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Optical Scanning Interferometer for Measuring Surface Flatness

IP.com Disclosure Number: IPCOM000052623D
Original Publication Date: 1981-Jun-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 3 page(s) / 48K

Publishing Venue

IBM

Related People

Rabedeau, ME: AUTHOR

Abstract

This scanning interferometer is arranged for measuring surface flatness with an error of less than Plus or minus 5 nanometers. The interferometer is sh schematically; the components shown in the smaller rectangle 6 are fixed relative to one another but move as a unit vertically between the components shown in the larger rectangle 8, which moves all of the components horizontally to impart an X-Y scanning motion to the beam from the laser 10. The beam from the stationary laser 10 is reflected upward by a mirror 11 and to the left by a mirror 12. As can be seen, if the mirrors 11 and 12 are moved in the X-scan direction noted in the figure, the beam will move in the X-direction, and if the mirror 12 is moved vertically, the beam will move in the Y-direction.

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Optical Scanning Interferometer for Measuring Surface Flatness

This scanning interferometer is arranged for measuring surface flatness with an error of less than Plus or minus 5 nanometers. The interferometer is sh schematically; the components shown in the smaller rectangle 6 are fixed relative to one another but move as a unit vertically between the components shown in the larger rectangle 8, which moves all of the components horizontally to impart an X-Y scanning motion to the beam from the laser 10. The beam from the stationary laser 10 is reflected upward by a mirror 11 and to the left by a mirror
12. As can be seen, if the mirrors 11 and 12 are moved in the X-scan direction noted in the figure, the beam will move in the X-direction, and if the mirror 12 is moved vertically, the beam will move in the Y-direction. A lens 21 and a polarization beam splitter 24 project a focused beam onto the part 20 to be scanned. A lens 22 and mirrors 28 and 29 cause the diverging and moving beam that exits the interferometer to be recollimated and reduced to a stationary condition so that it can be projected on to the stationary detector 30.

The laser used in this system is preferably the type that emits two orthogonal plane polarized waves that differ in frequency by a few megahertz. The laser and mirrors 11 and 12 are arranged so that the electric vectors of both of the two orthogonally polarized waves emitted by the laser are either parallel or perpendicular to the plane of incidence of the mirrors. The beam is then divided into the two polarization azimuths by the polarization beam splitter, and the beams are reflected from the part to be measured and the reference mirror, and recombined at the beam splitter, as is the practice with interferometers of this type. The scanning beam that exits the beam splitter is made stationary by the upper portion of the scanner, as described above, and directed on to the stationary detector which employs a polarization analyzer oriented at 45 degrees to the electric vectors of the laser beam in order to obtain interference of the orthogonal waves.

Since the system employs two frequencies, a beat frequency is generated at the detector and this beat frequency is compared with the beat frequency generated by the two laser waves in the laser housing. Any frequency difference in these two beat f...