The historical method of calling an end of a layer in optical coating was to monitor a wavelength and when a condition was met calling the end of the layer. As discussed in a previous application note the NVision optical monitor supports this method with good success. There is one problem with this technology; the measurement of only one wavelength limits the potential precision as we are typically waiting until a condition has been exceeded such as a quarter wave minima or maxima. In most cases the end point is called at some time after the actual meeting of the end point criteria condition. This requires the process engineer to design the algorithm to compensate for the delay in the decision.
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