An electric condenser includes a fixed film 110 including a conductive film 118 to be an upper electrode, a vibrating film 112 including a lower electrode 104 and a silicon oxide film 105 to be an electric film, and a silicon oxide film 108 provided between the fixed film 110 and the vibrating film 112 and including an air gap 109. Respective parts of the fixed film 110 and the vibrating film 112 exposed in the air gap 109 are formed of silicon nitride films 106 and 114.

RELATED APPLICATION

This application is a national phase of PCT/JP2005/001765 filed on Feb. 7, 2005, which claims priority from Japanese Application No. 2004-061987 filed Mar. 5, 2004, the disclosures of which Applications are incorporated by reference herein. The benefit of the filing and priority dates of the International and Japanese Applications is respectfully requested.

TECHNICAL FIELD

The present invention relates to an electric condenser including a vibrating electrode and a fixed electrode, and more particularly relates to an electric condenser formed using a MEMS (Micro Electro mechanical Systems) technology.

BACKGROUND ART

Conventionally, in electric condensers, which are usually applied to elements for condenser microphones and the like, a structure is employed which includes an electric film as dielectrics having a permanent electric polarization and an air gap (cavity) layer between a fixed electrode and a movable electrode which compose a parallel-plate condenser.

In such electric condensers, the thickness of the air gap layer has a direct relationship with a capacitance value of the condenser and involves significant influence on performance of the microphone and the like. Specifically, as the air gap layer is thin, the sensitivity of the microphone or the like increases. In contrast, when variation in thickness of the air gap layer in the manufacturing process is large, variation in sensitivity increases in the microphone and the like. Accordingly, the air gap layer provided in the electric condenser is desired to be thin and has less variation in thickness in the manufacturing process.

In recent years, in order to reduce the thickness of the air gap layer and variation in the thickness thereof in the manufacturing process, a structure of an air gap layer and a method for manufacturing it which utilize a microfabrication technology have been proposed. Specifically, for example, a technique has been proposed in which part of a Si (silicon) substrate is removed by wet etching using potassium hydroxide to form a recess (see Patent Document 1).

Patent Document 1: Japanese Patent Application Laid Open Publication No. 200-345088 A.

DISCLOSURE OF INVENTION

Problems That the Invention is to Solve

However, in order to realize small-size and high-performance appliances in a recent tendency, smaller-size and higher-performance electric condensers are desired of which characteristic variation is small.

Under the circumstances, formation of an electric condenser including a fixed electrode and a movable electrode is being tried with the use of the MEMS technology. However, this presents a problem that in forming an air gap between the fixed electrode and the movable electrode by wet etching, the fixed electrode and the movable electrode stick to each other by surface tension of an etching solvent or a cleaning solvent, that is, a problem that an air gap layer of a desired thickness cannot be formed.

The present invention has been made in view of the foregoing and has its object of preventing electrodes from sticking to each other in forming an air gap in an electric condenser to control the thickness of an air gap layer with high precision.

Means for Solving the Problems

To attain the above object, an electric condenser of the present invention includes: a first film including a first electrode; a second film including a second electrode and an electric film; a first insulting film formed between the first film and the second film; and an air gap formed by removing part of the first insulating film, wherein respective parts of the first film and the second film exposed in the air gap are formed of a second insulating film.

The present invention renders it possible to implement a highly-reliable, small-size, and high-performance microphone. It becomes further possible to widely supply various practical devices equipped with the microphone to the public.

21printed circuit board

22case for ECM

23internal circuit of ECM

102silicon oxide film

105silicon oxide film

108silicon oxide film

BEST MODE FOR CARRYING OUT THE INVENTION

An electric condenser according to one embodiment of the present invention will by referring to a case applying it to an ECM as an example with reference to the accompanying drawings.

First, the ECM as an element employing the electric condenser of the present embodiment will be described.

FIG. 1(a) andFIG. 1(b) are constitutional diagrams of the ECM according to the present embodiment, whereinFIG. 1(a) is a plan view of the ECM, andFIG. 1(b) is a section of the ECM.

As shown inFIG. 1(a) andFIG. 1(b), the ECM of the present embodiment is so composed that an electric condenser18, an SMD (surface mounting device)19, such as a condenser, and an FET (filed effect transistor) portion20are mounted on a printed circuit board21. Though not shown inFIG. 1(a), the printed circuit board21on which the electric condenser18, the SMD19, and the FET portion20are mounted is protected with a case22.

FIG. 2is a circuit block diagram of the ECM of the present embodiment.

As shown inFIG. 2, an internal circuit23of the ECM of the present embodiment includes the electric condenser18, which is the electric condenser of the present embodiment as will be described later, the SMD19, and the FET portion20. Signals are output from an output terminal24and an output terminal25of the internal circuit23to an external terminal26and an external terminal27, respectively. In an actual operation, when a signal at a voltage of, for example, approximately 2 V is input from a terminal28connected to the external terminal26via a resistor, a singal having an AC voltage of, for example, several tens of microvolts is output to a terminal29connected to the external terminal26via a condenser. The external terminal27and a terminal30connected thereto are connected to the output terminal25serving as a GND terminal in the internal circuit23of the ECM.

The electric condenser of the present embodiment will be described below.FIG. 3is a section of the electric condenser of the present embodiment.

As shown inFIG. 3, the electric condenser of the present embodiment is a parallel plate condenser which includes, on a semiconductor substrate101having a region (hereinafter referred to as a membrane region113) removed to leave the peripheral part thereof, a vibrating film112formed so as to cover the membrane region113and a fixed film110arranged above the vibrating film112as electrodes with an air gap109interposed therebetween. The vibrating film112includes a lower electrode104while the fixed film110includes a conductive film (upper electrode)118.

Specifically, a silicon oxide film102is formed on the semiconductor substrate101on which the electric condenser of the present embodiment is mounted, and the membrane region113is formed in such a manner that the semiconductor substrate101and the silicon oxide film102are removed partially so that the respective peripheral parts thereof are left. In other words, the membrane region113is a region formed by partially removing the semiconductor substrate101so as to leave the peripheral part thereof for allowing the vibrating film112to vibrate upon receipt of pressure from outside.

A silicon nitride film103is formed on the silicon oxide film102so as to cover the membrane region113. On the silicon nitride film103, a lower electrode104and a lead wire115are formed which are made of the same conductive film. The lower electrode104is formed on the silicon nitride film103covering the membrane region103and a surrounding region thereof (part of an external region of the membrane region113) while the lead wire115is formed so as to be in contact with the lower electrode104on part of the silicon nitride film103located outside the membrane region113.

On each of the silicon nitride film103, the lower electrode104, and the lead wire115, a silicon oxide film105and a silicon nitride film106are formed in this order. Thus, the vibrating film112is formed of the silicon nitride film103, the lower electrode104made of the conductive film, the silicon oxide film105, and the silicon nitride film106which are located within the membrane region113. In the vibrating film112, a plurality of leak holes107are formed to communicate with the air gap109. The silicon nitride film103and the silicon nitride film106are formed so as to cover the entire surfaces of the lower electrode104and the silicon oxide film105including the inner wall faces of the leak holes107. The silicon oxide film105is an electric film that accumulates charge.

Further, as shown inFIG. 3, the fixed film110, which is formed of the conductive film118covered with the lower layer of a silicon nitride film114and the upper layer of a silicon nitride film119, is provided above the vibrating film112, that is, above the silicon nitride film106. The air gap109is formed between the vibrating film112and the fixed film110in the membrane region113and the surrounding region thereof (part of an external region of the membrane region113). In the other region, the silicon oxide film108is formed between the silicon nitride film106or the silicon oxide film102and the fixed film110. In other words, the air gap109is formed above a region including at least the entirety of the membrane region113while the fixed film110is supported above the vibrating film112by the silicon oxide film108. The air gap109is formed by partially removing the silicon oxide film108formed on the semiconductor substrate101and the membrane region113.

In sum, as a significant feature of the present embodiment, respective parts of the fixed film110and the vibrating film112which are exposed in the air gap109are formed of the silicon nitride films (the silicon nitride film114of the fixed film110and the silicon nitride film106of the vibrating film112, respectively), as shown inFIG. 3.

A plurality of acoustic holes111communicating with the air gap109are formed in the fixed film110located above the air gap109. Also, an opening116is formed in the silicon oxide film108and the fixed film110including the silicon nitride film114, so as to partially expose the lead wire115. The lower electrode104is electrically connected to a gate of the FET portion20shown inFIG. 2via the lead wire115. Further, an opening117is formed in the silicon nitride film119composing the fixed film110so that the conductive film118composing the fixed film110is exposed therethrough, whereby the conductive film118is electrically connected to the GND terminal25inFIG. 2.

FIG. 4is a plan view showing the lower electrode104and the lead wire115of the electric condenser in the present embodiment. As described above, the lower electrode104and the lead wire115are formed of the same conductive film. Further, as shown inFIG. 4, the lower electrode104is formed within the membrane region103, and the plurality of leak holes107are formed in the peripheral part of the lower electrode104. The lead wire115is formed for electrically connecting the lower electrode104to the outside.

The reason why the lower electrode104is formed within the membrane region113will be described below. The capacitance of the condenser in the ECM depends on a capacitance component that varies with the vibration of the vibrating film and a capacitance component that does not vary with the vibration of the vibrating film. When a parasitic capacitance is increased, the capacitance component that does not vary with the vibration of the vibrating film increases disadvantageously, so that the performance of the ECM is largely influence thereby. In contrast, in the present embodiment, the lower electrode104of the electric condenser is provided within the membrane region113. This eliminates a region where the lower electrode104overlaps the semiconductor substrate101, eliminating a MOS (Metal Oxide Semiconductor) capacitance of a large area composed of the lower electrode104, the silicon oxide film102, and the semiconductor substrate101. More specifically, the parasitic capacitance is limited only to a MOS capacitor of a small area composed of the lead wire115, the silicon oxide film102, and the semiconductor substrate101. Consequently, the capacitance component (parasitic capacitance) that does not vary in the condenser is prevented from increasing, attaining a small-size and high-performance electric condenser.

Further, in the present embodiment, of the constitutional elements of the vibrating film112, namely, of the silicon nitride film103, the lower electrode104formed of the conductive film, the silicon oxide film105, and the silicon nitride film106, the silicon nitride film103, the silicon oxide film105, and the silicon nitride film106which cover the membrane region113overlap the semiconductor substrate101. In other words, each edge of the silicon nitride film103, the silicon oxide film105, and the silicon nitride film106is located on the semiconductor substrate101. On the other hand, the lower electrode104formed of the conductive film in the vibrating film112is formed within the membrane region113so as not to overlap the semiconductor substrate101. In short, the edge of the lower electrode104is located within the membrane region113. This enables control of the resonance frequency characteristic of the vibrating film112by adjusting each thickness of the silicon nitride film103, the silicon oxide film105, and the silicon oxide film106. In other words, the capacitance component that varies upon receipt of pressure from outside in the condenser can be controlled easily, attaining a small-size and highly-sensitive electric condenser.

A description will be given below to the reason why the silicon nitride film103and the silicon nitride film106are formed so as to cover the lower electrode104and the silicon oxide film105. When an electric formed of a silicon oxide film comes in contact with a liquid, the charge in the electric is reduced significantly. In the present embodiment, in order to control such reduction in charge of the electric, at least the surfaces (the upper surface, the lower surface, and the side surface) of the silicon oxide film105serving as the electric are covered with the silicon nitride film103and the silicon nitride film106. In detail, the silicon nitride film106covers completely even the inner wall faces of the leak holes107formed in the vibrating film112so as not to expose the silicon oxide film (electric)105in the leak holes107. This realizes an electric condenser excellent in moisture resistance and thermal resistance.

FIG. 5is a plan view of the silicon nitride film114composing the fixed film110in the electric condenser of the present embodiment. As described above, the plurality of acoustic holes111are formed in the fixed film110formed above the semiconductor substrate101and the membrane region113. The acoustic holes111are arranged in the membrane region113and the surrounding region thereof (part of an external region of the membrane region113).

An operation of the electric condenser of the present embodiment will be described below. In the electric condenser of the present embodiment shown inFIG. 3, upon receipt of sound pressure from above through the acoustic holes111and the air gap109, the vibrating film112vibrates up and down mechanically in response to the sound pressure. The electric condenser of the present embodiment is a parallel-plate condenser which uses the lower electrode104of the vibrating film112and the conductive film118of the fixed film110as electrodes. Accordingly, vibration of the vibrating film112changes the distance between the lower electrode104and the conductive film118to change the capacitance (C) of the condenser. The charge (Q) capable of being accumulated in the condenser is fixed, and therefore, change in capacitance (C) of the condenser causes the voltage (V) between the lower electrode104and the fixed film110(the conductive film118) to change. The reason for this is that the condition given by the following expression (1) must be satisfied physically.
Q=C·V(1)

Further, since the lower electrode104is electrically connected to the gate of the FET portion20inFIG. 2, change in voltage (V) between the lower electrode104and the fixed film110(the conductive film118) changes the gate potential of the FET portion20. Thus, the gate potential of the FET portion20changes in response to the vibration of the vibrating film112, and the change in gate potential of the FET portion20is output to the external output terminal29inFIG. 2as a voltage change.

Incidentally, large variation in capacitance of a condenser in an air gap formation region of the ECM causes significant influence on the performance of the ECM.

In contrast, in the present embodiment, respective parts of the fixed film110and the vibrating film112exposed in the air gap109are formed of the insulating films, specifically, the silicon nitride films (the silicon nitride film114and the silicon nitride film106), which have tensile stress. In other words, the silicon nitride films cover the upper surface and the lower side of the silicon oxide film108in which the air gap109is formed. This prevents the vibrating film112and the fixed film110from sticking to each other by surface tension in forming the air gap109. Accordingly, the thickness of the air gap109, which determines the capacitance of the condenser in the region where the air gap109is to be formed, can be determined according to the film thickness of a thin film (the silicon oxide film108in the present embodiment) formed by a semiconductor microfabrication technique or the like, so that the air gap109can have a desired thickness. This attains a smaller-size and higher-performance electric condenser with less characteristic variation.

Hence, according to the present embodiment, a highly-reliable, small-size, and high-performance microphone can be contemplated. Further, various practical devices equipped with the microphone can be supplied widely to the public.

It should be noted that the silicon nitride films (the silicon nitride films106and114) are used at the respective parts of the fixed film110and the vibrating film112exposed in the air gap109, but any other kind of insulating films may be used only if it has tensile stress.

Further, in the present embodiment, any of silicon or polysilicon doped with an impurity, gold, refractory metal, aluminum, and aluminum-containing alloy, and the like may be used as a conductive material of the lower electrode104.

As well, in the present embodiment, any of silicon or polysilicon doped with an impurity, gold, refractory metal, aluminum, and aluminum-containing alloy, and the like may be used as a material of the conductive film118composing the fixed film110.

Moreover, in the present embodiment, a substrate made of an insulating material may be used rather than the semiconductor substrate101.

In addition, in the present embodiment, the silicon oxide film108is used as an insulting film (sacrificial layer) for forming the air gap109but the kind of the sacrificial layer is not limited especially. Also, the sacrificial layer may be a lamination layer of a plurality of insulating films made of the same material. This minimizes variation in thickness of the sacrificial layer, in turn, minimizes variation in thickness of the air gap, compared with a case using as the sacrificial layer a single-layer insulating film having the same thickness, with a result of further minimization of characteristic variation of the electric condenser.

INDUSTRIAL APPLICABILITY

The present invention relates to an electric condenser including a vibrating electrode and a fixed electrode. When applied to an ECM or the like formed using a MEMS technology, the present invention can realize a high-performance and highly-reliable ECM, and therefore, is very useful.