Apparatus and method for autofocus

Abstract

A laser generates a collimated laser beam which passes through a lens off-axis. The beam is focused at a focal plane on a substrate surface. A first position sensitive detector receives the laser beam reflected from the substrate surface through the lens to generate a first signal proportional to lateral beam offset. A beam splitter may be provided to direct a portion of the laser beam before passing through the lens toward a second position sensitive detector to generate a second signal proportional to laser beam pointing instability. Apparatus computes the difference between the first and second signals, the difference being a defocused error signal. It is preferred that the first position sensitive detector be located at a distance from the lens that is at least twice the lens focal length.

What is claimed is:  
     
         1 . Autofocus apparatus, comprising: 
 a laser for generating a collimated laser beam;    a scan lens positioned to receive the laser beam off-axis and to focus the beam at a focal plane on a substrate surface;    a position sensitive detector for receiving the laser beam reflected from the substrate surface through the scan lens to generate a defocus error signal proportional to lateral beam offset; and    means for reducing the defocus error signal to achieve autofocus during scanning by varying the distance between the scan lens and the position sensitive detector.    
     
     
         2 . The autofocus apparatus of  claim 1  wherein the position sensitive detector is located at a distance from the scan lens that is at least twice a focal length of the scan lens.  
     
     
         3 . The apparatus of  claim 1  wherein a proportionality factor linking defocus and lateral displacement also varies.  
     
     
         4 . The apparatus of  claim 3  further including computational circuitry to correct the varying proportionality factor.  
     
     
         5 . Autofocus method, comprising: 
 generating a collimated laser beam;    positioning a lens to receive the laser beam off-axis to focus the beam at a focal plane on a substrate surface;    detecting the laser beam lateral offset after reflection from the substrate surface through the lens to generate a lateral offset defocus error signal; and    reducing the defocus error signal to achieve autofocus during scanning by varying the distance between the lens and the position sensitive detector.    
     
     
         6 . The method of  claim 5  wherein a proportionality factor linking defocus and lateral displacement also varies.  
     
     
         7 . Autofocus apparatus, comprising: 
 a laser for generating a collimated laser beam;    a lens positioned to receive the laser beam off-axis and to focus the beam at a focal plane on a substrate surface;    a position sensitive detector for receiving the laser beam reflected from the substrate surface through the lens to generate a defocus error signal;    an actuator adapted to move the substrate or the lens to decrease the defocus error signal; and    means for reducing the defocus error signal to achieve autofocus during scanning by varying the distance between the lens and the position sensitive detector.    
     
     
         8 . The apparatus of  claim 7  wherein a proportionality factor linking defocus and lateral displacement also varies.  
     
     
         9 . The apparatus of  claim 8  further including computational circuitry to correct the varying proportionality factor.