Low-power MEMS wakeup system
There is provided a near-zero-power wakeup system in which a MEMS sensor for mechanical or acoustic signals is coupled to a very-low-power complementary metal oxide semiconductor (CMOS) application-specific integrated circuit (ASIC). Power consumption can be minimized by operating the ASIC with sub-threshold gate voltages.
1. Apparatus comprising:
a resonant MEMS device; and
an integrated circuit electrically connected to the resonant MEMS device;
wherein:
the resonant MEMS device comprises a mass element suspended solely by flexures from a substrate;
each of the flexures comprises a piezoelectric layer;
the flexures are electrically connected to the integrated circuit such that in use, mechanically induced voltages in the flexures are coupled as input to the integrated circuit; and
the integrated circuit comprises a comparator-latch circuit that produces a latched output voltage in response to an input signal that exceeds a threshold.
2. The apparatus of claim 1 , wherein each of the tethers comprises a piezoelectric aluminum nitride layer overlying a layer of structural material.
3. The apparatus of claim 1 , wherein each of the tethers comprises a piezoelectric aluminum nitride layer overlying a structural layer of single-crystalline silicon.
4. The apparatus of claim 1 , wherein:
the resonant MEMS device is conformed as a microphone in which the mass element is a diaphragm suspended by the flexures over a back cavity;
the flexures are conformed to permit out-of-plane vibration of the suspended diaphragm; and
the suspended diaphragm has a resonant frequency of vibration that lies within an operating frequency range of the apparatus.
5. The apparatus of claim 4 , wherein the flexures are arranged circumferentially around the diaphragm, and wherein a vent is defined between each sequential pair of flexures.
6. The apparatus of claim 5 , wherein the diaphragm is mass-loaded to reduce the resonant frequency.
7. The apparatus of claim 6 , wherein the diaphragm is mass-loaded, at least in part, with additive material.
8. The apparatus of claim 1 , wherein:
the resonant MEMS device is conformed as an accelerometer in which the mass element is a suspended proof mass;
the flexures are conformed to permit in-plane vibration of the suspended proof mass; and
the suspended proof mass has a resonant frequency of vibration.
9. The apparatus of claim 8 , wherein each of the flexures is folded at least twice.
10. The apparatus of claim 1 , wherein the comparator-latch circuit comprises a comparator having an input, an output, and a positive feedback loop that latches the comparator by feeding the output back to the input.
11. The apparatus of claim 1 , wherein the comparator-latch circuit comprises a plurality of long-channel CMOS transistors.
12. The apparatus of claim 1 , wherein the resonant MEMS device is unpowered.
13. The apparatus of claim 1 , wherein the resonant MEMS device and the integrated circuit constitute one of a plurality of sensor units, each of the sensor units has a latched output, and the latched outputs of the sensor units are combined in a logic circuit.