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JPH11337538

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DESCRIPTION JPH11337538
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
ultrasonic probe, and more particularly to an ultrasonic probe having a structure suitable for
increasing the frequency to be used.
[0002]
2. Description of the Related Art Conventionally, an inspection method using ultrasonic waves is
known as one of nondestructive inspection methods. Ultrasonography can detect microdefects
present inside the inspection object. Today, development of chip size package (CSP) and the like
is in progress, and further development of surface mount devices (SMD) is also in progress.
Under these current developments, the need for detection of the above minute defects is
extremely high. In addition, the minute degree of the minute defect is also extremely small
corresponding to the miniaturization of the object to be inspected, and therefore, it is used when
detecting the minute defect in the object to be inspected using ultrasonic inspection. Increasing
the frequency of ultrasonic waves is essential.
[0003]
In the above-mentioned ultrasonic inspection method, an ultrasonic probe (or ultrasonic probe) is
used as a device configuration. The ultrasonic probe has an ultrasonic transducer, ie, a
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piezoelectric vibrator, and is excited by applying a pulse alternating electric signal to the
piezoelectric vibrator so as to generate high frequency ultrasonic waves. The ultrasonic waves
generated by the piezoelectric transducer propagate in the acoustic lens and are further emitted
into a medium that propagates the ultrasonic waves. The frequency of the ultrasonic wave output
from the piezoelectric vibrator conventionally used in the ultrasonic nondestructive inspection
was generally in the range of 1 to 15 MHz. However, in the case of detecting an extremely
minute defect as described above, the frequency of the ultrasonic wave must be made higher.
However, various problems occur when the frequency of ultrasonic waves is higher than 15 MHz.
Problems arise, for example, the attenuation of ultrasound in the coupling medium between the
piezoelectric transducer and the object to be examined, or the exponential attenuation of
ultrasound in the object to be examined itself. Therefore, for example, in the case of using a
piezoelectric vibrator that generates ultrasonic waves having a high frequency of 40 MHz or
more, it is required that the generated signal strength of the piezoelectric vibrator is high, that is,
the sensitivity attenuation is small.
[0004]
As a study in consideration of the above problems regarding the piezoelectric vibrator (ultrasonic
transducer) used for an ultrasonic probe for ultrasonic nondestructive inspection, the effect of · · ·
described in Ultrasonic 1994 Vol 32 No. 2 The paper on Active Diameter and Damping on the
Performance of Ultrasonic Transducers was submitted earlier, where Are discussed. In the
contents of this paper, in order to increase the S / N ratio of a device operating at a high
frequency, the optimum electrode diameter d of the electrode attached to the piezoelectric
vibrator is determined by the equation (1) in the paper.
[0005]
According to the contents described in the above-mentioned article, when experimentally tried, it
is true that the S / N ratio is good in the range where the frequency of ultrasonic waves is smaller
than 40 to 50 MHz. The piezoelectric vibrator having the above-mentioned structure can be
obtained, that is, a piezoelectric vibrator that outputs an ultrasonic wave of high signal strength.
However, in the case of generating an ultrasonic wave of a further higher frequency, for example,
a frequency higher than 50 MHz, attenuation of the ultrasonic wave becomes remarkable in
terms of detection sensitivity, and a piezoelectric vibrator that achieves sufficiently good
detection sensitivity is obtained. I had the problem that I could not do it.
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[0006]
The object of the present invention is to solve the above-mentioned problems, and in particular
to provide a highly practical ultrasonic probe which can achieve sufficient detection sensitivity
even by using high frequency ultrasonic waves of 50 MHz or higher. is there.
[0007]
SUMMARY OF THE INVENTION In order to achieve the above object, the ultrasonic probe
according to the present invention is configured as follows.
The first ultrasonic probe (corresponding to claim 1) has an acoustic lens, and a piezoelectric
vibrator (an upper electrode and a lower electrode are sandwiched between an upper electrode
and a lower electrode on an extraction electrode provided on one surface of the acoustic lens).
And the diameter d of the upper electrode is determined so as to satisfy the equation d = Cx [F0 t
/ (ζr F2 Xc)] 1/2. It is characterized by In the above equation, Cx is a constant, F0 t is a
frequency constant that is an acoustic characteristic of the piezoelectric vibrator, ζr is a
combined dielectric constant of the piezoelectric vibrator and the bonding material, F is a
frequency used in the probe, Xc Is the measurement system impedance. In the above-mentioned
ultrasonic probe, when an exciting voltage is applied between the upper electrode and the lower
electrode from a well-known ultrasonic pulser, the piezoelectric vibrator is excited to generate an
ultrasonic wave. The generated ultrasonic waves propagate in the acoustic lens, and the
ultrasonic waves are emitted from the lower lens surface (ultrasonic emission surface) to the
inspection object. In the above configuration, since the above equation is satisfied with respect to
the electrode diameter (diameter) of the upper electrode, it is possible to use a value ζ r which is
a combination of the dielectric constant of the piezoelectric vibrator and the dielectric constant of
the bonding material Therefore, the diameter of the upper electrode can be optimized. This
enables ultrasonic detection with a good S / N ratio even if the frequency of the ultrasonic waves
is 50 MHz or more. The above-mentioned bonding material is used for low temperature bonding
of the piezoelectric vibrator and the acoustic lens, and relieves unevenness of the bonding
surface. Further, the dielectric constant component of the bonding material contributes to the
improvement of detection sensitivity. A second ultrasonic probe (corresponding to claim 2) is
characterized in that, in the first configuration, the bonding material is preferably lead. The lead
is in the form of a film and preferably has a thickness of about 3 μm. When lead is used,
bonding is performed at a low temperature of about 150 ° C. The third ultrasonic probe
(corresponding to claim 3) is, in the second configuration, the thin film preferably formed by a
sputtering method. A film of lead is formed by sputter deposition, which produces a desired
dielectric constant in the bonding material.
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[0008]
BEST MODE FOR CARRYING OUT THE INVENTION Preferred embodiments of the present
invention will be described below with reference to the attached drawings.
[0009]
First, a typical structure of an ultrasonic probe will be described with reference to FIG.
FIG. 1 is a side view showing the main part of the ultrasonic probe in cross section. In this figure,
11 is a housing of the ultrasonic probe, and the housing 11 has a substantially cylindrical shape.
An acoustic lens 12 is provided on the lower side inside the housing 11. The acoustic lens 12 has
a cylindrical shape as a whole, and the lower surface 12 a thereof is an ultrasonic wave emitting
surface (lens surface). The acoustic lens 12 is preferably a lens made of quartz. The acoustic lens
12 is inserted from the lower opening of the cylindrical housing 11 and fixed in the housing 11
by interposing a fixing resin (resin) 13 with the inner surface of the housing 11.
[0010]
On the other hand, the piezoelectric vibrator 14 is provided on the upper surface of the acoustic
lens 12. The piezoelectric transducer 14 is an ultrasonic transducer that performs mutual
conversion between an electric signal and an ultrasonic wave. The structure for mounting the
piezoelectric vibrator 14 on the acoustic lens 12 is as follows. First, the lower extraction
electrode 15 is provided on the upper surface of the acoustic lens 12. On the lower extraction
electrode 15, the above-described piezoelectric vibrator 14 provided with the upper electrode 16
and the lower electrode 17 for excitation is provided. A bonding material 18 is provided between
the lower electrode 17 of the piezoelectric vibrator 14 and the lower extraction electrode 15. The
piezoelectric vibrator 14 is fixed on the acoustic lens 12 by the bonding material 18.
[0011]
In the above structure, the planar shapes of the lower extraction electrode 15, the piezoelectric
vibrator 14, the upper electrode 16, and the lower electrode 17 are circular. The piezoelectric
vibrator 14 has a flat plate-like (disk-like) form, and the bonding material 18 has a very thin film-
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like form. In FIG. 1 or FIG. 2 described later, the thickness of each part is exaggerated for
convenience of explanation. Furthermore, the lower extraction electrode 15, the upper electrode
16 and the lower electrode 17 are made of a conductive material.
[0012]
A connector portion 19 is provided outside the upper end portion of the housing 11. The
connector unit 19 is connected to a pulsar (not shown) that outputs a pulsating electrical signal
of a required frequency necessary for generating an ultrasonic wave. The circuit 20 provided in
the housing 11 is a waveform shaping circuit. The waveform shaping circuit 20 is connected to
the connector portion 19 by the ground lead 21 and the signal lead 22 while being connected to
the housing 11 by the other ground lead 23 and further connected to the upper electrode 16 by
the signal lead 24. ing. The housing 11 and the lower extraction electrode 15 are connected by
the lead wire 25 and both are at the same potential. In FIG. 1, 26 is a solder connection, and 27 is
a conductive paste connection.
[0013]
According to the structure of the ultrasonic probe shown in FIG. 1, the excitation electric signal
(pulsed AC signal of the required frequency) for generating the ultrasonic wave sent from the
pulsar has the connector portion 19, waveform shaping The upper electrode 16 is provided
through the circuit 20. When a pulse-like alternating voltage signal for excitation drive is applied
between the upper electrode 16 and the lower electrode 17, the piezoelectric vibrator 14
generates a vibration to convert an electrical signal into an ultrasonic wave. The ultrasonic wave
generated by the piezoelectric vibrator 14 propagates to the acoustic lens 12 via the bonding
material 18 and is emitted downward from the ultrasonic wave emitting surface 12 a while the
phase is controlled in the acoustic lens 12. At the time of normal inspection, an inspection object
is present below, and a coupling medium (such as water) for propagating ultrasonic waves is
present between the acoustic lens 12 and the inspection object. Therefore, the ultrasonic wave
emitted from the ultrasonic wave emitting surface 12a of the acoustic lens 12 travels toward the
inspection object, and receives the ultrasonic wave reflected from the inspection object again.
The received ultrasonic waves are extracted by the lower extraction electrode 15 and given to
the piezoelectric vibrator 14 via the bonding material 18. The piezoelectric vibrator 14 converts
the vibration due to the ultrasonic wave into an electric signal and outputs the electric signal, as
opposed to the action described above. The output electric signal is taken out by the signal lead
wire 24, is shaped by the waveform shaping circuit 20, is further taken out through the signal
lead wire 22, and is outputted from the connector unit 19 to the electric circuit unit for signal
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processing. The general structure and operation of the ultrasonic probe used for the abovementioned ultrasonic nondestructive inspection are what is known conventionally.
[0014]
Next, the characteristic structure of the ultrasonic probe according to the present embodiment
will be described. The features are shown in FIG. The structure shown in FIG. 2 is a structure of a
portion to which the piezoelectric vibrator 14 described above is attached. As a structure of a
portion to which the piezoelectric vibrator 14 is attached, the upper electrode 16 is provided on
the upper surface side of the piezoelectric vibrator 14 and the lower electrode 17 is provided on
the lower surface side, and between the lower extraction electrode 15 provided on the upper
surface of the acoustic lens 12. The structure has a bonding material 18 interposed
therebetween. Here, the planar shapes of the piezoelectric vibrator 14, the lower electrode 17,
and the bonding material 18 are circular and substantially the same in size. On the other hand,
the planar shape of the upper electrode 16 is also circular, and its diameter (d) is set to be
smaller than the diameter of the piezoelectric vibrator 14. The upper electrode 16 and the
piezoelectric vibrator 14 are in concentric positional relationship in which their centers coincide
in their planar positional relationship, and only the diameter of the upper electrode 16 is set
smaller than the diameter of the piezoelectric vibrator 14 There is. What is important here is how
to determine the diameter d of the upper electrode 16. By determining how to determine the
diameter d of the upper electrode in accordance with the equation as described later, the
maximum detection sensitivity can be achieved with respect to the high frequency of ultrasonic
waves (high frequency of 50 MHz or more) which is a feature of the present invention. By using
the upper electrode 16 formed to have such a diameter d, it becomes possible to detect a minute
defect with extremely high detection sensitivity even if the frequency of the ultrasonic wave is
increased.
[0015]
In the laminated structure including the piezoelectric vibrator 14 and the like shown in FIG. 2,
the bonding material 18 is an indispensable member for bonding the piezoelectric vibrator 14
and the acoustic lens 12 at a low temperature. For example, a film of lead (for example, 3 μm in
thickness) is used as the bonding material 18. The film-like lead as such a bonding material 18 is
preferably formed by a sputtering film forming method. The bonding material 18 is made to have
a desired dielectric constant by a sputter deposition method, and the dielectric constant
component of the bonding material 18 is an important factor for determining the electrode
diameter d as apparent from the following equation. The lower extraction electrode 15 formed
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on the upper surface of the acoustic lens 12 is formed, for example, by vapor deposition of
chromium gold.
[0016]
Next, a formula for determining the diameter d (electrode diameter) of the upper electrode 16
will be shown.
[0017]
[Equation 1] d = Cx [F0 t / (ζr F2 Xc)] 1 /2
[0018]
In the above equation, Cx is a constant, F0 t is a frequency constant that is an acoustic
characteristic of the piezoelectric vibrator 14, ζr is a combined dielectric constant of the
piezoelectric vibrator 14 and the bonding material 18, F is a frequency at the ultrasonic probe,
Xc is the measurement system impedance.
[0019]
The electrode diameter represented by the above equation, that is, the above-mentioned d, shows
the relationship between the frequency and the electrode at which the maximum sensitivity can
be obtained in the ultrasonic probe shown in FIG.
In determining the diameter of the upper electrode 16, that is, the electrode diameter d in the
above equation, an optimal electrode diameter d can be obtained by determining the dielectric
constant of each of the piezoelectric vibrator 14 and the bonding material 18.
Here, the synthetic dielectric constant ζ r will be described.
As the piezoelectric vibrator 14, for example, a piezoelectric vibrator having a trade name PC11
manufactured by Hitachi Metals, Ltd. is used. The dielectric constant in this case is the dielectric
constant of the ceramic alone. The bonding material 18 is provided below the piezoelectric
vibrator 14. The electrostatic capacitance determined by the piezoelectric vibrator 14 and the
bonding material 18 is determined by a relationship in which two electrostatic capacitances are
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connected in series in consideration of an equivalent circuit. When two capacitances in such a
relationship are combined, it is possible to think of a combined capacitance based on an equation
of series connection of normal capacitances. The value of the capacitance is given as the product
of the dielectric constant and the area divided by its thickness, so the area and thickness are
known, and the resulting composite capacitance is used to determine When the calculation is
performed according to the equation, the combined dielectric constant can be determined. In any
case, the piezoelectric vibrator 14 has an inherent dielectric constant, and the lead bonding
material 18 formed by sputtering deposition as described above has an inherent dielectric
constant due to its film characteristics, as shown in FIG. By making the illustrated structure, the
combined dielectric constant is determined based on the dielectric constants of the piezoelectric
vibrator 14 and the bonding material 18. As a result, the resultant synthetic dielectric constant
acts as an important factor for determining the diameter d of the upper electrode 16 as ζ r in
the above equation. By forming the upper electrode 16 with the electrode diameter determined
based on this equation, it is possible to obtain high detection sensitivity for high frequency
ultrasonic waves of 50 MHz or higher in the ultrasonic probe.
[0020]
Here, when the piezoelectric vibrator 14 is the PC 11 and the bonding material 18 is a layer (PP)
of lead, the respective dielectric constants and the specific numerical values of the combined
dielectric constants are shown in the table in FIG. The specific numerical values shown in FIG. 6
are measured at a frequency of 88 MHz.
[0021]
Next, referring to the graphs shown in FIG. 3 to FIG. 5, according to the ultrasonic probe based
on the present invention characterized by the above equation (Equation 1), high frequency
formation of ultrasonic waves (50 MHz or more) It is explained that sufficient sensitivity
characteristics can be obtained for It is FIG. 3 which showed the graph based on the relational
expression (1) described in the paper mentioned above. The graph shown in FIG. 3 shows the
relationship between the frequency at which the maximum sensitivity is obtained and the
electrode diameter. According to this graph, it can be seen that the electrode diameter decreases
as the frequency increases. Then, the graph which measured the receiving voltage by changing
an electrode diameter in each of 90 MHz (frequency at the time of manufacture of a 75 MHz
ultrasonic probe), 80 MHz, and 110 MHz is shown in FIG. In FIG. 4, the graph 41 is a graph near
80 MHz, the graph 42 is a graph near 90 MHz, and the graph 43 is a graph near 110 MHz. In
each of the graphs of FIG. 4, a portion indicated by a broken line is an approximate curve. It was
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found that the maximum sensitivity can not be obtained according to the electrode diameter
obtained by the equation for obtaining the electrode diameter of the above-mentioned article in
the region of three frequencies shown in FIG. Therefore, in the ultrasonic probe according to the
present invention, the equation for obtaining the electrode diameter of the above-mentioned
article was reviewed by considering the bonding material 18, and the above equation (Equation
1) was found. A graph based on the above equation (Equation 1) is shown in FIG. In the graph
shown in FIG. 5, the graph 51 is a graph in the case of the piezoelectric vibrator alone, and the
graph 52 is a graph regarding the ultrasonic probe as a whole in consideration of the
piezoelectric vibrator 14 and the bonding material 18. The graph 52 takes into consideration
that the dielectric constant of the piezoelectric vibrator 14 and the dielectric constant of the
bonding material 18 are combined. Thus, according to the equation (equation 1) set up using the
synthetic dielectric constant of the piezoelectric vibrator 14 and the bonding material 18, the
experimental result shown in the graph of FIG. 5 (experimental value “•”) It was confirmed
that they match well. In this way, the electrode diameter d of the upper electrode 16 is given by
the above equation (Equation 1) with respect to increasing the frequency of ultrasonic waves (50
MHz or more), that is, the combined permittivity of the piezoelectric vibrator 14 and the bonding
material 18 Based on the equation, it has been confirmed that an ultrasonic probe capable of
obtaining a highly practical maximum sensitivity can be realized.
[0022]
Although the bonding material 18 is formed by the sputter deposition method in the above
embodiment, the present invention is not limited to this method. Although the electrode diameter
d is the diameter of the upper electrode 16, it is not limited to this.
[0023]
As is apparent from the above description, according to the present invention, the electrode
diameter of the piezoelectric vibrator provided on the acoustic lens contained in the ultrasonic
probe is the dielectric constant of the piezoelectric vibrator itself. And the bonding constant for
fixing the piezoelectric vibrator, and by using the combined dielectric constant of the two, it is
possible to obtain the maximum sensitivity at high frequency of 50 MHz or more, and also to
maximize the detection sensitivity. By this, it is possible to effectively detect minute defects and
to realize a highly practical ultrasonic probe.
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