MEMS microphone with improved sensitivity The present invention refers to MEMS microphones with im ^¬ proved sensitivity, e.g. to MEMS microphones with a reduced temperature dependence of its sensitivity.

MEMS microphones comprise a MEMS capacitor with a variable capacitance where variances in capacitance reflect a received audio signal. Further, MEMS microphones comprise electrical and electronic circuitry in an ASIC (Application-Specific In ^¬ tegrated Circuit) . To protect the MEMS capacitor and the cir ^¬ cuitry in the ASIC in an ASIC chip from negative environmen- tal influences, a MEMS microphone comprises packaging compo ^¬ nents. The MEMS capacitor, the ASIC and the influence of the packaging components on the signal of the MEMS microphone are subject to temperature-induced variations in electrical prop ^¬ erties. In particular, an increase in temperature deterio- rates the microphone signal quality and due to the complex construction of the microphone, the individual contributions of the microphone's components to the total deterioration is complex, too. Thus, what is needed is a MEMS microphone with improved sig ^¬ nal quality, especially with improved sensitivity, e.g. with a reduced or eliminated temperature dependence of the sensi ^¬ tivity. MEMS microphones or components of such microphones improving the signal quality are known, e.g. from US 7,889,030, US 2011/0200212, US 8,067,992. However, improvements in sensitivity appear still possible.

A MEMS microphone according to independent claim 1 provides a microphone with improved sensitivity. The dependent claims provide preferred embodiments.

A MEMS microphone comprises a MEMS capacitor with a variable capacitance, a charging circuit provided for charging the MEMS capacitor, and bias circuit provided for creating a bias voltage applied to the charging circuit. The bias circuit comprises a closed loop control circuit. The bias voltage ap ^¬ plied to the charging circuit has a temperature dependence.

The closed loop control circuit of the bias circuit, thus provides a temperature dependent bias voltage that controls the charging circuit.

It was found that different MEMS microphones have different temperature dependent sensitivity curves that describe the temperature induced deterioration of the sensitivity of a mi ^¬ crophone. As explained above, the temperature induced deteri ^¬ oration of a complex microphone depends on a plurality of factors working together in a complex manner. Technical means eliminating one of the plurality of factors may improve a mi- crophone signal quality. However, even such an improved mi ^¬ crophone still has a temperature dependence of its sensitiv ^¬ ity.

However, with the present microphone, the temperature depend- ence of the bias voltage may result in a MEMS microphone with practically no temperature dependent sensitivity, even in a wide temperature range. As the sensitivity of a MEMS capacitor in the MEMS microphone is mainly proportional to the electrical voltage applied to the MEMS capacitor, the control of the voltage of the capaci ^¬ tor - which is fed by a charging circuit - allows to compen- sate a temperature induced sensitivity deterioration of the whole microphone without the need of compensating each of the microphone's components individually.

The closed loop control circuit of the bias circuit control- ling the charging circuit - and thus controlling the voltage applied to the MEMS capacitor - may monitor regularly the operation temperature of the microphone and adaptively compen ^¬ sate a temperature induced deterioration by selecting the op ^¬ timal bias voltage.

It is possible that the bias voltage has a piece-wise linear temperature dependence.

The complex temperature dependent sensitivity of a microphone may be approximated by piece-wise linear segments over a wide temperature range. The number of segments may be chosen to obtain a temperature compensation that fulfills the needed requirements. The temperature intervals defining the segments are represented by threshold temperatures. The threshold tem- peratures may be chosen at small temperature differences when the temperature dependent sensitivity has larger slopes. The segments can be wider if the temperature dependent sensitiv ^¬ ity has a mainly small slope. Thus, with a small number of segments of a piece-wise linear approximation of the tempera- ture dependent sensitivity, a corresponding set of piece-wise linear bias voltages having the same threshold temperatures can be obtained for providing a temperature compensated MEMS microphone with an improved sensitivity over a wide tempera ^¬ ture range without the need for compensating the contribu ^¬ tions of the individual components of the microphone. It is possible that the MEMS microphone comprises a tempera ^¬ ture sensor. The temperature sensor may be part of the closed loop control circuit of the bias circuit.

It is possible that the temperature sensor provides a temper- ature signal that is proportional to the absolute tempera ^¬ ture. Such a temperature signal may be a PTAT voltage V _S i _n (PTAT = Proportional To Absolute Temperature) .

Such a voltage is well suited to be utilized by circuit com- ponents of the closed loop control circuit having a rela ^¬ tively simple circuit topology.

It is possible that the closed loop control circuit comprises a slope generator. The slope generator may produce the slope of the bias voltage at a given operation temperature, e.g. within a temperature interval if a bias circuit providing a piece-wise linear temperature dependence is utilized.

It is possible that the slope generator comprises a compara- tor and a plurality of serial connections of a resistor and a switch .

The resistor can have a variable resistance. By setting the resistances of the resistors of the slope generator, the tem- perature dependence of the bias voltage can be modeled to provide a temperature compensated microphone. Especially, the resistance of the respective resistor can be chosen to deter ^¬ mine the corresponding slope of a segment of the piece-wise linear temperature dependence of the bias voltage. It is possible that the closed loop control circuit comprises a slope control circuit controlling the slope generator. The slope control circuit may provide the slope generator with a signal or a plurality of signals informing the slope genera ^¬ tor about the actual temperature or about the actual tempera- ture range or temperature interval.

It is possible that the slope control circuit comprises a plurality of comparators. Each of the comparators of the slope control circuit may be directly connected to one of the switches of the plurality of serial connections of the slope generator.

It is possible that the charging circuit comprises a charge pump. The charge pump may be a conventional charge pump such as a Dickson charge pump.

It is possible that the slope generator generates a voltage V _sg linear over temperature. Further, the slope of V _sg is mainly proportional to the negative slope of the temperature dependence of the sensitivity of the microphone, -S (t) , at a given temperature or within a given temperature interval.

The PTAT voltage V _sg generated by the slope generator re- fleets the amount of sensitivity deterioration and provides the corresponding countermeasure as it is responsible for the according additional electrical charge transferred to the MEMS capacitor. The PTAT voltage V _sg may - in addition to another offset voltage - establish the bias voltage conducted to the charg ^¬ ing circuit. It is possible that the bias voltage is the sum of a plural ^¬ ity of voltages selected from temperature independent volt ^¬ ages and temperature dependent voltages.

It is further possible that the MEMS microphone comprises an ASIC where the bias circuit is a part of the ASIC and at least partially arranged in an ASIC chip.

It is further possible that the MEMS microphone additionally comprises a non-volatile memory element for storing linearity parameters.

The linearity parameters may encode the resistances of the resistors having a variable resistance of the slope genera ^¬ tor. The precise values of the resistances may be transferred in a one-time programming step into the microphone and deter ^¬ mined in a calibration process after manufacturing the microphone .

The calibration process may comprise the steps of increasing the operation temperature of the microphone and simultane ^¬ ously monitoring the temperature dependent sensitivity of the microphone. The temperature dependent sensitivity may be ap ^¬ proximated by a piece-wise linear curve and the linearity pa ^¬ rameters may be directly obtained by inverting the sine of the slope of the piece-wise linear segments. The present invention, basic working principles and preferred embodiments are described in the accompanying drawings, wherein FIG. 1 shows a basic equivalent circuit diagram of the mi ^¬ crophone, shows a possible temperature dependent sensitivity curve with decreasing slope, shows a possible temperature dependent sensitivity curve with increasing absolute value of the sensi ^¬ tivity, shows different piece-wise linear segments of the voltage generated by the slope generator V _sg and the bias voltage V _bias . The bias voltage may be the sum of the voltage generated by the slope generator and an offset voltage, shows a possible output of a temperature sensor providing a PTAT voltage, shows a more detailed equivalent circuit diagram of the microphone where the bias circuit comprises a temperature sensor, a slope control circuit and a slope generator within the closed loop control cir ^¬ cuit, FIG. 7 shows another embodiment further comprising a voltage buffer in the bias circuit, FIG. 8 shows another embodiment further comprising an amplifier,

FIG. 9 shows embodiments of a slope control circuit, a

temperature sensor, and a slope generator,

FIG. 10 shows a possible embodiment of a charging circuit comprising elements of a charge pump. FIG. 1 shows an equivalent circuit diagram of the basic em ^¬ bodiment of a MEMS microphone MM. The microphone MM comprises a MEMS capacitor MCAP that may be arranged in a MEMS device, e.g. a MEMS chip MEMS. The microphone further comprises a bias circuit BC and a charging circuit CC . The bias circuit BC provides a control signal controlling the charging circuit CC . The charging circuit CC generates the operation voltage for the MEMS capacitor MCAP and transfers the corresponding electrical charge to the capacitor. The capacitor may com ^¬ prise two or more electrodes. At least one of the electrodes is connected to a signal output SO where an electrical signal encoding the received audio signal can be obtained for fur ^¬ ther processing.

The bias circuit BC comprises the closed loop control circuit CLCC creating the control signal controlling the charging circuit. The control signal may be a voltage signal applied to the control circuit CC .

Within the closed loop control circuit, a closed loop circuit monitoring the actual operation temperature and adaptively adjusting the control signal is contained. FIGs. 2 and 3 show different possible temperature dependent sensitivity curves of a MEMS microphone. In FIG. 2, a sensi ^¬ tivity curve and corresponding piece-wise linear approxima ^¬ tions are shown where the sensitivity is reduced with in- creasing temperature. However, the absolute value of the slope of the sensitivity is reduced with increasing tempera ^¬ ture. The threshold temperatures Tl, TN are chosen such that the approximation of the sensitivity curve by the piece- wise linear segments is as good as possible for a given num- ber of threshold temperatures. Thus, at sections where the absolute value of the slope of the temperature dependent sen ^¬ sitivity curve is relatively large, the temperature intervals may be chosen quite narrow. In regions where the absolute value of the slope of the temperature dependent sensitivity curve is quite low, wider temperature intervals can be cho ^¬ sen. Thus, at lower temperatures, the temperature intervals of the approximation in FIG. 2 are narrower than at higher temperatures . In contrast, FIG. 3 shows the situation where the absolute value of the slope increases with increasing temperature. Correspondingly, the temperature intervals for the piece-wise linear segments can be relatively wide at low temperatures. The width of the temperature intervals may be reduced at higher temperatures.

FIG. 4 shows a possible output V _sg of the voltage generated by the slope generator. Depending on the actual operation temperature and the corresponding temperature interval, respec- tively, a different slope with a different offset is pro ^¬ vided. An additional offset voltage V _{0ff Set} may be added to the output of the slope generator to obtain the bias voltage V _bias provided to the charging circuit. The additional offset volt ^¬ age V _{0ff S} et may be in a range between 8 to 16 V where the vari ^¬ ation of the voltage generated by the slope generator may be in a range from 1 to 2 V.

FIG. 5 shows the output of a PTAT temperature sensor provid ^¬ ing a voltage scaling with the absolute temperature.

FIG. 6 shows an embodiment of a MEMS microphone where the bias circuit BC comprises a closed loop control circuit CLCC with a temperature sensor TS, a slope control circuit SCC, and a slope generator SG. The temperature sensor TS provides a temperature signal, e.g. a voltage being proportional to the absolute temperature, to the slope control circuit SCC and to the slope generator SG. The slope control circuit SCC provides one or a plurality of control signals to the slope generator SC.

The output of the slope generator SC may be routed to the charging circuit CC . The signal provided to the charging cir ^¬ cuit CC may be a control voltage V _sc or a control voltage V _sg in addition to an additional offset voltage V _offset .

FIG. 7 shows a more detailed embodiment of the closed loop control circuit CLCC as the output of the slope generator SC is connected to a voltage buffer VB which feeds a control signal to the charging circuit CC . However, the voltage buffer is optional and can be omitted. FIG. 8 shows another embodiment where the output of the MEMS capacitor is connected to an amplifier AMP. The amplifier AMP may be integrated into an ASIC chip together with other cir- cuit components of the bias circuit and/or the charging cir ^¬ cuit. The voltage buffer shown in FIG. 8 is optional and can be omitted. FIG. 9 illustrates the closed loop of the closed loop control circuit. The temperature sensor TS may provide a voltage gen ^¬ erating element generating a voltage proportional to absolute temperature which may be connected to ground via a resistor R. Further, the output of the PTAT voltage generating element may be connected to the slope control circuit SCC and to the slope generator SC. The slope control circuit comprises a plurality of comparators where the embodiment presented in FIG. 9 has three comparators. Each comparator has two inputs where one of the two inputs is an inverted input. The non-in- verted inputs of the comparators are connected to the temper ^¬ ature sensor TS. The output of each comparator is fed via a corresponding signal line to a switch of the slope generator. The slope generator comprises a plurality of serial connec ^¬ tions of a resistive element and a switch. The embodiment shown in FIG. 9 comprises three serial connections where the first serial connection has a resistor Rl and a switch SI. The second serial connection has a resistor R2 and a switch S2 and the third serial connection has a resistor R3 and a switch S3. Further, the slope generator has an additional re- sistive element connected in parallel to the serial connec ^¬ tions and a further additional element connected to ground. Connected in parallel to the serial connections, a comparator is comprised in the slope generator SG. The inverted input of the comparator is connected to the serial connections, i.e. to the resistor's side of the serial connections while the non-inverted input of the comparator is connected to the tem ^¬ perature sensor TS. Reference voltages, e.g. reference voltages VI, V2, V3 in FIG. 9, are applied to the inverted input of the comparators of the slope control circuits. The references voltages VI, V2, V3 are compared to the voltage provided by the tempera- ture sensor TS. Depending on the actual temperature, the voltage provided by the temperature sensor TS causes a cer ^¬ tain number of the comparators of the slope control circuit to create an activation signal activating the corresponding switch of the slope generator. Thus, the switching state of the switches of the slope generator depends on the tempera ^¬ ture and on the reference voltages applied to the slope con ^¬ trol circuit. Accordingly, the output voltage V _sg as shown in FIG. 4 with piece-wise linear segments can be obtained. FIG. 10 shows a possible implementation of the charging cir ^¬ cuit CC which may be realized as a Dickson charge pump comprising a serial connection of basic elements, each comprising a diode and a storage capacitor. Between the output of the slope generator SC and the input of the

charging device CC, a further voltage follower VF may be provided. The output of the charging circuit CC may be connected to one or more electrodes of the MEMS capacitor.

The present MEMS microphone is not limited to the embodiments described above or shown in the figures. Microphones compris ^¬ ing further circuit elements such as further amplifier circuits or a higher number of comparators or serial connections in the slope control circuit and in the slope generator, re ^¬ spectively, are also comprised by the present invention. List of reference signs

AMP: amp1 i fier

BC: bias circuit

CC: charging circuit

CLCC closed loop control circuit

MCAP: MEMS capacitor

MEMS MEMS component comprising the MEMS capacitor MM: MEMS microphone

R: resistor

Rl, R2, R3: resistor

S : sensitivity

SI, S2, S3: switch

SCC: slope control circuit

SG: slope generator

SO: signal output

T: temperature

Ti, T ₂ , T threshold temperature

TS : temperature sensor

V: voltage

Vi, V ₂ , V ₃ : reference voltages

VB: voltage buffer

^" ^bias · bias voltage controlling the charging circuit

V ₀ f f set ^: offset voltage

V _sg : output voltage of a slope generator