Development and calibration of wavelength scanning interferometry for surface topography measurement

Modern advanced manufacturing is capable of generating complex structures on large area substrates while maintaining high feature resolution and small defects. Examples of these products include photovoltaic cells, OLED displays, and printed sensors, manufactured at high speed on roll-to-roll (R2R) processes. A challenge for manufacturers is to ensure that the quality of products is not compromised by the faster manufacturing process, therefore inspection should be in-line, fast, and should meet the accuracy needs of the product. A possible candidate for nanoscale surface measurement on large areas in an industrial environment is the wavelength scanning interferometer (WSI) technique for its higher speed when compared to other surface topography measurement techniques.

In this thesis, traceability for a WSI instrument is established, i.e. a procedure is defined to estimate the measurement uncertainty according to recent development in ISO standards for surface texture measurement. An estimation of measurement uncertainties associated with each of the metrological characteristics (MCs) and combined uncertainty is reported. It is shown that the WSI instrument is capable of measuring surface height with an uncertainty in the order of tens of nanometres. The larger uncertainty contribution is due to the linearity deviation of the vertical axis due to variable performance of the phase demodulating algorithm to fringe patterns with a large range of frequencies.

And alternative method is proposed to estimate the amplification factor and the linearity deviation which are usually estimated with the step height standard (SHS) method. The amplification factor can be estimated which lower uncertainty via the wavelength standard (WS) method and the linearity deviation via the measurement of a tilted-flat.

A technique variation, namely ’phase WSI’ is proposed that improves the measurement performance of the WSI. Rather than determining the conventional fringe frequency-derived height directly, the method uses the frequency to resolve the fringe order ambiguity, and combine this information with the more accurate and repeatable fringe phase derived z height. A theoretical model to evaluate the method’s performance in the presence of additive noise is derived and shown to be in good agreement with experiments. The linearity deviation is reduced by approximately an order of magnitude, reaching amplitudes of few nanometres. The measurement noise is also reduced by an order of magnitude, reaching the sub-nanometre range.

A complementary technique, quadrature WSI (QWSI), is also proposed which extends the measurement range by more than double, allowing positive and negative optical path differences (OPD) to be distinguished and making accessible for measurement the range around the zero OPD position. A theoretical explanation of the achieved improvement and the origin of possible phase estimation error is also provided.