Optical gap measuring apparatus and method having two-dimensional grating mark with chirp in one direction

A gapping apparatus for measuring the gap between first and second plates, comprising:

• a gapping mark on a face of said first plate including an array of similar elements in a two-dimensional pattern, having uniform period in a first direction and a non-uniform period in the transverse direction;
• illuminating and viewing optics configured so that the incident and viewing angles are inclined from the normal, bearing a correct relationship to said gapping mark such that interference patterns will be imaged by the viewing optics;
• a detector configured as an image-sensor to measure said first and second periods and first and second phases of said interference patterns, and thus to measure the gap between the said first and second surfaces and to determine the difference between said measured gap and a predetermined value;
• and
• a position adjuster for adjusting the relative position of said first and second plates until said detector detects that said gap is within an acceptable tolerance from the said predetermined value.

A method of measuring the gap between first and second plates, comprising:

• providing a gapping mark on a first surface of said first plate including a first array of similar elements in a two-dimensional pattern, having uniform period in a first direction and a non-uniform period in the transverse direction;
• illuminating a second surface of said second plate through said gapping mark on said first plate to produce a first interference pattern having a first period and a first spatial phase, and to produce a second interference pattern having a second period and a second spatial phase;
• measuring said first and second periods and first and second phases of said interference patterns, and thus the gap between said first and second surfaces in order to determine the difference between said measured gap and a predetermined value;
• and
• adjusting the relative position of said first and second plates until said gap is within an acceptable tolerance from said predetermined value.