JP4244885B2 - Electret condenser - Google Patents

Description

  The present invention relates to an electret capacitor having a vibrating electrode and a fixed electrode, and more particularly to an electret capacitor formed using MEMS (Micro Electro Mechanical Systems) technology.

  Conventionally, in electret capacitors, which are dielectric materials having permanent electric polarization applied to elements such as condenser microphones, an electret film and an air gap (cavity) layer are disposed between a fixed electrode and a movable electrode constituting a parallel plate capacitor. It has a structure provided with.

In recent years, in order to reduce the thickness of the air gap layer and reduce variations, a structure and manufacturing method of the air gap layer using a microfabrication technique have been proposed. As a specific example, a part of the Si substrate as shown in Patent Document 1 is removed by wet etching using potassium hydroxide to form a recess. Further, by providing a silicon nitride film, which is a planarizing film, on a polyimide surface that serves as a spacer as shown in Patent Document 2, variations in polyimide film thickness are suppressed.
JP 2002-345088 A JP 2002-315097 A

  However, in order to realize downsizing and high performance of recent devices, it is desired to realize an electret capacitor having a small parasitic capacity while being smaller and higher performance.

  It is an object of the present invention to provide an electret capacitor that is small and has high performance and has a small parasitic capacitance.

  In order to solve the above-described problem, a first electret capacitor according to the present invention is formed on a semiconductor substrate so as to cover a region in which a semiconductor substrate is selectively removed and a region in a central portion of the semiconductor substrate. An insulating film and an electrode film formed on the surface of the insulating film are provided, and the electrode film is formed in the region so as not to overlap the semiconductor substrate.

  According to the first electret capacitor of the present invention, since the electrode film is provided so as not to overlap the semiconductor substrate, the parasitic capacitance can be reduced.

  In the first electret capacitor according to the present invention, it is preferable that a wiring led out from the electrode film is formed so as to overlap the semiconductor substrate.

  A second electret capacitor according to the present invention is formed on a semiconductor substrate having a region removed so as to leave a peripheral portion, an insulating film formed on the semiconductor substrate so as to cover the region, and an insulating film surface An electrode film, and the electrode film is formed in the region so as not to overlap the semiconductor substrate.

  According to the second electret capacitor of the present invention, since the electrode film is provided so as not to overlap the semiconductor substrate, the parasitic capacitance can be reduced.

  In the second electret capacitor according to the present invention, it is preferable that a wiring led out from the electrode film is formed in a peripheral portion overlapping the semiconductor substrate.

  In the first or second electret capacitor according to the present invention, the end face of the insulating film is preferably formed in a region overlapping with the semiconductor substrate outside the region.

  As described above, according to the present invention, it is possible to realize a highly sensitive electret capacitor with a small parasitic capacitance. Furthermore, various application devices equipped with them can be widely supplied to society.

  First, an electret condenser microphone (hereinafter referred to as ECM), which is an application example of an electret condenser, will be described.

  FIG. 1 shows a configuration diagram of the ECM. 1A shows a top view of the ECM, and FIG. 1B shows a cross-sectional view of the ECM. In FIG. 1A, a microphone 18, an SMD (surface mount component) 19 such as a capacitor, and an FET (field effect transistor) 20 are mounted on a printed circuit board 21. In FIG. 1B, an ECM case 22 is shown.

  FIG. 2 is a circuit block diagram of the ECM. The internal circuit 23 of the ECM includes a microphone unit 18, an SMD 19, and an FET 20, and is configured to output signals from the output terminal 24 and the output terminal 25 to the external terminal 26 and the external terminal 27. As an actual operation, an input signal of about 2 V is made from the terminal 28 and an AC signal output of several tens of mV is made to the terminal 29. The terminals 27 and 30 are connected to an output terminal 25 that is a GND terminal in the ECM internal circuit 23.

  Hereinafter, embodiments of the electret condenser of the present invention will be described in detail.

  FIG. 3 is a cross-sectional view of the electret condenser of the present invention. FIG. 4 is a plan view of the lower electrode and the lead wiring of the electret capacitor.

  As shown in FIG. 3, a silicon oxide film 102 is formed on a semiconductor substrate 101, and a membrane region 113 is formed by removing the periphery of the semiconductor substrate 101 and the silicon oxide film 102 so as to leave them. Here, the membrane region 113 is a region where the semiconductor substrate 101 is partially removed so as to leave the periphery in order to allow the vibration film 112 to vibrate under pressure from the outside. A silicon nitride film 103 is formed on the silicon oxide film 102 and in the membrane region 113. On top of this, the lower electrode 104 and the extraction wiring 115 are formed from one conductive film. A silicon oxide film 105 and a silicon nitride film 106 are formed on the silicon nitride film 103, the lower electrode 104, and the lead wiring 115. Here, a leak hole 107 is formed in the lower electrode 104 and the silicon oxide film 105. The silicon nitride film 103 and the silicon nitride film 106 are formed so as to cover the lower electrode 104 and the silicon oxide film 105 in the membrane region 113 in which the leak hole 107 is formed. Here, the silicon nitride film 103 located in the membrane region 113, the lower electrode 104 made of a conductive film, the silicon oxide film 105, and the silicon nitride film 106 become the vibration film 112. The silicon oxide film 105 is an electret film that stores electric charges. Further, a fixed film 110 made of a conductive film covered with a silicon nitride film 114 is formed above the silicon nitride film 106. An air gap layer 109 is formed between the vibration film 112 and the fixed film 110, and a silicon oxide film 108 is formed between the other silicon nitride film 106 and the fixed film 110. The air gap layer 109 is formed so as to include at least the membrane region 113. A plurality of acoustic holes 111 are formed in the fixed film 110 above the air gap layer 109. An opening 116 is provided in the silicon nitride film 114, the fixed film 110, and the silicon oxide film 108 so that the lead wiring 115 is exposed. The lower electrode 104 is electrically connected to the gate of the FET 20 shown in FIG. Further, the fixed film 110 is exposed through an opening 117 provided in the silicon nitride film 114 and is connected to the GND terminal 25 in FIG.

  Here, the lower electrode 104 and the extraction wiring 115 will be described with reference to FIG. The lower electrode 104 and the extraction wiring 115 are formed from one conductive film. The lower electrode 104 is formed in the membrane region 113, and a leak hole 107 is formed in the periphery. A lead-out wiring 115 is formed so that the lower electrode 104 is electrically connected to the outside.

  Here, the reason why the lower electrode 104 is formed in the membrane region 113 will be described. The capacitance of the capacitor in the ECM is determined by a capacitance component that changes due to vibration of the diaphragm and a capacitance component that does not change. When the capacitance component that does not change due to the parasitic capacitance increases, it greatly affects the performance of the ECM. In the present invention, the lower electrode 104 of the electret condenser is provided inside the membrane region 113. With this configuration, the area of the MOS capacitor formed by the lower electrode 104, the semiconductor substrate 101, and the silicon oxide film 102 can be eliminated, and the area of the MOS capacitor formed by the lead wiring 115, the semiconductor substrate 101, and the silicon oxide film 102 can be reduced. be able to. That is, it is possible to prevent an increase in the capacitance component (parasitic capacitance) that does not change in the capacitor and to realize a small and high-performance electret capacitor.

  Further, of the silicon nitride film 103 to be the vibration film 112, the lower electrode 104 made of a conductive film, the silicon oxide film 105, and the silicon nitride film 106, the silicon nitride film 103 formed so as to cover the membrane region 113, silicon oxide End surfaces of the film 105 and the silicon nitride film 106 are formed so as to overlap the semiconductor substrate 101. The lower electrode 104 made of a conductive film in the vibration film 112 is formed inside the membrane region 113 so as not to overlap the semiconductor substrate 101.

  Thereby, the resonance frequency characteristic of the vibration film 112 can be controlled by adjusting the film thicknesses of the silicon nitride film 103, the silicon oxide film 105, and the silicon nitride film 106. That is, it is possible to easily control a capacitor capacity component that changes in response to an external pressure, and to realize a small and highly sensitive electret capacitor.

  Note that the conductive film for forming the lower electrode 104 can be formed using a metal such as polysilicon doped with impurities, gold, aluminum, or an aluminum-based alloy.

  Finally, the operation of the electret condenser will be described. In FIG. 3, when the vibrating membrane 112 receives sound pressure from above through the acoustic hole 111, the vibrating membrane 112 mechanically vibrates up and down according to the sound pressure. In FIG. 3, a parallel plate type capacitor structure is formed using the lower electrode 104 and the fixed film 110 as electrodes. When the vibration film 112 vibrates, the distance between the electrodes of the lower electrode 104 and the fixed film 110 changes, so that the capacitance (C) of the capacitor changes. Since the charge (Q) stored in the capacitor is constant, a change occurs in the voltage (V) between the lower electrode 104 and the fixed film 110. This is because it is necessary to physically satisfy the condition of the following formula (1).

Q = CV (1)
Since the fixed film 110 is electrically connected to the gate of the FET 20 in FIG. 2, the gate potential of the FET 20 changes due to vibration of the vibration film. The change in the potential of the gate of the FET 20 is output to the external output terminal 29 as a change in voltage.

  As described above, the electret capacitor of the present invention can reduce the parasitic capacitance, and thus is useful for realizing a high-performance and small-sized ECM.

Configuration diagram of electret condenser microphone Circuit diagram of electret condenser microphone Sectional drawing of the electret capacitor | condenser of embodiment of this invention The top view of the lower electrode of the electret capacitor | condenser of embodiment of this invention, and extraction wiring

Explanation of symbols

18 Microphone part 19 SMD
20 FET
21 Printed Circuit Board 22 Case of ECM 23 Internal Circuit of ECM 24 Output Terminal 25 Output Terminal 26 External Terminal 27 External Terminal 28 Terminal 29 Terminal 30 Terminal 101 Semiconductor Substrate 102 Silicon Oxide Film 103 Silicon Nitride Film 104 Lower Electrode 105 Silicon Oxide Film 106 Silicon Nitride film 107 Leak hole 108 Silicon oxide film 109 Air gap layer 110 Fixed film 111 Acoustic hole 112 Vibration film 113 Membrane region 114 Silicon nitride film 115 Lead wire 116 Opening 117 Opening

Claims (5)

In the central portion of the semiconductor substrate, a region where the semiconductor substrate is selectively removed,
A vibrating membrane having an electrode film formed on the semiconductor substrate so as to cover the region;
A fixed film formed on the vibration film;
An air gap formed by removing a portion of the sacrificial layer, which exists between the vibrating membrane and the fixed membrane;
The electrode film is formed in the region so as not to overlap the semiconductor substrate ,
The vibration film includes an insulating film in which a silicon nitride film, a silicon oxide film, and a silicon nitride film are stacked in this order so as to overlap the semiconductor substrate at an end surface . An insulating layer exists between the vibrating membrane and the fixed membrane,
The condenser microphone according to claim 1, wherein the insulating layer is a portion other than the air gap in the sacrificial layer. The condenser microphone according to claim 2, wherein the insulating layer is a silicon oxide film. The fixing film condenser microphone according to any one of claims 1 to 3, characterized in that it has a sound hole that penetrates the fixing film. It said to overlap with the semiconductor substrate, the condenser microphone according to any one of claims 1 to 4, characterized in that the wire drawn from the electrode film is formed.

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