A transparent chuck is used to force a semiconductor wafer to take a prescribed shape. Any gap which is formed between the chuck and the wafer can be imaged through the transparent chuck. An interferometer is used to illuminate the gap with a narrow band illumination to create interference fringes in...http://www.google.co.uk/patents/US5515167?utm_source=gb-gplus-sharePatent US5515167 - Transparent optical chuck incorporating optical monitoring
