A dual interferometer system with a short reference flat distance for wafer shape and thickness variation measurement

An interferometer system, comprising:

• first and second interferometer devices, the first and second interferometer devices including first and second spaced apart reference flats having corresponding first and second parallel reference surfaces forming a cavity therebetween for placement of a polished opaque plate, wherein the first and second reference surfaces of the first and second reference flats are positioned approximately 5 millimeters or less from each other uniformly throughout the parallel surfaces, allowing first and second surfaces of the polished opaque plate to be approximately 2.5 millimeters or less from the corresponding first and second reference surfaces of the first and second reference flats when the polished opaque plate is placed in the cavity, and wherein the first and second interferometer devices are located on diametrically opposite sides of the cavity to map the opposite first and second surfaces of the polished opaque plate;
• a light source optically coupled to the first and second interferometer devices, the light source comprising an illuminator configured for producing light of multiple wavelengths and an optical amplitude modulator configured for stabilizing power of the light produced by the illuminator;
• first and second interferogram detectors;
• at least one computer coupled to receive the outputs of the first and second interferogram detectors for determining thickness variations of the plate;
• and
• a temperature controlled enclosure configured for providing a thermally stable environment for at least the first and second reference flats, the first and second interferometer devices and the first and second interferogram detectors.

An apparatus for measuring the thickness variation and shape of a polished opaque plate, the apparatus comprising:

• first and second interferometer devices, the first and second interferometer devices including first and second spaced apart reference flats having corresponding first and second parallel reference surfaces forming a cavity therebetween for placement of the polished opaque plate, wherein the first and second reference surfaces of the first and second reference flats are positioned approximately 5 millimeters or less from each other uniformly throughout the parallel surfaces, allowing first and second surfaces of the polished opaque plate to be approximately 2.5 millimeters or less from the corresponding first and second reference surfaces of the first and second reference flats when placed within the cavity, and wherein the first and second interferometer devices are located on diametrically opposite sides of the cavity to map the opposite first and second surfaces of the polished opaque plate;
• a light source optically coupled to the first and second interferometer devices, the light source comprising an illuminator configured for producing light of multiple wavelengths and an optical amplitude modulator configured for stabilizing power of the light produced by the illuminator;
• first and second interferogram detectors;
• at least one computer coupled to receive the outputs of the first and second interferogram detectors for determining thickness variations of the plate;
• a temperature controlled enclosure configured for providing a thermally stable environment for at least the first and second reference flats, the first and second interferometer devices and the first and second interferogram detectors;
• and
• a mechanical arm configured for handling and transferring the polished opaque plate in and out of the cavity.