Calibrated microelectromechanical microphone

Abstract

A MEMS microphone comprising a MEMS transducer having a back plate and a diaphragm as well as controllable bias voltage generator providing a DC bias voltage between the back plate and the diaphragm. The microphone also has an amplifier with a controllable gain, and a memory for storing information for determining a bias voltage to be provided by the bias voltage generator and the gain of the amplifier.

Description

BRIEF DESCRIPTION OF THE INVENTION

In the following, a preferred embodiment of the invention will be described with reference to the drawing, wherein:

FIG. 1 illustrates a general diagram of important elements of the preferred embodiment of a microphone of the invention, and

FIG. 2 illustrates a manner of determining a bias voltage.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiment of a microphone 10 of the invention comprises a MEMS condenser microphone/transducer 12 with an integrated circuit portion 14 which comprises a microphone (pre)amplifier 16, a DC bias voltage generator 18 and is built into a microphone housing/package 20. In addition, the microphone has a voltage supply 111 and an output 15.

The amplifier 16 comprises an input for data 22 for adjusting the gain thereof, and the bias voltage generator 18 comprises a diode set-up 26 and a Dickson pump 24 (see e.g. EP-A-1 599 067 which is herein incorporated by reference in its entirety) having an input for data 28 for regulating the voltage output of the generator 18. The operation of the Dickson pump 24 is a direct conversion of the information of the M bits to a voltage.

The gain of the microphone preamplifier 16 is adjusted by the use of calibration data 22 that are loaded into and stored in a portion of a non-volatile memory 30 of the integrated circuit 14 during a final test step in the production process of the MEMS condenser microphone 10. Additionally, the data for use in the generator 18 are stored in another portion of the memory 30.

Preferably, the non-volatile memory 30 comprises One-Time-Programmable (OTP) memory such as EPROM, fuse-based memory or similar types of electronic memory. However, multi-programmable memory types such as EEPROM and/or Flash memory may be utilized in other embodiments of the invention, especially if these types of memory devices are already in use for other purposes on the integrated circuit.

The programming process of the MEMS condenser microphone 10 may in practice proceed along the below-mentioned steps:

A well defined sound pressure of predetermined level (e.g. 94 dB SPL/1 kHz sine-wave) is applied to the individually packaged microphone 10 while the electrical output signal of the MEMS condenser microphone transducer 12 is measured. The MEMS condenser microphone 10 may advantageously be located in a suitable test jig inside an acoustical test box.

In the preferred embodiment according to FIG. 1, the gain of the microphone preamplifier 16 is adjusted or calibrated by varying the ratio of either a set of resistors or a set of capacitors thereof that are coupled as a feedback network of a microphone preamplifier configuration. The feedback microphone preamplifier 16 can be either single-ended or differential.

The sensitivity of the MEMS transducer assembly is adjusted by adjusting the value of the DC bias voltage (see below in relation to FIG. 2).

In the present embodiment, the sensitivity of the MEMS transducer assembly 10 is measured, recorded and tracked in a test computer to the stage of final microphone assembly and test where the present calibration process is carried out. Based on the known sensitivity of the MEMS transducer assembly 10, an appropriate value for the DC bias voltage is determined/calculated by the test computer and thereafter programmed into the OTP memory 30 by choosing the appropriate code for example through a pre-stored lookup table.

FIG. 2 illustrates a particularly useful manner of estimating or determining a desired bias voltage for the MEMS transducer 12. A varying voltage is provided between the back plate and diaphragm of the MEMS transducer 12, whereby the air gap height (the distance between the diaphragm and back plate) will vary. This height may be estimated on the basis of a capacitance built between these elements. This capacitance value, however, is not linear with the distance but will increase drastically when the distance is close to zero. Zero distance is a so-called “collapse” where the diaphragm touches the back plate.

FIG. 2 illustrates the capacitance C as a function of a voltage V applied between the diaphragm and back plate. It is seen that C increases drastically, when V is close to the collapse voltage, Vcollapse, which is the lowest voltage required for having the back plate and diaphragm touch.

Thus, from this graph, Vcollapse may be estimated even without bringing the voltage V applied between the back plate and diaphragm to Vcollapse.

However, using a bias voltage close to Vcollapse will not provide the desired sensitivity of the microphone 10 in that once a sound pressure acts on the diaphragm, this will force the diaphragm toward the back plate and may cause collapse. Thus, the theoretically largest bias voltage should be Vcollapse subtracted a voltage which corresponds to the largest variation of the diaphragm-back plate distance caused by the largest sound pressure (or other phenomenon, such as acceleration caused by the microphone being dropped) which the microphone should be able to sense. This variation is illustrated by a varying curve illustrating a voltage variation required to simulate the variation caused by the sound, which may be, for example, 120-130 dB.

Thus, half this Vp-p should be subtracted from Vcollapse, and preferably a margin voltage, Vmargin, is also subtracted in order to ensure that collapse is not encountered during normal or expected operation.

As a result of this analysis, Vbias may be determined as Vcollapse subtracted Vmargin and half of Vp-p.

Once the OTP memory 30 has been programmed with the appropriate code, the test process is preferably halted for a short moment to allow the microphone output signal to settle to its correct bias point after the programming of the DC bias voltage.

Thereafter, the MEMS condenser microphone sensitivity is measured and the target and appropriate preamplifier gain is calculated on the basis of the measured sensitivity and a pre-stored reference sensitivity. Finally, from the target preamplifier gain, an appropriate code is determined and programmed into the corresponding OTP memory area. Optionally, a final calibration procedure step may be executed that comprises re-measuring the sensitivity of the MEMS condenser microphone to confirm that the actual measured value is within the expected sensitivity range that may have a band of +/−1 or 2 dB around the nominal sensitivity value.

The programming of the non-volatile memory 30 can be done with a very simple serial data interface 32 that may comprise a clock and a data signal or single signal line with composite data/clock signals that are accessible on respective external programming pin(s) of the microphone assembly 10. A state machine inside the integrated circuitry 14 is adapted to decode the incoming data stream and handle the writing of memory data to the OTP memory 30.

In the case of a digital microphone assembly, the external programming pin(s) 32 may be shared with already provided digital input/output pins such as Left/Right signal or other digital signals. For MEMS microphones 10 that are packaged in a SMD mountable package, the extra space and solder connections required by additional external programming pin(s) is a minor concern.

For an analogue microphone assembly it will normally be necessary to add the external programming pin(s) 32 to the already existing external pins. This addition can however be done at substantially no extra cost.

While the present invention has been described with reference to one or more particular embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the present invention.

Claims

1. A MEMS microphone assembly including a microphone housing, the MEMS microphone assembly comprising: a sound inlet;a MEMS transducer element having a back plate and a diaphragm displaceable in relation to the back plate;a controllable bias voltage generator adapted to provide a DC bias voltage between the diaphragm and the back plate;a memory for storing information including amplifier gain setting information;a controllable amplifier for receiving an electrical signal from the MEMS transducer element and providing an output signal, the controllable amplifier being adapted to amplify the electrical signal from the MEMS transducer in accordance with an amplifier gain setting; anda processor adapted to retrieve information from the memory and to control the gain of the amplifier in accordance with the amplifier gain setting information from the memory, andcontrol the bias voltage generator to provide a DC bias voltage in accordance with the information from the memory.

2. A MEMS microphone assembly according to claim 1, wherein the MEMS transducer element has a distance from the back plate to the diaphragm in the range of 1-10 μm, such as 2-5 μm.

3. A MEMS microphone assembly according to claim 1, wherein the controllable bias voltage generator is adapted to generate a DC bias voltage in the range of 5-20 V.

4. A MEMS microphone assembly according to claim 1, wherein the memory comprises memory circuitry of a type of the group consisting of: RAM, PROM, EPROM, EEPROM, flash, one-time-programmable memories, and memories based on fuse-link technology.

5. A method of calibrating a MEMS microphone assembly comprising a MEMS transducer element, the method comprising the steps of: measuring or estimating a collapse voltage of the MEMS transducer element;determining a DC bias voltage for the MEMS transducer element on the basis of the measured or estimated collapse voltage; andwriting information relating to the determined DC bias voltage to a memory of the microphone assembly.

6. A method according to claim 5, wherein measuring/estimating step comprises the steps of: applying a DC bias voltage to the MEMS transducer element,applying a predetermined sound pressure to the MEMS transducer element,measuring an acoustic sensitivity of the MEMS transducer element during the application of the DC bias voltage and the predetermined sound pressure, anddetermining the collapse voltage based on the measured sensitivity and the applied DC bias voltage.

7. A method according to claim 5, wherein the measuring/estimating step comprises the steps of: increasing a voltage provided between the back plate and the diaphragm of the MEMS transducer element while monitoring a capacitance value between the back plate and the diaphragm, until, at a first voltage, a predetermined increase in the capacitance value is detected, and estimating the collapse voltage on the basis of the first voltage.

8. A method according to claim 5, further comprising the steps of: applying a DC voltage corresponding to the determined DC bias voltage to the MEMS transducer element,applying a predetermined sound pressure to the MEMS transducer element,amplifying, in an amplifier, a signal output of the MEMS transducer element in response to the sound pressure, and outputting an amplified signal,determining, on the basis of the amplified signal and a predetermined signal parameter, an amplifier gain setting, andwriting information relating to the determined amplifier gain setting to the rmemory.

9. A method according to claim 8, further comprising the step of electrically interconnecting the MEMS transducer element and the amplifier permanently on a common substrate carrier before performing the step of determining the amplifier gain setting.

10. A method according to claim 5, wherein the step of measuring or estimating a collapse voltage of the MEMS transducer element is performed on a MEMS microphone wafer comprising a plurality of MEMS microphones.

11. A method according to claim 10, wherein the collapse voltage of the MEMS transducer element is estimated from a MEMS transducer subset of the plurality of MEMS transducers.

12. A method of calibrating a plurality of MEMS microphone assemblies, the method comprising: providing a plurality of MEMS transducer elements from a single wafer batch or a single wafer,providing a MEMS transducer element in each microphone assembly;calibrating a subset of the plurality of MEMS microphone assemblies in accordance with the method of claim 5 and deriving DC bias voltage information there from; andwriting at least the derived DC bias voltage information to respective memories of the remaining MEMS microphone assemblies of the plurality of MEMS microphone assemblies.