Abstract:
New baffles and methods of using these baffles are provided. The baffles comprise a body having an edge wall configured to direct the flow of a composition against a substrate (e.g., silicon wafer) edge. The edge wall comprises a vertical surface, a curved sidewall coupled to the vertical surface, and a lip coupled to the curved sidewall. A preferred baffle is annular in shape and formed from a synthetic resinous composition. Even more preferably, the baffle is not formed of a metal. The inventive methods comprise positioning the baffle adjacent a substrate during a spin coating process so that the edge wall causes the composition to cover the edges of the substrate and preferably a portion of the back side of the substrate.

Description:
RELATED APPLICATIONS 
     This application claims the priority benefit of U.S. Provisional Patent Application No. 60/626,034, filed Nov. 8, 2004, incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is broadly concerned with a baffle useful for directing a coating composition towards and over the edges of a substrate during microelectronic fabrication. 
     2. Description of the Prior Art 
     Microelectronic devices such as integrated circuits and microelectromechanical (MEMs) devices are typically formed by applying layers of coatings onto a substrate and forming those layers into the shapes and sizes needed for the particular device design. These layers are typically formed by spin coating a liquid composition onto the substrate, however, the coating does not typically flow to and over the edges of the substrate, thus leaving them unprotected. The substrates are subsequently subjected to etching and polishing processes. These are relatively harsh processes. As a result, the coatings often experience lift-off problems at their edges, i.e., the edges of the coatings pull away from the substrate. This leaves the substrate unprotected at its edges and vulnerable to subsequent processing conditions. The substrate will often become thinner and susceptible to cracking and breaking. This results in a reduction in wafer yield, thus increasing cost. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes these problems by broadly providing a novel baffle for affecting the flow of a composition during application of the composition to a substrate and directing that flow to and over the edge, and possibly back side, of the substrate. 
     In more detail, the baffle preferably comprises a body that is annular in configuration and includes an inner edge wall defining an opening and configured to direct the flow of the composition to the substrate edge. The edge wall comprises a vertical surface, a curved sidewall coupled with the vertical surface, and a lip coupled with the curved sidewall. 
     In use, the baffle and substrate are positioned so that the substrate edges are adjacent to, but preferably not in contact with, the baffle edge wall. The composition is then applied to the substrate via a typical spin coat process, with the centrifugal forces causing the composition to flow to the outer periphery of the substrate, and thus towards the baffle. The baffle edge wall causes the composition to cover the edges of the substrate. The wafer can then be subjected to further processing (e.g., baking/curing, etching, further layer application, etc). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         FIG. 1  is a schematic drawing depicting the problems associated with the prior art processes; 
         FIG. 2  is a perspective view of a baffle according to the present invention; 
         FIG. 3  is a sectional view taken along line  3 - 3  of the baffle of  FIG. 2 ; 
         FIG. 4  is a schematic drawing illustrating a coating process utilizing the inventive baffle; 
         FIG. 5  is a Scanning Electron Microscope (SEM) photograph depicting the edge view of a virgin silicon wafer; 
         FIG. 6   a  is an SEM showing the edge of a silicon wafer after being coated with a protective material according to a prior art process; 
         FIG. 6   b  is an enlarged view of the edge of  FIG. 6   a;    
         FIG. 7  is an SEM depicting the “knife-edge” obtained after the wafer of  FIG. 6   a  was subjecting to a wet etching process; 
         FIG. 8   a  is an SEM illustrating the edge of a silicon wafer after being coated with a protective material while using the inventive baffle; 
         FIG. 8   b  is an enlarged view of  FIG. 8   a , showing the edge of the protective coating on the wafer; 
         FIG. 9  is an SEM showing the edge of the wafer of  FIG. 8   a  after it was subjected to a wet etching process; 
         FIG. 10  is an SEM illustrating the back of a wafer after it was coated with a protective material; 
         FIG. 11  is an SEM of the wafer of  FIG. 10  after the wafer was subjected to a wet etching process; 
         FIG. 12  is an enlarged view of the SEM of  FIG. 11 ; and 
         FIG. 13  is an SEM photograph illustrating the back of the wafer of  FIGS. 9-12  after etching and after the protective coating has been removed. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a process according to the prior art. A substrate  10  having edge  12  is provided. A protective material  14  is applied to the substrate  10  via dispense nozzle  16  and formed into a film  18  via a spin coating process. The substrate  10  is then heated on a hot plate  20  and subjected to further processing. As shown in  FIG. 1 , the film  18  does not cover edge  12 , thus leaving it unprotected during subsequent processing steps such as wet etching. 
     The baffle according to the present overcomes this problem with its novel design. Referring to  FIGS. 2 and 3 , a baffle  22  is provided. Baffle  22  comprises an annular base  24  and support member  26 . Annular base  24  includes upper and lower surfaces  28 ,  30  and circumferential outer and inner edges  32 , 34 , respectively. Upper surface  28  slopes slightly towards surface  30  as it approaches edge  34 . 
     Inner edge or edge wall  34  defines an opening  36 . Edge  34  includes vertical surface  38  and a lower lip  40 . Vertical surface  38  and outer edge  32  are substantially parallel to one another. Vertical surface  38  and lip  40  are joined via a bight or curved sidewall  42 . It will be appreciated that lower lip  40  extends beyond vertical surface  38 , however, this distance should be limited to allow the composition to collect within curved sidewall  42 , but still allow the composition to drain past the lower lip  40 . Thus, the length of “L” should be from about 1.5-4 times, and more preferably from about 2-3 times. The length of “l ,” where “l” is the shortest distance from the innermost point (i.e., apex) in curved sidewall  42  to a line  44  that extends vertically from vertical surface  38  towards lower surface  30 , and “L” is the distance from line  44  to the end  46  of lower lip  40 , as shown in  FIG. 3 . 
     Lip  40  comprises an upper lip surface  48  that preferably slopes in a downward direction towards lower surface  30 . The angle of slope of upper lip surface  48  is preferably from about 1-15°, and even more preferably from about 2-10°, with a perfectly horizontal line being a slope of 0. 
     Support member  26  of baffle  22  comprises an upright member  50  and a flange  52 , with flange  52  preferably being substantially perpendicular to upright member  50 . In a preferred embodiment, flange  52  will comprise at least  2 , and preferably at least  3 , leveling devices  54 . Preferred devices  54  comprise protrusions  56  extending therefrom, with each of these protrusions  56  comprising an opening  58  configured to receive an adjustable fastener such as a set screw (not shown) that can be used to adjust the baffle  22  as necessary. It will be appreciated that the above specifications allow the baffle to be sized for the particular process conditions (e.g., substrate size, equipment being used, etc.), making the inventive baffle quite versatile. 
     Baffle  22  can be formed of a number of different materials, depending upon the equipment, process conditions, etc. to which it will be exposed. However, it is preferred that the baffle  22  be formed of a synthetic resin composition that is resistant to (i.e., will not react with or be eroded by) solvents that are typically utilized during microelectronic manufacturing. Such solvents include those selected from the group consisting of propylene glycol monomethyl ether acetate, propylene glycol monomethyl ether, ethyl lactate, methyl isoamyl ketone, n-methyl-2-pyrrolidone, and isopropyl alcohol. 
     It is also preferred that the baffle  22  be formed of a non-metallic material. That is, the material forming baffle  22  would comprise less than about 5% by weight, preferably less than about 1% by weight, and more preferably 0% by weight metal, based upon the total weight of the baffle taken as 100% by weight. 
     Particularly preferred materials of which baffle  22  can be formed include those selected from the group consisting of polytetrafluoroethylene (TEFLON®), polyethylene (preferably high density), polypropylene, polyphenylene sulfide, acetals, polyether ether ketone (available under the name PEEK from Tangram Technology Ltd.), and mixture of the foregoing. 
       FIG. 4  illustrates the use of the inventive baffle  22 . First, the substrate  10  is positioned on a chuck (not shown) within a spin coating apparatus. Examples of typical substrates  10  include those selected from the group consisting of silicon, silicon dioxide, silicon nitride, aluminum gallium arsenide, aluminum indium gallium phosphide, gallium nitride, gallium arsenide, indium gallium phosphide, indium gallium nitride, indium gallium arsenide, aluminum oxide (sapphire), glass, quartz, polycarbonates, polyesters, acrylics, polyurethanes, papers, ceramics, and metals (e.g., copper, aluminum, gold). 
     The inventive baffle  22  is then placed in the apparatus so that flange  52  rests upon a surface of the apparatus (not shown) and so that the edge  12  of the substrate  10  is approximately centered adjacent curved sidewall  42  within opening  36 . The baffle  22  is adjusted as necessary via the height-adjusting devices  54  (not shown in  FIG. 4 ) so that the baffle  22  is level, and the substrate  10  is centered properly. It is preferred that the baffle is sized so that the distance “D” from the innermost point or apex in curved sidewall  42  to the substrate edge  12  is a distance of from about 0.85-4 mm, and more preferably from about 0.85-2 mm. Thus, the substrate and baffle are preferably not in contact with one another. 
     After the baffle  22  and substrate  10  are properly positioned, the substrate is rotated while dispensing material  14  via dispense nozzle  16 . Unlike the prior art processes, however, the material  14  will accumulate in the baffle  22  in the recess created by curved sidewall  42 , and the material  14  will be directed towards the substrate edge  12  and even to the back surface  60  as shown in area  62 . Thus, the substrate edge  12  and at least part of the back surface  60  will be covered with the protective film  18 , protecting the substrate  10  from etching, thinning, and other damage during subsequent processing steps. More particularly, when using the inventive baffle, at least about 90%, preferably at least about 95%, and even more preferably about 100% of the surface area of the substrate edge  12  will be covered by the composition being applied to the substrate. 
     EXAMPLES 
     The following examples set forth preferred methods in accordance with the invention. It is to be understood, however, that these examples are provided by way of illustration and nothing therein should be taken as a limitation upon the overall scope of the invention. 
     Example 1 
     Comparative Example 
     A prior art process was carried out without using the inventive baffle in order to illustrate the drawbacks of that process. 
     Virgin silicon wafers have smooth, rounded edges.  FIG. 5  is an SEM photograph depicting such an edge. An etch protective film comprising ProTEK primer and ProTEK B (available from Brewer Science Inc.) was spin coated onto a virgin silicon water using a standard spin coating process.  FIG. 6   a  shows the etch protective film on the wafer. As shown in  FIG. 6   b , the edge was not completely covered. The silicon wafer was then subjected to a wet etch process using potassium hydroxide. As shown in  FIG. 7 , the etch process resulted in a “knife-edge” at the edge of the wafer. The etching process resulted in lifting of the film, thereby providing poor protection to the wafer edge. Such a thin wafer edge presents significant handling problems as it generally leads to the development of cracks on the wafer, leading to breakage and a reduction in yield. This problem is even more pronounced in thin-wafer handling. 
     Example 2 
     Use of Inventive Baffle 
     The process described in Example 1 was repeated except that a baffle according to the invention, sized according to the wafer size, was used. During spinning, the baffle collected the excess etch protective material and coated it on the edge and back of the wafer. As a result, the etch protective material applied to the top, edge, and under-side rim of the wafer, thus encapsulating the wafer and its edges and preventing lifting of the film at the wafer edge. 
       FIG. 8   a  shows the etch protective material wrapping around and covering the edge of the wafer and continuing to the back of the wafer.  FIG. 8   b  further illustrates that the edge of the etch protective coating is well from the edge on the back side of the wafer. 
     The wafer with protective coating in place was then subjected to a wet chemical etching process using potassium hydroxide.  FIG. 9  depicts the wafer after chemical etching, and the edge of the wafer is intact. 
     Example 3 
     Use of Inventive Baffle with Backside Dispense 
     The process of Example 2 was repeated except that a backside dispense was added to the spin bowl to create a larger area of protection against wet chemical etching on the wafer. The wafer was coated with a protective material as described in Example 2. With the backside dispense being utilized, the protective material was coated on the back of the wafer from the edge to about 5 mm from the wafer edge.  FIG. 10  shows the back of the silicon wafer after having been coated in this example. 
     The coated wafer was then subjected to a wet etch process using potassium hydroxide.  FIGS. 11 and 12  illustrate the wafer after etching. The protective material lifted less than 1 mm from the edge of the coating. The protective material was then removed from the wafer, and the SEM photograph of  FIG. 13  of the back of the wafer was taken. The entire outer ring of the wafer was protected from etching and remained intact.