Autofocus system using common path interferometry

Abstract

An autofocus system for use for instance in a photolithography machine (but not so limited) uses oblique incident optics which transform a defocus of the plane of the wafer whose location is being detected to a lateral shear of the beam. A common path interferometer measures the lateral shear. The common path interferometer is for instance a triangular path interferometer. Since both the signal (object) light beam and the reference light beams transverse almost exactly the same optical path, the system is thereby independent of mechanical vibration or other environmental influence such as temperature variations. Compensation is provided for refractive effects caused by local atmospheric heating immediately above the wafer surface.

What is claimed is:  
     
       1. A method of measuring a deviation between a reflective surface and a nominal position of the reflective surface, comprising: 
       directing a first beam and a second beam onto the reflective surface; the first beam and second beam being parallel and spaced apart;  
       detecting the first beam and second beam as reflected from the reflected surface; and  
       determining the deviation as a function of a distance, in a plane not parallel to that of the reflective surface, between axes defined by respectively the reflected second beam, the reflected first beam, and the incident second beam.  
     
     
       2. The method of claim  1 , where the reflected second beam and reflected first beam define triangular paths moving in an opposite direction to that of the incident second beam. 
     
     
       3. The method of claim  1 , wherein the second beam and first beam follow substantially the same optical path. 
     
     
       4. The method of claim  1 , wherein the determining the deviation includes transforming the deviation into a change of a spatial frequency of an interference fringe. 
     
     
       5. An apparatus for measuring a deviation between a reflective surface and a nominal position of the reflective surface, comprising: 
       an illumination source;  
       a beam splitter located to split illumination from the source into a first beam and a second beam and to direct one of the first beam or second beam onto the reflective surface;  
       a reflector located to direct the other of the first beam or second beam onto the reflective surface;  
       a detector located to detect the first beam and second beam reflected from the reflective surface, and to determine the deviation as a function of a distance in a plane not parallel to that of the reflective surface, between axes defined by respectively the reflected second beam, the reflected first beam, and the illumination from the source.  
     
     
       6. The apparatus of claim  5 , where the reflected second beam and reflected first beam define triangular paths moving in an opposite direction to that of the incident second beam. 
     
     
       7. The apparatus of claim  5 , wherein the second beam and first beam follow substantially the same optical path. 
     
     
       8. The apparatus of claim  5 , further comprising means for transforming the deviation into a change of a spatial frequency of an interference fringe. 
     
     
       9. A common path interferometer for measuring a distance to a reflective surface, comprising: 
       a beam splitter which directs a first beam and a second beam onto the reflective surface, wherein the first beam and second beam are parallel to one another but travel in opposite directions; and  
       a detector located to detect the first beam and second beam reflected from the reflective surface and to determine the distance to the reflective surface as a function of a distance, in a plane orthogonal to that of the reflective surface, between the reflected first beam and the reflected second beam.  
     
     
       10. A method of interferometrically measuring a distance to a reflective surface, comprising: 
       directing three beams of light onto the surface, there being a differential temperature layer overlying the surface, the three beams of light having different angles of incidence onto the surface;  
       receiving the beams of light reflected from the surface; and  
       interferometrically determining from the received beams a distance to the surface, a thickness of the layer, and an index of refraction of the layer.  
     
     
       11. The method of claim  10 , wherein the receiving is by common path interferometry. 
     
     
       12. The method of claim  10 , wherein the surface is a surface of a semiconductor wafer, and further comprising heating the wafer by incident exposing illumination, thereby heating the layer. 
     
     
       13. The method of claim  10 , wherein the layer is a layer of heated air.