Abstract:
A method of fabricating a silicon integrated circuit on a glass substrate includes preparing a glass substrate; fabricating a silicon layer on the glass substrate; implanting ions into the active areas of the silicon layer; covering the silicon layer with a heat pad material; activating the ions in the silicon layer by annealing while maintaining the glass substrate at a temperature below that of the thermal stability of the glass substrate; removing the heat pad material; and completing the silicon integrated circuit.

Description:
FIELD OF THE INVENTION 
   This invention relates to silicon on glass fabrication technology, and specifically to a method of annealing a silicon-on-glass wafer without exceeding the thermal stability temperature of the glass. 
   BACKGROUND OF THE INVENTION 
   Prior art silicon-on-glass fabrication processes require bonding a silicon thin film to glass, while the device fabrication process is similar to that of SOI process, except the highest temperature is limited by the thermal stability of glass, which is generally 650° C. or less. At temperatures lower than 650° C., the implanted ions cannot thoroughly be activated. 
   Colace et al.,  Efficient high - speed near - infrared Ge photodetectors integrated on Si substrates , Applied Physics Letters, Vol. 76, No. 10, pp 1231–1233 (2000), describes a process whereby pure germanium may be grown directly on a silicon wafer and used in low-cost monolithic transceivers for optical communications applications. 
   SUMMARY OF THE INVENTION 
   A method of fabricating a silicon integrated circuit on a glass substrate includes preparing a glass substrate; fabricating a silicon layer on the glass substrate; implanting ions into the active areas of the silicon layer; covering the silicon layer with a heat pad material; activating the ions in the silicon layer by annealing while maintaining the glass substrate at a temperature below that of the thermal stability of the glass substrate; removing the heat pad material; and completing the silicon integrated circuit. 
   It is an object of the invention to provide a method that may thoroughly activate implanted ions without over heating a glass substrate. 
   Another object of the invention is to provide a method of RTA a silicon-on-glass substrate through use of a heat pad deposited on a silicon layer. 
   This summary and objectives of the invention are provided to enable quick comprehension of the nature of the invention. A more thorough understanding of the invention may be obtained by reference to the following detailed description of the preferred embodiment of the invention in connection with the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of the method of the invention. 
       FIG. 2  depicts a complete the front end process up to source/drain ion implantation. 
       FIG. 3  depicts the structure after deposition and selective etching of a germanium thin film 
       FIGS. 4 and 5  depicts a comparison of the activation of amorphous silicon on glass and silicon wafers. 
       FIG. 6  depicts the extinction coefficient of germanium and silicon, showing high absorption of germanium in the NIR range. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The method of the invention provides a method to activate implanted ions, such as channel, source and drain ions, without exceeding a temperature which will render a glass substrate thermally unstable. 
   Referring initially to  FIGS. 1 and 2 , the method of the invention is depicted generally at  10 . The method of the invention includes use of a glass substrate  12 , which is prepared  14  for a silicon oxide layer  16 , which is formed  18  on glass substrate  12  by PECVD, thereby coating glass substrate  12  with a layer of silicon oxide. A silicon thin film  18 , either strained, non-strained, amorphous, or polycrystalline, is bonded  22  onto silicon oxide coated glass substrate  16 / 12 . The thickness of silicon oxide layer  16  is between about 10 nm to 1000 nm, depending on the application. Silicon oxide layer  16  is required to enhance the adhesion of a silicon thin film  20  to the glass substrate, which is bonded  22  to silicon oxide layer  16 , to prevent out diffusion of glass elements, and to buffer the heat during activation of implanted ions. The thickness of silicon thin film  20  is between about 5 nm to 500 nm, depending on the circuit requirement. The silicon film at the field region is removed  24  and any active device areas isolated  26 , followed by channel  28  ion implantation  30 , gate oxidation  32 , gate sidewall oxide/nitride passivation  36 , gate electrode  38  formation  40 , ion implantation  42 , using any suitable state-of-the-art process, to provide a source  44  and a drain  46 . 
   After the front end IC fabrication and source/drain ion implantation, as shown in  FIG. 2 , a thin layer, e.g., between about 500 nm to 2000 nm, of germanium  48  is deposited  50  on the glass substrate. The germanium film is patterned and etched, such that only the active area of the device is covered with a germanium layer, as shown in  FIG. 3 . Optionally, a thin layer, e.g., between about 10 nm to 50 nm, of silicon oxide may be deposited prior to the germanium thin film deposition. The wafer is than annealed  52  in a rapid thermal annealing (RTA) chamber, wherein the wavelength of the heating light includes light in the IR and near IR region, although other wavelengths may be suitable. The germanium layer absorbs light, which increases the temperature of the germanium layer. The heat is then transferred to the silicon beneath the germanium blanket, thereby activating the implanted ions. 
   After the annealing process is completed, the germanium film is selectively removed, preferably by a wet etch process. Wet etching solutions are available which etch germanium but not silicon or SiO x , and include diluted or undiluted H 2 O 2 , Piranhe (H 2 SO 4 :H 2 O 2 ), SC1 (NH 4 OH:H 2 O 2 :H 2 O), SC2 (HCl:H 2 O 2 :H 2 O), and other solutions which are well known to those of ordinary skill in the art. 
   The resulting structure is the same as that of  FIG. 2 , except the implanted ions in silicon source, drain, and channel regions are activated. The RTA power and length of annealing depends on the thickness of the silicon film as well as the thickness of Ge film. After the germanium thin film is removed, passivation oxide is deposited, followed by photoresist deposition and patterning for contact etch and metallization, and the device is completed by state-of-the-art processes  56 . 
   A thorough annealing and activation of the implanted n+ and p+ ions in the source/drain requires a temperature which is beyond the thermal stability of the glass substrate. As a result, as described in connection with prior art processes, the resulting device is not able to reduce the resistivity of silicon at the source/drain regions to a satisfactorily low level for the performance of the transistor. Salicidation may be used to reduce the source/drain series resistance, however, if the activated doping density is too low, the contact resistance of the silicide will be too high. In addition, the resistance at the gap between silicide and the channel will be too high. Therefore, a high degree of activation of the implanted ions in the device is necessary to bring the performance of the device to an acceptable level. 
   Low power laser annealing may be used to activate the implanted ions in the source and drain region. However, the laser annealing process is time consuming and may over heat the glass in the field area. Selective laser heating of the active area is too slow to be economically feasible. 
   The method of the invention uses RTA with a heat pad, e.g., germanium layer  48 . The heat pad is heated during RTA process by absorbing light in the near-IR range. Heat from the heat pad passes into the silicon by conduction to activate the implanted ions in the silicon. The lamp power, usually from a tungsten halogen lamp, depends on the arrangement and lamp design of the RTA system, however, the power may be specified by comparison with a silicon wafer under the same power. The temperature of the silicon wafer may be determined as a reference. A typical temperature for an equivalent silicon wafer is in the range of between about 650° C. to 1000° C. The anneal time is between about 5 seconds to 30 minutes. 
   As an example, a 121 nm-thick amorphous silicon (a-Si) film was deposited on a TEOS coated glass wafer, where TEOS is tetraethylorthosilicate oxide, also know as oxane. The a-Si layer was implanted with phosphorus at 20 Kev for a dose of 2×10 15  cm −2 . This structure was coated with a thin TEOS layer and covered with about 1000 nm of germanium film. RTA annealing experiments demonstrated that the sheet resistance reduction of the a-Si on glass was more pronounce than a similar film deposited on an oxidized silicon wafer. This is shown in  FIG. 4  as a function of lamp power, and in  FIG. 5  as a function of anneal time. 
   The data clearly demonstrates the effectiveness of germanium layer  48  as a heat absorbing blanket. Proper RTA procedure are designed to maximize the heat input to the silicon region, while keeping the glass at a relatively lower temperature. An alternate method of the invention is to use a pulse RTA anneal, thereby annealing the sample with a short pulse and at a higher temperature, wherein the desired pulse repetition will supply enough thermal energy for activation, but will keep the glass at a temperature below 650° C. 
   Germanium demonstrates high light absorption in the near-IR region, as shown in  FIG. 6 . Glass and silicon oxide are very transparent to light having wavelengths in the 1000 nm to 2000 nm spectrum. When a silicon oxide coated glass substrate, having germanium mesa islands, is heated in a RTA furnace, the germanium islands are heated while there is only a minimal increase in the glass temperature. This phenomenon may be used to locally heat the silicon to activate implanted ions without over heating the glass substrate. 
   Thus, a method of fabricating silicon integrated circuit on glass has been disclosed. It will be appreciated that further variations and modifications thereof may be made within the scope of the invention as defined in the appended claims.