JP5459925B2 - MEMS structure for gas sensor - Google Patents

Description

  The present invention relates to the structure of a gas sensor using micromachining technology. More specifically, the present invention relates to a MEMS structure of a gas sensor in which a bridge structure is formed using micromachining technology and an electrode, a heater, and a gas sensitive layer are provided on the bridge structure.

  In general, a ceramic gas sensor is used in a device that detects a target gas and outputs a concentration display output, an alarm output, or a control signal. When the gas sensor is exposed to a specific gas, a change in electrical conductivity or generation of an electromotive force occurs in the ceramic, which is a gas sensitive substance, and the presence and gas concentration of the target gas can be detected from the value. This type of gas sensor consumes much more power than other sensors. Therefore, in order to expand the application range of the ceramic gas sensor, it is very significant to provide a low power ceramic gas sensor that can maintain the sensor temperature with a small energy.

  A conventional gas sensor has a structure in which an electrode and a heater are formed using one or both surfaces of an insulating ceramic substrate, and a sensor element on which a gas-sensitive material is printed or applied is suspended by a lead wire. With this structure, it has been difficult to achieve sufficiently low power consumption.

  On the other hand, the conventional ceramic sensor has recently been miniaturized, and the print type and thin film type have also appeared. At the same time, an ultra-low power type in which gas-sensitive ceramic powder is applied to an extremely fine noble metal wire coil has also appeared. However, the conventional sensor in which ceramic powder is applied to a three-dimensional coil has drawbacks such as peeling between the powder and the electrode coil. In addition, since it is still suspended, it is vulnerable to vibration and impact.

  As another development trend, a diaphragm was formed using silicon micromachining technology, a microheater was provided on it, a detection electrode was further provided on the insulating layer, and a gas sensitive layer was further provided. Gas sensors having a MEMS structure have also appeared. A gas sensor having a MEMS type structure is suitable for mass production and can be realized at a low cost. However, it has not yet reached the stage of practical use because of its mechanical characteristics, thermal shock resistance, and durability. For example, gas sensors have recently been used to control the air quality of automobiles, but conventional ceramic gas sensors are still used because the reliability of MEMS gas sensors has reached the level of reliability of automobiles. Because it is not.

When mechanical characteristics, thermal shock resistance, durability, and other characteristics are required, such as in-vehicle applications, in view of the above-mentioned situation of existing MEMS gas sensor structures, Application 2005-325455 was invented. As a result, it has a MEMS structure that can withstand in-vehicle use. However, since four bridges are located on each diagonal line, the structure is not designed to relieve stress due to expansion and contraction generated in the bridge direction.
Japanese Patent Application No. 2005-325455

  FIG. 1 shows the configuration of Japanese Patent Application No. 2005-325455. However, only the configuration of the four bridges 3, the stage 2 supported by them and the silicon frame 1 is shown here, and the heaters, electrodes, etc. are omitted. A heater and electrodes are formed on the stage 2, and a gas sensor signal and heater power are supplied through a wiring pattern on the bridge 3.

  When the heater power is supplied to the stage 2 through the bridge 3 and heated, the stage 2 and the bridge 3 expand, and when the power supply is stopped and returned to room temperature, it contracts and returns to its original state. In FIG. 1, B1 and B3 and B2 and B4 are positioned on a straight line, respectively. Therefore, when the stage 2 is supplied with power and heated as described above, the opposing bridges B1 and B3 or B2 and B4 press against each other with the stage 2 interposed therebetween, and as a result, the silicon frame 1 that maintains the normal temperature is pressed. The power to try works. The stress generated here is concentrated near the boundary between the bridge 3 and the silicon frame 1. Therefore, if heating / cooling is repeated many times, the bridge 3 and the stage 2 may be damaged due to repeated generation and relaxation of stress. The challenge is to reduce the fatigue caused by stress generation and relaxation as much as possible.

  In order to solve the problem, as shown in FIG. 2, the bridge 3 is provided so as to be positioned in a parallel relationship while avoiding being arranged in a straight line with respect to the opposing bridge.

  In the configuration of the present invention, in order to avoid the concentration of stress on this boundary, the bridge 3 facing the bridge 3 is provided with an interval substantially corresponding to the width of the stage 2 and arranged in a parallel positional relationship. The bridges 3 that are sometimes opposed to each other expand, but by being expanded, they are converted into a force that rotates the stage 2, and the above-described stress is relieved.

  As shown in FIG. 2, the opposing bridges 3 are provided so as to be located in parallel at intervals corresponding to the width of the stage 2, thereby changing the stress in expansion during heating into a force for rotating the stage 2, The stress concentrated on the boundary of the bridge 3 can be relaxed.

  FIG. 1 shows the configuration of Japanese Patent Application No. 2005-325455, and FIG. 2 shows the configuration of the present invention. The four bridges 3 are shown with identification symbols B1, B2, B3, and B4, respectively. In FIG. 1, B1 and B3 or B2 and B4 are opposed bridges and are located on a straight line. In FIG. 2, B3 does not exist on a straight line of B1, and it is provided at a position moved by the width of the stage 2 from the position where B3 is located in FIG. The positional relationship between B2 and B4 is the same.

  As shown in FIG. 2b, the positional relationship of the four bridges 3 is the same even when the shape of the silicon frame 1 and the stage 2 is changed from a square to a circle. Even if minor changes are made to the shape of the bridge 3, stage 2 and silicon frame 1 by other design, the position of the bridge arranged in a straight line is changed in this way to convert the stress into rotational force By doing so, the stress, which is the above-mentioned problem, can be relaxed.

  Prior art patents Japanese Patent Application No. 2005-325455 can realize stress relaxation in the bridge direction, which can be further improved in reliability of the MEMS gas sensor as an in-vehicle gas sensor.

Patent of prior application Structure of Japanese Patent Application No. 2005-325455. The configuration of the present invention in which the bridge 3 is positioned in parallel.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Silicon frame 2 ... Stage 3 ... Bridge B1 ... Bridge (opposite or parallel to B3)
B2 ... Bridge (opposite or parallel to B4)
B3 ... Bridge (opposite or parallel to B1)
B4 ... Bridge (opposite or parallel to B2)

Claims (1)

In the diaphragm structure comprising a silicon substrate and the SiO2 layer and SiNX layer, a bridge structure having four bridges which are formed by removing a portion of the diaphragm portion, the bridge each for independently it is entirely A MEMS structure for a gas sensor, wherein the bridge is formed in a straight line and has a parallel positional relationship with a pair of opposing bridges on a diagonal line.

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