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JP2010045430

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DESCRIPTION JP2010045430
The present invention provides an electrostatic transducer capable of achieving high
performance by increasing the vacuum of a vacuum layer. A silicon substrate 10, a vacuum
sealing porous layer 19 formed of a porous polysilicon layer formed on one surface side of the
silicon substrate 10, and the silicon substrate 10 side of the vacuum sealing porous layer 19. And
the vacuum layer 21 is formed between the porous layer 19 for vacuum sealing and the one
surface of the silicon substrate 10, and the silicon substrate 10 side of the vacuum layer 21 is
formed. A fixed electrode 13 to be one electrode of the capacitor is formed, and a movable
electrode 24 to be the other electrode of the capacitor is formed on the side of the vacuum layer
21 opposite to the silicon substrate 10 side. The vacuum sealing porous layer 19 is formed by
forming a polysilicon layer 18 on the one surface side of the silicon substrate 10 such that the
polysilicon layer 18 is separated from the one surface, and a part of the polysilicon layer 18 is
used as an anode. It is formed by oxidation. [Selected figure] Figure 1
Electrostatic transducer
[0001]
The present invention relates to electrostatic transducers.
[0002]
Conventionally, electrostatic transducers formed using micromachining technology and the like
are known (see, for example, non-patent documents 1 and 2).
[0003]
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1
Here, as shown in FIG. 10, the electrostatic transducer disclosed in the non-patent document 1 is
a silicon oxide film (hereinafter referred to as a first silicon oxide film) 11 on one surface of a
silicon substrate 10 ′. Is formed, and a fixed electrode (lower electrode) 13 'made of a
conductive polysilicon layer patterned in a predetermined shape is formed on the side of the first
silicon oxide film 11 A silicon oxide film (hereinafter referred to as a second silicon oxide film) 32
'covering the fixed electrode 13' is formed on the opposite side of the silicon oxide film 11 'of the
first silicon oxide film 11' to the silicon substrate 10 'side. A silicon nitride film 33 'is formed on
the film 32', and is patterned into a predetermined shape on the silicon nitride film 33 ', and a
plurality of through holes 33a' of the silicon nitride film 33 'are partially formed. A vacuum
sealing porous layer 19 ′ made of a conductive porous polysilicon layer and a cap layer 22 ′
made of an Al film laminated on the vacuum sealing porous layer 19 ′, A movable electrode 24
'(upper electrode) is constituted by the sealing porous layer 19' and the cap layer 22 ', and a
vacuum layer 21' is formed between the fixed electrode 13 'and the silicon nitride film 33'. .
[0004]
In the above-described electrostatic transducer, since a capacitor having the fixed electrode 13
'and the movable electrode 24' as electrodes is formed, the movable electrode 24 'receives the
sound wave to thereby form the fixed electrode 13' and the movable electrode 24 '. And the
distance between them changes, and the capacitance of the capacitor changes.
Therefore, if a DC bias voltage is applied between the fixed electrode 13 'and the movable
electrode 24', a minute voltage change between the fixed electrode 13 'and the movable
electrode 24' according to the sound pressure of the sound wave As a result, the vibration energy
of the movable electrode 24 ′ generated by the sound pressure of the sound wave can be used
as an acoustic sensor for converting it into an electrical signal.
[0005]
In the above-described electrostatic transducer, when a voltage is applied between the fixed
electrode 13 ′ and the movable electrode 24 ′, the movable electrode 24 is generated by the
electrostatic attractive force generated between the fixed electrode 13 ′ and the movable
electrode 24 ′. Since 'is displaced in the direction approaching the fixed electrode 13', the
acoustic wave is generated by vibrating the movable electrode 24 'by changing the voltage
applied between the fixed electrode 13' and the movable electrode 24 '. Can be used as a
speaker.
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[0006]
Hereinafter, a method of manufacturing the electrostatic transducer having the configuration
shown in FIG. 10 will be briefly described with reference to FIG.
[0007]
First, a first silicon oxide film 11 'having a thickness of 1000 nm is formed on the one surface of
the silicon substrate 10', and then a polysilicon layer having a thickness of 400 nm is formed on
the first silicon oxide film 11 '. And forming an electroconductive polysilicon layer by performing
ion implantation and then annealing to form the electroconductive polysilicon layer, and
patterning the electroconductive polysilicon layer by patterning the electroconductive polysilicon
layer. A fixed electrode 13 'is formed, and then a second silicon oxide film 32' having a thickness
of 300 nm is formed on the one surface side of the silicon substrate 10 'by the CVD method
using TEOS as a raw material. The structure shown in 11 (a) is obtained.
[0008]
Thereafter, a silicon nitride film 33 'having a film thickness of 200 nm and a tensile stress of 280
MPa to 1089 MPa is formed on the one surface side of the silicon substrate 10 by LPCVD
method, and subsequently, the silicon nitride film 33' has an inner diameter of about 2 μm. A
plurality of through holes 33a 'are formed by plasma etching, and subsequently, a porous
polysilicon layer capable of transmitting hydrofluoric acid is formed by the LPCVD method on
the one surface side of the silicon substrate 10', and the porous polysilicon layer The structure
shown in FIG. 11B is obtained by performing ion implantation and annealing for forming the
conductive porous polysilicon layer 19a '.
In addition, in forming the porous polysilicon layer by the LPCVD method, process conditions are
set such that a large number of pinholes are formed at the time of film deposition, and the
pinhole size is 1 to 10 nm and the pinhole density Is about 10 to 100 [mu] m <-2>, and the size
and density of pinholes can be adjusted by changing the process parameters of the LPCVD
method.
[0009]
After forming the conductive porous polysilicon layer 19a 'described above, a hydrofluoric acid
solution (for example, BHF or the like) is introduced through the pinholes of the conductive
porous polysilicon layer 19a' to form the second silicon oxide film 32. The structure shown in
FIG. 11C is obtained by forming a vacuum layer 21 'by etching a part of'.
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[0010]
Thereafter, conductive porous polysilicon layer 19a 'is patterned to form vacuum sealing porous
layer 19', and then contact holes 34 for exposing portions of fixed electrode 13 'and silicon
substrate 10', respectively. By forming ', 35', the structure shown in FIG. 11 (d) is obtained.
Thereafter, an Al film is formed on the entire surface of the silicon substrate 10 'and then
patterned to form a cap layer 22' and the like, thereby obtaining a structure shown in FIG. 11 (e).
[0011]
Further, as shown in FIG. 12, the electrostatic transducer disclosed in Patent Document 2
includes a silicon substrate 10 ′ ′ serving as a fixed electrode 13 ′ ′ and a silicon nitride
film formed on one surface side of a silicon substrate 10 ′ ′. And a membrane 42 "made of
Si3N4 formed on the side of the insulating film 41" opposite to the silicon substrate 10 ", and a
movable electrode 24 formed on the membrane 42" and patterned in a predetermined shape.
And a passivation film 44 ′ ′ made of a silicon oxide film formed on the surface side of the
membrane 42 ′ ′ in a form covering the movable electrode 24 ′ ′, and a vacuum layer 21 ′
′ between the membrane 42 ′ ′ and the insulating film 41 ′ ′. Is formed, and a capacitor
having the fixed electrode 13 "and the movable electrode 24" as electrodes is formed.
[0012]
Hereinafter, a method of manufacturing the electrostatic transducer having the configuration
shown in FIG. 12 will be briefly described with reference to FIG.
[0013]
First, an insulating film 41 "made of a silicon nitride film is formed on the one surface of the
silicon substrate 10" also serving as the lower electrode 13 ", then an amorphous silicon film is
formed on the insulating film 41" and the amorphous silicon film is formed. By forming a
sacrificial layer 15 ′ ′ for forming a vacuum layer by patterning, a structure shown in FIG. 13A
is obtained.
[0014]
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Thereafter, a silicon nitride film 42 ′ ′ is formed on the one surface side of the silicon
substrate 10 ′ ′ to obtain a structure shown in FIG. 13 (b).
[0015]
Thereafter, etching holes 42b ′ ′ are formed in the silicon nitride film 42 ′ ′ to expose a
part of the surface of the sacrificial layer 15 ′ ′, and the sacrificial layer 15 ′ ′ is selectively
etched with KOH through the etching holes 42a ′ ′. The structure shown in (c) is obtained.
[0016]
Thereafter, a silicon nitride film is deposited under reduced pressure so that the etching holes
42a ′ ′ are sealed on the one surface side of the silicon substrate 10 ′ ′, and the silicon
nitride film and the silicon nitride film 42 ′ ′ are formed. A membrane 43 ′ ′ made of Si 3 N
4 is formed, and a vacuum layer 21 ′ ′ is formed immediately below the membrane 43 ′ ′.
Subsequently, a movable electrode 24 ′ ′ having a predetermined shape is formed on the one
surface side of the silicon substrate 10. By forming a passivation film 44 ′ ′ made of a film, a
structure shown in FIG. 13D is obtained.
Saarilahti et al, "A Novel Method for cMUT fabrication", 3rd International Workshop on
Micromachined Ultrasonic Transducers (2003), June, 2003 Omer Oralkan, et al, "Capacitive
Micromachined Ultrasonic Transducers: Next-Generation Arrays for Acoustic Imaging?", IEEE
TRANSACTIONS OFULTRAPONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 49, NO.
11, p159-1610, NOVEMBER 2002
[0017]
By the way, in the electrostatic transducer of the configuration shown in FIG. 10, at the time of
manufacture, a hydrofluoric acid solution (for example, BHF etc.) is introduced through the
pinholes of the conductive porous polysilicon layer 19a 'to form the second silicon oxide film.
The vacuum layer 21 'is formed by etching a part of 32', but when the second silicon oxide film
32 'is etched, the silicon nitride film 33' has a low etching rate (hydrofluoric acid based) from
both sides in the thickness direction. If the solution is BHF, although it is about 1 nm / min), the
adhesion between the conductive porous polysilicon layer 19a 'and the silicon nitride film 33' is
lowered, so the vacuum layer 21 'is produced. It is thought that it is difficult to achieve high
performance by increasing the degree of vacuum, and design characteristics can not be obtained.
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In addition, since the amount of change in thickness of the silicon nitride film 33 'before and
after the formation of the vacuum layer 21' varies depending on the size of the vacuum layer 21
', the degree of freedom in device design is low.
[0018]
In the electrostatic transducer of the configuration shown in FIG. 12, an etching hole 42a 'is
formed in the silicon nitride film 42 "at the time of manufacture, and the sacrificial layer 15" is
selectively etched through the etching hole 42a ". A silicon nitride film is deposited under a
reduced pressure so that the etching holes 42a ′ ′ are sealed on the one surface side of the
silicon substrate 10 ′ ′, thereby forming a Si 3 N 4 composed of the silicon nitride film and the
silicon nitride film 42 ′ ′ And the vacuum layer 21 ′ ′ is formed immediately under the
membrane 43 ′ ′, so it is difficult to achieve high performance by increasing the degree of
vacuum of the vacuum layer 21 ′, and to form the sacrificial layer 15 ′ ′. It is considered that
design characteristics can not be obtained because a silicon nitride film is also deposited on the
insulating film 41 ′ ′ after etching. That.
[0019]
The present invention has been made in view of the above-described problems, and an object
thereof is to provide an electrostatic transducer capable of achieving high performance by
increasing the vacuum of a vacuum layer.
[0020]
According to the invention of claim 1, a silicon substrate, a porous layer for vacuum sealing
comprising a porous polysilicon layer formed on one surface side of the silicon substrate, and a
silicon substrate side of the porous layer for vacuum sealing A cap layer is laminated on the
opposite side, a vacuum layer is formed between the vacuum sealing porous layer and the one
surface of the silicon substrate, and one electrode of a capacitor is formed on the silicon
substrate side of the vacuum layer. A fixed electrode is formed, and a movable electrode to be the
other electrode of the capacitor is formed on the side opposite to the silicon substrate side of the
vacuum layer, and the porous layer for vacuum sealing is formed on the one surface side of the
silicon substrate. It is characterized in that the silicon layer is formed in such a manner that the
polysilicon layer is separated from the one surface, and a part of the polysilicon layer is anodized.
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[0021]
According to the present invention, the vacuum sealing porous layer formed of the porous
polysilicon layer formed on one surface side of the silicon substrate, and the vacuum sealing
porous layer are laminated on the opposite side to the silicon substrate side. And forming a
vacuum layer between the porous layer for vacuum sealing and the one surface of the silicon
substrate, and the porous layer for vacuum sealing is a polysilicon on the one surface side of the
silicon substrate. Since the layer is formed by separating the polysilicon layer from the one
surface and anodizing a part of the polysilicon layer, vacuum sealing is performed on the one
surface side of the silicon substrate during manufacturing. After forming the porous layer, the
silicon oxide film, which is a sacrificial layer formed directly under the vacuum sealing porous
layer on the one surface side of the silicon substrate, is passed through the fine pores of the
vacuum sealing porous layer. Selected by hydrofluoric acid solution The space to be a vacuum
layer is formed by etching, and then the micropores of the vacuum sealing porous layer are
sealed by laminating a cap layer on the vacuum sealing porous layer, and the vacuum layer is
formed. A manufacturing process that achieves a desired degree of vacuum can be employed,
and high performance can be achieved by increasing the vacuum of the vacuum layer.
[0022]
In the invention of claim 2, according to the invention of claim 1, the fixed electrode is composed
of a region partially doped in a non-doped polysilicon layer formed on the one surface side of the
silicon substrate, and the movable electrode is A conductive porous polysilicon layer formed by
anodizing a partially doped region in the polysilicon layer is also used as the vacuum sealing
porous layer.
[0023]
According to the present invention, the fixed electrode and the movable electrode face each other
across the vacuum layer, and the fixed electrode and the movable electrode can be prevented
from being etched when the vacuum layer is formed. The gap length of the vacuum layer
between the fixed electrode and the vacuum sealing porous layer can be made more precise, and
the movable electrode anodizes the partially doped region in the polysilicon layer. And the
parasitic capacitance can be reduced as compared to the case where the entire polysilicon layer
is doped, and the above-described conductive layer is formed separately from the porous layer
for vacuum sealing. The movable electrode can be displaced more easily than when the movable
electrode is formed, and the sensitivity can be improved, for example, when used as an acoustic
sensor.
[0024]
According to the invention of claim 3, in the invention of claim 1 or claim 2, a pad is formed at
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each end of the movable electrode, and a sound wave is generated in accordance with a
temperature change of the movable electrode accompanying energization between the pads. It
can also be used as a thermally excited sound source that generates
[0025]
According to the present invention, since it is possible to receive an ultrasonic wave generated
from a thermally excited sound source and reflected by an object, it is possible to realize a
compact, low-cost ultrasonic transducer and ultrasonic range finder. In addition, since the heat
excitation type sound source generates sound waves without using the resonance phenomenon,
ultrasonic waves having short reverberation and short generation period can be transmitted, so
the dead zone can be shortened. The ultrasonic transducer element and the ultrasonic
measurement meter can be improved.
[0026]
The invention of claim 4 is characterized in that, in the invention of claims 1 to 3, a plurality of
the capacitors are formed on the one surface side of the silicon substrate.
[0027]
According to the present invention, when used as an acoustic sensor, it is possible to obtain the
arrival direction of a sound wave.
[0028]
The invention of claim 5 is characterized in that in the invention of claims 1 to 4, a signal
processing circuit for processing an output of the capacitor is formed on the one surface side of
the silicon substrate.
[0029]
According to the present invention, the wiring length between the capacitor and the signal
processing circuit can be shortened, and the size and cost can be reduced compared to the case
where the signal processing circuit is formed on another substrate. Can be
[0030]
According to the invention of claim 1, there is an effect that it is possible to improve the
performance by increasing the vacuum of the vacuum layer.
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[0031]
(Embodiment 1) As shown in FIG. 1, the electrostatic transducer according to the present
embodiment is a porous for vacuum sealing comprising a silicon substrate 10 and a porous
polysilicon layer formed on one surface side of the silicon substrate 10. Porous layer 19 and a
cap layer 22 laminated on the opposite side of the vacuum sealing porous layer 19 to the silicon
substrate 10 side, and the vacuum sealing porous layer 19 and the one surface of the silicon
substrate 10 The vacuum layer 21 is formed between them, the fixed electrode 13 to be one
electrode of the capacitor is formed on the silicon substrate 10 side of the vacuum layer 21, and
the other of the capacitors is formed on the opposite side of the vacuum layer 21 to the silicon
substrate 10 side. A movable electrode 24 to be an electrode is formed.
[0032]
As described later, the porous layer 19 for vacuum sealing is formed with the polysilicon layer
18 on the one surface side of the silicon substrate 10 with the polysilicon layer 18 being
separated from the one surface. It is formed by anodizing a part.
Here, in the present embodiment, the polysilicon layer 18 is formed of the impurity-doped
polysilicon layer, and the porous polysilicon layer to be the vacuum sealing porous layer 19 is
formed of the conductive porous polysilicon layer. In addition to the configuration, the cap layer
22 is formed of a conductive film (for example, an Al film, an Al-Si film, etc.), and the movable
electrode 24 is configured by the vacuum sealing porous layer 19 and the cap layer 22. There is.
[0033]
In the electrostatic transducer of this embodiment, the silicon oxide film 11 is formed on the one
surface of the silicon substrate 10, and the fixed electrode 13 is formed on the non-doped
polysilicon layer 12 formed on the silicon oxide film 11. The conductive polysilicon layer is
formed of an enclosed conductive polysilicon layer, and the conductive polysilicon layer is
formed by forming the non-doped polysilicon layer 12 entirely on the one surface side of the
silicon substrate 10 as described later. The polysilicon layer 12 is formed by partially doping an
impurity.
Here, the fixed electrode 13 and the polysilicon layer 18 are insulated by the silicon nitride film
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14 formed between the non-doped polysilicon layer 12 and the polysilicon layer 18 on the one
surface side of the silicon substrate 10.
[0034]
Further, in the electrostatic transducer according to the present embodiment, the passivation film
23 made of a silicon nitride film is formed on the outermost surface side of the silicon substrate
10 on the one surface side, and is formed on the exposed portion of the fixed electrode 13 and
fixed. A pad 25 electrically connected to the electrode 13 and a pad 26 formed on the exposed
portion of the movable electrode 24 and electrically connected to the movable electrode 24 are
provided.
[0035]
In the electrostatic transducer according to the present embodiment, a capacitor having the fixed
electrode 13 and the movable electrode 24 as electrodes is formed. Therefore, when the movable
electrode 24 receives a sound wave, the space between the fixed electrode 13 and the movable
electrode 24 is received. The distance of changes, and the capacitance of the capacitor changes.
Therefore, if a DC bias voltage is applied between the pad 25 electrically connected to the fixed
electrode 13 and the pad 26 electrically connected to the movable electrode 24, an acoustic
wave is generated between the two pads 25 and 26. Since a minute voltage change occurs in
accordance with the sound pressure of (1), it can be used as an acoustic sensor that converts the
vibration energy of the movable electrode 24 generated by the sound pressure of the sound wave
into an electric signal.
Moreover, the electrostatic transducer of the present embodiment can also be used as a pressure
sensor.
[0036]
Further, in the electrostatic transducer according to the present embodiment, when a voltage is
applied between the pad 25 electrically connected to the fixed electrode 13 and the pad 26
electrically connected to the movable electrode 24, the fixed electrode 13 and the fixed electrode
13 are electrically Since the movable electrode 24 is displaced toward the fixed electrode 13 by
the electrostatic attractive force generated between the movable electrode 24 and the movable
electrode 24, the movable electrode 24 is vibrated by changing the voltage applied between the
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10
both pads 25 and 26. As a result, sound waves can be generated, so that they can be used as
speakers.
[0037]
Hereinafter, a method of manufacturing the electrostatic transducer according to the present
embodiment will be described with reference to FIGS. 2 and 3.
[0038]
First, a silicon oxide film 11 is formed on the entire surface of the single crystal silicon substrate
10 by the CVD method, thermal oxidation method or the like to obtain the structure shown in
FIG. 2A.
[0039]
Thereafter, a non-doped polysilicon layer 12 is formed on the entire surface of the silicon
substrate 10 by the CVD method or the like to obtain a structure shown in FIG. By forming the
fixed electrode 13 by doping an impurity, a structure shown in FIG. 2C is obtained.
When the fixed electrode 13 is formed, the fixed electrode 13 is formed by ion-implanting and
annealing an impurity in a region of the non-doped polysilicon layer 12 where the fixed electrode
13 is to be formed.
[0040]
After the fixed electrode 13 described above is formed, a silicon nitride film 14 is formed on the
entire surface of the silicon substrate 10 on the one surface side to obtain a structure shown in
FIG.
[0041]
Thereafter, a silicon oxide film made of an NSG film is formed on the entire surface of the silicon
substrate 10, and then the silicon oxide film is patterned to form a vacuum layer 21 formed of a
part of the silicon oxide film. By forming the sacrificial layer 15, a structure shown in FIG. 2 (e) is
obtained.
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[0042]
Next, a polysilicon layer 16 is formed on the entire surface of the silicon substrate 10 by the CVD
method or the like, and then a PSG film 17 is formed on the polysilicon layer 16 as shown in FIG.
Obtain the structure shown.
[0043]
Thereafter, annealing is performed to dope the polysilicon layer 16 with impurities in the PSG
film 17 to form the polysilicon layer 18 having conductivity, and then the PSG film 17 is
removed, as shown in FIG. Obtain the structure shown.
[0044]
Thereafter, a resist layer 20 patterned to form a vacuum sealing porous layer 19 is formed on
the one surface side of the silicon substrate 10, and then, a portion of the polysilicon layer 18 is
anodized to be porous. By forming the vacuum sealing porous layer 19 made of a polysilicon
layer, the structure shown in FIG. 3B is obtained.
As an electrolytic solution for anodizing the part of the polysilicon layer 18, 55 wt% of hydrogen
fluoride is used as a solution for etching and removing SiO2, which is an oxide of Si which is a
constituent element of the polysilicon layer 18. Although a hydrofluoric acid solution in which an
aqueous solution and ethanol are mixed at 1: 1 is used, the concentration of the aqueous
hydrogen fluoride solution and the mixing ratio of the aqueous hydrogen fluoride solution and
ethanol are not particularly limited.
Further, the liquid to be mixed with the hydrogen fluoride aqueous solution is not limited to
ethanol, and is not particularly limited as long as it is a liquid that can remove air bubbles
generated in the anodic oxidation reaction, such as alcohols such as methanol, propanol and
isopropanol (IPA). .
[0045]
After the above-described porous layer 19 for vacuum sealing is formed, the resist layer 20 is
removed, and subsequently, the sacrificial layer 15 formed of a silicon oxide film immediately
below the porous layer 19 for vacuum sealing is porous for vacuum sealing. A space 21a to be a
vacuum layer is formed by selectively etching the fine holes of the layer 19 with a hydrofluoric
acid solution (for example, an aqueous solution of hydrogen fluoride or the like) to obtain a
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structure shown in FIG. 3C.
[0046]
Thereafter, a cap layer 22 made of a conductive film (for example, an Al film, an Al-Si film, etc.) is
laminated on the vacuum sealing porous layer 19 on the one surface side of the silicon substrate
10, and the cap layer 22 is patterned. Then, a passivation film 23 made of a silicon nitride film is
formed on the entire surface of the silicon substrate 10, and then the passivation film 23 is
patterned, and then the pads 25 and 26 are formed. The structure shown in 3 (d) is obtained.
[0047]
In the electrostatic transducer according to the present embodiment described above, the
vacuum sealing porous layer 19 formed of the porous polysilicon layer formed on the one
surface side of the silicon substrate 10, and the vacuum sealing porous layer 19. A cap layer 22
is stacked on the side opposite to the silicon substrate 10 side, and a vacuum layer 21 is formed
between the porous layer 19 for vacuum sealing and the one surface of the silicon substrate 10,
for vacuum sealing A porous layer 19 is formed by forming a polysilicon layer 18 on the one
surface side of the silicon substrate 10 such that the polysilicon layer 18 is separated from the
one surface, and anodizing a part of the polysilicon layer 18. Since the vacuum sealing porous
layer 19 is formed on the one surface side of the silicon substrate 10 as described above at the
time of manufacture, the vacuum sealing is performed on the one surface side of the silicon
substrate 10 as described above. A space to be the vacuum layer 21 by selectively etching the
sacrificial layer 15 made of a silicon oxide film formed directly under the porous layer 19 with
the hydrofluoric acid solution through the fine pores of the vacuum sealing porous layer 19 21a
is formed, and then the micropores of the vacuum sealing porous layer 19 are sealed by
laminating the cap layer 22 on the vacuum sealing porous layer 19 to make the vacuum layer 21
have a desired degree of vacuum. Such a manufacturing process can be adopted, and high
performance can be achieved by increasing the vacuum of the vacuum layer 21.
Further, in the electrostatic transducer according to the present embodiment, since the vacuum
layer 21 can be made higher in vacuum, it is possible to prevent the characteristics from being
deteriorated due to the effects of sticking or dust caused by condensation or the like.
[0048]
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Second Embodiment The basic configuration of the electrostatic transducer according to the
present embodiment is substantially the same as that of the first embodiment, and the cap layer
22 is formed of a silicon oxide film, and the movable electrode 24 is as shown in FIG. And the
movable electrode 24 is exposed without being covered by the passivation film 23 and the pads
26, 27 are formed on both ends of the movable electrode 24, respectively. The difference is that
it can also be used as a thermal excitation type sound source that generates a sound wave with
the temperature change of the movable electrode 24 accompanying energization.
Here, in the case of using as a heat excitation type sound source, the movable electrode 24
functions as a heat generating body layer, and the cap layer 22, the vacuum sealing porous layer
19 and the vacuum layer 21 are a heat generating body layer and the silicon substrate 10.
Function as a thermal insulation layer to thermally insulate.
In addition, since the other structure is the same as Embodiment 1, description is abbreviate ¦
omitted.
[0049]
Therefore, according to the electrostatic transducer of the present embodiment, since it is
possible to receive an ultrasonic wave generated from a thermally excited sound source and
reflected by an object, a compact, low-cost ultrasonic wave transmitting / receiving element or It
is possible to realize an acoustic distance meter, and furthermore, since a thermally excited
sound source generates an acoustic wave without using a resonance phenomenon, it is possible
to transmit an ultrasonic wave or an impulse-like acoustic wave having a short reverberation and
a short generation period. Therefore, the dead zone can be shortened, and the accuracy of the
ultrasonic transducer and ultrasonic meter can be improved.
[0050]
Third Embodiment The basic configuration of the electrostatic transducer according to the
present embodiment is substantially the same as that of the first embodiment, and as shown in
FIG. 5, the one surface of the silicon substrate 10 is in direct contact with the vacuum layer 21
and silicon is A back surface electrode 13a made of a conductive film (for example, an Al film, an
Al-Si film, etc.) is formed on the other surface side of the substrate 10, and a fixed electrode 13 is
formed by the silicon substrate 10 and the back surface electrode 13a. The points are different.
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14
In addition, the same code ¦ symbol is attached ¦ subjected to the component similar to
Embodiment 1, and description is abbreviate ¦ omitted.
[0051]
In the electrostatic transducer according to the present embodiment, the silicon nitride film 14
(see FIG. 1A) described in the first embodiment is not provided. The gap length between 13 and
the movable electrode 24 is determined with high accuracy.
A thermally excited sound source similar to that of the second embodiment may be formed in the
electrostatic transducer of this embodiment.
[0052]
Fourth Embodiment The basic configuration of the electrostatic transducer according to the
present embodiment is substantially the same as that of the first embodiment, and the movable
electrode 24 is formed on the one surface of the silicon substrate 10 as shown in FIG. The
difference is that it is made of a conductive porous polysilicon layer formed by anodizing a
partially doped region in the polysilicon layer 16 and also serving as a vacuum sealing porous
layer 19.
In addition, the same code ¦ symbol is attached ¦ subjected to the component similar to
Embodiment 1, and description is abbreviate ¦ omitted.
[0053]
Hereinafter, although the manufacturing method of the electrostatic transducer of this
embodiment is demonstrated, referring FIG.7 and FIG.8, description is suitably abbreviate ¦
omitted about the process similar to Embodiment 1. FIG.
[0054]
First, the silicon oxide film 11 is formed on the entire surface of the silicon substrate 10 by the
CVD method, thermal oxidation method or the like to obtain the structure shown in FIG. 7A.
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[0055]
Thereafter, the non-doped polysilicon layer 12 is formed on the entire surface of the silicon
substrate 10 by the CVD method or the like to obtain the structure shown in FIG. By forming the
fixed electrode 13 by doping an impurity, a structure shown in FIG. 7C is obtained.
[0056]
After the fixed electrode 13 described above is formed, a silicon nitride film 14 is formed on the
entire surface of the silicon substrate 10 on the one surface side, whereby a structure shown in
FIG. 7D is obtained.
Thereafter, a silicon oxide film made of an NSG film is formed on the entire surface of the silicon
substrate 10, and then the silicon oxide film is patterned to form a vacuum layer 21 formed of a
part of the silicon oxide film. By forming the sacrificial layer 15, a structure shown in FIG. 7 (e) is
obtained.
[0057]
Next, a polysilicon layer 16 is formed on the entire surface of the silicon substrate 10 by the CVD
method or the like, and then a PSG film 17 is formed on the polysilicon layer 16 and the PSG film
17 is patterned. Thus, the structure shown in FIG. 7 (f) is obtained.
[0058]
Thereafter, annealing is performed to dope the polysilicon layer 16 with impurities in the PSG
film 17 to form the polysilicon layer 18 having conductivity, and then the PSG film 17 is
removed, as shown in FIG. 8A. Obtain the structure shown.
[0059]
Thereafter, a resist layer 20 patterned to form a vacuum sealing porous layer 19 is formed on
the one surface side of the silicon substrate 10, and then, a portion of the polysilicon layer 18 is
anodized to be porous. By forming the vacuum sealing porous layer 19 made of a polysilicon
layer (conductive porous polysilicon layer), the structure shown in FIG. 8B is obtained.
[0060]
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After the above-described porous layer 19 for vacuum sealing is formed, the resist layer 20 is
removed, and subsequently, the sacrificial layer 15 formed of a silicon oxide film immediately
below the porous layer 19 for vacuum sealing is porous for vacuum sealing. A space 21a to be a
vacuum layer is formed by selectively etching the fine holes of the layer 19 with a hydrofluoric
acid solution (for example, an aqueous solution of hydrogen fluoride or the like) to obtain a
structure shown in FIG. 8C.
[0061]
Thereafter, a cap layer 22 made of a conductive film (for example, an Al film, an Al-Si film, etc.) is
laminated on the vacuum sealing porous layer 19 on the one surface side of the silicon substrate
10, and the cap layer 22 is patterned. Then, a passivation film 23 made of a silicon nitride film is
formed on the entire surface of the silicon substrate 10, and then the passivation film 23 is
patterned, and then the pads 25 and 26 are formed. The structure shown in 8 (d) is obtained.
[0062]
According to the electrostatic transducer of the present embodiment described above, the fixed
electrode 13 and the movable electrode 24 face each other with the vacuum layer 21 interposed
therebetween, and when the vacuum layer 21 is formed, the fixed electrode 13 and the movable
electrode 24 are hydrofluoric acid solution As a result, it is possible to prevent the etching of the
gap between the fixed electrode 13 and the porous layer 19 for vacuum sealing, and to increase
the precision of the gap length of the vacuum layer 21. Since the conductive porous polysilicon
layer is formed by anodizing the partially doped region (polysilicon layer 18) in the silicon layer
16, compared to the case where the entire polysilicon layer 16 is doped. Parasitic capacitance
can be reduced, and the movable electrode 24 can be displaced more easily than when the
movable electrode 24 is formed separately from the vacuum sealing porous layer 19. For
example it is possible to improve the sensitivity when used as an acoustic sensor or a pressure
sensor.
Further, in the electrostatic transducer according to the present embodiment, the porous layer 19
for vacuum sealing is partially formed on the polysilicon layer 16 facing the fixed electrode 13
with the vacuum layer 21 interposed therebetween. As compared with the above, the region
where the vacuum sealing porous layer 19 is formed can be made smaller, and the structural
strength can be enhanced compared to the first embodiment.
A thermally excited sound source similar to that of the second embodiment may be formed in the
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electrostatic transducer of this embodiment.
[0063]
Fifth Embodiment The basic configuration of the electrostatic transducer according to the
present embodiment is substantially the same as that of the first embodiment, and as shown in
FIG. 9, the fixed electrode 13 and the movable electrode are movable on the one surface side of
the silicon substrate 10. A difference is that a signal processing circuit 40 is provided which
performs signal processing on the output of a capacitor having the electrode 24 as an electrode.
Here, the signal processing circuit 40 is configured of a MOSFET or the like, and includes an
amplification circuit that amplifies the output signal of the capacitor.
In addition, the same code ¦ symbol is attached ¦ subjected to the component similar to
Embodiment 1, and description is abbreviate ¦ omitted.
[0064]
Thus, in the electrostatic transducer according to the present embodiment, the wiring length
between the capacitor and the signal processing circuit 40 can be shortened, noise can be
reduced, and the signal processing circuit 40 can be formed on another substrate. Compared
with the case where it is formed, miniaturization and cost reduction can be achieved.
A thermally excited sound source similar to that of the second embodiment may be formed in the
electrostatic transducer of this embodiment.
Further, the signal processing circuit 40 may be formed also in the other first to fourth
embodiments.
[0065]
In the above embodiments, if a plurality of the capacitors are formed on the one surface side of
the silicon substrate 10, it is possible to obtain the arrival direction of the sound wave when used
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as an acoustic sensor, for example, an ultrasonic wave transmitting element By using it in
combination with the thermally excited sound source of Embodiment 2 or the like, it becomes
possible to measure a three-dimensional shape of an object or the like.
[0066]
1st Embodiment is shown, (a) is a schematic sectional drawing, (b) is a schematic plan view.
It is principal process sectional drawing for demonstrating a manufacturing method same as the
above.
It is principal process sectional drawing for demonstrating a manufacturing method same as the
above.
5 is a schematic cross-sectional view showing Embodiment 2. FIG.
FIG. 10 is a schematic cross-sectional view showing Embodiment 3.
FIG. 10 is a schematic cross-sectional view showing Embodiment 4;
It is principal process sectional drawing for demonstrating a manufacturing method same as the
above.
It is principal process sectional drawing for demonstrating a manufacturing method same as the
above.
FIG. 10 is a schematic cross-sectional view showing Embodiment 5;
It is a schematic sectional drawing which shows a prior art example.
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It is principal process sectional drawing for demonstrating a manufacturing method same as the
above.
It is a schematic sectional drawing which shows another prior art example.
It is principal process sectional drawing for demonstrating a manufacturing method same as the
above.
Explanation of sign
[0067]
10 silicon substrate 11 silicon oxide film 12 non-doped polysilicon layer 13 fixed electrode 14
silicon nitride film 18 polysilicon layer 19 porous layer for vacuum sealing 21 vacuum layer 22
cap layer 24 movable electrode
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