A phase-sensitive scanning optical microscope is described which can measure surface height changes of less than 3 angstroems. The system is based on a heterodyne version of the Michelson interferometer, and has been designed to reject phase noise caused by vibration in the optics and the sample. A specially constructed objective lens is used to direct two laser beams on to the object surface. The first beams forms a tightly focused spot to probe the sample structure, and the second remains collimated, acting as a large-area on-sample reference beam. The configuration allows the relative areas illuminated by the two beams to be varied both arbitrarily and independently, thus guaranteeing an accurate absolute phase measurement. This is an important advantage over existing techniques, in which the range of suitable samples is restricted by the limited size of the on-sample reference beam. The two beams reflected from the sample are interfered with a third, frequency-shifted, beam, so forming two heterodyne Michelson interferometers in parallel. The phase from each interferometer is then compared to provide the object-surface phase structure. Path-length fluctuations due to microphonics are common to both interferometers and are cancelled by this comparison. Results from a bench-top version of the system are presented which demonstrate the principle of the technique, and the factors limiting the sensitivity are discussed.
|Number of pages||10|
|Journal||Transactions of the Institute of Measurement and Control|
|Publication status||Published - 1 Jan 1991|
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