Electronic device for the analysis and computation of the fourier coefficients of a periodic function, including such a device and ellipsometer

摘要

The output of a laser is directed through a polariser mounted on a step motor to permit variation of the polarisation. The polarised light is reflected from the surface of a material being analysed. An analyser rotated by a toothed wheel onto which an optical coder is directed produces an output for a photomultiplier. The photomultiplier output is a periodic sinusoidal function which is passed to an analysis and processing unit where samples of the signal are produced comprising 16 bit words. These are passed to a microprocessor which calculates the Fourier coefficients for the periodic function. The microprocessor also controls the peripheral units for storage and for display of the information on an X-Y platter. An analogue to digital converter operates on the sample and feeds a summing circuit which forms partial sums for each time interval corresponding to a quarter of the period of the function. The system is used in real time ellipsometry for analysing layers obtained by vapour deposition, e.g. semiconductors.