Sensors and Actuators A 145–146 (2008) 283–290 Contents lists available at ScienceDirect Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna In-plane MEMS-based nano-g accelerometer with sub-wavelength optical resonant sensor U. Krishnamoorthy a,∗ , R.H. Olsson III a , G.R. Bogart b , M.S. Baker a , D.W. Carr b , T.P. Swiler a , P.J. Clews a a b Advanced MEMS Technologies, Sandia National Laboratories, Albuquerque, NM 87185, USA Symphony Acoustics, 103 Rio Rancho Drive, Suite B-4, Rio Rancho, NM 87124, USA a r t i c l e i n f o Article history: Received 30 June 2007 Received in revised form 11 March 2008 Accepted 11 March 2008 Available online 28 March 2008 Keywords: Nano-grating MEMS optical accelerometer Optical resonant detection In-plane accelerometer Optical sensing Low-g accelerations a b s t r a c t We have successfully demonstrated a series of results that push the limits of optical sensing, acceleration sensing and lithography. Previously, we √ built some of the most sensitive displacement sensors with displacement sensitivities as low as 12 fm/ Hz at 1 kHz. Using reference detection circuitry in conjunction with correlated double sampling methods, we lowered the 1/f noise √ ﬂoor to 10 mHz, hence improving the detection limit at low frequencies (10 mHz) by 77 dB to 50 fm/ Hz. We converted these highly sensitive displacement sensors to highly sensitive acceleration sensors through a direct mass integration processes. √ Our accelerometers have resonant frequencies as low as 36 Hz and thermal noise ﬂoors as low as 8 nG/ Hz (where 1 G = 9.8 m/s2 ). We have pushed the limits of shaker table experiments to indepen√ dently verify acceleration measurements as low as 10 G/ Hz. Direct measurements with our integrated sub-wavelength optical nano-grating accelerometers have shown device sensitivities of 590 V/G and noise √ ﬂoors corresponding to 17 nG/ Hz (at 1 Hz). © 2008 Elsevier B.V. All rights reserved. 1. Introduction There is growing interest in extremely high-sensitivity, compact accelerometers (nano-g sensitivity where 1 nanog = 9.81 × 10−9 m/s2 ) for a growing list of applications including high-precision inertial navigation and seismic sensing for geophysical and oil-ﬁeld applications. Such applications √ require measurement of extremely small acceleration signals (nG/ Hz) at very low frequencies (<100 Hz). Currently there are no chip-scale, compact accelerometers that meet these high-sensitivity and lowbandwidth requirements. The fundamental challenges in building such a device include designing very high sensitivity displacement sensors with low thermo-mechanical noise and minimizing 1/f noise. A practical challenge in building these accelerometers on a micro-scale is in integrating a large enough proof mass to the displacement sensor and meet the sensitivity requirements for the device (since larger mass relates to better sensitivity). Our ﬁrst step in building this accelerometer was to identify and demonstrate a high-sensitivity optical displacement/motion sensing mechanism. Previously, optical sensing mechanisms based-on ∗ Corresponding author. Tel.: +1 505 844 6254. E-mail address: email@example.com (U. Krishnamoorthy). 0924-4247/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2008.03.017 diffraction-gratings have been used to demonstrate sub-angstrom scale sensitivity . Advantages of optical detection techniques compared to capacitive or piezoresistive methods include high sensitivity and performance close to the Brownian noise limits of the mechanical structure [2,3]. An in-plane nanophotonic resonant sensor based on multilayer sub-wavelength optical gratings was developed [2–4]. This nano-optic sensor comprised of two sets of nano-gratings that modulate the near-ﬁeld intensity and polarization of an incident light source based on the relative lateral motion of the two gratings. The in-plane sensing mechanism offers the advantage of single-chip solutions to multiple axis motion/acceleration detection. We have previously published results from these sensors √ demonstrating displacement sensitivities as low as 12 fm/ Hz at 1 kHz [2,3]. Further, we improved sensor performance at lower frequencies to suit our target applications such as precision inertial navigation and seismic applications. We dramatically improved sensor resolution by reducing 1/f noise limits in our detection circuitry through design and implementation of specialized reference detection circuitry and correlated double sampling methods . Next, we added masses to these delicate sub-wavelength nano-gratingbased displacement sensors to build extremely high-resolution accelerometers and compared them to similar accelerometers reported in the past [7,8]. 284 U. Krishnamoorthy et al. / Sensors and Actuators A 145–146 (2008) 283–290 Fig. 2. Readout circuit used to cancel laser relative intensity noise for nano-grating optical sensors . Fig. 1. MEMS optical near-ﬁeld resonant displacement sensor based on vertically stacked sub-wavelength nano-gratings. The nano-gratings are attached to electrostatic actuators to control their motion and characterize their displacement sensitivity . 2. MEMS optical displacement sensor Previously, we demonstrated a new class of nano-grating based optical MEMS resonant sensors√with extremely high lateral displacement sensitivities (12 fm/ Hz at 1 kHz) and greater than 120 dB of open loop dynamic range . As mentioned earlier, these sensors consist of two vertically offset layers of sub-wavelength polysilicon nano-gratings separated by an air gap. They modulate the near-ﬁeld intensity and polarization of an incident light source in response to relative motion of the nano-gratings [2,3]. The reﬂected/transmitted optical beam intensity from the nanogratings is measured as a function of the relative lateral positions of the nano-gratings. Electrostatic actuators were integrated into the sensor to control nano-grating motion and measure the displacement sensitivity. Typical devices showed 10%/nm change in reﬂectance vs. lateral position. . A detailed description and principle of operation of this near-ﬁeld MEMS optical resonant sensor can be found in . An SEM of this device is shown in Fig. 1. The high sensitivity of this sensor made it a natural choice to be the basis for our accelerometer design. 3. 1/f noise Above several kHz, we can achieve the shot noise limit with commercially available reference detection circuitry. However, at the lower frequencies needed for our applications outlined earlier, 1/f components of both laser relative intensity noise (RIN) and ampliﬁer noise dominate the response. This degrades the sensitivity of the nano-grating sensors in applications that require excellent sensitivity at frequencies below 100 Hz. Reducing the 1/f noise in these optical MEMS sensors is critical in extending their range to these ultra-low frequencies. In order to reduce 1/f noise, we designed and built reference detection low noise readout circuitry that cancels laser relative intensity noise (RIN) to frequencies as low as 0.7 Hz . The low frequency bandwidth of the sensor system was further reduced to <10 mHz using a new MEMS correlated double sampling technique that canceled low frequency RIN, drift, and circuit 1/f noise by greater than 77 dB . The reference detection circuit (Fig. 2) implements the architecture presented in  but optimized for low frequency performance. An additional reference diode is included in the circuit that can- cels laser RIN. Details of this laser noise cancellation circuitry and related analysis are published in . While the reference detection circuitry rejects over 60 dB of laser RIN, more rejection is needed to achieve the low frequency performance requirements for our target applications. To further reduce the 1/f corner and allow ultra-precise position measurement at very low frequencies a micromechanical correlated double sampling (CDS) scheme was implemented . The correlated double sampling technique uses the MEMS grating element to perform the sampling. First, the electrostatic actuators drive the nano-grating (shown in Fig. 1) to a known position (Ex: zero displacement). This ﬁrst sample position measurement is quantized. This measurement is used as a reference for the actual displacement measurement. The two signals are subtracted digitally to further reduce the 1/f laser noise at the output. This method requires the bandwidth of the electrostatically driven MEMS device and the associated readout circuit to be at least three times the sampling frequency to implement this scheme. Because of aliased RIN, shot, and circuit noise the white noise ﬂoor is increased √ by 6. However, this technique does cancel any low frequency noise in the laser, position sensor, photodiodes, or reference detection circuitry due to 1/f noise, drift, or thermal effects. Hence, combining custom reference detection circuitry and correlated double sampling methodology, we reduced the 1/f noise corner to 10 mHz and improved √ the detection limit at low frequency (10 mHz) by 77 dB to 50 fm/ Hz (Fig. 3). 4. Nano-g accelerometer 4.1. Operating principle Building accelerometers from these high sensitivity displacement sensors involves the integration of an appropriate proof mass. The acceleration experienced by of this proof mass is directly proportional to its displacement as shown in the following equation: a = ωo2 x (1) where a is acceleration experienced by the proof mass, x is its displacement and ωo is resonant frequency of the proof mass given by ωo2 = k m where k is its mechanical spring constant and m is its mass. (2) U. Krishnamoorthy et al. / Sensors and Actuators A 145–146 (2008) 283–290 285 Fig. 5. Integration concept: nano-g accelerometer showing accelerometer chip with integrated nano-grating displacement sensor and proof mass integrated with an optical source (VCSEL) and detector (photodetector) chips. Fig. 3. Displacement power spectral density of the reference detection circuit with correlated double sampling scheme when interfaced with a nano-grating MEMS √ device . The detection limit, 50 fm/ Hz extends down below 10 mHz. The reference detection and correlated double sampling techniques described above have improved the displacement detection limit at low frequencies (10 mHz) by 77 dB. 4.2. Accelerometer design For initial design simplicity, the accelerometer design we chose was an open loop system without any feedback controls. To meet our compact size requirements, our ideal choice was to build a MEMS accelerometer using standard IC fabrication methodologies. However, measuring nano-G accelerations requires displacement sensitivities in the femto-meter scale (where 1 fm = 10−15 m) and low resonant frequencies (∼100 Hz) based on Eq. (1). This translates to a large mass, m, and a weak spring constant, k, from Eq. (2). For example: An acceleration of 4 nG produces a displacement of 100 fm for a resonant frequency of 100 Hz. For our accelerometer design (Fig. 4), we started with the optical displacement sensor based on sub-wavelength in-plane√ nanogratings as mentioned earlier. The high resolution (12 fm/ Hz) of this displacement sensor was essential to obtain the high sensitivities we needed for our compact low-frequency accelerometer design. To meet our low resonant frequency requirements (<100 Hz) with our open-loop accelerometer design, we needed a large proofmass with very weak springs (see Eq. (2)). Here we had to push our fabrication limits using a combination of surface micromachining and Deep Reactive Ion Etching (DRIE) to create through-wafer structures that maximized the mass, m, while minimizing the spring constant, k. In addition, the mass springs were designed for large motion in one direction (y-axis) with very high crossaxis rejection. Our through-wafer etch process and lithographically deﬁned mass springs provided greater stiffness in the z-axis and x-axis. Considering the high sensitivity of our device, we needed to provide isolation of the mass and optical sensor from the thermally induced stresses caused by the mismatch in coefﬁcient of thermal expansion between the silicon MEMS device and the attached package or substrate. We have designed a gimbal around the proof mass whose springs help isolate higher frequency inputs and thermally induced stresses. We distributed multiple nano-grating sensors symmetrically around the proof mass to enable differential sensing (Fig. 4). This allows cancellation/rejection of both off-axis displacement errors and common mode noise including (Ex: from thermal effects). Both analytical and ﬁnite-element models were used to optimize the gimbaled mass–spring system design. Fig. 4. (Top-view) Accelerometer chip layout detail showing proof mass and device isolation platform (i.e. gimbal) with associated springs. Note that substrate section not shown in layout. This design shows multiple nano-gratings located between the mass and gimbal for common-mode noise rejection through differential sensing. The insets show details of the mass spring, gimbal spring and nano-grating models. 286 U. Krishnamoorthy et al. / Sensors and Actuators A 145–146 (2008) 283–290 Finally, we adapted our design for simple and robust integration of optical sources and detectors. We incorporated through-wafer holes under the nano-grating-based displacement sensors to accommodate transmission detection schemes with the facility to package VCSELs and photodetectors above/below the device (Fig. 5). 4.2.1. Spring design A ﬁrst order analytical model was generated for both the mass and gimbal springs (Fig. 4). The mass springs were designed to be very compliant in the y direction and relatively stiff in the x and z directions. To simplify the analysis, we made a few assumptions including a linear system approximation, small deﬂections and very stiff spring trusses. Using basic beam theory, the equations that deﬁne the springs for the device are given by:Effective spring constant in y-direction, ky eff = Ehw3 2L3 (3) 20 m. Cross-axis displacement coupling was negligible. The xdisplacement for 1 G acceleration in the x-direction (worst case) was ∼0.1 nm. The z-displacement was not simulated but the analytical model predicted it to be ∼0.15 m at 1 G and ∼0.15 fm at 1 nG accelerations. ANSYS simulations of thermal effects predicted DC shifts of ∼0.8 nm and ∼40 nm in the y-deﬂection for temperature differentials of 1 ◦ C and 50 ◦ C, respectively. 4.2.2. Optical grating design The two-layer optical gratings are deﬁned in two vertically offset polysilicon layers suspended in air and separated by an air gap. The bottom grating was attached to the compliant mass and moved laterally with it. The upper grating layer was attached to the relatively stiff gimbal. A rigorous coupled wave analysis (RCWA) was used to simulate and optimize the optical response of the grating for normal incidence [3,4]. The following design parameters were chosen for our optical nano-gratings (Fig. 4): Effective spring constant in x-direction, kx eff = 2Ehw L (4) Effective spring constant in z-direction, kz eff = Ewh3 2L3 (5) where E = Young’s modulus (∼160 GPa for silicon) and h = spring thickness. Speciﬁcations for our lower mass device were: Proof mass: 4500 m × 4500 m × 400 m. Mass springs: 4 sets of folded ﬂexures with each spring dimension: 20 m × 4300 m × 400 m. Gimbal springs: 4 sets of folded ﬂexures with each spring dimension: 30 m × 1100 m × 400 m. Mass, m, for the gimbal and proof mass were calculated to be ∼21.9 mg and 18.9 mg, respectively (based on device layout). Deﬂections were calculated from the force balance equation: F = ma = yk (6) a where k Therefore, deﬂectiony = m × k is the appropriate spring constant in the speciﬁed direction for the mass or gimbal element. The ratio of spring constants for the mass are: kx ky eff mass kx ky eff mass eff mass eff mass = 1.85 × 105 = 4.00 × 102 The ratio of the spring constants between the gimbal and the mass in the y-direction is: ky ky eff gimbal eff mass ∼ 200 Hence, this design should provide adequate decoupling between modes in the x, y and z direction for both the device proof mass and gimbal. We developed a 2D ﬁnite element model for this spring-mass model using ANSYS® . Modal simulations predicted dominant resonant frequencies at 108 Hz (y translation of mass) and at 406 Hz (y translation of gimbal). The relative motion between the gimbal and mass is recorded by the nano-grating. The simulation predicted a y-displacement of ∼20 fm corresponding to 1 nG applied acceleration. The ydisplacement for a 1 G acceleration in the y-direction was ∼ incident wavelength, = 850 nm; grating period, 0.84 m; grating width, w = 0.415 m; air gap, ga = 0.38–0.5 m; thickness of lower grating, t = 0.83 m; thickness of upper grating, t = 0.83 m. 4.3. Fabrication The accelerometers were fabricated at the Microsystems Development Laboratory (MDL) and Compound Semiconductor Research Laboratory (CSRL) within Sandia National Labs in Albuquerque, New Mexico. The fabrication process combined surface micromachining and deep reactive ion etching (DRIE) techniques. The devices were fabricated in silicon. The two-layer polysilicon subwavelength nano-gratings for the optical displacement sensor were fabricated in a surface micromachining process that has been published previously . This involved fabrication of lithographically deﬁned sub-micron structures with tight tolerances . Following the fabrication of the optical displacement sensors, the wafers underwent a high-aspect ratio Deep Reactive Ion Etch (DRIE) of the substrate silicon to structurally deﬁne the proof-mass and the mechanical springs for both the proof-mass and its gimbal. A layer of silicon dioxide was deposited over the optical gratings to protect them from the silicon DRIE processing that followed. The silicon wafers were patterned and etched from the backside. The oxide layer under the nano-gratings provided an etch-stop for the DRIE silicon etch. The DRIE process had to be optimized to ensure uniform etching of the springs and through-wafer vias with target aspect ratios >1:20. This was followed by a hydroﬂuoric acid (HF) dip to remove buried oxide layers and release the optical sensors. Finally, a CO2 supercritical drying step is used to prevent stiction of the fragile nano-gratings to the substrate. Fig. 6 outlines the fabrication process for both the optical sensors and the integrated proof mass. Due to the high sensitivity of these devices, our yield from the release and dicing steps was very low. We had to develop special operating procedures for both steps to improve the yield of our devices. Two accelerometer designs with integrated proof-masses were fabricated. Large proof-masses (18.9 mg and 33.6 mg, respectively) were chosen to lower the resonant frequency of the device. U. Krishnamoorthy et al. / Sensors and Actuators A 145–146 (2008) 283–290 287 Fig. 6. Process ﬂow for integrated nano-grating accelerometer fabrication. (a) Deposit oxide isolation layer (∼2 m). (b) Deposit and pattern ﬁrst polysilicon layer (0.82 m). (c) Deposit and pattern ﬁrst sacriﬁcial oxide layer (0.5 m). (d) Deposit and pattern second polysilicon layer (0.82 m). (e) Deposit second sacriﬁcial oxide layer (2 m) to protect structures. (f) Grind and polish backside followed by patterning springs and mass. Table 1 Nano-grating accelerometer dynamic characteristics measured in air Device 1 2 Resonant frequency (Hz) Q Mass Gimbal Mass Gimbal 44 43 1101 1131 22 18 83 113 4.4. Experimental results 4.4.1. Device set 1 (18.9 mg proof mass device) Our ﬁrst set of released devices were the lower mass (18.9 mg) accelerometers. Although the large proof masses and weak mechanical springs of this design survived the release process, the nano-grating sensors did not survive. Fig. 7. Phase/amplitude frequency response characteristics (in air) for nano-grating accelerometer with lower-mass device (18.9 mg mass). We successfully veriﬁed/tested the mechanical characteristics of this device with a Laser Doppler Vibrometer (LDV) and a shaker table setup. The device was mounted on a shaker table. Speciﬁc accelerations were applied to the device and the corresponding relative motion of the mass that would be measured by the integrated displacement sensor was externally measured with a LDV. We performed both dynamic and static mechanical tests on these smaller mass accelerometers to verify our calculations. We measured mass resonances as low as 43 Hz and a Q = 20 on these lower proof-mass devices (Table 1) (Fig. 7). We veriﬁed static displacement of the device for applied accelerations within the measurement system limits. A linear relationship between the nano-grating displacements and applied acceleration was observed (Fig. 8). The measured results were 8× better than the simulation results. This can be attributed to the mechanical Fig. 8. Veriﬁcation of static characteristics of lower-mass nano-grating accelerometer using LDV setup on shaker table. Device performance is better than predicted because fabricated mass springs were weaker than original design. 288 U. Krishnamoorthy et al. / Sensors and Actuators A 145–146 (2008) 283–290 The results of these mechanical system tests for the lower mass device gives us the mechanical operating limits for this accelerometer. They also give us performance veriﬁcation for the accelerometer independent of the nano-grating sensor. Due to the high sensitivity of these devices to very low accelerations/forces, they were inherently sensitive to handling and require specialized packaging. Our fragile optical gratings were very susceptible to breakage during release/packaging. We designed and developed a specialized package to provide necessary stops to protect the device from high shock conditions. These packages were made by laser machining Quartz followed by careful alignment and epoxy-based adhesion of the device chip to the package. Care was taken to minimize the bond line to improve robustness and prevent breakage of the gratings during shock conditions. Fig. 9. Phase/amplitude frequency response characteristics (in air) for packaged higher mass nano-grating accelerometer (33.6 mg mass) springs being weaker than the original designs due to overetching in the DRIE process. We measured displacements of 1 nm for accelerations of 10 G and 1 m displacements for accelerations of 10 mG. This yields √ an electronic noise ﬂoor of 0.5 nG/ Hz at frequencies as low as 10 mHz. The proof mass thermal √ noise ﬂoor calculated from the following equation is 11.7 nG/ Hz: anoise,rms = 4kb Tωo mQ (7) where kb = Boltzmann constant, and T = absolute temperature (K). We observed large mode separation between adjacent modes with large spring constant ratios: ky /kz > 100 and ky /kx > 105 . Spring constant ratio between gimbal and mass, ky gimbal /ky mass = 625. Hence, this design provided signiﬁcant decoupling between modes in the x, y and z directions for the sensor. 4.4.2. Device set 2 (33.6 mg proof mass device) With careful packaging and handling, we successfully released and tested the larger mass (33.6 mg) accelerometers with integrated nano-grating sensors intact. Similar to the previous device set, we used a Laser Doppler Vibrometer (LDV) and shaker table to perform dynamic mechanical tests on these larger mass devices. We measured resonant frequencies as low as 36 Hz (Fig. 9). To the best of our knowledge, these are the lowest resonances for MEMS accelerometers measured to date. Considering the complexity in fabrication and handling of these large MEMS “micro-structures”, this is a signiﬁcant achievement for such devices that need to operate in open-loop without the beneﬁt of feedback systems. To obtain direct measurements from our integrated sensor, we built an experimental setup to measure the optical output from our nano-gratings sensor (Fig. 10). This setup included an 850 nm laser diode source, several beam splitters to direct/re-direct the optical path and custom reference detection circuitry  to lower the noise ﬂoor of the low-noise photodiode. We used a high precision 3-axis piezo-controlled ﬂexure stage (NanomaxTM from Thorlabs Inc., with 5 nm resolution) to provide the extremely small displacements/accelerations needed. The intensity of the optical beam reﬂected from the integrated nano-grating sensor was measured with the photodetector Fig. 10. Experimental setup for optical testing of nano-grating accelerometers showing packaged device, optical setup, 5 nm resolution piezo-actuation stage and photodetector with reference detection electronics. U. Krishnamoorthy et al. / Sensors and Actuators A 145–146 (2008) 283–290 289 sensitive accelerometers. We have built integrated masses with resonant frequencies as low as 36 Hz. Due to the extreme sensitivity of these devices, we developed custom packaging to compensate for the inherent fragility of these devices. We measured sensitivity as high as 590 V/G √ for the 33.6 mg mass device. With −5 V/ Hz, corresponding to a resolution of noise ﬂoors of 10 √ 17 nG/ Hz. This was about two √times higher than the theoretical thermal noise ﬂoor of 8 nG/ Hz. To the best of our knowledge these are the most sensitive MEMS accelerometers built to date. Acknowledgements Fig. 11. Spectral noise characteristics of nano-grating accelerometer (33.6 mg mass) with 10 Hz signal input. circuitry described earlier and correlated with the applied acceleration. We directly measured output signals of 3.80 mV for applied accelerations as small as 6.45 G. This corresponds to √a sensitivity of 589 V/G. The noise ﬂoor was measured at 10 √V/ Hz (Fig. 11). This corresponds to a noise ﬂoor of 17 nG/ Hz. The theoretical thermal noise ﬂoor was calculated, based on Eq. (7), to be √ 8 nG/ Hz. The measured dynamic range of our device spans 6 orders of magnitude (106 ). To the best of our knowledge, these are the most sensitive MEMS accelerometers built to date and are 40 dB more sensitive than the best reported in-plane MEMS accelerometers [7,8]. 5. Discussion These nano-grating based optical accelerometers demonstrate some of the highest sensitivities achievable to date. The strengths of this device are the use of a high-sensitivity optical displacement sensing technique along with a low noise photodetection circuitry to lower the 1/f noise ﬂoor. An inherent weakness of this device is its high-sensitivity combined with an open-loop scheme. This requires a large mass (from a microsystem perspective) and weak springs along with high cross-axis rejection for successful operation. Our through wafer fabrication allowed us to build such a device successfully. However, the device is also inherently susceptible to random accelerations during routine handling. Careful packaging of our device was critical and necessary in improving its robustness and obtaining our results. Alternatively, an integrated package or a closed loop approach could be used in improving the robustness of such a device. 6. Conclusions In this work, we successfully demonstrated a highly sensitive nano-grating-based optical accelerometer.√ We have previously shown displacement measurements of 12 fm/ Hz at 1 kHz based on this optical transduction mechanism. We used previously demonstrated circuit techniques√ to improve sensitivity at very low frequencies by 77 dB to 50 fm/ Hz at 10 mHz. We successfully developed capabilities to integrate large masses with fragile surface micromachined optical nano-gratings and create extremely This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia National Laboratories. Sandia National Laboratories is a multiprogram laboratory operated by the Sandia Corporation, Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. We would also like to acknowledge David Epp for shaker table measurements of the nano-grating accelerometers. We would also like to acknowledge the staff of the Sandia Microsystems Development Laboratory (MDL) and Compound Semiconductor Research Laboratory (CSRL) who contributed to the fabrication of these devices. References  N.C. Loh, M.A. Schmidt, S.R. Manalis, Sub-10 cm3 interferometric accelerometer with nano-g resolution, J. Microelectromech. Syst. 11 (June (3)) (2002) 182–187.  B.E.N. Keeler, D.W. Carr, J.P. Sullivan, T.A. Friedmann, J.R. Wendt, Experimental demonstration of a laterally deformable optical nanoelectromechanical system grating transducer, Opt. Lett. 29 (11) (2004) 1182–1184.  D.W. Carr, G.R. Bogart, B.E.N. 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Najaﬁ, A monolithic three-axis micro-g micromachined silicon capacitive accelerometer, J. Microelectromech. Syst. 14 (2) (2005) 235–242. Biographies Uma Krishnamoorthy received the MS and PhD degree in electrical engineering from University of California, Davis, in 1999 and 2002. She was a Postdoctoral Researcher at E. L. Ginzton Laboratory, Stanford University, CA, from 2002 to 2004. In the course of her research, she has developed enabling technologies for design of reliable optical microscale components in optical switching, scanning and spectroscopy applications. She has authored/co-authored one book chapter and >25 journal and conference papers. Currently, she is a Senior Member of Technical Staff at Sandia National Laboratory in Albuquerque, NM, where she works on the design of low-g MEMS optical acceleration sensors and satellite components. Roy H. Olsson III received the MS and PhD degrees in electrical engineering from the University of Michigan, Ann Arbor, in 2001 and 2004. In July 2004, he joined the Advanced MEMS Group at Sandia National Laboratories, Albuquerque, NM, USA, where he is currently a Principal Member of the Technical Staff. His research interests include: RF MEMS resonators, micromachined acoustic bandgap devices, microinertial sensors and MEMS neural interfaces, where he has authored in excess of 20 journal and conference papers. 290 U. Krishnamoorthy et al. / Sensors and Actuators A 145–146 (2008) 283–290 Greg R. Bogart received his BS degree in chemistry from St. Cloud State University in St. Cloud, MN, and his PhD in chemistry from Colorado State University in Fort Collins, CO. His research interests involve development and transferring to manufacturing new technologies. He holds 12 US patents and is currently employed at Symphony Acoustics, Inc., in Rio Rancho, NM, where he is Vice President of Engineering. Michael S. Baker received his BS and MS degrees in mechanical engineering from Brigham Young University, Provo, UT, in 1999 and 2002. His research efforts were in the area of on-chip actuation of MEMS bistable mechanisms. He is currently a Senior Member of the Technical Staff in the Advanced MEMS Technologies department at Sandia National Laboratories, Albuquerque, NM. His research interests include compliant mechanism design and methods of actuation in MEMS. Dustin W. Carr received his PhD in physics from Cornell University in 2000, and his BS in mathematics from Oklahoma State University in 1994. While a graduate student his thesis research was on nanomechanical silicon structures. He was also a staff member at the Cornell Nanofabrication Facility before joining Bell Laboratories, Lucent technologies in Murray Hill, NJ, in 1999. While at Lucent, he worked on MEMS fabrication for very large-scale optical cross-connects for use in telecom networks. He managed a group that helped to transition a world class 200 mm CMOS research fab into one of the world’s only 200 mm MEMS fabrication facilities. In 2003, he left Lucent to join the technical staff at Sandia National Laboratories in Albuquerque. While at Sandia, he studied ways to integrate nanophotonics and MEMS structures for sensor applications. This work served as the motivation for the founding of Symphony Acoustics in 2006, where he is currently the chief technology ofﬁcer, and is working on optoelectronics and MEMS integration for acoustic and vibration sensing. Dr. Carr is widely published in peer-reviewed publications across a range of topics in optics, MEMS, and nanofabrication. He is an inventor on 8 issued patents with several more patents pending. In 2004 he was recognized by MIT’s Technology Review Magazine as one of the TR100 top 100 young innovators. Thomas P. Swiler is a Senior Member of the Technical Staff at Sandia National Laboratories in the Thin Films, Vacuum, and Packaging Department. He received a BS in ceramic science/BA in physics from Alfred University in 1986, and an MS and PhD in materials science and engineering from University of Florida in 1988 and 1994, respectively. He is currently employed in microelectronics and MEMS packaging, and his interests include devising novel package designs using lowtemperature coﬁred ceramics and modeling thermal and mechanical issues in packages. Peggy J. Clews is a Senior Member of the Technical Staff at Sandia National Laboratories in the MESAFab Operations Department. She received a BS in chemistry from Nebraska Wesleyan University in 1980 and a BS in chemical engineering from Washington University in St. Louis in 1981. She is currently employed as a process engineer in the MESA Fab complex which processes both CMOS and MEMS technologies. She is interested in developing release and dry processes for unique MEMS devices and applications.