Abstract:
A method of forming a heterojunction bipolar transistor (HBT) device is disclosed. The method includes forming an intrinsic base layer over a collector layer; forming a sacrificial block structure over the intrinsic base layer; formina a sacrificial spacer layer surrounding top and side surfaces of the sacrificial block structure; forming an extrinsic base layer over the intrinsic layer and adjacent the sacrificial spacer layer; forming a protective layer over the extrinsic base layer; removing the sacrificial spacer layer and implanting a ring shaped dopant profile within an upper portion of the collector layer, wherein the ring shaped collector implant structure corresponds to a pattern of the removed protective layer; removing the sacrificial block structure so as to expose an emitter opening; forming sidewall spacers within the emitter opening; and forming an emitter within the emitter opening, wherein the ring shaped dopant profile is disposed so as to be aligned beneath a perimeter portion of the emitter.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This non-provisional U.S. patent application is a continuation of U.S. patent application Ser. No. 11/538,848, which was filed Oct. 5, 2006 now U.S. Pat. No. 7,390,720, and is assigned to the present assignee. 

   BACKGROUND 
   The present invention relates generally to bipolar transistor technology, and, more particularly, to an improved local collector implant structure for heterojunction bipolar transistors (HBT) and method of forming the same. 
   The improvement in transistor performance, especially the operation speed, is an essential requirement for increased bandwidth and data rate for network communications. As silicon based technology enables large scale integration, an increase in the operation of silicon based devices is a key to achieving a low cost implementation of such systems. The heterojunction bipolar transistor (HBT) is an improvement of the bipolar junction transistor (BJT) that can handle signals of very high frequencies up to several hundred GHz. HBT technology is commonly found in modern ultrafast circuits, such as radio-frequency (RF) systems. 
   The principal difference between the BJT and HBT is the use of differing semiconductor materials (e.g., silicon, germanium) for the emitter and base regions, thus creating a heterojunction. The effect is to limit the injection of holes into the base region, since the potential barrier in the valance band is so large. Unlike BJT technology, this allows for high doping to be used in the base, thereby reducing the base resistance while maintaining gain. 
   There are several different critical features in the design of high performance bipolar transistors, including HBT devices. Such features include, for example, the vertical dimension of the transistor, the collector doping, base-collector and base-emitter capacitances, and collector and base resistances. To achieve higher performance, it is generally desirable to reduce the vertical dimension of the transistor, which reduces the transit time and thus can increase performance. Also, it is generally desirable to increase the collector doping concentration. This reduces collector resistance and thus can also increase performance. These goals are generally compatible, as a vertical dimension reduction is achieved partly by the increase in collector doping concentration since the base-collector space-charge region shrinks with higher doping concentrations. 
   However, simply reducing transistor vertical dimension and increasing the collector doping concentration has the negative result of increasing base-collector capacitance. The increase in base-collector capacitance in turn has a negative impact on the performance of the device, and thus can negate the benefits of reducing the vertical dimension and increasing collector doping concentration. As SiGe HBT switching speeds begin to exceed 350 GHz, it becomes increasingly important to reduce the parasitic base-collector capacitance while also delaying the Kirk effect. 
   HBT devices usually achieve their peak AC performance at relatively high collector current densities, which are needed to load the parasitic capacitance in a short time frame. As the collector current is increased, a larger portion of the current travels at the perimeter of the emitter due to current crowding. The Kirk effect is due to the high current density, which forces the space charge region of the base-collector junction to get pushed into the collector region, thus reducing the frequency response of the transistor. Accordingly, it would be desirable to construct an HBT device that provides a further reduction in base-collector capacitance, while also delaying the Kirk effect and reducing collector resistance. 
   SUMMARY 
   The foregoing discussed drawbacks and deficiencies of the prior art are overcome or alleviated by, in an exemplary embodiment, a bipolar transistor structure including an intrinsic base layer formed over a collector layer, an emitter formed over the intrinsic base layer, and an extrinsic base layer formed over the intrinsic layer and adjacent the emitter. A ring shaped collector implant structure is formed within an upper portion of the collector layer, wherein the ring shaped collector implant structure is disposed so as to be aligned beneath a perimeter portion of the emitter. 
   In one embodiment, the bipolar transistor structure may be made by a method of forming an intrinsic base layer over a collector layer; forming a sacrificial block structure over the intrinsic base layer; forming a sacrificial spacer layer surrounding top and side surfaces of the sacrificial block structure; forming an extrinsic base layer over the intrinsic layer and adjacent the sacrificial spacer layer; forming a protective layer over the extrinsic base layer; removing the sacrificial spacer layer and implanting a ring shaped dopant profile within an upper portion of the collector layer, wherein the ring shaped collector implant structure corresponds to a pattern of the removed protective layer; removing the sacrificial block structure so as to expose an emitter opening; forming sidewall spacers within the emitter opening; and forming an emitter within the emitter opening, wherein the ring shaped dopant profile is disposed so as to be aligned beneath a perimeter portion of the emitter. 
   In one embodiment, the method may further include forming a lower collector implant structure within a lower portion of the collector layer, in electrical contact with the ring shaped dopant profile, wherein the lower collector implant structure is disposed so as to be aligned beneath an interior, center portion of the emitter. 
   In another embodiment, the lower collector implant structure is formed by implantation through the emitter opening following formation of the sidewall spacers and prior to formation of the emitter. 
   In a further embodiment, the collector layer is formed over a buried sub-collector region. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
       FIG. 1  is a cross sectional view of a conventional HBT structure; 
       FIG. 2  is a schematic diagram illustrating the current-crowding effect in an HBT device; 
       FIG. 3  is a cross sectional view of one existing example of an HBT structure having a local collector implant structure for reducing collector resistance and collector-base capacitance; 
       FIG. 4  is a cross sectional view of another existing example of an HBT structure having a local collector implant structure with a stepped dopant profile; 
       FIG. 5  is a schematic diagram illustrating various parasitic capacitance components of the HBT structures shown in  FIGS. 3 and 4 ; 
       FIG. 6  is a cross sectional view of an HBT structure having a local ring shaped collector implant structure, in accordance with an embodiment of the invention; 
       FIG. 7  is a schematic diagram illustrating various parasitic capacitance components of the HBT structure shown in  FIG. 6 ; and 
       FIGS. 8(   a ) through  8 ( k ) illustrate a sequence of exemplary process flow diagrams for forming the HBT structure shown in  FIG. 6 . 
   

   DETAILED DESCRIPTION 
   Disclosed herein is an improved local collector implant structure for heterojunction bipolar transistors and method of forming the same that reduces base-collector capacitance while also delaying the Kirk effect, thereby achieving high AC performance. Briefly stated, a ring shaped region of increased collector dopant is introduced beneath the perimeter region of the emitter, where the majority of collector current travels at high frequencies due to current crowding. The ring is self-aligned to the emitter, and as a result, collector resistance is reduced with a minimal increase in base-collector capacitance, as the increased collector doping is localized within the ring configuration at the periphery of the emitter, and not beneath the central, interior portion of the emitter (or under the extrinsic base). 
   Referring initially to  FIG. 1 , there is shown a cross sectional view of a conventional HBT structure  100 . As is shown, the exemplary N-type HBT structure  100  includes an N++ doped, buried sub-collector region  102  formed within a substrate, and an N+ doped collector region  104  formed over the sub-collector region  102 . One skilled in the art will also recognize intrinsic base layer  106  formed, for example, by low temperature epitaxial (LTE) growth of single crystal layer (such as SiGe) and doped with a P-type dopant (such as boron). Both the (P-type) raised extrinsic base structure  108  and the (N-type) emitter  110  are formed from a polysilicon material, and are shown with silicide contacts  112  for providing ohmic contact to metal filled vias  114  formed within the interlevel dielectric layer (ILD)  116 . Also illustrated in the HBT structure  100  of  FIG. 1  are shallow trench isolation (STI) regions  118  (e.g., oxide), oxide layer  120 , and nitride spacers  122 . For purposes of illustration, arrows are shown to depict the current path from the collector through the emitter in  FIG. 1 . 
   As will be noted, the entire collector region  104  is formed with the N+ dopant. Although this universally doped collector structure reduces the degree of collector resistance (and hence tends to increase the maximum oscillating frequency, f max , and the cut-off frequency, f T , of the HBT), the resulting increase in parasitic base-collector capacitance due to the N+ dopant tends to offset such gains.  FIG. 2  schematically depicts the current crowding (Kirk) effect of a bipolar device, wherein the base current (horizontal arrows) encounters resistance as it travels laterally under the emitter  110 . At high frequencies, the current density of the emitter (vertical arrows) is greater at the periphery of the emitter than in the center of the emitter, with the emitter-base junction bias being larger at the edge than the center of the emitter. 
   Accordingly, one way to reduce the Kirk effect while also reducing the base-collector parasitic capacitance outside the collector current path is to provide a localized collector implant in the collector region, self-aligned to the emitter structure. In this manner, only the local implant is provided with the higher concentration of N+ dopant, instead of the entire collector region. An example of such a local collector implant is illustrated in HBT structure  300   FIG. 3 . As can be seen, the collector layer  104  is not provided with N+ dopant in its entirety, but rather, a local N+ type implant  302  is formed therein, corresponding to (and self-aligned with) the emitter region  110 . Thus configured, the higher doped implant region  302  reduces the Kirk effect and collector resistance, while outside the collector current path, the lower doped N-type collector material keeps the collector-base capacitance in this region low. 
     FIG. 4  illustrates another type of collector structure for an HBT device  400 , in which the collector implant has a stepped profile, such that a lower region  402  of the implant structure has a higher dopant concentration than an upper region  404 . As is the case with implant structure  300  of  FIG. 3 , the stepped collector implant structure of  FIG. 4  is self-aligned to the emitter structure by dopant implantation through the emitter opening. The deeper implant (region  402 ) links the buried collector  104  to the sub-collector  102 , providing a low resistance path to the sub-collector  102 . The shallow implant (region  404 ) reduces the base-collector space charge region width and tailors the collector-base breakdown characteristics. As is the case with implant structure  300  of  FIG. 3 , base-collector capacitance is reduced outside the collector current path. Additional information concerning the stepped collector implant structure of  FIG. 4  may be found in U.S. Pat. No. 6,506,656 to Freeman, et al., assigned to the assignee of the present application, and the contents of which are incorporated herein by reference in their entirety. 
   However, notwithstanding the benefits of a local collector implant structure as described above, the location/structure of the collector dopants is still not optimized, in view of the current crowding depicted in  FIG. 2 . In other words, the dopant of the above-described local collector implant structures is the same within the collector current path, with respect to the cross-sectional length of the emitter structure. Accordingly, the collector-base capacitance of the interior portions of the collector current path (i.e., beneath the inner region of the emitter) is substantially the same as the exterior portions of the collector current path (i.e., beneath the perimeter of the emitter), notwithstanding the increased current density at the perimeter of emitter at high frequencies. Schematically, this is depicted in  FIG. 5 . As is shown, the local collector implant structure reduces parasitic capacitance in the extrinsic base regions (denoted by C PB ) and in the extrinsic base regions outside the collector current path (denoted by C RX ). On the other hand, the parasitic capacitance is increased in the collector current path at both the peripheral region of the emitter (denoted by C N3 ) and the inner region of the emitter (denoted by C int ). 
   Therefore, in accordance with an embodiment of the invention,  FIG. 6  is a cross sectional view of an HBT structure  600  having a local, ring shaped collector implant structure  604  (also referred to as a ring shaped dopant profile herein) formed over a deeper collector implant  602 . The location of the ring shaped implant structure  604  coincides with the larger current density associated with the perimeter of the emitter  110  due to current crowding. The dopant concentration of the ring shaped implant  604  can be made lower or higher than the collector implant structure  602 , but is at least larger than the dopant concentration in collector region  104  as needed to best optimize the device performance. As such, the increase in base-collector capacitance at region  606  of the collector (beneath the center of the emitter  110 ) is minimized due to the lower collector dopant concentration therein. On the other hand, the increased collector doping of the ring structure  604  reduces the collector resistance at a precisely tailored location corresponding to the bulk of the current density location, so as to delay the Kirk effect. The resulting improvement in parasitic base-collector capacitance is schematically depicted in  FIG. 7 . Whereas the component of base-collector capacitance in the collector current path is substantially the same at both the peripheral region of the emitter (C N3 ) and the inner region of the emitter (C int ) in  FIG. 5 , the absence of the increased dopant beneath the inner region of the emitter leads to an even further decreased value of C int . 
   An exemplary process flow for forming the ring-shaped collector implant structure  604  for an HBT device is depicted in  FIGS. 8(   a ) through  8 ( l ). At the point of processing shown in  FIG. 8(   a ), the shallow trench isolation regions  118  are formed within the lightly doped collector region  104 , which has the LTE intrinsic base layer  106  grown thereon. For purposes of simplicity, the sub-collector layer beneath the collector region  104  is not illustrated in  FIGS. 8(   a ) through  8 ( l ). The oxide layer  120  has a sacrificial polysilicon block  802  formed thereon, which in turn is covered by a sacrificial spacer (e.g., nitride) layer  804 . The polysilicon block  802  is dimensioned so as to correspond to the interior region of the subsequently formed emitter layer. As then shown in  FIG. 8(   b ), the polysilicon material used for the raised extrinsic base layer  108  is formed. 
   Proceeding to  FIG. 8(   c ), a protective oxide layer  806  is formed over the extrinsic base layer  108 , after which the sacrificial spacer layer  804  is removed in  FIG. 8(   d ). This exposes a portion of the collector layer  104  between the extrinsic base layer  108  and the sacrificial block  802  to a dopant implant step that results in the formation of the ring shaped collector implant structure  604 , as shown in  FIG. 8(   e ). Thus, the outer diameter of the ring shaped collector implant structure  604  coincides with an emitter opening defined between the edges of the extrinsic base layer  108 . 
   In  FIG. 8(   f ), the sacrificial block  802  is removed, such as by an etch process selective to polysilicon, in order to prepare the device for emitter formation. First, in  FIG. 8(   g ), nitride sidewall spacers  122  are formed, the etching of which also results in the etching of oxide layer  120  so as to expose the intrinsic base layer  106 . Then, as shown in  FIG. 8(   h ), the lower collector implant structure  602  is formed through a dopant implantation (e.g., at a dopant concentration exceeding that of collector layer  104 ). Due to the formation of spacers  122 , it will be noted that the lower implant structure  602  is essentially aligned beneath the center portion of the subsequently formed emitter, while the ring shaped implant structure  604  is aligned beneath the periphery of the subsequently formed emitter. As such, the outer diameter of the ring shaped implant structure  604  exceeds the outer diameter of the lower implant structure. 
   Referring to  FIG. 8(   i ), the polysilicon material for the emitter  110  is deposited, planarized and doped with a suitable N-type dopant (e.g., phosphorous). Upon formation of the emitter  110 , the protective oxide layer  806  may be removed, down to the top of the extrinsic base layer  108 , as shown in  FIG. 8(   j ). At this point, the exposed polysilicon of the extrinsic base layer  108  and emitter  110  are silicided in accordance with known techniques to form silicide contacts  112 , as also shown in  FIG. 8(   j ). 
   Finally, the HBT structure  600  is provided with an interlevel dielectric layer  116 , as shown in  FIG. 8(   k ), and the ILD layer  116  is patterned to form the metal filled vias  114  for connecting the terminals of the HBT device to upper wiring levels. In order to prevent shorting between the emitter  110  and the extrinsic base layer  108 , the spacers  122  are widened in  FIG. 8(   k ) to provide for a greater alignment tolerance for vias  114 . Additional information concerning spacer widening adjacent an emitter region may be found in U.S. Pat. No. 6,979,884 to Ahlgren, et al., assigned to the assignee of the present application, and the contents of which are incorporated herein by reference in their entirety. It will also be noted that additional vias (not shown) would be used to make electrical contact to the collector layer  104  as well. 
   In addition to silicon based bipolar transistors including Si/SiGe heterojunction bipolar transistors, it will be appreciated that the invention embodiments can also be applied to other types of heterojunction bipolar transistors including, but not limited to: AlGaAs/GaAs, InGaP/GaAs, InP/InGaAs, InAlAs/InGaAs, and InAlGaAs/InGaAs single and double heterojunction bipolar transistors with GaAs, InGaAs, InP, AlGaAs, InGaP, InAlAs, or a combination thereof as the collector material. 
   While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.