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JP2006180082

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DESCRIPTION JP2006180082
The present invention provides a pressure wave generating element and a method of
manufacturing the same, which are less likely to cause a temporal change in the waveform and
sound pressure of a generated pressure wave than in the prior art. A support substrate 1 made of
a silicon substrate, a heating element layer 3 formed on one surface side of the support substrate
1, and a space between the support substrate 1 and the heating element layer 3 on the one
surface side of the support substrate 1 And a pair of pads 4 and 4 electrically connected to both
ends of the heat generating body layer 3, respectively, and the electricity to the heat generating
body layer 3 through the pair of pads 4 and 4 is provided. The pressure wave is generated by the
heat exchange between the heating element layer 3 and the medium accompanying the heat
treatment. The heat generating layer 3 is formed of a metal thin film subjected to annealing at a
temperature higher than the operating temperature. [Selected figure] Figure 1
Pressure wave generating element and method of manufacturing the same
[0001]
The present invention relates to, for example, a pressure wave generating element for generating
a pressure wave such as an acoustic wave intended for a speaker or an ultrasonic wave or a
single pulse compression wave, and a method of manufacturing the same.
[0002]
Conventionally, an ultrasonic wave generating element using mechanical vibration by a
piezoelectric effect is widely known.
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As this type of ultrasonic wave generating element, for example, one having a structure in which
electrodes are provided on both sides of a crystal made of a piezoelectric material such as barium
titanate is known. In this ultrasonic wave generating element, the space between both electrodes
is known. By applying electrical energy to generate mechanical vibration, air can be vibrated to
generate ultrasonic waves.
[0003]
The ultrasonic wave generating element utilizing mechanical vibration as described above has
problems such as a narrow frequency band and being susceptible to external vibration and
fluctuations in external pressure since it has an inherent resonance frequency.
[0004]
On the other hand, in recent years, as a pressure wave generating element capable of generating
a pressure wave such as an ultrasonic wave by thermal excitation without mechanical vibration,
the porous surface is formed on one surface side of a supporting substrate made of a single
crystal silicon substrate. A heat insulating layer formed of a high-quality silicon layer, a heat
generating layer formed of a metal thin film (for example, an aluminum thin film) is formed on
the heat insulating layer, and electrically connected to the heat generating layer on the one
surface side of the support substrate. It is proposed that a pair of the formed pads are formed,
and that a pressure wave such as an ultrasonic wave is generated by heat exchange between the
heating element layer and the air as the medium accompanying the energization of the heating
element layer through the pair of pads. (See, for example, Patent Document 1).
It is desirable to make the thermal conductivity and thermal capacity of the thermal insulation
layer smaller than the thermal conductivity and thermal capacity of the support substrate in
Patent Document 1 above, and the product of the thermal conductivity of the thermal insulation
layer and the thermal capacity is It is stated that it is preferable to make it sufficiently smaller
than the product of the thermal conductivity and the heat capacity.
[0005]
In addition, in the production of the pressure wave generating element having the abovementioned configuration, for example, after forming a thermal insulation layer made of a porous
silicon layer by making one surface side of the silicon substrate porous by anodizing treatment. A
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heating element layer made of a metal thin film may be formed by vapor deposition or
sputtering, and then a pair of pads may be formed by sputtering or vapor deposition.
[0006]
In the above-described pressure wave generating element, for example, while the heat generating
body layer generates heat by applying an alternating voltage or alternating current to the heat
generating body layer, the heat insulating layer is formed immediately below the heat generating
body layer. Is thermally isolated from the support substrate, so efficient heat exchange occurs
with the air in the vicinity of the heating element layer, and pressure waves such as ultrasonic
waves are generated as a result of expansion and compression of the air.
Japanese Patent Application Publication No. 11-300274
[0007]
By the way, in the above-mentioned pressure wave generating element, for example, the
frequency of the pressure wave to be generated is changed over a wide range by adjusting the
frequency of the driving voltage consisting of an alternating current voltage supplied to the
heating element layer or the driving current consisting of alternating current. It is possible to
change the sound pressure by changing the power supplied to the heating element layer. In FIG.
3, when the inventors of the present invention prototype the pressure wave generating element
similar to the conventional one and evaluate the characteristics, the driving voltage is a sine wave
voltage with a frequency of 60 kHz, and the peak value of the sine wave voltage is changed.
When the input electric power to a heat generating body layer is changed, the result of having
investigated the change of the sound pressure which generate ¦ occur ¦ produces, and the
temperature of a heat generating body layer is shown. The horizontal axis in FIG. 3 is the input
power when the peak value of the drive voltage consisting of a sine wave voltage with a
frequency of 60 kHz is variously changed, and the vertical axis on the left is a position 30 cm
away from the surface of the heating element layer. The sound pressure measured by the vertical
axis on the right is the surface temperature (maximum temperature) of the heat generating body
layer, and B in FIG. 3 is the measured value of the sound pressure and B is the measured
value of the temperature. Is shown.
[0008]
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However, in the pressure wave generating element described above, as the input power is higher,
the temperature of the heat generating body layer rises and falls over a wide temperature range
for each drive, so the resistance value of the heat generating body layer changes with time and
the pressure wave generated There was a problem that the waveform and the sound pressure of
the Although a charge / discharge circuit used for a flash circuit of a camera or the like can be
considered as a drive circuit for driving the above-described pressure wave generating element,
in such a charge / discharge circuit, the time constant of the response waveform at the time of
discharge is a load. As it depends on the impedance on the side (in the pressure wave generating
element, the resistance value of the heating element layer), if the resistance value of the heating
element layer changes with time, the response waveform changes and the frequency of the
pressure wave and the sound pressure level change. . Such a characteristic change occurs, for
example, when the pressure wave generating element is used as a wave transmitting device
(ultrasonic wave generating element) of an ultrasonic sensor for determining the distance to an
object using the time from transmission to reception of ultrasonic waves. And the like, which
causes a malfunction of the device incorporating the pressure wave generating element.
[0009]
The present invention has been made in view of the above-described problems, and its object is
to provide a pressure wave generating element which is less likely to cause a temporal change in
generated pressure wave waveform and sound pressure as compared with the prior art, and a
method of manufacturing the same. It is to provide.
[0010]
According to the first aspect of the present invention, there is provided a support substrate, a
heat generating body layer formed on one surface side of the support substrate, and a thermal
insulation layer interposed between the support substrate and the heat generating body layer on
the one surface side of the support substrate. A pair of pads electrically connected to each end of
the heat generating body layer, and a pressure wave is generated by heat exchange between the
heat generating body layer and the medium accompanying the energization of the heat
generating body layer through the pair of pads In the pressure wave generating element, the heat
generating body layer is made of a metal thin film which has been subjected to annealing
treatment at a temperature higher than the operating temperature.
[0011]
According to the present invention, since the heat generating body layer is formed of the metal
thin film annealed at a temperature higher than the operating temperature, the change with time
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of the resistance value of the heat generating body layer can be suppressed as compared with the
conventional case. Changes in pressure wave waveform and sound pressure over time are less
likely to occur.
[0012]
According to the second aspect of the present invention, there is provided a support substrate, a
heat generating body layer formed on one surface side of the support substrate, and a thermal
insulation layer interposed between the support substrate and the heat generating body layer on
the one surface side of the support substrate. A pair of pads electrically connected to each end of
the heat generating body layer, and a pressure wave is generated by heat exchange between the
heat generating body layer and the medium accompanying the energization of the heat
generating body layer through the pair of pads A method of manufacturing a pressure wave
generating element, comprising: forming a heat insulation layer on the one surface of a support
substrate; forming a heat generation layer formed of a metal thin film on the heat insulation
layer; And a pad forming step of forming a pair of pads in contact with both ends of the heat
generating body layer, and further annealing the heat generating body layer at a temperature
higher than the operating temperature later than the pad electrode forming step. An annealing
process is provided.
[0013]
According to the present invention, the heat generating body layer is formed of the metal thin
film annealed at a temperature higher than the operating temperature, and the temporal change
in the waveform of the pressure wave and the sound pressure which are generated as compared
with the prior art. Can be provided.
[0014]
According to the invention of claim 3, a support substrate, a heat generating body layer formed
on one surface side of the support substrate, and a thermal insulation layer interposed between
the support substrate and the heat generating body layer on the one surface side of the support
substrate A pair of pads electrically connected to each end of the heat generating body layer, and
a pressure wave is generated by heat exchange between the heat generating body layer and the
medium accompanying the energization of the heat generating body layer through the pair of
pads A method of manufacturing a pressure wave generating element, comprising: forming a
heat insulation layer on the one surface of a support substrate; forming a heat generation layer
formed of a metal thin film on the heat insulation layer; And forming a pair of pads in contact
with both ends of the heat generating body layer, wherein the heat generating body layer is
heated to a temperature higher than the operating temperature between the heat generating
body layer forming process and the pad forming process. And an annealing treatment step of
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annealing treatment.
[0015]
According to the present invention, the heat generating body layer is formed of the metal thin
film annealed at a temperature higher than the operating temperature, and the temporal change
in the waveform of the pressure wave and the sound pressure which are generated as compared
with the prior art. Can be provided.
In addition, since the annealing process is performed prior to the pad forming process, the
temperature of the annealing process can be set regardless of the material of the pad, and there
is an advantage that the choice of the material of the pad is increased compared to the invention
of claim 2.
[0016]
According to the invention of claim 1, it is possible to suppress the change with time of the
resistance value of the heat generating body layer compared to the prior art, and it is effective to
make the change of the waveform of the pressure wave and the sound pressure which are
generated less likely to occur. .
[0017]
The inventions of claims 2 and 3 have the effect of being able to provide a pressure wave
generating element in which the waveform of the pressure wave to be generated and the
temporal change in sound pressure are less likely to occur than in the prior art.
[0018]
(Embodiment 1) As shown in FIGS. 1 (a) and 1 (b), the pressure wave generating element of this
embodiment has one surface (FIG. 1 (b) of a support substrate 1 made of a single crystal p-type
silicon substrate. A thermal insulation layer (heat insulation layer) 2 formed of a porous silicon
layer is formed on the upper surface side), a heating body layer 3 is formed on the thermal
insulation layer 2, and a heating body layer on the one surface side of the support substrate 1 A
pair of pads 4, 4 electrically connected to 3 are formed.
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Here, in the pressure wave generating element of the present embodiment, the heat generating
layer 3 and the air serving as the medium are changed according to the temperature change of
the heat generating layer 3 according to the drive voltage waveform or the drive current
waveform applied to the heat generating layer 3. Heat exchange generates pressure waves.
The planar shape of the support substrate 1 is a rectangular shape, and the planar shapes of the
heat insulating layer 2 and the heat generating layer 3 are also formed in a rectangular shape.
[0019]
In the present embodiment, as described above, a p-type silicon substrate is used as the support
substrate 1 and the heat insulating layer 2 is formed of a porous silicon layer having a porosity
of approximately 70%. A porous silicon layer to be the heat insulating layer 2 can be formed by
anodizing a part of the silicon substrate to be used in a hydrogen fluoride aqueous solution.
The porosity and thickness of the porous silicon layer to be the heat insulating layer 2 can be set
to desired values by appropriately setting the conditions of the anodizing treatment (for example,
current density, current passing time, etc.) here. .
In the porous silicon layer, the thermal conductivity and the thermal capacity decrease as the
porosity increases. For example, the thermal conductivity is 148 W / (m · K) and the thermal
capacity is 1.63 × 10 <6> J / (m < The porous silicon layer having a porosity of 60% formed by
anodizing a single crystal silicon substrate of 3> · K) has a thermal conductivity of 1 W / (m · K)
and a heat capacity of 0.7 × 10 It is known that <6> J / (m <3> · K).
In the present embodiment, as described above, the heat insulating layer 2 is formed of a porous
silicon layer having a porosity of approximately 70%, and the heat conductivity of the heat
insulating layer 2 is 0.12 W / (m · K), The heat capacity is 0.5 × 10 <6> J / (m <3> · K).
Moreover, although aluminum is employ ¦ adopted as a material of the pads 4 and 4, it does not
limit to aluminum and materials other than aluminum may be employ ¦ adopted.
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In the pressure wave generating element of this embodiment, the thickness of the support
substrate 1 is 525 μm, the thickness of the heat insulating layer 2 is 10 μm, the thickness of
the heat generating layer 3 is 50 nm, and the thickness of each pad 4 is 0. The thickness is 5
μm, but these thicknesses are an example and not particularly limited.
[0020]
By the way, the heat generating body layer 3 is formed of a metal thin film which has been
subjected to the annealing process at a temperature higher than the operating temperature (peak
temperature of the surface of the heat generating body layer 3 when the driving power is applied
to the heat generating body layer 3). It is done. For example, referring to the characteristics
shown in FIG. 3, when using the pressure wave generating element as an ultrasonic wave
generating element that generates an ultrasonic wave with a sound pressure of 6 Pa, it is
necessary to apply 800 W of power to the heating element layer 3 Since the peak temperature of
the heat generating body layer 3 when power of 800 W is applied to the heat generating body
layer 3 is about 200 ° C., the annealing treatment is performed at a temperature higher than
200 ° C. (eg 300 ° C.) You should be included. Note that tungsten, which is a type of high
melting point metal, is used as the material of the heat generating body layer 3, and the thermal
conductivity is 174 W / (m · K), and the heat capacity is 2.5 × 10 <6> J / ( m <3> · K). The
material of the heat generating body layer 3 is not limited to tungsten, and, for example,
tantalum, molybdenum, iridium or the like may be employed.
[0021]
Thus, in the pressure wave generating element according to the present embodiment, since the
heat generating body layer 3 is formed of a metal thin film annealed at a temperature higher
than the operating temperature, the resistance value of the heat generating body layer 3 is
changed with time. The change can be suppressed, and the waveform of the generated pressure
wave and the change with time of the sound pressure hardly occur.
[0022]
Hereinafter, the manufacturing method of the pressure wave generation element of this
embodiment is explained.
[0023]
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First, a conductive electrode (not shown) used for anodizing treatment is formed on the other
surface (lower surface in FIG. 1B) side of the silicon substrate used as the support substrate 1,
and then thermal insulation on one surface side of the silicon substrate The heat insulating layer
forming step of forming the heat insulating layer 2 made of porous silicon is performed by
making the formation planned portion of the layer 2 porous by anodizing treatment.
Here, in the heat insulating layer forming step, for example, as shown in FIG. 2, an electrolytic
solution (for example, a 55 wt% hydrogen fluoride aqueous solution) in which the object C mainly
composed of a silicon substrate is placed in the treatment tank A Immersing in a mixed solution
(mixed solution B) in which ethanol and ethanol are mixed at 1: 1, and then connecting the
platinum electrode 21 connected to the negative side of the current source 20 through the
wiring in the electrolyte solution B Arrange so as to face the side.
Subsequently, a current-carrying electrode is an anode, a platinum electrode 21 is a cathode, and
a current of a predetermined current density (here, 20 mA / cm <2>) is supplied from the current
source 20 to the anode and the cathode 21 for a predetermined time (here Then, the heat
insulating layer 2 having a predetermined thickness (here, 10 μm) is formed on the one surface
side of the support substrate 1 by performing anodizing treatment by flowing for 8 minutes. The
conditions at the time of anodizing treatment are not particularly limited, and the current density
may be appropriately set, for example, in the range of about 1 to 500 mA / cm <2>. It may be
appropriately set according to the predetermined thickness.
[0024]
After the above-described heat insulation layer formation step, a heating element formation step
of forming the heating element 3 is performed, and thereafter, a pad formation step of forming
the pads 4 and 4 is performed. In the heating element layer forming step, the heating element
layer 3 may be formed by sputtering or evaporation using a metal mask or the like, and in the
pad forming step, the sputtering or evaporation may be performed using a metal mask or the
like. The pads 4 and 4 may be formed by the like.
[0025]
After the pad formation step, an annealing treatment step is further carried out in which the
heating element layer 3 is annealed in vacuum or in an inert gas under the conditions of a
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predetermined annealing temperature higher than the operating temperature and a
predetermined annealing time. The dicing step may be performed. Here, the condition of the
annealing process may be appropriately set based on the operating temperature of the heating
element layer 3 at the time of generating the pressure wave of the designed sound pressure from
the pressure wave generating element, for example, 100 ° C. than the operating temperature. If
the temperature is set to a high temperature, when the operating temperature is 200 ° C., the
annealing temperature may be set to 300 ° C., but these numerical values are not particularly
limited. The annealing time may be, for example, about 30 minutes, but the annealing time is not
particularly limited.
[0026]
According to the manufacturing method of the pressure wave generating element of the present
embodiment described above, the heat generating body layer 3 is formed of the metal thin film
annealed at a temperature higher than the operating temperature. The pressure wave generating
device according to the present embodiment can be provided, for example, using the time from
transmission of ultrasonic waves to reception of the pressure waves. It is possible to improve the
reliability of the ultrasonic sensor when it is used as a transmission device (ultrasonic wave
generating element) of the ultrasonic sensor for determining the distance to the object.
[0027]
Here, the pressure wave generating element of the comparative example manufactured by
omitting the annealing process step from the above manufacturing method generates a single
pulse input voltage waveform (driving voltage waveform) that generates a sound pressure of 6
Pa at the start of driving. When the body layer 3 was continuously driven at a rate of 60 times
per second for a total of 30 million times, the resistance value of the heat generating body layer
3 decreased from about 30 Ω to about 12 Ω.
For this reason, when driven by the charge / discharge circuit, the time constant of discharge
decreases due to the decrease in resistance value, the frequency of the generated pressure wave
rises from 40 kHz to 71 kHz, and the sound pressure level from 6 Pa to 4.5 Pa Diminished.
[0028]
On the other hand, in the pressure wave generating element of the embodiment manufactured by
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the above-described manufacturing method, the resistance value of the heating element layer 3
almost changes even if a single pulse input voltage waveform (drive voltage waveform) is applied
30 million times. When the battery was driven by the charge and discharge circuit, neither the
frequency of the generated pressure wave nor the sound pressure level changed. In the pressure
wave generating element of the example, the resistance value before annealing was about 30 Ω
during the manufacture thereof, and the resistance value after annealing was reduced to about 8
Ω.
[0029]
Second Embodiment The basic configuration of the pressure wave generating element of the
present embodiment is the same as that of the first embodiment, and only the manufacturing
method is different. Therefore, only the manufacturing method will be described below. However,
the description of the same steps as the manufacturing method described in the first embodiment
will be appropriately omitted.
[0030]
The manufacturing method of the pressure wave generating element of this embodiment is
substantially the same as the manufacturing method described in the first embodiment, and in
the first embodiment, the annealing process is performed after the pad forming process. The only
difference is that the annealing process is performed between the heating element layer forming
process and the pad forming process.
[0031]
In the present embodiment, in the heating element layer forming step, the heating element layer
3 made of a tungsten thin film which is a metal thin film is formed into a film by an RF sputtering
apparatus whose substrate temperature can be controlled.
In the RF sputtering apparatus, the wafer to be film-formed (the supporting substrate 1 on which
the heat insulating layer 2 is formed) is introduced into the chamber to recover the degree of
vacuum in the chamber to a predetermined degree of vacuum. The frequency of the applied high
frequency voltage is 13.56 MHz, the substrate temperature is 300 ° C., the sputtering gas is
argon gas, the target material is tungsten, and a tungsten thin film is formed. In the annealing
process, the above-mentioned RF sputtering apparatus is used Annealing is performed. However,
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in the annealing process, after the introduction of the sputtering gas into the chamber of the RF
sputtering apparatus is stopped and the degree of vacuum in the chamber is recovered to a
predetermined degree of vacuum, the high frequency voltage is not applied to the substrate
holder Then, the substrate temperature is maintained at the above annealing temperature (for
example, 300 ° C.) for the annealing time, and then the substrate temperature is decreased.
[0032]
Thus, according to the method of manufacturing a pressure wave generating element of the
present embodiment, as in the first embodiment, the heating element layer 3 is formed of a metal
thin film that has been annealed at a temperature higher than the operating temperature.
According to the present invention, it is possible to provide a pressure wave generating element
in which the waveform of the generated pressure wave and the temporal change in sound
pressure are less likely to occur than in the prior art. Further, in the manufacturing method of the
present embodiment, since the annealing is performed prior to the pad forming step, the
temperature of the annealing can be set regardless of the material of the pads 4 and 4, and the
pads compared to the manufacturing method of the first embodiment. There is an advantage that
the choice of materials of 4, 4 is increased. Further, in the present embodiment, since the
annealing process is performed in the chamber of the sputtering apparatus used in the heating
element layer forming process, it is not necessary to separately prepare an annealing apparatus
dedicated for the annealing process, and the manufacturing cost is increased. It can be reduced.
[0033]
By the way, although the single crystal p-type silicon substrate is adopted as the support
substrate 1 in each of the above embodiments, the support substrate 1 is not limited to the single
crystal p-type silicon substrate, and a polycrystalline or amorphous p-type silicon substrate Not
only p-type but also n-type or non-doping may be used, and the conditions of the anodizing
treatment may be appropriately changed according to the type of the support substrate 1. In
each of the above-described embodiments, Si is used as the material of the support substrate 1.
However, the material of the support substrate 1 is not limited to Si. For example, Ge, SiC, GaP,
GaAs, InP, etc. are anodized. Other semiconductor materials that can be made porous can also be
used. Therefore, the porous layer constituting the heat insulating layer 2 is not limited to the
porous silicon layer, and, for example, is formed of a porous polycrystalline silicon layer formed
by anodizing polycrystalline silicon, or a semiconductor material other than silicon. It may be a
porous semiconductor layer.
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[0034]
The pressure wave generation element in Embodiment 1 is shown, (a) is a schematic plan view,
(b) is a D-D 'schematic cross section of (a). It is explanatory drawing of the manufacturing method
same as the above. It is an input-output characteristic view of a conventional example.
Explanation of sign
[0035]
1 support substrate 2 thermal insulation layer 3 heating element layer 4 pad
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