FIELD OF THE INVENTION

The present disclosure relates to microelectromechanical (“MEMS”) force sensing dies, MEMS switches, and related methods of manufacture. The MEMS force sensing dies and/or MEMS switches can be used for converting force into a digital output code.

BACKGROUND

Current technology MEMS force dies are based on linking the applied force to a sensing diaphragm having strain gauges located on a surface of two or more stacked silicon or glass die. Wire bond pads are positioned around the sensing diaphragm and the resulting structure is packaged, which makes the device relatively large compared to more modern chip-scale packaged sensors and electronics. In addition, current MEMS force dies produce an analog output that must often be routed through an often noisy electrical environment before it is converted to a digital signal.

Current electromechanical switches consist primarily of conductive dome structures that deform to complete an electrical circuit. These switches are limited in their durability as the conductive material, often metal, wears over time. These switches are also incapable of being configured for multiple levels of actuation, which is becoming more desirable as software applications are growing in complexity and require more versatility from the user interfaces designed to control them.

Accordingly, there is a need in the pertinent art for a small, low-cost, digital force sensor.

SUMMARY

A MEMS force sensor including a plurality of sensing elements and digital circuitry positioned on a surface of the force sensor die is described herein. Each sensing element can include a flexure and a sensing element (e.g., piezoresistive strain gauge). In one implementation, four sensing elements can be employed, although additional or fewer sensing elements can also be used. The inclusion of MEMS in a standard complementary metal-oxide-semiconductor (“CMOS”) process allows the sensor to convert its analog output into digital codes and transmit them without loss of signal integrity due to electrical noise.

The MEMS force sensors described herein can be manufactured by bonding a cap wafer to a base wafer (e.g., a force sensor die) that has both the sensing element(s) (e.g., piezoresistive strain gauge(s)) and CMOS power, processing, and communication circuitry. Sensing elements can be formed by etching flexures on the top side of the base wafer. The bond between the base and cap wafers can include a gap produced by protrusions sculptured either on the top of the base wafer and/or on the bottom of the cap wafer. The gap can be designed to limit the displacement of the cap wafer in order to provide force overload protection for the MEMS force sensors. The protrusions and outer wall of the base wafer deflect with applied force, straining the sensing element(s) and producing an analog output signal. The analog output signal can be digitized and stored in on-chip registers of the CMOS circuitry until requested by a host device.

A wafer level MEMS mechanical switch including a base and a cap is also described herein. The mechanical switch employs at least one sensing element. The at least one sensing element is electrically connected to integrated CMOS circuitry on the same substrate. The CMOS circuitry can amplify, digitize, and calibrate force values, which are compared to programmable force thresholds to modulate digital outputs.

A MEMS switch including a plurality of sensing elements positioned on the surface of the switch die is also described herein. In one implementation, four sensing elements can be employed, although additional or fewer sensing elements may also be used. The sensing elements can have their analog outputs digitized and compared against multiple programmed force levels, outputting a digital code to indicate the current state of the switch.

The MEMS switch can be made compact as to only require a small number of input/output (“I/O”) terminals. The outputs of the device can be configured to indicate 2Ninput force levels, where N is the number of output terminals, which can be programmed by the user. In addition, the device's response can optionally be filtered such that only dynamic forces are measured. The resulting device is a fully-configurable, multi-level, dynamic digital switch.

An example MEMS force sensor is described herein. The MEMS force sensor can include a sensor die configured to receive an applied force. The sensor die has a top surface and a bottom surface opposite thereto. The MEMS force sensor can also include a sensing element and digital circuitry arranged on the bottom surface of the sensor die. The sensing element can be configured to convert a strain on the bottom surface of the sensor die to an analog electrical signal that is proportional to the strain. Additionally, the digital circuitry can be configured to convert the analog electrical signal to a digital electrical output signal.

Additionally, the sensing element can be a piezoresistive, piezoelectric, or capacitive transducer.

Alternatively or additionally, the MEMS force sensor can further include a plurality of electrical terminals arranged on the bottom surface of the sensor die. The digital electrical output signal produced by the digital circuitry can be routed to the electrical terminals. For example, the electrical terminals can be solder bumps or copper pillars.

Alternatively or additionally, the MEMS force sensor can further include a cap attached to the sensor die. The cap can be bonded to the sensor die at a surface defined by an outer wall of the sensor die. In addition, a sealed cavity can be formed between the cap and the sensor die.

Alternatively or additionally, the sensor die can include a flexure formed therein. The flexure can convert the applied force into the strain on the bottom surface of the sensor die. Optionally, the flexure can be formed in the sensor die by etching. The sensing element is arranged on the flexure.

Alternatively or additionally, a gap can be arranged between the sensor die and the cap. The gap can be configured to narrow with application of the applied force such that the flexure is unable to deform beyond its breaking point.

Alternatively or additionally, the digital circuitry can be further configured to provide a digital output code based on a plurality of predetermined force thresholds.

An example method for manufacturing a MEMS force sensor is described herein. The method can include forming at least one sensing element on a surface of a force sensor die, and forming complementary metal-oxide-semiconductor (“CMOS”) circuitry on the surface of the force sensor die. The at least one sensing element can be configured with a characteristic that is compatible with a downstream CMOS process.

Alternatively or additionally, the at least one sensing element can be formed before forming the CMOS circuitry.

Alternatively or additionally, the characteristic can be a thermal anneal profile of the at least one sensing element.

Alternatively or additionally, the method can further include etching an opposite surface of the force sensor die to form an overload gap, etching the opposite surface of the force sensor die to form a trench, and bonding of a cap wafer to the opposite surface of the force sensor die to seal a cavity between the cap wafer and the force sensor die. The cavity can be defined by the trench.

Alternatively or additionally, the method can further include forming of a plurality of electrical terminals on the opposite surface of the force sensor die.

Alternatively or additionally, the force sensor die can be made of p-type or n-type silicon.

Alternatively or additionally, the at least one sensing element can be formed using an implant or deposition process.

Alternatively or additionally, the CMOS circuitry can be configured to amplify and digitize an analog electrical output signal produced by the at least one sensing element.

Alternatively or additionally, the trench can be configured to increase strain on the at least one sensing element when a force is applied to the MEMS force sensor.

Alternatively or additionally, a depth of the overload gap can be configured to provide overload protection for the MEMS force sensor.

Alternatively or additionally, the electrical terminals can be solder bumps or copper pillars.

An example MEMS switch is also described herein. The MEMS switch can include a sensor die configured to receive an applied force. The sensor die has a top surface and a bottom surface opposite thereto. The MEMS switch can also include a sensing element and digital circuitry arranged on the bottom surface of the sensor die. The sensing element can be configured to convert a strain on the bottom surface of the sensor die to an analog electrical signal that is proportional to the strain. Additionally, the digital circuitry can be 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.

Alternatively or additionally, the digital circuitry can be further configured to compare the digital signal to the predetermined force thresholds. Optionally, the predetermined force thresholds are relative to a baseline. Alternatively or additionally, the digital circuitry can be further configured to update the baseline at a predetermined frequency. For example, the baseline can be updated by comparing the digital signal to an auto-calibration threshold.

Other systems, methods, features and/or advantages will be or may become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features and/or advantages be included within this description and be protected by the accompanying claims.

