Optical metrology plays a vital role in a wide range of research fields and for inspection in manufacturing industries. At present, the market offers a variety of optical metrology instruments, such as interferometers, confocal microscopes, and focus variation (FV) instruments. Although interferometers have the highest precision among optical metrology instruments, they are very sensitive to vibrations/environmental disturbances. On the other hand, focus variation technology is widely recognised for its robustness to vibrations (compared to interferometers), but so far, there is no study in place investigating the vibration frequency and amplitude limits within which focus variation instruments operate optimally in the presence of vibrations. To our knowledge, this article is the first study that aims to estimate quantitatively the immunity of focus variation instruments to vibration. This paper presents theoretical simulations to investigate how vibrations affect the FV principle of evaluating the best-focused images to calculate surface topography. The simulations were verified with practical results from an experimental FV setup built in the lab, and the two results match very well. Afterwards, vibration experiments were performed using the state-of-the-art focus variation instrument, Alicona InfiniteFocus G5, to measure the surface roughness of the Microusurf 334 comparator from Rubert & Co. LTD under vibrations induced by the P-840.2 piezoelectric actuator from Physik Instrumente (PI). The experiments were performed at different frequencies by incrementally changing the vibration amplitudes, pre-planned as a function of the depth of field (DoF) of each magnification lens (10x to 100x). It is observed that the FV system generates 100% “bad-data” when the vibration amplitude exceeds three times the DoF of the used objective lens at low frequencies (i.e. as early as 5 Hz).

Optical metrology plays a vital role in a wide range of research and inspection areas in the industry. At present, the market offers a variety of optical metrology instruments, among which the focus variation microscope stands out for its capability of measuring steep surfaces with high slopes. The traditional focus variation (FV) instrument mechanically scans the surface by sweeping the focal plane of the objective lens using linear motion stages and simultaneously capturing images at different scanning positions, forming a stack of images. The mechanical motion stages require regular maintenance and calibration to ensure accuracy over time. Another issue associated with the mechanical scanning methods is their physical size, which creates a limiting factor to compactness for in-situ measurement applications. This work proposes a chromatic focus variation (CFV) method that replaces mechanical scanning with a wavelength scanning mechanism to overcome the above limitations. Unlike traditional focus variation, the CFV system employs a dispersive objective lens (i.e. chromatically aberrated objective lens) to axially shift the focus along the optical axis to provide vertical/depth scanning. This approach brings significant enhancements in measurement speed and reduces the instrument size for on-machine metrology tasks. In this paper, a detailed analysis of the optical performance of the dispersive objective lens is conducted, and then the measurement performance of the proposed CFV system is validated using samples including a step height of 30 µm and a sine wave shape with a peak-to-valley amplitude of 19 µm. The experiment results were compared to those from the state-of-the-art commercial instrument (Alicona G5), which showed a good agreement between the two. Furthermore, a detailed analysis and discussions are provided to investigate the measurement's accuracy.

Optical metrology is an essential measurement technology across various research and inspection domains. Focus variation instruments are widely used in the industry due to their ability to measure steep and high-slope surfaces. The focus variation method extracts an areal surface map by analysing neighbouring pixels' contrast to determine the image focus position in a stack of images. Traditional focus variation instruments mechanically scan the sample surface by attaching the objective lens to a piezoelectric actuator (PZT) for image stack capture. The slow mechanical movement of the PZT poses a limitation for fast inspection in modern advanced manufacturing applications. Additionally, the PZTs' bulky mass and size, and their nonlinear hysteresis motion are other issues associated with the mechanical scanning process. To address these limitations, we present a chromatic focus variation approach that replaces mechanical scanning with a wavelength scanning mechanism. The system employs a chromatic objective lens that determines the image focus by axially shifting the light source wavelength without any mechanical movement. This paper presents the measurement of a 30 µm step height sample using the proposed system. The results show that wavelength scanning is capable of surface measurements, but the current version of the system has a low vertical resolution, which should be enhanced in future versions.

Thin film flexible electronics refer to a class of electronic devices built on flexible substrates. Examples includes printed Li batteries, Thin film flexible electronics refer to a class of electronic devices manufactured by multiple layering and scribing on flexible polymer substrates. Examples of such devices includes printed Li batteries, flexible photovoltaic cells and light emitting diodes. These devices are often mass manufactured by Roll-to-Roll processing (R2R). Whilst the basic technology is well established, the increasing demands on precision environmental protection and multi layering of devices means that in-process measurement of printed surface features is a critical bottle neck in terms of developing R2R as a process route. The purpose of the present paper is to review the current critical dimensional metrology needs in R2R manufacture and in particular to highlight the development of a new inprocess surface metrology system based on Multi-wavelength Polarizing Interferometry (MPI). The system is capable of measurement in real time, is environmentally robust and has nanometre resolution. The paper concludes by highlighting an example of the first trial implementation of the MPI on a production level R2R machine and discussed issues with quantification of film dimensions and associated signal processing

Wavelength scanning interferometry is an interferometric technique for measuring surface topography without the well-known 2π phase ambiguity limitation. The measurement accuracy and resolution of this technique depends, among other factors, on the algorithm used to evaluate its sinusoidal interference pattern. The widely used fast Fourier transform analysis experiences problems such as waviness error across the measured surface due to spectral leakage. This paper introduces a new fringe analysis method based on the Carré phase shifting algorithm combined with a least squares fitting approach. Numerical simulation was carried out to assess the performance of the Carré algorithm in comparison to fast Fourier transform analysis, and the same was validated by presenting four experimental case study examples (a surface flat, a ceramic ball bearing, a flexible thin film, and a discontinuous step height sample). The analysis results show that the proposed Carré algorithm with least squares fitting can significantly eliminate the waviness error, especially when measuring steep surfaces.

Optical coherence tomography (OCT) is an imaging technique which can provide sub-surface evaluation of defects in optically compliant components such as those manufactured by polymeric selective laser sintering. In OCT systems, achieving lateral imaging resolutions of <10 mm means that full-depth imaging requires multiple scans due to the limited depth of focus (DOF). We present a study on the application of 'non-diffracting' Bessel beams to extend system DOF and enable deeper imaging with a single scan. Such capability expands the potential for OCT as a rapid tool for sub-surface assessment, either in-line or in-process, by greatly reducing acquisition times. (C) 2020 The Authors. Published by Elsevier Ltd on behalf of CIRP.

The line-scan dispersive interferometry (LSDI) benefits from single-shot measurement in nature and has potential to perform in-line surface metrology. In this technique, the interference beam produced by the two arms of the interferometer is spatially dispersed by a diffraction grating along the rows (or columns) of the CCD pixels. In which case, a two-dimensional spectral interferogram is generated. In this paper, fringe order determination is carried out to retrieve the more accurate phase information along the chromaticity axis of the interferogram and then the height map of the tested profile can be calculated with high resolution. Two standard artefacts have been evaluated using the developed LSDI and the experimental results are compared with that of phase slope method as well as the commercial instrument (Talysurf CCI 3000), which shows that better performance in measurement noise is achieved. Additionally, the measurement repeatability is significantly improved and demonstrated within sub-nanometer range.

Non-destructive testing and online measurement of surface features are pressing demands in manufacturing. Thus optical techniques are gaining importance for characterization of complex engineering surfaces. Harnessing integrated optics for miniaturization of interferometry systems onto a silicon wafer and incorporating a compact optical probe would enable the development of a handheld sensor for embedded metrology applications. In this work, we present the progress in the development of a hybrid photonics based metrology sensor device for online surface profile measurements. The measurement principle along with test and measurement results of individual components has been presented. For non-contact measurement, a spectrally encoded lateral scanning probe based on the laser scanning microscopy has been developed to provide fast measurement with lateral resolution limited to the diffraction limit. The probe demonstrates a lateral resolution of ∼3.6 μm while high axial resolution (sub-nanometre) is inherently achieved by interferometry. Further the performance of the hybrid tuneable laser and the scanning probe was evaluated by measuring a standard step height sample of 100 nm.

By integrating photonic devices on a silicon wafer containing etched waveguides it is possible to create a complete optical system-on-chip. Such a device can contain all the elements required for implementing a wide range of interferometry techniques including wavelength scanning and phase shifting. In this paper we introduce a hybrid photonics based sensor for surface metrology applications containing the following ‘on-chip’ components: tunable laser, phase-shifter, wavelength de-multiplexer, and wavelength encoder. This paper presents the design of the system-on-chip as a miniaturised sensor. Initial experimental results are shown which prove the potential of this device as a viable surface measurement tool.

Embedded metrology is the provision of metrology on the manufacturing platform, enabling measurement without the removal of the work piece. Providing closer integration of metrology upon the manufacturing platform can lead to the better control and increased throughput. In this work we present the development of a high precision hybrid optical chip interferometer metrology device. The complete metrology sensor system is structured into two parts; optical chip and optical probe. The hybrid optical chip interferometer is based on a silica-on-silicon etched integrated-optic motherboard containing waveguide structures and evanescent couplers. Upon the motherboard, electro-optic components such as photodiodes and a semiconductor gain block are mounted and bonded to provide the required functionality. The key structure in the device is a tunable laser module based upon an external-cavity diode laser (ECDL). Within the cavity is a multi-layer thin film filter which is rotated to select the longitudinal mode at which the laser operates. An optical probe, which uses a blazed diffracting grating and collimating objective lens, focuses light of different wavelengths laterally over the measurand. Incident laser light is then tuned in wavelength time to effectively sweep an ‘optical stylus’ over the surface. Wavelength scanning and rapid phase shifting can then retrieve the path length change and thus the surface height. We give an overview of the overall design of the final hybrid photonic chip interferometer, constituent components, device integration and packaging as well as experimental test results from the current version now under evaluation.

In this paper, we explore Bessel features to determine the number of speakers from multispeaker speech signals collected simultaneously from a pair of spatially separated microphones. The arrival of the speech signals from speaker to microphones gives the time delays of given speaker. The time delays can be estimated by performing the cross-correlation to the band limited multispeaker signals collected at the two microphones. Signals are band limited using a finite number of Bessel coefficients. The computer simulation results demonstrate the proposed method is efficient compared to existing methods.