<title>Photomultiplier Linearization And System Stabilization For Photometric Ellipsometers And Polarimeters</title>

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AbstractModifications to standard photometric polarimeter systems that reduce measurement errorsdue to drift, digital noise, and stray light are described. For instruments using photo -multiplier (PM) detectors, a regulation and control circuit is discussed that also allowsthe linearity of the detector to be adjusted. The measured performance of a rotating -analyzer ellipsometer (RAE) using these modifications is as follows: Linearity to 5x10 -4over more than three orders of magnitude of incident light intensity, and stability to 5x10-5. IntroductionIt is generally recognized that null ellipsometers are superior in accuracy to photo-metric ellipsometers, but that photometric ellipsometers and polarimeters are capable ofsignificantly better precision.11,2) The principal source of loss of accuracy in photo-metric systems follows from their direct reliance on the properties of detectors, becausein photometric systems information is obtained by analyzing the time -dependent behavior of flux transmitted through the instrument. By contrast, in null ellipsometers the detectoris used only to verify a minimum -intensity condition.In order to improve the accuracy and stability of photometric instruments, it isnecessary to eliminate or to correct the data for three major sources of error. These arefirst, the possible nonlinearity of the detector and associated circuitry, including indigital systems the sample -hold amplifier (SHA) and analog -to- digital converter (ADC),second, the drift of the zero level due to thermal or relaxation effects anywhere in thesystem, and third, the stray light which may enter the optical path externally or as aresult of special operating conditions of the sample such as high temperature. Correctionof the first type of systematic error in data processing was discussed for RAE systems byHunter.(3) Such an approach may be feasible in this simplest photometric system where onlythe dc and one ac component is analyzed, but it is not practical in other polarimeterswhere several harmonics may be analyzed. Further, drift is not treated. We have adopted adifferent approach, that of adding or modifying system components as necessary to eliminateor reduce as far as possible these systematic errors at their sources.In th s paper, we discuss these modifications as applied to our previously described RAE