US 8,942,389 B2Granted

Trim method for CMOS-MEMS microphones

AssigneeRobert Bosch GmbH

InventorsSucheendran Sridharan, John Matthew Muza, Philip Sean Stetson

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Abstract

Systems and methods for adjusting a bias voltage and gain of the microphone to account for variations in a thickness of a gap between a movable membrane and a stationary backplate in a MEMS microphone due to the manufacturing process. The microphone is exposed to acoustic pressures of a first magnitude and a sensitivity of the microphone is evaluated according to a predetermined sensitivity protocol. The bias voltage of the microphone is adjusted when the microphone does not meet the sensitivity protocol. The microphone is then exposed to acoustic waves of a second magnitude that is greater than the first magnitude and a stability of the microphone is evaluated according to a predetermined stability protocol. The bias voltage and the gain of the microphone are adjusted when the microphone does not meet the stability protocol.

Claims23

Dependent claims

A method of adjusting a MEMS microphone to account for variations in manufacturing processes, the MEMS microphone including a gap of an undetermined thickness between a movable membrane and a stationary backplate, the method comprising:

applying a first voltage to a power supply voltage pad to enter a normal operation mode;
applying a first serial binary input to a second pad while in the normal operation mode, wherein a logic layer of the MEMS microphone performs a first operation unrelated to adjusting the bias voltage and the gain based on the first serial binary input while operating in the normal operation mode;
applying a second voltage, the second voltage higher than the first voltage, to the power supply voltage pad to enter a trim mode,
applying a first sound level to the MEMS microphone;
evaluating a sensitivity of the MEMS microphone based on a digital output of the MEMS microphone when the first sound level is applied;
adjusting a bias voltage of the MEMS microphone by applying a binary trim code to the second pad as a second serial binary input when operating in the trim mode when the sensitivity does not meet a defined sensitivity protocol, wherein the binary trim code indicates an adjustment to the bias voltage of the MEMS microphone and wherein the trim module adjusts the bias voltage of the MEMS microphone according to the binary trim code;
applying a second sound level to the MEMS microphone, the second sound level having a greater amplitude than the first sound level;
evaluating a stability of the MEMS microphone based on a digital output of the MEMS microphone when the second sound level is applied;
and
when the stability does not meet a defined stability protocol, lowering the bias voltage and making a corresponding increase in a gain applied to the movable membrane and the stationary backplate by applying a second binary trim code as a third serial binary input to the second pad when operating in the trim mode, wherein the trim module adjusts the gain and the bias voltage of the MEMS microphone according to the second binary trim code.

7 more elements

A MEMS microphone system comprising:

a membrane that moves relative to a MEMS microphone in response to acoustic pressures applied to the MEMS microphone;
a stationary backplate positioned a distance from the membrane;
a bias voltage module applying a bias voltage on the membrane and the stationary backplate;
a trim adjustment system including a trim module integrated into a logic layer of the MEMS microphone and a signal measurement module that is selectively connectable to the MEMS microphone configured to
evaluate a sensitivity of the MEMS microphone based on a digital output of the MEMS microphone when a first sound level is applied,
adjust the bias voltage applied to the membrane and the stationary backplate when the sensitivity does not meet a defined sensitivity protocol,
evaluate a stability of the MEMS microphone based on a digital output of the MEMS microphone when a second sound level is applied, the second sound level having a greater amplitude than the first sound level, and
when the stability does not meet a defined stability protocol, lower the bias voltage and make a corresponding increase in a gain applied to the movable membrane and the stationary backplate;
a power supply voltage pad, the power supply voltage pad receiving a first voltage during a normal operation mode;
and
a second pad, the second pad receiving a first serial binary input during normal operation of the MEMS microphone, wherein the logic layer of the MEMS microphone performs a first operation unrelated to adjusting the bias voltage and the gain based on the first serial binary input,
wherein the MEMS microphone operates in a trim mode when a second voltage is applied to the power supply voltage pad, the second voltage being greater than the first voltage,
wherein, when operating in the trim mode, the signal measurement module evaluates the sensitivity of the MEMS microphone and evaluates the stability of the MEMS microphone and further
generates a binary trim code indicating an adjustment to at least one of the bias voltage and the gain of the MEMS microphone, and
transmits the binary trim code as a second serial binary input to the trim module through the second pad, and
wherein, when operating in the trim mode, the trim module adjusts the bias voltage and the gain of the MEMS microphone according to the binary trim code.

13 more elements

Dependencies are inferred from the claim text.