This paper reports the development and integration of (Poly‐Si/Air) n Distributed BRAGG Reflectors (DBR) in a MOEMS Fabry‐Pérot‐Interferometer (FPI) concept. The realized reflectors constitute a promising and resource‐efficient alternative to complex Ion‐Assisted Deposition based DBRs while maintaining their advantages. Compared to state of the art MOEMS FPIs the (Poly‐Si/Air) n DBRs can be integrated into two moveable reflector carriers based on two individually fabricated wafers which are bonded. The (Poly‐Si/Air) n DBRs are investigated as (HL) and (HL) 2 reflector stacks showing a reflectance above 91% within the wavelength range of 2.8–5.7 μm. © 2023 The Authors. IEEJ Transactions on Electrical and Electronic Engineering published by Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.

A static filter array is developed based on Fabry-Perot cavities operating in the mid-infrared range from 2 to 4.5 mu m, matching the absorption region of several gases relevant for air quality monitoring. The filters include two distributed Bragg reflectors and a high-index tuning element with varying thickness in between. The filters display high peak transmittance (>80%$>80\%$) and controllable peak width. The robustness, ease of fabrication and the possibility to tune the optical response to a specific application make the integrated filter arrays suitable for compact sensing systems.

Abstract. Pyroelectric detectors are used for gas analysis and flame detection because of their fast response and excellent performance. Most pyroelectric devices are based on monocrystalline lithium tantalate or pyroelectric lead zirconate titanate thin films deposited on a silicon (Si) substrate. In comparison, recently discovered pyroelectric-doped hafnium oxide (HfO2) offers the possibility of manufacturing completely complementary metal-oxide-semiconductor (CMOS)-compatible devices on large Si wafers. This is a promising approach to simplifying mass production of the sensor element and realizing new sensor structures with a high performance. Si substrates were structured with trenches and filled with thin-doped HfO2 layers by atomic layer deposition to multiply the pyroelectric current responsivity. An effective pyroelectric coefficient of up to 1300  μC  /  m2  /  K was measured. Micromechanical structuring of the 6-in Si wafers was used to improve the thermal conversion of the sensor element. The applied plasmonic absorbers increase the infrared light absorption to >80  %   for the spectral range of 3 to 5  μm, which was determined using Fourier transform infrared reflection measurements. In the first step, the performance of the sensor element was evaluated with an analog transimpedance amplifier with a feedback resistance of 5  GΩ. A specific detectivity D  *    >  1  ·  107  cm√Hz  /  W was measured for the frequency range of 1 to 10 Hz. In addition, an application-specific integrated circuit was designed for the electrical signal conditioning to build a fully CMOS-compatible pyroelectric detector. It offers a simple to manufacture, flexible configuration, and digital communication interface with a signal-to-noise performance close to analog detectors. We present the measurement results of different sensor elements and detector types.

Pyroelectric detectors are widely used thermal infrared detectors due to their simple construction, robustness, and excellent performance. Most pyroelectric devices are based on monocrystalline lithium tantalate (LT) or pyroelectric lead zirconate titanate (PZT) thin films deposited on silicon (Si) substrate. In comparison, recently discovered pyroelectric doped hafnium oxide (HfO2) offers the possibility to manufacture completely CMOS-compatible devices on large silicon wafers. Three-dimensional substrates with trench structures were applied for detectors to multiply the pyroelectric current responsivity of a thin, doped hafnium oxide layer deposited by atomic layer deposition. Micromechanical structuring of the 6" silicon wafers was used to improve the thermal conversion and effective plasmonic absorbers for the spectral range 3 - 5 μm. An effective pyrocoefficient up to 1300 μC/m²/K was measured, depending on various dopants (Si, Al, La), layer thicknesses and polarization conditions. A high infrared light absorption > 80% of the plasmonic absorber in the relevant spectral range was determined using FTIR reflection measurements. The performance of the sensor element has been evaluated with a conventional analog transimpedance amplifier with a feedback resistance of 5 GΩ. A specific detectivity D* > 1 ∙ 107 cm√Hz/W (black body 1000 K, 3 - 5 μm) was measured for the frequency range 1 - 10 Hz. Additionally, a new application-specific integrated circuit (ASIC) was used for the electrical signal conditioning to realize the first fully CMOS-compatible pyroelectric detector. This detector offers a flexible configuration, digital communication interface and achieves a similar signal-to-noise performance as the analog detectors.

The results developed in a research project resulted from the cooperation between Fraunhofer IKTS in the field of ceramic multilayer technology and InfraTec GmbH in the field of infrared sensors and measurement technology and were generated within the project’s duration of 4 years. New types of miniature heaters embedded in ceramics were developed and their properties regarding modulation, spectral emission, long term and high temperature stability were characterized and optimized. A new process chain based on ceramic multilayer technology was developed to produce the IR emitter, which is based on a combination of sacrificial paste technology using screen or stencil printing and Low Temperature Cofired Technology (LTCC) processing. The generated dielectric layer thicknesses of 15-20 µm are half as thick as with conventional LTCC tape manufacturing processes. To optimize the emission coefficient towards a black body, new ceramic pastes suitable for screen printing have been developed. In this way, black surfaces could be created on the radiators, which are characterized by high temperature stability and low thermal mass. First long-term studies of the IR emitter up to 2000 hours show functionality at 650 °C with low degradation.

This paper reports on the progress in miniaturization of bulk micromachined FP filters and tunable detector modules for the mid infrared. The chip size was reduced from 7×7 mm 2 to 5×5 mm 2 , enabling integration into TO5-size detector packages, which is about only one quarter of the formerly used TO8. At the same time a high optical throughput is maintained. A MEMS design with two movable reflectors is used, which allows for lower actuation voltages and provides negligible acceleration sensitivity.

This paper presents a compact handheld gas analyzer, optimized for the multicomponent analysis of hydrocarbon gases, based on a micromachined, tunable Fabry-Perot filter (mu FPF) as a key element. The tuning range from 3.0 mu m to 3.7 mu m and spectral resolution of 30 to 40 nm (FWHM) cover a region of characteristic absorption of many combustible hydrocarbons and allow an accurate measurement of the concentration of each individual gas. The analyzer functions like a microspectrometer and outperforms the current state of the art for small gas detection handheld devices, using pellistors and NDIR-infrared detectors. An innovative MEMS design with two movable reflectors is insensitive to acceleration forces. Due to the handheld application, especially the vibration response in the frequency range of 10 to 100 Hz was carefully characterized. An uncooled photodiode is integrated as fast detector, together with a transimpedance amplifier and a sapphire lens for light concentration. The photodiode is working in photovoltaic mode without supply voltage. In comparison to a photoresistor this gives the advantages of less power consumption and compatibility to explosion protection regulations. A demonstrator was developed by designing a smart sensor module including complex analog and digital signal processing. Integrating this into a commercial, handheld, explosion-proofed Multi-Gas-Detector model results in a robust instrument, which enables field tests under harsh environmental conditions, typical for fire brigade missions. Spectral analysis of combustible gases (methane, ethane, propane and acetylene) is demonstrated with focus on safety-relevant LEL (lower explosion level) detection.

Electrostatically actuated MEMS Fabry-Pérot filters are an innovative component for infrared microspectrometers and miniature gas sensors. Fabricated in a bulk micromachining process, difficulties can arise from the mass of moving elements making the device sensitive to acceleration forces. To counteract this problem the capacitance of the electrostatic actuator can be measured and used in a closed control loop circuit. This paper gives a short overview on how such a closed-loop control system was implemented and subsequently focuses on experiments for evaluating the command and disturbance response. To investigate the latter, the behavior of the whole system was measured in a vibration test system.

MEMS tunable Fabry-Pérot filters are a key component for infrared microspectrometers and miniature gas sensors. Compared to other technologies, they have several advantages particularly with regard to miniaturization and optical throughput. This paper gives a short summary of the technology, packaging, system integration and closed loop operation. Several types with different spectral ranges and resolutions will be presented. Possible applications in gas analysis, based on own work and with reference to the literature, are also discussed.

A novel micro-machined chip carrier system for pyroelectric detectors was developed that integrates the sensor elements as well as the infrared filters into one assembly.Based on these chip carriers miniaturized multi-channel detectors with attractive electro-optical parameters especially for gas analysis measurements were engineered.

This report presents recent advances in the design and fabrication of a tunable Fabry-Pérot interferometer (FPI) with subwavelength grating (SWG) reflectors, as well as measurement results and applications. The FPI is designed as wavelength selecting element for highly miniaturized mid-wave infrared spectrometers. The optical resonator of the FPI is built between two highly reflecting mirrors. The mirrors are integrated in a supporting MEMS structure with one electrostatically movable and one fixed mirror carrier. The FPI is fabricated in a bulk micromachining batch process on wafer level from two silicon substrates. The substrates are bonded together with an intermediate SU-8 layer. The reflectors are made of aluminum subwavelength gratings, structured on a thin LP-Si3N4 membrane by nanoimprint lithography. The subwavelength structures build a frequency selective surface with high reflectance and low absorbance in a defined spectral range. Simulations and optimization of the design were done using finite element method with a 3D EM frequency domain solver. Comparison of simulation results and measurements of fabricated reflectors and FPIs are in very good agreement. The FPIs are used in the 5th interference order and can be tuned from 3.5 μm to 2.9 μm electrically. The measured maximum transmittance is between 70 % and 50 % and the measured FWHM bandwidth is lower than 50 nm. The new subwavelength grating reflectors can be integrated in a MEMS batch process more cost-efficient than previously used reflectors of dielectric layer stacks.

Microspectrometers based on MEMS Fabry-Pérot (FP) interferometers as optical band-pass filters are a recently emerging solution. InfraTec is one of the leading suppliers of tunable infrared MEMS filters for gas analysis. Different FP filters for the wavelength range 3-11 µm were designed and fabricated. The FP reflector thin film design based on conventional optical thin films was introduced to cover the whole MWIR and LWIR range. Beside this, a completely new so-called Dual-Band design has been developed, in which the 1st interference order in the LWIR and the 2nd order in the MWIR can be used simultaneously. To compensate temperature drift and stabilize sensitivity against vibration two different solutions were engineered. Furthermore FP filters of higher orders are developed to decrease the spectral bandwidth up to 20 nm in the MWIR. Combined with PbSe detectors fast tunable FP devices have been created with both a high signal-to-noise ratio and a high spectral resolution for the detection of hydrocarbons.

Many application fields of infrared spectroscopy require small, robust and transportable spectrometers, which are considerable less costly than existing products. Therefore microspectrometer technologies are rapidly emerging and many research groups spend effort on this. Compared to other kinds of devices, micromachined tunable Fabry-Pérot filters are best suited in terms of miniaturization and optical throughput. This paper gives a review of μFP filters for infrared spectroscopy. Different approaches from several groups are compared. Optical performance parameters like wavelength tuning range, spectral resolution and aperture size as well as complexity of fabrication and costs are discussed.

This paper presents a toolset for the efficient control of the indoor air quality and thermal comfort in retrofitted buildings. The refurbishment of existing buildings, compliant to actual regulations, often leads to airtightness and the consequent poor conditions for the occupants that could cause low productivity and even sickness. For this reason, the CETIEB (Cost Effective Tools for Better Indoor Environment in Retrofitted Energy Efficient Buildings) project developed innovative low-cost solutions to monitor and control the indoor air quality and thermal comfort. Among the technologies developed, this paper presents ad-hoc sensors for the monitoring of Total Volatile Organic Components (TVOC), CO2 and thermal comfort together with a control logic that, using measured data, provides the optimal rules to actuate the control devices (ventilation, heating/cooling, windows opening, shutters operation and so on). The application and validation of the integrated solution, monitoring plus control logic, was performed in a laboratory building to compare the performance of the proposed solution with the traditional system employed in buildings. The results turned out to show sensors performances comparable with commercial solutions but with a significant reduction of costs. Moreover, the application of the integrated solution showed an improvement of the indoor air quality and comfort with a 15% of energy saving, compared to the traditional thermostatic control.

A new fast infrared microspectrometer based on a MEMS tunable Fabry-Pérot filter (μFPF) is reported. In contrast to previous designs the acquisition time for a complete spectrum is decreased 50 times and the spectral resolution is improved 3 times. This is achieved by the use of higher interference orders and the combination with a room temperature operated PbSe photodetector. Such a system will be especially useful for the measurement of hydrocarbon gases in the spectral range of (3.1∶3.8) µm. The μFPF is tuned sinusoidally over the entire wavelength range in about 100 ms. Methods for offset elimination and drift compensation were developed as well.

A tunable MEMS Fabry-Perot infrared filter with subwavelength structured reflectors substituting state of the art Bragg reflectors is presented. The reflectors consist of a disc resonator array arranged on a membrane carrier. The subwavelength structures were simulated and optimized by finite difference time domain analysis in order to achieve high peak transmittance for a desired wavelength. Nano imprint lithography was used to fabricate the subwavelength structures at wafer level. Tuning voltages up to 80 V change the distance between both reflectors by electrostatic forces. Measurement results show high peak transmittance of more than 50 % within a wavelength range from 2.5 mu m to 6.5 mu m. (C) 2015 Published by Elsevier Ltd.

Micromachined tunable FABRY-PEROT filters (mu FPF) are key elements in a new class of miniature spectrometers and analyzers. Different groups all over the world are working on mu FPF for spectral ranges from the visible up to the long wave infrared. In order to achieve a large tuning range, the filters are normally operated in the first interference order. At the same time the spectral resolution is limited due to a limited effective finesse. A variety of applications demand for higher resolutions. This is particularly true for the multicomponent analysis of hydrocarbon gases, because the individual absorption bands are very similar and widely overlapping. In this paper mu FPF in 3rd and 4th order configuration with a spectral resolution of about (20 ... 30) nm and a tuning range of (3.1 ... 3.7) mu m are presented. For the measurement of additional gases in adjacent ranges (e. g. CO2 around 4.3 mu m) a dualband configuration with simultaneous use of different orders is proposed. A largely reduced damping of the mu FPF and the combination with a lead selenide photoresistor instead of a thermal detector allows for a fast acquisition of spectra.