U.S. patent application number 11/286284 was filed with the patent office on 2006-04-27 for ferroelectric film, ferroelectric capacitor, ferroelectric memory, piezoelectric element, semiconductor element, method of manufacturing ferroelectric film, and method of manufacturing ferroelectric capacitor.
This patent application is currently assigned to Seiko Epson Corporation. Invention is credited to Yasuaki Hamada, Takeshi Kijima, Eiji Natori, Koji Ohashi.
Application Number | 20060088731 11/286284 |
Document ID | / |
Family ID | 32180737 |
Filed Date | 2006-04-27 |
United States Patent
Application |
20060088731 |
Kind Code |
A1 |
Kijima; Takeshi ; et
al. |
April 27, 2006 |
Ferroelectric film, ferroelectric capacitor, ferroelectric memory,
piezoelectric element, semiconductor element, method of
manufacturing ferroelectric film, and method of manufacturing
ferroelectric capacitor
Abstract
A ferroelectric film is formed by an oxide that is described by
a general formula AB.sub.1-xNb.sub.xO.sub.3. An A element includes
at least Pb, and a B element includes at least one of Zr, Ti, V, W,
Hf and Ta. The ferroelectric film includes Nb within the range of:
0.05.ltoreq.x<1. The ferroelectric film can be used for any of
ferroelectric memories of 1T1C, 2T2C and simple matrix types.
Inventors: |
Kijima; Takeshi;
(Matsumoto-shi, JP) ; Hamada; Yasuaki; (Suwa-shi,
JP) ; Natori; Eiji; (Chino-shi, JP) ; Ohashi;
Koji; (Chino-shi, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 19928
ALEXANDRIA
VA
22320
US
|
Assignee: |
Seiko Epson Corporation
Tokyo
JP
|
Family ID: |
32180737 |
Appl. No.: |
11/286284 |
Filed: |
November 25, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10690021 |
Oct 22, 2003 |
|
|
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11286284 |
Nov 25, 2005 |
|
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Current U.S.
Class: |
428/701 ;
257/E21.009; 257/E21.271; 257/E21.272; 257/E21.664; 257/E27.104;
427/372.2; 427/58; 428/702 |
Current CPC
Class: |
H01L 41/318 20130101;
C01G 35/006 20130101; H01L 41/1876 20130101; C01P 2006/80 20130101;
H01L 21/02197 20130101; H01G 4/1254 20130101; H01L 41/0805
20130101; C01P 2006/40 20130101; C01P 2004/04 20130101; C01P
2004/80 20130101; H01G 4/33 20130101; C01P 2002/77 20130101; H01L
21/31691 20130101; H01L 27/11502 20130101; C01P 2002/72 20130101;
H01L 21/02282 20130101; H01L 28/55 20130101; H01B 1/08 20130101;
H01L 21/316 20130101; H01L 28/57 20130101; H01L 27/11507 20130101;
C01G 33/006 20130101 |
Class at
Publication: |
428/701 ;
428/702; 427/058; 427/372.2 |
International
Class: |
B05D 5/12 20060101
B05D005/12 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 24, 2002 |
JP |
2002-309487 |
Mar 19, 2003 |
JP |
2003-076129 |
Mar 26, 2003 |
JP |
2003-085791 |
Aug 18, 2003 |
JP |
2003-294072 |
Aug 27, 2003 |
JP |
2003-302900 |
Claims
1. A method of manufacturing a ferroelectric film, the method
comprising: forming a ferroelectric film described by
A(B,Nb)O.sub.3 over a substrate by using a mixture of sol-gel
solutions, wherein an A element comprises at least Pb, wherein a B
element comprises at least Zr and Ti, and wherein the mixture
comprises at least a first sol-gel solution for PbZrO.sub.3, a
second sol-gel solution for PbTiO.sub.3, and a third sol-gel
solution for PbNbO.sub.3.
2. The method of manufacturing a ferroelectric film as defined by
claim 1, wherein the mixture further comprises a fourth sol-gel
solution for PbSiO.sub.3.
3. The method of manufacturing a ferroelectric film as defined by
claim 1, wherein when the stoichiometric composition of Pb as a
constituent element in an A site is assumed to be 1, the
ferroelectric film is formed by using the mixture in which Pb is
included within the range of 0.9 to 1.2.
4. The method of manufacturing a ferroelectric film as defined by
claim 1, further comprising: forming a metal film on the substrate,
and wherein the ferroelectric film is formed on the metal film.
5. The method of manufacturing a ferroelectric film as defined by
claim 4, wherein the metal film comprises a platinum-group metal,
and wherein the platinum-group metal is at least one of Pt and
Ir.
6. A method of manufacturing a ferroelectric capacitor, the method
comprising: forming a lower electrode on a substrate; forming a
ferroelectric film on the lower electrode, the ferroelectric film
being formed of a PZTN complex oxide including Pb, Zr, Ti and Nb as
constituent elements; forming an upper electrode on the
ferroelectric film; forming a protective film so as to cover the
lower electrode, the ferroelectric film, and the upper electrode;
and performing thermal processing for crystallizing the PZTN
complex oxide, at least after forming the protective film.
7. The method of manufacturing a ferroelectric capacitor as defined
by claim 6, wherein a preliminary thermal processing is performed
on the ferroelectric film in an oxidizing atmosphere during the
formation of the ferroelectric film, to put the PZTN complex oxide
into an amorphous state until thermal processing for crystallizing
the PZTN complex oxide is performed.
8. The method of manufacturing a ferroelectric capacitor as defined
by claim 6, wherein the protective film is a silicon dioxide film
and is formed using trimethylsilane.
9. The method of manufacturing a ferroelectric capacitor as defined
by claim 6, wherein the thermal processing for crystallizing the
PZTN complex oxide is performed in a non-oxidizing atmosphere.
10. A ferroelectric capacitor manufactured by the method as defined
by claim 6.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of application Ser. No.
092,800, filed Mar. 8, 2002, which application is incorporated
herein by reference in its entirety.
[0002] The disclosures of Japanese Patent Application No.
2002-309487, filed on Oct. 24, 2002, Japanese Patent Application
No. 2003-76129, filed on Mar. 19, 2003, Japanese Patent Application
No. 2003-85791, filed on Mar. 26, 2003, Japanese Patent Application
No. 2003-294072 filed on Aug. 18, 2003, and Japanese Patent
Application No. 2003-302900 filed on Aug. 27, 2003 are hereby
incorporated by reference in their entirety.
BACKGROUND OF THE INVENTION
[0003] The present invention relates to a ferroelectric film, a
ferroelectric capacitor, a ferroelectric memory, a piezoelectric
element, a semiconductor element, a method of manufacturing a
ferroelectric film, and a method of manufacturing a ferroelectric
capacitor.
[0004] It has recently become popular to perform research and
development into ferroelectric films of PZT or SBT or the like, as
well as devices such as ferroelectric capacitors and ferroelectric
memory devices that use such films. The configurations of
ferroelectric memory devices are categorized into 1T type, 1T1C
type, 2T2C type, and simple matrix type. Of these, the structure of
the 1T type leads to the generation of internal electrical fields
which shorten the retention (data preservation) to one month, so it
is thought to be impossible to provide a guarantee of ten years,
which is generally requested of semiconductors. The 1T1C type and
the 2T2C type have mostly the same configuration as DRAM and have
selection transistors, so DRAM fabrication techniques can be used
therefor. Since the 1T1C type and the 2T2C type implement write
speeds similar to those of SRAM, they are currently being used in
small-capacitance capacitance products of 256 Kbit or less.
[0005] The ferroelectric materials used up until now have mainly
been Pb(Zr, Ti)O.sub.3 (PZT). With PZT, the ratio of Zr to Ti is
52/48 or 40/60 and the composition that is used is a region that is
a mixture of trigonal and tetragonal crystals, or the vicinity
thereof. With PZT, materials that have also been doped with
elements such as Lz, Sr, or Ca are used. These regions are used
because they guarantee the reliability that is most necessary for a
memory element. A tetragonal region that is rich in Ti has a
favorable hysteresis shape, but Schottky defects caused by the
ionic crystal structure occur therein. For that reason, failures
occur in the leakage current characteristic or imprint
characteristic (measures of hysteresis distortion), making it
difficult to ensure reliability.
[0006] A simple matrix type of memory cell, on the other hand, has
a cell size smaller than those of the 1T1C and 2T2C types, and it
is also possible to stack capacitors, so it is promising for high
integration, inexpensive applications.
[0007] Details of a conventional simple matrix type of
ferroelectric memory device are given in Japanese Patent Laid-Open
No. 9-116107. This publication discloses a drive method by which a
voltage that is 1/3 of the write voltage is applied to non-selected
memory cells when data is written to the memory cells.
[0008] However, details concerning the hysteresis loop of the
ferroelectric capacitor, which is necessary for operation, are not
specifically disclosed therein. Good squareness of hysteresis loop
is essential for obtaining a simple matrix type of ferroelectric
memory device that can operate in practice. Ti-rich tetragonal PZT
can be considered as a candidate for the ferroelectric material
that can be applied thereto, but the guaranteeing of reliability is
the most important technical concern therewith, in a similar manner
to the 1T1C and 2T2C types of ferroelectric memory.
[0009] PZT tetragonal crystals exhibit a hysteresis characteristic
that has the squareness suitable for memory applications, but they
lack reliability and cannot be used in practice. The reasons for
this are discussed below.
[0010] First of all, a PZT tetragonal thin film tends to have a
high leakage current density after crystallization, which increases
as the ratio of Ti contained therein increases. In addition, static
imprint testing in which data is written once in either the
positive or negative direction and the memory device is heated and
held at 100.degree. C. has shown that most of the written data
disappears after 24 hours. These problems are intrinsic to the
ionic crystals of PZT and to the Pb and Ti that are constituent
elements of PZT, and create the greatest technical problem relating
to PZT tetragonal thin film in which large proportions of the
constituent elements are Pb and Ti. This technical problems is
great because PZT Perovskite is ionic crystals, and is intrinsic to
PZT.
[0011] A list of the main energies involved in the bonds between
the constituent elements of PZT is shown in FIG. 44. It is known
that PZT includes many oxygen vacancies after crystallization. In
other words, it can be expected from FIG. 44 that Pb--O bonds have
the smallest bond energy among the constituent elements of PZT and
will simply break during baking or polarization inversions. In
other words, if Pb escapes, O will also escape for reasons of
charge neutrality.
[0012] During sustained heating such as imprint testing, the
constituent elements of PZT vibrate and collide repeatedly, and the
Ti that is the lightest constituent element of PZT can easily be
knocked out by these vibrational collisions during high-temperature
retention. Therefore, if Ti escapes, O will also escape for reasons
of charge neutrality. Since the maximum valence of +2 for Pb and +4
for Ti contribute towards bonding, there is no way to maintain
charge neutrality other than allowing O to escape. In other words,
two negative 0 ions escape readily for every positive ion of Pb or
Ti, so that Schottky defects easily form.
[0013] The description now turns to the mechanism of the generation
of leakage currents due to oxygen lack in PZT crystals. FIGS. 45A
to 45C illustrate the generation of leakage currents in oxide
crystals having a Brownmillerite type of crystal structure
described by the general formula ABO.sub.2.5. As shown in FIG. 45A,
the Brownmillerite type of crystal structure is a crystal structure
having an oxygen insufficiency in comparison with the Perovskite
type of crystal structure of PZT crystals having the general
formula ABO.sub.3. As shown in FIG. 45B, since oxygen ions appear
in the vicinity of positive ions in the Brownmillerite type of
crystal structure, positive ion defects make it difficult for
excessive leakage current to increase. However, oxygen ions link
the entire PZT crystal in series as shown in FIG. 45C, and leakage
currents increase accordingly in the case of a Brownmillerite type
of crystal structure, in which the oxygen vacancy is larger than
the above description.
[0014] In addition to the above-described generation of leakage
currents, insufficiencies of Pb and Ti and the concomitant
insufficiency of O, which are lattice defects, cause spatial charge
polarization such as that shown in FIG. 46. When that happens,
reverse electrical fields due to lattice defects are created by the
electrical fields of ferroelectric polarization can occur, causing
a state in which the bias potential is impeded in the PZT crystals,
and hysteresis shift or collapse can occur as a result.
Furthermore, these phenomena are likely to occur quicker as the
temperature increases.
[0015] The above problems are intrinsic to PZT and it is considered
difficult to analyze these problems in pure PZT, so that up until
now it has not been possible to implement suitable characteristics
for a memory element made by using tetragonal PZT.
[0016] In ferroelectric memory, one factor that determines the
characteristics of the device is the crystallization state of the
ferroelectric film included within the ferroelectric capacitor. The
process of manufacturing ferroelectric memory has processes for
forming an interlayer dielectric and a protective film, and
processes that generate large quantities of hydrogen are used.
Since the ferroelectric film at this point is mainly formed of
oxides, the oxides are reduced by the generated hydrogen during the
fabrication process, which has an undesirable effect on the
characteristics of the ferroelectric capacitor.
[0017] For that reason, a resistance to reduction is secured for
the ferroelectric capacitor in the conventional ferroelectric
memory by covering the capacitor with a barrier film such as an
aluminum oxide layer or an aluminum nitride layer, to prevent
deterioration of the characteristics of the ferroelectric
capacitor. However, such a barrier film necessitates the use of
extra real estate during the integration of the ferroelectric
memory, making it desirable to find a method that enables the
manufacture of ferroelectric memory by a simpler process, from the
productivity point of view as well.
BRIEF SUMMARY OF THE INVENTION
[0018] The present invention may provide a 1T1C, 2T2C, or simple
matrix type of ferroelectric memory including a ferroelectric
capacitor having a hysteresis characteristic that can be used in
any of a 1T1C, 2T2C, or simple matrix type of ferroelectric memory.
The present invention may also provide a ferroelectric film that is
suitable for the above-described ferroelectric memory, together
with a method of manufacturing the same. The present invention may
further provide a piezoelectric element and semiconductor element
in which the above-described ferroelectric film is used. The
present invention may still further provide a ferroelectric
capacitor, a method of manufacture thereof, and a ferroelectric
memory in which the ferroelectric capacitor is used, wherein
satisfactory characteristics are maintained by a simple process
that does not necessitate a barrier film.
[0019] A ferroelectric film according to one aspect of the present
invention is described by a general formula
AB.sub.1-xNb.sub.xO.sub.3, an A element includes at least Pb, a B
element includes at least one of Zr, Ti, V, W, Hf and Ta, and Nb is
included within the range of: 0.05.ltoreq.x<1.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a schematic section through a ferroelectric
capacitor;
[0021] FIG. 2 is a flowchart of the formation of a PZTN film by a
spin-coating method
[0022] FIG. 3 is a hysteresis curve of polarization (P) versus
voltage (V) of the ferroelectric capacitor;
[0023] FIGS. 4A to 4C show the surface morphologies of PZTN films
in accordance with a first embodiment;
[0024] FIGS. 5A to 5C show the crystallinities of PZTN films in
accordance with the first embodiment;
[0025] FIGS. 6A to 6C show the relationship between film thickness
and surface morphology of PZTN films in accordance with the first
embodiment;
[0026] FIGS. 7A to 7C show the relationship between film thickness
and crystallinity of PZTN films in accordance with the first
embodiment;
[0027] FIGS. 8A to 8C show the hysteresis characteristics for film
thicknesses of PZTN films in accordance with the first
embodiment;
[0028] FIGS. 9A to 9C show the hysteresis characteristics for film
thicknesses of PZTN films in accordance with the first
embodiment;
[0029] FIGS. 10A and 10B show the leakage current characteristics
of PZTN films in accordance with the first embodiment;
[0030] FIG. 11A shows the fatigue characteristic of a PZTN film in
accordance with the first embodiment, and FIG. 11 shows the static
imprint characteristic of a PZTN film in accordance with the first
embodiment;
[0031] FIG. 12 shows the configuration of a ferroelectric capacitor
in accordance with the first embodiment in which a SiO.sub.2
protective film is formed by ozone TEOS;
[0032] FIG. 13 shows the hysteresis characteristic of the
ferroelectric capacitor in accordance with the first embodiment in
which a SiO.sub.2 protective film is formed by ozone TEOS;
[0033] FIG. 14 shows the leakage current characteristics of a
conventional PZTN film;
[0034] FIG. 15 shows the fatigue characteristic of a ferroelectric
capacitor using a conventional PZTN film;
[0035] FIG. 16 shows the static imprint characteristic of a
ferroelectric capacitor in accordance with the first embodiment,
which uses a conventional PZT film;
[0036] FIGS. 17A and 17B show the hysteresis characteristics of
PZTN films in accordance with a second embodiment.
[0037] FIGS. 18A and 18B show the hysteresis characteristics of
PZTN films in accordance with a second embodiment.
[0038] FIGS. 19A and 19B show the hysteresis characteristics of
PZTN films in accordance with a second embodiment.
[0039] FIG. 20 shows X-ray diffraction patterns of PZTN films in
accordance with the second embodiment;
[0040] FIG. 21 shows the relationship between Pb insufficiency and
Nb compositional ratio in a PZTN crystal in accordance with the
second embodiment;
[0041] FIG. 22 is illustrative of the WO.sub.3 crystal structure
that is a Perovskite crystal;
[0042] FIGS. 23A to 23C are schematic sections illustrating the
process of manufacturing a PZTN film in accordance with a third
embodiment;
[0043] FIGS. 24A and 24B are illustrative of changes in lattice
constant in a PZTN film in accordance with the third
embodiment;
[0044] FIG. 25 is illustrative of changes in lattice mismatch ratio
between PZTN films and Pt metal films in accordance with the third
embodiment;
[0045] FIG. 26 is a flowchart of the formation of a conventional
PZT film by a spin-coating method, as a reference example;
[0046] FIGS. 27A to 27E show the surface morphologies of PZTN
films, as a reference example;
[0047] FIGS. 28A to 28E show the crystallinities of PZTN films, as
a reference example;
[0048] FIGS. 29A and 29B show the hysteresis loops of tetragonal
PZT films, as reference examples;
[0049] FIG. 30 shows the hysteresis loop of a conventional
tetragonal PZT film, as a reference example;
[0050] FIGS. 31A and 31B show the results of degassing analysis on
tetragonal PZT films as reference examples;
[0051] FIGS. 32A to 32C show a process of manufacturing a
ferroelectric capacitor;
[0052] FIGS. 33A and 33B show the hysteresis characteristics of
ferroelectric capacitors;
[0053] FIG. 34 shows the electrical characteristics of
ferroelectric capacitors
[0054] FIG. 35A is a schematic plan view of a simple matrix type of
ferroelectric memory device and FIG. 35B is a schematic section
through the simple matrix type of ferroelectric memory device;
[0055] FIG. 36 is a section through an example of a ferroelectric
memory device in which a memory cell array and a peripheral circuit
are integrated together on the same substrate;
[0056] FIG. 37A is a schematic section through a 1T1C type of
ferroelectric memory and FIG. 37B is an equivalent circuit
schematically showing the 1T1C type of ferroelectric memory;
[0057] FIGS. 38A to 38C show the process of manufacturing
ferroelectric memory;
[0058] FIG. 39 is an exploded perspective view of a recording
head;
[0059] FIG. 40A is a plan view of the recording head and FIG. 40B
is a section through the recording head;
[0060] FIG. 41 is a schematic section through the layer structure
of a piezoelectric element;
[0061] FIG. 42 is a schematic view of an example of an inkjet-type
recording device;
[0062] FIG. 43A shows the hysteresis characteristic of a
ferroelectric film in which Ta has been added to PZT and FIG. 43B
shows the hysteresis characteristic of a ferroelectric film in
which W has been added to PZT;
[0063] FIG. 44 lists characteristics relating to bonds of
constituent elements of PZT-family ferroelectric materials;
[0064] FIGS. 45A to 45C are illustrative of Schottky defects in the
Brownmillerite crystal structure; and
[0065] FIG. 46 is illustrative of ferroelectric spatial charge
polarization.
DETAILED DESCRIPTION OF THE EMBODIMENT
[0066] 1) A ferroelectric film according to an embodiment of the
present invention is described by a general formula
AB.sub.1-xNb.sub.xO.sub.3, an A element includes at least Pb, a B
element includes at least one of Zr, Ti, V, W, Hf and Ta, and Nb is
included within the range of: 0.05.ltoreq.x<1.
[0067] The A element may include Pb.sub.1-yLn.sub.y
(0<y.ltoreq.0.2). Ln includes at least one of La, Ce, Pr, Nd,
Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.
[0068] 2) A ferroelectric film according to an embodiment of the
present invention is described by a general formula
(Pb.sub.1-yA.sub.y)(B.sub.1-xNb.sub.x)O.sub.3 and an A element
includes at least one of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy,
Ho, Er, Tm, Yb and Lu, a B element includes at least one of Zr, Ti,
V, W, Hf and Ta, and Nb is included within the range of:
0.05.ltoreq.x<1 (more desirably 0.1.ltoreq.x.ltoreq.0.3).
[0069] 3) With a PZT-family ferroelectric film according to an
embodiment of the present invention, a Ti composition is greater
than a Zr composition, and at least 2.5 mol % and not more than 40
mol % (more desirably at least 10 mol % and not more than 30 mol %)
of the Ti composition is substituted by Nb. This PZT-family
ferroelectric film may have a crystal structure of at least one of
tetragonal and rhombohedral systems. This PZT-family ferroelectric
film may include Si, or Si and Ge of at least 0.5 mol % (more
desirably at least 0.5 mol % and less than 5 mol %).
[0070] This PZT-family ferroelectric film may be formed by using a
sol-gel solution.
[0071] 4) A PZT-family ferroelectric according to an embodiment of
the present invention is described by a general formula ABO.sub.3,
Pb is included as a constituent element in an A site and at least
Zr and Ti are included as constituent elements in a B site. Amount
of Pb vacancy in the A site is equal to or less than 20 mol % of
the stoichiometric composition of the ABO.sub.3. This PZT-family
ferroelectric film may include Nb in the B site with a
compositional ratio equivalent to twice the Pb vacancy in the A
site. With this PZT-family ferroelectric film, a Ti composition may
be higher than a Zr composition in the B site, and also the
ferroelectric may have a crystal structure of rhombohedral system.
This PZT-family ferroelectric film may be formed by using a sol-gel
solution.
[0072] 5) With a method of manufacturing the above ferroelectric
film according to an embodiment of the present invention, a mixture
of at least a sol-gel solution for PbZrO.sub.3, a sol-gel solution
for PbTiO.sub.3, and a sol-gel solution for PbNbO.sub.3 is used as
the sol-gel solution for forming the ferroelectric film.
[0073] With this method of manufacturing a ferroelectric film, a
sol-gel solution for forming PbSiO.sub.3 may be further mixed into
the mixture to be used as the sol-gel solution for forming the
ferroelectric film.
[0074] 6) With a method of manufacturing the above ferroelectric
film according to an embodiment of the present invention, when the
stoichiometric composition of Pb that is a constituent element of
the A site is assumed to be 1, the ferroelectric film is formed by
using a sol-gel solution in which Pb is included within the range
of 0.9 to 1.2.
[0075] 7) With this method of manufacturing the ferroelectric film,
the PZT-family ferroelectric film may be formed on a metal film
formed of a platinum-group metal.
[0076] 8) With this method of manufacturing the ferroelectric film,
the platinum-group metal may be at least one of Pt and Ir.
[0077] 9) The ferroelectric memory in accordance with an embodiment
of the present invention includes a first electrode leading up from
the source or drain electrode of a CMOS transistor that has been
formed on an Si wafer previously, a ferroelectric film formed on
the first electrode, and a second electrode formed on the
ferroelectric film. A capacitor formed of the first electrode, the
ferroelectric film and the second electrode is a ferroelectric
memory that performs a selection operation by the CMOS transistor
that was formed on an Si wafer in advance. The ferroelectric film
is formed from tetragonal PZT having a Ti ratio of at least 50%, at
least 5 mol % but not more than 40 mol % of the Ti composition is
substituted by Nb, and Si and Ge of at least 1 mol % is included
therein.
[0078] 10) The ferroelectric memory in accordance with an
embodiment of the present invention is a ferroelectric memory
including a previously formed first electrode, a second electrode
arranged in a direction intersecting the first electrode, and a
ferroelectric film disposed in at least an intersecting region
between the first electrode and the second electrode. Capacitors
form of the first electrode, the ferroelectric film, and the second
electrode are disposed in a matrix. The ferroelectric film is
formed from tetragonal PZT having a Ti ratio of at least 50%, at
least 5 mol % but not more than 40 mol % of the Ti composition is
substituted by Nb, and Si and Ge of at least 1 mol % is included
therein.
[0079] 11) A method of manufacturing a ferroelectric memory in
accordance with an embodiment of the present invention includes
crystallizing a sol-gel solution for forming PbZrO.sub.3 that is a
first raw material solution, a sol-gel solution for forming
PbTiO.sub.3 that is a second raw material solution, a sol-gel
solution for forming PbNbO.sub.3 that is a third raw material
solution, and sol-gel solution for forming PbSiO.sub.3 that is a
fourth raw material solution, after the first to fourth solutions
have been coated. The first, second, and third raw material
solutions are liquids of raw materials for forming the
ferroelectric layer and the fourth raw material solution is a
liquid of a raw material for forming an ordinary paraelectric layer
having a catalytic effect that is essential for forming the
ferroelectric layer from the first, second, and third raw material
solutions.
[0080] 12) A method of manufacturing a ferroelectric capacitor
according to an embodiment of the present invention includes:
[0081] forming a lower electrode on a given substrate;
[0082] forming a ferroelectric film on the lower electrode, the
ferroelectric film being formed of a PZTN complex oxide including
Pb, Zr, Ti and Nb as constituent elements;
[0083] forming an upper electrode on the ferroelectric film;
[0084] forming a protective film so as to cover the lower
electrode, ferroelectric film, and upper electrode; and
[0085] performing thermal processing for crystallizing the PZTN
complex oxide, at least after forming the protective film.
[0086] This embodiment uses a PZTN complex oxide that includes Pb,
Zr, Ti and Nb as constituent elements as the material of the
ferroelectric film, and this PZTN complex oxide is crystallized
after the formation of a protective film. Thus, even if the
ferroelectric film should become damaged by hydrogen generated
during the processing in the formation of the protective film, the
thermal processing for crystallization is performed subsequently so
that the PZTN complex oxide is crystallized while any such damage
is repaired. It is therefore possible to omit the process of
forming a barrier layer to protect the ferroelectric film from
reductive reactions, which is necessary in the prior art, thus
enabling an increase in productivity and a reduction in production
costs.
[0087] 13) With this method of manufacturing a ferroelectric
capacitor, preliminary thermal processing may be performed on the
ferroelectric film in an oxidizing atmosphere during the formation
of the ferroelectric film, to put the PZTN complex oxide into an
amorphous state until thermal processing for crystallizing the PZTN
complex oxide is performed.
[0088] This feature enables an amorphous state until the
ferroelectric film has been crystallized. This makes it possible to
prevent deterioration of the crystal quality due to grain boundary
diffusion by keeping the ferroelectric film in the amorphous state
until the protective film is formed. Since the ferroelectric film
in this amorphous state is subjected to the preliminary thermal
processing in an oxidizing atmosphere, oxygen can enter the film.
For that reason, it is possible to crystallize the PZTN complex
oxide during the thermal processing for crystallization without
depending on the gases included within the atmosphere therefor.
[0089] 14) With this method of manufacturing a ferroelectric
capacitor, the protective film may be a silicon dioxide film and is
formed by using trimethylsilane.
[0090] Since this feature makes it possible to form the protective
film of silicon dioxide film by using trimethylsilane (TMS) that
does not generate much hydrogen during the processing, in
comparison with the tetraethyl orthosilicate (TEOS) that is
generally used in the formation of silicon dioxide films, it is
possible to reduce damage due to reductive reactions in the
ferroelectric film.
[0091] 15) With this method of manufacturing a ferroelectric
capacitor, the thermal processing for crystallizing the PZTN
complex oxide may be performed in a non-oxidizing atmosphere.
[0092] Since this feature makes it possible to perform the thermal
processing for crystallization in a non-oxidizing atmosphere, it
makes it possible to prevent oxidation damage due to
high-temperature thermal processing on peripheral components
outside the capacitor (such as metal wiring), even if such
peripheral components are included in the device being
processed.
[0093] 16) A ferroelectric capacitor according to an embodiment of
the present invention is manufactured by using the above
manufacture method of the ferroelectric capacitor.
[0094] 17) The above ferroelectric film and ferroelectric capacitor
can be applied to a ferroelectric memory, piezoelectric element,
and semiconductor element using the same. Preferred embodiments of
the present invention are described below in detail with reference
to the accompanying figures.
[0095] 1. Ferroelectric Film, Ferroelectric Capacitor, and Method
of Manufacture Thereof
[0096] A schematic section through a ferroelectric capacitor 100
that uses a ferroelectric film 101 in accordance with an embodiment
of the present invention is shown in FIG. 1.
[0097] As shown in FIG. 1, the ferroelectric capacitor 100 is
formed of the ferroelectric film 101, a first electrode 102, and a
second electrode 103.
[0098] The first electrode 102 and the second electrode 103 are
either formed of a precious metal such as Pt, Ir, or Ru alone, or a
compound material in which that precious metal is the main part.
Since the diffusion of ferroelectric elements into the lower
electrode 102 or the upper electrode 103 would cause variations in
the composition of the interface between that electrode and the
ferroelectric film 101, which would adversely affect the squareness
of the hysteresis loop, a compact structure that does not permit
the diffusion of ferroelectric elements into the lower electrode
102 or the upper electrode 103 is desired. Among methods of
increasing the compactness of the lower electrode 102 and the upper
electrode 103 is a method of forming the films by sputtering by a
gas having a large mass, or a method of dispersing an oxide of a
substance such as Y or L into a precious metal electrode.
[0099] The ferroelectric film 101 is formed by using a PZT-family
ferroelectric formed of an oxide including Pb, Zr and Ti as
constituent elements. This embodiment is particularly characterized
in the use of Pb(Zr, Ti, Nb)O.sub.3 (PZTN) obtained by doping Nb
into Ti sites of this ferroelectric film 101.
[0100] Nb is substantially the same size as Ti (the ionic radii
thereof are close and the atomic radii are the same) but is twice
the weight thereof, so the atoms thereof are unlikely to escape
from the lattice even if there are collisions between atoms due to
lattice vibration. The valence of Nb is stable at +5, so that even
if the Pb escapes, the atomic weight after the Pb has escaped can
be compensated for by the Nb.sup.5+. During the crystallization,
even if Pb escape occurs, it is simpler for the small-sized Nb to
enter than the large-sized O to escape.
[0101] Since there are also some Nb atoms of valence +4, it is
possible that the substitution of Ti.sup.4+ will be performed
sufficiently. In addition, it is thought that the covalence of Nb
is extremely strong in practice, making it difficult for Pb to
escape (refer to H. Miyazawa, E. Natori, S. Miyashita; Jpn. J.
Appl. Phys. 39 (2000) 5679).
[0102] Up to now, the Nb doping into PZT has been mainly performed
into Zr-rich trigonal crystal regions and is extremely small, on
the order of 0.2 to 0.025 mol % (refer to J. Am. Ceram. Soc, 84
(2001) 902 and Phys. Rev. Let, 83 (1999) 1347). The reason why it
has not been possible to dope large quantities of Nb in this manner
is considered to be because the addition of 10 mol % of Nb, for
example, would require an increase in crystallization temperature
to at least 800.degree. C.
[0103] In such a case, it is preferable to further add PbSiO.sub.33
silicate in the proportion of 1 to 5 mol %, for example, during the
formation of the ferroelectric film 101. This makes it possible to
reduce the crystallization energy of the PZTN. In other words, if
PZTN is used as the material of the ferroelectric film 101, the
addition of PbSiO.sub.3 silicate makes it possible to design a
reduction in the crystallization temperature of the PZTN.
[0104] The description now turns to an example of a film formation
method for the PZTN ferroelectric film 101 employed in the
ferroelectric capacitor 100 of this embodiment.
[0105] The ferroelectric film 101 can be obtained by preparing
mixed solutions formed of first to third raw material solutions
including at least one of Pb, Zr, Ti and Nb, then subjecting the
oxides included within these mixed liquids to thermal processing or
the like, to cause them to crystallize.
[0106] An example of the first raw material solution could be a
solution in which a condensation polymer for forming PbZrO.sub.3
Perovskite crystals by Pb and Zr, from among constituent metal
elements for the PZTN ferroelectric phase, is dissolved in a
non-aqueous state in a solvent such as n-butanol.
[0107] An example of the second raw material solution could be a
solution in which a condensation polymer for forming PbTiO.sub.3
Perovskite crystals by Pb and Ti, from among constituent metal
elements for the PZTN ferroelectric phase, is dissolved in a
non-aqueous state in a solvent such as n-butanol.
[0108] An example of the third raw material solution could be a
solution in which a condensation polymer for forming PbNbO.sub.3
Perovskite crystals by Pb and Nb, from among constituent metal
elements for the PZTN ferroelectric phase, is dissolved in a
non-aqueous state in a solvent such as n-butanol.
[0109] When the ferroelectric film 101 is formed from
PbZr.sub.0.2Ti.sub.0.8Nb.sub.0.2O.sub.3 (PZTN) by using the above
first, second, and third raw material solutions, by way of example,
the ratios of (the first raw material solution):(the second raw
material solution):(the third raw material solution) could be
2:6:2. However, any attempt to use these mixed solutions for
crystallization as they are would necessitate a high
crystallization temperature for the manufacture of the PZTN
ferroelectric film 101. In other words, since the mixing in of Nb
will cause the crystallization temperature to rise abruptly, making
crystallization impossible within the temperature range that
enables the creation of the component at not more than 700.degree.
C., a substitution for Ti by Nb has not been used more than 5 mol %
in the conventional art and it has been used only as an additive.
In addition, there have been absolutely no examples of PZT
tetragonal crystals in which there is more Ti than Zr. This is
discussed in the previously cited J. Am. Ceram. Soc, 84 (2001) 902
and Phys. Rev. Let, 83 (1999) 1347.
[0110] This embodiment makes it possible to solve the
above-described technical problems by further adding to the
above-described mixed solution at least 1 mol % but less than 5 mol
% of a fourth raw material solution in which a condensation polymer
for forming PbSiO.sub.3 crystals is dissolved in a non-aqueous
state in a solvent such as n-butanol.
[0111] In other words, the use of the above-described mixture of
the first, second, third, and fourth solutions makes it possible to
move the crystallization temperature of the PZTN to a practicable
temperature range of not more than 700.degree. C.
[0112] More specifically, the ferroelectric film 101 is formed in
accordance with the flowchart of FIG. 2. The ferroelectric film 101
is formed by repeating a mixed solution painting process (step
ST11) then a series of an alcohol removal process, a dry thermal
process, and an absorbent thermal process (steps ST12 and ST13) a
desired number of times, followed by baking by crystallization
annealing (step ST14).
[0113] Examples of the conditions for these processes are given
below.
[0114] First of all, the film for the lower electrode is formed to
cover a precious metal for the electrode, such as Pt, on a Si
substrate (step ST10). A mixed liquid is then painted thereon by a
method such as spin-coating (step ST11). More specifically, the
mixed solution is dropped onto the Pt-covered substrate. After
spinning at approximately 500 rpm with the objective of spreading
the dropped solution over the entire surface of the substrate, the
angular velocity is dropped to not more than 50 rpm for about 10
seconds. The dry thermal processing is done at 150.degree. C. to
180.degree. C. (step ST13). The dry thermal processing is done by
using a hot-plate or the like in the atmosphere. Similarly, the
absorbent thermal processing is done in the atmosphere on the
hot-plate, which is held at 300.degree. C. to 350.degree. C. (step
ST13). The baking for crystallization is done by using rapid
thermal annealing (RTA) or the like in an oxygen atmosphere (step
ST14).
[0115] The film thickness after baking can be on the order of 100
to 200 nm. After the upper electrode has been formed by sputtering
or the like (step ST15), post-annealing is done with the objective
of forming an interface between the second electrode and the
ferroelectric thin film and improving the crystallinity of the
ferroelectric thin film, using RTA or the like in an oxygen
atmosphere in a similar manner to the baking (step ST16), to
achieve the ferroelectric capacitor 100.
[0116] The effects of the use of the PZTN ferroelectric film 101 in
the ferroelectric capacitor 100 on the hysteresis characteristic
are discussed below.
[0117] A hysteresis curve of electric polarization (P) versus
voltage (V) of the ferroelectric capacitor 100 is shown
schematically in FIG. 3. First of all, when a voltage +Vs is
applied, the polarization magnitude is P(+Vs), then when the
voltage becomes 0 the polarization magnitude becomes Pr. When the
voltage changes to -1/3Vs, the polarization magnitude is P(-1/3Vs).
When the voltage then becomes -Vs, the polarization magnitude
becomes (-Vs), and when the voltage is again 0, the polarization
magnitude becomes -Pr. When the voltage becomes +1/3Vs, the
polarization magnitude becomes P(+1/3Vs), and when the voltage is
again +Vs, the polarization magnitude returns to P(+Vs).
[0118] The ferroelectric capacitor 100 also has the features
described below, with respect to the hysteresis characteristic. If
a voltage Vs is applied and the polarization magnitude has gone to
P(+Vs) then the applied voltage goes to 0, the hysteresis loop
follows the path indicated by the arrow A in FIG. 3 and the
polarization magnitude holds the stable value PO(0). If a voltage
-Vs is applied and the polarization magnitude has gone to P(-Vs)
then the applied voltage goes to 0, the hysteresis loop follows the
path indicated by the arrow B in FIG. 3 and the polarization
magnitude holds the stable value PO(1). Thorough utilization of
this difference between the polarization magnitude PO(0) and the
polarization magnitude PO(1) makes it possible to operate a simple
matrix type of ferroelectric memory device by the drive method
disclosed in Japanese Patent Laid-Open No. 9-116107.
[0119] The ferroelectric capacitor 100 of this embodiment enables a
reduction in the crystallization temperature, an improvement in the
squareness of hysteresis loop, and an improvement in Pr. The
improvement in squareness of hysteresis loop achieved by the
ferroelectric capacitor 100 has the obvious effect of stabilizing
major disturbances in the driving of the simple matrix type of
ferroelectric memory device. In a simple matrix type of
ferroelectric memory device, a voltage of .+-.1/3Vs is applied even
to cells that are not being written to or read, so it is necessary
to have a stable disturbance characteristic to ensure that the
polarization does not change with these voltage changes. The
present inventors have confirmed for a general PZT that a
deterioration of approximately 80% of the polarization magnitude is
seen when a 1/3Vs pulse is applied 108 times in the direction
opposite to the polarization from a stable polarization state, but
the deterioration in the ferroelectric capacitor 100 of this
embodiment is not more than 10%. The use of the ferroelectric
capacitor 100 of this embodiment in a ferroelectric memory device
therefore makes it possible to realize a simple matrix type of
memory.
[0120] A detailed description of these embodiments is given
below.
First Embodiment
[0121] This embodiment compares the PZTN of the present invention
and the PZT of the conventional art. The entire film formation flow
shown in FIG. 2 was used.
[0122] Ratios of Pb:Zr:Ti:Nb=1:0.2:0.6:0.2, 1:0.2:0.7:0.1, and
1:0.3:0.65:0.5 were used. In other words, the total quantity of
added Nb is 5 to 20 mol %. In this case, 0 to 1% of PbSiO.sub.3 is
added.
[0123] The surface morphologies of the films in this case are shown
in FIGS. 4A to 4C. When the crystallinity of these films were
measured by an X-ray diffraction method, the results were as shown
in FIGS. 5A to 5C. With the 0% (none) case shown in FIG. 5A, only
ordinary paraelectric pyrochlore is obtained, even when the
crystallization temperature rises to 800.degree. C. With the 0.5%
case shown in FIG. 5B, PZT and the pyrochlore are mixed. With the
1% case shown in FIG. 5C, a single orientated film of PZT (111) is
obtained. The crystallinity thereof is also good, of a quality that
can not be achieved up to now.
[0124] Next, the crystallinity of comparative examples of PZTN thin
films with 1% of PbSiO.sub.3 added thereto, for different film
thicknesses of 120 to 200 nm, are shown in FIGS. 6A to 6C and FIGS.
7A to 7C. Note that FIGS. 6A to 6C are electron microphotographs of
the surface morphologies for film thicknesses 120 nm to 200 nm and
FIGS. 7A to 7C shown the results of measurements done by an X-ray
diffraction method to demonstrate the crystallinity of PZTN thin
films of film thicknesses 120 nm to 200 nm. As shown in FIGS. 8A to
8C and FIGS. 9A to 9C, hysteresis characteristics with good
squareness were obtained over the entire range of film thickness of
120 nm to 200 nm. Note that FIGS. 9A to 9C are enlargements of the
hysteresis curves of FIGS. 8A to 8C. These results confirmed that
the hysteresis curves clearly opened up and also reached saturation
at low voltages of less than or equal to 2 V, in the ZPTN thin
films of these examples.
[0125] The leakage characteristics were also extremely good at
5.times.10.sup.-8 to 7.times.10.sup.-9 A/cm.sup.2 when 2 V
(saturation) was applied thereto, regardless of the film
composition and film thickness, as shown in FIGS. 10A and 10B.
[0126] The results of measurements of fatigue characteristics and
static imprinting of PbZr.sub.0.2Ti.sub.0.8Nb.sub.0.2 thin films
were also good, as shown in FIGS. 11A and 11B. In particular, the
fatigue characteristic of FIG. 11A is extremely good, regardless of
whether Pt was used in the upper and lower electrodes.
[0127] Tests were also performed on an SiO.sub.2 film 605 formed by
ozone TEOS on a ferroelectric capacitor 600 in which a lower
electrode 602, a PZTN ferroelectric film 603 of this embodiment,
and an upper electrode 604 are formed on a substrate 601, as shown
in FIG. 12. It is known in the art that, if an SiO.sub.2 film is
formed by ozone TEOS on PZT, the hydrogen emitted by the TEOS
passes through the upper Pt and reduces, and the PZT crystal is so
destroyed that the hysteresis phenomenon does not occur.
[0128] With the PZTN ferroelectric film 603 of this embodiment,
however, favorable hysteresis is maintained with substantially no
deterioration, as shown in FIG. 13. In other words, it is clear
that the PZTN ferroelectric film 603 of this embodiment also has a
strong resistance to reduction. If the proportion of Nb in the
tetragonal PZTN ferroelectric film 603 of the present invention
does not exceed 40 mol %, favorable hysteresis is obtained in
proportion to the quantity of Nb added.
[0129] Evaluation with a conventional PZT ferroelectric film was
done for comparison, the conventional PZT samples had Pb:Zr:Ti
ratios of 1:0.2:0.8, 1:0.3:0.7, and 1:0.6:0.4. The leakage
characteristics thereof are such that the leakage characteristics
deteriorate with increasing T1 content, as shown in FIG. 14, so
that it is clear that when Ti is 80% and 2 V was applied, the
characteristic was 10.sup.-5 A/cm.sup.2, making it unsuitable for
memory applications. Similarly, the fatigue characteristic
deteriorated with increasing Ti content, as shown in FIG. 15. After
imprinting, it was clear that most of the data could not be read,
as shown in FIG. 16.
[0130] As is clear from the above description, the PZTN
ferroelectric film in accordance with this embodiment has simply
solved the problem of the increase in leakage current together with
the deterioration in the imprint characteristic, which are thought
to be intrinsic to PZT in the conventional art, making it possible
to use tetragonal PZT in memory applications without concern of
memory type or configuration. For the same reason, this material
can also be used in piezoelectric component applications in which
tetragonal PZT could not be used before.
Second Embodiment
[0131] This embodiment is a comparison of the ferroelectric
characteristics obtained when the amount of Nb added to the PZTN
ferroelectric film was varied to 0, 5, 10, 20, 30, 40 mol %. 5 mol
% of PbSiO.sub.3 was added to all the testpieces. In addition,
methyl succinate was added to the sol-gel solutions for forming the
ferroelectric films, includes of raw materials for film formation,
to adjust the pH to 6. The entire film formation flow shown in FIG.
2 was used therefor.
[0132] Measured hysteresis characteristics of PZTN ferroelectric
films in accordance with this embodiment are shown in FIGS. 17 to
19.
[0133] FIG. 17A shows that when the quantity of added Nb is 0,
leaky hysteresis is obtained, whereas FIG. 17B shows that when the
quantity of added Nb is 5 mol %, a good hysteresis characteristic
with a high level of insulation is obtained.
[0134] FIG. 18A shows that substantially no change is seen in the
ferroelectric characteristic until the quantity of added Nb reaches
10 mol %. Even when the quantity of added Nb is 0, it is leaky by
no change is seen in the ferroelectric characteristic. FIG. 18B
shown that when the quantity of added Nb is 20 mol %, a hysteresis
characteristic with an extremely good squareness is obtained.
[0135] However, it has been confirmed that if the quantity of Nb
added exceeds 20 mol %, the hysteresis characteristic changes
greatly and tends to deteriorate, as shown in FIGS. 19A and
19B.
[0136] Comparisons of X-ray diffraction patterns are shown in FIG.
20. When the quantity of added Nb is 5 mol % (Zr/Ti/Nb=20/75/5),
the (111) peak position does not change from that of a PZT film of
the conventional art in which no Nb is added, but the (111) peak
does shift towards the low-angle side in accordance with the
increases in the quantity of added Nb to 20 mol %
(Zr/Ti/Nb=20/60/20) and 40 mol % (Zr/Ti/Nb=20/40/40). In other
words, it is clear that the actual crystal is trigonal, regardless
of whether there are Ti-rich tetragonal regions in the PZT
composition. It is clear that the ferroelectric characteristics
change as the crystal composition changes.
[0137] In addition, when the quantity of added Nb reaches 45 mol %,
a sufficient hysteresis loop could not be obtained and it was not
possible to confirm the ferroelectric characteristics (not shown in
the figures).
[0138] It has already been stated that the PZTN of the present
invention has an extremely high level of insulation, but FIG. 21
shows this from the viewpoint of obtaining conditions that ensure
that the PZTN is a dielectric.
[0139] In other words, the PZTN of the present invention has an
extremely high level of insulation and this effect can be achieved
by ensuring that Nb is added to Ti sites in compositional ratio
equivalent to twice an insufficiency of Pb.
[0140] With PZTN, therefore, the addition of Nb enables active
control of Bb insufficiency, and also control over the crystal
configuration.
[0141] This shows that the PZTN of this embodiment would be
extremely useful when applied to piezoelectric element. In general,
when PZT is applied to piezoelectric elements, a trigonal crystal
region with a Zr-rich composition is used. In this case, Zr-rich
PZT is called soft PZT. This literally means that the crystal is
soft. Soft PZT is used in a nozzle that ejects ink in an inkjet
printer, but since it is excessively soft, ink that is too viscous
would impart stress thereto, making it impossible to push out.
[0142] Ti-rich tetragonal PZT, on the other hand, is called hard
PZT, which means it is hard and brittle. While the PZTN film of the
present invention is hard, the crystals can be changed into
trigonal crystals by artificial means. Since it is also possible to
change the crystal form arbitrarily by the quantity of added Nb and
since a Ti-rich PZT-family ferroelectric film has a small relative
permittivity, it is possible to drive such a component at a low
voltage.
[0143] This makes it possible to use hard PZT in applications in
which it could not be used previously, such as in the ink ejection
nozzles of an inkjet printer. In addition, since Nb makes the PZT
softer, it is possible to provide a PZT that is suitably hard, but
not brittle.
[0144] Finally, it is also possible to reduce the crystallization
temperature of this embodiment by adding not just Nb, but a
silicate simultaneously with the addition of the Nb.
Third Embodiment
[0145] This embodiment investigates the validity of using a PZTN
film from the viewpoint of lattice regularity, when the PZTN film
has been formed on a metal film formed of a platinum-group metal
such as Pt or Ir as an electrode material for a ferroelectric
capacitor that forms a memory cell portion of ferroelectric memory
or a piezoelectric actuator that configures an ink ejection nozzle
portion of an inkjet printer, by way of example. Platinum-group
metals act as underlayer films that determine the crystal
orientation of ferroelectric films, and are also useful as
electrode materials. However, since the lattice regularities of the
two materials are not sufficient, a problem arises concerning the
fatigue characteristics of ferroelectric films when applied to
elements.
[0146] In this case, the present inventors have developed a
technique designed to ameliorate lattice mismatches between a
PZT-family ferroelectric film and a platinum-group metal thin film,
by incorporating Nb into the constituent elements of the PZT-family
ferroelectric film. The process of manufacturing this PZT-family
ferroelectric film is shown in FIGS. 23A to 23C.
[0147] First of all, a given substrate 11 was prepared, as shown in
FIG. 23A. A TiOx layer formed on an SOI substrate was used as the
substrate 11. Note that a preferred material could be selected from
known materials as this substrate 11.
[0148] Next, as shown in FIG. 23B, a metal film (first electrode)
102 is formed by sputtering Pt, by way of example, onto the
substrate 11, then a PZTN film is formed as the ferroelectric film
101 on the metal film 102, as shown in FIG. 23C. sol-gel solutions
can be used as the materials for forming the PZTN film. More
specifically, a mixture of a sol-gel solution for PbZrO.sub.3, a
sol-gel solution for PbTiO.sub.3, and a sol-gel solution for
PbNbO.sub.3 is used, with a sol-gel solution for PbNbO.sub.3 added
thereto. Note that since a constituent element of the PZTN film is
Nb, the crystallization temperature thereof is high. For that
reason, it is preferable to further add the sol-gel solution for
PbSiO.sub.3, to reduce the crystallization temperature. With this
embodiment, the abovementioned sol-gel mixed solution is painted
onto the Pt metal film 102 by a spin-coating method, then is
subjected to predetermined thermal processing to crystallize it.
The film formation flow was similar to shown in FIG. 2.
[0149] When an X-ray diffraction method was used to measure the
crystal lattice constants of PZTN films obtained by this embodiment
of the invention, wherein the quantity of added Nb ranged from 0
mol % to 30 mol %, the results were as shown in FIGS. 24A and 24B.
It is clear from FIGS. 24A and 24B that the lattice constant along
the a axis (or the b axis) became closer to the lattice constant
along the c axis as the quantity of added Nb increased. In
addition, V(abc) in FIG. 24A is an index of volumetric changes in
lattice constant (a,b,c). The ratio V/V.sub.0 in FIG. 24A is the
ratio of the volume V(abc) of the PZTN crystal with respect to an
index V.sub.0 which is a volumetric change of the lattice constant
of a PZT crystal to which no Nb was added. It can be confirmed from
the V(abc) or V/V.sub.0 column that the crystal lattice of the PZTN
crystal becomes smaller as the quantity of added Nb increases.
[0150] The lattice mismatch ratios with respect to a lattice
constant (a,b,c=3.96) for Pt metal film were calculated from the
lattice constants of PZTN films formed with the addition of Nb in
this manner, and the quantity of added Nb (mol %) was plotted along
the horizontal axis in FIG. 25. It was confirmed from FIG. 25 that
the effects of including Nb into a PZT-family ferroelectric film
are not limited to the effect of improving the ferroelectric
characteristic of each of the above described embodiments, but they
also include the effect of approximating the lattice constant
thereof to the lattice constant of crystals of platinum-group
metals such as Pt. It was confirmed that this effect is
particularly obvious in the region in which the quantity of added
Nb is greater than or equal to 5 mol %.
[0151] It has therefore been confirmed that use of the method of
the present invention reduces lattice mismatches between the metal
film that is the electrode material and the ferroelectric film,
such that the lattice mismatch ratio is improved to the order of 2%
at a quantity of added Nb of 30 mol %, by way of example. This is
considered to be because strong bonds having both ionic bonding
between Nb atoms that have substituted for Ti atoms at the B sites
in the PZTN crystal structure and O atoms and covalence, these
bonds act in directions that compress the crystal lattice, causing
changes in the direction in which the lattice constant
decreases.
[0152] In addition, platinum-group metals such as Pt are chemically
stable substances that are ideal as the electrode material for
ferroelectric memory or a piezoelectric actuator, so that the
method of this embodiment makes it possible to alleviate lattice
mismatches more than in the conventional art, even when a PZTN film
is formed directly on this Pt metal film, and also improve the
interface characteristic thereof. The method of this embodiment
therefore makes it possible to reduce fatigue deterioration of
PZT-family ferroelectric films, making it suitable for application
to elements such as ferroelectric memory or piezoelectric
actuators.
Reference Example
[0153] For this example, PbZr.sub.0.4Ti.sub.0.6O.sub.3
ferroelectric films were manufactured.
[0154] A solution including approximately 20% excess Pb is used for
the conventional method, but this is to restrain volatile Pb and
reduce the crystallization temperature. However, since it is
unclear what happens to excess Pb in the completed thin films,
excessive quantities of Pb should be suppressed to a minimum.
[0155] In this case, a 10 wt % density of a sol-gel solution for
PbZr.sub.0.4Ti.sub.0.6O.sub.3 (solvent: n-butanol) having 0, 5, 10,
15, or 20% excess Pb was used, to which was added 1 mol % of 10 wt
% density of a sol-gel solution for forming PbSiO.sub.3 (solvent:
n-butanol), was used to form 200-nm PbZr.sub.0.4Ti.sub.0.6O.sub.3
ferroelectric films by the processes of steps ST20 to ST25 of FIG.
26. The surface morphologies in this case are shown in FIGS. 27A to
27C and XRD patterns thereof are shown in FIGS. 28A to 28C.
[0156] Although an excess of approximately 20% Pb is necessary in
the conventional art, it is clear that crystallization proceeds
sufficiently with a 5% excess of Pb. This shows that the addition
of just 1 mol % of the PbSiO.sub.3 catalyst lowers the
crystallization temperature of PZT, so that most of the excess Pb
is not needed. Thereonafter the solutions for forming PZT,
PbTiO.sub.3, and PbZrTiO.sub.3 all had 5% excess Pb.
[0157] Next, a mixed solution of 10 wt % density of a sol-gel
solution for forming PbZrO.sub.3 (solvent: n-butanol) and 10 wt %
density of a sol-gel solution for forming PbTiO.sub.3 (solvent:
n-butanol) in the ratio 4:6, to which was added 1 mol % of 10 wt %
density of a sol-gel solution for forming PbSiO.sub.3 (solvent:
n-butanol), was used in accordance with the flow shown in FIG. 2 to
manufacture 200-nm PbZr.sub.0.4Ti.sub.0.6O.sub.3 ferroelectric
films. The hysteresis characteristics in this case were favorable,
as shown in FIGS. 29A and 29B. However, it was clear they were
simultaneously leaky.
[0158] For comparison, a 200-nm PbZr.sub.0.4Ti.sub.0.6O.sub.3
ferroelectric thin film was manufactured by a conventional method
and the previously-described flow of FIG. 26, using a mixed
solution of 10 wt % density of a sol-gel solution for forming
PbSiO.sub.3 (solvent: n-butanol) in 10 wt % density of a sol-gel
solution for PbZr.sub.0.4Ti.sub.0.6O.sub.3 (solvent: n-butanol).
The hysteresis characteristic in this case was not particularly
impressive, as shown in FIG. 30.
[0159] When degassing analysis was performed on each of these
ferroelectric films, the results were as shown in FIGS. 31A and
31B.
[0160] As shown in FIG. 31A, it was confirmed that the conventional
ferroelectric film manufactured by PZT sol-gel solutions always
degases with respect to H and C, as the temperature rises from room
temperature to 1000.degree. C.
[0161] With the ferroelectric film of the present invention formed
by using a solution that is a 4:6 mixture of 10 wt % density of a
sol-gel solution for forming PbZrO.sub.3 (solvent: n-butanol) and
10 wt % density of a sol-gel solution for forming PbTiO.sub.3
(solvent: n-butanol), however, analysis showed that degassing was
mostly not seen.
[0162] This is thought to be because the use of a solution that is
a 4:6 mixture of 10 wt % density of a sol-gel solution for forming
PbZrO.sub.3 (solvent: n-butanol) and 10 wt % density of a sol-gel
solution for forming PbTiO.sub.3 (solvent: n-butanol) ensures that
PbTiO.sub.3 crystallizes on the Pt from the 10 wt % density of the
sol-gel solution for forming PbTiO.sub.3 (solvent: n-butanol)
within the initial mixed solution and this acts as initial
crystallization seeds, and also that lattice mismatches between the
Pt and the PZT disappear, facilitating the crystallization of the
PZT. The use of a mixed solution is also considered to form a
suitable interface between the PbTiO.sub.3 and the PZT, which is
linked to favorable squareness of hysteresis loop.
[0163] 2. Method of Manufacturing Ferroelectric Capacitor
[0164] Sections showing an example of a method of manufacturing a
ferroelectric capacitor in accordance with this embodiment of the
invention are shown schematically in FIGS. 32A to 32C.
[0165] 1) First of all, as shown in FIG. 32A, a lower electrode
102, the ferroelectric film 101, and an upper electrode 103 are
formed in sequence as a stack on a given substrate 110.
[0166] The substrate 110 is not particularly limited and thus any
preferred substance can be used therefore, depending on the
application of the ferroelectric capacitor, such as a semiconductor
substrate or a resin substrate, by way of example.
[0167] Either a precious metal such as Pt, Ir, or Ru alone or a
compound material having such a precious metal as a main component
can be employed as the lower electrode 102 and the upper electrode
103. A known film formation method could be used for forming the
lower electrode 102 and the upper electrode 103, such as sputtering
or vapor deposition. Since the diffusion of ferroelectric elements
into the lower electrode 102 or the upper electrode 103 would cause
variations in the composition of the interface between that
electrode and the ferroelectric film 101, which would adversely
affect the squareness of the hysteresis loop, a compact structure
that does not permit the diffusion of ferroelectric elements into
the lower electrode 102 or the upper electrode 103 is desired. In
this case, a method of forming the films by sputtering by a gas
having a large mass, or a method of dispersing an oxide of a
substance such as Y or L into a precious metal electrode could be
employed in order to increase the compactness of the lower
electrode 102 and the upper electrode 103.
[0168] The ferroelectric film 101 includes Pb, Zr, Ti, and Nb as
constituent elements, and thus is called a PZTN complex oxide. The
ferroelectric film 101 can be formed by using a spin-coating method
or the like to paint sol-gel solutions including Pb, Zr, Ti, and Nb
onto the lower electrode 102. Mixtures of a first sol-gel solution
in which a condensation polymer for forming PbZrO.sub.3 Perovskite
crystals by Pb and Zr is dissolved in a non-aqueous state in a
solvent such as n-butanol; a second solution in which a
condensation polymer for forming PbTiO.sub.3 Perovskite crystals by
Pb and Ti, from among constituent metal elements for the PZTN
ferroelectric phase, is dissolved in a non-aqueous state in a
solvent such as n-butanol; and a third sol-gel solution in which a
condensation polymer for forming PbTiO.sub.3 Perovskite crystals by
Pb and Ti, from among constituent metal elements for the PZTN
ferroelectric phase, is dissolved in a non-aqueous state in a
solvent such as n-butanol could be used as these sol-gel solutions.
In addition, during the formation of the ferroelectric film 101, a
sol-gel solution including a silicate or germanate for reducing the
crystallization temperature of the PZTN complex oxide could be
added. More specifically, at least 1 mol % but less than 5 mol % of
a fourth sol-gel solution in which a condensation polymer for
forming PbSiO.sub.3 crystals is dissolved in a non-aqueous state in
a solvent such as n-butanol could be further added to the
above-described mixture of sol-gel solutions. The mixing in of this
fourth sol-gel solution makes it possible for the crystallization
to occur within a temperature range that enables the creation of
elements at a crystallization temperature for the PZTN complex
oxide of 700.degree. C., although the inclusion of Nb as a
constituent element would normally increase the crystallization
temperature.
[0169] It is preferable that the painted film for the ferroelectric
film 101 is subjected to preliminary thermal processing at a
temperature (such as not more than 400.degree. C.) that does not
cause crystallization of the PZTN complex oxide in an oxidizing
atmosphere, to put the PZTN complex oxide into an amorphous state.
This enables the advance of the previously described process while
preventing the diffusion of constituent elements in a state in
which the ferroelectric film 101 is in an amorphous state, with no
grain boundaries. The performing of this preliminary thermal
processing in an oxidizing atmosphere has the effect of introducing
into the ferroelectric film 101 the oxygen component that is
necessary for the crystallization of the PZTN complex oxide after
the formation of a protective film, which will be described
layer.
[0170] 2) Next, as shown in FIG. 32B, the lower electrode 102, the
ferroelectric film 101, and the upper electrode 103 are etched to a
desired shape, and a protective film 104 of silicon dioxide
(SiO.sub.2) is formed to cover them. The protective film 104 in
this case can be formed by a CVD method, using trimethylsilane
(TMS). With trimethylsilane (TMS), there is a smaller quantity of
hydrogen generated during the CVD process, in comparison with the
tetraethyl orthosilicate (TEOS) that is generally used for forming
a silicon dioxide film. If trimethylsilane (TMS) is used for that
reason, it is possible to reduce processing damage to the
ferroelectric film 101 due to the reductive reaction. Since the
process of using trimethylsilane (TMS) to form the protective film
104 can be done at a lower temperature (from room temperature to
350.degree. C.) than the process using TEOS (a film-formation
temperature of at least 400.degree. C.), it is possible to maintain
the amorphous state achieved by the process of (1), preventing
crystallization of the PZTN complex oxide by the heat generated by
this process of forming the protective film 104.
[0171] 3) Next, as shown in FIG. 32C, thermal processing is
performed to crystallize the PZTN complex oxide that configures the
ferroelectric film 101, making it possible to obtain a
ferroelectric capacitor having a PZTN ferroelectric crystal film
101a. This thermal processing could be done, not in an oxygen
atmosphere, but in an atmosphere of a non-oxidizing gas such as Ar
or N.sub.2 or in air, to enable the crystallization of the PZTN
complex oxide.
[0172] FIGS. 33A and 33B show results obtained by measuring the
hysteresis characteristics of capacitors in which the manufacture
method of this embodiment was employed to form a SiO.sub.2
protective film by using TMS over a ferroelectric capacitor formed
of a Pt lower electrode, a PZTN ferroelectric film, and a Pt upper
electrode, when the PZTN ferroelectric film was subjected to
thermal processing in an oxygen atmosphere or air after this
SiO.sub.2 protective film was formed. FIG. 33A shows the results of
thermal processing in an oxygen atmosphere and FIG. 33B shows the
results of thermal processing in air. FIGS. 33A and 33B show that
hysteresis characteristics with good squareness were obtained,
regardless of whether the thermal processing was done in an oxygen
atmosphere or air, even though a hydrogen-resisting barrier film
was not formed. This is because preliminary thermal processing was
performed in an oxidizing atmosphere during the formation of the
ferroelectric film 101 so that the oxygen necessary for the
crystallization had previously entered the film. In other words,
the manufacture method of this embodiment makes it possible to
crystallize the ferroelectric film without being dependent on the
atmosphere for thermal processing. In addition, when the thermal
processing for crystallization is performed in a non-oxidizing gas
atmosphere, it is possible to prevent oxidation damage due to
high-temperature thermal processing on peripheral components (for
example, metal wiring) other than the capacitor, when applied to a
method of manufacturing a ferroelectric memory that will be
described later. Note that since the thermal processing for
crystallizing the PZTN complex oxide in this process is not very
dependent on the type of gas in the atmosphere, contact holes for
forming metal wiring for connecting the upper electrode 103 to the
exterior can be formed after the protective film 104 is formed.
[0173] FIG. 34 shows the results of measurements obtained by
measuring the hysteresis characteristic for examples in which the
manufacture method of this embodiment was employed to form a
SiO.sub.2 protective film by using TMS over a ferroelectric
capacitor formed of a Pt lower electrode, a PZTN ferroelectric
film, and a Pt upper electrode, and the PZTN ferroelectric film was
crystallized after the formation of this SiO.sub.2 protective film,
where the formation temperature of the SiO.sub.2 protective film
was room temperature, 125.degree. C., and 200.degree. C.; and the
hysteresis characteristic of a reference example in which the PZTN
ferroelectric film was crystallized without the SiO.sub.2
protective film being formed, and calculating the corresponding
change in residual polarization magnitude 2Pr. From FIG. 34 it can
be seen that there was no change in residual polarization magnitude
2Pr, whether the SiO.sub.2 protective film was formed at room
temperature, 125.degree. C., or 200.degree. C., which confirms that
the formation of the SiO.sub.2 protective film does not result in
an inferior product. In other words, by performing the thermal
processing for crystallizing the PZTN complex oxide even after
damage is done by hydrogen during the processing of the
ferroelectric film 101 in the formation of the protective film 104,
the manufacture method of this embodiment ensures that the PZTN
complex oxide is crystallized while any such damage is repaired.
This makes it possible to omit the process of forming a barrier
film for protecting against reductive reactions of the
ferroelectric film 101, which is necessary in the conventional art,
enabling an increase in productivity and a reduction in production
costs.
[0174] 3. Ferroelectric Memory
[0175] The configuration of a simple matrix type of ferroelectric
memory device 300 in accordance with an embodiment of the present
invention is shown in FIGS. 35A and 35B. FIG. 35A is a plan view
thereof and FIG. 35B is a section taken along a line A-A in FIG.
35A. The ferroelectric memory device 300 has a predetermined array
of word lines 301 to 303 and a predetermined array of bit lines 304
to 306 formed on a substrate 308. A ferroelectric film 307 formed
of the PZTN described with respect to this embodiment is inserted
between the word lines 301 to 303 and the bit lines 304 to 306, and
ferroelectric capacitors are formed at the intersection regions
between the word lines 301 to 303 and the bit lines 304 to 306.
[0176] In the ferroelectric memory device 300 in which memory cells
configured of this simple matrix are arrayed, the operations of
writing to and reading from the ferroelectric capacitors formed at
the intersections between the word lines 301 to 303 and the bit
lines 304 to 306 are done by peripheral drive circuits and a read
amplifier circuit (called "peripheral circuit" although not shown
in the figures). This peripheral circuit could be formed of MOS
transistors on another substrate than the memory cell array, or the
peripheral circuit could be integrated on the same substrate as the
memory cell array.
[0177] FIG. 36 is a section through an example of the ferroelectric
memory device 300 in which the memory cell array is integrated on
the same substrate as the peripheral circuit.
[0178] In FIG. 36, a MOS transistor 402 is formed on a
monocrystalline silicon substrate 401, and this transistor
formation region supports a peripheral circuit. The MOS transistor
402 is formed of the monocrystalline silicon substrate 401, a
source/drain region 405, a gate isolation film 403, and a gate
electrode 404.
[0179] The ferroelectric memory device 300 includes an element
separation oxide layer 406, a first interlayer dielectric 407, a
first wiring layer 408, and a second interlayer dielectric 409.
[0180] The ferroelectric memory device 300 has a memory cell array
formed of ferroelectric capacitors 420, where each ferroelectric
memory cell is formed of a lower electrode (first electrode or
second electrode) 410 that becomes a word line or bit line, a
ferroelectric film 411 including a ferroelectric phase and an
ordinary paraelectric phase, and an upper electrode (second
electrode or first electrode) 412 that becomes a bit line or a word
line.
[0181] This ferroelectric memory device 300 also has a third
interlayer dielectric 413 on the ferroelectric capacitor 420, and
the memory cell array and the peripheral circuit are connected by a
second wiring layer 414. Note that a protective film 415 is formed
over the third interlayer dielectric 413 and the second wiring
layer 414 of the ferroelectric memory device 300.
[0182] The ferroelectric memory device 300 having the configuration
described above makes it possible to integrate a memory cell array
and a peripheral circuit on the same substrate. Note that the
ferroelectric memory device 300 shown in FIG. 36 is configured of a
memory-cell array on top of the peripheral circuit, but the
configuration could equally well be such that the memory cell array
is connected to the peripheral circuit in a planar manner, without
disposing the memory cell array on the peripheral circuit.
[0183] Since the ferroelectric capacitor 420 used in this
embodiment is configured of the PZTN described above, the
squareness of the hysteresis loop is extremely good and it has a
stable disturbance characteristic. In addition, the reduction in
the processing temperature for this ferroelectric capacitor 420
reduces damage to peripheral circuits and other components, and
also reduces processing damage (particularly that due to hydrogen
reduction), so that any deterioration in the hysteresis loop due to
damage can be suppressed. The use of this ferroelectric capacitor
420 therefore enables practicable application of the simple matrix
type of ferroelectric memory device 300.
[0184] A configurational view of a 1T1C type of ferroelectric
memory device 500 that is a variant example is shown in FIG. 37A.
an equivalent circuit diagram of the ferroelectric memory device
500 is shown in FIG. 37B.
[0185] The ferroelectric memory device 500 is a memory device of a
configuration that closely resembles DRAM, having a capacitor 504
(1C) formed of a lower electrode 501, an upper electrode 502
connected to a plate line, and a ferroelectric film 503 to which
the PZTN ferroelectric of this embodiment is applied; and a
transistor element 507 (1T) for switching wherein either the source
or the drain electrode is connected to a data line 505 and a gate
electrode 506 is connected to a word line, as shown in FIG. 37A.
There are hopes that this structure will replace SRAM since writing
and reading with respect to a 1T1C type of memory can be done at
high speeds of not more than 100 ns and also the data written
thereto is non-volatile.
[0186] 4. Method of Manufacturing Ferroelectric Memory
[0187] The description now turns to a case in which the manufacture
method described in "2. Method of Manufacturing Ferroelectric
Capacitor" is applied to a method of manufacturing ferroelectric
memory.
[0188] FIGS. 38A to 38C are schematic sections showing an example
of the process of manufacturing the ferroelectric memory in
accordance with this embodiment.
[0189] With this embodiment, the lower electrode 102, the PZTN
ferroelectric film 101, and the upper electrode 103 of the
ferroelectric capacitor 100 are formed sequentially on the
substrate 110, as shown in FIG. 38A. During this time, the
ferroelectric film 101 is subjected to preliminary thermal
processing in an oxidizing atmosphere, to put it into an amorphous
state. Note that the substrate 110 could have a configuration such
that a transistor 116 for cell selection is formed on a
semiconductor substrate 111, as shown by way of example in FIG.
38A. This transistor 116 could be configured of a source and drain
113, a gate oxide layer 114, and a gate electrode 115. A plug
electrode 117 formed of tungsten or the like is formed over either
the source or the drain 113, enabling the use of a stack structure
that is formed to enable connection to the lower electrode 102 of
the ferroelectric capacitor 100. The cells are isolated by an
element separation region 112 in the substrate 110 between the
cells, and the transistor 116 can have an interlayer dielectric 118
formed of an oxide layer or the like above the transistor 116.
[0190] The fabrication process in accordance with this embodiment
patterns the ferroelectric capacitor 100 to the desired size and
shape, as shown in FIG. 38B. The SiO.sub.2 protective film 104 is
formed by using trimethylsilane (TMS) to cover the ferroelectric
capacitor 100, a contact hole 105 for connection to the exterior is
formed, and then thermal processing is preformed to crystallize the
PZTN ferroelectric and form the ferroelectric film 101a. During the
crystallization of the PZTN ferroelectric, the thermal processing
for the crystallization could be performed in a non-oxidizing
atmosphere, This makes it possible to prevent any oxidation damage
due to high-temperature thermal processing to peripheral components
(such as metal wiring) outside of the ferroelectric capacitor
100.
[0191] Finally, a contact hole for connecting the transistor 116 to
the exterior is formed in the SiO.sub.2 protective film 104 and the
ferroelectric memory is completed by the formation of metal wiring
layers 191 and 192, as shown in FIG. 38C. The fabrication process
of this embodiment enables the omission of the process of forming a
barrier film for protecting the ferroelectric film 101 from
reductive reactions, which is necessary in the conventional art,
thus enabling an increase in productivity and a reduction in
production costs. Since this enables the formation of the
ferroelectric capacitor 100 that has a favorably square hysteresis
characteristic even although the process of forming that barrier
layer is omitted, it makes it possible to obtain ferroelectric
memory with superlative characteristics.
[0192] Note that the descriptions above dealt with the process of
manufacturing a 1T1C type of ferroelectric memory but the method of
manufacturing a ferroelectric capacitor in accordance with this
embodiment can also be applied to methods of manufacturing
ferroelectric memory that use other types of cell, such as the 2T2C
type or the simple matrix type (crosspoint-type).
[0193] 5. Piezoelectric Element and Inkjet-type Recording Head
[0194] The description now turns to details of an inkjet type of
recording head in accordance with an embodiment of the present
invention.
[0195] In an inkjet-type recording head wherein part of a stress
generating chamber that communicates with a nozzle aperture that
ejects ink droplets is formed of a vibrating plate, where ink in
the stress generating chamber is pressurized by distortions of this
vibrating plate by a piezoelectric element and is ejected as ink
droplets from a nozzle aperture, there are two methods of
implementation: one using a piezoelectric actuator having
longitudinal resonance mode by which a piezoelectric element
resonates longitudinally to expand and contract in the axial
direction and one using an piezoelectric actuator having a flexural
resonance mode.
[0196] It is known to form a uniform piezoelectric layer by a
film-formation technique over the entire surface of a vibrating
plate, for use as an actuator of a flexural resonance mode, then
divide that piezoelectric layer by a lithography method into shapes
corresponding to stress generating chambers and form a
piezoelectric element independently for each stress generating
chamber.
[0197] A partial perspective view of parts of an inkjet-type
recording head in accordance with an embodiment of the present
invention is shown in FIG. 39, a plan view and a section taken
along the line A-A' of FIG. 39 are shown in FIG. 40A and FIG. 40B,
and a schematic view of the layer structure of a piezoelectric
element 700 is shown in FIG. 41. As shown in these figures, a flow
path shaping substrate 10 is formed of a (110)-orientation silicon
monocrystalline substrate in accordance with this embodiment, and
an elastic film 50 of thickness 1 to 2 .mu.m is formed of silicon
dioxide by previous thermal oxidation on one surface thereof. A
plurality of stress generating chambers 12 are arrayed in the
widthwise direction of the flow path shaping substrate 10. A
connective portion 13 is formed in the longitudinal direction of a
region on the outer side of the stress generating chambers 12 of
the flow path shaping substrate 10 and the connective portion 13
and the stress generating chambers 12 communicate through an ink
supply path 14 provided for each stress generating chamber 12. Note
that the connective portion 13 forms part of a reservoir 800 that
forms a common ink chamber for the stress generating chambers 12
communicating with a reservoir portion of a sealing substrate 30
that will be described later. Each ink supply path 14 is formed to
width that is narrower than the stress generating chamber 12, to
keep the resistance of ink flowing into the stress generating
chamber 12 from the ink supply path 14 constant.
[0198] On an aperture surface side of the flow path shaping
substrate 10, a nozzle plate 20 is affixed by adhesive or thermal
bonding film. Nozzle apertures 21 pierce the nozzle plate 20 and
communicate with an edge portion on the opposite side from the ink
supply paths 14 of the stress generating chambers 12.
[0199] On the opposite side from the aperture surface of the flow
path shaping substrate 10, the elastic film 50 of a thickness of
approximately 1.0 .mu.m, by way of example, is formed as mentioned
previously, and a dielectric film 55 of a thickness of
approximately 0.4 .mu.m, by way of example, is formed on that
elastic film 50. In addition, a lower electrode film 60 of a
thickness such as approximately 0.2 .mu.m, a piezoelectric layer 70
of a thickness such as approximately 1.0 .mu.m, and an upper
electrode film 80 of a thickness such as approximately 0.05 .mu.m
are formed in a stack on the dielectric film 55 by processing that
will be described later, to form the piezoelectric element 700. In
this case, the piezoelectric element 700 is the portion including
the lower electrode film 60, the piezoelectric layer 70, and the
upper electrode film 80. In general, one electrode of the
piezoelectric element 700 is a common electrode and the other
electrode and the piezoelectric layer 70 are patterned to form each
stress generating chamber 12. A portion formed of the
thus-patterned electrode and the piezoelectric layer 70 that
generates piezoelectric strain by the application of a voltage to
the two electrodes is called an active piezoelectric portion. With
this embodiment, the lower electrode film 60 is the common
electrode of the piezoelectric element 700 and the upper electrode
film 80 is the other electrode of the piezoelectric element 700,
but there is no obstacle to reversing these roles to suit the
circumstances of the drive circuit or wiring. In either case, an
active piezoelectric portion is formed for each stress generating
chamber. In this case, the combination of the piezoelectric element
700 and the vibrating plate in which displacements are generated by
the driving of that piezoelectric element 700 is called a
piezoelectric actuator. Note that the piezoelectric layer 70 is
provided independently for each stress generating chamber 12 and is
configured of a plurality of layers of ferroelectric film 71 (71a
to 71f).
[0200] An inkjet-type recording head forms part of a recording head
unit that provides an ink flow path communicating with an ink
cartridge or the like, and is mounted in an inkjet-type recording
device. A schematic view of an example of this inkjet-type
recording device is shown in FIG. 42. As shown in FIG. 42,
recording head units 1A and 1B having inkjet-type recording heads
are provided with removable cartridges 2A and 2B that form ink
supply means, and a carriage in which these recording head units 1A
and 1B are mounted is provided so as to be able to move freely in
the axial direction of a carriage shaft 5 that is attached in a
device body 4. These recording head units 1A and 1B are designed to
eject substances that compose black ink and color ink,
respectively. The carriage 3 in which the recording head units 1A
and 1B are mounted is made to move along the carriage shaft 5 by
the transfer of a driving force of a drive motor 6 to the carriage
3 through a plurality of gearwheels (not shown in the figure) and a
timing belt 7. A platen 8 is also provided on the carriage shaft 5
in the device body and a sheet S that is a sheet of a recording
medium such as paper supplied by paper-supply rollers (not shown in
the figure) is transferred onto the platen 8.
[0201] Note that the description above relates to one example of an
inkjet-type recording head that ejects ink as a liquid ejection
head, but the present invention can also be applied widely to
liquid ejection heads and liquid ejection devices in which
piezoelectric elements are used. Examples of such a liquid ejection
head include a recording head used in an image recording device
such as a printer, a color jet head used for forming a color filter
for a liquid-crystal display or the like, an organic EL display, an
electrode material ejection head used for forming electrodes for a
field emission display (FED) or the like, and a living organic
ejection head used in the formation of biochips.
[0202] Since the piezoelectric element of this embodiment uses a
PZTN film in accordance with this embodiment as described above as
the piezoelectric layer, it achieves the following effects.
[0203] 1) Since covalence is increased in the piezoelectric layer,
the piezoelectric constant is increased.
[0204] 2) It is easy to apply an electrical field that suppresses
the generation of faults in the interface between the piezoelectric
layer and the electrode, for suppressing any insufficiency of PbO
in the piezoelectric layer, enabling an increase in the efficiency
thereof as a piezoelectric element.
[0205] 3) Since leakage currents in the piezoelectric layer are
suppressed, it is possible to make a thin film of the piezoelectric
layer.
[0206] A liquid ejection head and liquid ejection device in
accordance with this embodiment makes use of a piezoelectric
element including the above-described piezoelectric layer, enabling
it to achieve the following effect in particular.
[0207] 4) Since a reduction in fatigue deterioration of the
piezoelectric layer is enabled, time-related changes in the
displacement magnitude of the piezoelectric layer can be
suppressed, enabling an increase in reliability.
[0208] The present invention has been described above with
reference to preferred embodiments thereof but the present
invention is not limited thereto and thus is it possible to
implement other types of distortion within the range of the
invention as laid out herein.
[0209] For example, the substitution of Ta, W, V, or Mo for the Nb
in the PZT of the ferroelectric film 101, or the addition thereof,
would have similar effects. The use of Mn as an addition would have
effects similar to those of Nb. Similar thinking would lead to the
substitution of elements of a valence of +3 or greater, to prevent
the escape of Pb, and candidates therefor could be of the
lanthanoide series, such as La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy,
Ho, Er, Tm, Yb and Lu. In addition the additive that promotes
crystallization could be a germanate (Ge) instead of a silicate.
The hysteresis characteristic of a material in which 10 mol % Ta is
added to the PZT in place of Nb is shown in FIG. 43A. The
hysteresis characteristic of a material in which 10 mol % W is
added to the PZT in place of Nb is shown in FIG. 43B. It is clear
that the use of Ta would have a similar effect to that obtained by
the addition of Nb. Similarly, the use of W have also a similar
effect to that obtained by the addition of Nb, from the viewpoint
of a hysteresis characteristic has good insulating properties.
* * * * *