Abstract:
A method for spin coating high viscosity materials. Two dispense steps are used. The first dispense step dispenses a small amount ( 102 ) of high viscosity material at the center of the wafer ( 100 ). A high-speed wafer rotation spreads the material to form a thin layer ( 104 ) to prime the surface of the wafer ( 100 ) and lower the surface tension without regard to uniformity. A second dispense step occurs at lower RPM and coats ( 108 ) the wafer ( 100 ) more uniformly.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention is generally related to the field of high viscosity materials for semiconductor processing and more specifically to photolithography using high viscosity materials.  
         BACKGROUND OF THE INVENTION  
         [0002]    High viscosity materials are applied to semiconductor devices for the purpose of buffer dielectric of photoresist coating. The term high viscosity refers to materials with greater than 1000 centipoise. These viscous materials are difficult to coat with good thickness uniformity. Thickness uniformity becomes more difficult as the diameter of starting wafers gets larger. The centrifugal on the puddle of the material causes internal shear forces that disrupt the flow of fluid. It is difficult to uniformly move the material from the center of the wafer to the edges.  
           [0003]    A first method of spin coating, uses a center dispense with high spin speeds. Normally, a high rate of dispense is combined with high RPM (revolutions per minute) spin speeds to force the material from the center dispense point to the edge of the wafer. High viscosity materials have a limited dispense rate typically around 0.5 ml/second maximum. As the dispense RPM increases, the material  12  has a tendency to “ball up”( 14 ) at the edges and not completely cover the wafer  10 , as shown in FIGS. 1A and 1B. If a lower dispense speed is used a “dome” profile in resist thickness results, as shown in FIG. 2.  
           [0004]    Another method of spin coating uses a swirl dispense method. In this method, a dispense arm is scanned across the wafer as the wafer is rotated. After nearly one revolution, the arm is moved towards the center a precise amount. Another revolution is performed and the arm is moved in again. The dispense is finished at the center and the water rotation is stopped. The wafer is then spun until the material is somewhat uniformly distributed. The wafer is then cast to final thickness. Problems with this method include: (1) timing difference between the pump spin motor and dispense arm, (2) variations in the swirl caused by batch to batch viscosity differences, and (3) high maintenance is required.  
         SUMMARY OF THE INVENTION  
         [0005]    The invention is a method for spin coating high viscosity materials. Two dispense steps are used. The first dispense step dispenses a small amount of high viscosity material at the center of the wafer. A high-speed wafer rotation spreads the material to form a thin layer to prime the surface of the wafer and lower the surface tension without regard to uniformity. A second dispense step occurs at lower RPM and coats the wafer more uniformly.  
           [0006]    An advantage of the invention is providing a simple method for spin coating a high viscosity material more uniformly across a wafer surface.  
           [0007]    This and other advantages will be apparent to those of ordinary skill in the art having reference to the specification in conjunction with the drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    In the drawings:  
         [0009]    [0009]FIG. 1A is a cross-sectional diagram of a prior art dispense method;  
         [0010]    [0010]FIG. 1B is a top view of resist dispensed using a prior art method;  
         [0011]    [0011]FIG. 2 is a 3-D view of resist dispensed using a prior art swirl method;  
         [0012]    FIGS.  3 A- 3 D are top views of a high viscosity material dispensed according to an embodiment of the invention at various stages in the process; and  
         [0013]    [0013]FIG. 4 is a 3-D view of a high viscosity material dispensed according to an embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0014]    The invention will now be described in conjunction with a photoresist process for forming patterns on semiconductor wafers. It will be apparent to those of ordinary skill in the art that the benefits of the invention may be applied in general to dispensing high viscosity materials over a semiconductor wafer such as polyamide buffer coating or thick film dielectrics.  
         [0015]    Photoresist patterns are used at many stages in a semiconductor fabrication process. Photoresist patterns are mainly used to mask implants or to mask various layers during an etch. At each stage a coating material must be applied. The preferred embodiment of the invention may be used to dispense photoresist material at any of these stages.  
         [0016]    A preferred embodiment of the invention will now be discussed with reference to FIGS.  3 A- 3 D. A semiconductor wafer  100  is processed to the point where a coating is desired. For example, if the desired coating is for forming an interconnect pattern, semiconductor wafer  100  may have been processed through the formation of transistors, isolation, and interlevel and intrametal dielectrics. In a first step of the invention, a small puddle  102  of high viscosity material, such as a high viscosity organic coating material, is dispensed in the center of wafer  100 , as shown in FIG. 3A. Approximately 10-40% of the total high viscosity material to be used is dispensed at this point. For example, approximately 2 cc of a 6 cc dispense may be used for a 200 mm wafer.  
         [0017]    In a second step of the invention, the wafer  100  is spun rapidly to move the material of puddle  102  to the edge of the wafer  100 . As a result, a thin layer  104  of high viscosity material covers wafer  100 , as shown in FIG. 3B. No high viscosity material is dispensed during this step. The wafer may be spun at a high speed in the range of 2000-4000 RPMs during this step. Low, medium and high speed designations are a function of the wafer size, the substrate, and the material being coated. Generally, the higher speed in this disclosure refers to a speed of 60-100% of maximum. The uniformity of the coverage at this point is of no concern. The purpose of thin layer  104  is to ‘prime’ the surface of wafer  100  and lower the surface tension for the second dispense step.  
         [0018]    Next, the rotation of the wafer is slowed down to between 0%-30% of maximum or 0%-50% of the casting speed. As an example, this may be on the order of 0-1500 RPMs. A second puddle  106  is formed in the center of the wafer with the remaining portion of the high viscosity material, as shown in FIG. 3C. As indicated in the ranges above, this may be a static dispense. For example, the remaining portion may be approximately 4 cc of a 6 cc dispense.  
         [0019]    Using a medium rotational speed, the high viscosity material  108  of second puddle  106  spreads evenly across the wafer  100 , as shown in FIG. 3D. The medium rotational speed is in the range of 20-60% of maximum or 30-50% of the casting speed. As an example, this may be in the range of 100-1500 RPMs. Uniformity is obtained because (1) the first thin layer  104  lowers the surface tension and primes the surface of wafer  100  and (2) the slower rotational speed results in a more even distribution. The centrifugal forces are reduced which results in a more uniform coat and a reduction in internal shear forces.  
         [0020]    Finally, the wafer is spun to the casting speed to set the thickness of the high viscosity material  108 . Processing then continues to form the desired pattern in high viscosity material  108 .  
         [0021]    The invention uses two distinct dispense steps separated by a high speed spin. The invention provides improved uniformity with the simplicity of a center dispense. FIG. 4 is a 3-D view of high viscosity resist deposited according to the invention. The thickness variation was 1265 Å for an approximately 80,000 Å film. This compares to a variation of 4737 Å for the prior art center dispense method.  
         [0022]    In addition to improved uniformity, a complicated, high maintenance dispense pattern, such as in the prior art swirl dispense, is avoided. The dispense method of the invention also uses less high viscosity material than a swirl dispense pattern (e.g., 6 cc vs. 8 cc) or a single puddle center dispense.  
         [0023]    While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.