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DESCRIPTION JP2001339796

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DESCRIPTION JP2001339796
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
capacitor type microphone manufactured by utilizing semiconductor manufacturing technology.
[0002]
2. Description of the Related Art A capacitor type small microphone (hereinafter referred to as an
IC microphone) manufactured by utilizing micromachine processing technology making full use
of semiconductor manufacturing technology has high quality stability, small size, since the
sensing portion and the first stage amplification circuit can be manufactured integrally.
Lightweight, suitable for mass production.
[0003]
FIG. 1 shows an example of the structure of an IC microphone and how to take out a signal
(Reference 1).
[0004]
The part of the IC microphone that detects sound is a capacitor formed of a thin and flat
vibrating electrode 1 and a back electrode 2.
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Reference numeral 3 denotes a connection wiring (contact) electrically connected to the back
electrode 2, and 4 denotes a case.
[0005]
The displacement of the vibrating electrode 1 caused by the change in sound pressure due to the
sound wave is detected as the change in capacity of the capacitor.
The signal voltage ΔV that can be taken out from this microphone is Vb for the bias voltage, q
for the stored charge, C for the capacity of the capacitor, s for the distance between the vibrating
electrode and the back electrode, C for change caused by sound pressure, and s for change
Assuming that ΔC, Δs, ΔV = −q (ΔC / C2) = (Δs / s) Vb (1) (Reference 2).
[0006]
This voltage change was read from the electrode.
[0007]
SUMMARY OF THE INVENTION The conventional measures to improve the sensitivity of the IC
microphone make Vb larger than the equation (1).
Reduce s. By using a soft material for the vibrating electrode so as to increase Δs, the
deformation to the sound pressure is increased. That is, the stiffness of the vibrating electrode is
reduced. In order to increase ΔC, the area of the electrode is increased. というものであった。
[0008]
However, in the condenser type microphone, it is necessary to prevent the vibrating electrode
and the back electrode from being stuck by electrostatic attraction. The stability μ of the
following equation is defined as an index for holding both electrodes without sticking (Reference
3).
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[0009]
In the equation (2), sm: stiffness of vibrating electrode, εa: dielectric constant of air, S: area of
back electrode, and is usually designed so that μ> 7.
[0010]
From equation (2), the measures of to improve the sensitivity contradict with the improvement of
the stability.
In particular, in the conventional IC microphone, since the distance s between the vibrating
electrode and the back electrode is extremely small such as several μm, there has been a limit to
high sensitivity.
[0011]
An object of the present invention is to provide a condenser type microphone which solves the
above problems.
[0012]
According to the invention of claim 1, there is provided a capacitor whose capacitance changes in
response to a change in sound pressure, an LC circuit having the capacitor as a part of the
capacitance, and a capacitance change of the capacitor. And output means for reading out as a
change in the oscillation frequency of the LC circuit.
[0013]
The invention of claim 2 is characterized in that in the claim 1, the output means comprises
means for FM-modulating and transmitting a carrier wave in response to a capacity change due
to a sound pressure of the capacitor.
[0014]
The invention of claim 3 is characterized in that, in claim 2, the capacitor is formed on the same
substrate as at least a part of the output means.
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[0015]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, an LC
oscillation circuit (oscillation frequency is determined by a coil and a capacitor) having this
microphone as C (capacitance) according to the change in capacitance of the capacitor
microphone due to sound pressure. Circuit) oscillation frequency change.
According to this method, since it is not necessary to apply a bias voltage to the capacitor
section, the degree of freedom in selecting a measure for increasing sensitivity is increased.
Furthermore, if the detected oscillation frequency change is regarded as FM modulation of the
carrier frequency and transmitted, and it is demodulated on the receiving side to obtain an audio
signal, a wireless microphone can be configured.
In this case, only the voltage necessary for the oscillation circuit to operate is applied to the
capacitor section, and the voltage can be very low compared to the bias voltage of a normal IC
microphone. A large degree of stability can be obtained, and the problem of sticking between the
vibrating electrode and the back electrode is less likely to occur.
[0016]
The oscillation frequency f of a general LC oscillation circuit is determined by the following
equation, where L is the inductance of the coil.
[0017]
Assuming that the change in f caused by the sound pressure is Δf,
[0018]
Therefore, the above-described measures are effective as a method of increasing the frequency
shift Δf in order to improve the sensitivity.
[0019]
The conventional capacitor type IC microphone requires a bias voltage Vb of at least several
volts, and several volts to obtain practical sensitivity (Reference 4).
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[0020]
On the other hand, in the IC microphone of the present invention, using the oscillation circuit
operating at 1 to 2 V, from Equation (2), the stability can be several to several hundreds times as
high as that of the conventional IC microphone.
This greatly increases the degree of freedom in choosing a strategy for high sensitivity.
[0021]
FIG. 2 shows the configuration of an embodiment of the present invention in which the
capacitance change of the capacitor is read out as the frequency change.
1 is a vibrating electrode, 2 is a back electrode, and these constitute an IC microphone (for the
detailed structure, see FIG. 3).
By vibrating the vibrating electrode 1 in response to the sound pressure, the distance between
the vibrating electrode 1 and the back electrode 2 changes, and the capacitance between the
vibrating electrode 1 and the back electrode 2 changes.
An oscillation / modulation circuit 5 has an LC oscillation circuit configured by connecting to the
vibrating electrode 1 and the back electrode 2 through the connection wiring 3.
The oscillation output signal of this LC oscillation circuit is supplied to the antenna (coil antenna)
6 and transmitted therefrom. The oscillation frequency of this LC oscillation circuit is determined
by equation (3).
[0022]
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5
Therefore, when the capacitance between the vibrating electrode 1 and the back electrode 2
changes in response to the change in the sound pressure, the oscillation frequency of the LC
oscillation circuit of the oscillation / modulation circuit 5 changes. That is, the oscillation output
signal of the LC oscillation circuit is frequency-modulated in response to the change in sound
pressure, and transmitted from the antenna 6.
[0023]
FIG. 3 shows a specific structure of the IC microphone, wherein (a) is a perspective view and (b)
is a cross-sectional view. The illustrated IC microphone was manufactured using a known
technique (Reference 1). The vibrating electrode 1, the back electrode 2, the connection wiring 3,
the oscillation / modulation circuit 5 and the coil antenna 6 are formed on the substrate 7, and
the back electrode 2 is covered by the case 4.
[0024]
As described above, since it is not necessary to apply a bias voltage between the vibrating
electrode 1 and the back electrode 2 that constitute the capacitor, the degree of freedom in
selecting a measure for increasing sensitivity is increased. Furthermore, it is considered that the
oscillation frequency change of the LC oscillation circuit detected by the capacitance change
between the vibrating electrode 1 and the back electrode 2 is regarded as the FM modulation of
the carrier frequency and transmitted, and it is demodulated on the receiving side to obtain an
audio signal In such a wireless microphone, only the voltage necessary for the oscillation /
modulation circuit to operate is applied to the capacitor section, and a large bias is generated as
in a normal IC microphone. Since it is not necessary to apply a voltage, the problem of sticking
between the vibrating electrode and the back electrode is less likely to occur.
[0025]
(References) Reference 1: J. Mol. Bergvist. F.Rudolf,“A
Siliconcondenser microphone using bond
andetch−back technology” Sensors and Actuators
A45,pp.115−124(1994). Reference 2: Dennis S. Greywall, "Micromachined
optical-interference microphone", Sensors and Actuators A75, pp 257-268 (1999). Reference 3:
Akio Mizoguchi, "Design for Miniaturization of Directional Capacitors and Microphones", Journal
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of the Acoustical Society of Japan, Vol. 31, No. 10, pp. 593-601 (1975). Reference 4: Gerhard M.
Sessler, "NEW ACOUSTIC SENSORS", Proc. 15th International Congress on Acoustics, pp. 253260 (1995).
[0026]
As described above, according to the present invention, a highly sensitive capacitor type
microphone can be provided. In addition, it is easy to make wireless.
[0027]
Brief description of the drawings
[0028]
1 is a diagram showing the structure of a conventional IC microphone and how to take out a
signal.
[0029]
2 is a diagram showing a configuration of an embodiment of the present invention in which the
capacitance change of the capacitor is read out as a frequency change.
[0030]
3 shows a specific structure of the IC microphone, (a) is a perspective view of a wireless
microphone according to the present invention, (b) is a cross-sectional view of the same.
[0031]
Explanation of sign
[0032]
Reference Signs List 1 vibration electrode 2 back electrode 3 connection wiring 4 case 5
oscillation / modulation circuit 6 coil antenna 7 substrate
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