JP4505672B2 - Pressure wave generator and manufacturing method thereof - Google Patents

Description

  The present invention relates to a pressure wave generating apparatus that generates pressure waves such as sound waves, ultrasonic waves, and monopulse dense waves targeted at a speaker, and a method for manufacturing the same.

  2. Description of the Related Art Conventionally, an ultrasonic generator using mechanical vibration due to a piezoelectric effect is widely known. In an ultrasonic generator using mechanical vibration, for example, electrodes are provided on both sides of a crystal of a piezoelectric material such as barium titanate, and electrical vibration is applied between both electrodes to generate mechanical vibration. An ultrasonic wave is generated by vibrating a medium such as air. However, an ultrasonic generator using mechanical vibration has a unique resonance frequency, so that the frequency band is narrow, and it is easily affected by external vibration and fluctuations in external pressure.

  On the other hand, for example, as described in Patent Document 1 or Patent Document 2, as a device capable of generating ultrasonic waves without mechanical vibration, heat is applied to a medium, and air density is increased by heat induction. There has been proposed a pressure wave generator using a method of forming the.

  As shown in FIG. 14 and FIG. 15, the pressure wave generator using thermal induction is directed to the inside of the semiconductor substrate 1 from the semiconductor substrate 1 of a single crystal silicon substrate and one surface in the thickness direction of the semiconductor substrate 1. And a heat insulating layer 2 formed at a predetermined depth and a heating element 3 of a metal thin film (for example, an Al thin film) formed on the heat insulating layer 2. The thermal insulating layer 2 is formed of a porous silicon layer and has a sufficiently low thermal conductivity and volumetric heat capacity as compared with the semiconductor substrate 1.

  When an alternating current is applied to the heating element 3 from the AC power source Vs, the heating element 3 generates heat, and the temperature (or the amount of generated heat) of the heating element 3 changes according to the frequency of the alternating current to be supplied. On the other hand, since the heat insulating layer 2 is formed immediately below the heat generating element 3 and the heat generating element 3 is thermally insulated from the semiconductor substrate 1, efficient heat is generated between the heat generating element 3 and the air in the vicinity thereof. An exchange occurs. Then, the air repeatedly expands and contracts in accordance with the temperature change of the heating element 3 (or the change in the heat generation amount), and as a result, a pressure wave such as an ultrasonic wave is generated (the upward arrow in FIG. 14 indicates the pressure). Shows the direction of wave travel).

  The pressure wave generator using such heat induction can change the frequency of the generated ultrasonic wave over a wide range by changing the frequency of the AC voltage (drive voltage) applied to the heating element 3. Therefore, for example, it can be used as an ultrasonic sound source or a sound source of a speaker.

  According to Patent Document 1, it is desirable to make the thermal conductivity and volumetric heat capacity of the thermal insulating layer 2 smaller than the thermal conductivity and volumetric heat capacity of the semiconductor substrate 1, and the thermal conductivity of the thermal insulating layer 2 and It is preferable to make the product of the volume heat capacity sufficiently smaller than the product of the thermal conductivity of the semiconductor substrate 1 and the volume heat capacity. For example, when the semiconductor substrate 1 is formed of a single crystal silicon substrate and the thermal insulating layer 2 is formed of a porous silicon layer, the product of the thermal conductivity and the volumetric heat capacity of the thermal insulating layer 2 is the semiconductor substrate 1. This value is about 1/400 of the product of the thermal conductivity and the volumetric heat capacity.

In order to form the thermal insulating layer 2 of the porous silicon layer on the one surface side of the semiconductor substrate 1 of the single crystal silicon substrate, for example, as shown in FIG. A mask layer in which a portion corresponding to a region where 2 is to be formed is opened is formed. Then, the current-carrying electrode 4 formed on the entire other surface of the semiconductor substrate 1 is used as an anode, and an electric current is passed between the cathode disposed so as to face one surface of the semiconductor substrate 1 in the electrolytic solution, Anodizing is performed.
Japanese Patent Laid-Open No. 11-3000274 JP 2002-186097 A

  By the way, in the pressure wave generator using the conventional thermal induction, an alternating current is applied between both longitudinal ends of the heating element 3 as shown in FIG. The heating element 3 repeats expansion and contraction as it is turned on / off. Since the heating element 3 is thermally insulated from the semiconductor substrate 1, there is a possibility that the heating element 3 is damaged due to a thermal stress generated in the heating element 3 due to a rapid temperature change of the heating element 3.

  In designing a pressure wave generator using thermal induction, the size of the pressure wave generator is about 15 mm × 15 mm, which is a general size of an ultrasonic generator using mechanical vibration that has been widely used conventionally. The temperature of the heating element 3 was examined by generating a sound pressure equivalent to that of an ultrasonic generator using mechanical vibration (for example, about 20 Pa at a frequency of 40 kHz and 30 cm away). As a result, it was found that the temperature of the heating element 3 instantaneously became a very high temperature exceeding 1000 degrees.

  An object of the present invention is to provide a pressure wave generator utilizing thermal induction that hardly causes damage to a heating element due to thermal stress, and a method for manufacturing the same.

  The invention of claim 1 includes a substrate, a porous thermal insulation layer formed on one surface in the thickness direction of the substrate, and a thin film heating element formed on the thermal insulation layer. A pressure wave generator that generates a pressure wave by heat exchange between a heating element and a medium, in which the temperature of the heating element changes according to a waveform of an electric input, Within the range in the width direction defined by the reference thickness of the center portion in the width direction of the heat insulating layer toward the inside, the average thermal conductivity in the thickness direction of the inner portion from the outer periphery of the heating element is αin, the average volumetric heat capacity And Cin, the average thermal conductivity in the thickness direction of the outer part of the outer periphery of the heating element is αout, the average volumetric heat capacity is Cout, and the condition of αin × Cin <αout × Cout is satisfied, In the vicinity of the boundary with the outer portion, the value of αin × Cin increases toward the outer side.

  According to a second aspect of the present invention, in the pressure wave generating device of the first aspect, the boundary of the region where the value of αin × Cin changes is substantially coincided with the outer periphery of the heating element, or is located inside the outer periphery of the heating element. It is characterized by.

  According to a third aspect of the present invention, in the pressure wave generating device according to the first or second aspect, in the region where the value of αin × Cin changes, at least one of the thermal conductivity and the heat capacity of the material forming the thermal insulating layer itself is outside. It is characterized by being continuously changed so as to increase toward.

  According to a fourth aspect of the present invention, in the pressure wave generating device according to the third aspect, the material composition substantially coincides at the boundary between the thermal insulating layer and the substrate where αin × Cin = αout × Cout.

  The present invention is based on the following relational expression, based on the viewpoint that the heat radiation amount per unit time can be increased by increasing the product of the thermal conductivity and the volumetric heat capacity of the thermal insulation layer. This is based on the technical idea of relaxing the temperature gradient of the outer peripheral portion of the heating element by increasing the heat radiation amount so as to suppress the temperature rise of the portion.

  In the above equation, α is the thermal conductivity of the thermal insulation layer, C is the volumetric heat capacity of the thermal insulation layer, ω is the angular frequency of the AC voltage input across the heating element, and q (ω) is the heating element. The input electric energy, T (ω), is the temperature of the heating element.

  According to the invention of claim 1, the average thermal conductivity in the thickness direction of the inner part from the outer periphery of the heating element is αin, the average volumetric heat capacity is Cin, and the average of the outer part in the thickness direction from the outer periphery of the heating element. Assuming that the thermal conductivity is αout and the average volumetric heat capacity is Cout, the condition of αin × Cin <αout × Cout is satisfied, and the value of αin × Cin increases near the boundary between the inner part and the outer part. Therefore, in the outer periphery of the heating element, the amount of heat radiated along the thickness direction of the substrate is larger than the amount of heat radiated in the central part of the heating element, so that compared to the conventional pressure wave generator The thermal stress applied to the heating element can be reduced. Therefore, compared with the conventional pressure wave generator, the heating element is less likely to be damaged due to thermal stress, and the life of the pressure wave generator can be extended. That is, when driving the pressure wave generator, even if thermal stress occurs due to expansion and contraction of the heating element accompanying the temperature rise and temperature drop of the heating element, the heating element is hardly damaged, and over a long period of time, Ultrasound can be generated stably.

  According to the invention of claim 2, in the pressure wave generator of claim 1, the boundary of the region where the value of αin × Cin changes substantially coincides with the outer periphery of the heating element, so that the outer main portion of the heating element A decrease in the amplitude of the pressure wave due to an increase in the amount of heat radiated to the substrate can be suppressed. In addition, the temperature gradient at the outer periphery of the heating element can be made gentler by positioning it inside the outer periphery of the heating element, and the thermal stress applied to the heating element is further reduced compared to the conventional pressure wave generator. can do.

  According to the invention of claim 3, in the pressure wave generator of claim 1 or 2, in the region where the value of αin × Cin changes, at least one of the thermal conductivity and the heat capacity of the material itself forming the thermal insulation layer Therefore, the discontinuity of the material composition or thickness can be eliminated by continuously changing either the thermal expansion coefficient or the volumetric heat capacity. It is possible to prevent discontinuities due to changes with time by eliminating discontinuous points in the mechanical strength.

  According to the invention of claim 4, in the pressure wave generator of claim 3, the material composition is made to substantially coincide at the boundary between the thermal insulating layer and the substrate where αin × Cin = αout × Cout, so αin × Cin There is no discontinuous portion of the thermal expansion coefficient in the portion where = αout × Cout. As the temperature of the heating element rises, the temperature of the heat insulation layer also rises due to some heat conduction. Therefore, a temperature difference occurs between the thermal insulating layer and the substrate where αin × Cin = αout × Cout, and the thermal stress at the boundary may increase depending on the difference in thermal expansion coefficient between the thermal insulating layer and the substrate. However, by eliminating the discontinuous portion of the thermal expansion coefficient in this way, it is possible to prevent damage at the boundary between the thermal insulating layer and the substrate due to the difference in thermal expansion coefficient.

(First embodiment)
A first embodiment of the present invention will be described. Fig.1 (a) is a top view of the pressure wave generator concerning 1st Embodiment, (b) is AA sectional drawing in Fig.1 (a).

  As shown in FIG. 1B, the pressure wave generator of the first embodiment includes a semiconductor substrate (substrate) 1 of a single crystal p-type silicon substrate, and one surface (first surface) of the semiconductor substrate 1 in the thickness direction. Surface) 1a, a heat insulating layer 2 of a porous silicon layer (porous body) formed toward the inside of the semiconductor substrate 1, and a thin film (for example, a metal thin film such as an aluminum thin film) formed on the heat insulating layer 2 And the like. As shown in FIG. 1A, the planar shape of the semiconductor substrate 1 is rectangular (for example, rectangular), and the planar shapes of the heat insulating layer 2 and the heating element 3 are also rectangular (for example, rectangular). Has been. As an example, the heating element 3 has a long side length of 12 mm and a short side length of 10 mm. Further, the thickness of the semiconductor substrate 1 is set to 525 μm, the thickness of the thermal insulating layer 2 is set to 10 μm, and the thickness of the heating element 3 is set to 50 nm. These dimensions are not particularly limited.

  As shown in FIG. 1B, the heat insulating layer 2 has a heating element in the width direction (including both the long side direction and the short side direction of the rectangle) perpendicular to the thickness direction of the semiconductor substrate 1. 3 is formed to have a substantially uniform thickness so as to reach a predetermined depth except for a portion facing the outer peripheral portion. In addition, in the portion facing the outer peripheral portion of the heating element 3, the inclined portion 2 a is formed so that the thickness of the heat insulating layer 2 gradually decreases toward the outside.

  In the pressure wave generator, the heating element 3 is caused to generate heat by energizing (supplying electrical energy) an electrical input (for example, alternating current) whose voltage and / or current changes with time. The temperature (or heat generation amount) of the heating element 3 is changed with time. And a pressure wave (for example, an ultrasonic wave etc.) is generated by heat exchange with exothermic body 3 and a medium (for example, air). When, for example, a sinusoidal AC voltage as shown in FIG. 3A is applied between the AC power source (see Vs in FIG. 14) and the longitudinal ends of the heating element 3, the temperature of the heating element 3 is increased. It changes as shown in FIG. 3B by the generation of Joule heat. Further, a pressure wave (sound wave) having a waveform as shown in FIG.

  The porous silicon 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, as will be described later in the manufacturing method. Moreover, the porosity of the heat insulating layer 2 can be changed by appropriately changing the conditions of the anodizing treatment. The porous silicon layer has a lower thermal conductivity and heat capacity as the porosity increases. Therefore, by setting the porosity appropriately, the thermal conductivity of the porous silicon layer can be made sufficiently smaller than that of single crystal silicon.

となる。熱絶縁層２の厚さは、熱拡散長Ｌの０．５〜３倍程度の厚さであることが望ましい。 The thermal conductivity of the thermal insulating layer 2 immediately below the heating element 3 is α, the volumetric heat capacity is C, the angular frequency of the sinusoidal AC voltage applied to the heating element 3 is ω, and the temperature of the heating element 3 is T (ω) ( The temperature T is a function of ω, and the distance (depth) from the surface of the thermal insulation layer 2 in the thickness direction of the semiconductor substrate 1 is 1 / e times the temperature of the surface of the thermal insulation layer 2 (e is If the distance that becomes the base of the natural logarithm is defined as the thermal diffusion length L,

It becomes. The thickness of the thermal insulating layer 2 is desirably about 0.5 to 3 times the thermal diffusion length L.

  In the pressure wave generator of the first embodiment, as shown in FIG. 1 (b), in the thermal insulating layer 2, the thickness of the portion facing the vicinity of the outer peripheral portion of the heating element 3 becomes thinner toward the outside. An inclined portion 2a is formed. In this pressure wave generator, when the heating element 3 is energized (when electric energy is applied), the surface of the thermal insulating layer 2 in the vicinity of the outer periphery of the heating element 3 (the thermal insulating layer 2 and the heating element 3 The temperature distribution in the plane including the boundary) and the first surface 1a of the semiconductor substrate 1 was simulated by the finite element method. The result is shown by curve A in FIG. Moreover, the result of having performed the same simulation about the prior art example shown in FIG. 14 is shown in the curve B of FIG.

Curves A and B in FIG. 7 indicate the heat insulating layer 2 and the heat generating element 3 in the cross section in the short side direction (A-A cross section) of the heat generating element 3, respectively, as shown in FIGS. 1 (c) and 15 (c). The first surface 1a of the semiconductor substrate 1 is defined as the origin O and the direction away from the thermal insulation layer 2 (the right direction in FIGS. 1C and 15C) as the positive direction of the X axis. It is the result of having performed the simulation of the temperature distribution of the plane containing. In addition, as data of thermal conductivity and volumetric heat capacity at the time of simulation, the numerical data disclosed in Patent Document 1 is used, and the thermal conductivity of the semiconductor substrate 1 made of a single crystal silicon substrate is 168 W / ( m · K), the heat capacity is 1.67 × 10 ⁶ J / (m3 · K), and the thermal conductivity of the thermal insulating layer 2 made of a porous silicon layer having a porosity of 60% is 1 W / (m · K). The heat capacity was set to 0.7 × 10 ⁶ J / (m ³ · K).

  As can be seen from FIG. 7, in both the pressure wave generator of the first embodiment and the conventional pressure wave generator, there is a temperature gradient (−dT / dx) along the X-axis direction. The pressure wave generator of the embodiment has a gentler temperature gradient than the conventional pressure wave generator. The reason is that, in the portion facing the outer peripheral portion of the heating element 3 of the pressure wave generating device of the first embodiment, the inclined portion 2a is formed so that the thickness of the heat insulating layer 2 becomes thinner toward the outside. This is because the amount of heat radiated along the thickness direction of the semiconductor substrate 1 is larger than that of the central portion of the heating element 3.

  In other words, in the pressure wave generator of the first embodiment, as shown in FIG. 2, thermal insulation is performed from one surface (first surface) 1 a in the thickness direction D of the semiconductor substrate 1 toward the inside of the semiconductor substrate 1. Within the range of the width direction W defined by the reference thickness t at the center of the layer 2 in the width direction, the thickness of the heat insulating layer 2 in FIG. The average thermal conductivity in the direction is αin, the average volumetric heat capacity is Cin, the outer part of the outer periphery of the heating element 3, that is, the average thermal conductivity in the thickness direction of the semiconductor substrate 1 is αout, and the average volumetric heat capacity is Cout. The condition of αin × Cin <αout × Cout is satisfied, and the value of αin × Cin increases toward the outside near the boundary between the inner part and the outer part. In short, the greater the product of thermal conductivity and volumetric heat capacity, the higher the heat dissipation and the greater the amount of heat dissipation per unit time. In the first embodiment, in the immediate vicinity of the outer periphery of the heating element 3. By making the heat dissipation property of the heat insulating layer 2 greater than the heat dissipation property of the heat insulating layer 2 immediately below the central portion of the heat generating element 3, the temperature gradient in the vicinity of the outer peripheral portion of the heat generating element 3 is made gentle.

  Thus, in the pressure wave generator of the first embodiment, the amount of heat radiated along the thickness direction of the semiconductor substrate 1 in the outer peripheral portion of the heating element 3 is larger than the amount of heat radiated in the central portion of the heating element 3. Therefore, the thermal stress applied to the heating element 3 can be reduced as compared with the conventional pressure wave generator, and the heating element 3 is less likely to be damaged due to the thermal stress. Can be

  Further, within the range of the width direction W defined by the reference thickness t, the boundary of the region where the value of αin × Cin changes (that is, the outer peripheral end of the inclined portion 2a) is made to substantially coincide with the outer periphery of the heating element 3. Therefore, while maintaining the physical property value of the outer peripheral portion of the thermal insulating layer 2 and the physical property value of the central portion, that is, while maintaining the physical properties of the porous silicon layer forming the thermal insulating layer 2 uniform, the heating element The decrease in the amplitude of the pressure wave can be suppressed without increasing the amount of heat radiated from the outer peripheral portion 3 to the semiconductor substrate 1 so much.

  Next, the manufacturing method of the pressure wave generator in 1st Embodiment is demonstrated, referring FIGS. 4-6. As shown in FIG. 4A, a current-carrying electrode 4 having a rectangular planar shape used during anodization is formed on the other surface (second surface) 1b in the thickness direction of the semiconductor substrate 1 of a p-type silicon substrate. To do. As shown in FIG. 5, the center of the energizing electrode 4 is a region (heating element forming region) 3 a where the rectangular heating element 3 is to be formed in a plane parallel to the first surface 1 a of the semiconductor substrate 1. It almost coincides with the center of. Further, the length of each side of the energizing electrode 4 is set to be shorter than the length of each corresponding side of the heating element forming region 3a by a predetermined reduction dimension.

  In the process of forming the energizing electrode 4, for example, a conductive layer is formed on the second surface 1b of the semiconductor substrate 1 by sputtering or vapor deposition, and the conductive layer is utilized by using photolithography technology and etching technology. Of these, unnecessary portions other than the portion used for the energizing electrode 4 may be removed. In the first embodiment, the long side of the heating element forming region 3a is 12 mm, the short side is 10 mm, and the reduction dimension is set to 1 mm. That is, the energizing electrode 4 is smaller than the heating element forming region 3a, and the long side is set to 11 mm and the short side is set to 9 mm. These numerical values are not particularly limited.

  After the energization electrode 4 is formed, one end of an energization lead wire (not shown) is attached to the energization electrode 4, and the attachment portion of the energization electrode 4 and one end of the lead wire is used for an anodizing treatment. It is covered with a sealing material having hydrofluoric acid resistance so as not to touch. Thereafter, an anodizing process is performed using an anodizing apparatus as shown in FIG. 6 to form a thermal insulating layer 2 made of a porous silicon layer as shown in FIG. 4B on the semiconductor substrate 1. The Thereafter, a heating element forming step is performed on the heating element forming region 3a of the first surface 1a of the semiconductor substrate 1 to obtain a structure having the heating element 3 as shown in FIG.

In the manufacturing method of the pressure wave generator of the first embodiment, as described above, the thermal insulating layer 2 is formed by anodizing. In the anodic oxidation process, as shown in FIG. 6, an object to be processed 24 mainly composed of the semiconductor substrate 1 is immersed in an electrolytic solution 23 in a processing tank 22. Next, in the electrolytic solution 23, the platinum electrode 21 is disposed so as to face the first surface 1 a of the semiconductor substrate 1. Further, the lead wire attached to the energizing electrode 4 is connected to the plus side of the current source 20, and the platinum electrode 21 is connected to the minus side of the current source 20. Then, with the energizing electrode 4 as an anode and the platinum electrode 21 as a cathode, a predetermined current density (for example, 20 mA / cm ² ) is applied between the energizing electrode 4 and the platinum electrode 21 from the current source 20. Run for a time (eg, 8 minutes).

  By such anodizing treatment, the thermal insulating layer 2 having a substantially constant thickness (for example, 10 μm) is formed on the first surface 1 a side of the semiconductor substrate 1 except for the outer peripheral portion. Thereafter, the processing object 24 is taken out from the processing tank 22, the sealing material of the processing object 24 is peeled off, and the lead wire connected to the energizing electrode 4 is removed.

In addition, the conditions at the time of an anodizing process are not specifically limited, For example, what is necessary is just to set an electric current density within the range of about 1-500 mA / cm < ² > suitably. The predetermined energization time may be set as appropriate according to the thickness of the heat insulating layer 2.

  Moreover, as an electrolytic solution used for the anodizing treatment, for example, a mixed solution in which a 55 wt% aqueous solution of hydrogen fluoride and ethanol are mixed at a ratio of 1: 1 is used. Moreover, as a sealing material, the sealing material which consists of fluororesins like Teflon (trademark) can be used, for example.

  In forming the heating element 3, a metal thin film (for example, an Al thin film) for the heating element 3 is formed on the first surface 1a of the semiconductor substrate 1 by a sputtering method or the like. Thereafter, a photoresist is applied on the metal thin film, and a resist layer (not shown) patterned for forming the heating element 3 is formed by a photolithography technique. Then, by using the resist layer as a mask, unnecessary portions of the metal thin film are removed by a dry etching process, whereby the heating element 3 is formed. Finally, the structure shown in FIG. 4C is obtained by removing the resist layer.

  Generally, when the size of the heat insulating layer 2 to be formed is made slightly smaller than the size of the heat insulating layer 2 as described above, and the size of the platinum electrode 21 is made larger than the size of the heat insulating layer 2. The direction of the electric field becomes oblique at the outer peripheral portion of the heat insulating layer 2 to be formed, and the electric field strength becomes weaker toward the outer side. Therefore, if anodizing is performed under such conditions, the current flowing in the outer peripheral portion of the oxide film formed on the first surface 1a side of the semiconductor substrate 1, that is, the thermal insulating layer 2, is reduced, and the film thickness is more outward. Thinly formed. Accordingly, at the outer peripheral portion of the thermal insulating layer 2 formed on the first surface 1a side of the semiconductor substrate 1, as shown in FIG. 1B and the like, the inclined portion 2a so that the thickness gradually decreases toward the outer side. Is formed. Here, if the heating element is formed in conformity with the inclined portion 2a, the thermal stress applied to the heating element 3 can be reduced as compared with the conventional pressure wave generator, and the heating element 3 is damaged due to the thermal stress. Is less likely to occur.

  As a result of observing the cross-sectional shape of the thermal insulating layer 2 with a scanning electron microscope, referring to FIG. 2, the outer peripheral portion of the thermal insulating layer 2 is from the first reference plane including the first surface 1a of the semiconductor substrate 1. The boundary between the heat insulating layer 2 and the semiconductor substrate 1 is inclined so that the distance d in the width direction from the second reference plane including the end face (outer periphery) 3e of the heating element 3 increases as the depth of the heat generating element 3 increases. I found out. Specifically, it was confirmed that the distance from the second reference plane of the heating element 3 is approximately 0.5 mm at a position where the depth from the first reference plane is 10 μm.

  Further, as described above, by making the energizing electrode 4 smaller than the heating element formation region 3a, the outer periphery of the inclined portion 2a of the heat insulating layer 2 is made substantially coincident with the outer periphery of the heating element 3, or the heating element 3 It can be located inside the outer periphery. Specifically, when the length of each side of the energizing electrode 4 is shortened by 1 mm as compared with each side of the heating element forming region 3a as described above (when the reduced size is 1 mm), the thermal insulation is performed. The outer periphery of the inclined portion 2 a of the layer 2 substantially coincides with the outer periphery of the heating element 3. On the other hand, when the length of each side of the energizing electrode 4 is shorter by 2 mm than each side of the heating element formation region 3 a (when the above-mentioned reduction dimension is 2 mm), the thermal insulating layer 2 is formed of the heating element 3. It is formed inside the outer periphery.

  In the latter case, the projection region of the heat insulating layer 2 onto the heat generating element 3 is located inside the outer periphery of the heat generating element 3, so that the outer peripheral portion of the heat generating element 3 is in direct contact with the first surface 1 a of the semiconductor substrate 1. Thus, when the outer periphery of the heat insulating layer 2 is formed inside the outer periphery of the heating element 3, as shown in FIG. 8, the thickness of the outer peripheral portion of the heat insulating layer 2 is set to the thickness of the central portion (the above reference). You may form so that it may become substantially the same as (thickness).

  Also in this case, the thermal conductivity and volumetric heat capacity of the single crystal silicon that is the material of the semiconductor substrate 1 are αout and Cout, respectively, and the thermal conductivity and volumetric heat capacity of the porous silicon that is the material of the thermal insulating layer 2 are respectively Since αin and Cin described above, the magnitude relationship between the products of thermal conductivity and heat capacity satisfies the condition of αin × Cin <αout × Cout. In addition, since the boundary of the region where the value of αin × Cin changes within the reference thickness range is located on the inner side of the outer periphery of the heating element 3, the temperature gradient at the outer periphery of the heating element 3 is more moderate. The thermal stress applied to the heating element 3 can be further reduced as compared with the conventional pressure wave generator.

  Further, as shown in FIG. 16, even when the energizing electrode 4 is formed on the entire second surface 1b of the semiconductor substrate 1, the heat insulating layer 2 can be formed in the same manner as described above. In that case, when the thermal insulating layer 2 is formed by anodization, a mask layer 5 may be provided on the first surface 1a of the semiconductor substrate 1 to define a region where the thermal insulating layer 2 is formed.

  In the first embodiment, a single crystal p-type silicon substrate is used as the semiconductor substrate 1, but the semiconductor substrate 1 is not limited to a single crystal p-type silicon substrate, and is a polycrystalline or amorphous p-type silicon substrate. But you can. The semiconductor substrate 1 is not limited to a p-type substrate, and may be an n-type substrate or a non-doped substrate. Then, depending on the type of the semiconductor substrate 1, the anodizing conditions may be changed as appropriate. Therefore, the porous body constituting the heat insulating layer 2 is not limited to the porous silicon layer. For example, a porous polycrystalline silicon layer formed by anodizing polycrystalline silicon or a porous material made of a semiconductor material other than silicon is used. It may be a quality semiconductor layer. The material of the heating element 3 is not limited to Al, and a metal material (for example, W, Mo, Pt, Ir, etc.) having higher heat resistance than Al may be used.

(Second Embodiment)
Next, a second embodiment of the present invention will be described. The basic configuration of the pressure wave generator of the second embodiment is the same as that of the first embodiment, except that a single crystal n-type silicon substrate is employed as the semiconductor substrate 1. Therefore, illustration and description of the structure of the pressure wave generator are omitted, and only the manufacturing method will be described with reference to FIG.

  As shown in FIG. 9A, a current-carrying electrode 4 used for anodization is formed on the entire second surface 1b in the thickness direction of the semiconductor substrate 1 made of an n-type silicon substrate. In addition, what is necessary is just to form a conductive layer on the 2nd surface 1b of the semiconductor substrate 1 by the sputtering method, a vapor deposition method etc. as the electrode 4 for electricity supply.

  After the energization electrode 4 is formed, one end of an energization lead wire (not shown) is attached to the energization electrode 4, and the attachment portion of the energization electrode 4 and one end of the lead wire is used for an anodizing treatment. It is covered with a sealing material having hydrofluoric acid resistance so as not to touch. Thereafter, anodization is performed using an anodizing apparatus as shown in FIG. 10A, so that the thermal insulating layer 2 made of a porous silicon layer as shown in FIG. Formed. Thereafter, a heating element forming step is performed on the heating element forming region 3a of the first surface 1a of the semiconductor substrate 1, whereby a structure having the heating element 3 as shown in FIG. 9C is obtained.

Also in the manufacturing method of the pressure wave generator of the second embodiment, as described above, the thermal insulating layer 2 is formed by the anodizing process. In the anodizing process, as shown in FIG. 10A, an object to be processed 24 mainly composed of the semiconductor substrate 1 is immersed in an electrolytic solution 23 in a processing tank 22. Next, in the electrolytic solution 23, the light shielding plate 30 formed of a material resistant to the electrolytic solution 23 is disposed so as to face the first surface 1a of the semiconductor substrate 1, and further, the light shielding plate 30 and the semiconductor are arranged. The platinum electrode 21 is disposed so as to face the first surface 1 a of the substrate 1. Further, the lead wire attached to the energizing electrode 4 is connected to the plus side of the current source 20, and the platinum electrode 21 is connected to the minus side of the current source 20. Then, the first surface 1a of the semiconductor substrate 1 is energized from the current source 20 with the energizing electrode 4 as the anode and the platinum electrode 21 as the cathode while irradiating light with a light source (not shown) such as a tungsten lamp. A current having a predetermined current density (for example, 20 mA / cm ² ) is allowed to flow between the working electrode 4 and the platinum electrode 21 for a predetermined energization time (for example, 8 minutes).

  By such anodizing treatment, the thermal insulating layer 2 having a substantially constant thickness (for example, 10 μm) is formed on the first surface 1 a side of the semiconductor substrate 1 except for the outer peripheral portion. Thereafter, the processing object 24 is taken out from the processing tank 22, the sealing material of the processing object 24 is peeled off, and the lead wire connected to the energizing electrode 4 is removed.

In addition, the conditions at the time of an anodizing process are not specifically limited, For example, what is necessary is just to set an electric current density within the range of about 1-500 mA / cm < ² > suitably. The predetermined energization time may be set as appropriate according to the thickness of the heat insulating layer 2.

  Moreover, as an electrolytic solution used for the anodizing treatment, for example, a mixed solution in which a 55 wt% aqueous solution of hydrogen fluoride and ethanol are mixed at a ratio of 1: 1 is used. Moreover, as a sealing material, the sealing material which consists of fluororesins like Teflon (trademark) can be used, for example.

The light shielding plate 30 is formed in a planar shape as shown in FIG. 10B by a material (for example, silicon) having resistance to the electrolytic solution 23. Specifically, the aperture ratio of the portion 32 corresponding to the central portion of the region (thermal insulating layer forming region) where the thermal insulating layer 2 is to be formed in the semiconductor substrate 1 of the light shielding plate 30 is set to 100%. The opening ratio of the portion 31 corresponding to the outer side of 2 is set to 0%, and the opening ratio of the portion 33 facing the outer peripheral portion of the heat insulating layer 2 is changed so as to decrease from the inside toward the outside.

  The process of forming the heating element 3 is the same as in the first embodiment, and a metal thin film (for example, an Al thin film) for the heating element 3 is formed on the first surface 1a of the semiconductor substrate 1 by sputtering or the like. Form. Thereafter, a photoresist is applied on the metal thin film, and a resist layer (not shown) patterned for forming the heating element 3 is formed by a photolithography technique. Then, by using the resist layer as a mask, unnecessary portions of the metal thin film are removed by a dry etching process, whereby the heating element 3 is formed. Finally, the structure shown in FIG. 9C is obtained by removing the resist layer.

  According to the manufacturing method of the pressure wave generating device of the second embodiment, the outer periphery of the heat insulating layer forming region on the first surface 1 a of the semiconductor substrate 1 using the light shielding plate 30 in the step of forming the heat insulating layer 2. The anodizing treatment is performed while irradiating light so that the intensity of the light applied to the part is smaller than the intensity of the light applied to the central part and becomes weaker toward the outside. For this reason, the porous formation speed in the outer peripheral portion of the heat insulating layer forming region on the first surface 1a of the semiconductor substrate 1 is slower than the porous formation speed in the central portion, and therefore, as shown in FIG. As described above, the inclined portion 2a is formed on the outer peripheral portion of the thermal insulating layer 2 formed on the first surface 1a side of the semiconductor substrate 1 so that the thickness gradually decreases toward the outer side. As a result, the thermal stress applied to the heating element 3 can be reduced as compared with the conventional pressure wave generator, and the heating element 3 is less likely to be damaged due to the thermal stress.

(Third embodiment)
Next, a third embodiment of the present invention will be described. The basic configuration of the pressure wave generator of the third embodiment is almost the same as that of the first embodiment, but as shown in FIG. 11, the thickness of the outer peripheral portion of the thermal insulating layer 2 is set to the thickness of the central portion (the above-mentioned The difference is that the porous silicon layer constituting the thermal insulating layer 2 is configured so that the porosity of the porous silicon layer gradually increases from the central portion toward the peripheral portion. In addition, the same code | symbol is attached | subjected to the component similar to 1st Embodiment, and description is abbreviate | omitted.

  In the pressure wave generator of the third embodiment, the outer periphery of the thermal insulating layer 2 and the outer periphery of the heating element 3 are substantially coincident (that is, the boundary of the region where the value of αin × Cin varies within the reference thickness range). And the average thermal conductivity and average heat capacity at the outer peripheral portion of the thermal insulating layer 2 while setting the thickness of the thermal insulating layer 2 to be substantially the same at the central portion and the outer peripheral portion. Is made larger than the product of the average thermal conductivity and the average volumetric heat capacity at the center. That is, the physical property values of the heat insulating layer 2 are made non-uniform so that the porosity per unit volume in the outer peripheral portion of the heat insulating layer 2 is smaller than the porosity per unit volume in the central portion.

  Also in the pressure wave generator of the third embodiment, it is possible to increase the amount of heat radiated from the outer peripheral portion of the heating element 3 along the thickness direction of the semiconductor substrate 1, and to reduce the thermal stress applied to the heating element 3. be able to. On the other hand, a decrease in the amplitude of the pressure wave can be suppressed without increasing the amount of heat radiated from the outer peripheral portion of the heating element 3 to the semiconductor substrate 1.

  Next, the manufacturing method of the pressure wave generator of 3rd Embodiment is demonstrated, referring FIG.12 and FIG.13. First, a predetermined thickness (for example, as shown in FIG. 12A) is formed in a region (thermal insulating layer forming region) where the thermal insulating layer 2 is to be formed on the first surface 1a of the semiconductor substrate 1 of the p-type silicon substrate. 2 μm) impurity-doped regions 11 are formed by doping using an ion implantation method, a thermal diffusion method, or the like. The impurity doping region 11 has an impurity concentration distribution in which the specific resistance at the outer peripheral portion is smaller than the specific resistance at the central portion (in the third embodiment, the specific resistance decreases from the central portion toward the outer peripheral portion). Is formed.

  The long side in the planar size of the heating element 3 is set to 12 mm, the short side is set to 10 mm, the specific resistance of the central portion of the impurity doping region 11 is set to about 30 Ω · cm, and the specific resistance of the outer peripheral portion is set to about 2 Ω · cm. Yes. Further, doping is performed so that the specific resistance gradually changes between the central portion and the outer peripheral portion. These numerical values are merely examples and are not particularly limited.

  Next, a silicon nitride film for forming a mask at the time of anodization is formed on the entire first surface 1a of the semiconductor substrate 1 by a plasma CVD method or the like, and the silicon nitride film is formed by using a photolithography technique and an etching technique. Of these, a portion overlapping the heat insulating layer forming region is opened. As a result, as shown in FIG. 12B, the mask layer 5 made of the remaining silicon nitride film is formed on the first surface 1 a of the semiconductor substrate 1.

  Next, as shown in FIG. 12C, a current-carrying electrode 4 used for anodic oxidation is formed on the entire second surface 1b of the semiconductor substrate 1 of the p-type silicon substrate. In addition, what is necessary is just to form the electroconductive layer on the 2nd surface 1b of the semiconductor substrate 1 as the electricity supply electrode 4, for example by a sputtering method or a vapor deposition method.

  After the energization electrode 4 is formed, one end of an energization lead wire (not shown) is attached to the energization electrode 4, and the attachment portion of the energization electrode 4 and one end of the lead wire is used for an anodizing treatment. It is covered with a sealing material having hydrofluoric acid resistance so as not to touch. Thereafter, an anodizing process is performed using an anodizing apparatus as shown in FIG. 6, thereby forming a thermal insulation layer 2 of a porous silicon layer having different porosities at the central part and the outer peripheral part. Subsequently, the structure shown in FIG. 12D is obtained by removing the mask layer 5. Thereafter, a heating element forming step is performed on the heating element forming region 3a of the first surface 1a of the semiconductor substrate 1 to obtain a structure having the heating element 3 as shown in FIG.

The anodizing process using the anodizing apparatus as shown in FIG. 6 is basically the same as that in the first embodiment. Using the current-carrying electrode 4 as an anode and the platinum electrode 21 as a cathode, a current having a predetermined current density (for example, 20 mA / cm ² ) is supplied from the current source 20 between the current-carrying electrode 4 and the platinum electrode 21 for a predetermined time (for example, 2 As a result, the thermal insulating layer 2 having a predetermined thickness (for example, 2.5 μm) is formed on the first surface 1a side of the semiconductor substrate 1. The porosity of the central portion of the heat insulating layer 2 is approximately 60%, and the porosity of the outer peripheral portion is approximately 0%.

In addition, the conditions at the time of an anodizing process are not specifically limited, For example, what is necessary is just to set an electric current density within the range of about 1-500 mA / cm < ² > suitably. The predetermined energization time may be set as appropriate according to the thickness of the heat insulating layer 2.

  Moreover, as an electrolytic solution used for the anodizing treatment, for example, a mixed solution in which a 55 wt% aqueous solution of hydrogen fluoride and ethanol are mixed at a ratio of 1: 1 is used. Moreover, as a sealing material, the sealing material which consists of fluororesins like Teflon (trademark) can be used, for example.

  The process of forming the heating element 3 is the same as in the first embodiment, and a metal thin film (for example, an Al thin film) for the heating element 3 is formed on the first surface 1a of the semiconductor substrate 1 by sputtering or the like. Form. Thereafter, a photoresist is applied on the metal thin film, and a resist layer (not shown) patterned for forming the heating element 3 is formed by a photolithography technique. Then, by using the resist layer as a mask, unnecessary portions of the metal thin film are removed by a dry etching process, whereby the heating element 3 is formed. Finally, the structure shown in FIG. 12E is obtained by removing the resist layer.

  According to the manufacturing method of the pressure wave generator of the third embodiment, the thickness of the thermal insulating layer 2 formed on the semiconductor substrate 1 is made substantially uniform, and the porosity of the central portion in the width direction of the thermal insulating layer 2 is determined. Also, the porosity of the outer peripheral portion can be lowered. That is, the product of the average thermal conductivity and the average volumetric heat capacity in the outer peripheral portion of the heat insulating layer 2 is larger than the product of the average thermal conductivity and the average volumetric heat capacity in the central portion, so that compared with the conventional pressure wave generator. Thus, the thermal stress applied to the heating element 3 can be reduced, and the heating element is less likely to be damaged due to the thermal stress.

  Further, in the width direction, if the thermal insulation layer 2 is formed at the boundary between the outer peripheral portion of the thermal insulation layer 2 and the outer portion of the semiconductor substrate 1 outside the thermal insulation layer 2, the thermal expansion coefficients of the two match each other. Discontinuous parts of thermal expansion coefficient disappear. In short, in a region where the value of αin × Cin changes, at least one of the thermal conductivity and the heat capacity of the material itself forming the thermal insulating layer 2 is changed so as to increase outward, and αin × Cin = αout × If the material composition is matched in the portion where Cout, the discontinuous portion of the thermal expansion coefficient in the portion where αin × Cin = αout × Cout is eliminated. As a result, cracks are less likely to occur in the thermal insulation layer 2 due to stress resulting from the difference in thermal expansion coefficient between the outer peripheral portion of the thermal insulation layer 2 and the semiconductor substrate 1.

  As shown in FIG. 13, if the planar shape of the energizing electrode 4 is formed so as to match the heating element forming region 3a on the first surface 1a of the semiconductor substrate 1, the first surface 1a of the semiconductor substrate 1 is formed. Without providing the mask layer 5 on top, only the impurity doping region 11 can be made porous to form the thermal insulation layer 2 made of a porous silicon layer.

(A) is a top view which shows the structure of the pressure wave generator concerning 1st Embodiment of this invention, (b) is AA sectional drawing in (a), (c) is the surface of a thermal insulation layer, and a semiconductor substrate Explanatory drawing which shows the reference point at the time of simulating the temperature distribution of the plane containing this 1st surface by the finite element method. The figure which shows notionally the structure of the pressure wave generator which concerns on 1st Embodiment. (A) is a waveform diagram showing a waveform of an AC voltage applied to the pressure wave generator, (b) is a waveform diagram showing a temperature change of the heating element, and (c) is a pressure wave generated by the pressure wave generator ( The wave form diagram which shows the waveform of a sound wave. (A)-(c) is process drawing which shows the manufacturing method of the pressure wave generator which concerns on 1st Embodiment. Process drawing which shows the other process of the manufacturing method of the pressure wave generator which concerns on 1st Embodiment. The figure which shows the anodizing apparatus used for the manufacturing method of the pressure wave generator concerning 1st Embodiment. The graph which shows the temperature distribution characteristic of the pressure wave generator which concerns on 1st Embodiment, and the conventional pressure wave generator. Sectional drawing which shows another structural example of the pressure wave generator which concerns on 1st Embodiment. (A)-(c) is process drawing which shows the manufacturing method of the pressure wave generator which concerns on 2nd Embodiment of this invention. The figure which shows the anodizing apparatus used for the manufacturing method of the pressure wave generator which concerns on 2nd Embodiment. Sectional drawing which shows the structure of the pressure wave generator which concerns on 3rd Embodiment of this invention. (A)-(e) is process drawing which shows the manufacturing method of the pressure wave generator concerning 3rd Embodiment. Process drawing which shows the other process of the manufacturing method of the pressure wave generator which concerns on 3rd Embodiment. Sectional drawing which shows the structure and operation | movement of the conventional pressure wave generator. (A) is a top view which shows the structure of the conventional pressure wave generator, (b) is AA sectional drawing of (a), (c) is a plane containing the surface of a heat insulating layer, and the 1st surface of a semiconductor substrate. Explanatory drawing which shows the reference point at the time of simulating the temperature distribution of this by the finite element method. (A) is a top view which shows 1 process of the manufacturing method of the conventional pressure wave generator, (b) is AA sectional drawing of (a).

Explanation of symbols

1 Semiconductor substrate 1a First surface (one surface) of a semiconductor substrate
1b Second surface (other surface) of semiconductor substrate
2 Thermal insulating layer 2a Inclined portion 3 Heating element 3a Heating element formation region 3e Outer periphery or end face of heating element

Claims (4)

A substrate, a porous thermal insulating layer formed on one surface in the thickness direction of the substrate, and a thin film heating element formed on the thermal insulating layer, according to the waveform of electric input to the heating element A pressure wave generating device that generates a pressure wave by heat exchange between the heating element and the medium when the temperature of the heating element changes,
The thickness direction of the inner part from the outer periphery of the heating element within the range of the width direction defined by the reference thickness of the center part in the width direction of the thermal insulation layer from one surface in the thickness direction of the substrate toward the inside of the substrate The average thermal conductivity is αin, the average volumetric heat capacity is Cin, the average thermal conductivity in the thickness direction of the outer portion of the heating element is αout, the average volumetric heat capacity is Cout,
αin × Cin <αout × Cout
And a value of αin × Cin increases toward the outer side in the vicinity of the boundary between the inner part and the outer part.   2. The pressure wave generator according to claim 1, wherein the boundary of the region where the value of αin × Cin changes is substantially coincident with the outer periphery of the heating element, or is located inside the outer periphery of the heating element. .   In the region where the value of αin × Cin changes, at least one of the thermal conductivity and the heat capacity of the material forming the thermal insulation layer is continuously changed so as to increase outward. Item 3. The pressure wave generator according to Item 1 or 2. In αin × Cin = αout × Cout become thermally insulating layer and the substrate of the boundary, the pressure wave generator of the mounting serial to claim 3, characterized in that the material composition substantially coincide.

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