Binary pseudorandom grating standard for calibration of surface profilometers

We suggest and describe the use of a binary pseudorandom (BPR) grating as a standard test surface for measurement of the modulation transfer function (MTF) of interferometric microscopes. Knowledge of the MTF of a microscope is absolutely necessary to convert the measured height distribution of a surface undergoing metrology into an accurate power spectral density (PSD) distribution. For an ideal microscope with an MTF function independent of spatial frequency out to the Nyquist frequency of the detector array and with zero response at higher spatial frequencies, a BPR grating would produce a flat 1-D PSD spectrum, independent of spatial frequency. For a real instrument, the MTF is found to be the square root of the ratio of the PSD spectrum measured with the BPR grating to the ideal, spatial-frequency-independent PSD spectrum. We present the results from a measurement of the MTF of a MicromapTM-570 interferometric microscope, demonstrating high efficiency for the calibration method.