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DESCRIPTION JP2013225546

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DESCRIPTION JP2013225546
Abstract: Even when forming a piezoelectric thin film having an orientation direction different
from that of an electrode on an electrode, both the orientation and crystallinity of the
piezoelectric thin film are easily satisfied to improve the piezoelectric characteristics. At least an
electrode is formed on a substrate, and a piezoelectric thin film having an orientation direction
different from that of the electrode is formed on the electrode. The surface of the electrode is
roughened by roughening the surface below the electrode. [Selected figure] Figure 4
Piezoelectric element and method of manufacturing the same
[0001]
The present invention relates to a piezoelectric element in which at least an electrode is formed
on a substrate and a piezoelectric thin film is formed on the electrode, and a method of
manufacturing the piezoelectric element.
[0002]
Heretofore, a piezoelectric material such as PZT (lead zirconate titanate) has been used as an
electromechanical transducer such as a drive element or a sensor.
On the other hand, MEMS (Micro Electro Mechanical Systems) devices using a Si substrate are
increasing in response to recent demands for downsizing, high density, cost reduction and the
like of devices. If a piezoelectric body is applied to the MEMS element, various devices such as an
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inkjet head, an ultrasonic sensor, an infrared sensor, a frequency filter, and the like can be
manufactured.
[0003]
Here, when applying a piezoelectric body to a MEMS element, it is desirable to thin the
piezoelectric body. This is because the following advantages can be obtained by thinning the
piezoelectric body. That is, high-precision processing using semiconductor process technology
such as film formation and photolithography becomes possible, and miniaturization and high
densification can be realized. The cost can be reduced because the piezoelectric bodies can be
processed collectively to a large-area wafer. The conversion efficiency of the electric machine is
improved, and the characteristics of the drive element and the sensitivity of the sensor are
improved.
[0004]
As a method of forming a film on a substrate such as Si, a chemical method such as CVD
(Chemical Vapor Deposition) method, a physical method such as sputtering method or ion plating
method, liquid phase such as sol-gel method Growth methods are known.
[0005]
Piezoelectric substances such as PZT are generally ABO3 type oxides, and it is known that when
the crystals have a perovskite type structure, they exhibit a good piezoelectric effect.
FIG. 9 schematically shows the crystal structure of PZT. The perovskite structure is, for example,
a tetragonal system of Pb (Zrx, Ti1-x) O3, in which a Pb atom is located at each vertex of the
tetragonal system, a Ti atom or a Zr atom is located in the body center, and O in each face center.
It is a structure where an atom is located.
[0006]
In addition, PZT is a solid solution of PTO (PbTiO3; lead titanate) and PZO (PbZrO3; lead
zirconate) both having a perovskite structure, but when the ratio of PTO is high, the entire PZT
becomes tetragonal, and PZO When the ratio is high, the entire PZT becomes rhombohedral.
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[0007]
FIG. 10 shows the relationship between the composition ratio of PTO and PZO and the crystal
system.
When the composition ratio of PTO to PZO is around 48/52 to 47/53, the crystal system
changes from tetragonal to rhombohedral or from rhombohedral to tetragonal. The boundary at
which the crystal system changes in this manner is referred to as a composition phase boundary
(MPB; Morphotropic phase boundary), and is hereinafter simply referred to as a phase boundary.
At around room temperature, the crystal structure of PZT is tetragonal, rhombohedral or a mixed
crystal of these (phase boundary), but at temperatures above the Curie point, the crystal
structure of PZT is the composition ratio of PTO to PZO In any case, it is cubic.
[0008]
FIG. 11 shows the relationship between the composition ratio of PTO and PZO and the
characteristics (dielectric constant, electromechanical coupling coefficient). At the phase
boundary described above, both the relative dielectric constant and the electromechanical
coupling coefficient are specifically high. There is a positive correlation between the relative
dielectric constant and the piezoelectric constant (displacement amount per unit electric field),
and the piezoelectric constant becomes high due to the high relative dielectric constant. In
addition, the electromechanical coupling coefficient is an index indicating the efficiency at the
time of converting an electrical signal into mechanical distortion, or the efficiency at the time of
converting the signal, and this coefficient becomes high. Conversion efficiency is high. The fact
that the piezoelectric body is deformed by applying an electric field to the piezoelectric body, or
conversely, the electric field (potential difference) is generated in the piezoelectric body by
deforming the piezoelectric body is referred to herein as the piezoelectric effect.
[0009]
FIG. 12 schematically shows the difference in piezoelectric effect due to the difference in crystal
orientation of the piezoelectric body. When the piezoelectric body has (100) orientation, that is,
when the polarization direction P of the piezoelectric body is the (100) direction and this
direction is perpendicular to the substrate, applying an electric field in the direction
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perpendicular to the substrate, the piezoelectric body Since the direction of polarization P and
the direction of application of the electric field E are aligned, the magnitude of the electric field is
completely converted to the force of deformation of the piezoelectric body, and the piezoelectric
body is efficiently deformed in the direction perpendicular to the substrate. On the other hand,
when the piezoelectric body has (111) orientation, the (100) direction which is the polarization
direction P of the piezoelectric body intersects the application direction E of the electric field, so
the magnitude of the electric field is completely converted to the force of deformation As a result,
the amount of deformation of the piezoelectric body in the direction perpendicular to the
substrate is reduced.
[0010]
Note that FIG. 12 shows the case where the crystal of the piezoelectric body is tetragonal, but
even in the case of rhombohedral (the polarization direction is the (111) direction), the
piezoelectric effect is generated by the rotation of the domain. ) Orientation is preferred.
[0011]
The crystal orientation of the piezoelectric body is determined mainly by the film formation
temperature, film formation pressure, stress applied to the film, lattice constant and orientation
of the base, etc. Among them, the surface state of the base is dominant It becomes an important
factor.
In general, a metal material such as Pt (platinum) or Ir (iridium) is often used as the base of the
piezoelectric body in order to provide the function of the electrode. In order to enhance the
crystallinity of the piezoelectric body, it is desirable to flatten the surface of the electrode to be
the base, and the ideal state is a cleavage plane of a single crystal.
[0012]
On the other hand, Pt and Ir have the property of being easily self-aligned in the (111) direction
with respect to the substrate surface. Therefore, when PZT is deposited as a piezoelectric on an
ideal Pt base, the PZT is also oriented in the (111) direction, and the requirement for enhancing
the piezoelectric characteristics can not be satisfied.
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[0013]
Therefore, conventionally, in order to achieve both the orientation of the piezoelectric body (to
make the orientation direction the desired orientation direction) and the crystallinity of the
piezoelectric body (to make the crystal structure be a perovskite type) in order to improve the
piezoelectric characteristics. There are various ideas for
[0014]
For example, in Patent Document 1, titanium or titanium oxide is deposited on an electrode (a
protrusion is 2 nm or less), and an orientation control layer such as PLZT (lead lanthanum
zirconate titanate) is thereon (100) or (001) orientation The perovskite type piezoelectric
material (for example, PZT) is formed on the orientation control layer in a desired orientation
direction.
[0015]
Further, in Patent Document 2, a seed layer containing Ti and a lower electrode are formed in
this order on a substrate, and the element (Ti) of the seed layer is precipitated on the surface of
the lower electrode (surface roughness Ra = 0 The crystal orientation and piezoelectric
performance of the piezoelectric body are improved by forming the piezoelectric body on the
surface of the lower electrode (5 to 30 nm).
In addition, surface roughness Ra is a parameter of surface roughness calculated | required
based on JIS (Japanese Industrial Standards) B0601: 1994.
[0016]
Furthermore, in Patent Document 3, a hillock made of Pt is formed on the lower electrode, a seed
layer made of PTO with a (100) orientation is formed, and a piezoelectric material made of PZT is
formed on the seed layer. The crystal orientation of the piezoelectric body is appropriately
controlled without using a special substrate such as MgO.
At this time, the surface roughness Ra of the lower electrode is 0.75 nm before the formation of
hillocks, and 1.015 nm after the formation of hillocks.
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[0017]
Patent No. 3481235 (see claim 1, paragraphs [0027], [0028], FIG. 1, FIG. 2, etc.) JP 2007281238 (claims 1, 5, 6, paragraph [0024], FIG. See, for example, JP-A-2011-238774 (see claim
1, paragraphs [0007], [0010], [0017], [0020], FIG. 1, FIG. 2, etc.)
[0018]
However, in Patent Documents 1 and 2, Ti deposited on the electrode is a constituent element of
PZT as a piezoelectric material and mutually diffuses with PZT, so the ratio of PTO to PZO in PZT
changes. It is conceivable that the characteristics deteriorate.
Further, in Patent Document 3, it is difficult to control the density and height of the hillocks and
to control the surface roughness (surface shape) of the electrodes, so it is difficult to achieve both
orientation and crystallinity of PZT. is there.
[0019]
The present invention has been made to solve the above-mentioned problems, and its object is to
form the orientation and crystallinity of the piezoelectric thin film even when the piezoelectric
thin film having an orientation different from that of the electrode is formed on the electrode. It
is an object of the present invention to provide a piezoelectric element and a method of
manufacturing the same that can satisfy both easily and improve the piezoelectric characteristics.
[0020]
The method of manufacturing a piezoelectric element according to the present invention is a
method of manufacturing a piezoelectric element in which at least an electrode is formed on a
substrate and a piezoelectric thin film having an orientation direction different from that of the
electrode is formed on the electrode. It is characterized in that the surface of the electrode is
roughened by roughening the surface of the lower layer.
The piezoelectric element of the present invention is a piezoelectric element in which at least an
electrode is formed on a substrate and a piezoelectric thin film having an orientation direction
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different from that of the electrode is formed on the electrode, and a surface lower than the
electrode The surface of the electrode is roughened by roughening the surface of the electrode.
[0021]
Since the surface of the electrode is roughened by roughening the surface of the lower layer than
the electrode, the surface of the electrode is roughened without depositing the metal of the same
element as the constituent element of the piezoelectric thin film on the electrode from the lower
layer. Can be Thereby, it is possible to avoid the deterioration of the piezoelectric characteristics
due to the mutual diffusion of the metals. In addition, since the surface of the electrode can be
roughened without forming hillocks on the electrode, control of the surface roughness of the
electrode is facilitated, and a piezoelectric thin film formed on the electrode by controlling the
surface roughness. It is easy to enhance the orientation and crystallinity of
[0022]
Therefore, even in the case where a piezoelectric thin film having an orientation direction
different from that of the electrode is formed on the electrode, both the orientation and
crystallinity of the piezoelectric thin film can be easily satisfied, and the piezoelectric
characteristics can be improved.
[0023]
In the said manufacturing method, it is desirable to roughen the surface of a lower layer rather
than the said electrode so that the surface roughness RMS of the said electrode may be 1 nm or
more and 10 nm or less.
Further, in the piezoelectric element, it is desirable that the surface roughness RMS of the
electrode is 1 nm or more and 10 nm or less.
[0024]
When the surface roughness RMS of the electrode falls below the lower limit, the surface of the
electrode becomes flatter, and it becomes difficult to form a piezoelectric thin film in a different
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orientation direction (for example, (100) direction) from the electrode (piezoelectric thin film Is
likely to be deposited in the (111) orientation, for example, following the electrode in the lower
layer). On the other hand, when the surface roughness RMS of the electrode exceeds the upper
limit, the crystallinity of the piezoelectric thin film is easily broken, and it becomes difficult to
obtain a piezoelectric thin film having a perovskite structure. Therefore, when the surface
roughness RMS of the electrode is in the above range, the orientation and the perovskite
crystallinity of the piezoelectric thin film can be compatible, and good piezoelectric
characteristics can be obtained.
[0025]
In the above manufacturing method, the surface of the electrode may be roughened by
roughening the surface of the substrate. Further, in the piezoelectric element, the surface of the
electrode may be roughened by roughening the surface of the substrate.
[0026]
By roughening the surface of the substrate to be the base of the electrode, it is possible to
roughen the surface of the electrode following the surface of the substrate.
[0027]
In the above manufacturing method, it is desirable to roughen the surface of the substrate such
that the surface roughness RMS is 1 nm or more and 10 nm or less.
Further, in the piezoelectric element, it is preferable that the surface roughness RMS of the
substrate is 1 nm or more and 10 nm or less.
[0028]
In this case, the surface roughness RMS of the electrode formed on the substrate can be made 1
nm or more and 10 nm or less following the surface roughness of the substrate surface.
[0029]
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In the above manufacturing method, when the thermal oxide film and the electrode are formed in
this order on the substrate, the surface of the electrode may be roughened by roughening the
surface of the thermal oxide film.
In the piezoelectric element, a thermal oxide film and the electrode are formed in this order on
the substrate, and the surface of the thermal oxide film is roughened, whereby the surface of the
electrode is roughened. It may be standardized.
[0030]
By roughening the surface of the thermal oxide film to be the base of the electrode, it is possible
to roughen the surface of the electrode following the surface of the thermal oxide film.
[0031]
In the above manufacturing method, it is desirable that the surface of the thermal oxide film is
roughened such that the surface roughness RMS is 1 nm or more and 10 nm or less.
Further, in the piezoelectric element, it is preferable that the surface roughness RMS of the
thermal oxide film is 1 nm or more and 10 nm or less.
[0032]
In this case, the surface roughness RMS of the electrode can be made 1 nm or more and 10 nm
or less following the surface roughness of the surface of the thermal oxide film.
[0033]
In the above manufacturing method, a first piezoelectric thin film is formed as the piezoelectric
thin film, and a second piezoelectric thin film having a perovskite structure is formed as an
intermediate layer between the first piezoelectric thin film and the electrode. May be
Further, in the piezoelectric element, the piezoelectric thin film includes a first piezoelectric thin
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film and a second piezoelectric thin film having a perovskite structure, and the second
piezoelectric thin film is the first piezoelectric thin film and the first piezoelectric thin film. You
may form the intermediate | middle layer between electrodes.
[0034]
Even when the first and second piezoelectric thin films are formed as the piezoelectric thin film,
the second piezoelectric thin film of the perovskite structure has a desired orientation direction
different from that of the electrode by roughening the electrode surface. For example, the film
can be formed in the (100) direction, and the first piezoelectric thin film can be formed thereon
with a perovskite structure in a desired orientation (for example, the (100) direction).
[0035]
Even when a piezoelectric thin film having an orientation direction different from that of the
electrode is formed on the electrode, both the orientation and crystallinity of the piezoelectric
thin film can be easily satisfied to improve the piezoelectric characteristics.
[0036]
(A) is sectional drawing which shows the outline | summary structure of the piezoelectric
element concerning one Embodiment of this invention, (b) is sectional drawing which expands
and shows a part of said piezoelectric element.
It is a graph which shows the result of the X-ray diffraction of the piezoelectric thin film of the
said piezoelectric element.
It is a graph which shows the relationship between the surface roughness of the lower electrode
of the said piezoelectric element, and the perovskite crystallinity and (100) orientation of PZT
which comprise the said piezoelectric thin film. It is a flowchart which shows the flow of the
manufacturing process of the said piezoelectric element. It is sectional drawing which shows the
general | schematic structure of the sputter apparatus which forms the said piezoelectric thin
film into a film. It is sectional drawing which shows the other structure of the said piezoelectric
element. It is a top view which shows the structure when the said piezoelectric element is applied
to a diaphragm. It is an A-A 'line arrow directional cross-sectional view of FIG. It is explanatory
drawing which shows the crystal structure of PZT typically. It is a graph which shows the
relationship between the composition ratio of PTO and PZO, and a crystal system. It is a graph
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which shows the relationship between the composition ratio of PTO and PZO, and a
characteristic. It is explanatory drawing which shows typically the difference of the piezoelectric
effect by the difference of the crystal orientation of a piezoelectric material.
[0037]
It will be as follows if one embodiment of the present invention is described based on a drawing.
[0038]
〔1.
Configuration of Piezoelectric Element] FIG. 1A is a cross-sectional view showing a schematic
configuration of the piezoelectric element 10 according to the present embodiment, and FIG. 1B
is a cross-sectional view showing a part of the piezoelectric element 10 in an enlarged manner.
FIG. The piezoelectric element 10 of the present embodiment is configured by laminating the
thermal oxide film 2, the lower electrode 3, the piezoelectric thin film 4 and the upper electrode
5 in this order on the substrate 1.
[0039]
The substrate 1 is formed of, for example, a semiconductor substrate or an SOI (Silicon on
Insulator) substrate made of single crystal Si (silicon) alone having a thickness of about 300 to
500 μm. The thermal oxide film 2 is made of, for example, SiO 2 (silicon oxide) having a
thickness of about 0.1 μm, and is formed for the purpose of protecting and insulating the
substrate 1.
[0040]
The lower electrode 3 is configured by laminating a Ti (titanium) layer 3a and a Pt (platinum)
layer 3b. The Ti layer 3a is formed to improve the adhesion between the thermal oxide film 2 and
the Pt layer 3b. The thickness of the Ti layer 3a is, for example, about 0.02 μm, and the
thickness of the Pt layer 3b is, for example, about 0.1 μm. The Pt layer 3 b is an electrode on the
surface of which the piezoelectric thin film 4 is formed, that is, an electrode serving as a base of
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the piezoelectric thin film 4. Hereinafter, the Pt layer 3b is referred to simply as an electrode. As
shown in FIG. 1B, the surface of the Pt layer 3b is roughened by roughening the surface of the
lower layer (for example, the substrate 1) than the Pt layer 3b. Details will be described later.
[0041]
The piezoelectric thin film 4 is made of PZT (lead zirconate titanate) whose crystal structure is a
perovskite type. Although the thickness of PZT varies depending on the application, it is, for
example, 1 μm or less for memory and sensor applications, and for example, 3 to 5 μm for an
actuator.
[0042]
The upper electrode 5 is configured by laminating a Ti layer 5a and a Pt layer 5b. The Ti layer 5a
is formed to improve the adhesion between the piezoelectric thin film 4 and the Pt layer 5b. The
thickness of the Ti layer 5a is, for example, about 0.02 μm, and the thickness of the Pt layer 5b
is, for example, about 0.2 μm.
[0043]
〔2. Regarding Surface Roughness and Piezoelectric Properties of Electrode] FIG. 2 is a graph
showing the results of X-ray diffraction (XRD) of the formed piezoelectric thin film 4. Here, a Si
substrate of (100) orientation, that is, a Si substrate in which the (100) plane is parallel to the
lamination plane, is used as the substrate 1, but between the substrate 1 and the Pt layer 3b of
the lower electrode 3 Since an amorphous SiO 2 layer (thermal oxide film 2) exists and the Pt
layer 3b has self-orientation, Pt in the Pt layer 3b is strongly oriented in the (111) direction.
[0044]
When PZT as the piezoelectric thin film 4 is formed on the Pt layer 3b, the PZT is strongly
affected by the base, so even if the film forming conditions are optimized, the PZT is more
oriented in the (111) direction than in the (100) direction. It becomes a film that is strongly
oriented. Here, the polarization direction of PZT is the (100) direction.
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[0045]
FIG. 3 is a graph showing the relationship between the surface roughness of the electrode and
the perovskite crystallinity and (100) orientation of PZT. In the figure, the horizontal axis
indicates the surface roughness RMS (nm) of the electrode (Pt layer 3b) which is the base of PZT.
The surface roughness RMS (Root Mean Square) is one of the indices indicating the surface
roughness, and is a square root of a value obtained by averaging the squares of the deviation
from the average line to the measurement curve (the curve indicating the surface unevenness).
The value to be determined (the root mean square roughness), which can be measured by an
atomic force microscope (AFM). The surface roughness RMS is also referred to as Rq in the JIS
standard.
[0046]
Further, the vertical axis on the left side indicates the crystallinity of the perovskite, and is
represented by the sum of the intensity of the peak indicating the perovskite (100) of PZT and
the intensity of the peak indicating the perovskite (111) in FIG. The relationship between the
surface roughness RMS of the electrode and the crystallinity is shown by the solid line in FIG.
The larger the value indicating the crystallinity of the perovskite, the more crystals of the ABO 3 type perovskite structure exist, which means that the piezoelectric characteristics become higher.
From the figure, it can be seen that when the surface roughness RMS of the electrode is
increased and exceeds 10 nm, the perovskite crystallinity of the PZT is reduced and the required
piezoelectric characteristics can not be obtained.
[0047]
In addition, although the intensity of the above-mentioned peak is the diffraction intensity at the
time of X-ray irradiation, it is shown by the count rate (cps; count per second) of X-rays per
second, but in FIG. In order to compare perovskite crystallinity relative to each RMS, it is shown
in arbitrary units (au; arbitrary units) rather than absolute values (counting rates).
[0048]
On the other hand, the vertical axis on the right side of FIG. 3 indicates the degree of (100)
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orientation of PZT.
Assuming that the (100) orientation degree is I (%), in FIG. 2, the intensity of the peak showing
the perovskite (100) is I (100) and the intensity of the peak showing the perovskite (111) is I
(111) Sometimes, the degree of orientation I is determined by: I = {I (100) / (I (100) + I (111))} ×
100. The relationship between the surface roughness RMS of the electrode and the (100)
orientation is shown by the broken line in the same figure.
[0049]
According to FIG. 3, when the surface roughness RMS of the electrode increases, the (100)
orientation degree of the PZT improves, but when the surface roughness RMS of the electrode
becomes smaller than 1 nm, the (100) orientation degree of the PZT decreases. The required
piezoelectric characteristics can not be obtained.
[0050]
Thus, the perovskite crystallinity of PZT and the orientation ((100) orientation degree) are in a
contradictory relationship, and in order to make them compatible, the surface roughness RMS of
the electrode is controlled in the range of 1 nm to 10 nm. There is a need to.
More preferably, the surface roughness RMS of the electrode is 1 nm or more and 5 nm or less.
In order to realize such surface roughness RMS of the electrode, in the present embodiment, for
example, the surface of the substrate 1 (Si substrate) underlying the electrode is roughened to
the same extent as described above. Since the electrode made of Pt is very thin and formed to
follow the surface of the lower layer such as Si substrate, the surface roughness of Pt is
controlled within a desired range by adjusting the surface roughness RMS of the Si substrate. be
able to.
[0051]
〔3. Method of Manufacturing Piezoelectric Element Hereinafter, a method of manufacturing
the piezoelectric element 10 of the present embodiment including the roughening of the surface
of the substrate 1 will be described based on the flowchart of FIG. 4 with reference to FIG.
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[0052]
First, the surface of the substrate 1 made of Si is polished so that the surface roughness RMS
becomes 1 to 10 nm, and the surface of the substrate 1 is roughened (S1). As a method of
polishing the surface of the substrate 1, for example, a known chemical mechanical polishing
(CMP) can be used. Incidentally, in this chemical mechanical polishing method, the object to be
polished is held by a member called a carrier, the object to be polished is pressed against a flat
plate on which a polishing pad or a polishing pad is stretched, and various chemical components
and hard fine abrasive grains are included. In this method, polishing is performed by causing
relative motion (for example, rotational motion) together while flowing slurry. The surface
roughness of the substrate 1 which is the object to be polished can be controlled by adjusting the
material and size of the abrasive used, the number of rotations at the time of polishing, the
pressure, the time and the like. For example, the surface roughness RMS of the substrate 1 was
about 0.3 nm before polishing but increased to 4.8 nm after polishing.
[0053]
Next, a thermal oxide film 2 made of SiO 2 for insulation and protection is formed on the surface
of the substrate 1 (S 2). The thermal oxide film 2 can be formed by heating the substrate 1 at
about 1000.degree. Since the surface roughness of the thermal oxide film 2 follows the surface
roughness of the lower substrate 1, the surface roughness RMS of the thermal oxide film 2 is 1 to
10 nm, but here the substrate 1 is oxidized at a high temperature Since the thermal oxide film 2
was formed, the surface roughness RMS of the thermal oxide film 2 slightly decreased to 4.0 nm.
[0054]
Subsequently, Ti and Pt are sequentially formed by sputtering on the thermal oxide film 2 of the
substrate 1 to form the lower electrode 3 composed of the Ti layer 3a and the Pt layer 3b (S3,
S4). At this time, Pt constituting the Pt layer 3 b has a self-orientation property, and thus is
oriented in the (111) direction. The surface roughness of the Pt layer 3b follows the surface
roughness of the lower layer, so the surface roughness RMS of the Pt layer 3b is 4.0 nm, which is
the same as the surface roughness RMS of the thermal oxide film 2, but the surface of the lower
layer is It is controlled in the range of 1 to 10 nm depending on the roughness.
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[0055]
Next, the substrate 1 is heated to about 600 ° C., and the piezoelectric thin film 4 made of PZT
is formed on the lower electrode 3 by sputtering (S5). The film formation conditions of the
piezoelectric thin film 4 at this time are film formation conditions in which PZT has (100)
orientation. The details of the method of forming the piezoelectric thin film 4 will be described
later. Since the Pt layer 3b is formed on the substrate 1 so as to be self-oriented in the (111)
direction, the PZT formed thereon is likely to be oriented in the (111) direction, but as described
above Since the surface is roughened, it is possible to deposit PZT in a (100) orientation on part
of the surface of the Pt layer 3b. If PZT is partially oriented in the (100) direction at the initial
stage of film formation of PZT, then the film thickness of PZT can be increased to broaden the
region of (100) orientation of PZT, as described above. The percentage of areas can be increased.
[0056]
Finally, Ti and Pt are sequentially formed by sputtering on the piezoelectric thin film 4 to form
the upper electrode 5 composed of the Ti layer 5a and the Pt layer 5b (S6, S7), and the
piezoelectric element 10 is completed. .
[0057]
〔4.
Details of PZT Film Forming Method] FIG. 5 is a cross-sectional view showing a schematic
configuration of a sputtering apparatus for forming PZT as the piezoelectric thin film 4. The
piezoelectric thin film 4 can be formed, for example, by high frequency magnetron sputtering.
[0058]
First, as a target material, a powder of a PZT material prepared by blending PTO and PZO at a
composition ratio of 50:50, which can obtain high characteristics, is fired, crushed, filled in the
target plate 12, and added by a press By pressing, the target 11 is produced. Then, the target
plate 12 is placed on the magnet 13 and the cover 14 is placed thereon. The magnet 13 and the
high frequency electrode 15 under the magnet 13 are insulated from the vacuum chamber 17 by
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the insulator 16. The high frequency electrode 15 is connected to a high frequency power supply
18.
[0059]
Next, the substrate 1 is placed on the substrate heater 19. Then, the inside of the vacuum
chamber 17 is exhausted and the substrate 1 is heated to 600 ° C. by the substrate heater 19.
After heating, the valves 20 and 21 are opened, and Ar and O2 as sputtering gases are
introduced into the vacuum chamber 17 from the nozzle 22 at a predetermined ratio to keep the
degree of vacuum at a predetermined value. By supplying high frequency power to the target 11
from the high frequency power supply 18 and generating plasma, a PZT layer as the piezoelectric
thin film 4 can be formed on the substrate 1.
[0060]
〔5. Regarding Effect of Roughening As described above, since the surface of the Pt layer 3 b
is roughened by roughening the surface of the substrate 1 located below the Pt layer 3 b as the
electrode, as in the prior art The surface of the Pt layer 3 b can be roughened without depositing
a metal (for example, Ti) of the same element as the constituent element of the piezoelectric thin
film 4 on the Pt layer 3 b from the lower layer. As a result, the ratio of PTO to PZO in the
piezoelectric thin film 4 is changed by the interdiffusion between the metal piezoelectric thin
films 4 and the deterioration of the piezoelectric characteristics can be avoided.
[0061]
In addition, since the surface roughening of the surface lower than the Pt layer 3b can be easily
performed by the known method (for example, CMP) as described above, compared to the
conventional method of forming hillocks on the Pt layer Thus, the surface roughness (surface
shape) of the Pt layer 3b can be easily controlled, and the (100) orientation and the perovskite
crystallinity of the piezoelectric thin film 4 can be enhanced.
[0062]
Therefore, even when the piezoelectric thin film 4 having the (100) orientation direction different
from that of the electrode is formed on the Pt layer 3b oriented in the (111) direction, the (100)
orientation of the piezoelectric thin film 4 and the perovskite crystallinity Both can be easily
satisfied to improve the piezoelectric characteristics.
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[0063]
Further, in the present embodiment, the surface lower than the Pt layer 3 b is roughened so that
the surface roughness RMS of the Pt layer 3 b is 1 nm or more and 10 nm or less.
When the surface roughness RMS of the Pt layer 3b falls below the lower limit of 1 nm, the
surface of the Pt layer 3b becomes flatter, so that the (100) orientation of the piezoelectric thin
film 4 is lowered as shown in FIG.
That is, the piezoelectric thin film 4 is easily formed in the (111) orientation following the lower
Pt layer 3b, and it becomes difficult to form the piezoelectric thin film 4 in the (100) orientation.
Conversely, when the surface roughness RMS of the Pt layer 3b exceeds the upper limit of 10
nm, as shown in FIG. 3, the perovskite crystallinity of the piezoelectric thin film 4 collapses, and
the piezoelectric thin film 4 with good characteristics is formed. Is difficult. Therefore, when the
surface roughness RMS of the Pt layer 3b is within the above range, the (100) orientation of the
piezoelectric thin film 4 and the perovskite crystallinity can be compatible, and good
piezoelectric characteristics can be obtained.
[0064]
Further, in the present embodiment, the surface of the Pt layer 3b is roughened by roughening
the surface of the substrate 1 which is the base of the Pt layer 3b, so that the roughened surface
of the substrate 1 is copied. The surface of the Pt layer 3b can be easily roughened in the form.
In particular, by roughening the surface of the substrate 1 so that the surface roughness RMS is
1 nm or more and 10 nm or less, the surface roughness RMS of the Pt layer 3 b formed on the
substrate 1 is 1 nm or more and 10 nm or less It can be easily controlled.
[0065]
In the above, the example using the Si substrate as the base substrate 1 of the Pt layer 3b has
been described, but even when using a substrate other than the Si substrate, an electrode is
formed on the substrate and the electrode is When depositing piezoelectric thin films having
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different orientation directions, the electrode surface can be roughened to obtain the abovedescribed effect by polishing the substrate surface to have a desired surface roughness. As a
substrate other than Si, for example, MgO (magnesium oxide), STO (SrTiO3; strontium titanate),
glass, or a metal substrate can be used.
[0066]
〔6. Another Example of Control of Surface Roughness of Electrode] The surface roughness of
the substrate 1 can be controlled by a dry etching method, whereby the surface roughness of the
Pt layer 3b can also be controlled. For example, as a result of performing reactive ion etching
(RIE) under the conditions of SF6: 20 sccm, O2: 9 sccm, pressure: 3 Pa, RF power: 150 W, etching
time: 5 minutes using a Si substrate as the substrate 1 Surface roughness RMS was 0.3 nm before
etching but increased to 5.5 nm after etching. At this time, the surface roughness RMS of the
thermal oxide film 2 was 4.2 nm, and the surface roughness RMS of the Pt layer 3 b after
forming the Ti layer 3 a and the Pt layer 3 b was 3 nm. Thus, even if the surface of the substrate
1 is dry-etched and roughened, the surface roughness RMS of the Pt layer 3b can be controlled in
the range of 1 to 10 nm.
[0067]
Also, the surface roughness of the substrate 1 can be controlled by a wet etching method,
whereby the surface roughness of the Pt layer 3b can also be controlled. For example, a Si
substrate is used as the substrate 1, and the substrate 1 is immersed in an aqueous solution of
tetramethylammonium hydroxide in a wet bath and etched under the conditions of a temperature
of 45 ° C. and an etching time of 15 minutes. The surface roughness RMS in the plane where
the substrate orientation of (100) is (100) was 0.3 nm before etching but increased to 9.2 nm
after etching. At this time, the surface roughness RMS of the thermal oxide film 2 was 6.3 nm,
and the surface roughness RMS of the Pt layer 3 b after forming the Ti layer 3 a and the Pt layer
3 b was 4.5 nm. Thus, even if the surface of the substrate 1 is wet-etched and roughened, the
surface roughness RMS of the Pt layer 3b can be controlled in the range of 1 to 10 nm. The
etching solution may be another alkaline solution such as KOH (potassium hydroxide aqueous
solution).
[0068]
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The surface roughness of the Pt layer 3b can also be controlled by roughening the surface of the
thermal oxide film 2 formed on the substrate 1. For example, a Si substrate is used as the
substrate 1, this substrate 1 is oxidized at high temperature to form a thermal oxide film 2, and
then CHF3: 60 sccm, pressure: 0.3 Pa, RF power: 250 W, etching time: 2 minutes, As a result of
performing reactive ion etching (RIE) under the conditions, the surface roughness RMS of the
thermal oxide film 2 was 0.3 nm before dry etching, but increased to 3.5 nm after dry etching.
Then, the surface roughness RMS of the Pt layer 3 b after forming the Ti layer 3 a and the Pt
layer 3 b on the thermal oxide film 2 was 2.7 nm.
[0069]
As described above, even if the surface of the thermal oxide film 2 is dry-etched and roughened,
the surface of the Pt layer 3b can be roughened. In particular, by roughening the surface of the
thermal oxide film 2 so that the surface roughness RMS is 1 nm or more and 10 nm or less, the
surface roughness RMS of the Pt layer 3 b is equal to the surface roughness of the surface of the
thermal oxide film 2 Can be 1 nm or more and 10 nm or less.
[0070]
〔7. Another Configuration of Piezoelectric Element FIG. 6 is a cross-sectional view showing
another configuration of the piezoelectric element 10. As shown to the same figure, the
piezoelectric thin film 4 may be comprised including the 1st piezoelectric thin film 4b and the
2nd piezoelectric thin film 4a. The second piezoelectric thin film 4a forms an intermediate layer
between the first piezoelectric thin film 4b and the Pt layer 3b. That is, the second piezoelectric
thin film 4a and the first piezoelectric thin film 4b are stacked in this order on the Pt layer 3b.
The first piezoelectric thin film 4 b is made of, for example, PZT.
[0071]
The second piezoelectric thin film 4a is a piezoelectric thin film having a perovskite structure,
and is made of, for example, PLT (lead lanthanum titanate). The second piezoelectric thin film 4a
is provided to enhance the perovskite crystallinity of the first piezoelectric thin film 4b formed
thereon. The PLT as the second piezoelectric thin film 4a is formed with a film thickness of 45
nm on the Pt layer 3b by, for example, sputtering, and the film forming conditions at that time
are, for example, Ar flow rate: 19.5 sccm, O2 flow rate: 0 .5 sccm, pressure: 0.5 Pa, substrate
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temperature: 640 ° C., RF power applied to target: 150 W.
[0072]
The second piezoelectric thin film 4a as an intermediate layer easily forms a crystal nucleus of
perovskite on the Pt layer 3b, and the crystal growth of the first piezoelectric thin film 4b (PZT)
is promoted with this as a trigger. When the second piezoelectric thin film 4a is provided, the
crystallinity and the orientation of the first piezoelectric thin film 4b follow the crystallinity and
the orientation of the second piezoelectric thin film 4a as the base. Therefore, in the same
manner as described above, the surface of the underlayer of the Pt layer 3b is roughened to
control the surface roughness of the Pt layer 3b, and the perovskite crystallinity of the second
piezoelectric thin film 4a formed on the Pt layer 3b By enhancing the (100) orientation, it is
possible to form the first piezoelectric thin film 4b with high characteristics on the second
piezoelectric thin film 4a.
[0073]
The second piezoelectric thin film 4a as the intermediate layer is not limited to the abovedescribed PLT, and in addition, LNO (LaNiO3; nickel oxide lanthanum), SRO (SrRuO3; ruthenium
ruthenate), PTO An oxide such as PbTiO3 (lead titanate) can be used as a constituent material of
the second piezoelectric thin film 4a.
[0074]
〔8.
Application Example of Piezoelectric Element] FIG. 7 is a plan view showing a configuration when
the piezoelectric element 10 manufactured in the present embodiment is applied to a diaphragm
(diaphragm), and FIG. 8 is a line AA 'in FIG. It is arrow sectional drawing. The piezoelectric thin
films 4 are arranged in a two-dimensional staggered manner in the necessary area of the
substrate 1. A region corresponding to the formation region of the piezoelectric thin film 4 in the
substrate 1 is a concave portion 1a in which a part in the thickness direction is removed with a
circular cross section, and is formed on the upper portion (bottom side of concave portion 1a) of
the concave portion 1a in the substrate 1 The thin plate-like area 1b remains. The lower
electrode 3 and the upper electrode 5 are connected to an external control circuit by a wire not
shown.
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[0075]
By applying an electric signal to the lower electrode 3 and the upper electrode 5 sandwiching the
predetermined piezoelectric thin film 4 from the control circuit, only the predetermined
piezoelectric thin film 4 can be driven. That is, when a predetermined electric field is applied to
the upper and lower electrodes of the piezoelectric thin film 4, the piezoelectric thin film 4
expands and contracts in the left and right direction, and the piezoelectric thin film 4 and the
region 1b of the substrate 1 are curved upward and downward by the effect of the bimetal.
Therefore, when the concave portion 1a of the substrate 1 is filled with gas or liquid, the
piezoelectric element 10 can be used as a pump, which is suitable for an inkjet head, for example.
[0076]
In addition, the amount of deformation of the piezoelectric thin film 4 can also be detected by
detecting the charge amount of the predetermined piezoelectric thin film 4 via the lower
electrode 3 and the upper electrode 5. In other words, when the piezoelectric thin film 4 vibrates
due to sound waves or ultrasonic waves, an electric field is generated between the upper and
lower electrodes by the opposite effect to the above, so piezoelectricity is detected by detecting
the magnitude of the electric field and the frequency of the detection signal at this time. The
element 10 can also be used as a sensor (ultrasonic sensor). Furthermore, as long as the
piezoelectric thin film 4 exerts a pyroelectric effect, the piezoelectric element 10 can also be used
as a pyroelectric sensor (infrared sensor).
[0077]
In addition, by utilizing the piezoelectric effect of the piezoelectric thin film 4, the piezoelectric
element 10 can also be used as a frequency filter (surface elastic wave filter), and when the
piezoelectric thin film 4 is formed of a ferroelectric, the piezoelectric element 10 is non-volatile It
can also be used as sex memory.
[0078]
In the present embodiment, the piezoelectric thin film 4 is formed by sputtering, but as the film
forming method of the piezoelectric thin film 4, not only the sputtering method described above,
but also the vapor deposition method, which is physical vapor deposition, It is also possible to
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use other methods such as a CVD method which is a vapor phase growth method and a sol-gel
method which is a liquid phase method.
[0079]
The present invention is applicable to various devices such as, for example, an inkjet head, an
ultrasonic sensor, an infrared sensor, a frequency filter, and a non-volatile memory.
[0080]
Reference Signs List 1 substrate 2 thermal oxide film 3 b Pt layer (electrode) 4 piezoelectric thin
film 4 a second piezoelectric thin film 4 b first piezoelectric thin film 10 piezoelectric element
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