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Microelectronics, Electrical Engineering, Mathematics and Computer Science

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., Department of ME/EI, Electrical Engineering, Mathematics and Computer Science

This paper presents spectral measurements using a linear variable optical filter. A LVOF has been developed for operation in the 530 nm–720 nm spectral band and has been fabricated in an IC-compatible process. The LVOF has been mounted on a CMOS camera. A Least Mean Square algorithm has been implemented to calculate the spectrum of light from the recorded image on the CMOS camera. A spectral resolution of 0.5 nm has been achieved using the algorithm. The spectral resolution is limited by the monochromator accuracy used for calibration., Microelectronics, Electrical Engineering, Mathematics and Computer Science

This paper reports on the IC-compatible fabrication of vertically tapered optical layers for use in linear variable optical filters (LVOF). The taper angle is fully defined by a mask design. Only one masked lithography step is required for defining strips in a photoresist with trenches etched therein of a density varying along the length of the strip. In a subsequent reflow, this patterned photoresist is planarized, resulting in a strip with a local thickness defined by the initial layer thickness and the trench density at that position before reflow. Hence a taper can be flexibly programmed by the mask design to be from 0.001º to 0.1º, which enables the simultaneous fabrication of tapered layers of different taper angles. The 3D pattern of resist structures is subsequently transferred into Si or 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. Design principle, processing and simulation results plus experimental validation of the technique on the profile in the resist and after transfer of the taper into Si and SiO2 are presented., Department of ME/EI, Electrical Engineering, Mathematics and Computer Science

The spectral resolution of a MEMS-based IR microspectrometer critically depends on the thermal cross-talk between adjacent TE elements in the detector array. Thermal isolation between elements is realized by using bulk micromachining directly following CMOS processing. This paper reports on the characterization results of bridge-shaped TE detector elements that are cut out of a membrane. Elements with dimensions of 650 × 36 ?m2 are separated by 10 ?m wide gaps in order to minimize the thermal cross-talk by heat conduction through the support structure. The static and dynamic aspects of thermal cross-talk have been evaluated with an emphasis on the effect of the thermal conductivity of air as a function of the package pressure., Department of ME/EI, Electrical Engineering, Mathematics and Computer Science

An analytical model has been developed and applied to explore the limits in the design of a highly miniaturized planar optical microspectrometer based on an imaging diffraction grating. This design tool has been validated as providing the smallest possible dimensions while maintaining acceptable spectral resolution. The resulting planar spectrometer is composed of two parallel glass plates, which contain all components of the device, including a reflective slit and an imaging diffraction grating. Fabrication is based on microelectromechanical system technology and starts with a single glass wafer; IC-compatible deposition and lithography are applied to realize the parts in aluminum, which makes the microspectrometer highly tolerant for component mismatch. The fabricated spectrometer was mounted directly on top of an image sensor and takes up a volume of only 50mm3. The measured spectral resolution of 6nm (FWHM) in the 100nm operating wavelength range (600–700 nm) is in agreement with a model calculation., Micro Electronics, Electrical Engineering, Mathematics and Computer Science

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 x 1.5 x 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., Electronic Instrumentation Laboratory, Electrical Engineering, Mathematics and Computer Science

Electrical Engineering, Mathematics and Computer Science

A concept for a highly miniaturized spectrometer featuring a two-component design is presented. The first component is a planar chip that integrates an input slit and aberration-correcting diffraction grating with an image sensor and is fabricated using microelectromechanical systems (MEMS) technologies. Due to the fabrication in a simple MEMS batch process the essential elements of the spectrometer are automatically aligned, and a low fabrication cost per device can be achieved. The second component is a spherical mirror, which is the only external part. The optimized grating structure compensates for aberrations within the spectrometer operating range, resulting in a diffraction-limited performance of the spectrometer optics. The prototype of the device has been fabricated and characterized. It takes a volume of 0:5cm³ and provides a FWHM spectral resolution of 0:7nm over a 350nm bandwidth from 420nm to 770nm combined with an etendue of 7:4 × 10?5 mm² sr., Micro Electronics, Electrical Engineering, Mathematics and Computer Science

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., Electronic Instrumentation Laboratory, Electrical Engineering, Mathematics and Computer Science

Electrical Engineering, Mathematics and Computer Science

We have investigated the possibility of using transparent stretchable diffraction gratings for spectrometric applications. The gratings were fabricated by replication of a triangular-groove master into a transparent viscoelastic. The sample length, and hence the spatial period, can be reversibly changed by mechanical stretching. When used in a monochromator with two slits, the stretchable grating permits scanning the spectral components over the output slit, converting the monochromator into a scanning spectrometer. The spectral resolution of such a spectrometer was found to be limited mainly by the wave-front aberrations due to the grating deformation. A model relating the deformation-induced aberrations in different diffraction orders is presented. In the experiments, a 12-mm long viscoelastic grating with a spatial frequency of 600 line pairs/mm provided a full-width at half-maximum resolution of up to ~1.2 nm in the 580-680 nm spectral range when slowly stretched by a micrometer screw and ~3 nm when repeatedly stretched by a voice coil at 15 Hz. Comparison of aberrations in transmitted and diffracted beams measured by a Shack-Hartmann wave-front sensor showed that astigmatisms caused by stretch-dependent wedge deformation are the main factors limiting the resolution of the viscoelastic-grating-based spectrometer., Micro Electronics, Electrical Engineering, Mathematics and Computer Science