Abstract:
A bipolar transistor compatible with CMOS processes utilizes only a single layer of polysilicon while maintaining the low base resistance associated with conventional double-polysilicon bipolar designs. Dopant is implanted to form the intrinsic base through the same dielectric window in which the polysilicon emitter contact component is later created. Following poly deposition within the window and etch to create the polysilicon emitter contact component, large-angle tilt ion implantation is employed to form a link base between the intrinsic base and a subsequently-formed base contact region. Tilted implantation enables the link base region to extend underneath the edges of the polysilicon emitter contact component, creating a low resistance path between the intrinsic base and the extrinsic base. Fabrication of the device is much simplified over a conventional double-poly transistor, particularly if tilted implantation is already employed in the process flow to form an associated structure such as an LDMOS.

Description:
This application is a divisional of application Ser. No. 09/312,879, filed May 17, 1999 now U.S. Pat. No. 6,262,472. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a bipolar transistor having a polysilicon emitter and, in particular, to a bipolar device formed utilizing a single polysilicon layer and a tilted ion-implanted link base region that may readily be incorporated into existing CMOS processes. 
     2. Description of the Related Art 
     The majority of integrated circuit designs employ some form of digital signal processing. These conventional logic structures dictate the use of complementary MOS (CMOS) device architectures. In the pursuit of enhanced performance and greater flexibility, IC designers have increasingly relied upon circuits that combine both bipolar and MOS transistor technologies. For maximum process efficiency, a polysilicon bipolar transistor should be formed in a process flow that shares as many processing steps as possible with a standard CMOS process flow. However, creating a process flow that successfully integrates bipolar and MOS architectures can pose a significant challenge. 
     FIG. 1A shows a cross-sectional view of a conventional NPN bipolar transistor device which includes a diffused polysilicon emitter structure formed from a single polysilicon layer. Conventional single-poly NPN bipolar transistor  100  lies within N-well  102  formed in P-type silicon  104 . Conventional single-poly bipolar transistor  100  is electrically isolated from the effects of adjacent semiconductor devices by inter-device isolation structures  106 . 
     NPN transistor  100  further includes a buried N+ collector layer  108  connected to collector contact  110  by N+ sinker structure  112 . Collector contact  110  may be conveniently formed during the source/drain implant steps of an associated CMOS process. Collector contact  110  and sinker  112  are electrically insulated from the remainder of transistor  100  by intra-device isolation structure  114 . 
     Bipolar transistor  100  also includes P-type base  116  having a P+ base contact  118 . Base  116  consists of intrinsic base region  116   a  and extrinsic base region  116   b . As used in this patent application, the term “intrinsic base” refers to that portion of the base directly underneath the collector. The term “extrinsic base” refers to that portion of the base region of the bipolar transistor which is not directly underneath the collector. 
     Base  116  may be formed within N-well  102  during implantation of dopant to form the lightly doped drain (pldd) regions of associated CMOS devices. Base contact  118  may be formed during the source/drain implant of associated CMOS devices. 
     Bipolar transistor  100  further includes diffused polysilicon emitter structure  122 . Emitter  122  includes a polysilicon contact component  122   a  and a diffused single crystal component  122   b . Polysilicon contact component  122   a  may be formed from the same N-type polysilicon layer used to create the gates of associated CMOS devices. Diffused single crystal component  122   b  is formed by thermal diffusion of N-type dopant from polysilicon contact component  122   a  into base  116 . 
     Conventional single-poly bipolar transistor  100  also includes an overlying dielectric material  124 . 
     While the conventional single-poly bipolar transistor shown in FIG. 1A is useful in many applications, it suffers from the disadvantage of exhibiting a relatively high base resistance. 
     FIG. 1B shows an enlarged cross-sectional view of an edge portion of the emitter base junction of the device of FIG.  1 A. During operation of bipolar transistor  100 , the bulk of the charge traveling between base contact  118  and intrinsic base  116   a  must traverse conductive path  126 . 
     Because the overlying interconnect must make contact with both polysilicon emitter component  122   a  and base contact region  118 , regions  116   a  and  118  must be separated to ensure electrical isolation between the contacts. Therefore, conductive path  126  traverses a relatively long distance. The length of path  126  in turn creates high electrical resistance. 
     The elevated high base resistance acts to degrade device performance. In particular, equation (I) determines the maximum frequency of switching of the transistor:              (   I   )                     f   MAX       =       f   T     /     (     8      π                   C   JBC          R   B       )         ,     where        :                       f   MAX     =     frequency                 at                 which                 unilateral                 power                 gain                 is                 unity                   f   T     =     unity                 gain                 cutoff                 frequency                   C   JBC     =     base        -        collector                 junction                 capacitance                   R   B     =     base                 resistance                   (     intrinsic   +   extrinsic     )                                    
     Thus, a higher overall base resistance will reduce the switching frequency of the transistor. A low f MAX  is particularly problematic given the extremely rapid switching frequencies demanded by modern, high-speed digital applications. 
     In order to reduce base resistance and thereby overcome this disadvantage, device engineers have implemented a double-polysilicon bipolar transistor design. FIG. 2A shows a cross-sectional view of a conventional double-poly NPN bipolar transistor device. 
     Double-poly NPN bipolar transistor  200  lies within N-well  202  formed within P-type silicon  204 . Conventional double-poly bipolar transistor  200  is electrically isolated from the effects of adjacent semiconductor devices by inter-device isolation structures  206 . 
     Bipolar transistor  200  includes a buried N+ collector layer  208  connected to collector contact  210  by N+ sinker structure  212 . Collector contact  210  may conveniently be formed during the source/drain implant steps of an associated CMOS process. Collector contact  210  and sinker  212  are electrically isolated from remainder of transistor  200  by intra-device isolation structure  214 . 
     Bipolar transistor  200  further includes a doped base layer  216 , which includes an intrinsic base region  216   a . Diffused polysilicon base  218  overlies and is separated from doped base layer  216  by a first dielectric layer  219 . Diffused polysilicon base  218  includes a polysilicon contact component  218   a  and a diffused single crystal silicon component  218   b.    
     Polysilicon base contact  218   a  is formed from a P-type polysilicon layer. Single crystal base component  218   b  is formed by diffusion of P type dopant from polysilicon base contact  218   a  into doped base layer  216 . 
     Bipolar transistor  200  further features diffused polysilicon emitter structure  222  which is formed over and separated from polysilicon base  218   a  by second dielectric layer  224 . Diffused polysilicon emitter structure  222  includes polysilicon emitter contact component  222   a  and single crystal diffused emitter component  222   b.    
     Polysilicon emitter contact  222   a  is formed from a second doped polysilicon layer of N-type conductivity, as could be used to form the gates of associated CMOS transistors Single crystal emitter component  222   b  is formed by diffusion of N-type dopant from polysilicon component  222   a  into doped base layer  216 . 
     FIG. 2B is an enlarged view of an edge portion of the emitter/base junction of the device of FIG.  2 A. FIG. 2B reveals that because single crystal base component  218   b  is self-aligned to single crystal emitter component  222   b , regions  218   b  and  222   b  are separated by only the width of second dielectric layer  224  (typically less than 0.4 μm). This configuration is made possible by the presence of polysilicon base contact component  218   a , and single crystal base component  218   b , which provide highly doped, low-resistance conductive path  226  to intrinsic base  216   a.    
     While the conventional double-poly bipolar transistor structure addresses significant performance disadvantages of the conventional single-poly bipolar transistor, this design suffers from a serious disadvantage in the form of more complex processing. Specifically, the double-poly bipolar transistor depicted in FIG. 2A requires additional polysilicon deposition, implant, masking, and etching steps to create the diffused polysilicon base structure. Each of these added steps confers a process penalty in the form of reduced yield and increased cost. 
     In particular, utilizing separate polysilicon layers can introduce a “poly stringer” problem. This “poly stringer” problem is a result of deposition of a second polysilicon layer occurring over sharp corners of raised features of a first polysilicon layer. Such a double-polysilicon structure is difficult to etch without leaving behind filaments (“stringers”) from the second polysilicon layer. 
     This “poly stringer” problem can be eliminated by forming an intervening dielectric layer between the two polysilicon layers. However, this solution requires additional processing steps to form the dielectric layer. These additional steps further degrade overall throughput and thus increase expense. 
     The enhanced complexity in fabricating a double-poly bipolar transistor is particularly troublesome when the processing requirements of associated CMOS devices are taken into account. 
     Therefore, there is a need in the art for a bipolar transistor structure compatible with a CMOS process flow that maintains low base resistance while preserving process simplicity. 
     SUMMARY OF THE INVENTION 
     The present invention proposes a bipolar transistor compatible with CMOS processes that utilizes only a single layer of polysilicon. The design in accordance with the present invention maintains lowered base resistance and superior performance associated with conventional double-poly bipolar transistors, while preserving process simplicity. 
     The present invention utilizes implantation of dopant to form the intrinsic base through the same nitride window in which a diffused polysilicon emitter structure will later be created. Following polysilicon deposition, implant, and etching, a tilted implant is used to form a link base region. 
     This link base creates a short, highly doped, low resistance path between the base contact and the diffused polysilicon emitter. 
     Because the present invention employs only a single polysilicon layer, the process flow is much simpler than that associated with formation of the conventional double-poly structure. This advantage is even more apparent where tilted ion implantation must already be employed in the process to form an LDMOS or some other associated semiconductor structure. 
     A first embodiment of a process for forming a bipolar transistor in accordance with the present invention comprises the steps of forming a well of a first conductivity type in a semiconductor material, forming a buried highly doped collector region of the first conductivity type in the well, and forming a dielectric layer having a window over the collector region in the well. Next, dopant of a second conductivity type opposite the first conductivity type is introduced through the window to form an intrinsic base region. A polysilicon layer is then formed over the dielectric layer and within the window. Dopant of the first conductivity type is then introduced into the polysilicon layer, and the polysilicon layer is etched to form a polysilicon emitter contact component structure extending at least over the window. Dopant of the first conductivity type is then introduced into the semiconductor material directly underneath the polysilicon emitter contact component and above the intrinsic base to form a single crystal emitter component, and dopant of the second conductivity type is ion implanted at an angle of less than 90° to the semiconductor material to form a link base region, the link base region extending underneath the polysilicon emitter contact component and overlapping the intrinsic base region. 
     A first embodiment of a single-poly bipolar transistor in accordance with the present invention comprises a well of a first conductivity type formed in a semiconductor material, a subsurface collector region of the first conductivity type located in the well, and an intrinsic base region of the second conductivity type formed in the well above the collector region. A single crystal emitter component of the first conductivity type is formed in the well above the intrinsic base region. A polysilicon emitter contact component of the first conductivity type is formed over the intrinsic emitter. A base contact region is formed in the well adjacent to the polysilicon emitter component, and a link base region of the second conductivity type is formed in the well by tilted ion implantation, the link base extending laterally underneath the polysilicon emitter contact component and overlapping with the intrinsic base region and the base contact region. 
     The features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A shows a cross-sectional view of a conventional NPN bipolar transistor formed from a single polysilicon layer. 
     FIG. 1B shows an enlarged cross-sectional view of an edge portion of the emitter base junction of the device of FIG.  1 A. 
     FIG. 2A shows a cross-sectional view of a conventional NPN bipolar transistor formed from two polysilicon layers. 
     FIG. 2B shows an enlarged cross-sectional view of an edge portion of the emitter/base junction of the device of FIG.  2 A. 
     FIG. 3A shows a cross-sectional view of one embodiment of a NPN bipolar transistor formed from a single polysilicon layer in accordance with the present invention. 
     FIG. 3B shows an enlarged cross-sectional view of an edge portion of the emitter/base junction of the device of FIG.  3 A. 
     FIGS. 4A-4I show cross-sectional views of one embodiment of a process flow for forming the NPN transistor of FIGS.  3 A- 3 B. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3A shows a cross-section view of one embodiment of an NPN bipolar transistor formed from a single polysilicon layer in accordance with the present invention. NPN bipolar transistor  300  lies within N-well  302  formed within P-type silicon  304 . 
     Bipolar transistor  300  is electrically isolated from the effects of adjacent semiconductor devices by inter-device isolation structures  306 . Isolation structures  306  could comprise oxide structure formed by LOCOS processes, dielectric-filled deep trenches, doped isolation regions, or various combinations of these structures. 
     Bipolar transistor  300  includes a buried N+ collector layer  308  connected to a collector contact  310  by N+ sinker structure  312 . Collector contact  310  and sinker  312  are electrically isolated from remainder of transistor  300  by intra-device isolation structure  314 . Isolation structure  314  could comprise an oxide structure formed by a LOCOS process or a dielectric-filled shallow trench. 
     Bipolar transistor  300  further includes intrinsic and link P-type base regions  320  and  322  respectively, as well as base contact region  316 , all formed within N-well  302 . Link base  322  is contiguous with both intrinsic base  320  and base contact region  316 . 
     Bipolar transistor  300  also includes diffused polysilicon emitter structure  324 . Emitter  324  includes a polysilicon contact component  324   a  and a diffused single crystal component  324   b . Polysilicon emitter component  324   a  bears a thin dielectric spacer  327  along its sides. Polysilicon emitter component  324   a  may be formed from the same N-type polysilicon layer used to create the gates of associated CMOS devices. 
     Diffused single crystal component  324   b  is formed by thermal diffusion of N-type dopant from polysilicon emitter component  324   a  into intrinsic base  320 . Dielectric material  328  lies above portions of the single crystal silicon, including between the edge of polysilicon emitter component  324   a  and intrinsic base  320 . 
     FIG. 3B shows an enlarged cross-sectional view of an edge portion of the base-emitter junction of the bipolar device of FIG.  3 A. FIG. 3B illustrates the low resistance conductive path  330  between base contact region  316  and intrinsic base  320 . The lowered resistance of conductive path  330  is directly attributable to the configuration of base regions  316 ,  320 , and  322 . Specifically, as shown below in FIG. 4C, intrinsic base  320  is the product of ion-implantation through the same window in which the polysilicon emitter component is formed. 
     Because link base  322  is the product of tilted ion-implantation, link base  322  includes a highly doped region  322   a  which extends an appreciable lateral distance beneath diffused polysilicon emitter structure  324 . Highly doped link base portion  322   a  thus overlaps intrinsic base  320  in region  320   a , thereby creating conductive path  330  having uniformly high dopant concentration between base contact  316  and intrinsic base  320 . 
     FIGS. 4A-4I show cross-sectional views of one embodiment of a process flow for forming the single-poly bipolar transistor of FIGS. 3A-3B. FIG. 4A shows the starting point for the process, wherein N-well  302 , buried N+ collector layer  308 , and N+ sinker  312  are formed in P-type silicon  304 . Inter-device isolation structures  306  are then formed to electrically isolate the transistor from the electromagnetic fields of adjacent devices. Intra-device isolation structure  314  is formed to insulate sinker  312  from other portions of the device. 
     FIG. 4B shows the next step, wherein dielectric material  328  is formed over the silicon and the isolation structures, and first photoresist mask  332  is patterned over dielectric layer  328 . Dielectric material  328  may be composed of silicon oxide, silicon nitride, or even a plurality of dielectric layers. Unmasked portions of dielectric material  328  are then etched to produce window  334 . 
     FIG. 4C shows implantation of P-type dopant through window  334  to form intrinsic base region  320 . P-type dopant may be implanted to form intrinsic base  320  in the same step that dopant is implanted into pldd regions of associated PMOS devices. 
     FIG. 4D shows etching of dielectric layer  328  in window  334 , followed by removal of first photoresist mask  332 . Polysilicon layer  323  is then formed and implanted with N-type dopant. Polysilicon layer  323  may be deposited and implanted in the same step that polysilicon comprising the gates of associated CMOS transistors are formed. 
     FIG. 4E further shows subsequent heating of implanted polysilicon layer  323 , causing diffusion of N-type dopant out of polysilicon layer  323  into underlying intrinsic base  320  and forming single crystal emitter component  324   b.    
     FIG. 4F shows patterning of second photoresist mask  338 , followed by etching of N-doped polysilicon layer  323  in unmasked regions to create polysilicon emitter contact component  324   a . This masking and etching step can be the same used to define the gates of associated CMOS transistors. 
     FIG. 4G shows the stripping of second photoresist mask  338 , followed by patterning of third photoresist mask  340 . Third photoresist mask  340  excludes regions adjacent to diffused polysilicon emitter  324 . 
     FIG. 4H shows tilted ion implantation of P-type dopant into regions exposed by third photoresist mask  340 , creating link base regions  322 . In this step, P-type dopant is implanted at an angle of less than 90° to the underlying silicon. This tilted implant is masked by diffused polysilicon emitter  324 , such that the resulting link base  322  extends far enough underneath the edge of emitter  324  to overlap with intrinsic base  320 . This creates a short, highly doped, low-resistance conductive path between intrinsic base  320  and the subsequently-formed base contact region. A brief thermal processing step is performed after the tilted implant to anneal implant damage and to further drive-in the implanted dopant. 
     Although not shown in FIG. 4H, at this point in the process additional P type dopant may be vertically implanted into unmasked regions, as a result of formation of pldd regions of associated MOS transistors. This step merely serves to further increase surface base dopant concentration. 
     FIG. 4I shows subsequent formation of a thin dielectric film over the entire surface, followed by carefully controlled anisotropic etching to yield lateral spacers  327  along sidewalls  324   c  of diffused polysilicon emitter  324 . This step may coincide with creation of lateral spacers along the gate sidewalls of associated CMOS devices. 
     FIG. 4I also shows subsequent implantation of P-type dopant masked by extrinsic polysilicon emitter  324  and lateral spacers  327 , to form self-aligned base contact region  316 . This implantation of P-type dopant may coincide with implant of source/drain regions of associated PMOS devices. 
     Fabrication of the single-poly NPN bipolar transistor device in accordance with the present invention is completed by forming contacts with the device. A collector contact is formed by etching through dielectric material  328  above sinker  312 . 
     The single-poly bipolar transistor in accordance with the present invention offers a number of important advantages over previous designs. First, the base resistance of the device is lowered relative to conventional single-poly bipolar designs. This is due to the presence of the link base region created by tilted ion implantation. The link base elevates the dopant concentration and hence lowers the resistance of the conductive path between the base contact and the intrinsic base. 
     A second important advantage of the present invention is that the process flow is substantially less complex as compared with fabrication of a conventional double-poly bipolar transistor device. A second polysilicon layer is not required to form a diffused polysilicon base structure. This avoids the yield loss and increased cost associated with depositing, implanting, masking, and etching a second polysilicon layer. In addition, the “poly-stringer” problem discussed at length above is entirely avoided. 
     The processing advantage afforded by the present invention becomes even more apparent when a single-poly bipolar transistor in accordance with the present invention is fabricated in conjunction with a device already requiring tilted ion implantation, such as an LDMOS. Under such circumstances, implantation of the link base can be coincident with implantation of the LDMOS body. 
     Although the invention has been described in connection with one preferred embodiment, it should be understood that the invention should not be unduly limited to this specific embodiment. Various other modifications and alterations in the process of this invention will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. 
     For example, while an NPN bipolar transistor has been described above in connection with FIGS. 3-4I, the present invention is also applicable to fabricate a PNP bipolar transistor. Because P-type polysilicon exhibits a higher sheet resistance than N-type polysilicon, a silicided emitter contact would be required to overcome this increased resistance, adding complexity to the process flow. However, the high diffusion coefficient of boron (P-type dopant) relative to arsenic (N-type dopant) could result in migration of dopant from the polysilicon emitter contact component during the anneal and drive in following the tilted base implant, thereby obviating the need for a separate thermal diffusion step. 
     Moreover, the process in accordance with the present invention should not be limited to the specific order of steps depicted above in connection with FIGS. 4A-4I. Specifically, while in FIG. 4D the doping of the polysilicon layer is described as occurring by ion implantation, this is not required. The polysilicon could also be doped in situ during chemical vapor deposition. Similarly, while FIG. 4D shows etching of the polysilicon layer after doping has been performed, this is also not required and polysilicon doping could occur after etching and remain within the province of the present invention. 
     Another possible processing variation leading to an alternative embodiment would be creation of a polysilicon emitter contact structure having tapered, rather than vertical, sidewalls. This sidewall profile could result from careful control of etching conditions during etching of the polysilicon layer. The presence of tapered polysilicon emitter sidewalls would affect the characteristics of the link base, as less polysilicon would be available to mask the tilted ion implant. 
     An additional processing variation leading to another alternative embodiment would be to form the single crystal emitter component directly in the silicon substrate by ion-implantation, rather than by thermal diffusion of dopant out of the polysilicon emitter contact component. In such an alternative embodiment, dopant of the first conductivity type could be ion-implanted through the dielectric window prior to formation of the polysilicon layer. 
     A further processing variation leading to another alternative embodiment would be the use of multiple dielectric layers during fabrication. For example, dielectric layer  328  shown in FIGS. 3A-4F could be a single layer as depicted, or could be composed of multiple layers such as the nitride/pad oxide combination commonly used to mask LOCOS formation. In the event that such a combination is utilized, the nitride component in the window region would likely need to be removed prior to ion implant of the intrinsic base as shown in FIG.  3 C. 
     Finally, while the well/buried collector layer configuration is conventionally formed by 1) introducing dopant into the surface of a silicon substrate, 2) forming epitaxial silicon over the silicon substrate, and then 3) forming the well within the epitaxial silicon, this sequence of steps is not required by the present invention. The buried collector layer and well could be formed directly in the substrate by ion implantation. 
     Given the multitude of embodiments described above, it is intended that the following claims define the scope of the present invention, and that methods and structures within the scope of these claims and their equivalents be covered hereby.