Abstract:
A wafer comprising at least one emitter-up Heterojunction Bipolar Transistor (HBT) and at least one emitter-down HBT on a common InP based semiconductor wafer. Isolation and N-type implants into the device layers differentiate an emitter-down HBT from an emitter-up HBT. The method for preparing a device comprises forming identical layers for all HBTs and performing ion implantation to differentiate an emitter-down HBT from an emitter-up HBT.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. patent application Ser. No. 11/052,935, filed on Feb. 7, 2005, and is related to U.S. Provisional Application No. 60/603,480, filed on Aug. 20, 2004 for “Group III-V Compound Semiconductor Based Heterojunction Bipolar Transistors with Various Collector Profiles on a Common Wafer” by Mary Chen and Marko Sokolich, the disclosure of which is incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The present invention was made with support from the United States Government under Grant No. F33615-02-C-1286 awarded by DARPA. The United States Government has certain rights in the invention. 
    
    
     FIELD 
     This invention relates to a new design with InP based Heterojunction Bipolar Transistors (HBTs) with emitter-down profiles, including those for high Ft HBTs, and emitter-up profiles, including those for high breakdown voltage (BVceo), on a common wafer and to a method of producing the same. 
     BACKGROUND AND PRIOR ART 
     InP based HBT Integrated Circuit (IC) technologies have demonstrated great potential in high-speed digital and mixed-signal applications because of superior speed and bandwidth properties over the SiGe based HBT technology. Although C. R. Bolognesi et al, “Non-blocking collector InP/GaAs 0.51 Sb 0.49 /InP double heterojunction bipolar transistor with a staggered lined up base-collector junction”, IEEE Electron Device Letters, Vol., 20, No. 4, April, 1999, pp. 155-157 suggests that a symmetry of InP/GaAsSb/InP DHBT band structure may have the potential for integration of collector-up and emitter-up devices, present invention implements selective ion implantation technology for integration of high Ft HBT (collector-up HBTs) and high BVceo HBT (emitter-up HBTs) on same chip. 
     SiGe based HBT technology of various collector concentrations available on the same chip has been described in the prior art. See, for example, G. Freeman et al, “Device scaling and application trends for over 200 GHz SiGe HBTs”, 2003 Topical Meetings on Silicon Monolithic Integrated Circuits in RF Systems, pp. 6-9, Digest of papers. The SiGe based HBT technology enables high F t  to be traded for high BVceo on the same chip. However, IC designers up to now could not trade high F t  for high BVceo or vice versa on the same InP. 
     The ability to provide high F t  HBTs and high BVceo HBTs on the same chip is particularly useful in smart Power Amplifiers (PAs) in millimeter wave image radar. Increased power provides longer distance of operation. Smart PAs with digital electronics to control the PAs can be realized by high speed signal processes for regular logic and high BVceo (breakdown voltage) for large swing at output stage. However, presently, when high BVceo HBTs are used in logic circuits lower speed may occur as compensation due to inability to serve as high F t  HBTs in logic circuits on the same chip. 
     The ability to provide high F t  HBTs and high BVceo HBTs on a common chip substrate may also be useful in the front-end stage of an analog to digital (A/D) converter. Having high F t  HBTs and high BVceo HBTs on common chip substrate may provide increased dynamic range and larger input to analog converter which may be advantageous for higher signal/noise (S/N) ratio and resolution. However, A/D technologies of today cannot provide significantly higher peak-to-peak input signal than 1V with good linearity. Better dynamic range may improve this technology. 
     Accordingly there is a need for fabricating and integrating high F t  HBTs and high BVceo HBTs on the common non-silicon based wafer. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES AND THE DRAWINGS 
         FIG. 1  depicts a side view of an emitter-up HBT; 
         FIG. 2  depicts a side view of an emitter-down HBT; 
         FIG. 3  depicts a wafer with HBTs on the wafer; 
         FIGS. 4-23  depict a process of forming HBTs based on an exemplary embodiment; 
         FIGS. 24-29  depict a process of forming HBTs based on another exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, like reference numbers are used to identify like elements. Furthermore, the drawings are intended to illustrate major features of exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of every implementation nor relative dimensions of the depicted elements, and are not drawn to scale. 
     The present disclosure describes new designs with InP based HBTs with emitter-up (collector-down) and emitter-down (collector-up) profiles including those for high F t  HBTs and high BVceo HBTs on a common wafer. Specially designed epi-taxial layer structures with selective area doping by ion implantation can integrate HBTs with emitter-up and emitter-down profiles, including those HBTs for high F t  and HBTs for high BVceo on the same InP wafer without backside processing. 
     Referring to  FIGS. 1 and 2 , in one exemplary embodiment, a cutaway side view is shown of two out of hundreds of thousands (for example) of HBTs  10  and  11  of the presently disclosed technology that may be grown as part of individual circuits  30  separated by streets  40  on a substrate of wafer  20  (See FIG.  3 ). For clarity reasons the HBTs  10  and  11 , individual circuits  30  and wafer  20 , as depicted in  FIGS. 1 ,  2  and  3 , are not to scale. 
     According to the presently disclosed technology, HBTs, as shown in  FIGS. 1 and 2 , may be grown having either a high BVceo or a high F t  by performing ion implantation in layer  90  or layer  70 , as shown in  FIGS. 1 and 2 . 
     Referring to  FIGS. 1-19 , individual HBTs  10  and  11  may be grown on the substrate  50  of the wafer  20 , wherein the substrate layer  50  may be a Semi-Insulating (S.I.) InP wafer. The thickness of the substrate layer  50  may be about 0.5 mm. For clarity and example purposes  FIGS. 3-19  depict the process of forming at least one emitter-up HBT  10  and at least one emitter-down HBT  11  on the single wafer  20 , as shown in  FIG. 3 . 
     Referring to  FIG. 4 , layer  60  may be formed, for example by epitaxial growth, on top of the substrate  50 . The layer  60  may comprise, for example, N-type InGaAs (N+) material that is heavily doped with silicon or N-type InP (N+) material that is heavily doped with silicon. The thickness of the layer  60  can vary from about 100 Å to about 5000 Å. Layer  60  may function as a sub-collector layer for emitter-up HBT  10  or as a sub-emitter layer for emitter-down HBT  11 . 
     Referring to  FIG. 5 , layer  70  may be formed, for example, by epitaxial growth, on top of the layer  60 . The layer  70  may comprise, for example, N-type InP (N−) undoped material. The thickness of the layer  70  may be determined by the emitter-up HBTs in the wafer  20  with the highest BVceo requirement. Layer  70  may be formed uniformly across layer  60  to a maximum thickness that is required to yield the emitter-up HBT with the highest BVceo requirement. The profile of the layer  70  for emitter-up HBTs may be varied as described in the U.S. Provisional Application No. 60/603,480, incorporated herein by reference. Layer  70  may function as a collector layer for emitter-up HBT  10  or as an emitter layer for emitter-down HBT  11 . 
     Referring to FIGS.  2  and  6 - 11 , to form an emitter layer for emitter-down HBT  11 , an ion implantation may be performed on layer  70  to create N-type doped (N) regions  75  and isolated regions  78 . Isolation regions  78  may prevent parasitic current injection through the extrinsic base-emitter junction area under forward bias. The ion implantation of region  75  in the individual emitter-down HBTs  11  may be performed by: 1) applying and forming an implant mask  71  on top of the layer  70  so as to expose only the portion of the layer  70  for one or more of the emitter-down HBTs, as shown in  FIG. 6 ; 2) performing ion implantation until region  75  is formed, as shown in  FIG. 7 ; 3) removing implant mask  71  and annealing the structure in  FIG. 8  for implant activation and damage removal wherein N region  75  is formed. 
     This disclosure is not limited to a shape of implant region  75  as depicted in FIGS.  2  and  6 - 8 . There may be single or multiple implants forming individual region  75  depending on the performance requirements for the emitter-down HBTs  11 . The thickness and doping level of region  75  may be formed by varying the energy and dose of the ion implantation process. 
     Referring to FIGS.  2  and  9 - 11 , the ion implantation of regions  78  for isolation in the individual emitter-down HBTs  11  may be performed by: 1) applying and forming an implant mask  72  on top of the layer  70  so as to expose only the portions of the layer  70  for one or more of the emitter-down HBTs, as shown in  FIG. 9 ; 2) performing ion implantation until regions  78  are formed, as shown in  FIG. 10 ; 3) removing implant mask  72 , as shown in  FIG. 11 . This disclosure is not limited to shape of implant isolation regions  78  as depicted in FIGS.  2  and  9 - 11 . 
     The ion implantation of regions  75  may follow ion implantation of regions  78 . If regions  78  are implanted before regions  75 , regions  75  may be subjected to rapid thermal annealing to avoid possible thermal instability in regions  78 . 
     The ion implantation of regions  75  and  78  may be performed by regular masked implantation or by stencil mask ion implantation technology. See for example Takeshi Shibata et al, “Stencil mask ion implantation technology”, IEEE Transactions on semiconductor manufacturing, Vol, 15, No. 2, May 2002, pp. 183-188. 
     Upon completion of the ion implantation, an optional smoothing layer (not shown) may be formed by epitaxial growth on top of the layer  70 . The smoothing layer may enable smoothing of the epitaxial growth surface prior to deposition of the base-collector interface and emitter-base interface. The smoothing layer may comprise, for example, N-type InP (N−) material. The thickness of the smoothing layer may, for example, be about 200 Å. 
     Referring to  FIG. 12 , a base layer  80  may be formed, for example, by epitaxial growth, on top of the layer  70  or on top of the optional layer referred to above. The base layer  80  may comprise, for example, P-type GaAsSb (P+) material. The thickness of the base layer  80  may, for example, be about 400 Å. 
     Referring to  FIG. 13 , a layer  90  may be formed, for example by epitaxial growth, on top of the base layer  80 . The layer  90  may comprise, for example, N-type InP (N) material doped with silicon. The thickness of the layer  90  may, for example, be about 1500 Å. Layer  90  may function as an emitter layer for emitter-up HBT  10  or as a collector layer for emitter-down HBT  11 . 
     Referring to  FIG. 14 , a layer  100  may be formed, for example by epitaxial growth. The emitter cap layer  9  may comprise, for example, N-type InGaAs (N+) material that is doped heavily with silicon. The thickness of the layer  100  may, for example, be about 1000 Å. Layer  100  may function as a collector cap for emitter-down HBT  11  or as an emitter cap for emitter-up HBT  10 . 
     Referring to  FIG. 1 , an optional implantation of region  95  may be performed to increase doping of the emitter layer (layer  90 ) and lower emitter resistance Re for emitter-up HBTs  10 . 
     Referring to FIGS.  1  and  15 - 17 , to form an emitter layer for emitter-up HBT  10 , an ion implantation may be performed on layer  90  to create a heavily doped (N+) region  95 . The ion implantation of region  95  in the individual emitter-up HBTs  10  may be performed by: 1) applying and forming an implant mask  91  on top of the layer  100  so as to expose only the portions of the layers  90  and  100  for one or more of the emitter-up HBTs, as shown in  FIG. 15 ; 2) performing ion implantation until region  95  may be formed, as shown in  FIG. 16 ; 3) removing implant mask  91  and performing rapid thermal annealing of the structure in  FIG. 17  for implant activation and damage removal wherein N+ region  95  may be formed. 
     Ion implantation of region  95  may be performed on layer  90  prior to formation of layer  100 . 
     This disclosure is not limited to a shape of implant region  95  as depicted in FIGS.  1  and  15 - 17 . There may be single or multiple implants forming individual region  95  depending on the performance requirement for emitter-up HBTs  10 . The thickness and doping level of region  95  may be formed by varying the energy and dose of the ion implantation process. 
     In one exemplary embodiment, the process of HBT fabrication may include: providing metal contacts  110  through lithography and metal deposition as shown in  FIG. 18 ; etching emitter mesas  150  for emitter-up HBT  10  and collector mesas  160  for emitter-down HBT  11 , as shown in  FIG. 19 ; providing base metal contacts  120  through lithography and metal deposition, as shown in  FIG. 20 ; etching base mesas  170 , as shown in  FIG. 21 ; providing metal contacts  130  through lithography and metal deposition, as shown in  FIG. 22 ; and etching isolation mesas  180 , as shown in  FIG. 23 . As shown in  FIGS. 18-23 , the substrate  50  has the same thickness at locations under the emitter-up HBT  10  and the emitter-down HBT  11 . The substrate  50  may also have the same thickness at locations between the emitter-up HBT  10  and the emitter-down HBT  11 . 
     Metal contacts  110  may function as emitter contacts for emitter-up HBTs  10  or as a collector contact for emitter-down HBTs  11 . Metal contacts  130  may function as collector contacts for emitter-up HBTs  10  or as emitter contacts for emitter-down HBTs  11 . The electrically conducting metal contacts  110 ,  120 ,  130  may comprise, for example, Ti/Pt/Au, Pt/Ti/Pt/Au, AuGe or AuGe/Ni/Au. 
     Referring to  FIGS. 24-29 , in another exemplary embodiment, the process of HBT fabrication may include formation of self-aligned base metal contacts  120  so as to lower base resistance. Referring to  FIG. 24 , formation of self-aligned metal contacts  120  may include providing metal contacts  110  through lithography and metal deposition. Referring to  FIG. 25 , emitter mesas  210  for emitter-up HBT  10  and collector mesas  220  for emitter-down HBT  11  may be etched. Using metal contacts  110  as a mask, etching of mesas  210  and  220  may be performed. Due to over etching, lateral overhang of the metal contacts  110  may be expected. 
     Referring to  FIG. 26 , layer  200  may be formed to at least partially cover metal contacts  110 . Layer  200  may comprise, for example, positive tone photo definable polyimide (PDPI) or positive tone photo sensitive interlayer dielectric (ILD). 
     Referring to  FIG. 27 , layer  200  may be soft baked, may be flood exposed (maskless) and may be developed so as to remove most of the exposed layer  200  material except for the portions  201 ,  202 ,  203  and  204  protected by the lateral overhang of the metal contacts  110 . Flooding of the layer  200  may be performed with g-line (436 nm) or I-line (365 nm) lithography tools. Portions  201 ,  202 ,  203  and  204  may further be cured so as to avoid damage from sequential solvents or other processes. 
     Referring to  FIGS. 28-29 , formation of self-aligned metal contacts  120  may be performed by: providing a photoresist layer  205  (patterned for base contact metal) so as to expose portions of layer  80 , as shown in  FIG. 28 ; depositing metal contacts  120  and removing photoresist layer  205  through lift off process, as shown in  FIG. 29 ; removing any metal flaxes that may be deposited on portions  201 ,  202 ,  203 ,  204  by performing slight Argon (Ar) ion milling etch process. 
     The process of HBT fabrication may further include: etching base mesas (not shown); providing metal contacts  130  through lithography and metal deposition; and etching isolation mesas (not shown). 
     The foregoing Detailed Description of exemplary and preferred embodiments is presented for purposes of illustration and disclosure in accordance with the requirements of the law. It is not intended to be exhaustive nor to limit the invention to the precise form(s) described, but only to enable others skilled in the art to understand how the invention may be suited for a particular use or implementation. The possibility of modifications and variations will be apparent to practitioners skilled in the art. No limitation is intended by the description of exemplary embodiments which may have included tolerances, feature dimensions, specific operating conditions, engineering specifications, or the like, and which may vary between implementations or with changes to the state of the art, and no limitation should be implied therefrom. Applicant has made this disclosure with respect to the current state of the art, but also contemplates advancements and that adaptations in the future may take into consideration those advancements, namely in accordance with the then current state of the art. It is intended that the scope of the invention be defined by the claims as written and equivalents as applicable. Reference to a claim element in the singular is not intended to mean “one and only one” unless explicitly so stated. Moreover, no element, component, nor method or process step in this disclosure is intended to be dedicated to the public regardless of whether the element, component, or step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for . . . ” and no method or process step herein is to be construed under those provisions unless the step, or steps, are expressly recited using the phrase “comprising the step(s) of . . . ”