Patent Publication Number: US-2011076858-A1

Title: Methods for Coating the Backside of Semiconductor Wafers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Patent Application No. PCT/US2009/046866 filed Jun. 10, 2009, which claims the benefit of U.S. Provisional Patent Application No. 61/060,286 filed Jun. 10, 2008, the contents of both of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to coating methods for semiconductor wafers, and particularly to methods for coating the backside of semiconductor wafers with minimal coating material waste. 
     BACKGROUND OF THE INVENTION 
     Due to the ever-increasing demand for smaller, more powerful, lightweight electronic devices, electronic manufacturers have been required to use very thin wafers for the production of active microchips. During fabrication of a semiconductor device, various processes are performed on a semiconductor wafer to form microelectronic components on the wafer. One such process involves coating of the backside (inactive face) of the thin wafer with an adhesive or support material prior to dicing. This process is commonly referred to as wafer backside coating (WBC). 
     Typically, the backside of a wafer is coated by one of three methods: screen printing, stencil printing, or spin coating. Each method has its advantages and disadvantages. Screen printing provides an even coating thickness with fast coating speeds, but the coating cannot be dispensed all the way to the edge of the wafer. This can result in the chip flying during dicing (die fly), as well as wafer breakage and blade breakage. Stencil printing provides various coating thicknesses with fast coating speeds, but, as with screen printing, the coating can not be dispensed all the way to the edge of the wafer, and it is difficult to obtain an even coating thickness over the entire area of the wafer. Spin coating does result in entire coverage of the wafer, but it is much slower than stencil printing or screen printing and suffers from a great deal of wasted coating, up to 40% by weight. Accordingly, there is a continuing need for improved wafer back coating processes. 
     SUMMARY OF THE INVENTION 
     This invention is a method for depositing a coating onto the entire backside of a semiconductor wafer using the combination of stencil or screen printing and spin coating, which corrects the deficiencies associated with using only one of the typical deposition processes for coating the backside of semiconductor wafers. The stencil or screen printing operation deposits the majority of the coating, and then the remainder of the coating is deposited to the edge of the wafer with spin coating. 
     Thus, in one embodiment of the invention, the method comprises (a) providing a semiconductor wafer, (b) depositing the coating onto the backside of the wafer, wherein the coating is not deposited at the edge of the wafer, and thereafter (c) spinning the wafer so that the coating deposited in step (b) flows to the edge of the wafer, thereby depositing a coating on the entire backside of a semiconductor wafer. 
     In another embodiment of the invention, the method comprises (a) providing a semiconductor wafer, (b) stencil or screen printing the coating onto the backside of the wafer, wherein the radial extension of the stencil or screen printed coating is less than the radius of the wafer, and thereafter (c) spinning the wafer so that the coating deposited in step (b) flows to the edge of the wafer, thereby depositing a coating onto the entire backside of the semiconductor wafer. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention claimed. As used herein, the use of the singular includes the plural unless specifically stated otherwise. As used herein, “or” means “and/or” unless stated otherwise. Furthermore, use of the term “including” as well as other forms, such as “includes,” and “included,” is not limiting. The section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. 
     As used herein, the term “coating” refers to any material that can be dispensed via stencil or screen printing onto the backside of a wafer. 
     As used herein, the phrase “coating waste” refers to the amount of coating material that is lost from the backside of the wafer after performing the methods of the invention. Coating waste can be readily determined by weighing the amount of coating on the backside of the wafer after stencil or screen printing but before spinning, and then weighing the amount of coating on the backside of the wafer after spinning. The difference is “coating waste”, measured in % by weight. 
     Semiconductor wafers used in the methods of the invention are typically 0.025 mm to 1 mm thick and range in diameter from 1 inch (25 mm) to 12 inch (300 mm). 
     In some embodiments of the invention, the coating is an adhesive. In some embodiments the adhesive is selected from the group consisting of maleimides, polyesters, (meth)acrylates, urethanes, epoxies, vinyl esters, olefinics, styrenics, oxetanes, benzoxazines, oxazolines, and the like. 
     In addition to screen printing or stencil printing, it is to be understood that in step (b) of the method of the invention, the coating may be deposited by any procedure that does not adequately cover the entire backside of the wafer, although screen printing and stencil printing are the two most widely used methods currently. As will be shown in the Examples, this combination of screen or stencil printing with spin coating is an efficient way to coat the entire backside surface of a semiconductor wafer without the accompanying waste attendant on using only one method. 
     EXAMPLES 
     Comparative Example 
     In a traditional spin coating process, the coating material is deposited in the center of backside of the wafer and spun at different speeds (in revolutions per minute, “rpm”) for various time periods (in seconds, “s”). The data in the following tables were generated during a traditional spin coating process using a seven step protocol, with a different speed and time interval for each. 
     
       
         
           
               
             
               
                   
               
               
                 SPIN COATING PROTOCOL 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Step 1 
                 Step 2 
                 Step 3 
                 Step 4 
                 Step 5 
                 Step 6 
                 Step 7 
               
               
                   
               
               
                 300 rpm 
                 400 rpm 
                 500 rpm 
                 700 rpm 
                 1000 rpm 
                 1250 rpm 
                 150 rpm 
               
               
                  20 s 
                  20 s 
                  20 s 
                  20 s 
                  40 s 
                  30 s 
                  5 s 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Comparative 
                 Weight (grams) 
                 Weight (grams) 
                   
               
               
                 Example 
                 Before Step 1 
                 After Step 7 
                 Percent Waste 
               
               
                   
               
               
                 1 
                 3.905 
                 2.435 
                 37.6 
               
               
                 2 
                 3.945 
                 2.330 
                 40.9 
               
               
                 3 
                 3.895 
                 2.330 
                 40.2 
               
               
                 4 
                 3.992 
                 2.363 
                 40.8 
               
               
                 5 
                 3.704 
                 2.326 
                 37.2 
               
               
                   
               
            
           
         
       
     
     Invention Example 
     The data in the following table were generated using the methods of the invention. It can be seen that invention methods produce significantly less coating material waste than traditional WBC methods. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                   
               
               
                   
                   
                   
                   
                 WBC 
                 WBC 
                   
               
               
                   
                   
                 After 
                 After 
                 weight 
                 weight 
               
               
                   
                 Bare 
                 stencil 
                 spin 
                 after 
                 after spin 
               
               
                   
                 wafer wt 
                 printing 
                 coat 
                 printing 
                 coating 
                 Percent 
               
               
                   
                 (grams) 
                 (grams) 
                 (grams) 
                 (grams) 
                 (grams) 
                 waste 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Sample 1 
                 29.04 
                 30.73 
                 30.58 
                 1.69 
                 1.54 
                 8.6 
               
               
                 Sample 2 
                 29.11 
                 30.49 
                 30.39 
                 1.38 
                 1.28 
                 7.2 
               
               
                 Sample 3 
                 29.16 
                 30.68 
                 30.54 
                 1.52 
                 1.38 
                 8.9 
               
               
                   
               
            
           
         
       
     
     The invention provides novel methods for depositing a coating onto the entire backside of a semiconductor wafer. The methods of the invention result in wafers coated all the way to the edge, thereby minimizing problems such as chip flying and wafer breakage during dicing of the wafer. In addition, the methods of the invention result in typically less than 10% coating waste, compared to 30-40% seen with traditional spin coating methods.