Abstract:
An improved Silicon-On-Insulator (SOI) device structure with a thin SOI silicon layer maintains excellent I off  DC characteristics without degrading device AC speed and characteristics. The device structure comprises double gate sidewall spacers including an inner polysilicon spacer and an outer dielectric (nitride or oxide) sidewall spacer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/260,484, filed on Jan. 9, 2001. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to semiconductor device structures, and in particular to ultra-thin Silicon-On-Insulator (SOI) devices structures. 
     BACKGROUND OF THE INVENTION 
     As integrated circuit dimensions decrease and circuit speed increases, new transistor structures have been developed in order to yield good performance at the smaller dimensions. In particular, Silicon-On-Insulator (SOI) devices are known and are generally undoped or very lightly doped silicon on a low K dielectric. SOI devices are characterized by having the active device region insulated from the bulk substrate, generally by a buried oxide layer. The active device region is thereby said to be floating. SOI devices have been developed which consume less power than do bulk CMOS devices, and which operate at higher speeds than do Ct bulk CMOS devices. FIG. 1 shows a prior art SOI device, including bulk substrate  2 , buried oxide layer  4 , SOI silicon layer  6 , field oxide regions  7 , gate dielectric layer  8 , conducting gate  10 , gate sidewall spacers  11 , doped source and drain regions  12 , and channel region  14 . Source and drain regions may overlap the gate region, or gate sidewall spacer technology may be used to provide separation, or underlap, between the gate and the source-drain regions. 
     For SOI devices having channel lengths below about 50 nm, it is very difficult to achieve good short-channel control, i.e., to effectively shut off the transistors in the off state, without significantly thinning down the thickness of the buried layer and the thickness of the SOI silicon layer, which is technically very challenging. For a device with SOI silicon thickness of less than 20 nm, an underlap of the source/drain regions with the gate is needed in order to be able to turn off the device. Accordingly, the details of the gate sidewall spacer technology used in the fabrication of such devices are critical to their performance. By way of example, it is known that if doped polysilicon spacers are used in place of nitride spacers (termed a “straddled gate device”), the device DC characteristics improve significantly. The polysilicon spacer which is also doped during source/drain implant serves as a side gate with a lower work function It behaves like a longer gate when there is no bias applied on the gate (i.e., in the I off  condition). The polysilicon side gate causes the surface beneath it to invert at much lower applied voltage than the voltage necessary to invert the main channel region, due to the lower work function of the polysilicon. This causes the device to behave like a very short channel device during I on  conditions. The result is a much improved I on  and I off . However, when single layer polysilicon gate sidewall spacers are used, the source/drain extension regions reach under the poly spacers and cause an increased overlap capacitance which slows down the AC device performance. 
     BRIEF SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a Silicon-On-Insulator device structure with a thin SOI silicon layer which maintains excellent I off  DC characteristics without degrading device AC speed and characteristics. 
     These objects are met by providing double gate sidewall spacers including an inner polysilicon spacer and an outer dielectric (nitride or oxide) sidewall spacer. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows a prior art SOI device structure. 
     FIG. 2 a  shows the intermediate inventive device structure having polysilicon gate sidewall spacers. 
     FIG. 2 b  shows the intermediate inventive device structure having inner dielectric spacers and extension implanted regions. 
     FIG. 2 c  shows the final inventive device structure having outer dielectric spacers and source/drain implanted regions. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 2 a - 2   c  show the inventive device structures. FIG. 2 a  shows buried oxide layer  16 , usually undoped SiO 2  with thickness of approximately 50-60 nm, atop bulk substrate  18 . Thin undoped silicon layer  20  of the SOI has a thickness of approximately 5-20 nm. Gate dielectric  22  is comprised of silicon dioxide or alternately silicon nitride, aluminum oxide, tantalum pentoxide or hafnium oxide with equivalent oxide thickness of 0.8-1.4 nm. Conducting metal gate  24  is deposited and patterned from TiN, TaN, TaW. W. Al, Ni, Ta, Mo, or Cr, and has thickness of approximately 2.5-25 nm, with gate length of 30-60 nm. Polysilicon encapsulation layer  26  with thickness of 50-100 nm is deposited on and patterned with gate  24 . (Encapsulation layers in general are necessary to prevent cross-contamination of the fab line during post-gate formation processing.) 10-15 nm polysilicon is deposited and anisotropically etched to form poly spacers  28  which when doped function as side gates. 
     FIG. 2 b  shows first dielectric spacers  30 , formed by depositing 10-20 nm silicon nitride or silicon dioxide followed by anisotropic etch. The dielectric spacer etch removes the exposed portions of gate dielectric  22 . Extension implanted regions  32  are formed following formation of first dielectric spacers  30 . The extension implants may be comprised of 1-2e14/cm 2  BF 2  at 10-15 keV for p-channel, or 0.5-2e14/cm 2  As at 3-5 keV for n-channel, by way of example. RTA anneal for 5-10 seconds at 900-950 C follows. 
     FIG. 2 c  shows second dielectric spacers  34 , formed by depositing 50-90 nm of silicon nitride or silicon dioxide followed by anisotropic etch. Sidewalls  36  of spacers  34  may be vertical or sloped. Source/drain implanted regions  38  are formed following formation of second spacers  34 . Polysilicon spacers  28  are also doped during source/drain implantation. The source/drain implants may be comprised of 1-2e15/cm 2  of B at 2-3 keV for p-channel, or 1-2e15/cm 2  of P at 7.5-12.5 keV for n-channel, by way of example. RTA anneal for 5-10 seconds at 950-1025 C follows. Formation of approximately 5 nm of nickel silicide may follow. 
     My inventive structure, which comprises double spacers, one set of polysilicon spacers, and at least one set of dielectric spacers, enables the source/drain extension implanted regions to be moved away from under the doped polysilicon side gate, thereby reducing overlap capacitance. Device simulations of the inventive device structure using the Medici simulation program by Avanti show that the DC I off  characteristics are comparable to those achieved with only polysilicon spacers; the I on  characteristics are within 2-3% of those achieved with only polysilicon spacers. The AC characteristics, specifically the inverter ring oscillator stage delay, are significantly improved compared to a device having only polysilicon spacers. The AC characteristics are within 10% of the values achieved with devices having only silicon nitride spacers, but with much improved short channel control. 
     It is not intended that the invention be restricted to the exact embodiments described herein. For example, the processing details, including temperatures, times, implant energies and doses, and exact metal and dielectric materials used, may be altered without departing from the inventive concept. Additionally, the dielectric spacers may be comprised of a single first set of oxide or nitride spacers rather than the first and second dielectric spacers disclosed herein. The scope of the invention should be construed in view of the claims. 
     With this in mind,