In the past, an ultrasonic wave generator using mechanical vibrations due to the piezoelectric effect has been widely known. As this kind of ultrasonic wave generator, for example, there is a structure where electrodes are formed on both surfaces of a crystal of a piezoelectric material such as barium titanate. The mechanical vibrations obtained by applying an electric energy between the electrodes generate the ultrasonic wave in a surrounding medium (e.g., air). However, since the above-mentioned ultrasonic wave generator has a characteristic resonance frequency, there are problems that the frequency band becomes narrow, and it is susceptible to external vibrations or fluctuations of outside air pressure.
On the other hand, in recent years, a pressure wave generator capable of generating a pressure wave such as ultrasonic wave in a medium without using mechanical vibrations is attractive. For example, a pressure wave generator disclosed in Japanese Patent Early Publication No. 11-300274 is equipped with a single crystal silicon used as a substrate, a porous silicon layer formed as a heat insulating layer on the substrate, an aluminum film formed as a heat generating layer on the heat insulating layer, and a pair of pads electrically connected to the heat generating layer. In this pressure wave generator, when an electric energy is applied to the heat generating layer through the pads, a temperature change occurs in the heat generating layer in response to a driving input waveform, i.e., a driving voltage waveform or a driving current waveform. This temperature change of the heat generating layer causes, through a heat exchange between the heat generating layer and a medium (e.g., air) in the vicinity of the device, expansion and contraction of the medium in a thermally induced manner. As a result, the pressure wave is generated in the medium.
However, in the case of using this kind of thermally induced type pressure wave generator in the air, it is known that there is a phenomenon that an efficiency defined as a ratio of sound pressure of the generated compression wave relative to the input power reduces over time. That is, when oxidation of the porous silicon layer proceeds by the influence of oxygen and moisture in the air, the heat insulating property of the porous silicon layer deteriorates, so that a reduction in the aforementioned efficiency happens.
In this regard, when it is assumed that a condition for driving the above pressure wave generator (i.e., an input power applied to the heat generating layer) is constant, the sound pressure of the generated compression wave reduces due to an increase over time in heat conductivity of the heat insulating layer or an increase over time in heat capacity per unit volume thereof. Therefore, when the pressure wave generator is used as a wave sending device for a reflection-type ultrasonic sensor, the maximum measurable distance reduces (i.e., the detection area becomes narrow). As a result, there is a case that an object can not be detected. In addition, when the pressure wave generator is used as a speaker, there is a problem that the sound pressure reduces. The above-described change over time of the porous silicon layer is a phenomenon caused irrespective of conditions for forming the porous silicon layer.
In addition, since the heat generating layer that is an electrical resistive element is formed on the porous silicon layer, the heat generating layer partially reacts with the porous silicon layer when the pressure wave generator is used for an extended time period, so that a leak current may locally flow through a resistance reduced portion. Furthermore, when a conductive path is formed through the silicon substrate, an electric current having a very large current density locally flows. This phenomenon easily happens in the case of increasing the input power applied to the pressure wave generator to obtain a large sound pressure. As a result, the pressure wave generator may have a breakdown due to burn out of the heat generating layer.
In the above, it was explained about the case characteristics of the heat insulating layer of porous silicon deteriorate due to a reaction with oxygen in the air. On the other hand, even when the heat insulating layer is made of an inactive material such as porous silica and porous alumina, it is expected that a change over time in heat conductivity or heat capacity per unit volume of the heat insulating layer is caused by adsorption or adherence of the moisture in the air and the other impurities.
Thus, from the viewpoint of solving various kinds of defects caused by diffusion of components (principally air) of the surrounding medium into the heat insulating layer, conventional pressure wave generators still have plenty of room for improvement.