MOEMS Vibration Sensor for Advanced Low-frequency … · 838 W. Hortschitz et al. / Procedia...
Transcript of MOEMS Vibration Sensor for Advanced Low-frequency … · 838 W. Hortschitz et al. / Procedia...
Procedia Engineering 87 ( 2014 ) 835 – 838
Available online at www.sciencedirect.com
1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Peer-review under responsibility of the scientific committee of Eurosensors 2014doi: 10.1016/j.proeng.2014.11.282
ScienceDirect
EUROSENSORS 2014, the XXVIII edition of the conference series
MOEMS Vibration Sensor for Advanced Low-Frequency
Applications with pm Resolution
W. Hortschitza,∗, A. Kainzb, H. Steinera, M. Stiftera, F. Kohla, J. Schalkoa,b, T. Sautera,
F. Keplingerb
aCenter for Integrated Sensor Systems, Danube University Krems, Wiener Neustadt, AustriabInstitute of Sensor and Actuator Systems, Vienna University of Technology, Vienna, Austria
Abstract
The majority of MEMS vibration sensors requires relatively high resonance frequencies of several kilohertz toavoid mechanical contact of moving parts such as electrode plates. In contrast to that, our hybrid micro-opto-electro-mechanical system (MOEMS) enables sensor applications at low frequencies. This work describes a distinct MOEMSfeaturing extremely low resonance frequencies of below 200 Hz. It is operated ambient air without closed loop feed-back, extensive electronics and cooling. Due to the soft suspension of the micro-mechanical sub-system the funda-mental limit, the Brownian noise floor, is reached. The resulting noise equivalent displacement for frequencies above
the resonance is 1.9 pm/√
Hz, which is equivalent to 0.29 μg/√
Hz below the resonance. We describe the design spacefor sensors with further enhanced sensitivity and resonance frequencies far below 100 Hz.
© 2014 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific commitee of Eurosensors 2014.
Keywords: MOEMS, vibration, sensor, low frequency, high resolution
1. Introduction
Micro-mechanical vibration sensors or accelerometers were used for decades in automotive industry, to monitor
bearings in fabrication lines, and more recently, in consumer electronics like cameras for image stabilization or game
consoles [5]. Nowadays, capacitive MEMS sensors are applied in almost all of these cases. This implies, however,
relatively stiff systems and, therefore, high resonance frequencies to avoid damage of the sensor components in case
of a mechanical shock [1]. Furthermore, most of these sensors are operated in a closed loop mode to extend the
dynamic range at the expense of a high power consumption. The presented competitive sensor principle depicted
in Fig. 1 features laterally deflectable openings which modulate a perpendicularly introduced light flux. The light
flux is generated by an LED (Osram SFH 4680) and received by a photo-transistor (Osram SFH 3600). Therefore,
the seismic mass of the MEMS can be deflected in-plane almost freely without any risk of damage or destruction of
electrodes or readout elements.
∗Corresponding author. Tel.: +43 2622 2342 0; fax: +43 2622 23 42 0 99
E-mail address:[email protected]
© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Peer-review under responsibility of the scientific committee of Eurosensors 2014
836 W. Hortschitz et al. / Procedia Engineering 87 ( 2014 ) 835 – 838
Figure 1: Schematic cross-section of the sensor for a fully encapsulated
device. The emitted light flux is modulated by two micro-structured grat-
ings. The first is fabricated as chromium layer fixed to the MEMS frame
while the second one is suspended via springs inside device layer of an
SOI wafer. The output signal of the phototransistor is proportional to the
deflection of the movable mass.
Figure 2: Micrographs of the fabricated MEMS components of the
MOEMS sensor. The perforated seismic mass is ~2.5x2.5 mm² in
size and was fabricated into a 20 μm thick device layer of an SOI
wafer. The inset shows the fixed Cr layer on glass, bonded to the
SOI chip, forming a light modulation aperture.
2. Constraints for the sensitivity
The MOEMS incorporates two main subsystems, i. e. the mechanical and the opto-electrical one. Both subsys-
tems exhibit constraints regarding the resolution limits of the combined sensor system. The main limitation for the
mechanical subsystem is the Brownian motion noise of the seismic mass while the optoelectronics is constrained by
the dark current of the receiver element, e.g., the photo-transistor. For the presented sensor, the Brownian noise is the
dominating restriction for the sensitivity. Hence, it will be discusse subsequently in greater detail.
The microstructure depicted in Fig. 2 was fabricated into the 20 μm thick device layer of an SOI wafer. The
perforated, oscillating mass of 2.5x2.5 mm² is suspended by four folded beams. The low total stiffness of the structure
of c = 0.23 N/m results in a relatively large Brownian noise equivalent deflection NEDBr where the underlying
Brownian noise force exhibits a white noise characteristic. The transfer characteristic Hn between an input force
and the differential deflection xd can be approximated by Hn(ω � ω0) ≈ 1/c for frequencies far below the angular
resonance frequency ω0 of the MEMS. The noise equivalent deflection induced by the Brownian force is, therefore,
given by
NEDBr = Fn · Hn = Fn/c =√
4kBTd/c , (1)
where kB is the Boltzmann constant, T is the absolute temperature and d is the damping coefficient. Hence, the noise
equivalent deflection does not depend on the seismic mass m but on the damping coefficient d and the stiffness c,
for frequencies far below the angular resonance frequency. The damping can be minimized by reducing the ambient
pressure, requiring a hermetical package of the MEMS component. Such sealings are challenging to fabricate and
hence not considered in this paper. The work focuses on the impact of the stiffness instead. The sensor design allows
to increase the stiffness c in order to reduce the influence of the Brownian noise. This leads to a higher natural angular
frequency, which can be compensated by increasing the seismic mass.
837 W. Hortschitz et al. / Procedia Engineering 87 ( 2014 ) 835 – 838
Metal housing
with shaker unit
Lock-in for
circuit output
Lock-in for output
of laser-vibrometer
Shaker
ampli er
Waveform-
generator
Laser-vibrometer
sensing head
Laser-vibrometer-
controller and PC
(a) Overall measurement set-up for characterizing the MOEMS vibration
sensors. The laser-Doppler-vibrometer is part of a Polytec MSA-400 mi-
cro system analyzer. The sensor is mounted inside the metal housing in
the middle. A detailed view of the sensor mounting is depicted in Figure
(b). The shaker unit is excited via a waveform generator and an appropi-
ate piezo amplifier.
Micro system analyzer
laservibrometer
Excitation via
shaker unit
Sensor
mounting
Am
pli
er
Supply
Exci
tation v
ia
shaker
unit
MEMS
(b) Detailed vibration measurement set-up for transducer character-
ization. In the depicted set-up the optical transmission axis of the
transducer is oriented horizontally, whereas the vertically aligned
laser beam of the Doppler-vibrometer probes the motion of the foot-
plate through a hole in the sample holder.
Figure 3: Measurement set-up with detailed view on the sensor mounting.
3. Setup and Results
The devices were characterized with the set-up depicted in Fig. 3. The MEMS is operated in ambient air without
closed loop feedback, extensive electronics or cooling. The results were obtained by exciting the sensor with a custom
made piezo-electical shaker unit (Fig. 3b). Excitation amplitude and phase were determined with a laser Doppler-
vibrometer (Polytec MSA-400) while simultaneously recording the output signal of the sensor’s transimpedance am-
plifier with a Stanford Research Lock-In amplifier (SR830). The Lock-In amplifier was also used to estimate the
electrical noise floor of the opto-electronics while the LED was switched off. Further details of the measurement
set-up and the data evaluation can be found in [4] and [2]. The measurement results depicted in Fig. 4 were fitted
with an iterative least squares algorithm to gain the mechanical parameter of the sensor. The resolution was computed
with the mechanical parameters listed in Fig. 4 and Eq. 1 to be NEDBr = 1.9 pm/√
Hz. The dark current noise of the
phototransistor and the output noise of the transimpedance amplifier were determined and recalculated with the sensi-
tivity from Fig. 4 to be equivalent to NEDLED,off = 0.0146 pm/√
Hz. Hence, Brownian noise is currently limiting the
sensitivity of the sensor. This limitation can easily be circumvented for a constant resonance frequency by increasing
the stiffness via a thicker device layer [3].
4. Conclusion
The discussed sensor is operated in ambient air without closed loop feedback, extensive electronics, or cooling.
Due to the soft suspension of the seismic mass in the micro-mechanical sub-system of the sensor of 0.23 N/m, one
fundamental limit, the Brownian noise floor, is reached. The resulting noise equivalent displacement for frequencies
above the resonance frequency is 1.9 pm/√
Hz, which is equivalent to 0.29 μg/√
Hz below the resonance. A future
work will concentrate on further increasing the sensitivity by adjusting the mechanical parameters stiffness and mass
as described in this paper while simultaneously decreasing the resonance frequency to gain more bandwidth for
displacement sensing.
The presented extremely sensitive low frequency (MOEMS) vibration sensor with low resonance frequency paves
the way for micro-mechanical systems to be used in new fields of applications such as seismology or novel medical
applications.
838 W. Hortschitz et al. / Procedia Engineering 87 ( 2014 ) 835 – 838
Figure 4: Transfer characteristics of the MOEMS with a resonance frequency of 196 Hz; The excitation amplitude x̃f is 6 nm. The mechanical
parameters for stiffness, mass and damping which were fitted with a least squares fit are listed inside the plot. The phase of the sensor system is
ϕS = ϕOut − ϕf , the phase of the sensor foundation is ϕf while ϕOut is the phase of sensor output.
References
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[3] Hortschitz, W., Encke, J., Kohl, F., Sauter, T., Steiner, H., Stifter, M., Keplinger, F., oct. 2012. Optimized hybrid MOEMS sensors based on
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