1. Technical Field
One or more embodiments of the present invention relate to acoustic sensors and microphones, and specifically to an MEMS (Micro Electro Mechanical Systems) type acoustic sensor manufactured by using the MEMS technique, and a microphone using such acoustic sensor.
2. Related Art
A capacitance type acoustic sensor is disclosed in Japanese Patent Publication No. 4338395 and Japanese Unexamined Patent Publication No. 2009-89097. In the capacitance type acoustic sensor, a diaphragm (movable electrode film) is arranged on a front surface of a silicon substrate, a back plate is fixed on the front surface of the silicon substrate so as to cover the diaphragm, and a capacitor is configured by the fixed electrode film and the diaphragm of the back plate. The diaphragm is vibrated with the acoustic vibration, and the change in electrostatic capacitance between the fixed electrode film and the diaphragm in such case is output. A great number of acoustic holes are opened in the back plate because the acoustic vibration needs to be introduced to an air gap between the fixed electrode film and the diaphragm in order to vibrate the diaphragm with the acoustic vibration.
In such acoustic sensor, the opening size of the acoustic hole needs to be made large to enhance the S/N ratio. However, the acoustic hole that is opened in the back plate is opened not only at the plate portion having a relatively thick film thickness but also at the fixed electrode film having a thin thickness. Therefore, if the opening size of the acoustic hole is made large, the extraction wiring portion of the fixed electrode film may easily break or the parasitic resistance may increase.
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