**3.1. Experimental set-up**

**Figure 6.** Measurement results of a semiconductor daughterboard sample with vibration compensation: (a) measured

From the above case study, we can conclude that by using a common-path reference interferometer together with an active environmental noise compensation system, a wavelength

Roll-to-roll (R2R) processing is a fast and economic processing method for manufacturers that produce high-volume products using large area foils such as packaging products, photovoltaic films and emerging market sectors such as flexible electronics. However, there is an increased risk of defects forming as the number of interfaces increases in the multi-layer films, and the size and nature of those defects change as the layer thicknesses shrink to the nanoscale [22, 23]. Because of the nature of these practices, the inspection methods have to be in non-contact with the film surfaces. Effective surface inspection is the key for further processes

such as applying local repair techniques to eliminate the defects from the film surface.

inspection and has a vertical measurement range over a few hundred micrometers.

The above system has been implemented into a R2R system for demonstration of online surface inspection [24]. Nevertheless, a measurement can only be achieved if the measured sample is at a standstill. However, the tested sample surface is in constant movement during inspection, due to the nature of R2R processing. In order to achieve dynamic inspection, all the measurement information must be sampled in just one sample, and the sampling rate should be greater than a few hundred Hz to reduce the effect of mechanical vibration. In this case, a dynamic interferometer [8–10] is the solution. However, these approaches have one drawback for the application of online surface measurement: the 2µ phase ambiguity for phase-shift interferometer, which restricts the vertical measurement range to a few hundred nanometres. It is short of the demands of most surface measurements and inspections. We have introduced a single-shot line-scan dispersive interferometer [25] which is able to perform dynamic surface

scanning interferometer can be used for shop floor surface measurement.

surface and (b) cross-sectional profile.

50 Optical Interferometry

The basic configuration of the proposed line-scan dispersive interferometer (LSDI) is shown in **Figure 7**. A halogen bulb with broadband spectrum is used to generate the white light illumination for the system. This low-coherence white light is coupled into a multi-mode optical fibre patch cable with a numerical aperture of 0.39. The tested surface is observed through a 4 × Michelson interferometric objective. The interfered beam is focused using a tube lens with a focal length of 250 mm and split into two parts by a pellicle beamsplitter coated with a 45:55 (reflectance: transmission) split ratio. The reflectance part is received by camera 1 which provides real-time imaging of the observed surface. The other part is brought to a spectrometer to generate spectral interferograms for surface measurement. More specifically, this transmission beam first passes through a slit to block the light redundant for measurement, after that a narrow line of light which represents an interference signal of a surface profile is selected and diffracted by the grating and finally received on camera 2. The direction of the slit is set to be parallel to the columns of camera pixels, so that the dispersion axis is along the rows.

**Figure 7.** Schematic diagram of the optical set-up.
