Patent Application
20140307909
The invention generally relates to microphone systems and, more particularly, the invention relates to transducers.
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.
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.
Those skilled in the art should more fully appreciate advantages of various embodiments of the invention from the following “Description of Illustrative Embodiments,” discussed with reference to the drawings summarized immediately below.
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.
A board aperture 16 (shown in phantom) extends upwardly through the printed circuit board 12 to the bottom of the microphone package (identified by reference number 18, discussed in detail below). To ensure proper receipt of the acoustic signal, the microphone package 18 may be sealed to the top surface of the printed circuit board 12 by 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 board 12, the gasket, and an inlet aperture 30 in the bottom surface of the package 18.
Those skilled in the art can mount the packaged microphone 10 onto the printed circuit board 12 using 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 microphone 10 and two other miscellaneous circuit components 14 are shown for simplicity. The circuit board 12 thus 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 microphone 10 of this embodiment has a microphone package 18 that contains both a MEMS microphone (hereinafter “microphone die 20”) and application-specific internal circuit (“ASIC 22” or “circuit die 22”). Illustrative embodiments may use a variety of different types of MEMS microphone dies, such as that shown in cross-section by example in
To those ends,
The packaged microphone 10 shown in those figures has a package base 24 that, together with a corresponding lid 26, forms an interior chamber 28 (shown in
As shown in
In alternative embodiments, however, the inlet aperture 30 is at another location, such as through the top of the lid 26, or through one of the side walls of the lid 26. For example, the inlet aperture 30 can extend through the lid 26 with a connection to the microphone die 20. The package 18 also may have two or more ports/apertures 30. For example, the package 18 could have a second input port (not shown) for directional sound purposes. Accordingly, discussion of a package 18 having its inlet aperture 30 through the base 24 is but one example of a variety of different embodiments.
In illustrative embodiments, the package base 24 shown in
The package 18 may have selective metallization to protect it from electromagnetic interference. For example, the lid 26 could be formed from stainless steel, while the base 24 could include printed circuit board material, such as metal layers and FR-4 substrate material. Alternatively, the lid 26 could 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 die 20. Moreover, various embodiments may form the base 24 and lid 26 from similar or the same materials. For example, both can be formed from a laminate, or the lid 26 can be formed from a laminate, while the base 24 can be formed from a carrier or pre-molded leadframe.
The interior chamber 28 can contain any of a variety of different types of microphone dies 20. To that end,
Among other things, the microphone die 20 includes a single static backplate 34 that supports and forms a variable capacitor with a flexible diaphragm 36. In illustrative embodiments, the backplate 34 is formed from single crystal silicon (e.g., the top layer of a silicon-on-insulator wafer; a “SOI” wafer), while the diaphragm 36 is formed from deposited polysilicon. Other embodiments, however, use other types of materials to form the backplate 34 and the diaphragm 36. For example, a single crystal silicon bulk wafer, or some deposited material may form the backplate 34. 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 diaphragm 36. To facilitate operation, the backplate 34 has a plurality of through-holes 38 that lead to a backside cavity 40. As discussed below, these through-holes 38 have a secondary function of acting as a filter that helps prevent debris from contacting the diaphragm 36.
Springs 42 movably connect the diaphragm 36 to the static portion of the microphone die 20, which includes the backplate 34. Other embodiments have no springs. Audio/acoustic signals cause the diaphragm 36 to vibrate, thus producing a changing capacitance. On-chip or off-chip circuitry (e.g., the circuit die 22, among other things) receive and convert this changing capacitance into electrical signals that can be further processed. For example, the diaphragm 36 may oscillate about an equilibrium position (i.e., the rest position for the diaphragm 36) in a direction that is generally orthogonal to its top and bottom faces. Normally, this oscillation should be minimal. Undesirably, however, the springs 42 may permit a much greater diaphragm swing about the equilibrium position. This greater swing could be so large as to damage the diaphragm 36 and/or springs 42 (discussed below).
The microphone shown in
It should be noted that discussion of the specific microphone die 20 shown in
The positioning of the diaphragm 36 and backplate 34 presents a balance between having a sufficiently high variable capacitance signal and potential interference with free diaphragm movement. Specifically, while the diaphragm 36 typically is positioned very close to the backplate 34 to provide a strong variable capacitance signal, it preferably is spaced far enough away to not clip the signal by frequently contacting the backplate 34. For example, a diaphragm 36 spaced about 3 to 4 microns away from the backplate 34 should provide sufficient clearance for normal operation of certain microphone dies 20. In that case, that diaphragm 36 normally may vibrate up to about three microns about its equilibrium position (i.e., as noted above, the position of the diaphragm 36 when no acoustic signal is received).
Undesirably, however, the microphone die 20 may be subjected to sudden and sometimes short high pressure events (e.g., pressure spikes) that forcefully move the diaphragm 36 far away from the backplate 34, 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 diaphragm 36 away from the backplate 34 a substantial distance, which can damage the fragile microstructure (e.g., the diaphragm 36 and springs 42) and disable the microphone die 20. For example, simulations of a specific microphone die 20 similar to that discussed above showed that such shocks can move the diaphragm 36 fifteen 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 diaphragm 36. In other words, although it does not always damage the diaphragm 36 and/or springs 42, such a large displacement is expected to often damage the diaphragm 36 and/or springs 42—it is outside of the rated range for the microphone die 20.
The inventors responded to this problem by positioning a stop member 44 relatively close to the side of the diaphragm 36 that is opposite the backplate 34. To mitigate the risk of damaging the microphone die 20, the stop member 44 can be positioned a distance from the diaphragm 36 that is less than the maximum distance the diaphragm 36 can travel before being highly likely to break or damage the microstructure. For example, the stop member 44 can be placed between five and fifteen microns from the diaphragm 36 in its equilibrium position—preventing it from exceeding rated distances. Accordingly, the stop member 44 limits diaphragm movement to a distance that should not damage the microphone die components (e.g., the diaphragm 36 and/or the springs 42).
To that end,
The microphone die 20 preferably is mounted directly over and covering the inlet aperture 30. Accordingly, incoming acoustic signals enter the interior chamber 28 and pass through the backside cavity 40 and backplate 34 through-holes 38 before striking the diaphragm 36. As known by those skilled in the art, this region between the inlet aperture 30 and the diaphragm 36 is known as the “front volume” of the microphone die 20, or the front volume of the interior chamber 28. Other embodiments, however, may position the microphone die 20 in other regions of the interior chamber 28. Accordingly, discussion of this embodiment is for exemplary purposes only.
In accordance with illustrative embodiments of the invention, the packaged microphone 10 also has the above noted stop member 44 to protect the structural integrity of the microphone die 20. To that end, the stop member 44 may be directly secured to the microphone die 20 in the back volume of the interior chamber 28. In this embodiment, the stop member 44 is considered to be in a “stacked configuration” with the microphone die 20. Specifically, in this stacked configuration, the stop member 44 is stacked upon the top, generally planar surface of the microphone die 20 within the interior chamber 28. The stop member 44 thus has a generally planar bottom face (from the perspective of these drawings) that is generally parallel with the generally planar top face of the diaphragm 36. Alternative embodiments of the stop member 44, however, may be non-planar, with dimples, curved portions, or other similar features (discussed below).
The stop member 44 may be an integral part of the microphone die 20—formed on the die 20 during the die fabrication/micromachining process. Alternatively, the stop member 44 may be formed as a separate component secured to the microphone die 20 in a post-fabrication processing step (discussed in greater detail below with regard to
Illustrative embodiments ensure that the stop member 44 does not appreciably impede the intended movement of the diaphragm 36. To that end, as noted above, the stop member 44 preferably is positioned relatively far from the diaphragm 36 in its equilibrium position. Among other ranges, this gap can range from slightly more the normal range of motion of the diaphragm 36 to multiple times that range. For example, the above discussed microphone having a diaphragm 36 that normally moves about three microns above and below its equilibrium point may position the stop member 44 between about four and about fifteen microns from the diaphragm 36 above the equilibrium position. As such, this embodiment should prevent the diaphragm 36 from moving a distance that could potentially damage the fragile microstructure of the microphone die 20. Moreover, alternative embodiments space the stop member 44 a distance that is the same or closer to the diaphragm 36 than the spacing between the backplate 34 and the diaphragm 36.
To further reduce its impact on normal microphone operation, the stop member 44 also may have one or more relief holes 46 or other similar features to relieve squeeze film damping effects it may produce.
To ensure proper microphone performance, illustrative embodiments mitigate the electrostatic impact of the stop member 44 on the diaphragm 36. For example, the stop member 44 may 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 diaphragm 36. The stop member 44 nevertheless cannot be considered to be a backplate 34 and thus, does not form a variable capacitance that is used in any manner by the ASIC or packaged microphone 10. Instead, the stop member 44 generally is a substantially inert, generally electrically irrelevant member with a principal function of limiting the maximum distance that the diaphragm 36 may move. As known by those skilled in the art, incidental electrostatic interaction with the diaphragm 36 does not transform it into a backplate 34, especially where it does not perform such a function within the packaged microphone 10.
Alternative embodiments do not form the stop member 44 directly on the microphone die 20.
Discussion of the specific stop member configurations of
In illustrative embodiments, the packaged microphone 10 is 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 bases 24 of each of the packaged microphones 10. 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 step 800, which secures the microphone die 20 and ASIC die 22 to the base 24. More specifically, the panel is considered to have a two dimensional array of individual bases 24 that each ultimately form a portion of a single packaged microphone 10. Each base 24 has its pre-formed inlet aperture 30 and configuration of contacts/pads 32 on 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 dies 20 in designated location over their respective inlet apertures 30, and an array of ASICs in their designated locations next to the microphone dies 20. 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 step 802, which electrically connects the microphone dies 20 and ASICs 22 to their bases 24, and couples the stop members 44 to the microphone dies 20. Specifically, this step first applies a conductive adhesive to certain pads 21 on the top surfaces of the microphone dies 20 and the ASIC dies 22. This step also applies the conductive adhesive to pads 23 on the top face of the panel. As shown in
Next, as shown in
To provide more precision in the spacing between the stop members 44 and microphone dies 20, some embodiments may place protruding features (e.g., fillets) on the stop member 44 or the microphone die 20 to more precisely position the stop members 44. For example, such embodiments may have downwardly protruding fillets or other protrusions from the stop member 44 that contact but do not adhesively couple with the microphone die 20—they only make contact with the microphone die 20. Accordingly, the stop adhesive 45 can 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 diaphragm 36, this stop adhesive 45 preferably does not contact the diaphragm 36 or springs 42 of any microphone die 20. If used in the embodiment in which the stop member 44 has a controlled voltage, then this adhesive optionally may be conductive and positioned over additional pads 21 on the top surfaces of the microphone dies 20. After dispensing the stop adhesive 45, this step then places the stop members 44 directly on their respective microphone dies 20 (e.g., see
It should be noted that the stop adhesive application and stop member placement portions of this step may be omitted if the stop member 44 was formed directly on the microphone die 20 during the fabrication process.
Step 804 then secures the lids 26 to the panels by conventional means. For example, the process may apply a plurality of rings of adhesive about each base 24 on the panel. Some embodiments may use a conductive adhesive to appropriately control the potential of the lids 26. For example, such embodiments may normally ground the potential of the lid 26 during use.
At this point, the panel may be considered to have a plurality of independently functional packaged microphones 10. Accordingly, the process concludes at step 806, which dices the panel along prescribed lines in the panel to form the plurality of independent packaged microphones 10. Just prior to dicing, however, some embodiments may test the devices using conventional testing/probe processes.
Accordingly, using one or more simple stop members 44 as ruggedizing reinforcement, illustrative embodiments significantly enhance the robustness and potential usable lifespan of a microphone die 20 mounted with its backplate 34 in the front volume. Expensive flip-chip equipment is not required to protect the diaphragm 36. In fact, regardless of its mounting within the interior chamber 28, such a design is expected to better withstand undesired high pressure acoustic signals than those designs that do not have a stop member 44.
Although the above discussion discloses various exemplary embodiments of the invention, it should be apparent that those skilled in the art can make various modifications that will achieve some of the advantages of the invention without departing from the true scope of the invention.
