JP2006013962A - Pressure wave generating element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pressure wave generating element capable of preventing reduction of output due to oxidization of a thermal insulating layer consisting of a porous silicon layer. <P>SOLUTION: The pressure wave generating element is provided with a heating element layer 3 consisting of a metal film on one surface side of a supporting substrate 1 consisting of a single crystal silicon substrate, is provided with a thermal insulating layer 2 consisting of a porous silicon layer between the supporting substrate 1 and the layer 3, and allows an oxidation preventing layer 5 for preventing oxidation of the layer 2 to intervene between the layer 3 and the layer 2. This pressure wave generating element generates a pressure wave by heat exchange between the layer 3 and air in response to a temperature change according to the waveform of electrical input to be given to the layer 3. The layer 3 is formed of W, and the layer 5 comprises a high melting point film formed of HfC, one kind of a material of melting point higher than that of a silicon, and its film thickness is set at thermal diffusion or low which is decided by a thermal conductivity, a thermal capacitance and a waveform. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a pressure wave generating element that generates a pressure wave such as a sound wave targeted for a speaker, an ultrasonic wave, or a monopulse density wave.

  2. Description of the Related Art Conventionally, an ultrasonic wave generating element using mechanical vibration due to a piezoelectric effect is widely known. As this type of ultrasonic 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. By applying electrical energy to generate mechanical vibration, air can be vibrated to generate ultrasonic waves.

  The ultrasonic generating element using the mechanical vibration as described above has a problem that the frequency band is narrow because it has a specific resonance frequency, and it is easily affected by external vibration and fluctuations in external pressure.

  On the other hand, as an element that can generate pressure waves such as ultrasonic waves by thermal excitation without mechanical vibration, as shown in FIG. A heat insulating layer 2 having a sufficiently small thermal conductivity and heat capacity as compared to the formed support substrate 1 and a heat generating layer 3 formed on the heat insulating layer 2 are provided, and an AC current is supplied to the heat generating layer 3. A pressure wave generating element that generates a pressure wave by heat exchange between the heating element layer 3 and air has been proposed (Patent Documents 1, 2, and 3).

  In the pressure wave generating element having the configuration shown in FIG. 3, the heat insulating layer 2 is formed immediately below the heat generating body layer 3. While the temperature of the heating element layer 3 changes according to the input waveform energized to 3, efficient heat exchange occurs with the air in the vicinity of the heating element layer 3. Pressure waves such as sound waves are generated. Note that the pressure wave generating element having the configuration shown in FIG. 3 can change the frequency of the generated pressure wave over a wide range by adjusting the frequency of the alternating current supplied to the heating element layer 3. It can be used as a sound source or a sound source of a speaker. In short, in the pressure wave generating element having the configuration shown in FIG. 4, the waveform of the electrical input (voltage applied to the heating element layer 3 or current supplied to the heating element layer 3) given to the heating element layer 3 is a periodic wave ( For example, by changing the waveform by changing the period of the periodic wave as a sine wave, square wave, etc., the frequency of the generated pressure wave can be changed over a wide range, and the electricity applied to the heating element layer 3 can be changed. If the input waveform is a solitary wave, a single-pulse dense wave (impulse sound wave) can be generated as a pressure wave.

  Here, in the infrared radiation elements described in Patent Documents 1 and 2, the support substrate 1 is composed of a single crystal silicon substrate, and the heat insulating layer 2 is made porous by anodizing a part of the silicon substrate. It is comprised by the porous silicon layer formed by forming. Further, in Patent Document 1, it is desirable to make the thermal conductivity and heat capacity of the heat insulating layer 2 smaller than the heat conductivity and heat capacity of the support substrate 1, and the product of the heat conductivity and the heat capacity of the heat insulating layer 2 is calculated. It is described that it is preferable to make it sufficiently smaller than the product of the thermal conductivity and the heat capacity of the support substrate 1.

In the pressure wave generating elements described in Patent Documents 1 and 2, the heating element layer 3 is positioned on the heat insulating layer 2 on the inner side of the outer periphery of the heat insulating layer 2, and the surface of the heat generating layer 3 (see FIG. 3 is employed, and the surface of a part of the heat insulating layer 2 (the portion where the heat generating layer 3 is not laminated) is exposed. There is also described a structure in which the heat insulating layer 2 is formed by subjecting the porous silicon layer to a rapid thermal oxidation treatment instead of forming the layer 2 with a porous silicon layer. Further, the pressure wave generating element described in Patent Document 3 employs a structure in which the surface of the heating element layer 3 is covered with an insulating protective layer made of a SiO ₂ film. Patent Documents 1 and 2 describe examples in which the heating element layer 3 is made of an aluminum thin film, and Patent Document 3 describes an example in which the heating element layer 3 is made of a tantalum nitride film. . In the example of Patent Document 3, the film thickness of the tantalum nitride film constituting the heating element layer 3 is set to 0.5 μm, and the film thickness of the SiO ₂ film constituting the insulating protective layer is set to 1.5 μm. Has been.
Japanese Patent Laid-Open No. 11-3000274 JP 2002-186097 A JP-A-3-140100

  By the way, when the pressure wave generating element having the configuration shown in FIG. 3 generates ultrasonic waves having a frequency of 60 kHz, the inventors of the present application have a sound pressure of about 15 Pa at a position 30 cm away from the pressure wave generating element. In order to obtain the above, it is necessary to raise the temperature of the heating element layer 3 to about 400 ° C., and in order to obtain the sound pressure of about 30 Pa, it is necessary to raise the temperature of the heating element layer 3 to a high temperature exceeding 1000 ° C. The experimental results were obtained. However, in the pressure wave generating elements described in Patent Documents 1 and 2, even if a metal having a high melting point and excellent oxidation resistance as compared with aluminum is used as the material of the heating element layer 3 in order to increase the output, Since a part of the surface of the heat insulating layer 2 is exposed and exposed to air, the heat insulating layer 2 is oxidized, the heat capacity of the heat insulating layer 2 is increased, and the output is lowered.

  In addition, when the structure in which the heat insulating layer 2 is formed by subjecting the porous silicon layer to rapid thermal oxidation instead of the heat insulating layer 2 made of a porous silicon layer, stability over time is improved. Since the initial heat capacity of the heat insulating layer 2 is increased, there is a problem that the output is low.

Therefore, as in the pressure wave generating element described in Patent Document 3, the insulating protective layer that covers the heating element layer 3 and the heat insulating layer 2 where the heating element layer 3 is not formed on the one surface side of the support substrate 1. However, since the insulating protective layer is composed of a SiO ₂ film, oxygen in the air diffuses into the insulating protective layer and insulates from the heat insulating layer 2 when the temperature of the heating element layer 3 becomes high. It reaches the interface with the protective layer and the heat insulating layer 2 is oxidized.

Further, in the pressure wave generating element described in Patent Document 3, a SiO ₂ film is adopted as the heat insulating layer 2, tantalum nitride is adopted as the material of the heating element layer 3, and the heating element layer 3 is oxidized. However, since tantalum nitride has a higher resistance than metals such as aluminum, it generates heat compared to the pressure wave generating elements described in Patent Documents 1 and 2 when driven at a constant voltage. There is a problem in that it is necessary to apply a high voltage to the body layer 3 and the input power becomes high (that is, it is difficult to reduce power consumption). In the pressure wave generating element described in Patent Document 3, the heat capacity of the heating element layer 3 is larger than that of the pressure wave generating elements described in Patent Documents 1 and 2, so The response of the temperature change to a typical input waveform is delayed, the temperature of the heating element layer 3 is difficult to rise, and it is difficult to increase the output and increase the response speed.

  The present invention has been made in view of the above-described reasons, and an object thereof is to provide a pressure wave generating element capable of preventing a decrease in output due to oxidation of a heat insulating layer made of a porous silicon layer.

  According to the first aspect of the present invention, there is provided a heat insulating layer made of a porous silicon layer between a silicon substrate and a heating element layer provided on one surface side of the silicon substrate, and a waveform of electrical input given to the heating element layer. A pressure wave generating element that generates a pressure wave by heat exchange between the heating element layer and air in accordance with the temperature change of the heating element layer according to the condition, wherein the thermal insulation layer is oxidized between the heating element layer and the thermal insulation layer. It is characterized by interposing an antioxidant layer to prevent. Here, the electrical input given to the heating element layer means a voltage applied to the heating element layer or a current supplied to the heating element layer.

  According to this invention, an oxidation preventing layer that prevents oxidation of the heat insulating layer is interposed between the heat generating layer and the heat insulating layer made of the porous silicon layer, thereby oxidizing the heat insulating layer made of the porous silicon layer. It is possible to prevent the decrease in output due to the oxidation of the porous silicon layer.

  The invention according to claim 2 is the invention according to claim 1, wherein the heating element layer is made of a metal having a higher melting point than silicon, and the antioxidant layer is made of a material having a higher melting point than silicon and has a thickness. Is less than or equal to the thermal diffusion length determined by the thermal conductivity, the heat capacity, and the waveform.

  According to the present invention, the heating element layer is formed of a metal having a melting point higher than that of silicon, and the antioxidant layer is formed of a material having a melting point higher than that of silicon. Can be raised to the maximum use temperature of silicon (the melting point of silicon is 1410 ° C.), so that higher output can be achieved compared to the case where the heating element layer is formed of a metal material having a relatively low melting point such as aluminum. Moreover, since the thickness of the antioxidant layer is equal to or less than the thermal diffusion length, it is possible to suppress a decrease in output due to the provision of the antioxidant layer.

According to a third aspect of the present invention, in the first or second aspect of the present invention, the antioxidant layer is made of a material selected from the group consisting of carbide, nitride, boride, and silicide. . Here, examples of the carbide include TaC, HfC, NbC, ZrC, TiC, VC, WC, ThC, and SiC, and examples of the nitride include HfN, TiN, TaN, BN, Si ₃ N _{4, and the} like. Examples of borides include HfB, TaB, ZrB, TiB, NbB, WB, VB, MoB, and CrB. Examples of silicide include WSi, MoSi, and TiSi.

  According to the present invention, the antioxidant layer can be formed by a general thin film forming method used in a semiconductor manufacturing process such as sputtering, vapor deposition, or CVD.

  According to the first aspect of the invention, there is an effect that it is possible to prevent a decrease in output due to oxidation of the porous silicon layer.

  As shown in FIGS. 1A and 1B, the pressure wave generating element of the present embodiment is provided with a heating element layer 3 made of a metal film on one surface side of a support substrate 1 made of a single crystal silicon substrate. In addition, a heat insulating layer 2 is provided between the support substrate 1 and the heating element layer 3, and both end portions (left and right end portions in FIG. 1 (b)) of the heating element layer 3 on the one surface side of the support substrate 1. A pair of pads 4 and 4 are formed in contact with each other, and an antioxidant layer 5 for preventing oxidation of the heat insulating layer 2 is interposed between the heating element layer 3 and the heat insulating layer 2. Here, the pressure wave generating element of the present embodiment is a heating element layer corresponding to the waveform of an electrical input (voltage applied to the heating element layer 3 or current supplied to the heating element layer 3) applied to the heating element layer 3. A pressure wave is generated by heat exchange between the heating element layer 3 and air in accordance with the temperature change of 3. In addition, the planar shape of the support substrate 1 is a rectangular shape, and the planar shapes of the heat insulating layer 2, the heating element layer 3, and the antioxidant layer 5 are also formed in a rectangular shape. However, the length dimension of each of the long side and the short side in the planar shape of the antioxidant layer 5 is set to be larger than the length dimension of each of the long side and the short side in the planar shape of the heat insulating layer 2. The surface of the part where the heating element layer 3 is not laminated is covered with the antioxidant layer 5.

By the way, in this embodiment, since the single crystal silicon substrate is used as the support substrate 1 as described above, and the heat insulating layer 2 is composed of a porous silicon layer having a porosity of approximately 70%, the support substrate 1 A porous silicon layer serving as the heat insulating layer 2 can be formed by anodizing a part of the silicon substrate used as a hydrogen fluoride aqueous solution. Here, by appropriately setting the conditions for anodizing treatment (for example, current density, energization time, etc.), the porosity and thickness of the porous silicon layer to be the heat insulating layer 2 can be set to desired values, respectively. The porous silicon layer has a smaller thermal conductivity and heat capacity as the porosity increases. For example, the thermal conductivity is 148 W / (m · K), and the heat capacity is 1.63 × 10 ⁶ J / (m ³ · K. The porous silicon layer having a porosity of 60% formed by anodizing a single crystal silicon substrate of) has a thermal conductivity of 1 W / (m · K) and a heat capacity of 0.7 × 10 ⁶ J / ( m ³ · K). In this embodiment, as described above, the heat insulating layer 2 is composed of a porous silicon layer having a porosity of approximately 70%, and the heat conductivity of the heat insulating layer 2 is 0.12 W / (m · K), The heat capacity is 0.5 × 10 ⁶ J / (m ³ · K).

The heating element layer 3 is made of tungsten which is a kind of high melting point metal. The heating element layer 3 has a thermal conductivity of 174 W / (m · K) and a heat capacity of 2.5 × 10 ⁶ J /. (M ³ · K). The material of the heating element layer 3 is not limited to tungsten, but may be any metal having a melting point higher than that of silicon. For example, tantalum, molybdenum, iridium, or the like may be employed.

Although the antioxidant layer 5 is formed of HfC having a melting point higher than that of silicon, the material of the antioxidant layer 5 may be a material selected from the group of carbide, nitride, boride, and silicide. TaC, HfC, NbC, ZrC, TiC, VC, WC, ThC, SiC, and the like can be adopted as the carbide having a higher melting point than silicon, and HfN, TiN, TaN as the nitride having a higher melting point than silicon. , BN, Si ₃ N _4, etc. can be adopted, and HfB, TaB, ZrB, TiB, NbB, WB, VB, MoB, CrB, etc. can be adopted as borides having a melting point higher than that of silicon. As the silicide having a higher melting point, WSi, MoSi, TiSi, or the like can be used.

  In the pressure wave generating element of this embodiment, the thickness of the silicon substrate before the formation of the heat insulation layer 2 is 525 μm, the thickness of the heat insulation layer 2 is 2 μm, the thickness of the heating element layer 3 is 50 nm, and each pad 4, Although the thickness of 4 is 0.5 μm and the thickness of the antioxidant layer 5 is 50 nm, these thicknesses are merely examples and are not particularly limited.

  Hereinafter, the manufacturing method of the pressure wave generating element of this embodiment will be briefly described.

  First, after a current-carrying electrode (not shown) used for anodizing treatment is formed on the other surface (the lower surface in FIG. 1B) side of the silicon substrate used as the support substrate 1, a heat insulating layer on one surface side of the silicon substrate is formed. An anodizing treatment step for forming a heat insulating layer 2 made of porous silicon is performed by making a portion to be formed 2 porous by anodizing treatment. Here, in the anodizing process, 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 as an electrolytic solution, and an object to be processed mainly composed of a silicon substrate can be placed in a processing tank. The electrode is immersed in the electrolyte, the current-carrying electrode is the anode, the platinum electrode facing the one surface side of the silicon substrate is the cathode, and a current of a predetermined current density is supplied from the power source to the anode and the cathode for a predetermined time. The heat insulating layer 2 made of porous silicon is formed by flowing.

  After the above-described anodizing process, an antioxidant layer forming process for forming the antioxidant layer 5, a heating element layer forming process for forming the heating element layer 3, and a pad forming process for forming the pads 4 and 4 are sequentially performed. The pressure wave generating element is completed. In the antioxidant layer forming step, the heating element layer forming step, and the pad forming step, the film may be formed by, for example, various sputtering methods, various vapor deposition methods, various CVD methods, or the like.

  By the way, as a comparative example of the pressure wave generating element of the present embodiment, an output sound is produced when an element in which the antioxidant layer 5 is not provided in the structure of FIG. 1 is prototyped and the input power to the heating element layer 3 is varied. The results of measuring the pressure and the temperature of the heating element layer 3 are shown in FIG. Here, the horizontal axis in FIG. 2 indicates the peak value of the input power when the peak value is variously changed by inputting the voltage of a sine wave having a frequency of 30 kHz, and the vertical axis on the left side is only 30 cm from the surface of the heating element layer 3. The sound pressure (output sound pressure) of the ultrasonic wave having a frequency of 60 kHz measured at a distant position is shown, and the vertical axis on the right side shows the surface temperature of the heating element layer 3, respectively. A change in sound pressure is indicated, and “B” indicates a change in temperature of the heating element layer 3.

  From FIG. 2, the sound pressure and the temperature of the heating element layer 3 tend to increase as the input power to the heating element layer 3 increases. To obtain a sound pressure of about 15 Pa, the temperature of the heating element layer 3 is set to 400. It can be seen that it is necessary to increase the temperature of the heating element layer 3 to a high temperature exceeding 1000 ° C. in order to obtain a sound pressure of about 30 Pa. However, in the structure in which a part of the surface of the heat insulating layer 2 made of a porous silicon layer is exposed as in this comparative example, when the temperature of the heating element layer 3 reaches about 400 ° C., the heat insulating layer 2 in the air Since oxidation begins to occur, the heat capacity of the heat insulating layer 2 increases. In general, the porous silicon layer has a larger surface area than bulk silicon having the same thickness and is very active. Therefore, the porous silicon layer is easily oxidized in the air. When heated by the heat of the heating element layer 3, the porous silicon layer is oxidized. Can be accelerated.

On the other hand, in the pressure wave generating element of the present embodiment, an anti-oxidation layer 5 that prevents oxidation of the heat insulating layer 2 is interposed between the heat generating layer 3 and the heat insulating layer 2 so that the heat generating layer 2 has a heat generating element. The surface of the portion where the layer 3 is not laminated is not exposed. Here, if the film thickness (thickness) of the high melting point film constituting the antioxidant layer 5 is too thick, the heat capacity of the antioxidant layer 5 becomes too large, the function of the heat insulating layer 2 is not exhibited, and the output decreases. Resulting in. Therefore, in the present embodiment, the film thickness of the high melting point film allowed as the antioxidant layer 5 is set to be equal to or less than the thermal diffusion length determined by the thermal conductivity, the heat capacity, and the waveform of the electric input given to the heating element layer 3. It is. When the electric input waveform applied to the heating element layer 3 is, for example, an alternating sine wave having a frequency of f ′ [Hz], the thermal diffusion length L is set to f ( = 2f ′), the angular frequency of the temperature change waveform of the heating element layer 3 is ω (= 2πf), and the thermal conductivity and heat capacity of the antioxidant layer 5 are α [W / (m · K)], C [J / (M ³ · K)], it is expressed by the following formula derived from the heat conduction equation.

  Note that the thermal diffusion length L described above is 1 / e times the temperature of the interface between the antioxidant layer 5 and the heating element layer 3 in the thickness direction of the antioxidant layer 5 (e is the base of the natural logarithm). The distance between the position and the interface. The frequency of the pressure wave generated from the heating element layer 3 is equal to the frequency f.

  Here, to give a numerical example when ultrasonic waves are generated from the pressure wave generating element of the present embodiment, when the material of the antioxidant layer 5 is HfC, when the frequency f is 20 kHz (that is, the frequency is higher than 20 kHz). When the sound wave is generated), the thermal diffusion length L is 11 μm, so the thickness of the antioxidant layer 5 may be set to 11 μm or less, and when the frequency f is 100 kHz (that is, when the ultrasonic wave having a frequency of 100 kHz is generated). ), The thermal diffusion length L is 5.1 μm, so the thickness of the antioxidant layer 5 should be 5.1 μm or less (in this embodiment, HfC is used as the material of the antioxidant layer 5 as described above. The thickness of the antioxidant layer 5 is set to 50 nm). When the antioxidant layer 5 is TaN, the thermal diffusion length L = 5.9 μm when the frequency f is 20 kHz, so the thickness of the antioxidant layer 5 may be 5.9 μm or less, and when the frequency f is 100 kHz. Since the thermal diffusion length L is 2.6 μm, the thickness of the antioxidant layer 5 may be 2.6 μm or less. In short, even in the pressure wave generating element of the present embodiment, the waveform is changed by changing the period of the periodic wave as the waveform of the electrical input given to the heating element layer 3 as a periodic wave (eg, sine wave, square wave, etc.). The frequency of the generated pressure wave can be changed over a wide range, and if the waveform of the electrical input applied to the heating element layer 3 is a solitary wave, a single-pulse coarse wave (as a pressure wave) Impulse sound waves) can be generated.

  In the pressure wave generating element of the present embodiment described above, the antioxidant layer 5 for preventing oxidation of the heat insulating layer 2 is interposed between the heat generating layer 3 and the heat insulating layer 2 made of a porous silicon layer. Further, it is possible to prevent oxidation of the heat insulating layer 2 made of a porous silicon layer, and to prevent a decrease in output due to oxidation of the porous silicon layer. In addition, the heating element layer 3 is made of a metal having a melting point higher than that of silicon, and the antioxidant layer 5 is made of a material having a melting point higher than that of silicon. Since the temperature can be raised to the use temperature (the melting point of silicon is 1410 ° C.), it is possible to increase the output as compared with the case where the heating element layer 3 is formed of a metal material having a relatively low melting point such as aluminum. In addition, since the film thickness of the antioxidant layer 5 is equal to or less than the above-described thermal diffusion length L, it is possible to suppress a decrease in output due to the provision of the antioxidant layer 5.

  Further, by adopting any of the above-mentioned carbides, nitrides, borides, and silicides as the material of the antioxidant layer 5, the antioxidant layer 5 can be used in semiconductor manufacturing processes such as sputtering, vapor deposition, and CVD. It can be formed by a general thin film forming method.

Embodiment is shown, (a) is a schematic plan view, (b) is a schematic sectional drawing. It is characteristic explanatory drawing of a comparative example same as the above. It is a schematic sectional drawing which shows a prior art example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Support substrate 2 Heat insulation layer 3 Heat generating body layer 4 Pad 5 Antioxidation layer

Claims (3)

  A heat insulating layer made of a porous silicon layer is provided between the silicon substrate and the heating element layer provided on one surface side of the silicon substrate, and the heating element layer corresponding to the waveform of the electrical input applied to the heating element layer is provided. A pressure wave generating element that generates a pressure wave by heat exchange between the heating element layer and air in accordance with a temperature change, and an anti-oxidation layer is provided between the heating element layer and the heat insulation layer to prevent oxidation of the heat insulation layer. A pressure wave generating element characterized by being made.   The heating element layer is formed of a metal having a melting point higher than that of silicon, the antioxidant layer is formed of a material having a melting point higher than that of silicon, and the thickness is determined by the thermal conductivity, the heat capacity, and the waveform. The pressure wave generating element according to claim 1, wherein the pressure wave generating element is not longer than the length.   3. The pressure wave generating element according to claim 1, wherein the antioxidant layer is formed of a material selected from the group consisting of carbide, nitride, boride, and silicide.

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