We present an experimental demonstration of a novel, integrated readout approach for measuring the suspended height of micro-electro-mechanical systems (MEMS) structures. The approach is based on creating a resonant optical cavity between the suspended MEMS structure and the substrate that the MEMS structure is anchored to. The resulting interferometric effect causes modulation of an optical laser signal which is strongly dependent on the position of the MEMS device.
A miniature Fabry-Perot etalon was designed and fabricated to provide spectral filtering capability at the resonance
wavelength of 10 μm. A high transmission peak of 85% and a relatively broad bandwidth of 500 nm are expected based
on optical modeling. Optimal deposition conditions for process durable thin film materials were developed and optical
constants of these materials were characterized. Fabrication of devices was accomplished using standard surface
micromachining technique. Released mirrors exhibited a deflection of 400 nm over a length of 150 μm.
A microfluidics based targeted etchant delivery and masking approach to wet etching has been used to control the etch
progression of a MEMS sacrificial layer during the release of silicon nitride (SiN<sub>x</sub>) microbeams. A reusable 3-input
open-channel polydimethylsiloxane (PDMS) microfluidic cassette was used to form a dynamically controllable fluid etch
mask to control the location of the etchant during the wet release process. In contrast conventional release techniques
which use solid masking and homogeneous etching environments, microfluidic devices can utilise laminar flows to
generate heterogeneous etching conditions which can be controlled in real-time by altering the composition and flow
rates of the fluids passing through specific inlets. The fluid nature of the heterogeneous flow can be used to target etch
specific areas of sacrificial material or conversely, dynamically mask specific areas both above and below suspend
structures. As a result of this control, structures with anchor geometries not achievable using conventional release
techniques were created. Not only does this method require small volumes of etchant fluid, it is also suitable for use on
samples which may be sensitive to the chemical and/or physical rigors of photolithographic patterning, such as porous
silicon. Microfluidic based release etching, using dynamically controlled fluid masks, provides a valuable addition to the
suite of microchannel based fabrication techniques.
This paper reports on the modeling and experimental investigation of optical excitation of silicon cantilevers.
In this work, the silicon cantilevers fabricated have dimensions with width of 15 μm, thickness of 0.26 μm,
and variable length from 50 to 120 μm. In order to investigate the effect of the laser modulation frequency
and position on the temperature at the anchor edge and displacements at the tip of cantilevers, a transient
thermal ANSYS simulation and a steady-state static thermal mechanical ANSYS simulation were undertaken
using a structure consisting of silicon device layer, SiO<sub>2</sub> sacrificial layer and silicon substrate. The dynamic
properties of silicon cantilevers were undertaken by a series of experiments. The period optical driving signal
with controlled modulation amplitude was provided by a 405 nm diode laser with a 2.9 μW/μm<sub>2</sub> laser power
and variable frequencies. The laser spot was located through the longitude direction of silicon cantilevers. In
factor, simulation results well matched with experimental observation, including: 1) for untreated silicon
cantilevers, the maximum of displacement is observed when the laser beam was located half a diameter way
from the anchor on the silicon suspended cantilever side; 2) for the both cantilevers, maximum displacement
occurs when the optical actuation frequency is equal to the resonant frequency of cantilevers. Understanding
the optical excitation on silicon cantilevers, as waveguides, can potentially increase sensing detection
sensitivity (ratio of transmission to cantilever deflection).
This paper reports work on the development of rugged micro-electromechanical systems (MEMS)-based
microspectrometers for real-time applications in agriculture. The devices are electrostatically actuated, first order Fabry-
Perot tuneable optical filters, hybridised with InGaAs photodiode detectors. Tuning range and resolution of the devices
are 1615 nm to 2425 nm and 52 nm (FWHM) at 2000 nm, respectively. To our knowledge, this tuning range is the
largest reported for a MEMS-based Fabry-Perot filter. Three-layer distributed Bragg reflectors are used for the Fabry-
Perot mirrors, and consist of e-beam evaporated layers of germanium - silicon monoxide - germanium. The moveable
mirror also includes two silicon nitride layers that act as the MEMS flexures, stress compensation layers, and as an
encapsulant for the mirror layers. The spectral resolution matches the theoretical expected for the mirror structures used
when the residual bowing of the mirror (~15 nm across a diameter of 70 μm) is included, and can be improved to ~10 nm
if five layer mirrors are used. The out of band rejection is approximately 20 dB. Experimental results show that the
throughput of the device is sufficient to allow transmittance, specular reflectance and diffuse reflectance spectra to be
measured. The primary outstanding issue is wavelength calibration, and is being addressed using a number of
approaches including incorporation of wavelength calibration standards in the hybrid structure and accurate, real-time
measurement of the separation of the two mirrors.
A method to create microfluidic devices by utilizing hot imprinting stamps formed using printed circuit boards is demonstrated. Very large microfluidic devices (15×15 cm<sup>2</sup>) can be created with lateral features down to 100 microns and depths of nominally 17-70 μm. Room temperature solvent bonding was found to be a simple method of sealing the channels. The work also decribes the fabriation and operation of thermally actuated microvalves with sub-second switching and micropumps based on the imprinting techniques described.
Determining the lactose concentration in human breast milk (HBM) via standard assay techniques requires fat removal from the milk (defatting), followed by lactose detection in the remaining skim milk. This work focuses on methods of defatting which can be subsequently integrated in the same Lab-on-Chip (LOC) as the lactose measurement. One method under study for defatting HBM is the use of a cross-flow microfiltration structure. This kind of microfiltration prevents clogging and separates the large fat globules from the smaller nutrition constituents of milk, of which lactose is amongst the smallest. To test if large fat globules may clog the channel or not, the biocompatibility of PMMA and HBM was studied. The weight of absorbed fat on the surface of PMMA was found to be 3-orders of magnitude lower than that of the total fat in HBM. Photolithgraphy using SU-8 was applied for mold fabrication; however, hot-embossing using SU-8 mold has not been successful due to the high stress resulting in the demolding process. To improve mold strength, nickel molds were fabricated by electroplating using different current densities. As expected, the deposition rates were found to have a linear relationship with applied current density, while the smaller features have a higher deposition rate than larger features.
The production of high quality optical devices based on porous silicon relies on having precise control over the refractive
index and thickness of each porous silicon layer. Until now this has been achieved by pre-calibrating each growth
system and making sure that parameters such as wafer doping, electrolyte concentration and temperature are kept constant
with each fabrication. However low doped silicon required for IR based silicon photonics has significant non-uniformity
in the index and growth rate during formation of the porous silicon. The solution we have developed is based on realtime
in-situ monitoring of low-doped silicon during porous silicon growth. This process rapidly measures the optical
interference between the porous silicon film and the backside silicon surface. The optical light source comes from six
coarse-wavelength-division-multiplexed lasers, with rapid switching between wavelengths achieved using a
microelectromechanical switch. The system permits rapid measurement (<1 sec) of the reflection spectra from all lasers,
enabling real-time thickness and refractive index of each layer to be determined during growth. Our aim is to enable
growth of high quality multi-layer films such as those required for Bragg Reflectors and high-Q Fabry-Perot microcavities.
In this paper we briefly describe the instrument, the numerical models developed to gather the measurements,
and show preliminary results gathered from this instrument during growth. The results show a good agreement with
theoretical optical modelling, and also direct measurements of the porous silicon layers.
A palm-size interdigital impedance sensor incorporating a 10 μL sample reservoir, temperature sensor and hybrid heater was fabricated to determine the feasibility of measuring macronutrients in ultra-small volumes of human breast milk. Comparisons with previous measurements of homogenized cows milk show excellent agreement with fat measurement. Human breast milk however shows no correlation with fat but a surprising correlation with protein. Our investigations
and proposed methods to improve the correlation and measurement accuracy are discussed.
There is an increasing need for infrared spectroscopic instrumentation that is low-cost and extremely robust for
applications in agriculture, environmental monitoring, food science and medicine. This paper describes a MEMS-based
tunable Fabry-Perot filter that can be directly integrated on a detector. The fabrication process is detector independent,
and has been demonstrated on Si as well as one of the most unforgiving detector material systems, HgCdTe. Results are
presented that show that the technology is applicable for coverage of a wide spectral range, with examples of tuning from
~1600nm to ~2300nm and ~3800nm to ~4800nm using voltages <20V with line widths < 100nm and tuning speeds of
50kHz. Modeling shows that the device should be stable to shocks up to 250G. Line widths and tuning speeds can be
significantly improved using different actuator designs and removal of squeezed-film damping effects. The process uses
a maximum process temperature of 125°C, and is therefore compatible with a wide range of detector materials including
Si, Ge, InGaAs, InSb, as well as more specialized detector materials such as InAs quantum dots and InAs/GaSb
superlattices. Work is currently underway to demonstrate application of microspectrometers fabricated using this
technology in real-time testing of soils for agricultural applications.
We have developed a microspectrometer based on monolithic integration of a Fabry-Perot optical filter directly with a
Hg<sub>x</sub>Cd<sub>1-x</sub>Te-based infrared detector. The tunable Fabry-Perot is created by a parallel plate MEMS fabricated from two
dielectric mirror stacks separated by an initial air gap of 1.4 μm. We have measured linewidths as low as 55 nm,
switching times of 40 μs and a tuning range of 380 nm. However this tuning corresponds to only 42% of the desired
tuning range, from 1.6-2.5 μm (900 nm). The tuning range is limited by a process called "snap down" which occurs
when the MEMS is drive by a voltage source. It can be shown that for a parallel plate snap down occurs at 1/3 the
initial gap; complete tuning across the SWIR band requires a physical deflection of at least 60% of the gap. We have
developed a modified actuator design which allows 60% tuning of the moveable mirror. Further, the method minimizes
actuation-induced stress gradients which can lead to substantial bowing of the mirror and subsequently broad optical
linewidths. We will compare the results of our current microspectrometer with our new extended tuning designs. These
designs are based on Coventorware and analytical mechanical models combined with optical models for the Fabry-
We have previously developed a SWIR microspectrometer based on monolithic integration of a parallel plate Micro-
Electro-Mechanical Systems (MEMS) optical filter directly with a Hg<sub>x</sub>Cd<sub>1-x</sub>Te-based infrared detector. The primary
technical challenge in achieving the integration of a MEMS Fabry-Perot filter with the Hg<sub>x</sub>Cd<sub>1-x</sub>Te detector is to keep the processing temperature less than 150°C, as the performance of Hg<sub>x</sub>Cd<sub>1-x</sub>Te based photoconductors degrade at higher process temperatures. In this work we present our results to extend the operation into the 3-5 μm (MWIR) wavelength
range. For our preliminary results, the MWIR microspectrometer was based on a hybrid packaging approach, fabricating
the MWIR filter separately from the Hg<sub>x</sub>Cd<sub>1-x</sub>Te detector; however the key process parameters relating to temperature
control were maintained during fabrication of the MWIR filter, ensuring we can migrate this technology into an
integrated solution. Linewidths of 210 nm, switching times of 20 μs and a tuning range of 900 nm have been achieved.
The tuning speed is limited by squeezed film damping due to the physically narrow gap (&lgr;/2) between the Fabry-Perot
Hyperspectral imaging in the infrared bands is traditionally performed using a broad spectral response focal plane array,
integrated in a grating or a Fourier transform spectrometer. This paper describes an approach for miniaturizing a
hyperspectral detection system on a chip by integrating a Micro-Electro-Mechanical-System (MEMS) based tunable
Fabry Perot (FP) filter directly on a photodetector. A readout integrated circuit (ROIC) serves to both integrate the
detector signal as well as to electrically tune the filter across the wavelength band. We report the first such
demonstration of a tunable MEMS filter monolithically integrated on a HgCdTe detector. The filter structures, designed
for operation in the 1.6-2.5 μm wavelength band, were fabricated directly on HgCdTe detectors, both in photoconducting
and high density vertically integrated photodiode (HDVIP) detectors. The HDVIP detectors have an architecture that
permits operation in the standard photodiode mode at low bias voltages (≤0.5V) or in the electron avalanche photodiode
(EAPD) mode with gain at bias voltages of ~20V. In the APD mode gain values of 100 may be achieved at 20 V at 200
K. The FP filter consists of distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity
and a silicon nitride spacer membrane for support. Mirror stacks fabricated on silicon, identical to the structures that will
form the optical cavity, have been characterized to determine the optimum filter characteristics. The measured full width
at half maximum (FWHM) was 34 nm at the center wavelength of 1780 nm with an extinction ratio of 36.6. Fully
integrated filters on HgCdTe photoconductors with a center wavelength of approximately 1950 nm give a FWHM of
approximately 100 nm, and a peak responsivity of approximately 8 × 10<sup>4</sup> V/W. Initial results for the filters on HDVIP
detectors exhibit FWHM of 140 nm.
In this article the design, fabrication and characterization of micro-Fabry-Perot filters operating in the mid-wavelength infrared range is presented. Using surface micromachining techniques, low temperature silicon nitride based structures with distributed Bragg mirrors made of Ge/SiO/Ge layers have been fabricated and tested, both mechanically and optically. The membrane/mirror deflection has been measured using an optical profilometer and is estimated to be of the order of 800nm with voltage bias up to 17V while still preserving good mirror parallelism. The respective optical transmission peak shifted from 4.5μm to 3.6μm. Without antireflection coating at the back of the silicon substrate ~50% maximum transmission has been measured at the resonance peaks. The FWHM was measured to be 210+/-20nm, which is ~20% larger than estimated theoretically. In agreement with theoretical modeling, after crossing 1/3 of the cavity length, the membrane/mirror structure has been found to enter into an unstable region followed by snap-down to the bottom mirror surface. In order to prevent this detrimental effect, membranes with anti-stiction bumps have been fabricated demonstrating repeatable structure recovery from the stage of full collapse.
A low temperature MEMS process integrated with an infrared detector technology has been developed. The integrated microsystem is capable of electrically selecting narrow wavelength bands in the range from 1.6 to 2.5 μm within the short-wavelength infrared (SWIR) region of the electromagnetic spectrum. The integrated fabrication process is compatible with two-dimensional infrared focal plane array technology. The demonstration prototypes consist of both HgCdTe SWIR photoconductive as well as high density vertically integrated photodiode (HDVIP®) detectors, two distributed Bragg mirrors formed of Ge-SiO-Ge, an air-gap optical cavity, and a silicon nitride membrane for structural support. The tuning spectrum from fabricated MEMS filters on photoconductive detectors indicates a wide tuning range and high percentage transmission. Tuning is achieved with a voltage of only 7.5 V, and the FWHM ranged from 95-105 nm over a tuning range of 2.2 μm to 1.85 μm. The same MEMS filters, though unreleased, and with the sacrificial layer within the optical cavity, have been fabricated on planarised SWIR HDVIP® photodiodes with FWHM of less than 60 nm centred at a wavelength of approximately 1.8 μm. Finite element modelling of various geometries for the silicon nitride membrane will also be presented. The modelling is used to optimize the filter geometry in terms of fill factor, mirror displacement versus applied voltage, and membrane bowing.
A monolithically integrated low temperature MEMS and HgCdTe infrared detector technology has been implemented and characterised. The MEMS-based optical filter, integrated with an infrared detector, selects narrow wavelength bands in the range from 1.6 to 2.5 μm within the short-wavelength infrared (SWIR) region of the electromagnetic spectrum. The entire fabrication process is compatible with two-dimensional infrared focal plane array technology. The fabricated device consists of an HgCdTe SWIR photoconductor, two distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity, which is then removed to leave an air-gap, and a silicon nitride membrane for structural support. The tuning spectrum from fabricated MEMS filters on photoconductive detectors shows a wide tuning range and high percentage transmission is achieved with a tuning voltage of only 7.5 V. The FWHM ranged from 95-105 nm over a tuning range of 2.2 μm to 1.85 μm. Finite element modelling of various geometries for the silicon nitride membrane will also be presented. The modelling is used to determine the best geometry in terms of fill factor, voltage displacement prediction and membrane bowing.