System and method for compensating instability in an autofocus system

Abstract

An autofocus system and method designed to account for instabilities in the system, e.g. due to instabilities of system components (e.g. vibrating mirrors, optics, etc) and/or environmental effects such as refractive index changes of air due to temperature, atmospheric pressure, or humidity gradients, is provided. An autofocus beam is split into a reference beam component (the split off reference channel) and a measurement beam component, by a beam splitting optic located a predetermined distance from (and in predetermined orientation relative to) the substrate, to create a first space between the beam splitting optic and the substrate. A reflector is provided that is spaced from the beam splitting optic by the predetermined distance, to create a second space between the reflector and the beam splitting optic. The measurement beam component is directed at the substrate and a reflected measurement beam component through the first space between the substrate and the beam splitting optic, while the reference beam component is directed at the reflector and a reflected reference beam component is directed from the reflector through the second space between the beam splitting optic and the reflector. The reflected reference and measurement beam components are returned to the beam splitting optic, and emerge substantially collinear from the beam splitting optic. The reference and measurement beam components are then detected, and provide information that enables compensation for changes in the z position of the substrate that are due to instabilities in the autofocus system components and/or environmental factors.

1 . A method of compensating instability in an optical autofocus system that uses light directed at and reflected from a substrate to determine changes in the z position of the substrate, comprising
 a. splitting the light directed at the substrate into a reference beam component and a measurement beam component, by a beam splitting optic located a predetermined distance from the substrate, to create a first space between the beam splitting optic and the substrate,   b. providing a reflector spaced from the beam splitting optic by the predetermined distance, to create a second space between the reflector and the beam splitting optic,   c. directing the measurement beam component at the substrate and directing a reflected measurement beam component through the first space between the substrate and the beam splitting optic, while directing the reference beam component at the reflector and directing a reflected reference beam component from the reflector through the second space between the beam splitting optic and the reflector,   d. returning the reflected reference and measurement beam components to the beam splitting optic, so that the reference and measurement beam components emerge substantially collinear from the beam splitting optic, and   e. detecting the reference and measurement beam components and producing from the detected reference and measurement beam components information to enable compensation for changes in the z position of the substrate that are due to instabilities in system components and/or environmental factors.   
     
     
         2 . The method of  claim 1 , wherein the position and orientation of the reflector relative to the beam splitting optic is maintained substantially fixed. 
     
     
         3 . The method of  claim 1 , wherein the optical autofocus system is of the slit detection type, where light reflected from the substrate is passed through a slit before impinging on a slit detector, and wherein detection of the reflected reference and measurement beam components comprises directing the reflected reference and measurement beam components through a beam splitter disposed between the slit and respective detectors for the reflected reference beam component and the reflected measurement beam component. 
     
     
         4 . The method of  claim 1 , wherein the substrate is moveable in a first direction and the reflector has a reflective surface in a plane oriented substantially transverse to the first direction. 
     
     
         5 . The method of  claim 1 , wherein the substrate is moveable in a first direction and the reflector has a reflective surface in a plane oriented substantially parallel to the first direction. 
     
     
         6 . The method of  claim 5 , wherein the beam splitting optic comprises a pair of adjacent 30-60-90 prisms with respective sides that are adjacent and offset from each other in the direction transverse to the first direction. 
     
     
         7 . The method of  claim 1 , including adjusting for index differences in the non common first and second air spaces between the substrate and the beam splitting optic and between the reflector and the beam splitting optic. 
     
     
         8 . The method of  claim 7 , wherein adjusting for index differences in the non common first and second air spaces comprises directing a dedicated air supply to the non common first and second air spaces. 
     
     
         9 . The method of  claim 1 , including adjusting for index gradients in the non common first and second air spaces between the substrate and the beam splitting optic and between the reflector and the beam splitting optic. 
     
     
         10 . The method of  claim 9 , wherein the optical autofocus system is of the slit detection type, where light reflected from the substrate is passed through a slit before impinging on a slit detector, wherein the reference and measurement beams are detected by means of respective detection slits, and adjusting for index gradients in the first and second non common air spaces comprises changing the orientations of the respective detection slits. 
     
     
         11 . The method of  claim 10 , further including adjusting for changes in index gradients with time in the first and second non common air spaces. 
     
     
         12 . The method of  claim 11 , wherein adjusting for changes in index gradients with time comprises adjusting the sizes of the respective detection slits. 
     
     
         13 . The method of  claim 1 , including adjusting for changes in index gradients with time in the non common air spaces between the reflector and the beam splitting optic and between the substrate and the beam splitting optic. 
     
     
         14 . The method of  claim 13 , wherein the optical autofocus system is of the slit detection type, where light reflected from the substrate is passed through a slit before impinging on a slit detector, and becoming insensitive to changes in index gradients with time comprises adjusting the sizes of respective detection slits for the reflected measurement and reference beams. 
     
     
         15 . An optical autofocus system that uses light directed at and reflected from a substrate to determine changes in the z position of the substrate, and which is configured to compensate for instability due to environmental factors and/or mechanical factors involving system components, comprising
 a. a beam splitting optic located a predetermined distance from the substrate, creating a first space between the beam splitting optic and the substrate,   b. a reflector spaced from the beam splitting optic by the predetermined distance, and creating a second space between the beam splitting optic and the reflector,   c. wherein the beam splitting optic is configured to split light directed at the substrate into a reference beam component and a measurement beam component, direct the reference beam component at the reflector and direct a reflected reference beam component from the reflector through the second space between the reflector and the beam splitting optic, direct the measurement beam component at the substrate and direct a reflected measurement beam component from the substrate through the first space between the beam splitting optic and the substrate, and to transmit the reflected reference and measurement beam components in a manner that causes the reference and measurement beam components to emerge substantially collinear from the beam splitting optic, and   d. detectors for the reference and measurement beam components and a processing device configured to produce from the detected reference and measurement beam components information to enable compensation for changes in the z position of the substrate that are due to instabilities in system components and/or environmental factors.   
     
     
         16 . The optical autofocus system of  claim 15 , wherein the position and orientation of the reflector relative to the beam splitting optic is maintained substantially fixed. 
     
     
         17 . The optical autofocus system of  claim 15 , wherein the optical autofocus system is of the slit detection type, where light reflected from the substrate is passed through a slit before impinging on a slit detector, and wherein detection of the reflected reference and measurement beam components comprises directing the reflected reference and measurement beam components through a beam splitter disposed between the slit and respective detectors for the reflected reference beam component and the reflected measurement beam component. 
     
     
         18 . The optical autofocus system of  claim 15 , wherein the substrate is moveable in a first direction and the reflector has a reflective surface in a plane oriented substantially transverse to the first direction. 
     
     
         19 . The optical autofocus system of  claim 15 , wherein the substrate is moveable in a first direction and the reflector has a reflective surface in a plane oriented substantially parallel to the first direction. 
     
     
         20 . The optical autofocus system of  claim 19 , wherein the beam splitting optic comprises a pair of adjacent 30-60-90 prisms with respective sides that are adjacent and offset from each other in the direction transverse to the first direction. 
     
     
         21 . The optical autofocus system of  claim 15 , further including one or more fluid conduits are provided for directing a dedicated air supply to the non common first and second spaces between the substrate and beam splitting optic and between the reflector and the beam splitting optic, to minimize index differences in the non common first and second spaces. 
     
     
         22 . The optical autofocus system of  claim 21 , wherein the optical autofocus system is of the slit detection type, where light reflected from the substrate is passed through a slit before impinging on a slit detector, wherein the detectors comprise respective slit detectors for the measurement and reference beams, and the orientations of the respective detection slits are oriented to reduce sensitivity to index gradients in the non common first and second air spaces. 
     
     
         23 . The optical autofocus system of  claim 22 , wherein the sizes of the respective detection slits are adjustable to adjust for changes in index gradients with time in the non common first and second spaces. 
     
     
         24 . The optical autofocus system of  claim 15 , wherein the optical autofocus system is of the slit detection type, where light reflected from the substrate is passed through a slit before impinging on a slit detector, wherein respective detection slits are provided for the measurement and reference beams, and the sizes of the respective detection slits are adjustable to adjust for changes in index gradients with time in the non common first and second spaces.