MEMS-based Compact FT-Spectrometers - A Platform for Spectroscopic Mid-Infrared Sensors

Martin Kraft, Andreas Kenda Carinthian Tech Research AG

Villach, Austria martin.kraft@ctr.at

Thilo Sandner, Harald Schenk Fraunhofer Institute for Photonic Microsystems

Dresden, Germany

Abstract — Novel low-voltage driven translational MEMS mi-cro-mirror components are used to build a fully portable Fourier-Transform spectrometer. Applying nearly inertia-less MEMS devices to modulate the IR light allows building highly compact spectrometers with sub-millisecond scan times that are immune to shock and vibrations. Compact, energy efficient and robust, MEMS-based spectrometers can thus act as trans-ducers for spectroscopic IR sensors deployable in the field. The devices shown can be used to qualitatively and quantitatively measure absorbance and/or emission spectra, even of transient signals and in real time, over a wide spectral range. This allows making full use of the inherent multi-analyte detection capa-bilities of spectroscopic sensors, giving this spectroscopy plat-form a significant advantage over single-wavelength IR sen-sors. When combined with already available fibre-optic sensor probes, the resulting sensors could open new application fields for smart sensors.

I. INTRODUCTION Over the last years, the potential merit of spectroscopic

mid-IR sensors has clearly been proven [1,2], in particular for applications requiring multi-analyte and/or interferent detection capability. The key advantage of IR sensors is their ability to directly detect and evaluate a characteristic inherent property of the analyte(s), rather than the change of some property of a chemo- or bio-selective layer in response to the presence of an analyte. The fact that, despite their obvious advantages, the practical realisation and implementation of such sensors still lags behind is to a large extent due to a lack of suitable and affordable spectrometers translating spectro-scopic information into sensor readings. For a widespread use of IR analysers and sensors, a small, robust, cost-effec-tive and easy to use spectrometer system would be needed.

A key problem in designing such a spectrometer device for the mid-IR is the wide spectral range (typically 2.5 µm - 20 µm) that has to be covered. When designing a sensor for a specific compound this range can be reduced, allowing to either use a non-dispersive (NDIR) device or resort to an IR scanning grating instrument covering the (limited) spectral range of interest. Both these technologies are available in

various embodiments, some even using micro-devices. Yet, as soon as a wider spectral range needs to be covered, thus e.g. to allow for a more flexible use of the sensor system, a Fourier-transform infrared (FT-IR) spectrometer would be the instrument of choice.

The problem with using FT-IR instruments in real-world sensing application is that currently available spectrometers are costly, susceptible to environmental inferences and often bulky. Typically, the sample has to be brought to the instru-ment rather than the instrument to the sample, a procedure that is clearly incompatible with the concept of a sensor. An-other limitation is the achievable data acquisition speed, which is typically in the tens of seconds to minutes range. As control systems usually require the sensors to deliver several consistent readings before actuating an intervention, this may be too slow for e.g. real-world process sensing.

To enable a widespread use of mid-IR sensors, the goal thus is the development of a possibly compact mid-infrared broadband spectrometer device with sensor-oriented spectral properties, sufficient robustness for use in process environ-ments and the potential of being produced in larger quantities at competitive costs. With such devices, IR sensor may be-come a viable option for many practical applications, ranging from industrial process control and environmental real-time monitoring to autonomous patient surveillance and security monitoring.

II. MEMS FT SPECTROMETER SYSTEM DESIGN The core component of Michelson-type FT-spectrome-

ters is a translational mirror effecting a variable optical path difference in the interferometer. In standard FT instruments, these mirrors are large and heavy, thus limiting miniaturisa-tion and scan frequencies, requiring high-precision scanning mechanisms for the mirrors and contributing to the high cost of such instruments. When aiming at building a compact spectrometer, this component hence needs to be replaced with a suitable micro-mirror element capable of forward-backward motion.

Initial stages of this research were financially supported by the Kärnt-ner Wirtschafts-Förderungs-Fonds (KWF-4127/10036/12282); ongoing work is co-funded by the European Union within the FP7-ICT project “MEMFIS”

1-4244-2581-5/08/$20.00 ©2008 IEEE 130 IEEE SENSORS 2008 Conference

On the micro-element scale, these requirements translate favourably into an electrostatically driven translational mi-cro-electro-mechanical mirror recently developed by the Fraunhofer IPMS [3]. Around this translational micro-mir-ror element, a suitably optimised micro-optical layout based on a shrunken Michelson-layout has been designed. Using a modular approach here allows adapting a standardised spec-trometer core to various applications and sensing accessories. The third essential component are control and data evalua-tion electronics, responsible for driving the spectrometer and for translating the spectral information into sensor readings.

A. Translational MEMS Mirror Component Two different types of translational MEMS mirrors were

tested as to their suitability as the moving member in a MEMS-based compact FT-spectrometer. In the first tested layout (type A) of the translational micromechanical mirror, the actual mirror element is suspended on two long bending springs (Figure 1) [4].

Figure 1. Layout and SEM micrograph of a type A translational micro-mirror component, using two long bending springs as mirror suspension

While allowing for high oscillation frequencies, this de-sign showed a number of critical limitations. First, while outstanding for a device of that size and certainly sufficient for a proof-of-principle study, an oscillation amplitude of ± 100 µm severely limits the achievable spectral resolution. Second, the springs caused the actual mirror element to elas-tically deform during the oscillatory movement, causing problems with the dynamic symmetry of the interferograms.

Based on the experiences with the first prototype, an im-proved (type B) layout using an innovative pantograph sus-pension principle (Figure 2) was realised [5]. Having a sig-nificantly larger mirror area shape-optimised for optical ap-plications, higher amplitudes (Table I) and a significantly re-duced deformation during oscillation, this type of MEMS elements became the device of choice for the ongoing system development.

Common to both designs is the use of a proprietary ac-tuation principle using vertical comb drive actuation [6]. When applying pulsed voltages of typically < 100 V, inter-locking comb-structured electrodes located around the springs and the actual mirror area induce electrostatic forces that actuate a sinusoidal forward-backward oscillation of the mirror plate. Thus, such components are capable of oscillat-ing at design-dependent frequencies in the range from ~ 500

Hz up to several kHz with mechanical amplitudes exceeding ± 100 µm at ambient pressures of a few Pa.

Figure 2. SEM micrograph and suspension layout of a type B translational micro-mirror component, using pantograph-type suspensions

TABLE I. DESIGN AND PERFORMANCE PARAMETERS

Translational MEMS mirror devices Parameter Type A Type B

suspension design bending springs pantograph resonance frequency typ. 1 – 10 kHz typ. 500 / 1000 Hz

mirror area 1.1 x 1.5 mm² ∅ 3 mm

design amplitude ± 100 µm @ 10 Pa ± 250 µm @ 30 Pa

B. Spectrometer Layout Around these MEMS devices, a suitably optimised opti-

cal layout has been designed and built. The layout optimisa-tion resulted in a classical 90° Michelson arrangement, as illustrated in Figure 3.

Figure 3. Basic layout of the MEMS-based Fourier-Transform mid-infrared spectrometer core

In this design the IR spectrometer acts as a post-probe analyser, which is advantageous since the hot radiation source can be separated from the potentially sensitive micro-element. The IR radiation to be analysed is transferred into

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the Michelson interferometer using suitable interface optics accommodating for optically divergent (fibre optics) or par-allel (open beam) beam coupling. The radiation is then modulated in the interferometer, and the modulated output radiation is directed to an integrated TE-cooled mid-IR de-tector with high bandwidth and sensitivity.

The MEMS element itself is contained in a suitable vac-uum housing providing the necessary reduced pressure around the MEMS element, as listed in Table I. The housing is equipped with an IR transparent front and a VIS-transpar-ent rear window, the latter being necessary to allow access-ing the rear surface of the forward-backward oscillating MEMS component with a laser reference interferometer.

The practical implementation of this concept into a pro-totype (Figure 4) followed a modular design philosophy. Each functional part, i.e. spectrometer core, detector, radia-tion source and sensing probe connector – has clearly de-fined, standardised interfaces. This will allow e.g. using dif-ferent detectors according to the wavelength range of inter-est, or connecting different sensor probes, like fibre optics, integrated planar waveguides, hollow waveguides, open beam path cells, etc., to the spectrometer device.

Figure 4. Prototype hardware of the 2nd generation MEMS FT-IR spectrometer

C. Control Electronics The control electronics have three main roles to fulfil: i)

controlled resonant driving of the MEMS component, ii) acquisition of the detector data, followed by correlation with the mirror position and the FFT-transformation of the re-corded interferograms back to the spectral domain and iii) application specific chemometric processing of the resulting spectra, yielding clear-text sensor readings rather than spec-tra requiring expert knowledge to interpret.

Following the modular design approach also in the elec-tronics, the device can be flexibly adapted specific applica-tions, simply by connecting a suitable probe and uploading the necessary software/firmware into an integrated dedicated data processor.

III. SYSTEM PERFORMANCE With system characterisation ongoing, initial results us-

ing chalcogenide IR fibre probes show that a prototypes cov-ering the spectral range 4500 cm-1 to 1450 cm-1 (2.2 - 7 µm) have a spectral resolution better than 30 cm-1 and a wave-length stability better than ± 4 cm-1. As shown in Figure 5, the bands below 2000 cm-1, spaced some 70 cm-1 apart, are baseline separated, while the spectral lines around 3000 cm-1 spaced only about 10 cm-1 apart are resolved only as spectral shoulders. While obviously not suitable for highly resolved gas phase spectra, this performance is acceptable when dealing with condensed phases, i.e. liquid or solid samples.

Figure 5. Spectral performance of the MEMS-FT-prototype, shown for a 1.5 mm polystyrene standard; MEMS-FTIR: fibre-optically coupled trans-mission setup with a 4 W source; 500 scans / 0.2 s acquisition time; refer-ence: open path transmission measurement using a DTGS detector and a 20

W source; 50 scan / 118 s acquisition time

Besides the compact size, a key advantage of a MEMS-based spectrometer is the high scanning speed. Utilising the high mirror oscillation frequencies allows measuring FT-IR spectra at a time resolution in the millisecond regime, or co-adding a large number of spectral scans in a short time pe-riod to improve the signal/noise characteristics. With a type A mirror modulated at 5000 Hz, a single scan could be ac-quired within 0.2 ms every 0.4 ms. Alternatively, a type B MEMS mirror oscillating at e.g. 550 Hz would allow to ac-quired at least 500 scans within 1 s, resulting in a spectrum with better signal/noise characteristics than spectra acquired with a high-quality laboratory spectrometer in the same time. It thus appears feasible to build process analysers enabling multi-parameter mid-IR monitoring even in highly dynamic processes.

IV. REAL-TIME GAS EMISSION MONITORING To demonstrate the capabilities of the MEMS-FT proto-

type, a simple fibre-optically coupled open path gas absorp-tion / emission measurement setup was designed. A 6 W tungsten filament source in a parabolic collimation mirror was positioned 60 mm from an off-axis parabolic mirror fo-cussing the parallel beam onto the endface of a 900 µm chal-

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cogenide fibre transferring the radiation to the MEMS-FTIR instrument. The sample, in this case the gas stream respec-tively a flame from a propane/butane burner, was positioned in the open parallel path of this setup, with the lower bound-ary of the measurement path aligned to the upper end of the burner nozzle.

Figure 6. Absorption / emission sensing of a Bunsen burner flame using an open path setup; semi-quantitative analysis of (cold, hence absorbing)

hydrocarbons and (hot, hence emitting) carbon dioxide in the flame, measured at 20 ms time resolution; 50 scans co-added per spectrum

As can be seen from Figure 6, the spectrometer is capable of monitoring the emissions in real time with 20 ms time resolution. With the opening of the gas valve, the absorptions in the 3000 cm-1 region rise until reaching a non-stable equi-librium influenced e.g. by ambient air movement. Upon ig-nition of the flame, the previously emitted gas spontaneously combusts, producing hot carbon dioxide that acts as an IR emitter (negative absorption peaks), in contrast to the IR absorption caused by the cold hydrocarbon gas. When oper-ating the burner with a closed air valve and hence little oxy-gen in the resulting “cold” yellow flame, only a part of the gas fuel is burned, while the remainder causes stable CHx-absorptions. Only when changing the settings of the valve, introducing more oxygen and resulting in a hot, bright blue flame, almost the entire gas burns. Corresponding, the CO2-levels increase while the CHx-trace approaches zero.

V. CONCLUSION AND OUTLOOK The results of this proof-of-concept study on the feasi-

bility of using translational MEMS micro-mirror in compact FT-IR spectrometers have exceeded expectations. The novel approach explored here enables for the first time the con-struction of thermally and mechanically robust, reliable FT-spectrometers that are actually applicable in realistic mid-IR sensing applications. Capable of covering a wide spectral range at good spectral resolution, the MEMS-based design has proven to be basically suitable for both qualitative and (semi-)quantitative spectroscopic measurement of condensed phases. Compact size, low weight, and significantly lower

costs in comparison to laboratory systems make this techno-logical approach an interesting alternative to existing FT-IR instruments and open a range of applications, including such in chemical sensing.

The ongoing development of this approach will be con-tinued within the European collaborative research FP7-ICT project MEMFIS. Goals of this project include the develop-ment of a flexibly applicable, compact MEMS-FTIR spec-trometer module with a size about a tenth of that of present instruments, bringing the effective dimensions to those of a smaller cigar box. A concurrent goal is to extend the spectral range up to 18µm to cover the most relevant part of the mid-IR range, i.e. the range 2.5 – 18 µm while improving the spectral resolution to 10 cm-1 in a first step and 5 cm-1 on the long run. The resulting devices will thus cover the range from OH, NH and CH-stretch vibrations down to the finger-print region, something that is well beyond state of the art with compact systems. Making full use of the high informa-tion content contained in mid-IR spectra, this will allow im-proving IR-analyses in terms of sensitivity, selectivity and analytical stability.

The use of microsystem technology, integration of a maintenance-free micro-optical interferometer system for the mid-IR range, high scanning frequency, compactness and low power will contribute to a wide applicability. In combi-nation with improved MEMS control systems and an inte-grated processor for smart (chemometric) data evaluation, this development can be expected to result in a rugged and reliable device.

ACKNOWLEDGEMENT The authors express their gratitude to their co-workers at

CTR and the IPMS and to their project partners within the European FP7-ICT research project “MEMFIS”.

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