U.S. patent application number 13/960412 was filed with the patent office on 2013-12-05 for scalable lead zirconium titanate (pzt) thin film material and deposition method, and ferroelectric memory device structures comprising such thin film material.
This patent application is currently assigned to Advanced Technology Materials, Inc.. The applicant listed for this patent is Advanced Technology Materials, Inc.. Invention is credited to Thomas H. Baum, Steven M. Bilodeau, Stephen T. Johnston, Jeffrey F. Roeder, Michael W. Russell, Peter C. Van Buskirk, Daniel J. Vestyck.
Application Number | 20130324390 13/960412 |
Document ID | / |
Family ID | 22953832 |
Filed Date | 2013-12-05 |
United States Patent
Application |
20130324390 |
Kind Code |
A1 |
Van Buskirk; Peter C. ; et
al. |
December 5, 2013 |
SCALABLE LEAD ZIRCONIUM TITANATE (PZT) THIN FILM MATERIAL AND
DEPOSITION METHOD, AND FERROELECTRIC MEMORY DEVICE STRUCTURES
COMPRISING SUCH THIN FILM MATERIAL
Abstract
A novel lead zirconium titanate (PZT) material having unique
properties and application for PZT thin film capacitors and
ferroelectric capacitor structures, e.g., FeRAMs, employing such
thin film material. The PZT material is scalable, being
dimensionally scalable, pulse length scalable and/or E-field
scalable in character, and is useful for ferroelectric capacitors
over a wide range of thicknesses, e.g., from about 20 nanometers to
about 150 nanometers, and a range of lateral dimensions extending
to as low as 0.15 .mu.m. Corresponding capacitor areas (i.e.,
lateral scaling) in a preferred embodiment are in the range of from
about 10.sup.4 to about 10.sup.-2 .mu.m.sup.2. The scalable PZT
material of the invention may be formed by liquid delivery MOCVD,
without PZT film modification techniques such as acceptor doping or
use of film modifiers (e.g., Nb, Ta, La, Sr, Ca and the like).
Inventors: |
Van Buskirk; Peter C.;
(Newtown, CT) ; Roeder; Jeffrey F.; (Brookfield,
CT) ; Bilodeau; Steven M.; (Oxford, CT) ;
Russell; Michael W.; (New Milford, CT) ; Johnston;
Stephen T.; (Bethel, CT) ; Vestyck; Daniel J.;
(Wilkes Barre, PA) ; Baum; Thomas H.; (New
Fairfield, CT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Advanced Technology Materials, Inc. |
Danbury |
CT |
US |
|
|
Assignee: |
Advanced Technology Materials,
Inc.
Danbury
CT
|
Family ID: |
22953832 |
Appl. No.: |
13/960412 |
Filed: |
August 6, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12978393 |
Dec 23, 2010 |
8501976 |
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13960412 |
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12768374 |
Apr 27, 2010 |
7862857 |
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12978393 |
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11924716 |
Oct 26, 2007 |
7705382 |
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12768374 |
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11328582 |
Jan 10, 2006 |
7344589 |
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11924716 |
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09928860 |
Aug 13, 2001 |
6984417 |
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11328582 |
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09251890 |
Feb 19, 1999 |
6316797 |
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09928860 |
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Current U.S.
Class: |
501/134 |
Current CPC
Class: |
C04B 2235/449 20130101;
C07F 7/003 20130101; H01G 4/1245 20130101; H01L 21/02205 20130101;
H01L 21/02271 20130101; H01L 27/11502 20130101; C04B 2235/441
20130101; C07C 49/92 20130101; C23C 16/409 20130101; H01L 27/11507
20130101; C04B 35/491 20130101; H01G 4/33 20130101; H01L 28/55
20130101; H01L 21/31691 20130101; C04B 2235/963 20130101; H01L
21/02197 20130101 |
Class at
Publication: |
501/134 |
International
Class: |
H01L 27/115 20060101
H01L027/115 |
Claims
1. A precursor composition for forming a PZT film on a substrate by
liquid delivery MOCVD, said precursor composition comprising lead,
zirconium and titanium precursors each in a solvent medium so that
the precursors when vaporized form precursor vapor which when
contacted with the substrate forms a film characterized by at least
one of the characteristics of: (i) said film having an A/B ratio of
from 0.612 to 1.53, wherein the A/B ratio is Pb to (Zr+Ti); and
(ii) said film being at least one of dimensional scalable, pulse
length scalable and E-field scalable.
2. The precursor composition of claim 1, wherein the lead precursor
is selected from the group consisting of Pb(thd).sub.2 and
Pb(thd).sub.2pmdeta.
3. The precursor composition of claim 1, wherein the zirconium
precursor is selected from the group consisting of Zr(thd).sub.4
and Zr(O-i-Pr).sub.2(thd).sub.2.
4. The precursor composition of claim 1, wherein the titanium
precursor is Ti(O-i-Pr).sub.2(thd).sub.2.
5. The precursor composition of claim 1, wherein the lead,
zirconium and titanium precursors are selected from the group
consisting of: lead bis(2,2,6,6-tetramethyl-3,5-heptanedionate);
lead bis(2,2,6,6-tetramethyl-3,5-heptanedionate)
N,N',N''-pentamethyl diethylenetriamine; zirconium
tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionate); zirconium
bis(isopropoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionate);
Zr.sub.2(O-i-Pr).sub.6(2,2,6,6-tetramethyl-3,5-heptanedionate).sub.2;
and Ti(O-i-Pr).sub.2(2,2,6,6-tetramethyl-3,5-heptanedionate).sub.2,
wherein O-i-Pr is isopropoxy.
6. The precursor composition of claim 1, wherein each of the lead,
zirconium and titanium precursors is in a same solvent medium.
7. The precursor composition of claim 1, wherein each of the lead,
zirconium and titanium precursors is in a different solvent
medium.
8. The precursor composition of claim 1, wherein the solvent
mediums of the respective lead, zirconium and titanium precursors
include solvent selected from the group consisting of glymes,
diglymes, hydrocarbon solvents, aryl solvents, aliphatic
hydrocarbons, aromatic hydrocarbons, isopropanol, organic ethers,
organic esters, alkyl nitriles, alkanols, alcohols, organic amines,
polyamines, glymes having from 1 to 20 ethoxy --(C.sub.2H.sub.4O)--
repeat units; C.sub.2-C.sub.12 alkanols, dialkyl ethers comprising
C.sub.1-C.sub.6 alkyl moieties, C.sub.4-C.sub.8 cyclic ethers;
C.sub.12-C.sub.60 crown O.sub.4-O.sub.20 ethers; C.sub.6-C.sub.12
aliphatic hydrocarbons; C.sub.6-C.sub.18 aromatic hydrocarbons,
tetrahydrofuran, alkyl acetate, tetraglyme, C.sub.3-C.sub.8
alkanols, butyl acetate, 12-crown-4, 15-crown-5, and 18-crown-6
ethers, acetone, dimethoxyethane (DME), dimethylformamide (DMF),
polyether alcohols, tetrathiocyclodecane, tetrahydrofuranacetate,
octane, decane, and mixtures of two or more of the foregoing.
9. The precursor composition of claim 1, wherein at least one of
the solvent mediums of the respective lead, zirconium and titanium
precursors includes a solvent selected from the group consisting of
tetrahydrofuran, glyme solvents, alcohols, hydrocarbon solvents,
aryl solvents, amines, polyamines, and mixtures of two or more of
the foregoing.
10. The precursor composition of claim 1, wherein at least one of
the solvent mediums of the respective lead, zirconium and titanium
precursors includes a solvent selected from the group consisting of
tetrahydrofuran, isopropanol, tetraglyme, octane, decane, and
polyamine, and mixtures of two or more of the foregoing.
11. The precursor composition of claim 1, wherein at least one of
the solvent mediums of the respective lead, zirconium and titanium
precursors includes a solvent selected from the group consisting of
glyme, diglyme, isopropanol, tetrahydrofuran, tetraglyme, butyl
acetate, acetone, dimethoxyethane, dimethylformamide,
tetrathiocyclodecane, tetrahydrofuranacetate, octane, decane, and
mixtures of two or more of the foregoing.
12. The precursor composition of claim 1, wherein the lead,
zirconium and titanium precursors are selected from the group
consisting of: (i) lead
bis(2,2,6,6-tetramethyl-3,5-heptanedionate),
Ti(O-i-Pr).sub.2(2,2,6,6-tetramethyl-3,5-heptanedionate).sub.2, and
zirconium tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionate) as
respective lead, titanium and zirconium precursors; (ii) lead
bis(2,2,6,6-tetramethyl-3,5-heptanedionate) N,N',N''-pentamethyl
diethylenetriamine,
Ti(O-i-Pr).sub.2(2,2,6,6-tetramethyl-3,5-heptanedionate).sub.2, and
zirconium tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionate) as
respective lead, titanium and zirconium precursors; and (iii) lead
bis(2,2,6,6-tetramethyl-3,5-heptanedionate) N,N',N''-pentamethyl
diethylenetriamine,
Ti(O-i-Pr).sub.2(2,2,6,6-tetramethyl-3,5-heptanedionate).sub.2, and
zirconium
bis(isopropoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionate) as
respective lead, titanium and zirconium precursors; wherein O-i-Pr
is isopropoxy.
13. A precursor composition for forming a PZT film on a substrate
by liquid delivery MOCVD, said precursor composition comprising
lead, zirconium and titanium precursors each in a solvent medium so
that the precursors when vaporized form precursor vapor which when
contacted with the substrate forms a film having an A/B ratio of
from 0.612 to 1.53, wherein the A/B ratio is Pb to (Zr+Ti).
14. The precursor composition of claim 13, wherein at least one of
the solvent mediums of the respective lead, zirconium and titanium
precursors include solvent selected from the group consisting of
glymes, diglymes, hydrocarbon solvents, aryl solvents, aliphatic
hydrocarbons, aromatic hydrocarbons, isopropanol, organic ethers,
organic esters, alkyl nitriles, alkanols, alcohols, organic amines,
polyamines, glymes having from 1 to 20 ethoxy --(C.sub.2H.sub.4O)--
repeat units; C.sub.2-C.sub.12 alkanols, dialkyl ethers comprising
C.sub.1-C.sub.6 alkyl moieties, C.sub.4-C.sub.8 cyclic ethers;
C.sub.12-C.sub.60 crown O.sub.4-O.sub.20 ethers; C.sub.6-C.sub.12
aliphatic hydrocarbons; C.sub.6-C.sub.18 aromatic hydrocarbons,
tetrahydrofuran, alkyl acetate, tetraglyme, C.sub.3-C.sub.8
alkanols, butyl acetate, 12-crown-4, 15-crown-5, and 18-crown-6
ethers, acetone, dimethoxyethane (DME), dimethylformamide (DMF),
polyether alcohols, tetrathiocyclodecane, tetrahydrofuranacetate,
octane, decane, and mixtures of two or more of the foregoing.
15. The precursor composition of claim 13, wherein at least one of
the solvent mediums of the respective lead, zirconium and titanium
precursors includes solvent selected from the group consisting of
tetrahydrofuran, glyme solvents, alcohols, hydrocarbon solvents,
aryl solvents, amines, polyamines, and mixtures of two or more of
the foregoing.
16. The precursor composition of claim 13, wherein at least one of
the solvent mediums of the respective lead, zirconium and titanium
precursors includes a solvent selected from the group consisting of
glyme, diglyme, isopropanol, tetrahydrofuran, tetraglyme, butyl
acetate, acetone, dimethoxyethane, dimethylformamide,
tetrathiocyclodecane, tetrahydrofuranacetate, octane, decane, and
mixtures of two or more of the foregoing.
17. A precursor composition for forming a PZT film on a substrate
by liquid delivery MOCVD, said precursor composition comprising
lead, zirconium and titanium precursors each in a solvent medium so
that the precursors when vaporized form precursor vapor which when
contacted with the substrate forms a film that is at least one of
dimensional scalable, pulse length scalable and E-field
scalable.
18. The precursor composition of claim 17, wherein at least one of
the solvent mediums of the respective lead, zirconium and titanium
precursors includes a solvent selected from the group consisting of
glymes, diglymes, hydrocarbon solvents, aryl solvents, aliphatic
hydrocarbons, aromatic hydrocarbons, isopropanol, organic ethers,
organic esters, alkyl nitriles, alkanols, alcohols, organic amines,
polyamines, glymes having from 1 to 20 ethoxy --(C.sub.2H.sub.4O)--
repeat units; C.sub.2-C.sub.12 alkanols, dialkyl ethers comprising
C.sub.1-C.sub.6 alkyl moieties, C.sub.4-C.sub.8 cyclic ethers;
C.sub.12-C.sub.60 crown O.sub.4-O.sub.20 ethers; C.sub.6-C.sub.12
aliphatic hydrocarbons; C.sub.6-C.sub.18 aromatic hydrocarbons,
tetrahydrofuran, alkyl acetate, tetraglyme, C.sub.3-C.sub.8
alkanols, butyl acetate, 12-crown-4, 15-crown-5, and 18-crown-6
ethers, acetone, dimethoxyethane (DME), dimethylformamide (DMF),
polyether alcohols, tetrathiocyclodecane, tetrahydrofuranacetate,
octane, decane, and mixtures of two or more of the foregoing.
19. The precursor composition of claim 17, wherein at least one of
the solvent mediums of the respective lead, zirconium and titanium
precursors includes solvent selected from the group consisting of
tetrahydrofuran, glyme solvents, alcohols, hydrocarbon solvents,
aryl solvents, amines, polyamines, and mixtures of two or more of
the foregoing.
20. The precursor composition of claim 17, wherein at least one of
the solvent mediums of the respective lead, zirconium and titanium
precursors includes a solvent selected from the group consisting of
glyme, diglyme, isopropanol, tetrahydrofuran, tetraglyme, butyl
acetate, acetone, dimethoxyethane, dimethylformamide,
tetrathiocyclodecane, tetrahydrofuranacetate, octane, decane, and
mixtures of two or more of the foregoing.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a division of U.S. patent application
Ser. No. 12/978,393 filed Dec. 23, 2010, which is a continuation of
U.S. patent application Ser. No. 12/768,374 filed Apr. 27, 2010 and
issued Jan. 4, 2011 as U.S. Pat. No. 7,862,857, which is a
continuation of U.S. patent application Ser. No. 11/924,716 filed
Oct. 26, 2007, and issued Apr. 27, 2010 as U.S. Pat. No. 7,705,382,
which is a continuation of U.S. patent application Ser. No.
11/328,582 filed Jan. 10, 2006, and issued Mar. 18, 2008 as U.S.
Pat. No. 7,344,589, which is a continuation of U.S. patent
application Ser. No. 09/928,860 filed Aug. 13, 2001, and issued
Jan. 10, 2006 as U.S. Pat. No. 6,984,417, which in turn is a
division of U.S. patent application Ser. No. 09/251,890 filed Feb.
19, 1999, and issued Nov. 13, 2001 as U.S. Pat. No. 6,316,797. The
disclosures of all of the foregoing patents and patent applications
are hereby incorporated herein by reference in their respective
entireties, for all purposes, and the priority of all such
applications is hereby claimed under the provisions of 35 USC
120.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates generally to a novel lead zirconium
titanate (PZT) material having unique properties and application
for PZT thin film capacitors, as well as to a deposition method for
forming PZT films of such material, and ferroelectric capacitor
structures employing such thin film material.
[0004] 2. Description of the Related Art
[0005] There is a major effort by semiconductor companies
throughout the world to commercialize high dielectric constant and
ferroelectric thin films in advanced dynamic random access memories
(DRAMs) and ferroelectric random access memories (FeRAMs),
respectively.
[0006] While the majority of current efforts are directed to the
commercial development of relatively large capacitors (e.g. 5
.mu.m.sup.2 area), the ultimate goal is to adapt ferroelectric
random access memory technology for future generations of
integrated circuit devices in which capacitor areas, cell sizes and
operating voltages are scaled downward as the technology
evolves.
[0007] For FeRAM devices, most research is currently being directed
to either ferroelectric SrBi.sub.2Ta.sub.2O.sub.9 (SBT) or
Pb(Zr,Ti)O.sub.3 (PZT) thin films. Each material has relative
advantages and disadvantages. Pt/SBT/Pt capacitors, for example,
have been shown to have excellent fatigue and retention
characteristics, although processing temperatures in excess of
750.degree. C. pose integration issues. For PZT, phase-pure thin
films can be deposited at temperatures in the 450-650.degree. C.
range, although Pt/PZT/Pt capacitors are known to suffer from poor
fatigue and retention. In the prior art usage of previously known
PZT materials, doping and/or oxide electrodes have been needed to
produce satisfactory capacitor electrical properties.
[0008] Much of the previous work in the field that has established
the feasibility of PZT and SBT for memory applications has focused
on films that switch at 3V and above. Given the inexorable trends
towards smaller circuit elements and lower operating voltages, it
is extremely desirable to achieve high reliability and performance
for thin films at low operating voltages, especially below 2V.
[0009] Low operating voltage requires a combination of adequately
low coercive field (E.sub.c) and film thickness. SBT films have
been shown to have low E.sub.c (.apprxeq.35 kV/cm) at thicknesses
on the order of 140 nm, resulting in coercive voltages of 0.5V.
SBT, however, is handicapped by a low value of switched
ferroelectric polarization (P.sub.sw), typically less than 25
.mu.C/cm.sup.2. Furthermore, the thermal processing (800.degree.
C.) required for improvement of thin film properties is considered
severe and undesirable.
[0010] Several studies have presented thickness scaling data for
PZT films as thin as .about.150 nm. See, for example, P. K. Larsen,
G. J. M. Dormans, and P. J. Veldhoven, J. Appl. Phys., 76, (4),
1994; and A. K. Tagantsev, C. Pawlaczyk, K. Brooks, and N. Setter,
Integrated Ferroelectrics, 4, (1), 1994. These studies have shown
that as the film thickness is reduced, the coercive field increases
and the polarization decreases. Such effects have been attributed
to depletion and depolarizing phenomena (A. K. Tagantsev, C.
Pawlaczyk, K. Brooks, M. Landivar, E. Colla and N. Setter,
Integrated Ferroelectrics, 6, 309 (1995)).
[0011] The foregoing effects have been considered by the art to be
intrinsic to thin film PZT, and thus low voltage and thickness
scaling of PZT was discouraged.
[0012] The high ferroelectric polarization and low processing
temperatures of PZT (compared to other materials) provide a
compelling motivation to identify a form of PZT and a deposition
process that allows scaling the material to low operating
voltages.
[0013] It would therefore be a major advance in the art, and
accordingly is an object of the present invention, to provide a
form of PZT and a deposition process that allows scaling of the PZT
material to low operating voltages.
[0014] It is another object of the invention to provide a PZT
material that is scalable in lateral dimensions (i.e. dimensions
parallel to the film surface) without incorporating in the material
acceptor dopants or modifiers, e.g., Nb, Ta, La, Sr, Ca, etc.
[0015] It is a further object of the invention to provide a PZT
material of the foregoing type, which is useful for ferroelectric
capacitors over a broad range of thicknesses, particularly in the
range of from about 20 to about 150 nanometers.
[0016] A still further object of the invention is to provide a PZT
material that is useful for ferroelectric capacitors over a broad
range of pulse lengths, particularly in the range of from about 5
nanoseconds to about 200 microseconds.
[0017] Other objects and advantages of the invention will be more
fully apparent from the ensuing disclosure and appended claims.
SUMMARY OF THE INVENTION
[0018] The present invention relates generally to a novel lead
zirconium titanate (PZT) material, as well as to a deposition
method for forming PZT thin films of such material, and
ferroelectric capacitor structures employing such thin film
material.
[0019] As used hereinafter, the following terms shall have the
following definitions:
[0020] "Remanent polarization," P.sub.r, is the polarization at
zero volts after passing through V.sub.op.
[0021] "Ferroelectric switched polarization," P.sub.sw=P*-P ,
wherein P* is the polarization transferred out of the capacitor
traversing from zero to V.sub.op volts when the capacitor starts at
P.sub.r(-V.sub.op), and P is the polarization transferred out of
the capacitor traversing from zero to V.sub.op volts when the
capacitor starts at P.sub.r(V.sub.op). The pulse length is 0.23
milliseconds. The measuring instrument used to determine the values
hereinafter set forth was a Radiant 6000 unit.
[0022] "Coercive E-field," E.sub.c is the electric field at which
the polarization is zero during a polarization versus voltage
hysteresis loop. The E-field frequency is 50 Hertz for this
purpose.
[0023] "E.sub.max" is the maximum E-field for the hysteresis loop
measured with E.sub.max=3E.sub.c.
[0024] "Leakage current density," J, is measured at the operating
voltage, V.sub.op, and a step voltage response at 5 seconds.
[0025] "Retention" is the remanent polarization as measured by the
method described in Integrated Ferroelectrics, Vol. 16 [669], No.
3, page 63 (1997).
[0026] "Cycling fatigue P.sub.sw" is the ferroelectric polarization
measured with a frequency of 0.5 MegaHertz or slower square pulse,
at a 50% duty cycle, and a capacitor area of .ltoreq.10.sup.-4
cm.sup.2.
[0027] "Dimensionally-scalable PZT" material means a PZT material
that is un-doped and has useful ferroelectric properties for PZT
thin film capacitors over a range of thickness of from about 20 to
about 150 nanometers, and with lateral dimensions as low as 0.15
.mu.m and lower, and corresponding capacitor areas from about
10.sup.4 to about 10.sup.-2 .mu.m.sup.2.
[0028] "E-field scalable PZT" material means a PZT material that is
un-doped and has useful ferroelectric properties for PZT thin film
capacitors over the range of film thickness of 20 to 150
nanometers, at a voltage below 3 Volts.
[0029] "Pulse length scalable PZT" material means a PZT material
that is un-doped and has useful ferroelectric properties over a
range of excitation (voltage) pulse length from 5 nanoseconds to
200 microseconds.
[0030] "Ferroelectric operating voltage" means a voltage that when
applied to a PZT thin film in a capacitor causes the material to be
dielectrically switched from one to another of its orientational
polar states.
[0031] "Plateau effect determination" means establishing a
correlative empirical matrix of plots of each of ferroelectric
polarization, leakage current density and atomic percent lead in
PZT films, as a function of each of temperature, pressure and
liquid precursor solution A/B ratio, wherein A/B ratio is the ratio
of Pb to (Zr+Ti), and identifying from the plots the "knee" or
inflection point of each plot as defining a region of operation
with respect to the independent process variables of temperature,
pressure and liquid precursor solution A/B ratio, and conducting
the MOCVD process at a corresponding value of the temperature,
pressure and liquid precursor solution A/B ratio selected from such
region of operation, as hereinafter described.
[0032] "Type 1 properties" means, collectively, a ferroelectric
polarization P.sub.sw greater than 20 microCoulombs (.mu.C) per
square centimeter, a leakage current density (J) less than
10.sup.-5 amperes per square centimeter at operating voltage, a
dielectric relaxation defined by J.sup.-n log (time) wherein n is
greater than 0.5 and a cycling fatigue defined by P.sub.sw being
less than 10% lower than its original value after 10.sup.10
polarization switching cycles.
[0033] "Type 2 properties" means, collectively, the following
dimensionally scaled properties of ferroelectric polarization,
coercive E-field, leakage current density, retention and cycling
fatigue:
TABLE-US-00001 Basic property Thickness Scaling (t) Lateral
dimension scaling (l) Ferroelectric P.sub.sw > 40 .mu.C/cm.sup.2
for t > 90 nm P.sub.sw > 30 for l > 1 .mu.m polarization
(P.sub.sw) P.sub.sw > 30 .mu.C/cm.sup.2 for t > 50 nm
P.sub.sw > 20 for l > 0.05 .mu.m P.sub.sw > 20
.mu.C/cm.sup.2 for t > 20 nm Coercive E-field (E.sub.c) E.sub.c
< 100 kV/cm for t > 50 nm E.sub.c < 100 kV/cm for l >
0.05 .mu.m E.sub.c < 150 kV/cm for t > 20 nm Leakage current
J < 10.sup.-5 A/cm.sup.2 for t > 90 nm J < 10.sup.-4
A/cm.sup.2 for l > 0.05 .mu.m density (J) J < 10.sup.-4
A/cm.sup.2 for t > 50 nm Retention <3%/natural log decade (t
in <3%/natural log decade (t in hours) hours) at 150.degree. C.,
per procedure at 150.degree. C., per procedure for t > 50 nm for
l > 0.05 .mu.m Cycling fatigue P.sub.sw <10% decrease after
10.sup.10 cycles <10% decrease after 10.sup.10 cycles for t >
50 nm for l > 0.05 .mu.m <10% decrease after 10.sup.8 cycles
for t > 20 nm
[0034] "Un-doped" in reference to PZT film material means that the
dopants and modifiers (heterologous atomic species that are added
into the crystal structure of the PZT material and are responsible
for the observed or enhanced ferroelectric properties of the
material) are present in the material at concentrations of less
than 1 atomic percent.
[0035] In one aspect, the present invention relates to a
dimensionally scalable, pulse length scalable and/or E-field
scalable ferroelectric PZT material.
[0036] Another aspect of the invention relates to a ferroelectric
PZT material having Type 1 and/or Type 2 characteristics.
[0037] In another aspect, the invention relates to a ferroelectric
PZT material that has at least one, more preferably at least two,
still more preferably at least three, and most preferably all four,
of the Type 1 properties.
[0038] In a further aspect, the present invention relates to a
ferroelectric PZT material having at least one of the Type 2
properties and progressively more preferably having two, three,
four or five of such properties.
[0039] The invention relates in another aspect to a ferroelectric
PZT material capacitor comprising a ferroelectric PZT material of a
dimensionally-scalable, pulse length scalable and/or E-field
scalable character, and a capacitor area of from about 10.sup.4 to
about 10.sup.-2 .mu.m.sup.2.
[0040] Another aspect of the invention relates to a FeRAM device,
including a capacitor comprising a ferroelectric PZT material of a
dimensionally-scalable, pulse length scalable and/or E-field
scalable character, and a capacitor area of from about 10.sup.4 to
about 10.sup.-2 .mu.m.sup.2.
[0041] In another aspect, the invention relates to a method of
fabricating a ferroelectric PZT film on a substrate, comprising
forming the film by liquid delivery MOCVD on the substrate under
MOCVD conditions producing a ferroelectric film on the substrate
having Type 1 and/or Type 2 characteristics.
[0042] A further aspect of the invention relates to a method of
fabricating a ferroelectric PZT film on a substrate, comprising
forming the film by liquid delivery MOCVD on the substrate under
MOCVD conditions including nucleation conditions producing a
dimensionally scalable, pulse length scalable and/or E-field
scalable PZT film on the substrate.
[0043] Another aspect of the invention relates to a method of
fabricating a ferroelectric PZT film on a substrate, comprising
forming the film by liquid delivery MOCVD on the substrate under
MOCVD conditions including temperature, pressure and liquid
precursor solution A/B ratio determined by plateau effect
determination from a correlative empirical matrix of plots of each
of ferroelectric polarization, leakage current density and atomic
percent lead in PZT films, as a function of each of temperature,
pressure and liquid precursor solution A/B ratio, wherein A/B ratio
is the ratio of Pb to (Zr+Ti).
[0044] A further aspect of the invention relates to a method of
fabricating a ferroelectric PZT film on a substrate, comprising
forming the film by liquid delivery MOCVD on the substrate under
MOCVD conditions including Correlative Materials or Processing
Requirements, to yield a ferroelectric PZT film having PZT
Properties, wherein such Correlative Materials or Processing
Requirements and PZT Properties comprise:
TABLE-US-00002 Correlative Materials or Processing PZT Properties
Requirements Basic properties: Ferroelectric polarization Film Pb
concentration > threshold level; P.sub.sw > 20 .mu.C/cm.sup.2
operation on A/B plateau above the knee region, and with
temperature, pressure and gas phase A/B concentration ratio defined
by plateau effect determination Leakage current density Film Pb
concentration within a range (between J < 10.sup.-5 A/cm.sup.2
at operating voltage the minimum and maximum) on the A/B plateau,
and with temperature, pressure and gas phase A/B concentration
ratio defined by plateau effect determination Dielectric relaxation
Zr/Ti ratio <45/55 For characteristic J.sup.-n .varies. log
(time), n > 0.5 Deposition P > 1.8 torr and J < 1%
ferroelectric switching current from 0-100 ns. Retention Operation
within ranges of temperature, pressure Maintenance of ferroelectric
properties and gas phase A/B concentration ratio defined by
(ferroelectric domains) plateau effect determination Avoidance of
cycling fatigue Use of Ir-based electrodes P.sub.sw < 10%
decrease after 10.sup.10 cycles E-field scalability Operation
within ranges of temperature, pressure and gas phase A/B
concentration ratio defined by plateau effect determination Surface
smoothness Nucleation-growth conditions during film formation
within ranges of temperature, pressure and gas phase A/B
concentration ratio defined by plateau effect determination Grain
size Nucleation-growth conditions during film formation within
ranges of temperature, pressure and gas phase A/B concentration
ratio defined by plateau effect determination
[0045] A still further method aspect of the present invention
relates to a method of fabricating a FeRAM device, comprising
forming a capacitor on a substrate including a ferroelectric PZT
material of a dimensionally-scalable, pulse length-scalable and/or
E-field scalable character, wherein the PZT material is deposited
by liquid delivery MOCVD under processing conditions yielding a
ferroelectric film on the substrate having Type 1 and/or Type 2
characteristics as such ferroelectric PZT material.
[0046] Yet another aspect of the invention relates to a mixture of
liquid precursor solutions comprising precursors of lead, zirconium
and titanium dissolved or suspended in a solvent medium, wherein
the precursors of lead, zirconium and titanium do not undergo
ligand exchange.
[0047] A further aspect of the invention relates to a precursor
vapor composition comprising precursors of lead, zirconium and
titanium, wherein the precursors of lead, zirconium and titanium do
not undergo ligand exchange.
[0048] Other aspects, features and embodiments of the invention
will be more fully apparent from the ensuing disclosure and
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0049] FIG. 1 is a graph of film composition (A/B).sub.f as a
function of gas-phase composition (A/B).sub.g for a PZT thin film
material, showing the insensitivity of the stoichiometry to the
precursor concentration.
[0050] FIG. 2 is a model data matrix derived for empirically
determined values of the logarithm of the leakage current density
(Log J), the ferroelectric polarization (P.sub.sw) and atomic % Pb
in the film, as a function of pressure (P), temperature (T) and
solution A/B ratio.
[0051] FIG. 3 is a schematic cross-section of a semiconductor
device utilizing a stack capacitor configuration.
[0052] FIG. 4 is a plot of TGA data comparing Pb and Ti compounds
with Zr(thd).sub.4 and Zr(O-i-Pr).sub.2(thd).sub.2.
[0053] FIG. 5 is a plot of current density versus E-field for PZT
films varying thickness showing the current density is only weakly
dependent on film thickness above .about.125 nm.
[0054] FIG. 6 is a plot of coercive field (E.sub.c) versus film
thickness showing that E.sub.c is nearly independent of film
thickness.
[0055] FIG. 7 is a plot of a) P.sub.sw versus voltage; and b)
P.sub.sw versus electric field.
[0056] FIG. 8 is a plot of P*-P versus number of cycles for PZT
films showing that fatigue is nearly independent of film
thickness.
[0057] FIG. 9 is a plot of imprint behavior for PZT films of
varying thickness.
[0058] FIG. 10 shows the test configuration for the shunt method of
ferroelectric pulse testing.
[0059] FIG. 11 is a plot of test signal and response signal versus
time for a 10 .mu.m.times.10 .mu.m capacitor tested with a 1 .mu.s
pulse.
[0060] FIG. 12 is a plot of Q.sub.sw versus pulse length for
various applied voltages showing Q.sub.sw to be independent of
pulse length over the range investigated.
[0061] FIG. 13 is a plot of Q.sub.sw versus capacitor dimension
showing Q.sub.sw to be independent of area over the range
investigated.
DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS
THEREOF
[0062] The present invention provides a unique PZT material that is
scalable in character and provides major advantages over the prior
art in application to the formation and use of PZT in ferroelectric
thin film capacitor structures.
[0063] In contrast to the PZT materials previously used in the
prior art, the PZT material of the present invention is useful for
ferroelectric capacitors over a wide range of thicknesses, e.g.,
from about 20 nanometers to about 150 nanometers, and a range of
lateral dimensions extending to as low as 0.15 .mu.m. Corresponding
capacitor areas (i.e., lateral scaling) in a preferred embodiment
are in the range of from about 10.sup.4 to about 10.sup.-2
.mu.m.sup.2.
[0064] The foregoing properties provide a dimensional scaling
capability that is achievable without PZT film modification
techniques such as acceptor doping or the use of film modifiers
such as Nb, Ta, La, Sr, Ca and the like.
[0065] The novel PZT material of the present invention in one
embodiment has Type 1 properties, viz., a ferroelectric
polarization P.sub.sw greater than 20 .mu.C/cm.sup.2, a leakage
current density J less than 10.sup.-5 A/cm.sup.2 at V.sub.op, a
dielectric relaxation defined by J.sup.-n log(time) wherein n is
greater than 0.5, and a cycling fatigue defined by P.sub.sw being
less than 10% lower than its original value after 10.sup.10
polarization switching cycles.
[0066] Such PZT material is a substantial advance over the PZT
materials of the prior art lacking such Type 1 properties, and the
present invention provides a reproducible method of fabricating
such material by liquid delivery metalorganic chemical vapor
deposition.
[0067] The present invention relates in one aspect to a method to
deposit thin film ferroelectric materials by MOCVD utilizing a
liquid delivery technique. This technique affords precise
compositional control by virtue of mixing liquid precursor
solutions and flash vaporizing them. Flash vaporization has the
added benefit of preventing unwanted premature decomposition of the
precursor species. Further, tailored precursor chemistries may be
employed that are compatible for forming the thin film material
because the precursors do not undergo ligand exchange (or they have
degenerate exchange) which prevents the formation of involatile
species.
[0068] For thin film PZT and related materials, precise and
repeatable compositional control is required in order to produce
films of high quality. Physical deposition methods (e.g.,
sputtering, evaporation) of thin film deposition are deficient in
this regard, as are traditional approaches to MOCVD involving the
use of bubblers.
[0069] A process therefore is desired for the formation of thin
films of PZT and related materials, which affords compositional
control, provides uniformity of the thin film material over large
areas, and achieves a high degree of conformality on the substrate
structure as well as a high deposition rate. The deposited material
should also be free of pinholes, since in capacitive structures and
in many other devices, pinholes will result in an electrically
shorted, useless device.
[0070] In accordance with the invention, the metalorganic
precursors of the component metals of the desired
PbZr.sub.xTi.sub.1-xO.sub.3 film are introduced in liquid form,
either as neat liquids or dilute solutions if the precursor is a
liquid at ambient temperature and pressure (e.g., 25.degree. C. and
atmospheric pressure) conditions, or if the precursor composition
is a solid at such ambient conditions, then as a solution of the
precursor in a compatible solvent medium. The solvent medium may be
of any suitable type that is compatible with the specific precursor
composition employed, as is known and understood by those skilled
in the art of liquid delivery MOCVD, and may be constituted by
single-component solvent species, or alternatively by
multicomponent solvent mixtures.
[0071] The metalorganic precursors utilized for such liquid
delivery technique may for example comprise lead
bis(2,2,6,6-tetramethyl-3,5-heptanedionate), [Pb(thd).sub.2], as a
Pb precursor; titanium
bis(isopropoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionate),
[Ti(O-i-Pr).sub.2(thd).sub.2], as a Ti precursor; and zirconium
tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionate), [Zr(thd).sub.4],
as a Zr precursor. Alternatively, the lead precursor may comprise
lead bis(2,2,6,6-tetramethyl-3,5-heptanedionate)
N,N',N''-pentamethyl diethylenetriamine, [Pb(thd).sub.2pmdeta], and
the zirconium precursor may alternatively comprise zirconium
bis(isopropoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionate),
[Zr(O-i-Pr).sub.2(thd).sub.2].
[0072] The solvent media used in the liquid delivery MOCVD process
of the invention may suitably comprise by way of example the
solvent compositions that are disclosed in U.S. patent application
Ser. No. 08/414,504 filed Mar. 31, 1995 in the names of Robin A.
Gardiner, et al., and issued on Oct. 13, 1998 as U.S. Pat. No.
5,820,664, U.S. patent application Ser. No. 08/484,654 filed Jun.
7, 1995 in the names of Robin A. Gardiner, et al., and issued on
Aug. 29, 2000 as U.S. Pat. No. 6,110,529, and U.S. patent
application Ser. No. 08/975,372 filed Nov. 20, 1997 in the names of
Thomas H. Baum, et al., and issued on Jun. 29, 1999 as U.S. Pat.
No. 5,916,359, which are compatible with the specific metalorganic
precursors used for forming the PbZrTiO.sub.3 thin film materials
and efficacious in the constituent liquid delivery and chemical
vapor deposition process steps. The disclosures of the foregoing
patent applications and corresponding patents based thereon are
hereby incorporated herein by reference in their entireties.
[0073] In one preferred embodiment the solvent media is comprised
of a solution containing approximately 8 parts tetrahydrofuran
(THF), 2 parts isopropanol and one part tetraglyme (parts by
volume). In another embodiment, such as the case where
Pb(thd).sub.2 is used, one or both of the isopropanol and
tetraglyme would be excluded from the preferred solvent media.
Other illustrative multicomponent solvent compositions include a
solvent medium comprising octane:decane:polyamine in an 5:4:1
ratio, and a solvent medium comprising octane:polyamine in an 9:1
ratio. One particularly preferred single component solvent medium
is tetrahydrofuran (THF).
[0074] The liquid precursor composition once formulated is
introduced into a vaporization zone, in which the liquid is rapidly
vaporized, e.g., by flash vaporization on a foraminous vaporization
element (e.g., a porous frit element, or a wire, grid or other high
surface area structural element) heated to suitable temperature, to
produce a corresponding precursor vapor.
[0075] The precursor vapor then is transported to the chemical
vapor deposition chamber, which may for example comprise a CVD
reactor of known or conventional type. The CVD system may be
suitably equipped to introduce the precursor vapor into the
deposition chamber for contact with a heated substrate, at a
temperature that is suitable to effect deposition of the metal
constituents of the vapor onto the substrate element. For this
purpose, the substrate may be mounted on a heated susceptor or
other substrate mounting structure, with the spent vapor from the
process being discharged from the deposition chamber and subjected
to further treatment or processing in a known and conventional
manner.
[0076] Further, the film as deposited may be further processed in
any suitable manner, e.g., by annealing according to a specific
time/temperature relationship, and/or in a specific atmosphere or
environment, to produce the final desired thin film PbZrTiO.sub.3
material.
[0077] In one embodiment of the present invention, the precursors
for the metal components of the product film are dissolved in a
solvent and flash vaporized at temperatures between about 100 to
about 300.degree. C. and transported into the MOCVD reactor with a
carrier gas (e.g., Ar, N.sub.2, H.sub.2, He, or NH.sub.3). The
resulting carrier gas/precursor vapor mixture then may be mixed
with an oxidizing co-reactant gas (e.g., O.sub.2, N.sub.2O, O.sub.3
or mixtures thereof) and transported to the deposition chamber to
undergo decomposition at a substrate heated to a temperature of
from about 400.degree. C. to about 1200.degree. C. at chamber
pressures in the range of from about 0.1 to about 760 torr. Other
active oxidizing species may be used to reduce deposition
temperature, as through the use of a remote plasma source.
[0078] Investigation in the field of PZT materials research has
established the existence of regimes for CVD process parameters in
which the film Pb composition is insensitive to changes in
precursor concentrations (see, for example, M. De Keijser, P. Van
Veldhoven and G. Dormans, Mat. Res. Symp. Proc., Vol. 310 (1993) p.
223-234; and J. Roeder, B. A. Vaartstra, P. C. Van Buskirk, H. R.
Beratan, "Liquid delivery MOCVD of ferroelectric PZT," Mat. Res.
Symp. Proc., Vol. 415 (1996) p. 123-128).
[0079] This characteristic is highly desirable in application to
the design and optimization of manufacturing processes, and the
inventors have investigated film properties for films deposited
over a range of precursor concentrations within this
self-correcting regime. It has been discovered that although the
PZT composition remains substantially independent of precursor
concentration, the associated PZT film microstructure and
properties can vary significantly. Furthermore, the inventors have
discovered that proximity to the "edge" of the self-correcting
regime is of primary importance rather than proximity to the
stoichiometric composition. This is a significant finding relative
to the prior art conventional wisdom that a few percent
stoichiometric excess Pb is optimal.
[0080] The present inventors have demonstrated that MOCVD processes
for deposition of PZT have the desirable property of the
stoichiometry being largely insensitive to process conditions over
a fairly wide range of process conditions. However, it has also
surprisingly been found that the properties of deposited films with
the same stoichiometric composition are not functionally
equivalent. The present invention resolves this incongruity by an
approach that identifies process conditions with optimum electrical
properties.
[0081] In a primary aspect, the invention relates to a methodology
for selection of CVD process conditions that result in PZT films
with superior properties. The methodology exploits the "A/B plateau
effect" to achieve the fabrication of capacitive PZT films whose
electrical properties are congruent with optimum requirements of
ferroelectric non-volatile (NV) memories such as ferroelectric
random access memories (FeRAMs). The "A/B plateau effect" is
described below, and is based on the concept that smoothness and
grain size can be controlled by modifying specific nucleation and
growth phenomena. The complete ensemble of deposition conditions
and principles for selecting processing parameters, as described
hereafter, result in PZT film properties that were previously
considered by the art to be unachievable, especially low imprint
for un-doped PZT, and E-field scaling to low thicknesses, e.g., to
thicknesses as low as 20 nm.
[0082] The matrix of PZT properties and correlative materials or
processing requirement(s) for achieving such properties is set out
in Table A below.
TABLE-US-00003 TABLE A Correlative Materials or Processing PZT
property Requirement(s) Basic properties: Ferroelectric
polarization Film Pb concentration > threshold level; P.sub.sw
> 20 .mu.C/cm.sup.2 operation on A/B plateau above the knee
region; the A/B plateau determines 3 process parameters: P, T and
gas phase A/B concentration ratio Leakage current density Film Pb
concentration within a range (between J < 10.sup.-5 A/cm.sup.2
at operating voltage the minimum and maximum) on the A/B plateau;
A/B plateau determines 3 process parameters: P, T and gas phase A/B
concentration ratio Dielectric relaxation Zr/Ti ratio <45/55 For
characteristic J.sup.-n .varies. log (time), n > 0.5 Deposition
P > 1.8 torr and J < 1% ferroelectric switching current from
0-100 ns. Retention Operation within ranges of P, T and gas phase
Maintenance of ferroelectric properties A/B concentration ratio
determined above (ferroelectric domains) Avoidance of cycling
fatigue Use of Ir-based electrodes P.sub.sw < 10% decrease after
10.sup.10 cycles E-field scaling Operation within ranges of P, T
and gas phase Achieving qualitative capacitor performance for A/B
concentration ratio determined above films with reduced thickness,
and at reduced voltages Surface smoothness Nucleation-growth
conditions during film formation within ranges of P, T and gas
phase A/B concentration ratio determined above Grain size
Nucleation-growth conditions during film formation within ranges of
P, T and gas phase A/B concentration ratio determined above
[0083] An illustrative specific process embodiment that may be used
in connection with the Table A process parameters is set out below,
as "Process Set A." As shown, Process Set A utilizes a specific
precursor chemistry including precursor reagents and solvent
compositions, substrate and barrier layer materials (the barrier
layer being deposited or otherwise provided between the substrate
and the PZT material layer), to provide an electrical environment
suitable for the achievement of optimum electrical and performance
properties of the PZT material, electrode materials of
construction, carrier gas species and oxidant species.
TABLE-US-00004 Process Set A: Process Condition/Material Precursor
Chemistry Reagents: [Pb(thd).sub.2, Ti(O--i-Pr).sub.2(thd).sub.2;
Zr(thd).sub.4] Solvents: tetrahydrofuran, glyme solvents, alcohols,
hydrocarbon and aryl solvents, amines, polyamines, and mixtures of
two or more of the foregoing; examples of illustrative
multicomponent solvent compositions are
tetrahydrofuran:isopropanol:tetraglyme in a 8:2:1 volume ratio
and/or octane:decane:polyamine in a volume ratio of 5:4:1.
Electrodes and Barriers Bottom Electrode: Ir bottom electrode on
TiAIN barrier layer on substrate deposition via sputtering using a
collimator and deposition-etch processing Top Electrode: Structures
containing Ir and IrO.sub.x for top electrode Carrier Gas, Oxidant:
Ar, He, H.sub.2 or other inert or non-adverse carrier gas; O.sub.2,
O.sub.3, N.sub.2O, O.sub.2/N.sub.2O, etc. as oxidant medium
Deposition Conditions: Temperature, Pressure, Precursor Ratio:
operate to exploit the plateau effect, in combination with
appropriate gas delivery, oxidizer constituents, ratios, flow
rates, liquid delivery, liquid flow rate, mixing and deposition
time Vaporizer: operate the vaporizer of the liquid delivery system
to achieve the foregoing process conditions, as appropriate to the
specific vaporizer apparatus employed; examples of vaporizer
operating parameters that may be involved include: back pressure,
delivery tube ID, frit porosity, vaporizer temperature, gas
co-injection, delivery tube/frit composition, and tube/frit
installation procedure
[0084] It will be appreciated that the Process Set A elements are
by way of exemplification only, and that the specific precursor
chemistries, carrier gas species, device structure layers, etc. may
be widely varied in the broad practice of the present invention to
achieve a scalable PZT film material within the scope of the
present invention.
[0085] The thickness of the ferroelectric film material in the
practice of the present invention may be widely varied. A preferred
thickness for FeRAM applications is typically in the range of from
about 20 to about 150 nanometers. Operating voltages for such PZT
material films in FeRAM applications are typically below 3.3 Volts,
down to much lower voltage levels.
[0086] The general formula for perovskite oxides is ABO.sub.3,
where specific metal elements occupy the A and B sites in the
crystal lattice, and O is oxygen. For PZT, Pb is on the A site and
Zr and Ti share the B site. Because the vapor pressure of PbO is
lower when its incorporated in the perovskite structure, fairly
wide ranges of CVD process parameters result in PZT films with the
same or very slightly varying A/B site ratio.apprxeq.1.00. The
existence of this A/B "self-correcting effect" is utilized to
advantage in the present invention to achieve the formation of PZT
material with the properties of: a ferroelectric polarization
P.sub.sw greater than 20 .mu.C/cm.sup.2, a leakage current density
J less than 10.sup.-5 A/cm.sup.2 at V.sub.op, a dielectric
relaxation defined by J.sup.-n log(time) wherein n is greater than
0.5, and a cycling fatigue defined by P.sub.sw being less than 10%
lower than its original value after 10.sup.10 polarization
switching cycles (Type 1 properties).
[0087] FIG. 2 is a model data matrix derived for empirically
determined values of the logarithm of the leakage current density
(Log J), the ferroelectric polarization (P.sub.sw) and atomic % Pb
in the film, as a function of pressure (P), temperature (T) and
solution A/B ratio.
[0088] Model data from the matrix show the basic relationships
between the independent (process variables) P, T and
(A/B).sub.solution, on the dependent variables: atomic % Pb in the
film, ferroelectric polarization (P.sub.sw) and leakage current
density (log J). Among these variables, (A/B).sub.solution is
equivalent to (A/B).sub.gas since the precursor liquid reagent
solution is vaporized to achieve a same gas-phase composition as is
present in the liquid solution of the precursor.
[0089] Visual inspection of the various curves generated for the
dependent variables (including average values for the central and
edge regions of the ferroelectric films in the model data matrix of
FIG. 2) shows a "knee" or inflection point beyond which the curve
flattens in the direction of increasing value of the given
independent process variables P, T and (A/B).sub.solution. By
operating at or in the vicinity of the knee point, the superior PZT
material of the invention is produced. The "vicinity" of the knee
point will vary with the independent process variable; in the case
of the solution A/B ratio and pressure, the vicinity is preferably
within .+-.25% of the knee point, and for the temperature, the
vicinity is preferably within .+-.5% of the knee point.
[0090] For the specific data shown in FIG. 2, this "knee" point is
1.02 for the solution A/B ratio, 1750 millitorr for the deposition
pressure, and 575.degree. C. for the deposition temperature. By
selection of these independent variable values, the corresponding
dependent values producing the superior PZT material of the
invention produced with such A/B solution ratio, pressure and
temperature may be readily determined, including a Log J.sub.avg
center value of -4.35 amperes per square centimeter at operating
voltage, Log J.sub.avg edge value of -6.77 amperes per square
centimeter at operating voltage, a P.sub.sw edge value of 35.1
.mu.C per square centimeter, a P.sub.sw center value of 33.7 .mu.C
per square centimeter, and an atomic % Pb of 52.3%.
[0091] The present invention thus encompasses a "plateau effect
determination" comprising the steps of establishing a correlative
empirical matrix of plots of each of ferroelectric polarization,
leakage current density and atomic percent lead in PZT films, as a
function of each of temperature, pressure and liquid precursor
solution A/B ratio, wherein A/B ratio is the ratio of Pb to
(Zr+Ti), and identifying from the plots the "knee" or inflection
point of each plot as defining a region of operation with respect
to the independent process variables of temperature, pressure and
liquid precursor solution A/B ratio, and conducting the MOCVD
process at a corresponding value of the temperature, pressure and
liquid precursor solution A/B ratio selected from such region of
operation, as hereinafter described.
[0092] Set out in Table B below is a tabulation listing of the most
preferred material properties (Type 2 properties) of the
thickness-scalable and dimensionally-scalable PZT material of the
invention, wherein t is the film thickness of the PZT material, and
1 is the effective lateral dimension, defined as side of a square
with an area that is the same as the capacitor area.
TABLE-US-00005 TABLE B Basic property Thickness Scaling (t) Lateral
dimension scaling (l) Ferroelectric P.sub.sw > 40 .mu.C/cm.sup.2
for t > 90 nm P.sub.sw > 30 for l > 1 .mu.m polarization
(P.sub.sw) P.sub.sw > 30 .mu.C/cm.sup.2 for t > 50 nm
P.sub.sw > 20 for l > 0.05 .mu.m P.sub.sw > 20
.mu.C/cm.sup.2 for t > 20 nm Coercive E-field (E.sub.c) E.sub.c
< 100 kV/cm for t > 50 nm E.sub.c < 100 kV/cm for l >
0.05 .mu.m E.sub.c < 150 kV/cm for t > 20 nm Leakage current
J < 10.sup.-5 A/cm.sup.2 for t > 90 nm J < 10.sup.-4
A/cm.sup.2 for l > 0.05 .mu.m density (J) J < 10.sup.-4
A/cm.sup.2 for t > 50 nm Retention <3%/natural log decade
(time in <3%/natural log decade (time in hours) at 150.degree.
C., for t > 50 nm hours) at 150.degree. C., for l > 0.05
.mu.m Cycling fatigue P.sub.sw <10% decrease after 10.sup.10
cycles <10% decrease after 10.sup.10 cycles for t > 50 nm for
l > 0.05 .mu.m <10% decrease after 10.sup.8 cycles for t >
20 nm
[0093] The invention contemplates the use of the "plateau effect"
to achieve ferroelectric properties with the scaling properties
specified in Table B, as well as the use of nucleation/smoothness
methods to achieve ferroelectric properties with the scaling
properties that are specified in Table B.
[0094] The ferroelectric PZT material of the invention thus is a
dimensionally scalable material, and suitably comprises at least
one of the Table A and/or Table B properties. The ferroelectric PZT
material of the invention is also E-field scalable and pulse length
scalable in character.
[0095] The PZT material of the present invention may be used to
form capacitive structures such as FeRAM devices, as well as other
microelectronic devices and precursor structures in which PZT may
be used to advantage. The invention thus contemplates the provision
of a microelectronic device structure including the PZT material of
the invention, e.g., a pulse length scalable PZT material of the
invention in combination with a power supply and associated power
circuitry including such PZT material, as a microelectronic
structure arranged for excitation of the PZT material, wherein the
excitation is characterized by an excitation (voltage) pulse length
in the range of from 5 nanoseconds to 200 nanoseconds.
[0096] By way of example, the PZT material may be used to fabricate
a ferroelectric capacitor device structure, by forming a
ferroelectric stack capacitor comprising a PZT ferroelectric
material of the present invention as a capacitor element on a
substrate containing buried transistor circuitry beneath an
insulator layer having a via therein containing a conductive plug
to the transistor circuitry. Such fabrication process may comprise
the steps of patterning, deposition, etch, diffusion, ion
implantation, ion bombardment, chemical modification, etc.
[0097] A more complete understanding of the present invention is
enabled by the following detailed description, including specific
reference to an illustrative device structure comprising a PZT
material according to the invention.
[0098] Referring now to FIG. 3, there is shown the cross-section of
an integrated circuit semiconductor device 200, which is in the
process of fabrication. Device 200 includes a semiconductor
substrate 202 that may include active device structures, not shown,
and an insulator layer 204. The semiconductor substrate 202 may be
silicon, doped silicon, or another semiconductor material. The
insulator layer 204 is deposited on the substrate 202 by any
suitable deposition process. The insulator layer 204 may be, for
example, silicon dioxide, silicon nitride, or some combination
thereof.
[0099] A conductive diffusion barrier layer 210, such as titanium
aluminum nitride TiAlN is deposited over the insulator layer 204. A
layer of conductive material 212, such as iridium, iridium oxide,
platinum or combinations thereof, is deposited over the conductive
diffusion barrier layer 210. Next, a layer of high dielectric
constant material 214, such as PZT, is deposited by MOCVD over the
conductive layer 212. A second layer of conductive material 216,
such as iridium, iridium oxide, platinum, or combinations thereof,
is shown deposited over the layer of high dielectric constant
material 214.
[0100] A diffusion barrier material such as titanium aluminum
nitride (TiAlN) will substantially reduce the possibility of
diffusion of oxygen during subsequent processing steps that require
high temperatures in excess of 500.degree. C. Other materials can
be used for the diffusion barrier, such as those disclosed in U.S.
patent application Ser. No. 08/994,089 filed Dec. 19, 1997 in the
names of Peter S. Kirlin, et al., and issued on Nov. 20, 2001, as
U.S. Pat. No. 6,320,213, the disclosure of which hereby is
incorporated herein by reference in its entirety.
[0101] FIG. 3 shows the portion of the device 200 after the device
has been patterned with photoresist and etched. Desired portions of
the conductive diffusion barrier layer, upper and lower layers of
iridium or other conductive material and of the high dielectric
constant material are left to form the upper electrode 216,
capacitor dielectric 214, lower electrode 212, and lower electrode
barrier layer 210.
[0102] A layer of interlevel dielectric 218 such as silicon dioxide
or silicon nitride, is deposited over all. The layer of interlevel
dielectric is patterned with photoresist and etch to form contact
plug holes 221, 222, and 223. The insulator is etched down at the
contract plug hole locations 221 and 222 until the iridium or other
conductor of the lower electrode 212 and the upper electrode 216,
respectively, are reached. Similarly the contact plug hole 223 is
etched down through the insulator layers 218 and 204 until the
semiconductor substrate is reached. Once the contact plug openings
are prepared, the device 200 is ready for deposition of a layer of
oxidation-barrier material.
[0103] FIG. 3 shows the semiconductor device 200 following an
overall etch of the diffusion barrier layer leaving a diffusion
barrier layer 232 in contact with the lower capacitor electrode
212, a diffusion barrier layer 234 in contact with the upper
capacitor electrode 216, and a diffusion barrier layer 236 in
contact with the semiconductor substrate 202. A transfer transistor
of the memory cell may be located below the diffusion barrier layer
236, but it is not shown. As an alternative to the aforementioned
diffusion barrier deposition scheme, the barrier layers 232, 234,
and 236 could be deposited as a single continuous layer prior to
the capacitor stack etch and deposition of insulating layer 218.
According to this alternative configuration, the barrier layer
could be patterned and used as a hardmask for the subsequent
patterning of the capacitor stack. The alternate process flow would
continue with the deposition and patterning of the insulating layer
218.
[0104] A conductive material, or metallization, is deposited over
the interlevel dielectric 218 and the diffusion barrier layers 232,
234, and 236. The conductive material 238 makes contact with the
diffusion barrier layers 232, 234, and 236. The conductive material
238 may be selected from a group of conductive materials such as
aluminum, aluminum alloys, tungsten, tungsten alloys, iridium, and
iridium alloys. The diffusion barrier layers 232, 234, and 236
significantly reduce the possibility of any diffusion of the layer
of conductive material 238 to the capacitor electrodes 212 and 216
of the semiconductor substrate 202.
[0105] FIG. 3 shown the semiconductor device 200 after the layer of
conductive material 238 is patterned and etched to form desired
lead lines in the layer of conductive material. The pattern is
formed of photoresist material. Etching is accomplished in
accordance with well-established practices known to those of
ordinary skill in the semiconductor manufacturing arts.
[0106] A layer of passivation dielectric 240 is deposited over the
conductive material layer 238 and the interlevel dielectric 218.
The passivation dielectric may be a material such as silicon
dioxide, silicon nitride, or other insulator that can provide
mechanical and electrical protection for the top surface of the
semiconductor device. Material of the passivation dielectric layer
240 is deposited by well-known techniques.
[0107] The invention contemplates as an aspect thereof a
microelectronic device structure comprising a PZT material of the
present invention. While the invention has been describe herein
with reference to specific features, aspects and embodiments, it
will be appreciated that the utility of the invention is not thus
limited, and that the invention contemplates variations,
modifications and embodiments other than those shown and described
herein. The aforementioned capacitor geometry may comprise a
recessed capacitor geometry, for example, or other structures and
conformations that will be readily apparent to those with ordinary
skill in the art. Accordingly, the invention is to be broadly
interpreted and construed to encompass all such variations and
modifications
[0108] The features and advantages of the invention are more fully
shown with respect to the following illustrative examples.
Example 1
[0109] The lead precursor chosen was lead
bis(2,2,6,6-tetramethyl-3,5-heptanedionate) [Pb(thd).sub.2]. This
compound has no appreciable vapor pressure at room temperature,
which makes it much safer to handle than tetra-alkyl lead reagents.
However, the low volatility of Pb(thd).sub.2 (0.05 Torr at
180.degree. C.) requires the use of liquid precursor delivery.
Titanium
bis(isopropoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionate)
[Ti(O-i-Pr).sub.2(thd).sub.2] was used as the titanium precursor.
Zirconium tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionate)
[Zr(thd).sub.4] was used as the Zr source reagent. These compounds
are extremely soluble in organic media and no possible detrimental
ligand exchange occurs since the titanium atom is coordinatively
saturated.
[0110] The following process conditions were applied:
TABLE-US-00006 Operating Parameter Process Condition Substrate
temperature 550~610.degree. C. Bottom electrode Ir/SiO.sub.2/Si
Total reactor pressure 1~10 Torr Reactor wall temperature
~210.degree. C. Carrier Ar flow ~200 sccm O.sub.2 flow 500 sccm
N.sub.2O flow 500 sccm Total reagent solution concentration 0.29M
Reagent solution flow rate 0.1~0.2 ml/min
[0111] In a representative run, the film was deposited at
565.degree. C. on Ir/SiO.sub.2/Si. The pressure was 1.2 Torr, the
oxidizer flow was a mixture of 500 sccm O.sub.2 and 500 sccm
N.sub.2O, and the reagent flow rate was 0.14 ml/min for 32.5
minutes. XRF analysis gave the following thickness and composition
data for the resulting PbZrTiO.sub.3 film:
TABLE-US-00007 Thickness (.mu.m) Pb (at. %) Zr (at. %) Ti (at. %)
0.13 52.0 23.0 25.0
Example 2
[0112] Solutions with a range of different Pb/(Zr+Ti) ratios were
used over a series of deposition runs. This ratio is hereafter
defined as (A/B).sub.g, denoting the conventional assignment of Pb
to the "A" site, and Zr and Ti each to the "B" site in the
perovskite cell, ABO.sub.3. The subscript g denotes the gas-phase
concentration in the reaction chamber, while (A/B).sub.f denotes
the equivalent ratio in the film.
[0113] The gas phase ratio of Zr/(Zr+Ti) was held constant at
0.612. Under the conditions given above in Example 1 and for the
specific CVD reactor employed, the gas phase ratio of
Zr/(Zr+Ti)=0.612 resulted in films with Zr/Ti.about.40/60, which
for bulk materials yielded a tetragonal crystal structure and
ferroelectric properties. This is a common composition chosen for
FeRAM applications because of its high P.sub.r and the relative
ease in forming the perovskite phase for lower Zr/Ti ratios.
[0114] Next, a series of PZT films was deposited with fixed
deposition time, under the process conditions set out in Table 1
below; the effect on (A/B).sub.f of (A/B).sub.g determined in this
empirical work is shown in FIG. 1. Nominal film thickness was 100
nm. For low (A/B).sub.g, (A/B).sub.f increased monotonically with
mole fraction of lead in the gas phase. Over the range
0.93<(A/B).sub.g<1.53 a plateau was observed in (A/B).sub.f,
in the range of 1.10 to 1.15. For these films the perovskite phase
was the only crystalline phase present.
[0115] The appearance of this processing window, where film
composition is insensitive to changes in the composition of Pb in
the gas-phase, is rationalized in term of two competing processes:
the formation of perovskite PZT via decomposition of Pb, Zr and Ti
precursors, and the desorption of excess PbO from the growth
surface. While we do not wish to be bound by any theory or
mechanism in explanation for this processing window, it is
hypothesized that the vapor pressure of PbO over PZT is
significantly lower than it is for solid PbO, as is the case with
bulk PZT (see the compiled bulk PZT material properties identified
in K. H. Hartl and H. Rau, Solid State Comm., 7, 41 (1969)).
TABLE-US-00008 TABLE 1 CVD deposition conditions Precursors
Pb(thd).sub.2, Zr(thd).sub.4, Ti(O--i-Pr).sub.2 (thd).sub.2
Solution molarity 0.29M Liquid flow rate 0.14 ml/min Substrate
temperature 550.degree. C. Pressure 1.2 torr Deposition rate 3.5
nm/min Substrate Ir/MgO/SiO.sub.2/Si
[0116] Under process conditions in which the kinetics of PZT
formation are fast, and PbO volatility is high, single-phase,
stoichiometric PZT can be formed.
[0117] The presence of the plateau at (A/B).sub.f values exceeding
1.00 may be influenced by inaccuracy in the XRF measurement or by
excess Pb diffused into the bottom electrode. Analysis of
incorporation efficiencies for the metallic constituents revealed a
decrease in Pb efficiency for (A/B).sub.g>0.83, while Zr and Ti
efficiencies remained nearly constant over the same range. This is
consistent with the appearance of the plateau, and the absence of
PbO from XRD analysis of the films. Film thickness decreased
slightly with increased (A/B).sub.g. This corresponded to an
approximate growth rate decrease from 3.8 to 3.2 nm/min.
[0118] The as-deposited films were all smooth, dense, and
fine-grained. The roughness and grain size values calculated from
these images are given in Table 2 below. The measured film
roughness was insensitive to gas phase composition and was
approximately double the starting surface roughness of the Ir films
used as substrates.
[0119] With increasing (A/B).sub.g, the grain size increased as did
the extent of faceting, suggesting an enhanced surface mobility.
This is believed to be a consequence of the higher PbO surface
coverage that must be present during growth at higher gas-phase Pb
concentrations if the previously described growth model is valid.
The faceting in the high-Pb sample revealed predominately square
features indicative of the presence of PZT (001)-type orientations,
a conclusion also supported by the x-ray diffraction results.
TABLE-US-00009 TABLE 2 Summary of AFM data for PZT/Ir/MgO layers.
Film thickness is nominally 100 nm. (A/B).sub.g (A/B).sub.f RMS
roughness (nm) Grain size (nm) 0.631 0.59 6.6 67 0.731 0.97 8.4 72
0.831 1.01 7.8 86 1.031 1.08 7.7 91
[0120] X-ray diffraction analysis revealed single perovskite phase
for films deposited with (A/B).sub.g.gtoreq.0.83. For
(A/B).sub.g<0.83, an additional peak was observed at
2.THETA.=29.9.degree.. The intensity of this peak decreased with
increasing (A/B).sub.g and was attributed to formation of the
undesirable pyrochlore phase under lead-deficient deposition
conditions.
[0121] PZT films on Ir/MgO displayed dominant (001) and (101) PZT
orientations; furthermore, the (001)/(101) ratio of PZT peak
intensities increased with increased (A/B).sub.g, i.e., oriented
toward the tetragonal c-axis with a c-axis lattice constant of
0.406 nm. No appreciable (111) PZT texture was observed on Ir/MgO.
X-ray diffraction of the as-received substrates revealed
principally (111) oriented Ir; however, a considerable (200) Ir
peak was present.
[0122] The best electrical properties were found for films with
(A/B).sub.g just above the knee in the curve shown in FIG. 1. Films
with much higher or much lower (A/B).sub.g were electrically
shorted. For 3V operation, the remanent polarization (2P.sub.r) and
coercive voltage (V.sub.c) were measured to be 85 .mu.C/cm.sup.2
and 0.77 V, respectively, for a 150 nm thick film deposited at
(A/B).sub.g=0.93. This high value of remanent polarization is
attributable to the strong preferred (001) orientation and the high
degree of crystallization obtained on the Ir substrate.
Example 3
[0123] A central-composite-design experiment was used to probe a
large volume of process space and assess interactions between
principal process variables. Deposition temperature (550, 575,
600.degree. C.) and pressures (500, 1750, 3000 mTorr) were
independently varied at five different values of (A/B).sub.g:
(0.53, 0.73, 0.93, 1.13, 1.53). A constant deposition time of 1660
seconds was used.
[0124] Compositionally, a plateau onset in (A/B).sub.f was observed
at (A/B).sub.g=0.93. For a given pressure, film lead content was
observed to decrease with increased deposition temperature.
Furthermore, incorporation efficiencies of both lead and titanium
decreased with increasing (A/B).sub.g for all temperatures.
[0125] Electrically, the best samples arose from the conditions at
the center of the design (575.degree. C., 1.75 Torr,
(A/B).sub.g.about.1). Most of the good samples displayed a (-)
voltage offset. There was no appreciable center-to-edge (wafer)
effect. Leakage seemed to behave better for lower Pb and for
thicker samples (.about.130 nm up to .about.2.5 V).
Example 4
[0126] Alternative zirconium oxide precursors are required, which
are more volatile than Zr(thd).sub.4 and which also have lower
thermal stability to be more compatible with the surface
decomposition of Pb(thd).sub.2, for example.
[0127] FIG. 4 shows comparative TGA data for Pb(thd).sub.2,
Ti(O-i-Pr).sub.2'(thd).sub.2 and two selected Zr compounds:
Zr(thd).sub.4 and Zr(O-i-Pr).sub.2(thd).sub.2. Although the
Zr(O-i-Pr).sub.2(thd).sub.2 compound possesses an undesirable
residuals content of nearly 20% at 400.degree. C., it possesses a
desirable thermal stability match with the Pb and Ti compounds
commonly used for MOCVD PZT.
[0128] MOCVD PZT depositions were conducted on standard Ir/TiAlN
bottom electrodes (BEs) using the novel Zr(O-i-Pr).sub.2(thd).sub.2
(Zr-2-2) compound with a vaporizer temperature set to 200.degree.
C. Additional depositions were processed using the standard
Zr(thd).sub.4 (Zr-0-4) compound at a vaporizer temperature of
203.degree. C. All other deposition conditions were held constant.
Following PZT deposition, Pt top electrodes (TEs) were e-beam
evaporated, and the samples were annealed at 650.degree. C. in
flowing argon for 30 min.
[0129] Composition and electrical data for the Zr-2-2 samples are
presented along with data collected from the Zr-0-2 samples. The
Zr-2-2 samples, though appreciably lower in Pb and Zr content than
the Zr-0-4 samples, were electrically comparable.
[0130] In addition to the Zr(O-i-Pr).sub.2(thd).sub.2 precursor,
MOCVD PZT may be prepared using still other novel Zr source
precursors. For example, Zr.sub.2(O-i-Pr).sub.6(thd).sub.2 has good
ambient stability, high volatility and excellent thermal
compatibility with Pb and Ti precursors.
TABLE-US-00010 TABLE 3 Composition and electrical data for MOCVD
PZT films deposited using Zr(O--i-Pr).sub.2(thd).sub.2 and
Zr(thd).sub.4 Zr(O--i-Pr).sub.2(thd).sub.2 Zr(thd).sub.4 % Pb (at.
%) 49.5 .+-. 0.07 51.8 .+-. 0.12 Zr: (Zr .+-. Ti) 0.322 .+-. 0.001
0.443 .+-. 0.008 efficiency Pb 5.99 .+-. 0.049 7.43 .+-. 0.15
efficiency Zr 6.54 .+-. 0.078 4.36 .+-. 0.16 efficiency Ti 9.31
.+-. 0.035 10.7 .+-. 0.13 t (nm) 125 128 P.sub.sw (.mu.C/cm.sup.2)
(2 V) 36 38 .+-.J (A/cm.sup.2) (2 V) 7.4 .times. 10.sup.-6; -1.5
.times. 10.sup.-6 <3 .times. 10.sup.-6 for both polarities
Example 5
[0131] A number of samples are made up using PZT material formed in
accordance with the present invention. The samples are comprised of
bottom electrodes formed by sputtering techniques, deposition on
the electrodes of PZT material in accordance with the present
invention, followed by deposition of top electrodes through a
shadow mask by e-beam deposition. The PZT deposition time was
varied between 165 sec and 4065 sec to target film a thickness
between 10 nm and 260 nm.
[0132] Electrical testing of the samples provides electrical
characteristics of the capacitor structures that validate the
thickness, pulse length and area scaling properties, including
ferroelectric polarization, cycling fatigue, etc., of the
ferroelectric PZT material of the present invention.
[0133] Leakage current density versus E-field is given in FIG. 5.
The high "leakage" for the 77 nm film was directly confirmed. The
qualitatively different electrical behavior for films below a
thickness threshold may be due to the high relative roughness
(roughness/thickness), which results in some extremely thin regions
in thinner films. Locally high E-fields are expected in those
cases. Current density for the thicker films in the set (>77 nm)
showed remarkably consistent E-field dependence. Leakage in both
polarities was in the 10.sup.-6 to 10.sup.-7 A/cm.sup.2 range for
150 kV/cm. (150 kV/cm corresponds to 1.9 V for the 125 nm film, for
example.)
[0134] Leakage was one of four properties that was insensitive to
thickness when plotted as a function of E-field; the other
properties were coercive voltage (V.sub.c), polarization saturation
(P.sub.sw versus V.sub.op), and fatigue endurance.
[0135] The coercive voltage for each sample was determined from the
relation 3V.sub.c (measured)=V.sub.op and the calculated values of
E.sub.c based on that method are given in FIG. 6. For PZT films
greater than 77 nm thick, E.sub.c was approximately 50 kV/cm.
[0136] Pulse measurements were used to investigate polarization
saturation. FIG. 7 shows the dependence of switched polarization on
voltage and E-field. The data shows the expected increase of
polarization for higher voltages. Clear saturation behavior is seen
for the thicker films, which can withstand higher fields. For 125
nm and thicker films, saturated Psw>40 .mu.C/cm.sup.2.
Normalized to P.sub.sw near saturation (300 kV/cm), P.sub.sw versus
E is nearly independent of film thickness. For all of the samples
P.sub.sw reaches nearly 90% of it maximum value at 3E.sub.c
(.about.150 kV/cm).
[0137] Fatigue properties for this sample set also displayed
consistent properties in terms of E-field scaling (FIG. 8). Fatigue
measurements were made at 150 kV/cm, which corresponds to 3E.sub.c.
The fatigue waveform was a square wave with a period of 10.sup.-5
seconds. Fatigue is nearly independent of PZT thickness with a
reduction in P.sub.sw by .about.50% at 10.sup.9 cycles.
[0138] Static imprint is manifested as an asymmetry in coercive
voltage, and is defined as [V.sub.c(-)+V.sub.c(+)]/2. Imprint
voltage was probed via repeated measuring, poling, and annealing of
a single capacitor. Following an initial measurement of V, (+ and
-) at time=0; capacitors were poled positive or negative to 2.5 V
and then annealed at 150.degree. C. (air) for 20 min. After
cooling, the same (poled) capacitors were re-measured. Additional
measurements were repeated following anneals of 24 min. (i.e. 45
min. elapsed time) and 45 min. (90 min. elapsed time).
[0139] FIG. 9 show a semi-log plot of imprint voltage versus time.
The samples had nearly identical imprint at time=0 (.about.-0.1 V).
It is evident the imprint increases significantly at some time
prior to the first measurement at 20 minutes. Thinner films were
observed to have a lower imprint rate than thicker films. This
trend suggests analyzing the data in terms of an imprint
E-field.
Example 6
[0140] In order to simplify test equipment and sample preparation
during the development of ferroelectric materials, electrical
testing has traditionally been conducted with pulse widths on the
order of 1 mS and capacitor areas on the order of 10.sup.4
.mu.m.sup.2. A disadvantage of this testing regimen is its
inability to demonstrate the viability of electrical properties at
scales appropriate for devices (i.e. 25 nS pulses and 10.sup.2
.mu.m.sup.2). The market trend is toward devices that operate on
increasingly shorter time scales and with smaller capacitor areas,
hence scaling issues will be critical to future device success.
[0141] The sample used for these measurements originated with a
6-inch diameter Si wafer with an Ir bottom electrode, standard PZT,
and a top electrode that consisted of 40 nm of IrO.sub.2 and 60 nm
of Ir, as described in Process Set A. Individual capacitors were
defined using patterned photoresist and reactive ion etching in a
Cl.sub.2/Ar/O.sub.2 mixture. Following patterning the sample was
annealed at 650.degree. C. in flowing oxygen for 30 min.
[0142] The test system consisted of an SRS DS345 arbitrary waveform
generator, a Tektronix 620B digital storage oscilloscope, and a
shunt resistor as shown in FIG. 10. Details of this testing
protocol are further described in P. K. Larsen, G. Kampschoer, M.
Ulenaers, G. Spierings and R. Cuppens, Applied Physics Letters,
Vol. 59, Issue 5, pp. 611-613 (1991). A standard square-pulse
ferroelectric pulse train was used. This pulse train consisted of
one negative polarity pulse, followed by two positive pulses and
two negative pulses (set, positive, up, negative, down). A typical
drive signal, as measured at position X in FIG. 10, and response
signal as measured at position Y in FIG. 10, are shown in FIG.
11.
[0143] The total charge passing from the ferroelectric capacitor to
ground within each response pulse can be calculated from:
Charge: Q=(1/R.sub.s).intg.VdV
[0144] Ferroelectric switching occurs when a ferroelectric
capacitor that has been previously poled by a negative (positive)
pulse is poled up (down) by a subsequent positive (negative) pulse.
Q.sub.sw is defined to be the difference in total charge contained
within a switching pulse and a non-switching pulse:
Q.sub.sw=(S.sub.0-S.sub.1)-(P.sub.0+P.sub.1)
where S.sub.0 and S.sub.1 are the leading and trailing edge
response pulses for the first switching pulse and P.sub.0 and
P.sub.1 are the leading and trailing edge pulses for the first
non-switching pulse (FIG. 11).
[0145] Measurements were made as described above with 1, 2 and 3 V
pulses and pulse lengths between 25 nS and 0.22 mS. The
polarization was found to be independent of pulse length over the
range investigated (FIG. 12).
[0146] Additionally, area scaling was investigated using 1 .mu.S
pulses and square capacitors from 33 .mu.m.times.33 .mu.m down to 4
.mu.m.times.4 .mu.m. Q.sub.sw was also found to be independent of
capacitor dimension over the range investigated (FIG. 13).
[0147] While the invention has been illustratively described herein
with reference to specific aspects, features and embodiments, it
will be appreciated that the utility and scope of the invention is
not thus limited and that the invention may readily embrace other
and differing variations, modifications and other embodiments. The
invention therefore is intended to be broadly interpreted and
construed, as comprehending all such variations, modifications and
alternative embodiments, within the spirit and scope of the ensuing
claims.
* * * * *