Patent Publication Number: US-6670288-B1

Title: Methods of forming a layer of silicon nitride in a semiconductor fabrication process

Description:
RELATED PATENT DATA 
     This patent resulted from a divisional application of U.S. patent application Ser. No. 09/057,153 which was filed on Apr. 7, 1998 now U.S. Pat. No. 6,316,372. 
    
    
     TECHNICAL FIELD 
     The invention pertains to silicon nitride layers and to methods of forming silicon nitride layers. Additionally, the invention pertains to photolithography and etching. 
     BACKGROUND OF THE INVENTION 
     Silicon nitride has many applications in modern semiconductor devices, as well as in modern semiconductor device fabrication processes. For instance, silicon nitride is electrically insulating, and is frequently utilized as a dielectric layer to separate electrically conductive circuit components. Additionally, silicon nitride is selectively etchable to other materials frequently used in semiconductor fabrication processes, such as, for example, polysilicon and silicon dioxide, and therefore makes an effective etch stop during etching of such other materials. It would be desirable to develop methods for altering chemical properties of silicon nitride to vary its insulative properties and etch properties for suitability to particular applications. 
     In addition to the above-discussed applications in which silicon nitride is commonly utilized, there are a number of semiconductor processing applications for which silicon nitride is not considered appropriate, or has less than desired attributes, for example, Si 3 N 4  often requires discrete antireflective coating layers intermediate it and an overlying photoresist layer. Accordingly, Si 3 N 4  does not have very good inherent antireflective properties. Antireflective coatings are utilized during photolithographic processing of photoresist layers to absorb light passing through the photoresist layers. Antireflective coatings can thereby prevent light from being reflected from beneath the photoresist layer to constructively and/or destructively interfere with other light propagating through the photoresist layer. 
     Although Si 3 N 4  generally requires discrete antireflective coating layers intermediate it and an overlying photoresist layer, silicon enriched silicon nitride layers (i.e., silicon nitride layers having a greater concentration of silicon than Si 3 N 4 , such as, for example, Si 4 N 4 ) frequently do not. However, silicon enriched silicon nitride is difficult to pattern due to a resistance of the material to etching. Silicon enriched silicon nitride layers are formed to have a substantially homogenous composition throughout their thicknesses, although occasionally a small portion of a layer (1% or less of a thickness of the layer) is less enriched with silicon than the remainder of the layer due to inherent deposition problems. 
     It would be desired to develop alternative methods of utilizing silicon nitride in wafer fabrication processes. As silicon nitride is relatively ubiquitous in semiconductor processing methods, it would be convenient if methods could be developed wherein the silicon nitride could function as an antireflective coating, and yet could also be relatively easily subsequently patterned. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention encompasses a method of forming a silicon nitride layer. A first portion of a silicon nitride layer is formed, and a second portion of the silicon nitride layer is formed. One of the first and second portions has a greater stoichiometric amount of silicon than the other of the first and second portions. The one portion comprises less than or equal to about 95% of a thickness of the layer of silicon nitride. 
     In another aspect, the invention encompasses a semiconductor fabrication process. A substrate is provided. A layer of silicon nitride is formed over the substrate. The silicon nitride layer has a thickness. A refractive index of a first portion of the thickness of the silicon nitride layer is increased relative to a refractive index of a second portion of the silicon nitride layer. The first portion comprises less than or equal to about 95% of the thickness of the silicon nitride layer. 
     In yet another aspect, the invention encompasses a semiconductor wafer assembly. The assembly includes a semiconductor wafer substrate and a layer of silicon nitride over the substrate. The silicon nitride layer comprises a thickness and two portions elevationally displaced relative to one another. A first of the two portions has less resistance than a second of the two portions, and comprises less than or equal to about 95% of the thickness of the silicon nitride layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention are described below with reference to the following accompanying drawings. 
     FIG. 1 is a diagrammatic, fragmentary, cross-sectional view of a semiconductor wafer fragment at a preliminary step of a fabrication process of the present invention. 
     FIG. 2 is a view of the FIG. 1 wafer fragment shown at a processing step subsequent to that of FIG.  1 . 
     FIG. 3 is a view of the FIG. 1 wafer fragment shown at a processing step subsequent to that of FIG.  2 . 
     FIG. 4 is a view of the FIG. 1 wafer fragment shown at a processing step subsequent to that of FIG.  3 . 
     FIG. 5 is a view of the FIG. 1 wafer fragment shown at a processing step subsequent to that of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This disclosure of the invention is submitted in furtherance of the constitutional purposes of the U.S. Patent Laws “to promote the progress of science and useful arts” (Article 1, Section 8). 
     The invention encompasses silicon nitride layers comprising at least two portions. One of the at least two portions is a first portion and another is a second portion. The first portion has a greater stoichiometric amount of silicon than the second portion. Preferably, the first portion comprises less than or equal to about 95% of a thickness of the layer of silicon nitride, more preferably it comprises less than or equal to about 30% of said thickness, and most preferably it comprises from about 10% to about 30% of said thickness. The silicon enriched first portion of the layer silicon nitride can comprise, for example, at least 10% more silicon than the less enriched second portion. As another example, the silicon enriched first portion can comprise at least 35% more silicon than then less enriched second portion. The relative orientation of the first and second portions can vary depending on the application of the silicon nitride layer. Accordingly, the first portion can be formed either above, below, or beside the second portion. 
     An example method of forming the above-described silicon nitride layer is a chemical vapor deposition (CVD) process utilizing a silicon precursor gas (for example SiH 2 Cl 2  (dichlorosilane)) and a nitrogen precursor gas (for example, NH 3  (ammonia)). A substrate is provided within a CVD reaction chamber, together with a first ratio of the silicon precursor gas to the nitrogen precursor gas. One of the first and second portions of the silicon nitride layer is then deposited. Subsequently, the ratio of the silicon precursor gas to the nitrogen precursor gas is altered and the other of the first and second portions of the silicon nitride layer is deposited. Exemplary processing conditions for the CVD process include a pressure of from about 100 mTorr to about 1 Torr, and a temperature of from about 700° C. to about 800° C. 
     In an exemplary process of forming a silicon nitride layer in which the less enriched second portion is to be formed first, and in which the silicon precursor is dichlorosilane and the nitrogen precursor is ammonia, the initial ratio of the silicon precursor gas to the nitrogen precursor gas is about 0.33. Such ratio forms second portion of the silicon nitride layer having a stoichiometry of about Si 3 N 4 . After depositing the Si 3 N 4  the ratio of silicon precursor gas to nitrogen precursor gas is increased to, for example, about 6 to form a silicon enriched first portion of the silicon nitride layer. The silicon enriched first portion has a stoichiometry of Si X N y , wherein the ratio of x to y is greater than or equal to 1. The silicon enriched first portion can comprise, for example, Si 4 N 4 , Si 7 N 4 , Si 10 N 1 , or, if the flow of nitrogen precursor gas is effectively ceased, Si. 
     In the above-described example process, the ratio of silicon precursor gas to nitrogen precursor gas utilized to form the first portion is about 18 times greater than that utilized to form the second portion. Preferably, the ratio of silicon precursor gas to nitrogen precursor gas utilized for formation of a silicon enriched portion of a silicon nitride layer will be at least about 15 times greater than that utilized for formation of a less silicon enriched portion. 
     The respective depositions of the first and second portions of the silicon nitride layer preferably occur in a common and uninterrupted deposition process. By “common deposition process” it is meant a deposition process wherein a wafer is not removed from a reaction chamber between the time that an initial portion of a silicon nitride layer is formed and the time that a final portion of the silicon nitride layer is formed. By “uninterrupted deposition process” it is meant a process wherein the flow of at least one of a silicon precursor gas and a nitrogen precursor gas does not stop during the deposition process. 
     A silicon nitride layer having a portion more enriched with silicon than another portion can be better suited for a number of uses than the prior art silicon nitride layers. For instance, the susceptibility of a silicon nitride layer to etching, particularly wet etching, generally decreases as the silicon nitride layer becomes more enriched with silicon. Accordingly, a heavily silicon enriched portion of a silicon nitride layer will generally function as a better etch stop then will a less enriched portion of the silicon nitride layer. By having both heavily silicon enriched portions and less enriched portions present within the same silicon nitride layer, the location of the heavily silicon enriched portion can be precisely adjusted within the silicon nitride layer to adjust the location of the etch stop. Also, the removability of the silicon nitride layer can be adjusted. Specifically, silicon nitride layers having a higher proportion of less silicon enriched portions to heavily silicon enriched portions can be easier to remove in subsequent wafer processing. Example etch conditions which proceed slower with respect to an silicon enriched silicon nitride relative to a silicon nitride having a lower stoichiometric amount of silicon are etching in hot phosphoric acid, or etching in dilute hydrofluoric acid (wherein “dilute” refers to a hydrofluoric acid solution comprising less than 50% hydrofluoric acid (by weight)). 
     Additionally, it is found that a portion of a silicon nitride layer having a higher silicon enrichment has a greater refractive index than a portion having a lower stoichiometric amount of silicon. As discussed in the “background” section of this disclosure, silicon nitride is not utilized as an antireflective coating because conventional silicon nitride layers have too low of a refractive index. Specifically, conventional silicon nitride (Si 3 N 4 ) has refractive index of less than 2. In contrast, a silicon-enriched portion of a silicon nitride layer can have a refractive index of 2.2. or greater. Example refractive indexes for varying stoichiometries of silicon nitride are listed in Table 1. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Approximate Stoichiometry of 
               
               
                   
                 Refractive Index 
                 Silicon Nitride Layer 
               
               
                   
                   
               
             
            
               
                   
                 less than 2.0 
                 Si 3 N 4   
               
               
                   
                 about 2.2 
                 Si 4 N 4   
               
               
                   
                 about 2.5 
                 Si 7 N 4   
               
               
                   
                 about 3.0 
                 Si 10 N 1   
               
               
                   
                 about 3.5 
                 SiN 0   
               
               
                   
                   
               
            
           
         
       
     
     As can be seen in Table 1, a greater stoichiometric amount of silicon within a silicon nitride layer leads to a higher refractive index of the layer. Silicon nitride layers having refractive indices of 2.2 or greater can be utilized as antireflective coatings. 
     It is also found that a conductivity of a silicon nitride layer can be adjusted by adjusting silicon enrichment within the layer. Specifically, higher silicon enrichment leads to greater conductivity within a silicon nitride layer. Provision of a heavily silicon enriched portion of a silicon nitride layer over a less silicon enriched portion of the silicon nitride layer can effectively provide a conductive channel within the silicon nitride layer. Specifically, the heavily silicon enriched portion of the nitride layer is the conductive channel, and it can be electrically insulated from adjacent conductive components by the less silicon enriched portion of the nitride layer. 
     An example processing method incorporating a silicon nitride layer of the present invention is described with reference to FIGS. 1-5. Referring to FIG. 1, a semiconductor wafer fragment  10  comprises a substrate  12  and a first material layer  14  over substrate  12 . Substrate  12  can comprise, for example, monocrystalline silicon lightly doped with a p-type dopant. To aid in interpretation of the claims that follow, the term “semiconductive substrate” is defined to mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure, including, but not limited to, the semiconductive substrates described above. 
     First material layer  14  is a layer which is ultimately to be covered with silicon nitride and patterned. Layer  14  can comprise, for example, silicon dioxide, such as utilized in local oxidation of silicon (LOCOS) to grow field oxide regions. As another example, material  14  can comprise polysilicon, such as utilized during formation of conductive lines. If layer  14  comprises silicon dioxide for LOCOS, it will typically be much thinner relative to silicon nitride layer  16  than shown in FIG.  1 . Specifically, the silicon dioxide will be utilized as a pad oxide to protect substrate  12  from stress induced by silicon nitride layer  16 , and will have a thickness that is less than about one-half of the thickness of silicon nitride layer  16 . 
     A silicon nitride layer  16  is formed over first material layer  14 . Silicon nitride layer  16  comprises an innermost portion  18  and an outermost portion  20  elevationally displaced from portion  18 . An interface between portions  18  and  20  is illustrated in FIG. 1 as dashed line  22 . One of portions  18  and  20  is more heavily enriched with silicon then is the other of the portions. In a preferred embodiment, outermost portion  20  comprises a higher stoichiometric amount of silicon then innermost portion  18 . This permits outermost portion  20  to function as both an antireflective surface and an etch-stop layer. The portion  18  having a lower stoichiometric amount of silicon enables silicon nitride layer  16  to be more readily removed by etching methods than it would be if an entirety of the layer were formed of the heavily silicon enriched silicon nitride. 
     A layer of photoresist  24  is formed over silicon nitride layer  16 . Preferably, no antireflective coating layer is formed between silicon nitride layer  16  and photoresist layer  24 . 
     Referring to FIG. 2, photoresist layer  24  is patterned to form photoresist blocks  26 . Patterning of photoresist layer  24  comprises exposing photoresist layer  24  to a patterned light beam to render portions of photoresist layer  24  more soluble in a solvent than the portions which ultimately form blocks  26 . During the exposure to the light, silicon nitride layer  16  functions as an antireflective surface. In prior art processes, a separate antireflective coating would typically have been formed between silicon nitride layer  16  and photoresist layer  24 . The method of the present invention can eliminate the need for such separate antireflective coating layer. 
     After the exposure to the light, photoresist layer  24  is exposed as to the solvent to selectively remove portions of layer  24  and leave photoresist blocks  26 . 
     Referring to FIG. 3, a pattern is transferred from photoresist blocks  26  to layers  14  and  16 . The pattern is transferred by etching layers  14  and  16 . Such etch can comprise, for example, a plasma-enhanced etch utilizing NF 3  and HB r . After the pattern is transferred, layers  14  and  16  comprise stacks  28 . 
     Referring to FIG. 4, photoresist blocks  26  are removed from over stacks  28  and a second material layer  30  is formed over substrate  12  and over stacks  28 . In applications in which first material  14  is to be utilized for formation of conductive lines, second material  30  can comprise an insulative material, such as, for example, silicon dioxide. The insulative material can electrically isolate conductive lines  14  from one another. In applications in which stacks  28  are utilized for LOCOS, there would typically be no deposition of a second material  30 . Instead, portions of substrate  12  between stacks  28  would be oxidized to grow field oxide regions. 
     Referring to FIG. 5, second layer  30  is planarized, such as, for example, by chemical-mechanical polishing. During the planarization of second material layer  30 , silicon nitride layers  16  function as etch stops. 
     In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means la herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.