IEEE TRANSACTIONS ON MAGNETICS, VOL. 38, NO. 5, SEPTEMBER 2002
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MEMS Microbridge Vibration Monitoring Using Spin-Valve Sensors Haohua Li, João Gaspar, Paulo P. Freitas, Virginia Chu, and João P. Conde
Abstract—Spin-valve sensors were used to measure on chip, the vibration of microelectromechanical systems microbridges excited 6.5 ) by dc ac voltages. A spin-valve sensor (10 2 m2 , MR was placed 3 m away and 2.6 m below the central region of an a-Si:H /Al bridge, with a 1- m air gap. This sensor detects the fringe field created by a Co78 Pt22 micromagnet deposited on top of the bridge. The bridge movement was controlled by an applied voltage on the gate. An unforeseen capacitance coupling effect was found between the control gate and the spin-valve sensor, affecting all ac measurements. Once this coupling was isolated and taken into account, bridge oscillation at joint dc ac excitation voltages was monitored. Bridge vibration amplitudes of several tens of angstroms peak to peak were detected for dc and ac gate voltages up to 30 V.
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Index Terms—MEMS, micromagnets, spin-valve sensors.
I. INTRODUCTION
sensitivity of mV/Oe. They are thus well suited for this application, as has been shown before. In this experiment, a spin-valve sensor was placed 3 m away and 2.6 m below the central region of a microbridge, with a 1- m air gap. The bridge movement results from an electrostatic force controlled by an applied voltage between gate and bridge. Details about spin-valve sensor properties, as well as the calculated fringe field, and calibration of deflection have been reported in [3]. This paper is mainly concerned with bridge response to simultaneous ac dc gate voltages. As soon as an ac voltage is applied to the gate, a spurious capacitive coupling between gate and spin-valve sensor is found, irrespective of bridge deflection. This leads to erroneous bridge deflection values if not properly taken into account. Once this coupling is characterized and ac fields is isolated, bridge movement under combined dc characterized.
M
ICROELECTROMECHANICAL systems (MEMS) are being used for a variety of applications [1], [2]. In particular, microcantilevers can be used in biochip applications, either as detectors or as actuators. In conventional MEMS structures fabricated on Si, membrane/microbridge/cantilever motion is often monitored by capacitance measurements. In [3], the deflection of MEMS microbridges was determined with nanometer resolution using magnetoresistive (MR) spin-valve sensors, measuring the fringe field coming out of integrated micromagnets placed on top of the moving bridge structure. The choice of magnetoresistive sensor [4]–[7] was dictated by the fringe field created by the micromagnet at the sensor location. For Co Pt micromagnets (15 m 16 m 0.2 m dimensions defined by the bridge width) placed 2–3 m away from the MR sensor, a fringe field of 30–40 Oe is created. Unshielded spin-valve sensors with aspect ratios 5 1 (sensor width 10 m, sensor height 2 m) have a linear response to the transverse field, with a linear range of 40–50 Oe and a Manuscript received February 13, 2002; revised May 20, 2002. This work was supported by the PRAXIS P/CTM/10195/1998 and SAPIENS/34155/99 Projects. The work of H. Li was supported by the FCT under Grant PRAXIS/BD/19896/99. H. Li and P. P. Freitas are with the Instituto de Engenharia de Sistemas e Computadores (INESC MN), 1000 Lisbon, Portugal, and also with the Department of Physics, Instituto Superior Técnico (IST), 1096 Lisbon, Portugal (e-mail:
[email protected];
[email protected]). J. Gaspar is with the Instituto de Engenharia de Sistemas e Computadores (INESC MN), 1000 Lisbon, Portugal, and also with the Department of Materials Engineering, Instituto Superior Técnico (IST), 1096 Lisbon, Portugal (e-mail:
[email protected]). V. Chu is with the Instituto de Engenharia de Sistemas e Computadores (INESC MN), 1000 Lisbon, Portugal (e-mail:
[email protected]). J. P. Conde is with the Department of Materials Engineering, Instituto Superior Técnico (IST), 1096 Lisbon, Portugal (e-mail:
[email protected]). Digital Object Identifier 10.1109/TMAG.2002.802288.
II. EXPERIMENT Device fabrication started with the spin-valve sensor fabrication on 7059 Corning glass substrates. The spin-valve sensor was made out of a top-pinned spin-valve structure, Ta 20 /NiFe 30 /CoFe 20 /Cu 22 /CoFe 25 /MnIr 60 /Ta 30 prepared in an automated Nordiko 3000 ion-beam deposition system, a structure similar to that described in [3]. Spin-valve sensors with dimensions of 10 2 m were patterned by photolithography and ion-beam milling. Al contacts of 3000- thick were fabricated by liftoff. A 2600thick Al O layer was deposited after sensor fabrication to protect this sensing structure, followed by the Al gate electrode (3000 thick) deposition and patterning. 18 m wide MEMS microbridges (with length ranging from 30 to 160 m) were subsequently fabricated, consisting of a bilayer made of Al (top layer, 0.3 m thick) and a-Si:H (bottom layer, 0.6 m thick). Photoresist was used as sacrificial layer ( 1 m thick) between gate and bridge, and was wet etched away at the end of the process releasing the structure. A Co Pt film was deposited by radio-frequency magnetron sputtering from a mosaic target (Pt pieces on a Co target) [8] on top of the structure and patterned into micromagnets by photolithography and liftoff (magnet fabrication is done prior to bridge release). The magnet was then saturated in a direction transverse to the bridge length, creating a transverse fringe field in the sensor. Fig. 1(a) shows the schematic side view for this integrated device. All the processing steps for fabrication were performed 110 C . Fig. 1(b) shows critical dimenat low temperature sions near the gate and sensor, where the capacitive coupling is found. The separation between gate and sensor is 3 m
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 38, NO. 5, SEPTEMBER 2002
Fig. 1. (a) Schematic drawing of the integrated spin-valve sensor and MEMS bridge with micromagnet. (b) Schematic drawing of the gate, spin-valve sensor, and lead environment.
horizontally and 0.23 m vertically. The bridge is excited by dc and ac voltages. Two experiments were performed. In the first experiment, the gate was subjected to a variable dc voltage plus constant ac modulation at a frequency . In this case, the spin-valve output is measured by locking at the excitation frequency . The dc voltage produces a certain bridge deflection position, while the ac voltage is responsible for an oscillation around the dc voltage deflection point. The second experiment uses ac exfrequency. To citation only. The spin-valve output is read at identify the capacitive coupling between the gate and sensor circuit, a third set of measurements was performed before bridge release, thereby preventing bridge motion upon applying an ac voltage to the gate. In this case, any signal in the sensing circuit must come from capacitive coupling (current leakage across 3 m of oxide is negligible). III. RESULTS AND DISCUSSIONS With ac voltage applied on gate, a capacitance effect is formed between gate and spin-valve sensor leads through the insulating Al O layer (one side of the spin-valve sensor was kept at ground). Suppose the applied ac voltage is (1) Then, the coupling current should be (2) Thus, the induced voltage in the spin-valve sensor will be (3) where is an effective capacitance constant and is the sensor resistance. From (3), the measured coupling output should be and to the frequency. proportional to Fig. 2 shows the experimental results. For these measurements, the 1- m photoresist sacrificial layer used to define
Fig. 2. (a) Peak-to-peak coupling output versus gate ac voltage at two different 4 kHz and f = 40 kHz. (b) Peak-to-peak coupling output frequencies: f versus frequency at two different gate ac voltages: V = 3 V and V = 6 V.
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Fig. 3. Peak-to-peak sensor output and calculated bridge vibration versus dc voltage for two different structures measured at 1f frequency.
the air gap was not removed, so that the bridge cannot deflect upon applying voltages to the gate. Fig. 2(a) displays the peak-to-peak sensor output voltage versus ac gate voltage at two different frequencies (4 and 40 kHz). As can be seen in Fig. 2(a), sensor output increases linearly with ac voltage at fixed frequency, and increases linearly with frequency at fixed ac excitation [Fig. 2(b)]. These results are consistent with the capacitive coupling model described above. Several devices were measured, and this capacitive coupling is essentially similar for all devices, since gate and sensor/lead dimensions are kept constant. From here on, this coupling effect will be subtracted when determining bridge deflections. Fig. 3 shows measured sensor output (after capacitive coupling subtraction for measurements) and the calculated bridge vibration versus dc applied voltage for two different bridge strucm and m). This extures (with lengths periment was done by applying a variable dc voltage plus ac modulation on the gate and connecting the bridge to ground.
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LI et al.: MEMS MICROBRIDGE VIBRATION MONITORING USING SPIN-VALVE SENSORS
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Fig. 4 shows the sensor output and bridge vibration for pure ac excitation gate voltage. Three different bridge structures were analyzed (with bridge length and m). Since only ac excitation is used, the sensor output is detected at according to (5), avoiding the measurement of the coupling that is only present at . Signal output and, thus, bridge deflection always increases with ac excitation voltage with a quadratic dependence, as expected. The dependence of bridge deflection on bridge length is what is expected from a simple mechanical [9]), as shown in Fig. 4 model (deflection should vary as (see the inset plot). The noise level measured by the lock-in amplifier is 20–30 nV, which corresponds to 0.2 bridge deflection. Fig. 4. Sensor output and calculated bridge vibration versus ac voltage for three different structures measured at 2f frequency. The inset shows bridge vibration versus length at ac voltage 20 V.
Suppose the applied voltage is (4) Then, the bridge deflection
can be written as
(5) In (5), the first term corresponds to the dc deflection. The second and third terms are related to the ac excitation, with a component when dc is also applied, and a component that is presented even if no dc voltage is applied. For the structures shown in Fig. 3, a fixed ac voltage of 2.8 V is used and dc voltage ranges from 0 to around 10 V. Spin-valve sensor output frequency Hz and is measured by lockin at the peak-to-peak output data is plotted. Coupling level in this are constant), meaexperiment is constant (because and with 0 V. Similar experimental coupling sured also at values are obtained with 1.02 V for a bridge length of 60 m and 1.17 V for a bridge length of 100 m . The output shown in Fig. 3 refers to the data after subtracting the coupling level. As can be seen clearly, sensor output increases with the dc voltage. Bridge deflection amplitudes corresponding to this spin-valve sensor output are then calculated using [3] (6) and refer to the sensor output and bridge where mV/Oe is the sensor deflection, respectively. Oe m is the calculated rate sensitivity, and of change of the fringe field with vertical magnet motion, at the sensor site.
IV. CONCLUSION Magnetic spin-valve sensors and micromagnets have been successfully integrated with MEMS microbridges and used to monitor the bridge vibration. A capacitive coupling effect between the control gate and the sensor circuit was found for any measurements involving ac gate voltages. Bridge vibration was monitored for pure ac gate voltages, and for mixed dc ac excitation voltages. Peak-to-peak vibration amplitude of hundreds of angstroms was calculated from the sensor output. The bridge deflection was also monitored optically with an HeNe laser focused on top of bridge and reflected to a light detector. Magnetic and optical measurement results were compared and the same bridge response was obtained. Detailed comparison results are described elsewhere. REFERENCES [1] J. W. Judy and R. S. Muller, “Magnetically actuated addressable microstructures,” IEEE J. Microelectromech. Syst., vol. 6, pp. 249–256, Sept. 1997. [2] M. Boucinha, P. Brogueira, V. Chu, and J. P. Conde, “Amorphous silicon air-gap resonators on large-area substrates,” Appl. Phys. Lett., vol. 77, pp. 907–909, Aug. 2000. [3] H. Li, M. Boucinha, P. P. Freitas, J. Gaspar, V. Chu, and J. P. Conde, “Microelectromechanical system microbridge deflection monitoring using intergrated spin-valve sensors and micromagnets,” J. Appl. Phys., vol. 91, May 2002. [4] D. E. Heim, R. E. Fontana, C. Tsang, V. S. Speriosu, B. A. Gurney, and M. L. Williams, “Design and operation of spin-valve sensors,” IEEE, Trans. Magn., vol. 30, pp. 316–321, Mar. 1994. [5] J. K. Spong, V. S. Speriosu, R. E. Fontana, M. M. Dovek, and T. L. Hylton, “Giant magnetoresistive spin-valve bridge sensor,” IEEE Trans. Magn., vol. 32, pp. 366–371, Mar. 1996. [6] J. Heremans, “Solid state magnetic field sensors and applications,” J. Phys. D: Appl. Phys., vol. 26, pp. 1149–1168, 1993. [7] P. P. Freitas, F. Silva, N. J. Oliveira, L. V. Melo, L. Costa, and N. Almeida, “Spin-valve sensors,” Sens. Actuators, vol. 81, pp. 2–8, Jan. 2000. [8] W. Ku, F. Silva, J. Bernardo, and P. P. Freitas, “Integrated GMR bridge sensors with transverse permanent magnet biasing,” J. Appl. Phys., vol. 87, pp. 5353–5355, May 2000. [9] J. Gaspar, M. Boucinha, V. Chu, and J. P. Conde, “Electromechanical properties of amorphous and microcrystalline silicon micromachined structures,” in Proc. Materials Research Society Symp., vol. 664, 2001, pp. A 26.4.1–6.
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