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
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.
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.
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 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.