Vertical axis non-linearities in wavelength scanning interferometry

The uncertainty of measurements made on an areal surface topography instrument is directly influenced by its metrological characteristics. In this work, the vertical axis deviation from linearity of a wavelength scanning interferometer is evaluated. The vertical axis non-linearities are caused by the spectral leakage resulting from the Fourier transform algorithm for phase slope estimation. These non-linearities are simulated and the results are compared with experimental measurements. In order to reduce the observed non-linearities, a modification of the algorithm is proposed. The application of a Hamming window and the exclusion of edge points in the extracted phase are shown to increase the accuracy over the whole instrument range

Feature-based characterisation of laser powder bed fusion surfaces

A novel algorithmic pipeline for the automated identification and dimensional/geometric characterisation of topographic formations of interest (surface features) is proposed, specifically aimed at the investigation of signature features left by laser powder bed fusion of metallic surfaces. Unmelted and partially-melted particles, as well as spatter formations and weld tracks, are automatically identified and extracted from topography datasets obtained via state-of-the-art areal topography measurement instruments, and then characterised in terms of their size and shape properties. Feature -based characterisation approaches, such as the one proposed in this work, allow for development of new solutions for the study of advanced manufacturing processes through the investigation of their surface fingerprint

Laser ultrasound measurement of diaphragm thickness, Young’s modulus and Poisson’s ratio in a MEMS device

Laser-generated Lamb waves, coupled with a large bandwidth Michelson interferometer, have been demonstrated to accurately measure the thickness of a MEMS pressure sensor diaphragm in the [110] direction of a silicon wafer. Using the reassigned Gabor time-frequency method to produce group velocity dispersion curves, the technique facilitates the measurement of thickness, Young’s modulus and Poisson’s ratio from just one non-contact measurement. In this investigation, thickness was determined to be 35.01 μm ± 0.18 μm. For comparison, the thickness was measured using an independent optical technique; obtaining a value of 34.60 μm ± 0.27 μm. Values for Young’s modulus and Poisson’s ratio were also determined to be 163 GPa ± 11.7 GPa and 0.351 respectively and these are in good agreement with values found in the literature

High-precision lateral distortion measurement and correction in coherence scanning interferometry using an arbitrary surface

Lateral optical distortion is present in most optical imaging systems. In coherence scanning interferometry, distortion may cause field-dependent systematic errors in the measurement of surface topography. These errors become critical when high-precision surfaces, e.g. precision optics, are measured. Current calibration and correction methods for distortion require some form of calibration artefact that has a smooth local surface and a grid of high-precision manufactured features. Moreover, to ensure high accuracy and precision of the absolute and relative locations of the features of these artefacts, requires their positions to be determined using a traceable measuring instrument, e.g. a metrological atomic force microscope. Thus, the manufacturing and calibration processes for calibration artefacts are often expensive and complex. In this paper, we demonstrate for the first time the calibration and correction of optical distortion in a coherence scanning interferometer system by using an arbitrary surface that contains some deviations from flat and has some features (possibly just contamination), such that feature detection is possible. By using image processing and a self-calibration technique, a precision of a few nanometres is achieved for the distortion correction. An inexpensive metal surface, e.g. the surface of a coin, or a scratched and defected mirror, which can be easily found in a laboratory or workshop, may be used. The cost of the distortion correction with nanometre level precision is reduced to almost zero if the absolute scale is not required. Although an absolute scale is still needed to make the calibration traceable, the problem of obtaining the traceability is simplified as only a traceable measure of the distance between two arbitrary points is needed. Thus, the total cost of transferring the traceability may also be reduced significantly using the proposed method

Towards uncertainty in dimensional metrology of surface features for advanced manufacturing

In previous work, an original approach was developed for the dimensional characterisation of surface features on parts and test artefacts, aimed at supporting researchers involved in the study of advanced manufacturing processes. In the approach, methods and algorithms from image processing, coordinate metrology, surface metrology and reverse engineering are merged into an original framework for feature identification, extraction and dimensional characterisation, starting from areal topography data. With the ultimate goal of associating uncertainty to the results obtained in dimensional characterisation, this paper focuses on specifically investigating reproducibility and repeatability of dimensional characterisation results obtained on a test dataset consisting of a step-like feature manufactured by material jetting