A No-Power MEMS Shock Sensor

Luke CurranoU.S. Army Research Laboratory

lcurrano@arl.army.milSeptember 12, 2005

Motivation

Shock monitoring is important for condition-based maintenance There are many MEMS accelerometers available, but all require

some constant operating power– Electrostatic accelerometers work by monitoring capacitance between a

fixed electrode and a spring-mounted electrode » Some circuitry is required to monitor capacitance changes and convert them

into voltages

– Piezoelectric accelerometers produce a charge as a result of acceleration-induced deformation

» No power needed to monitor deflections, but conditioning circuits which consume power are required to use the sensor output

Eliminating constant power consumption by MEMS accelerometer could increase battery lifetime significantly– 8μA constant current draw (100% duty cycle) at ~3V, for 24μW

continuous power draw Health monitoring of long-shelflife or long-lifetime systems without

changing batteries is needed– Some Army systems have 20-year shelflife combined with limited space

Requirements

Very low power/no power sensing of shock events Non-destructive (i.e. must be reusable/resetable) 3-axis sensing required, bidirectional (+/-) in each axis 5 levels desired over the range of 10g-150g

Designed and fabricated functional no-power MEMS shock sensors– Up to 7 acceleration threshold levels (one axis, bidirectional) per

1cm2 chip– Latching demonstrated between 25g and 150g

Designed and fabricated functional thermal reset actuators

Major Accomplishments

Shock Sensor Design

Mechanical latching threshhold sensor design approach Silicon MEMS fabrication process allows for very small devices

and very tight tolerances

Resettable latching no-power MEMS shock sensor Latch and release mechanism closeup.

Design Details Design set 4

– 4-spring design to make stiffer in z-axis– Narrowed springs to lower spring

constant– Added anti-stiction bumps to springs– Version with metal-coated latch to lower

resistivity in process– Pyrex cap wafer in process (this is main

impediment to getting test data)

Design Set 4 Shock Sensor (latched)

Platinum coated latch for lower resistivity

Shock Sensor Usage

Designed to be used as either:– Wakeup sensor

» Power supply connected to processor and other sensors through shock sensor

» Traditional high-resolution accelerometer used to record shock pulse after shock sensor wakes system up

» One or more trigger levels

– Mechanical memory» Shock event triggers device, device “remembers” event

» Interrogate sensor periodically or just before use (go / no go)

» The more trigger levels, the better the resolution

Either way power savings comes from having system off most of the time– Shock sensor itself does not draw any power except small

amount when interrogating/waking up system

Fabrication Process Flow for MEMS Shock Sensor

Starting material – SOI wafer with 20μm thick device layer, 2 μm oxide

1. Pattern and liftoff Cr/Au bondpads

2. Deep reactive ion etch device layer to define spring, mass, and latches

3. Isotropically etch the oxide layer to release the mass

Analytical Modeling Force balance:

Integrate equation 2, using the fact that: a dy = v dv:

Set v = 0 to find maximum travel:

For a given level of shock, two of three variables (mass spring constant and desired deflection to latch) picked by designer, the third is solved from (6)

Time to latch due to an impulse is determined by natural frequency of device:

Result: response time is dictated solely by latching distance given a threshold level

– Caveat: adding damping to the system allows for slowing the response time but not speeding it up

Response times of ARL designs 2.2ms or lower

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Experimental Results - Latch

Centrifuge test of devices designed to latch at static levels of 10G -75G Visually and electrically confirmed latching during centrifuge tests

– Factor of ~2 between designed trigger level and actual level– This is attributed to simplification in model – not including interaction of mass and

latch (friction and normal force both contribute to resist motion of mass once in contact)

– Complete nonlinear model is under development Shock table tests

– Large amount of out-of-plane vibration– Out-of-plane vibration caused devices to reset themselves– Cap chip needed – packaging process under development

Designed Trigger Level (G’s)

Centrifuge Actual Trigger Level (G’s)

10 25

25 57

50 95

75 142

Thermal Reset Actuator

• 15V, 125mA currently required to reset the devices• Pulse duration 10ms• Vacuum packaging or removing the substrate underneath the device will

decrease the power required by 75%• 20mA, 10.1V for 15μm deflection in air• 10mA, 5.1V for 15μm deflection at 250mT

Future Work

Packaging– Wafer-level encapsulation of devices is critical to produce chips

with 5 level, three axis shock sensors with a small enough form factor

– Also critical to produce any reasonable number of testable devices, since devices often fracture when cleaving wafer

– Lower voltage and power requirements of reset actuators through vacuum packaging

Modeling– Complete contact/friction model to more accurately predict

trigger level

Refine design for more robust, smaller sensors