Thermopile Detector Research Articles

Contrast Enhancement of Wavelength-Selective Detection of Mid-Infrared Using Localized Atmospheric-Pressure Plasma Treatment

A new processing method to enhance the signal contrast of a mid-infrared (MIR) detector integrated with a wavelength-selective function is studied. Using the hydrophilic characteristic of an IR absorber solution, an absorber material is selectively deposited onto a hydrophilically modified area over the hot junctions in the diaphragm of a thermopile detector. The hydrophilic modification of the chip-mounted detector is realized using localized atmospheric Ar + O2 plasma treatment through a stencil mask. Using a thermograph, we measured thermal distributions over a previously fabricated detector, whose absorber material is deposited using a manual manipulator without a position-selective mechanism, and the newly fabricated detector for comparison. The newly fabricated detector exhibited a larger temperature difference between hot and cold junctions than that of the previous detector. The detector has an increased signal contrast of 100% from the baseline at the absorption peak.

Thin film absorbers for visible, near-infrared, and short-wavelength infrared spectra

Two thin film absorbers are presented in this paper: one for the visible (VIS) and the near-infrared (NIR) spectra in the wavelength range of 350…1000 nm, the other for the short-wavelength infrared (SWIR) spectrum in the wavelength range of 1200…2000 nm. First, the refractive indices were determined for Al 2O 3 films prepared with atomic layer deposition (ALD), and amorphous Mo–Si–N films prepared with reactive sputter deposition. The measurements were made by spectroscopic reflectometry, ellipsometry, gonioreflectometry, and double-beam transfer standard spectrometry. The results were utilised in the design of the absorbers. The absorbers were manufactured as well, and they proved to have high absorption over their whole working spectra varying from 93.4% at the minimum to 99.9% at the maximum. The absorbers are applicable, e.g. to MEMS thermopile detectors.

Integrated micromachined thermopile IR detectors with an XeF2 dry-etching process

Based on the Seebeck effect, the CMOS compatible micromachined thermopile is widely used in infrared detection for its advantages of low-cost, high batch yield, broad spectral response and insensitivity to ambient temperature. We present two integrated thermopile IR detectors on stacked dielectric layers realized by a standard P-well CMOS technology, followed by one CMOS compatible maskless XeF2 isotropic dry-etching step. Characterizations of CMOS devices, before and after XeF2 etching, respectively, were performed to investigate the effects of XeF2 etching on the CMOS devices. With a 2.5 µm thick stacked silicon oxide–nitride–oxide layer as an absorber, the rectangular thermopile detector and the circular thermopile detector provided responsivity of 14.14 and 10.26 V W−1, specific detectivity of 4.15 × 107 and 4.54 × 107 cm Hz1/2 W−1, and time constant of 23.7 and 14.6 ms, respectively. Compared with the rectangular thermopile detector, the circular thermopile detector is mechanically more stable, because its circular structure design eases the internal stress problem in the CMOS layers. After XeF2 etching, the maximum changes of threshold voltage, maximum transconductance and switching threshold voltage were 0.97%, 1.25% and 0.08%, respectively. Experimental results show that the effects of XeF2 etching on the CMOS devices are insignificant, and XeF2 etching is suitable for post-CMOS micromachining.

Fabrication and characterization of a design optimized SU-8 thermopile with enhanced sensitivity

In the infrared wavelength region, thermopiles are an important type of detector. A major advantage of thermopiles is their non-cooling requirement. Depending on the applied absorption layer, their responsivity is often rather flat within a large wavelength region. This work presents the fabrication and characterization of a sensitivity and design optimized thermopile detector with a 4 µm self-supported SiO2/SU-8 membrane. The structure consists of 240 series interconnected thermocouple junctions obtained by metal evaporation and lift-off. Two metal combinations have been evaluated, namely, nickel/titanium and aluminium/bismuth. Series resistances of 76 kΩ and 283 kΩ were measured for the Ni/Ti thermopile and the Al/Bi thermopile respectively. For the Al/Bi thermopile a responsivity of 60 V W−1 was achieved using a 1.56 µm fibre coupled diode laser with a power of 3.5 mW. Using a white light source with a radiation flux of 0.45 W mm−2 a voltage response of 68 V mm2 W−1 was measured for the Al/Bi thermopile. The time constant of the characterized detectors was calculated to be 70 ms, using the pulsed IR laser.

Thin film absorbers for visible, near-infrared, and short-wavelength infrared spectra

In this study, the refractive indices were determined for Al2O3 films deposited with ALD, and amorphous Mo-Si-N films deposited with reactive sputtering. The measurements were made by spectroscopic reflectometry, ellipsometry, gonioreflectometry, and double-beam transfer standard spectrometry. Based on the results, two thin film absorbers were designed and manufactured: one for wavelengths of 350…1000 nm, the other for wavelengths of 1200…2000 nm. The manufactured absorbers showed high absorption over their whole working spectra varying from 93.4 % at the minimum to 99.9 % at the maximum. One of the absorbers was applied to a MEMS thermopile detector with successful process integration.

Static and Dynamic Analysis of Thermal Cross-Talk in an Thermopile Detector Array for use in an Microspectrometer

The spectral resolution of a MEMS-based Infrared (IR) microspectrometer with a plainer grating for projection of dispersed light on a detector array critically depends on the thermal cross-talk between adjacent Thermo-Electric (TE) elements in the array. Bridge-shaped TE detector elements are cut out of a membrane using MEMS process. The air gap between pixels is the cause of the cross-talk. The static and dynamic characterizations of thermal cross-talk have been evaluated, with an emphasis on the effect of the thermal conductivity of air as the function of the package pressure. The developed models accurately predict the behaviour of the thermal cross-talk under different package pressure.

Characterization of thermal cross-talk in a MEMS-based thermopile detector array

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 µm2 are separated by 10 µm 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.

Thermal simulation and design optimization of a thermopile infrared detector with an SU-8 membrane

Simulation and optimization tools are commonly used in the design phase of advanced electronics devices. In this work, we present a thermal simulation and design optimization tool for infrared thermopile detectors based on a closed membrane structure. The tool can be used to simulate and optimize thermopile detectors with an arbitrary number of design parameters. The optimization utilizes the Nelder–Mead and the adaptive simulated annealing optimization algorithms to maximize the system performance. A thermopile detector with an SU-8-based closed membrane and metal–metal thermocouples has been simulated and optimized. Based on the results generated by the tool, an optimized detector has been fabricated and characterized. The results from the measurements presented are in good agreement with the simulation results.

Experimental Evaluation of a Thermopile Detector With SU-8 Membrane in a Carbon Dioxide Meter Setup

Continuous control of the carbon dioxide levels in the ventilation systems in office buildings and public schools has been shown to increase productivity and save money. However, these measurement systems require further developments in order to be more cost effective. This paper presents an evaluation of an Al/Bi thermopile detector with a 4 mum thin SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SU-8 membrane in a CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> meter application using the nondispersive infrared technology (NDIR). The system consists of an 11 cm aluminum tube, used as the sample chamber and in which a light source and a thermopile detector with a 4.26 mum optical bandpass filter are positioned on its opposite sides. The light source is pulsed with a frequency of 0.5 Hz. The voltage response of the Al/Bi thermopile is measured for different CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentrations, and, as expected according to the Lambert-Beer law, there is an exponential decrease in the measured intensity. The absolute response is about 50% lower than for a commercial HMS J21 thermopile from Heimann Sensor GmbH. In relative terms, on the other hand, the Al/Bi thermopile is more sensitive for changes in the CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentration. At 7500 ppm, the voltage response has decreased to 40% of the reference response measured in the nitrogen atmosphere.

A thermopile detector array with scaled TE elements for use in an integrated IR microspectrometer

The design and fabrication of a thermopile detector array for use in a fully integrated infrared optical spectrometer are described. IC-compatible MEMS technologies are used for fabrication of the spectrometer components, such as the slit, planar imaging diffraction grating and detector array. The IR micro-spectrometer was designed for operation in the 1.5–3 µm wavelength range with the size of the largest dimension about 8 mm. The imaging properties of the diffraction grating result in non-uniform dispersion, which imposes special requirements on the dimensions of each single detector in the array. The result is an array of unequally sized elements. The design considers technological constraints, sensitivity and cross-talk between elements. Simulation results, final design, fabrication technique and fabricated devices are presented.

A compact optical multichannel system for ethylene monitoring

Precise and continuous ethylene detection is needed in various fruit ripening applications. The aim of this work is the development of a miniaturised mid-infrared filter spectrometer for ethylene detection at 10.6 μm wavelength. For this reason optical components and signal processing electronics were developed, tested and integrated in a compact measurement system. The main optical components, their integration of the optical system, as well as a description of the developed electronics and the first results of gas measurements are described in this paper. In fact the application conditions demand not a single channel system but a multichannel one. A silicon-based macroporous IR-emitter, a miniaturised absorption cell and a detector module for the simultaneous measurement with four channels including, ethylene, two interfering gases and the reference signal were integrated in the optical system. The new inner architecture of the detector module, consisted of optical filters which were directly attached by flip-chip technology onto the thermopile-arrays, allowing silicon-based Fresnel multilenses to be attached to the cap of the detector housing. Because of the high reflection losses at the silicon-air surface the Fresnel lenses were coated with zinc sulphide antireflection layers. For the signal processing electronics a preamplification stage and a DSP-based lock-in-amplifier has been developed. Although some of this work is still on-going, first ethylene measurements with the miniaturised gas cell, silicon-based IR-emitter, a commercial thermopile detector and the self-developed system electronics showed a detection limit better than 20 ppm.

Non-selective NDIR array for gas detection

A miniaturized non-selective optical gas detection system is proposed in this work. The measurement method combines the advantages of optical systems, i.e. aging and drift effects can be corrected, and the versatility of the non-selective gas sensors arrays that make up an electronic nose. The detection module is composed of two matching arrays, one of thermopile detectors and the other of non-substance-oriented filters. Both arrays are fabricated using CMOS compatible processes and bonded together by means of flip-chip techniques. This detection module is part of a measurement setup that also includes a gas cell closed by IR-transparent lenses and a commercial IR source. First measurements with volatile compounds (acetone, ethanol and isopropanol) are reported. Experimental results show the ability of the system to discriminate these substances as far as their infrared spectra differ.

Tuning D* with Modified Thermal Detectors

We report on the fabrication and characterization of a modified thermopile detector that has a spectral detectivity, D*, primarily determined by the absorbance of a polymer film. This was done by coating the detector with a metal mirror, followed by the polymer film, so that the film absorbances are responsible for most thermal conversion. The detector is designed to tailor the spectral response of optical systems more specifically to analytes in order to improve precision in methods such as multivariate optical computing and simple photometry. Interference effects in the thin-film response are eliminated by the textured surface of the silicon thermopile, which makes the spectral response relatively simple. The maximum detectivity due to a 1-micrometer-thick film is found to be 20% of the detectivity of the original wide-band detector at 10 Hz modulation frequency. We estimate the thermal diffusion length in the polymer at 10 Hz to be 40 micrometers. We also suggest that the detectivity of the modified detector can be approximated as the product of the D* of the underlying thermal detector and the absorbance of the modifying film, provided the modulation frequency is low and interference effects are defeated.

High accuracy measurement of the absolute spectral responsivity of Ge and InGaAs trap detectors by direct calibration against an electrically calibrated cryogenic radiometer in the near-infrared

This paper presents measurements of the absolute spectral responsivity of Ge and InGaAs trap detectors carried out by direct comparison with an electrically calibrated cryogenic radiometer (ECCR) in the near-infrared spectral range. Two tunable laser diodes were used as radiation sources operating between 1260 nm and 1360 nm and between 1460 nm and 1620 nm. The measurements of the cavity absorption and the Brewster-angled window transmission of the ECCR in these wavelength ranges are presented. A detailed analysis of the total measurement uncertainty is performed. The relative standard uncertainty of the responsivity measurement achieved is less than 5 × 10−4 for all wavelengths investigated. The measurements are also discussed in terms of external spectral quantum efficiency, which is up to 90% for the Ge trap and up to 93% for the InGaAs trap. The determined spectral responsivities are compared with the existing scale for spectral responsivity at the Physikalisch-Technische Bundesanstalt (the German National Metrology Institute) based on a thermopile detector. The deviation between the scales is at most ± 0.15% for the InGaAs trap detector and at most ± 0.30% for the Ge trap detector. The agreement is therefore within the combined standard uncertainties.

Exploration of the metrological performance of a gas detector based on an array of unspecific infrared filters

The feasibility of a non-dispersive infrared (NDIR) gas detection system based on an array on non-specific detectors is studied in this work. The measurement system, that is intended to be low cost and of reduced dimension, is aimed to the simultaneous quantitative detection of different gases with IR absorption bands in the region of the 1000–4000 cm −1. The detector is a single module that has been already fabricated. It is composed by an array of broadband infrared filters and a matching array of thermopile detectors joint together by means of flip chip. The elements of the filter array are not tuned for the specific measurement of any gas, in opposition to traditional NDIR setups, making necessary the use of multivariate regression techniques (Partial least squares) to predict the gases concentration. A preliminary evaluation of the metrological characteristics of the complete NDIR system is carried out using a realistic model of the system. Two arrays sizes, 3 × 3 and 4 × 4 elements, are studied for the discrimination of a mixture composed by methane, carbon monoxide and carbon dioxide at different concentrations. Prediction errors about tens of ppm are found for an optical path length of 10 cm. In traditional NDIR systems infrared source aging is compensated by means of a reference channel where no absorptions bands are present. Instead, in this in this work these effects are removed using a digital component correction technique.

Spatial non-uniformity analyses of radiometric detectors to identify suited transfer standards for optical radiometry

The non-uniformity of responsivity of photodiodes and thermal detectors limits the accuracy of their calibration. Spatial non-uniformity properties of a pyroelectric radiometer, a thermopile detector, silicon, germanium and indium gallium arsenide photodiodes are investigated in the scope of our work. Silicon type photodiodes showed best uniformity characteristics among the analysed photodiodes and detectors. Significant changes have been found in the spatial responsivity close to the band gap of silicon. The spatial responsivity of thermopile detector was measured at two different wavelengths and no wavelength dependence was observed.

Modular fibre optic sensor for the detection of hydrocarbons in water

A novel modular, mid-infrared, evanescent wave fibre optic sensor for the detection of hydrocarbon pollutants in water has been designed, constructed and tested. The key sensor design feature, its modularity, utilises simple, low cost, optical components and provides the potential for low cost production of rugged field based sensors. In addition, novel optical design upgrades, such as tapered, coiled and bevelled fibres, can be incorporated to improve detection limits. The current set-up uses a broadband source with backreflecting optics coupled to a fibre optic sensing element, coated with an analyte-enriching polymer that concentrates the analyte in the sensing region. The fibres output end is filtered at an analyte absorption peak and coupled to a thermopile detector whose signal is read into a computer for analysis. Results show the system's use as a pollutant detector, with benzene quantified down to 500 ppm using a PVC polymer coating. Response times for a range of benzene concentrations are sufficient for real time quantification. Other analytes can be quantified using an appropriate enriching polymer and bandpass filter centred on an absorption peak for the analyte in question or an analyte family. Ongoing component developments allow for the evolution of the sensor system's specifications.

Exploring The Saturn System In The Thermal Infrared: The Composite Infrared Spectrometer

The Composite Infrared Spectrometer (CIRS) is a remote-sensing Fourier Transform Spectrometer (FTS) on the Cassini orbiter that measures thermal radiation over two decades in wavenumber, from 10 to 1400 cm− 1 (1 mm to 7μ m), with a spectral resolution that can be set from 0.5 to 15.5 cm− 1. The far infrared portion of the spectrum (10–600 cm− 1) is measured with a polarizing interferometer having thermopile detectors with a common 4-mrad field of view (FOV). The middle infrared portion is measured with a traditional Michelson interferometer having two focal planes (600–1100 cm− 1, 1100–1400 cm− 1). Each focal plane is composed of a 1× 10 array of HgCdTe detectors, each detector having a 0.3-mrad FOV. CIRS observations will provide three-dimensional maps of temperature, gas composition, and aerosols/condensates of the atmospheres of Titan and Saturn with good vertical and horizontal resolution, from deep in their tropospheres to high in their mesospheres. CIRS’s ability to observe atmospheres in the limb-viewing mode (in addition to nadir) offers the opportunity to provide accurate and highly resolved vertical profiles of these atmospheric variables. The ability to observe with high-spectral resolution should facilitate the identification of new constituents. CIRS will also map the thermal and compositional properties of the surfaces of Saturn’s icy satellites. It will similarly map Saturn’s rings, characterizing their dynamical and spatial structure and constraining theories of their formation and evolution. The combination of broad spectral range, programmable spectral resolution, the small detector fields of view, and an orbiting spacecraft platform will allow CIRS to observe the Saturnian system in the thermal infrared at a level of detail not previously achieved.

A Simple High-Sensitivity Radiometer in the Infrared for Measurements of the Directional Total Emissivity of Opaque Materials at Near-Ambient Temperatures

A novel device for direct determination of the total directional emissivity of solid surfaces at near-ambient temperatures is described. The method of operation is based on comparing the radiation emitted by the surface with that emitted by a reference. The primary sensor is a solid-state, thermopile detector, and the signal recovery is achieved by phase-sensitive detection techniques, using a mechanical chopper. The device is simple and does not use any cooling. Experimental results are presented using a test sample of clear float glass, kept at 323 K (50°C), with black paint on a float glass substrate as the reference. These results, likely the first for the directional total emissivity of float glass at such low temperatures obtained without cooling, compare well with values in the literature.

Thermopile detectors: spatial non-uniformity measurements and correction methods

The spatial non-uniformity of a newly developed electrically calibrated radiometer system is characterized. The detector is a thin-film thermopile with an integral electrical heater. This paper shows that the local responsivities of these detectors have a rotational symmetry and a quadratic dependence on the distance to the centre of the detector. Typical measurement errors, which can be caused by applying different beam sizes, different power distributions of the beams, and different beam positions impinging on the detector, are investigated. A significant reduction of measurement uncertainties is achieved using specific corrections. The influence of spatial non-uniformity on responsivity measurements is experimentally checked with various types of beam.