KR20130112804A - Self test of mems accelerometer with asics integrated capacitors - Google Patents

Abstract

PURPOSE: A self test of a micro-electro-mechanical systems (MEMS) accelerometer with application specific integrated circuit (ASIC) integrated capacitors is provided to simplify the design of an MEMS sensor. CONSTITUTION: An apparatus comprises an MEMS sensor (105) and an integrated circuit (IC) (110). The MEMS sensor includes first and second capacitive elements. The IC includes a switch network circuit (120) and a capacitance measurement circuit (125). The switch network circuit electrically decouples the first capacitive element of the MEMS sensor from the first input unit of the IC and electrically couples the second capacitive element to the second input unit of the IC. The capacitance measurement circuit measures the capacitance of the second capacitive element of the MEMS sensor duration application of a first electrical signal to the decoupled first capacitive element. [Reference numerals] (120) Switch network circuit; (130) Test circuit

Description

SELF TEST OF MEMS ACCELEROMETER WITH ASICS INTEGRATED CAPACITORS}

FIELD OF THE INVENTION The present invention relates to self test of micro electromechanical system accelerometers with ASIC integrated capacitors.

Micro-electromechanical systems (MEMS) include micromechanical devices that perform electronic and mechanical functions fabricated using photolithography techniques similar to those used to fabricate integrated circuits. Some MEMS devices are sensors that can detect motion, such as accelerometers, or angular speeds, such as gyroscopes. An accelerometer is a device that receives a measurable change in response to an accelerometer operating on the device. MEMS accelerometers may include piezoelectric, piezo-resistive, capacitive accelerometers. Capacitive accelerometers undergo a change in capacitance in response to acceleration. Fabrication of MEMS accelerometers involves testing steps that must quickly detect any defects in the fabricated device.

This document describes other components, devices, systems, and methods for testing MEMS sensors.

An embodiment device comprises a microelectromechanical system (MEMS) sensor comprising a first capacitive element and a second capacitive element, and an integrated circuit (IC). The IC includes a switch network circuit and a capacitance measurement circuit. The switch network circuit is configured to electrically disconnect the first capacitive element of the MEMS sensor from the first input of the IC and electrically connect the second capacitive element to the second input of the IC. The capacitance measurement circuit can be configured to measure the capacitance of the second capacitive element of the MEMS sensor while applying the first electrical signal to the disconnected first capacitive element.

It is herein intended to provide an overview of the subject matter of the patent application and is not intended to provide a limiting description of the invention. The detailed description is described with the intention of providing further information about the present patent application.

The number of circuit components and circuit contacts required for the MEMS sensor is reduced, so that the design of the MEMS sensor can be simplified.

In the drawings, scales are not necessarily shown, and the like and the like may depict similar parts at various times. The drawings generally illustrate the various embodiments described herein by way of example, but are not limited thereto.
1 shows a block diagram of a portion of one embodiment of a MEMS sensor and integrated circuit.
2 is a flowchart of one embodiment of a method of implementing a self test of a MEMS sensor.
3 shows one embodiment of a circuit for testing a MEMS sensor.
4 shows another embodiment of a circuit for testing a MEMS sensor.
5 illustrates one embodiment of a specific test circuit.

1 shows a block diagram of a portion of one embodiment of an integrated circuit 110 (IC) and MEMS sensor 105 for monitoring changes in sensor output. MEMS sensor 105 may be a capacitive accelerometer in which the IC monitors a change in capacitance of the sensor in response to an acceleration affecting the sensor.

Typical MEMS capacitive accelerometers include a proof mass of mobility with capacitive elements attached via mechanical suspension to a reference frame. Two capacitive elements are shown in FIG. 1 as circuit capacitors, denoted by CaP and CaN. The actual capacitive element may consist of multiple plates electrically connected (eg, in parallel) to provide the total capacitance, shown as capacitor CaP or CaN in the figure. As shown in the figure, the capacitor forms a bridge from the two outputs of the MEMS sensor 105 to the general circuit node 115, which can represent the circuit connection with the proof mass of mobility. One plate or set of plates of each capacitor can be attached to the proof mass of mobility, while the other set of plates or plates is stationary.

Acceleration applied to the MEMS accelerometer results in mobility of the proof mass. Movement of the proof mass changes the space between the plates of the capacitor. This shift is approximately proportional to the result of the difference in capacitance between the two capacitive elements. Modeling the proof mass and the mechanical suspension as elastic allows acceleration to be determined from the movement in accordance with Hook's Law.

In general, the change in capacitor for a capacitor pair is related to the acceleration in one direction. Adding additional capacitor pairs arranged to be orthogonal to the first pair allows acceleration in the determined second direction, which can be used as a two axis accelerometer. Three capacitor pairs enable three-axis or three-dimensional (3D) accelerometers.

For testing accelerometers, the advantage is the fact that capacitive MEMS sensors can be used as actuators. In general, capacitors are added to MEMS sensors that are used to add electrostatic charge and drive the proof mass in test mode. A better approach is to use sense capacitive elements in the test. This simplifies the design of the MEMS sensor by eliminating parts dedicated to testing.

2 is a flow diagram of one embodiment of a method 200 of implementing a self test of a MEMS capacitive sensor. In normal mode of operation, the MEMS sensor is electrically connected to an IC (eg, an application specific integrated circuit or ASIC) as in FIG. 1. The IC measures the capacitance at the output of the MEMS sensor in normal mode. At block 205, the first capacitive element of the MEMS sensor is electrically disconnected from the IC in the test mode. At block 210, a first electrical signal is applied to the disconnected capacitive element. The application of the first electrical signal causes the proof mass to shift, changing the capacitance of the second capacitive element measured at block 215. Similarly, the first capacitive element can be measured.

Returning to FIG. 1, IC 110 includes switch network circuit 120. The switch network circuit 120 can operate in normal mode and test mode. In the normal mode of operation, the switch network may connect the first and second capacitive elements (eg, CaP and CaN) of the MEMS sensor 105 as capacitive element pairs. Capacitive element pairs change capacitance in response to acceleration, thereby forming an acceleration-to-capacitance sensor.

In the test mode, the switch network circuit 120 may electrically disconnect the first capacitive element of the MEMS sensor 105 from the first input of the IC, and the second capacitive to the second input of the IC 110. The elements can be electrically connected. IC 110 also includes a capacitance measurement circuit 125 that measures the capacitance of the second capacitive element of the MEMS sensor while applying the first electrical signal to the disconnected first capacitive element.

3 illustrates this test approach. To test the capacitor CaN, the capacitor CaP of the MEMS sensor 305 is electrically disconnected or otherwise disconnected from the circuits of the IC 310. External electrical connection to the proof mass is possible at circuit node 315 (denoted as node A) common to both capacitors. The electrical signal is applied to the leg of the disconnected capacitor CaP (denoted as node B). Node B can be driven within Node A's phase or out of Node A's phase to test the sensor. With the opposite phase, the proof mass is electrostatically pulled so that a change in acceleration force is emulated. Then, while driving NodeB, the capacitance of CaN is measured. It can be demonstrated that the capacitive element changes as the proof mass moves. When driving Node A and Node B with the same phase, it can be proved that the capacitance changes little or not at all.

Capacitor CaP can be measured in a similar manner. This is shown in FIG. To measure the capacitor element CaP, the switch network circuit electrically disconnects the capacitor CaN of the MEMS sensor 405 from the IC 410 and electrically connects the capacitor CaP to the IC 410. An electrical signal is applied to the leg of the disconnected capacitor CaN (denoted as node B). Node B may be driven within the phase of node A or out of phase of node A to test the sensor. The MEMS sensor is a multidimensional sensor and the test can be repeated for multiple capacitive element pairs.

According to some embodiments, the electrical signal used to drive the disconnected capacitor is a square wave. Returning to FIG. 1, applying a first square wave to a disconnected first capacitive element (eg, CaP in FIG. 3), and applying a second square wave to an external circuit node (eg, circuit node 315 in FIG. 3). To do this, the test circuit 130 can be used. The external circuit node is common to the first capacitive element and the second capacitive element and can be electrically connected to the proof mass.

During the application of the first and second square wave signals, the capacitance measurement circuit 125 measures the capacitance of the second capacitive element (eg, CaN in FIG. 3). In a particular embodiment, the second square wave signal has an opposite phase of the phase of the first square wave signal to emulate a change in acceleration. The test of the device may simply be a pass / drop test or a test that quantifies the change in capacitance to compare the amount of change over a target capacitance value or range of values. In a particular embodiment, the second square wave signal is in phase with the first square wave signal. Then, it can be proved that the capacitance measured by this test is less than the target capacitance value.

According to some embodiments, capacitance measurement circuit 125 includes a differential input analog-to-digital converter (ADC) circuit configured to generate a digital value representing the capacitance of the capacitive element (eg CaP or CaN) being measured. In some embodiments, capacitance measurement circuit 125 includes differential input sigma delta ADC circuits.

5 is a test circuit diagram of FIG. 4 showing a specific embodiment of how the capacitive element of the MEMS sensor is connected to the sigma delta ADC circuit 525. This embodiment includes a primary integrator and a capacitor. In certain embodiments, the integrator may be a higher order integrator. The output of the sigma delta ADC circuit 525 produces a number representing the capacitance of the MEMS sensor. In this way, the capacitive element of the MEMS sensor forms a capacitance-to-voltage sensor circuit with a sigma delta ADC. The digital low pass filter circuit 135 may be followed by the output of the sigma delta ADC circuit. In the embodiment of FIG. 5, the output of the sigma delta ADC circuit 525 is used to generate a number representing the capacitance of CaP when the switch network circuit is operating in test mode.

The IC may include a self test capacitor pair consisting of CstP and CstN. In a particular embodiment, the capacitor has the same value as the capacitance. The first capacitive element of the MEMS sensor is electrically disconnected from the IC, and the switch network circuit is capable of electrically connecting the second capacitive element of the MEMS sensor to the first input of the ADC circuit and to the second input of the ADC circuit. Self-test capacitor pairs can be electrically connected. Thus, the switch network circuitry constitutes a self test capacitor pair as part of the capacitance-to-voltage sensor of the IC.

In some embodiments, the IC includes one or more offset capacitors (eg, CofP and / or CofN) that are used to cancel any common mode offset of the ADC circuit. When the first capacitive element of the MEMS sensor is electrically disconnected from the IC, the switch network circuit applies a second capacitive element and an offset capacitor to the first input of the differential input ADC circuit while the second capacitive element is measured. Electrically connected In the embodiment of FIG. 5, the MEMS capacitor CaP and the offset capacitor CofP are integrated into the voltage sensor to form part of the capacitor to form the input of the ADC circuit.

It should be noted that only the capacitive element of the MEMS sensor used to measure the acceleration is used for the test and the MEMS sensor does not require an additional test capacitor. Thus, the smaller number of circuit components and smaller number of circuit contacts are required for the MEMS sensor, so that the design can be simplified.

Additional Example

Embodiment 1 may include or use a microelectromechanical system (MEMS) sensor having a first capacitive element and a second capacitive element, and an object (eg, a device) that includes an IC. The IC is configured to electrically disconnect a first capacitive element of a MEMS sensor from a first input of the IC and to electrically connect a second capacitive element to a second input of the IC, and disconnection. And a capacitance measurement circuit configured to measure the capacitance of the second capacitive element of the MEMS sensor while applying the first electrical signal to the first capacitive element.

 In Example 2, the subject matter of Example 1 can optionally include a switch network configured to electrically disconnect the second capacitive element of the MEMS sensor from the IC, and electrically connect the first capacitive element to the IC. . The capacitance measurement circuit can optionally be configured to measure the capacitance of the first capacitive element of the MEMS sensor while applying the second electrical signal to the disconnected second capacitive element.

In Embodiment 3, the object of one or any combination of Embodiments 1 and 2 applies a first square wave signal to the disconnected first capacitive element and is common to the first capacitive element and the second capacitive element. And optionally, a test circuit configured to apply a second square wave signal to an external circuit node. The second square wave signal may optionally have a phase opposite to that of the first square wave signal, and the capacitance measurement circuitry is selectively configured to measure the capacitance of the second capacitive element while applying the first and second square wave signals. Can be.

In Embodiment 4, the object of one or any combination of Embodiments 1 and 2 applies a first square wave signal to a disconnected first capacitive element and is common to the first capacitive element and the second capacitive element. And optionally, a test circuit configured to apply a second square wave signal to an external node. The second square wave signal may be selectively in phase with the first square wave signal, and the capacitance measurement circuit may be selectively configured to measure capacitance while applying the first and second square wave signals.

In Embodiment 5, the subject matter of one or any combination of Embodiments 1-4 may optionally include a capacitance measurement circuit comprising a differential input ADC circuit configured to generate a digital value representing the capacitance of the second capacitive element. Can be.

In Example 6, the subject matter of Example 5 may optionally include an IC having a self test capacitor pair. The switch network circuit electrically connects the second capacitive element of the MEMS sensor to the first input of the ADC circuit, configures a self-test capacitor pair as a capacitance-to-voltage sensor inside the IC, and the second input of the ADC circuit. It may optionally be configured to electrically connect a self test capacitor pair to the device.

In embodiment 7, the subject matter of one or any combination of embodiments 5 and 6 can optionally include an IC having one or more offset capacitors configured to cancel any common mode offset of the ADC circuit. The switch network circuit can optionally be configured to electrically connect the second capacitive element and offset capacitor to the first input of the differential input ADC circuit while measuring the second capacitive element.

In embodiment 8, the subject matter of one or any combination of embodiments 5-7 may optionally include a differential input sigma delta ADC circuit.

In Embodiment 9, the subject matter of one or any combination of Embodiments 1-8 may optionally include switch network circuitry that may be selectively configured to operate in a test mode and a normal mode. In the test mode, the switch network may be selectively configured to electrically disconnect at least one of the first capacitive element or the second capacitive element from the IC, and in the normal mode the switch network circuit is configured as a first capacitive element pair. It may optionally be configured to connect the first and second capacitive elements of the MEMS sensor. The first capacitive element pair can optionally be configured to change the capacitance in response to acceleration in the first direction.

In Embodiment 10, the subject matter of one or any combination of Embodiments 1-9 can optionally include a capacitance-to-voltage sensor circuit.

In Example 11, one or any combination of Examples 1-10 may optionally include a MEMS sensor that includes an accelerometer.

Embodiment 12 includes electrically disconnecting a first capacitive element of a MEMS sensor from an IC, applying a first electrical signal to the disconnected capacitive element, and applying the first electrical signal. One or any combination of objects (eg, a method, means for performing the method, or a machine when performed by a machine) of one or any combination of Examples 1 to 11 to include an object having a measurement of the capacitance of a second capacitive element of Machine-readable media containing instructions for causing the machine to perform the method) or optionally combine the subjects. Such an object may comprise means for electrically disconnecting a first capacitive element, and the illustrated embodiment of this means may comprise one or more switch circuits or a switch network. Such an object may include means for applying a first electrical signal to a disconnected capacitive element, the illustrated embodiment of which means may comprise a test signal circuit. Such object may include means for measuring the capacitance of the second capacitive element of the MEMS sensor while applying the first electrical signal, the illustrated embodiment of which means includes a capacitance measurement circuit, an ADC circuit, and a differential sigma It may include a delta ADC circuit.

In Example 13, the subject of Example 12 electrically disconnected the second capacitive element of the MEMS sensor from the IC, applying the second electrical signal to the second capacitive element, and applying the second electrical signal. And optionally measuring the capacitance of the first capacitive element of the MEMS sensor.

In Embodiment 14, the subject of one or any combination of Embodiments 12 and 13 applies a first square wave signal to the first capacitive element, an external node common to the first capacitive element and the second capacitive element. And selectively applying a second square wave signal having a phase opposite to that of the first square wave signal, and measuring capacitance of the second capacitive element while applying the first and second square wave signals. can do.

In Embodiment 15, the subject of one or any combination of Embodiments 12 and 13 applies a first square wave signal to the first capacitive element, an external node common to the first capacitive element and the second capacitive element. And, optionally, applying a second square wave signal that is in phase with the first square wave signal, and measuring the capacitance of the second capacitive element measured while applying the first and second square wave signals. have.

In Embodiment 16, the subject of one or any combination of Embodiments 12-15 optionally uses a differential input sigma delta analogue digital converter (ADC) circuit to generate a digital value representing the capacitance of the second capacitive element. It may include.

In Example 17, the subject of Embodiment 16 electrically connects a second capacitive element to a first input of a differential input ADC circuit, and connects a pair of self test capacitors internal to the IC to the second input of the ADC circuit. Electrically connecting, the pair of self-test capacitors can optionally include forming a charge-to-voltage sensor inside the IC.

In Example 18, the subject of one or any combination of Examples 12-17 can optionally include measuring the first and second capacitive elements. At this time, the first and second capacitive elements are measured during the test mode, and in the normal operation mode, the first and second capacitive elements include the first capacitive element, and the capacitance is changed in response to the acceleration in the first direction. Configured to change.

In Example 19, the subject of one or any combination of Examples 12-18 cancels any common mode using one or more offset capacitors in the normal mode of operation, and cancels the second capacitive element during the test mode. When measuring, the method may optionally include electrically connecting an offset capacitor and a second capacitive element to the first input of the differential input ADC circuit.

In Example 20, the subject of one or any combination of Examples 12-19 can optionally include measuring the capacitance of the second capacitive element of the acceleration-to-capacitance MEMS sensor.

Embodiment 21 includes means for performing any one or more functions of Embodiments 1-20, or instructions for causing the machine to perform any one or more functions of Embodiments 1-20 when performed by a machine. Any portion or combination of any portions of any one or more embodiments of Examples 1-20 may be included or optionally combined to include subjects that may have a machine readable medium.

Each of these non-limiting embodiments may be satisfied on its own, or may be combined with one or more other embodiments in various substitutions or combinations.

The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are also referred to herein as "embodiment" or "example ". All publications, patents, and patent documents mentioned in this specification are incorporated herein by reference in their entirety, as if incorporated by reference in their entirety. In the event of any inconsistency in use between this specification and the documents contained herein by reference, use in the included document should be considered as a supplement to that part of the specification; In case of incompatible inconsistencies, the use of this specification takes precedence.

In this specification, the expression "work" or "one" is intended to encompass one or more than one, regardless of the usage of the phrase "at least one" Is used. In the present specification, the expression "or" means that the expression " A or B "includes" not A or B, " Used to refer to. In the appended claims, the words "including" and "in which" are used as "common" equivalents of "comprising" and "wherein". Furthermore, in the following claims, the expressions "including" and "having" have an open-eneded meaning. That is, a system, device, article, or process that includes elements other than those listed before this expression in the claims is also considered to be within the scope of the claims. Moreover, in the following claims, the expressions "first "," second ", and "third ", etc. are used merely as labels and are not intended to impose numerical requirements on such objects.

The foregoing description is for the purpose of illustration and is not to be construed as limiting the present invention. For example, the above-described embodiments (or one or more features of such embodiments) may be used in combination with each other. Other embodiments may be utilized by one of ordinary skill in the art by reviewing the above description. Further, in the detailed description of the present invention, the disclosure may be simplified by grouping various features together. This should be construed as intention that the non-claimed published feature is not essential to any claim. Rather, the subject matter of the invention may be less than all features of a particular disclosed embodiment. Accordingly, the following claims are hereby incorporated into the Detailed Description, and each claim represents a separate embodiment. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

105: MEMS sensor
110: IC
115: common circuit node
120: switch network circuit
125: capacitance measurement circuit
130: test circuit
135: digital low pass filter circuit

Claims (15)

A micro-electromechanical system (MEMS) sensor having a first capacitive element and a second capacitive element, and an integrated circuit (IC),
In the IC,
A switch network circuit configured to electrically disconnect the first capacitive element of the MEMS sensor from a first input of the IC and to electrically connect the second capacitive element to a second input of the IC; And
And a capacitance measurement circuit configured to measure the capacitance of a second capacitive element of the MEMS sensor while applying a first electrical signal to the disconnected first capacitive element. The method of claim 1,
The switch network circuit is configured to electrically disconnect the second capacitive element of the MEMS sensor from the IC, and electrically connect the first capacitive element to the IC,
The capacitance measurement circuitry is configured to measure the capacitance of the first capacitive element of the MEMS sensor while applying a second electrical signal to the disconnected second capacitive element. The method of claim 1,
A test configured to apply a first square wave signal to the disconnected first capacitive element and to apply a second square wave signal to an external circuit node common to the first capacitive element and the second capacitive element. Including circuits,
The second square wave signal has a phase opposite to that of the first square wave signal,
The capacitance measurement circuitry is configured to measure the capacitance of the second capacitive element while applying the first square wave signal and the second square wave signal. The method of claim 1,
A test circuit configured to apply a first square wave signal to the disconnected first capacitive element, and to apply a second square wave signal to an external node common to the first capacitive element and the second capacitive element;
The second square wave signal is in phase with the first square wave signal;
The capacitance measurement circuitry is configured to measure the capacitance while applying the first square wave signal and the second square wave signal. The method of claim 1,
Wherein the capacitance measurement circuit comprises a differential input analog to digital converter (ADC) circuit configured to generate a digital value representing the capacitance of the second capacitive element. The method of claim 5,
The IC comprises a self test capacitor pair,
The switch network circuit,
And electrically connect the second capacitive element of the MEMS sensor to a first input of the ADC circuit,
Configure the self test capacitor pair as a capacitance-to-voltage sensor inside the IC to electrically connect the self test capacitor pair to a second input of the ADC circuit. The method of claim 5,
The IC comprises one or more offset capacitors configured to cancel any normal mode offset of the ADC circuit,
The switch network circuit is configured to electrically connect the second capacitive element and the offset capacitor to a first input of the differential input ADC circuit while measuring the second capacitive element. The method of claim 5,
The ADC circuit is a differential input sigma delta ADC circuit. The method of claim 1,
The switch network circuitry is configured to operate in a test mode and a normal mode,
In the test mode, the switch network is configured to electrically disconnect one or more of the first capacitive element or the second capacitive element from the IC,
The switch network circuit in the normal mode is configured to connect the first capacitive element and the second capacitive element of the MEMS sensor as a first capacitive element pair,
And the first capacitive element pair is configured to change capacitance in response to acceleration in a first direction. The method of claim 1,
The IC comprises a capacitance-to-voltage sensor circuit. 11. The method according to any one of claims 1 to 10,
And the MEMS sensor comprises an accelerometer. Electrically disconnecting the first capacitive element of the MEMS sensor from the IC;
Applying a first electrical signal to the disconnected capacitive element; And
Measuring the capacitance of a second capacitive element of the MEMS sensor while applying the first electrical signal
/ RTI > The method of claim 12,
Electrically disconnecting the second capacitive element of the MEMS sensor from the IC;
Applying a second electrical signal to the second capacitive element; And
Measuring the capacitance of the first capacitive element of the MEMS sensor while applying the second electrical signal. The method according to claim 12 or 13,
Applying the first electrical signal comprises applying a first square wave signal to the first capacitive element,
Measuring the capacitance of the second capacitive element includes applying a second square wave signal to an external node common to the first capacitive element and the second capacitive element,
The capacitance of the second capacitive element is measured while applying the first square wave signal and the second square wave signal,
And the second square wave signal has a phase opposite to that of the first square wave signal. The method according to claim 12 or 13,
Applying the first electrical signal comprises applying a first square wave signal to the first capacitive element,
Measuring the capacitance of the second capacitive element includes applying a second square wave signal to an external node common to the first capacitive element and the second capacitive element,
The capacitance of the second capacitive element is measured while applying the first square wave signal and the second square wave signal,
And the second square wave signal is in phase with the first square wave signal.

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