Integrated circuit structure having intermediate metal silicide layer and method of making same
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to integrated circuit structures. More particularly, this invention relates to an integrated circuit structure having an intermediate metal silicide layer. 
     2. Background Art 
     In the production of integrated circuit structures, such as, for example, static RAM chips, metal step coverage of contacts in the application of the final layers or rear end of the process is less than satisfactory when an intermediate metal layer is used. The presence of such an intermediate metal layer complicates the step coverage problem because higher temperature processing cannot be used to smooth out the step coverage since, if sufficient heat is used to soften the outer oxide layer to smooth out the step, the heat may adversely affect the intermediate metal layer. 
     Furthermore, it is customary to provide an undoped oxide layer over a polycrystalline silicon (hereinafter referred to as polysilicon or poly) layer having lightly doped portions to provide resistive paths or poly loads. This second, undoped, oxide layer is applied to prevent diffusion of the dopants from getting to the poly load area of the polysilicon layer. The use of an additional doped oxide layer over this oxide layer to provide a lower melting point oxide whereby the step coverage can be smoothed out means that contact openings must penetrate three layers of oxide before contacting the desired portion of the silicon substrate, such as, for example, the drain or source of an MOS transistor. This, in turn, can provide inadequate metal coverage in the contact opening resulting in thinning of portions of the metal on the wall of the contact opening. 
     While the use of an intermediate conductive layer of metal silicide is not unknown, prior uses of such a metal silicide have been made by concurrently sputtering silicon and a refractory metal to form a metal silicide layer which, after selective etching, is covered by a polysilicon layer. Subsequent doping of the poly layer results in the formation of resistive paths between the metal silicide portions. However, this use of two conductive layers provides a further step which must be covered during the subsequent application of further layers. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an improved integrated circuit structure and a process for making the structure wherein a metal silicide layer replaces a metal layer to permit higher temperature processing of subsequently applied layers. 
     It is an another object of the invention to provide an improved integrated circuit structure and a process for making the structure wherein a metal silicide layer is used beneath a doped oxide layer instead of a metal layer whereby a subsequently applied doped oxide layer may be softened at high temperature to provide better step coverage without interfering with the underlying conductive layer of metal silicide. 
     It is yet another object of the invention to provide an improved integrated circuit structure and a process for making the structure wherein the use of an intermediate metal silicide layer eliminates an intermediate oxide layer from the contact area thereby reducing the thickness of oxide layers which must be removed by a subsequent contact etch whereby better step coverage is provided. 
     It is a further object of the invention to provide an improved integrated circuit poly load structure and a process for making the structure wherein a metal silicide interconnection layer is formed in situ by selective reaction with a polysilicon layer in areas unmasked by oxide. The metal silicide layer is connected to the poly load structure masked by the oxide, eliminating the need for an additional interconnect layer. 
     It is yet a further object of the invention to provide an improved integrated circuit structure and a process for making the structure wherein an intermediate metal silicide layer is provided having rounded oxide corners formed by anisotropic etching to provide better step coverage when subsequent layers are applied. 
     It is a still further object of the invention to provide an improved integrated circuit structure wherein conductive paths are formed in polysilicon, previously doped to provide a preselected resistivity and patterned to form strips, by the in situ formation of metal silicide with a portion of said polysilicon strips whereby poly load resistors are formed which are connected at their ends with portions of the conductive metal silicide. 
     These and other objects of the invention will be apparent from the drawings and description of the preferred embodiments. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a sectional view of a portion of an integrated circuit device constructed in accordance with the invention. 
     FIG. 1B is a sectional view of an integrated circuit device constructed in accordance with the prior art. 
     FIGS. 2A-2G are sectional views of the process steps performed to arrive at the structure of FIG. 1A. 
     FIG. 3 is a sectional view of a portion of a CMOS device constructed in accordance with the invention. 
     FIG. 4 is sectional end view of the structure of FIG. 1A taken along lines IV--IV. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1A illustrates an integrated circuit structure utilizing the invention in contrast to an integrated circuit structure having the same active and passive components formed therein in accordance with the prior art as shown in FIG. 1B. 
     In both FIGS. 1A and 1B, a silicon wafer or substrate 4 has formed thereon an enhancement mode field effect transistor (FET) 6 and a depletion mode FET 8. FET 6 has a gate 6a, a source 6b, and a drain 6c.  FET 8 has a gate 8a, a source 8b, and a drain 8c. Metal silicide segments 60a, 60b, and 60c of FIG. 1A, in accordance with the invention, overlay poly silicon segments 10a, 10b, and 10c which overlaying segments together make connection to various portions of the circuit. Segments 10a/60a make connection to a polysilicon segment 12 which may, for example, be a word line in a static RAM cell. Segments 10b/60b make connection to a portion 14 of the N+ layer contiguous with source 6b. Portion 14 may, for example, be a voltage supply line with segments 10b/60b providing a low resistance strap. Segments 10c/60c provides a low resistance quasi-buried contact (QBC) of a polysilicon segment 18 to a portion 20 of the N+ layer contiguous with drain 6c. It will be immediately noticed that oxide layer 50 in prior art FIG. 1B has been eliminated in FIG. 1A except in the poly load area where this layer is designated as 50a. 
     It will also be noted that polysilicon segments 10a&#39;, 10b&#39;, and 10c&#39; respectively overlay metal silicide segments 60a&#39;, 60b&#39;, and 60c&#39; in prior art FIG. 1B while the metal silicide segments 60a, 60b, and 60c overlay polysilicon segments 10a, 10b, and 10c in FIG. 1A. This is because the metal silicide layer, in the system of the invention, is formed in situ by depositing, on the surface of the polysilicon, a layer of a metal, such as titanium capable of reacting with the silicon, and then subsequently, sintering the materials to form titanium silicide in situ on the surface of the polysilicon layer. In the prior art practice exemplified by FIG. 1B, the metal silicide layer is first applied by codepositing silicon and the metal portion of the silicide to form the metal silicide layer after which the polysilicon layer is applied. Thus, in the prior art, two distinct layers are applied to the substrate; first the metal silicide layer and then the polysilicon layer. In the practice of the invention, the metal silicide forms in situ on the surface of the previously laid down and patterned polysilicon segments or strips without forming an extra layer from the standpoint of additional thickness. 
     In accordance with the invention, the metal silicide layer is formed by first patterning a deposited layer of polysilicon over the substrate and any previous layers thereon followed by application of a metal, such as a refractory metal, capable of reacting with the polysilicon to form the metal silicide. Examples of such metals include titanium, molybdenum, and tungsten. 
     The metal is preferably also one that is easily applied to the polysilicon as by sputtering or vapor deposition. 
     The metal used should be further capable of reacting with the polysilicon at a temperature of from about 600°-800° C., i.e., high enough to permit diffusion of the silicon into the metal reactant to form silicide, yet low enough to inhibit unwanted diffusion of the silicon to other parts of the device (e.g., the gap between two patterned polysilicon lines). 
     After the application of the metal coating over the silicon, the device is heated in a heat pulsed sintering environment to a temperature of about 600°-800° C. and held at this temperature for from 10 to 20 seconds to permit reaction to occur forming the metal silicide while inhibiting the side diffusion of silicon. 
     The resultant metal silicide should have a resistivity of less than 5 ohms per square. 
     It shouild be noted here that it is preferable that not all of the polysilicon on which the metal deposits, react with the metal to form the desired metal silicide layer. It has been found that a small remaining layer of poly beneath the metal silicide serves to strengthen the metal silicide layer. Preferably, after reaction, the remaining poly should have a thickness of about 0.25 to 0.33 that of the silicide. To achieve this, the ratios of amounts of poly to metal deposited thereon must be controlled. The thickness ratio of silicon to metal should be kept to a 3:1 ratio or higher to provide the desired final ratio of silicide to poly. 
     FIGS. 2A-2G are cross-sectional views which illustrate the processing steps used in fabricating a typical N channel device utilizing the invention such as the device shown in FIG. 1A. As will be shown in FIG. 3, other devices can also be constructed utilizing the invention;, the devices illustrated in FIGS. 1A, 2A-2G, and 3 being only representative of the invention. Representative process and characteristic parameters such as dopant concentration, implantation depth, layer thickness, and electrical resistance are tabulated in Appendices 1-3 below. 
     FIG. 2A shows wafer 4 after it has been subjected to the following steps: 
     preoxidation, 
     barrier oxidation, 
     nitride depostion 
     source/drain mask, and 
     field doping. 
     More particularly, a thin barrier oxide layer is deposited by heating the wafer in an oxygen atmosphere, and a silicon nitride layer is then deposited by standard techniques. The source/drain regions are masked by selectively removing portions of the nitride layer and underlying oxide layer leaving nitride/oxide layer segments 30a and 30b over respective regions 32 and 33 to define the source/drain mask. The region 35 that is not masked is then subjected to the ion implantation of a P-type impurity to define a channel stop region 36. 
     FIG. 2B shows wafer 4 after it has been subjected to the following additional steps: 
     field oxidation, 
     implant oxidation I, 
     enhancement mask and doping, 
     depletion mask and doping, and 
     gate oxidation. 
     More particularly, a field oxide layer 37 is formed from region 35 by heating the wafer in an atmosphere of steam. The nitride/oxide mask that defines region 35 is then removed by a chemical wet etch and Kooi ribbon oxidation. Prior to doping regions 32 and 33, a very thin oxide layer (not shown) is grown to randomize the ion implantation. In a series of mask layer deposition and definition steps, region 32 then has P-type impurities implanted while region 33 has N-type impurities implanted. Portions of the implanted layers within regions 32 and 33 will ultimately define the channels for FETs 6 and 8. After these enhancement and depletion region definition and implantation steps, a gate oxide layer is grown over regions 32 and 33 by heating the wafer in an atmosphere of oxygen and hydrogen chloride. This gate oxide layer, being very thin, is not illustrated. 
     FIG. 2C shows wafer 4 after it has been subjected to the following additional steps: 
     polysilicon (gate) deposition and doping, 
     polysilicon (gate) definition, 
     implant oxidation II, and 
     source/drain doping and annealing. 
     More particularly a polysilicon layer is deposited by a chemical vapor deposition (&#34;CVD&#34;) process, and phosphorus added by deposition and diffusion by heating in an atmosphere of phosphorus oxychloride or by ion implantation. Selected portions are then removed to leave segments 12 and 18 and polysilicon gates 6a and 8a. The portions of regions 32 and 33 not occupied by gates 6a and 8a are then subjected to the implantation of-N-type impurities, such as arsenic, to define the source and drain of each of FETs 6 and 8. 
     FIG. 2D shows wafer 4 after it has been subjected to the following additional steps: 
     CVD oxide deposition and densification 1, 
     and QBC mask. 
     More particularly, a layer 40 of silicon dioxide is deposited by a CVD process, densified to remove pinholes, and has portions removed over regions 45a, 45b, and 45c to define contact regions. These regions are defined by an etching step which removes the CVD oxide (as well as the thin layer of oxide over N+ layer portions 14 and 20). The opening over region 45a exposes a portion of polysilicon segment 12; the opening over region 45b exposes N+ layer portion 14 that is contiguous with source 6b; and the opening over region 45c exposes a portion of polysilicon layer segment 18 and N+ layer portion 20 in the substrate to define a QBC region. 
     FIG. 2E shows wafer 4 after being subjected to the following steps: 
     deposition of polysilicon layer, 
     doping of polysilicon layer, and 
     masking and selective removal of the polysilicon. 
     A 1000-3000 angstrom layer of polysilicon is deposited by a chemical vapor deposition (CVD) process. The polysilicon layer is then doped, for example, by ion implantation, with phosphorous, boron, or arsenic to control the resistance of the poly load resistor which will be formed from a portion of this layer. The layer is then masked to define a poly load resistor 10d and contact segments 10a, 10b, and 10c. 
     In FIG. 2F a further oxide layer has been deposited on wafer 4 by a CVD process and then masked and selectively etched away to leave only a portion 50a over poly load resistor 10d. It will be particularly noted here that the almost complete removal of this layer of oxide is in contrast to the prior art oxide layer 50 shown in FIG. 1B which is only etched away in the contact areas. 
     It should be further noted here that a high selectivity etch will be used on this oxide film which will stop at the densified oxide layer 40 beneath. Using an anisotropic plasma etch, the rounded oxide portions 58 as best seen in FIG. 4 will be formed adjacent the edges of the polysilicon segments or strips, thus further smoothing out the geometry of the layers resulting in better step coverage. 
     In FIG. 2G a layer of a refractory metal, such as titanium, has been applied, for example, by sputtering. In accordance with the invention, the wafer is then sintered causing the titanium to react with a majority of the underlying polysilicon segments to form a metal silicide, e.g. titanium silicide segments 60a, 60b, and 60c, in situ on the surface of the remaining polysilicon segments 10a, 10b, and 10c. In those portions where there is no underlying polysilicon, e.g., portion 50a of CVD oxide over poly load resistor 10d, no metal silicide will form, and the metal will remain in metallic form. This metal is then selectively removed from the substrate with an acid and/or base etch, such as a solution of ammonium hydroxide, hydrogen peroxide, and water. 
     Thus, the formation over all of the polysilicon, except for the portion covered by the CVD oxide, of a silicide layer or strip above this polysilicon layer (which has been previously lightly doped to provide a preselected resistivity and patterned to provide polysilicon strips) results in the effective segregation or separation of one or more portions of the entire doped polysilicon strip or line to form poly load resistor 10d out of only that portion of the doped polysilicon under CVD oxide 50a. In essence, then the remainder of the high resistance polysilicon strip is shorted out by the application of the low resistance silicide (conductive) layer over it. 
     The final product, as illustrated in FIG. 1A, includes the application of a further oxide layer 62 of PVX which is doped to lower its melting point. This layer serves as a dielectric between the metal silicide and a further metal layer 65 which may be aluminum. Before masking, the device may be heated to a temperature of from about 900°-950° C. to soften the PVX oxide to smooth out the step coverage. The PVX oxide layer is then masked and selectively etched to provide contacts 65b and 65c between polysilicon segment 12 and metal silicide/polysilicon segments 60b/10b as well as provide contact 65a to the N+ region adjacent source 8b of depletion mode FET 8. A metal layer 65 of aluminum or aluminum silicon is then applied and, after masking, etched to form contacts 65a, 65b and 65c. A protective oxide layer 80 is then applied in a final fabrication step prior to testing, mounting and attachment of leads. 
     FIG. 3 illustrates the invention as embodied in a CMOS device having an N well 24 in which a P channel device 8&#39; is formed having a source 8b&#39;, a drain 8c&#39;, and a gate 8a&#39;. Parameters for such a device are also listed in the Appendices. 
     If an N channel process is used rather than CMOS technology, the lower melting point of the PVX oxide can also be used to permit the rounding or sloping of the edge 62a of the etched contact openings for contacts 65a, 65b and 65c to provide better metal fill in and avoid thinning of the sidewalls of the metal contact. This also provides better step coverage and planarization of the surface. However, the dopant in the PVX layer must be the same as that used for the source and drain since heating the PVX after opening the contacts can result in some of the dopant from the PVX entering the openings and depositing on the doped substrate. 
     It should also be noted in this regard that the overall topology with respect to contacts 65a, 65b and 65c is further enhanced by the elimination of the intermediate silicon dioxide layer 50 of prior art FIG. IB except for CVD portion 50a over the poly load resistor 10d. This results in contacts 65a, 65b and 65c needing to only penetrate two thicknesses of oxide rather than three. 
     Thus, the invention provides an improved process and resulting device wherein the use of an intermediate silicide layer formed in situ in the surface of a polysilicon layer eliminates stepping between a metal layer and a silicide layer and permits subsequent higher temperature processing to provide better step coverage. Furthermore, by providing the silicide layer above the polysilicon layer or strips, and therefore the silicide portion of the contacts for the poly load resistor above the polysilicon, the CVD layer, used in the prior art to protect the poly load resistor from external doping, can be, in essence, eliminated by leaving the CVD oxide, after the anisotropic etching, only above the poly load resistor (except for the rounded portions of CVD oxide adjacent the polysilicon strips). This was not feasible when the silicide, including the silicide interface between the metal contacts and the poly load resistor, was beneath the polysilicon layer. This elimination of an intermediate layer of oxide, except over the poly load resistor, further enhances the topology adjacent the contact areas by reducing the number of oxide layers, and hence the total thickness, which the metal contacts must penetrate. 
     While the invention has been described with respect to its preferred embodiments, minor modifications may be made which do not depart from the scope of the invention which is defined by the appended claims. 
     
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Appendix 1 - Implantation Parameters                                      
                            Dopant                                        
Region          Depth       (molecules/cm.sup.3)                          
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channel stop 36 1 micron    10.sup.16 --10.sup.17 B                       
enhancement 32  1 micron    10.sup.16 B                                   
depletion 33    0.3-0.6 micron                                            
                            10.sup.16 -10.sup.17 As                       
source/drain 8b, 8c                                                       
                0.2-0.4 micron                                            
                            10.sup.20 As                                  
                            (N dopant)                                    
*source/drain 8b&#39;, 8c&#39;                                                    
                0.3-0.5 micron                                            
                            10.sup.20 B                                   
                            (P dopant)                                    
*N well 24      4.0 micron  10.sup.16 P                                   
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 *for CMOS device                                                         
 
    
     
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Appendix 2 - Layer Parameters                                             
                          Dopant                                          
Layer        Thickness    (molecules/cm.sup.3)                            
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barrier oxide                                                             
             200-700 A                                                    
nitride 30a, 30b                                                          
             500-1000 A                                                   
field oxide 37                                                            
             0.7-1 micron                                                 
implant oxide                                                             
             100 A                                                        
gate oxide   300 A                                                        
polysilicon                                                               
gate 7a, 8a  0.2-0.5 micron                                               
                          10.sup.20 -10.sup.21 P                          
*gate 8a&#39;    0.2-0.5 micron                                               
                          10.sup.20 -10.sup.21 P                          
CVD oxide 40 0.2-0.5 micron                                               
metal silicide                                                            
             0.1-0.3 micron                                               
60a, 60b, 60c                                                             
polysilicon  0.1-0.3 micron                                               
                          10.sup.19 As                                    
10a, 10b, 10c                                                             
PVX oxide 62 0.5-1.0 micron                                               
                          4-8% P                                          
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 *for CMOS device                                                         
 
    
     
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Appendix 3 - Electrical Characteristics                                   
Layer/Region     Resistivity (ohms/square)                                
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source/drain     40                                                       
polysilicon gates                                                         
                 30                                                       
metal silicide 60a, 60b, 60c                                              
                  5                                                       
polysilicon 10a, 10b, 10c                                                 
                 .sup. 10.sup.9                                           
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