An in-process, non-contact surface finish sensor for high quality components generated using diamond turning

The object of this Ph.D. project was to design and construct an in-process, non contact surface finish sensor for high quality components generated using diamond turning. For this application the instrument must have the following properties: i rapid acquisition of data. ii capability of measuring translating and or rotating surfaces. iii ruggedness for in-process use. iv insensitivity to moderate vibrations. v remoteness from the surfaces to be measured. The remoteness requirement virtually excludes the otherwise ubiquitous stylus instrument, while the rapid gathering of data from rotating surfaces excludes other profiling techniques. The above mentioned properties strongly suggest an optical method. An optical diffraction technique has been chosen, since it produces an optical Fourier Transform of the surface. This transform is produced at the speed of light, since the optical system has the property of parallel data processing, unlike a typical electronic computer. With the aid of a microprocessor various surface finish parameters can be extracted from the optical transform. These parameters are respectively the rms surface roughness, slope and wavelength. The actual sensor consists of a measuring head and a minicomputer. It fulfils the above mentioned requirements. Its only limitations are: i limited to surface finishes up to 100nm ii presence of cutting fluids has to be avoided, although certain modern lubricating fluids can be tolerated. The algorithms devised to extract the surface finish parameters from the optical transforms have initially been tested on optical spectra produced by Thwaite. Comparison of the optical roughness values and the values quoted by Thwaite show close agreement. Thwaite's values are obtained by a stylus instrument. Rqopt (um) Rqstylus (um) 0.16 0.156 0.38 0.37 0.44 0.40 In addition a computer program has been devised which simulates the optical sensor head. The input data can be obtained by a profiling instrument, or generated by a computer program. This last option enables the creation of surface profiles with "controllable" machining errors. This program can be utilised to create an atlas, which maps optical diffraction patterns versus machine-tool errors

Design and Development of an Optical Chip Interferometer For High Precision On-Line Surface Measurement

Advances in manufacturing and with the demand of achieving faster throughput at a lower cost in any industrial setting have put forward the need for embedded metrology. Embedded metrology is the provision of metrology on the manufacturing platform, enabling measurement without the removal of the workpiece. Providing closer integration of metrology upon the manufacturing platform will improve material processing and reliability of manufacture for high added value products in ultra-high-precision engineering. Currently, almost all available metrology instrumentation is either too bulky, slow, destructive in terms of damaging the surfaces with a contacting stylus or is carried out off-line. One technology that holds promise for improving the current state-of-the-art in the online measurement of surfaces is hybrid photonic integration. This technique provides for the integration of individual optoelectronic components onto silicon daughter boards which are then incorporated on a silica motherboard containing waveguides to produce a complete photonic circuit. This thesis presents first of its kind a novel chip interferometer sensor based on hybrid integration technology for online surface and dimensional metrology applications. The complete metrology sensor system is structured into two parts; hybrid photonic chip and optical probe. The hybrid photonic 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. Optical probe is a separate entity attached to the integrated optic module which serves as optical stylus for surface scanning in two measurement modes a) A single-point for measuring distance and thus form/surface topography through movement of the device or workpiece, b) Profiling (lateral scanning where assessment of 2D surface parameters may be determined in a single shot. Wavelength scanning and phase shifting inteferometry implemented for the retrival of phase information eventually providing the surface height measurement. The signal analysis methodology for the two measurement modes is described as well as a theoretical and experimental appraisal of the metrology capabilities in terms of range and resolution. The incremetal development of various hybrid photonic modules such as wavelength encoder unit, signal detection unit etc. of the chip interferometer are presented. Initial measurement results from various componets of metrology sensor and the surface measurement results in two measurement modes validate the applicability of the described sensor system as a potential metrology tool for online surface measurement applications

High precision optical surface topometry

For contactless measurement of the surface topography optical techniques are promissing. A lot of progress was made by the development of laser supported methods. It is due to the development of new different lasers, the introduction of solid state detectors and powerful computers for the information processing. Time of flight, phase measurements, active and passive triangulation and projected fringes are very robust techniques for industrial applications. A lot of progress was made in the application of interferometry to surface and vibration measurements. One or more wavelength together with diode lasers are used. Although interferometry requires polished surfaces, however, by using synthetic wavlengths, it can also be applied for optically rougher surfaces. In addition the unambiguity range will be extended when longer synthetic wavelengths are used. Furthermore scanning and whole field confocal microscopy can be used for macro- and microstructure analysis. New techniques will be described to extend the field of view. Some limits especially with respect to resolution and industrial application will be discussed for the methods presented together with some experimental results and future developments

Improving coherence scanning interferometry signal modelling and topography measurement for complex surfaces

This thesis presents work on advanced optical surface metrology methods that enable extending the range of surface slopes that can be reliably measured by optical surface topography measurement instruments, and on investigating the reliability of the current capability. Optical instruments can only capture a limited portion of light scattered from an object’s surface, determined by the instrument’s numerical aperture. As the surface measured becomes steeper, less scatter is captured until all specular scatter is lost, referred to as the specular reflection limit (SRL). While surface measurement of slopes beyond the SRL by modern instruments is possible via the capture and detection of non-specular scatter, the instrument response to these slopes is not well understood. In addition, as the non-specular scatter has a low signal-to-noise ratio, data dropout can occur. Topography measurement of steep and complex surfaces using optical methods can therefore be challenging and have an unknown reliability, and can have significant errors when multiple scattering is present. The instrument modelling and experimental work focussed on coherence scanning interferometry (CSI). Through use of an approximate linear model the instrument response of a CSI instrument to various slope angles and spatial frequencies was described by a three-dimensional (3D) surface transfer function (STF). This theory was experimentally verified by demonstrating that an experimental 3D STF obtained from measurement of microspheres can be used to generate a filter that can compensate for the effect of lens aberration at a fundamental level and consequently reduce errors in the topographies obtained, especially from surface slopes just below the SRL. Second, a rigorous two-dimensional boundary element method (BEM) model of electromagnetic surface scatter was verified through multiple comparisons including an exact analytical Mie scatter solution and through experimental comparison to measurement data from a laser scatterometer, providing evidence of the BEM model’s capability to accurately predict scatter from complex surfaces, including those that linear models cannot accurately model. A CSI model based on this BEM scattering model was then developed and verified, demonstrating the model’s capability to accurately model the CSI signal for complex surfaces which contain steep surfaces, including those that produce multiple scattering. Using this BEM-CSI model and experimental measurement, the capability of optical surface topography measurement methods for measurement of steep surfaces was investigated, illustrated for the first time with both fringe data and the resulting height estimates for a series of surfaces at slopes steeper than the SRL. At high tilt angles it was found that sharp edges with undercuts still provide strong signals which appear as plateaus in the topography data, with a width corresponding to the width of the point spread function of the instrument. While phase information was lost, part of the topography could still be obtained from the non-specular scatter. The BEM-CSI model’s results were accurate even for challenging surfaces beyond the capabilities of linear models, providing a tool for future investigation of other complex surfaces and providing progress towards evaluating the measurement uncertainty of complex surface measurements by optical instruments

Calibration and adjustment of coherence scanning interferometry

Coherence scanning interferometry (CSI) is a non-contacting optical technique which is widely used for the measurement of surface topography. CSI combines the lateral resolution of a high power microscope with the axial resolution of an interferometer. As with any other metrology instrument, CSI is calibrated to define measurement uncertainty. The traditional calibration procedure, as recommended by instrument manufacturers, consists of calibration of the axial and lateral scales of the instrument. Although calibration in this way provides uncertainties for the measurement of rectilinear artefacts, it does not give information about tilt-related uncertainty. If an object with varying slope is measured, significant errors are observed as the surface gradient increases. In this thesis a novel approach of calibration and adjustment for CSI using a spherical object is introduced. This new technique is based on three dimensional linear filtering theory. According to linear theory, smooth surface measurement in CSI can be represented as a linear filtering operation, where the filter is characterised either by point spread function (PSF) in space domain or by transfer function (TF) in spatial frequency domain. The derivation of these characteristics usually involves making the Born approximation, which is strictly only applicable for weakly scattering objects. However, for the case of surface scattering and making use of the Kirchhoff approximation, the system can be considered linear if multiple scattering is assumed to be negligible. In this case, the object is replaced by an infinitely thin foil-like object, which follows the surface topography and, therefore, is called the foil model of the surface. For an ideal aberration free instrument, the linear characteristics are determined by the numerical aperture of the objective lens and the bandwidth of the source. However, it is found that the PSF and TF of a commercial instrument can depart significantly from theory and result in a significant measurement error. A new method, based on modified inverse filter to compensate the phase and amplitude-related errors in the system PSF/TF, is demonstrated. Finally, a method based on de-warping to compensate distortion is discussed. The application of the linear theory as well as modified inverse filter is dependent on the assumption of the shift invariance. As distortion introduces a field dependent magnification, the presence of distortion for CSI with relatively large field of view, restricts the applicability of the linear theory. Along with this restriction, distortion also introduces erroneous height measurement for objects with gradients. This new approach, based on de-warping, resolves the problems associated with distortion

Improving coherence scanning interferometry signal modelling and topography measurement for complex surfaces

This thesis presents work on advanced optical surface metrology methods that enable extending the range of surface slopes that can be reliably measured by optical surface topography measurement instruments, and on investigating the reliability of the current capability. Optical instruments can only capture a limited portion of light scattered from an object’s surface, determined by the instrument’s numerical aperture. As the surface measured becomes steeper, less scatter is captured until all specular scatter is lost, referred to as the specular reflection limit (SRL). While surface measurement of slopes beyond the SRL by modern instruments is possible via the capture and detection of non-specular scatter, the instrument response to these slopes is not well understood. In addition, as the non-specular scatter has a low signal-to-noise ratio, data dropout can occur. Topography measurement of steep and complex surfaces using optical methods can therefore be challenging and have an unknown reliability, and can have significant errors when multiple scattering is present. The instrument modelling and experimental work focussed on coherence scanning interferometry (CSI). Through use of an approximate linear model the instrument response of a CSI instrument to various slope angles and spatial frequencies was described by a three-dimensional (3D) surface transfer function (STF). This theory was experimentally verified by demonstrating that an experimental 3D STF obtained from measurement of microspheres can be used to generate a filter that can compensate for the effect of lens aberration at a fundamental level and consequently reduce errors in the topographies obtained, especially from surface slopes just below the SRL. Second, a rigorous two-dimensional boundary element method (BEM) model of electromagnetic surface scatter was verified through multiple comparisons including an exact analytical Mie scatter solution and through experimental comparison to measurement data from a laser scatterometer, providing evidence of the BEM model’s capability to accurately predict scatter from complex surfaces, including those that linear models cannot accurately model. A CSI model based on this BEM scattering model was then developed and verified, demonstrating the model’s capability to accurately model the CSI signal for complex surfaces which contain steep surfaces, including those that produce multiple scattering. Using this BEM-CSI model and experimental measurement, the capability of optical surface topography measurement methods for measurement of steep surfaces was investigated, illustrated for the first time with both fringe data and the resulting height estimates for a series of surfaces at slopes steeper than the SRL. At high tilt angles it was found that sharp edges with undercuts still provide strong signals which appear as plateaus in the topography data, with a width corresponding to the width of the point spread function of the instrument. While phase information was lost, part of the topography could still be obtained from the non-specular scatter. The BEM-CSI model’s results were accurate even for challenging surfaces beyond the capabilities of linear models, providing a tool for future investigation of other complex surfaces and providing progress towards evaluating the measurement uncertainty of complex surface measurements by optical instruments

Investigations into a multiplexed fibre interferometer for on-line, nanoscale, surface metrology

Current trends in technology are leading to a need for ever smaller and more complex featured surfaces. The techniques for manufacturing these surfaces are varied but are tied together by one limitation; the lack of useable, on-line metrology instrumentation. Current metrology methods require the removal of a workpiece for characterisation which leads to machining down-time, more intensive labour and generally presents a bottle neck for throughput. In order to establish a new method for on-line metrology at the nanoscale investigation are made into the use of optical fibre interferometry to realise a compact probe that is robust to environmental disturbance. Wavelength tuning is combined with a dispersive element to provide a moveable optical stylus that sweeps the surface. The phase variation caused by the surface topography is then analysed using phase shifting interferometry. A second interferometer is wavelength multiplexed into the optical circuit in order to track the inherent instability of the optical fibre. This is then countered using a closed loop control to servo the path lengths mechanically which additionally counters external vibration on the measurand. The overall stability is found to be limited by polarisation state evolution however. A second method is then investigated and a rapid phase shifting technique is employed in conjunction with an electro-optic phase modulator to overcome the polarisation state evolution. Closed loop servo control is realised with no mechanical movement and a step height artefact is measured. The measurement result shows good correlation with a measurement taken with a commercial white light interferometer

Development and Characterization of a Dispersion-Encoded Method for Low-Coherence Interferometry

This Open Access book discusses an extension to low-coherence interferometry by dispersion-encoding. The approach is theoretically designed and implemented for applications such as surface profilometry, polymeric cross-linking estimation and the determination of thin-film layer thicknesses. During a characterization, it was shown that an axial measurement range of 79.91 µm with an axial resolution of 0.1 nm is achievable. Simultaneously, profiles of up to 1.5 mm in length were obtained in a scan-free manner. This marked a significant improvement in relation to the state-of-the-art in terms of dynamic range. Also, the axial and lateral measurement range were decoupled partially while functional parameters such as surface roughness were estimated. The characterization of the degree of polymeric cross-linking was performed as a function of the refractive index. It was acquired in a spatially-resolved manner with a resolution of 3.36 x 10-5. This was achieved by the development of a novel mathematical analysis approach