Microphone system with a stop member

A microphone system has a package with an interior chamber and an inlet aperture for receiving an acoustic signal, and a microphone die having a backplate and a diaphragm. The microphone is positioned within the package interior to form a front volume between the diaphragm and the inlet aperture. Accordingly, the microphone is positioned to form a back volume defined in part by the diaphragm within the interior chamber. The system also has a stop member positioned in the back volume so that the diaphragm is between the stop member and the backplate.

FIELD OF THE INVENTION

The invention generally relates to microphone systems and, more particularly, the invention relates to transducers.

BACKGROUND OF THE INVENTION

MEMS microphones (i.e., microelectromechanical system microphones) typically are secured within the interior chamber of a package to protect them from the exterior environment. An integrated circuit chip, typically mounted within the interior chamber and having active circuit elements, processes electrical signals to and from the microphone. One or more apertures through some portion of the package permit acoustic signals to reach the microphone. Receipt of the acoustic signal causes the microphone, with its corresponding integrated circuit chip, to produce an electronic signal representing the acoustic qualities of the received signal.

Since they are exposed to the exterior environment through their apertures(s), MEMS microphones often are subject to high pressure events that can damage their fragile microstructure.

SUMMARY OF VARIOUS EMBODIMENTS

In accordance with one embodiment of the invention, a microphone system has a package with an interior chamber and an inlet aperture for receiving an acoustic signal, and a single backplate microphone die having a backplate and a diaphragm. The microphone is positioned within the package interior to form a front volume between the diaphragm and the inlet aperture. Accordingly, the microphone is positioned to form a back volume defined in part by the diaphragm within the interior chamber. The system also has a stop member positioned in the back volume so that the diaphragm is between the stop member and the backplate.

The stop member may be spaced a given distance (e.g., between about 5 and about 16 microns) from the generally planar top surface of the diaphragm to limit orthogonal movement of the diaphragm in a direction that is generally normal to the top surface of the diaphragm. The maximum orthogonal movement is about the same as the given distance. That distance may be greater than the distance between the diaphragm and the backplate. Moreover, the stop member may have a floating potential, or a potential that is substantially the same as the potential of the diaphragm.

Some embodiments secure the stop member to the microphone die. For example, the stop member and microphone die may be secured together in a stacked configuration. Other embodiments couple the stop member to at least one interior wall that defines the interior chamber.

To facilitate diaphragm movement, the stop member may be formed as a generally planar member (e.g., a laminate) having at least one opening therethrough. To form the front volume and back volume in the requisite manner, the microphone die may be positioned to substantially cover the inlet aperture.

The package may include a base forming the inlet aperture, and a lid secured to the base. The lid and base also may form the interior chamber and have a plurality of pads on the base (e.g., in the interior chamber, on the exterior package surface, or both surfaces).

In accordance with another embodiment of the invention, a microphone system has a lid that, at least with a base, forms a package having an interior chamber and an inlet aperture for receiving an acoustic signal. The system also has a single backplate microphone die within the interior chamber, and a stop member proximate to the microphone die. The microphone die is mounted over and covers the inlet aperture, and is positioned between the inlet aperture and the stop member.

In accordance with other embodiments of the invention, a microphone system has a lid that, at least with a base, forms a package having an interior chamber and an inlet aperture for receiving an acoustic signal. The system also has a single plate microphone die within the interior chamber—mounted over and covering the inlet aperture. The microphone die has a diaphragm suspended by at least one spring, and a backplate that forms a variable capacitor with the diaphragm. The spring permits the diaphragm to move a maximum distance in a direction that is generally orthogonal to the top generally planar face of the diaphragm. The maximum distance is a distance that would damage the microphone die. Accordingly, the system also has a stop member positioned proximate to and spaced a given distance from the diaphragm in a direction that is generally orthogonal to the top face of the diaphragm. The given distance is less than the maximum dimension. The stop member is positioned between the diaphragm and the lid to prevent the diaphragm from moving more than the given distance in the direction of the lid.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In illustrative embodiments, a MEMS microphone is configured to maintain its structural integrity when subjected to sudden high pressure acoustic signals. To that end, the MEMS microphone has a stop member that limits the distance its flexible diaphragm may travel away from its local backplate. Specifically, the stop member ensures that the diaphragm cannot move a distance that could potentially damage the diaphragm and/or its springs (among other things). Details of illustrative embodiments are discussed below.

FIG. 1schematically shows one application of a microphone system that can implement illustrative embodiments of the invention. Specifically,FIG. 1schematically shows a printed circuit board12supporting and electrically interconnecting a packaged microphone10with additional electronic components14. The packaged microphone10cooperates with on-board and off-board circuitry to convert and deliver audio/acoustic signals to a larger system, such as a mobile telephone or public announcement system.

A board aperture16(shown in phantom) extends upwardly through the printed circuit board12to the bottom of the microphone package (identified by reference number18, discussed in detail below). To ensure proper receipt of the acoustic signal, the microphone package18may be sealed to the top surface of the printed circuit board12by means of a gasket (e.g., formed from an elastomeric or other sealing material, not shown) or without a gasket, such as with some foam or elastomeric material. Accordingly, this arrangement produces an acoustic signal path through the printed circuit board12, the gasket, and an inlet aperture30in the bottom surface of the package18.

Those skilled in the art can mount the packaged microphone10onto the printed circuit board12using any of a variety of different techniques. For example, surface mount technology or lead-through-board technologies (e.g., gull wing mounting) should suffice. Moreover, it should be noted that only the packaged microphone10and two other miscellaneous circuit components14are shown for simplicity. The circuit board12thus may have a number of other components, such as additional microphones, resistors, capacitors, transistors, application-specific integrated circuits, traces, contact pads, etc. . . .

Indeed, the packaged microphone10of this embodiment has a microphone package18that contains both a MEMS microphone (hereinafter “microphone die20”) and application-specific internal circuit (“ASIC22” or “circuit die22”). Illustrative embodiments may use a variety of different types of MEMS microphone dies, such as that shown in cross-section by example inFIGS. 4 and 5.

To those ends,FIG. 2schematically shows a top, perspective view of a packaged microphone10(also referred to as a “packaged microchip10” or “microphone system10”) that may be configured in accordance with illustrative embodiments of the invention. In a corresponding manner,FIG. 3schematically shows a bottom, perspective view of the same packaged microphone10.

The packaged microphone10shown in those figures has a package base24that, together with a corresponding lid26, forms an interior chamber28(shown inFIGS. 6A and 7) containing the noted microphone die20and, if desired, the noted separate circuit die22. Alternatively, the microphone die20has on-chip circuitry, thus obviating the need for separate microphone circuitry within the chamber28. The lid26in this embodiment is a cavity-type lid, which has four walls extending generally orthogonally from a top, interior face to form a cavity. The lid26secures to the top face of the substantially flat package base24to form the interior chamber28. In alternative embodiments, the lid26and base24combine with other components (e.g., an intervening wall between the lid26and the base24) to form the interior chamber28. Other embodiments may implement the base24as a cavity package (with a bottom and walls extending from a flat surface), and/or the lid26in a generally flat planar shape.

As shown inFIG. 3, the base24has an audio/acoustic input port30(also referred to as an “input aperture30” or “inlet aperture30”) that enables ingress of audio/acoustic signals into the interior chamber28. Acoustic signals entering the interior chamber28interact with the microphone die20to produce an electrical signal that, with additional (exterior) components (e.g., a speaker and accompanying on-chip or off-chip circuitry), produce an output audible signal corresponding to the input audible/acoustic signal.

In alternative embodiments, however, the inlet aperture30is at another location, such as through the top of the lid26, or through one of the side walls of the lid26. For example, the inlet aperture30can extend through the lid26with a connection to the microphone die20. The package18also may have two or more ports/apertures30. For example, the package18could have a second input port (not shown) for directional sound purposes. Accordingly, discussion of a package18having its inlet aperture30through the base24is but one example of a variety of different embodiments.

FIG. 3also shows a number of base contacts32for electrically (and physically, in many anticipated uses) connecting the microphone die20with a substrate, such as the printed circuit board12ofFIG. 1or other electrical interconnect apparatus. For example, the base contacts32may be surface mountable pads or leads. The packaged microphone10may be used in any of a wide variety of applications. For example, the packaged microphone10may be used with mobile telephones, land-line telephones, computer devices, video games, biometric security systems, two-way radios, public announcement systems, camcorders, and other devices that transduce signals.

In illustrative embodiments, the package base24shown inFIGS. 2 and 3is a premolded, leadframe-type package (also referred to as a “premolded package”). Other embodiments may use different package types, such as, among other types, ceramic cavity packages, substrate packages, or laminate base (e.g., BT) packages. Accordingly, discussion of a specific type of package base is for illustrative purposes only.

The package18may have selective metallization to protect it from electromagnetic interference. For example, the lid26could be formed from stainless steel, while the base24could include printed circuit board material, such as metal layers and FR-4 substrate material. Alternatively, the lid26could be formed from an insulator, such as plastic, with an interior conductive layer. Other embodiments contemplate other methods for forming an effective Faraday cage that reduces electromagnetic interference with the internal microphone die20. Moreover, various embodiments may form the base24and lid26from similar or the same materials. For example, both can be formed from a laminate, or the lid26can be formed from a laminate, while the base24can be formed from a carrier or pre-molded leadframe.

The interior chamber28can contain any of a variety of different types of microphone dies20. To that end,FIG. 4schematically shows a perspective view of one type of microphone die20that may be used in illustrative embodiments. For more detail,FIG. 5schematically shows a cross-sectional view of the microphone die20ofFIG. 4.

Among other things, the microphone die20includes a single static backplate34that supports and forms a variable capacitor with a flexible diaphragm36. In illustrative embodiments, the backplate34is formed from single crystal silicon (e.g., the top layer of a silicon-on-insulator wafer; a “SOI” wafer), while the diaphragm36is formed from deposited polysilicon. Other embodiments, however, use other types of materials to form the backplate34and the diaphragm36. For example, a single crystal silicon bulk wafer, or some deposited material may form the backplate34. In a similar manner, a single crystal silicon bulk wafer, part of a silicon-on-insulator wafer, or some other deposited material may form the diaphragm36. To facilitate operation, the backplate34has a plurality of through-holes38that lead to a backside cavity40. As discussed below, these through-holes38have a secondary function of acting as a filter that helps prevent debris from contacting the diaphragm36.

Springs42movably connect the diaphragm36to the static portion of the microphone die20, which includes the backplate34. Other embodiments have no springs. Audio/acoustic signals cause the diaphragm36to vibrate, thus producing a changing capacitance. On-chip or off-chip circuitry (e.g., the circuit die22, among other things) receive and convert this changing capacitance into electrical signals that can be further processed. For example, the diaphragm36may oscillate about an equilibrium position (i.e., the rest position for the diaphragm36) in a direction that is generally orthogonal to its top and bottom faces. Normally, this oscillation should be minimal. Undesirably, however, the springs42may permit a much greater diaphragm swing about the equilibrium position. This greater swing could be so large as to damage the diaphragm36and/or springs42(discussed below).

The microphone shown inFIGS. 4 and 5often is referred to as a “single backplate microphone.” Specifically, this type of microphone has only one backplate34; namely, the backplate34between the diaphragm36and the backside cavity40. Alternative embodiments (not shown in the figures) may divide the backplate34into a plurality of sub-backplates34that are in the same plane and/or on the same side of the diaphragm36and thus, they are still considered a single backplate34. Accordingly, some backplate embodiments may produce a single variable capacitance, while others produce a single variable capacitance from multiple variable sub-capacitances with one or more diaphragms36. Because they both use single backplates34to reproduce the incoming acoustic signal, both examples thus may be considered to effectively form a single plate variable capacitance. This is in contrast to a double backplate MEMS microphone design, which has backplates on both sides of the diaphragm36.

It should be noted that discussion of the specific microphone die20shown inFIGS. 4 and 5is for illustrative purposes only. For example, as noted above, the microphone die20may have multiple sub-diaphragms36facing multiple-sub-backplates34, or be formed from a bulk silicon wafer and not from an SOI wafer. Other microphone configurations thus may be used with illustrative embodiments of the invention.

The positioning of the diaphragm36and backplate34presents a balance between having a sufficiently high variable capacitance signal and potential interference with free diaphragm movement. Specifically, while the diaphragm36typically is positioned very close to the backplate34to provide a strong variable capacitance signal, it preferably is spaced far enough away to not clip the signal by frequently contacting the backplate34. For example, a diaphragm36spaced about 3 to 4 microns away from the backplate34should provide sufficient clearance for normal operation of certain microphone dies20. In that case, that diaphragm36normally may vibrate up to about three microns about its equilibrium position (i.e., as noted above, the position of the diaphragm36when no acoustic signal is received).

Undesirably, however, the microphone die20may be subjected to sudden and sometimes short high pressure events (e.g., pressure spikes) that forcefully move the diaphragm36far away from the backplate34, beyond its normal range. For example, when used within a mobile telephone, a high-pressure event may occur when closing a car door, or positioning the device inside a sealed strong box that is suddenly closed. This can cause a shock or sudden pressure that can forcefully move the diaphragm36away from the backplate34a substantial distance, which can damage the fragile microstructure (e.g., the diaphragm36and springs42) and disable the microphone die20. For example, simulations of a specific microphone die20similar to that discussed above showed that such shocks can move the diaphragm36fifteen or more microns from the equilibrium point. Such simulations of that specific microphone die also demonstrated that displacements of greater than about seventeen microns could actually break the diaphragm36. In other words, although it does not always damage the diaphragm36and/or springs42, such a large displacement is expected to often damage the diaphragm36and/or springs42—it is outside of the rated range for the microphone die20.

The inventors responded to this problem by positioning a stop member44relatively close to the side of the diaphragm36that is opposite the backplate34. To mitigate the risk of damaging the microphone die20, the stop member44can be positioned a distance from the diaphragm36that is less than the maximum distance the diaphragm36can travel before being highly likely to break or damage the microstructure. For example, the stop member44can be placed between five and fifteen microns from the diaphragm36in its equilibrium position—preventing it from exceeding rated distances. Accordingly, the stop member44limits diaphragm movement to a distance that should not damage the microphone die components (e.g., the diaphragm36and/or the springs42).

To that end,FIG. 6Aschematically shows a cross-sectional view of the packaged microphone10ofFIGS. 2 and 3in accordance with illustrative embodiments of the invention. As noted above, the interior chamber28has the noted microphone die20for receiving incoming acoustic signals, and an ASIC die22electrically controlling the microphone die20(e.g., biasing its plates and managing signal transmission to and from the package18). The interior chamber28may have additional components that are not shown, such as passive components and integrated passive devices.

The microphone die20preferably is mounted directly over and covering the inlet aperture30. Accordingly, incoming acoustic signals enter the interior chamber28and pass through the backside cavity40and backplate34through-holes38before striking the diaphragm36. As known by those skilled in the art, this region between the inlet aperture30and the diaphragm36is known as the “front volume” of the microphone die20, or the front volume of the interior chamber28. Other embodiments, however, may position the microphone die20in other regions of the interior chamber28. Accordingly, discussion of this embodiment is for exemplary purposes only.

In accordance with illustrative embodiments of the invention, the packaged microphone10also has the above noted stop member44to protect the structural integrity of the microphone die20. To that end, the stop member44may be directly secured to the microphone die20in the back volume of the interior chamber28. In this embodiment, the stop member44is considered to be in a “stacked configuration” with the microphone die20. Specifically, in this stacked configuration, the stop member44is stacked upon the top, generally planar surface of the microphone die20within the interior chamber28. The stop member44thus has a generally planar bottom face (from the perspective of these drawings) that is generally parallel with the generally planar top face of the diaphragm36. Alternative embodiments of the stop member44, however, may be non-planar, with dimples, curved portions, or other similar features (discussed below).

The stop member44may be an integral part of the microphone die20—formed on the die20during the die fabrication/micromachining process. Alternatively, the stop member44may be formed as a separate component secured to the microphone die20in a post-fabrication processing step (discussed in greater detail below with regard toFIG. 8). Among other things, the stop member44may be formed from a heterogeneous or homogeneous material, such as one or more of single crystal silicon, polysilicon, laminate (e.g., BT laminate), circuit board material (e.g., BT laminate or FR-4 circuit board material), metal, ceramic, or other material that may be used in semiconductor or packaging processes.

Illustrative embodiments ensure that the stop member44does not appreciably impede the intended movement of the diaphragm36. To that end, as noted above, the stop member44preferably is positioned relatively far from the diaphragm36in its equilibrium position. Among other ranges, this gap can range from slightly more the normal range of motion of the diaphragm36to multiple times that range. For example, the above discussed microphone having a diaphragm36that normally moves about three microns above and below its equilibrium point may position the stop member44between about four and about fifteen microns from the diaphragm36above the equilibrium position. As such, this embodiment should prevent the diaphragm36from moving a distance that could potentially damage the fragile microstructure of the microphone die20. Moreover, alternative embodiments space the stop member44a distance that is the same or closer to the diaphragm36than the spacing between the backplate34and the diaphragm36.

To further reduce its impact on normal microphone operation, the stop member44also may have one or more relief holes46or other similar features to relieve squeeze film damping effects it may produce.FIG. 6B, for example, shows a top view of the stop member44and its plurality of pressure relief holes46. Moreover, this view also shows that the stop member44does not necessarily cover the entire surface area of the diaphragm36. Instead, the stop member44may have portions around its periphery that, from a plan view, do not cover the diaphragm36. In fact, some embodiments may implement the stop member44to have a very small surface area. For example, such embodiments may implement the stop member44as a mesh or other structure that, if necessary, merely provides point or line contact to the diaphragm36. To improve manufacturability, some embodiments may form a single large hole46through the stop member44. This single hole46may have a diameter that is slightly smaller than the diameter of the diaphragm36and, in some embodiments, is generally concentric with the diaphragm36. Accordingly, the stop member44contacts the outer periphery of the diaphragm36only during a high pressure event.

To ensure proper microphone performance, illustrative embodiments mitigate the electrostatic impact of the stop member44on the diaphragm36. For example, the stop member44may have a floating voltage, a negligible voltage (e.g., if it were formed from a non-conductive material), or have a controlled bias voltage, such as a voltage that is substantially equal to that of the diaphragm36. The stop member44nevertheless cannot be considered to be a backplate34and thus, does not form a variable capacitance that is used in any manner by the ASIC or packaged microphone10. Instead, the stop member44generally is a substantially inert, generally electrically irrelevant member with a principal function of limiting the maximum distance that the diaphragm36may move. As known by those skilled in the art, incidental electrostatic interaction with the diaphragm36does not transform it into a backplate34, especially where it does not perform such a function within the packaged microphone10.

FIGS. 6C and 6Dschematically show additional plan views of the microphone die20before the stop member44is secured to its top surface. As noted above, details of this process are discussed below with regard toFIG. 8.

Alternative embodiments do not form the stop member44directly on the microphone die20.FIG. 7schematically shows one such embodiment, in which the stop member44extends downwardly (from the perspective of the drawings) from the interior surface of the lid26, but does not contact the microphone die20in any manner when the diaphragm36is in its equilibrium position. Like the embodiment described above with regard toFIG. 6A, this embodiment also is configured to minimize its impact on diaphragm movement. As such, like the stop member44inFIG. 6A, this stop member44also has features (e.g., apertures) that permit airflow through its body, and may be spaced far enough from the diaphragm36to reduce its damping effect on the diaphragm36. In addition, also like the stop member44discussed above with regard toFIG. 6A, this stop member44also may have a minimum potential contact area with the diaphragm36, and minimal electrostatic interaction with the diaphragm36(e.g., the stop member44also does not form or perform the function of a backplate34).

Discussion of the specific stop member configurations ofFIGS. 6A and 7are but two of a number of different potential implementations. Those skilled in the art therefore can configure the stop member44in any number of additional manners, such as a stop member44that is merely around at least a portion of the periphery of the diaphragm36, or only at certain locations (e.g., a collection of spaced apart components that effectively form a single stop member44).

FIG. 8shows a process of forming the microphone system/packaged microphone10in accordance with illustrative embodiments of the invention. It should be noted that for simplicity, this described process is a significantly simplified version of an actual process used to form the microphone system10. Accordingly, those skilled in the art would understand that the process may have additional steps and details not explicitly shown inFIG. 8. Moreover, some of the steps may be performed in a different order than that shown, or at substantially the same time. Those skilled in the art should be capable of modifying the process to suit their particular requirements.

In illustrative embodiments, the packaged microphone10is formed in a batch process that simultaneously forms dozens, hundreds, or even thousands of packaged microphones at the same time. To that end, this process is described as using panels of packaging material (e.g., laminate, FR-4, ceramic substrate material, or pre-molded leadframe packaging) that ultimately form the bases24of each of the packaged microphones10. It nevertheless should be noted that those skilled in the art can apply these techniques to other batch processes, or processes that form only one microphone at a time.

The process begins at step800, which secures the microphone die20and ASIC die22to the base24. More specifically, the panel is considered to have a two dimensional array of individual bases24that each ultimately form a portion of a single packaged microphone10. Each base24has its pre-formed inlet aperture30and configuration of contacts/pads32on its upper surface. Accordingly, the process first may apply adhesive to the panel at prescribed locations on the upper panel surface. This adhesive may be a conductive or non-conductive epoxy commonly used in the MEMS packaging space. Next, this step may place an array of microphone dies20in designated location over their respective inlet apertures30, and an array of ASICs in their designated locations next to the microphone dies20. The step also may position passive components or other devices onto prescribed portions of the panel. The cured adhesive effectively secures each of these components to the panel.

After the components are secured to the panel, the process continues to step802, which electrically connects the microphone dies20and ASICs22to their bases24, and couples the stop members44to the microphone dies20. Specifically, this step first applies a conductive adhesive to certain pads21on the top surfaces of the microphone dies20and the ASIC dies22. This step also applies the conductive adhesive to pads23on the top face of the panel. As shown inFIG. 6C, this step next secures wire bonds48between the microphone dies20and their respective ASIC dies22, and between the ASIC dies22and those pads23on the top face of the panel with the adhesive. Alternatively, some embodiments may directly connect the microphone die20to the panel. In that case, the ASIC die22and microphone die20electrically communicate through electrical traces or conductive paths within the base24.

Next, as shown inFIG. 6D, this step dispenses stop adhesive45onto the top surfaces of the array of microphone dies20. Those skilled in the art should understand that the stop adhesive45is carefully dispensed and selected, and the stop members44are placed in a specific manner (e.g., with a specified downward pressure) to ensure that the distance between the stop member44and the diaphragm36is a certain prescribed distance from the diaphragm36. Of course, this distance is subject to certain manufacturing tolerances commonly associated with conventional packaging processes.

To provide more precision in the spacing between the stop members44and microphone dies20, some embodiments may place protruding features (e.g., fillets) on the stop member44or the microphone die20to more precisely position the stop members44. For example, such embodiments may have downwardly protruding fillets or other protrusions from the stop member44that contact but do not adhesively couple with the microphone die20—they only make contact with the microphone die20. Accordingly, the stop adhesive45can more coarsely couple the members together while the protrusions provide the precise spacing and planar relationships.

To minimize any interference with the movement of the diaphragm36, this stop adhesive45preferably does not contact the diaphragm36or springs42of any microphone die20. If used in the embodiment in which the stop member44has a controlled voltage, then this adhesive optionally may be conductive and positioned over additional pads21on the top surfaces of the microphone dies20. After dispensing the stop adhesive45, this step then places the stop members44directly on their respective microphone dies20(e.g., seeFIG. 6B).

It should be noted that the stop adhesive application and stop member placement portions of this step may be omitted if the stop member44was formed directly on the microphone die20during the fabrication process.

Step804then secures the lids26to the panels by conventional means. For example, the process may apply a plurality of rings of adhesive about each base24on the panel. Some embodiments may use a conductive adhesive to appropriately control the potential of the lids26. For example, such embodiments may normally ground the potential of the lid26during use.

At this point, the panel may be considered to have a plurality of independently functional packaged microphones10. Accordingly, the process concludes at step806, which dices the panel along prescribed lines in the panel to form the plurality of independent packaged microphones10. Just prior to dicing, however, some embodiments may test the devices using conventional testing/probe processes.

Accordingly, using one or more simple stop members44as ruggedizing reinforcement, illustrative embodiments significantly enhance the robustness and potential usable lifespan of a microphone die20mounted with its backplate34in the front volume. Expensive flip-chip equipment is not required to protect the diaphragm36. In fact, regardless of its mounting within the interior chamber28, such a design is expected to better withstand undesired high pressure acoustic signals than those designs that do not have a stop member44.