System and method for run-time hermeticity detection of a capped MEMS device

Patent metadata

Field	Value
Publication number	US-9442131-B2
Application number	US-201313799382-A
Country	US
Kind code	B2
Filing date	Mar 13, 2013
Priority date	Mar 13, 2013
Publication date	Sep 13, 2016
Grant date	Sep 13, 2016

Abstract

Determining if a hermetically sealed MEMs device loses hermeticity during operation. In one embodiment, the MEMs device is an accelerometer. A test signal having an associated frequency above an operational frequency range for the accelerometer is provided to the accelerometer at an input during operation of the accelerometer for sensing an acceleration. The output signal of the accelerometer is filtered at least above the operational frequency range of the accelerometer producing a test output signal. The test output signal is then compared to a predetermined threshold to determine if the amplitude of the test output signal differs from the threshold. If the amplitude of the test output signal differs from the predetermined threshold, an error signal is produced indicating that hermeticity of the accelerometer has been lost.

First claim

What is claimed is:

1. A method for determining if a hermetically sealed micro-electrical mechanical system linear accelerometer measuring linear accelerations about one or more axes containing a hermetically sealed gas maintains its hermeticity during operation of the accelerometer under acceleration by an external stimulus, the method comprising:
providing the linear accelerometer during operation with a test signal having an associated frequency above an operational frequency range for the linear accelerometer producing a test portion of an output signal from the linear accelerometer;
filtering from the output signal the accelerometer frequencies at least below the operational frequency range of the linear accelerometer producing a test output signal, the test output signal not including frequencies within the operational frequency range of the linear accelerometer;
determining if the amplitude of the test output signal differs from a predetermined threshold for a predetermined frequency wherein the predetermined threshold is based at least in part on the viscosity of the gas within the linear accelerometer; and
if the amplitude of the test output signal differs from the predetermined threshold by a predefined range,
producing an error signal indicating that hermeticity of the linear accelerometer has been lost otherwise hermeticity is maintained.

2. A method according to claim 1 wherein the test signal is an alternating current test signal.

3. A method according to claim 1 wherein the test signal is a periodic step function.

4. A method according to claim 1 , further comprising:
generating a stimulus portion of the output signal when the accelerometer is provided with the external stimulus.

5. A method according to claim 4 wherein the output signal of the accelerometer is composed of the stimulus portion and the test portion.

6. A method according to claim 5 , further comprising:
filtering the output signal with a filter to remove the test portion of the output signal and providing the filtered output signal to an output wherein the filtered output signal represents the linear acceleration due to the external stimulus.

7. A method according to claim 1 , wherein the filtering, determining, and producing an error signal are performed on circuitry external to an accelerometer package.

8. A method according to claim 1 , wherein the filtering, determining, and producing an error signal are performed on circuitry internal to the accelerometer package.

9. A method according to claim 1 wherein filtering the output signal of the accelerometer above the operational frequency range is performed by a high-pass filter.

10. A method according to claim 1 wherein filtering the output signal of the accelerometer above the operational frequency range is performed by a notch filter.

11. The method according to claim 6 , wherein filtering the output signal with a filter to remove the test portion of the output signal is performed by a low pass filter.

12. A system for determining if hermeticity of a hermetically capped linear accelerometer measuring linear acceleration about one or more axes containing a gas has been lost during operation under acceleration by an external stimulus, the system comprising:
an input for receiving an output signal of the linear accelerometer;
a first filter coupled to the input for filtering the output signal of the accelerometer to remove the test signal from the output signal so that only an acceleration signal representative of linear acceleration about one or more axes remains as the result of the external stimulus remains;
a second filter coupled to the input for filtering the output signal of the accelerometer to remove a stimulus portion of the output signal from the output signal so that only the test portion of the output signal remains wherein frequencies of the test portion are outside of the operational frequency range of the linear accelerometer;
a comparison module for comparing the test portion of the output signal to a threshold range wherein the predetermined threshold is based at least in part on the viscosity of the gas within the accelerometer and wherein if the amplitude of the test output signal differs from the predetermined threshold, hermeticity is found to be lost if the amplitude of the test output signal is greater than the predetermined threshold for gases with a viscosity greater than that of air and if the amplitude is not greater than the predetermined threshold for gases with a viscosity greater than that of air hermeticity is maintained and wherein hermeticity is found to be lost if the amplitude of the test output signal is less than the predetermined threshold for gases with a viscosity less than that of air and if the amplitude is not less than the predetermined threshold for gases with a viscosity less than that of air hermeticity is maintained;
and an output for outputting an error signal if the hermeticity is found to be lost by the comparison module.

13. The system according to claim 12 further comprising:
a test generation signal module for generating a test signal for the linear accelerometer at a frequency above an operational range for the accelerometer.

14. The system according to claim 12 wherein the first filter is a low pass filter.

15. The system according to claim 12 wherein the second filter is a notch filter.

16. The system according to claim 12 wherein the second filter is a high pass filter.

17. The system according to claim 12 wherein the system includes a hermetically capped accelerometer.

18. The system according to claim 12 wherein the comparison module is a comparator.

19. The system according to claim 12 wherein the comparison module includes logic for identifying an amplitude for at least one frequency of the output signal corresponding to a known test signal and comparing the amplitude to the threshold.

20. A computer program product including a tangible computer-readable medium with computer code thereon for determining if a hermetically sealed linear accelerometer measuring linear acceleration about one or more axes has lost hermeticity during operation of the linear accelerometer under acceleration by an external stimulus, the computer code comprising:
computer code for providing the accelerometer during operation under acceleration by the external stimulus with a test signal having an associated frequency above an operational frequency range for the linear accelerometer producing a test portion of an output signal from the accelerometer;
computer code for filtering from the output signal the linear accelerometer frequencies at least below the operational frequency range of the linear accelerometer producing a test output signal wherein the test signal does not include frequencies within the operational frequency range of the linear accelerometer;
computer code for determining if the amplitude of the test output signal differs from a predetermined threshold by a predetermined range for a predetermined frequency wherein the predetermined threshold is based at least in part on the viscosity of the gas within the accelerometer; and
computer code for producing an error signal indicating that hermeticity of the accelerometer has been lost if the amplitude of the test output signal differs from the predetermined threshold by the predefined range otherwise hermeticity is maintained.

21. The method according to claim 1 , wherein if the amplitude of the test output signal differs from the predetermined t

Assignees

• Analog Devices Inc

Inventors

• Hazel Matthew Aaron
• Wilson James
• Prendergast Colm
• Boyko Daniel
• Dufort Benoit

Classifications

• G01P15/125

  by capacitive pick-up · CPC title

• G01P21/00 Primary

  Testing or calibrating of apparatus or devices covered by the preceding groups · CPC title