JP4931389B2 - Pressure wave generator and driving method of pressure wave generator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pressure wave generator having high durability and quality, and a method for driving the pressure wave generator. <P>SOLUTION: The pressure wave generator according to the present invention is a pressure wave generator 1 for generating a pressure wave, and comprises a silicon substrate 11, a heat insulating layer 13 disposed on the silicon substrate 11, a heating element film 14 formed on the heat insulating layer 13, and a driving unit 17 connecting to the heating element film 14 and supplying a driving current to the heating element film 14, wherein the driving current is composed of a pulse train with intervals of amplitudes and pulses, or a pulse width being modulated. Accordingly, the pressure wave generator according to the present invention generates and outputs a pressure wave corresponding to the modulated pulse train. <P>COPYRIGHT: (C)2007,JPO&amp;INPIT

Description

  The present invention relates to a pressure wave generator and a method for driving the pressure wave generator, and more particularly to a pressure wave generator for generating ultrasonic waves or the like as a pressure wave and a method for driving the pressure wave generator.

  Conventionally, a piezoelectric element or the like has been used in a pressure wave generator that generates pressure waves such as ultrasonic waves. An apparatus that generates a pressure wave by mechanical vibration such as a piezoelectric element has a unique resonance frequency, and thus it is difficult to widen the frequency range of generated ultrasonic waves. Furthermore, since this pressure wave generator is difficult to integrate with a circuit and can only be manufactured as a single product, it has been difficult to add functions such as downsizing of the device and an array structure.

  In order to solve the problems of the mechanical vibration type pressure wave generator, a heat-induced pressure wave generator is formed by forming a heating element thin film on a thermal insulating layer and electrically driving the heating element thin film. Has been proposed. Patent Document 1 discloses an example of a heat-induced pressure wave generator. FIG. 9 is a schematic diagram showing a partial cross-section of an example of the configuration of this heat-induced conventional pressure wave generator. In a conventional pressure wave generator, a thermal insulating layer 902 made of a porous material such as nanocrystalline silicon is formed on a substrate 901. On the heat insulating layer 902, a heating element thin film 903 is formed as a heating electrode, and a driving current is supplied to the heating element thin film 903.

  In this pressure wave generator, an alternating current is supplied to the heating element thin film 903 functioning as an electrode to output a pressure wave. FIG. 10 shows a timing chart showing the output timing of this pressure wave. 10A shows the input AC current, FIG. 10B shows the heat generation timing, FIG. 10C shows the change timing of the surface of the heating element thin film, and FIG. 10D shows the output timing of the pressure wave.

  When the alternating current (I) shown in FIG. 10A is supplied to the pressure wave generator, heat generation (q) due to Joule heat is generated in the heating element thin film 903 as shown in FIG. 10B. Then, as shown in FIG. 10C, a temperature change (T) occurs in the heating element thin film 903 formed on the surface of the pressure wave generator at a frequency twice as high as the input alternating current. Since the heat insulating layer 902 is disposed under the heating element thin film 903, the heating element thin film 903 that changes in temperature efficiently exchanges heat with the air immediately above. As a result, air density is produced, and this air density is transmitted as a pressure wave (P) as shown in FIG. Thus, since the pressure wave generator is a non-linear system, a pressure wave is output at a frequency twice that of the input alternating current. Therefore, it is difficult to apply the pressure wave generator to a speaker or the like that outputs a sound wave as a pressure wave.

に比例し、右辺の第２項に、入力の交流電流の２倍の周波数成分cos２ωtが生じている。 The case where such a pressure wave generator is applied to a speaker will be described using mathematical expressions. If the input alternating current is asinωt, the output acoustic signal is

In the second term on the right side, a frequency component cos2ωt twice as large as the input AC current is generated.

  As a method for solving the problem of generating such a double frequency component, there is a method of applying a DC bias together with an input AC current. FIG. 11 shows a current waveform in this case. As shown in FIG. 11, when a direct current of magnitude b is added to the input alternating current, the waveform of the input signal slides in the positive direction of the input alternating current asinωt by the amount of the direct current bias b. Will be.

に比例し、入力の交流電流と同じ周波数ωの成分が見られる。このとき、直流バイアスｂを入力の交流電流の振幅ａよりも大きくすれば、事実上、２ωの周波数成分は無視できるほど小さくすることができる。具体的には、ｂ≫ａとすると、出力される音響信号は、

となり、２ωの周波数成分が実質的に除去される。 When the output in this case is shown by a mathematical expression, the output acoustic signal is

And a component having the same frequency ω as that of the input alternating current can be seen. At this time, if the DC bias b is made larger than the amplitude a of the input AC current, the frequency component of 2ω can be effectively reduced to a negligible level. Specifically, if b >> a, the output acoustic signal is

Thus, the frequency component of 2ω is substantially removed.

In this case, as the magnitude b of the DC bias increases, the DC component of the current waveform increases, and the current flowing through the heating element thin film 903 on the thermal insulating layer 902 made of nanocrystalline silicon increases. To do. The heating element thin film 903 is desirably formed to be extremely thin in order to reduce the heat capacity value as much as possible. Therefore, the load on the heating element thin film 903 is large, and the heating element thin film 903 may be cut by a thermal shock. Furthermore, the resistance value in the heating element thin film 903 may change due to electromigration, and it may be difficult to realize the drive as designed in the long term.
Japanese Patent Laid-Open No. 11-3000274

As described above, in the conventional pressure wave generator, since the thickness of the heating element thin film as an electrode is thin for the purpose of reducing the heat capacity value, the heating element thin film is cut by the drive current in some cases, and the pressure is high. There was a problem that it was difficult to realize a wave generator.
The present invention has been made to solve such a problem, and an object thereof is to provide a high-quality pressure wave generator having high durability and a driving method of the pressure wave generator.

  A pressure wave generator according to the present invention is a pressure wave generator that generates a pressure wave, and includes a substrate, a thermal insulation layer disposed on the substrate, and a heat generation disposed on the thermal insulation layer. And an electrode and a drive unit that is connected to the heat generating electrode and generates a drive current composed of a modulated pulse train. With such a configuration, it is possible to avoid the continuous application of current to the heating electrode, so that the load on the heating electrode can be reduced. Therefore, a high-quality pressure wave generator with high durability can be realized.

  Further, the modulation can be amplitude modulation of the pulse train or modulation of the interval between the pulse trains. Furthermore, the modulation may be pulse width modulation of the pulse train.

  Preferably, the thermal insulating layer can be a nanocrystalline silicon layer.

  A method of driving a pressure wave generator according to the present invention is a method of driving a pressure wave generator that generates a pressure wave, and includes a step of generating a modulated pulse train and a step of applying the generated pulse train. It is provided. With such a configuration, it is possible to avoid the continuous application of current to the heating electrode, so that the load on the heating electrode can be reduced. Therefore, a high-quality pressure wave generator with high durability can be realized.

  Furthermore, in the step of generating the pulse train, amplitude modulation of the pulse train is performed, and in the step of generating the pulse train, the interval of the pulse train is modulated. Furthermore, in the step of generating the pulse train, pulse width modulation of the pulse train may be performed.

  According to the present invention, it is possible to provide a high-quality pressure wave generator with high durability and a driving method of the pressure wave generator. In other words, the principle of the present invention focuses on the fact that the pressure wave generator according to the structure of the present invention has a substantially flat frequency characteristic in a wide frequency range, so that the source waveform can be reproduced by the modulated pulse signal, This realizes the highly durable pressure wave generator.

The pressure wave generator according to the present invention is preferably a sound source using the thermal insulation of nanocrystalline silicon, to a speaker capable of reproducing an acoustic signal with high quality and high sound pressure with little damage to the heating electrode. Application.
The best mode for carrying out the present invention will be described below with reference to the drawings.

Embodiment 1 of the Invention
In the first embodiment of the invention, a case where the amplitude of a pulsed drive current is modulated will be described.
First, the configuration of the pressure wave generator according to the present invention will be described with reference to FIG. 1A and 1B are schematic views showing a configuration example of a pressure wave generator according to the present invention, in which FIG. 1A is a cross-sectional view and FIG. 1B is a top view. Moreover, in FIG. 1, the main structures of the pressure wave generator which concerns on this invention are shown.

As shown in FIG. 1, the pressure wave generator 1 according to the present invention includes a silicon substrate 11, a mask layer 12, a thermal insulating layer 13, a heating element thin film 14, pads 151 and 152, wires 161 and 162, and a driving unit 17. Have.
The silicon substrate 11 is formed by dicing the wafer.
The mask layer 12 is a layer using a high-resistance material and plays a role of electrical insulation between pads. The mask layer 12 is not a porous thermal insulating layer but a silicon substrate, and therefore has a chip structure that prevents adhesion failure due to escape of ultrasonic waves during wire bonding. This makes it possible to connect wires with high reliability. The mask layer 12 is made of, for example, SiC or silicon nitride (silicon nitride). The mask layer 12 is disposed on the silicon substrate 11, and in detail, is disposed on the periphery of the four sides of the silicon substrate 12. The mask layer 12 also serves as a dicing region on the wafer, and also serves to prevent the porous layer from being broken by dicing.

The thermal insulating layer 13 is made of porous silicon (PS) including nanocrystalline silicon constituting the porous layer, and is formed by anodizing the surface of the silicon substrate 1. Further, the heat insulating layer 13 can be composed of a polymer material or a glass-based material. The thermal insulating layer 13 is disposed inside the outer peripheral portion surrounded by the mask layer 12.
The heating element thin film 14 is made of a metal material such as Al and functions as a heating electrode. The heating element thin film 14 is disposed on the heat insulating layer 13 and is formed in a band shape as an example. Specifically, the heating element thin film 14 is relatively thin compared to the heat insulating layer 13 and has a thickness of, for example, 30 nm. Since the heat insulating thin film 14 is formed in the recessed portion of the mask layer 12, the heating element thin film 14 is laid on the mask layer 12 in close contact with the heat insulating layer 13.

  The pads 151 and 152 are disposed so as to be in electrical contact with both end portions of the heating element thin film 14. The pads 151 and 152 extend to the outer peripheral portion of the mask layer 12. The wires 161 and 162 are bonded to the pads 151 and 152, respectively. The drive unit 17 generates a drive current and inputs the drive current to the heating element thin film 14.

Next, the driving operation of the pressure wave generator 1 according to the present invention will be described.
The drive unit 17 of the pressure wave generator 1 generates a drive current by performing signal processing such as AD conversion on an input signal input from a signal input unit (not shown in FIG. 1). More specifically, the drive unit 17 generates a pulse train modulated by the input signal. As will be described later, the drive current is composed of, for example, a pulse train whose amplitude is modulated.

  The drive unit 17 supplies drive current to the pads 151 and 152 via the wires 161 and 162, and supplies drive current to the heating element thin film 14. When a driving current is applied to the heating element thin film 14, a temperature change occurs in the heating element thin film 14, so that a temperature change occurs in the air layer in contact with the surface of the heating element thin film 14. Due to this temperature change, the air layer becomes dense, and pressure waves are generated by this density.

The waveform diagram of FIG. 2 shows an example of the drive current applied to the pressure wave generator according to the present invention. Here, a description will be given using FIG. 11 showing an example of a conventional driving current as appropriate.
As shown in FIG. 2, this drive current is a pulsed drive current, and more specifically, is composed of a pulse train whose amplitude is modulated. Accordingly, the drive unit 17 supplies a current in the form of the heating element thin film 14 via the wires 161 and 162 by supplying a pulsed drive current to the pads 151 and 152. By sufficiently shortening the pulse width, a desired signal can be output without applying a large load to the heating element thin film 14 which is a heating electrode.

  As shown in FIG. 2, the amplitude of the driving current pulse supplied to the pressure wave generator 1 is specifically a partial reproduction of a conventional driving current composed of an AC component and a DC component. is there. Therefore, the envelope at the top of this amplitude has a sine wave signal waveform that is substantially the same as the conventional drive current shown in FIG. That is, a sinusoidal signal is reproduced by a pulse train whose amplitude is modulated.

Next, an ultrasonic signal when the pressure wave generator 1 according to the present invention generates an ultrasonic wave as a pressure wave will be described with reference to FIGS. 3 and 4.
The graph of FIG. 3 shows an example of the frequency and sound pressure level of an ultrasonic wave that is generated when the same level of input is applied over a frequency region in the pressure wave generator 1 according to the present invention. In FIG. 3, the horizontal axis represents the input frequency, and the vertical axis represents the ultrasonic sound pressure level. FIG. 3 shows a flat frequency characteristic in the region of 30 kHz to 80 kHz, which means that the pressure wave generator 1 according to the present invention can generate ultrasonic waves with substantially the same magnitude for each frequency. ing.

FIG. 4 shows the results of frequency and sound pressure level when a sine wave is input by the driving operation of the pressure wave generator 1 according to the present invention. 4A shows the result when the input frequency is 50 kHz, and FIG. 4B shows the result when the input frequency is 40 kHz. Here, a pulse train having a pulse width of 0.25 μs is repeatedly applied at 400 kHz as a drive current.
As shown in FIGS. 4 (a) and 4 (b), in the pressure wave generator 1 according to the present invention, the sound pressure level of 70 dB is almost the same regardless of whether the input frequency is 40 kHz or 50 kHz. It is output.

Here, with reference to FIG. 5 and FIG. 6, the output characteristics when an ultrasonic wave is generated by supplying a driving current according to the present invention to a general pressure wave generator will be described. Here, an ultrasonic signal was measured using a piezoelectric element as a general pressure wave generator.
The graph of FIG. 5 shows the sound pressure level with respect to the frequency of ultrasonic waves generated by a general piezoelectric element when the same level of input is given over a certain frequency region. Unlike FIG. 3, resonance points are seen in the frequency characteristics, and the sound pressure level varies greatly with frequency.
FIG. 6 shows the result of applying the driving method according to the present invention to this piezoelectric element. Here, since the piezoelectric element is a linear system, a pulse whose amplitude corresponding to the AC component is modulated is applied instead of a driving current composed of a DC component and a sinusoidal AC component. FIG. 6A shows the result when the frequency of the AC component is 50 kHz, and FIG. 6B shows the result when the frequency of the AC component is 40 kHz.

  As shown in FIG. 5, the piezoelectric element used here has a resonance point at a frequency of 50 kHz. Therefore, as shown in FIG. 6A, this pressure wave generator outputs an ultrasonic wave having a maximum sound pressure level when the frequency of the AC component is 50 kHz. On the other hand, as shown in FIG. 6B, the piezoelectric element has the sound pressure level shown in FIG. 6A when the frequency of the AC component is 40 kHz which is shifted from the resonance frequency 50 kHz. In comparison, the output sound pressure level is drastically reduced.

  As can be seen from FIGS. 3 to 6, in the pressure wave generator 1 according to the present invention, it is shown that the waveform of the drive current is composed of a pulse train, and a sinusoidal signal waveform can be reproduced by this pulse train. This can be realized because the pressure wave generator 1 has a flat frequency characteristic and an impulse response is possible. Further, since it is a non-linear system, it is necessary to apply a large DC bias component when applied as a speaker. However, with the driving method according to the present invention, it is continuously driven to the heating element thin film 14 serving as a heating electrode. Since no current is applied, the load on the heating element thin film 14 can be significantly reduced. In the pressure wave generator 1 according to the present invention, the heating element thin film 14 is formed as thin as possible in order to reduce the heat capacity. Therefore, such a reduction in the load on the heating element thin film 14 has a remarkable effect.

  In the pressure wave generator 1 according to the present invention, a pressure wave having a high frequency corresponding to the repetition of a pulse is simultaneously output together with a pressure wave having a desired frequency. The high-frequency pressure wave that is output at the same time can be ignored because it is sufficiently higher than the pressure wave that is actually desired to be output. Further, in the ultrasonic range, a high frequency is attenuated by absorption. Therefore, if the distance is increased, this high frequency can be ignored.

Embodiment 2 of the Invention
In the second embodiment of the invention, a case where the pulse interval of the drive current is modulated will be described. Note that the pressure wave generator 1 is also used in the present embodiment, but since it is the same as in the first embodiment of the present invention, the description thereof is omitted here.
FIG. 7 shows an example of a pulsed drive current in the present embodiment. As shown in FIG. 7, the pulse-shaped drive current in this embodiment is composed of a pulse train in which the interval between the pulses is modulated. In other words, the density of the pulse of the pulse train is modulated, and the density of the pulse train reproduces the density of the pressure wave itself, which is a longitudinal wave resulting from the vibration of air with respect to atmospheric pressure. Therefore, a portion where the pulse interval of the pulse train is narrow corresponds to a longitudinal wave having a large amplitude, and conversely, a portion where the pulse interval is wide corresponds to a small amplitude. At this time, the amplitude of the drive current is not modulated, and a pulsed drive current is applied with a constant amplitude. Also in this embodiment, if the pulse width is sufficiently shortened, a desired signal can be output without applying a large load to the heating element thin film 14 that is a heating electrode.

  More specifically, the interval of each pulse of the drive current is modulated in a sine wave shape as in the conventional case, and is modulated like a longitudinal wave. This driving current reproduces a conventional driving current composed of an AC component and a DC component. Therefore, when the modulation of the interval between pulses is viewed as a longitudinal wave, the envelope of the peak of the amplitude becomes a sine wave signal waveform that is substantially the same as the conventional drive current shown in FIG. That is, a sinusoidal signal waveform is reproduced by a pulse train in which the interval between pulses is modulated.

  As described above, in the pulsed drive current in the present embodiment, the sinusoidal drive current is reproduced by the pulse train in which the interval between pulses is modulated. Even in such a case, the same effect as the drive current in the first embodiment can be obtained.

Embodiment 3 of the Invention
In the third embodiment of the present invention, a case where the pulse width of the drive current is modulated will be described. Note that the pressure wave generator 1 is also used in the present embodiment, but since it is the same as in the first embodiment of the present invention, the description thereof is omitted here.
FIG. 8 shows an example of a pulsed drive current in the present embodiment. As shown in FIG. 8, the pulsed drive current in the present embodiment is composed of a pulse train in which the pulse width of each pulse is modulated. In other words, the pulse width of the pulse train is modulated, and the density of the pressure wave itself, which is a longitudinal wave generated from the vibration of air with respect to atmospheric pressure, is reproduced by the magnitude of the pulse width. Accordingly, a portion where the pulse width of the pulse train is large corresponds to a longitudinal wave having a large amplitude, and conversely, a portion where the pulse width is small corresponds to a small amplitude. At this time, the amplitude of the drive current is not modulated, and a pulsed drive current is supplied with a constant amplitude.

  More specifically, the pulse width of each pulse of the drive current is modulated in a sine wave shape as in the prior art, and is modulated like a longitudinal wave. This drive current is a reproduction of a conventional drive current composed of an AC component and a DC component. Therefore, when the modulation of the pulse width is viewed as a longitudinal wave, the envelope at the peak of the amplitude becomes a sine wave signal waveform that is substantially the same as the conventional driving current shown in FIG. 11, and the pulse width is modulated. A sinusoidal signal waveform is reproduced by the pulse train.

  As described above, in the pulsed drive current in the present embodiment, the sinusoidal drive current is reproduced by the pulse train whose pulse width is modulated. Even in such a case, the same effect as the drive current in the first embodiment can be obtained.

  In the present embodiment, since modulation is performed with the width of each pulse, the pulse width may not be sufficiently reduced depending on the input. In this case, since a load may be applied to the heating element thin film 14 that is an electrode, it may be necessary to adjust the interval of each pulse, the change rate of the modulation width of the pulse width, and the like.

It is a schematic diagram which shows one structural example of the pressure wave generator which concerns on this invention. It is a graph which shows an example of the drive current waveform of the pressure wave generator concerning the present invention. It is a graph which shows an example of the output characteristic of the pressure wave generator concerning the present invention. It is a graph which shows an example of the sound pressure level of the pressure wave generator which concerns on this invention. It is a graph which shows an example of the output characteristic of the pressure wave generator using a piezoelectric element. It is a graph which shows an example of the sound pressure level of the pressure wave generator using a piezoelectric element. It is a graph which shows another example of the drive current waveform of the pressure wave generator which concerns on this invention. It is a graph which shows another example of the drive current waveform of the pressure wave generator which concerns on this invention. It is a partial schematic diagram which shows an example of the conventional pressure wave generator. It is a graph which shows an example of the drive current waveform of the conventional pressure wave generator. It is a graph which shows another example of the drive current of the conventional pressure wave generator.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Silicon substrate, 12 ... Mask layer, 13 ... Thermal insulation layer, 14 ... Heat generating body thin film,
151, 152 ... Pad, 161, 162 ... Wire, 17 ... Drive unit

Claims (2)

A pressure wave generator for generating pressure waves,
A substrate,
A thermal insulation layer disposed on the substrate;
A heating electrode disposed on the thermal insulation layer;
A drive unit connected to the heat generating electrode and generating a drive current composed of a modulated pulse train, and
The drive unit takes in an input signal composed of an AC component and a DC component,
The modulation converts a pulse train having the same height as the DC component into a pulse train based on the AC component by any one of amplitude modulation, density modulation, and width modulation,
A pressure wave generator characterized in that a load on the heating electrode is reduced by avoiding continuous current application to the heating electrode . A method of driving a pressure wave generator for generating a pressure wave,
Capturing an input signal composed of an AC component and a DC component;
Converting a pulse train having the same height as the direct current component into a pulse train based on the alternating current component by any one of amplitude modulation, density modulation and width modulation;
Applying the modulated pulse train to a heating electrode,
A method of driving a pressure wave generator, wherein a load on the heating electrode is reduced by avoiding continuous application of current to the heating electrode .

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