A Multiple Height Transfer Interferometric Technique and Its Applications

Abstract

Absolute distance interferometric metrology is one of the most useful techniques for dimensional measurements. Without movement, measurements can be made without ambiguity, by using either one or several synthetic wavelengths. Synthetic wavelengths result from the beating of two or more wavelengths in multiple-wavelength interferometry (MWI), or a wavelength scan in wavelength-scanning interferometry (WSI). However, conventional MWI requires accurate wavelength information for a large measurement range, while WSI is limited by a mode-hop-free laser tuning range. A multiple height-transfer interferometric technique (MHTIT) is proposed based on concepts from both MWI and WSI. Using multiple accurately calibrated reference heights, this technique preserves the capabilities to determine the optical path difference (OPD) unambiguously without accurate wavelength information, and yet does not require the laser to be continuously tuned. A multiple reference height calibration artifact is proposed and installed in a holographic measuring system. Applying the MHTIT with the calibration artifact, the metrology system measurement range is increased from 5 mm to over 100 mm without accurate wavelength information. Three-dimensional images of discontinuous surface heights obtained from a variety of automotive parts demonstrate the applicability of the MHTIT in workshop environments.

We present an uncertainty analysis, analyzing the primary sources of uncertainties that limit the performance of the MHTIT and discuss how errors can be minimized. The measurement uncertainty is experimentally demonstrated to be about 0.3 µm for 50.8 mm at a confidence level of 95% for two discontinuous surfaces under lab environments. Another application of the MHTIT for measuring the thickness of a transparent plate is investigated. WSI can measure the thickness of transparent plates by differentiating OPDs from multiple surface interferences in the Fourier domain. However, nonlinear laser tuning deviates the measurement result from the correct value. We propose a wavelength-stepping method for application to thickness measurements of transparent plates. Systematic errors caused by nonlinearity in laser source stepping are reduced with accurate synthetic wavelengths measured by the MHTIT. A 10-µm step height standard etched on a 25 mm × 25 mm × 3 mm quartz block is measured to demonstrate the proposed method with sub-micron accuracy.