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A new vertical suspension inertial grade gyroscope

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A new vertical suspension inertial grade gyroscope
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  A NEW VERTICAL SUSPENSION INERTIAL GRADE GYROSCOPE  A. Abdel Aziz  1  , H. H. Tawfik  1  , A. Sharaf  1,3  , A. M. Elshurfa 4  , M. Serry 1  , and S. Sedky 2   1 The American University in Cairo, New Cairo, EGYPT 2 Zewail City of Science and Technology, Giza, EGYPT 3 Egyptian Atomic Energy Authority, Cairo, EGYPT 4 King Abdullah University of Science and Technology, Thuwal, SAUDI ARABIA ABSTRACT This paper presents two novel architectures for realizing high performance bulk micromachined gyroscopes based on Silicon-on-Insulator (SOI) technology. The designs have a simulated Brownian noise floor of to 0.014 o /hr, a sensitivity 115 mV/ o /s, and a signal to noise ratio (SNR) of 1,437 mV/º/hr. These  performance characteristics were achieved at resonance frequencies of few kilohertz. The backbone idea behind the proposed gyroscopes was the utilization of an out-of- plane suspension configuration comprising vertical  beams, in which the latter can suspend a seismic proof mass of 1.6 mg. Measurements of the fabricated gyroscopes prove, for the first time, the concept of  building MEMS devices using a vertical suspension arms. KEYWORDS Bulk Micromachining, Gyroscopes, Inertial Sensors, SOI, Vertical suspension. INTRODUCTION Micromachined vibroatory gyroscopes (MVG) have received wide research interest because they enable the realization of high precision miniature inertial navigation systems, which can be used in pico-satellites and  planetary landers [1]. Despite that many designs cover rate-grade specifications; they are still short on meeting the specifications of inertial rate applications [2]. The architectures of reported MVG share a common concept, which is primarily relying on in-plane suspensions designs, i.e. springs [1-5]. Although the in- plane suspension suppresses, to a great extent, the out-of- plane deflections of the proof mass, it limits the area fill factor of the proof mass. When the proof mass is reduced, the gyroscope becomes noisier, and other figures of merit including the minimum detectable angular rate, the output signal sensitivity, signal-to-noise ratio (SNR), are all affected. This work reports, for the first time, an approach that employs the use of an out-of-plane suspension configuration to realize a MVG. The out-of-plane suspension bestows a number of benefits including for example, higher Sensitivity, SNR, and lower Brownian  Noise at the expense of a more complex fabrication  process. NOVEL ARCHITECTURES The novelty of both reported architectures relies on replacing the commonplace in-plane beams used for realizing the suspension system with vertical ones [6]. The use of vertical beams inherently means that the area fill factor of the proof mass will be significantly improved (~80% in our case), which enlarges the proof mass, and consequently reduces the resonance frequency, reduces Brownian noise floor, improves the mechanical quality factor, and increases SNR of the sensor. We note that if the fabrication employs bulk micromachining technology, a suspension beams with lengths reaching up to 400-500µm are feasible. In turn, the thermo-elastic and support losses, which are the dominant sources of losses in high-quality vacuum operated inertial sensors, are also suppressed [3]. Another inherent advantage of using vertical suspension beams lies in the fact that the spring length, and accordingly the resonance frequency, is determined by the deep reactive ion etching (DRIE) time rather than the mask features. In other words, with the same patterning mask, several resonance frequencies can  be achieved. Moreover, the mode matching was achieved  by designing the sensor to be 3-fold symmetric [4]. Figure 1 shows the first architecture which utilizes 8 vertical  beams for each mode.  Figure 1: 3D model of vertical suspension gyroscope. In the second architecture, the outer set of vertical  beams was replaced with horizontal ones as shown in Figure 2. These in-plane beams ensure direct electrical connections between the movable part of the comb-drive and the anchors. Such scheme simplifies the fabrication  process flow as will be shown later. In Figure 2, the movable part of the comb-drive design was modified to accommodate the horizontal beam inside of it. This modification creates more space that allows the presence of a longer beam, and hence a smaller spring constant without reducing the area fill factor of the proof mass or reducing the number of fingers (i.e driving/ sense capacitances). Another advantage this architecture offers is the drastic reduction of parasitic capacitances. In the vertical suspension, the electrical connections between the T3P.010978-1-4673-5983-2/13/$31.00 ©2013 IEEE956Transducers 2013, Barcelona, SPAIN, 16-20 June 2013  anchors and the drive/sense electrodes are made through the bulk Si below the intermediate SiO 2  film of the SOI wafer. Therefore, a relatively large parasitic capacitance  between the comb-drives and the proof mass is generated. This is avoided completely with the hybrid architecture. The reduction of the parasitic capacitance enhances the electrical decoupling between the drive and sense electrodes.  Figure 2: 3D model of hybrid suspension architecture. SIMULATION RESULTS COMSOL Multiphysics®, the commercial Finite Element Analysis (FEA) package, was used to analyze the mode shape and verify the decoupling of both gyroscope designs. The mechanical coupling between the sense and the drive modes was calculated to be 6.5%. In addition, the resonance frequencies of the drive and sense modes, of 3.1 KHz, are only 2 Hz apart (as shown in Figure 3). (a) (b)  Figure 3: Vertical suspension gyroscope: (a) Drive mode resonance frequency 3117 Hz. (b) sense mode resonance  frequency 3119 Hz. Figure 4 shows FEA simulations of the drive and sense modes of the hybrid suspension gyroscope. Mechanical coupling was calculated to be 2.8%. A slight increase was observed at the resonance frequency of the drive and sense mode (of 4.95 KHz) but also the mode matching is still present. (a) (b)  Figure 4: Hybrid suspension gyroscope: (a) Drive mode resonance frequency 4964 Hz. (b) sense mode resonance  frequency 4953 Hz. A Numerical model was built in Matlab/Simulink environment developed in [7] was utilized to investigate the behavior of both devices under different excitation values (DC and AC voltages). This modeling was essential to determine the maximum excitation that  provides maximum driving amplitudes without causing nonlinearities. It was found that the vertical suspension architecture can achieve driving amplitudes of 3 μ m without entering the nonlinear region while the hybrid suspension architecture can reach to only half that value. In Table 1, we compare the simulated performance of the proposed gyroscope with another gyroscope fabricated using the same technology [5]. FABRICATION PROCESS FLOW Vertical Gyroscope Process An SOI wafer of 100µm device layer, 1µm SiO 2  layer, and 500µm handle layer was used. First, a film of  photoresist (AZ 9260) was spun (Figure 5a) and MASK#1 was printed on the front-side. This mask defines the vias which are filled later with a conductor to electrically connect the anchors with the actuating electrodes through the handle layer. Further, MASK#1 defines one of the four sides of the vertical beams. MASK#1 pattern was used to etch through device layer intermediate SiO2, and 470µm inside the Si handle wafer (Figure 5b). 957  Table 1: Comparison of the proposed gyroscopes’ performance with state of the art reported in [5]. Performance Measure State of the art [5] Vertical Design Hybrid Design Overall Area 2mm 2  2 mm 2  2mm 2  Proof Mass Area Fill Factor 17% 80% Proof Mass (  M  e ) 30 µg 1.6mg Resonance Frequency 17.4 KHz 3.1 KHz 4.9 KHz Drive/Sense Quality Factor (respectively) 81,000/64,000 •   21,000 (for both) •   With advanced etching technology (aspect ratio = 1:10) Q equal 340,000 Max drive Amplitude for the SAME Voltage 3 µm 4 µm 0.6 µm Sense Amplitude 3 nm 150 nm 115 nm Mechanical Noise Equivalent Angular Rate 0.3 º/hr 0.08 º/hr Drive/Sense Capacitances 0.16pF 2.2pF Output Sensitivity 1.25mV/º/s 150 mV/º/s 115 mV/º/s Signal to Noise Ratio 4.17mV/ º/hr 1,875mV/ º/hr 1,437mV/ º/hr Then, high conductivity (boron-doped) SiGe was deposited using LPCVD to fill the previously etched via (Figure 5c). Then, photoresist was spray-coated on the front-side of the wafer and MASK#2 is printed. This mask defines the proof mass, fingers, and comb-drives. The SiGe film was etched in DRIE followed by Si etching for the device layer till the SiO 2  layer was reached (Figure 5d). (a) (b) (c) (d) (e) (f)    Figure 5: Fabrication Process of vertical suspension  gyroscope. Afterwards, a layer of photoresist (AZ 9260) was spun on the backside of the wafer and MASK#3 was  printed. The purpose of MASK#3 is to create a different level between the moving proof mass and the bulk of the handle layer to prevent friction during testing. Hence, this  print is used to etch less than 30µm of Si (Figure 5e). Also, it is important to etch all the silicon beneath the fixed comb-drives to reduce the parasitic capacitances  between the comb-drives and the movable proof-mass. Finally, photoresist was spray-coated on the back-side of the wafer and MASK#4 was printed, which defines the other three parts of the 400 μ m long vertical beams; this is done by etching the Si in the handle layer for 470 µm. Hybrid Gyroscope Process This is a more simplified two-mask version of the  process in which the biasing of the fixed electrodes  problems is fixed by adding horizontal connections that have minimum effect on the resonance frequency, sensitivity, and overall performance of the device. The same type of SOI wafer (as with the vertical suspension gyroscope) was used (Figure 5.a). First, a film of  photoresist (AZ 9260) was spun and MAKS#1 was  printed on the front-side. MASK#1 defines the horizontal  beams and one of the four sides of the vertical beams. In addition, MASK#1 defines the proof mass, fingers, and comb-drives. Then, a DRIE etch was for the device layer Si, SiO 2  layer, and 400µm inside the handle wafer (Figure 6.a). (a) (b)    Figure 6: Fabrication Process of hybrid suspension  gyroscope. After that, a layer of photoresist (AZ 9260) was spun on the backside of the wafer and MASK#2 was printed. The  purpose of this mask is to define the other three parts of the 400 μ m long vertical beams by etching Si of the handle layer for 500µm (i.e. etching through the handle wafer and stops by reaching the intermediate SiO 2 ) as seen in Figure 6.b. RESULTS & DISCUSSION Scanning Electron Microscope (SEM) images of the fabricated devices are shown in Figure 7. Measurements results for the drive and sense modes of the fabricated hybrid suspension gyroscope show a resonant frequency of ~28 KHz (Figure 8). It is clear that the mode matching capability of the proposed gyrosocpe was achieved with only 0.5 KHz mismatch. 958   (a) (b) (c) (d)  Figure 7: (a-b) SEM of vertical suspension gyroscope. (c-d) SEM of hybrid suspension gyroscope. The low quality factor measured (~6) was due to  performing the measurements in ambient pressure. Due to limitations with the deep Si etching, the realized vertical  beams were only 350µm long instead of the desired 400 µm. Furthermore, the fabricated cross section of the beam was 50 µm x 80 µm rather than the intended 10 µm x 40 µm. The shorter and thicker vertical beams resulted in a stiffer suspension configuration (affected the quality factor), and increased the value of the drive/sense modes resonance frequeny. (a) (b)  Figure 8: (a) Drive mode frequency response. (b) Sense mode frequency response. CONCLUSION Two novel architectures were reported for the realization of MEMS inertial grade gyroscopes using standard SOI bulk micromachining process. MEMS gyroscopes utilizing out-of-plane mass-spring-damping system (using vertical beams) were fabricated for the first time, and proof-of-concept measurements were  performed. The out-of-plane suspension configuration allows the accommodation of a seismic proof-mass (1.6-mg) with long beams (400-500 µm) in a total device area of 2 mm 2 . REFERENCES [1] C. Acar and A. M. Shkel. “MEMS Vibratory Gyroscopes: Structural Approaches to Improve Robustness”.  No. 738. Springer   2009 [2] N. Yazdi, F. Ayazi, K. Najafi, “Micromachined Inertial Sensors”,  Proc. IEEE   86 (8), 1998, pp. 1640-1659. [3] A. Sharma, M. F. Zaman and F. Ayazi. “A sub-0.2 o /hr  bias drift micromechanical silicon gyroscope with automatic CMOS mode-matching”. Solid-State Circuits, IEEE Journal of   44(5), pp. 1593-1608. 2009. [4] S. E. Alper and T. Akin. “Symmetrical and decoupled nickel microgyroscope on insulating substrate”. Sensors and Actuators A: Physical   115(2), pp. 336-350. 2004. [5] M. Zaman, A. Sharma and F. Ayazi. “High  performance matched-mode tuning fork gyroscope”.  Micro Electro Mechanical Systems, 2006 IEEE 19th  International Conference on , vol., no., pp.66-69, 2006. [6] A. K. S. A. Aziz, A. H. Sharaf, M. Y. Serry and S. S. Sedky. “MEMS mass-spring-damper systems using an out-of-plane suspension scheme”, US patent application publication 2011/0030472A1 February 10, 2011 2010. [7] A. M. Elshurafa, K. Khirallah, H. H. Tawfik, A. Emira, A. K. S. Abdel Aziz and S. M. Sedky. “Nonlinear dynamics of spring softening and hardening in folded-MEMS comb drive resonators”.  Microelectromechanical Systems,  Journal of   (99), pp. 1-16. 2011. 959
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