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JP2007037005

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DESCRIPTION JP2007037005
The present invention provides an ultrasonic sensor with improved detection sensitivity as
compared to the prior art. A diaphragm (1) formed into a thin film shape by processing a
semiconductor substrate, a frame (2) made of a semiconductor substrate and surrounding and
supporting the periphery of the diaphragm (1), and a thin film provided closely to the surface of
the diaphragm (1) Of the lower electrode 3, the thin film piezoelectric film 4 provided in close
contact with the surface of the lower electrode 3 opposite to the diaphragm 1, and the thin film
provided in close contact with the surface of the lower electrode 3 opposite the piezoelectric film
4. And an upper electrode 5. Since the upper electrode 5, the piezoelectric film 4 and the lower
electrode 3 are provided on the surface of the diaphragm 1 in a narrower range than the
diaphragm 1, the residual stress of the upper electrode 5, the piezoelectric film 4 and the lower
electrode 3 affects the diaphragm 1 The smaller the diaphragm 1 is, the smaller the diaphragm 1
is. As a result, the detection sensitivity can be improved as compared with the prior art. [Selected
figure] Figure 1
Ultrasonic sensor
[0001]
The present invention relates to an ultrasonic sensor formed by processing a semiconductor
substrate, and more particularly to a piezoelectric ultrasonic sensor utilizing the piezoelectric
effect.
[0002]
Conventionally, as an ultrasonic sensor for detecting an ultrasonic wave, a piezoelectric sensor
formed by processing a semiconductor substrate has been provided (see, for example, Patent
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Document 1).
[0003]
FIG. 3 is a side sectional view showing an example of a conventional ultrasonic sensor.
In this conventional example, a diaphragm 1 formed into a thin film shape by processing a
semiconductor substrate, a frame 2 made of a semiconductor substrate and surrounding and
supporting the periphery of the diaphragm 1, and a thin film provided in close contact with the
surface of the diaphragm 1 Lower electrode 3, a thin film piezoelectric film 4 provided in close
contact with the surface of the lower electrode 3 opposite to the diaphragm 1, and a thin film
provided in close contact with the surface of the lower electrode 3 opposite the piezoelectric film
4 And the upper electrode 5 of
The piezoelectric film 4 is formed by depositing a material generating the piezoelectric effect, for
example, lead zirconate titanate (PZT) on the surface of the lower electrode 3 by a sol-gel
method.
[0004]
Thus, the diaphragm 1 and the piezoelectric film 4 provided on the diaphragm 1 are bent in
response to an ultrasonic wave propagating through a medium such as air, and the voltage
(charge) generated in the piezoelectric film 4 according to the bending is lowered. The intensity
(sound pressure) of the ultrasonic wave can be detected by taking it out through the electrode 3
and the upper electrode 5. Japanese Patent Application Laid-Open No. 10-256570
[0005]
The thermal expansion coefficient, Young's modulus, Poisson's ratio, etc. of the semiconductor
(for example, silicon) constituting the diaphragm 1 and the thin films of the lower electrode 3,
the piezoelectric film 4, and the upper electrode 5 formed on the diaphragm 1 differ from each
other. Furthermore, since these thin films are formed in a relatively high temperature process,
residual stress inherent to each thin film is generated. It has been found that such residual stress
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is proportional to the temperature at the time of thin film formation, the difference in thermal
expansion coefficient of each thin film, and the area of the interface. In particular, since the step
of forming a thermal oxide film on a silicon substrate and the step of forming a piezoelectric film
4 are performed at about 1100 ° C. and 600 ° C. respectively, they are the main cause of
residual stress generation. Then, the rigidity of each thin film becomes high due to such residual
stress, and as a result, the diaphragm 1 becomes difficult to bend, resulting in a problem that the
sensitivity is lowered.
[0006]
The present invention has been made in view of the above-described circumstances, and an
object thereof is to provide an ultrasonic sensor with improved detection sensitivity as compared
to the prior art.
[0007]
According to a first aspect of the present invention, in order to achieve the above object, a
diaphragm formed by processing a semiconductor substrate into a thin film, a frame portion
made of a semiconductor substrate and surrounding and supporting the periphery of the
diaphragm, and a surface of the diaphragm A thin film-like lower electrode closely attached, a
thin film-like piezoelectric film closely attached to the lower electrode surface opposite to the
diaphragm, and a thin film-like film closely attached to the lower electrode opposite to the
piezoelectric film An ultrasonic sensor comprising: an upper electrode; and extracting a voltage
generated in the piezoelectric film when the diaphragm is flexed by receiving ultrasonic waves
through the lower electrode and the upper electrode, the upper electrode and the piezoelectric
film on the surface of the diaphragm And the lower electrode is provided in a narrower range
than the diaphragm.
[0008]
The invention of claim 2 is characterized in that, in the invention of claim 1, the piezoelectric film
is made of lead zirconate titanate (PZT), and the area of the surface of the diaphragm is at least
33 times the area of the piezoelectric film on the surface of the diaphragm. I assume.
[0009]
According to the present invention, the upper electrode, the piezoelectric film and the lower
electrode are provided in a narrower range than the diaphragm on the surface of the diaphragm,
so the upper electrode, the piezoelectric film and the lower electrode are provided in the same or
a wider range as the diaphragm. Compared with the conventional example, the residual stress of
the upper electrode, the piezoelectric film, and the lower electrode exerts less influence on the
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diaphragm and the diaphragm is more easily bent. As a result, the detection sensitivity can be
improved as compared with the conventional case.
[0010]
Hereinafter, embodiments of the present invention will be described in detail with reference to
the drawings.
However, the same reference numerals are given to constituent elements common to those in the
conventional example, and the description will be appropriately omitted.
[0011]
In this embodiment, as shown in FIG. 2, a diaphragm 1 (a portion surrounded by a broken line in
FIG. 1A) formed in a thin film by processing a semiconductor substrate (SOI substrate) and a
semiconductor substrate is a diaphragm A frame portion 2 surrounding and supporting the
periphery of 1, a thin film lower electrode 3 provided in close contact with the surface of the
diaphragm 1, and a thin film provided in close contact with the surface of the lower electrode 3
on the opposite side of the diaphragm 1 Provided in the frame portion 2 including the surface of
the frame portion 2 and the back surface of the diaphragm 1 provided in close contact with the
surface of the piezoelectric film 4 and the surface of the piezoelectric film 4 opposite to the lower
electrode 3 And an oxide film 6 in the form of a thin film.
Further, a window 4 a is provided in the piezoelectric film 4 on the surface of the frame 2, and
the land 3 a joined to the surface of the lower electrode 3 in the window 4 a and the land 5 a
joined to the surface end of the upper electrode 5. The voltage (charge) generated in the
piezoelectric film 4 is extracted to the outside through the
[0012]
Next, the manufacturing process of the present embodiment will be briefly described.
[0013]
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First, the lower electrode 3 is formed by depositing a multilayer thin film of platinum and
titanium on the oxide film 6 on the surface of the semiconductor substrate by RF magnetron
sputtering.
Then, photolithography of the silicon anisotropic etching window is performed on the back
surface (the lower surface in FIG. 2) of the semiconductor substrate, and the oxide film on the
back surface of the semiconductor substrate is etched with buffered hydrofluoric acid (BHF)
using the resist as a mask. The substrate is immersed in an aqueous solution of TMAH
(tetramethyl ammonium hydroxide) (concentration 25%, 90 ° C.) to perform anisotropic silicon
etching to form the thin film diaphragm 1 and the rectangular frame 2.
Subsequently, a PZT thin film is formed on the surface of the lower electrode 3 by a sol-gel
method to form a piezoelectric film 4.
In this film forming process, the film thickness of the piezoelectric film 4 is made about 0.8 μm
by applying 12 layers of PZT, and while drying for 10 minutes each at 400 ° C. for each layer,
600 ° C. for each 4 layers. Sinter for 10 minutes each under an oxygen atmosphere.
[0014]
Then, after forming a multilayer thin film of platinum and titanium on the surface of the
piezoelectric film 4 by RF magnetron sputtering, the upper electrode 5 is processed to a desired
size by photolithography and dry etching using an ICP type RIE device. Form Furthermore, the
piezoelectric film 4 is processed to a desired size by wet etching using photolithography and
dilute hydrofluoric nitric acid, and the lower electrode 3 is processed to a desired size by dry
etching using photolithography and an ICP type RIE apparatus. Thus, the present embodiment is
completed.
[0015]
Here, as described in the prior art, residual stress (film stress) is generated in each thin film of
the lower electrode 3, the piezoelectric film 4, and the upper electrode 5 in the above-described
manufacturing process. When this residual stress is a compressive stress, the rigidity of the
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diaphragm 1 decreases and the resistance to mechanical vibration decreases, but when the
residual stress is a tensile stress, the rigidity of the diaphragm 1 increases and the resistance to a
mechanical vibration As a result, the diaphragm 1 becomes difficult to bend and the detection
sensitivity decreases. If the residual stress generated in the piezoelectric film 4 is a tensile stress,
the unit cell is compressed and the charged particles (electrons or ions) inside the unit cell, which
determine the polarization characteristics, become very difficult to move. If the residual stress
generated in 4 is a compressive stress, the unit cell expands and the charged particles in the unit
cell move easily, so that the polarization characteristic is improved and the residual polarization
Pr [C / m <2>]. And the value of the coercive electric field Ec [V / m] is significantly increased. In
the conventional example shown in FIG. 3, since the lower electrode 3, the piezoelectric film 4
and the upper electrode 5 on the surface of the diaphragm 1 are formed in a size that is wider
(area) than the diaphragm 1, they are very large. A tensile stress is generated in the piezoelectric
film 4 to significantly reduce the polarization characteristic.
[0016]
On the other hand, in the present embodiment, as shown in FIG. 1A, the lower electrode 3, the
piezoelectric film 4 and the upper electrode 5 on the surface of the diaphragm 1 Since it is
formed in the desired size, the tensile stress generated in the piezoelectric film 4 is reduced and
the compressive stress is increased as compared with the conventional example, and the
polarization characteristic of the piezoelectric film 4 is improved and the machine of the
diaphragm 1 is Since the resistance to the dynamic vibration is reduced, the detection sensitivity
can be greatly improved as compared with the conventional example.
[0017]
Here, several types of samples (sample numbers 1 to 5) having different ratios Ar (= Am / Ap) of
the area Ap of the piezoelectric film 4 and the area Am of the diaphragm 1 on the surface of the
diaphragm 1 are prepared. When the present inventors conducted an experiment to check the
detection sensitivity [μV / Pa], the results shown in Table 1 below were obtained.
However, the outer shapes of the diaphragm 1 and the piezoelectric film 4 are regarded as
square.
[0018]
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[0019]
According to the above experimental results, the detection sensitivity of sample No. 3 is the
highest at 50 to 60 [μV / Pa], and then the detection sensitivity of sample No. 4 is the second
highest at 30 [μV / Pa], The detection sensitivity of the samples (sample numbers 1, 2, and 5) is
considerably low at 10 [μV / Pa] or less.
That is, if the area of the diaphragm 1 is the same, the detection sensitivity is higher for the
sample having a smaller area of the piezoelectric film 4, and if the area of the piezoelectric film 4
is the same, the sample having a larger area of the diaphragm 1 is Is found to be high. Here,
when the area ratios Ar1 to Ar5 of the sample numbers 1 to 5 are determined, Ar1 = 950 <2> /
200 <2> ≒ 22.6, Ar2 = 1050 <2> / 200 <2 >> 27. 6, Ar3 = 1150 <2> / 200 <2> ≒ 33.1, Ar4 =
1150 <2> / 300 <2> ≒ 14.7, Ar5 = 1150 <2> / 400 <2> ≒ 8.3 and Become. Therefore, when the
piezoelectric film 4 is formed of PZT, if the area Am of the surface of the diaphragm 1 is 33 times
or more of the area Ap of the piezoelectric film 4 on the surface of the diaphragm 1, the
detection sensitivity can be significantly improved. .
[0020]
In the present embodiment, the outer shape of the diaphragm 1 and the piezoelectric film 4 is
regarded as a square, but it may be a polygon other than a square, or a circle or an ellipse.
Alternatively, the piezoelectric film 4 may be formed of a material other than PZT, and the ratio
Ar of the area Ap of the piezoelectric film 4 to the area Am of the diaphragm 1 on the surface of
the diaphragm 1 is the case where the piezoelectric film 4 is formed of the other material Choose
the optimal value of.
[0021]
The embodiment of this invention is shown, (a) is a top view of the principal part containing a
diaphragm, (b) is a X-X 'line cross section arrow view of (a). FIG. It is a sectional side view which
shows a prior art example.
Explanation of sign
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[0022]
1 diaphragm 2 frame portion 3 lower electrode 4 piezoelectric film 5 upper electrode
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