The use of masked UV (i-line) lithography in a MEMS foundry for CMOS-compatible fabrication of large-area metasurface-based absorbers for the mid-infrared is demonstrated. The challenges are in: (a) the limited number of acceptable metals, (b) the thickness tolerance of the layers used in the CMOS process, and (c) the imaging capabilities of i-line lithography as compared to e-beam. The claimed throughput advantage with manageable distortion of masked lithography and the suitability of the layers used in a CMOS-compatible process in the fabrication of mid-IR absorbers was tested. Issues investigated are: (a) the impact of aluminium as the preferred metal in the MIM patch on the plasmonic response, (b) the influence of SiO₂ as preferred dielectric material, (c) the effect of corner rounding and horizontal-vertical bias of the masked lithography and (d) measures that can be taken during the design phase to mitigate any such detrimental effect. Based on the findings, disk-shaped patches are identified as the most suitable for shape-tolerant design. Metasurfaces with a unit-cell side-length of 3μm were fabricated over a chip area larger than 10⁵μm². Measurements do confirm that (a) aluminium is a suitable CMOS-compatible material for mid-IR metamaterial absorber fabrication, (b) a large surface roughness results in widening of the absorption peaks and (c) the typical layer thickness tolerance used in a MEMS foundry is also acceptable for mid-IR metasurface fabrication. Masked lithography limits the minimum design wavelength to about 3.5μm, while the surface roughness Rq ~ 5nm results in a bandwidth up to FWHM = 400nm.

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In this paper, the growth of optimized vertically aligned multi-walled carbon nanotube (VA-MWCNT) forests by LPCVD method for use in a large-area absorber in infrared detectors is presented. The effect of synthesis temperature (500−700 °C) and time (1−10 min) on the optical absorption coefficient in the infrared (2−20 μm) is investigated by FT-IR measurement at various incident angles (15-80°). The structural properties of VA-MWCNT are characterized by SEM, TEM and Raman spectroscopy. Spectral measurements show an increasing absorption with the height of the forest that results at increased synthesis time and temperature. However, the absorption coefficient decreases with increasing synthesize time and temperature, while it is also affected by other properties, such as diameter, density, alignment, and uniformity. Moreover, the reduction in absorption at oblique incident angles demonstrates the relevance of surface properties. Finally, a circular graphite waveguide system is used to model the absorption characteristics of an MWCNT forest.

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A suspended polySi heater is presented for use as a mid-IR light source in a microspectrometer based on a linearly variable optical filter (LVOF). Distributed electrical powering of a segmented structure with a specially designed suspension system is used for obtaining a temperature profile that is constant over the length of the element with a peak temperature significantly higher as compared to the conventional on-chip hot-wires. The integrated LVOF design results in an enhanced spectral emission and facilitates the use in the composition measurement of liquids and gases by absorption spectroscopy.

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Exhaust gas measurement in the harsh environment of the tailpipe by optical techniques is a highly robust technique, provided that optical access is maintained in the presence of soot. The design, fabrication, and testing of membranes in SiC-on-Si with integrated heaters to serve as a regenerable MEMS optical window into the tailpipe are presented. Membranes at slightly elevated temperatures are demonstrated to keep the surface transparent by thermophoresis, while surface regeneration is achieved at pulsed high temperatures, which allows long-term optical measurement in the exhaust.

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Silicon waveguide structures with SiO<inf>2</inf> cladding and tapered on-chip couplers are fabricated on Si wafers for use in water-containing biofuel composition measurement in the 2.4- 2.6 μm respectively 3.5-3.8 μm water and ethanol dominated absorption bands.@en

The design and fabrication of wideband mid-infrared metamaterial absorbers are presented. The emphasis is put on the shape-tolerant design for using masked UV (i-line) lithography and CMOS-compatible fabrication to enable on-chip co-integration with detector and readout circuits in a MEMS foundry while maintaining wafer throughput. The CMOScompatibility implies the use of aluminum rather than the commonly used high conductivity metals. The use of masked lithography rather than e-beam lithography in the fabrication of metamaterial absorbers for the mid-infrared range between 3 and 4 μm introduces the challenge of the shape-tolerant design of the unit cell. Moreover, the sensitivity of the fabricated metamaterials to the surface roughness and exposure dose were investigated in this paper. The throughput advantage of masked lithography has been exploited in the fabrication of mid-infrared absorbers over an area of several mm². The measurements confirm the theoretical spectral response and a 98% peak absorption at an angle close to perpendicular incidence. Measurements at different angles show that the absorption spectrum only deviates marginally from normal incidence for angles up to 30°. The combined CMOS-compatibility and masked lithography enable batch fabrication and the on-chip integration of the metamaterial absorbers with MEMS devices and sensors.

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Continuous monitoring of the composition of E85 biofuel is essential for a quick start and clean and efficient operation of Flex-Fuel Vehicles. The actual ethanol concentration in E85 fuel is in the range 50%–85% and fuel-line sensors are used for ethanol-gasoline composition measurement. However, also a small amount of water is typically present, which cannot be reliably detected using state-of-the-art capacitive fuel-line sensors. Thermal impedance spectroscopy has been investigated as a non-destructive technique to determine the composition of ternary mixtures of biofuels. The principle of the thermal conductivity detector has been extended for measuring both the thermal conductivity and the thermal capacity of biofuel in the range up to 10 kHz using an AC-operated polysilicon heater for injecting a sinusoidal heat flux, and another polysilicon strip at a well-defined spacing or thermopile sensors for measuring the in-phase and quadrature components of the resulting AC temperature difference. Measurements on the components are in reasonable agreement with simulations, with a −3 dB cut-off frequency at 422.5 Hz and 340.8 Hz for ethanol and gasoline, respectively. However, the cut-off frequency of water was found to be significantly lower than simulations due to its high surface tension, thus limiting access to the detector.@en

Integration of an array of differently tuned mid-infrared metamaterial-based absorbers on top of thermopile detector arrays in a compatible fabrication process is presented. UV lithography is used for patterning over large areas with high throughput.@en

Metamaterial absorbers are photonic structures composed of an array of sub-wavelength metallic patterns. Results in literature are usually based on structures of nominal dimensions, despite the significant effect of fabrication tolerances on performance. This research aims to identify the main sources of uncertainty and to investigate their effect, notably that of an irregular surface quality (i.e., roughness) of the thin metallic layer and the lithography related variations in size and shape. The effect of the shape and positioning of the resonance peak was investigated and validated using mid-infrared metamaterial absorbers. This sensitivity analysis is essential to the batch fabrication of metamaterial absorbers for MEMS applications.@en

This paper demonstrates the functional integration of a linear variable optical filter (LVOF) and a gas cell at the wafer level, i.e. a gas-filled LVOF, where the resonator cavity of the filter is also used for storing the gas sample. A mm-level effective optical absorption path length is achieved from a μm-level physical path length, by exploiting multiple reflections from highly reflective Bragg mirrors and the high-order operation of the filter. The wideband spectral response of the device is ensured by using a tapered cavity, where the cavity length changes linearly along the length of the filter. Therefore, combined with a detector array and a light source, the gas-filled LVOF enables wideband operation and long absorption path in a single MEMS device, thus ensuring a highly sensitive on-chip gas absorption microspectrometer.@en

Measuring the ethanol/water ratio in biofuel of high ethanol content, such as E85, is important when used in a flex-fuel engine. A capacitive probe is generally used for measuring the ethanol/gasoline ratio. However, the water content in E85 biofuel cannot be disregarded or considered constant and full composition measurement of biofuel is required. Electric impedance spectroscopy with a customized coaxial probe operating in the 10 kHz to 1 MHz frequency range was investigated. An in-depth investigation of the electrical impedance domain has led to the conclusion that additional information is required to unambiguously determine the composition of the ternary biofuel mixture. Among the different options of measurement domains and techniques, optical absorption spectroscopy in the UV spectral range between 230 and 300 nm was found to be the most appropriate. The typical absorbance in the UV range is highly dominated by gasoline, while ethanol and water are almost transparent. This approach is experimentally validated using actual fuels.@en

Thermal impedance spectroscopy has been investigated as a non-destructive technique to determine the composition of ternary mixtures of biofuels. The principle of the thermal conductivity detector has been extended for measuring both the thermal conductivity and the thermal capacity of biofuel in the range between 1 to 100 Hz, using an AC-operated polysilicon heater for injecting a sinusoidal heat flux, and another polysilicon strip at a well-defined spacing or thermopile sensors for measuring the in-phase and quadrature components of the resulting AC temperature difference.@en

A capacitive probe is generally used in a flex-fuel engine for measuring the ethanol content in biofuel. However, the water content in biofuel of high ethanol content cannot be disregarded or considered constant and the full composition measurement of ethanol, gasoline and water in biofuel is required. Electrical impedance spectroscopy with a customized capacitive probe operating in the 10 kHz to 1 MHz frequency range is combined with optical absorption spectroscopy in the UV spectral range between 230 and 300 nm for a full composition measurement. This approach is experimentally validated using actual fuels and the results demonstrate that electrical impedance spectroscopy when supplemented with optical impedance spectroscopy can be used to fully determine the composition of the biofuel and applied for a more effective engine management. A concept for a low-cost combined measurement system in the fuel line is presented.

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This paper presents the design, fabrication, and characterization of MEMS airgap-based optical filters designed for the ultraviolet (UV) spectral range. Optical design, process conditions, and material choice have been optimized for obtaining large-area and optically flat membranes. Several distributed Bragg reflectors centered at λ=400 nm were designed using silicon oxide and nitride layers as the membrane materials. Airgap Bragg reflectors with up to 80% fill-factor were fabricated in standard Si technology and the spectral reflectance of the reflectors were compared. The optical characterization demonstrated a high reflectance (up to 80%) with only a few number of layers over a bandwidth of up to 175 nm in the UV spectral range.@en

A generic method to reduce the in-line flow dependence of thermal conductivity detectors (TCDs) is presented. The principle is based on a dual-MEMS device configuration. Two thin-film sensors on membranes in parallel in the gas stream on the same chip are differentially operated. Both micro-TCDs are designed to be identical in terms of contact with the main gas flow, however a different depth of the detection chamber results in a different response to the thermal conductivity of the sample gas. Static and dynamic simulations have been performed to characterize the design of the fabricated structures. Devices have been fabricated in a MEMS process using a combined surface- and bulk micromachining process. The devices have been characterized statically and dynamically. Measurements on prototypes show that depending on the range of gases, device size and flow range device the effect of flow on the thermal conductivity can be reduced by a factor 4–15.@en

This paper investigates the mechanical and optical properties of thin PECVD silicon-oxide layers for optical applications. The different deposition parameters in PECVD provide a promising tool to manipulate and control the film structure. Membranes for use in optical filters typically are of ~λ/4n thickness and should be slightly tensile for remaining flat, thus avoiding scattering. The effect of the thermal budget of the process on the mechanical characteristics of the deposited films was studied. Films with compressive stress ranging from  −100 to 0 MPa were deposited. Multiple thermal annealing cycles were applied to wafers and the in situ residual stress and ex situ optical properties were measured. The residual stress in the films was found to be highly temperature dependent. Annealing during the subsequent process steps results in tensile stress from 100 to 300 MPa in sub-micron thick PECVD silicon-oxide films. However, sub-100 nm thick PECVD silicon-oxide layers exhibit a lower dependence on the thermal annealing cycles, resulting in lower stress variations in films after the annealing. It is also shown that the coefficient of thermal expansion, hence the residual stress in layers, varies with the thickness. Finally, several free-standing membranes were fabricated and the results are compared.@en

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. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.@en

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. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.@en

A robust and highly miniaturized optical gas sensor based on optical absorption spectroscopy is presented. By using the resonator cavity of a linear variable optical filter (LVOF) also as a gas chamber, a compact and robust optical sensor is achieved. The device operates at the 15th order in 3.2–3.4 μm wavelength range for distinguishing hydrocarbons. The physical cavity length at the μm-level is translated into an effective optical absorption path length at the mm-level by the use of highly reflective (R > 98%) Bragg mirrors. The optical design using the Fizeau interferometer approach is described. Moreover, the CMOS-compatible fabrication method is explained. In addition to the wideband and single wavelength filter characterization, absorption of methane in the LVOF cavity is demonstrated at 3392 nm and 3416.60 nm wavelengths.@en