Abstract:
A method of fabricating a semiconductor device structure, includes: providing a substrate, providing an electrode on the substrate, forming a recess in the electrode, the recess having an opening, disposing a small grain semiconductor material within the recess, covering the opening to contain the small grain semiconductor material, within the recess, and then annealing the resultant structure.

Description:
BACKGROUND OF INVENTION 
   The present invention relates to semiconductor device structures, such as a CMOS device structure including both FET and pFET devices. 
   Mobility enhancements are important to future semiconductor, e.g. CMOS device technologies. Performance improvements from conventional process technologies are becoming extremely difficult to achieve. Methods to stress Si channels include: using SiGe which imparts stress from the bottom of the channel; different shallow trench isolation (STI) material choices which impart stresses from various sides, and SiN etch stop layers which also impart longitudinal stress from the sides. Drawbacks from the SiGe buffer layer or implanted-anneal-buffer approach with a strained Si cap layer are well known. Drawbacks include dislocations that impact yield severely, along with significant difficulty controlling As diffusion enhancements. Further, the process is quite complicated and costly. The STI approach is less costly but is not self-aligned to the gate and has RX size sensitivity. The less costly approach of using nitride etch stop layers to create stress does produce some benefit, but the benefit is believed to be relatively marginal. 
   SUMMARY OF INVENTION 
   The present invention improves device performance using channel mobility enhancement. The present invention improves mobility from the top of the channel by using the stress properties of properly modulated polysilicon gate stacks. Prior to the prevent invention these stress properties were very difficult to control. However, the present invention includes a method and structure which use small grain polysilicon to control stress properties. The present invention provides a method and a structure to impart compressive stress to the pFET channel and a tensile stress to the FET channel. Other embodiments include imparting compressive stress to the pFET channel while preventing compressive stress from being imparted to the FET channel. Another embodiment of this invention includes imparting tensile stress to the FET channel while preventing the tensile stress from being imparted to the pFET channel. 
   The method of the present invention disposes (e.g., deposits) small grain poly Si films in gate electrode stacks, which films become highly tensile as a result of annealing. The tensile film stress can be suppressed by annealing with a low temperature deposited SiN film disposed on top of the poly Si film. 
   It is a principal object of the present invention to enhance channel mobility in semiconductor devices. 
   According to the present invention, a method of fabricating a semiconductor device structure includes providing a substrate; providing an electrode on the substrate; forming a recess in the electrode, the recess having an opening; disposing a small grain semiconductor material within the recess; covering the opening to contain the small grain semiconductor material within the recess, and then annealing the resultant structure. The present invention also includes a semiconductor device structure. 
   Further and still other objects of the present invention will become more readily apparent in light of the following detailed description when taken in conjunction with the accompanying drawing figures. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIGS. 1–9  are side schematic views of process steps according to a preferred embodiment of the present invention. 
       FIG. 10  is a diagram showing simulated stress contours in the gate and channel of the preferred inventive structure. 
   

   DETAILED DESCRIPTION 
   Turning now to the figures, and  FIG. 1  in particular, a semiconductor substrate  1  is provided. The semiconductor substrate is a bulk Si substrate, an SOI substrate, or a stressed (strained) Si substrate. Alternatively, the substrate is a hybrid substrate which includes more than one surface orientation. The substrate alternatively includes a semiconductor material other than Si, such as Ge or any combination of Group III–V elements or Group II–V elements. 
   After an initial substrate cleaning procedure (conventional), an isolation scheme is carried out. As is well known in semiconductor manufacturing, the isolation scheme is used to separate selected devices electrically from each other. The isolation scheme may be a standard or a modified shallow trench isolation (STI) scheme. The STI  2  is shown in  FIG. 1 . Alternatively, the isolation is accomplished using a LOCOS process or mesa isolation scheme, as is well known in the art of fabricating semiconductor devices. For various known or conventional processes for fabricating semiconductor devices, see VLSI Technology, 2 nd  Edition, by S. M. Sze, (McGraw Hill Publishing Co., 1988). 
   After isolation  2  is formed, a conventional gate oxide pre-cleaning process is performed. As is the case in high performance logic fabrication processes, various conventional gate oxide processes may be used to fabricate devices having different gate oxide thicknesses. The gate oxide  3  is formed, for example, using a conventional thermal oxidation process. The oxide  3  is formed using N 2 0, NO, O 2  or any combination of them. The oxide may be nitridized using a conventional plasma process. Alternatively, the gate oxide may be formed using a base oxide followed by deposition of a high k gate dielectric such as aluminum oxide, or hafnium oxide, or another high k gate dielectric. The gate dielectric material  3  has an approximately (+10%) uniform thickness in the range of about (+10%) 0.6 nm to about 7 nm. 
   Next, a film  4  is deposited over the entire wafer structure  1 ,  2 ,  3 . The film  4  is used as a disposable (removable) or semi-disposable gate electrode material. In a preferred embodiment, the film  4  includes a polysilicon (poly Si) material having an approximately uniform thickness or height (T) in the range of about 80 nm to about 150 nm. A deposition technique such as low pressure chemical vapor deposition (LPCVD) or rapid thermal chemical vapor deposition (RPCVD) is used to deposit the removable gate electrode material  4 . The resulting structure is shown in  FIG. 1 . The poly Si layer  4  preferably has a standard grain size in a range of about one nm to about 40 nm. 
   Next, a conventional lithography process is used to pattern photoresist images on the top of the removable gate electrode material  4 . The photoresist images, not shown in the figures, are used to transfer desired features into the removable gate electrode material  4  by using a conventional dry etching process. The dry etching process includes or several chemistries capable of etching the removable gate electrode material  4  selectively with respect to the gate oxide material  3 . The structure shown in  FIG. 2  shows the fully patterned removable gate electrode  5  for a FET gate stack  3 , 5  and a removable gate electrode  6  for a pFET gate stack  3 , 6 . 
   A conventional gate reoxidation process, not shown in the figures, is then used, as is commonly done in high performance logic manufacturing processes. The reox is formed by using a thermal oxidation process to achieve an approximately uniform thickness from about one nm to about 7 nm. Following the reox process, a block mask is patterned over the pFET regions, using a conventional photolithography process. The block (e.g., resist) mask (not shown in the figures) is used to block or prevent the pFET regions from being implanted, while the appropriate FET regions are being implanted. The FET extensions  7  and halos (not shown) are implanted using a low energy As and B implant, respectively. The resist mask is then removed using a dry or wet process. Another block mask (not shown) is patterned over the FET regions. The pFET extensions  8  and halos (not shown) are implanted using a low energy BF 2  or B implant and As implant, respectively. The extension implant profiles  7 , 8  for the FET and the pFET are shown in  FIG. 2 . 
   After the extension and halo implantation, a dielectric liner layer  9  is formed over the entire wafer structure (FIG.  3 ). The dielectric film to be used as the liner layer  9  is preferably SiN which is deposited by CVD or RTCVD or any other suitable deposition technique. A purpose of the liner layer  9  is to provide a CMP stop layer for the next process. Additionally, the SiN liner  9  will be etched, at a later point in the process flow, to form a source-drain spacer. The resulting structure is shown in  FIG. 3 . 
   The next step in the process flow is to deposit an oxide film  10 . The oxide film  10  is deposited and planarized using chemical mechanical polishing (CMP). The film  10  is deposited using, for example a high density plasma (HDP) process. The top of the liner  9  over the removable gate electrode  5  is removed by using a dry etching process that is capable of etching silicon nitride but does not etch appreciable amounts of oxide or poly Si. The planarized oxide film  10  and SiN liner  9  structure is shown in  FIG. 4 , after the top portion of the liner layer  9  has been removed. 
   An important aspect of this invention is that, at this point in the inventive process flow, a gate recess process is used to remove the poly Si, either completely or partially, from the gate electrodes  5 ,  6 . A preferred embodiment in which the poly Si is partially removed is shown in  FIG. 5 . The poly Si is recessed using any suitable dry or wet etch process. A portion  12  and a portion  13  of the original poly Si is left remaining, and has an approximately uniform thickness in a range of about one nm to about 20 nm. A recessed portion  12  of the FET gate electrode and a recessed portion  13  of the pFET gate electrode are shown in  FIG. 5 . In another embodiment (not shown), the poly Si is completely removed. If this embodiment is used, then a conventional gate oxide pre-clean process followed by a conventional gate oxidation process is next performed. 
   Another important aspect of this invention is that, following the gate recess process, a small grain poly Si is deposited over the entire wafer. Small grain polysilicon is known from Shimizu, S. et al. Proceedings of the 1997 Symposium on VLSI Technology, Kyoto, Japan 10–12 Jun. 1997, and also from Silicon Processing for The VLSI Era, Vol 1—Process Technology, by S. Wolf, 1999. The grain structure preferably is in a range of about one nm to about 50 nm. A more preferable grain size is a substantially uniform size in a range of about five nm to 30 nm. The polySi is deposited by RTCVD or LPCVD. Next, the polySi is planarized and recessed from the top of the oxide layer  10  using, for example, CMP and a dry etch. Both CMP and dry etch processes are capable of removing the poly Si selectively relative to the SiO 2  layer  10 . After CMP and dry etch, the inventive gate electrode structure  14 ,  12  for the FET and the inventive gate electrode structure  13 ,  18  for the pFET are shown in  FIG. 6 . 
   At this point in the inventive method, a hard mask  15  shown in  FIG. 7  is patterned over the FET region. The mask  15  is patterned using a dielectric film including SiN and a conventional photolithography procedure. The hard mask  15  is deposited using a low temperature deposition process such as plasma enhanced chemical vapor deposition with a temperature in the range of about 350° C. to about 700° C. Preferably, the material which forms the hard mark  15  is deposited at a temperature below 550° C. or lower, to prevent re-growth of the poly Si grains. The whole structure is next subjected to an anneal using a temperature from about 500° C. to about 600° C. for about one hour. Alternatively, a rapid thermal anneal may be carried out at about 700° C. to about 1000° C. for about one second to about seven seconds. In another embodiment, the structure  12 ,  14  is annealed at a future point in the process. The small grain poly Si remains small, in the range preferably of about five nm to about 30 nm for the FET gate electrode portion  17  because the SiN cap layer  15  is present. However, the grains in the pFET gate electrode portion  18  grow significantly to grain sizes of greater than about 30 nm. 
   The present inventors believe that the inventive process (e.g., with respect to  FIG. 7 ) represents a notable departure from conventional process technology. If the small grain poly Si is subjected to the standard thermal budget, then the poly Si grain growth causes a severe increase in tensile stress. The tensile stress creates a compressive stress in the channel region which degrades electron mobility and limits the performance for the FET. See the simulations of  FIG. 10 . The inventors believe that the grain growth and resulting severe increase in tensile stress is almost completely eliminated by annealing with the SiN hard mask  15  disposed over the FET region. The grain structure can be optimized for each device independently by depositing the small grain poly Si into the recess and annealing the FET with the SiN hard mask. This step appears to result in a notable improvement in FET device performance. 
   The next step in the inventive process is to remove the SiN hard mask  15  from the entire horizontal part of the oxide film  10  or from the entire horizontal part (except over the portion  17 , as shown in  FIG. 8 ). Because the poly Si was recessed as described previously, a portion  19  of the hard mask  15  is left remaining (disposed) in the recessed hole as shown in  FIG. 8 . The purpose of the structure  19  is to prevent the grains from growing during subsequent conventional thermal cycles common in standard state-of-the art semiconductor manufacturing process technology. After the SiN etch, the oxide film  10  is removed using a suitable dry or wet etch process capable of removing the SiO 2  film selectively relative to the SiN and the poly Si materials. A dry directional etch process is next performed on the liner layer  9  to form the spacers  20 ,  25  as shown in  FIG. 8 . Although not essential for the present invention, the spacers  20  of the pFET and the spacers  25  of the FET may have differing heights as shown. A similar block mask and implant process as was used to form the FET and pFET extension regions  7 ,  8  is used to form the FET source-drain regions  21  and the pFET source-drain region  22  shown in  FIG. 8 . A rapid thermal anneal is next performed to activate the junctions. Because the FET continues to have the SiN layer  19  present, grain growth in the FET gate is suppressed, thereby minimizing the tension in the gate electrode stack  3 ,  12 ,  17  and subsequent compression in the channel region below the stack. 
   The remaining portion  19  over the gate electrode stack is next removed using a wet or dry etch process. Next, a suicide pre-cleaning process is carried out followed by a conventional silicide process. See  FIG. 9  and silicide  23 . Standard back-end-of-line processing is done including pre-metal dielectric deposition and planarization, contact etch, contact liner formation and contact formation, followed by metal wiring and final chip fabrication, all not shown. 
   Simulation results indicate that the tensile stress level in uncapped polySi increases by about 600 MPa to about 1200 MPa as a result of annealing for about one hour at a temperature of about 600° C., while the capped polySi increases in tensile stress by only about 10 MPa. Our simulation results show that about 33% to about 50% of the stress in the gate material (with the opposite sign with respect to the gate stress) can be translated into the channel region. Thus, the uncapped gate stack imparts−200 MPa to −300 MPa, while the capped gate stack translates little or no stress into the channel region. One simulation of stress contours is shown in the diagram of  FIG. 10 . 
   While there has been shown and described what is at present considered the preferred embodiments of the present invention, it will be apparent to those killed in the art that various changes and modifications may be made therein without departing from the spirit and scope of the present invention which shall be limited only by the appended claims.