Apparatus and method for holding and planarizing thin workpieces

Abstract

An apparatus and method for holding a thin workpiece such as a semiconductor wafer for operations which require the workpiece to have a high degree of planarity such as photolithographic printing includes positioning the workpiece onto a planar holding face comprising the points of a multiplicity of regularly spaced-apart substantially parallel pins with a thin rim encompassing all pins to contain a vacuum in the region adjacent to the workpiece. The small abutting area of each pinpoint abutment reduces the probability of dirt particles collecting on the holding face and provides a high thrust pressure to dislodge dirt particles interposed between the abutment and the workpiece. A small amount of lateral motion is imparted to the workpiece when it first contacts the holding face to brush off any dirt particles on the abutments.

I claim: 
     
       1. A vacuum apparatus for holding and planarizing a thin workpiece on a holding face which minimizes the interposition of dirt particles between said workpiece and said holding face, said apparatus comprising: (a) a baseplate having a rigid raised rim which includes a smooth planar edge surface adapted to support a workpiece having front and back surfaces, said edge surface being adapted to contact the periphery of said back surface to make a substantially airtight seal thereto, thus forming an evacuable chamber enclosed by said baseplate, said rim, and said back surface;   (b) a plurality of .[.regularly.]. spaced-apart rigid support members disposed within said chamber, each providing a localized pinpoint abutment with said back surface, said abutments together with said edge surface forming a holding face which is substantially planar, each of said abutments having an area sufficiently small to reduce the probability of dirt particles collecting thereon, evacuation of said chamber producing a pressure difference between said front and back workpiece surfaces forcing said back surface to conform to said holding face, the spacing between adjacent ones of said abutments and the spacing between said abutments nearest said rim and said edge-surface being sufficiently small to prevent significant distortion of the unsupported regions of said workpiece.   
     
     
       2. An apparatus as in claim 1 wherein the area of each of said abutments is in the range of 2.0×10 -5  to 2.0×10 -3  square inches. 
     
     
       3. An apparatus according to claim 2 wherein said support members comprise substantially parallel pins mounted in said baseplate and wherein said abutments comprise the points of said pins. 
     
     
       4. An apparatus according to claim 3 wherein said baseplate is made from aluminum, said rim is made from steel, and said pins are made from alumina. 
     
     
       5. In the fabrication of semiconductive devices, a method for positioning and planarizing a substrate wafer having a front and a back surface, said method comprising the steps of: loosely positioning said wafer in a general position;   applying substantially uniformly distributed forces along said front surface;   applying a multiplicity of localized forces along said back surface, said back surface forces counterbalancing said front surface forces and being sufficient to planarize said wafer;   applying each of said localized forces to an area of said back surface which area is in the range of 2×10 -5  to 2×10 -3  square inches;   initially moving said wafer in a direction transverse to that of said localized forces so as to dislodge any dirt particles in contact with said back surface.   
     
     
       6. In the fabrication of semiconductive devices, a method for positioning and planarizing during pattern exposure a substrate wafer having a front surface and a back surface, comprising the steps of: loading said wafer onto a wafer holding face with wafer loading means, said back surface of said wafer contacting said holding face, .Iadd.said loading .Iaddend.providing lateral motion to said wafer .[.with said loading means.]. when said back surface initially contacts said holding face,   supporting said back surface of said wafer with a multiplicity of .[.regularly.]. spaced apart rigid pinpoint abutments comprising said holding face, said abutments providing a multiplicity of localized support regions all lying in one plane, each of said support regions having a maximum dimension which is less than the extent of said lateral motion of said wafer whereby dirt particles situated on said abutments are brushed off said abutments when said wafer is loaded onto said holding face,   while said wafer is so supported, evacuating a region adjacent to said back surface to create a force to hold said wafer against said holding face in a manner that ensures that said front surface exhibits a high degree of planarity, and   while said wafer is so positioned and planarized, exposing a high resolution pattern onto said front surface of said wafer.