U.S. patent application number 12/390882 was filed with the patent office on 2009-06-25 for piezoelectric device, process for producing the piezoelectric device, and inkjet recording head.
Invention is credited to Takamichi FUJII.
Application Number | 20090160914 12/390882 |
Document ID | / |
Family ID | 38574496 |
Filed Date | 2009-06-25 |
United States Patent
Application |
20090160914 |
Kind Code |
A1 |
FUJII; Takamichi |
June 25, 2009 |
PIEZOELECTRIC DEVICE, PROCESS FOR PRODUCING THE PIEZOELECTRIC
DEVICE, AND INKJET RECORDING HEAD
Abstract
A process for producing a piezoelectric device constituted by a
first electrode, at least one second electrode, and a piezoelectric
film sandwiched between the first electrode and the at least one
second electrode so that an electric field can be applied to the
piezoelectric film. First, a seed layer of a material containing at
least one element is formed on a substrate, and then the first
electrode is formed on the seed layer. Next, the at least one
element is diffused through the first electrode so that the at
least one element precipitates on a surface of the first electrode
on the opposite side to the seed layer, and then the piezoelectric
film is formed on the first electrode.
Inventors: |
FUJII; Takamichi;
(Ashigarakami-gun, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
38574496 |
Appl. No.: |
12/390882 |
Filed: |
February 23, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11783221 |
Apr 6, 2007 |
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12390882 |
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Current U.S.
Class: |
347/68 ; 310/328;
310/358 |
Current CPC
Class: |
B41J 2/1628 20130101;
H01L 41/319 20130101; B41J 2/1646 20130101; H01L 41/1876 20130101;
H01L 41/316 20130101; Y10T 29/42 20150115; B41J 2/161 20130101;
H01L 41/0477 20130101; H01L 41/0973 20130101 |
Class at
Publication: |
347/68 ; 310/358;
310/328 |
International
Class: |
B41J 2/045 20060101
B41J002/045; H01L 41/187 20060101 H01L041/187 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 7, 2006 |
JP |
2006-106256 |
Claims
1-14. (canceled)
15. An inkjet recording head comprising: a piezoelectric device
constituted by a first electrode, at least one second electrode,
and a piezoelectric film sandwiched between the first electrode and
the at least one second electrode so that an electric field can be
applied to the piezoelectric film, the piezoelectric device being
produced by forming on a substrate a seed layer of a material
containing at least one element; forming said first electrode on
said seed layer; diffusing said at least one element though said
first electrode so that the at least one element precipitates on a
surface of the first electrode one a side opposite to the said seed
layer; and forming said piezoelectric film on said first electrode;
and an ink hold-and-discharge member being attached to said
piezoelectric device and having, one or more ink chambers which
hold ink, and one or more ink outlets through which said ink is
externally discharged from said one or more ink chambers.
16. An inkjet recording apparatus comprising said inkjet recording
head according to claim 15.
17. A piezoelectric device comprising: a substrate; a first
electrode formed over said substrate; a piezoelectric film formed
on said first electrode; and at least one second electrode formed
on said piezoelectric film; said piezoelectric film is sandwiched
between said first electrode and said at least one second electrode
so that an electric field an be applied to the piezoelectric film,
at least one element constituting the piezoelectric film
precipitates on a surface of the first electrode on a side opposite
to the said substrate, and the surface has an arithmetic average
surface roughness (Ra) of 0.5 to 30 nm.
18. A piezoelectric device according to claim 17, wherein said
piezoelectric film has a piezoelectric constant d.sub.31 of 150
pm/V or greater.
19. An inkjet recording head comprising: said piezoelectric device
according to claim 17; and an ink hold-and-discharge member being
attached to said piezoelectric device and having, one or more ink
chambers which hold ink, and one or more ink outlets through which
said ink is externally discharged from said one or more ink
chambers.
20. An inkjet recording apparatus comprising said inkjet recording
head according to claim 19.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a piezoelectric device for
use in an inkjet recording head, a process for producing the
piezoelectric device, and an inkjet recording head using the
piezoelectric device.
[0003] 2. Description of the Related Art
[0004] Currently, piezoelectric devices constituted by a
piezoelectric film and electrodes are used as, for example,
actuators installed in inkjet recording heads, where the
piezoelectric film expands and contracts according to increase and
decrease in the strength of an electric field applied from the
electrodes to the piezoelectric film in a predetermined direction.
For example, perovskite oxides such as PZT (lead titanate
zirconate) are known as materials suitable for the piezoelectric
film.
[0005] When the vector direction of the spontaneous polarization
axis in the piezoelectric film coincides with the direction of the
electric field applied to the piezoelectric film, the expansion and
contraction according to increase and decrease in the strength of
the electric field effectively occur, so that the piezoelectric
constants become great. It is most preferable that the direction of
the spontaneous polarization axis in the piezoelectric film
completely coincide with the direction of the electric field
applied to the piezoelectric film. In addition, in order to
suppress the variations of the ink discharge amount, it is
desirable that the variations in the piezoelectric performance of
the piezoelectric film over the surface of the piezoelectric film
be small. In consideration of the above circumstances, it is
desirable that the piezoelectric film have high degree of crystal
orientation.
[0006] For example, Japanese Unexamined Patent Publication No.
2004-186646 (hereinafter referred to as JPP 2004-186646) discloses
a technique of growing a PZT film which is preferredly oriented
along the (001) plane on a surface of a lower electrode of noble
metal containing titanium, where the lower electrode is formed by
concurrent sputtering of titanium and noble metal such as platinum,
and titanium is insularly precipitated on the surface of the lower
electrode.
[0007] In addition, Japanese Unexamined Patent Publication No.
2004-262253 (hereinafter referred to as JPP 2004-262253) discloses
that it is possible to form a PZT film having high degree of
crystal orientation by using a MgO substrate.
[0008] According to the technique disclosed in JPP 2004-186646, it
is possible to form a PZT film exhibiting crystal orientation.
However, as indicated in the embodiments 1 to 5 in JPP 2004-186646,
the values of the piezoelectric constant d.sub.31 of the PZT films
obtained according to the embodiments 1 to 5 in JPP 2004-186646
range from 122 to 138 pm/V (i.e., -122 to -138 pC/N), and no PZT
film having the piezoelectric constant d.sub.31 of 150 pm/V or
greater is reported in JPP 2004-186646.
[0009] On the other hand, since the MgO substrate, which is
required to be used in the technique disclosed in JPP 2004-262253,
is expensive, the use of the technique disclosed in JPP 2004-262253
increases the manufacturing cost.
[0010] In addition, according to the techniques disclosed in JPP
2004-186646 and JPP 2004-262253, it is basically necessary that the
film-formation temperature at which the PZT film is formed be
600.degree. C. or higher, as indicated in the embodiments 1, 2, 4,
and 5 in JPP 2004-186646. In addition, the lowest film-formation
temperature indicated in JPP 2004-186646 is 580.degree. C. (in the
embodiment 3 in JPP 2004-186646). Thus, it is conjectured that the
film-formation temperature is required to be 580.degree. C. or
higher according to the techniques disclosed in JPP 2004-186646.
However, when the PZT film is formed at high temperatures, lead
loss can occur in the PZT film, so that the piezoelectric
performance can deteriorate.
SUMMARY OF THE INVENTION
[0011] The present invention has been made in view of such
circumstances.
[0012] The first object of the present invention is to provide a
process for producing a piezoelectric device exhibiting high degree
of crystal orientation and high piezoelectric performance, at a
relatively low temperature without use of an expensive substrate
such as the MgO substrate.
[0013] The second object of the present invention is to provide a
piezoelectric device which is produced by the above process, and
exhibits high degree of crystal orientation and high piezoelectric
performance.
[0014] The third object of the present invention is to provide an
inkjet recording head having the above piezoelectric device.
[0015] The fourth object of the present invention is to provide an
inkjet recording apparatus having the above piezoelectric
device.
[0016] (I) In order to accomplish the above first object, according
to the first aspect of the present invention, a process for
producing a piezoelectric device is provided, where the
piezoelectric device is constituted by a first electrode, at least
one second electrode, and a piezoelectric film sandwiched between
the first electrode and the at least one second electrode so that
an electric field can be applied to the piezoelectric film. The
process according to the first aspect of the present invention
comprises the steps of: (a) forming on a substrate a seed layer of
a material containing at least one element; (b) forming the first
electrode on the seed layer; (c) diffusing the at least one element
through the first electrode so that the at least one element
precipitates on a surface of the first electrode on the opposite
side to the seed layer; and (d) forming the piezoelectric film on
the first electrode.
[0017] That is, in the process according to the first aspect of the
present invention, the seed layer and the first electrode are
successively formed on the substrate, and then the at least one
element contained in the seed layer is diffused so that the at
least one element precipitates on the upper surface of the first
electrode. Thereafter, the piezoelectric film is formed on the
first electrode. Therefore, it is possible to form the
piezoelectric film exhibiting high degree of crystal orientation
and high piezoelectric performance, at a relatively low temperature
without use of an expensive substrate such as the MgO substrate.
Specifically, in the process according to the first aspect of the
present invention, it is possible to form the piezoelectric film at
temperatures in the range of 400.degree. C. to 700.degree. C., and
produce a piezoelectric device with a piezoelectric film which has
a piezoelectric constant d.sub.31 of 150 pm/V or greater. Further,
in the process according to the first aspect of the present
invention, it is possible to produce a piezoelectric device with a
piezoelectric film which has a tetragonal or rhombohedral crystal
structure and crystal orientation along a (100) or (001) face.
[0018] Preferably, the above process according to the first aspect
of the present invention may further comprise one or any possible
combination of the following additional features (i) to (ix).
[0019] (i) The at least one element diffused in the step (c)
includes at least one element constituting the piezoelectric
film.
[0020] (ii) The piezoelectric film is formed in the step (c) at a
temperature equal to or higher than 400.degree. C. and lower than
600.degree. C.
[0021] (iii) The seed layer has a thickness of 5 to 50 nm, and the
first electrode has a thickness of 50 to 500 nm.
[0022] (iv) The at least one element is diffused in the step (c) so
that the surface has an arithmetic average surface roughness Ra of
0.5 to 30.0 nm after the step (c) is completed. The values of the
arithmetic average surface roughness Ra in this specification are
obtained in accordance with JIS (Japanese Industrial Standard)
B0601-1994.
[0023] (v) The at least one element includes titanium.
[0024] (vi) In the process having the additional feature (v),
titanium as the at least one element is thermally diffused by heat
treatment which is performed at a temperature in the range of
400.degree. C. to 700.degree. C.
[0025] (vii) In the process having the additional feature (vi), the
thermal diffusion of titanium is performed for 10 minutes to 2
hours.
[0026] (viii) The piezoelectric film has a tetragonal or
rhombohedral crystal structure and crystal orientation along a
(100) or (001) face.
[0027] In this specification, the expression "the piezoelectric
film has crystal orientation" means that the piezoelectric film
exhibits a degree F. of orientation equal to or greater than 80%
obtained by the Lotgerling technique. The degree F. of orientation
is defined by the formula,
F(%)=(P-P0)/(1-P0)X100, (1)
where P is the ratio of the total XRD (X-ray diffraction) intensity
from an orientation plane to the total XRD intensity from all the
crystal planes, and P0 is the value of P in the case where the
sample is completely randomly oriented. In the case of the (001)
orientation, P=.SIGMA.I(001)/.SIGMA.I(hkl) where I(hkl) is the XRD
intensity from the crystal plane (hkl), .SIGMA.I(001) is the total
XRD intensity from the crystal plane (001), and .SIGMA.I(hkl) is
the total XRD intensity from all the crystal planes (hkl). For
example, in the case of the (001) orientation in a perovskite
crystal, P=I(001)/{I(001)+I(100)+I(101)+I(110)+I(111)}. When the
sample is completely randomly oriented, F=0%. When the sample is
completely oriented, F=100%.
[0028] (ix) The piezoelectric film is formed of at least one
perovskite oxide or at least one mixed crystal of perovskite
oxides, and the at least one perovskite oxide and the perovskite
oxides are one or more of lead titanate, lead titanate zirconate,
barium titanate, bismuth sodium titanate, bismuth potassium
titanate, sodium niobate, potassium niobate, and lithium niobate.
(The piezoelectric film may contain inevitable impurities.)
[0029] (II) In order to accomplish the aforementioned second
object, a piezoelectric device according to the second aspect of
the present invention is provided. The piezoelectric device
according to the second aspect of the present invention is a
piezoelectric device produced by the process according to the first
aspect of the present invention.
[0030] In order to accomplish the aforementioned second object, a
piezoelectric device according to the third aspect of the present
invention is also provided. The piezoelectric device according to
the third aspect of the present invention comprises: a substrate; a
first electrode formed over the substrate; a piezoelectric film
formed on the first electrode; and at least one second electrode
formed on the piezoelectric film. In the piezoelectric device
according to the third aspect of the present invention, the
piezoelectric film is sandwiched between the first electrode and
the at least one second electrode so that an electric field can be
applied to the piezoelectric film, at least one element
constituting the piezoelectric film precipitates on a surface of
the first electrode on the opposite side to the substrate, and the
surface has an arithmetic average surface roughness Ra of 0.5 to 30
nm.
[0031] Preferably, in the piezoelectric devices according to the
second and third aspects of the present invention, the
piezoelectric film has a piezoelectric constant d.sub.31 of 150
pm/V or greater.
[0032] In this specification, the piezoelectric constant d.sub.31
is obtained as follows.
[0033] The amount of displacement of the piezoelectric film is
measured by using a laser Doppler vibrometer under the condition
that rectangular pulses of the electric field with the field
strength of 60 kV/cm are applied to the piezoelectric film at the
frequency of 30 kHz, which is a typical condition under which the
piezoelectric device according to the present invention is driven
in practical use. The above measurement is performed at intervals
of 10 mm along each of the x and y directions parallel to the
substrate. At the same time, the driving frequency at which the
displacement is maximized is obtained as the resonance frequency by
varying the driving frequency. Then, the piezoelectric constant
d.sub.31 at each measurement point is obtained by analysis (using
the structural analysis software "ANSYS") of the results of the
above measurement, and an average of the values of the
piezoelectric constant d.sub.31 at all the measurement points is
obtained as the value of the piezoelectric constant d.sub.31 of the
piezoelectric film.
[0034] (III) In order to accomplish the aforementioned third
object, an inkjet recording head according to the fourth aspect of
the present invention is provided. The inkjet recording head
according to the fourth aspect of the present invention comprises:
the piezoelectric device according to the second aspect of the
present invention; and an ink hold-and-discharge member attached to
the piezoelectric device. The hold-and-discharge member has one or
more ink chambers which hold ink, and one or more ink outlets
through which the ink is externally discharged from the one or more
ink chambers.
[0035] In order to accomplish the aforementioned third object, an
inkjet recording head according to the fifth aspect of the present
invention is also provided. The inkjet recording head according to
the fifth aspect of the present invention comprises: the
piezoelectric device according to the third aspect of the present
invention; and an ink hold-and-discharge member attached to the
piezoelectric device. The hold-and-discharge member has one or more
ink chambers which hold ink, and one or more ink outlets through
which the ink is externally discharged from the one or more ink
chambers.
[0036] (IV) In order to accomplish the aforementioned fourth
object, an inkjet recording apparatus according to the sixth aspect
of the present invention is provided. The inkjet recording
apparatus according to the sixth aspect of the present invention is
an inkjet recording apparatus which comprises the inkjet recording
head according to according to the fourth aspect of the present
invention.
[0037] In order to accomplish the aforementioned fourth object, an
inkjet recording apparatus according to the seventh aspect of the
present invention is provided. The inkjet recording apparatus
according to the seventh aspect of the present invention is an
inkjet recording apparatus which comprises the inkjet recording
head according to according to the fifth aspect of the present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] FIG. 1 is a cross-sectional view schematically illustrating
a cross section of an essential portion of an inkjet recording head
having a piezoelectric device according to an embodiment of the
present invention.
[0039] FIGS. 2A, 2B, 2C, 2D, and 2E are diagrams illustrating
structures in respective steps in a process for producing the
piezoelectric device constituting the inkjet recording head of FIG.
1.
[0040] FIG. 3 is a schematic diagram of an example of an inkjet
recording apparatus having the inkjet recording head of FIG. 1.
[0041] FIG. 4 is a top view of a portion of the inkjet recording
apparatus of FIG. 3.
[0042] FIG. 5A is an AFM (atomic force microscope) photograph of a
surface of a lower electrode in a comparison example 1.
[0043] FIG. 5B is an AFM photograph of a surface of a lower
electrode in a concrete example 2.
DESCRIPTION OF PREFERRED EMBODIMENT
[0044] A preferred embodiment of the present invention is explained
in detail below with reference to drawings.
[0045] Piezoelectric Device and Inkjet Recording Head
[0046] Hereinbelow, the structures of a piezoelectric device and an
inkjet recording head according to a preferred embodiment of the
present invention are explained with reference to FIG. 1, which is
a cross-sectional view schematically illustrating a cross section
of an essential portion of the inkjet recording head having the
piezoelectric device, where the vertical direction corresponds to
the thickness direction of the piezoelectric device, and the
respective elements are illustrated with such dimensions (different
from the actual dimensions) as to clearly indicate the structures
of the piezoelectric device and the inkjet recording head.
[0047] The piezoelectric device 1 is constituted by a lower
electrode 30, a piezoelectric film 40, and upper electrodes 50,
which are laminated on a substrate 10 in this order. The
piezoelectric film 40 is formed of a piezoelectric inorganic
material so that an electric field in the thickness direction can
be applied to the piezoelectric film 40 between the lower electrode
30 and the upper electrodes 50.
[0048] A seed layer 20, which contains at least one of constituents
elements of the piezoelectric film 40, is formed between the
substrate 10 and the lower electrode 30. As explained in detail
later, the piezoelectric film 40 is formed after an element
constituting the piezoelectric film 40 which is contained in the
seed layer 20 is (preferably thermally) diffused so that the
element precipitates on an upper surface of the lower electrode 30.
The seed layer 20 can also have the function of an adhesion layer
which makes the lower electrode 30 satisfactorily adhere to the
substrate 10.
[0049] In the piezoelectric device 1, the seed layer 20 and the
lower electrode 30 are formed on the approximately entire upper
surface of the substrate 10, the piezoelectric film 40 is
constituted by a plurality of piezoelectric elements 41 which
protrude from the upper surface of the lower electrode 30, extend
in the direction perpendicular to the plane of FIG. 1, and are
arranged in a predetermined stripelike pattern, and the upper
electrodes 50 are formed on the tops of the piezoelectric elements
41. Alternatively, the pattern of the piezoelectric film 40 is not
limited to the above stripelike pattern, and the pattern of the
piezoelectric film 40 may be designed as needed. Further
alternatively, the piezoelectric film 40 may be realized by a
continuously formed film. However, it is preferable that the
piezoelectric film 40 be formed of the plurality of piezoelectric
elements 41, which are separated from each other, since each
piezoelectric element can expand or contract smoothly, so that
greater displacement occurs in the piezoelectric film 40.
[0050] According to the present embodiment, no special limitation
is imposed on the substrate 10. For example, the substrate 10 may
be made of silicon, glass, stainless steel, YSZ (yttrium stabilized
zirconia), alumina, sapphire, silicon carbide, or the like. In
addition, the substrate 10 may be realized by a laminated substrate
such as the SOI (silicon-on-insulator) substrate, which is produced
by forming an oxide film of SiO.sub.2 on a surface of a silicon
substrate. According to the present embodiment, it is possible to
form the piezoelectric film 40 exhibiting high degree of crystal
orientation and high piezoelectric performance without use of an
expensive substrate such as the MgO substrate.
[0051] In addition, the main component of the lower electrode 30 is
not specifically limited, and may be, for example, one or a
combination of metals such as Pt or Ir and metal oxides such as
IrO.sub.2, RuO.sub.2, LaNiO.sub.3, or SrRuO.sub.3. In the case
where the main component of the lower electrode 30 is one or a
combination of the above materials, it is possible to make an
element constituting the piezoelectric film 40 and being contained
in the seed layer 20 precipitate on the upper surface of the lower
electrode 30.
[0052] Further, the main component of the upper electrodes 50 is
not specifically limited, and may be, for example, one or a
combination of the materials indicated before as the examples of
the main component of the lower electrode 30 and the materials such
as Al, Ta, Cr, or Cu which are used in electrodes in the general
semiconductor processes.
[0053] Furthermore, although the material for the piezoelectric
film 40 is not specifically limited, it is preferable that the
piezoelectric film 40 be formed of one or more perovskite oxides,
although the piezoelectric film 40 may contain inevitable
impurities. The perovskite oxides may be:
[0054] (1) lead compounds such as lead titanate, lead titanate
zirconate (PZT), lead zirconate, lead lanthanum titanate, lead
lanthanum titanate zirconate, zirconium magnesium niobate-lead
titanate, zirconium nickel niobate-lead titanate, and mixed
crystals of such lead compounds; and
[0055] (2) nonlead compounds such as barium titanate, bismuth
sodium titanate, bismuth potassium titanate, sodium niobate,
potassium niobate, and lithium niobate, and mixed crystals of such
nonlead compounds.
[0056] The above examples of the perovskite oxides are
ferroelectric materials which exhibit spontaneous polarization when
no electric field is applied to the ferroelectric materials.
[0057] Moreover, the thickness of the piezoelectric film 40 is not
specifically limited, is normally 1 micrometer or greater, and is,
for example, 1 to 5 micrometers.
[0058] As mentioned before, the composition of the seed layer 20 is
determined so as to contain one or more elements constituting the
piezoelectric film 40.
[0059] The thicknesses of the seed layer 20 and the lower electrode
30 are determined so that an element constituting the piezoelectric
film 40 and being contained in the seed layer 20 can be diffused
and precipitate on the upper surface of the lower electrode 30, and
no other limitation is imposed on the thicknesses of the seed layer
20 and the lower electrode 30. However, it is preferable that the
thickness of the seed layer 20 be 5 to 50 nm, and the thickness of
the lower electrode 30 be 50 to 500 nm. When the thicknesses of the
seed layer 20 and the lower electrode 30 are too small, it is
difficult to form the seed layer 20 and the lower electrode 30. On
the other hand, when the thicknesses of the seed layer 20 and the
lower electrode 30 are too great, the time for and the cost of
formation of the seed layer 20 and the lower electrode 30 having
such great thicknesses unnecessarily increase. In addition, when
the thickness of the seed layer 20 is too small, stable supply of
the aforementioned element constituting the piezoelectric film 40
and being contained in the seed layer 20 to the lower electrode 30
can become difficult. Further, when the thickness of the lower
electrode 30 is too great, it becomes difficult to diffuse the
element contained in the seed layer 20 through the lower electrode
30 and make the element reach the upper surface of the lower
electrode 30.
[0060] According to the present embodiment, as mentioned before,
the piezoelectric film 40 is formed after an element constituting
the piezoelectric film 40 and being contained in the seed layer 20
is diffused so as to precipitate on the upper surface of the lower
electrode 30, so that it is possible to form the piezoelectric film
40 exhibiting high degree of crystal orientation since the
precipitate on the upper surface behaves as nuclei in the formation
of the piezoelectric film 40.
[0061] It is preferable to form the seed layer 20 containing at
least one element constituting the piezoelectric film 40, and make
the at least one element diffuse through the lower electrode 30 and
precipitate on the upper surface of the lower electrode 30.
Alternatively, the element which is diffused from the seed layer 20
to the upper surface of the lower electrode 30 may not be an
element constituting the piezoelectric film 40, and therefore the
seed layer 20 may not contain any of one or more elements
constituting the piezoelectric film 40, as long as the precipitate
on the upper surface has an affinity for the piezoelectric film 40
and behaves as nuclei in the formation of the piezoelectric film 40
so that the piezoelectric film 40 has high degree of crystal
orientation.
[0062] The crystals of PZT and the like can have any of the cubic,
tetragonal, and rhombohedral structures, and barium titanate and
the like can have any of the cubic, tetragonal, triclinic, and
rhombohedral crystal structures. Since the cubic crystals do not
exhibit piezoelectricity, the piezoelectric film 40 is required to
be realized by one of the tetragonal, triclinic, and rhombohedral
crystals.
[0063] The spontaneous polarization axes of the tetragonal,
triclinic, and rhombohedral crystals realizing the structures of
ferroelectric materials are <001>, <110>, and
<111>, respectively.
[0064] As mentioned before, when the vector direction of the
spontaneous polarization axis in the piezoelectric film 40
coincides with the direction of the electric field applied to the
piezoelectric film (which is perpendicular to the surface of the
substrate according to the present embodiment), the expansion and
contraction according to increase and decrease in the strength of
the electric field effectively occur, so that the piezoelectric
constants become great. It is most preferable that the direction of
the spontaneous polarization axis in the piezoelectric film
completely coincide with the direction of the electric field
applied to the piezoelectric film.
[0065] Specifically, it is preferable that the piezoelectric film
40 have one of the tetragonal, triclinic, and rhombohedral crystal
structures, and be a film having crystal orientation along the
(100), (001), or (111) face.
[0066] In consideration of the coincidence of the direction of the
electric field applied to the piezoelectric device 1 with the
spontaneous polarization axis, it is most preferable that the
piezoelectric film 40 be preferredly oriented along the (001) plane
when the piezoelectric film 40 has a tetragonal crystal structure,
along the (001) plane when the piezoelectric film 40 has a
triclinic crystal structure, and along the (111) plane when the
piezoelectric film 40 has a rhombohedral crystal structure.
[0067] For example, it is possible to form a seed layer 20
containing titanium so as to diffuse titanium through the lower
electrode 30 and make titanium precipitate on the upper surface of
the lower electrode 30 before formation of the piezoelectric film
40. In particular, in the case where the piezoelectric film 40
contains a titanium-containing perovskite oxide such as lead
titanate, lead titanate zirconate (PZT), or barium titanate, it is
preferable to form a seed layer 20 containing titanium so as to
diffuse titanium through the lower electrode 30 and make titanium
precipitate on the upper surface of the lower electrode 30 before
formation of the piezoelectric film 40.
[0068] The present inventor has found that the piezoelectric film
40 having a tetragonal or rhombohedral crystal structure and
crystal orientation along the (100) or (001) face can be formed by
forming the seed layer 20 as above.
[0069] The inkjet recording head 2 of FIG. 1 is formed by attaching
an ink nozzle 70 to the lower surface of the substrate 10 through a
diaphragm 60. The ink nozzle 70 is a member for holding and
discharging ink, and comprises a plurality of ink chambers 71 and a
plurality of ink outlets 72, which are arranged in correspondence
with the arrangement of the plurality of piezoelectric devices 41
on the lower electrode 30. Each ink chamber 71 holds ink, and the
ink held in each ink chamber 71 is discharged out of the ink
chamber 71 through the ink outlet 72 connected to the ink chamber
71.
[0070] In the above inkjet recording head 2, the strength of the
electric field applied to each piezoelectric device 41 is increased
or decreased so as to expand or contract the piezoelectric element,
and control the discharge and the discharge amount of the ink.
[0071] Production Process
[0072] Hereinbelow, a process of producing the piezoelectric device
1 and the inkjet recording head 2 is explained with reference to
FIGS. 2A, 2B, 2C, 2D, and 2E, which are diagrams illustrating
structures in respective steps in a process for producing the
piezoelectric device constituting the inkjet recording head of FIG.
1.
[0073] <Step A>
[0074] The structure produced in the step A in the process for
producing the piezoelectric device and a magnified view of a
portion of the upper surface of the lower electrode 30 after the
step A are illustrated in FIG. 2A.
[0075] First, the substrate 10 is prepared. For example, the
substrate 10 is a substrate made of silicon, glass, or stainless
steel, or an SOI substrate which is made by forming an SiO.sub.2
oxide film on a surface of a silicon substrate. Next, the seed
layer 20 and the lower electrode 30 are formed in this order over
the substrate 10. As illustrated in the magnified view of the
portion of the upper surface of the lower electrode 30 in FIG. 2A,
in this step, the upper surface of the lower electrode 30 is
approximately flat, and the arithmetic average surface roughness Ra
is typically less than 0.5 nm. The manners of production of the
seed layer 20 and the lower electrode 30 are not specifically
limited, and the seed layer 20 and the lower electrode 30 may be
formed by the sputtering or the like. It is possible to form the
seed layer 20 and the lower electrode 30 by using an identical film
formation system or different film formation systems. It is
preferable that the seed layer 20 have a thickness of 5 to 50 nm,
and the lower electrode 30 have a thickness of 50 to 500 nm.
[0076] <Step B>
[0077] The structure produced in the step B in the process for
producing the piezoelectric device and a magnified view of a
portion of the upper surface of the lower electrode 30 after the
step B are illustrated in FIG. 2B.
[0078] An element constituting the piezoelectric film 40 and being
contained in the seed layer 20 is diffused through the lower
electrode 30 so as to precipitate on the upper surface of the lower
electrode 30 as illustrated in the magnified view of the portion of
the upper surface of the lower electrode 30 in FIG. 2B, in which
the precipitate is indicated by the reference 21. The diffusion may
be thermal diffusion by heating, which may be realized by normal
heat treatment using a heater, heating by irradiation of light, or
the like.
[0079] Examples of the thermal diffusion are explained below.
[0080] The thermal diffusion temperature and the thermal diffusion
time (i.e., the time for which the thermal diffusion is performed)
are not specifically limited, and it is sufficient to determine the
thermal diffusion temperature and the thermal diffusion time within
such ranges that the element can be thermally diffused to a
satisfactory degree and other problems such as the deterioration of
the substrate 10 do not occur.
[0081] The thermal diffusion temperature is preferably 400 to
700.degree. C., and particularly preferably 450 to 650.degree. C.
In addition, it is preferable that the thermal diffusion
temperature be equal to the film-formation temperature of the
piezoelectric film 40, since the operation of changing the
temperature at the start of the formation of the piezoelectric film
40 can be dispensed with, and therefore the efficiency in the
formation of the piezoelectric film 40 can be increased.
[0082] The thermal diffusion time is preferably 10 minutes to 2
hours, although the preferable thermal diffusion time depends on
various conditions such as the thermal diffusion temperature and
the thickness of the lower electrode 30.
[0083] The present inventor has found that fine irregularity occurs
at the upper surface of the lower electrode 30 after completion of
the operation in the step B. The upper surface of the lower
electrode 30 after completion of the operation in the step B has an
irregularity like a number of ranging, gently-sloping mountains,
and insular precipitate 21 is deposited at the positions
corresponding to the valleys between the gently-sloping mountains.
(See the AFM photograph of the surface of the lower electrode in
the concrete example 2 indicated in FIG. 5B.)
[0084] The present inventor has also found that the piezoelectric
film 40 exhibiting high degree of crystal orientation can be formed
in the following step C in the case where the diffusion is
performed under the condition that the arithmetic average surface
roughness Ra of the lower electrode 30 after the operation in the
step B is 0.5 nm to 30.0 nm. (See the concrete examples 1 to 4,
which are explained later.)
[0085] <Step C>
[0086] The structures produced in the first and second substeps in
the step C in the process for producing the piezoelectric device
are respectively illustrated in FIGS. 2C and 2D.
[0087] First, as illustrated in FIG. 2C, a piezoelectric film 40 is
formed on the approximately entire upper surface of the lower
electrode 30 after the precipitate 21 is deposited on the upper
surface of the lower electrode 30. The manner of the formation of
the piezoelectric film 40 is not specifically limited, and it is
preferable to use a vapor deposition technique such as sputtering,
metal organic chemical vapor deposition (MOCVD), or pulsed laser
deposition for the formation of the piezoelectric film 40.
Alternatively, an aerosol deposition technique or a chemical
solution deposition (CSD) technique such as the sol gel technique
or the metal organic decomposition (MOD), may be used for the
formation of the piezoelectric film 40.
[0088] Thereafter, as illustrated in FIG. 2D, patterning of the
piezoelectric film 40 is performed by using a known technique such
as dry etching so that the aforementioned stripelike pattern of the
piezoelectric film 40 in which the plurality of piezoelectric
elements 41 are arranged is formed on the lower electrode 30.
[0089] Since, according to the present embodiment, the element is
diffused through the lower electrode 30 so that the element
precipitates on the upper surface of the lower electrode 30 and is
distributed over the upper surface of the lower electrode 30 before
formation of the piezoelectric film 40, the precipitate 21
distributed over the upper surface of the lower electrode 30
behaves as nuclei in crystal growth during the formation of the
piezoelectric film 40. Thus, it is possible to form the
piezoelectric film 40 so as to exhibit high degree of crystal
orientation and high piezoelectric performance.
[0090] As mentioned in the "Description of the Related Art," JPP
2004-186646 discloses a technique of growing a PZT film which is
preferredly oriented along the (001) plane on a lower electrode of
noble metal containing titanium, where the lower electrode is
formed by concurrent sputtering of titanium and noble metal such as
platinum, and titanium is insularly precipitated on a surface of
the lower electrode. However, the values of the piezoelectric
constant d.sub.31 of the PZT films obtained by the techniques
disclosed in JPP 2004-186646 range from 122 to 138 pm/V (i.e., -122
to -138 pC/N), and no PZT film having the piezoelectric constant
d.sub.31 of 150 pm/V or greater is reported in JPP 2004-186646.
[0091] On the other hand, in the piezoelectric device 1 according
to the present embodiment, it is possible to achieve the
piezoelectric constant d.sub.31 of 150 pm/V or greater, and further
180 pm/V or greater, as indicated later in the concrete examples 1,
2, and 4.
[0092] Since titanium is not diffused in the techniques disclosed
in JPP 2004-186646, the upper surface of the lower electrode is
considered to be approximately flat. Although JPP 2004-186646 does
not disclose concrete values of the surface roughness, the surface
roughness of the upper surface of the lower electrode disclosed in
JPP 2004-186646 can be estimated to be approximately equal to the
surface roughness of the lower electrode 30 before the
precipitation of titanium. Therefore, for example, the arithmetic
average surface roughness of the upper surface of the lower
electrode disclosed in JPP 2004-186646 is estimated to be less than
0.5 nm. In addition, the paragraph No. 0059 in JPP 2004-186646
indicates that the thickness of the lower electrode disclosed in
JPP 2004-186646 is preferably in the range of 0.02 to 0.2
micrometers, and the crystal orientation in the PZT film is
facilitated when the thickness of the lower electrode is in this
range since the irregularity of the surface of the electrode
becomes 5 nm or smaller. That is, the description of JPP
2004-186646 can be understood to mean that a lower electrode having
a smoother upper surface is more preferable.
[0093] On the other hand, according to the present embodiment, an
element constituting the piezoelectric film 40 and being contained
in the seed layer 20 is diffused through the lower electrode 30 so
as to precipitate on the upper surface of the lower electrode 30.
Therefore, the arithmetic average surface roughness (Ra) of the
upper surface of the lower electrode 30 after the diffusion is
greater than the arithmetic average surface roughness of the upper
surface of the lower electrode in JPP 2004-186646, and is, for
example, 0.5 to 30.0 nm. The precipitate 21 containing the element
constituting the piezoelectric film 40 and being contained in the
seed layer 20 is distributed over the upper surface of the lower
electrode 30, so that the upper surface of the lower electrode 30
has irregularity which is suitable for formation of the
piezoelectric film 40.
[0094] After the diffusion of the element constituting the
piezoelectric film 40 and being contained in the seed layer 20, an
irregularity like a number of ranging, gently-sloping mountains is
produced on the upper surface of the lower electrode 30, and the
insular precipitate 21 is deposited at the positions corresponding
to the valleys between the gently-sloping mountains. Therefore, the
precipitate 21 deposited at the recessed portions of the upper
surface of the lower electrode 30 behaves as nuclei in the growth
of the piezoelectric film 40. Thus, it is possible to consider that
the upper surface of the lower electrode 30 according to the
present embodiment is more suitable for the base for crystal growth
than the approximately flat upper surface of the lower electrode in
JPP 2004-186646.
[0095] As mentioned in the "Description of the Related Art,"
according to the techniques disclosed in JPP 2004-186646,
conventionally, it is basically necessary to raise the temperature
to 600.degree. C. or higher during formation of the PZT film as
indicated in the embodiments 1, 2, 4, and 5 in JPP 2004-186646, and
it is conjectured that the film-formation temperature is required
to be 580.degree. C. or higher since the lowest film-formation
temperature indicated in JPP 2004-186646 is 580.degree. C. (in the
embodiment 3 in JPP 2004-186646). However, when the PZT film is
formed at high temperatures, lead loss can occur in the PZT film
and the piezoelectric performance can deteriorate.
[0096] On the other hand, according to the present embodiment, the
upper surface of the lower electrode 30 is suitable for use as a
base for growth of the piezoelectric film 40. Therefore, it is
possible to form the piezoelectric film 40 at a relatively low
temperature. Specifically, the film-formation temperature is in the
range equal to or greater than 400.degree. C. and lower than
600.degree. C., so that it is possible to form the piezoelectric
film 40 exhibiting high degree of crystal orientation.
[0097] In the case where the piezoelectric film 40 is formed of a
material containing lead such as PZT, the film-formation
temperature is preferably 450 to 580.degree. C., and particularly
preferably 500 to 580.degree. C. When the piezoelectric film 40 of
the lead-containing material is formed under the above condition,
the piezoelectric film 40 of the lead-containing material can be
stably formed, and the formed piezoelectric film 40 has
satisfactory crystal orientation. In addition, the loss of lead can
be suppressed, so that the formed piezoelectric film 40 exhibits
high performance.
[0098] As explained above, according to the present embodiment, the
upper surface of the lower electrode 30 has a structure suitable
for use as a base for growth of the piezoelectric film 40, and the
piezoelectric film 40 can be formed at a relatively low
temperature, so that both of these advantages contribute to
formation of the piezoelectric film 40 having higher performance
than the piezoelectric film formed by the techniques disclosed in
JPP 2004-186646.
[0099] Further, the seed layer 20 may be dispensed with in the
piezoelectric device 1 as long as the following conditions (a) and
(b) are satisfied.
[0100] (a) At least one element constituting the piezoelectric film
40 precipitates on the upper surface of the lower electrode 30.
[0101] (b) The upper surface of the lower electrode 30 has the
arithmetic average surface roughness (Ra) of 0.5 to 30 nm.
[0102] Generally, a piezoelectric device being constituted by a
lower electrode, a piezoelectric film, and an upper electrodes
formed in this order over a substrate and satisfying the above
conditions (a) and (b) is not conventionally known.
[0103] <Step D>
[0104] The structure produced in the step D in the process for
producing the piezoelectric device is illustrated in FIG. 2E.
[0105] After the patterning of the piezoelectric film 40 is
performed in the step C, the upper electrodes 50 are formed on the
tops of the piezoelectric element 41 as illustrated in FIG. 2E. In
addition, when necessary, the bottom surface of the substrate 10 is
etched so as to reduce the thickness of the substrate 10. Thus, the
production of the piezoelectric device 1 is completed.
[0106] <Step E>
[0107] Although not shown, the diaphragm 60 and the ink nozzle 70
are attached to the piezoelectric device 1 produced as above. Thus,
the production of the inkjet recording head 2 is completed.
Alternatively, it is possible to process portions of the substrate
10 into the diaphragm 60 and the ink nozzle 70, instead of
attaching the diaphragm 60 and the ink nozzle 70 to the
piezoelectric device 1. For example, in the case where the
substrate 10 is realized by a laminated substrate such as an SOI
substrate, the ink chambers 71 can be formed by etching the
corresponding portions of the bottom surface of the substrate 10,
and the diaphragm 60 and the other structures of the ink nozzle 70
can be formed by processing the substrate 10 per se.
[0108] As explained above, in the process for producing a
piezoelectric device according to the present embodiment, the
piezoelectric film 40 is formed on the upper surface of the lower
electrode 30 after the formation of the seed layer 20 and the lower
electrode 30 in this order on the substrate 10 and the diffusion of
the element contained in the seed layer 20 through the lower
electrode 30 for precipitation of the element at the upper surface
of the lower electrode 30. Therefore, it is possible to form the
piezoelectric film 40 exhibiting high degree of crystal orientation
and high piezoelectric performance at a relatively low temperature
without using an expensive substrate such as the MgO substrate.
Specifically, the piezoelectric film 40 can be formed at the
film-formation temperature equal to or higher than 400.degree. C.
and lower than 600.degree. C. According to the process for
producing a piezoelectric device according to the present
embodiment, it is possible to produce a piezoelectric device 1
having the piezoelectric constant d.sub.31 of 150 pm/V or
greater.
[0109] Inkjet Recording Apparatus
[0110] Hereinbelow, an example of an inkjet recording apparatus
having the inkjet recording head 2 is explained with reference to
FIGS. 3 and 4. FIG. 3 is a schematic diagram illustrating an
outline of an example of an inkjet recording apparatus having the
inkjet recording head 2 of FIG. 1, and FIG. 4 is a top view of a
portion of the inkjet recording apparatus of FIG. 3.
[0111] As schematically illustrated in FIG. 3, the inkjet recording
apparatus 100 comprises a printing unit 102, an ink
loading-and-holding unit 114, a sheet feeding unit 118, a decurling
unit 120, a suction-type belt conveyer 122, a print detection unit
124, and a sheet output unit 126. The printing unit 102 comprises a
plurality of inkjet recording heads 2K, 2C, 2M, and 2Y
corresponding to inks of different colors (specifically, black (K),
cyan (C), magenta (M), and yellow (Y)). Hereinafter, the inkjet
recording heads may be referred to as heads. The ink
loading-and-holding unit 114 holds the inks to be supplied to the
heads 2K, 2C, 2M, and 2Y. The sheet feeding unit 118 feeds a
recording sheet 116. The decurling unit 120 eliminates curl of the
recording sheet 116. The suction-type belt conveyer 122 is arranged
to face the nozzle faces (ink-discharge faces) of the printing unit
102, and conveys the recording sheet 116 while maintaining the
flatness of the recording sheet 116. The print detection unit 124
reads an image printed on the recording sheet 116 by the printing
unit 102. The sheet output unit 126 externally outputs a printed
recording sheet 116.
[0112] Each of the heads 2K, 2C, 2M, and 2Y constituting the
printing unit 102 corresponds to the inkjet recording head 2
explained before.
[0113] The decurling unit 120 performs decurling of the recording
sheet 116 by heating the recording sheet 116 with a heating drum
130 so as to eliminate the curl produced in the sheet feeding unit
118.
[0114] In the case where the inkjet recording apparatus 100 uses
roll paper, a cutter 128 for cutting the roll paper into desired
size is arranged in the stage following the decurling unit 120. The
cutter 128 is constituted by a fixed blade 128A and a round blade
128B. The fixed blade 128A has a length equal to or greater than
the width of the conveying path of the recording sheet 116, and is
arranged on the side opposite to the print side of the recording
sheet 116. The round blade 128B is arranged opposite to the fixed
blade 128A on the print side of the recording sheet 116, and moves
along the fixed blade 128A. In the inkjet recording apparatuses
using cut paper, the cutter 128 is unnecessary.
[0115] After the roll paper is decurled and cut into the recording
sheet 116, the recording sheet 116 is transferred to the
suction-type belt conveyer 122. The suction-type belt conveyer 122
is constituted by rollers 131 and 132 and an endless belt 133. The
rollers 131 and 132 are placed apart and the endless belt 133 is
looped around the rollers 131 and 132 in such a manner that at
least portions of the endless belt 133 which face the nozzle faces
of the printing unit 102 and the sensor face of the print detection
unit 124 are flat and horizontal.
[0116] The endless belt 133 has a width greater than the width of
the recording sheet 116, a great number of suction pores (not
shown) are formed through the endless belt 133. A suction chamber
134 is arranged inside the loop of the endless belt 133 at the
position opposite to the nozzle faces of the printing unit 102 and
the sensor face of the print detection unit 124, and suctioned by a
fan 135, so that a negative pressure is generated in the suction
chamber 134, and the recording sheet 116 on the endless belt 133 is
held by suction.
[0117] Power of a motor (not shown) is transmitted to at least one
of the rollers 131 and 132 so that the endless belt 133 is driven
clockwise in FIG. 3, and the recording sheet 116 held on the
endless belt 133 is moved from left to right in FIG. 3.
[0118] In the case of borderless printing, ink can be deposited on
the endless belt 133. Therefore, in order to clean the endless belt
133, a belt cleaning unit 136 is arranged at a predetermined
(appropriate) position outside the loop of the endless belt 133 and
the printing region.
[0119] A heating fan 140 is arranged on the upstream side of the
printing unit 102 above the conveying path of the recording sheet
116 (which is realized by the suction-type belt conveyer 122). The
heating fan 140 blows heated air to the recording sheet 116 before
printing so as to heat the recording sheet 116 and facilitate
drying of deposited ink.
[0120] Each of the heads 2K, 2C, 2M, and 2Y in the printing unit
102 is a so-called full-line type head, which is a linear head
having a length corresponding to the maximum width of the recording
sheet 116, and being arranged across the width of the recording
sheet 116 (i.e., in the main scanning direction perpendicular to
the feeding direction of the recording sheet 116) as illustrated in
FIG. 4. Specifically, each of the heads 2K, 2C, 2M, and 2Y is a
linear head in which a plurality of ink-discharge outlets (nozzles)
are arrayed over a length exceeding the maximum length of a side of
the largest recording sheet 116 on which the inkjet recording
apparatus 100 can print an image. The heads 2K, 2C, 2M, and 2Y
corresponding to the inks of the different colors are arrayed
upstream in this order along the feeding direction as illustrated
in FIG. 4. Thus, a color image can be printed on the recording
sheet 116 by discharging the inks of the different colors while
conveying the recording sheet 116.
[0121] The print detection unit 124 may be constituted by, for
example, a line sensor which takes an image formed of spots of the
inks discharged from the printing unit 102, and detects, from the
image taken by the line sensor, incomplete discharge, which can be
caused by clogging of a nozzle or the like.
[0122] A rear drying unit 142 for drying the printed surface of the
recording sheet 116 is arranged in the stage following the print
detection unit 124. For example, the rear drying unit 142 is
realized by a heating fan or the like. Since it is preferable to
avoid contact with the printed surface before the ink on the
printed surface is completely dried, it is preferable that the rear
drying unit 142 dry the ink on the printed surface by blowing
heated air.
[0123] In order to control the glossiness of the image printed on
the recording sheet 116, a heating-and-pressurizing unit 144 is
arranged in the stage following the rear drying unit 142. The
heating-and-pressing unit 144 comprises a pressure roller 145 with
a surface having predetermined projections and depressions, and
transfers the predetermined projections and depressions to the
printed surface of the recording sheet 116 by pressing the printed
surface with the pressure roller 145 while heating the printed
surface.
[0124] Finally, the printed recording sheet 116 produced as above
is outputted from the sheet output unit 126. It is preferable to
separately output test prints and prints for practical use.
Therefore, the sheet output unit 126 includes an output unit 126A
for the prints for practical use and an output unit 126B for the
test prints. Although not shown, the inkjet recording apparatus 100
further comprises a sorting unit which sorts the printed recording
sheets 116 into the test prints and the prints for practical use,
and sends the test prints to the output unit 126B, and the prints
for practical use to the output unit 126A.
[0125] Further, in the case where both of a test image and an image
for practical use are concurrently printed on a recording sheet
116, it is possible to arrange a cutter 148, and separate a first
portion of the recording sheet 116 on which the test image is
printed and a second portion of the recording sheet 116 on which
the image for practical use is printed.
CONCRETE EXAMPLES OF THE PRESENT INVENTION
[0126] The present inventor has produced concrete examples 1 to 3
of the piezoelectric device according to the present invention and
a comparison example 1 of a conventional piezoelectric device as
indicated below. The conditions under which the concrete examples 1
to 3 and the comparison example 1 are produced are summarized in
Table 1.
Concrete Example 1
[0127] The concrete example 1 of the piezoelectric device according
to the present invention is produced as follows.
[0128] First, an SOI substrate having a diameter of 6 inches
(approximately 150 mm) is prepared. The SOI substrate is produced
by forming an SiO2 film having a thickness of approximately 300 nm
and an active layer of silicon having a thickness of 15 micrometers
in this order on a (100) silicon substrate.
[0129] Next, a seed layer of titanium having a thickness of 30 nm
and a lower electrode of iridium having a thickness of 150 nm are
formed in this order on an approximately entire upper surface of
the above SOI substrate at the substrate temperature of 200.degree.
C. in an argon atmosphere with a vacuum degree of 0.5 Pa by using a
sputtering system. After the formation of the lower electrode is
completed, in order to thermally diffuse titanium from the seed
layer, the SOI substrate on which the seed layer and the lower
electrode are formed is left in the sputtering system, and the
substrate temperature is raised to 525.degree. C. and maintained
for 2 hours.
[0130] Thereafter, a piezoelectric film of PZT having a thickness
of 5 micrometers is formed on an approximately entire upper surface
of the lower electrode by using a target of Pb1.3Zr0.52Ti0.48O3 in
the same sputtering system while maintaining the substrate
temperature at 520.degree. C. in an atmosphere of a mixture of Ar
and 3.0 volume percent O2 with a vacuum degree of 0.5 Pa.
[0131] Further, upper electrodes of platinum having a thickness of
100 nm are formed on the above piezoelectric film of PZT by using
the same sputtering system. Finally, the bottom surface of the SOI
substrate is etched by RIE (reactive ion etching) so that ink
chambers having the dimensions of 500.times.500 micrometers with
the ink outlets are formed, and the active layer of silicon is used
as the aforementioned diaphragm. Thus, the diaphragm and the ink
nozzles each having an ink chambers and an ink outlet are formed by
processing the SOI substrate per se, and the production of the
concrete example 1 of the piezoelectric device according to the
present invention is completed.
Concrete Examples 2 and 3
[0132] The concrete examples 2 and 3 of the piezoelectric device
according to the present invention are produced in similar manners
to the concrete example 1 except that the substrate temperatures
during the thermal diffusion and the formation of the piezoelectric
film are different from the concrete example 1 as indicated in
Table 1.
Comparison Example 1
[0133] The comparison example 1 of the piezoelectric device is
produced as follows.
[0134] The piezoelectric film is formed immediately (specifically
at most 5 minutes) after the formation of the lower electrode,
i.e., the thermal diffusion of titanium is not performed. The other
conditions are similar to the concrete example 1 as indicated in
Table 1.
[0135] Evaluation 1
[0136] The concrete examples 1 to 3 of the piezoelectric device
according to the present invention and the comparison example 1 are
evaluated with respect to the following evaluation items (i) to
(v).
[0137] (i) Surface Observation of the Lower Electrode by AFM
[0138] The upper surface of the lower electrode immediately before
the formation of the piezoelectric film (after the thermal
diffusion of titanium in the cases of the concrete examples 1 to 3,
and immediately after the formation of the lower electrode in the
cases of the comparison example 1) is observed by an AFM (atomic
force microscope).
[0139] (ii) Surface Roughness of the Lower Electrode
[0140] The arithmetic average surface roughness Ra of the lower
electrode immediately before the formation of the piezoelectric
film is measured in accordance with JIS (Japanese Industrial
Standard) B0601-1994.
[0141] (iii) Cross-sectional Observation of Interface between Lower
Electrode and Piezoelectric Film
[0142] The interface between the lower electrode and the
piezoelectric film and the vicinity of the interface in a cross
section perpendicular to the interface is observed by a TEM
(transmission electron microscope) after the formation of the
piezoelectric film.
[0143] (iv) XRD Measurement
[0144] Measurement by powder X-ray diffraction (XRD) is performed
after the formation of the piezoelectric film.
[0145] (v) Piezoelectric Constant d31
[0146] The piezoelectric constant d31 of the piezoelectric device
produced as each of the concrete examples 1 to 3 and the comparison
example 1 is measured.
[0147] The measured values of the arithmetic average surface
roughness Ra of the lower electrodes and the piezoelectric constant
d31 of the concrete examples 1 to 3 and the comparison example 1
are also shown in Table 1. In addition, FIG. 5A shows an AFM
photograph of the upper surface of the lower electrode in the
comparison example 1, and FIG. 5B shows an AFM photograph of the
upper surface of the lower electrode in the concrete example 2.
[0148] In the comparison example 1 (in which titanium is not
thermally diffused although the titanium seed layer is formed), the
upper surface of the lower electrode observed by AFM before the
formation of the piezoelectric film (immediately after the
formation of the lower electrode) is approximately flat as
indicated in FIG. 5A, and the arithmetic average surface roughness
Ra of the upper surface of the lower electrode is measured to be
0.3 nm. The condition of the upper surface of the lower electrode
in the comparison example 1 before the formation of the
piezoelectric film is similar to the conditions of the upper
surfaces of the lower electrodes in the concrete examples 1 to 3
before the thermal diffusion of titanium.
[0149] In addition, no indication of titanium precipitate is
observed in the cross section of the comparison example 1 which
contains the interface between the lower electrode and the
piezoelectric film and is observed by the TEM.
[0150] The XRD measurement of the piezoelectric film in the
comparison example 1 indicates that the piezoelectric film does not
exhibit the crystal orientation of the perovskite structure, and
contains only the pyrochlore phase, i.e., the degree of crystal
orientation of the piezoelectric film in the comparison example 1
is almost zero. Therefore, the piezoelectric film in the comparison
example 1 does not exhibit piezoelectricity, and the piezoelectric
constant d31 cannot be measured.
[0151] On the other hand, in each of the concrete examples 1 to 3
(in which titanium is thermally diffused from the titanium seed
layer), the upper surface of the lower electrode after the thermal
diffusion of titanium, observed by the AFM, indicates an
irregularity like a number of ranging, gently-sloping mountains as
indicated in FIG. 5B. The measured values of the arithmetic average
surface roughness Ra of the upper surfaces of the lower electrodes
in the concrete examples 1 to 3 are 0.5 to 8.0 nm, and greater than
the measured value of the comparison example 1. In addition, the
measurement results indicate a tendency that rise in the thermal
diffusion temperature increases the arithmetic average surface
roughness Ra.
[0152] Further, the cross sections of the concrete examples 1 to 3
each of which contains the interface between the lower electrode
and the piezoelectric film and is observed by the TEM indicate that
insular titanium precipitate is deposited at the positions
corresponding to the valleys between the gently-sloping mountains
on the upper surface of the lower electrode.
[0153] Furthermore, the XRD measurement of the piezoelectric film
in each of the concrete examples 1 to 3 indicates that the
piezoelectric film exhibits prominent crystal orientation along the
(100) face, and the degree of crystal orientation is 99.0% or
higher. In addition, the measured values of the piezoelectric
constant d31 of the concrete examples 1 to 3 are 110 to 182 pm/V,
i.e., indicate high piezoelectric performance. In particular, in
the concrete examples 1 and 2, in which the film-formation
temperatures for the piezoelectric film are in the range of 500 to
580.degree. C., it is possible to obtain a piezoelectric device
which has the piezoelectric constant d31 of 150 pm/V or greater,
i.e., a piezoelectric device which exhibits higher piezoelectric
performance than the piezoelectric element disclosed in JPP
2004-186646.
[0154] In addition to the concrete examples 1 to 3 and the
comparison example 1, the present inventor has further produced a
concrete example 4 of the piezoelectric device according to the
present invention and a comparison example 2 of a conventional
piezoelectric device as indicated below. The conditions under which
the concrete example 4 and the comparison example 2 are produced
are summarized in Table 2.
Concrete Example 4
[0155] The concrete example 4 of the piezoelectric device according
to the present invention is produced in a similar manners to the
concrete example 1 except that the substrate temperatures during
the thermal diffusion and the formation of the piezoelectric film
are different from the concrete example 1 as indicated in Table
2.
Comparison Example 2
[0156] The comparison example 2 of the piezoelectric device is
produced as follows.
[0157] The piezoelectric film is formed immediately (specifically
at most 5 minutes) after the formation of the lower electrode so
that the thermal diffusion of titanium is not performed. The other
conditions are similar to the concrete example 4 as indicated in
Table 2.
[0158] Evaluation 2
[0159] The concrete example 4 of the piezoelectric device according
to the present invention and the comparison example 2 are also
evaluated with respect to the aforementioned evaluation items (i)
to (v). The measured values of the arithmetic average surface
roughness Ra of the lower electrodes and the piezoelectric constant
d31 of the concrete example 4 and the comparison example 2 are also
shown in Table 2. As explained in detail below, the concrete
example 4 exhibits similar performance to the concrete examples 1
to 3, and the comparison example 2 exhibits similar performance to
the comparison example 1.
[0160] In the comparison example 2 (in which titanium is not
thermally diffused although the titanium seed layer is formed), the
piezoelectric film does not have the perovskite structure, and has
only the pyrochlore phase, which does not exhibit piezoelectricity.
Therefore, the piezoelectric constant d31 cannot be measured.
[0161] On the other hand, in the concrete example 4 (in which
titanium is thermally diffused from the titanium seed layer), the
upper surface of the lower electrode after the thermal diffusion of
titanium, observed by the AFM, indicates an irregularity like a
number of ranging, gently-sloping mountains. In addition, the
measured value of the arithmetic average surface roughness Ra of
the upper surface of the lower electrode in the concrete example 4
is 0.5 nm, and greater than the measured value of the comparison
example 2.
[0162] Further, the cross section of the concrete example 4 which
contains the interface between the lower electrode and the
piezoelectric film and is observed by the TEM indicates that
insular titanium precipitate is deposited at the positions
corresponding to the valleys between the gently-sloping mountains
on the upper surface of the lower electrode.
[0163] Furthermore, the XRD measurement of the piezoelectric film
in the concrete example 4 indicates that the piezoelectric film
exhibits prominent crystal orientation along the (100) face, and
the degree of crystal orientation is 99.0% or higher. In addition,
the measured value of the piezoelectric constant d31 of the
concrete example 4 is 170 pm/V, i.e., indicates high piezoelectric
performance.
[0164] Although the thermal diffusion is performed for two hours in
each of the concrete examples 1 to 4, the present inventor has
confirmed that similar results are obtained even when the time for
which the thermal diffusion is performed is varied within the range
of 10 minutes to 2 hours. For example, similar results are obtained
even in the case where the thermal diffusion is performed at
650.degree. C. for 10 minutes, and the formation of the
piezoelectric film is started after the temperature is dropped to
450 to 575.degree. C.
[0165] In addition, the present inventor has confirmed that similar
results are obtained even when a substrate of glass or stainless
steel is used instead of the silicon substrate.
[0166] The present invention is not limited to the embodiment
explained above. All suitable modifications and equivalents which
will readily occur to those skilled in the art are regarded as
falling within the scope of the invention.
TABLE-US-00001 TABLE 1 Ra(nm) of Lower Film-Formation Electrode
before Temperature Piezo- Seed Lower Thermal Formation of (.degree.
C.) of Electric Sub- Layer Electrode Diffusion Piezo-electric
Piezo-electric Consist d.sub.31 strate (Thickness) (Thickness)
Condition Film Film (pm/V) Concrete Si Ti(30 nm) Ir(150 nm)
525.degree. C. 2 h 1.0 525.degree. C. 182 Example 1 Concrete Si
Ti(30 nm) Ir(150 nm) 575.degree. C. 2 h 8.0 575.degree. C. 150
Example 2 Concrete Si Ti(30 nm) Ir(150 nm) 450.degree. C. 2 h 0.5
450.degree. C. 110 Example 3 Comparison Si Ti(30 nm) Ir(150 nm) No
0.3 525.degree. C. Unmeasurable Example 1 (Pyrochlore Phase)
TABLE-US-00002 TABLE 2 Ra(nm) of Lower Film-Formation Electrode
before Temperature Piezo- Seed Lower Thermal Formation of (.degree.
C.) of Electric Sub- Layer Electrode Diffusion Piezo-electric
Piezo-electric Consist d.sub.31 strate (Thickness) (Thickness)
Condition Film Film (pm/V) Concrete Si Ti(30 nm) Pt(150 nm)
525.degree. C. 2 h 0.5 525.degree. C. 170 Example 4 Comparison Si
Ti(30 nm) Pt(150 nm) No 0.4 525.degree. C. Unmeasurable Example
2
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