Surface and thickness measurement of a transparent film using wavelength scanning interferometry

A wavelength scanning interferometer for measuring the surface
and thickness of a transparent film has been studied. A halogen light source
combined with an acousto-optic tuneable filter is used to generate a
sequence of filtered light in a Linnik interferometer, which leads to a
sequence of interferograms captured by a CCD camera. When a transparent
thin film is measured, the reflection signals from both the top and bottom
surfaces of the film will interfere with the reference signal. At the same
time, the multiple reflection signals between the two film surfaces will also
interfere with each other. Effective separation of the interference signals
from each other is the key to achieving a successful measurement. By
performing a frequency-domain analysis, these interference signals can be
separated. An optimized Fourier transform method is used in the analysis.
Measurements of the top and bottom surface finishes of the film, as well as
the film thickness map, have been achieved. The film needs to be more than
3 μm in optical path length, and must transparent with no absorption of
light. The film’s refractive index needs to be known as a function of
wavelength. In this paper, the theoretical analysis and simulation study of
wavelength scanning interferometry for transparent film measurement is
discussed. Experiments on thin film layers of Parylene N coated on a glass
slide surface are studied and analyzed. Comparison study results with other
contact and non-contact methods are also presented