M4A.006
A SILICON MICROSEISMOMETER FOR MARS
W.T. Pike1*, I.M.Standley2, and S. Calcutt3
1
Department of Electrical and Electronic Engineering, Imperial College London, UK
2
Kinemetrics Inc., Pasadena, USA
3
Atmospheric, Oceanographic and Planetary Physics, University of Oxford, UK
ABSTRACT
of the microseismometer from 0.05 to 25 Hz. The
technology of the two seismometers is quite different:
each axis of the VBB has a metal spring and proof mass
with a very-low-frequency resonant frequency which can
not be tested in its Mars configuration under terrestrial
gravity; in contrast the microseismometer uses a
monolithic silicon suspension and proof mass and the aim
of the design is to allow operation under both Earth and
Mars gravity.
The design, fabrication and testing of a
microseismometer for the InSight mission to Mars and
described. Particular challenges include accommodation
of Martian gravity while allowing testing on Earth and
surviving the 2000g shock profile of the mission while
maintaining a nano-g sensitivity.
KEYWORDS
Micromachined
planetary
accelerometer
suspension
shock
DESIGN
Each axis of the microseismometer detects motion
through the capacitive measurement of the displacement
of a suspended mass, with electromagnetic feedback used
to recenter the proof mass, the feedback signal providing
the acceleration output (1). The ultimate sensitivity of the
microseismometer is determined by a combination of the
noise of the displacement transducer and the
thermomechanical noise of the suspension. The transducer
noise is minimised by reducing the resonant frequency, fo,
of the suspension, while the thermomechanical noise is
also minimised by reducing fo, but in addition increasing
the proof mass, M, and reducing the dissipative losses in
the suspension to maximize Q, the quality factor. Taken
together these maximize the MTQ product (2) that is
inversely proportional to the thermomechanical noise,
where T = 1/fo.
With these considerations, a die size of 21 mm has
been selected, into which a suspension with a resonant
frequency of 10 Hz has been micromachined using
through-wafer deep reactive ion etching, producing a
proof mass of 0.4 g, the majority of the die area. Energy
dissipation is minimised by packaging between machined
glass dies, relieved aside from the displacement
transducer electrodes, to reduce gas damping.
A particular issue with a seismometer designed for
another planet is the different gravity offset for the
vertical sensing axis: Mars has 37% of Earth’s gravity and
so the proof mass will settle to a different position, which
can be simulated by tilting the microseismometer by 68°
to the vertical. However, this tilt also produces 2.5 times
Martian gravity perpendicular to the compliant direction
of the suspension, which can test the cross-axis
compliance of suspensions with low resonant frequencies.
To be able to test a seismometer designed for Mars on
Earth, the displacement transducer has to be able to
operate over large displacements while maintaining
sufficient gain, and the suspension has to have sufficient
cross-axis rigidity to maintain all operational tolerances
with large cross-axis gravitational fields.
In addition, the microseismometer has to withstand
the vibration, and particularly shock levels of delivery to
the surface of Mars, up to 2000 g, and operate at
Figure 1: Artist’s rendition of the InSight mission to Mars
lander and payload showing the SEIS package within
which the microseismometers are housed.
INTRODUCTION
The InSight mission to Mars, selected for launch in
2016, will deliver the first payload focused on the
investigation of the interior of another planet. InSight fills
a longstanding gap in the scientific exploration of the
solar system by performing, for the first time, an in-situ
investigation of the interior of a truly Earth-like planet.
InSight would provide unique and critical information
about the fundamental processes of terrestrial planet
formation and evolution. To achieve this the prime
payload will be a suite of seismic instrument, SEIS (see
fig. 1), incorporating a three-axis micromachined
seismometer capable of detecting the seismicity of Mars
at the nano-g level. The microseismometer will
accompany a larger instrument, know as the very-broadband (VBB) seismometer, with the two instruments
tasked to cover different seismic bandwidths, in the case
978-1-4673-5983-2/13/$31.00 ©2013 IEEE
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Transducers 2013, Barcelona, SPAIN, 16-20 June 2013
deployment angles of up to 15 degrees.
Three aspects of the microseismometer design have
been developed to accommodate the relative sag, and
allow the testing of the flight microseismometer on Earth
prior to launch. Firstly, the displacement transducer
produces a periodic output (200 µm period) over a large
displacement allowing the output to repeatedly cycle over
the full output range. Secondly, the suspensions are
etched with the beams taking on the shape expected under
reversed Martian gravity, and hence when deployed on
Mars (or tested tilted on Earth) the suspensions are
unbent. Thirdly, by incorporating a series of frames in the
suspension, off-axis rigidity can be considerably increased
(3): we have improved by a factor of two on our previous
approaches with the first spurious mode designed to be
twenty times the fundamental. This additional rigidity
gives a corresponding maximum cross-axis displacement
of 3µm under Mars testing (Table 1), which is sufficiently
small to avoid compromising the performance of the
microseismometer. With this new frame geometry, the
required performance should be demonstrable on Earth
prior to launch.
bonds are formed by reflow of solder balls on metal pads
ensuring lateral alignment as well as the controlled
spacing of the dies which sets the gap between the
electrodes of the transducer. A second reflow forms the
solder buffers that are used to protect the suspension from
shock. Finally, a glass backing die, with precision
machined cavities, was mounted on the back of the silicon
die to provide protection while providing a further path
for gas flow around the proof mass. Fig. 2 shows a
completed microseismometer die.
Table 1: Dynamics of the microseismometer suspension
showing the rejection ratio for the spurious modes. The
spurious mode is indicated in bold.
Mode
fo
f1
f2
f3
f4
f5
Frequency/Hz
10.2
210
220
202
240
460
Rejection
ratio
21
22
20
24
46
Figure 2: The microseismometer packaged die showing
the machined glass capping dies which carry the fixed
plates of the displacement transducer. The die dimensions
are 21 mm x 21 mm.
FABRICATION
The proof mass and suspension of the
microseismometer are fabricated from a single-crystal
silicon wafer with a thermally grown 300nm oxide layer.
A metal-dielectric-metal-multilayer structure is used to
provide the topology required for multiple-turn coils as
well as guard electrodes between the capacitance
transducer electrodes and the substrate. This multilayer
structure was fabricated by patterning first a sputtered
metal, then a dielectric and finally an electrodeposited
metal layer. Electrodeposition ensures that the coils have
sufficient cross-sectional area to carry the levitating
currents required to hold the proof mass at one of the
operating points. Connections to the transducer electrodes
and coils on the proof mass are made with
electrodeposited traces routed as pairs along each spring.
The through-wafer DRIE subsequently forms the
suspension as well as singulating the dies. The DRIE used
a halo mask with an etch width of 40 µm to optimize the
side-wall geometry of the flexures [4].
The glass capping dies which carry the fixed
electrodes of the displacement transducer are fabricated
by plated metal followed by precision machining to
produce both the relief for gas flow and singulation of the
dies. Solder bonding of the silicon and capping dies routes
all the electrical connections on to the glass die. The
TESTING AND RESULTS
Suspension dynamics
The rigidity of the suspensions was determined using
a scanning electron microscope as a vibrometer and also
with an optical microscope to measure the out-of-plane
sag. SEM was performed in a LEO VP 1400 (Leica
Electron Optics) operated at 30 kV. The dynamic
response of the suspension to an external impulse was
imaged in the SEM by aligning the fast-scan direction of
the beam raster to the compliant. The suspensions were
excited by either an impulse from the stage drive or an
external impulse on the microscope column. The scan
speed of the raster was then adjusted so that the time
period, or several time periods of the dynamics of interest
corresponded to the frame acquisition time. Images were
taken at UHV, which for the very high quality factors of
the suspensions ensured that a very-nearly-constantamplitude sinusoid was evident in the image. Fig. 3 shows
the fundamental and first spurious on-axis mode imaged
in the SEM, and confirms the suspension design approach
with excellent separation of the lowest spurious mode
from the fundamental and associated cross-axis rigidity.
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Figure 3: SEM imaging of the dynamics of the suspension
as the proof mass oscillates under the rastering electron
beam. The fundamental frequency is 10.2 Hz with the first
on-axis spurious mode at a multiple of 20 times the
fundamental.
Figure 4: Two pairs of solder bumpers used to armour the
microseismometer from impacts within the die resulting
from the 2000 g shock of landing. Inset is shown one
bumper after drop testing, with the deformation of the
solder evident as a flattened portion at the area of contact
with the opposing bumpers.
Shock Testing
Shock testing of the devices was carried out on a
drop-test rig with the magnitude of the shock pulse varied
by altering the base or adjusting the drop height. A highshock piezoelectric accelerometer connected to a charge
amplifier recorded the magnitude of each shock event
which ranged from 1000 to 6000g, of approximate halfsine duration of 200-300µs. A Photron FASTCAM SA-3
(monochrome) high speed camera was used together with
a high magnification lens and a 60W metal halide light
source to capture images of the suspensions and bumpers
before, during and after the moment of impact at 100µs
intervals to identify the failure modes of the suspensions,
with SEM used to examine the bumpers post impact.
Fig. 4 shows the solder bumper geometry and the
deformation resulting from a 2000g impact. Two pair of
bumpers have been fabricated facing each other for
impact on their protruding portions. The geometry of the
conduit was chosen to maximize the volume of the solder
available after reflow extruding from the silicon sidewalls
and hence maximizing the kinetic energy that might be
absorbed through plastic deformation.
With the bumpers, the suspension was able to survive
this shock level with the plastic deformation of the solder
absorbing the energy of the collision. Such plastic
deformation occurs when the the kinetic energy of the
collision is large enough, in which case kinetic energy
prior to impact is converted into plastic strain energy,
elastic strain energy and elastic stress waves, with plastic
strain energy being by far the dominant mode of energy
dissipation.
The overall deformation of the bumper is less than 5
µm from the drop test, so although the 2000-g shock is a
single event associated with backshell separation during
entry into the Martian atmosphere, the microseismometer
is able survive repeated shocks at a comparable level.
Sensitivity determination
The sensitivity of a single-axis microseismemeter
was tested in a tilted configuration that corresponds to
Martian gravity within the deployment uncertainty on for
the mission of ±15 degrees. The self noise of the
microseismometer was determined by coincidence testing
alongside an STS-2 conventional seismometer (fig. 5).
This enables the self noise of the microseismometer to be
calculated for levels below the ambient seismic noise of
the terrestrial test location down a lower limit set by the
self noise of the reference sensor. Such an approach is
required to ensure that the microseismometer has the
necessary sensitivity to determine the seismicity of Mars,
expected to be considerably less active than the Earth due
to predicted absence of plate tectonics and the known
absence of contributions to the seismic background from
the oceanic and atmospheric contributions seen on Earth.
First, two simultaneous traces were taken from the
STS-2 and the microseismometer and two axes of the
STS-2 mixed as a function of angle to give the highest
level of coherence between the two signals (fig. 5b). Such
an approach will be used during the InSight mission to
determine the relative angle between the two
seismometers, the VBB and the microseismometer. At
this angle the coherence between the STS-2 and the
microseismometer seismic signals as a function of
frequency is then used to allocate the incoherent
remainder of the difference as a measure of the
microseismometer noise. Such an approach will tend to
exaggerate the microseismometer noise, as a portion of
the difference will be due to the noise on the reference
seismometer, but in this case this contribution can be
neglected.
Fig. 5a shows the derived noise profile for the
microseismometer together with a model of the noise
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(a)
(b)
Figure 5 (a) Noise testing of the microseismometer to below the terrestrial ambient seismic level and (b) coherence
determination of relative angle between the tilted microseismometer and the reference seismometer
floor with a white noise of 4 ng/√(Hz) above a corner
frequency of 0.05 Hz.
micromachined
lateral
suspensions
using
intermediate frames,” J. Micromech. MicroEng. Vol.
17 pp 1680-1694, 2007.
[4] W. T. Pike, W. J. Karl, S. Kumar, S. Vijendran, T.
Semple, “Analysis of sidewall quality in throughwafer deep reactive-ion etching” Microelectron.Eng.
73-74 pp. 340-345, 2004
CONCLUSIONS
A microseismometer has been fabricated capable of
deployment on Mars while demonstrating performance on
Earth. Through-wafer etching incorporating solder
bumpers is capable of armouring low-frequency
suspensions to the high shock levels anticipated. Although
the microseismometer design has been developed to meet
the needs of a planetary mission, on Earth such an
approach should allow the installation of high
performance seismic stations under a range of
orientations, and allowing remote deployments of such
microseismometers
in
inaccessible
or
hostile
environments.
CONTACT
W. T. Pike, tel: +44-020-7594-6207;
[email protected]
ACKNOWLEDGEMENTS
We would like to thank Guangbin Dou and Anisha
Mukerjee for fabricating the suspensions, Aifric
Delahunty for solder-bumper formation and shock testing
and Huafeng Liu for finite element analysis of the
suspensions. This work has been supported by the UK
Space Agency under a grant from the Science and
Technology Facilities Council.
REFERENCES
[1] W.T. Pike, I. M. Standley, W.J. Karl, S. Kumar, T.
Semple, S. J. Vijendran and T. Hopf, “Design,
fabrication and testing of a micromachined
seismometer with nano-g resolution” in Digest Tech.
Papers Transducers‘08 Conference, Colorado, June
8-12, 2008, pp. 668-671.
[2] T. B. Gabrielson, “Mechanical–thermal noise in
micromachined acoustic and vibration sensors” IEEE
Trans Electron Dev 40 pp. 903–909, 1993
[3] W. T. Pike and S. Kumar, “Improved design of
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