Abstract. 1. Introduction. 2. Sensor design

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1 M. Haueis, J. Dua, C. Cavaoni *, M. Gnieka*, R. Buser ** - ETH Zürich, Institute of Mechanics, Tannenstrasse 3, CH-89 Zürich, Switzerand *Kister Instrumente AG, CH-848 Wintherthur, Switzerand **NTB Buchs, CH-947 Buchs, Switzerand Abstract We present a packaged microresonator for static oad measurement under high temperatures, performing with high precision and a resoution better than ppm. There is an industria need a for such measurement tasks, however, such sensing ces are not avaiabe so far. To minimize temperature stress we deveoped an a-in-siicon soution, in difference to micromachined resonant force sensors, which have been pubished. We propose a force sensor where oad couping, the excitation and detection of the vibration of the microresonator are integrated in one and the same singe crysta siicon package. The compete singe crysta design together with a fiber-optica on-chip detection method wi aow measurements at high temperatures. A considerabe degree of freedom for the resonator s shape design, as needed for the investigation of fiter mechanisms, is given by a DRIE fabrication method. Microresonator, Resonant sensor, oad ce, Fabry-Perot Interferometer, MEMS, DRIE. Introduction This paper describes design, fabrication and characterization of a force measuring unit with quasi digita output, which can be fabricated in batch processes such as used in microengineering's technoogy. The focus is a sensor part exhibiting a high sensitivity to appied stress with reiabe performance in a high temperature environment. Buk resonators and surface acoustic wave resonators are exampes for resonating systems with quasidigita output []. Their response to varying boundary conditions is measurabe through their change of resonant frequency and phaseshift. Much effort has been invested in the utiization of these features for stress, mass and temperature measurements as we as their use for fiters in communication devices [],[3],[4]. Strong industria interests are focused on resonant sensors for oad and pressure measurement [5], [6], [7]. Resonators based on singe crystas such as siicon and quartz are of particuar interest because singe crystas combine high accuracy and repeatabiity and potentiay ow power consumption [8]. Materias ike SiC, Diamond [9], Gaiumorthophosphate have advantages for appications under high temperatures but have on the contrary the major drawback that they ack of advanced technoogy such as for siicon is avaiabe at this time.. Sensor design The presented sensing ce consists of a SOI (siicon on insuator) hande wafer where the bending resonator, the excitation and the detection means are integrated and which is protected by a siicon id. The design of our resonant structure is based on the required sensitivity (frequency shift versus force), the ampitude at resonance and the requirements of the readout eectronics. For thermoeastic oss minimization the resonator was made to vibrate in a near isotherma mode. A inear eectrostatic force is generated by an integrated comb shaped eectrode, a principe which is compatibe to high temperature. Corresponding author: ETH Zurich, Institute of Mechanica Systems, Tannenstrasse 3, CLA H., te: fax: , e-mai: martin.haueis@imes.mavt.ethz.ch

2 appications. Two packaged chip designs were reaized: one, where the vibration is detected capacitivey (Type CAP: Figure ), and one where the vibration is measured opticay (Type OPT: Figure ). For capacitive detection the probem of cross tak was soved by operating in cosed oop with a switched ock-in ampifier [], which resuted in a stabe resonance behavior (Figure ). Aternativey, for optica detection a Fabry-Perot-Interferometer is formed by a mirror-cut of a singe mode fiber end face which is passivey aigned to a high quaity dry etched siicon side wa. In contrast to pressure sensors the crucia issue of the force sensor is the couping of the force. We soved this by guiding the oad from the meta housing via bots to the siicon chip (Figure ). On the siicon chip the force is transated from the bots through a spring mechanism to the resonant structure. No guing and a singe materia soution minimizes the temperature stress and makes the performance robust to high temperatures. 5HVRQDWRU *ODVVÃOLG *HWWHU ([FLWDWLRQ *OXH 5HVRQDWRU *ODVVÃOLG *HWWHU ([FLWDWLRQ 'HWHFWLRQ *URXQG *ODVVÃILEHU *URXQG +RXVLQJÃSDUW +RXVLQJÃSDUW 6, 6, Schematic cross section perpendicuar to the axis of a sensor with capacitive detection Schematic cross section perpendicuar to the axis a of sensor with optica detection Resonator a.) b.) c.) Fabricated resonant structures with capacitive detection: a.) inner side of housing part, b.) inner side of SOI with resonator in the center, c.) bonded package with eectrodes and camping hoes 3. Resonant structure The sensitivity to the measurand is a function of the resonator shape and the shunt force of the package. On the siicon chip the force is transated from the bots through a spring mechanism to the resonant structure in order to match the maximum stress specifications of the resonant structure and to protect the ce against overoad. The measurand force introduced to the sensing chip, in particuar the frame, is spit in a shunt force in the frame and a force oading the resonant structure itsef. The stress experienced by the resonant structure is determined by the frame it is camped on. The ce design presented has comparativey high shunt force so that the sampes are safe for processing and for mounting into the meta housing. We optimized a resonant beam-mass system in the foowing way. First, we considered the boundary conditions for geometry set by technoogy, especiay the aspect ratio of DRIE and the sensitivity to geometrica toerances. Second, the resonator ayout is determined by the desired Quaity factor (Q-factor). A high Q-factor improves the frequency stabiity for a phase-ocked osciator oop, the ampitude and therefore the output signa of the detection circuit. Provided the resonator operates in a vacuum, where moecuar damping is eiminated, the oss is governed by thermoeastic friction [],[]. Depending on the mode of operation, isotherma or adiabatic oss characteristics are distinguished. Two different beam dimensions satisfy the

3 3 required Q factor in these oss regimes. To match the frequency of avaiabe eectronics and to be we under khz (caibration frequency for Quartz resonators) we designed a resonator operating in the isotherma mode at 5 khz. We chose for the beam thickness 5 micrometer, whie the height of the beam has to be in accordance to the technoogicay achievabe aspect ratio (DRIE: about 8). For a 5 micrometer comb drive distance the etchabe height becomes 8 micrometer. The mass dimensions and its perforation are chosen to match the required frequency, the oad sensitivity and to aow space for the necessary number of combs. The number of combs is cacuated from the desired defection which is necessary for our capacitive sensing eectronics (greater than nm in resonance). The stabe operation of a resonant sensor furthermore depends on its materia properties which shoud exhibit a inear eastic behavior, no creeping, ideay no temperature dependence of its eastic properties and no interna stress. We seected Siicon as a singe crysta materia with a wide range of processing technoogies avaiabe at this time. Load sensitivity The resonance frequency for a mass suspended by two beams (Figure 4) is governed by the genera expression for a beam with stiffness, mass inertia, rotationa inertia and shear effects ( Timoshenko beam ) [3]: 4 m I E m yxt [, ] + E I yxt [, ] x 4 - A + κ - yxt [, ] = G x t t, where m = mass per unit ength, E = Youngs moduus, I= moment of bending inertia, A = area, G = shear moduus, κ = 5 6 for a rectancuar cross section. Negecting higher order terms and the contribution of shear forces and rotationa inertia this equation is simpified to: 4 m yxt [, ] + E I yxt [, ] =. x 4 t The dependence of resonance frequency on appied oad is approximated by the Rayeigh-Ritz Method which for a singe beam was investigated in [5],[4] under consideration of different boundary conditions [5]. For the resonance frequency of a mass suspended by two beams vibrating in its first bending mode (Figure 5) we derived (see Appendix I): ω o 7 E I = - -, C, m A m C y = - y 98 3 where = ength of the suspending beams, m= mass of the suspended mass. The appication of a oad F generates an axia stress in the suspending beams and shifts the resonance frequency of the first bending mode from to ω: ω o.5 F. ω = ω o + - E I The sensitivity ratio between a beam having a suspended mass (denoted A ) and a simpe beam (denoted B ), both assumed to have identica resonance frequencies, becomes: r S = m B - m A 5 m m A A

4 4 where m B = mass per unit ength of singe beam B, m A = mass per unit ength of suspending beams A, A = ength of both suspending beams A. This expressions say, that for two beams with the same materia properties, momentum of inertia for bending, cross section and resonance frequency, a these are parameters which are given by technoogy and eectronics, the added mass decreases sensitivity as shorter the beams and as heavier the mass of the mesa become. Eectrodes Eectrodes Frame Resonator Eectrodes Resonant structure: perforated mass suspended on two beams First bending mode at 5.9 khz (Moda anaysis, FEM-IDEAS) 4. Fabrication The of the packaged resonant structure is done by micromachining processes (Figure 6). Chip Type CAP and chip Type OPT were reaized on one and the same wafer. Type CAP and Type OPT sensing ces consist each of three parts: a resonator with hande part (Figure 7), a housing part and a gass id. The resonator with hande is a SOI wafer with a micrometer oxide and 8 micrometer p-doped device ayer. On the device ayer the resonator, the eectrodes, the connectors from the eectrodes to the housing, the aignment springs for gass fibers (Type OPT) and the vacuum channes are integrated. A structures are etched by Deep Reactive Ion Etching (DRIE), which aowed the critica dimension, the minimum comb spacing, to be 5 µm. That is required for keeping the excitation drive votage ow and for maximizing the capacity change during vibration. The siicon sidewa mirror for optica detection in Type OPT was fabricated with an optimized DRIE process. A ow surface roughness was achieved by specia baancing of the microoading during DRIE through shieding structures in front of the beam at the intended mirror position. After competition of the SOI wafer etching the remova of the µm oxide ayer by 49% HF reeased the movabe parts. Sticking at the µm gap between the mass and the hande wafer is avoided through a subimation drying step after wet processing. The hande is structured by DRIE as we. The eectrodes for circuit connections and the housing ayout are determined in this step. The cover part has 5 nm Spin-on-Gass (SOG) on the side, which wi face the resonator after bonding [6]. The HF-etch roughened the siicon surface consideraby. SOG eves out this roughness and aows bonding. The HFetch can be avoided, if the movabe structures are underetched prior to the Siicon-Direct-Bonding of the SOI. With such a process the resonator and housing can be bonded with therma oxide instead of SOG in between. The SOG on the housing part was opened by a RIE-step with a subsequent DRIE step 5 micrometer deep, where spacer and trenches for optica fibers (Type OPT) are formed. The backside of the housing was aso DRIE through the wafer, given shape to the housing. The resonator part and the housing part were ceaned in Pirania, activated in an oxygen pasma, aigned, mated and underwent a h C step. The resuting bond connected the housing and the resonator part. The generation of the vacuum is possibe through a vacuum anodic-bonding step of a prestructured gass pate on the siicon housing. To guarantee good vacuum stabiity a getter can be paced in the vacuum cavity prior to the bonding step. In Type OPT, the optica sensor type, the surface roughness of the sidewa where the ight from the optica fiber is refected on has be sufficient ow for a ow noise Fabry- Perot Interferometer (FPI). The vaue of roughness improves if overetching in DRIE is minimized. But the optica fiber requires a wide trench, whie the eectrode combs trenches are ony 5 micrometer wide. To overcome this probem the reso-

5 5 nator sidewa which buids the FPI therefore was shieded by a wa which was removed after etching. The improvement of the sidewa roughness in shown in Figure 8. The sidewa roughness can furthermore be reduced by an oxidation foowed by oxide remova. The oxidation rate on convex surfaces is faster than on concave surface since the oxidation rate depends on oca stress. Loca stress is concentrated on convex and concave corners [7]. Finay, the chip is connected to the eectronics by wire bonding. The optica fiber is inserted in the fiber groove and gued. The process steps for the gass id, the housing part, the resonator on the hande (SOI) and their assemby mm Resonator mass Resonator spring Resonator mass Combs Detai Optica fiber aignment Eectrodes Resonant structure with eectrostatic excitation and optica detection

6 6 4 µm 4 µm Wa surface at a 5 mm trench Wa surface in a µm trench 5. Characterization A with a commercia reference force sensor in a vacuum chamber (Figure 3) was used for the investigation of the resonator stabiity, impact of the fabrication toerances and the repeatabiity. The pressure in the chamber is ess than. mbar, so that, according to our theoretica mode, the vibration is not further affected by moecuar damping. A oad sensitivity was as high as 4 Hz/N (Figure 4). The resonant structure vibrated with an ampitude of nm in resonance and a Q of 3. A stiffer package increases shunt force and therefore reduces sensitivity with the benefit of increased robustness when mounting the structures in a standard stee housing by Kister Instrumente AG. Currenty we work on an optimization of the shunt force to a minimum necessary for handing and packaging. The difference of the measured resonance frequency from anaytica and FEM simuations can be expained by considering the fabrication toerances in a Rayeigh-Ritz approach. The excitation by an inear eectrostatic force generated in a comb drive worked reiabe. For the detection we compared capacitive detection and optica detection. Capacitive detection used a switched ock-in ampifier patented at our Institute [] and introduced for MEMS by Cormann [8]. The signa from the siicon chip is ampified on eectronics in SMD technoogy, brought very cose to the chip, connected by wire bonding and mounted together in the housing by Kister Instrumente AG. The cross tak between the eectrodes and the resonator does not affect the measurements, because the excitation is switched off during detection (Figure ). In contrast to eectrica detection the setup for the optica detection is not imited to C. Light from a temperature stabiized aser diode (635 nm) passes a Faraday fiter and a fused spitter reaching the resonator. The fiber endface has a nm god ayer, so that refractive index of the fiber end matches the refractive index of the siicon [9]. The back refected ight is couped by the fused spitter to a photo detector. The resuts a shown in Figure. Excitation U Detection U Excitation Detection Capacitive detection in cosed oop operation with switched ock-in ampifier t Optica detection of resonance in cosed oop t

$testing with optica detection in opened vacuum chamber 5 UHVRQDQFHÃIUHTXHQF\ÃDVÃIXQFWLRQÃRIÃDSSOLHGÃIRUFHÃVDPSOHÃ y = 3.$ 8653x + 344 45 resonance frequency [Hz] 4 35 3 5 measured cacuated Linear (measured) 8 4 6 8 3 3 34 force [mn] Load-sensitivity: theory and experiment 6.

8653x + 344 45 resonance frequency [Hz] 4 35 3 5 measured cacuated Linear (measured) 8 4 6 8 3 3 34 force [mn] Load-sensitivity: theory and experiment 6.

7 7 Vacuum chamber oad ce Piezo actuator Siicon oad ce Reference force sensor Singe mode fiber Camping unit with siicon oad ce, piezo for oad appication and reference force sensor Test setup for testing with optica detection in opened vacuum chamber 5 UHVRQDQFHÃIUHTXHQF\ÃDVÃIXQFWLRQÃRIÃDSSOLHGÃIRUFHÃVDPSOHÃ y = x resonance frequency [Hz] measured cacuated Linear (measured) force [mn] Load-sensitivity: theory and experiment 6. Concusion In, an integrated siicon resonator used as oad ce for commercia appication and for microstructure characterization was deveoped. For a temperature up to C a switched ock in ampifier aows off chip charge ampifiers and therefore makes integrated eectronics not necessary. Aternativey, optica detection together with the quasi digita output of the

8 8 resonator aows safe signa transmission as required by investigations under high temperature and harsh environments. In our experiments we observed on some sampes a hysteresis-ike behavior. Possibe reasons are memory effects in the test setup itsef, especiay the meta string which connects the siicon ce with the reference force sensor. A stiffer setup wi improve that. An other cause we currenty ook at is the variation of the oading point during stress appication []. In this case the frame needs to be made insensitive to the contact point between bot and siicon frame. A possibe approach to this is expained in []. Acknowedgments The presented research has been made possibe by the Swiss Priority Program in Micro & Nano System Technoogy. We thank Dr. Gobrecht at PSI (Viigen, CH), Dr. Hunziker at ETH Zürich (Institute of Quantenphysics), Dr. Reiche at MPI (Hae, D), Mr. Cerc at University of Neuchate (CH) and Dr. Johnston at STS (Newport, UK) for their vauabe contributions. References [] Langdon R.M, Resonator sensors - a review, J. Phys. E. Sci. Instrumen.,Vo 8, 985, 3 [] Eernisse E.O., Ward R., Wiggins R. B., Survey of Quartz buk resonator sensors technoogies, IEEE Transactions on utrasonics, ferroeectronics and frequency contro, 35, no. 3, May 988, [3] Stemme G., Resonant siicon sensors, J. of Micromech. and Microeng.,, 99, 3-5 [4] Nguyen CT., Micromachining technoogies for miniaturized communication devices, Proceedings of SPIE The Internationa Society for Optica-Engineering. v 355, 998, SPIE, Beingham, WA, USA [5] Bom F.R., Resonant siicon beam force sensor, Ph.D. Thesis, University of Twente, 989 [6] Timans H., Micromechanica sensors using encapsuated buit-in resonant strain gauges, Ph.D. Thesis, University of Twente, 993 [7] Weham Ch., Greenwood J., Bertioi M., A high accuracy resonant pressure sensor by fusion bonding and trench etching, Sensors & Actuators 76, 999, [8] Buser R., Dissertation, University of Neuchate, 989 [9] Bates H., Göpe W., Hesse J., Sensors, Wiey-VCH, Weinheim, 999, 4-9 [] Goodbread, J., K. Häuser, M. B. Sayir and J. Dua: Verfahren und Vorrichtung zum Messen dercharakteristik eines Schwingungssystems. Schweiz. Patentanmedung 657/94-, 994 [] Roszhart T.V., The effect of thermoeastic interna friction on the Q of micromachined siicon resonators, Technica Digest, IEEE - Soid State Sensor and Actuator Workshop 9, Piscataway, USA, 99, 3-6 [] Zener C., Interna Friction in Soids, Phys. Rev., 5, 937, 3-35 [3] Harris C., Crede Ch., Shock and vibration handbook, McGraw-Hi Book Company, New York, 976, [4] Abert W. C., Vibrating crysta beams acceerometers, Proc. 8.th ISA Int. Instrument. Symp. Las Vegas, NV, USA, May 98, [5] Bouwstra S., Geiijseaers B., On the resonance frequencies of microbridges, Transducers-'9, Int. Conf. Soid State Sens. Actuators, Piscataway, 99, [6] Private communication with Dr. Reiche, Max-Panck-Institut Hae [7] Microithography, micromachining, and microfabrication, SPIE optica engineering press, London, 997, 34-5 [8] Cormann T., Enoksson P., Noren K., Stemme G., Nove burst technoogy for cosed oop detection and excitation of resonant siicon sensors, Transducers 99, Sendai, 999, CD-ROM [9] Mayer H., Physik dünner Schichten, Wissenschaftiche Veragsgeseschaft m.b.h., Stuttgart, 959 [] Debnam R.C., Jenkins R.F., The infuence of end oading conditions on the performance of strain gauge oad ces, nd internationa discussion meeting of the IMEKO sub-committee 'Measurement of force and weight', VDI-Verag, Dussedorf, West Germany; 97, 53-6 [] Robins G., Load ce shape optimization using genetic agorithms and finite eement anaysis, State of the art in force an mass measurement, Proceedings of the 4 th IMEKO TC3 Conference, Warsaw, Poand, 995, 67-7

9 9 Appendix I The space soution of is: y n C Sinh k n x C Cosh k n x C 3 Sin k n x C 4 Cos k n = x where k n = wave number of mode n, C, C, C 3, C 4 = constants. Fexura potentia energy E pm, Axia Force potentia energy E pf, Shear potentia energy E pv, Linear kinetic energy E k :: E pm E pv with ρ = density and rotationa energy : E I - ( y ) F = dx,, E pf = ( y ) dx where θ = moment of rotationa inertia. For the tota energy in the system we receive: E pf + E pm + E pv = E kr + E k 3( E I) (, 4 G A y ) ω A ρ = dx E k = - y dx + ω m - 4 y ( ) E kr ρ I ω E kr = ( y ) dx + ω θ 4 y ( ) In [4] was shown that ony for high ratios of beam thickness/ength and for high modes the shear and rotationa energy needs to be considered. If therefore shear potentia energy and rotationa energy are negected in the first approximation for the first bending mode and the resonant frequency is determined by: F ( y ) dx ω = ω o +, ω o m ω o A ρ y dx + - y ( ) = A ρ E I y dx ( y ) dx + m - y ( ) The integra expressions are: y ( ) C C 4 - Cosh[ ] + Cos[ ] = = C and y C 4 C y =.8 C 4 ( y ) dx 3. C 4 = -, ( y ) x, d 7 C 4 = - y dx =.54 C 4 3

10 The resonance frequency for a oaded beam - mass structure becomes:.5 F ω = ω o + -, ω E I o 7 - E I = - m A m C y 3 The sensitivity ratio r S between a singe beam and a mass suspended by beams is: The resonance freqency for a singe beam is: ( y ) dx ( y ) dx - r E I beama S = - - or simpified r S = - - E I E I ( y ) dx E I ( y ) beamb dx ω ob beama λ - = E I - B m B beamb where λ = mode number and B.= ength of beam B. Assuming for both, the beam with the mass in its center and the simpe camped beam same materia and momentum of bending inertia and that the mesa mass is much greater than the beam mass we receive for the simpe beam enght: m 4 B.63 λ A - 4 m = 7 m B A + 5 C y - m A Finay, the sensitivity ratio becomes for the first bending mode: A 3 4 r S = m B -. m A 5 m m A A

Technical Data for Profiles. Groove position, external dimensions and modular dimensions

Technical Data for Profiles. Groove position, external dimensions and modular dimensions Technica Data for Profies Extruded Profie Symbo A Mg Si 0.5 F 25 Materia number.206.72 Status: artificiay aged Mechanica vaues (appy ony in pressing direction) Tensie strength Rm min. 245 N/mm 2 Yied point