Patent Publication Number: US-4550069-A

Title: Positive photoresist compositions with o-quinone diazide, novolak, and propylene glycol alkyl ether acetate

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to radiation sensitive positive photoresist compositions and particularly to compositions containing novolak resins together with naphthoquinone diazide sensitizing agents. 
     It is well known in the art to produce positive photoresist formulations such as those described in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470. These include alkali-soluble phenol-formaldehyde novolak resins together with light-sensitive materials, usually a substituted naphthoquinone diazide compound. The resins and sensitizers are dissolved in an organic solvent or mixture of solvents and are applied as a thin film or coating to a substrate suitable for the particular application desired. 
     The novolak resin component of these photoresist formulations is soluble in alkaline aqueous solution, but the naphthoquinone sensitizer acts as a dissolution rate inhibitor with respect to the resin. Upon exposure of selected areas of the coated substrate to actinic radiation, however, the sensitizer undergoes a radiation induced structural transformation and the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in alkaline developing solution while the unexposed areas are largely unaffected, thus producing a positive relief pattern on the substrate. 
     In most instances, the exposed and developed substrate will be subject to treatment by a substrate-etchant solution. The photoresist coating protects the coated areas of the substrate from the etchant and thus the etchant is only able to etch the uncoated areas of the substrate, which, in the case of a positive photoresist, correspond to the areas that were exposed to actinic radiation. Thus, an etched pattern can be created on the substrate which corresponds to the pattern of the mask, stencil, template, etc., that was used to create selective exposure patterns on the coated substrate prior to development. 
     The relief pattern of photoresist on substrate produced by the method described above is useful for various applications including, for example, as an exposure mask or a pattern such as is employed in the manufacture of miniaturized integrated electronic components. 
     The properties of a photoresist composition which are important in commercial practice include the photospeed of the resist, development contrast resist resolution, and resist adhesion. 
     Increased photospeed is important for a photoresist, particularly in applications where a number of exposures are needed, for example, in generating multiple patterns by a repeated process, or where light of reduced intensity is employed such as, in projection exposure techniques where the light is passed through a series of lenses and mono-chromatic filters. Thus, increased photospeed is particularly important for a resist composition employed in processes where a number of multiple exposures must be made to produce a mask or series of circuit patterns on a substrate. These optimum conditions include a constant development temperature and time in a particular development mode, and a developer system selected to provide complete development of exposed resist areas while maintaining a maximum unexposed resist film thickness loss not exceeding about 10 percent of initial thickness. 
     Development contrast refers to a comparison between the percentage of film loss in the exposed area of development with the percentage of film loss on the unexposed area. Ordinarily, development of an exposed resist coated substrate is continued until the coating on the exposed area is substantially completely dissolved away and thus, development contrast can be determined simply by measuring the percentage of the film coating loss in the unexposed areas when the exposed coating areas are removed entirely. 
     Resist resolution refers to the capability of a resist system to reproduce the smallest equally spaced line pairs and intervening spaces of a mask which is utilized during exposure with a high degree of image edge acuity in the developed exposed spaces. 
     In many industrial applications, particularly in the manufacture of miniaturized electronic components, a photoresist is required to provide a high degree of resolution for very small line and space widths (on the order of one micron or less). 
     The ability of a resist to reproduce very small dimensions, on the order of a micron or less, is extremely important in the production of large scale integrated circuits on silicon chips and similar components. Circuit density on such a chip can only be increased, assuming photolithography techniques are utilized, by increasing the resolution capabilities of the resist. Although negative photoresists, wherein the exposed areas of resist coating become insoluble and the unexposed areas are dissolved away by the developer, have been extensively used for this purpose by the semiconductor industry, positive photoresists have inherently higher resolution and are utilized as replacements for the negative resists. 
     A problem with the use of conventional positive photoresists in the production of miniaturized integrated circuit components is that the positive resists generally have slower photospeed than the negative resists. 
     Various attempts have been made in the prior art to improve the photospeed of positive photoresist compositions. For example, in the U.S. Pat. No. 3,666,473, a mixture of two phenol formaldehyde novolak resins was utilized together with a typical sensitizer, said novolak resins being defined by their solubility rates in alkaline solutions of a particular pH and by their cloud points. In U.S. Letters No. 4,115,128, a third component consisting an organic acid cyclic anhydride was added to the phenolic resin and naphthoquinone diazide sensitizer to provide increased photospeed. 
     The present invention provides an improved positive working photoresist composition which demonstrates substantially increased photospeed while retaining or improving the resist erosion rate, plasma etch rate, and contrast. It has been unexpectedly found that such an improved resist can be formulated when the novolak resin and quinone diazide photosensitizer are blended with a propylene glycol alkyl ether acetate component. This component also acts as a solvent for the resin and photosensitizer to facilitate the application of the resist to a substrate. This component demonstrates lower toxicity over other solvents useful for forming photoresists. 
     SUMMARY OF THE INVENTION 
     The invention provides a positive working photosensitive composition which comprises a novolak resin, a quinone diazide photosensitizer and a sufficient amount of a propylene glycol alky ether acetate to increase the photospeed of said composition. Most preferably the acetate is propylene glycol methyl ether acetate. 
     The photoresist compositions of the present invention, in addition to exhibiting increased photospeed in comparison with prior art positive photoresists, exhibit a high degree of resolution and good development contrast and adhesion properties. These properties are in marked contrast to some prior art resins which achieve moderately increased photospeed while at the same time sacrificing resolution and contrast. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As hereinbefore mentioned, the invention provides a composition containing a novolak resin, a quinone diazide photosensitizer and a propylene glycol alkyl ether acetate. A preferred acetate compound is propylene glycol methyl ether acetate. 
     The production of novolak resins, which may be used for preparing photosensitive compositions, is well known in the art. A procedure for their manufacture is described in Chemistry and Application of Phenolic Resins, Knop A. and Scheib, W.; Springer Verlag, New York, 1979 in Chapter 4 which is incorporated herein by reference. Similarly, the use of o-quinone diazides is well known to the skilled artisan as demonstrated by Light Sensitive Systems, Kosar, J.; John Wiley &amp; Sons, New York, 1965 in Chapter 7.4 which is also incorporated herein by reference. These sensitizers which comprise a component of the present U.V. resist compositions of the present invention are selected from the group of substituted naphthoquinone diazide sensitizers which are conventionally used in the art in positive photoresist formulations. Such sensitizing compounds are disclosed, for example, in U.S. Letters Pat. Nos. 2,797,213; 3,106,465; 3,148,983; 3,130,047; 3,201,329; 3,785,825; and 3,802,885. Useful photosensitizers include naphthoquinone-(1,2)-diazide-5-sulfonyl chloride, and naphthoquinone-(1,2)-diazide-4-sulfonyl chloride condensed with phenolic compounds such as hydroxy benzophenones. 
     In the preferred embodiment, the solid parts of the photoresist composition, that is the novolak and diazide preferably ranges from 15% to about 99% novolak resin and from about 1% to about 85% quinone diazide. A more preferred range of novolak resin would be from about 50% to about 90% and most preferably from about 65% to about 85% by weight of the solid resist parts. A more preferred range of the diazide would be from about 10% to about 50% and more preferably from about 15% to about 35% by weight of the solid resist parts. In manufacturing the resist composition the novolak and diazide are mixed with the propylene glycol alkyl ether acetate such that the acetate is present in an amount of from about 40% to about 90% by weight of the overall resist composition. A more preferred range is from about 60% to about 83% and most preferably from about 65% to about 70% by weight of the overall resist composition. 
     Additives such as colorants, dyes, anti-striation agents, plasticizers, adhesion promoters, speed enhancers, solvents and such surfactants as non-ionic surfactants may be added to the solution of novolak resin sensitizer and acetate before the solution is coated onto a substrate. 
     Examples of dye additives that may be used together with the photoresist compositions of the present invention include Methyl Violet 2B (C.I. No. 42535), Crystal Violet (C.I. 42555), Malachite Green (C.I. No. 42000), Victoria Blue B (C.I. No. 44045) and Neutral Red (C.I. No. 50040) at one to ten percent weight levels, based on the combined weight of novolak and sensitizer. The dye additives help provide increased resolution by inhibiting back scattering of light off the substrate. 
     Anti-striation agents may be used up to five percent weight level, based on the combined weight of novolak and sensitizer. 
     Plasticizers which may be used include, for example, phosphoric acid tri-(β-chloroethyl)-ester; stearic acid; dicamphor; polypropylene; acetal resins; phenoxy resins; and alkyl resins at one to ten percent weight levels, based on the combined weight of novolak and sensitizer. The plasticizer additives improved the coating properties of the material and enable the application of a film that is smooth and of uniform thickness to the substrate. 
     Adhesion promoters which may be used include, for example, β-(3,4-epoxy-cyclohexyl)-ethyltrimethoxysilane; p-methyl-disilanemethyl methacrylate; vinyltrichlorosilane; and γ-amino-propyl triethoxysilane up to a 4 percent weight level, based on the combined weight of novolak and sensitizer. 
     Speed enhancers that may be used include, for example, picric acid, nicotinic acid or nitrocinnamic acid at a weight level of up to 20 percent, based on the combined weight of novolak and sensitizer. These enhancers tend to increase the solubility of the photoresist coating in both the exposed and unexposed areas, and thus they are used in applications when speed of development is the overriding consideration even though some degree of contrast may be sacrificed; i.e., while the exposed areas of the photoresist coating will be dissolved more quickly by the developer, the speed enhancers will also cause a larger loss of photoresist coating from the unexposed areas. 
     Solvents may include xylene, butyl acetate and Cellosolve® acetate and may be present in the overall composition in an amount of up to 95% by weight although preferably no additional solvents are employed in the composition. 
     Non-ionic surfactants that may be used include, for example, nonylphenoxy poly(ethyleneoxy) ethanol; octylphenoxy (ethyleneoxy) ethanol; and dinonyl phenoxy poly(ethyleneoxy) ethanol at up to 10 percent weight levels, based on the combined weight of novolak and sensitizer. 
     The prepared resist solution, can be applied to a substrate by any conventional method used in the photoresist art, including dipping, spraying, whirling and spin coating. When spin coating, for example, the resist solution can be adjusted as to the percentage of solids content in order to provide coating of the desired thickness given the type of spinning equipment utilized and the amount of time allowed for the spinning process. Suitable substrates include silicon, aluminum or polymeric resins, silicon dioxide, doped silicon dioxide, silicon nitride, tantalum, copper, polysilicon, ceramics and aluminum/copper mixtures. 
     The photoresist coatings produced by the above described procedure are particularly suitable for application to thermally grown silicon/silicon dioxide-coated wafers such as are utilized in the production of microprocessors and other miniaturized integrated circuit components. An aluminum/aluminum oxide wafer can be used as well. The substrate may also comprise various polymeric resins especially transparent polymers such as polyesters. 
     After the resist composition solution is coated onto the substrate, the substrate is baked at approximately 80° to 105° C. Until substantially all the solvent has evaporated and only a thin coating of photoresist composition on the order of a micron in thickness remains on the substrate. The coated substrate can then be exposed to actinic radiation especially ultraviolet radiation in any desired exposure pattern, produced by use of suitable masks, negatives, stencils, templates, etc. 
     The exposed resist-coated substrates are next substantially immersed in alkaline developing solution. The solution is preferably agitated, for example, by nitrogen burst agitation. 
     The substrates are allowed to remain in the developer until all, or substantially all, of the resist coating has dissolved from the exposed areas. 
     After removal of the coated wafers from the developing solution, a post-development heat treatment or bake may be employed to increase the coating&#39;s adhesion and chemical resistance to etching solutions and other substances. The post-development heat treatment can comprise the oven baking of the coating and substrate below the coating&#39;s softening point. In industrial applications, particularly in the manufacture of microcircuitry units on silicon/silicon dioxide-type substrates, the developed substrates may be treated with a buffered, hydrofluoric acid base etching solution. The resist compositions of the present invention are resistant to acid-base etching solutions and provide effective protection for the unexposed resist-coating areas of the substrate. 
    
    
     The following specific examples will provide detailed illustrations of the methods of producing and utilizing compositions of the present invention. These examples are not intended, however, to limit or restrict the scope of the invention in any way and should not be construed as providing conditions, parameters or values which must be utilized exclusively in order to practice the present invention. 
     EXAMPLE 1 
     Two photoresist formulations are prepared as follows: 
     
         ______________________________________                                    
Resist A           Resist B                                               
______________________________________                                    
Novolak Resin  23.8%   Novolak Resin 23.8                                 
Photoactive            Photoactive                                        
component (Lot A)                                                         
                7.0%   component (Lot A)                                  
                                      7.0%                                
Control solvent                                                           
               69.2%   Propylene glycol                                   
                                     69.2%                                
composition comprising methyl ether                                       
10% xylene, 10% butyl  acetate                                            
acetate and 80% Cello-                                                    
solve acetate                                                             
______________________________________                                    
 
    
     The photoactive component is the condensation product of naphthoquinone-(1,2)-diazido-(2)5-sulfonyl chloride with trihydroxybenzophenone. The absorptivity of Resist A is 1.31 and the absorptivity of Resist B is 1.33, confirming that the photoactive component level in each case is the same. The novolak resin and photoactive component used in each case are the same. The resin is dissolved into the solvent by overnight stirring, the photoactive component is then dissolved and the solution passed through 0.2 um Millipore Teflon filters. 
     Evaluation for Photosensitivity 
     The photoresist is spin-coated on several wafers at a constant, predetermined spinspeed. The wafers are then baked at 90° C. for 30 minutes. The initial film thicknesses of coated resist are measured by a Rudolf film Thickness Monitor. Photosensitivity is measured by generating a contrast curve as described (G. C. Willson, in &#34;Introduction to Microlithography,&#34; Chapter 3, p. 105, American Chemical Society, Washington, D.C. 1983). One plots film thickness loss after one minute development versus the logarithm of the UV exposure dose. Film thickness is monitored by laser interferometry using the technique described. The resist was then developed using AZ Developer, available from American Hoechst Corporation, Somerville, New Jeresy, diluted 1:1 with deionized water, at 25.0°±0.5° C. By extrapolation of the plot to total film thickness loss, one obtains the photosensitivity value (mJ/cm 2 ) and the slope of this plot is the contrast. The results below were obtained for Resists A and B. 
     
         ______________________________________                                    
                 Resist A                                                 
                        Resist B                                          
______________________________________                                    
Photosensitivity (mJ/cm.sup.2)                                            
                   111.6    97.2                                          
Contrast           1.52     1.54                                          
Initial Film Thickness                                                    
                   1.811    1.797                                         
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     The results show that Resist B gives 15% improvement in photospeed in a formulation for comparable film thicknesses. There is no appreciable loss in contrast. 
     Erosion Rate 
     The rate of loss of the resist film in the unexposed areas upon development (erosion rate) should be kept low to retain edge acuity. The lower the erosion rate, the lower the amount of light required for exposure. Erosion rate is measured by developing an unexposed, baked (90° for 30 minutes) resist film for ten minutes in the developer diluted 1:1 with deionized water, and calculating the film thickness loss per minute. Below are the results for the A and B Resist. 
     
         ______________________________________                                    
                 Resist A                                                 
                        Resist B                                          
______________________________________                                    
Erosion Rate (°A/min )                                             
                   17       10                                            
______________________________________                                    
 
    
     Resist B has an erosion rate at least comparable to Resist A. 
     EXAMPLE 2 
     Example 1 is repeated using a different lot of the same photoactive component (Lot B). 
     
         ______________________________________                                    
Resist C           Resist D                                               
______________________________________                                    
Novolak Resin  23.3%   Novolak Resin 23.3%                                
Photoactive            Photoactive                                        
component (Lot B)                                                         
                6.9%   component (Lot B)                                  
                                      6.9%                                
Control solvent                                                           
               69.8%   Propylene glycol                                   
                                     69.8%                                
composition comprising methyl ether                                       
10% xylene, 10% butyl  acetate                                            
acetate and 80% Cello-                                                    
solve acetate                                                             
______________________________________                                    
 
    
     The bake time of the wafers after spin-coating is varied and photosensitivity and contrast measured as indicated in Example 1 above. 
     
         ______________________________________                                    
Resist C                                                                  
Bake times,                                                               
        Photo-              Resist D                                      
90° C.                                                             
        sensitivity         Photosensitivity                              
(minutes)                                                                 
        (mJ/cm.sup.2)                                                     
                  Constrast (mJ/cm.sup.2)                                 
                                      Contrast                            
______________________________________                                    
10       92       1.72      84        1.74                                
25      --        --        92        1.63                                
30      103       1.66      93        1.64                                
30      103       1.59      89        1.65                                
______________________________________                                    
 
    
     The photospeed of Resist D is improved, from 9 to 16% in the cases cited. 
     Flash Points of Resists 
     A higher flash point is a desirable property for safety. The flash points of Resist C and D were compared by ASTM Test (D93, Method A). 
     
         ______________________________________                                    
            Flash Point                                                   
______________________________________                                    
       Resist C                                                           
              106° F.                                              
       Resist D                                                           
              118° F.                                              
______________________________________                                    
 
    
     The Resist D shows a considerable improvement in flash point. 
     Plasma Etch Rate 
     The plasma etch rate of a resist film should be kept as low as possible for dry processing. For this plasma etch rate test, the coated resist samples are baked 140° C. for 30 minutes, as is typically by called a &#34;hard bake&#34;. Below are the plasma etch rates comparing Resists C and D. Conditions are 150 w. power, 0.6 Torr pressure, 95/5 CF 4  /O 2  ratio, 60 SCCM O 2  flow rate. 
     
         ______________________________________                                    
          Etch Rate (°A/min.)                                      
______________________________________                                    
Resist C    334                                                           
Resist D    313                                                           
______________________________________                                    
 
    
     The plasma etch rate of Resist D is at least comparable to Resist C. 
     Depth of Penetration Test 
     Another measure of positive resist sensitivity is the Depth of Penetration (DOP) Test, described by D. J. Elliot, in &#34;Integrated Circuit Fabrication Technology,&#34; Chapter 13, p. 325, McGraw Hill Book Company, New York (1982). The test measures the resist film thickness removed after development with a measured dose of UV light. The greater the film thickness removed for a given dose of UV light, the higher the sensitivity. Below are the results using AZ Developer, diluted 1:1 with deionized water. 
     
         ______________________________________                                    
Dose (mJ/cm.sup.2)                                                        
               Resist A (um)                                              
                          Resist B (um)                                   
______________________________________                                    
15.0           0.45       0.53                                            
22.0           0.75       0.80                                            
26.0           0.95       1.07                                            
31.0           0.10       1.44                                            
36.5           1.35       1.58                                            
______________________________________                                    
 
    
     Resist B is shown to have greater photosensitivity by this test. 
     Spin Curve Characteristics 
     Spin curves are used to predetermine the thickness of baked resist film from the spin speed of application. These curves must therefore be regular, generating a straight line when ploteed as log film thickness versus log spin speed. The two resists, Resists C and D, are each spun at various spin speeds as indicated below, baked at 90° C. for 30 minutes, and resist film thickness measured. 
     
         ______________________________________                                    
               Resist C Resist D                                          
RPM            (um)     (um)                                              
______________________________________                                    
2,000          2.47     2.79                                              
3,000          1.99     2.28                                              
4,000          1.77     1.95                                              
5,000          1.51     1.74                                              
6,000          1.38     1.60                                              
7,000          1.28     1.47                                              
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     When plotted as indicated, each resist generates a linear curve with high correlation. Since Resist D has a higher viscosity than Resist C (29.6 cSt versus 37.8 cSt), the two curves are displaced from each other. However, each resist generates a well-behaved spin curve required for predicting coating thickness. 
     EXAMPLE 3 
     Example 1 is repeated using a mixed solvent system for Resist B. 
     
         ______________________________________                                    
Resist A        Resist B                                                  
______________________________________                                    
Novolak Resin                                                             
           23.8%    Novolak Resin   23.8%                                 
Photoactive                                                               
            7.0%    Photoactive      7.0%                                 
component           component                                             
Control solvent                                                           
           69.2%    Solvent composition                                   
                                    69.2%                                 
composition         comprising 10% xylene, 10%                            
comprising                                                                
10% xylene, 10% butyl                                                     
                    butyl acetate and 80%                                 
acetate and 80%     Propylene glycol methyl                               
Cellosolve acetate  ether acetate                                         
______________________________________                                    
 
    
     
         ______________________________________                                    
                 Resist A                                                 
                        Resist B                                          
______________________________________                                    
Photosensitivity (mJ/cm.sup.2)                                            
                   111.6    98.2                                          
Contrast           1.52     1.54                                          
Initial Film Thickness                                                    
                   1.811    2.170                                         
______________________________________                                    
 
    
     The results show that Resist B gives a 12% improvement in photospeed in a formulation even when the propylene glycol methyl ether acetate is used in conjunction with other solvents. This is especially unexpected since Resist B has a significantly greater film thickness. 
     EXAMPLE 4 
     Example 1 is repeated except the use of an additional plasticizer is demonstrated. 
     
         ______________________________________                                    
Resist A           Resist B                                               
______________________________________                                    
Polymethyl vinyl ether                                                    
              15.43%   Polymethyl vinyl                                   
                                    15.43%                                
                       ether                                              
Novolak Resin 20.12%   Novolak Resin                                      
                                    20.12%                                
Photoactive   5.96%    Photoactive  5.96%                                 
component              component                                          
Control solvent                                                           
              58.5%    Propylene glycol                                   
                                    58.5%                                 
                       methyl                                             
composition            ether acetate                                      
comprising                                                                
10% xylene, 10% butyl                                                     
acetate and 80% Cello-                                                    
solve ether acetate                                                       
______________________________________                                    
 
    
     
         ______________________________________                                    
                Resist A      Resist B                                    
______________________________________                                    
Initial Film Thickness                                                    
                  6.05    um      6.74 um                                 
Photosensitivity (mJ/cm.sup.2)                                            
                  114.85          100.0                                   
______________________________________                                    
 
    
     The results show that Resist B gives a 14.85% improvement in photospeed in comparable formulations containing a plasticizer (polymethyl vinyl ether).