White light interferometry produces short coherence interferometry hose fringe visibility is narrowly localized in the spatial domain so that the optical path difference between the test and reference beams can be scaled without 2π-ambiguity. The position of thr test surface is determined from the peak of envelope of white light interferogram. This paper present a new computational method of white light scanning interferometry for 3-D surface mapping called Quadratic Polynomial Fitting Algorithm. This method accomplishes the task of detecting the true peak of the interference fringe in two steps. The first step is global search locating the envelope peak by fitting sampled intensity data directly to a symmertric quadratic polynomial. The second step is fine-tuning to precisely determine the fringe peak by compensating for phase shift on reflection using the absolute the fringe order identified by the envelope peak obtained in the first step. This two-step method offers an efficient means of computation to provide a goodmeasuring accuracy with high noise immunity owing to its inherent reliance on least squares principles.
On the other hand, there remain many problems to be tackled in order for white light interferometry to be a more general tool of precision surface metrology. One of the problemsis to find out an effective compensation method fo the phase change upon reflection from the target surface;otherwise no accurate surface mapping is achieved when multiple dissmilar materials comprises the target surface.This paper present a compensation technique of the phase chage upon reflection in white light interferometry s0 that precise 3-D profile mapping becomes possible for composite target surfacescomprising multiple dissimilar materials. The variation of phase change with the spectral distribution of the light source is estimated through first-order approximation, which can then be directly determuned by additional quasi-monochromatic phase-measuring interferometic measurements or two-wavelength white light scanning interferometry. Experimental results prove that the proposed compensation technique is capable of reducing the measurement error in step height gauging within ±2nm.
The other problem is the effect of aberrations in optical system. Inconventional two-beam interferometry, the aberration is canceled out by the subtraction between the reference and the test rays. However, that effect is showed up in case that the test surface is tilted evoking the area that the reference ray passes only. The aberration mainly occurred from both the beam splitter in optical microscope and alignments in optical system shift the fringe peak of white light interferogram. Experimental results prove that the aberration effects would cause measurement errors of 100nm on fringe peak.
