Abstract:
A method is taught for forming shallow LDD diffusions using polysilicon sidewalls as a diffusion source. The polysilicon sidewalls are formed along side squared-off silicon nitride sidewall spacers which have an essentially rectangular cross section and are in direct contact with the subjacent silicon wherein the shallow LDD elements are formed. The method is applied to the formation of a p-channel MOSFET with salicide contacts wherein the polysilicon sidewalls can be made full size because the essentially flat tops of the nearly rectangular silicon nitride sidewalls provide ample gate-to-source drain spacing to prevent silicide bridging and thereby reduce gate-to-source/drain shorts. In addition, the squared-off silicon nitride sidewalls are formed with parallel vertical sides. This permits improved control of their width, reduced lateral encroachment of boron dopant under the gate, and reduced gate-to-source drain silicide bridging. The reduced boron encroachment results in reduced source/drain series resistance as well as inhibition of short channel effects in the p-channel MOSFET. The full sized polysilicon sidewall also permits greater silicide contact area which fosters results in better contact to the LDD elements and overall lower contact resistance. The source/drain contacts are made not only to the single crystalline silicon of the main source/drain regions, as in conventional contacts, but also directly to the LDD region through the large area silicide region over the polysilicon sidewall. The process is applied to the formation of a CMOS structure having shallow, high concentration, p-type LDD elements on the p-channel device.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The invention relates to processes for the manufacture of semiconductor devices and more particularly to processes for forming self-aligned polysilicon gate field effect transistors. 
     (2) Background to the Invention and Description of Related Art 
     Complimentary metal oxide semiconductor(CMOS) field effect transistor(FET) technology involves the formation n-channel FETs(NMOS) and p-channel FETs(PMOS) in combination to form low current, high performance integrated circuits. The complimentary use of NMOS and PMOS devices, typically in the form of a basic inverter device, allows a considerable increase of circuit density of circuit elements by reduction of heat generation. The increase in device density accompanied by the shrinkage of device size has resulted in improved circuit performance and reliability as well as reduced cost. For these reasons CMOS integrated circuits have found widespread use, particularly in digital applications. 
     The basic MOSFET (Metal Oxide Semiconductor Field Effect Transistor), whether it be an NMOS or PMOS is typically formed by a self-aligned polysilicon gate process wherein source and drain regions are formed adjacent to the polysilicon gate by ion implantation using the gate as a mask. The source/drain is thereby self-aligned to the gate electrode. A channel region directly under the polysilicon gate is thereby also defined by the gate electrode. In order to reduce hot electron injection into the channel region, a low concentration of source/drain dopant is first implanted with the gate as a mask. This is commonly referred to as a lightly doped drain (LDD) implant. Sidewalls are then formed alongside the gate electrode and a second substantially higher dosage implant is then applied to form the main source/drain regions which are spaced laterally away from the edge of the polysilicon gate by the sidewall thickness. The completed source/drain regions then each consist of a main heavily doped portion to which external contact is made and a lightly doped (LDD) portion next to the channel region. 
     As device dimension continue to shrink, short channel effects become significant and begin to affect device performance. In conventional LDD processes short channel effects are compensated by implanting shallower junctions which come at the expense of high impurity concentrations. As a consequence, the resultant lower impurity concentrations cause undesirably high source and drain series resistance. It would be beneficial to be able to form shallow LDD regions with high impurity concentration. The process of the present invention, which introduces LDD dopant from a silicon spacer, overcomes these shortcomings which are characteristic of ion implanted LDD regions. 
     Diffusion of dopants into active regions, in particular LDD regions, using a polysilicon spacer as a dopant source is not new. However the method of the present invention overcomes some of the shortcomings of the prior methods. Wu, U.S. Pat. No. 6,136,636 discloses a method for simultaneously forming source/drain and LDD regions by ion implanting dopant ions into a substrate where the LDD regions are covered by an amorphous silicon layer on a pad oxide. The source/drain regions are uncovered and therefore receive a greater dose than the covered LDD regions. The amorphous silicon layer is then oxidized and the dopant implanted therein is driven into the substrate through the subjacent pad oxide. The pad oxide also forms a sidewall on the polysilicon gate. Thus the pad oxide sidewall determines the final spacing of the LDD edge with respect to the edge of the gate electrode. There are several disadvantages of this technique. One disadvantage is that the final spacing of the LDD edge and the LDD diffusion are both dependent upon the pad oxide thickness and thereby also upon each other. Although shallow LDD regions may be formed by this method, the dopant concentration and profile is nevertheless determined by the non-uniform implanted dopant profile which is further dispersed after transit through the pad oxide resulting in a weakened or less abrupt dopant profile. In addition, the segregation coefficients of the dopants between oxide and silicon are different for—and p-type dopants. 
     In a related patent, Wu, U.S. Pat. No. 5,930,617 diffuses nitrogen from an amorphous silicon layer (not a sidewall) through an intermediate pad oxide to form shallow LDD regions. The diffusion takes place concurrently with the oxidation of the amorphous layer. While slightly different than Wu &#39;636, the method suffers from the same disadvantages cited above. 
     Gardner, et.al., U.S. Pat. No. 5,710,054 shows a method for diffusing shallow LDD regions from doped polysilicon spacers wherein the spacers are separated from the main polysilicon gate electrode by an oxide spacer which is thermally grown on the gate electrode. Not only is the oxide spacer difficult to form because it is trimmed by wet etching, but also the structure is subject to gate-to-source/drain shorts/leakage from the lower edge of the polysilicon gate to the polysilicon spacer. Further, if contacts are later formed by a salicide process, gate-to-source drain shorts are likely to occur by silicide bridging over the thin oxide from the upper edge of the gate electrode to the polysilicon sidewall. In another embodiment of the same reference an oxide sidewall spacer is formed, using conventional spacer formation methodology, alongside the gate electrode to control the lateral diffusion from the polysilicon sidewall source towards the channel region. This conventional spacer presents a high risk of silicide bridging during contact formation because of it&#39;s narrow upper section. 
     Son, U.S. Pat. No. 5,759,885 shows the formation of a CMOS structure wherein n-type LDD regions are formed for both the n-channel and p-channel devices by diffusing phosphorous from doped polysilicon spacers into the silicon substrate through an intermediate oxide layer. The p-type source/drain regions of the p-channel device are formed by out diffusion from a blanket BSG (borosilicate glass) layer. Like Wu, the method of Son has the disadvantages of diffusing LDD dopant through an intermediate oxide. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a method for forming shallow LDD diffusions. 
     It is another object of this invention to provide a method for forming a CMOS having shallow p-type LDD elements on the p-channel device. 
     It is yet another object of this invention to provide a method for forming an ultra narrow diffusion source which is in direct contact with the subjacent silicon which is to be the diffusion recipient. 
     It is still another object of this invention to provide a method for forming an ultra narrow silicon diffusion source positioned alongside a polysilicon gate electrode and spaced therefrom by an essentially rectangular dielectric spacer. 
     It is still another object of this invention to provide a method for forming an ultra shallow, high concentration diffused element in a silicon substrate. 
     It is another object of this invention to provide a method for reducing source/drain series resistance of a polysilicon gate MOSFET. 
     It is another object of this invention to provide a method for reducing lateral encroachment of boron dopant under the gate of a p-channel MOSFET. 
     It is another object of this invention to provide a method for reducing short channel effects in a p-channel MOSFET. 
     It is yet another object of this invention to provide a method for reducing gate-to-source/drain shorts in MOSFETs formed by a salicide process. 
     It is still another object of this invention to provide a method for forming a source/drain contact which is made to both to the single crystalline silicon of the main source/drain region but also to the LDD region. 
     These objects are accomplished by forming essentially rectangular silicon nitride sidewalls alongside a polysilicon gate stack. The gate stack comprises a gate oxide, a polysilicon gate over the gate oxide and a silicon oxide top cap. The silicon nitride sidewall spacer is made essentially rectangular by a CMP (chemical mechanical planarization) and etching process. 
     Amorphous silicon sidewalls are then formed alongside the nitride sidewalls by conventional sidewall practices wherein a blanket silicon layer is deposited. The layer is then anisotropically etched leaving a silicon sidewall which is spaced laterally away from the polysilicon gate by the thickness of the nitride spacer. The silicon sidewalls, after implantation with dopant ions, serves as a diffusion source to form an LDD region in the subjacent active single crystalline silicon. A combination of furnace and rapid thermal annealing (RTA) is then used to drive the implanted dopant into the silicon surface and crystallizing and activating the silicon sidewall. Because the diffusion source is in direct contact with the active silicon, the LDD regions can be made shallow and the junctions abrupt. Such a junctions result in improved device performance and are relatively immune to short channel effects as compared to conventionally formed LDD junctions. In addition, there is less encroachment of the shallow LDD region under the gate electrode. 
     LDD junctions formed using the silicon source according to the teaching of this invention are shallower and permit higher drive currents than those formed using conventional BSG sources. While LDD diffusion from a BSG source results in much less tailing than conventional ion implantation, the tailing from the doped silicon source outlined in this invention is even less than that from the BSG source. This results in a more abrupt junction and still higher performance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A through 1F are cross sections showing the process steps for the formation of MOSFET according to a first embodiment of this invention. 
     FIGS. 2A through 2H are cross sections showing the process steps for the formation of a complimentary MOSFET pair (CMOS) according to a second embodiment of this invention. 
     FIGS. 3A through 3F are cross sections showing the process steps for the formation of a squared-off silicon nitride sidewall spacer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In a preferred embodiment of this invention a p-type monocrystalline &lt;100&gt; oriented silicon wafer is provided. The first embodiment addresses the formation of a polysilicon gate MOSFET by a salicide process. Referring to FIG. 1A there is shown a cross section of an in-process, self-aligned polysilicon gate MOSFET on the silicon wafer  10  at a point in the process wherein a polysilicon gate stack  8  has been patterned by anisotropic dry etching. The dry etching may be either reactive ion etching (RIE) or plasma etching. Both methods are well known by those in the art and are widely used. The silicon wafer  10  has been provided with shallow trench field isolation (STI)  12  and a thin gate oxide  14  using conventional methods. Alternatively the field isolation may be formed by the well known LOCOS (local oxidation of silicon) method. The polysilicon gate stack  8  comprises a thin gate oxide  14 , a polysilicon layer  16  and a silicon oxide cap  18 . The stack layers  16  and  18  are blanket deposited onto a thermally grown gate oxide by a CVD method. Methods for forming a gate stack with an oxide cap are well known in the art. In the present embodiment, the gate oxide  14  is between about 1.5 and 2 nm. thick and the polysilicon layer  16  is between about 120 and 150 nm. thick. The polysilicon gate  16  may consist of a single uniformly doped polysilicon layer or it may have an upper heavily doped portion and a lower undoped portion. The oxide cap  18  is between about 10 and 20 nm. thick. Alternately the oxide cap  18  may be formed of a doped silicon oxide, for example, PSG (phosphosilicate glass), BSG (borosilicate glass), or BPSG (borophosphosilicate glass). A particular advantage of these doped silicate glasses, is that, because of their higher etch rate, they can be subsequently etched away with negligible loss of the isolation  12 . 
     Rectangular or “squared” sidewalls  20  are next formed alongside the polysilicon gate stack  8 . The sidewalls  20  are formed of silicon nitride by depositing a blanket silicon nitride layer over the wafer  10  and then selectively removing material. In conventional sidewall formation, the blanket nitride layer is anisotropically dry etched until the planar portions of the wafer are reached. Sidewalls formed by this conventional method are highly tapered and are not suitable to the present process. Instead the silicon nitride sidewalls are formed by the process described by Chen, et. al., in U.S. Pat. No. 6,358,827 B1, hereafter referred to as Chen, &#39;827. Wherein “squared” or “nearly square” polysilicon spacers with essentially flat tops and parallel vertical sides are formed by anisotropic dry etching using a hardmask. Whereas Chen, &#39;827 forms a squared off polysilicon spacer using an oxide hardmask patterned by CMP or RIE etchback, the vertical side of the silicon nitride spacer distal to the polysilicon gate of the present invention can be similarly formed by RIE using a self-alignment mask scheme to create an oxide hardmask over the silicon nitride. This requires a modification of the Chen, &#39;827 procedure for forming the oxide RIE mask. A workable process for forming an oxide hardmask for forming a squared off or rectangular silicon nitride spacer for the current embodiments, using the method of Chen, &#39;827, is illustrated in FIGS. 3A through 3F. 
     Referring to FIG. 3A, there is shown a cross section of one side of structure of FIG.  1 A. The silicon nitride layer  20  is conformaly deposited over the wafer  10  according to the procedure of Chen, &#39;827. Next a thin layer of polysilicon  22  is deposited over the nitride layer  20  followed by a second thin silicon nitride layer  124 . An SOG (spin-on-glass) layer  126  is deposited to a level  128 . Next, referring to FIG. 3B, the exposed silicon nitride layer  124  is removed by dip etching in hot phosphoric acid, exposing the underlying polysilicon layer  122  over the gate electrode  16 . Referring to FIG. 3C, the SOG  126  is next removed and the exposed polysilicon layer  122  is oxidized to form an oxide layer  128  using the silicon nitride segment  124   a  as an oxidation mask. Finally, in FIG. 3D, the segments  124   a  and  124   b  are removed by dip etching in hot phosphoric acid and a selective silicon etch, for example amine/pyrocatechol, leaving the silicon oxide hardmask  128 . Anisotropic etching of the silicon nitride layer then proceeds according to the two step process of Chen, &#39;827 which includes the removal of the hardmask  128  (FIGS.  3 E,  3 F) to achieve the final squared off nitride spacer  20  shown in FIG.  3 F and in the corresponding Figures of the first embodiment. 
     FIG. 1A shows the silicon nitride-sidewall spacers  20  having slight rounding on the upper corners away from the gate stack  8 . This slight rounding illustrates what is meant by a “nearly square” spacer and is formed by a method of Chen, &#39;827. The width of the silicon nitride spacers  20  is between about 10 and 20 nm. 
     Referring now to FIG. 1B, silicon sidewalls  22  are formed by the conventional sidewall formation method whereby amorphous silicon is blanket deposited on the wafer  10  by LPCVD at a substrate temperature of between about 300 and 400° C. and then anisotropically blanket etched back to substrate silicon by RIE using a gas mixture containing Cl 2  or HBr. The significantly greater taper of the resulting silicon sidewalls  22  formed by the conventional method is illustrated in FIG.  1 B. If the integrated circuit design contains only one type of device (NMOS or PMOS), the blanket silicon may optionally be in-situ doped with the corresponding impurity during deposition. For circuits containing both types of devices, for example CMOS integrated circuits, the appropriated impurities are ion implanted after the sidewalls are formed using appropriate masking. In the present embodiment, only a p-channel device is formed, and the boron impurity is ion implanted. Optionally the blanket silicon could have been in-situ boron doped also. 
     After the amorphous silicon sidewalls  22  have been formed the wafer  10  is subjected to a furnace anneal in a nitrogen ambient for a period of between about 30 and 40 minutes at a wafer temperature of between about 600 and 700° C. During this annealing period the amorphous silicon sidewalls  22  crystallize to form polycrystalline silicon. 
     The squared-off top of the nitride spacer  20  provides an increased surface path length between the silicon spacer and the polysilicon gate  16  without having to compromise contact area on the outer surface of the silicon spacer which contacts the LDD region. The additional surface path  19  over the top of the nitride spacer decreases the chance of gate-to-source/drain shorts cause by silicide bridging during the subsequent salicidation step. If the top were not squared-off the silicon spacer would have to be driven further down alongside the nitride spacer in order to achieve the same upper surface path over insulator. Shortening the spacer height in this way reduced the contact area on the outer surface of the silicon spacer. 
     Referring now to FIG. 1C, BF 2   +  ions are implanted into the wafer  10  surface at a dose of between about 1×10 12  and 2×10 16  ions/cm 2  and at energies between about 12 and 18 keV. After the implantation, the wafer is subjected to an RTA in a nitrogen ambient at a wafer surface temperature of between about 1,000 and 1,100° C. for a period of between about 8 and 13 seconds. During this annealing period the boron implanted into the exposed active regions diffuses in the substrate wafer  10  to form source/drain regions  24  while boron from the boron doped polycrystalline silicon sidewalls  22  diffuses to into the subjacent single crystalline silicon to form shallow, high concentration LDD regions  26 . It can be seen in FIG. 1C that the thickness of the silicon nitride spacer  20  determines the distance between the lateral edge of the LDD region and the effective channel region under the polysilicon gate. The presence of the nitride spacer  20  suppresses boron encroachment into the channel region under the gate  16 . The silicon nitride spacers  20  have essentially vertical sides because their outer edges (distal to the polysilicon gate) are formed by anisotropic etching. The width of the spacer and consequently, the LDD-to-channel spacing “d” can be controlled to a high degree of precision. 
     Referring now to FIG. 1D, the oxide cap  18  is etched away using a calibrated etchant, such as dilute HF, preferably 50:1, or, if the cap is formed of a silicate glass, a etchant containing H 2 O 2  may be used. 
     A laminar Ti/TiN layer  28  having a thickness of between about 475 and 600 Angstroms is deposited over the wafer, preferably by sputtering. The Ti/TiN layer  28  is formed by first sputtering a titanium layer  28 A having a thickness of between about 275 and 400 Angstroms or thereabout onto the wafer and immediately thereafter, without breaking vacuum, sputtering a TiN  28 B layer having a thickness of 200 Angstroms or thereabout over the titanium layer. This may be accomplished by sputtering a titanium target, first with argon to form the Ti layer  28 A, and then with nitrogen to form the TiN layer  28 B. Alternately interchangeable targets of titanium and of titanium nitride may be used in the same chamber. Refractory metals other than titanium may also be used to form the metal layer  28 A over the exposed silicon surfaces. Suitable metals include cobalt, molybdenum, and tungsten. Similarly the TiN layer  28 B may be either omitted or substituted by an alternative protective layer. 
     A first RTA is performed at a temperature of between about 675 and 750° C. or thereabout for a period of between about 10 and 40 seconds in a nitrogen ambient. During this period titanium silicide (TiSi 2 ) forms in the regions where the Ti/TiN layer  28  is deposited over silicon by reaction of the silicon with the Ti layer  28 A. During the RTA step, nitrogen from the ambient, diffuses through the TiN layer  28 B and reacts with the upper surface of the Ti layer  28 A to form TiN, thereby consuming un-reacted Ti over the sidewalls  20  and over the field isolation  12 . This inhibits the transport of silicon over the sidewall regions and thereby prevents bridging of the TiSi 2  layer between the gate electrode  16  and the polysilicon spacers  22 , which, in turn connect to the source/drain regions  24  through the LDDs regions  26 . Referring now to FIG. 1E The wafer is next subjected to an aqueous etching procedure wherein the TiN and residual titanium of the Ti/TiN layer  28  are selectively removed leaving the TiSi 2    30 A over the polysilicon gate  18 ,  30 B on the polysilicon sidewalls  22 , and  30 C over the source/drain active areas  24 . A suitable and commonly used aqueous etchant contains H 2 O 2  and NH 4 OH. A second RTA, performed at between about 800 and 950° C., for a period of between about 10 and 40 seconds, completes the formation of the TiSi 2  contacts. 
     Processing of the MOSFET then proceeds by the deposition of an insulative layer  32  over the wafer (FIG.  1 F). This layer  32 , typically of a flowable glass such as borophosphosilicate glass, is thermally flowed to planarized the surface topology. Alternately, the layer  32  may be planarized by chemical mechanical planarization which is a well known planarization process. Contact openings  34  the source/drain regions  24  are then patterned and etched with RIE. A comparable contact opening (not shown) to the TiSi 2  layer  30 A over the polysilicon gate electrode  16  is simultaneously formed elsewhere, preferably in a regions above or below the plane of the page where the gate electrode  16  passes over field oxide. Using well known procedures, a preferred barrier metallurgy  36 , comprising Ti/TiN is formed over the wafer followed by the formation of tungsten plugs  38 . In the figure, the tungsten barrier metallurgy  36  are shown overlapping the polysilicon sidewalls  22  to illustrate that the contact may extend in this manner to provide a maximum region of contact with the source/drain regions including the connection to the LDD portions  16 . This additional region of contact results in a low source/drain series resistance with improved contact to the LDD regions  26  through the silicided sidewalls  22 . 
     In a second embodiment, a complimentary n-channel/p-channel MOSFET pair are formed wherein the p-channel device has LDD regions formed by the method of this invention an the n-channel device does not. The device pair can comprise a CMOS pair. 
     Referring to FIG. 2A, a silicon wafer  40  is provided. Using well known procedures,—and p-wells are formed in the wafer surface in regions where the CMOS device pair is to be formed. Field isolation  46  is formed to define an active silicon region for each device. LOCOS field oxide is illustrated in the diagram although the isolation may alternatively be STI. The n-channel device will be formed in the p-well  42  and the p-channel device in the n-well  44 . 
     As in the first embodiment a polysilicon gate stack  48 A and  48 B is formed for each device in the respective well. The dry etching may be either reactive ion etching (RIE) or plasma etching. Both methods are well known by those in the art and are widely used. The silicon wafer  40  has been provided with polysilicon gate stacks  48 A and  48 B each of which comprise a thin gate oxide  54 , a polysilicon gate electrode  56 A and  56 B respectively and a silicon oxide cap  58 . The stack layers from which these stack components are formed are blanket deposited onto a thermally grown gate oxide by a conventional CVD method. Methods for forming a gate stack with an oxide cap are well known in the art. In the present embodiment, the gate oxide  54  is between about 1.5 and 2.0 nm. thick and the polysilicon layer from which the gate electrodes  56 A and  56 B are patterned is between about 120 and 150 nm. thick. The polysilicon gates  56 A and  56 B may consist of a single uniformly doped polysilicon layer or may have an upper heavily doped portion and a lower undoped portion. The oxide caps  58  are between about 10 and 20 nm. thick. Alternately the oxide caps  58  may be formed of a doped silicon oxide or silicate glass, for example, PSG (phosphosilicate glass), BSG (borosilicate glass), or BPSG (borophosphosilicate glass). A particular advantage of these doped silicate glasses, is that, because of their higher etch rate, they can be subsequently etched away with negligible loss of the field isolation  46 . 
     Rectangular or “squared” sidewall spacers  60  are next formed alongside the polysilicon gate stacks  48 A. The sidewall spacers  60  are formed of silicon nitride by depositing a blanket silicon nitride layer over the wafer  40  and then selectively removing portions of the layer. In conventional sidewall formation, the blanket nitride layer is anisotropically dry etched until the planar silicon surface of the wafer is reached. Sidewalls formed by this conventional method are highly tapered and are not suitable to the present process. Instead, as in the first embodiment, the silicon nitride sidewall spacers  60  in this embodiment are formed by the process described by Chen, &#39;827 as detailed in the first embodiment and illustrated by FIGS. 3A through 3F. The width of the silicon nitride spacers  60  is between about 15 and 20 nm. 
     Referring next to FIG. 2B, photoresist  62  is patterned to form a block-out mask covering the region of where the p-channel device is being formed. The wafer  40  is then ion implanted with arsenic ions at a dose of between about 3×10 13  and 4×10 13  ions/cm 2  and an energy of between about 12 and 16 keV forming n-type LDD regions  64  for the n-channel device. Referring next to FIG. 2C, the photoresist mask  62  is stripped, either by plasma ashing or with a liquid stripper, and amorphous silicon sidewalls  66 A and  66 B are formed alongside both the gate stacks  48 A and  48 B using the conventional sidewall formation method whereby amorphous silicon is blanket deposited on the wafer  40  by LPCVD, and then anisotropically blanket etched back to substrate silicon by RIE using a gas mixture containing a halogen species. 
     After the amorphous silicon sidewalls  66 A and  66 B have been formed the wafer  40  is subjected to a furnace anneal in a nitrogen ambient for a period of between about 30 and 40 minutes at a wafer temperature of between about 600 and 700° C. During this annealing period the amorphous silicon sidewalls  66 A and  66 B crystallize to form polycrystalline silicon. 
     The squared-off tops of the nitride spacers  60  provide an increased surface path length between the silicon spacer and the polysilicon gates  56 A and  56 B without having to compromise contact area on the outer surface of the silicon spacer which contacts the LDD region. The additional surface path over the top of the nitride spacer decreases the chance of gate-to-source/drain shorts cause by silicide bridging during the subsequent salicidation step. If the top were not squared-off the silicon spacer would have to be driven further down alongside the nitride spacer in order to achieve the same upper surface path over insulator. Shortening the spacer height in this way reduced the contact area on the outer surface of the silicon spacer. 
     Referring next to FIG. 2D, a second photoresist block-out mask  70  is patterned over the region of the gate stack  48 A where the p-channel device is to be formed and the source/drain regions  72  of that device are ion implanted with arsenic at a dose of between about 3×10 13  and 4×10 13  ions/cm 2  and an energy of between about 12 and 16 keV forming n-type LDD regions  64  for the n-channel device. 
     Referring next to FIG. 2E, the photoresist mask  70  is stripped, either by plasma ashing or with a liquid stripper, and a third photoresist block-out mask  74  is patterned to cover the region of where the n-channel device is to be formed. BF 2   +  ions are then implanted into the region of the p-channel device under the polysilicon gate  48 A at a dose of between about 1×10 16  and 2×10 16  ions/cm 2  and at energies between about 12 and 18 keV. After the implantation, the wafer is subjected to an RTA in a nitrogen ambient at a wafer surface temperature of between about 1,000 and 1,100° C. for a period of between about 8 and 13 seconds. During this annealing the boron implanted into the exposed active regions diffuses into the substrate wafer  40  to form source/drain regions  76  while boron from the polysilicon sidewalls  66 A diffuses to into the subjacent single crystalline silicon to form shallow, high concentration LDD regions  78 . It can be seen in FIG. 2E that the thickness of the silicon nitride spacers  60  determines the distance between the lateral edge of the LDD regions and the effective channel regions under the respective polysilicon gates. The presence of the nitride spacer  60  in the p-channel device suppresses boron encroachment into the channel region under the gate  56 A. The silicon nitride spacers  60  have essentially vertical sides because their outer edges (distal to the polysilicon gate) are formed by the anisotropic etching process of Chen, &#39;827 as set forth in the first embodiment. The width of the spacers and consequently, the LDD-to-channel spacing can be controlled to a high degree of precision. 
     Referring now to FIG. 2F, the photoresist mask  74  is stripped, either by plasma ashing or with a liquid stripper, and the oxide caps  58  are etched away using a calibrated etchant, such as dilute HF, or if the caps are formed of a doped oxide or silicate glass, a etchant containing H 2 O 2  may be used. 
     A laminar Ti/TiN layer  80  having a thickness of between about 475 and 600 Angstroms is deposited over the wafer, preferably by sputtering. The Ti/TiN layer  80  is formed by first sputtering a titanium layer  82  having a thickness of between about 275 and 400 Angstroms or thereabout onto the wafer and immediately thereafter, without breaking vacuum, sputtering a TiN  84  layer having a thickness of 200 Angstroms or thereabout over the titanium layer. This may be accomplished by sputtering a titanium target, first with argon to form the Ti layer  82 , and then with nitrogen to form the TiN layer  84 . Alternately interchangeable targets of titanium and of titanium nitride may be used in the same chamber. Refractory metals other than titanium may also be used to form the layer  82  over the exposed silicon surfaces. Suitable metals include cobalt, molybdenum, and tungsten. Similarly the TiN layer  84  may be either omitted or substituted by an alternative protective layer. 
     A first silicide RTA is performed at a temperature of between about 675 and 750° C. or thereabout for a period of between about 10 and 40 seconds in a nitrogen ambient. During this period titanium silicide (TiSi 2 ) forms in the regions where the Ti/TiN layer  80  is deposited over silicon by reaction of the silicon with the Ti layer  82 . During the RTA step, nitrogen from the ambient, diffuses through the TiN layer  84  and reacts with the upper surface of the Ti layer  82  to form TiN, thereby consuming un-reacted Ti over the sidewalls  60  and over the field isolation  46 . This inhibits the transport of silicon over the sidewall regions and thereby prevents bridging of the TiSi 2  layer between the gate electrodes  56 A and  56 B and the corresponding polysilicon spacers  60 A and  60 B respectively. Referring now to FIG. 2G The wafer  40  is next subjected to an aqueous etching procedure wherein the TiN and any residual titanium of the Ti/TiN layer  80  are selectively removed leaving the TiSi 2    86  over the polysilicon gates  56 A and  56 B on the polysilicon sidewalls  66 A and  66 B, and over the source/drain active areas  72  and  76 , leaving behind the TiSi 2  contacts  88 A over the source/drain regions of each device and  88 B over the polysilicon gates. A suitable and commonly used aqueous etchant contains H 2 O 2  and NH 4 OH. A second RTA, performed at between about 800 and 950° C., for a period of between about 10 and 40 seconds, completes the formation of the TiSi 2  contacts. 
     Processing of the MOSFET pair then proceeds by the deposition of an insulative layer  90  over the wafer (FIG.  1 H). This layer  90 , typically of a flowable glass such as borophosphosilicate glass, is thermally flowed to planarized the surface topology. Alternately, the layer  90  may be planarized by chemical mechanical planarization which is a well known planarization process. Contact openings  92  the source/drain regions  72  and  76  are then patterned and etched with RIE. Comparable contact openings (not shown) to the TiSi 2  layers  88 B over the polysilicon gate electrodes  56 A and  56 B are simultaneously formed elsewhere, preferably in regions above or below the plane of the page where the gate electrodes  56 A and  56 B pass over field oxide. Using well known procedures, a preferred barrier metallurgy  94 , comprising Ti/TiN is formed in the contact openings followed by the formation of tungsten plugs  96 . In the figure, the tungsten plugs  96  and the barrier metallurgy  94  are shown overlapping the polysilicon sidewalls  66 A and  66 B to illustrate that the contacts may extend in this manner to provide a maximum region of contact with the source/drain regions including the LDD regions  64  and  78 . This additional region of contact results in a low source/drain series resistance with improved contact to the LDD regions  64  and  78  through the silicided polysilicon sidewalls  66 A and  66 B. 
     While this invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention. While the embodiments of this invention are directed towards the formation of integrated circuit elements in a silicon based technology, the principles and procedures practiced therein are understood to be applicable to technologies using other semiconductor materials.