Pressure sensor and manufacturing method therefor

This application is a divisional of U.S. patent application Ser. No. 11/825,797, filed Jul. 9, 2007, which in turn claims priority on Japanese Patent Application No. 2006-189021, Japanese Patent Application No. 2006-196578, and Japanese Patent Application No. 2006-211889, the contents of which are incorporated herein by reference.

1. Field of the Invention

The present invention relates to pressure sensors such as condenser microphones (or silicon capacitor microphones), which are manufactured by way of semiconductor device manufacturing processes. The present invention also relates to manufacturing methods of pressure sensors.

2. Description of the Related Art

Conventionally, various types of pressure sensors such as pressure sensors of silicon capacitor types and condenser microphones, which can be manufactured by way of semiconductor device manufacturing processes, have been developed. A typical example of a pressure sensor of a silicon capacitor type is constituted of a diaphragm that vibrates due to pressure variations, a plate that is positioned opposite to the diaphragm via a dielectric such as air, and an air chamber (or a cavity). Electrostatic capacitance formed between the diaphragm and the plate varies due to vibration of the diaphragm. The pressure sensor converts variations of electrostatic capacitance into electric signals. The air chamber releases variations of internal pressure disturbing vibration of the diaphragm. Therefore, it is possible to improve output characteristics of the pressure sensor by increasing the volume of the air chamber.

Japanese Patent Application Publication No. 2004-537182 teaches a miniature silicon condenser microphone in which a cavity is formed between a recess of a substrate and a diaphragm covering the recess, wherein the internal wall of the recess is formed perpendicular to the diaphragm; hence, the opening of the recess cannot be increased to be larger than a thin film forming the diaphragm, and it is very difficult to form the cavity having a relatively large volume. Japanese Unexamined Patent Application Publication No. 2004-356707 teaches a condenser microphone in which a cavity is formed by means of a diaphragm and an internal wall of a through-hole formed in a substrate, wherein the through-hole is formed in a tapered shape whose diameter is increased in a direction opposite to a plate, so that the volume of the cavity can be increased to be larger than the volume of the cavity of the aforementioned miniature silicon condenser microphone. The tapered shape of the through-hole is formed using the lattice plane of silicon; hence, the tapered angle thereof is constant. This limits the volume of the cavity depending upon the size of a thin film forming the diaphragm; hence, it is very difficult to increase the volume of the cavity without increasing the size of the pressure sensor.

In the condenser microphone taught in Japanese Patent Application Publication No. 2004-537182 in which the cavity has a constant volume, high-frequency characteristics are degraded when the volume of the cavity is increased in order to improve low-frequency characteristics, while low-frequency characteristics are degraded when the volume of the cavity is decreased in order to improve high-frequency characteristics.

In the condenser microphone taught in Japanese Unexamined Patent Application Publication No. 2004-356707, a through-hole is formed in conformity with the two-dimensional shape of the diaphragm. This is because the through-hole serves as an introduction path for introducing an etching solution during the wet etching process, in which the prescribed portion of a sacrifice film between the thin film forming the diaphragm and the substrate in proximity to the through-hole is removed so as to form the diaphragm above the through-hole of the substrate. In the wet etching process, a part of the sacrifice film formed on the substrate is selectively removed from the substrate by way of wet etching, thus forming prescribed parts of the condenser microphone.

Due to the shape of an opening formed in the backside of the substrate opposite to the diaphragm, bubbles may occur to entirely cover the opening of the backside of the substrate during the wet etching process, thus preventing the etching solution from entering into the through-hole. Since the opening of the backside of the substrate of the aforementioned condenser microphone has a circular shape in conformity with the two-dimensional shape of the diaphragm, bubbles may easily remain in the opening due to surface tension exerted uniformly on bubbles having semispherical shapes. This makes it necessary to artificially break remaining bubbles in the aforementioned condenser microphone, which thus suffers from the complexity of the manufacturing process.

It is an object of the present invention to provide a pressure sensor, i.e., a condenser microphone that can be manufactured by way of manufacturing processes of semiconductor devices.

It is another object of the present invention to provide a pressure sensor having a relatively large volume of a cavity and a manufacturing method therefor.

It is a further object of the present invention to provide a pressure sensor, which is improved in low-frequency characteristics and high-frequency characteristics, and a manufacturing method therefor.

It is a further object of the present invention to provide a pressure sensor that can be manufactured with a simple manufacturing method.

In a first aspect of the present invention, a pressure sensor includes a plate having a fixed electrode, a diaphragm having a moving electrode positioned opposite to the fixed electrode, which is subjected to displacement due to pressure variations applied thereto, and a support having a first interior wall forming a first cavity, in which end portions of the plate are fixed, and a second interior wall, in which a step portion is formed in a thickness direction of the diaphragm in relation to the first interior wall and which forms a second cavity whose cross-sectional area is larger than the cross-sectional area of the first cavity in the plane direction of the diaphragm. That is, the cross-sectional area of the second cavity is discontinuously enlarged to be larger than the cross-sectional area of the first cavity in the plane direction of the diaphragm, and the end portions of the plate are fixed to the first interior wall forming the first cavity. This makes it possible to increase the overall volume of the cavities of the pressure sensor without bearing limitation due to the size of the plate in its plane direction and without enlarging the overall size of the pressure sensor.

In the above, the cross-sectional area of the second cavity in the plane direction of the diaphragm is enlarged in the direction opposite to the plate by way of the step portion, which the second interior wall forms in the thickness direction of the diaphragm; hence, it is possible to increase the overall volume of the cavities of the pressure sensor.

In the manufacturing method of the pressure sensor, a thin film forming the plate having the fixed electrode and a thin film forming the diaphragm are deposited on a first surface of a substrate; a first mask having a first opening is formed on a second surface opposite to the first surface of the substrate; a second mask having a second opening is formed on the second surface of the substrate, wherein the second mask covers the first mask so that the prescribed portion of the substrate just above the thin film forming the plate is exposed in the second opening; a recess is formed by performing anisotropic etching on the substrate exposed in the second opening by use of the second mask; the second mask is removed; then, anisotropic etching is performed using the first mask on the substrate exposed in the first opening such that the bottom of the recess is removed, thus forming a through-hole forming the cavity in the substrate.

When anisotropic etching is performed using the second mask on the substrate having the second opening, the recess is formed in the substrate. When anisotropic etching is performed using the first mask on the recess of the substrate, which is exposed in the first opening, and its peripheral portion such that the bottom of the recess is removed, the through-hole having the step portion is formed in the substrate in its thickness direction. The second opening is subjected to patterning such that the prescribed portion of the substrate just above the thin film forming the plate is exposed; hence, the remaining portion of the thin film forming the plate is still deposited on the substrate after completion of the formation of the through-hole. As a result, it is possible to produce a pressure sensor having a relatively large volume of cavities by way of the formation of the through-hole having the step portion in the substrate.

The manufacturing method can be modified in such a way that after the deposition of the thin film forming the plate and the thin film forming the diaphragm on the first surface of the substrate, a mask is formed on the second surface opposite to the first surface of the substrate, wherein the mask has a first opening for exposing the prescribed portion of the substrate just above the thin film forming the plate and a second opening having a slit-like shape that lies along the periphery of the first opening; anisotropic etching is performed using the mask on the substrate exposed in the first and second openings, thus forming a hole corresponding to the first opening and a recess corresponding to the second opening in the substrate; then, a wall between the hole and the recess is removed, thus forming a through-hole forming the cavity in the substrate.