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Optical Glass Properties

Fizeau interferometer limitations

Flatness Measurements with Fizeau Interferometer
Measuring the flatness of wafers, substrates, windows, vacuum chucks and other similar parts is challenging, because when placed in a horizontal position, they are affected by gravitational bending forces. The thinner the part the more it is affected by gravity. The result is a distorted and inaccurate flatness reading.

The logical approach is to measure with an interferometer that can accommodate the vertical placement of parts, and to position them in stress-free holding fixtures to minimize gravitational bending forces.

When measuring transparent thin wafers, windows and substrates of 0.5mm and thinner, on a Fizeau Interferometer, the application of opaque coatings on the backside is required, which distorts the flatness of the surface.

The Indars Interferometer addresses both of the issues which can produce inaccurate flatness readings of thin transparent wafers. Firstly, the wafers are held in a vertical position, eliminating the bending components introduced by gravitational force. Secondly, instead of a laser, a reference optical flat is used to read the interference fringes relevant to the sample to be measured. This eliminates the need for the application of flatness distorting films to the backside of thin transparent wafers to be measured.

For additional flatness measurement related information see:

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