MEMS pressure sensor

1. A MEMS pressure sensor comprising:

a MEMS resonator comprising:

a substrate;

a vibrator comprising a mechanically vibrating part and a fixed part;

at least one electrode that is disposed close to the vibrator and has an area overlapping with the vibrator across a gap in a direction perpendicular to a surface of the substrate; and

a pressure transferring mechanism to displace the at least one electrode according to externally-applied pressure so as to change the gap; and

a detection circuit to detect transmission characteristics of an AC signal from an input electrode to an output electrode, the input and output electrodes being one and the other of the vibrator and the at least one electrode,

wherein the pressure is detected based on the transmission characteristics of the AC signal that is detected by the detection circuit.

2. The MEMS pressure sensor according to claim 1 , wherein the MEMS resonator further comprises a diaphragm that displaces according to externally-applied pressure,

the pressure transferring mechanism is composed of the diaphragm, and

at least part of the diaphragm is formed by the at least one electrode.

3. The MEMS pressure sensor according to claim 1 , wherein the MEMS resonator comprises a diaphragm that displaces according to externally-applied pressure, and

the pressure transferring mechanism is composed of the diaphragm and at least one connector that connects the diaphragm with the at least one electrode.

4. The MEMS pressure sensor according to claim 1 , wherein the MEMS resonator comprises a diaphragm that displaces by externally-applied pressure,

the vibrator and the at least one electrode are disposed between the diaphragm and the substrate,

a first cavity and a second cavity are formed between the substrate and the diaphragm, the first and second cavities being separated by the at least one electrode,

the first cavity is located at a side of the vibrator in a direction perpendicular to the surface of the substrate when viewed from the at least one electrode at the area where the vibrator and the electrode overlap with each other,

the second cavity is located at an opposite side of the vibrator in a direction perpendicular to the surface of the substrate when viewed from the at least one electrode at the area where the vibrator and the electrode overlap with each other,

an inner surface of a barrier wall A is located at an outer side than an inner surface of a barrier wall B in a direction parallel to the surface of the substrate, the barrier wall A defining the first cavity and being in contact with the electrode, the barrier wall B defining the second cavity and being contact with the electrode, and

the pressure transferring mechanism is composed of the diaphragm, the barrier wall A and the barrier wall B.

5. The MEMS pressure sensor according to claim 1 , wherein the fixed part of the vibrator is fixed on the substrate.

6. The MEMS pressure sensor according to claim 5 , wherein the fixed part of the vibrator is fixed by a barrier wall that is fixed on the substrate and surrounds a space that allows the vibrator to vibrate mechanically.

7. The MEMS pressure sensor according to claim 1 , wherein the vibrator is a beam structure in which the vibrating part of the vibrator is a beam that is provided parallel to the surface of the substrate between the fixed parts of the vibrator,

the beam has a triangular or trapezoidal cross section of which base is parallel to the surface of the substrate, and

the beam vibrates mechanically in a torsional resonance mode with its center being a longitudinal axis of the beam.

8. The MEMS pressure sensor according to claim 1 , wherein the detection circuit comprises a feedback circuit with an amplifier along a path,

a signal of the feedback circuit is partially sent to a frequency-voltage converting circuit, and

the detection circuit detects an oscillation frequency of the vibrator as the transmission characteristics of the AC signal by vibrating the vibrator at a predetermined vibration mode while applying a DC potential between the input and output electrodes and outputs pressure information as an electrical signal by converting a change of the oscillation frequency to a voltage change by the frequency-voltage converting circuit, in which the change of the oscillation frequency is caused by a change of a resonance frequency of the vibrator that occurs when the gap changes according to the externally-applied pressure.

9. The MEMS pressure sensor according to claim 1 , wherein the detection circuit inputs an AC signal of a predetermined frequency to the input electrode and monitors a voltage change of the AC signal that is output from the output electrode; and

the detection circuit detects a resonance frequency of the vibrator as the transmission characteristics of the AC signal when the predetermined AC signal is input to the input electrode while a DC potential is applied between the input and output electrodes, and outputs pressure information as an electrical signal by converting a change of the resonance frequency of the vibrator to a voltage change of the AC signal by a frequency-voltage converting circuit, in which the change of the resonance frequency occurs when the gap changes according to the externally-applied pressure.