METHOD FOR OPERATING AN INTEGRATED MEMS MICROPHONE DEVICE AND INTEGRATED MEMS MICROPHONE DEVICE

Abstract

A method for operating an integrated MEMS microphone device is proposed. The integrated MEMS microphone device comprises a package housing enclosing an interior cavity, wherein an integrated MEMS microphone die with a movable membrane, at least one environmental sensor and a thermal decoupling circuit are arranged inside the cavity. The method comprising the steps of repeatedly operating the environmental sensor in a measurement mode and activating the thermal decoupling circuit for a transition phase preceding and/or succeeding the measurement mode of the environmental sensor. During the transition phase a heat dissipation into the cavity is gradually adjusted.

Claims

1. A method for operating an integrated MEMS microphone device comprising a package housing enclosing an interior cavity, wherein an integrated MEMS microphone die with a movable membrane, at least one environmental sensor and a thermal decoupling circuit are arranged inside the cavity, the method comprising: repeatedly operating the environmental sensor in a measurement mode; activating the thermal decoupling circuit for a transition phase preceding and/or succeeding the measurement mode of the environmental sensor; and during the transition phase, gradually adjusting a heat dissipation into the cavity.

2. The method according to claim 1, wherein in the measurement mode, the environmental sensor is operated at an operating condition defining a first level of heat dissipation into the cavity; and wherein: in the transition phase preceding the measurement mode, the heat dissipation is gradually adjusted to reach the first level, and/or in the transition phase succeeding the measurement mode, the heat dissipation is adjusted to drop below the first level.

3. The method according to claim 1, wherein the heat dissipation is adjusted depending on a transition function with a characteristic time constant.

4. The method according to claim 3, wherein the transition function is arranged such that a pressure change due to adjusting the heat dissipation inside the cavity induces a vibration frequency in the movable membrane which is inaudible to a human, for example, a vibration frequency which is smaller than 50 Hz or which is smaller than 20 Hz.

5. The method according to claim 3, wherein the transition function for the transition phase preceding the measurement mode increases as a function of time, and/or the transition function for the transition phase succeeding the measurement mode decreases as a function of time.

6. The method according to claim 1, wherein the measurement mode is initiated by switching the environmental sensor on, and/or the measurement mode is terminated by switching the environmental sensor off.

7. The method according to claim 6, wherein heat dissipation meets the first level when the measurement mode is initiated, and/or heat dissipation drops from the first level when the measurement mode is terminated.

8. The method according to claim 6, wherein the thermal decoupling circuit is switched on when the measurement mode is initiated, and/or the thermal decoupling circuit is switched off when the measurement mode is terminated.

9. The method according to claim 7, wherein the thermal decoupling circuit comprises an adjustable current source or an adjustable current sink to dissipate heat into the cavity and the thermal decoupling circuit is separate from the environmental sensor, and wherein: a dissipation current of the thermal decoupling circuit is adjusted depending on the transition function, and wherein in the transition phase preceding the measurement mode, the dissipation current is adjusted until it meets an operating current, wherein the operating current is defined by the operating condition of the measurement mode, and/or in the transition phase succeeding the measurement mode, the dissipation current is adjusted to drop from operating current.

10. The method according to claim 6, wherein the thermal decoupling circuit is comprised by the environmental sensor and remains switched on in the measurement mode.

11. The method according to claim 10, wherein in the transition phase preceding the measurement mode, the operating current is increased until the operating current reaches a value defined by the operating condition of the measurement mode, and/or in the transition phase succeeding the measurement mode, the operating current is decreased until the environmental sensor is off.

12. An integrated MEMS microphone device, comprising: a package housing enclosing a cavity; an integrated MEMS microphone die arranged inside the cavity, wherein the MEMS microphone die comprises a movable membrane; at least one environmental sensor arranged inside the cavity, wherein the environmental sensor is configured to be activated in a measurement mode; and a thermal decoupling circuit arranged inside the cavity, wherein the thermal decoupling circuit can be activated in a transition phase preceding and/or succeeding the measurement mode, and wherein the thermal decoupling circuit is configured to gradually dissipate power in the transition phase.

13. The device according to claim 12, wherein the thermal decoupling circuit is separate from the environmental sensor and comprises at least one of: one or more adjustable current sources to dissipate heat into the cavity, one or more adjustable current sinks to dissipate heat into the cavity, one or more a digitally adjustable analog-to-digital converters one or more a digitally adjustable current analog-to-converters.

14. The device according to claim 12, wherein the thermal decoupling circuit is integrated into the integrated MEMS microphone die and/or the environmental sensor.

15. The device according to claim 12, wherein the environmental sensor comprising at least one of: a temperature sensor, a pressure sensor, a humidity sensor, a gas sensor and/or an air quality sensor.

16. A method for operating an integrated MEMS microphone device comprising a package housing enclosing an interior cavity, wherein an integrated MEMS microphone die with a movable membrane, at least one environmental sensor and a thermal decoupling circuit are arranged inside the cavity, the method comprising: repeatedly operating the environmental sensor in a measurement mode; activating the thermal decoupling circuit for a transition phase preceding and/or succeeding the measurement mode of the environmental sensor; and during the transition phase, gradually adjusting of temperature inside the cavity due to the flow of electrical current through the thermal decoupling circuit; wherein: the measurement mode is initiated by switching the environmental sensor on, and/or the measurement mode is terminated by switching the environmental sensor off, heat dissipation due to the flow of electrical current through the thermal decoupling circuit meets a first level when the measurement mode is initiated, and/or heat dissipation drops from the first level when the measurement mode is terminated, and the thermal decoupling circuit comprises an adjustable current source or an adjustable current sink to dissipate heat into the cavity and the thermal decoupling circuit is separate from the environmental sensor, and wherein: a dissipation current of the thermal decoupling circuit is adjusted depending on the transition function, and wherein in the transition phase preceding the measurement mode, the dissipation current is adjusted until it meets an operating current, wherein the operating current is defined by the operating condition of the measurement mode, and/or in the transition phase succeeding the measurement mode, the dissipation current is adjusted to drop from operating current.

17. An integrated MEMS microphone device, comprising: a package housing enclosing a cavity; an integrated MEMS microphone die arranged inside the cavity, wherein the MEMS microphone die comprises a movable membrane; at least one environmental sensor arranged inside the cavity, wherein the environmental sensor is configured to be activated in a measurement mode; and a thermal decoupling circuit arranged inside the cavity, wherein the thermal decoupling circuit can be activated in a transition phase preceding and/or succeeding the measurement mode, and wherein the thermal decoupling circuit is configured to gradually adjust, in the transition phase, the temperature inside the cavity due to the flow of electrical current through the thermal decoupling circuit, the thermal decoupling circuit comprises an adjustable current source or an adjustable current sink to dissipate heat into the cavity and the thermal decoupling circuit is separate from the environmental sensor; and wherein the device is operable to: repeatedly operate the environmental sensor in a measurement mode; activate the thermal decoupling circuit for a transition phase preceding and/or succeeding the measurement mode of the environmental sensor; and during the transition phase, gradually adjust a heat dissipation into the cavity; wherein: the measurement mode is initiated by switching the environmental sensor on, and/or the measurement mode is terminated by switching the environmental sensor off, heat dissipation meets a first level when the measurement mode is initiated, and/or heat dissipation drops from the first level when the measurement mode is terminated, and a dissipation current of the thermal decoupling circuit is adjusted depending on the transition function, and wherein in the transition phase preceding the measurement mode, the dissipation current is adjusted until it meets an operating current, wherein the operating current is defined by the operating condition of the measurement mode, and/or in the transition phase succeeding the measurement mode, the dissipation current is adjusted to drop from operating current.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] FIG. 1 shows an exemplary embodiment of an integrated microelectromechanical systems, MEMS, microphone device,

[0047] FIG. 2 shows an exemplary embodiment of a method to operate an integrated MEMS microphone device,

[0048] FIGS. 3A to 3C show examples of transient functions and an exemplary embodiment of a thermal decoupling circuit,

[0049] FIG. 4 shows an exemplary embodiment of a method to operate an integrated MEMS microphone device,

[0050] FIG. 5 shows an exemplary embodiment of an integrated microelectromechanical systems, MEMS, microphone device, and

[0051] FIG. 6 shows an exemplary embodiment of a method to operate an integrated MEMS microphone device from the prior art.

DETAILED DESCRIPTION

[0052] FIG. 1 shows an exemplary embodiment of an integrated microelectromechanical systems, MEMS, microphone device (MEMS microphone for short). The MEMS microphone device 1 comprises a package housing 10, a lid 11, an integrated MEMS microphone die 12, an integrated circuit die 17, at least one environmental sensor 13 and a thermal decoupling circuit 14.

[0053] The package housing 10 has a sound hole 15 which is arranged in and through the housing, leaving the housing open to its environment. For example, the sound hole communicates sound from outside the package housing to an interior of the package housing. In fact, an interior cavity 16 is defined by a lid 11 which is mounted to a substrate 18 of the package housing. The lid 11 encapsulated the interior of the package housing and thereby forms the interior cavity.

[0054] The integrated MEMS microphone die 12 is mounted on the substrate and resides inside the interior cavity. The MEMS microphone die comprises a movable membrane 19, e.g. a MEMS diaphragm. The movable membrane is configured to receive sound from outside the package housing through the sound hole. In fact, the movable membrane is responsive to pressure changes inside the interior cavity, which may originate from sound received from the outside via the sound hole or by interior sources, for example. Pressure can also be induced by changes in temperature inside the cavity.

[0055] The (analog or digital) integrated circuit die 17 is arranged inside the cavity next to the MEMS microphone die. The integrated circuit die 17 is arranged to process data from the MEMS microphone die, e.g. as an application specific integrated circuit (ASIC). For example, the integrated circuit die comprises control logic, analog-to-digital converters, a microprocessor, driver units and interfaces which are integrated into the same die. Alternatively some or all of these components can be arranged in the integrated MEMS microphone die to effect operation, control and processing of the MEMS microphone. Electrical connections 21, such as wire bond interconnects or through-silicon-vias, may be arranged between the integrated MEMS microphone die, the integrated circuit die and the substrate. The substrate comprises a printed circuit board, laminate, ceramic or lead frame, for example.

[0056] At least one environmental sensor 13 is mounted on the substrate and resides also inside the interior cavity. Environmental sensors include temperature sensors, pressure sensors, humidity sensors, gas sensors or air quality sensors, for example. Such sensors can be arranged inside the package housing in order to determine environmental parameters of the air or gas present inside the cavity, e.g. temperature, pressure, humidity, gas content or air quality. The environmental sensor can be complemented with additional circuitry to effect operation, control and pre-processing, for example. In fact, as will be discussed further with respect to FIG. 4 the thermal decoupling circuit may be comprised by the environmental sensor or its additional circuitry.

[0057] FIG. 2 shows an exemplary embodiment of a method to operate an integrated MEMS microphone device. Only the thermal decoupling circuit 14 and the environmental sensor 13 are shown for easier representation. In this example embodiment the thermal decoupling circuit is represented by an adjustable current source coupled to the supply voltages VDD, VSS. The environmental sensor is connected in parallel to the current source.

[0058] The environmental sensor is operated in a measurement mode MM. The measurement mode can be activated repeatedly and/or by user interaction, for example. In the measurement mode the environmental sensor is switched on for a certain duration and generates its characteristic sensor signal. In other words, the measurement mode is initiated by switching the environmental sensor on and the measurement mode is terminated by switching the environmental sensor off. For the duration of the measurement mode the environmental sensor is operated at an operating current Iq.

[0059] Initiating (switching on) and terminating (switching off) the measurement mode MM induce essentially instantaneous power transitions, respectively. For example, when the measurement mode is initiated, the environmental sensor is switched on from zero to reach the operating current Iq. When the measurement mode is terminated, the environmental sensor is switched off and the operating current Iq drops back to zero. These sudden changes induce respective temperature changes on a similar time scale. This is due to Joule heating caused by heat dissipation into the cavity due to the current flow associated with the environmental sensor, for example. As temperature equals particle movement in the cavity volume there is a thermal coupling path that causes a change in temperature to change pressure. The pressure change eventually actuates the movable membrane of the MEMS microphone. In other words there is a thermal coupling path between the electronic components and the air or gas arranged inside the cavity. Power consumption of the electronic components, for example, of the environmental sensor in measurement mode, modulates temperature, hence pressure, inside the package and may be detected by the MEMS microphone as audible signal or noise.

[0060] The thermal decoupling circuit 14, e.g. the adjustable current source in this embodiment, is activated for a first transition phase TP1 preceding initiating the measurement mode. The thermal decoupling circuit is activated again for a second transition phase TP2 succeeding terminating the measurement mode. During the first transition phase a dissipation current Id is gradually adjusted (increased) to reach the operating current Iq. In fact, the operating current Iq is met in the moment (or close to said moment) when the measurement mode is initiated and the first transition phase ends. Similarly, during the second transition phase the dissipation current Id is gradually adjusted (decreased) to drop from the operating current Iq. In fact, the dissipation current Id is decreased in the moment (or close to said moment) when the measurement mode is terminated. The second transition phase ends when the dissipation current Id reaches zero (or a constant) level.

[0061] The drawing depicts both the operating current Iq=Iq(t) and the dissipation current Id=Id(t) as functions of time t, respectively. On the scale of the cavity and considering both the environmental sensor and the thermal decoupling circuit as heaters, a net current In causes heat dissipation into the cavity volume. The net current constitutes a superposition of the operating current Iq and the dissipation current Id which is depicted in the drawing as a function In=In(t). The current functions introduced above are objects of the electric domain. The drawing also depicts a pressure function p(t) which is located in the mechanical domain. The pressure function shows how the heat dissipation caused by the net current In changes (or modulates) pressure inside the cavity.

[0062] The dissipation current Id=Id(t) can be considered a mathematical function (denoted transition function hereinafter) which can be characterized by a time constant and slope, for example. In this example embodiment the transition function is linear. The slope can be arranged such that the induced temperature and pressure changes are slow enough to induce an inaudible signal in the MEMS microphone.

[0063] This is to say that the induced pressure change due to adjusting the heat dissipation inside the cavity according to the transition function induces a vibration frequency in the movable membrane 19 which is inaudible to a human. Typically, a vibration frequency which is smaller than 50 Hz may not be sensed. A vibration frequency smaller than 20 Hz typically is not audible at all. In terms of characteristic time constant of the transient function, noise due to operation of the environmental sensor can be suppressed or even eliminated when a transient of the induced pressure change is slower than about 0.02 to 0.05 seconds.

[0064] FIGS. 3A to 3C show examples of transient functions and an exemplary embodiment of a thermal decoupling circuit. As discussed above the dissipation current Id is gradually adjusted according to a transient function. Thus, heat dissipation into the cavity depends on the transition function as well.

[0065] The transition function can be characterized by a characteristic time constant. For example, adjusting heat dissipation during the transition phase TP1, TP2 should not be too fast and should avoid occurrence of transients, especially audible transients. Thus, the characteristic time constant can be arranged such that the gradual adjustment of heat dissipation into the cavity is slow enough to avoid a transient in the audio signal, e.g. an audible transient in the audio signal. For example, the time constant determines that the induced pressure change is slower than about 0.02 to 0.05 seconds.

[0066] FIG. 3A shows a transition function which is a continuous function of time, e.g. including an exponential function. Depending on the chosen time constant, the transition function has a steeper slope at the beginning of the transition phase and evens out the closer it comes to initiating the measurement mode, i.e. towards reaching the level of the operating current Iq. Such a transient function can be implemented by an adjustable transistor or diode based current source (or current sink) as thermal decoupling circuit, for example. Another example includes a RC circuit characterized by the characteristic time constant.

[0067] FIG. 3B shows a transition function which is a discontinuous function of time. This exemplary function is continuous only in a stepwise manner. However, the time scale may resemble the time constant of FIG. 3A to determine that the induced pressure change is slow enough, e.g. slower than about 0.02 to 0.05 seconds. Such a stepwise transition function can be implemented by a current analog-to-digital converter as the one depicted in FIG. 3C, for example. The current analog-to-digital converter comprises a current mirror having a first and a second branch M1, M2. A current source CS connected to a transistor T1 is located in the first branch and mirrors a current into the second branch. The second branch comprises a number of diode-connected transistors T2, T3, T4, T5 which are connected in parallel via respective switches S2, S3, S4, S5. The switches can be utilized to connect the transistors one after the other. Depending on the number of transistors actively connected together the current analog-to-digital converter outputs a current which ramps up in steps depending on the number of transistors connected. The individual transistors constitute current sources of their own.

[0068] The transition functions shown in FIGS. 3A and 3B can be applied during the transition phase TP1 preceding the measurement mode, for example. Inverse functions can be applied during the transition phase TP2 succeeding the measurement mode, for example. The transition functions and their implementation may also be applied to the method discussed in FIG. 4.

[0069] FIG. 4 shows an exemplary embodiment of a method to operate an integrated MEMS microphone device. In fact, the integrated MEMS microphone device can be the same as the one discussed with respect to FIG. 1. However, instead the thermal decoupling circuit 14 is comprised by the environmental sensor or by parts of its circuitry. Thus, the environmental sensor, or parts thereof, function as thermal decoupling circuit as well.

[0070] In order to implement the thermal decoupling functionality the environmental sensor is operated with an operating current changing in time Iq=Ig(t). In fact, the function Iq(t) can be considered a superposition of a first transition phase TP1 during which the operating current gradually increases, an essentially constant operating current at an operating level (similar to FIG. 2) and a second transition phase TP2 during which the operating current gradually decreases, e.g. back to a zero level. The operating current, considered as a function of time, may have the shape of the net current In(t) shown in FIG. 2, as the transition phases in operating current take over the role of the dissipation current Id of FIG. 2.