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JP2011124771

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DESCRIPTION JP2011124771
An object of the present invention is to provide a microphone capable of achieving downsizing
and cost reduction while maintaining sensitivity to sound pressure. A supporting frame (2)
comprising a lower Si layer (30), which is a first layer of a substrate, in a microphone (1) formed
by processing a substrate comprising a plurality of layers by a semiconductor substrate
processing technique. And the movable portion 5 as the movable plate elastically supported by
the elastic support portion 4 at a position facing the back chamber 7 and formed from the upper
Si layer 10, which is the second Si layer of the substrate. And the fixed portion 3 facing the side
surface of the movable portion 5 via the predetermined gap 8a. Then, when the velocity of sound
at a predetermined temperature in the working temperature range is c and the dynamic viscosity
coefficient of air is ν, the dimension h of the predetermined gap 8a in the thickness direction of
the movable plate and the distance between the side circumferential surface and the fixed
portion facing surface d is set to satisfy h ≧ πcd / 6ν. [Selected figure] Figure 1
マイクロフォン
[0001]
The present invention relates to a microphone used in a mobile phone, an IC recorder, and the
like.
[0002]
Conventionally, a condenser microphone manufactured by micromachining using a silicon wafer
is known.
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1
The microphone is provided with a membrane that is a diaphragm that detects sound pressure
and a back plate that faces the membrane, and is configured to electrically detect a change in the
gap between the membrane and the back plate ( For example, refer to Patent Document 1).
[0003]
Japanese Patent Application Laid-Open No. 2003-163998
[0004]
By the way, in microphones used for mobile phones and IC recorders, it is required to reduce the
volume required for mounting and to reduce the cost while satisfying certain performance.
However, conventionally, since the multilayer structure has a diaphragm and a back plate facing
each other, a plurality of spacers and an electrode structure are required, which is an obstacle to
volume reduction and cost reduction. Moreover, in the case of a capacitor microphone
manufactured by micromachining, in order to form a capacitor satisfying the performance, a gap
of several μm or less is required in the above-described multilayer structure, and a high height
corresponding to surface micromachining It requires manufacturing technology and equipment.
[0005]
The invention according to claim 1 is a microphone formed by processing a substrate comprising
a plurality of layers by a semiconductor substrate processing technique, comprising a first layer
of the substrate, a support frame on which a back chamber is formed, and a substrate A fixed
member formed of the second layer, the movable plate elastically supported by the elastic
support portion at a position facing the back chamber, and the second layer and facing the side
surface of the movable plate with a predetermined gap therebetween The velocity h at a
predetermined temperature within the working temperature range is c and the dynamic viscosity
coefficient of air is .nu., The dimension h of the predetermined gap in the thickness direction of
the movable plate and the side circumferential surface and the fixed portion opposing surface
And a distance d between them and h.gtoreq..pi.cd <2> /6.nu. According to the invention of claim
2, in the microphone according to claim 1, the movable plate and the fixed portion form a
capacitor, and the sound pressure is detected based on a change in capacitance when the
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2
movable plate is displaced. It is a thing. The third aspect of the invention is the microphone
according to the second aspect, wherein the movable plate is disposed so as to be displaced in
the thickness direction of the movable plate with respect to the position where the capacitance of
the capacitor is maximum. It is characterized by According to the invention of claim 4, when the
thickness of the movable plate and the fixed portion is H and the maximum amplitude of the
movable plate when sound pressure acts is ξ1 in the microphone according to claim 3, the
positional deviation 記載 0 is It is set to satisfy ξ1 / 2 ≦ ξ0 <h−ξ1 / 2. The invention
according to claim 5 is characterized in that, in the microphone according to any one of claims 2
to 4, each opposing surface of the movable plate and the fixed portion has a comb-like shape
which is indented in a concavo-convex shape. The invention according to claim 6 is characterized
in that, in the microphone according to any one of claims 2 to 5, at least one of the fixed portion
and the movable plate is divided into a plurality to form a plurality of capacitors. The invention
according to claim 7 is the microphone according to any one of claims 2 to 6, wherein a
permanent charge film for applying a bias voltage to the capacitor is formed on the surface of the
fixed portion or the movable plate. . The invention according to claim 8 relates to the microphone
according to any one of claims 2 to 6, wherein the permanent charge film formed on the surface
of the support frame is electrically connected to one of the permanent charge film and the fixed
portion or the movable plate. And a conducting portion. The invention according to claim 9 is
characterized in that, in the microphone according to claim 1, the piezoresistive element is
disposed on the elastic support portion, and the sound pressure is detected based on the
resistance change of the piezoresistive element when the movable plate is displaced. I assume.
The invention according to claim 10 is characterized in that, in the microphone according to any
one of claims 1 to 9, a groove is formed in the movable plate, or a through hole having a
diameter equal to the distance d is formed. I assume.
[0006]
According to the present invention, downsizing and cost reduction of the microphone can be
achieved while maintaining the sensitivity to the sound pressure.
[0007]
It is a figure which shows 1st Embodiment of the microphone by this invention, (a) is a top view
which shows schematic structure of the microphone 1, (b) is AA sectional drawing.
FIG. 2 is a schematic view showing an operation principle of the microphone 1; It is a figure
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which shows the output circuit of the microphone 1. FIG. It is a figure explaining a pressure
difference, and (a) shows the case where the back side of diaphragm 230 serves as a sealed
space, and (b) shows the case where the space by the side of the diaphragm back is in a semisealed state. It is a figure explaining propagation of a pressure difference in a gap. It is a figure
which shows the graph of h = picd <2> / 6 (nu). It is a figure which shows a sensitivity
characteristic. It is a figure explaining the manufacturing process of the microphone 1. FIG. It is a
figure explaining the manufacturing process of the microphone 1, and shows the process of
following the process shown in FIG. It is a figure explaining the manufacturing process of the
microphone 1, and shows the process of following the process shown in FIG. It is a figure which
shows 2nd Embodiment of the microphone by this invention, (a) is a top view, (b) is AA sectional
drawing. It is a figure which shows the BB cross section of Fig.11 (a). It is a figure explaining the
case where initial stage shift ξ0 is ξ0 = 0, (a) shows the sound pressure change of an input
sound wave, (b) shows the positional relationship between fixed part 3 and movable part 5, (c)
Indicates a change in capacitance. It is a figure explaining the case where initial stage shift ξ0 is
ξ0 ≠ 0, (a) shows the case of ξ0ξξ1 / 2, (b) shows the case of ξ0 <ξ1 / 2. It is a figure
which shows a 1st modification. It is a figure which shows a 2nd modification. It is a figure which
shows a 3rd modification. It is a figure which shows 3rd Embodiment of the microphone by this
invention, (a) is a top view, (b) is AA sectional drawing. It is a figure explaining the directivity of a
microphone. It is a figure which shows the modification of 3rd Embodiment, (a) is a top view, (b)
is AA sectional drawing. It is a top view which shows the microphone 1 by which the circuit area
| region was provided on the board | substrate. FIG. 6 is a plan view of a microphone 1 provided
with an electret portion 33. FIG. 6 is a diagram for explaining setting of an initial displacement
wedge 0 according to the shapes of the fixed portion 3 and the movable portion 5;
[0008]
Hereinafter, embodiments of the present invention will be described with reference to the
drawings. -1st Embodiment-FIG. 1: is a figure which shows schematic structure of the
microphone of this Embodiment, (a) is a top view, (b) is AA sectional drawing. The microphone 1
includes a fixed portion 3 fixed on the support frame 2 and a movable portion 5 connected to the
support frame 2 via the elastic support portion 4. In the present embodiment, the microphone 1
uses an SOI (Silicon on Insulator) substrate having a three-layer structure of a lower Si layer 30,
an SiO 2 layer 20, and an upper Si layer 10 by micromachining technology or photolithography
technology. It is made.
[0009]
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The support frame 2 is formed of the lower Si layer 30, and the fixed portion 3, the movable
portion 5 and the elastic support portion 4 are formed of the upper Si layer 10. A SiO 2 layer 20
as an insulating layer is interposed between the fixing portion 3 and the elastic support portion 4
and the support frame 2. The support frame 2 is formed with a back chamber 7 having a
rectangular cavity, and the movable portion 5 is elastically supported by the elastic support
portion 4 above the back chamber 7. Electrodes 6a and 6b are formed on the fixed portion 3 and
the movable portion 5, and a DC voltage is applied between them as described later.
[0010]
The fixed portion 3 and the movable portion 5 have a comb-tooth shaped portion, and the fixed
portion 3 and the movable portion 5 are arranged such that the teeth of the movable portion 5
enter between the teeth of the fixed portion 3 and the teeth. . In the present embodiment, a gap
8a between the fixed portion 3 and the movable portion 5, a gap 8d between U-shaped elastic
support portions 4, a gap 8b between the movable portion 5 and the elastic support portion 4
and fixation The gap dimension of the gap 8c between the portion 3 and the elastic support
portion 4 is set to the same dimension. The gap dimension is set to a predetermined dimension
as described later. Here, the back chamber 7 side of the movable portion 5 and the fixed portion
3 is referred to as the back surface side, and the opposite side is referred to as the front surface
side. The sound wave to be detected is incident on the surface side.
[0011]
FIG. 2 is a schematic view showing the operation principle of the microphone 1. The gap 8 a
described above is formed between the side surface 500 of the movable portion 5 and the
opposing surface 300 of the fixed portion 3 opposed thereto, and the fixed portion 3 and the
movable portion 5 form a capacitor C. When the sound wave reaches the surface side of the
movable portion 5 and the sound pressure acts, the elastic support portion 4 elastically
supporting the movable portion 5 is bent, and the movable portion 5 is displaced to the back
chamber 7 side (z-axis minus direction). As a result, the capacitance of the capacitor C changes.
Thus, the microphone 1 of the present embodiment constitutes a condenser type microphone,
and the sound pressure is read by converting the change in capacitance of the condenser C due
to the vibration of the movable portion 5 into an electrical signal.
[0012]
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FIG. 3 is a diagram showing an output circuit of the microphone 1. A portion surrounded by a
broken line represents the capacitance of the microphone 1, C is a capacitance between the fixed
portion 3 and the movable portion 5, Cs1 is a parasitic capacitance on the fixed portion 3 side,
and Cs2 is It is a parasitic capacitance on the movable part 5 side. A DC bias voltage Vb is applied
between the fixed portion 3 and the movable portion 5. An electret may be formed on the fixed
portion side, and a bias voltage may be applied by the electret. Cp is a bypass capacitor.
[0013]
When the movable portion 5 is displaced with respect to the fixed portion 3, the charge of the
movable portion 5 on the ground side slightly changes. The movable portion 5 is connected to
the gate G of the FET 41, and a drain-source voltage Vdc for operating the FET 41 is applied
between the drain and source of the FET 41. Note that by applying the DC bias voltage Vb to the
fixed unit 3, the fluctuation of the charge of the movable unit 5 can be increased. The charge
fluctuation of the movable portion 5 is amplified as a current change, and by reading the voltage
of the resistor R, the change of the capacitance C due to the sound pressure can be detected as a
voltage change. Although in FIG. 3 the movable portion side is the ground and the FET 41 is
connected to the movable portion side, conversely, the fixed portion side may be the ground and
the FET 41 may be provided on the fixed portion side.
[0014]
Description of Gap Dimensions Conventional condenser microphones have a diaphragm and a
back plate for reading pressure fluctuations due to acoustic waves. The diaphragm shuts off air
and the diaphragm is vibrated by the pressure difference generated on the front and back of the
diaphragm. Then, the sound pressure is read by converting the change in capacitance caused by
the diaphragm vibrating with respect to the back plate into an electric signal. On the other hand,
in the present embodiment, the sound pressure is detected by replacing the diaphragm and back
plate constituting the capacitor with the conventional condenser microphone with the fixed
portion 2 and the movable portion 5.
[0015]
In general, in a microphone that detects sound by vibration of the diaphragm, a pressure
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6
difference generated between the front and back of the diaphragm as a diaphragm becomes
important. In the present embodiment, in the movable portion 5 to which the sound pressure is
incident, the surface side and the back side are in communication via the gap. In such a structure,
in order to generate a sufficient pressure difference between the front and back of the movable
portion 5, it is necessary to set the gap size of the gap to an optimum value.
[0016]
For example, as shown in FIG. 4 (a), when the back side of the diaphragm 230 is a sealed space,
and as shown in FIG. 4 (b), the hole 230a is formed in the diaphragm 230, and the space on the
diaphragm back side We consider the case where is semi-sealed.
[0017]
Since air is fluid and has viscosity, it can be considered that the velocity of air is equal to 0 at the
surface of a substance, and the velocity gradually returns to its original speed of sound as it gets
away from the surface.
The region where air is reduced to less than the speed of sound (99% or less of the speed of
sound) from the surface of the material is called the velocity boundary layer. If the surface of the
object placed in the fluid sound field is not perpendicular to the vibration velocity of the fluid
particle, a velocity boundary layer of thickness δ = (2ν / ω) <0.5> is formed on the surface of
the object. Here, the velocity boundary layer is a region where the velocity of sound is slowed by
the influence of the wall. ν is the kinematic viscosity of air, ω is the angular frequency of the
sound wave.
[0018]
As shown in FIG. 4 (b), in the case of a semi-open system having a sufficiently large hole or gap
acoustically, pressure fluctuation is transmitted to the front and back at the speed of sound. And
the pressure difference between the front and back of the flat plate which received the sound
wave in one cycle of the sound wave contributes only to the thickness of the flat plate to the
degree of density of the sound, and it is very weak as a force. Assuming that the pressure
fluctuation P of air due to sound pressure is 1 Pa (at 94 dB), the pressure applied to the flat plate
is the same as in the case of a sealed or nearly closed state as shown in FIG. It shows quasisinusoidal variation up to 1 Pa. However, in the case of a semi-open system having a sufficiently
03-05-2019
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large gap as shown in FIG. 4 (b), the pressure difference between the front and back is simply the
difference by which the sound wave travels by the thickness.
[0019]
For example, if the sound pressure is 2 Pa and the frequency is 1 kHz at normal temperature and
pressure, the sound is 1 ms per period, and the pressure fluctuation advances by one wavelength,
that is, about 34 cm during that period. The time taken for the pressure fluctuation of the sound
velocity of sound to pass through the thickness 20 μm of the flat plate is 59 ns. Assuming that
the pressure fluctuation sinusoidally fluctuates, let each frequency of the density fluctuation be
ω and time be t, then the pressure difference δP on the front and back is expressed as follows:
δP = (P0 + Psinω (t + 59ns)) − (P0 + Psinωt) Be done. 59Since the value of ns is about 17000
for 1 ms and about 0.021 ° in phase, δP is less than about 2500 at most for P under this
condition, and is half open It can be seen that the pressure difference is clearly small relative to
the closed state. Therefore, in general, when the hole is open, the sensitivity is too low for the
vibrator to receive the sound wave, and the reaction is performed only in a special state where
the input sound wave has a resonance frequency.
[0020]
On the other hand, if one half of the hole radius or gap spacing on the flat plate is equal to or less
than the velocity boundary layer, it can be said that the sound wave passing through the gap is
propagated later than originally expected. FIG. 5 is a diagram for explaining the propagation of
the pressure difference in the gap. When the dimension d of the gap 8a is sufficiently narrow,
and the thickness h of the fixed portion 2 and the movable portion 5 is sufficiently thick relative
to the gap dimension d, the propagation of the pressure difference in the gap 8a is delayed with
respect to a wide free space Will occur. Therefore, pressure fluctuations are not transmitted at
the speed of sound.
[0021]
If one half of the gap dimension d is equal to or less than the thickness of the velocity boundary
layer, the inside of the gap is in the range of the velocity boundary layer. Inside the velocity
boundary layer, the velocity distribution u of air is calculated by the following equation (1). a is a
coefficient depending on viscosity and the like. u is the maximum value umax when x = d / 2.
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8
u=ax(d−x) …(1)
[0022]
Since the thickness δ of the velocity boundary layer is the distance from the wall surface to the
velocity becoming the sound velocity c, when the gap dimension d is 2δ or more, x = δ (= (2ν /
ω) <0.5> Since umax = c at the time of), the value a can be obtained from this condition. c = aδ
(2δ−δ) = aδ <2> (2) a = c / δ <2> = ωc / 2ν (3) u = ωcx (d−x) / 2ν (4)
[0023]
When the gap dimension d is smaller than 2δ, the flow velocity u is maximum at x = 0.5 d but
smaller than the sound velocity c. Therefore, in the gap, the position at which the pressure
fluctuation due to the sound wave reaches the value shown by the following equation (6) per one
cycle, where t is the cycle of the sound wave. z = ut = ctx (d−x) / δ <2> = (ωc / 2ν) tx (d−x)
(5) zmax = (ωc / 2ν) t · d / 2 · d / 2 = ωctd < 2> / 8ν (6)
[0024]
When the value zmax of the equation (6) exceeds the flow path length (the dimension in the
thickness direction of the gap) h shown in FIG. 5, the pressure fluctuation reaches the back
surface side for each cycle. Further, not only the front end of the pressure fluctuation can reach
the back side, but also if it is sufficient to displace air in the gap volumetrically, it diffuses and
starts to wrap around on the back side. Considering the depth (y direction) unit length of the gap
8a in FIG. 5, the volume Vs occupied by the sound pressure fluctuation per one cycle becomes as
shown in the equation (7) by integrating the equation (5). On the other hand, the volume Vd of
the portion of the gap 8a is Vd = dh in terms of depth (y direction) per unit length, so the
condition that the sound wave does not penetrate the gap 8a is as shown in equation (8). Vs =
πcd <3> / 6> (7) h ≧ πcd <2> / 6ν (8)
[0025]
FIG. 6 is a graph of h = πcd <2> / 6ν. The condition of equation (8) is the upper region (OK) of
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the curve h = πcd <2> / 6/6, and the combination of (h, d) falls in the region (NG) below the
curve. Will cause the sound wave to penetrate the gap 8a. Since all the gaps 8a to 8d shown in
FIG. 1 penetrate from the front surface side to the back surface side (back chamber 7), the
dimensions h, h, and h satisfy the formula (8) for all the gaps 8a to 8d. Set d.
[0026]
In equation (8), the speed of sound c and the dynamic viscosity coefficient は are quantities
dependent on temperature, but in microphone design, the predetermined temperature within the
working temperature range of the microphone is the design temperature, and the speed of sound
c at the design temperature And the dynamic viscosity coefficient ν. A typical use condition of
the microphone is considered to be a temperature of 0 to 40 ° C. and 1 atm (± 10%). For
example, the kinematic viscosity ν at 0 ° C. is 13.2 × 10 <-6> (m <2> / s), and at 50 ° C. 17.7
× 10 <-6> (m <2> / s) It becomes.
[0027]
FIG. 7 is a view showing sensitivity characteristics, and a curve L1 shows the sensitivity
characteristics of the microphone 1 of the present embodiment. In this case, the dimension h and
the gap dimension d are set so as to satisfy the equation (8). On the other hand, the curve L2 is
the case where the gap dimension is the same as that of the general comb-tooth actuator, that is,
the case where the equation (8) is not satisfied. Curve L3 shows the characteristics of a general
condenser microphone.
[0028]
In the case of the curve L2 in which the gap size does not satisfy the equation (8), the sensitivity
increases near the resonance point of the movable portion, but hardly at other frequencies. On
the other hand, in the case of the curve L1, in the audible band (10 Hz to 20 kHz), the same
sensitivity as that of a general condenser microphone is obtained. The mass of the movable
portion 5 and the spring constant of the elastic support portion 4 are set so that the resonance
point does not enter the audible band.
[0029]
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10
Next, with reference to FIGS. 8-10, the method to manufacture the microphone 1 from a SOI
(Silicon on Insulator) board | substrate is demonstrated. First, as shown in FIG. 8A, the mask layer
11 and the resist layer 12 are sequentially formed on the surface of the Si layer 10 of the SOI
substrate 100. The SOI substrate 100 has a three-layer structure of a lower Si layer 30, an SiO 2
layer 20, and an upper Si layer 10, the fixed portion 3 and the movable portion 5 are formed on
the upper Si layer 10, and the back chamber is formed on the lower Si layer 30. A support frame
2 having 7 is formed. The mask layer 11 is a film such as aluminum or silicon nitride formed by a
known film forming method such as sputtering or vacuum evaporation.
[0030]
Next, the resist layer 12 is exposed and developed by photolithography to form a resist pattern
12P as shown in FIG. In the resist pattern 12P, the shapes of the fixed portion 3, the movable
portion 5 and the elastic support portion 4 shown in FIG. 1 are patterned. Thereafter, using the
resist pattern 12P as a mask, the mask layer 11 is etched by a mixed acid solution to expose the
upper Si layer 10 as shown in FIG. 8C.
[0031]
Next, as shown in FIG. 8D, the upper Si layer 10 is anisotropically etched in the vertical direction
by ICP-RIE (inductively coupled plasma-reactive ion etching) to expose the SiO 2 layer 20.
Thereafter, the resist pattern 12P and the mask layer 11 are removed by a mixed solution of
sulfuric acid and hydrogen peroxide.
[0032]
Then, as shown in FIG. 9A, a thick film resist layer 15 for protection is formed so as to cover the
etched upper Si layer 10 and the exposed SiO 2 layer 20. Thereafter, as shown in FIG. 9B, the
substrate 100 is turned upside down, and an aluminum (Al) layer 13 is formed on the lower Si
layer 30 by sputtering or vacuum evaporation. Then, a resist layer 14 is formed on the Al layer
13, and a resist pattern 14P for back chamber formation is formed by photolithography.
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11
[0033]
Next, using the resist pattern 14P as a mask, the Al layer 13 is etched with a mixed acid solution
to form an Al pattern 13P (see FIG. 9C). Thereafter, lower resist layer 14 is etched by ICP-RIE
using resist pattern 14P and Al pattern 13P as a mask (see FIG. 10A). The lower Si layer 30 is
etched vertically by anisotropic etching, and etching is performed until the SiO 2 layer 20 is
exposed.
[0034]
After the etching is completed, the resist layers 14 and 15 and the Al layer 13 are removed by a
mixed solution of sulfuric acid and hydrogen peroxide (see FIG. 10B). Thereafter, an aluminum
film is formed on the portion to be the fixed portion 3 of the upper Si layer 10 by vacuum
evaporation or the like to form the electrodes 6a and 6b (see FIG. 10C). Thus, the microphone 1
shown in FIG. 1 is formed. In the above-described example, the aluminum film electrodes 6a and
6b are formed in the connection terminal portion, but the aluminum film may be formed on the
entire surface side of the fixed portion 3 and the movable portion 5 in order to enhance
conductivity. good.
[0035]
(Modification) FIGS. 15-17 is a figure which shows the modification of 1st Embodiment. In the
first modification shown in FIG. 15, a pair of fixed portions 3a, 3b and movable portions 5a, 5b
are provided. The movable portion 5a supported by the pair of elastic support portions 4b and
the movable portion 5b supported by the pair of elastic support portions 4c are connected at a
central portion, and the connection portion is supported by the pair of elastic support portions
4d. ing. The capacitor formed of the movable portion 5a and the fixed portion 3a is connected in
parallel to the capacitor formed of the movable portion 5b and the fixed portion 3b. Thus, the
sensitivity of the microphone 1 can be improved by increasing the areas of the fixed portion and
the movable portion.
[0036]
FIG. 16 is a view showing a second modification. FIG. 16 (b) is an enlarged view of the circular
03-05-2019
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area of FIG. 16 (a). Smaller comb teeth 31 and 51 are formed on the respective teeth of the fixed
part 3 and the movable part 5, and the convex parts of the comb teeth 31 formed on each tooth
of the fixed part 3 are movable parts It has entered into the recess of the comb teeth 51 formed
on each of the five teeth. With such a structure, the capacitance can be increased without
increasing the areas of the fixed portion and the movable portion as in the first modification, and
the sensitivity of the microphone 1 can be improved. it can.
[0037]
FIG. 17 is a view showing a third modification. In the third modification, as shown in FIG. 17 (b),
the inclined grooves 5 a are formed on each tooth of the movable portion 5. When such an
inclined groove 5a is formed, the movable portion 5 can easily move not only in the vertical
direction but also in the horizontal direction when receiving a sound wave, and in particular, with
respect to the direction perpendicular to the inclination of the inclined groove 5a. The sensitivity
is improved. Further, by providing the inclined grooves 5a, the weight of the movable portion 5 is
reduced, the resonance frequency (see FIG. 7) moves to a higher frequency, and the
characteristics are stabilized. In addition, you may form the through-hole which makes d
diameter which satisfy | fills Formula (8) in the whole region of the movable part 5 in order to
reduce movable part weight.
[0038]
Second Embodiment FIGS. 11 to 12 show a second embodiment of a microphone according to
the present invention. FIG. 11 is a view similar to FIG. 1 described above, and FIG. 12 is a crosssectional view taken along the line B-B of FIG. The microphone of the second embodiment is also
the same as the first embodiment in that the fixed portion 3 and the movable portion 5 of the
comb-tooth structure are provided, but as shown in the B-B cross-sectional view of FIG. The
difference is that a film having a compressive stress (hereinafter referred to as a stress film) 4 a
is formed on the surface side of the elastic support 4.
[0039]
Generally, if there is compressive stress in the film, tensile stress will occur on the silicon side to
offset it. As a result, the surface with the membrane is extended (outer side of the arc), and the
membrane surface is bent as a convex. That is, when a film having a compressive stress is formed
03-05-2019
13
as the stress film 4a on the elastic support portion 4, the elastic support portion 4 as a whole is
bent toward the back surface side of the substrate, and the movable portion 5 is a substrate as
shown in FIG. It will shift to the back side.
[0040]
In the example shown in FIGS. 11 and 12, although the stress film 4a has compressive stress, it
may have tensile stress. In that case, the elastic support portion 4 bends to the surface side, and
the movable portion 5 shifts to the surface side with respect to the fixed portion 3.
[0041]
Examples of the stress film 4 a include a polysilicon film in a film having compressive stress, and
a chromium film, a silicon nitride film, and the like in a film having tensile stress. However, these
films may also have different directions of stress depending on the manufacturing method. For
example, by using the difference in thermal expansion coefficient for each material, film
formation can be performed in a high-temperature atmosphere of about several hundred degrees
Celsius, and the stress difference can be generated or reduced by cooling to normal temperature.
In addition, even with the same metal film, the direction of stress may change depending on the
film forming temperature and the orientation. There is also a method of controlling stress by
forming a plurality of different metal films in place of forming the stress film 4a with a single
layer film.
[0042]
The formation of the stress film is performed after the movable portion 5 is formed as shown in
FIG. 8 (d), but may be performed after the back chamber 7 is formed as shown in FIG. 10 (b). It
may be before forming the chamber 7. In any case, before forming the back chamber 7, an
insulating film is formed on the entire surface of the comb teeth (the fixed portion 3 and the
movable portion 5) by CVD or oxidation. And after removing the insulating film of the part of the
elastic support part 4, the stress film | membrane 4a is formed in the part. Sputtering or plating
is used to form the stress film 4a.
[0043]
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By the compressive stress of the stress film 4a, the elastic support portion 4 is bent to the back
surface side, and as shown in FIG. 11B, the fixed portion 3 and the movable portion 5 are
mutually shifted in the z direction. As described above, the present embodiment is characterized
in that the fixed portion 3 and the movable portion 5 are shifted in the vibration direction of the
movable portion 5, and thereby, the following effects can be obtained. In the example shown in
FIGS. 11 and 12, the movable portion 5 is shifted from the fixed portion 3 in the negative z
direction by ξ0, but the direction of the initial shift ξ0 may be the positive z direction. .
[0044]
FIG. 13 is a diagram for explaining the case where there is no deviation between the fixed part 3
and the movable part 5 as in the microphone 1 shown in FIG. 1 (図 0 = 0). In FIG. 13, (a) shows
the change of the sound pressure of the input sound wave, (b) shows the positional relationship
between the fixed part 3 and the movable part 5 when the sound pressure acts, (c) shows the
fixed part 3 And the change of the capacitance between the movable portion 5 and the movable
portion 5 is shown.
[0045]
When the instantaneous sound pressure in free space is p 0 = A sin ωt, the force by the sound
pressure is expressed by the following equation (9). D is an air pressure coefficient, which
represents the degree of reduction due to the gap 8a and the like, and indicates the ratio of the
sound pressure actually applied to the movable portion 5. Further, Sa is the area on the surface
side of the movable portion 5. F = DSa · Asin ωt (9)
[0046]
At t = t1 in FIG. 13A, since F = 0, the movable portion 5 is not displaced, and the capacitance at
that time is C0 as represented by the following equation (10). In the equation (10), LH is the area
of the surface (side surface) of the movable portion 5 facing the fixed portion 3, H is the
thickness of the fixed portion 3 and the movable portion 5, and L is a comb-like surface It is the
length of the edge of the surface (outline of the comb teeth in FIG. 11A). C0 = ε0LH / d (10)
03-05-2019
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[0047]
The amplitude ξ 1 of the movable portion 5 that vibrates due to the sound pressure is expressed
by the following equation (11). Kz is a spring constant of the movable portion 5 in the z direction.
When a sound wave as shown in FIG. 13A is input, the movable portion 5 is displaced in the
minus z direction at t = t1 to t3 and is displaced in the plus z direction at t = t3 to t5. The
displacement of t = t2 is (−ξ1⁄2), and the displacement of t = t4 is (+ ξ1⁄2). ξ 1 = F / Kz =
ADSa / Kz (11)
[0048]
The change in electrostatic capacitance between the fixed part 3 and the movable part 5 is the
same as long as the movable part 5 is displaced in either positive or negative direction of the zaxis if the displacement amounts are equal. Therefore, the change of the electrostatic capacitance
(dynamic electrostatic capacitance C1) is as shown in FIG. 13 (c), and it can be expressed by the
following equation (12). As a result, an irregular harmonic component is generated. C1 = C0−C2
(1−cos <2> ωt) <0.5> (12) C2 = ε0L01 / 2d
[0049]
Next, as shown in FIG. 14, a case where there is an initial displacement ξ0 between the fixed
portion 3 and the movable portion 5 will be considered. In this case, as shown in (a) and (b) of
FIG. 14, the initial deviation ξ0 is ξ0ξ1 / 2 with respect to the amplitude ξ1 and 場合 0 <ξ1
/ 2. , The state of change of the dynamic capacitance C1 is different.
[0050]
When the initial displacement ξ0 is ξ0ξ1⁄2, as shown in FIG. 14A, the movable portion 5 does
not protrude in the plus z direction with respect to the fixed portion 3 even at t = t4. That is, the
displacement direction of the movable portion 5 with respect to the fixed portion 3 is zero or
always in the negative z direction. Since the capacitance at t = t1 in FIG. 14A is C3 = ε0L
(H−ξ0) / d, the dynamic capacitance C1 in this case is expressed by the following equation
(13), and FIG. It becomes a sine curve as shown in a). Since ξ0ξξ1 / 2, C0−C3 = ε0Lξ0 /
03-05-2019
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dεε0Lξ1 / d = C2. C1 = C3-C2 · sin ωt (13)
[0051]
On the other hand, when the initial deviation ξ0 is ξ0 <ξ1 / 2, C0−C3 <C2, and as shown in
FIG. 14B, in the range F including t = t4, the movable portion with respect to the fixed portion 3
The displacement direction of 5 is the plus z direction. Therefore, the curve of the dynamic
capacitance C1 does not become a sine curve, and as shown in FIG. 14 (b), in the range F, the sine
curve has an inverted shape with respect to the straight line of C0. That is, frequency
components due to the influence of the portion of the range F are generated.
[0052]
Because of this, it is necessary to set the initial deviation ξ 0 as ξ 0 ξ ξ 1/2 in order to
prevent the generation of an extra frequency component such as a harmonic component.
Furthermore, when the movable part 5 vibrates, it is necessary to always generate an
electrostatic capacitance, and therefore, if a part of the side face of the movable part 5 and a part
of the side face of the fixed part 3 do not necessarily face each other It does not. That is, it is
necessary to satisfy ξ0 + ξ1 / 2 <H. Therefore, the initial deviation ξ0 is set to satisfy the
following equation (14). In addition, when such an initial deviation ξ 0 is set, the relationship
between the thickness H of the movable portion 5 and the dimension h of the gap in the
thickness direction described in the first embodiment is h = ξ 0. ξ 1/2 ξ 0 <H-ξ 1/2 (14)
[0053]
The actual movable part 5 is not strictly on the same plane as the fixed part 3 as shown in FIG.
13 due to the influence of distortion or warpage due to residual stress of the wafer. If the
amplitude ξ1 of the vibration due to the sound pressure is in a range such that ξ0ξ1 / 2 with
respect to the displacement ξ0 due to the distortion, the change in capacitance is not a
harmonic and the same frequency as the input is output It will be. However, if the amplitude ξ1
of the movable portion 5 becomes large enough to satisfy ξ1 / 2> ξ0, overtones are output.
[0054]
The amplitude ξ1 of the vibration due to the sound pressure is about 1 μm or less at 120 dB
03-05-2019
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when the size of the movable portion 5 is several mm square.
[0055]
The equations used in the above description are simply equations, and there are more
parameters to be actually considered.
For example, as the applied bias voltage E is larger, the spring constant Kz is increased by the
electrostatic force, and the rate of increase is changed by the displacement. There is also negative
stiffness due to air resistance. On the other hand, a general diaphragm type microphone is
strongly influenced by air resistance due to the squeeze film damper effect in principle. However,
in the case of the microphone 1 using the vibration of the comb-teeth-type movable portion 5
described above, since the gap d does not move in the narrowing direction, it is difficult to
receive the squeeze film damper effect and the influence of air resistance is relatively small. .
[0056]
As described above, in the second embodiment, by forming the movable portion 5 in the
vibration direction with respect to the fixed portion 3, the generation of the harmonic component
as shown in FIG. 13 can be suppressed. In particular, by setting the initial deviation ξ0 so as to
satisfy the equation (14), the generation of the overtone component can be completely
prevented.
[0057]
Third Embodiment FIG. 18 is a view showing a third embodiment of the microphone according to
the present invention. In the first and second embodiments described above, the fixed portion 3
and the movable portion 5 have a comb shape, but in the present embodiment, the movable
portion 5 has a circular shape as in the conventional microphone. The movable portion 5 is
supported on the circular back chamber 7 by the four elastic supports 4. Arc-shaped fixed parts
3A to 3D are arranged on the side circumference of the movable part 5. The fixing portions 3A to
3D are independent of each other, and can detect each capacitance individually. That is, the fixed
portions 3A to 3D are set to the ground side, and the FETs 41 shown in FIG. 3 are provided in
each of the fixed portions 3A to 3D to detect changes in each electrostatic capacitance.
03-05-2019
18
[0058]
With such a configuration, a directional microphone can be obtained. For example, as shown in
FIG. 19, when there is a sound source in the oblique direction of the movable part 5, the arrival
time and sound pressure of the sound wave from the sound source differ depending on the
position on the surface of the movable part The movable part 5 is inclined so that the line is
directed to the sound source direction. Then, in the inclined state, the movable portion 5 vibrates
according to the output of the sound wave. Therefore, the change in electrostatic capacitance of
each of the fixed portions 3A to 3D becomes different values, and the sound source direction can
be estimated from the difference in the change in electrostatic capacitance.
[0059]
In FIG. 18, the fixed part side is divided into four fixed parts 3A to 3D, but as shown in FIG. 20,
the movable part side is also divided into a plurality of movable parts 5A to 5D so that each can
be displaced individually. Also good. In the example shown in FIG. 20, the opposing portions of
the movable portions 5A to 5D and the fixed portion 3 are in a comb shape. Each of the movable
parts 5A to 5D is supported by an elastic support 4 extending from the central portion of the
support column 42. In addition, the gap dimension d of the gap formed above the back chamber
7 is not limited to the gap around the movable parts 5A to 5D, and the gap dimension d of the
elastic supporting part 4 and the column 42 may It is set to satisfy).
[0060]
Although illustration is omitted, in the configuration of FIG. 20, only the movable portion may be
divided into four without dividing the fixed portion into four. At the place, the FET 41 is provided
for each of the divided movable parts 5A to 5D so as to detect a change in capacitance.
[0061]
In the case of the microphone of the conventional diaphragm structure, the diaphragm is
generally a circular membrane structure and requires a certain size, but in the case of the
03-05-2019
19
microphone according to the present invention, the gap dimension d is in the range satisfying the
equation (8) The shape of the movable portion 5 supported by the elastic support portion 4 can
be various shapes. For example, as shown in FIG. 21, the comb-shaped fixed portion 3 and the
movable portion 5 shown in FIG. 1 are formed in the L-shaped area 200 on the substrate, and the
circuit area 201 is provided in the remaining area. It becomes possible to easily form the circuit
shown in FIG. 3 and an application specific integrated circuit (ASIC) in the area 201.
[0062]
In the embodiment described above, the condenser type microphone is used to detect the
vibration of the movable part 5 based on the change in electrostatic capacitance. However, the
piezoresistive element is formed by ion implantation or the like on the elastic support part 4
shown in FIG. The sound pressure may be converted into an electrical signal by reading a change
in piezoresistance caused by the displacement of the movable portion 5.
[0063]
In addition, when the bias voltage Vb is applied by an electret (permanently charged film), an
insulating layer (for example, a silicon oxide film) is formed on the upper surface of the fixed
portion and the movable portion which is not on the ground side. The insulating layer is charged.
Alternatively, ions may be implanted. Further, as shown in FIG. 22, the electret portion 33
connected by the fixed portion 3 and the conductive portion 32 may be separately formed. The
same configuration can be applied to the electret for the movable portion 5 as well.
[0064]
In the above-described embodiment, the microphone is formed by processing the SOI substrate,
but instead of the SOI substrate, a two-layer substrate in which tempax glass is bonded by anodic
oxidation to a polished single crystal silicon layer may be used. .
[0065]
As described above, in the microphone 1 formed by processing the substrate 100 composed of a
plurality of layers by micromachining technology, the movable portion 5 and the fixed portion 3
03-05-2019
20
opposed to the side surface via the predetermined gap 8a are the same upper portion The
movable portion 5 is elastically supported by the elastic support portion 4 at a position facing
the back chamber 7 of the support frame 2 by the Si layer 10.
Then, assuming that the sound velocity is c and the dynamic viscosity coefficient of air is ν, the
dimension h of the predetermined gap 8a in the thickness direction and the distance d between
the side surface and the fixed portion facing surface are h ≧ πcd <2> / 6ν It was set to be
satisfactory. As a result, it is possible to miniaturize the microphone and reduce the cost while
maintaining the sensitivity to the sound pressure.
[0066]
The movable portion 5 and the fixed portion 3 may form a capacitor, and the sound pressure
may be detected based on a change in capacitance when the movable portion 5 is displaced. An
element may be disposed to detect the sound pressure based on a change in resistance of the
piezoresistive element when the movable portion is displaced.
[0067]
Further, by disposing the movable portion 5 so as to be displaced in the thickness direction of
the movable portion 5 from the position where the capacitance of the capacitor is maximum, the
generation of the harmonic component is suppressed.
In particular, assuming that the thickness of the movable portion 5 and the fixed portion 3 is H,
and the maximum amplitude of the movable portion 5 when the sound pressure is applied is ξ1,
the positional deviation ξ0 is ξ1 / 2 ≦ ξ0 <H−ξ1 / 2 By setting so as to be satisfied, it is
possible to prevent the generation of the overtone component.
[0068]
In the second embodiment described above, the initial displacement wedge 0 has been described
by way of example in which the movable portion 5 and the fixed portion 3 have the same
thickness. However, as shown in FIG. Even when the thickness of the part 3 is different, the initial
deviation ξ0 may be set similarly. That is, the movable portion 5 may be shifted downward by
an initial deviation ξ 0 from the position where the capacitance is maximized as shown by the
03-05-2019
21
broken line. Further, as shown in FIG. 23 (b), only the opposing portion of the movable portion 5
and the fixed portion 3 is made thinner than the entire thickness, and the thinned portion is
shifted up and down (thickness direction) by the initial displacement ξ0. It is good. In this case,
the thickness of the facing portion corresponds to the dimension H.
[0069]
Furthermore, by making each opposing surface of the movable portion 5 and the fixed portion 3
into a comb-like shape in which the concave and convex shapes are embedded, the capacitance
of the capacitor can be increased, and the sensitivity of the microphone can be improved.
Further, by forming at least one of the fixed portion 3 and the movable portion 5 into a plurality
of pieces to form a plurality of capacitors, directivity can be given to the microphone.
[0070]
The sensitivity can be improved by applying a bias voltage to the fixed part 3 or the movable part
5, but by forming a permanent charge film on the surface of the fixed part 3 or the movable part
5, the power supply for the bias voltage is omitted. be able to. Further, as shown in FIG. 22, an
electret portion 33 (permanently charged film) is formed on the surface of the support frame 2
so that the electret portion 33 and one of the fixed portion 3 or the movable portion 5 are
conducted by the conductive portion 32. Also good.
[0071]
A piezoresistive element may be disposed in the elastic support portion, and the sound pressure
may be detected based on a change in resistance of the piezoresistive element when the movable
portion 5 is displaced. Also, by forming a groove (slant groove 5a) or a through hole having the
same dimension as the distance d in the movable portion 5 to reduce the weight of the movable
portion 5, the resonance point of the movable portion 5 is increased, and resonance Points can
be taken far from the audible range.
[0072]
03-05-2019
22
Each of the embodiments described above may be used alone or in combination. This is because
the effects of the respective embodiments can be exhibited singly or synergistically. Further, the
present invention is not limited to the above embodiment as long as the features of the present
invention are not impaired.
[0073]
1: Microphone 2: Support frame 3: 3, 3A to 3D: Fixing portion 4, 4b to 4d: Elastic support portion
4a: Stress film 5, 5A to 5D: Movable portion 5a: Inclined groove 7: 7: Back Chambers 8a to 8d:
gap 10: upper Si layer 30: lower Si layer 31, 51: comb teeth 32: conductive portion 33: electret
portion 100: SOI substrate
03-05-2019
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