TECHNICAL FIELD 
     The present invention relates generally to silicon-on-insulator (SOI) structures, and more specifically to a fully depleted SOI structure formed on a silicon-on-insulator substrate which includes tungsten damascene contacts. 
     BACKGROUND OF THE INVENTION 
     Conventional or bulk semiconductor devices are formed in semiconductive material by implanting a well of either P-type or N-type conductivity silicon in a silicon substrate wafer of the opposite conductivity. Gates and source/drain diffusions are then manufactured using commonly known processes. These form devices known as metal-oxide-semiconductor (MOS) field effect transistors (FETs). When a given chip uses both P-type and N-type, it is known as a complimentary metal oxide semiconductor (CMOS). Each of these transistors must be electrically isolated from the others in order to avoid shorting the circuits. A relatively large amount of surface area is needed for the electrical isolation of the various transistors. This is undesirable for the current industry goals for size reduction. Additionally, junction capacitance between the source/drain and the bulk substrate and “off” state leakage from the drain to the source both increase power consumption. Junction capacitance also slows the speed at which a device using such transistors can operate. These problems result in difficulties in reducing the size, power consumption, and voltage of CMOS technology devices. 
     In order to deal with the junction capacitance and “off state” leakage problem as well as obtain reduced size, silicon-on-insulator technology (SOI) has been gaining popularity. SOI technology employs a thin silicon device layer of monocrystalline silicon material overlying an insulating layer on a bulk wafer. The structure can be formed by a number of well-known techniques, such as zone melting and recrystallization (ZMR), separation by implanted oxygen (SIMOX), or Bonded and Etchback (BESOI). 
     Field effect transistors fabricated in the silicon device layer have many advantages over bulk silicon FETs fabricated in the traditional bulk silicon substrates including resistance to short-channel effect, steeper subthreshold slopes, increased current drive, higher packing density, reduced parasitic capacitance, and simpler processing steps. 
     These advantages combined with the continually increasing cost of bulk silicon submicron integrated circuit processes and the lower complexity/cost of SOI integrated circuit processes, SOI technology shows great potential to become the low cost mainstream production technology. 
     Despite the advantages, there are problems with SOI technology which limit its performance. Unlike bulk silicon FETs, the body of an SOI FET is usually electrically floating. In a non-fully depleted FET (e.g. the silicon film thickness is greater than the maximum channel depletion width), carriers (holes in n-channel FETs and electrons in p-channel FETs) generated by impact ionization accumulate near the source/body junction of the FET. Eventually sufficient carriers will accumulate to forward bias the body with respect to the source thus lowering the threshold voltage through the body-bias effect. Extra current will start flowing resulting in a “kink” in the I-V characteristics. This reduces the achievable gain and dynamic swing in analog circuits, and gives rise to abnormality in the transfer characteristics in digital circuits. 
     In a fully-depleted SOI FET (e.g. silicon film thickness that is less than the maximum channel depletion width), the channel is depleted completely under normal operations. The source/channel junction has a lower potential barrier, and the carriers generated by impact ionization have smaller effect on the body and channel potential, thus the “kink” softens. 
     However, in fully-depleted FETs, the depletion charge is reduced for a given body doping concentration, leading to a smaller threshold voltage. Threshold voltage becomes very sensitive to variations in the silicon film thickness and therefore, fabrication of high performance circuits can be very difficult. Additionally, the reduction of silicon film thickness in a fully-depleted FET gives rise to high source/drain series resistance which in turn lowers the operating speed of the device. One solution to the series resistance problem is to selectively reduce the silicon film thickness over the channel region. However, the resulting recessed region and the polysilicon gate are not automatically aligned. To allow for the possible misalignment, the recessed thin silicon region must be made longer than the gate. This reduces the device performance and density, and results in asymmetrical devices. 
     Accordingly, there is a strong need in the art for a semiconductor circuit structure, and a method for forming such structure, that includes a fully depleted channel to eliminate the “kink” in the I-V curve but does not suffer the problems of poor output resistance and slowed operating speed. Further, there a need in the art for such a device which does not suffer possible misalignment caused by fabrication processes which do not automatically align the gate, source and drain regions of the FET. 
     SUMMARY OF THE INVENTION 
     A first aspect of the present invention is to provide a fully depleted field effect transistor (FET) with a tungsten damascene top gate. The FET is formed in a silicon on insulator substrate which includes a thin device layer positioned over an insulating layer. The FET comprises a body region formed in the silicon device layer and including a source region and a drain region of a first conductivity semiconductor on opposing sides of a central channel region of an opposite conductivity semiconductor. A base gate region is positioned above the central channel region and a wide top gate region comprised of tungsten damascene is spaced apart from the central channel region and positioned over the base gate region. The tungsten damascene forms a contact between the base gate region and metal layers of an integrated circuit utilizing the FET. 
     In the preferred embodiment, the central channel region is an inverted T-shaped central channel region with a narrow full depletion region adjacent the base gate region and a wide channel region adjacent the insulating layer. The narrow full depletion region may have a channel width that is less than a width of the base gate region and the wide channel region may have a width approximately equal to a width of the top gate region. 
     A second aspect of the present invention is to provide a semiconductor device comprising a plurality of fully depleted field effect transistors formed on a semiconductor substrate. Each fully depleted FET comprises: a) a body region including a source region and a drain region of a first conductivity semiconductor on opposite sides of a central channel region of an opposing conductivity semiconductor; b) an insulating layer positioned below the body region isolating the body region from the semiconductor substrate; c) a base gate region positioned above the central channel region; and d) a wide top gate region comprised of tungsten damascene and spaced apart from the central channel region. 
     The central channel region of each transistor may be an inverted T-shaped central channel region with a narrow full depletion region adjacent the base gate region and a wide channel region adjacent the insulating layer. The narrow full depletion region of each transistor may have a-channel width that is less than a width of the base gate region and the wide channel region of each transistor may have a channel width approximately equal to a width of the wide top gate region. 
     The semiconductor device may further include a plurality of isolation trench regions isolating the body region of each transistor from adjacent transistors and the substrate may be a semiconductor of the same first conductivity semiconductor as the central channel region. 
     A third aspect of the present invention is to provide a method of fabricating a fully depleted field effect transistor with a tungsten damascene top gate region. The fully depleted FET is formed in a silicon on insulator substrate including a thin device layer of a first conductivity semiconductor positioned over an insulating layer. The method comprises: a) isolating a body region in the device layer; b) forming a base gate region above the body region; c) doping a thin top portion of the body region on opposing sides of the base gate region to an opposite conductivity semiconductor as the first conductivity semiconductor; d) forming a layer of nitride over the base gate region and the body region of the device layer such that the nitride has a vertical thickness adjacent to the base region that is thicker than a vertical thickness across the surface of the body region; e) doping a deep portion of the body region on opposing sides of the base gate region to the opposite conductivity silicon to form an inverted T-shaped central channel region of the first conductivity semiconductor positioned between a source region and a drain region of the opposite conductivity semiconductor; and f) forming a wide tungsten damascene top gate region above the base gate region. 
     The step of forming the base gate region may include forming a polysilicon layer over the body region; forming a mask layer over the polysilicon layer; patterning the mask layer to define the base gate region; and etching the mask layer and the polysilicon layer to form the base gate region. The step of forming the wide tungsten damascene top gate region may include depositing a layer of TEOS over the layer of nitride; polishing the TEOS to expose an island of nitride over the base gate region; etching the nitride to form a well extending to the base gate region; and forming a diffusion barrier over the exposed base gate region and filling the well with tungsten damascene from the wide tungsten damascene top gate region. 
     The step of doping the deep portion may include implanting impurity ions utilizing a 15-25 KeV electric field and the step of doping the thin top portion may include implanting impurity ions utilizing a 10-20 KeV electric field. 
     The method of fabricating a fully depleted field effect transistor with a tungsten damascene top gate region may further include forming source and drain tungsten damascene contacts. Forming such contacts may include: a) etching the nitride to form a well over each of the source region and the drain region to expose the source region and the drain region; b) forming a titanium nitride diffusion layer over the exposed source region and drain region; and c) filling each of the wells with tungsten damascene to from the tungsten damascene contacts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross section diagram of a fully depleted field effect transistor (FET) in accordance with one embodiment of this invention; 
     FIG. 2 is a flow chart showing exemplary steps in fabricating the FET of FIG. 1; 
     FIG. 3 a  is a cross section diagram of showing the structure of the FET of FIG. 1 at a partially complete fabrication step; 
     FIG. 3 b  is a cross section diagram of showing the structure of the FET of FIG. 1 at a partially complete fabrication step; 
     FIG. 3 c  is a cross section diagram of showing the structure of the FET of FIG. 1 at a partially complete fabrication step; 
     FIG. 3 d  is a cross section diagram of showing the structure of the FET of FIG. 1 at a partially complete fabrication step; 
     FIG. 3 e  is a cross section diagram of showing the structure of the FET of FIG. 1 at a partially complete fabrication step; 
     FIG. 3 f  is a cross section diagram of showing the structure of the FET of FIG. 1 at a partially complete fabrication step; 
     FIG. 3 g  is a cross section diagram of showing the structure of the FET of FIG. 1 at a partially complete fabrication step; 
     FIG. 3 h  is a cross section diagram of showing the structure of the FET of FIG. 1 at a partially complete fabrication step; 
     FIG. 3 i  is a cross section diagram showing the structure of the FET of FIG. 1 at a partially complete fabrication step; and 
     FIG. 3 j  is a cross section diagram showing the structure of the FET of FIG. 1 at a partially complete fabrication step. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will now be described in detail with reference to the drawings. In the drawings, like reference numerals are used to refer to like elements throughout. 
     Referring to FIG. 1, a fully depleted silicon on insulator (SOI) field effect transistor (FET)  10  with a wide tungsten damascene gate cap is shown. FET  10  is formed in an SOI wafer  28  which includes a thin silicon device layer  26  formed on the top of an insulating oxide layer  22  which is on top of a bulk silicon substrate  24 . The FET  10  includes an active region  18  which comprises a source region  12 , a drain region  14 , and a channel region  16 . In the exemplary embodiment, the FET  10  is an n-type FET which includes an n-conductivity silicon source region  12  and an n-conductivity silicon drain region  14  separated by a p-conductivity silicon channel region  16 . However, in accordance with known semiconductor technology, the channel region  16  may be n-conductivity silicon while each of the source region  12  and drain region  14  may be p-conductivity silicon. 
     The active region  18  is isolated from the bulk silicon substrate  24  by the insulating oxide layer  22 . Insulating trenches  20  which extend through the silicon device layer  26  to the insulating oxide layer  22  isolate the active region  18  of FET  10  from other devices (not shown) that may be formed in the silicon device layer  26 . A gate oxide layer  30  is on the top surface of the active region  18  and isolates the active region  18  from a gate  32 . The gate  32  includes a polysilicon base gate region  34  adjacent to the gate oxide layer  30  and a tungsten damascene wide cap region  36 . The tungsten damascene wide cap region  36  is positioned to the top and sides of the base gate region  34  and forms a contact coupling the base gate region to the metal layers  38 . A thin titanium nitride diffusion barrier  40  prevents diffusion between the tungsten damascene cap region  36  and the base gate region  34 . 
     The channel region  16  includes an upper region  42  adjacent to the gate oxide layer  30  and a lower region  44  adjacent to the insulating oxide layer  22 . In the preferred embodiment, the upper region  42  has a width of less than 20 nm and the lower region  44  has a width of approximately 50 nm. The silicon device layer  26  has a thickness of 20-30 nm and the insulating oxide layer is approximately 30-70 nm thick. 
     It should be appreciated that the upper region  42  has a width less than the thickness of the silicon device layer  26  while the lower region  44  has a width greater than the thickness of the silicon device layer  26 . 
     As such, FET  10  is a combination of both a short channel and a long channel device. While in the “on” state (e.g. gate  32  is charged), the upper region  42  may fully deplete to enable the FET  10  to behave as a fully depleted FET to eliminate the “kink” in the I-V curve. However, while in the “off” state, the device behaves as a long channel device eliminating off-state current leakage. 
     FIG. 2 shows a flow chart of exemplary processing steps for forming the FET  10  structure of FIG.  1 . FIGS. 3 a  to  3   j  each show a cross section of the structure of the FET  10  at various partially complete processing steps as described in the flowchart of FIG.  2 . Therefore, referring to FIG. 2 in conjunction with FIGS. 3 a  to  3   j , fabrication of FET  10  is shown. 
     Step  50  represents forming the SOI wafer  28  which can be forming using techniques known in the art such as zone melting and recrystallization (ZMR), separation by implanted oxygen (SIMOX), or Bonded and Etchback (BESOI). FIG. 3 a  shows the SOI wafer  28  with the silicon device layer  26 , approximately 20-30 nm thick, above the insulating layer  22 , approximately 30-70 nm thick, above the bulk silicon  24 . 
     Step  52  represents isolating the active region  18  by forming insulating trenches  20  in the silicion device layer  26  as shown in FIG. 3 b . The insulating trenches  20  may be formed using known etch, fill, and polish techniques. More specifically, a silicon nitride mask may be formed to cover and protect the substrate in the area where the active region  18  of FET  10  is to be formed while leaving the area where the insulating trenches  20  are to be formed exposed. More specifically, the silicon nitride mask may be formed by depositing a layer of silicon nitride approximately 1,500-2,000A thick over an oxide on the surface of the silicon device layer and patterning and etching such silicon nitride layer using known photolithography techniques. Thereafter, the unmasked portions of the silicon device layer  26  (e.g. the portions corresponding to the insulating trenches  20  where the silicon nitride mask has been etched away) are etched away form open trenches through the silicon device layer  26  to the insulating oxide layer  22 . More specifically, the etching process for the silicon device layer may include an anisotropic dry etch using hydrogen bromide (HBr). Thereafter, the open trenches are filled with silicon dioxide to form insulating trenches  20 . Filling the open trenches preferably uses any known technique for forming silicon dioxide in an open trench such as filling the trench with SiH4 or TEOS and subsequently oxidizing either of such compounds to form insulating silicon dioxide. After filling the open trenches, the surface of the wafer  28  is polished using a chemical mechanical polish to remove any excess silicon dioxide layer and the remaining silicon nitride mask and yield an isolated active region  18  in the silicon device layer  26  as shown in FIG. 3 b.    
     Step  54  represents forming the polysilicon base region  34  of the gate  32 . Referring to FIG. 3 c , a gate oxide layer  30  is formed on the surface of the silicon device layer  26  over the active region  18 . Thereafter, the base gate region  34  of the gate  32  is formed on the surface of the gate oxide layer  30  to define the upper region  42  of the channel region  16  in a conventional CMOS self aligned gate, source, and drain process. The gate oxide layer  30  is typically grown on the surface of the active region  18  using a thermal oxidation process and a polysilicon layer is deposited on top of the gate oxide layer  30  using a low pressure chemical vapor deposition (LPCVD) process. The polysilicon layer is pre-doped by phosphorus (4e15 cm-2, 15 KeV) and annealed for 10 seconds at 950C in RTA. The polysilicon layer is then patterned and etched using conventional photolithography techniques to form the base gate region  42 . 
     Thereafter, at step  56 , halo implants are formed in the active region  18  by implanting boron (1-3e13 cm-2, 1-2 KeV) and annealed for 1 second at 950C in Nitrogen and portions of the silicon substrate on opposing sides of the base gate region  34  to form low dose extension of both the source region  12  and the drain region  14  with phosphorus (5e13-1e14 cm-2, 3-5 Kev) at zero degree tilt. These low does extensions do not extend through the entire thickness of the silicon device layer  26 . Because the ions cannot penetrate the polysilicon gate, the polysilicon gate effectively operates as a mask which results in doping only the exposed source region  12  and drain region  14  as shown in FIG. 3 c.    
     Step  58  represents applying an oxide layer  82 , approximately 10 nm thick, and a blanket silicon nitride layer  84 , approximately 40-60 nm thick over the base gate region  34  and over the entire surface of the wafer  28  as shown in FIG. 3 d . It should be appreciated that the thickness of the silicon nitride layer  84  extends to a thickness equal to or greater than the thickness of the base gate region  34  adjacent to the base gate region  34 . As such, at. step  60 , when the source region  12  and drain region  14  are further doped by implanting phosphorus (2-3e15 cm-2, 10-20 KeV) annealing for 5 to 10 seconds at 1020-1040C, the doping is masked by both the base gate region  34  and the thick regions of the nitride layer  84  adjacent to the base gate region  34 . Therefore, the implant of this step  60  forms the high does regions of the source region  12  and the drain region  14  and leaves the wide lower channel region  44  un-doped as shown in FIG. 3 e.    
     At step  62 , a layer of TEOS  86  is applied to cover the silicon nitride layer  84  as shown in FIG. 3 f . Thereafter, at step  64 , the surface is polished to a flat surface  88  wherein the nitride  84  is exposed as an island within the TEOS  86  above the base region  34  as shown in FIG. 3 g.    
     Step  66  represents etching the nitride  84  above the base region  34  to a depth wherein the base region  34  is exposed in a well within the TEOS  86  as is shown in FIG. 3 h . Preferably, the etch is a wet etch using phosphoric acid and the etch is deep enough such that the nitride  84  is recessed 30-60 nm below the top of the base region  34 . 
     At step  68 , a contact region on the surface of each of the source reason  12  and the drain region  14  are exposed by etching wells  88  in the TEOS  86 , nitride layer  84 , oxide layer  82 , and gate oxide  30 . Known photolithography processes are used to mask and pattern the wells  88  and chemicals of appropriate selectivity are used to perform the etching. 
     Step  70  represents depositing a titanium nitride diffusion layer  40  on the exposed silicon regions of the source region  12 , drain region  14  and polysilicon base gate region  39  as shown in FIG. 3 j . Thereafter, referring again to FIG. 1, tungsten damascene is deposited in each of the wells  88  to form the source and drain contacts coupling the source region  12  and the drain region  14  to the metal layers  38  and into the wide cap region to form the tungsten damascene wide cap region of the gate  32   
     These processing steps described with reference to the flowchart of FIG. 2 in combination with the diagrams of FIGS. 3 a  through  3   j  yield the fully depleted FET  10  as described with reference to FIG.  1 . 
     It should be appreciated that the FET  10  structure and processes of fabricating such a semiconductor result in a unique field effect transistor structure which behaves as a fully depleted FET when in the “on” state to reduce “kink” in the I-V curve yet behaves as a wide channel device in the “off” state to reduce off-state current leakage. 
     Although the invention has been shown and described with respect to certain preferred embodiments, it is obvious that equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalents and modifications, and is limited only by the scope of the following claims.
A fully depleted field effect transistor formed in a silicon on insulator (SOI) substrate includes a body region formed in a silicon device layer over an isolation layer of the SOI substrate. A gate is positioned above the body region and includes a base gate region adjacent the body region and a wide top gate region formed of tungsten damascene and spaced apart from the body region. An inverted T-shaped central channel region is formed between adjacent source regions and drain region in the body region.