Multi-Wavelength Polarising Interferometer for In-Process Metrology

Micro-scale and nano-scale surfaces are now fabricated to serve in many fields: from optics needed in telescope/microscope imaging to semiconductors integrated in electronic devices such as smart phones and micro sensors. The production of these surfaces is inspiring the development of new metrology instrumentation that can not only ensure the quality but also optimise the manufacturing process. However, the state-of-the-art offline metrology instruments suffer from a main limitation, namely the inability to operate in the manufacture environment. The industry evolution requires in-process and metric metrology instrumentation that can provide rich surface information within harsh manufacture environment. The specification of such instruments has to be non-destructive, fast, and highly accurate; such instruments have to be combined with a production line. Interferometers offer non-destructive and parallel fashion measurement with nanometre accuracy.

A well-established phase-shift interferometer (PSI) is widely used for high measurement accuracy; however, it has two limitations. Firstly, the height difference between two adjacent points on the sample should be smaller than quarter of wavelength (Λ/4), and secondly, a PSI is slow and not suitable for in-process measurements, if a mechanical scanning is used for phase shifting. In order to utilise PSI for in-process measurements, data capturing at single exposure should be used to overcome the environmental disturbances and advanced phase unwrapping methods need to be employed to extend the measurement range beyond (Λ/4).

This research aimed to develop a multi-wavelength polarising phase-shift interferometer (MPI) for surface measurement and to investigate the possibility of its use for in-process metrology applications. The target specifications of the proposed instrument are as follows: a vertical measurement range greater than (Λ/4) (i.e. greater than 1 μm) with the resolution of a single wavelength interferometer (i.e. less than 10 nm). The MPI requires no mechanical scanning to obtain the phase shift with an extended measurement range using a single shot technique. This represents an improvement over the conventional single wavelength interferometer in terms of the measurement range and speed.

The methodology followed to achieve this study’s aims included reviewing the literature and implementing proof-of-concept experiments using mechanical and non-mechanical methods to acquire phase-shifted colour interferograms, hence determining algorithms for fringe analysis. Finally a novel MPI setup using polarisation technique and Red-Green-Blue (RGB) illumination source was developed that can be used for in-process measurement with extended range.

An acousto-optics tuneable filter (AOTF) was successfully employed to simultaneously provide RGB wavelengths with approximately 2 nm linewidth. Several fringe analyses and phase unwrapping algorithms, such as fringe order and best-match methods, were explored to retrieve areal surfaces. Colour crosstalk between cameras’ pixels was also investigated. It was found that the crosstalk is significant. A mathematical model and AOTF tuning capability were used to achieve minimum crosstalk. A spatial two dimensional image filtration was used to enhance the interferograms, hence signal-to-noise improvement. The proposed MPI has successfully measured samples (from 40 nm to 4 μm) with few nanometres accuracy and with single exposure (less than 0.3 second). This MPI has the potential for use in the measurement of surfaces produced by ultra-fast manufacturing such as roll-to-roll (R2R) manufacturing process. In the R2R process, structured surfaces are fabricated on large-area substrates (on the scale of several metres squared) at high speed exceeding several meters per minute. As such, MPI can be potentially used to measure moving surfaces within the manufacturing environment at speed limited only to the single exposure of the cameras.