Abstract:
A method of manufacturing an accumulation mode n-channel Silicon On Insulator (SOI) transistor includes forming an intrinsic silicon body region implanted with two deep Boron and one shallow Phosphorous implants; forming source/drain regions each implanted with Arsenic; and forming p-type regions adjacent each of the source and drain regions and disposed along the transistor channel. The SOI transistor has a higher transconductance than known SOI devices.

Description:
FIELD OF THE INVENTION 
     The present invention relates to semiconductor devices and more particularly to semiconductor on insulator devices. 
     BACKGROUND OF THE INVENTION 
     Integrated Circuits (IC) containing Semiconductor On Insulator (SOI) devices are becoming increasingly important due to their speed. An SOI device (i.e., transistor) is typically formed in a layer of semiconductor material overlaying an insulating layer formed in a semiconductor substrate. 
     A prior art SOI transistor includes a source region and a drain region which are separated from each other by a channel region. Both the source and drain regions are of the same conductivity type and are of opposite conductivity type to that of the body region. For example, when the body region is of a p-type material, the source and drain regions are of an n-type material. The source and drain regions typically have a higher dopant concentration level than the body region. 
     The transconductance of currently known SOI devices decreases as the supply voltage decreases. Therefore, a need exits for an SOI device which exhibits higher transconductance than SOI devices known in the prior art at low supply voltages. 
     SUMMARY OF THE INVENTION 
     An accumulation mode n-channel Silicon On Insulator (SOI) transistor, in accordance with one embodiment of the present invention, includes: an intrinsic silicon body region which contains two deep Boron and one shallow Phosphorous implants; source/drain regions each including Arsenic implant; p-type regions adjacent each of the source and drain regions, and disposed along the channel. 
     The following processing steps are carried out to make the SOI device, in accordance with one embodiment of the present invention. After forming a shallow trench isolation, the top silicon layer receives deep Boron and shallow Phosphorous implants through a thin layer of an insulating material (e.g., oxide). Thereafter, gate oxide is grown, polysilicon gate is formed and a zero-tilt Arsenic implant is made to form the source/drain regions of the device. After a rapid thermal anneal, a tilted channel implant delivers BF2 impurities through an insulating layer (e.g., oxide liner) to the channel, thus creating p-type regions adjacent each of the source and drain regions. Thereafter, a shallow phosphorous implant is delivered to the channel and through oxide spacers formed adjacent the polysilicon gate to form n-type regions near each of the source and drain regions. A deep Boron implant is then performed to prevent punch-through. Next, a pair of second oxide spacers are formed adjacent the first oxide spacers and the wafer is subsequently salicided using a conventional salicidation process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of the various layers of a silicon-on-insulator (SOI) wafer, used to make an accumulation mode n-Channel SOI device, in accordance with one embodiment of the present invention. 
     FIG. 2 is a cross-sectional view of the SOI wafer of FIG. 1, after the top silicon layer of the SOI wafer has been etched following oxidation and nitride deposition steps to create trench isolation. 
     FIG. 3 is a cross-sectional view of the SOI wafer of FIG. 2, following formation of oxide liners, deposition of plasma TEOS and polishing the resulting structure down to the top surface of the nitride layer. 
     FIG. 4 is a cross-sectional view of the SOI wafer of FIG. 3, after removal of the oxide and nitride layers. 
     FIG. 5 is a cross-sectional view of the SOI wafer of FIG. 4, after growing sacrificial oxide and carrying out deep Boron and shallow Phosphorous implants. 
     FIG. 6 is a cross-sectional view of the SOI wafer of FIG. 5, after removing the sacrificial oxide, growing gate oxide and depositing polysilicon. 
     FIG. 7 is a cross-sectional view of the SOI wafer of FIG. 6, after forming the polysilicon gate and forming oxide liners, nitride spacers, and source/drain regions. 
     FIG. 8 is a cross-sectional view of the SOI wafer of FIG. 7, after removing the nitride spacers and performing tilted channel implants to create p-type regions near each of the source and drain regions. 
     FIG. 9 is a cross-sectional view of the SOI wafer of FIG. 8, after forming oxide spacers and performing a shallow Phosphorous implant. 
     FIG. 10 is a cross-sectional view of the SOI wafer of FIG. 9, after performing a deep Boron implant. 
     FIG. 11 shows the various regions of the SOI wafer of FIG. 10 having n-type impurities. 
     FIG. 12 is a cross-sectional view of the SOI wafer of FIG. 10 after forming oxide spacers adjacent existing oxide spacers. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows the three layers  2 ,  3  and  4  of silicon-on-insulator (SOI) wafer  100 . Layer  2  is a p-type silicon substrate. Layer  3  is a silicon dioxide layer and has a thickness of approximately 2000 angstroms. Layer  4  is intrinsic silicon layer and has a thickness of approximately 1000 to 1200 angstroms. Wafer  100  is commercially available from a number of manufacturers. 
     The first step in making an n-channel enhancement mode device in one embodiment of the present invention is to make a trench isolation. To make a trench isolation (see FIG.  2 ), a silicon dioxide layer  5  with a thickness of, for example, 90 angstrom is grown over the surface of silicon layer  4 . Next, a silicon nitride layer  6  with a thickness of, for example, 1800 angstrom is deposited over oxide layer  5 . Thereafter, wafer  100  is masked and patterned using conventional masking and etching steps such that layer  4  is etched in all regions except in the areas approximately underneath layers  5  and  6 , thereby forming structure  105  as shown in FIG.  2 . 
     Thereafter, as shown in FIG. 3, using a high temperature dry oxidation process (e.g. 1100° C.), an oxide liner  7 , which typically has a thickness of 150 angstrom is grown. Oxide liner  7  reduces dislocation defects occurring near the surface of layer  4 . Thereafter, plasma TEOS  8  (Tetra Ethyl Ortho Silicate) layer  8  having a thickness of 5000-6000 angstrom is deposited across the entire wafer. Next, wafer  100  is polished down to the top surface of silicon nitride layer  6 , thereby, forming structure  110 , as shown in FIG.  3 . 
     Next, nitride layer  6  and oxide layer  5  are removed. As seen from the resulting structure  115  of FIG. 4, silicon layer  4  contains sharp corners inside perimeter lines  4   —   a  and  4   —   b.    
     To taper and thereby reduce the electric field near the sharp corners  4   —   a  and  4   —   b , a layer of sacrificial oxide  9  (typically less than 100 angstrom) is grown on top of the wafer (see FIG.  5 ). Thereafter, silicon layer  4  is subjected to a deep Boron implant (with a typical Boron concentration of 10 17  to 2×10 17  cm −3 ) through the sacrificial oxide layer  9 , thereby, forming p −  region  10  inside silicon layer  4 . Next, layer  4  receives a shallow Phosphorous implant to form n −  region  11  near the surface of silicon layer  4  to thereby form structure  120 , as shown in FIG.  5 . The energy used to implant Phosphorous is typically around 5 to 10 kilo-electron volts (Kev). The Phosphorous dose is typically between 2.5×10 12  to 7.5×10 12  cm −2 , which is high enough to convert region  11  from p to n −  conductivity type. 
     Next, as shown in FIG. 6, sacrificial oxide  9  is removed, and gate oxide  12  (with a thickness of e.g. 10-20 Å) is grown. During the gate oxidation process the implanted Phosphorous atoms advantageously move closer to the silicon surface. Thereafter, polysilicon layer  13  is deposited over the wafer, thereby forming structure  125 , shown in FIG.  6 . Polysilicon layer  13  has a typical thickness of, for example, between 1200 to 1700 Å. 
     Next, as shown in FIG. 7, using conventional masking and etching steps, polysilicon gate  13 , oxide liner  14  and nitride spacer  15  are formed. Subsequently, wafer  100  receives an Arsenic implant to form n +  source/drain regions  16 , thereby forming structure  130 , as shown in FIG.  7 . The arsenic implant is performed at zero tilt and has an energy of 10-30 Kev and a dose of 3−5×10 −15  cm −2 . Thereafter, the wafer is annealed using a rapid thermal annealing process at a temperature of approximately 1030-1060 degrees centigrade for a period of approximately 5-10 seconds. The anneal process activates the implanted Arsenic atoms and causes the junction between source/drain regions  16  and silicon layer  4  to move deeper into silicon layer  4 . 
     After the anneal process, nitride spacer  15  is removed by placing wafer  100  in hot Phosphoric acid. Next, as shown in FIG. 8, a tilted channel implant (TCI) is performed to implant silicon layer  4  with BF2 (Boron-Fluoride) as indicated by arrows  17 . A typical energy, total dose and the tilt angle of the TCI are respectively, 30-50 Kev, 4−6×10 13  CM −2  and 7-20°. The BF2 dose of 4−6×10 13  cm −2  is delivered during four tilt rotations. The TCI forms p region  18  in n −  region  11  of silicon layer  4 , thereby forming structure  135 , as shown in FIG. 8. P regions  18  each have a dopant concentration that is approximately four times greater than that of p −  region  10 . 
     Next, as seen in FIG. 9, using conventional processing steps, oxide spacers  19  which are typically 100 to 200 angstroms wide are formed. Thereafter, a shallow zero-tilt Phosphorous implant is made as indicated by arrows  20 . A typical energy, and dose of the shallow Phosphorous implant are, 3-7 Kev and 5×10 14  to 1.2×10 15  CM −2 , respectively. The shallow Phosphorous implant allow n +  regions  16  to extend under the gate oxide  12  by forming shallow n regions  21  near the surface of silicon layer  4 . The resulting structure  140  is shown in FIG.  9 . 
     Next, as seen in FIG. 10, a deep Boron implant is carried out as indicated by arrows  22  (e.g. 0 to 15° tilt angle) to prevent punch-through. The deep Boron implant has an energy of 25-35 Kev and a total dose of 5×10 12  to 1×10 13 . Next an RTA is performed for a period of 2-5 seconds at a temperature of 990-1010° C. The boundaries  23  of the deep Boron implant—following the anneal process—are shown in structure  145  of FIG.  10 . 
     FIG. 11, shows the various regions having n-type impurities in silicon layer  4 . Regions  16  have a relatively very high n-type doping concentration. Regions  21  have an n-type doping concentration that is smaller than those of regions  16  but larger than that of region  21 . Region  24  is lightly doped with n-type dopants. 
     Next, as shown in FIG. 12, oxide spacer  25  are formed using conventional processing steps. Thereafter, salicides are selectively formed on the surface of wafer  100 . 
     The exemplary embodiments of the invention disclosed above are illustrative and not limiting. Other embodiments of this invention are possible within the scope of the appended claims.