US Patent 7283256 Method and apparatus for measuring wafer thickness

A system for non-contact measurement of thickness of a test object. A laser beam is split into two identical directly opposed input beams. A calibration object of known thickness causes beams to be reflected from sides of the test object. Each reflected beam passes through sensing means including a pinhole aperture and a photodiode sensor. Maximum sensor output defines first and second focal points a known distance apart. The calibration object is removed, and the test object is inserted into the path of the input beams, creating focus position intensity curves for the reflected beams. By determining the deviation, at maximum photodiode output, of the positions of the test object reflecting surfaces from the positions of the calibration object surfaces, the test object thickness can be readily and accurately determined.