Appel, K. Roland; (1990) Development of a scanning acousto-optic based differential intensity and phase system for optical metrology. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

An optical technique is described which simultaneously and independently measures the difference in intensity and phase of light reflected from two close areas on a sample surface. Line scans or images of the object are then formed by scanning the light across the surface. The key element is a Bragg cell which is used to divide a well collimated beam of light into two probing beams amplitude modulated in phase quadrature, with a relative frequency shift. A comprehensive review is given of current profilometry techniques, and in particular optical methods. Emphasis is given in developing a thorough description of the system. Analysis of the acousto-optic interaction is combined within a mathematical framework which gives a general description of how the differential phase and intensity responses depend on the form of the modulated electrical Bragg cell drive. The analysis is applied to determine the tolerances necessary in the construction of this type of system. The use of a scanning mirror in a differential phase system is discussed and analysis is presented to model the effects of lens aberrations. Three implementations of the technique have been built and experimental results are shown from prepared specimens which have, i) predominantly phase contrast - a silicon wafer which has had a series of parallel tracks etched, ranging in depth from 180 to 700Å; ii) predominantly intensity contrast - a silicon wafer half implanted with As+ ions; iii) a mixture of phase and intensity information - a deposited aluminium step on a silicon substrate. Unlike any previous method, this system can measure true differential phase and intensity simultaneously. The real advantage of this is clearly demonstrated in the case of the third specimen where there is a change in surface conductivity across the sample. The two measurements may then help resolve uncertainties present in interpreting topography from phase data.

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