Chromatic confocal gauging for high precision dimensional metrology

Abstract

Interest in the chromatic confocal microscope in the high precision dimensional metrology industry is growing rapidly. In fact, the chromatic confocal microscope offers a “stylus like” distance measurement applicable to various surface types. In addition, the chromatic confocal microscope can potentially compete in resolution with contact measurement probes, while significantly increasing the sampling rate to several kHz. Therefore, such technology is key to the Taylor Hobson Ltd. strategy. The work embodied in this thesis focuses on the design, development, and evaluation of a low cost and high resolution version of the chromatic confocal microscope. Both product are currently in pre-production phase. The low cost version the chromatic confocal microscope is designed to be a compact and cost effective system while exhibiting “state of the art” performances. In fact, the raw material cost of the system is below £500, this being achieved while exhibiting an optical head outer diameter of 8 mm, a working distance of more than 27 mm, and a resolution better than 100 nm over a measurement range exceeding 7 mm. The high resolution version of the chromatic confocal microscope is dedicated to precision. The aim of the design is to exhibit the highest achievable resolution while maintaining a measurement working distance exceeding 12 mm. By tailoring the chromatic dispersion to be appropriately low, a resolution of better than 10 nm is achieved. Both designs have been tailored within a few design cycles; this has been achieved through the development of two novel models, the spectral irradiance model and the measurement standard deviation model. The spectral irradiance model enables the analytical estimation of the chromatic confocal peak from only the optical parameters of the optical heads. Based on a geometric approach; this is more than 5 times more precise than the previously used model based on wave optics. Furthermore, by applying a fully integrated system design approach incorporating design, production, and test of optical systems, the resulting chromatic confocal system surpasses comparable commercially available chromatic confocal gauges in terms of cost, resolution, numerical aperture, range of measurement, and working distance. Such performance is further enhanced by the use of staircase diffractive surfaces. Staircase diffractive surfaces are special hybrid aspheric diffractive surfaces exhibiting insignificant manufacturing losses allowing the design to be only limited by the scalar diffraction theory used by ray tracing packages. In addition, staircase diffractive lenses permit the passively athermalisation of the optical head of the chromatic confocal microscope. The work encapsulated in this thesis extends the current understanding of the measurement environment impact on chromatic confocal gauges. This study includes the impact of the surface reflectivity, roughness, and slope onto the linearity of the chromatic confocal microscope. Using the previously described findings, methods to mitigate the linearity error induced by the surface roughness, reflectivity, and slope are presented and applied.