U.S. patent application number 15/523968 was filed with the patent office on 2017-12-14 for multilayered piezoelectric thin film element.
This patent application is currently assigned to XAAR TECHNOLOGY LIMITED. The applicant listed for this patent is XAAR TECHNOLOGY LIMITED. Invention is credited to Peter Mardilovich, Susan Trolier-Mckinstry.
Application Number | 20170358732 15/523968 |
Document ID | / |
Family ID | 54258782 |
Filed Date | 2017-12-14 |
United States Patent
Application |
20170358732 |
Kind Code |
A1 |
Mardilovich; Peter ; et
al. |
December 14, 2017 |
Multilayered Piezoelectric Thin Film Element
Abstract
A piezoelectric thin film element having a first electrode, a
second electrode and a piezoelectric thin film between the
electrodes, wherein the thin film comprises a laminate having two
or more piezoelectric thin film layers and wherein a first thin
film layer is doped by one or more dopants and a second film layer
is doped by one or more dopants and wherein at least one dopant of
the second thin film layer is different from the dopant or dopants
of the first thin film layer.
Inventors: |
Mardilovich; Peter;
(Cambridge, GB) ; Trolier-Mckinstry; Susan;
(University Park, PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
XAAR TECHNOLOGY LIMITED |
Cambridge |
|
GB |
|
|
Assignee: |
XAAR TECHNOLOGY LIMITED
Cambridge
GB
|
Family ID: |
54258782 |
Appl. No.: |
15/523968 |
Filed: |
November 4, 2015 |
PCT Filed: |
November 4, 2015 |
PCT NO: |
PCT/GB2015/053312 |
371 Date: |
May 3, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62075153 |
Nov 4, 2014 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B41J 2002/14266
20130101; H01L 41/1876 20130101; H01L 41/29 20130101; H01L 41/047
20130101; H01L 41/0805 20130101; H01L 41/18 20130101; B41J 2/14233
20130101; H01L 41/0973 20130101; H01L 41/318 20130101 |
International
Class: |
H01L 41/18 20060101
H01L041/18; H01L 41/318 20130101 H01L041/318; B41J 2/14 20060101
B41J002/14; H01L 41/29 20130101 H01L041/29; H01L 41/047 20060101
H01L041/047; H01L 41/09 20060101 H01L041/09 |
Claims
1. A piezoelectric thin film element having a first electrode, a
second electrode and a piezoelectric thin film between the
electrodes, wherein the thin film comprises a laminate having two
or more piezoelectric thin film layers and wherein a first thin
film layer is doped by one or more dopants and a second thin film
layer is doped by one or more dopants and wherein at least one
dopant of the second thin film layer is different from the dopant
or dopants of the first thin film layer.
2. A piezoelectric thin film element according to claim 1, in which
the dopants are selected from the group of dopant types consisting
of donor dopants, acceptor dopants and isovalent dopants.
3. A piezoelectric thin film element according to claim 1, in which
the thin film layers comprise a crystal or crystallites based on
metal oxides which have a perovskite crystal structure
(ABO.sub.3).
4. A piezoelectric thin film element according to claim 3, in which
the dopants occupy the same or different co-ordination sites
(A-site or B-site) in the perovskite crystal structure.
5. A piezoelectric thin film element according to claim 1, in which
the laminate further comprises one or more thin film layers which
are undoped.
6. A piezoelectric thin film element according to claim 1, in which
the laminate comprises one or more further thin film layers which
are doped by one or more dopants which are the same as or different
to the dopant or dopants of the first thin film layer and/or the
dopants of the second thin film layer.
7. A piezoelectric thin film element according to claim 1, in which
the first thin film layer is singly doped by a first dopant and the
second thin film layer is singly doped by a second dopant.
8. A piezoelectric thin film element according to claim 7, in which
the first dopant and the second dopant are of the same dopant type
or of a different dopant type.
9. A piezoelectric thin film element according to claim 7, in which
the laminate comprises one or more further thin film layers which
are singly doped by dopants of the same or different dopant
type.
10. A piezoelectric thin film element according to claim 7, in
which the laminate comprises two or more thin film layers which are
alternately doped by dopants of a first dopant type or by a dopant
of a first dopant type and a dopant of a second dopant type.
11. A piezoelectric thin film element according to claim 10, in
which any two adjacent thin film layers are alternately doped by a
dopant of the first dopant type and a dopant of the second dopant
type.
12. A piezoelectric thin film element according to claim 10, in
which any one thin film layer which is doped is adjacent a thin
film layer which is undoped or doped and any two sequential thin
film layers which are doped, are alternately doped by a dopant of
the first dopant type and a dopant of the second dopant type.
13. A piezoelectric thin film element according to claim 10, in
which the laminate comprises a first series of adjacent thin film
layers which are doped by a dopant of the first dopant type and a
second series of adjacent thin film layers which are doped by a
dopant of the second dopant type.
14. A piezoelectric thin film element according to claim 10, in
which similarly doped thin film layers define a gradient in dopant
concentration across thin film layers.
15. A piezoelectric thin film according to claim 14, in which the
dopant concentration increases or decreases from the first
electrode to the second electrode.
16. A piezoelectric thin film element according to claim 14, in
which the dopant concentration increases from the first electrode
and decreases to the second electrode.
17. A piezoelectric thin film element according to claim 1, in
which one or more thin film layers define a gradient in dopant
concentration within the thin film layer.
18. A method for manufacturing a piezoelectric thin film element
having a first electrode, a second electrode and a piezoelectric
thin film between the electrodes, which method comprises a first
step of forming a piezoelectric thin film layer on an electrode and
one or more further steps of forming a piezoelectric thin film
layer on the thin film layer whereby to form a laminate comprising
a plurality of piezoelectric thin film layers wherein a first thin
film layer is doped by one or more dopants and a second film layer
is doped by one or more dopants and wherein at least one dopant of
the second thin film layer is different from the dopant or dopants
of the first thin film layer.
19. (canceled)
20. A method according to claim 18, in which the dopants are
selected from the group of dopant types consisting of donor
dopants, acceptor dopants and isovalent dopants.
21. A method according to claim 18, in which the thin film layers
comprise a crystal or crystallites based on metal oxides which have
a perovskite crystal structure (ABO.sub.3).
22.-24. (canceled)
25. A method according to claim 18, in which the first and further
steps form a laminate in which the first thin film layer is singly
doped by a first dopant and the second thin film layer is singly
doped by a second dopant.
26.-34. (canceled)
35. An actuator for a printhead, which actuator comprises a
piezoelectric element according to claim 1.
36. A printhead, comprising the actuator of claim 35.
37. An ink-jet printer, comprising the printhead of claim 36.
Description
[0001] The present invention is generally concerned with a
piezoelectric thin film element suitable for use in actuators,
sensors, energy harvesting devices and multilayer capacitors as
well as a method of manufacturing the element. It is particularly,
although not exclusively, concerned with a piezoelectric thin film
element suitable for use as an actuator for a printhead in an
ink-jet printer, as well as with actuators including the element,
print heads including the actuator and ink-jet printers including
the printheads.
[0002] A typical thin film piezoelectric element suitable for use
as an actuator for a print head in an ink-jet printer comprises a
metal or metal oxide bottom electrode formed on a substrate, a
metal top electrode and a piezoelectric thin film interposed
between the top and bottom electrodes.
[0003] The piezoelectric thin film may be formed by a variety of
techniques including sputtering, physical vapour deposition (PVD),
chemical vapour deposition (CVD), pulsed laser deposition (PLD) and
atomic layer deposition (ALD) but it is conveniently formed by a
chemical solution deposition process such as a sol-gel process or
metallo-organic deposition (MOD). A sol-gel process is described in
US patent application 2003/0076007 A1 (incorporated by reference
herein).
[0004] In a sol-gel process, a sol-gel solution is applied to a
bottom electrode formed on a substrate, dried and then pyrolised to
form a first precursor layer. The precursor layer is annealed by
heating to form a first piezoelectric thin film layer. The sol-gel
solution is then applied to the first layer, dried and pyrolised to
form a second precursor layer. The second precursor layer is
annealed by heating to form a second piezoelectric thin film layer.
These latter steps are repeated so as to build up a laminate of
piezoelectric thin film layers of desired thickness and then a top
electrode is formed on the thin film (for example, by sputtering
gold or iridium). The thicknesses of the thin film layers may be
the same or different (as described in US patent application
2003/0076007 A1).
[0005] The performance of a piezoelectric thin film element depends
on a complex interplay of piezoelectric, electrical and mechanical
properties of the element which can be difficult to balance.
[0006] In general, it is desired that the element is easily poled
and shows a relatively large displacement response at conveniently
applied electrical fields with good electrical properties such as
low current leakage and high dielectric breakdown field.
[0007] It is further desired that the element shows good
degradation performance viz. low electrical and mechanical
performance losses over a large number of applications of these
fields (cycles).
[0008] In many cases, the onset of degradation of a piezoelectric
thin film typically begins after about 10.sup.9 cycles. A number of
disclosures are specifically concerned with improving fatigue (or
mechanical loss in performance prior to mechanical failure) in lead
zirconate titanate (PZT) piezoelectric elements and some report an
onset beyond 10.sup.9 cycles in monolayer films by lowering the
applied voltage or by doping with a single dopant (for example,
with La.sup.3+ or Nb.sup.5+).
[0009] The fatigue performance of six distinct PZT piezoelectric
thin film elements comprising four thin film layers interposed
between platinum and gold electrodes is reported by Meyers, T,
Banerjee, P., Bose, S. and Bandyopadhyay, A. in J. Mater. Res,
2002, 17, 9, 2379-2385 (incorporated by reference herein).
[0010] A correlation is found between thin film elements having
La.sup.3+ doped (L) or undoped (P) thin film layers adjacent each
electrode (LPPL and PLLP) and their end-layer monolithic
counterparts (L, P). The middle layers (PP, LL) and the thicknesses
of the layers are reported to have only a very small influence on
the ferroelectric properties of the elements as compared to the
influence of the end layers.
[0011] The present invention generally aims to provide a
piezoelectric thin film element of improved piezoelectric and
dielectric performance by exploiting complementary effects which
are obtained in a piezoelectric thin film element which is doped by
different dopants.
[0012] Accordingly, in a first aspect the present invention
provides a piezoelectric thin film element having a first
electrode, a second electrode and a piezoelectric thin film between
the electrodes, wherein the thin film comprises a laminate having
two or more piezoelectric thin film layers and wherein a first thin
film layer is doped by one or more dopants and a second thin film
layer is doped by one or more dopants and wherein at least one
dopant of the second thin film layer is different from the dopant
or dopants of the first thin film layer.
[0013] As used herein, a thin film comprising a laminate is a thin
film which is formed layer-by-layer, in particular, by any of the
aforementioned methods. The laminate may show defined interfaces
between adjacent thin film layers (for example, when it is formed
by a sol-gel process) or it may not show defined interfaces between
adjacent thin film layers (for example, when it is formed by a PVD
process.
[0014] The piezoelectric element may have an electrode
configuration in which the piezoelectric thin film is interposed
between the first and second electrodes. This configuration need
not include any intermediate or other electrode--but other
electrodes may be provided when, for example, a piezoelectric
element comprising a stack of piezoelectric thin films is desired.
Alternatively, the piezoelectric element may have an electrode
configuration in which the first and second electrodes are provided
on one surface of the piezoelectric thin film, for example, as an
adjacent or interdigitated pair.
[0015] The dopants may be selected from the group of dopant types
consisting of acceptor dopants, donor dopants and isovalent
dopants.
[0016] In one embodiment, the thin film layers comprise crystals or
crystallites based on metal oxides (for example, PZT) which have a
perovskite crystal structure (ABO.sub.3). In this embodiment, the
dopants may notionally occupy the same or different co-ordination
sites (A-site or B-site) in the perovskite crystal structure.
[0017] In particular, a first dopant may be a donor dopant which
occupies the B-site (for example, Nb.sup.5+) within the crystal
structure and a second dopant may be an acceptor dopant which also
occupies the B-site (for example, Fe.sup.3+ or Ni.sup.2+) within
the crystal structure.
[0018] Alternatively, a first dopant may be a donor dopant which
occupies the A-site (for example, La.sup.3+) and a second dopant
may be an acceptor dopant which occupies the B-site (for example,
Fe.sup.3+ or Ni.sup.2+).
[0019] Alternatively, a first dopant may be a donor dopant which
occupies the A-site (for example, La.sup.3+) and a second dopant
may be a donor dopant which occupies the B-site (for example,
Nb.sup.5+).
[0020] The laminate may further comprise one or more thin film
layers which are undoped and/or one or more further thin film
layers which are doped by one or more dopants which are the same as
or different to the dopant or dopants of the first thin film layer
and/or the dopant or dopants of the second thin film layer.
[0021] In one embodiment, the thin film element comprises a
laminate having a first thin film layer which is singly doped by a
first dopant and a second thin film layer which is singly doped by
a second dopant which is different to the first dopant.
[0022] The second dopant may be of the same dopant type as the
first dopant or it may be of a different dopant type to the first
dopant.
[0023] The laminate may, in particular, comprise a first thin film
layer which is singly doped by a first dopant of a first dopant
type (for example, a donor dopant D) and a second thin film layer
which is singly doped by a different, second dopant of the first
dopant type (for example, a donor dopant D').
[0024] The laminate may comprise a first thin film layer which is
singly doped by a dopant of a first dopant type (for example, a
donor dopant D) and a second thin film layer is singly doped by a
dopant of a different, second dopant type (for example, an acceptor
dopant A).
[0025] In these embodiments, the laminate may further comprise one
or more thin film layers which are undoped (P) or singly doped by a
dopant of the first dopant type (same or different dopant to the
first dopant) or singly doped by a dopant of the second dopant type
(same or different dopant to the second dopant).
[0026] Alternatively or additionally, the laminate may further
comprise one or more thin film layers which are singly doped by a
dopant of a different dopant type to both the first and the second
dopant types, for example, by an isovalent dopant (I).
[0027] Alternatively or additionally, the laminate may further
comprise thin film layers which are doubly doped by dopants of the
first and second dopant types (acceptor and donor dopants as
compensating dopants).
[0028] The laminate may adopt any arrangement of doped thin film
layers within these alternatives but it is preferred that they are
alternately doped.
[0029] The laminate may, in particular, comprise two or more thin
film layers which are alternately doped by dopants of the same
dopant type (for example, -D.sup.1-D.sup.2-D.sup.1-D.sup.2- in
which D.sup.1 and D.sup.2 are different dopants) or two or more
thin film layers which are alternately doped by dopants of
different dopant types (for example, -A-D-A-D- in which A and D are
different dopants).
[0030] The laminate may, in particular, comprise three, four, five,
six or seven or more singly doped thin film layers in which any two
adjacent thin film layers are singly doped by dopants of different
dopant type (for example, -A-D-A-D-A-D-).
[0031] Alternatively or additionally, the laminate may comprise
three or more singly doped thin film layers in which any one thin
film layer which is doped is adjacent a thin film layer which is
undoped and in which any two sequential thin film layers which are
singly doped, are alternately doped by a dopant of a different
dopant type (for example, -A-P-D-P-A-P-D-).
[0032] The laminate may also comprise four or more thin film layers
in which a first series of adjacent thin film layers are singly
doped by a dopant of a first dopant type and a second series of
adjacent thin film layers are singly doped by a dopant of a second
dopant type (for example, -A-A-D-D-). The laminate may include one
or more additional series in which adjacent thin film layers are
each singly doped by a dopant of the first dopant type or a dopant
of the second dopant type (for example, -A-A-D-D-A-A-).
[0033] The series may or may not be separated from one another by
an undoped thin film layer or by a doubly doped (A+D) thin film
layer.
[0034] Of course, the arrangement of alternately doped thin film
layers may be similar when one or other of the first thin film
layer and the second thin film layer is doubly doped and/or the
laminate comprises further thin film layers which are singly doped
and further thin film layers which are doubly doped.
[0035] In particular, the laminate may comprise three, four, five,
six or seven or more thin film layers in which any two adjacent
thin film layers include a singly doped thin film layer and a
doubly doped thin film layer and the doubly doped thin film layer
includes a dopant of a different type to the singly doped thin film
layer (for example, -A-A+D-A-A+D-A-A+D-).
[0036] Alternatively or additionally, the laminate may comprise
three or more doped thin film layers in which any one thin film
layer which is doubly doped is adjacent a thin film layer which is
undoped and in which any two sequential thin film layers which are
singly or doubly doped, are alternately doped and the doubly doped
thin film layers include a dopant of a different type to the singly
doped thin film layers (for example, -A+D-P-A+D-P-A+D-P-A+D-).
[0037] The laminate may also comprise four or more thin film layers
in which a first series of adjacent thin film layers are doubly
doped by dopants of a first dopant type and a second dopant type
and a second series of adjacent thin film layers are singly doped
by a dopant of the first dopant type (for example,
-A+D-A+D-A+D-D-D-D-).
[0038] The series may or may not be separated from one another by
an undoped thin film layer or by a singly doped thin film
layer.
[0039] In one embodiment, the laminate provides that the first thin
film layer is adjacent the first electrode and the second thin film
layer is adjacent the second electrode. In this embodiment, a first
dopant may be of the same or different dopant type to a second
dopant and the laminate may include one or more thin film layers
which separate the first and second thin film layers. The laminate
may, in particular, comprise thin film layers which are alternately
doped as described above.
[0040] The dopants may be chosen so that they provide complementary
effects in the piezoelectric thin film element. The doping of thin
film layers may, in particular, enable the properties of the
element to be tuned for a particular application such as actuating
(for example, in ink-jet printing), sensing and/or energy
harvesting.
[0041] Donor doping at the A-site replaces Pb.sup.2+ with a large
trivalent ion (or at the B-site by pentavalent or hexavalent ion)
and leads to cation vacancies at the A-site for charge
compensation. Consequently, the domain walls are easier to move and
a lower coercive field can often be applied to the element for
poling. The piezoelectric thin film element typically shows better
piezoelectric properties as compared with an element which is
undoped as well as better DC electrical resistance, but dielectric
losses due to domain wall motion are increased.
[0042] Acceptor doping at the B-site replaces Ti.sup.4+ or
Zr.sup.4+ with a lower valent ion and leads to oxygen vacancies for
charge compensation. The domain walls are more heavily pinned and
often a higher coercive field has to be applied to the element for
poling. The piezoelectric thin film element can show worse
piezoelectric properties as compared with an element which is
undoped and reduced DC electrical resistance but dielectric losses
due to domain wall motion are lower.
[0043] Isovalent doping at the A-site or the B-site (for example,
at the A-site by Sr.sup.2+ or Ba.sup.2+; at the B-site by Hf.sup.4+
or Sn.sup.4+) can broaden phase transitions and shift Curie
temperatures so that the temperature dependence of the dielectric
and piezoelectric response is changed.
[0044] Compensating doping at the B-site replaces Ti.sup.4+ and
Zr.sup.4+ with a combination of donor and acceptor dopants (for
example, Mg.sup.2+ and Sb.sup.5+ in mole ratio 1:2) and results in
good properties of the thin film element along the morphotropic
phase boundary.
[0045] The dopants may, in particular, be chosen to optimise
piezoelectric properties against dielectric losses and/or breakdown
strength in the piezoelectric thin film element. A thin film
element doped by a donor dopant which occupies the A-site may
provide a piezoelectric thin film element having better
piezoelectric properties as compared to an undoped piezoelectric
thin film element but at the disadvantage of having high dielectric
losses. However, a thin film element doped by a donor dopant and an
acceptor dopant may show better piezoelectric properties and low
dielectric losses.
[0046] The dopants may, in particular, be chosen to optimise
piezoelectric response against stability of the piezoelectric thin
film element to high applied fields. A thin film element doped by a
donor dopant may provide a large displacement at lower applied
fields as compared to an undoped thin film element but the thin
film element is less stable to high applied fields. However, a thin
film element doped by a donor dopant and an acceptor dopant may
provide a large displacement with comparable or better stability to
high applied fields.
[0047] The dopants may, in particular, be chosen to optimise the
mechanical robustness of the piezoelectric thin film element as
well as its piezoelectric performance. A piezoelectric thin film
element typically comprises first and second electrodes which are
the same or different to one another and dopants can be chosen for
one or both of the thin film layers adjacent the electrodes so as
to optimise the mechanical robustness of the interface with the
electrode and/or to minimise charge injection from the electrode to
the piezoelectric thin film element.
[0048] The dopants may, in particular, be chosen to relieve
internal stress in the piezoelectric thin film element by providing
a gradient in mechanical, piezoelectric or electrical properties
within the element.
[0049] The laminate may comprise two or more thin film layers (for
example, at least three, four, five thin film layers) which define
a gradient in dopant concentration across thin film layers. In the
arrangements of alternately doped thin film layers described above,
the gradient concentration may be defined in a first dopant type
across singly doped and/or doubly doped thin film layers.
[0050] In one embodiment, the laminate comprises two, three, four
or more thin film layers which are singly doped by a dopant of a
first dopant type and/or two, three, four or more thin film layers
which are singly doped by a dopant of a second type so that
similarly doped thin film layers define a gradient in dopant
concentration across thin film layers (for example
-A.sub.1-A.sub.2-A.sub.3-D.sub.1-D.sub.2-D.sub.3-).
[0051] The dopant concentration may increase or decrease from the
first electrode to the second electrode. Alternatively, the dopant
concentration may increase from the first electrode and decrease to
the second electrode. The gradient may be constant or it may
increase and/or decrease with dopant concentration (for example, in
the same way as described for dopant concentration).
[0052] In one embodiment, the thin film layers are sol gel derived
and one or more doped thin film layers define a gradient in dopant
concentration within the thin layer. In this embodiment, the
dopants should show very low mobility during annealing.
[0053] Suitable piezoelectric materials, dopants and dopant
precursors for the present invention will be apparent to those
skilled in the art. Preferred piezoelectric materials include PZT
and lead-free alternatives including, for example, potassium sodium
niobate (KNN) and those of binary or tertiary composition known in
the art as BNT-BT, BKT-BNT, BKT-BZT, BKT-BNT-BZT and
BKT-BNT-BT.
[0054] The dopants may, in particular, be any of those mentioned
above. Other suitable donor dopants include Ta.sup.5+, V.sup.5+,
U.sup.5+, W.sup.6+ and divalent or trivalent ions of the alkaline
earth and rare earth elements. Other suitable acceptor dopants
include Na.sup.+, K.sup.+, Cs.sup.+ and Rb.sup.+ as well as
Cr.sup.3+, Li.sup.+, Co.sup.2+, Ni.sup.2+, Cu.sup.2+, Cu.sup.+,
Y.sup.3+ and Ti.sup.4+, Zr.sup.4+ and Sn.sup.4+.
[0055] In one embodiment in which all the thin film layers comprise
singly doped PZT, the different dopants are Mn.sup.2+ or Mn.sup.3+
or Mn.sup.4+ and Nb.sup.5+ or Hf.sup.4+.
[0056] The concentration of dopant in a thin film layer may be
between 0.1 to 10.0 mole %. In particular, it may be between 0.1
mole % and 3.0 mole %, for example, 0.5 mole %, 1.0 mole %, 1.5
mole %, 2.0 mole % or 2.5 mole %.
[0057] The thickness of each thin film layer in the laminate may be
between 5 nm and 300 nm. In one embodiment, the thickness of the
thin film layer adjacent the first electrode or the second
electrode is between 10 and 100 nm and the thickness of each of the
other thin film layers is between 100 nm and 300 nm.
[0058] The thickness (t) of each thin film layer may be chosen so
that a plurality of thin film layers define a gradient in
thicknesses (for example, t.sub.1>t.sub.2>t.sub.3) across
thin film layers. In one embodiment, the gradient in thicknesses
may complement a gradient in dopant concentrations across thin film
layers. The gradient may, in particular, reinforce or oppose the
gradient in dopant concentration across thin film layers.
[0059] The first electrode and the second electrode may be a metal
and/or a metal oxide electrode as is known to the art. For example,
the first electrode may be platinum, copper, nickel, gold. It may
be iridium or an iridium-iridium dioxide composite. It may
alternatively be lanthanum nickel oxide or strontium ruthenate.
[0060] In some embodiments, the first electrode or the second
electrode may include a seed layer. The seed layer can control the
crystal orientation of the thin film layers and may comprise any
that are known to the art for that purpose, in particular, titanium
dioxide, strontium ruthenate, lead titanium oxide or lanthanum
nickel oxide.
[0061] In preferred embodiments of the present invention, the
piezoelectric thin film comprises piezoelectric thin film layers
which are all sol-gel derived thin film layers.
[0062] In a second aspect, the present invention provides a method
for manufacturing a piezoelectric thin film element having a first
electrode, a second electrode and a piezoelectric thin film between
the electrodes, which method comprises a first step of forming a
piezoelectric thin film layer on an electrode and one or more
further steps of forming a piezoelectric thin film layer on the
thin film layer whereby to form a laminate comprising a plurality
of piezoelectric thin film layers wherein a first thin film layer
is doped by one or more dopants and a second film layer is doped by
one or more dopants and wherein at least one dopant of the second
thin film layer is different from the dopant or dopants of the
first thin film layer.
[0063] The method may provide a piezoelectric element having an
electrode configuration in which the piezoelectric thin film is
interposed between the first and second electrodes. This
configuration does not include an intermediate or other electrode
when the laminate comprises only piezoelectric thin film layers.
Alternatively, the method may provide a piezoelectric element
having an electrode configuration in which the first and second
electrodes are provided on one surface of the piezoelectric thin
film, for example, as an adjacent or interdigitated pair.
[0064] The method may comprise forming the piezoelectric thin film
layers by any of the processes known to the art including
sputtering, physical vapour deposition (PVD), chemical vapour
deposition (CVD), pulsed laser deposition (PLD) and atomic layer
deposition (ALD) but it is conveniently formed by a chemical
solution deposition process such as a sol-gel process or
metallo-organic deposition (MOD).
[0065] It is preferred, therefore, that the method comprises a
sol-gel process. In a preferred embodiment, the present invention
provides a method for manufacturing a piezoelectric thin film
element having a first electrode, a second electrode and a
piezoelectric thin film between the electrodes which method
comprises a first step of depositing one or more precursor layers
onto an electrode and annealing the precursor layer or layers to
form a piezoelectric thin film layer and one or more further steps
of depositing one or more precursor layers onto the thin film layer
and annealing the precursor layer or layers to form a laminate
comprising a plurality of piezoelectric thin film layers wherein
the one or more precursor layers in each step are doped or undoped
so that the laminate comprises a first thin film layer doped by one
or more dopants and a second thin film layer doped by one or more
dopants and wherein at least one dopant of the second thin film
layer is different from the dopant or dopants of the first thin
film layer.
[0066] In this embodiment, the method comprises depositing the
precursor layers by applying a sol-gel solution onto an electrode
provided on a substrate or thin film layer followed by drying and
pyrolysis. The number of precursor layers in any one step may, in
particular, be one, two, three or four precursor layers.
[0067] The sol-gel solution can be applied by spin-coating or
dip-coating but any of the coating techniques known to the art may
also be used. In one embodiment, the drying comprises heating to a
temperature of between 100.degree. C. and 250.degree. C., for
example between 100.degree. C. and 150.degree. C. and the pyrolysis
comprises heating to a temperature of between 200.degree. C. and
500.degree. C., for example between 200.degree. C. and 400.degree.
C. and in particular, to 350.degree. C.
[0068] The method may comprise forming thin film layers comprising
a crystal or crystallites based on metal oxides and having a
perovskite crystal structure (ABO.sub.3). The crystal or the
crystallites may, in particular, comprise PZT and/or PZT doped by
any one of Mn.sup.2+ or Mn.sup.3+ or Mn.sup.4+ and Nb.sup.5+ or
Hf.sup.4+.
[0069] In the preferred embodiment, the method comprises annealing
the precursor layers by heating from below the substrate to a
temperature between 550.degree. C. and 800.degree. C., for example
between 600.degree. C. and 800.degree. C. and, in particular, to
700.degree. C. This heating, which may be accomplished by rapid
thermal processing (RTP), results in very good columnar growth of
crystallites and well-defined grain boundaries between grains.
[0070] The dopants may be selected from the group of dopant types
consisting of acceptor dopants, donor dopants and isovalent
dopants.
[0071] In one embodiment, the first and further steps forms thin
film layers in which the dopants notionally occupy the same or
different co-ordination sites (A or B) in the perovskite crystal
structure (see above).
[0072] The first and further steps may provide a laminate
comprising further thin film layers which are undoped. They may
alternatively or additionally provide a laminate comprising further
thin film layers which are doped by one or more dopants which are
the same as or different to the dopant or dopants of the first thin
film layer and/or the dopant or dopants of the second thin film
layer.
[0073] In one embodiment, the first and further steps may form a
laminate comprising a first thin film layer which is singly doped
by a first dopant and a second thin film layer which is singly
doped by a second dopant which is different to the first
dopant.
[0074] The second dopant may be of the same dopant type as the
first dopant or it may be of a different dopant type to the first
dopant.
[0075] The first and further steps may provide a laminate
comprising a first thin film layer which is singly doped by a
dopant of a first dopant type (for example, a donor dopant D) and a
second thin film layer which is singly doped by a different, second
dopant of the first dopant type (for example, a donor dopant
D).
[0076] The first and further steps may also provide a laminate
comprising a first thin film layer which is singly doped by a
dopant of a first dopant type (for example, a donor dopant D) and
the second thin film layer which is singly doped by a dopant of a
different, second dopant type (for example, an acceptor dopant
A).
[0077] In these embodiments, the first and further steps may also
provide a laminate further comprising one or more thin film layers
which are either undoped (P) or singly doped by a dopant of the
first dopant type (same or different dopant to the first dopant) or
singly doped by a dopant of the second dopant type (same or
different dopant to the second dopant).
[0078] Alternatively or additionally, the first and further steps
may provide a laminate further comprising thin film layers which
are singly doped by a dopant of a different dopant type to both the
first and the second dopant types.
[0079] Alternatively or additionally, the first and further steps
may provide a laminate further comprising thin film layers which
are doubly doped by dopants of the first and second dopant types
(as compensating dopants).
[0080] The first and further steps may provide any arrangement of
doped and/undoped layers within these alternatives but it is
preferred that they are alternately doped by the same or different
dopant types.
[0081] The first and further steps may provide a laminate
comprising two or more thin film layers which are alternately doped
by dopants of the same dopant type (for example,
-D.sup.1-D.sup.2-D.sup.1-D.sup.2- in which D.sup.1 and D.sup.2 are
different dopants) or two or more thin film layers which are
alternately doped by dopants of different dopant type (for example,
-A-D-A-D- in which A and D are different dopants).
[0082] The first and further steps may provide a laminate
comprising three or more singly doped thin film layers in which any
two adjacent thin film layers are alternately doped by dopants of a
different dopant type (for example, -A-D-A-D-A-D-).
[0083] Alternatively or additionally, the first and further steps
may form a laminate comprising three or more singly doped thin film
layers in which any one thin film layer which is doped is adjacent
a thin film layer which is undoped and in which any two sequential
thin films which are doped, are alternately doped by a dopant of a
different dopant type (for example, -A-P-D-P-A-P-D-).
[0084] The first and further steps may also provide a laminate
comprising four or more thin film layers in which a first series of
adjacent thin film layers are doped by a dopant of a first dopant
type and a second series of adjacent thin film layers are doped by
a dopant of a second dopant type (for example, -A-A-D-D-). This
laminate may include one or more additional series in which
adjacent thin film layers are singly doped by a dopant of the first
dopant type or the second dopant type (for example, -A-A-D-D-A-A-).
The series may or may not be separated from one another by one or
more undoped or doubly doped thin film layers.
[0085] Other embodiments of the method will be apparent from the
description of the laminate in which one or other of the first thin
film layer and the second film layer is doubly doped and/or the
laminate comprises further thin film layers which are undoped or
singly doped.
[0086] The first and further steps may provide a laminate of any
appropriate thickness (for example, about 1 .mu.m) having any
number of thin film layers (for example, four, eight, ten, twelve,
fourteen, sixteen or more).
[0087] The first and further steps may provide a laminate in which
the first thin film layer is adjacent the first electrode and the
second thin film layer is adjacent the second electrode. Here, the
first dopant may be of the same or different dopant type as the
second dopant and the laminate may include additional thin film
layers which separate the first and second thin film layers. The
first and further steps may form a laminate such as this in which
thin film layers are alternately doped as described above.
[0088] In one embodiment, the first and further steps provide a
laminate in which two or more thin film layers define a gradient in
dopant concentration across thin film layers. In the arrangements
of alternately doped thin film layers described above, the gradient
concentration may be defined in a first dopant type across singly
doped and/or doubly doped thin film layers.
[0089] In one embodiment, the first and further steps provide a
laminate comprising two, three, four or more thin film layers which
are doped by a dopant of a first dopant type and/or two, three,
four or more thin film layers which are doped by a dopant of a
second dopant type so that similarly doped thin film layers define
a gradient in dopant concentration across thin film layers (for
example -A.sub.1-A.sub.2-A.sub.3-D.sub.1-D.sub.2-D.sub.3-).
[0090] In this embodiment, the method may comprise depositing one
or more precursor layers doped by the first dopant (or a dopant of
a first dopant type) at a first concentration and annealing the
precursor layers to form a first doped thin film layer and
depositing one or more precursor layers doped by the first dopant
(or dopant of a first dopant type) at a second concentration which
is different from the first concentration and annealing the
precursor layers to form a second doped thin film layer.
[0091] Of course, the method may further comprise depositing one or
more precursor layers doped by the first dopant (or dopant of a
first dopant type) at a third concentration and annealing the
precursor layers to form a third piezoelectric thin film layer and
so on. The third and subsequent concentrations may, however, be the
same or different to the first concentration.
[0092] The first and further steps may provide a laminate in which
in dopant concentration increases or decreases from the first
electrode to the second electrode. Alternatively, they may form a
laminate in which dopant concentration increases from the first
electrode and decreases to the second electrode. The gradient may
be constant or it may increase and/or decrease with dopant
concentration (for example, in the same way as described for dopant
concentration).
[0093] In the preferred embodiment, the first and further steps may
form one or more thin film layers which define a gradient in dopant
concentration within the thin layer.
[0094] In this embodiment, the method may comprise depositing a
first precursor layer doped by a first dopant at a first
concentration followed by depositing on the first precursor layer a
second precursor layer doped by the first dopant but at a second
concentration which is different from the first concentration and
annealing the precursor layers to form the first thin film
layer.
[0095] Of course, the method may comprise depositing third and
fourth precursor layers doped by the first dopant before the
annealing. The concentration of the first dopant in the third and
fourth precursor layers may, however, be the same or different to
the concentration of the first dopant in the first precursor layer.
In any case, dopants should show low mobility during annealing to
form the thin film layer.
[0096] In one embodiment, the first and further steps may provide a
laminate comprising a plurality of thin film layers defining a
gradient in thin film layer thickness (t) across thin film layers
(for example, t.sub.1>t.sub.2>t.sub.3). The gradient may
complement a gradient in dopant concentrations across thin film
layers. The gradient in thickness may, in particular, reinforce or
oppose the gradient in dopant concentration across similarly doped
thin film layers.
[0097] In this embodiment, the method may comprise depositing one
or more precursor layers and annealing the precursor layers to form
a first thin film layer having a first thickness and depositing one
or more precursor layers to form a second thin film layer having a
second thickness which is different from the first thickness.
[0098] Of course, the number of precursor layers may be used to
control the thickness of a thin film layer and the method may
further comprise depositing one or more precursor layers and
annealing the precursor layers to form a third thin film layer
having a third thickness and so on. The third and subsequent
thicknesses may be the same or different to the first
thickness.
[0099] The first electrode and the second electrode may be a metal
and/or a metal oxide electrode as is known to the art. For example,
the first electrode may be platinum, copper, nickel, gold. It may
be iridium or an iridium-iridium dioxide composite. It may
alternatively be lanthanum nickel oxide or strontium ruthenate.
[0100] In some embodiments, the first electrode or the second
electrode may be provided with a seed layer. The seed layer can
control crystal orientation and may comprise any that are known to
the art for that purpose, for example, titanium dioxide, strontium
ruthenate or lanthanum nickel oxide. In these embodiments, the
method comprises a first step of depositing one or more precursor
layers onto the seed layer.
[0101] In a third aspect, the present invention provides an
actuator for a printhead, which actuator comprises a piezoelectric
element according to the first aspect.
[0102] In a fourth aspect, the present invention provides a
printhead, comprising the actuator according to the third
aspect.
[0103] In a fifth aspect, the present invention provides an ink-jet
printer, comprising the printhead according to the fourth
aspect.
[0104] Embodiments of the actuator, printhead and ink-jet printer
will be apparent from the first and second aspects of the
invention.
[0105] The present invention is disclosed in more detail as follows
and with reference to certain non-limiting embodiments and the
accompanying Drawings in which:
[0106] FIG. 1 is a graph showing a depth profile obtained by X-ray
photoelectron spectroscopy of a 1 .mu.m PZT film disposed on a
platinum electrode over a silicon dioxide substrate provided with a
zinc tin oxide adhesion layer (PZT/Pt/ZTO/SiO.sub.2);
[0107] FIG. 2 is a scheme illustrating in section view one
embodiment of the method of the present invention;
[0108] FIG. 3 is a section view illustrating one embodiment of the
piezoelectric thin film element of the present invention;
[0109] FIG. 4 is a section view illustrating another embodiment of
the piezoelectric thin film element of the present invention;
[0110] FIG. 5 is a section illustrating a further embodiment of the
piezoelectric thin film element of the present invention; and
[0111] FIG. 6 is a section view illustrating still another
embodiment of the piezoelectric thin film element of the present
invention;
[0112] FIG. 7 is a section view illustrating still another
embodiment of the piezoelectric thin film element of the present
invention;
[0113] FIG. 8 is a section view illustrating still another
embodiment of the piezoelectric thin film element of the present
invention;
[0114] FIG. 9 is a section view illustrating an embodiment of the
piezoelectric thin film element of the present invention and a
graph showing a gradient in dopant concentration across the
layers;
[0115] FIG. 10 is a section view illustrating another embodiment of
the piezoelectric thin film element of the present invention and a
graph showing another gradient in dopant concentration across the
layers; and
[0116] FIG. 11 is a section view of a drop ejector portion of an
ink-jet printhead showing use of one embodiment of the
piezoelectric thin film of the present invention as an
actuator.
[0117] FIG. 1 shows a graph reporting the result of an X-ray
photoelectron spectroscopy experiment examining the depth profile
of the concentration of elements within a PZT thin film of 1 .mu.m
on a platinum electrode provided on a glass substrate. The thin
film is formed by depositing a precursor layer and annealing the
precursor layer by heating from below the electrode and repeating
these steps to form a laminate of six crystallised thin film
layers.
[0118] The graph plots the elemental composition in the thin film
against sputter time (increasing depth of penetration of incident
radiation).
[0119] As may be seen, the Ti.sup.4+ concentration and the
Zr.sup.4+ concentration within the thin film follows a profile
(circled) which is consistent with faster crystallisation of lead
titanate as compared to lead zirconate and is repeated throughout
the film in a number corresponding to the number of crystallised
thin layers.
[0120] The graph shows not just six distinct crystallisations but
also that the Ti.sup.4+ concentration and the Zr.sup.4+
concentration within a first crystallised thin film layer is
unaltered by repeated annealing to form subsequent crystallised
thin film layers.
[0121] In other words, there is no migration of Ti.sup.4+ or
Zr.sup.4+ between layers during the formation of one or more
crystallised thin film layers on an already formed crystallised
thin film layer.
[0122] Referring now to FIG. 2, there is shown a scheme
illustrating a method for manufacturing a piezoelectric thin film
element 10 according to one embodiment of the present
invention.
[0123] A sol-gel layer comprising appropriate amounts of PZT and
acceptor dopant precursor is provided to an upper surface of an
electrode 11 (for example, of lanthanum niobate) provided on a
silicon substrate by spin coating. The sol-gel layer is dried by
heating the substrate and sol-gel layer to a temperature of between
120.degree. C. and 150.degree. C. After cooling, the dried layer is
pyrolised by heating the substrate and dried layer to a temperature
of about 350.degree. C. to provide an amorphous precursor layer
12a.
[0124] The spin coating, drying and pyrolysis are repeated so as to
provide further amorphous precursor layers 12b and 12c on the
substrate.
[0125] After cooling, the substrate and the three precursor layers
are heated rapidly to a temperature of about 700.degree. C. by
placing the substrate on or above a hot plate. The below substrate
heating anneals the three precursor layers into a first thin film
layer 13 comprising crystallites of PZT doped by an acceptor dopant
(A, for example, Mn.sup.2+ at the B-site).
[0126] After cooling, a sol-gel layer comprising appropriate
amounts of PZT and donor dopant precursor is provided to the thin
film layer 13 on the substrate and dried and pyrolised as before to
provide a precursor layer 14a on the thin film layer.
[0127] The spin coating, drying and pyrolysis are repeated so as to
provide further amorphous precursor layers 14b and 14c on the
substrate.
[0128] The substrate and its layers are again rapidly heated to a
temperature of about 700.degree. C. by placing the substrate on or
above a hot plate. The below substrate heating anneals the
precursor layers into a second thin film layer 15 comprising
crystallites of PZT doped by a donor dopant (D, for example
Nb.sup.5+, at the B-site or La.sup.3+ at the A-site).
[0129] A sol-gel layer comprising appropriate amounts of PZT and
donor dopant precursor is provided to the thin film layer 15 by
spin-coating, drying and pyrolysis in the same way it is applied to
the electrode. The precursor layers are annealed into a thin film
layer 16 comprising crystallites of PZT doped by an acceptor dopant
(A, for example, Mn.sup.2+ at the B-site).
[0130] The cycle can be repeated for so as to provide a laminate of
desired thickness (for example, 1 .mu.m) comprising alternately
doped PZT thin film layers (sixteen for example). Of course, the
process may use a sol-gel solution without a dopant precursor so
that the laminate includes one or more undoped thin film layers of
crystallites of PZT.
[0131] Finally an electrode (not shown in FIGS. 2 to 10, for
example of gold metal) is formed by sputtering (for example) on to
the top thin film layer.
[0132] In this embodiment, the first thin film and the third thin
film layer may be doped by an acceptor dopant (A, for example,
Mn.sup.2+, at the B-site) and the second thin film layer may be
doped by a donor dopant (D, for example Nb.sup.5+, at the B-site or
La.sup.3+ at the A-site).
[0133] In another embodiment (not shown), the first thin film layer
is undoped, the second thin film layer is doped by a donor dopant
(D, for example Nb.sup.5+, at the B-site or La.sup.3+ at the
A-site) and the third thin film layer is doped by an acceptor
dopant (A, for example, Mn.sup.2+, at the B-site).
[0134] FIG. 3 shows an embodiment of the present invention having
four thin film layers. The first thin film layer 13 and the fourth
thin film layer 17 are undoped whilst the second thin film layer 15
is doped by an acceptor dopant (A, for example, Mn.sup.2+, at the
B-site) and the third thin film layer 16 is doped by a donor dopant
(D, for example Nb.sup.5+, at the B-site or La.sup.3+ at the
A-site).
[0135] FIG. 4 shows an embodiment of the present invention also
having four thin film layers. The first thin film layer 13 and the
second thin film layer 15 are doped by acceptor dopants (A, for
example, Mn.sup.2+, at the B-site) whilst the third thin film layer
16 and the fourth thin film layer 17 are doped by donor dopants (D,
for example Nb.sup.5+ at the B-site or La.sup.3+ at the
A-site).
[0136] FIG. 5 shows an embodiment of the present invention having
seven thin film layers. The thin film layers 13 to 20 are
alternately doped by donor dopant and acceptor dopant. In
particular, the first, third, fifth and seventh thin film layers
13, 16, 18, 20 are doped by an acceptor dopant (A, for example,
Mn.sup.2+, at the B-site) and the second, fourth and sixth thin
film layers 15, 17, 19 are doped by a donor dopant (D, for example
Nb.sup.5+ at the B-site or La.sup.3+ at the A-site).
[0137] FIG. 6 shows an embodiment of the present invention also
having seven thin film layers. The thin film layers include a
series of layers which are similarly doped. The first thin film
layer 13 and the fourth and seventh thin film layers 17, 20 are
undoped whilst the second and third thin film layers 15, 16 are
doped by an acceptor dopant (A, for example, Mn.sup.2+, at the
B-site) and the fifth and sixth thin film layers 18, 19 are doped
by a donor dopant (D, for example Nb.sup.5+, at the B-site or
La.sup.3+ at the A-site).
[0138] FIG. 7 shows an embodiment of the present invention also
having seven thin film layers. The thin film layers are alternately
doped with donor dopant and acceptor dopant. In particular, the
first, third and sixth thin film layers 14, 16, 19 are doped by an
acceptor dopant (A, for example, Mn.sup.2+, at the B-site). The
second, fifth and seventh thin film layers 15, 18, 20 are doped by
a donor dopant (D, for example Nb.sup.5+, at the B-site or
La.sup.3+ at the A-site) whilst the fourth thin film layer 17 is
doped by compensating dopants comprising a combination of acceptor
and donor dopants (A and D, for example, Mg.sup.2+ and Sb.sup.5+ in
molar ratio 1:2 or Ni.sup.2+ and Nb.sup.5+ in molar ratio 1:2 or
Mg.sup.2+ and W.sup.6+ in molar ratio 1:1).
[0139] FIG. 8 shows an embodiment of the present invention also
having seven thin film layers. In this embodiment, the thin film
layers are alternately doped but have an intervening undoped layer.
In particular, the first thin film layer 13 and the fifth thin film
layer 18 are doped by an acceptor dopant, the third and seventh
thin film layers 16, 20 are doped by a donor dopant and the second,
fourth and sixth thin film layers 15, 17, 19 are undoped.
[0140] FIG. 9 shows a similar embodiment to that shown in FIG. 5.
In this embodiment, however, the concentration of dopant in each
thin film layer doped by an acceptor dopant differs and increases
toward the seventh layer (A.sub.1 to A.sub.4). The concentration of
dopant in each thin film layer doped by a donor dopant increases
toward the seventh layer (D.sub.1 to D.sub.3). The accompanying
graph (ordinate axis 0.5 mole % increments) particularly points out
acceptor and donor dopant concentrations which increase from 0.5
mole % to 2.0 mole % or from 0. 5 mole % to 1.5 mole %.
[0141] FIG. 10 also shows a similar embodiment to that shown in
FIG. 5. In this embodiment, however, the concentration of dopant in
each thin film layer doped by the acceptor dopant differs and
increases towards the seventh layer. The concentration of dopant in
each thin film layer doped by the donor dopant decreases to toward
the seventh layer (D.sub.1 to D.sub.3). The accompanying graph
(ordinate axis 0.5 mole % increments) particularly points out
acceptor dopant concentrations (A.sub.1 to A.sub.3) which increase
from 0.5 mole % to 2.0 mole % and donor dopant concentrations which
decrease from 1.5 mole % to 0.5 mole %.
[0142] FIG. 11 shows a section view of an ink-jet printhead 21
according to one embodiment of the present invention. A
piezoelectric thin film element 10 comprising first and second
electrodes 11 and 22 having a piezoelectric thin film comprising a
laminate of seven thin film layers interposed between the
electrodes is provided to a diaphragm 23 on top of a pressure
chamber 24, provided with a nozzle plate 25.
[0143] The pressure chamber 24 may comprise a silicon single
crystal of thickness about 200 pm and the diaphragm may comprise a
thin film comprising one or more of silicon dioxide, zirconium
oxide, tantalum oxide, and silicon nitride or aluminium oxide.
[0144] A buffer layer 26 of ultra-thin titanium film or chromium
film (about 10 nm thickness) is interposed between the diaphragm
and the first electrode.
[0145] In use, a predetermined drive voltage is applied between the
first and second electrodes by a signal from a control circuit. The
voltage causes the piezoelectric thin film element 10 to deform so
deflecting the diaphragm 23 into the pressure chamber 24 and
changing its volume. A sufficient increase in pressure within the
pressure chamber causes ink droplets to be ejected from the
nozzle.
[0146] The present invention has been described in detail with
reference to certain embodiments which are illustrated by the
drawings. However, it will be understood that other embodiments not
described in detail or illustrated by the drawings are also
included within the scope of the present invention.
[0147] It will be appreciated that similar alternate arrangements
are possible in singly doped thin film layers doped by two
different donor dopants or by two different acceptor dopants (for
example, doping at the A-site and the B-site) or by a plurality of
donor dopants including different dopants or by a plurality of
different acceptor dopants including different dopants.
[0148] It will also be appreciated that the first thin film layer
and the second thin film layer may be triply doped or doped by four
or more dopants. And that laminates comprising any reasonable
combination of thin film layers which are undoped, singly doped and
triply doped or doped by four or more dopants are possible.
* * * * *