The measurement of rough surface topography using coherence scanning interferometry

This guide describes good practice for the measurement and characterisation of rough surface topography using coherence scanning interferometry (commonly referred to as vertical scanning white light interferometry). It is aimed at users of coherence scanning interferometry for the optical measurement of surface texture within production and research environments. The general guidelines described herein can be applied to the measurement of rough surfaces exhibiting different types of surface topography. For the purpose of this guide, the definition of a rough surface is one that has features with heights ranging from approximately 10 nm to less than 100 µ

Surface measurement errors using commercial scanning white light interferometers

This paper examines the performance of commercial scanning white light interferometers in a range of measurement tasks. A step height artefact is used to investigate the response of the instruments at a discontinuity, while gratings with sinusoidal and rectangular profiles are used to investigate the effects of surface gradient and spatial frequency. Results are compared with measurements made with tapping mode atomic force microscopy and discrepancies are discussed with reference to error mechanisms put forward in the published literature. As expected it is found that most instruments report errors when used in regions close to a discontinuity or those with a surface gradient that is large compared to the acceptance angle of the objective lens. Amongst other findings, however, we report systematic errors that are observed when the surface gradient is considerably smaller. Although these errors are typically less than the mean wavelength they are significant compared to the instrument resolution and indicate that current scanning white light interferometers should be used with some caution if sub-wavelength accuracy is required

Tolerance on sphere radius for the calibration of the transfer function of coherence scanning interferometry

Although coherence scanning interferometry (CSI) commonly achieves a sub-nanometre noise level in surface topography measurement, the absolute accuracy is difficult to determine when measuring a surface that contains varying local slope angles and curvatures. Recent research has shown that it is possible to use a single sphere with a radius much greater than the source wavelength to calibrate the three-dimensional transfer function of a CSI system. A major requirement is the accurate knowledge of the sphere radius, but the three-dimensional measurement of a sphere with nanometre level uncertainty is a highly challenging metrology problem, and is not currently feasible. Perfect spheres do not exist and every measurement has uncertainty. Without having a quantitative understanding of the tolerance of the sphere radius, the calibration method cannot be used confidently for calibration of the transfer function of a CSI system that may be used in research laboratories or industry. In this paper, the effects of the tolerance of the radius of the calibration sphere on surface topography measurements are quantitatively analysed through a computational approach. CSI measurements of spherical, sinusoidal and rough surfaces are investigated in the presence of various degrees of radius error. A lookup table that relates the surface height error as a function of the radius error and surface slope angle is provided. The users may estimate the required tolerances of the sphere radius for their specific surface measurements if this calibration approach is used. The output of this paper provides a feasibility analysis for this calibration method for further development and applications

Coherence scanning interferometry: linear theory of surface measurement

The characterization of imaging methods as three-dimensional (3D) linear filtering operations provides a useful way to compare the 3D performance of optical surface topography measuring instruments, such as coherence scanning interferometry, confocal and structured light microscopy. In this way, the imaging system is defined in terms of the point spread function in the space domain or equivalently by the transfer function in the spatial frequency domain. The derivation of these characteristics usually involves making the Born approximation, which is strictly only applicable to weakly scattering objects; however, for the case of surface scattering, the system is linear if multiple scattering is assumed to be negligible and the Kirchhoff approximation is assumed. A difference between the filter characteristics derived in each case is found. However this paper discusses these differences and explains the equivalence of the two approaches when applied to a weakly scattering object

The assessment of areal surface texture parameters for characterizing the adhesive bond strength of copper plated micro-machined glass.

The micro-electronics industry is investigating glass as an alternative printed circuit board material and interposer. Electroless copper plating of glass is required for tracks and interconnects, but understanding of how the surface topography of the glass substrate affects the mechanics of the copper/glass bond quality is limited. Areal surface texture parameters provide the potential for characterizing key surface features associated with improving copper/glass bonding. Laser ablation techniques have been used to prepare glass surfaces with micro-scale structured features, and these features have been quantified using areal parameters. The copper/glass bond strength has been quantified using scratch testing techniques, with statistical analysis identifying strongly correlating areal parameters that may be used for predictive design of glass surfaces

Areal texture and angle measurements of tilted surfaces using focus variation methods

Optical instruments for areal surface topography measurement have seen significant commercial development in the last five years, along with the ISO 25178 areal standard. Providing the user with confidence in new instruments depends on understanding instrument behavior and sources of error. Focus variation techniques rely on the inherent micro- or nano-scale roughness of a surface to allow acquisition of topography data. The work reported here has been examining the sensitivity of the focus variation technique to surface slope, using areal parameters to characterize surface roughness at extended slope values. The results illustrate links between instrument variables and slope characterization

The use of areal surface texture parameters to characterize the mechanical bond strength of copper on glass plating applications

This report describe research into the role that surface topography plays in influencing the mechanical bond strength of the electroless copper plating of novel glass substrates. The work considers bespoke laser machining of glass substrates, electroless plating chemistry, areal surface topography analysis using non-contact optical techniques, paramaterization of the surfaces using ISO 25178 areal parameters, and scratch testing of plated copper to measure the adhesive bond strength. By correlating bond strength to appropriate areal parameters, it is anticipated that better mechanical adhesive potential of machined glass surfaces can be achieved

The assessment of residual flatness errors in focus variation areal measuring instruments

Optical instruments for areal surface topography measurement rely on high-precision lenses that guide the light from the object surface to the image plane. Lens aberrations may cause distortion of the transmitted image and consequently a residual flatness error in the measurement data. Previous work at NPL suggests using an averaging method for residual flatness error assessment for optical surface topography instruments. However, the averaging method does not apply to the focus variation technique, which relies on the nano-scale roughness of a surface to allow acquisition of topography data. This paper presents alternative methods for measuring residual flatness for focus variation instruments

Coherence scanning interferometry: measurement and correction of three-dimensional transfer and point-spread characteristics

When applied to the measurement of smooth surfaces, coherence scanning interferometry can be described by a three-dimensional linear filtering operation that is characterized either by the point-spread function in the space domain or equivalently by the transfer function (TF) in the spatial frequency domain. For an ideal, aberration-free instrument, these characteristics are defined uniquely by the numerical aperture of the objective lens and the bandwidth of the illumination source. In practice, however, physical imperfections such as those in lens aberrations, reference focus, and source alignment mean that the instrument performance is not ideal. Currently, these imperfections often go unnoticed as the instrument performance is typically only verified using rectilinear artifacts such as step heights and lateral grids. If an object of varying slope is measured, however, significant errors are often observed as the surface gradient increases. In this paper, a new method of calibration and adjustment using a silica micro-sphere as a calibration artifact is introduced. The silica microsphere was used to compute the point-spread and TF characteristics of the instrument, and the effect of these characteristics on instrument performance is discussed. Finally, a straightforward method to correct for phase and amplitude imperfections in the TF is described using a modified inverse filter

Determination of the lateral resolution of an interference microscope using a micro-scale sphere

Determination of the lateral resolution of an interference microscope using a micro-scale spher