MEMS and Piezo Actuator Based Fabry-Perot ...

MEMS and Piezo Actuator Based Fabry-Perot Interferometer Technologies and Applications at VTT Jarkko Antila*, Akseli Miranto, Jussi Mäkynen, Mari Laamanen, Anna Rissanen, Martti Blomberg, Heikki Saari and Jouko Malinen VTT Technical Research Centre of Finland, P.O. Box 1000, FI-02044 VTT, Finland ABSTRACT Miniaturized spectrometers covering spectral regions from UV to thermal IR are of interest for several applications. For these purposes VTT has for many years been developing tuneable MEMS-based and more recently piezo-actuated Fabry-Perot Interferometers (FPIs). Lately several inventions have been made to enter new wavelengths in the VIS range and enlarge apertures of MEMS devices and also extending the wavelength range of piezo-actuated FPIs. In this paper the background and the latest FPI technologies at VTT are reviewed and new results on components and system level demonstrators are presented. The two FPI technologies are compared from performance and application point of view. Finally insight is given to the further development of next generation devices. Keywords: miniaturized spectrometer, Fabry-Perot, MEMS, piezo, imaging, NIR, VIS

1. INTRODUCTION Spectroscopy is a technique for measuring concentration or amount of a given species. It is typically used in physical and analytical chemistry for the identification of substances through their emitted or absorbed spectrum. However, as most of the common instruments are bulky and expensive laboratory devices, there is an increasing interest in low-cost, handheld and inexpensive spectrometers in many on-site and in-situ applications. In addition to measuring gases and liquids, monitoring of our environment becomes more and more important and there remote sensing, not only from space but from Unmanned Aerial Vehicles (UAVs), is an emerging field. In Process Analytical Technologies (PAT) and in life sciences various NIR-spectroscopic and fluorescence-based phenomena are among the most interesting ones. In industrial processes such as in pulp and paper industry there is an interest in on-line sensing from multiple points. Figure 1 below shows graphically different application fields for microspectrometers. VTT has approached these problems by developing spectrometer devices and technologies based on Fabry-Perot Interferometers (FPIs), a well known invention from the turn of the 20th century. Two technology families are concentrated on: the microelectromechanical systems (MEMS) and piezo-actuated FPIs. The development of the MEMS technology at VTT began already in early 90’s whereas the modern piezo-actuated components originate from a project started in 2006. Looking at the Fabry-Perot field world-wide regarding patenting, there has been a clear rising trend in the annual amount of patents between years 1993 and 2007 (see Figure 2). This same trend applies to MEMS FPIs, although the annual amounts have stayed quite constant from 2001 to 2007. Despite this and the many scientific papers written on the subject MEMS-FPI has only very recently started to make its way to new products as the technologies have matured.

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[email protected]; tel.: +358-20-722-6819; fax: +358-20-722-7012 ; www.vtt.fi. Next-Generation Spectroscopic Technologies III, edited by Mark A. Druy, Christopher D. Brown, Richard A. Crocombe, Proc. of SPIE Vol. 7680, 76800U · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.850164

Proc. of SPIE Vol. 7680 76800U-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 03/23/2014 Terms of Use: http://spiedl.org/terms

Figure 1 Application fields for microspectrometers.

Figure 2 Year trends in the annual amounts of Fabry-Perot patents. Light red = all patents containing Fabry and Perot; blue = MEMS and MOEMS subset. CAS information is reprinted by permission of the American Chemical Society. STN, STN AnaVist, and STN AnaVist logo are registered trademarks of the American Chemical Society.

1.1 Background on MEMS FPIs VTT has started the development of MEMS-based Fabry-Perot Interferometers (MEMS FPIs) in the 90’s. The original scientific paper by Blomberg et al. was published in 1997, explaining the construction and usage of a surface micromachined Fabry-Perot Interferometer1. This component was applied to a CO2 sensor, which is still produced by Vaisala Oyj under the product name Carbocap®. Another version of the component was made for the European Space Agency, where miniaturized spectrometers were constructed for measuring C2H4 and H2O simultaneously2. These devices included a Peltier cooling element and a PbSe detector and measured wavelength ranges from 3150 to 3920 nm and 2380 to 2870 nm for C2H4 and H2O, respectively. The resolution values (Full Width at Half Maximum, FWHM) were from 27 to 77 nm. In all components, good reproducibility and stability was achieved.


Figure 3 MEMS-FPI spectrometer for C2H4 measurements.2

Significant research effort has been put on the MEMS FPI technology over the recent years. Correia et al.3 presented a visible range FPI using bulk-micromachining, Weidong et al.4 analyzed a MEMS FPI filter with dielectric mirrors for telecom applications and Calaza et al.5 developed a process and a component for Mid Infrared (MIR) FPI. Recently Dell et al. presented a technology for fabricating a MEMS FPI –based spectrometer for real-time testing of soils for agricultural applications6 and Milne et al. realized a structure for allowing a wider usable wavelength range7. Despite of these papers among many others, there are still only a few products utilizing MEMS FPI technology. Example products available in addition to the afore-mentioned Carbocap® come from Axsun Technologies consisting of two machined Silicon-On-Insulator wafers8,9 and InfraTec GmbH, who’s technology is based on two silicon chips separated by SU-8 spacers10,11. Other MEMS-based analyzer technologies exist, such as MEMS-FTIR technologies. Among the most interesting ones, Block Engineering LLC has announced to release their ChemPenTM product in 201112 and ARCoptix S.A already sell their MEMS-Lamellar grating -based FTIR spectrometers13. 1.2 Background on Piezo-Actuated Fabry-Perot Interferometers Fabry-Perot interferometers using piezo-elements for actuation is a well-known technology. VTT has been developing piezo-actuator –based Fabry-Perot interferometers for spectral measurement applications since 2006. These devices follow in many parts conventional techniques where three piezo actuators are used to move the mirrors and three plate capacitances are used for closed loop feedback control of the device. Such a principle has been presented e.g. by Rees et al. in the 80’s14. VTT’s piezo-actuated Fabry-Perot Interferometers (piezo-FPIs) have been constructed using silica substrates and Ti-Ag-SiO2 mirrors. A complete camera system utilizing the piezo-FPI tuneable filter component has been made for Unmanned Aerial Vehicles15 for the spectral range of 500 – 900 nm. The complete device weighed less than 350 g including batteries and it was used to identify diseases in pear trees. Another system that has been built is a prototype of a chemical imaging spectrometer operating in the wavelength range of 1000 – 2500 nm. The concentration distribution for caffeine, aspirin and acetaminophen in an Excedrin™ tablet was successfully measured. A third demonstrated device was an LED spectrometer where several LEDs where packaged together with a small piezo-FPI. This enables a tuneable light source that can be used in an LED array spectrometer. Figure 4 shows the developed three different piezo-FPI filter versions. These components enable development of spectrometers and spectral imagers at much lower cost and size than before.


Figure 4 Three types of developed piezo-FPI devices. a) A small version with an aperture of 2.8 mm dedicated for miniature tunable LED IR source; b) a 7-mm version for VIS/NIR imaging spectrometer; c) a 19-mm version for chemical imaging and multipoint measurements.

2. CURRENT FPI TECHNOLOGIES AT VTT 2.1 MEMS FPI and Detectors Within the past few years VTT has made new advances in the MEMS FPI development. Two new technologies have emerged: 1) novel fabrication process that is based on using polymeric sacrificial layer and dielectric mirrors made of atomic layer deposited Al2O3 and TiO2 thin films16 and 2) a new fabrication process based on low-stress silicon nitride layers. In addition new absorber construction for an inexpensive thermopile detector has been developed to be used with the NIR/MIR MEMS FPIs17. Two key features of the current VTT technologies can be mentioned when comparing with other techniques: a) the filters are ‘monolithic’ i.e. they are made entirely on one substrate in batch processes without assembling separate chips together and b) the upper mirror is formed as a pre-tensioned diaphragm. A third difference is that no beams or hinges are used to support the mirror. The tuning range of the devices is approx. ± 10 % around a selected centre wavelength, depending on the realization of the mirrors. The Full Width at Half Maximum (FWHM) values vary from 5 nm at visible to a few tens of nanometres in the MIR. The general structure of all MEMS FPI technologies at VTT is similar despite wavelength range selections (Figure 5). Two Bragg mirrors made of dielectric quarter wave layer stacks are formed on a silicon or silica substrate with an air gap between the mirrors. Electrodes over the air gap are used to ‘pull downwards’ the upper mirror thus decreasing the air gap. Very small holes are etched through the upper mirror in order to etch away the sacrificial layer thus releasing the upper mirror. The amount and size of the holes are not significant in the optical performance of the device. The devices have circular apertures from 0.5 up to 2 mm in diameter.

Figure 5 The general structure of VTT's MEMS FPI devices.


Usually the upper mirror has been bending as a stiff plate, experiencing a well-known bowing shape when deflected by even pressure. In VTT’s modern devices processes have been designed so that the upper mirror acts like a tensioned membrane, no longer following the traditional mechanically stiff case. In addition, when the electrode configuration is designed so that the electrostatic force acts only within an annular range near the diaphragm edges, one achieves a very parallel mirror situation in the optical aperture (see Figure 6). This technique has several key benefits: 1) the transmission peak of the FPI devices hold its FWHM throughout the tuning range, 2) very large parallel areas can be manufactured making higher amount of optical power and imaging applications possible and finally 3) the parallelity does not need active control as might be the case in beam-based solutions as the symmetricity of the membrane-like structure is quite easy to control in fabrication. Currently the largest manufactured aperture diameter is 1.2 mm but the next generation will already have that of 2 mm and in principle one can go up to several millimetres. This increases the light throughput significantly as the conventional MEMS FPI apertures tend to be on the order of few hundred micrometers. Axsun’s MEMS FPIs for example has an aperture of much less than 1 mm9. Infratec’s FPI filter has an aperture of 1.9 mm in diameter but their filter experiences warping of typically 35 nm11. Milne and co-workers manufactured devices with aperture diameters of 70 – 210 µm7 . The possibility for manufacturing large apertures with VTT’s MEMS technology make imaging applications appealing as over a million CCD or CMOS pixels can be fitted within the aperture.

Figure 6 Photographs of the ALD MEMS FPI aperture as the gap is changed. (a) Control voltage = 0 V, wavelength of the transmitted light = 501 nm; (b) Control voltage = 27.6 V, wavelength of the transmitted light = 550 nm. Corresponding gap change from a) to b) is ca. from 1250 nm to 900 nm. Diameter of the optical aperture is 1 mm. Transmitted light is first coming through a monochromator with a resolution of ca. 1 nm. The maximum intensity variation over the aperture (~ 70 %) therefore indicates that the overall gap length variation is below 10 nm.

In the first MEMS FPI type Atomic Layer Deposition (ALD) was used to create the mirrors. The mirror materials were Al2O3 and Ti2O. The substrate used was silica in order to have a transmissive component in the visible range – the device was designed for 450 – 550 nm. The chip dimensions were 3 x 3 mm. An AC control system was used to tune the device transmission; the electrical system in the device was designed so that there is a constant capacitor in series with the varying plate capacitance over the air gap, making extended travel distances possible beyond the pull-in point18. Performing a time response measurement by following the side of a transmission peak it was found that the -3 dB point for the high frequency cut-off was 5.1 kHz. The wavelength range of the component is ca. 100 nm and a typical FWHM value was 6 nm, therefore ca. 50 points should be measured to fully cover the spectrum. Scanning the whole range through these 50 points takes only ca. 2 ms and therefore it is clear that several spectra per second can be measured, depending on the light power available.


Figure 7 Photograph of the first ALD MEMS FPIs.16

The other realized MEMS FPI type was designed for the NIR wavelengths between 1 – 2 µm. The devices were constructed on a silicon substrate using silicon rich Si-N and polysilicon as the quarter wave layers. On contrary to the ALD-MEMS FPI where metal electrodes were used, doped polysilicon acted as conductors and electrodes. This component can be driven in AC or DC mode. Figure 8 shows a typical transmission of such a component for two different control voltages (DC). The peak transmission is > 75 % and FWHM values of 12 nm have been achieved. The resolution is also limited by the opening angle of the optical system and therefore this is usually limited to 5° - 7° as is also the case for the other FPI technologies in this paper.

Figure 8 Transmission measurements of a silicon nitride MEMS FPI tuned from 1600 to 1425 nm.19

VTT has also developed a MEMS thermopile detector with a special absorber17. Two types of absorbers were designed and manufactured for wavelength ranges of 350 – 1000 nm and 1200 – 2000 nm. The absorbers employed Al2O3 films fabricated by ALD and amorphous Mo-Si-N films fabricated by reactive sputtering. A detector device was manufactured for the range 1200 – 2000 nm, resulting in an inexpensive component that can be packaged together with the MEMS FPI. Absorption was more than 93 % over the whole wavelength range. Figure 9 shows a photograph of the thermopile component. The manufactured thermopile detectors were later compared with two commercial ones, HL-planar 8060-1 and GE ZTP-135SR and the responsivity of VTT’s thermopile devices was comparative to that of the commercial ones.


Figure 9 Microscope photograph of the manufactured thermopile detector device.17

To have the first evaluation on the performance of the sensor, a liquid sample was prepared and measured with a MEMS spectrometer setup and a commercial FTIR spectrometer. Figure 10 presents a comparison of the spectra measured with the MEMS spectrometer and the FTIR. The measured solution contained 16.7 vol-% of ethanol and the absorption length was 10 mm. There is an offset visible in the MEMS data, originating from the moderate contrast of this component, letting through some additional signal from the sidebands. Figure 11 shows a picture of a constructed spectrometer.

Figure 10 Comparison between absorption measurements of liquid solution containing 16.7 vol-% of ethanol done with a MEMS spectrometer and a Vector 22 FTIR spectrometer by Bruker Corp.19

Figure 11 Packaged NIR/MIR spectrometer.

2.2 Piezo-Actuated FPI Most of the effort for developing VTT’s piezo-FPI versions has been focused on the 7-mm aperture device, designed for the visible range. Two different complete devices have been built: one autonomously working spectral camera for UAV applications and a spectral imager module compatible with standard video and microscope lenses for hand held


measurements. The benefits of the new devices compared to e.g. Acousto-Optic Tuneable filter (AOTF) or Liquid Crystal Tuneable Filter (LCTF) devices are their small size and weight, speed of wavelength tuning, high optical throughput, independence of polarization state of incoming light and the capability to record three wavelengths simultaneously. Recently this novel method for utilizing three multiple orders of a Fabry-Perot simultaneously15,20 has been developed and demonstrated. Here the key is to use a sensor element with pixels dedicated to certain wavelength ranges. A common example is the RGB CMOS image sensor where three different filters are fabricated on top of the pixels. When three transmission peaks of a Fabry-Perot filter is matched with the sensitive bands of the Red (R), Green (G) and Blue (B) pixels one is able to extract the entire visible spectrum with one component. The wavelength calibration of such a device is done by guiding light through a scanning monochromator and recording an image for each scanned wavelength. Then the gap of the piezo-FPI is changed and the monochromator scan is repeated. Finally a calibration matrix is created containing calibration information for each pixel. This is required because even though the gap flatness is usually better than 10 nm, it has a spatial effect on the spectrum if not corrected for. Figure 12 shows example data obtained with the spectral imager in a microscope mode.

Figure 12 Example data extracted with a spectral imaging microscope (magnification x1). a) Original colour target (laserprinted stripes on a transparency), b) target at 652 nm, c) spectrum of a selected pixel, marked with a green cross in b).

Figure 13 The spectral imaging microscope concept.

The first stability and repeatability tests have been ongoing at VTT’s lab since February 2009 for one component (therefore giving only indicative results). The component has been constantly changed at a rate of 100 Hz for 12 months, which makes already 3·109 repetitions without significant degradation in the control performance. The test has been stopped 6 times over an 8-month period in order to make more detailed measurements. There the air gap has been changed by ca. 100 nm at a rate of 0.1 Hz. The measured maximum deviation from the average was 0.3 nm for the span. The absolute gap values were not compared as the measurement was done in 7 sections and therefore the used measurement spot position changed on the surface (this component is not actively balanced). Good results were achieved with the first UAV spectral imager: The complete spectral imaging device had a mass of 302 g and a power consumption of 1.6 W. The time required to change the air gap was 0.6 ms and a complete data cube was recorded and stored in ca. 1.5 s.15,21 Another device that has shown good performance is the Chemical Imager22. The working principle of the device is similar to that of Figure 13 but with a much larger aperture (19 mm) and a Mercury Cadmium Telluride (MCT) matrix detector. This device working in the wavelength range 1000 – 2500 nm with a FWHM of ca. 20 nm is capable of


measuring e.g. the distribution of certain ingredients on a tablet surface, when a suitable calibration model is used to process the measured data23. In comparative experiments between VTT’s Chemical Imager, SpectralDimensions’ MatrixNIRTM 24 and Specim’s SisuChema25 it was found out that the performance of these instruments were very similar. Figure 14 shows the obtained distribution charts for an ExedrinTM tablet.

Figure 14 ExedrinTM tablet images of predicted concentrations with spatial averages scaled to Caffeine: 9.8 %, Aspirin: 38 % and Acetaminophen 38 %. A third realized device type is the tuneable LED array –based spectrometer. In this approach the spectrum of the light source is controlled rather than performing filtering just before the detector. This patented26 technology enables one to make very efficient wavelength tuneable sources for e.g. surface inspection tools and moisture meters. In e.g. some medical applications it could be beneficial to radiate as little optical power on the target as possible – there the LED FPI technology has an advantage. A moisture meter device without an FPI was built and demonstrated showing (values present 2σ of moi-%) accuracy of 0.14, repeatability of 0.01 and noise down to 0.04 when measured at 1950 nm from a paper target with 4.9 % moisture content. Figure 15 shows the optical concept of the LED spectrometer.

Figure 15 Optical concept for a LED array spectrometer. *FP-filter may be added to the system for more precise spectral measurements.

Finally, cascaded FPI arrangements have been used in new ways. A method for replacing the filter wheel and the chopper in a conventional spectroscopic device with two FPIs has been demonstrated. There one selects with the first FPI the desired transmitted wavelength and with the transmission peak of the other scans or ‘chops’ over that of the first FPI. This creates a modulated signal at a certain wavelength. Extending this even further it has been demonstrated that given a certain time window for the integration of light at the detector, one can imitate a desired spectral transmission of a desired target by tuning the transmission peaks of the cascaded FPIs in a proper way. This means that online matched filtering can be performed: the system built this way can be used to monitor a certain chemical e.g. with an increased sensitivity and greatly reduced amount of post-processing. Using a cascaded setup also the contrast and resolution of these FPI devices can be improved.


3. COMPARING MEMS AND PIEZO-FPI TECHNOLOGIES The MEMS and piezo-FPI technologies, although similar in the optical principle, are complementary techniques. MEMS requires high development costs and long development times and requires large product volumes in order to be commercially profitable. On the other hand, spectrometer modules based on MEMS technology may be very compact and have low production costs, making them especially suitable for sensor networks and mobile measurement devices. VTT’s MEMS technology has so far been used for single point measurements, but this will be extended to imaging applications where most of the development has been concentrated around the piezo-FPI technology. The piezo-FPI technology does not require huge investments, is relatively fast to develop and is commercially feasible even at low volumes. Furthermore, most of the high-level instruments in this paper have been developed using piezo-FPI rather than MEMS technology thanks to the more straightforward approach to construct a complete analyzer without long lead times. The piezo-FPI has larger apertures and more possibilities for substrate and mirror material selection, making it most suitable and efficient in imaging and multipoint measurement applications. The piezo-FPI technique can also be used as a part of the development chain of a MEMS device – real application tests can be first performed using the piezoFPI while the MEMS development advances as a parallel task. This approach reduces development risks and decreases the time to market for an optimized MEMS based product. Table 1 Comparison table of some key parameters for current piezo-FPI and MEMS FPI technologies.

Parameter

Piezo-Actuated FPI

MEMS FPI

Wavelength range (with one component and a single broadband detector)

Limited by Free Spectral Range and choice of materials. Current versions exist between 0.35 and 2.5 µm.

Approximately ± 10 % around centre wavelength. Centre wavelength selectable between 0.4 and 4.5 µm.

Resolving power λ/∆λ

Typical 100 with Ag mirrors. 10’000 – 100’000 achievable with dielectric coatings

Typical 50 – 200

Contrast (with one component)

100 – 200 (with metallic mirrors)

Typical 80 – 500

Aperture diameter

Currently 2.8, 7 and 19 mm. Other sizes possible.

Currently 0.5 – 1.2 mm. Technology enables more than 2 mm.

Packaged module dimensions (typical)

30 x 30 x 15 mm3

10 x 10 x 5 mm3

Commercially feasible volumes per year for a product (approximated)

100 – 10’000

> 10’000

Cost per spectrometer module

Moderate

Low

Development costs

Moderate

High

Applications

Spectral imaging, multipoint spectral measurements, hand-held and aerial instruments, surface measurements

Hand-held gas analyzers and colour measurement devices, small spectral imagers, distributed sensor networks

4. NEXT GENERATION DEVICES There are several development projects ongoing at VTT around both of the FPI technologies. Currently new wavelength ranges around 670 nm for the ALD-MEMS FPI component are manufactured and tested and larger apertures up to 2 mm in diameter will be trialled. One goal is to develop the first imaging MEMS FPI device, enabling a very compact spectral imaging device. For the silicon rich Si-N technology new wavelength ranges and larger apertures are to be developed as


well. The first version of a novel, monolithic spectrometer chip for the visible range will be processed; this unique design consists of a photodiode on which an electrostatically tuneable MEMS FPI construction is made. There has been previous work on constructing monolithic IR spectrometers for NIR27 and IR28, but no successful monolithic tuneable devices have been reported for the UV/VIS ranges, where colour and fluorescence measurements are the potential applications. In addition the next generation MEMS devices will used to construct small spectrometer devices for field tests. Development of the piezo-FPI technology will be more application and analyzator oriented than that of the MEMS FPI. Instrumentation for enabling online spectral measurements in the fields of environment, food, agriculture, healthcare and process analytical technology will be developed and tested and more emphasis will be focused on rugged overall system design. Effort will therefore be directed also in the further development of hermetic packaging of the piezo-FPI. For both technologies investigations on using cascaded structures will continue to make the FPI technology more useful for applications requiring on-the-fly data reduction and wider wavelength ranges.

Figure 16 Next generation monolithic MEMS spectrometer chip.

Figure 17. Robust, hermetic packaging for the piezo-FPIs.

5. CONCLUSIONS VTT actively develops components and devices for spectroscopic applications based on MEMS and piezo-actuated FPI technologies. New type of monolithic MEMS FPI components have been built for the VIS and NIR ranges and characterized. Thermopile components have been developed as detectors for the NIR range. Spectral imaging instruments based on the piezo-FPI technology have been built and demonstrated in a remote environmental sensing camera, handheld microscope and chemical imaging of tablets. Different cascaded FPI configurations have been tested and demonstrated. The two FPI technologies are complementary with MEMS being targeted for high volume, low cost applications and piezo-FPI for moderate volume and moderate cost imaging applications. The next generation technologies will include large-aperture MEMS FPIs enabling MEMS spectral imagers and higher amount of light, a monolithic MEMS spectrometer chip for the VIS range and several piezo-FPI -based instruments for field tests for several applications.

ACKNOWLEDGEMENTS The research has been funded by Tekes (Finnish Funding Agency for Technology and Innovation) and VTT Strategic Research. We would also like to thank all the many researchers and technicians for their significant contribution in developing these technologies.


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