The preliminary results of a study on the effect of the membrane deformation on the optical response of the distributed Bragg reflector, that is based on a stack of such membranes, are presented. The analysis is applied to airgap-based optical filters, which offer an enhanced refractive index contrast and hence are highly promising for optical MEMS devices. The available methods and materials in MEMS technology would make fabrication of such devices feasible, but the optical requirements impose strict geometrical implications on the membrane structure. Although (an overall) tensile stress in membrane is expected to result in a flat structure after the release, a stress gradient results in a deformed structure. A combined finite element and finite-difference time- domain method has been utilized in this work to study the effects of a stress gradient in a distributed Bragg reflector. The results on the effects of both a linear and a non-linear stress gradient are presented. It is shown that a non-linear stress profile results in twice the deformation and a further reduction of optical performance.
CMOS-compatible fabrication of thin dielectric membranes for the ultraviolet and visible spectrum is presented for use in airgap/SiO2-based interference filter design. A typical optical design consists of multiple membranes of 50-100 nm thickness. Maintaining flatness over a large area, as required by the optical application, is challenging. In such a free-standing membrane, the residual stress is the main force acting on the structure. Although an overall tensile residual stress can effectively stretch the membrane, too much stress would exceed the yield strength of the material and results in fracturing. Furthermore, the presence of a residual stress gradient causes the membrane to deform. In this work, the effect of a stress profile in the thin film has is investigated. Although PECVD SiO2 layers with an average tensile stress level of 178 MPa are used for the fabrication of the membranes, the presence of a stress gradient of about 0:67 MPa=nm results in a deformation in the membrane. A simple straining method is applied to reduce flatness. The preliminary results and discusses the challenges in the fabrication of stacked membranes for optical filters are presented.
The increasing demand for handheld systems for absorption spectroscopy has triggered the development of microspectrometers at various wavelength ranges. Several MEMS implementations of the light source, interferometer/optical filter, and detector have already been reported in the literature. However, the size of microspectrometers is still limited by the required absorption path length in the sample gas cell. This paper presents a compact MEMS linear-variable optical filter (LVOF) where the resonator cavity of the filter is also used as a sample gas cell to measure the absorption of methane at 3392nm wavelength. The physical resonator cavity length is elongated 62.2-fold, using multiple reflections from highly reflective Bragg mirrors to achieve a sufficiently long effective optical absorption path. Although the LVOF would in principle enable operation as a robust portable microspectrometer, here it is used in a miniaturized NDIR methane sensor for wavelength selection and calibration.
The design of a metamaterial-based absorber for use in a MEMS-based mid-IR microspectrometer is reported. The microspectrometer consists of a LVOF that is aligned with an array of thermopile detectors, which is fabricated on a SiN membrane and coated with the absorber. Special emphasis is put on the CMOS compatible fabrication, which results in an absorber design based on Al disc resonators and an Al background plane that are separated by an SiO2 layer. Wideband operation over the 3-4 μm spectral range is achieved by staggered tuning of four Al disk resonators in one 1.5 x 1.5 μm2 unit cell, using four different values of the radius of the Al disk between 0.50 μm and 0.63 μm and an SiO2 layer thickness of 150 nm. Simulations reveal an average absorption of about 95% with a ±4% ripple at normal incidence, which reduces to about 80% absorption at a 20° incidence angle. The influence of material choice and dimensions on a single absorption peak was studied and the magnetic polariton was identified as the underlying mechanism of absorption.
A concept for a highly integrated and miniaturized gas sensor based on infrared absorption, a Fabry-Perot type linear variable optical filter with integrated gas cell, is presented. The sample chamber takes up most of the space in a conventional spectrometer and is the only component that has so far not been miniaturized. In this concept the gas cell is combined with the resonator cavity of the filter. The optical design, fabrication, and characterization results on a MEMSbased realization are reported for a 24-25.5 μm long tapered resonator cavity. Multiple reflections from highly reflective mirrors enable this optical cavity to also act as a gas cell with an equivalent optical absorption path length of 8 mm. Wideband operation of the filter is ensured by fabrication of a tapered mirror. In addition to the functional integration and significant size reduction, the filter contains no moving parts, thus enables the fabrication of a robust microspectrometer
The optical performance of a distributed Bragg reflector (DBR) is typically the determining factor in many optical MEMS devices and is mainly limited by the number of the periods (number of layers) and the refractive index contrast (RIC) of the materials used. The number of suitable available materials is limited and implementing a large number of periods increases the process complexity. Using air as a low-index material improves the RIC by almost 50% as compared with most conventional layer combinations and hence provides a higher optical performance at a given number of layers. This paper presents the design, fabrication, and optical characterization of multiple air-SiO2 Bragg reflectors with two airgap layers designed for the visible spectrum. Alternate polysilicon deposition and silicon-dioxide growth on the wafers followed by the selective etching of polysilicon layers in a TMAH-based solution results in a layer stack according to the optical design. However, unlike the conventional MEMS processes, fabrication of a blue-band airdielectric DBR demands several sacrificial layers in the range of 100 nm. Therefore, a successful release of the membrane after wet-etching is critical to the successful performance of the device. In this study, several DBRs with two periods have been fabricated using a CO2 supercritical drying process. The wide-area reflection measurements showed a peak reflectance of 65% and an FWHM of about 100 nm for a DBR centered at 500 nm. DBRs centered on 400 nm gave a much wider spectral response. This paper presents preliminary optical characterization results and discusses the challenges for a reflector design in the blue-visible range.
This paper presents the design, simulation, fabrication, and characterization of a thin-film Fabry-Perot resonator composed of titanium dioxide (TiO2) and silicon dioxide (SiO2) thin-films. The optical filter is developed to be integrated with a light emitting diode (LED) for enabling narrow-band imaging (NBI) in endoscopy. The NBI is a high resolution imaging technique that uses spectrally centered blue light (415 nm) and green light (540 nm) to illuminate the target tissue. The light at 415 nm enhances the imaging of superficial veins due to their hemoglobin absorption, while the light at 540 nm penetrates deeper into the mucosa, thus enhances the sub-epithelial vessels imaging. Typically the endoscopes and endoscopic capsules use white light for acquiring images of the gastrointestinal (GI) tract. However, implementing the NBI technique in endoscopic capsules enhances their capabilities for the clinical applications. A commercially available blue LED with a maximum peak intensity at 404 nm and Full Width Half Maximum (FWHM) of 20 nm is integrated with a narrow band blue filter as the NBI light source. The thin film simulations show a maximum spectral transmittance of 36 %, that is centered at 415 nm with FWHM of 13 nm for combined the blue LED and a Fabry Perot resonator system. A custom made deposition scheme was developed for the fabrication of the blue optical filter by RF sputtering. RF powered reactive sputtering at 200 W with the gas flows of argon and oxygen that are controlled for a 5:1 ratio gives the optimum optical conditions for TiO2 thin films. For SiO2 thin films, a non-reactive RF sputtering at 150 W with argon gas flow at 15 sccm results in the best optical performance. The TiO2 and SiO2 thin films were fully characterized by an ellipsometer in the wavelength range between 250 nm to 1600 nm. Finally, the optical performance of the blue optical filter is measured and presented.
This paper presents the design, fabrication and characterization of a linear variable optical filter (LVOF) that operates in the infrared (IR) spectral range. An LVOF-based microspectrometer is a tapered-cavity Fabry-Perot optical filter placed on top of a linear array of detectors. The filter transforms the optical spectrum into a lateral intensity profile, which is recorded by the detectors. The IR LVOF has been fabricated in an IC-compatible process flow using a resist reflow and is followed by the transfer etching of this resist pattern into the optical resonator layer. This technique provides the possibility to fabricate a small, robust and high-resolution micro-spectrometer in the IR spectral range directly on a detector chip. In these designs, the LVOF uses thin-film layers of sputtered Si and SiO2 as the high and low refractive index materials respectively. By tuning the deposition conditions and analyzing the optical properties with a commercial ellipsometer, the refractive index for Si and SiO2 thin-films was measured and optimized for the intended spectral range. Two LVOF microspectrometers, one operating in the 1.8-2.8 μm, and the other in the 3.0-4.5 μm wavelength range, have been designed and fabricated on a silicon wafer. The filters consist of a Fabry-Perot structure combined with a band-pass filter to block the out-of-band transmission. Finally, the filters were fully characterized with an FTIR spectrometer and the transmission curve widening was investigated. The measured transmittance curves were in agreement with theory. The characterization shows a spectral resolution of 35-60 nm for the short wavelength range LVOF and 70 nm for the long wavelength range LVOF, which can be further improved using signal processing algorithms.
An IC-Compatible Linear-Variable Optical Filter (LVOF) for application in the UV spectral range between 310 nm and
400 nm has been fabricated using resist reflow and an optimized dry-etching. The LVOF is mounted on the top of a
commercially available CMOS camera to result in a UV microspectrometer. A special calibration technique has been
employed that is based on an initial spectral measurement on a Xenon lamp. The image recorded on the camera during
calibration is used in a signal processing algorithm to reconstruct the spectrum of the Mercury lamp and the calibration
data is subsequently used in UV spectral measurements. Experiments on fabricated LVOF-based microspectrometer with
this calibration approach implemented reveal a spectral resolution of 0.5 nm.
This paper presents the design, fabrication and characterization of Infra-Red (IR) Linear Variable Optical Filter (LVOF)-
based micro-spectrometers. Two LVOF microspectrometer designs have been realized: one for operating in the 1400 nm
to 2500 nm wavelength range and another between 3000 nm and 5000 nm. The IR LVOFs have been fabricated in an ICCompatible
process using resist reflow. The LVOF provides the possibility to have a small size, robust and highresolution
micro-spectrometer in the IR on a detector chip. Such IR microspectrometers can be fabricated at low-cost in
high volume production and have huge potential in applications such as liquid identification (e.g. water in alcohol, water
in oil) and gas sensing.
This paper reports on the functional and spectral characterization of a microspectrometer based on a CMOS detector
array covered by an IC-Compatible Linear Variable Optical Filter (LVOF). The Fabry-Perot LVOF is composed of 15
dielectric layers with a tapered middle cavity layer, which has been fabricated in an IC-Compatible process using resist
reflow. A pattern of trenches is made in a resist layer by lithography and followed by a reflow step result in a smooth
tapered resist layer. The lithography mask with the required pattern is designed by a simple geometrical model and FEM
simulation of reflow process. The topography of the tapered resist layer is transferred into silicon dioxide layer by an
optimized RIE process. The IC-compatible fabrication technique of such a LVOF, makes fabrication directly on a
CMOS or CCD detector possible and would allow for high volume production of chip-size micro-spectrometers. The
LVOF is designed to cover the 580 nm to 720 spectral range. The dimensions of the fabricated LVOF are 5×5 mm2. The
LVOF is placed in front of detector chip of a commercial camera to enable characterization. An initial calibration is
performed by projecting monochromatic light in the wavelength range of 580 nm to 720 nm on the LVOF and the
camera. The wavelength of the monochromatic light is swept in 1 nm steps. The Illuminated stripe region on the camera
detector moves as the wavelength is swept. Afterwards, a Neon lamp is used to validate the possibility of spectral
measurement. The light from a Neon lamp is collimated and projected on the LVOF on the camera chip. After data
acquisition a special algorithm is used to extract the spectrum of the Neon lamp.
A thermopile-based detector array for use in a miniaturized Infrared (IR) spectrometer has been designed and fabricated
using CMOS compatible MEMS technology. The emphasis is on the optimal of the detector array at the system level,
while considering the thermal design, the dimensional constraints of a design on a chip and the CMOS compatibility.
The resolving power is maximized by spacing the Thermo-Electric (TE) elements at an as narrow as possible pitch,
which is limited by processing constraints. The large aspect ratio of the TE elements implies a large cross-sectional area
between adjacent elements within the array and results in a relatively large lateral heat exchange between
micromachined elements by thermal diffusion. This thermal cross-talk is about 10% in case of a gap spacing of 10 μm
between elements. Therefore, the detector array should be packaged (and operated) in vacuum in order to reduce the
cross-talk due to the air conduction through the gap. Thin film packaging is a solution to achieve an operating air
pressure at 1.3 mBar, which reduces the cross-talk to 0.4%. One of other advantages of having low operating pressure is
the increased sensitivity of single TE element. An absorber based on an optical interference filter design is also designed
and fabricated as an IC compatible post-process on top the detector array. The combination of the use of CMOS
compatible materials and processing with high absorbance in 1.5 - 5 μm wavelength range makes a complete on-chip
microspectrometer possible.
KEYWORDS: Control systems, Photodetectors, Switches, Signal detection, Signal to noise ratio, Sensors, Interference (communication), Standards development, Optical filters, CMOS technology
A linear array of 128 Active Pixel Sensors has been developed in standard CMOS technology and a Linear Variable
Optical Filter (LVOF) is added using CMOS-compatible post-process, resulting in a single chip highly-integrated highresolution
microspectrometer. The optical requirements imposed by the LVOF result in photodetectors with small pitch
and large length in the direction normal to the dispersed spectrum (7.2μ;m×300μm). The specific characteristics of the
readout are the small pitch, low optical signals (typically a photocurrent of 100fA~1pA) and a much longer integration
time as compared to regular video (typically 100μs~63s). These characteristics enable a very different trade-off between
SNR and integration time and IC-compatibility. The system discussed in this paper operates in the visible part of the
spectrum. The prototype is fabricated in the AMIS 0.35μm A/D CMOS technology.
This paper reports on a CMOS-Compatible Linear Variable Optical Filter (LVOF) visible micro-spectrometer. The
CMOS-compatible post process for fabrication of the LVOF has been used for integration of the LVOF with a CMOS
chip containing a 128-element photodiode array and readout circuitry. Fabrication of LVOF involves a process for
fabrication of very small taper angles, ranging from 0.001° to 0.1°, in SiO2. These layers can be fabricated flexibly in a resist layer by just one lithography step and a subsequent reflow process. The 3D pattern of the resist structures is
subsequently transferred into SiO2 by appropriate etching. Complete LVOF fabrication involves CMOS-compatible
deposition of a lower dielectric mirror using a stack of dielectrics on the wafer, tapered layer formation and deposition of
the top dielectric mirror. The LVOF has been optimized for 580 nm - 720 nm spectral operating range and has also been
mounted on a CCD camera for characterization. The design of LVOF micro-spectrometer, the fabrication and
characterization results are presented.
The miniaturized IR spectrometer discussed in this paper is comprised of: slit, planar imaging diffraction grating and
Thermo-Electric (TE) detector array, which is fabricated using CMOS compatible MEMS technology. The resolving
power is maximized by spacing the TE elements at an as narrow as possible pitch, which is limited by processing
constraints. The large aspect ratio of the TE elements implies a large cross-sectional area between adjacent elements
within the array and results in a relatively large lateral heat exchange between micromachined elements by thermal
diffusion. This thermal cross-talk is about 10% in case of a gap spacing of 10 μm between elements. Therefore, the
detector array should be packaged (and operated) in vacuum in order to reduce the cross-talk due to the air conduction
through the gap. Thin film packaging is a solution to achieve an operating air pressure at1.3 mBar, which reduces the
cross-talk to 0.4%. An absorber based on an optical interference filter design is also designed and fabricated as an IC
compatible post-process on top the detector array. The combination of the use of CMOS compatible materials and
processing with high absorbance in 1.5 - 5 μm wavelength range makes a complete on-chip microspectrometer possible.
The design and performance of a highly miniaturized spectrometer fabricated using MEMS technologies are reported in
this paper. Operation is based on an imaging diffraction grating. Minimizing fabrication complexity and assembly of the
micromachined optical and electronic parts of the microspectrometer implies a planar design. It consists of two parallel
glass plates, which contain all spectrograph components, including slit and diffraction grating, and can be fabricated on a
single glass wafer with standard lithography. A simple analytical model for determining spectral resolution from device
dimensions was developed and used for finding the optimal parameters of a miniaturized spectrometer as a compromise
between size and spectral resolution. The fabricated spectrometer is very compact (11 × 1.5 × 3 mm3), which allowed
mounting directly on top of an image sensor. The realized spectrometer features a 6 nm spectral resolution over a 100 nm
operating range from 600 nm to 700 nm, which was tested using a Ne light source.
This paper reports on the development and validation of a new technology for the fabrication of variable line-spacing
non-planar diffraction gratings to be used in compact spectrometers. The technique is based on the standard lithographic
process commonly used for pattern transfer onto a flat substrate. The essence of the technology presented here is the
lithographic fabrication of a planar grating structure on top of a flexible membrane on a glass or silicon wafer and the
subsequent deformation of the membrane using a master shape. For the validation of the proposed technology we
fabricated several reflection concave diffraction gratings with the f-numbers varying from 2 to 3.8 and a diameter in the
4 - 7 mm range. A glass wafer with circular holes was laminated by dry-film resist to form the membranes.
Subsequently, standard planar lithography was applied to the top part of the membranes for realizing grating structures.
Finally the membranes were deformed using plano-convex lenses in such a way that precise lens alignment is not
required. A permanent non-planar structure remains after curing. The imaging properties of the fabricated gratings were
tested in a three-component spectrograph setup in which the cleaved tip of an optical fiber served as an input slit and a
CCD camera was used as a detector. This simple spectrograph demonstrated subnanometer spectral resolution in the 580
- 720 nm range.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.