Integrated digital force sensors and related methods of manufacture
In one embodiment, a ruggedized wafer level microelectromechanical (“MEMS”) force sensor includes a base and a cap. The MEMS force sensor includes a flexible membrane and a sensing element. The sensing element is electrically connected to integrated complementary metal-oxide-semiconductor (“CMOS”) circuitry provided on the same substrate as the sensing element. The CMOS circuitry can be configured to amplify, digitize, calibrate, store, and/or communicate force values through electrical terminals to external circuitry.
1. A microelectromechanical (“MEMS”) switch, comprising:
a sensor die configured to receive an applied force, wherein the sensor die comprises a top surface and a bottom surface opposite thereto,
a sensing element arranged on the bottom surface of the sensor die, wherein the sensing element is configured to convert a strain to an analog electrical signal that is proportional to the strain, and
digital circuitry arranged on the bottom surface of the sensor die, wherein the sensing element and the digital circuitry are arranged on the same surface of the sensor die, and wherein the digital circuitry is configured to:
convert the analog electrical signal to a digital signal, and
provide a digital output code based on a plurality of predetermined force thresholds.
2. The MEMS switch of claim 1 , wherein the digital circuitry is further configured to compare the digital signal to the predetermined force thresholds.
3. The MEMS switch of claim 1 , wherein the predetermined force thresholds are relative to a baseline.
4. The MEMS switch of claim 3 , wherein the digital circuitry is further configured to update the baseline at a predetermined frequency.
5. The MEMS switch of claim 4 , wherein the baseline is updated by comparing the digital signal to an auto-calibration threshold.
6. The MEMS switch of claim 1 , wherein the sensing element is formed by diffusion or implantation.
7. The MEMS switch of claim 1 , wherein the sensing element is formed by a polysilicon process from an integrated circuit process.
8. The MEMS switch of claim 1 , further comprising a plurality of electrical terminals arranged on the bottom surface of the sensor die, wherein the digital output code provided by the digital circuitry is routed to the electrical terminals.
9. The MEMS switch of claim 8 , wherein the electrical terminals comprise solder bumps or copper pillars.
10. The MEMS switch of claim 1 , further comprising a cap attached to the sensor die at a surface defined by an outer wall of the sensor die, wherein a sealed cavity is formed between the cap and the sensor die.
11. The MEMS switch of claim 10 , wherein the sensor die comprises a flexure formed therein, and wherein the flexure converts the applied force into the strain on the bottom surface of the sensor die.
12. The MEMS switch of claim 11 , wherein the flexure is formed in the sensor die by etching.
13. The MEMS switch of claim 10 , wherein a gap is arranged between the sensor die and the cap, and wherein the gap is configured to narrow with application of the applied force such that the flexure is unable to deform beyond its breaking point.
14. The MEMS switch of claim 1 , wherein the sensing element comprises a piezoresistive, piezoelectric, or capacitive transducer.