Linear variable filter fabrication and its applications in miniaturized spectrometer

This thesis presents new designs for a linear variable filter (LVF) and discusses their
manufacture using drum-based microwave plasma-assisted pulsed DC magnetron reactive
sputtering. By using this deposition method, LVF samples for both the visible and nearinfrared
bands are manufactured, and spectroscopic measurements are used to characterize the
LVF samples. It is demonstrated that deposition times can be reduced to a few days rather
than the 6 weeks of alternative designs and methods described in the literature. The drumbased
tool used here is shown to allow multiple copies of the filters to be manufactured in one
run, increasing potential throughput by at least an order of magnitude over alternative
deposition techniques.
An innovative miniaturized spectrometer design is proposed using the LVFs described in this
thesis and several iterations are manufactured and tested. These miniaturized spectrometers
are suitable for portable and real-time monitoring applications. Compared with traditional
spectrometers, the manufacturing cost is relatively low, and the use of advanced
microelectronic technologies opens up applications in chemical testing, food safety, material
analysis, energy testing and other fields.
The design and analysis of the LVFs is achieved using thin film design software (TFCalc).
The designs are based on narrow-bandpass filters and wide blocking filters, deposited on front
and back surfaces of suitable substrates using the drum-based microwave plasma assisted DC
magnetron sputtering thin film deposition system (Microdyn). Two particular LVFs are
designed, one with a working range of 450 nm - 900 nm and linearity of 11.14 nm / mm for
use in the visible light band and another with a working range of 1.5 μm - 2.5 μm and
linearity of 42.68 nm / mm for use in the near-infrared band.
Nb2O5 and SiO2 materials are selected as high and low refractive index materials respectively
and their optical constants characterised by fitting to spectroscopic data for subsequent use in
TFCalc.
To fabricate linear variable filters, the deposited film thickness must vary linearly along the
length of the substrate. This is achieved by modelling the distribution of sputtered material
arriving at the substrate during the sputtering process used in the MicroDyn tool. Optimal
masking functions are designed using MathCad 15 scripts and successfully manufactured to fit the target holders. With this mask in place, single thin film layers of the selected materials
are shown to exhibit a linear trend in thickness over 44 mm lengths of glass and silicon
substrates allowing the successful manufacture of the designed LVFs. Spectroscopic
measurements at selected points along the lengths of the LVFs confirm the designed
performance, with the central wavelength deviating from linearity by less than 1% over the
450 nm - 900 nm range and varying by a very satisfactory 10.8 nm/mm.
The continuous nature of LVFs means that different sizes of the illuminating spot produce
different degrees of an averaging or blurring effect on spectral results and this effect is
simulated, analysed and quantified for various spot sizes using square-wave and Pearson
functions, with the Pearson VII function shown to give the best fit. This information can be
used to improve the resolution of measurements. The full width at half maximum with an
incident beam size of 0.4 mm is shown to be 11.2 nm.
Solidworks is used to design a miniaturized-spectrometer device incorporating the LVF as the
core component, and 3D printing is used to manufacture practical spectrometers. An MCU
control module is designed for the miniaturized spectrometer and an interface program is
written to be used in a PC with the Windows operating system. Finally, optical thin film
samples are measured using the manufactured spectrometers and compared with
measurements obtained with an expensive conventional spectrometer equipment. Best results
are obtained with the third iteration of spectrometer design, based on a threaded screw instead
of 3D printed gears to scan the LVF past the single detector.
Suggestions for future work and potential improvements are discussed.