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JP2006013961

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DESCRIPTION JP2006013961
A pressure wave generating element capable of preventing breakage of a heat generating
element by preventing generation of a crack in a heat insulating layer at the time of
manufacturing or driving, and a method of manufacturing the same. A heat generating body
comprising a semiconductor substrate 1 as a substrate, a heat insulating layer 2 formed of a
porous layer formed on one surface side in the thickness direction of the semiconductor
substrate 1, and a thin film formed on the heat insulating layer 2 3 and the pads 4 and 4 formed
on both end portions of the heating element 3, respectively, and the heating element 3 and
medium (for example, with the energization of the heating element 3 through the pair of pads 4
and 4) , And air) to generate pressure waves (eg, ultrasonic waves). The porous layer constituting
the heat insulating layer 2 is formed by anodizing a part of a p-type silicon substrate as the
semiconductor substrate 1 in an electrolytic solution, and the high porosity layer 21 on the
heating element 3 side is formed. And the low porosity layer 22 on the semiconductor substrate
1 side. [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]
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1
Conventionally, an ultrasonic wave generating element using mechanical vibration by a
piezoelectric effect is widely known.
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, a medium such as 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 an element capable of generating an ultrasonic wave
without mechanical vibration, a pressure wave generating element has been proposed which
utilizes a method of forming air density by thermal excitation which applies heat to a medium (
For example, Patent Documents 1 and 2).
[0005]
As shown in FIG. 9, this type of pressure wave generating element comprises a semiconductor
substrate 1 made of a single crystal silicon substrate, and a porous silicon layer formed to a
predetermined depth from one surface of the semiconductor substrate 1 in the thickness
direction. A heat insulating layer 2 'having a sufficiently small thermal conductivity and heat
capacity as compared with the semiconductor substrate 1, a heat generating body 3 formed of a
metal thin film (for example, an Al thin film etc.) formed on the heat insulating layer 2' A
pressure wave is generated by heat exchange between the heat generating body 3 and a medium
(for example, air) accompanied by the application of alternating current to the heat generating
body 3 with the pads 4 and 4 formed on both end portions of 3 respectively. It is a thing.
That is, in the pressure wave generating element having the configuration shown in FIG. 9, the
heating element 3 generates heat when the alternating current is supplied to the heating element
3 through the pair of pads 4 and 4 while the heating element 3 generates heat. Since the heat
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insulating layer 2 'is formed directly under the heat source to thermally insulate the heat
generating body 3 from the semiconductor substrate 1, efficient heat exchange occurs with the
air in the vicinity of the heat As a result of expansion and compression, pressure waves such as
ultrasonic waves are generated.
Here, the heat insulating layer 2 ′ is formed by making part of the semiconductor substrate 1
porous by anodizing treatment.
[0006]
In the pressure wave generating element having the configuration shown in FIG. 9, the frequency
of the pressure wave to be generated can be changed over a wide range by adjusting the
frequency of the AC voltage (driving voltage) applied to the heating element 3 For example, it can
be used as a sound source of an ultrasonic sound source or a speaker. JP-A-11-300274 JP-A2002-186097
[0007]
By the way, the inventors of the present application are general sizes of ultrasonic wave
generating elements utilizing mechanical vibration in which the size of pressure wave generating
elements is widely used with respect to the pressure wave generating element having the
configuration shown in FIG. If it is driven to generate a sound pressure (for example, about 20 Pa
at a position separated by 30 cm at a frequency of 40 kHz) equivalent to the above ultrasonic
wave generation element with about 15 mm □, the heating element 3 may be broken. We got
the experimental result.
[0008]
Therefore, as a result of intensive research on the pressure wave generating element having the
configuration shown in FIG. 9, the inventors of the present application apply a rectangular wave
voltage of 20 kHz in frequency to the heating element 3 to generate an ultrasonic wave of 40
kHz in frequency. In this case, it is possible to obtain the knowledge that the temperature of the
heating element 3 becomes a very high temperature exceeding 300 ° C., and the expansion and
contraction of the heating element 3 accompanying the temperature rise and fall of the heating
element 3 at high speed It has been found that a very large thermal stress is generated in the
heat insulating layer 2 'formed of a porous silicon layer formed of different materials.
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[0009]
Among the above two findings, the inventors of the present invention conducted various
simulations on the pressure wave generating element having the configuration shown in FIG. 9,
and as the heat insulating layer, uniform physical property values (thermal conductivity, heat
capacity) Assuming that a porous silicon layer having a porosity of 60% is applied to the heating
element 3, the temperature of the heating element 3 and the temperature in the depth direction
of the heat insulating layer 2 'when a rectangular wave voltage of 40 kHz is applied The
distribution is derived based on the result of simulation using the finite element method.
Here, with regard to the temperature distribution in the depth direction of the heat insulating
layer 2 ′, the result is obtained that the temperature distribution as shown in FIG. 10 is obtained
when the temperature of the heating element 3 reaches the maximum temperature.
In FIG. 10, the horizontal axis represents the depth from the interface between the heat
insulating layer 2 ′ and the heating layer 3, and the vertical axis represents the normalized
temperature obtained by normalizing the temperature of the heat insulating layer by the
temperature at the interface. is there.
[0010]
From the results of FIG. 10, when a rectangular wave voltage with a frequency of 40 kHz is
applied to the heating element 3 of the pressure wave generating element having the
configuration shown in FIG. 9, the region from the surface of the thermal insulation layer 2 ′ to
the depth of 2 μm sharply It can be seen that the rapid temperature distribution causes thermal
stress to be concentrated on a portion on the heat generating body 3 side in the thickness
direction of the heat insulating layer 2 due to this rapid temperature distribution. It has been
found that the cracks which are the cause of the cracks in the insulating layer 2 and the cracks
generated in the heat insulating layer 2 'contribute to the breakage of the heating element 3.
Further, in the pressure wave generating element having the configuration shown in FIG. 9, since
the thermal insulation layer 2 ′ is supported by the semiconductor substrate 1, the boundary
between the thermal insulation layer 2 ′ and the semiconductor substrate 1 due to the abovementioned thermal stress. It has been found that the load applied to the vicinity is large, and the
thermal insulating layer 2 'is likely to be peeled off.
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[0011]
Further, in the pressure wave generating element having the configuration shown in FIG. 9, the
inventors of the present application carried out an anodizing process on a part of the
semiconductor substrate in the electrolytic solution when forming the thermal insulation layer 2
composed of a porous silicon layer. The heat insulation layer 2 'is formed by making it porous,
and then the washing step and the drying step are sequentially carried out, but the porosity of
the heat insulation layer 2' is increased to increase the amplitude of the pressure wave to be
generated. When the thermal insulation layer 2 'is designed to improve the thermal insulation,
that is, when the porosity of the thermal insulation layer 2' is designed to have a relatively high
value, the cleaning step immediately before in the above-mentioned drying step is performed. In
the middle of the manufacturing process, we obtain experimental results that cracks may occur
in the thermal insulation layer 2 'or the thermal insulation layer 2' may be peeled off from the
semiconductor substrate 1 due to the effect of the surface tension of the liquid used in Cracks
generated in the thermal insulation layer 2 'are also heating elements 3 Found that one of the
causes of the rupture. In addition, there is a tendency that the generation of the crack in the heat
insulating layer 2 ′ and the peeling of the heat insulating layer 2 ′ from the semiconductor
substrate 1 are more likely to occur as the thickness dimension of the heat insulating layer 2 ′
increases. there were.
[0012]
The present invention has been made in view of the above-described problems, and an object
thereof is to prevent the generation of a crack in a heat insulating layer during manufacturing or
driving, thereby preventing breakage of the heating element. It is an object of the present
invention to provide a generating element and a method of manufacturing the same.
[0013]
The invention according to claim 1 comprises a substrate, a heat generating body formed of a
thin film formed on one surface side in the thickness direction of the substrate, and a thermal
insulation layer interposed between the substrate and the heat generating body, A pressure wave
generating element that generates a pressure wave by heat exchange between a heat generating
body and a medium accompanying energization, and in the heat insulating layer, the porosity of
the portion on the substrate side in the thickness direction is higher than the porosity of the
portion on the heat generating side It is also characterized by being small.
[0014]
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5
According to the present invention, it is possible to increase the mechanical strength in the
vicinity of the boundary of the substrate in the thermal insulating layer while suppressing the
deterioration of the thermal insulating performance in the portion on the heat generating body
side in the thermal insulating layer. The stress generated near the boundary with the substrate
can be alleviated, and the generation of a crack in the heat insulating layer during manufacturing
or driving can be prevented, and breakage of the heat generating body can be prevented.
As a result, manufacturing yield and reliability can be improved.
[0015]
In the invention of claim 2, according to the invention of claim 1, the heat insulation layer is
formed of a high porosity layer formed on the heat generating body side in the thickness
direction and a low porosity layer formed on the substrate side. It is characterized by becoming.
[0016]
According to the present invention, the thermal insulation performance of the thermal insulation
layer can be determined by the porosity and thickness dimension of the high porosity layer, and
the mechanical strength of the portion on the substrate side in the thermal insulation layer is
reduced. Since it is possible to design according to the porosity and thickness of the thermal
layer, the design of the thermal insulation performance of the thermal insulation layer is
facilitated, and the formation of the thermal insulation layer is facilitated.
[0017]
According to the invention of claim 3, according to the invention of claim 1, the heat insulating
layer has a high porosity layer formed on the heat generating body side in the thickness
direction, and a porosity formed on the substrate side closer to the substrate. Is characterized by
comprising a low porosity graded layer having a reduced size.
[0018]
According to the present invention, compared to the case where the porosity changes stepwise in
the thickness direction of the substrate as in the invention of claim 2, the vicinity of the boundary
between the heat insulating layer and the substrate and the heating element side The mechanical
strength of the portion of (1) can be enhanced, and the stress generated in the vicinity of the
boundary can be alleviated, and the peeling of the heat insulating layer from the substrate during
manufacturing or driving can be more reliably prevented. Can.
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[0019]
The invention according to claim 4 is characterized in that, in the invention according to claim 3,
the heat insulating layer is continuous in porosity at the boundary between the high porosity
layer and the low porosity gradient layer in the thickness direction. I assume.
[0020]
According to this invention, the stress generated near the boundary between the high porosity
layer and the low porosity gradient layer can be dispersed and reduced, and the mechanical
strength of the thermal insulation layer can be enhanced.
[0021]
The invention of claim 5 is characterized in that, in the invention of claims 2 to 4, the thickness
dimension of the high porosity layer is set to a value greater than the thermal diffusion length.
[0022]
According to this aspect of the invention, it is possible to prevent a significant decrease in the
amplitude of the pressure wave that is generated when the heat generating element is energized.
[0023]
The invention according to claim 6 is characterized in that, in the invention according to claim 1,
the porosity of the thermally insulating layer is continuously reduced from the heat generating
body side toward the substrate side in the thickness direction.
[0024]
According to the present invention, the mechanical strength of the heat insulating layer can be
further enhanced as compared with the invention of claim 2, and stress generated near the
boundary between the heat insulating layer and the substrate is alleviated. It is possible to
prevent generation of cracks in the heat insulating layer during manufacturing or driving,
breakage of the heat generating body due to cracks in the heat insulating layer, and peeling of
the heat insulating layer from the substrate more reliably. .
[0025]
According to the invention of claim 7, in the invention of claim 3 or claim 6, the low porosity
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7
gradient layer is formed such that the porosity becomes zero near the boundary with the
substrate in the thickness direction. It is characterized by
[0026]
According to the present invention, the mechanical strength in the vicinity of the boundary with
the substrate in the heat insulating layer can be further enhanced, and the stress generated in the
vicinity of the boundary can be further alleviated. It is possible to more reliably prevent the
generation of cracks in the heat insulating layer, breakage of the heat generating body due to the
cracks in the heat insulating layer, and peeling of the heat insulating layer from the substrate.
[0027]
The invention according to claim 8 is the method for producing a pressure wave generating
element according to claim 2, wherein a part of the substrate on the one surface side is made
porous by anodizing treatment to form a heat insulating layer. In the formation of the thermal
insulation layer, the anodic oxidation treatment is performed for the first predetermined time at
the first current density specified for the formation of the high porosity layer by the anodic
oxidation treatment, and then the low temperature by the anodic oxidation treatment Anodizing
treatment is performed for a second predetermined time at a second current density specified for
forming the porosity layer.
[0028]
According to this invention, it is possible to continuously form a thermal insulation layer
consisting of a high porosity layer and a low porosity layer at the time of production, and after
both the high porosity layer and the low porosity layer are formed. Since the cleaning step and
the drying step may be sequentially performed, it is possible to prevent the occurrence of a crack
in the thermal insulating layer or peeling of the thermal insulating layer from the substrate at the
time of manufacturing, and it is possible to improve the manufacturing yield. It is possible to
provide a pressure wave generating element that can prevent the occurrence of cracks in the
heat insulating layer and can prevent breakage of the heat generating body.
[0029]
The invention according to claim 9 is the method for producing a pressure wave generating
element according to claim 3, wherein a part of the one surface side of the substrate is made
porous by anodizing treatment to form a heat insulating layer. In the formation of the thermal
insulation layer, the anodic oxidation treatment is performed for the first predetermined time at
the first current density specified for the formation of the high porosity layer by the anodic
oxidation treatment, and then the low temperature by the anodic oxidation treatment Anodizing
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treatment is performed for a second predetermined time in a current density decreasing pattern
specified for forming the porosity graded layer.
[0030]
According to the present invention, it is possible to continuously form a thermal insulation layer
consisting of a high porosity layer and a low porosity gradient layer at the time of manufacture,
and to form both the high porosity layer and the low porosity gradient layer. After that, the
cleaning process and the drying process may be sequentially performed, so that it is possible to
prevent the thermal insulating layer from being cracked or peeling off the substrate at the time
of manufacturing, and it is possible to improve the manufacturing yield. It is possible to provide a
pressure wave generating element capable of preventing the generation of a crack in the heat
insulating layer at the time and preventing the breakage of the heat generating body.
[0031]
The invention according to claim 10 is the method for producing a pressure wave generating
element according to claim 6, wherein a part of the substrate on the one surface side is made
porous by anodizing treatment to form a heat insulating layer. In the formation of the heat
insulating layer, the current density at the time of anodizing treatment is characterized by being
continuously reduced with the passage of time.
[0032]
According to the present invention, it is possible to form a thermal insulating layer consisting of
a porous layer whose porosity is continuously changed by one anodizing treatment, and the
porosity is continuously reduced as it approaches the substrate. Since the cleaning step and the
drying step may be performed after forming the thermal insulation layer made of the porous
layer, it is possible to prevent the thermal insulation layer from cracking or peeling off the
thermal insulation layer from the substrate at the time of manufacture. It is possible to provide a
pressure wave generating element capable of improving the yield, preventing the generation of
cracks in the thermal insulation layer at the time of driving, and preventing the breakage of the
heating element.
[0033]
In the invention of claim 1, it is possible to increase the mechanical strength in the vicinity of the
boundary of the substrate in the heat insulating layer while suppressing the decrease in the heat
insulating performance in the portion on the heat generating body side in the heat insulating
layer. The stress generated near the boundary with the substrate can be relaxed, and the
generation of cracks in the thermal insulation layer during manufacturing or driving can be
05-05-2019
9
prevented, and breakage of the heat generating element can be prevented. It has the effect of
being able to
[0034]
The inventions of claims 8, 9 and 10 are capable of preventing the occurrence of cracks in the
thermal insulating layer and peeling of the thermal insulating layer from the substrate at the time
of production, which can improve the production yield, and heat at the time of driving. There is
an effect that it is possible to provide a pressure wave generating element which can prevent the
generation of a crack in the insulating layer and can prevent the breakage of the heating element.
[0035]
(Embodiment 1) As shown in FIG. 1, the pressure wave generating element of this embodiment
includes a semiconductor substrate 1 made of a single crystal p-type silicon substrate, and one
surface of the semiconductor substrate 1 in the thickness direction (upper surface in FIG. A heat
insulating layer 2 formed of a porous layer formed on the side, a heat generating body 3 formed
of a thin film (for example, a metal thin film such as an aluminum thin film) formed on the heat A
heating element 3 and a medium (e.g., for example, a heating element 3) accompanied by
energization (supply of electric energy) to the heating element 3 through the pair of pads 4 and 4
including the pads 4 and 4 formed on both ends. Heat exchange with air generates a pressure
wave (eg, ultrasonic waves).
Here, the planar shape of the semiconductor substrate 1 is rectangular, and the planar shapes of
the heat insulating layer 2 and the heating element 3 are also rectangular.
The heat generating element 3 is smaller in planar size than the heat insulating layer 2 (the heat
insulating layer 2 is formed inside the outer periphery of the heat generating element 3), and the
long side is 12 mm long and the short side is Although the length dimension is set to 10 mm,
these dimensions are not particularly limited.
Further, in the present embodiment, the semiconductor substrate 1 constitutes a substrate.
[0036]
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10
The porous layer constituting the heat insulating layer 2 is formed by anodizing a part of the ptype silicon substrate as the semiconductor substrate 1 in the electrolytic solution, and the high
porosity layer on the heating element 3 side ( For example, the porous silicon layer 21 having a
porosity of 70% and the low porosity layer (for example, a porous silicon layer having a porosity
of 40%) 22 on the substrate 1 side are provided.
Here, as the porosity of the porous silicon layer increases, the thermal conductivity and the
thermal capacity decrease, and the thermal conductivity can be sufficiently reduced as compared
to single crystal silicon by appropriately setting the porosity.
In Patent Document 1, a single crystal silicon substrate having a thermal conductivity of 168 W /
(m · K) and a heat capacity of 1.67 × 10 <6> J / (m <3> · K) is anodized. The porous silicon layer
having a porosity of 60% formed by treatment has a thermal conductivity of 1 W / (m · K) and a
heat capacity of 0.7 × 10 <6> J / (m <3> · K) It has been reported that
Further, in the pressure wave generating element of this embodiment, the thickness of the
semiconductor substrate 1 is 525 μm, the thickness of the high porosity layer 21 of the heat
insulating layer 2 is 5 μm, and the thickness of the low porosity layer 22 of the heat insulating
layer 2 Is 5 .mu.m and the thickness of the heating element 3 is 50 nm, but these thicknesses are
an example and are not particularly limited.
However, the thermal conductivity of the high-porosity layer 21 immediately below the heating
element 3 is α [W / (m · K)], and the heat capacity is C [J / (m <3> · K)]. The waveform of the
temperature change of the heating element 3 is f (= 2 f '), the heating element as the waveform of
the electric input of the electric input (voltage waveform or current waveform) as an alternating
current sine wave of frequency f' [Hz] Assuming that the angular frequency of the waveform of
the temperature change of 3 is ω (= 2πf) and the temperature of the heat generating body 3 is T
(ω) (that is, the temperature T is a function of ω) If a distance which is 1 / e times the
temperature of the surface of the high porosity layer 21 (e is the base of natural logarithms) with
respect to the distance from the surface to the depth direction is defined as the thermal diffusion
length L, L ≒ (2α / ωC) It is desirable that the thickness of the high porosity layer 21 be set to
a value of the thermal diffusion length L or more.
Here, the frequency of the pressure wave generated from the heating element 3 is equal to the
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frequency f.
As an example of use of the pressure wave generating element according to this embodiment, an
ultrasonic wave generating an ultrasonic wave with a frequency of 40 kHz and a pressure f of 20
kHz for the waveform of the electrical input to the heating element 3 is used. Assuming that the
thermal insulation layer 2 is a porous silicon layer having a porosity of 60%, the thermal
conductivity is assumed to be 1 W / (m · K), and the heat capacity is 0.7 × 10. The thickness of
the high-porosity layer 21 is set based on <6> J / (m <3> · K) and the thermal diffusion length L =
3.37 μm determined with the frequency f of 40 kHz.
[0037]
Hereinafter, the manufacturing method of the pressure wave generating element of the present
embodiment will be briefly described.
[0038]
First, a conductive electrode (not shown) for use in anodizing treatment is formed on the other
surface (lower surface in FIG. 1) of the semiconductor substrate 1, and then the high porosity
layer 21 is formed on the one surface of the semiconductor substrate 1. The anodizing treatment
step of forming a thermally insulating layer consisting of the high porosity layer 21 and the low
porosity layer 22 by making the planned portion and the formation scheduled portion of the low
porosity layer 22 porous by anodizing treatment is performed .
Here, in the anodizing treatment step, using a mixed solution of a 55 wt% hydrogen fluoride
aqueous solution and ethanol mixed at a ratio of 1: 1 as an electrolytic solution, an object to be
treated having the semiconductor substrate 1 as a main component is put in a treatment tank.
The power source is immersed in the electrolyte, and a current-carrying current is passed
between the anode and the cathode from the power source by using the current-carrying
electrode as the anode and the platinum electrode oppositely disposed on the one surface side of
the semiconductor substrate 1 as the cathode. Thus, the high porosity layer 21 and the low
porosity layer 22 are continuously formed.
However, when forming the heat insulating layer 2, as shown in FIG. 2, the first predetermined
time is set at the first current density J1 (for example, 100 mA / cm <2>) specified for the
formation of the high porosity layer 21. The second current density J2 (for example, 10 mA / cm
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<2>) specified for forming the low porosity layer 22 when forming the low porosity layer 22 by
performing anodizing treatment of T1 (for example, 2 minutes) Anodizing treatment is performed
for a second predetermined time T2 (for example, 15 minutes).
[0039]
After the energization of the above-mentioned anodizing treatment step, the object to be treated
is taken out of the electrolytic solution, and after the washing step and the drying step are
sequentially performed, the heating element forming step of forming the heating element 3 and
the pads for forming the pads 4 and 4 By sequentially performing the forming process, the
pressure wave generating element is completed.
In the drying step, various drying methods such as drying with nitrogen gas and drying with a
centrifugal dryer may be appropriately adopted.
In the heating element forming step, the heating element 3 may be formed by vapor deposition
using a metal mask or the like, and in the pad forming step, the pads 4 and 4 are formed by
vapor deposition using a metal mask or the like. do it.
[0040]
The heat insulation layer 2 in the pressure wave generating element of the present embodiment
described above is formed on the high porosity layer 21 formed on the heat generating body 3
side in the thickness direction of the semiconductor substrate 1 and the semiconductor substrate
1 side. And the porosity of the portion on the semiconductor substrate 1 side in the thickness
direction is smaller than the porosity of the portion on the heat generating body 3 side. The
mechanical strength in the vicinity of the boundary of the semiconductor substrate 1 in the heat
insulating layer 2 can be increased while suppressing the decrease in the thermal insulation
performance in the portion on the heat generating body 3 side. The stress generated in the
vicinity of the boundary of the heat insulating layer 2 can be alleviated, and the generation of
cracks in the thermal insulation layer 2 during manufacturing or driving can be prevented, and
breakage of the heating element 3 can be prevented. Of heat insulation layer 2 from water It is
possible to prevent.
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As a result, manufacturing yield and reliability can be improved.
[0041]
Here, in the pressure wave generating element according to the present embodiment, as
described above, the thermal insulation layer 2 is formed on the semiconductor substrate 1 side
with the high porosity layer 21 formed on the heating element 3 side in the thickness direction
of the semiconductor substrate 1 Since the thermal insulation performance of the thermal
insulation layer 2 can be determined by the porosity and thickness dimension of the high
porosity layer 21, the semiconductor substrate in the thermal insulation layer 2 can be
determined. The mechanical strength of the portion on the side 1 can be designed according to
the porosity and thickness of the low porosity layer 22, so the design of the thermal insulation
performance of the thermal insulation layer 2 becomes easy, and the thermal insulation layer 2
Formation of
Here, by setting the thickness of the high-porosity layer 21 of the thermal insulation layer 2 to a
value greater than the thermal diffusion length L as described above, the amplitude of the
pressure wave caused by the thermal conduction to the semiconductor substrate 1 side While the
high-porosity layer 21 can determine the thermal insulation performance, the low-porosity layer
21 can determine the mechanical strength of the thermal insulation layer 2. .
In other words, in the pressure wave generating element of the present embodiment, mechanical
strength at the time of manufacturing and driving does not decrease the thermal insulation
performance more than when making the porosity uniform in the depth direction of the thermal
insulation layer 2 It is possible to increase the strength.
[0042]
Therefore, in the pressure wave generating element of the present embodiment, the heat
resistance is improved as compared with the conventional case, so that the amplitude of the
pressure wave can be increased by increasing the power supplied to the heating element 3 at the
time of energization.
[0043]
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By the way, although a single crystal p-type silicon substrate is adopted as the semiconductor
substrate 1 in the present embodiment, the semiconductor substrate 1 is not limited to a single
crystal p-type silicon substrate but may be a polycrystalline or amorphous p-type silicon
substrate The conditions are not limited to the p-type but may be n-type or non-doped, and the
conditions of the anodizing treatment may be appropriately changed according to the type of the
semiconductor substrate 1.
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.
Further, the material of the heat generating element 3 is not limited to Al, and it is preferable to
use a metal material (for example, W, Mo, Pt, Ir, etc.) having higher heat resistance than Al.
[0044]
Second Embodiment The basic configuration of the pressure wave generating element of this
embodiment is substantially the same as that of the first embodiment, and as shown in FIG. 3, the
heat insulation layer 2 is on the heat generating body 3 side in the thickness direction of the
semiconductor substrate 1. The difference is that the formed high porosity layer 21 and the low
porosity inclined layer 23 which is formed on the semiconductor substrate 1 side and whose
porosity decreases continuously toward the semiconductor substrate 1 are different.
Here, the low porosity gradient layer 23 has a depth profile of porosity such that the porosity is
continuous at the boundary with the high porosity layer 21 and the porosity is zero near the
boundary with the semiconductor substrate 1. Yes.
Since the other configuration is the same as that of the first embodiment, the same components
as those of the first embodiment are denoted by the same reference numerals and the description
thereof will be omitted.
[0045]
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The method of manufacturing the pressure wave generating element of this embodiment is
substantially the same as the manufacturing method described in the first embodiment, and
when forming the heat insulating layer 2, as shown in FIG. 4, the high porosity layer 21 is
formed. Anodizing treatment for the first predetermined time T1 (for example, 2 minutes) at the
first current density J1 (for example, 100 mA / cm <2>) specified for the formation of the low
porosity gradient layer 23 The only difference is that the second predetermined time T3 (e.g., 10
minutes) of anodizing treatment is performed in the decreasing pattern of the current density
specified for the formation of the low porosity gradient layer 23.
The decreasing pattern here is a monotonically decreasing pattern in which the current density is
continuously reduced from the first current density J1 to the second current density J3 (for
example, 0 mA / cm <2>) over the second predetermined time T3. It is prescribed.
Although the monotonically decreasing pattern in FIG. 4 has a constant slope, the monotonically
decreasing pattern may be, for example, a monotonically decreasing pattern in which the slope
increases with time as shown in FIG. As shown in 5 (b), it may be a monotonically decreasing
pattern in which the slope decreases with the passage of time.
[0046]
Therefore, also in the pressure wave generating element of this embodiment, the porosity of the
portion on the semiconductor substrate 1 side in the thickness direction of the semiconductor
substrate 1 is smaller than the porosity of the portion on the heating element 3 side in the
thickness direction of the semiconductor substrate 1 Therefore, the mechanical strength of the
portion of the heat insulating layer 2 on the side of the semiconductor substrate 1 can be
increased while suppressing the decrease in the thermal insulation performance of the portion of
the heat insulating layer 2 on the heat generating body 3 side. The occurrence of cracks in the
heat insulating layer 2 during driving or the occurrence of breakage of the heat generating body
3 can be prevented, and peeling of the heat insulating layer 2 from the semiconductor substrate
1 can be prevented.
As a result, manufacturing yield and reliability can be improved.
[0047]
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Moreover, in the pressure wave generating element of the present embodiment, as compared
with the case where the porosity of the heat insulating layer 2 changes in a step shape in the
depth direction (the thickness direction of the semiconductor substrate 1) as in the first
embodiment, The mechanical strength of the heat insulating layer 2 in the vicinity of the
boundary with the semiconductor substrate 1 and the portion on the heating element 3 side can
be enhanced, and the stress generated in the vicinity of the boundary can be alleviated. Peeling
of the heat insulating layer 2 from the substrate 1 can be prevented more reliably.
[0048]
Further, in the pressure wave generating element of the present embodiment, the thermal
insulation layer 2 is continuous at the boundary between the high porosity layer 21 and the low
porosity inclined layer 23 in the thickness direction of the semiconductor substrate 1, so The
stress generated in the vicinity of the boundary between the porosity layer 21 and the low
porosity gradient layer 23 can be dispersed and reduced, and the mechanical strength of the
thermal insulation layer 2 can be enhanced. Furthermore, the low porosity gradient layer 23 is a
semiconductor Since the porosity is formed to be zero in the vicinity of the boundary with the
substrate 1, the mechanical strength in the vicinity of the boundary with the semiconductor
substrate 1 in the thermal insulating layer 2 can be further enhanced, and occurs near the
boundary. The stress can be further relieved, and the generation of a crack in the thermal
insulating layer 2 during manufacturing or driving, the breakage of the heat generating body 3
due to the crack in the thermal insulating layer 2 or the thermal insulating layer 2 from the
semiconductor substrate 1 More reliably prevent peeling It can be.
[0049]
(Third Embodiment) The configuration of the pressure wave generating element of this
embodiment is substantially the same as that of the first embodiment, and as shown in FIG. The
difference is that the porosity decreases continuously toward the semiconductor substrate 1 side.
In short, in the thickness direction of the semiconductor substrate 1, the heat insulating layer 2
has a higher porosity as it is closer to the heating element 3 and a lower porosity as it goes closer
to the semiconductor substrate 1.
Here, the thermal insulating layer 2 is set to have a depth profile of porosity such that the
porosity becomes zero in the vicinity of the boundary with the semiconductor substrate 1. The
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17
other configuration is the same as that of the first embodiment, and hence the description is
omitted.
[0050]
The method of manufacturing the pressure wave generating element of the present embodiment
is substantially the same as the manufacturing method described in the first embodiment, and
when forming the heat insulating layer 2, as shown in FIG. The only difference is that the
anodizing treatment for a predetermined time T4 (for example, 10 minutes) is performed in the
current density decreasing pattern defined in the above. The decreasing pattern here is
continuous from a first current density J4 (for example, 100 mA / cm <2>) to a second current
density J5 (for example, 0 mA / cm <2>) over a predetermined time T4. It is defined in
monotonically decreasing patterns to be reduced. Although the monotonically decreasing pattern
in FIG. 7 has a constant slope, the monotonically decreasing pattern may be, for example, a
monotonically decreasing pattern in which the slope increases with time as shown in FIG. As
shown in 8 (b), it may be a monotonically decreasing pattern in which the slope decreases with
the passage of time.
[0051]
Therefore, in the pressure wave generating element of the present embodiment, the porosity is
continuously decreased from the heat generating body 3 side toward the semiconductor
substrate 1 in the thickness direction of the semiconductor substrate 1 as compared with the
first embodiment. The mechanical strength of the thermal insulation layer 2 can be further
enhanced, and the stress generated near the boundary between the thermal insulation layer 2
and the semiconductor substrate 1 can be alleviated. It is possible to more reliably prevent the
generation of the cracks, the breakage of the heat generating body 3 caused by the cracks in the
heat insulating layer 2, and the peeling of the heat insulating layer 2 from the semiconductor
substrate 1. Moreover, in the pressure wave generating element of the present embodiment, the
thermal insulation layer 2 is formed so as to have zero porosity near the boundary with the
semiconductor substrate 1, so the boundary with the semiconductor substrate 1 in the thermal
insulation layer 2 is The mechanical strength in the vicinity can be further enhanced, and the
stress generated near the boundary can be further relieved, so that a crack is generated in the
thermal insulating layer 2 at the time of manufacturing or driving, or a crack of the thermal
insulating layer 2 It is possible to more reliably prevent the breakage of the heating element 3
and the peeling of the thermal insulation layer 2 from the semiconductor substrate 1 that are
caused.
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[0052]
By the way, although Si is adopted as a material of semiconductor substrate 1 in each abovementioned embodiment, the material of semiconductor substrate 1 is not restricted to Si, for
example, by anodic oxidation treatment of Ge, SiC, GaP, GaAs, InP, etc. Other semiconductor
materials that can be made porous can also be used.
[0053]
FIG. 2 is a schematic cross-sectional view of a pressure wave generating element in Embodiment
1.
It is explanatory drawing of the manufacturing method same as the above. FIG. 7 is a schematic
cross-sectional view of a pressure wave generating element in Embodiment 2. It is explanatory
drawing of the manufacturing method same as the above. It is explanatory drawing of the
manufacturing method same as the above. FIG. 10 is a schematic cross-sectional view of a
pressure wave generating element in Embodiment 3. It is explanatory drawing of the
manufacturing method same as the above. It is explanatory drawing of the manufacturing
method same as the above. It is a schematic sectional drawing of the pressure wave generation
element which shows a prior art example. It is a graph which shows an example of temperature
distribution of the depth direction of the heat insulation layer in the same as the above.
Explanation of sign
[0054]
Reference Signs List 1 semiconductor substrate 2 thermal insulation layer 3 heating element 4
pad 21 high porosity layer 22 low porosity layer
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