illustrates a vertical npn bipolar transistor  2 ′ using SOI. Usually, it is simply a vertical bipolar transistor, including its n+ type subcollector layer  6 , sitting on a buried oxide layer  9  and substrate  8  of the SOI. The SOI silicon layer has to be rather thick, thick enough to accommodate the various layers of a vertical bipolar transistor described above. 
   SOI BiCMOS obtained from the integration of a vertical bipolar transistor with CMOS devices using SOI has been described in a publication by Toshiro Hiramoto, et al., “A 27 GHz double polysilicon bipolar technology on bonded SOI with embedded 58 μm 2  CMOS memory cells for ECL-CMOS SRAM applications,” IEDM Technical Digest, pp. 39-42, 1992. 
   The thick silicon layer needed for the bipolar transistor results in the CMOS devices behaving like regular bulk CMOS devices, rather than like high-speed SOI CMOS devices. The silicon layer of high-speed SOI CMOS is usually rather thin, typically less than 200 nm, much too thin to accommodate present vertical bipolar transistor structures. 
   It is possible to significantly reduce the silicon thickness needed for making SOI vertical bipolar transistors  2 ″ by omitting the relatively thick n+ subcollector layer  6 . This structure is illustrated in FIG.  5 . The electrons still flows the same way as in a vertical bipolar transistor with a subcollector layer, namely vertically through the base layer and through the depletion layer of the base-collector diode to the quasi-neutral collector region. However, without the n+ subcollector layer  6 , electron current will have to be carried by the quasi-neutral collector layer which has very high sheet resistance because of its relatively light doping concentration and relatively small thickness compared to the n+ type subcollector layer  6 . The resulting collector series resistance is unacceptably large. If the n type collector thickness is increased significantly to reduce collector series resistance, the resultant SOI silicon layer will again be much to thick for integration with high-speed SOI CMOS devices. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, a bipolar transistor on SOI is described comprising a substrate, an insulating layer over the substrate, a first single crystal semiconductor layer positioned over the insulating layer having a lightly doped region of a first type and at least one contiguous heavily doped region of the first type, the lightly doped region and the contiguous heavily doped region functioning as a collector, a second patterned semiconductor layer of a second type formed over the lightly doped region of the first semiconductor layer to function as the base, and a third patterned semiconductor layer of the first type positioned over the second semiconductor layer to function as the emitter, the lightly doped region of the first type of the collector having a dopant concentration to fully deplete of mobile charge through the first semiconductor layer to the insulating layer of the SOI below. 
   The invention further provides an integrated circuit chip having both npn and pnp bipolar transistors of the above structure on SOI. 
   The invention further provides an integrated circuit chip having one or both npn and pnp bipolar transistors of the above structure on SOI and p-channel MOSFETs and n-channel MOSFETs wherein the source and drain regions of the MOSFETs extend downward to the insulating layer of the SOI. 
   The invention further provides a bipolar transistor on SOI comprising a substrate, an insulating layer over the substrate, a first single crystal semiconductor layer positioned over the insulating layer having a lightly doped region of a first type and at least one contiguous heavily doped region of the first type, the lightly doped region and the contiguous heavily doped region functioning as a collector, a top region of the lightly doped region is counter-doped to a second type to function as the base, a second patterned semiconductor layer of the second type formed over a region of the counter-doped region of the first semiconductor layer to function as the extrinsic base, and a third patterned semiconductor layer of the first type positioned over the counter-doped region of the first semiconductor layer to function as the emitter, the lightly doped region of the first type in the first semiconductor layer having a dopant concentration to fully deplete of mobile charge through the first semiconductor layer to the insulating layer of the SOI below. 
   The invention further provides an integrated circuit chip having both npn and pnp bipolar transistors of the above structure on SOI. 
   The invention further provides an integrated circuit chip having one or both npn and pnp bipolar transistors of the above structure on SOI and p-channel MOSFETs and n-channel MOSFETs wherein the source and drain regions of the MOSFETs extend downward to the insulating layer of the SOI. 
   The invention provides a fully-depleted-collector SOI vertical bipolar transistor which has a much smaller base-collector junction capacitance than conventional devices and does not require a heavily doped subcollector layer. 
   The invention provides an SOI bipolar transistor structure which uses thin-silicon SOI typically less than 200 nm, and is therefore readily compatible with high-speed SOI CMOS devices also using thin-silicon SOI for making high-speed SOI BiCMOS. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     These and other features, objects, and advantages of the present invention will become apparent upon consideration of the following detailed description of the invention when read in conjunction with the drawing in which: 
       FIG. 1  is a cross section schematic view of a vertical bipolar transistor of the prior art. 
       FIG. 2  is a cross section schematic view of a vertical bipolar transistor of the prior art with the electron path shown through the transistor. 
       FIG. 3  is a graph of the energy-band diagram along the electron current path shown in FIG.  2 . 
       FIG. 4  is a cross section schematic view of a vertical npn bipolar transistor using SOI of the prior art. 
       FIG. 5  is a cross section schematic view of an alternate vertical npn bipolar transistor structure on SOI illustrating deficiencies due to the high series resistance through the collector. 
       FIG. 6  is a cross section schematic view of one embodiment of the invention. 
       FIG. 7  is a cross section schematic view of the electron current path through the embodiment of FIG.  6 . 
       FIG. 8  shows an energy band diagram along the electron current path shown in FIG.  7 . 
       FIGS. 9-18  are cross section schematic views illustrating the steps for fabricating a fully-depleted-collector SOI vertical npn bipolar transistor having the base region formed in an epitaxially deposited silicon layer.  FIG. 18  is a cross section view along the lines  18 — 18  of FIG.  19 . 
       FIG. 19  is a top schematic view of the transistor shown in FIG.  18 . 
       FIGS. 20-27  are cross section schematic views illustrating the steps for fabricating a double-polysilicon self-aligned vertical npn bipolar transistor. 
       FIGS. 28-39  are cross section schematic views illustrating the steps for fabricating a vertical npn bipolar transistor structure shown in FIG.  18  and CMOS devices to provide SOI BiCMOS. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The basic structure of the fully-depleted-collector SOI npn bipolar transistor  10  is illustrated in FIG.  6 . Bipolar transistor  10  has an emitter  12  of n+ type semiconductor, a base  14  of p type semiconductor and a collector  16  having a depleted n collector region  18  and a quasi-neutral n collector region  20 . An n+ type semiconductor region  22  contiguous with quasi-neutral n collector region  20  provides a low impedance reachthrough for electrical connection to circuit wiring. As shown in  FIG. 6 , emitter  12  is over base  14 . Base  14  is over collector region  18  and does not contact collector region  20 . Collector region  20  is positioned between collector region  18  and n+ type semiconductor region or reachthrough  22 . Collector regions  18  and  20  and n+ type semiconductor region  22  are positioned over buried oxide layer  26 . Below buried oxide layer  26  is over substrate  28 . Silicon-on-Insulator  30  comprises semiconductor regions  14 ,  18 ,  20  and  22 , buried oxide  26  and substrate  28 . Electrical contact to bipolar transistor  10  may be made via contact to leads  32 ,  34  and  36  which are coupled, respectively, to the emitter  12 , base  14  and collector  16  via semiconductor region  22 . 
     FIG. 6  shows that this is no quasi-neutral collector region  18  underneath the base region  14 . The collector region  18  directly underneath base is fully depleted. There is only a space-charge region between the base region  14  and the buried oxide layer  26  of the SOI  30 . There is a small quasi-neutral collector region  20  horizontally between the fully depleted collector region  18  (the space-charge region) and the collector reachthrough region  22 . There is no need for an n+ type subcollector layer as used in the prior art. The n+ type reachthrough  22 , the quasi-neutral collector region  20 , and the fully depleted collector region  18  all lie on top of the buried oxide  26  of the SOI  30 . 
   The electron current path for this fully-depleted-collector SOI bipolar transistor  10  is indicated in  FIG. 7  by arrow  40 . Electrons from the emitter  12  enter the space-charge region  18  of the collector  16  after traversing the base  14 . Once inside the space-charge region  18 , these electrons follow the electric field in the space-charge region and drift more or less laterally towards the quasi-neutral region  20  of collector  16 . From there, electron current is carried by the n+ type reachthrough  22  to the semiconductor surface. Since the electrons traverse the space-charge region  18  more or less laterally, instead of vertically in a traditional vertical bipolar transistor as shown in  FIG. 2 , there is no need for an n+ type subcollector layer. The energy-band diagram along the electron path  40  is illustrated in FIG.  8 . Position A indicates the emitter contact  32 . Position A′ indicates where the electrons approach the buried oxide  26  of the SOI  30 . Position A″ indicates where the electrons reach the quasi-neutral region  20  of collector  16 . 
   For the traditional vertical bipolar transistor, such as the one shown in  FIGS. 1 ,  4 , or  5 , the base-collector junction capacitance is given by the usual base-collector diode space-charge layer capacitor. For the fully-depleted-collector SOI bipolar transistor  10 , the base-collector junction capacitance is given by two capacitors in series. They are the vertical space-charge layer capacitor and the buried oxide capacitor. For two capacitors in series, the combined capacitance is determined primarily by the smaller of the two capacitors. Thus, the base-collector junction capacitance of the fully-depleted-collector SOI transistor is determined primarily by the buried oxide capacitor. This capacitance is much smaller than the base-collector junction capacitance of a traditional vertical bipolar transistor. 
   As an example, consider a vertical bipolar transistor with a collector doping concentration of 2×10 17  cm −3  and a base-collector reverse bias of 3 V. The base-collector diode space-charge width is about 160 nm. This width is about the same as the thickness of the silicon layer of high-speed SOI CMOS devices. Thus the fully-depleted-collector bipolar transistor  10  is readily compatible with high-speed SOI CMOS for making high-speed SOI BiCMOS. 
   The fully-depleted-collector SOI bipolar transistor  10  can be implemented with the commonly used vertical bipolar structures and processes. Thus, it can be of the double-polysilicon self-aligned type, or double-polysilicon non-self-aligned type. The intrinsic base layer can be formed by epitaxial deposition of silicon or by ion implantation of the silicon layer over insulator  26  of SOI  30 . With epitaxially deposited silicon for the base  14 , germanium can also be added during deposition to form a SiGe-base bipolar transistor  10 . 
   A process for fabricating a fully-depleted-collector SOI bipolar transistor  10 , using a double-polysilicon non-self-aligned structure and epitaxial deposition of silicon, or silicon-germanium (SiGe) alloy, for forming the intrinsic base is outlined in  FIGS. 9  to  18 . The starting SOI wafer  30  shown in  FIG. 9 , can be prepared by any one of the common SOI preparation processes. Isolation oxide can be formed by the usual masked oxidation of silicon, or by the usual shallow-trench isolation processes which involve etching silicon trenches and then filling the trenches with oxide followed by planarization using chemical-mechanical polishing. The resulting structure is illustrated in  FIG. 10. A  blanket implantation step is then made to dope the n-type collector region  16 . The implantation step provides a concentration so that the n-type collector region  16  will be completely depleted when the transistor is used in circuit applications. For example, if the silicon layer thickness is 100 nm and the base-collector diode is reverse-biased at 3 V, full depletion of the collector region is assured if its average doping concentration is less than about 3×10 17  cm −3 . A masked implantation step is used to dope the reachthrough region  22 , which is shown as surrounding the collector region  16 , more heavily. This is illustrated in FIG.  11 . An insulator layer  50 , for example an oxide layer, is deposited or formed. A heavily p-type doped polysilicon layer  52 , which will form part of the base contact polysilicon layer, is deposited. This is illustrated in FIG.  12 . The base region window is etched open, as shown in  FIG. 13. A  layer of silicon  54  is then grown or deposited epitaxially over collector  16 . Over the single-crystal base window region, the deposited silicon layer  54  is crystalline and forms the intrinsic base of the bipolar transistor, but over the polysilicon region  52 , the deposited silicon layer  54  is polycrystalline and simply adds to the thickness of polysilicon layer  52 . The intrinsic base is doped p-type. The doping can be done by boron implantation of the deposited layer of silicon  54 , or by in situ doping of silicon layer  54  during deposition. If germanium is added to silicon layer  54  during deposition, the resulting transistor will be a SiGe-base bipolar transistor. A layer of insulator  56 , for example an oxide layer, is then deposited. This is shown in FIG.  14 . The base polysilicon layer  54  is patterned, followed by an oxide deposition and reactive-ion etching to form a sidewall oxide  57  on the vertical surface of the etched polysilicon  54 , as shown in FIG.  15 . The emitter window is etched open, as shown in FIG.  16 . An n+ polysilicon emitter  58  is formed as shown in FIG.  17 . Contact windows to the base and the collector are then etched open. The cross section view of a completed transistor is shown in  FIG. 18. A  top schematic view of the completed transistor is shown in FIG.  19 . 
   A process for fabricating a double-polysilicon self-aligned bipolar transistor, using ion-implantation for forming the intrinsic base region, is outlined in  FIGS. 20  to  27 . The starting SOI wafer  30  shown in  FIG. 20 , can be prepared by any of the common SOI preparation process. Isolation oxide can be formed by the usual masked oxidation of silicon process, or by the usual shallow-trench isolation processes which involve etching silicon trenches and then filling the trenches with oxide followed by planarization using chemical-mechanical polishing. This is illustrated in  FIG. 21. A  blanket implantation step is then made to dope the n-type collector region  16 . The implantation step provides a concentration so that the n-type collector region  16  will be completed depleted when the transistor is used in circuit applications. A masked implantation step is used to dope more heavily the reachthrough region  22 , which is shown as surrounding the collector region  16 . This is illustrated in FIG.  22 . An insulator layer, for example an oxide layer  60 , is deposited or formed. The base window is etched open. This is illustrated in  FIG. 23. A  heavily p-type polysilicon layer  62  is deposited. Polysilicon layer  62  forms the base polysilicon contact layer. It can be doped in situ during deposition, or doped by ion implantation of an undoped polysilicon layer. An insulator layer  64 , for example an oxide layer, is then deposited or formed. This is illustrated in FIG.  24 . The base polysilicon layer  62  is patterned and a sidewall insulator layer  66  is formed on the vertical etched surface. A thermal annealing process is carried out to drive the p-type dopant from the polysilicon layer  62  into the single crystal region to form p+ regions  68  and  69 . These p+ regions  68  and  69  are for connecting to the p-type intrinsic base region, to be formed later. This is illustrated in FIG.  25 . The intrinsic base region  70  is formed by boron implantation. The n+ polysilicon emitter  72  is then formed. This is illustrated in FIG.  26 . The base contact  74  and the collector contact  76  are then etched open. This completes formation of the double-polysilicon self-aligned implanted-base fully-depleted-collector SOI bipolar transistor  72 , shown in FIG.  27 . 
   It should be noted that fully-depleted-collector SOI pnp bipolar transistors can also be made by following the processes outlined in  FIGS. 9  to  19  and in  FIGS. 20  to  27  but using dopant impurities of the opposite type. Furthermore, both vertical npn and pnp bipolar transistors can be made on the same silicon layer of the SOI  30  for use in complementary bipolar circuits. 
   Any of the fully-depleted-collector SOI bipolar transistors can be integrated with SOI CMOS devices to form SOI BiCMOS. This is due to the fact that the silicon layer thickness for fully-depleted-collector SOI bipolar transistors can be the same as the silicon thickness for high-speed SOI CMOS devices. For simplicity of illustration, only the process for integrating a vertical npn bipolar transistor structure shown in  FIGS. 18 and 19  and CMOS devices is outlined here, in  FIGS. 28  to  39 . The starting SOI wafer  30 , shown in  FIG. 28 , can be prepared by any one of the common SOI preparation processes. Isolation oxide  80  can be formed by the usual masked oxidation of silicon, or by the usual shallow-trench isolation process which involves etching silicon trenches and then filling the trenches with oxide  80  followed by planarization using chemical-mechanical polishing. This is illustrated in FIG.  29 . The depleted n-type collector region  16  is formed by masked ion implantation. The n+ type reachthrough regions  22  of the bipolar transistor are formed by masked ion implantation. The regions  82  and  84  for the n-channel MOSFET and the p-channel MOSFET, respectively, are also doped by masked ion implantation. An insulator layer  86 , for example an oxide layer, is formed and patterned to insulate the bipolar transistor region. This is illustrated in FIG.  30 . The gate insulators  87  and  88  for the CMOS devices are then formed, as shown in  FIG. 31. A  layer  90  of undoped polysilicon is deposited, as shown in FIG.  32 . This polysilicon layer  90  serves to protect the gate insulators  87  and  88  while steps for forming the bipolar transistor are carried out. The polysilicon layer  90  is doped heavily p-type where it is used as the base polysilicon contact layer  91 . This is illustrated in FIG.  33 . The base window  92  is etched open, as shown in  FIG. 34A  silicon layer  94  is deposited epitaxially, forming single-crystalline silicon over the silicon in the base window  92 , and polycrystalline silicon over the polysilicon layer  90  and  91 . The single-crystalline part of this deposited silicon layer  94  forms the base layer of the bipolar transistor. The polycrystalline part simply adds to the thickness of the polysilicon layer  90 . This thickened polysilicon layer  90  and  94  forms the base polysilicon contact layer for the bipolar transistor and the gate polysilicon layer for the CMOS devices. This is illustrated in FIG.  35 . An insulator layer  96 , for example an oxide layer, is deposited, as shown in FIG.  36 . The insulator layer  96  and polysilicon layers  90  and  94  are then patterned by reactive-ion etching to form the polysilicon base contact structure. An oxide layer is deposited and then etched to form an oxide sidewall  98  to insulate the vertical polysilicon surfaces  91 ,  94  and  90 ,  94 . The emitter window  99  is then etched open, and an n+ doped polysilicon layer  102  is deposited and patterned to form the polysilicon emitter. This is illustrated in FIG.  37 . The gate polysilicon  90 ,  94  for the CMOS devices are then patterned, and sidewall insulator  104  is formed on the vertical surface of the gate polysilicon. The gate polysilicon  90 ,  94 , and the source regions  106 ,  108  and drain regions  107 ,  109 , are then doped by ion implantation. This is illustrated in FIG.  38 . Contacts  112  and  114  to the base and the collector respectively of the bipolar transistor are then etched open, as illustrated in  FIG. 39. A  top view of the bipolar transistor is similar to FIG.  19 . This completes formation of the BiCMOS devices. 
   It should be noted that in the drawing like elements or components are referred to by like and corresponding reference numerals. 
   While there has been described and illustrated a bipolar transistor structure having a fully depleted collector region on an insulator and without the need for an n+ subcollector, it will be apparent to those skilled in the art that modifications and variations are possible without deviating from the broad scope of the invention which shall be limited solely by the scope of the claims appended hereto.