Abstract:
A method for etching contact openings into a polycide layer including a metal silicide layer and a polysilicon layer comprises providing a substrate that includes a polycide layer, forming a patterned photoresist mask, and etching with a series of plasmas. The etches include a silicide etch, a polycide etch including chlorine gas and nitrogen gas where the nitrogen flow rate is between 20% and about 30% of the sum of the nitrogen flow rate plus the chlorine flow rate, and a poly overetch. A polycide etch with a composition in the specified range will have a polycide selectivity greater than one.

Description:
BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates to semiconductor processing and more particularly to a process for forming openings in layered structures such as tungsten silicide (WSi) over polycrystalline silicon (polysilicon). 
     2. The Relevant Art 
     Semiconductor processing to create integrated circuits and the like involves repeated cycles through a series of masking operations, processing operations, and cleaning operations. Some passes through the cycle create new layers, some form holes in existing layers, others remove layers, and still others modify exposed surfaces. FIG. 1 illustrates a cross-section of a typical semiconductor stack  10  of the prior art that is commonly fabricated in the manufacture of integrated circuits. The stack  10  includes a substrate  12 , a dielectric layer  14 , a polysilicon layer  16 , a WSi layer  18 , and a photomask  20 . An opening  22  is formed over a desired location on the dielectric layer  14  by first forming an opening in the photomask  20  by photolithography techniques that are well known in the art. The opening in the photomask  20  allows the exposed surface to be further processed, in this case to form an opening in the WSi layer  18  and the polysilicon layer  16 . Many different techniques are known in the art for etching semiconductor layers, but for etching WSi  18  over polysilicon  16  a frequently used technique involves plasma etching with a mixture of chlorine gas and oxygen gas. 
     Many semiconductor devices presently manufactured include numerous semiconductor stacks  10  in successive layers. The fabrication of such devices is made more complex by the topography that develops after several stacks  10  have already been created. FIG. 2 shows a cross-section of a more complex stack  24  during the formation of openings  22  in two separate locations. Stack  24  includes a polysilicon layer  26  with a topography that is conformal with the layer beneath it (not shown). Typically, this underlying layer is a dielectric layer such as a thermally grown silicon dioxide film that itself has a topography dictated by the layers beneath it. Formed over the polysilicon layer  26  is a WSi layer  28 . WSi layer  28  is commonly a thick layer that can be mechanically or chemically polished to form a flat surface  29 , as shown in FIG.  2 . Stack  24  also includes a photomask  20  disposed over the WSi layer  28 . The photomask  20  further includes openings  22  over portions of the WSi layer  28  having different thicknesses. 
     In FIG. 2, a plasma etch with a mixture of chlorine and oxygen gases is applied to the stack  24  to etch the WSi layer  28  where exposed by the openings  22  in the photomask  20 . This etch is referred to as a silicide etch. Openings  22  grow deeper at approximately the same rate until one advances to the polysilicon layer  26  as illustrated. At this point, because polysilicon generally etches at a faster rate than WSi when etched with a plasma of chlorine and oxygen, the first opening to reach the polysilicon layer  26  will etch through to the dielectric layer  14  not only well before the other opening, but possibly before the other opening even reaches the polysilicon layer  26 . Commonly, a silicide overetch is applied to remove any residual silicide remaining in the openings  22  at the completion of the initial silicide etch. The silicide overetch is typically nothing more than the initial silicide etch applied for an additional length of time. 
     Selectivity is a term commonly used in the art to represent the ratio between the etch rates of two different materials under common conditions. The ratio of the WSi etch rate to the polysilicon etch rate, commonly known as the polycide selectivity, is well known to be less than one for plasma etching with most ratios of chlorine to oxygen. The lower the polycide selectivity the greater the disparity between the depths of the several openings  22  where the thickness of the WSi layer  28  is not uniform. 
     Following the silicide overetch a second plasma etch using a mixture of hydrogen bromide (HBr) and chlorine gases, commonly referred to as a poly etch, is applied. The poly etch etches polysilicon well but etches WSi poorly. Thus, the poly etch has a low selectivity for WSi over polysilicon, necessitating the preceding silicide overetch to make sure residual WSi is removed as the later processing is not likely to remove it. Lastly, a poly overetch is applied to clean up any exposed residual portions of the polysilicon layer  26 . A poly overetch should have a high selectivity for polysilicon over the dielectric. Put another way, the poly overetch ideally should etch polysilicon but not silicon dioxide so that the ratio of their etch rates approaches infinity. A common poly overetch employs a plasma of HBr, He, and O 2 . 
     It will be apparent, therefore, that the prior art calls for four etching steps to form contact openings in a polycide layer where the layer thicknesses are not everywhere uniform, a first etch to substantially remove silicide, a second etch to complete the silicide removal, a third etch to substantially remove polysilicon, and a fourth etch to complete the polysilicon removal. 
     In addition to mixtures of chlorine and oxygen, other gas mixtures have been attempted. U.S. Pat. No. 5,914,276 to Shin et al. discloses using a mixture of chlorine and nitrogen gases at preferred volumetric flow rates where the nitrogen is 5% to 20% of the total. This range of flow rates was found to strike the proper balance between protecting the silicide/poly interface from lateral overetching and complete removal of the desired portions. A volumetric flow rate for nitrogen above 20% of the total was found to lead to difficulties controlling the critical dimension of the pattern where the metal silicide layer was titanium silicide. Shin, however, was not concerned with the problem of complex topographies where the silicide layer thickness is not everywhere the same at the start of the etch. Therefore, achieving polycide selectivities greater than one was not a goal of Shin. 
     What is desired, therefore, is a new etch with a polycide selectivity greater than one that is effective to replace the silicide overetch and the poly etch of the prior art and therefore simplify the formation of contact openings in a polycide layer that includes a silicide layer with a varying thickness. 
     SUMMARY OF THE INVENTION 
     The present invention discloses a method for etching a plurality of contact openings in a polycide layer where the result of underlying topography creates a silicide layer with a varying thickness. The method of the present invention includes providing a polycide layer disposed over a substrate, forming a patterned mask layer over the polycide layer, and selectively etching exposed portions of the polycide layer with a series of three different etches to form a plurality of contact openings that each expose the substrate. In some embodiments a dielectric layer is disposed between the substrate and the polycide layer and is exposed by the contact openings instead of the substrate. The polycide layer of the present invention includes a polysilicon layer disposed above the substrate and a metal silicide layer disposed above the polysilicon layer. In some embodiments the metal silicide is tungsten silicide. Processes for the formation of the various layers and the patterning of the photoresist layer are well known in the art. 
     According to the method of the invention the three etches that are applied to form the plurality of contact openings are a silicide etch, a polycide etch, and a poly overetch. The silicide etch defines a plurality of contact openings in the silicide layer where exposed by the patterned photoresist layer by substantially removing the exposed portions of the silicide layer. The silicide etch in some embodiments comprises a plasma of CF 4 , Cl 2 , and N 2 . 
     The polycide etch extends the plurality of contact openings towards the substrate and continues until the substrate has been exposed by at least one of the plurality of contact openings. The polycide etch employs a plasma of N 2  and Cl 2  where the N 2  is supplied at a flow rate between 20% and about 30%. Mixtures of chlorine and nitrogen in this range advantageously have polycide selectivities greater than one. A selectivity greater than one implies that a contact opening that has advanced into the polysilicon layer will etch more slowly than one still advancing through the silicide layer. 
     The poly overetch has a high selectivity for polysilicon compared to the substrate. Consequently, the poly overetch has little effect on exposed portions of the substrate but continues to remove polysilicon from those contact openings that are not fully defined. In those embodiments that include a dielectric layer the poly overetch has instead a high selectivity for polysilicon compared to the dielectric layer. The poly overetch is continued until each of the plurality of contact openings exposes the substrate or the dielectric layer. The poly overetch in some embodiments comprises a plasma of HBr, He, and O 2 , and in further embodiments further includes N 2 . These 
     These and other aspects and advantages of the present invention will become more apparent when the description below is read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, with like reference numerals designating like elements. 
     FIG. 1 is a cross-section of a semiconductor stack of the prior art; 
     FIG. 2 is a cross-section of semiconductor stack of the prior art having greater topography; 
     FIG. 3 shows a cross-section of a semiconductor stack as provided according to an embodiment of the method of the invention; 
     FIG. 4 shows a cross-section of a semiconductor stack partially etched according to an embodiment of the method of the invention; 
     FIG. 5 shows a cross-section of a semiconductor stack further partially etched according to an embodiment of the method of the invention; 
     FIG. 6 shows a cross-section of a semiconductor stack still further partially etched according to an embodiment of the method of the invention; and 
     FIG. 7 shows a cross-section of a semiconductor stack completely etched according to an embodiment of the method of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 have been described with reference to the prior art. FIGS. 3-7 illustrate etching a polycide layer according to an embodiment of the present invention. In FIG. 3 a semiconductor stack  30  includes a substrate  31 , a dielectric layer  32  disposed over the substrate  31 , a polycide layer  33  disposed over the dielectric layer  32 , and a patterned photoresist layer  40  disposed over the polycide layer  33 . The polycide layer  33  includes a polysilicon layer  34  and a metal silicide layer  36 . Methods for forming each of these layers are well known in the semiconductor fabrication arts. 
     Substrate  31  may be a semiconductor wafer such as single crystal silicon, but can also include a stack of layers fabricated on a semiconductor wafer. A stack of layers fabricated on a semiconductor substrate commonly will have a certain amount of topography associated with it, as is represented by the upward protrusion on substrate  31  in FIG.  3 . The topography is reflected in the dielectric layer  32 , disposed over substrate  31  and formed of a dielectric material such as SiO 2 . Next, a layer  34  of polycrystalline silicon (“polysilicon”) is formed above the dielectric layer  32 . The polysilicon layer  34  conforms to the topography of the dielectric layer  32 . 
     A metal silicide layer  36  is formed above the polysilicon layer  34 . The metal silicide layer  36  is preferably tungsten silicide, but can also be titanium silicide, for example. The metal silicide layer  36 , as formed, tends to be conformal and therefore to reflect the topography of the underlying layers. Preferably, the metal silicide layer  36  is planarized to create a substantially planar surface  38 . Planarization can be achieved, for example, by chemical mechanical polishing (CMP) as is well understood in the art. A substantially planar surface  38  is desirable because it aids in the formation of the patterned photoresist layer  40  that is disposed over surface  38 . 
     Patterned photoresist layer  40  is formed and patterned according to photolithography methods that are well known in the art. Accordingly, openings in layer  40  are formed in locations where it is desired to define contact openings through the polycide layer  33  down to the dielectric layer  32  or the substrate  31 . Because polysilicon layer  34  reflects the topography beneath it, while planarized silicide layer  36  does not, it follows that a first opening  42  can be located above a thinner portion of silicide layer  36  while a second opening  44  can be located above a thicker portion of silicide layer  36 . 
     According to the present invention the definition of contact openings  42 ,  44  is initiated by applying a silicide etch to exposed portions of surface  38 . The silicide etch is effective to substantially remove the portions of the silicide layer left exposed by the patterned photoresist mask  40 , and is preferably applied until approximately 90% to 95% of the exposed portions are removed. Stopping the silicide etch at this point is generally a matter of timing. In other words, the thickness of the silicide layer  36  is typically well known as is the etch rate to be expected from a given etch, so it is a simple matter to calculate the time necessary for a silicide etch to remove approximately 90% to 95% of the exposed portions. 
     Many silicide etches are known in the art. A preferred silicide etch comprises flowing CF 4  at a rate of 70 sccm, Cl 2  at a rate of 20 sccm, and N 2  at a rate of 18 sccm into a chamber operating at a pressure of 4 milliTorr (mT) while applying 600 W to a top electrode and 35 W to a bottom electrode. This silicide etch is particularly desirable as it is highly uniform across a wafer and has a good etch rate. It will be appreciated, of course, that the present invention may also be practiced without the silicide etch by starting immediately with the polycide etch. 
     FIG. 4 shows the stack  30  after the completion of the silicide etch. The method of the invention next applies a polycide etch to stack  30 . The polycide etch comprises a plasma formed of chlorine and nitrogen gases supplied to the chamber in a ratio chosen to have a polycide selectivity greater than one. It will be appreciated that the polycide etch will reach the polysilicon layer  34  in those areas where the silicide layer  36  is thin sooner than in those areas where the silicide layer  36  is thick. Thus, first opening  42  will etch into the polysilicon layer  34  while second opening  44  is still being defined into the silicide layer  36 . 
     FIG. 5 shows a cross-section of the semiconductor stack  30  as contact openings  42  and  44  are further defined by the polycide etch. Because the polycide etch has a polycide selectivity greater than one, it will be apparent that first opening  42  will be defined into polysilicon layer  34  more slowly than second opening  44  will continue to be defined into silicide layer  36 . Once both contact openings  42 ,  44  have reached the polysilicon layer  34  they will thereafter be defined into the polysilicon layer  34  at the same rate until first opening  42  reaches the dielectric layer  32 . 
     Table 1 gives the polycide selectivity for three nitrogen flow rates according to the method of the present invention. The greater the polycide selectivity, the smaller the penetration of first opening  42  into the polysilicon layer  34  before second opening  44  enters the polysilicon layer  34 . Consequently, the exposed residual polysilicon in second opening  44  remaining when first opening  42  is completed to the dielectric layer  32  is reduced as the polycide selectivity is increased. Thus, increasing the polycide selectivity has the effect of reducing the amount of polysilicon left to be removed by the subsequent poly overetch. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Selectivity vs. Nitrogen Flow Rate 
               
             
          
           
               
                   
                 N 2 /(N 2  + Cl 2 ) 
                 Selectivity 
               
               
                   
                   
               
             
          
           
               
                   
                 0.11 
                 1.05 
               
               
                   
                 0.2 
                 1.47 
               
               
                   
                 0.3 
                 1.7 
               
               
                   
                   
               
             
          
         
       
     
     A preferred polycide etch comprises flowing Cl 2  at a rate of 80 sccm and N 2  at a rate of 20 sccm into a chamber operating at a pressure of 4 mT while applying 450 W to a top electrode and 60 W to a bottom electrode. Another preferred polycide etch applies is 70 W to the bottom electrode. 
     FIG. 6 illustrates the end point of the polycide etch. The polycide etch endpoint can be detected by monitoring the reaction products for an indication that the dielectric layer  32  has been exposed. Such endpoint detectors are well known in the art. It can be seen that the polycide etch is completed when first opening  42  is etched to the dielectric layer  32 . At this point second opening  44  is not yet through to the dielectric layer  32 . Instead, an exposed residual amount of polysilicon with a thickness of x still remains to be removed from above the dielectric layer  32 . As previously described, this may be accomplished by switching to a poly overetch that has a high selectivity between polysilicon and the dielectric material. Clearly, the time needed to perform a poly overetch will be minimized when the thickness x is minimized, as is the case when the polycide selectivity is maximized. 
     FIG. 7 illustrates the end point of the poly overetch. It can be seen that at the completion of the poly overetch both contact openings  42 ,  44  have been defined through to the dielectric layer  32 . A preferred poly overetch comprises flowing HBr at a rate of 70 sccm and a mixture of He and O 2  at a rate of 10 sccm into a chamber operating at a pressure of 40 mT while applying 220 W to a top electrode and 50 W to a bottom electrode. Another preferred poly overetch adds 5 sccm of N2 to the other process gases flowing into the chamber. 
     Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.