MEMS MICROPHONE

A MEMS microphone includes: a glass substrate including an opening portion; a membrane provided on the glass substrate so as to cover the opening portion and including a first conductive layer; and a backplate provided above the membrane via a cavity, including a plurality of through holes through which sound waves pass, and including a second conductive layer. The first conductive layer is made of a metal or a conductive oxide. The second conductive layer is made of a metal or a conductive oxide.

FIELD

The present invention relates generally to a MEMS microphone for detecting sound.

BACKGROUND

In a general MEMS microphone, an element is formed on a semiconductor substrate (e.g., a silicon substrate). The MEMS microphone can be miniaturized by utilizing a semiconductor manufacturing technology. The MEMS microphone includes a backplate electrode having a plurality of through holes and a membrane electrode (diaphragm electrode) that vibrates in accordance with sound pressures caused by sound waves. When the membrane electrode vibrates due to the sound pressure, a capacitance between the backplate electrode and the membrane electrode changes. The MEMS microphone detects sound by detecting a voltage change corresponding to the capacitance change.

SUMMARY

According to an aspect of the present invention, there is provided a MEMS microphone comprising:

a glass substrate including an opening portion;

a membrane provided on the glass substrate so as to cover the opening portion and including a first conductive layer; and

a backplate provided above the membrane via a cavity, including a plurality of through holes through which sound waves pass, and including a second conductive layer,

wherein

the first conductive layer is made of a metal or a conductive oxide, and

the second conductive layer is made of a metal or a conductive oxide.

DETAILED DESCRIPTION

Hereinafter, embodiments will be described with reference to the drawings. Note that the drawings are schematic or conceptual, and the dimensions and proportions of the drawings are not necessarily the same as the actual ones. Furthermore, even when parts shown in the drawings indicate the same part, they may be expressed with different dimensional relationships or ratios. Several embodiments described below merely show exemplary apparatuses and methods for implementing the technical ideas of the present invention, and the technical ideas are not limited by the element shapes, structures, arrangements, etc. described below. In the description below, structural elements having the same functions and configurations will be denoted by the same reference symbol, and a repetitive description of such elements will be given only where necessary.

The following embodiment is a configuration example of a MEMS microphone. MEMS (Micro Electro Mechanical Systems) is a device in which an electric circuit and a fine mechanical structure are integrated on one substrate.

[1] First Embodiment

[1-1] Configuration of MEMS Microphone10

FIG. 1is a plan view of a MEMS microphone10according to the first embodiment.FIG. 2is a bottom view of the MEMS microphone10shown inFIG. 1.FIG. 3is a cross-sectional view of the MEMS microphone10taken along line A-A′ shown inFIG. 1.FIG. 4is a cross-sectional view of the MEMS microphone10taken along line B-B′ shown inFIG. 1.FIG. 5is a cross-sectional view of the MEMS microphone10taken along line C-C′ shown inFIG. 1.

A general MEMS is formed using a semiconductor material and a semiconductor manufacturing technology. In addition, in the general MEMS, a silicon substrate is used so that microfabrication is advantageous. In contrast, in the present embodiment, a glass substrate11is used as a substrate of the MEMS microphone10. The glass substrate11has an insulating property. As the glass substrate11, for example, alkali-free glass is used. The glass substrate11is approximately 0.5 mm thick. The planar shape of the glass substrate11is, for example, a quadrangle. The present embodiment uses the glass substrate11for the MEMS microphone10, so is different in laminated structure from the general MEMS using a silicon substrate.

The glass substrate11includes an opening portion11A. The opening portion11A has a circular shape. The diameter of the opening portion11A is approximately 1 mm. The opening portion11A has, for example, a tapered shape such that the diameter continuously decreases from a lower surface side toward an upper surface side. The opening portion11A shown inFIG. 2shows the diameter (i.e., the minimum diameter) of the upper surface side of the glass substrate11.

A protective layer12is provided on the glass substrate11. The protective layer12is formed so as to surround the periphery of the opening portion11A of the glass substrate11. InFIGS. 1 and 2, the protective layer12is not shown. The protective layer12is made of a metal such as chromium (Cr). The opening portion11A of the glass substrate11is formed by wet etching using a solution containing hydrofluoric acid. The protective layer12has a function of preventing a membrane to be described later from being eroded by the solution in a wet etching step.

A membrane (also referred to as a diaphragm)13is provided on the glass substrate11and the protective layer12so as to cover the opening portion11A. The membrane13has a circular shape. The size of the membrane13is larger than that of the opening portion11A of the glass substrate11. The size of the opening portion11A is of the upper surface side of the glass substrate11. The membrane13has a three-layered structure in which an insulating layer13A, a conductive layer13B, and an insulating layer13C are sequentially stacked. The conductive layer13B functions as an electrode (vibration electrode) of the membrane13. The conductive layer13B is made of a metal, for example, molybdenum (Mo) having a film thickness of about 35 nm. As the conductive layer13B, chromium (Cr), aluminum (Al), etc. may be used. Alternatively, conductive oxides such as indium tin oxides (ITO) may be used as the conductive layer13B. Each of the insulating layer13A and the insulating layer13C is made of, for example, silicon nitride (SiNx) having a film thickness of about 250 nm. “x” in the chemical formula means that the composition ratio is discretionary.

The membrane13includes at least one through hole14.FIG. 2shows two through holes14as an example. InFIG. 3, one through hole14is exemplified for the sake of simplicity. The through hole14has a function of, in a case where a sudden sound pressure is applied to the membrane13, releasing the sound pressure to prevent the membrane13from being damaged.

A backplate16is provided above the membrane13via a cavity15. The backplate16is formed by sequentially stacking a conductive layer16A and an insulating layer16B. The conductive layer16A functions as an electrode (fixed electrode) of the backplate16. The conductive layer16A is made of a metal, for example, molybdenum (Mo) having a film thickness of about 35 nm. As the conductive layer16A, chromium (Cr), aluminum (Al), etc. may be used. Alternatively, conductive oxides such as indium tin oxides (ITO) may be used as the conductive layer16A. The insulating layer16B is made of, for example, silicon nitride (SiNx) having a film thickness of about 2000 nm.

The above-described cavity15is provided between the membrane13and the backplate16. The cavity15is approximately 3 μm thick. The cavity15is surrounded by the insulating layer17. In other words, the insulating layer17includes the cavity15. The insulating layer17is provided on the entire region of the glass substrate11excluding the cavity15. The insulating layer17and the insulating layer16B of the backplate16are continuous layers, and are integrally formed. That is, the insulating layer17and the insulating layer16B are provided on approximately the entire surface of the glass substrate11.

The backplate16includes a plurality of through holes18. The through holes18are provided over the entire surface of the backplate16. InFIG. 3, three through holes18are exemplified for the sake of simplicity. The through holes18have a function of passing a sound wave applied from a side of the backplate16opposite to the membrane13to the cavity15. Since the backplate16includes many through holes18, vibration of the backplate16in accordance with the sound pressure can be suppressed. That is, the conductive layer16A of the backplate16functions as a fixed electrode.

A plurality of protrusions19are provided on the membrane13side of the backplate16. AlthoughFIG. 3shows two protrusions19as an example, the number of protrusions19can be set discretionarily. The protrusions19have a function of preventing the backplate16and the membrane13from coming into contact with each other. Contact between the backplate16and the membrane13may occur during operation of the MEMS microphone10and during the manufacturing process.

A protective layer20is provided on the backplate16and the insulating layer17. The protective layer20is made of, for example, amorphous silicon having a film thickness of about 150 nm. The protective layer20has a function of protecting the insulating layer16B of the backplate16when a sacrificial layer (e.g., silicon oxide (SiOx)) for forming the cavity15is removed by wet etching. The backplate16may include the protective layer20. In this case, the backplate16has a three-layered structure in which the conductive layer16A, insulating layer16B, and insulating layer (protective layer)20are sequentially stacked.

Next, a configuration of a terminal (membrane terminal) electrically connected to the membrane13will be described with reference toFIGS. 1, 2, and 4.

A wiring layer21extending in a discretionary direction (e.g., an obliquely lower left direction inFIG. 1) is electrically connected to the conductive layer13B of the membrane13. The wiring layer21and the conductive layer13B are continuous layers, and are integrally formed. Insulating layers are provided under and on the conductive layer13B, respectively. The two insulating layers under and on the conductive layer13B are respectively continuous with the insulating layers13A and13C of the membrane13. A membrane terminal22is electrically connected to one end of the wiring layer21. The membrane terminal22is exposed by an opening portion23formed in the insulating layer17.

Next, a configuration of a terminal (backplate terminal) electrically connected to the backplate16will be described with reference toFIGS. 1, 2, and 5.

On the glass substrate11, an insulating layer24extending in a discretionary direction (e.g., an obliquely upper left direction inFIG. 1) is provided. The insulating layer24is made of, for example, silicon oxide (SiOx). The insulating layer24is configured to overlap an end portion of the membrane13. The insulating layer24and the cavity are partitioned by an insulating layer25. The insulating layer25is made of the same material (silicon nitride (SiNx)) as that of the insulating layer17, and is a continuous layer with the insulating layer17.

A wiring layer26extending in the same direction as that of the insulating layer24is electrically connected to the conductive layer16A of the backplate16. The wiring layer26is provided along both side surfaces and a bottom surface of the insulating layer25and is provided on the insulating layer24.

A backplate terminal27is electrically connected to one end of the wiring layer26. The backplate terminal27is exposed by an opening portion28formed in the insulating layer17.

As described above, the insulating layer24is formed to overlap the end portion of the membrane13. A region where the insulating layer24and the membrane13overlap with each other is indicated by reference sign “OA” inFIGS. 1 and 5. Due to the presence of the region OA, an end portion of the conductive layer13B of the membrane13and the wiring layer26are spaced apart. Thus, the wiring layer26and the conductive layer13B of the membrane13can be prevented from being short-circuited.

In the region OA, a distance between the wiring layer26and the conductive layer13B in the thickness direction can be increased. Thus, a parasitic capacitance in the region OA can be reduced. InFIG. 5, by reducing the width of the insulating layer25, that is, by increasing the length of the region OA, a region where the wiring layer26and the conductive layer13B face each other with the insulating layer13C interposed therebetween can be reduced. Thus, the parasitic capacitance in the region OA can be reduced. The parasitic capacitance is a capacitance generated in a region where the membrane13does not vibrate.

[1-2] Operation of MEMS Microphone10

Next, an operation of the MEMS microphone10configured as described above will be described.

The MEMS microphone10receives sound waves (and a sound pressure caused by the sound waves) from a side of the backplate16opposite to the membrane13. The backplate includes a large number of through holes18, and the sound waves pass through the through holes18to the cavity15. In addition, since the backplate16includes the large number of through holes18, vibration due to a sound pressure is suppressed and the backplate16functions as a fixed electrode.

The membrane13vibrates in accordance with the sound pressure, and functions as a vibration electrode. When the membrane13vibrates due to the sound pressure, a capacitance (electrostatic capacitance) of a parallel plate capacitor (parallel plate condenser) formed by the backplate16and the membrane13changes. The MEMS microphone10detects sound by detecting a voltage change corresponding to the capacitance change. Specifically, the MEMS microphone10includes a power supply (not shown) that applies a bias voltage to the capacitor formed by the backplate16and the membrane13, and a detection circuit (not shown) that detects the voltage change of the capacitor.

Here, in the present embodiment, a metal (e.g., molybdenum (Mo)) is used as the conductive layer13B of the membrane13, and a metal (e.g., molybdenum (Mo)) is used as the conductive layer16A of the backplate16.

FIG. 6is a diagram for explaining resistivities (Ωm) of a plurality of substances (conductive materials).FIG. 6lists the resistivities of polysilicon (p-Si), ITO (indium tin oxide), chromium (Cr), molybdenum (Mo), and aluminum (Al).

Molybdenum (Mo) has a lower resistivity than polysilicon (p-Si), indium tin oxide (ITO), chromium (Cr), etc. Therefore, by using molybdenum (Mo) for the membrane13and the backplate16, the resistance of the membrane13and the backplate16can be further reduced. Thus, the capacitance change between the membrane13and the backplate16can be accurately detected, and sensitivity of the MEMS microphone10can be improved. From the viewpoint of resistivity, it is also effective to use aluminum (Al) for the membrane13and the backplate16.

Molybdenum (Mo) has a high corrosion resistance to an acidic solution such as hydrofluoric acid. Therefore, even when hydrofluoric acid is used in the step of forming the opening portion11A in the glass substrate11, corrosion of the membrane13and the backplate16can be suppressed, and deterioration of electrical characteristics of the membrane and the backplate16can be suppressed. From the viewpoint of corrosion resistance, molybdenum (Mo) is more suitable than aluminum (Al).

Molybdenum (Mo) and aluminum (Al) can be processed by dry etching. Thus, when the membrane13made of a laminated film is processed and the through holes are formed in the membrane13, the manufacturing process is facilitated and the processing accuracy can be improved. Similarly, when the backplate16made of a laminated film is processed and the through holes are formed in the backplate16, the manufacturing process is facilitated and the processing accuracy can be improved. The use of molybdenum (Mo), etc. for the membrane13and the backplate16is advantageous in the manufacturing process of the MEMS microphone10.

[1-3] Advantageous Effects of First Embodiment

As described above in detail, in the first embodiment, the MEMS microphone10includes the glass substrate11having the opening portion11A, the membrane13provided on the glass substrate11so as to cover the opening portion11A and including the conductive layer13B, and the backplate16provided above the membrane13via the cavity15, including a plurality of through holes through which sound waves pass, and including the conductive layer16A. The conductive layer13B is made of a metal or conductive oxide, and the conductive layer16A is made of a metal or conductive oxide.

Therefore, according to the first embodiment, the resistance between the membrane electrode (the conductive layer13B of the membrane13) and the backplate electrode (the conductive layer16A of the backplate16) can be reduced. Thus, the sensitivity of the MEMS microphone10can be improved. For example, according to the first embodiment, the sensitivity of the MEMS microphone10can be improved as compared with the case where polysilicon is used for the membrane electrode and the backplate electrode.

The glass substrate11having an insulating property is used as a substrate of the MEMS microphone10. Thus, a plurality of layers formed on the glass substrate11can be prevented from being short-circuited through the glass substrate11. The glass substrate11is less expensive than a semiconductor substrate (e.g., a silicon substrate). Thus, the cost of the MEMS microphone10can be reduced.

The MEMS microphone10includes the wiring layer26electrically connected to the conductive layer16A of the backplate16. The insulating layer24made of, for example, silicon oxide (SiOx) is provided between the wiring layer26and the end portion of the membrane13. Thus, the wiring layer26and the conductive layer13B of the membrane13can be prevented from being short-circuited. In the region where the insulating layer24is disposed, the distance between the wiring layer26and the conductive layer13B in the thickness direction can be increased. Thus, the parasitic capacitance can be reduced.

[2] Second Embodiment

In the second embodiment, the membrane13has a two-layered structure including the conductive layer13B and the insulating layer13C.

The plan view and the bottom view of the MEMS microphone10according to the second embodiment are the same asFIGS. 1 and 2described in the first embodiment.FIGS. 7 to 9are cross-sectional views of the MEMS microphone10according to the second embodiment.FIG. 7is a cross-sectional view taken along line A-A′ shown inFIG. 1,FIG. 8is a cross-sectional view taken along line B-B′ shown inFIG. 1, andFIG. 9is a cross-sectional view taken along line C-C′ shown inFIG. 1.

The membrane13is provided on the glass substrate11so as to cover the opening portion11A. The membrane13has a circular shape. The size of the membrane13is larger than that of the opening portion11A of the glass substrate11.

The membrane13has a two-layered structure in which the conductive layer13B and the insulating layer13C are sequentially stacked. The conductive layer13B functions as an electrode (vibration electrode) of the membrane13. The conductive layer13B is provided in contact with the glass substrate11. Since the glass substrate11has an insulating property, it is not a problem if the conductive layer13B is in direct contact with the glass substrate11. The conductive layer13B and the insulating layer13C are made of the material described in the first embodiment.

Also in the membrane terminal22shown inFIG. 8, an insulating layer corresponding to the insulating layer13A is deleted. The other structures are the same as those in the first embodiment.

According to the second embodiment, the conductive layer13B included in the membrane13can be directly formed on the glass substrate11, and the membrane13can have a two-layered structure. Thus, the cost of the MEMS microphone10can be reduced.

In addition, since the thickness of the membrane13can be reduced, the membrane13easily vibrates according to the sound pressure. Thus, the sensitivity of the MEMS microphone10can be improved.

When a capacitance of a portion where a membrane (diaphragm) vibrates is Ci and a capacitance (parasitic capacitance) of a portion where a membrane does not vibrate is Cp, a capacitance of an entire MEMS microphone is “Ci+Cp”. Thus, when a capacitance change between the vibrating membrane and a backplate is detected, the parasitic capacitance Cp becomes a factor in deteriorating the sensitivity of the MEMS microphone. Therefore, in order to improve the sensitivity of the MEMS microphone, it is desirable to reduce the parasitic capacitance Cp. The third embodiment is a configuration example for reducing the capacitance (parasitic capacitance) of the portion where the membrane does not vibrate.

Hereinafter, the conductive layer16A of the backplate16is referred to as a backplate electrode16A, and the conductive layer13B of the membrane13is referred to as a membrane electrode13B.FIG. 10is a plan view of the backplate electrode16A, the membrane electrode13B, and the opening portion11A of the glass substrate11according to the third embodiment. That is,FIG. 10(a)is a plan view of the backplate electrode16A,FIG. 10(b)is a plan view of the membrane electrode13B, andFIG. 10(c)is a plan view of the opening portion11A of the glass substrate11. The cross-sectional structure of the MEMS microphone10is the same as that of the first embodiment or the second embodiment.

The diameter of the backplate electrode16A is Db, the diameter of the membrane electrode13B is Dm, and the diameter of the opening portion11A of the glass substrate11is Dg. As described above, the diameter Dg of the opening portion11A is the diameter (i.e., the minimum diameter) on the upper surface side of the glass substrate11. The diameter Db of the backplate electrode16A and the diameter Dm of the membrane electrode13B satisfy a relationship of “Db<Dm”. For example, Db=1.1 mm and Dm=1.2 mm.

In addition, for example, the diameter Db of the backplate electrode16A and the diameter Dg of the opening portion11A satisfy a relationship of “Db>Dg”. For example, Dg=1.0 mm.

In the MEMS microphone10configured as such, the electrode portion of the non-vibrating region of the membrane electrode13B does not constitute a capacitance. Accordingly, since the parasitic capacitance can be reduced, the sensitivity of the MEMS microphone10can be improved.

The fourth embodiment is another configuration example for reducing the parasitic capacitance of the MEMS microphone.

FIG. 11is a cross-sectional view of the MEMS microphone10according to the fourth embodiment.FIG. 11is a cross-sectional view taken along line A-A′ ofFIG. 1.FIG. 12is a plan view of the backplate electrode16A, the membrane electrode13B, and the opening portion11A of the glass substrate11according to the fourth embodiment. That is,FIG. 12(a)is a plan view of the backplate electrode16A,FIG. 12(b)is a plan view of the membrane electrode13B, andFIG. 12(c)is a plan view of the opening portion11A of the glass substrate11.

The membrane13has a three-layered structure in which an insulating layer13A, a conductive layer13B, and an insulating layer13C are sequentially stacked. The diameter of the conductive layer13B is smaller than the diameters of the insulating layers13A and13C.

The diameter of the backplate electrode16A is Db, the diameter of the membrane electrode13B is Dm, and the diameter of the opening portion11A of the glass substrate11is Dg. The diameter Dm of the membrane electrode13B and the diameter Dg of the opening portion11A satisfy a relationship of “Dm<Dg”. For example, Dm=1.0 mm and Dg=1.1 mm.

In addition, for example, the diameter Db of the backplate electrode16A and the diameter Dg of the opening portion11A satisfy a relationship of “Db>Dg”. For example, Db=1.2 mm.

In the MEMS microphone10configured as such, the entire membrane electrode13B vibrates according to a sound pressure. In other words, the membrane electrode13B does not include a non-vibrating region that becomes a parasitic capacitance. Accordingly, since the parasitic capacitance can be reduced, the sensitivity of the MEMS microphone10can be improved.

The present invention is not limited to the above-described embodiments, and the constituent elements can be modified and embodied without departing from the scope of the invention. Furthermore, the embodiments described above include inventions at various stages, and various inventions can be configured by an appropriate combination of a plurality of components disclosed in one embodiment or an appropriate combination of components disclosed in different embodiments. For example, even if some structural elements are deleted from all the structural elements disclosed in the embodiments, in the case where the problem to be solved by the invention can be solved and the effect of the invention can be obtained, an embodiment from which these structural elements are deleted can be extracted as an invention.