Sensors and Actuators 82 Ž2000. 149–154 www.elsevier.nlrlocatersna
Laterally driven accelerometer fabricated in single crystalline silicon ) O. Ludtke , V. Biefeld, A. Buhrdorf, J. Binder ¨ Institut fur ¨ Mikrosensoren, -aktuatoren and -systeme (IMSAS), UniÕersity of Bremen, P.O. Box 330 440, Bremen D-28334, Germany Received 7 June 1999; received in revised form 25 October 1999; accepted 1 November 1999
Abstract The request for wireless sensor operations grows in medical and automotive applications. These sensors receive their energy and send their data by a telemetric unit. The wireless transferred energy restricts the power consumption of the sensor and signal processing to less than 3 mW. Therefore, the sensor has to be operated in open-loop. Furthermore, a main focus is directed to increase the sensitivity of the mechanical–electrical transducer. Considering both open-loop and sensitivity, the sensor has to be optimized by referring to the structure height. The way for realizing high structures, as described in this paper, is the micromachining of silicon wafers with a specified thickness. The superior mechanical properties of single crystalline silicon compared to electroplated metals or surface-micromachined devices confirm the use of silicon as sensor material. A laterally driven accelerometer is simulated, designed and fabricated comprising the technologies of deep reactive ion etching ŽDRIE. of silicon, silicon direct bonding ŽSDB. and chemical mechanical polishing ŽCMP.. Characterization results confirm the performance of this new technology. The open-loop sensor, which was characterized, had a height of 50 mm with damping constant greater than 0.1. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Capacitive accelerometer; Resonant sensors; DRIE; Bulk micromachining; High aspect ratio
1. Introduction In the field of wireless applications, there is a demand for acceleration measurement. In detail, these applications are settled in the biomedical in vivo monitoring of fracture zones or in automotive applications like acceleration measurement in the rotational part of a wheel bearing w1x, respectively. However, due to the telemetric unit, these applications require very low power consumption less than 3 mW, but a very high resolution on the other hand. Usual accelerometers fabricated in surface micromachining use closed-loop circuits for stabilization with a power consumption higher than 10 mW w2,3x. The mentioned surface micromachining technology realizes structure heights of a few microns. Hence, the open-loop transfer function of the mechanical structure shows a peak at the resonance frequency due to the low damping generated between the structure and the ambient air w4x. Another important reason to use the feedback-loop is the requirement for evacuated operation, as the noise of the mechanical structure in atmospheric pressure restricts the resolution caused by the small seismic mass w5x. )
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Considering these facts, the damping is able to be improved by increasing the structure height using viscous air damping. The high structure results in a seismic mass, which compensates the noise-generating effect of a high damping constant w5x. A device with a resolution of 0.5 pFrmm is developed, allowing the electronic read-out circuit to be not highly sophisticated in the resolution but, therefore, optimized in its power consumption. The behavior of the mechanical structure is derived from Eq. Ž1. with the displacement x, the resonance frequency v 0 , damping constant d and acceleration a. For a successful open-loop operation without external linearisation, the damping constant should be at least critical Ž d ) 0.7.: x¨ Ž t . q 2 d v 0 x˙ Ž t . q v 02 x Ž t . s a Ž t . .
Ž 1.
Analytical expressions for viscous squeeze-film damping are illustrated in Ref. w4x. Using these results, the expected damping constant d from Eq. Ž1. for structure heights up to 100 mm is shown in Fig. 1. The calculations are based on the principle design introduced in Section 2, is shown in Fig. 1. The maximum deflection is assumed to 0.4 mm. With structures higher than 40 mm, it is possible to achieve
0924-4247r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 3 2 8 - 3
O. Ludtke et al.r Sensors and Actuators 82 (2000) 149–154 ¨
150
Fig. 1. Effect of squeeze-film damping.
critical damping with a nonlinearity in the capacitance change less than 1%. In addition to the improved sensitivity, this low power consumption sensor enables new applications in the abovementioned fields.
2. Design and simulation Eq. Ž1. describes the dynamic behavior of a spring-mass system. The influence and variation of the damping constant have been explained in Section 1. In this section, the sensor design and the suspension of the sensor are presented. The displacement can be adjusted due to an appropriate design of the suspension. In this application, the mechanical–electrical interface is realized by a comb-finger arrangement. The following analysis will be concentrated on the design of the mechanical spring in the form of a folded-flexure suspension. The investigated mechanical structure is shown in Fig. 2. The spring constant k x for this suspension, on the assumption of a rigid truss element and negligible stress due to the deflection, is given by w6x: kxs
24 EI L3
,
Fig. 2. Design of the one-dimensional acceleration sensor.
well-known linear behavior of the folded-flexure suspension w6x and in fact, the small deflection. The non-linearity of the simulated deflection is less than 0.1% in the analyzed deflection range. Fig. 4 depicts the mentioned results of analytic and FEM simulation. As mentioned above, the displacement of the proof mass is used to obtain an electrical signal due to a capacitive change of an interdigital finger arrangement. The accelerometer shown in Fig. 2 is using the transversal deflection. In that way, a high sensitivity with respect to the displacement is achieved:
´A
dC s ds
s2
,
where A describes the area of the plate capacitor arrangement, ´ the permeability and s the gap between the plates. In contrast to the longitudinal principle, the sensitivity depends additionally on the variable plate distance. Off-axis
Ž 2.
where E is the Young’s modulus, I the moment of inertia and L the length of the beams. Finite element method ŽFEM. simulations with ANSYS, which takes stress due to the deflection into consideration, have been performed. A solution of the analyzed FEM model for a static acceleration is shown in Fig. 3. The deviation from the analytic expression in Eq. Ž2. and FEM simulation is less than 6%, justified on the
Ž 3.
Fig. 3. Contour plot of the deflected acceleration sensor.
O. Ludtke et al.r Sensors and Actuators 82 (2000) 149–154 ¨
151
Fig. 4. Comparison of analytical and simulated deflection.
acceleration causes equal changes in both capacitance Csq and Csy ŽFig. 2. and results in a common-mode signal, which can be rejected by an electronic circuitry w2x. Furthermore, the system benefits from the high structure height, regarding off-axis accelerations.
function of the structured, upper poly-silicon layer is to define a gap between the buried conducting lines and the
3. Fabrication Recently, more focus has been put on the use of silicon as bulk material in MEMS. The well-defined mechanical properties of single crystalline silicon are superior to, e.g., electroplated metals or alloys. The lack of intrinsic stress, the resistance against aging, as well as the high Young’s modulus make it a desirable material for sensor applications. The electric performance can be adjusted easily in a wide range. Fig. 5 shows the process flow of the proposed accelerometer in hybrid version. Starting point is the fabrication of the carrier substrate with buried conducting lines. It consists of a doped low pressure chemical vapour deposition ŽLPCVD. poly-silicon layer on top of a thermal oxidized wafer. This layer is patterned in order to define the buried conducting lines ŽFig. 5a.. Then a low temperature LPCVD oxide ŽLTO. is deposited and structured ŽFig. 5b. for generating a defined electrical feedthrough to a second LPCVD poly-silicon layer, which serves as bond surface for the subsequent steps ŽFig. 5c.. On account of the high surface roughness of LPCVD deposited layers and the extreme requirements on surfaces for silicon direct bonding ŽSDB. w7x, a chemical mechanical polishing ŽCMP. process has been developed to decrease the surface roughness of the poly-silicon layer. After CMP, the surface quality allows SDB of a silicon wafer onto the poly-silicon layer ŽFig. 5d.. The following structuring of the second poly-silicon layer realizes the contact from the bond areas of the mechanical element to the outer bond pad area ŽFig. 5d.. Another
Fig. 5. Principle of the process flow.
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O. Ludtke et al.r Sensors and Actuators 82 (2000) 149–154 ¨
later-on-applied single crystalline silicon structures to assure movement Žsee also Fig. 6.. In the next process sequence, a highly-doped, doublesided polished silicon wafer Žsystem wafer. is bonded onto the carrier wafer by SDB. Bond strengths in the magnitude of the intrinsic strength of silicon can be achieved w8x. In order to receive ohmic behavior of the bond w9x, the SDB is carried out under vacuum with hydrophobic surfaces, which is confirmed by measurements on test structures. A second benefit of the vacuum bonding of hydrophobic surfaces is the reduction of a resulting waferbow and stress, respectively. First experiments for the SDB were carried out with hydrophilic surfaces under atmosphere. With these bond processes, waferbows in the order of y50 to q80 mm were achieved, depending on contact force and contact direction, respectively. After changing the bond process to vacuum bonding with hydrophobic surfaces, the resulting waferbow after SDB was reduced to approximately y3 to y10 mm. Fig. 7 shows an infrared transmission photograph of a bonded wafer pair. The bright areas indicate the removed poly-silicon layer, realizing the gap between the upper silicon wafer and the polished, structured poly-silicon layer. Fig. 6 shows the bond interface and the resulting gap of approximately 1.1 mm. The shown interface serves as anchor for a single finger of the comb structure. In the next step, the system wafer is thinned down to a desired system thickness of 100 mm or less ŽFig. 5e.. This can be accomplished either by wet chemical etching or mechanical wafer polishing. The bond pads on the outer connection area are metalized by an aluminum layer. Afterwards, the free-standing mechanical structures are etched through the system wafer
Fig. 7. The IR transmission photograph of wafer bonded onto structured poly-silicon.
by a deep reactive ion etching ŽDRIE. process ŽFig. 5f.. For that step, a plasma-enhanced vapour deposition ŽPECVD. oxide is deposited and patterned on the system wafer, which serves as masking layer for the DRIE process. The DRIE trench process is carried out with an inductively coupled plasma ŽICP. source manufactured by Surface Technology Systems ŽSTS., set up with the licensed Advanced Silicon Etch ŽASE.. For this application, an aspect ratio of 25:1 with a silicon etch rate of 1.5 mmrmin for a feature size of 4 mm is achieved. The selectivity to PECVD oxide is approximately 130:1. The sidewall angles are 89.68. The typical RIE-lag of dry etch processes is minimized by defining design rules based on initial experiments. Finally, the wafer package is sealed by a structured cover wafer in order to protect the mechanical element ŽFig. 5g.. This can be carried out by standard packaging technologies like SDB, anodic bonding, adhering, etc. The hermetic sealed device can be fabricated by the use of seven mask layers. The combination of CMP, SDB and ASE enables the realization of movable structures with an aspect ratio of 25:1.
4. Characterization
Fig. 6. The SEM photograph of a bonded anchor.
After successfully fabricating prototypes with the explained process flow, first results of the devices are presented below. The characterization is carried out by an optical method, which uses one at the IMSAS-developed image processing unit to measure deflections w10x. For that purpose, the structure is excited by using the sense capacitors acting as a complimentary drive. The measurement is
O. Ludtke et al.r Sensors and Actuators 82 (2000) 149–154 ¨
accomplished in ambient atmosphere. For the evaluation, the measurement tool takes 24 frames per oscillation and frequency. In our case, a sweep is carried out with steps of 150 Hz from 1000 to 4000 Hz. Results of the characterized accelerometer, whose design is described in Section 2, are shown in Fig. 8. The calculated transfer function of the accelerometer derived from Eq. Ž1. meets the measured values and is described by Eq. Ž4.: 1
