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DESCRIPTION JPH07274289

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DESCRIPTION JPH07274289
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
ultrasonic transducer formed on a silicon substrate.
[0002]
2. Description of the Related Art Since ultrasonic waves have a slower propagation speed than
light, they can accurately measure information such as distance, position, and object shape
without contact, and because they have good dispersibility, they can be used for sensing over a
wide spatial area. Is also suitable. By taking advantage of such features, application to a wide
range of fields such as a biotomograph, a fish finder, a material damage, a thickness gauge, a
level gauge, an ultrasonic microscope, a monitoring alarm device, a remote switch and the like
has been made. With such sensing technology, there is an increasing technical demand for highspeed, high-accuracy measurement, and from point measurement to surface measurement and
stereo measurement.
[0003]
For this reason, an ultrasonic transducer having a silicon monolithic structure in which a
detection unit and a signal processing unit are integrated has been proposed (see, for example,
Japanese Patent Application Laid-Open No. 61-220596 (H04R 17/00)).
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[0004]
The element structure of a silicon monolithic ultrasonic transducer is anisotropically etching a
cantilever composed of silicon dioxide (SiO 2) having a length of several tens to several hundreds
of μm, a width of several tens of μm and a thickness of several thousand angstroms on a silicon
(Si) substrate Form by
A lower electrode such as Pt, etc., a piezoelectric such as AlN, PbTiO3, ZnO, PZT (lead zirconate
titanate), PLZT ((Pb1-x, Lax) (Zry, Ti1-y) 1-x / 3O3), etc. It is formed by sequentially laminating a
body film and an upper electrode such as Al.
[0005]
Then, when ultrasonic waves are applied from above, the cantilever made of the composite film
vibrates. The oscillations become very large at several mechanical resonance frequencies
determined by the structure of the cantilever. The vibration generates stress inside the
piezoelectric thin film, and the piezoelectric effect generates a voltage between the lower
electrode and the upper electrode, thereby detecting an ultrasonic wave.
[0006]
By the way, when forming a piezoelectric film, it is often formed by raising the substrate
temperature. Therefore, the lower electrode to be the base is a metal having low reactivity, in
order to avoid reaction with the base. Pt is used. In terms of characteristics, the piezoelectric film
is required to have orientation in its crystallinity.
[0007]
On the other hand, when AlN is used as the piezoelectric film, AlN can not be formed with good
adhesion on Pt, and there is a problem that fine processing such as patterning can not be
performed.
[0008]
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For this reason, considering only the adhesiveness with AlN, it is conceivable to use Al as the
lower electrode, but when Al is formed on a SiO 2 film, the orientation of Al is not good, and AlN
formed thereon is also There is a problem that the orientation deteriorates, and it can not
withstand practical use.
Furthermore, since Al is simultaneously etched when patterning an AlN film, there is a problem
that it is difficult to use as a lower extraction electrode.
[0009]
The present invention has been made to solve the above-mentioned conventional problems, and
has an object to provide an ultrasonic transducer having a piezoelectric film with a good
orientation and a good characteristic.
[0010]
SUMMARY OF THE INVENTION According to the present invention, there is provided a silicon
dioxide thin film supported on a silicon substrate in a cantilever manner, a piezoelectric thin film
laminated on the silicon dioxide thin film via a lower electrode, and the piezoelectric thin film An
ultrasonic transducer comprising an upper electrode laminated thereon, wherein platinum (Pt),
palladium (Pd), iridium (Ir) or iridium (Ir) which is easy to orient and has low reactivity on the
silicon dioxide thin film A lower electrode made of a metal selected from rhodium (Rh) is
provided, and a metal layer selected from aluminum (Al), zinc (Zn) or copper (Cu) reflects the
orientation of the lower electrode on the lower electrode. The piezoelectric thin film having the
orientation is formed on the metal layer reflecting the orientation of the metal layer.
[0011]
According to the present invention, Pt, Pd, Ir and Rh can be formed with good orientation on a
silicon dioxide thin film.
Then, Al, Zn and Cu can be formed on the lower electrode made of these metals, reflecting the
orientation of the lower electrode.
By forming the piezoelectric thin film on the Al, Zn, and Cu, the piezoelectric thin film can obtain
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a good orientation, and also has good adhesion to Al, Zn, and Cu.
[0012]
Embodiments of the present invention will be described below with reference to the drawings.
[0013]
FIG. 1 is a plan view showing an embodiment of the present invention, and FIG. 2 is a
longitudinal sectional view of the embodiment.
[0014]
On a (100) Si single crystal substrate 1, a SiO2 film 20 having a thickness of 0.15 to 1.2 .mu.m,
0.5 .mu.m in this embodiment, is provided by thermal oxidation.
This SiO2 film 20 is formed with a concave opening in the <110> direction by photolithography,
and is etched by an alkaline solution, for example, a mixed solution of ethylene diamine,
pyrocatechol and water, using the SiO2 film 20 as a mask. By anisotropic etching, a cantilever 2
composed of a SiO2 film supported on the Si substrate 1 in a cantilever manner is formed.
[0015]
On the SiO 2 film 20, platinum (Pt), palladium (Pd), iridium (Ir) or rhodium (Rh) which is easy to
be oriented to lower electrode 3 and has low reactivity is provided.
In this embodiment, a lower electrode 3 made of Pt is provided. This Pt can be formed on the SiO
2 film 20 with good orientation.
[0016]
Then, a metal layer 4 having a film thickness of 50 to 300 angstroms made of Al, Zn or Cu is
formed on the lower electrode 3 reflecting the orientation of the lower electrode. In this
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embodiment, a metal layer 4 made of Al is formed.
[0017]
A piezoelectric film 5 made of AlN, PbTiO3, ZnO, PZT, PLZT is provided on the metal layer 4. The
piezoelectric body 5 is formed with good orientation reflecting the orientation of the metal layer
4. In this embodiment, a piezoelectric film 5 made of AlN is formed. Then, in order to prevent a
short circuit with the lead-out portion 3a of the lower electrode 3, an upper electrode 7 made of
Al is provided via the SiO2 film 6. Thus, the SiO 2 film 20 is supported in a cantilever manner on
the Si substrate 1, and the lower electrode 3, the metal layer 4, the piezoelectric film 5, and the
upper electrode 6 are stacked thereon to form a vibrating body. The vibrator vibrates in the
etching groove provided in the Si substrate 1 up and down to transmit and receive ultrasonic
waves.
[0018]
As described above, the lower electrode 3 made of Pt, Pd, Ir, Rh is provided on the SiO 2 film 20.
These lower electrodes 3 can be formed on the SiO 2 film 20 with good orientation. Then, Al, Zn,
Cu can be formed on the lower electrode 3 reflecting the orientation of the lower electrode 3. By
forming the piezoelectric thin film 5 on Al, Zn, and Cu, the piezoelectric thin film 5 can obtain a
good orientation, and also has good adhesion to Al, Zn, and Cu.
[0019]
Next, a second embodiment of the present invention will be described in accordance with FIGS.
FIG. 3 is a plan view showing a second embodiment of the present invention, and FIG. 4 is a
longitudinal sectional view of that embodiment.
[0020]
In the second embodiment, Pt is used as the lead-out electrode of the lower electrode 3 in the
first embodiment, while the lower electrode 3 is taken out using Al of the metal layer 4. That is,
the window is opened in a part of the SiO 2 film 20, and the lead-out electrode portion 4c made
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of Al having good crystallinity is provided on the upper side of the window. The other
configuration is the same as that of the first embodiment, so the description is omitted here to
avoid the repetition of the description.
[0021]
Next, the ultrasonic transducer of the present invention will be described together with an
example of its production. 5 and 6 are sectional views showing the method of manufacturing the
ultrasonic transducer according to the present invention step by step.
[0022]
First, as shown in FIG. 5A, a SiO2 film 20 having a thickness of 0.15 to 1.2 .mu.m, 0.5 .mu.m in
this embodiment, is formed on a Si (100) single crystal substrate 1 by thermal oxidation. . The
SiO2 film 20 may be formed by RF sputtering, CVD or the like in addition to the thermal
oxidation method.
[0023]
The lower electrode 3 is provided on the SiO 2 film 20. The lower electrode 3 is formed of Pt by
0.1 to 0.4 μm by RF sputtering or ion beam sputtering. The lower electrode 3 may use Pd, Ir, Pd
or the like in addition to Pt.
[0024]
Thereafter, the lower electrode 3 made of Pt is patterned by photolithography. The patterning of
the lower electrode 3 is performed by ion beam milling using a resist of about 0.8 μm in
thickness as a mask. The conditions for the ion beam milling were an Ar gas pressure of
[email protected] Torr, an acceleration voltage of 500 V, an ion current of 0.3 mA / cm @ 2, and a
milling rate of 2000 angstrom / 10 minutes.
[0025]
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After completion of ion milling, the resist is removed by O 2 plasma or acetone, an organic
solvent such as methyl ethyl ketone, or a solution of sulfuric acid and hydrogen peroxide water.
[0026]
Thereafter, Al having a thickness of 50 to 300 angstroms is formed as the metal layer 4 by ion
beam sputtering.
[0027]
As the film forming conditions for this Al ion beam sputtering method, the target is aluminum of
99.999%, the substrate temperature is kept at room temperature to 300 ° C., the Ar gas
pressure is 2.8 × 10 -4 Torr, and the acceleration voltage is The ion current was set to 800 V,
the ion current was set to 0.76 mA / cm 2, and the deposition rate of Al was set to 35 angstroms
/ minute.
[0028]
Subsequently, as shown in FIG. 5B, a piezoelectric film 5 of 0.1 to 20 μm in thickness made of
AlN is continuously formed on the metal layer 4 by ion beam sputtering.
In this example, 0.7 μm of AlN was laminated.
[0029]
As the film forming condition of this AlN, in the case of using the ion beam sputtering method,
the target is aluminum of 99.999%, the substrate temperature is kept from room temperature to
800 ° C., 300 ° C. in this embodiment, and nitrogen gas Flow rate 8 CCM, Ar flow rate 4 CCM,
Kauffman ion gun 800 eV, 0.76 mA / cm 2, nitrogen ion beam energy 72 to 200 eV, 100 eV in
this example, nitrogen ion beam current 0.20 to 0.74 mA / cm 2 In this example, 0.32 mA / cm
2, ECR microwave power 200 to 500 W, in this example 300 W, ultimate pressure ~ 2 × 10 -7
Torr, pressure 1.4 × 10 -4 Torr during deposition. The deposition rate of AlN was 10 to 70
angstroms / minute, and in this example, 30 angstroms / minute.
[0030]
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The formed metal layer 4 made of Al and the piezoelectric film 5 made of AlN are patterned by
wet etching or ion milling.
Al and AlN films are not etched by most acids at low temperatures near room temperature, and it
is necessary to use an alkaline solution.
[0031]
The conditions for patterning the Al and AlN films by wet etching are as follows: after forming a
mask with a resist, etching is performed by heating an alkaline solution such as 1.0 N KOH to 40
to 70 ° C.
In the case of Al and AlN films formed by ion beam sputtering, the etching rate by such wet
etching is 70 angstroms / minute.
[0032]
The conditions for patterning the Al and AlN films by ion milling are as follows: mask with a
resist, Ar gas pressure is 2.8 × 10 −4 Torr, acceleration voltage is 500 V, and ion current is 0.3
mA / cm 2.
The milling rate is 40 angstroms / minute in the case of Al and AlN films formed by ion beam
sputtering.
[0033]
Then, the resist used in the etching is removed by O 2 plasma or an organic solvent such as
acetone or methyl ethyl ketone or a solution of sulfuric acid and hydrogen peroxide solution.
[0034]
Thereafter, as shown in FIG. 5C, an SiO2 film 6 is formed as an interlayer insulating film of the
lead-out electrode, and then, as shown in FIG. Al of 8 μm, 0.3 μm in this embodiment, is
selectively formed by ion beam sputtering.
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[0035]
As the film forming conditions for this Al ion beam sputtering method, the target is aluminum of
99.999%, the substrate temperature is kept at room temperature to 300 ° C., the Ar gas
pressure is 2.8 × 10 -4 Torr, and the acceleration voltage is The film formation rate of Al was set
to 35 angstroms / min.
[0036]
The upper electrode 7 may be made of a metal film such as Au or Pt, or a conductive oxide such
as InO or SnO2 ITO, instead of Al.
[0037]
Furthermore, in the case of providing a passivation film, it is sufficient to selectively form a
passivation film such as SiO 2 or a polyimide film.
When a plurality of chips are formed on the same substrate, half-cut dicing may be performed at
this time to perform dicing into each chip or to facilitate division.
[0038]
Thereafter, Si etching is performed to form a SiO2 cantilever. Since an etching solution used for
this Si etching is an alkaline solution, Cr is used as an etching mask to prevent damage to Al and
AlN.
As shown in FIG. 6B, the Cr film 8 is formed by ion beam sputtering or RF sputtering, and the Cr
film 8 in a portion to be subjected to Si etching is further removed by photolithography, and then
the Cr film 8 is removed. The SiO2 film 20 at the place where it is located is etched by buffered
hydrofluoric acid.
[0039]
When the Cr film 8 is formed, for example, by ion beam sputtering, the film forming conditions
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are such that the target is 99.999% Cr, the substrate temperature is kept at room temperature to
300 ° C., and the Ar gas pressure is 2.8 The acceleration voltage was 800 V, the ion current was
0.76 mA / cm 2, and the deposition rate of Al was 35 Å / min.
[0040]
The etching of the Cr film 8 uses, as an etching solution, a mixed solution of 25 g of ceric
ammonium nitrate, 6.5 ml of 70% perchloric acid, and 150 ml of water.
This etching solution selectively etches the Cr film 8 and hardly damages Al, Pt, AlN, SiO 2 and
the like.
The etching rate by this etching solution is 1300 angstroms / min.
[0041]
Subsequently, as shown in FIG. 6C, the Si substrate 1 is etched using the Cr film 8 as a mask to
form a SiO2 cantilever 2.
The etching is ended when the desired cantilever is obtained. The conditions for this Si etching
are carried out by heating a mixed solution of 75 ml of ethylene diamine, 12 ml of pyrocatechol
and 24 ml of water (abbreviated as EPW) to 116 ° C. as an etching solution. The SiO 2
cantilever 2 having a width of 40 μm and a length of 200 μm can be formed by Si etching for
about 2 hours. As an etching solution, an etching solution such as KOH, NaOH, hydrazine, NH 4
OH or the like may be used other than the above. These etching solutions have a very high
etching rate to the (100) plane of Si as compared to the (111) plane. For example, in the case of
EPW, anisotropic etching can be performed about 40 times or more larger. The SiO2 film is also
etched slightly by this etching solution, but it is very small compared to the etching rate of Si.
[0042]
When deep Si etching is performed on the Si substrate 1 having a (100) plane through the
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opening, a recess 11 surrounded by four sides arranged in the <110> direction is formed on the
(100) plane. As described above, although the etching of the (111) plane and the SiO2 film 20 is
slow, the etching rate of the (100) or (110) plane is fast, and the etching causes the Si substrate 1
under the cantilever 2 to be in the <110> direction. Arrayed and etched away.
[0043]
Finally, as shown in FIG. 6 (d), the Cr film 8 is removed, chips are divided, and assembly such as
bonding is performed to obtain the ultrasonic transducer according to the present invention.
[0044]
Although the AlN film is used as the piezoelectric film 5 in the embodiment described above,
PbTiO 3, ZnO, PZT, PLZT can be used other than the AlN film, and the piezoelectric film 5 of
these materials is used. Even in this case, the adhesion is improved and the characteristics are
improved as compared with the case where the metal layer 4 is not interposed.
[0045]
When Al is used as the metal layer 4, it is preferable to form the piezoelectric film 5 stacked
thereon at a temperature of 400 ° C. or less in consideration of the diffusion of Al and the like.
When the film formation temperature is 400 ° C. or more, by using Cu as the metal layer 4,
diffusion does not occur so much, and deterioration of the characteristics of the piezoelectric film
5 stacked thereon can be suppressed.
[0046]
Furthermore, in the case where ZnO is used as the piezoelectric film 5, if Zn is used as the metal
layer 4, deterioration of the characteristics can be prevented even if the Zn diffuses in the film
formation of the piezoelectric film 5.
[0047]
As described above, according to the present invention, the lower electrode made of Pt, Pd, Ir, Rh
is formed on the silicon dioxide thin film, and Al, Zn is formed on the lower electrode made of
these metals. By forming the piezoelectric thin film on the Al, Zn, and Cu by forming Cu on the
basis of the orientation of the lower electrode, the piezoelectric thin film can obtain a good
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orientation, and it is possible to use Al, Zn, and Cu. An ultrasonic transducer with good adhesion
and good characteristics can be obtained.
[0048]
Brief description of the drawings
[0049]
1 is a plan view showing a first embodiment of the present invention.
[0050]
2 is a longitudinal sectional view showing a first embodiment of the present invention.
[0051]
3 is a plan view showing a second embodiment of the present invention.
[0052]
4 is a longitudinal sectional view showing a second embodiment of the present invention.
[0053]
5 is a cross-sectional view showing the manufacturing example of the ultrasonic transducer of
the present invention step by step.
[0054]
6 is a cross-sectional view showing an example of manufacturing the ultrasonic transducer of the
present invention step by step.
[0055]
Explanation of sign
[0056]
Reference Signs List 1 Si substrate 2 cantilever 3 lower electrode 4 metal layer 5 piezoelectric
film 7 upper electrode 20 SiO 2 film
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