Abstract:
A Heterojunction Bipolar Transistor (HBT) is provided where the SiGe base region is formed through selective deposition, after the formation of the base electrode layer and the emitter window. A sacrificial oxide layer is deposited between the collector and base electrode. The contact to the SiGe base is made at an extrinsic area, underneath the base electrode, after removal of the sacrificial oxide. The SiGe is covered with a temporary oxide layer during further processes, and this protective layer is removed immediately before the deposition of the emitter material. The selective deposition of the SiGe at a relatively late stage of the fabrication process helps insure that the film remains free of the stresses which can degrade electron mobility. A process of fabricating the above-described HBT device is also provided.

Description:
This application is a divisional of U.S. patent application Ser. No. 09/517,093, filed Mar. 1, 2000 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention generally concerns Heterojunction Bipolar Transistors (HBT)s and, more particularly, a HBT device formed through the selective deposition of silicon germanium (SiGe) in the base region. 
     Rapid progress has occurred in the development of high performance bipolar and BiCMOS integrated circuits for applications such as high speed data and RF wireless communications. SiGe heterojunction bipolar technology offers economically feasible solutions with comparable performance characteristics to III-V technologies. The critical performance criteria include high frequency performance, low noise at both low and high frequency, sufficiently high intrinsic gain, and high breakdown voltages. 
     The integration of SiGe into the silicon bipolar base processing has been of interest because of the resulting improvements in electrical properties such as transmit frequency (Ft), Early voltage (Va), and collector-to-emitter breakdown (BVceo). The band gap at the collector side can be reduced by substituting germanium (Ge) for silicon (Si) in the base region of a bipolar transistor. This results in an electric field in the base, which reduces the majority carriers transit time through the base. SiGe films can be integrated into silicon processing with much less difficulty than other materials. However, even the use of structurally similar materials, such as Si and Ge, results in lattice mismatches on the crystal boundary area. Further, the formation of very thin base regions is complicated by the fact that boron implantation, even at an energy as low as 5 Kev, can still penetrate 1000 Å, into the base collector junction. 
     Different techniques have been proposed to integrate SiGe into the base of a bipolar device. These techniques are classified into two categories: blanket SiGe film deposition and selective SiGe film deposition. The blanket SiGe deposition method produces less silicon defects, and, therefore, higher yields. Thin, heavily doped, film can be produced with this method using growth rates of 25 to 100 Å per minute. However, blanket deposition processes are difficult to integrate into standard bipolar fabrication processes. Undesired areas of SiGe cannot easily be etched away without damaging the thin, intended base region. Although nonselective deposition is less complicated in terms of nucleation, microloading effects and faceting, it has to be done at an earlier state in the front-end fabrication sequence for patterning purposes. The stability of the film is frequently compromised due to the number of thermal cycling and etching steps, excessive dopant out-diffusion, and defect formation. 
     Alternately, selective deposition techniques can be used to form the base electrode and base region underlying the emitter. Selective deposition process can be used to grow SiGe only on silicon areas, so that the process is self-aligned. Selective deposition can be done at a later stage which makes its integration much less complicated. The process is less complicated because post-deposition patterning is not required, the process is self-aligned, and extraneous thermal cycles are avoided. Although selective SiGe film deposition is conceptually simple, there are problems concerning the connection of the SiGe base to the base electrodes, and with defect formation near the emitter-base junction. However, if these particular problems could be solved, the selective deposition of SiGe in the fabrication of HBTs would result in higher yields and better electrical performance. 
     It would be advantageous if an HBT base region could be reliably fabricated using a selective SiGe deposition, at a later stage in the fabrication sequence, to minimize exposure of the SiGe layer to undesired heat cycles and chemical processes. 
     It would be advantageous if a SiGe base could be self-aligned, and formed without the necessity of post-deposition patterning. 
     It would be advantageous if a selectively deposited SiGe base could be formed subsequent to the formation of the base electrode layer to avoid annealing and chemical etch processes which act to degrade a SiGe film. 
     It would be advantageous if a selectively deposited SiGe base could be protected during the formation of the emitter window to prevent defects along the emitter-base junction. 
     SUMMARY OF THE INVENTION 
     A Heterojunction Bipolar Transistor is provided comprising: 
     a collector region; 
     a silicon germanium (SiGe) base region overlying the collector region; 
     a silicon base electrode at least partially overlying an extrinsic region of the base region; and 
     an contact, connecting the extrinsic region of the base region to the base electrode. 
     A sacrificial oxide layer temporarily overlies the collector region and underlies the silicon base electrode. After etching, an extrinsic region of the base is formed in the region temporarily occupied by the sacrificial oxide layer. 
     In some aspects of the invention the base region has a SiGe bottom surface and a SiGe top surface, and further comprises: 
     a bottom silicon cap layer separating the collector region and the SiGe bottom surface; and 
     a top silicon cap layer separating the SiGe top surface from the emitter region. Then, the protective oxide layer is formed by oxidizing the top Si-cap. 
     The HBT also has dielectric sidewalls to define the emitter electrode, and include a window between the emitter electrode and the base region top surface. The temporary protective oxide layer is formed after the SiGe deposition, and is removed after the formation of the dielectric sidewalls, before the formation of the emitter region. 
     A method for fabricating an HBT is also provided comprising: 
     selectively depositing a silicon germanium (SiGe) composition to form a base region with a top surface; 
     forming a protective layer of oxide overlying the base region top surface; 
     forming dielectric sidewalls to define an emitter region; 
     etching to remove the protective oxide layer overlying the base region top surface; and 
     forming an emitter overlying the base region. 
     The contact from the SiGe base region to the base electrode is made by depositing a sacrificial layer of oxide overlying the gate oxide layer; 
     depositing a layer of silicon overlying the sacrificial layer of oxide, forming a base electrode; 
     depositing a layer of nitride overlying the silicon layer; 
     depositing a layer of TEOS overlying the nitride layer, forming a TEOS/nitride/silicon stack; 
     patterning an emitter window in the TEOS/nitride/silicon stack; and 
     etching the sacrificial oxide layer to form an undercut between the underlying silicon collector region, and the overlying base electrode. 
     The selective deposition of SiGe includes filling the undercut formed between the silicon collector region and the base electrode, forming an extrinsic contact with the overlying silicon layer. Typically, the SiGe layer is graded with respect to Ge content. 
     Some aspects of the invention further comprise: 
     depositing a silicon bottom cap layer to separate the collector region from the base region, and depositing a silicon top cap layer overlying the SiGe. The formation of the protective oxide layer typically involves oxidizing the silicon top cap overlying the SiGe base region. A high-pressure low-temperature (HIPOX) process oxidizes the base region top surface at a temperature in the range of 600 to 700 degrees C., and a pressure in the range of 10-25 atmospheres. The resulting oxide layer has a thickness in the range of 50 to 250 Angstroms (Å). Alternately, a Si-cap is not used, and the top surface of the SiGe base is oxidized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 through 3 illustrate a Heterojunction Bipolar Transistor (HBT) of the present invention. 
     FIGS. 4-11 depict detailed steps in the formation of a completed HBT device in accordance with the present invention. 
     FIG. 12 illustrates steps in a method for fabricating a Heterojunction Bipolar Transistor (HBT). 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In following description, manufacturing steps are described with enough detail to show relationships between elements of the completed device. Many fabrication details are omitted from this description, with the understanding that those skilled in the art may employ as many of those details as are called for in any particular design. Moreover, when description is given in this application of fabrication steps, those skilled in the art will realize that each such step may actually comprise one or more discrete steps and that other steps, not described herein, may be necessary to achieve specific applications of the invention. 
     FIGS. 1 through 3 b  illustrate a Heterojunction Bipolar Transistor (HBT) of the present invention. Referring to FIG. 1, the HBT  10  comprises a collector region  12 , and a base region  14  having a top surface  16  overlying the collector region  12 . The base region  14  includes silicon germanium (SiGe). A silicon base electrode  18  at least partially overlies an extrinsic region  20  of the base region  14 . A contact  22  connects the extrinsic region  20  of the base region  14  to the base electrode  18 . 
     A temporary protective oxide layer  24  overlies the base region top surface  16 . The protective oxide layer  24  is formed by oxidizing the base region top surface  16  at a temperature in the range of 600 to 700 degrees C., and a pressure in the range of 10-25 atmospheres. The low temperature oxidation of the base region top surface  16  includes forming the protective oxide layer  24  to a thickness  26  in the range of 50 to 250 Angstroms (Å). Alternately, the protective oxide layer  24  is formed by depositing a thin layer of oxide through a chemical vapor deposition (CVD) process. 
     Referring to FIG. 2, an emitter region  28  overlies the base region top surface  16 . As is explained in detail below, the temporary protective oxide layer  24  (see FIG. 1) is removed before the formation of the emitter region  28 . 
     FIG. 3 is a more detailed depiction of the HBT base region  14  of FIG.  2 . In some aspects of the invention the base region  14  has a SiGe bottom surface  30  and a SiGe top surface  32 . A bottom silicon cap layer  34  separates the collector region  12  and the SiGe bottom surface  30 . A top silicon cap layer  36  separates the SiGe top surface  32  from the emitter region  28 . The base region  14  has a thickness  40  in the range of approximately 500 to 1000 Å. 
     When the top Si-cap  36  is used, the top surface  16  of the base region is defined herein as the top surface of Si-cap  36 . Likewise, when the top surface  16  of the base region  14  is oxidized to form protective oxide layer  24 , it is the top Si-cap  36  that is oxidized. Si-caps  34  and  36  have thicknesses  42  in the range of 100 to 300 Å. Alternately, either the bottom  34 , or a top Si-cap  36  may be used without the other. 
     In some aspects of the invention the SiGe base region  14  is graded with respect to the Ge content, with less Ge content in progression from the SiGe bottom  30  to the SiGe top  32  surfaces. Typically, the Ge content of the SiGe base region  14  varies from 20 to 0%. In a preferred aspect of the invention, the Ge content progressively decreases from 12% at the SiGe bottom surface  30  to 0% at the SiGe top surface  32 . The addition of Ge acts to decrease the bandgap across the base. The grading of the Ge creates a bandgap that is not constant. The built-in drift field resulting from the grading speeds cars across the bandgap gradient. Further, the grading acts to reduce film stress by reducing lattice mismatch, especially along the emitter-base junction. Note, the above-described Ge grading of the SiGe base region  14  may occur with, or without the use of Si-caps  34  and  36 . 
     FIGS. 4-11 depict detailed steps in the formation of a completed HBT device  10  in accordance with the present invention. FIG. 4 depicts a conventional front-end bipolar process (prior art). An N+ buried layer is formed via implantation of a dopant such as arsenic or antimony and a high temperature drive. An N-Epi layer is deposited and doped in-situ with arsenic to form collector region. Device isolation is achieved by forming deep and shallow trenches  100 . Deep trench isolation is conducted first, and consists of forming deep and high aspect ratio grooves in the silicon 5-10 microns deep. The walls of deep trenches  100  are oxidized (500-1500 Å thermal oxide) and then the trenches are filled with poly-Si. Planarization is made via blanket etchback of the poly-silicon layer or using chemo-mechanical polish (CMP). The oxide is wet etched, and a stack of nitride (500-2000 Å)/oxide (250-1000 Å) layers is deposited and patterned. Shallow trenches  100  are etched (1-2 microns) and then the trench walls are oxidized (500-1500Å). Next, nitride is removed (wet etch) and the trenches  100  are filled with tetraethylorthosilicate (TEOS). Planarization is made by a combination of resist etchback and spin-on glass (SOG) etchback to expose the device areas. 
     FIG. 5 illustrates the HBT  10  of FIG. 4 following the deposition of a thin gate oxide  104  (prior art). The gate oxide  104  is grown to a thickness  106  of 50-200 Å. 
     FIG. 6 illustrates the HBT  10  of FIG. 5 following the formation of a sacrificial CVD oxide deposition  108 . Sacrificial oxide layer  108  has a thickness  110  in the range of 250 to 600 Å. The CVD oxide layer  108  is later etched to expose a cavity for SiGe deposition. First, however, a stack is deposited on oxide layer  108  of poly-silicon  112 , having a thickness  114  of 1000-3000 Å, followed by CVD nitride layer  116 , having a thickness  118  of 1000-4000 Å, followed by a TEOS layer  120 , having a thickness  122  of 1000-4000 Å. 
     FIG. 7 illustrates the HBT  10  of FIG. 6 following emitter window patterning. The TEOS  120 /nitride  116  stack is etched in plasma using CF 4 /CHF 3  chemistry. Then, the poly-silicon layer  112  is etched using Cl 2  chemistry. Photoresist is stripped and then a CVD nitride spacer  124 , having a thickness  126  of 250-750 Å, is formed through blanket deposition and etchback. The TEOS layer  120  on top of the stack acts as an etch stop layer during nitride spacer  124  etchback. 
     FIG. 8 illustrates the HBT  10  of FIG. 7 following an oxide etch. The sacrificial oxide layer  106  temporarily overlies the collector region  126  and underlies the silicon base electrode  112 . The sacrificial CVD oxide  108  over the device collector area  126  is set etched in 10:1 (DI:HF) and a cavity is exposed via oxide undercut  128 , to a distance  130  of 500 Å to 2500 Å, to realize overhanging in the base poly-silicon layer  112 . This is a very critical step of the whole fabrication which will determine the contact between the subsequently formed base SiGe layer and the extrinsic base poly-silicon layer  112 . Wet etching of the sacrificial oxide  108  immediately precedes the SiGe deposition to ensure passivation of the exposed silicon regions  126 . 
     FIG. 9 illustrates the HBT  10  of FIG. 8 following the deposition of SiGe. SiGe selective deposition  132  is conducted with a DCS (dichlorosilane) or DCS/Cl 2  chemistry, which prevents nucleation over dielectrics  124 . An extrinsic region  134  of the base  132  is formed in the region temporarily occupied by the sacrificial oxide layer  108 . Typically, the selective SiGe deposition includes depositing a stack which includes a bottom Si-cap  34 , an intrinsic SiGe, a boron doped SiGe, an intrinsic SiGe, and a top Si-cap layer  36 , see FIG.  3 . The SiGe layers are either graded or fixed (box) in germanium content. The thickness, germanium content, profile (box or graded) and the boron doping level are extremely important in determining the final electrical properties of the device. Above, is describes just one conventional technique of doping the base region. The present invention is enabled with all convention doping techniques used to form base electrodes. 
     When a top Si-cap layer  36  is used, it is oxidized to form an oxide layer  140  having a thickness  142  of 50 Å to 250 Å, using high pressure-low temperature oxidation (HIPOX, 10-25 atm, 600-700 C.) for protection purposes. Alternately, oxide layer  140  is formed by a thin CVD oxide deposition. In another alternative, oxide layer  140  is formed without Si-cap  36 . 
     FIG. 10 illustrates the HBT  10  of FIG. 9 following the formation of dielectric sidewalls. The dielectric sidewalls  150  define the subsequently deposited emitter electrode, and a window  152  between the emitter electrode and the base region top surface  154 . Referring briefly to FIG. 9, the temporarily protective oxide layer  140  is removed after the formation of the dielectric sidewalls  150 . An L-shaped nitride  158 /oxide  156  spacer avoids emitter plugging effects when using arsenic implant for doping the subsequently deposited emitter poly-silicon layer. 
     FIG. 11 illustrates the HBT  10  of FIG. 10 following the formation of the emitter. The HIPOX layer  140  is wet etched by a quick HF dip and emitter poly-silicon  160  is deposited to a thickness  162  of 1500-2500 Å. Arsenic is then implanted (1e16 to 2e16 at/cm 2  dose) followed by thin CVD nitride deposition (500-1500 Å). A furnace anneal is then performed to drive the emitter dopant (675-775 C.) followed by a rapid thermal anneal for dopant activation (900-975 C.). The emitter window is patterned and the poly-silicon/TEOS/nitride  160 / 120 / 116  stack is etched to expose the base poly-silicon layer  112 . This is followed by platinum deposition  164  and furnace silicidation anneal. The non-reacted platinum is wet etched in Aqua Regia solution. 
     FIG. 12 illustrates steps in a method for fabricating a Heterojunction Bipolar Transistor (HBT). Although the invention is presented as a series of numbered steps for the purpose of clarity, no order should be inferred from the numbering unless explicitly stated. Step  200  provides a substrate, including a lightly doped collector region. Step  202  selectively deposits a silicon germanium (SiGe) composition to form a base region with a top surface. Step  204  forms a protective layer of oxide overlying the base region top surface. Step  206  forms dielectric sidewalls to define an emitter region. Step  208  etches to remove the protective oxide layer overlying the base region top surface. Step  210  forms an emitter overlying the base region. Step  212  is a product, an HBT where the SiGe base has been selectively deposited to minimize film stress. 
     Preceding the selective SiGe deposition in Step  202 , Step  200   a  forms a gate oxide layer. Step  200   b  deposits a sacrificial layer of oxide overlying the gate oxide layer. Step  200   c  deposits a layer of silicon overlying the sacrificial layer of oxide, forming a base electrode. Step  200   d  deposits a layer of nitride overlying the silicon layer, and Step  200   e  deposits a layer of TEOS overlying the nitride layer, forming a TEOS/nitride/silicon stack. Step  200   f  patterns an emitter window in the TEOS/nitride/silicon stack. The formation of the dielectric sidewalls in Step  206  includes forming first nitride sidewalls adjoining the TEOS/nitride/silicon stack, following the patterning of the emitter window in the TEOS/nitride/silicon stack. Step  200   g  etches the sacrificial oxide layer to form an undercut between the underlying silicon collector region, and the overlying base electrode. An undercut is also formed between the collector region and the overlying first nitride sidewalls. 
     Some aspects of the invention comprise further steps. Step  201  deposits a silicon bottom cap layer to separate the collector region from the base region. Following the selective deposition of SiGe in Step  202 , Step  203  deposits a silicon top cap layer overlying the SiGe base region, forming a base region top surface. When a top Si-cap is provided in Step  203 , the formation of the protective oxide layer overlying the base region top surface in Step  204  includes oxidizing the silicon top cap overlying the SiGe base region. The Si-cap layers of Step  201  and  203  typically have a thickness of 100 to 300 Å, and are deposited with a low pressure (LP)CVD process. 
     The selective deposition of SiGe in Step  202  includes using a chemistry selected from the group consisting of dichlorosilane (DCS) and DCS/Cl 2  to fill the undercut formed between the silicon collector region and the base electrode, forming an extrinsic contact with the overlying silicon layer. As explained above in the description of FIG. 3, the selective deposition of SiGe includes forming a base region with area having no Ge content, or forming a SiGe base region that is graded with respect to Ge content. The selective deposition of SiGe in Step  202  includes the Ge content varying from 20% to 0%. In some aspects of the invention Step  200  provides the SiGe base region with a SiGe bottom surface adjacent the collector and a SiGe top surface. Then, the selective deposition of SiGe in Step  202  includes progressively varying the Ge content from 12% at the SiGe bottom surface to 0% at the SiGe top surface. The selectively deposited SiGe is deposited to a thickness in the range of 500 to 1000 Å. A boron dopant is typically used at a concentration of 2e18 to 1e19. The deposition temperature is typically in the range of 600 to 725 degrees C. 
     With respect to Step  204 , the formation of the protective oxide layer overlying the base region top surface includes oxidizing the base region top surface at a temperature in the range of 600 to 700 degrees C. and a pressure in the range of 10-25 atmospheres. The base region top surface is oxidized to form a layer having a thickness in the range of 50 to 250 Å. Alternately, the formation of the protective oxide layer overlying the base region top surface in Step  204  includes depositing a thin layer of oxide through a chemical vapor deposition (CVD) process. 
     Following the formation of the protective layer in Step  204 , the formation of dielectric sidewalls in Step  206  includes sub-steps. Step  206   a  isotropically deposits a sidewall oxide layer. Step  206   b  isotropically deposits a second nitride sidewall layer, forming a nitride/oxide stack. Step  206   c  anisotropically etches the nitride/oxide stack to form an emitter opening with nitride/oxide sidewalls. 
     The formation of the emitter in Step  210  includes sub-steps (not shown). Step  210   a  deposits silicon in the emitter well overlying the base region top surface. Step  210   b  implants dopant in the emitter silicon. Step  210   c  anneals the emitter, and Step  210   d  forms contacts to the base electrode. 
     While only certain preferred features of this invention have been shown by way of illustration, many changes and modifications will occur to those skilled in the art. Accordingly, it is to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit and scope of the invention.