DE102016105923A1 - MEMS microphone and method of operation - Google Patents

Abstract

A MEMS microphone has an ASIC (AS) with a programmable internal memory (IM) and a MEMS sensor (MS). In the internal memory operating parameters corresponding to a plurality of defined operating states (M1, M2, M3) for the control of the MEMS sensor (MS) and / or the processing of the sensor to the ASIC (AS) guided signal can be stored. Via a control line for the ASIC, the microphone can be switched to a selected operating state.

Description

MEMS microphones, which are designed as microelectromechanical systems, are used today for mobile phones and acoustic sensors. Particularly advantageous are their small size and their good acoustic performance. They are also characterized by their resistance to the conditions of soldering.

One problem encountered in the fabrication of MEMS microphones is the sensitivity dispersion after fabrication. Production tolerances and material fluctuations can lead to an inadmissibly high dispersion of the microphone characteristics. However, since only a small tolerance of the properties is tolerable for the use of MEMS microphones, many microphones do not meet the requirements of modern mobile phones directly after production.

The state of the art is therefore to subject MEMS microphones to a final test after their production and subsequently to return microphones with properties which deviate greatly from the desired values by changing the operating parameters to a desired range. It will be determined new operating parameters for the microphone and stored in a memory of the ASIC, which is part of each MEMS microphone stored. Such a microphone is for example from the EP1906704 A1 known. With the help of such a calibration it is possible in mass production to reduce the spread to a maximum deviation of +/- 1 dB.

With the help of the calibration, it is possible to optimally set a microphone to a desired sensitivity. One problem remains, however, that a microphone in different environments with different sound pressures or different levels of background noise actually requires a specific calibration in order to optimize it for operation in the respective environment. Due to the standardized calibration that has been practiced up to now, the MEMS microphone has been optimized for standardized and precisely defined environmental parameters.

Object of the present invention is therefore to provide a microphone and a method for operating a microphone, which is compatible with different external environment parameters and also provides there a good signal quality.

This object is achieved by a microphone and a method for operation according to the independent claims. Advantageous embodiments of the invention can be found in the dependent claims.

A known calibratable MEMS microphone already has a programmable internal memory. This is usually designed as OTP memory (One Time Programmable) and allows a one-time writing of the calibration parameters, which then remain constant over the entire life of the MEMS microphone.

The basic idea of the invention is now to use this memory to store operating parameters for several defined operating states so that the microphone can be switched to a desired or required operating state via a control line during later proper operation.

The operating parameters for the operating states are values calibrated to the respective MEMS microphone, which were determined in a final test after the production of the microphone.

However, it is also possible to redetermine operating parameters for individual desired operating states at any time and to re-store them in the internal memory and thus to recalibrate the MEMS microphone. This is particularly advantageous when the MEMS microphone has been subjected to aging, or thermal or mechanical stress, which in each case can lead to an internal strain on the membrane of the MEMS microphone and not least can result in a drift in the microphone characteristics.

The re-programming of operating parameters and the selection of stored operating states can be done via the same control line that connects the microphone to an external terminal.

The programming is preferably carried out by inputting signals at a voltage which is also available during normal operation of the MEMS microphone. For example, it is possible to input a bit sequence via the drive line, which comprises either a programming command or a data record.

Alternatively, it is possible to apply a voltage level intended for an operating state to the drive line in order to set the respective operating state associated with the voltage level for the desired operating state.

An operating parameter, which according to the invention can be stored for an operating state, is a gain factor, which represents a measure of the amplification by an amplifier integrated in the ASIC (pre-amplifier).

As a further operating parameter, a BIAS voltage typical for an operating state can be stored. The bias voltage can be used to influence the sensitivity or SNR level or THD behavior of the microphone at higher sound pressure levels. The operating state allows the sensitivity to be set to correspond to a prevailing noise level in the environment.

The various adjustable operating states can be provided for different environmental parameters. In addition to a noisy or quiet environment, a demanded high or low dynamics, a high or low signal-to-noise ratio (SNR), or high or low frequency interference signals to be filtered out, the respective operating states to be selected can take into account various other ambient conditions and guarantee optimum microphone behavior.

In a particular embodiment, the MEMS microphone is also designed to detect external environmental parameters and / or properties of the acoustic signals.

From the absolute height, the dynamic range and / or the frequency position of the signals to be recorded by the microphone, and optionally as a function of the ambient temperature, a matching stored operating state is selected and set automatically by means of an algorithm.

It is also possible that the MEMS microphone calculates and suggests an optimized operating state, which can then be set manually by a user. The setting values are again the stored operating states, or the operating parameters that define the respective stored operating state.

The MEMS microphone usually has a number of contacts, which serve both for fastening the microphone, for example by soldering, and as electrical connections of the microphone. One of the contacts may be connected to the drive line.

However, it is also possible to use an existing contact, e.g. for the initial calibration to store operating parameters for various operating conditions, to select and set an operating condition, or to recalibrate the operating parameters for the operating conditions. Of course, it is also possible to provide further contacts via which operating states according to the invention can be stored or selected.

As already stated, the operating parameters for the operating states can be determined once in a final test and stored in the internal memory of the MEMS microphone. However, it is also possible to recalibrate after certain events. It is also possible to carry out a recalibration at regular intervals. For a calibration, a normalized acoustic environment is usually required in order to set a desired microphone characteristic with standardized acoustic signals, the standardized acoustic signals serving as a benchmark.

In the following the invention will be explained in more detail with reference to an embodiment and the associated figures. The figures are only schematic and therefore not drawn to scale.

Show it:

1 a schematic block diagram of a microphone,

2 the physical structure of a microphone in schematic cross section,

3 the sensitivity measured over the frequency for different applied BIAS voltages and different amplification factors,

4 the distribution of sensitivity over a larger number of individual microphones measured after manufacture, after calibration, and after switching to another operating mode.

1 schematically shows the structure of a microphone according to the invention MIC. It comprises a microphone sensor MS which, in a preferably micromechanical structure, has at least one membrane MM and a back electrode BP, which together form a capacitive sensor element. Connected to the capacitive microphone sensor is an ASIC AS, which is arranged together with the microphone sensor MS on a common substrate and preferably under a common cover, for example a metal cap MC. The metal cap is glued eg with a sealing adhesive mass SM on the substrate SU.

Other design variants can also be used. Instead of flip-chip technology, e.g. the chips are also glued and the electrical connections are made by bonding wires.

At the capacitive element of the microphone sensor MS is applied to a BIAS voltage from a BIAS voltage generator BS is generated. At the capacitive element, the capacitance is now determined during operation of the microphone, converted into a voltage in the ASIC, amplified by an amplifier AMP and output as a measuring signal at an output OUT.

The microphone MIC can be designed as a digital or analog microphone. An analog microphone also has as outputs and inputs serving contacts for the supply voltage VDD, for ground GND and for a read / write access W / R.

For a digital microphone at least one input for Clock CLK is provided. In addition, the ASIC AS has an internal memory IM. In the internal memory, the operating parameters for example, three or more different selectable operating states M1, M2, M3 of the microphone MIC are stored.

To store the data in the internal memory IM, the read / write input W / R can be used. The operating state M _n can also be selected via this input.

However, it is also possible to provide a read / write input W / R different auxiliary input AI to select the operating state, which is also connected to the internal memory IM.

Depending on the selected operating state M, the corresponding operating parameters are routed from the internal memory IM to the BIAS voltage generator BS and / or to the amplifier AMP and thus set the properties of the microphone to match the desired operating state.

2 shows a conventional MEMS microphone in schematic cross section, as it can be used for the invention. On a common substrate SU, which is designed, for example, as a multilayer ceramic with integrated circuitry, a capacitive MEMS sensor MS and an ASIC AS are arranged. Both components can be arranged side by side as shown. However, it is also possible to arrange the ASIC above or below the MEMS sensor MS.

In the illustrated embodiment, MEMS sensor MS and ASIC AS are sealed with a polymeric film against the surface of the substrate. On the substrate, a cap MC, such as a metal cap, is placed, which seals against the substrate and includes a cavity below, are arranged in the microphone sensor MS and ASIC AS. Above the membrane MM and the rear electrode BP, the polymer film is removed, so that the membrane MM can communicate with the cavity under the cover MC.

Below the membrane, a sound opening SO is provided through the substrate SU, through which the membrane can communicate with the external environment and sound waves can penetrate to the membrane. Membrane MM and polymer film PF separate an acoustic front volume below the membrane and an acoustic back volume above the membrane and below the cover MC.

MEMS sensor MS and ASIC AS can be mounted on the substrate via solder connections, which are embodied, for example, in the form of bumps BU.

The ASIC is connected to the microphone sensor MS or its membrane MM and back electrode BP via the internal wiring within the multilayer substrate SU or via conductor tracks running on the surface of the substrate SU. Shown is a capacitive microphone sensor with two back electrodes BP, which provides a differential signal with a higher signal quality. However, it is also possible to switch only a single return electrode against the membrane.

3 shows different waveforms in which the sensitivity of an exemplary MEMS microphone is plotted against the applied frequency in Hertz. The different curves are determined with different operating parameters.

From the 3 It becomes clear that the sensitivity is significantly different depending on the applied operating parameters, which are representative of certain operating conditions. A first curve shows the sensitivity with minimum gain GF _min . A second curve shows the sensitivity at maximum gain GF _max . A third graph shows the sensitivity at minimum BIAS voltage BV _min , while a fourth graph shows the sensitivity at maximum applied BIAS voltage BV _max . It can be seen that the sensitivity can be set via both operating parameters, with a variation of the amplification factor GF leading to a greater maximum difference ΔGF _max .

Similarly, other microphone characteristics can be set via the two operating parameters, which can be defined in the form of combinations of operating parameters. For example, the signal-to-noise ratio SNR can be set.

Due to production tolerances, the properties of microphones scatter within a batch, and especially from batch to batch. 4 shows how within a larger amount of individually measured microphones the sensitivities fluctuate or how different the sensitivities are distributed over a larger number of microphones.

A first distribution 1 shows a relatively wide dispersion of the sensitivities and is obtained, for example, directly after the production of a larger number of microphones. However, this broad spread is inadmissible for a deployable product, so either only the appropriate microphones can be selected or, as already practiced, the properties are shifted back to a required narrow range by calibration.

The distribution shown 2 is for example the result of such a calibration with individually set operating parameters. The required operating parameters are determined separately for each microphone in a standardized acoustic environment and stored in the internal memory. Further alternative operating states, each with a different desired property profile, or the operating parameters required for this purpose, are also stored in the memory so that, for example, different sensitivities can be set.

The distribution 2 shows, for example, the distribution of sensitivity across different microphones, in which a first operating state is set. The distribution 3 on the other hand shows the sensitivity of the same microphones after the microphones are switched to a second operating state and the corresponding operating parameters associated with this operating state are set on the microphone. This results, for example, in a lower sensitivity for the microphones according to the distribution 3 , This also has the consequence that the microphone has a lower THD influence at higher sound pressures.

In a similar manner as shown here, the sensitivity can be set, for example, the signal-to-noise behavior and other properties of the microphone via suitable operating parameters. Since the operating conditions for each microphone are calibrated individually, there is also a slight deviation from the target value for the different operating states over a plurality of microphones, such as for example 4 clearly stated.

Although the invention has been explained only in one embodiment, it is not limited to this. The invention is preferably optimized for a capacitive MEMS microphone, but is not limited to capacitive microphones. For microphones with a different sensor principle, other operating parameters may possibly be set.

LIST OF REFERENCE NUMBERS

1

 first distribution

2

 second distribution

3

 third distribution

AI

 auxiliary input

AMP

 amplifier

AS

 ASIC

BP

 back electrode

BS

 BIAS voltage generator

BU

 soldered point

BV

 BIAS tension

CLK

 Clock

GF

 gain

GND

 Dimensions

IN THE

 Internal memory

M1

 first operating mode / mode

M2

 second operating state / mode

M3

 third operating state / mode

MC

 cap

MIC

 MEMS microphone

MM

 membrane

MS

 MEMS sensor

OUT

 microphone output

PF

 polymer film

SM

 sealant

SO

 sound hole

SU

 substratum

VDD

 supply voltage

W / R

 Read / write input

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 1906704 A1 [0003]

Claims (11)

microphone with an ASIC (AS) and a MEMS sensor (MS), where the ASIC has a programmable internal memory (IM), in which operating parameters corresponding to a plurality of defined operating states (M1, M2, M3) for the control of the MEMS sensor (MS) and / or the processing of the signal conducted by the sensor to the ASIC (AS) can be stored in the internal memory, in which the microphone can be switched to a selected operating state via a drive line for the ASIC. Microphone according to the preceding claim, in which the ASIC (AS) comprises an amplifier in which a gain factor (GF) for an amplifier can be stored as the operating parameter. Microphone according to one of the preceding claims, in which the MEMS sensor (MS) is a capacitive MEMS sensor in which an operating voltage to be applied to the capacitive MEMS sensor BIAS voltage can be stored. Microphone according to one of the preceding claims, in which the internal memory (IM) is designed to be programmable via the control line in order to store new or changed operating parameters for a new or a changed operating state (M) at any time. Microphone according to one of the preceding claims, in which the MEMS microphone (MIC) can detect external environmental parameters and is adapted to automatically set a suitable operating state (M _n ) in accordance with an external algorithm according to an algorithm. Microphone according to one of the preceding claims, comprising a number of contacts which serve both for mounting the microphone and as electrical connections of the microphone in which one of the contacts is connected to the control line. Microphone according to one of the preceding claims, wherein the internal memory is writable via an external terminal of the microphone. Method for operating a microphone, using a MEMS microphone (MIC) having an ASIC (AS) with a programmable internal memory (IM), wherein the operating parameters calibrated in the internal memory to the MEMS microphone for a plurality of defined operating states (M1, M2, M3) are stored, in which the operating parameters required to set desired characteristics of the MEMS microphone in different operating states are determined in an upstream test be read from the internal memory at the operating parameters for a the environment parameters adapted or a desired operating state by means of a control line and the corresponding operating state is set. Method according to the preceding claim, in which a BIAS voltage applied to the capacitive MEMS sensor (MS) for setting a desired sensitivity in the respective operating state (M1, M2, M3) is stored as an operating parameter in which after selection and adjustment of the desired operating state the BIAS voltage is adjusted according to the operating condition. Method according to one of the preceding claims, in which the operating parameters for an operating state (M1, M2, M3) are stored in each case a gain factor (GF) for setting a desired signal amplification, wherein after selection and adjustment of the desired operating state, the gain factor corresponding to Operating state is set. Method according to one of the preceding claims, where the MEMS Microphone (MIC) determines environmental parameters, wherein the MEMS microphone for the measured environmental parameters selects an optimized operating state (M1, M2, M3) according to an algorithm and adjusts automatically or on manual request.

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