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(Industrial field of application) The present invention is mounted on a robot or various industrial
devices to measure the distance to a target object at a relatively short distance, or to use an
ultrasonic distance signal of the target object by phased array processing. The present invention
relates to an ultrasonic transducer for recognizing its size and shape by visualizing the 2.
Description of the Related Art In recent years, inexpensive and high-performance
microcomputers have become widespread, and by using them, automation or robotization is
being promoted in various industrial fields. However, robots or automatic devices currently put
into practical use can only lift and carry objects of a certain shape, a certain size, or a certain
weight, or work such as processing 0 assembly only by means of a certain program. The current
situation is that it can not be done. On the other hand, the diversification of the consumer
segment has made the trend of multi-kind, low-place souvenirs stronger, and the development of
automation technology called FMS (Flexible Manufacturing System) has been called for. In such a
flow of automation, a robot or an automatic machine needs to be equipped with various sensors
that replace human sense organs, and perform appropriate actions based on such information.
When the robot thinks that it lifts an object, it first finds the object with the visual sensor, then
approaches the object with the proximity sensor, and finally grasps the hardness or the object of
the object based on the information of the tactile sensor Know what you are doing and lift the
object without dropping it. Among these sensors, a proximity sensor is for obtaining near
distance information that can not be captured by a visual sensor, and it is considered that
ultrasonic sensors are suitable for distance measurement accuracy. Furthermore, in order to
image an object in a full invisible environment such as smoke, an ultrasonic short distance sensor
is essential. There are many commercially available ultrasonic transducers of this type that use
resonance of a piezoelectric material such as PZT (lead zirconate titanate), but an array requiring
narrow frequency characteristics and uniform characteristics Has the disadvantage of being
difficult to As an electrostatic ultrasonic transducer, for example, Japanese Patent Application No.
60-289290 or the IEEE 1986 Ultrasonics Symposium Proceedings held in 1986, pages 559 to
562, and 1987. On pages 414 to 417 of Transducers '87 Digest of Technical Papers held, there
are new proposals by Higuchi, Suzuki, Tani, et al.
The typical structure is shown in FIG. In the figure, reference numeral 101 denotes a silicon
substrate, reference numeral 102 denotes a silicon oxide film, reference numeral 103 denotes a
lower electrode, and reference numeral 107 denotes a hole for a gas reservoir formed by Hdirectional etching. Reference numeral 104 denotes a CVD silicon oxide film, and reference
numeral 106 denotes a polyester film on which an aluminum film to be the upper electrode 105
is vapor-deposited. The operating principle is similar to so-called condenser speakers and
condenser microphones. That is, a direct current bias of about 30 V to 100 V is applied between
the lower electrode 103 and the upper electrode 105, and when an alternating current signal is
applied, the polyester film 106 vibrates and the gas (here, air) trapped in the gas reservoir 107.
Ultrasonic waves are transmitted into the air by the spring action. On the other hand, when an
ultrasonic wave comes from the outside, the polyester film 106 is vibrated by the pressure
change, and the capacitance of a pair of capacitors formed by the upper electrode 105 and the
lower electrode 103 is changed, and the change is converted into an electric signal. It is possible
to receive ultrasound signals. In particular, when the lower electrode 103 is an array electrode
and AC signals of different phases are manually applied to each electrode, ultrasonic waves of
different phases are emitted from each ultrasonic element and the composite wave can be
directed forward in any direction by its phase. it can. Also, the ultrasonic signal input from any
direction can be reproduced as a distance image by delay combining of the electric signals from
the respective electrodes. (Problems to be Solved by the Invention) In the above, the conventional
electrostatic ultrasonic transducer has been described using an example. In this type of ultrasonic
transducer, the amplification unit and the signal processing unit can be formed on the same
silicon substrate, and the frequency band can be controlled by changing the size of a large
number of holes precisely formed on the silicon surface. It has an excellent feature of being able
to However, the transmission / reception sensitivity of the sensor is proportional to the DC bias
voltage. For this reason, there is a disadvantage that a DC bias power supply having a higher
voltage than a few volts to a few tens volts used as a power supply of a normal IC or a transistor
is required. An object of the present invention is to provide a novel ultrasonic transducer having
a self-biasing function without requiring a DC bias power supply for driving an electrostatic
ultrasonic transducer, and a method of manufacturing the same. It is in. (Means for Solving the
Problems) According to the present invention, the insulating thin film between the first and
second electrodes is an engineering lefttret insulating film when there is a hole on the surface of
the semiconductor substrate. An ultrasonic transducer is obtained.
Furthermore, according to the present invention, the method includes the steps of injecting
charges and polarization by making holes on the surface of a semiconductor substrate, and
forming a second electrode formed of a conductive thin film thereon. A method of manufacturing
an ultrasound transducer is obtained. (Operation) In the ultrasonic transducer according to the
present invention, the surface potential is raised by injecting charges into the insulator thin film
between the lower electrode and the upper electrode to polarize them, and an effect of
equivalently giving a DC bias That is, to realize self-bias. Therefore, no DC bias power supply is
required. A problem in producing an electret insulating film is the lifetime of the electret
insulating film. If the injected charge moves immediately, it will not remain as a permanent
charge, and will be in an electrically neutral state. The electret insulating film produced by
electron beam irradiation or corona discharge has a long life. Although the reason for this is not
clear, it is presumed that the surface of the insulating film is damaged at the time of charge
injection, a very deep energy level is generated, and electrons are trapped there, so that the
electrons are difficult to move. (Embodiment) Hereinafter, an embodiment of the present
invention will be described using the drawings. First, as shown in FIG. 1 (a), a thermal oxide film
100 is formed on the surface of a p-type (ioo) oriented silicon substrate 101, and the thermal
oxide film in a portion to be a hole for air reservoir 107 is removed. Anisotropic etching of silicon
with hydrazine solution. After removing the thermal oxide film 100 on the surface, as shown in
FIG. 1B, a thermal oxide film 102 is newly formed, and a lower electrode 103 is formed of
aluminum or the like. Further, a CVD silicon oxide film 104 to be an interlayer insulating film
with the upper electrode is deposited. FIG. 1 (C) shows the most important manufacturing steps
for realizing the structure of the present invention. While heating the sample shown in FIG. 1 (b)
at 300 ° C., the surface is irradiated with an electron beam at an acceleration voltage of about 5
kV to inject electrons into the CVD silicon oxide film 104 for polarization. That is, an electret
silicon oxide film is formed. It was about 1000 V when the surface potential of the electret oxide
film was measured with a vibrating capacitance type surface voltmeter. After that, as shown in
FIG. 1D, a polyester film 106 on which an aluminum layer to be the upper electrode 105 is
vapor-deposited on the back surface is stretched on the electret silicon oxide film 108. The
ultrasonic transducer 200 of the present invention thus obtained was subjected to an experiment
of ultrasonic wave transmission by the drive circuit shown in FIG. 2 (a) to measure the frequency
dependency of the transmission sensitivity.
FIG. 2 (b) shows a drive circuit when driving a conventional ultrasonic transducer 204, and a DC
bias 206 is added via a resistor. The result shown by the solid line in FIG. 3 is the frequency
characteristic by the ultrasonic transducer 200 of the present invention, and the broken line is
the frequency characteristic when the conventional ultrasonic transducer 204 is driven under a
direct current bias of 100 V. . In the case of the ultrasonic transducer according to the present
invention, it has been found that not only the DC bias power source is not required at the time of
driving but also the sensitivity is improved. Accordingly, there is also an advantage that the
distance measurement range is broadened. A sensitivity increase of about 10 dB is obtained as
the transmission sensitivity. However, 1 μbar / IV was used as a reference here as O dB. The
reason why the sensitivity is improved is assumed to be that the surface potential of the electret
oxide film is about 1000 V as described above, and the same effect as obtained by using a DC
bias power supply of 100 OV equivalently obtained. Be done. If it is attempted to obtain the same
effect by adding a direct current bias as in the prior art, the power supply becomes extremely
large. The lifetime of the electret oxide film is 20 to 30 years, which is sufficiently long for
practical use. Although the CVD silicon oxide film is used as the electret insulating film in this
embodiment, the same effect can be obtained by using the CVD silicon nitride film. Furthermore,
although charge injection was carried out using an electron beam at the time of electret
formation, the sample may be put into corona discharge to carry out charge injection. Although
the lower electrode is described as the array electrode in this embodiment, it is obvious that the
lower electrode may be one connected electrode. (Effects of the Invention) As described above,
the ultrasonic array transducer according to the structure of the present invention can be
produced by an extremely simple process and is necessary for driving a conventional ultrasonic
transducer. The DC bias power supply is no longer needed.
Brief description of the drawings
FIG. 1 is a view showing an embodiment of the present invention.
1 (a) to 1 (d) are cross-sectional views showing the manufacturing process. FIG. 2 (a) is a diagram
showing an example of a drive circuit of the ultrasonic transducer obtained in this manner. FIG. 2
(b) is a diagram showing an example of a conventional ultrasonic transducer drive circuit. FIG. 3
is a diagram showing the frequency characteristic result. FIG. 4 is a cross-sectional view of a
conventional capacitive ultrasonic transducer. In the figure, reference numeral 101: silicon
substrate, 100. 102: thermal oxide film, 103: lower electrode, 104: CvD silicon oxide film, 105:
upper electrode, 106: polyester film, 108: electret silicon oxide film.
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