Abstract:
A method of forming an Si—Ge epitaxial layer comprising the following steps. A structure is provided and a doped Si—Ge seed layer is formed thereover. The doped Si—Ge seed layer having increased nucleation sites. A Si—Ge epitaxial layer upon the doped Si—Ge seed layer whereby the Si—Ge epitaxial layer lacks discontinuity.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to semiconductor fabrication and more specifically to semiconductor transistor fabrication. 
   BACKGROUND OF THE INVENTION 
   Silicon-germanium epitaxial (Si—Ge epi) technology is becoming the mainstream in the application of heterojunction bipolar transistors. Si—Ge epi layers are used as the base material in such transistors in BiCMOS applications where bi-polar (BI) and complementary metal-oxide semiconductor (CMOS) transistors are fabricated in different areas of the same wafer. The Si—Ge epi layer could provide higher emitter injection efficiency and lower base transit time. 
   However, the discontinuity of the Si—Ge epi layer occurs on different intermediate layers and becomes a major issue for subsequent process steps due to poor polysilicon (poly) sheet resistance connected with the base electrode. 
   U.S. Pat. No. 6,388,307 B1 to Kondo et al. describes a B-doped SiGe layer in a transistor process. 
   U.S. Pat. No. 5,976,941 to Boles et al. describes a SiGe epi process. 
   U.S. Pat. No. 5,273,930 to Steele et al. describes a SiGe epi process on a silicon seed layer. 
   U.S. Pat. No. 5,620,907 to Jalali-Farahani et al. describes a method for a heterojunction bipolar transistor. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of one or more embodiments of the present invention to provide a method of fabricating semiconductor transistors utilizing Si—Ge epi layers. 
   Other objects will appear hereinafter. 
   It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner. Specifically, a structure is provided and a doped Si—Ge seed layer is formed thereover. The doped Si—Ge seed layer having increased nucleation sites. A Si—Ge epitaxial layer upon the doped Si—Ge seed layer whereby the Si—Ge epitaxial layer lacks discontinuity. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which like reference numerals designate similar or corresponding elements, regions and portions and in which: 
       FIGS. 1 to 3  schematically illustrates a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Initial Structure— FIG. 1   
   As shown in  FIG. 1 , structure  10  has a seed layer  12  formed thereover. Structure  10  is preferably an intermediate substrate and may be a silicon substrate and is understood to possibly include a semiconductor wafer or substrate, active and passive devices formed within the wafer, conductive layers and dielectric layers (e.g., inter-poly oxide (IPO), intermetal dielectric (IMD), etc.) formed over the wafer surface. The term “semiconductor structure” is meant to include devices formed within a semiconductor wafer and the layers overlying the wafer. Structure  10  may also include silicon oxide and/or polysilicon. 
   Seed layer  12  is preferably a doped Si—Ge layer having a thickness of preferably from about 10 to 400 Å and more preferably from about 20 to 200 Å. Doped Si—Ge seed layer  12  is preferably doped with boron (B), C, P or As and is more preferably doped with boron (B). 
   When doping with boron, B 2 H 6  is introduced during the formation of Si—Ge seed layer  12  at a rate of preferably from about 0 to 100 sccm and more preferably from about 0 to 50 sccm under the following conditions:
         temperature: preferably from about 500 to 750° C. and more preferably from about 600 to 700° C.;   pressure: preferably from about 20 to 200 Torr and more preferably from about 50 to 150 Torr; and   time: preferably from about 10 to 120 seconds and more preferably from about 10 to 60 seconds.       

   The dopant within doped Si—Ge seed layer  12  preferably has a concentration of from about 1E18 to 1E20 atoms/cm 2  and more preferably about 1E19 cm 2 . 
   The addition of a dopant to the Si—Ge forms the doped Si—Ge seed layer  12  permitting much better step coverage and eliminates discontinuity by, the inventors believe, increasing the nucleation sites. 
   Formation of Si—Ge Epitaxial Layer  14 — FIG. 2   
   As shown in  FIG. 2 , a Si—Ge epitaxial (epi) layer  14  is formed upon the doped Si—Ge seed layer  12  to a thickness of preferably from about 100 to 700 Å and more preferably from about 200 to 500 Å. Si—Ge epi layer  14  is formed under the following conditions:
         Si precursor: preferably SiH 4 , SiH 2 Cl 2 , SiHCl 3  or SiCl 4  and more preferably SiH 4 ;   Ge precursor: preferably GeH 4  or GeCl 4  and more preferably GeH 4 ;   temperature: preferably from about 500 to 750° C. and more preferably from about 600 to 700° C.;   pressure: preferably from about 20 to 200 Torr and more preferably from about 50 to 150 Torr; and   time: preferably from about 20 to 400 seconds and more preferably from about 100 to 300 seconds.       

   The epi layer  14  could have Si—Ge epi film with graded or box Ge profile. The epi film  14  might also have other doping concentrations. 
   Formation of Optional Cap Layer  16 — FIG. 3   
   As shown in  FIG. 3 , a cap layer  16  may be optionally formed over the Si—Ge epitaxial layer  14  to a thickness of preferably from about 20 to 200 Å and more preferably from about 40 to 120 Å to finish the base process in a BiCMOS process flow. 
   Due to the doped Si—Ge seed layer, the discontinuity issue is eliminated. 
   Cap layer  16  is preferably comprised of silicon. 
   ADVANTAGES OF THE PRESENT INVENTION 
   The advantages of one or more embodiments of the present invention include:
         1. shorten the incubation time of seed layer;   2. improve film uniformity on different substrates; and   3. improve epi quality.       

   While particular embodiments of the present invention have been illustrated and described, it is not intended to limit the invention, except as defined by the following claims.