A compact and portable IR Analyzer: Progress of a MOEMS FT-IR System for mid-ir Sensing

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1 A compact and portable IR Analyzer: Progress of a MOEMS FT-IR System for mid-ir Sensing Andreas Kenda* a, Stephan Lüttjohann b, Thilo Sandner c Martin Kraft a, Andreas Tortschanoff a, Arno Simon b a Carinthian Tech Research AG, Europastrasse 4/1, 9524 Villach, Austria b Bruker Optik GmbH, Rudolf Plank Strasse 27, Ettlingen, Germany c Fraunhofer Institute for Photonic Microsystems, Maria-Reiche-Strasse 2, Dresden, Germany ABSTRACT We show results on the progress in the development of MOEMS based FT spectrometers dedicated to operate in the mid- IR. Recent research is performed within an EC-FP7 project with the goal to show the feasibility of miniaturized high performance infrared spectroscopic chemical analyzers. Exploiting the high analyte selectivity of the mid-ir paired with the inherent sensitivity of an FT-IR spectrometer, such devices could be used in a wide range of applications, from air monitoring over in-line real-time process control to security monitoring. For practical applicability in these fields, appropriate detection limits and spectral quality standards have to be met. The presented system aims at a performance to measure in the range between cm -1 at a spectral resolution better than 10 cm -1, which would clearly outmatch previous MOEMS based spectrometer approaches. A further technological advantage is the rapid-scan capability. The MOEMS devices oscillate at 500 Hz. A spectrometer based on this device can acquire 1,000 scans per second in forward-backward mode. The interplay of all these components with the challenges in system integration will be described in detail and experimental results will be shown, presenting a significant step forward in smart spectroscopic sensors, microsystems technology and vibrational spectroscopy instrumentation. Keywords: MOEMS, MEMS mirror, FT-IR, spectrometer, Michelson interferometer 1. INTRODUCTION Standard FT-IR spectrometers are large, usually static, quite expensive and require operation by qualified personnel. The presented development involves achievements in MOEMS technologies and electronics design to address size, speed and power requirements and develop a fully integrated miniaturized Fourier-Transform mid-ir spectrometer. A suitably matched interaction of multiple new components - source, interferometer, detector and control and data processing - develops unique MOEMS based spectrometers capable of reliable operation and finally results in compact, robust and economical analyzers with outstanding performance. Not only the development of a new MOEMS device capable of a piston-like movement of 1 mm in total, which enables the spectral performance, could be achieved, but also a dedicated miniature IR source and a highly sensitive thermoelectrically cooled MCT detector have been developed for this purpose. To complete the fully integrated spectrometer system, fast and intelligent electronics manage the entire operation, digital data acquisition and preprocessing as well as external communication. In recent years several MOEMS based FT-IR approaches have been published. Among them one can find different types including lamellar grating spectrometers [1], [2], [3], [4], standing wave interferometers [5], and Michelson spectrometers [6], [7]. The common aim of these efforts is to develop systems with integrated miniaturized optics. Hence, most of them are inherently limited in the accessible spectral range and also in terms of spectral resolution due to small actuator travel. Unlike these works, we present a dedicated mid infrared system with a high throughput infrared optical bench and precise signal sampling. An early approach already led to a prototype operating in the mid-ir which has been published in [8], [9]. This system has first shown the feasibility for MOEMS based FT-IR instrumentation, but the performance in terms of SNR and spectral resolution was also limited due to its smaller mirror aperture and actuator travel. The presented work now shows significant progress in several ways. *andreas.kenda@ctr.at; phone ; fax ;

2 First, heading towards a desired spectral resolution of at least 10 cm -1, this improvement will enable applying the devices for a large number of mid-ir analyzer and sensor applications. A spectral resolution of 13 cm -1 could already be demonstrated. In addition, signal-to-noise characteristics and spectrometer sensitivity have been dramatically improved by increasing the optical aperture of the MOEMS mirror and thus the optical throughput of the device. Second, the spectral range could be extended up to 14 μm from previously 10 µm. Now, covering μm, OH, NH and CH-stretch vibrations down to the fingerprint region can be observed. This will allow improving IR-analyses in terms of sensitivity, selectivity and analytical stability with compact systems. Third, MOEMS technology not only enables building ultra-compact but also ultra-rapid scanning spectrometers. Currently, the fastest commercially available rapid-scan FT-IR spectrometers are capable of acquiring a single scan in a few ms at 16 cm -1 spectral resolution. Our development aims at a time resolution of 1 ms at 10 cm -1 or even better spectral resolution. There will be tradeoffs in terms of signal-to-noise, since the beam diameter is still comparatively small, but the performance could be unmatched anyway. To benefit from this potential, data acquisition has to cope with high and, due to a quasi-harmonic mirror motion, modulated sampling rate. A FPGA device is used to acquire data from each scan with the forward and backward motion of the mirror. The same FPGA also controls the MOEMS device and the laser reference system. Further processing of the sampled data as well as communication tasks are performed by a Digital Signal Processor (DSP). Fast Fourier-transformation (FFT) of the interferogram, and later chemometric models and/or decision algorithms will be implemented on this device. Together, using Microsystems technology to replace delicate mechanics, infrared optimized optics, a miniaturised source, an improved miniaturised detector and powerful digital electronics are the presented components of an upcoming intelligent spectroscopic sensor for industrial applications. 2.1 Design 2. MOEMS DEVICE A novel translatory MOEMS actuator was specially designed for the FT-IR spectrometer. The Michelson interferometer design and the desired performance put several demands on the MOEMS device. Amongst these, a mirror travel of ± 500 μm and a minimal dynamic deformation of < λ/10 peak-to-peak in combination with a large mirror aperture of 5 mm were the most challenging goals. To realize a large stroke of the mirror plate, a pantograph-like suspension was chosen. Here, torsional springs are used as deflectable elements instead of bending springs, which reduces significantly parasitic mirror deformation due to mechanical stress. The new MOEMS device consists of four symmetric pantograph suspensions in contrast to two pantographs used for a first pantograph MEMS design, where only ± 140μm amplitude could be achieved due to parasitic tilt modes [10].As shown in Figure 1, the mirror plate is supported symmetrically by four pantograph suspensions. One single pantograph consists of six torsional springs and connected by stiff levers. torsional springs a) Figure 1: a) schematic design of the mirror plate and basic pantograph suspension, b) micrograph of the MOEMS device with a mirror diameter of 5 mm showing details of pantograph and spring structures. b)

3 The translatory MOEMS devices are manufactured in a CMOS-compatible SOI process using a highly p-doped device layer of 75 μm. The translatory MEMS actuators are driven electrostatically in resonant mode using in-plane vertical comb drives [11] located on each of the 4 pantographs for optimized driving efficiency. A detailed description and characterization of this device and its variants has been published previously [12]. In addition to the mentioned specifications in terms of maximal mirror travel and aperture, the MOEMS device has to suppress any parasitic oscillations, especially mirror tilts. This has been achieved by a consequent design-inherent mode separation of mirror plate and pantograph suspension structures. Figure 2 shows a FEM modal analysis of the MOEMS device. The first parasitic mode concerning a tilt of the mirror plate appears at a resonance frequency of 2238 Hz, which is well separated from the frequency of the desired piston mode at 500 Hz. Freq1 = 500Hz (translatory mode) Freq2 = 1413Hz Freq3 = 1413Hz Freq4 = 1633Hz Freq5 = 1681Hz Freq6 = 1938Hz Freq7 = 1938Hz Freq8 = 2238Hz (tilt mode) Figure 2: FEM modal analysis of the MOEMS mirror. Color scaling shows the absolutes of the out-of-plane deflection. For the operation in an interferometer parasitic tilt modes are critical; other parasitic modes are mainly relevant regarding reliability. 2.2 MOEMS device characterization The MOEMS device is driven in open-loop operation with pulsed driving voltages of 50% duty cycle and a pulse frequency twice the mechanical oscillation frequency. Furthermore, operation demands the MOEMS device to be encapsulated inside a vacuum package due to significant viscous gas damping in normal ambient. Consequently, for FT- IR system integration a long term stable optical vacuum package with a broadband IR window is required. Alternatively, an operation at atmospheric pressure would be advantageous in order to make the vacuum encapsulation obsolete. First results on this have been demonstrated in [12], where ± 80 μm travel was measured at atmospheric pressure and 40 V excitation voltage. However, at a reduced ambient pressure, which is used for this work, the actuator performs a precise piston movement with up to ± 500 μm travel as shown in Figure 3. Figure 3: Frequency response curves measured for different driving voltages under reduced ambient pressure of 30 Pa (left) and driving frequency versus travel measured at 40 V and varied ambient pressure levels (right).

4 The effective mirror tilt has been estimated to be 10 arcsec using a stroboscopic autocollimator setup [12]; a mirror tilt of 24 arcsec has been measured using the internal laser reference system of the FT-IR system. The interpretation of the differences demands more work, but it is most likely due to the different mounting of the MOEMS mirror for device characterization and for system integration. The maximal dynamic mirror deformation appearing at the turning points of the mirror oscillation could not be measured so far, but was simulated to be 433 nm peak-to-peak. The RMS value is 113 nm being less than λ/10 assuming 2.5 µm to be the lowest accessible wavelength of the spectrometer. As already mentioned, to achieve a practical mirror travel of ± 500 μm the MOEMS device has to be operated in reduced ambient pressure. Consequently, a sealed optical vacuum package is currently being developed. In this regard there are several challenges to cope with e.g. soldering of coated IR transparent windows, chip bonding and issues related to thermal stress. For this work a preliminary version of this package was used where the operating point in terms of pressure was set to 70 Pa. Applying an excitation voltage of 50 V, the optimal driving frequency was found at 930 Hz yielding a MOEMS mirror oscillation at 465 Hz. Following the frequency versus pressure and amplitude characteristics (Figure 3), a mirror travel of ± 380 μm could thus be achieved. 3.1 System Design 3. SYSTEM INTEGRATION The optical set-up is based on a classical Michelson interferometer design with the MOEMS mirror as the key element. With Figure 4 showing the schematic layout and Figure 5 showing a photograph and an exploded assembly drawing, the current prototype is presented. At this stage a compromise between miniaturization and accessibility of all components was found. A later system could even be built more compact. Figure 4: Schematics of the optical layout and block diagram of the signal path of the FTIR system. Heading for throughput maximization, a special miniaturized IR source with an emission area of only 500 µm in diameter and narrow angular emission characteristics has been developed. Accounting for the maximal acceptable beam divergence, this allows the focal length of the collimating mirror to be set to 5.8 mm only, giving the highest possible numerical aperture. AR coatings used for ZnSe beamsplitter and compensator window have been optimized for the desired wavelength range but also for a reasonable performance at the reference laser wavelength. As seen in Figure 4, compensator and IR window of the vacuum package coincide. Once the modulated IR radiation leaves the interferometer, it enters the sampling unit where it gets focused by an off-axis parabolic mirror onto the sample plane. The divergent light transmitted through the sample is then collimated again and focused onto a 4-stage thermoelectrically cooled MCT detector using a ZnSe lens and a hyperhemispherical GaAs lens. Other sampling configurations such as specular reflectance, diffuse reflectance or attenuated total reflection (ATR) would equally be possible.

5 sampling chamber IR detector interferometer IR source a) b) system control electronics MOEMS device in vacuum-package and optomechanics Figure 5: a) Photograph of the FTIR prototype system including IR source, MOEMS interferometer, sampling chamber, detector, analog-to-digital converter, system management and data preprocessing electronics as well as a LANmodule for external communication. b) Exploded assembly drawing. During scanning, the position of the MOEMS mirror is monitored using the interference signal of a laser-reference interferometer. The radiation of a temperature-stabilized 850 nm VCSEL diode enters the interferometer at a small angle so that no light of the IR is blocked by any optical elements of the reference interferometer. For acquisition, the reference interferogram is also used to precisely clock the IR interferogram sampling. In the context of this type of device with its harmonic motion, this method has been described previously [8]. Asymmetries in the interferometer setup itself, dispersion and electronic delays as well as residual parasitic modes and dynamic deformations of the MOEMS mirror cause small but observable differences in the sampled IR interferogram as the mirror travels forth or back. In order to exploit each scan, the reference interferometer not only encodes the absolute mirror position but also the actual moving direction, as shown in Figure 6. a) b) Figure 6: a) interferogram signals prior to AD conversion and the digital direction signal derived from the reference interferogram across an entire scan. b) IR and reference signals around the centerburst for the case of a symmetrically sampled interferogram.

6 The direction signal detects the turning point of the MOEMS motion and thus triggers the acquisition (start/stop) of the actual scan. As the reference interferogram (after digitization) clocks the ADC, single scans can be precisely averaged and the quasi-harmonic motion of the MOEMS mirror is compensated. For this MOEMS-based instrument, the special challenge regarding data acquisition is the high bandwidth paired with a significant modulation of the sampling rate. According to the Nyquist criterion, the sampling interval in the optical path difference (OPD) domain has to be less than ½ min, with min as the minimum wavelength measured. min = 2.5 µm as defined for this system, the sampling interval has to be smaller than 1.25 µm. Using a reference laser at ref = 850 nm, which triggers one sample per period of the laser interferogram, this criterion is met. For a given mirror velocity v, the sampling frequency is given by f = 2v/ ref in a continuous scan operation. Due to the nonlinear motion of the MEMS mirror, this calculation has to account for the fastest possible mirror velocity which occurs when the mirror passes the point of zero deflection. Describing the mirror motion based on a harmonic model and accounting for a mirror frequency of 500 Hz and a mirror travel of ± 500 μm, the highest frequency in the laser reference interferogram is derived as 2 MHz. This sampling frequency already meets the Nyquist criterion as shown above and is relevant for designing the detector and acquisition electronics. The ADC used to digitize the detector signal has a finite resolution yielding an uncertainty of 1-bit level in converting an analog signal. Therefore, the S/N of the digital interferogram, and thus the spectral S/N, is determined by the total number of bits. For a spectrum measured between max and min at a resolution of there are M resolution elements, where M = ( max - min)/ For a broadband source, the dynamic range of the interferogram is given, in a good approximation, by the dynamic range of the spectrum multiplied by ½ M [13]. The desired dynamic range of the spectrum of the MOEMS spectrometer has been specified by 500:1. Thus, at least a 14-bit ADC is needed to sample the interferogram. Based on these calculations a flexible multiprocessor FPGA/DSP platform has been developed to cope with data acquisition, MOEMS and system control, calculation and communication tasks. A detailed description of the electronics has been published previously [14].While the FPGA is used for real time processing tasks as data acquisition and MOEMS control, the DSP is used for computation intensive tasks and communications. In future, the FPGA should also do the FFT calculation of the spectra, so the DSP can cope with complex chemometrics and quantitative analysis algorithms on absorbance and/or emission spectra even of transient signals. Assuming an OPD of the MOEMS interferometer of 0.2 cm (single sided interferogram at ± 500 μm mirror travel), the maximum achievable spectral resolution, which is given by = OPD, yields 5 cm -1. This is a theoretical value assuming a perfect single sided acquisition of the interferogram meaning that zero path difference of the two interferometer arms is reached when the mirror is at its maximum deflection. This optimal configuration in terms of spectral resolution is not entirely applicable, since phase correction methods have to be performed on the interferogram. This requires a small symmetric section around the centerburst. Hence, the spectral resolution of the MOEMS FT-IR spectrometer is expected to reach 8 cm -1 in a final state. Currently, a mirror travel of ± 380 μm is achieved and the interferogram is sampled symmetrically yielding a calculated spectral resolution of 13 cm SYSTEM CHARACTERIZATION Figure 7a) shows single beam spectra illustrating the instrument function in terms of intensity vs. wavenumber. The intensity distribution results from the detector response, the emission characteristics of the source, the efficiency of the beamsplitter and the absorption of optical elements. Contributions from ambient gases (air) within the optical path, mainly from water vapor and CO 2 at 2340 cm -1, are also present. First results demonstrate that a spectral range between 5500 cm -1 (1.8 µm) and 750 cm -1 (13.3 µm) can be accessed. The signal-to-noise behavior (S/N) was characterized measuring the peak-to-peak noise between 2200 cm -1 and 2100 cm -1 at 100% transmission. The result is shown in Figure 7b) as a function of the number of scans averaged. The measured values correspond closely to the theoretical square root dependence based on the S/N of a single scan.

7 a) b) Figure 7: a) Single beam spectrum. Staggered plot showing the results of a single scan and 500 scans averaged. b) S/N performance of coaddition numbers and corresponding measurement times compared to the theoretical N ½ curve. A single scan yields a S/N of 25:1; averaging of 1000 scans improves the S/N to 750:1. Regarding the total measurement time, a S/N of 25:1 is achieved with 1 ms, while S/N improvement to 750:1 takes about 1 second. For S/N (RMS) values of 100:1 for a single scan and 2750:1 for 1000 averaged scans could be achieved. The S/N (RMS) are reported to compare the performance with previous prototypes[8], [9]. Figure 8: Staggered plot of 1.5 mil thickness Polystyrene transmission spectra acquired with the MOEMS based FT-IR spectra using 1000 scans averaged and a Bruker TENSOR TM FT-IR spectrometer as reference. A cutout between 3400 cm -1 and 2600 cm -1 visualizes resolved features in detail.

8 For spectral resolution characterization, transmission spectra of a clear 1.5 mil Polystyrene film have been obtained. The spectra shown in Figure 8, which were acquired with the MOEMS based FT-IR system show all sample features perfectly resolved and comparable to the reference measurement which was performed at a higher resolution. Analyzed features are in the order of 13 cm -1 FWHM, so the predicted resolution can be confirmed experimentally. 5. CONCLUSIONS With the characterization of a previous prototype, to which was referred several times in this work, the authors concluded several minimum specifications, namely a spectral resolution of 10 cm -1 and a SNR of 1000:1 to qualify a FT- IR system as a sensor for industrial applications e.g. process control. The purpose of the system, presented in this work, is to proof that this is feasible on the basis of MOEMS technology and it could be shown that these specifications are already within our grasp. Core components like the MOEMS device and the sampling and processing units are ready for even higher performance. Once a permanent vacuum package for the MOEMS device, as currently under development, is ready for integration the performance of the system will get another boost in terms of spectral resolution, stability and portability. This will mark another milestone towards a mobile analyzer deployable, e.g., as an all-purpose hazardous vapor sensor, as a sensor for air- and spaceborne based IR analysis, etc. The high scanning speed of about 1 ms makes further applications accessible. Tracing of fast chemical reactions and transient states, which is of research interest, fits very well to this strength. In spectral imaging and scanning applications this feature is also advantageous; hence one of the next steps will be the implementation of the presented system as a spectral sensor for IR spectral ellipsometry. ACKNOWLEDGMENTS This work is based on the results of the EC 7th framework project MEMFIS. The authors gratefully acknowledge the funding granted by the European Commission. The authors may thank all other peers involved in the development of the spectrometer prototype, especially Adam Piotrowski of VIGO System S.A., Norbert Rapp of Bruker Optik GmbH and Stephan Marofsky of RHe Microsystems GmbH, who developed essential parts of the presented system. REFERENCES [1] Manzardo, O., Herzig H. P., Marxer C. R. and Rooij N.F., "Miniaturized time-scanning Fourier transform spectrometer based on silicon technology", Optics Letters 24(23), (1999). [2] Manzardo, O., "Micro-sized Fourier Spectrometers", Ph.D. Thesis, University of Neuchatel, (2002). [3] Ataman, C., Urey, H. and Wolter, A., "MEMS-based Fourier Transform Spectrometer", J. Micromechanics and Microengineering 16, (2006). [4] Ataman, C. and Urey, H., "Compact Fourier Transform Spectrometers using FR4 Platform", Sensors and Actuators: A. Physical 151, 9-16 (2009). [5] Kung, H. L., et. al., "Standing-Wave Transform Spectrometer Based on Integrated MEMS Mirror and Thin- Film Photodetector", IEEE Journal on Selected Topics in Quantum Electronics 8(1), (2002). [6] Solf, C., Mohr, J. and Wallrabe, U., "Miniaturized LIGA Fourier transformation spectrometer", Proceedings of IEEE Sensors 2, (2003). [7] Collins, S. D., Smith, R. L., Gonzales, C., "Fourier-transform optical microsystem", Opt. Letters 24, 12, (1999). [8] Kenda, A., Drabe, C., Schenk, H., Frank, A., Lenzhofer, M. and Scherf, W., "Application of a micromachined translatory actuator to an optical FT-IR spectrometer", Proc. SPIE 6186, (2006). [9] Tortschanoff, A., Kenda, A., Kraft, M., Sandner, T., Schenk, H. and Scherf, W., "Improved MOEMS based ultra rapid Fourier transform infrared spectrometer", Proc. SPIE 7319, (2009). [10] Sandner, T., et. al., "Translatory MEMS actuators for optical path length modulation in miniaturized fouriertransform infrared spectrometers", Journal of micro/nanolithography MEMS and MOEMS 7(2), (2008). [11] Schenk, H., et. al., "Large Deflection Micromechanical Scanning Mirrors for Linear Scans and Pattern Generation", Journal of Selected Topics of Quantum Electronics 6, (2000). [12] Sandner, T., Grasshoff, T., Schenk, H. and Kenda, A., "Out-Of-Plane Translatory MEMS actuator with extraordinary large stroke for optical path length modulation", Proc. SPIE 7594, (2011). [13] Griffiths, P. R., De Haseth, J. A., [Fourier transform Infrared Spectroscopy], Chem. Analysis 83, Wiley, (1986). [14] Lenzhofer, M., Kenda, A., Mayer, C., Rapp, N., "Development of a Flexible Multiprocessor (FPGA/DSP) Platform for a Varity of Applications Shown on a MEMS Based FTIR Spectrometer", Austrochip Workshop on Microelectronics, (2010).