Phase Calculation of Spectral Interferograms using Template Matching

Embedded metrology is the measurement of components or assemblies upon the manufacturing platform. Successful implementation enables a reduction in the cost and manufacturing time of high-added-value and high-precision components. Measurement of high-value components in-situ eliminates the need to remove them from the manufacturing platform, and eliminate the problems associated with re-alignment if further manufacturing cycles are required. Increasing the coverage of embedded metrology in modern manufacturing requires the development of new sensor technologies with the requisite attributes such as high dynamic range, high speed, small size and robustness against adverse environmental conditions.
Dispersed reference interferometry (DRI) aims to improve upon existing commercial single point measurement techniques such as chromatic confocal microscopy (CCM) by eliminating the requirement of expensive front-end optics while improving dynamic range. DRI has previously been demonstrated with a resolution of 250 nm over a range of 200 µm [1,2]. However, DRI is an interferometric technique which means high resolution phase information is inherent in the generated spectral interferograms and nanometre resolution is achievable over an axial range of several hundred microns. Extraction of phase data will improve the measurement resolution of DRI, thus increasing the dynamic range of the technique over and above commercial alternatives such as CCM.
This paper describes a method of phase calculation using template matching which is a technique commonly used in image processing. Template matching is used to extract high resolution phase information from an experimental DRI apparatus. 800 spectral interferogram templates, representing axial measurement positions at 1 nm intervals, are generated by simulation. These templates are cross-correlated with a captured spectral interferogram from the DRI apparatus. The peak of the resulting correlogram indicates the relative measured position with high resolution. This novel phase extraction method is evaluated in terms of linearity, resolution and operating range.