U.S. patent application number 10/256446 was filed with the patent office on 2004-04-01 for temperature-compensated ferroelectric capacitor device, and its fabrication.
Invention is credited to Dougherty, T. Kirk, Drab, John J..
Application Number | 20040061990 10/256446 |
Document ID | / |
Family ID | 32029278 |
Filed Date | 2004-04-01 |
United States Patent
Application |
20040061990 |
Kind Code |
A1 |
Dougherty, T. Kirk ; et
al. |
April 1, 2004 |
Temperature-compensated ferroelectric capacitor device, and its
fabrication
Abstract
A temperature-compensated capacitor device has ferroelectric
properties and includes a ferroelectric capacitor using a
ferroelectric material such as a metal oxide ferroelectric
material, a negative-temperature-vari- able capacitor using a
negative-temperature-coefficient-of-capacitance material such as a
metal oxide paraelectric material, and an electrical series
connection between the negative-temperature-variable capacitor and
the ferroelectric capacitor. The temperature-compensated capacitor
device may be formed as an integrated layered structure, or as
separate capacitors with a discrete electrical connection
therebetween.
Inventors: |
Dougherty, T. Kirk; (Playa
del Rey, CA) ; Drab, John J.; (Santa Barbara,
CA) |
Correspondence
Address: |
Raytheon Company
Intellectual Property & Licensing, EO/E01/E150
2000 E.EI Segundo Blvd.
P.O. Box 902
EI Segundo
CA
90245
US
|
Family ID: |
32029278 |
Appl. No.: |
10/256446 |
Filed: |
September 26, 2002 |
Current U.S.
Class: |
361/272 ;
257/E21.009; 257/E21.664; 257/E27.048; 257/E27.104 |
Current CPC
Class: |
H01L 27/11502 20130101;
H01L 27/0805 20130101; H01L 28/55 20130101; H01L 27/11507
20130101 |
Class at
Publication: |
361/272 |
International
Class: |
H01G 002/00 |
Claims
What is claimed is:
1. A temperature-compensated capacitor device having ferroelectric
properties and comprising: a ferroelectric capacitor comprising a
ferroelectric material; a negative-temperature-variable capacitor
comprising a negative-temperature-coefficient-of-capacitance
material; and an electrical series connection between the
negative-temperature-vari- able capacitor and the ferroelectric
capacitor.
2. The temperature-compensated capacitor device of claim 1, wherein
the electrical series connection comprises a direct physical
contact between the ferroelectric capacitor and the
negative-temperature-variable capacitor.
3. The temperature-compensated capacitor device of claim 1, wherein
the ferroelectric material comprises a ferroelectric layer, and
wherein the negative-temperature-coefficient-of-capacitance
material comprises a paraelectric layer in direct, facing contact
with the ferroelectric layer.
4. The temperature-compensated capacitor device of claim 1, wherein
the electrical series connection comprises a discrete electrical
connection extending between the ferroelectric capacitor and the
negative-temperature-variable capacitor.
5. The temperature-compensated capacitor device of claim 1, wherein
the ferroelectric material is a metal oxide ferroelectric
material.
6. The temperature-compensated capacitor device of claim 1, wherein
the ferroelectric material is a metal oxide ferroelectric material
selected from the group consisting of lead titanate, lead zirconate
titanate, lead lanthanum zirconate titanate, barium titanate,
strontium bismuth tantalate, strontium bismuth niobate, strontium
bismuth tantalate niobate, and bismuth lead titanate.
7. The temperature-compensated capacitor device of claim 1, wherein
the ferroelectric material is strontium bismuth tantalate
niobate.
8. The temperature-compensated capacitor device of claim 1, wherein
the negative-temperature-coefficient-of-capacitance material is a
paraelectric material.
9. The temperature-compensated capacitor device of claim 1, wherein
the negative-temperature-coefficient-of-capacitance material is a
metal oxide negative-temperature-coefficient-of-capacitance
material.
10. The temperature-compensated capacitor device of claim 1,
wherein the negative-temperature-coefficient-of-capacitance
material is a metal oxide
negative-temperature-coefficient-of-capacitance material selected
from the group consisting of strontium titanate and barium
strontium titanate.
11. The temperature-compensated capacitor device of claim 1,
wherein the negative-temperature-coefficient-of-capacitance
material is barium strontium titanate.
12. A temperature-compensated capacitor device having ferroelectric
properties and comprising: a ferroelectric capacitor comprising a
first electrode layer, and a ferroelectric layer of a ferroelectric
material in direct physical contact with the first-electrode layer;
and a negative-temperature-variable capacitor comprising a
negative-temperature-variable layer of a
negative-temperature-coefficient- -of-capacitance material in
direct physical contact with the ferroelectric layer, and a second
electrode layer in direct physical contact with the
temperature-variable layer.
13. A method for fabricating a temperature compensated capacitor
having ferroelectric properties, comprising the steps of: providing
a first electrode layer; depositing a ferroelectric precursor layer
of a ferroelectric precursor material on the first electrode layer;
reacting the ferroelectric precursor layer to produce a
ferroelectric layer; depositing a negative-temperature-variable
precursor layer of a
negative-temperature-coefficient-of-capacitance material on the
ferroelectric layer; reacting the negative-temperature-variable
precursor layer to form a paraelectric layer; and placing a second
electrode layer on the paraelectric layer.
14. The method of claim 13, wherein the step of providing the first
electrode layer includes the step of depositing the first electrode
layer, and wherein the step of placing a second electrode layer
includes the step of depositing the second electrode layer.
15. The method of claim 13, wherein the step of depositing the
ferroelectric precursor layer includes the step of depositing a
precursor of a metal oxide ferroelectric material.
16. The method of claim 13, wherein the step of depositing the
temperature-variable precursor layer includes the step of
depositing a precursor of a metal oxide
negative-negative-temperature-coefficient-of-c- apacitance
material.
Description
[0001] This invention relates to ferroelectric capacitors and, more
particularly, to a ferroelectric capacitor device which is
temperature compensated to reduce its variation of ferroelectric
properties with temperature.
BACKGROUND OF THE INVENTION
[0002] Ferroelectric materials are used in a variety of
applications. One such application is a ferroelectric capacitor
used in a nonvolatile, random access memory whose information is
retained even after a power loss. A ferroelectric material is one
whose physical state changes upon the application of an electrical
field, in a manner analogous with the change undergone by
ferromagnetic materials to which a magnetic field is applied. A
memory cell may be constructed based upon the hysteresis effects
associated with the physical state change. The ferroelectric
material has the advantages that its physical state is controlled
by the application of a voltage rather than a magnetic field, a
measurable state is retained after a power loss, and small-size
memory elements may be constructed by microelectronics fabrication
techniques, resulting in memory elements that consume little
power.
[0003] One difficulty with using ferroelectric materials in some
applications of interest, such as ferroelectric nonvolatile memory,
is that some of the material properties such as permittivity change
substantially over relatively narrow temperature ranges. These
properties change so greatly, in some cases more than 100 percent
over a temperature range of less than 100.degree. C., that the
associate read/write electronics can be quite difficult to design
and implement.
[0004] Ferroelectric materials such as barium titanate, strontium
titanate, calcium titanate, calcium stannate, and calcium zirconate
are also used to produce discrete ceramic capacitors. For the
discrete capacitor application, the material composition is varied
to provide a relatively high permittivity over a specified
temperature range. While these devices are optimized to provide a
relatively constant capacitance value over a specified temperature
range, they are not useful to non-volatile memory applications due
to their lack of a remnant polarization component which can be used
for information storage.
[0005] There exists a need for an improved approach to the design
of electronic circuits that utilize ferroelectric properties, to
reduce the effects of temperature variations. The present invention
fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
[0006] The present invention provides a temperature-compensated
capacitor device having ferroelectric properties, but in which the
ferroelectric properties of the capacitor device have a reduced
dependence upon the ambient temperature. The temperature
compensation is built into the temperature-compensated capacitor
device, and does not require the use of separate compensation
devices. It may be fabricated with a relatively minor modification
to the fabrication procedure.
[0007] In accordance with the invention, a temperature-compensated
capacitor device having ferroelectric properties comprises a
ferroelectric capacitor comprising a ferroelectric material, a
negative-temperature-variable capacitor comprising a
negative-temperature-coefficient-of-capacitance material, and an
electrical series interconnection between the
negative-temperature-variab- le capacitor and the ferroelectric
capacitor. The negative-temperature-coe- fficient-of-capacitance
material, and thence the negative-temperature-vari- able capacitor,
exhibits decreased capacitance with increasing temperature over an
operational temperature range.
[0008] The electrical series connection may comprise a direct
physical contact between the ferroelectric capacitor and the
negative-temperature-variable capacitor. In one such embodiment,
the ferroelectric material comprises a ferroelectric layer, and the
negative-temperature-coefficient of capacitance material comprises
another layer in direct, facing contact with the ferroelectric
layer. In this case, the ferroelectric capacitor and the
negative-temperature-varia- ble capacitor are fabricated as an
integral unit.
[0009] The electrical series connection may instead comprise a
discrete electrical connection extending between the ferroelectric
capacitor and the negative-temperature-variable capacitor. In this
case, the ferroelectric capacitor and the
negative-temperature-variable capacitor are fabricated separately
and then linked in series with the electrical connection.
[0010] The ferroelectric material is preferably a metal oxide
ferroelectric material, such as lead titanate, lead zirconate
titanate, lead lanthanum zirconate titanate, barium titanate,
strontium bismuth tantalate, strontium bismuth niobate, strontium
bismuth tantalate niobate, or bismuth lead titanate. The presently
most-preferred ferroelectric material is strontium bismuth
tantalate niobate.
[0011] The negative-temperature-coefficient of capacitance material
is preferably a paraelectric material. One such
negative-temperature-coeffic- ient-of-capacitance material is a
metal oxide negative-temperature-coeffic- ient-of-capacitance
material, such as strontium titanate or barium strontium titanate.
The presently most-preferred negative-temperature-coe-
fficient-of-capacitance material is barium strontium titanate.
[0012] In a preferred structure, an integrated
temperature-compensated capacitor device has ferroelectric
properties and comprises a ferroelectric capacitor comprising a
first electrode layer, and a ferroelectric layer of a ferroelectric
material in direct physical contact with the first electrode layer.
A negative-temperature-variable capacitor comprises a
negative-temperature-variable layer of a
negative-temperature-coefficient-of-capacitance material, such as a
paraelectric material, in direct physical contact with the
ferroelectric layer, and a second electrode layer in direct
physical contact with the temperature-variable layer.
[0013] Such an integrated structure may be fabricated by providing
a first electrode layer, depositing a ferroelectric precursor layer
of a ferroelectric precursor material on the first electrode layer,
reacting the ferroelectric precursor layer to produce a
ferroelectric layer, depositing a temperature-variable precursor
layer of a negative-temperature-coefficient-of-capacitance material
on the ferroelectric layer, reacting the temperature-variable
precursor layer to form a paraelectric layer, and placing a second
electrode layer on the paraelectric layer. Compatible features
discussed elsewhere herein may be used in relation to this
fabrication procedure.
[0014] The temperature-compensated capacitor device takes advantage
of the different temperature dependencies in ferroelectric and
paraelectric materials so that the changes in permittivity and
coercive voltage with temperature are greatly diminished, as
compared with a conventional ferroelectric capacitor. The voltage
across the temperature-compensated capacitor is divided across the
ferroelectric capacitor and the negative-temperature-variable
capacitor, in either the discrete or integrated embodiments as
discussed herein.
[0015] The paraelectric (negative-temperature-variable) capacitor
has a relatively high capacitance at the lower temperatures in the
range of operation. Most of the voltage drop is therefore across
the ferroelectric capacitor, and a normal ferroelectric hysteresis
loop is observed. At higher temperatures within the operating
temperature range, the paraelectric material has a lower
permittivity so that the voltage drop is greater across the
negative-temperature-variable capacitor relative to the
ferroelectric capacitor. For small signal capacitance, the
temperature-compensated capacitor device exhibits less variation
over a selected temperature range than does the ferroelectric
capacitor taken by itself. Regarding the hysteresis loop, the
increased voltage across the paraelectric material at high
temperature serves to compensate the decrease in coercive voltage
for the ferroelectric material. Consequently, the change in
performance as a function of temperature is less for the
temperature-compensated capacitor device than for a conventional
ferroelectric capacitor.
[0016] The present approach provides a capacitor device having
ferroelectric properties which have a smaller dependence upon
temperature than conventional ferroelectric capacitors. It may be
used in any circuitry that requires a ferroelectric capacitor, such
as those described in U.S. Pat. No. 5,729,488, U.S. Pat. No.
5,487,030, and U.S. Pat. No. 4,853,893, whose disclosures are
incorporated by reference, and particularly those which are
expected to experience variations in the operating temperature
during their service lives. The need for associated
temperature-compensation electronics is reduced, and in some cases
eliminated.
[0017] The presence of the negative-temperature-variable capacitor
in the temperature-compensated capacitor device results in a
decrease in slope of the hysteresis loop at the coercive voltage,
yielding improved performance for non-destructive read
ferroelectric memories. For destructive-read memories, this slope
change is of little consequence as long as the voltage applied is
sufficient to saturate the polarization of the material.
[0018] Other features and advantages of the present invention will
be apparent from the following more detailed description of the
preferred embodiment, taken in conjunction with the accompanying
drawings, which illustrate, by way of example, the principles of
the invention. The scope of the invention is not, however, limited
to this preferred embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a schematic representation of a
temperature-compensated ferroelectric capacitor device using
discrete components;
[0020] FIG. 2 is a schematic representation of an integrated
temperature-compensated ferroelectric capacitor device;
[0021] FIG. 3 is a graph of the relative permittivity change with
temperature for ferroelectric and paraelectric materials;
[0022] FIG. 4 presents calculated capacitor performance curves of
an uncompensated and a compensated ferroelectric capacitor device;
and
[0023] FIG. 5 is a block diagram of a preferred approach for
fabricating the temperature-compensated ferroelectric capacitor
device.
DETAILED DESCRIPTION OF THE INVENTION
[0024] FIG. 1 depicts one preferred embodiment of a
temperature-compensated capacitor device 20 having ferroelectric
properties. The temperature-compensated capacitor device 20
comprises a ferroelectric capacitor 22, a
negative-temperature-variable capacitor 24, and an electrical
series connection 26 between the negative-temperature-variable
capacitor 24 and the ferroelectric capacitor 22. The ferroelectric
capacitor 22 includes a ferroelectric layer 28 of a ferroelectric
material, with electrodes 30 on either side of and contacting the
ferroelectric layer 28. The negative-temperature-va- riable
capacitor 24 includes a paraelectric layer 32 of a
negative-temperature-coefficient-of-capacitance material, with
electrodes 34 on either side of and contacting the paraelectric
layer 32. The electrical series connection 26 extends between one
of the electrodes 30 and one of the electrodes 34.
[0025] The temperature-compensated capacitor device 20 of FIG. 1
utilizes discrete capacitors 22 and 24, with the electrical series
connection 26 in the form of a discrete electrical connection
extending between the ferroelectric capacitor 22 and the
negative-temperature-variable capacitor 24.
[0026] An integrated embodiment is illustrated in FIG. 2, where the
ferroelectric capacitor 22 and the negative-temperature-variable
capacitor 24 are integrated into a single structure that forms the
temperature-compensated capacitor device 20. The integrated
embodiment of FIG. 2 is preferred to the discrete embodiment of
FIG. 1 because of its compact structure, for those cases where the
integrated embodiment of FIG. 2 may be manufactured.
[0027] In this integrated embodiment of FIG. 2, there is a direct
physical contact between the ferroelectric capacitor 22 and the
negative-temperature-variable capacitor 24. The ferroelectric
material comprises the ferroelectric layer 28, and the
negative-temperature-coeffi- cient of capacitance material
comprises the paraelectric layer 32 in direct, facing contact with
the ferroelectric layer 28. That is, the direct, facing contact
serves as the electrical series connection 26. A first electrode 38
and a second electrode 40 have the ferroelectric layer 28 and the
contacting paraelectric layer 32 sandwiched therebetween. In a
typical case, the ferroelectric layer 28 is from about 500
Angstroms to about 4000 Angstroms thick, and the paraelectric layer
32 is from about 75 Angstroms to about 3000 Angstroms thick. The
electrodes 30, 38 and 40 may be made of a metal such as platinum,
iridium, ruthenium, or palladium, or an electrically conductive
nonmetal such as iridium oxide or ruthenium oxide.
[0028] The ferroelectric material of the ferroelectric layer 28 is
preferably a metal oxide ferroelectric material such as lead
titanate, lead zirconate titanate, lead lanthanum zirconate
titanate, barium titanate, strontium bismuth tantalate, strontium
bismuth niobate, strontium bismuth tantalate niobate, or bismuth
lead titanate. Most preferably, the ferroelectric material is
strontium bismuth tantalate niobate.
[0029] For typical ferroelectric materials showing
polarization/voltage hysteresis below the Curie temperature, the
coercive voltage decreases and the permittivity increases as the
temperature is increased toward the Curie temperature from lower
temperatures. At the Curie temperature, the hysteresis diminishes
to zero, and the permittivity approaches an infinite value. Above
the Curie temperature, there is no hysteresis and the permittivity
decreases, as expected for a paraelectric material. FIG. 3
illustrates properties of typical ferroelectric and paraelectric
materials. The relative permittivity k of the ferroelectric
materials typically increases strongly with temperature, and the
relatively permittivity of the paraelectric materials typically
decreases with increasing temperature.
[0030] Because of these variations, it is difficult to design a
readout circuit that functions properly over a wide temperature
range wherein the ferroelectric permittivity and coercive voltage
change. This difficulty is particularly of concern where a
non-destructive read ferroelectric memory relies on accurate
control of the read voltage to be equal to the coercive voltage, to
assure proper non-destructive read characteristics while detecting
a small capacitance change in an environment where both the
capacitor value and the coercive voltage are a function of
temperature.
[0031] The negative-temperature-variable capacitor 24 therefore
desirably exhibits decreased capacitance with increasing
temperature over an operational temperature range. The
negative-temperature-coefficient of capacitance material of the
layer 32 is desirably a paraelectric material whose relative
permittivity decreases with increasing temperature. The
negative-temperature-coefficient of capacitance material is
preferably a metal oxide
negative-temperature-coefficient-of-capacitance material such as
strontium titanate or barium strontium titanate, and is most
preferably barium strontium titanate.
[0032] FIG. 4 depicts the calculated capacitance of a conventional,
uncompensated ferroelectric capacitor made of strontium bismuth
tantalate niobate (SBTN), whose total capacitance increases sharply
with temperature. Also shown in FIG. 4 are the similarly calculated
properties of the temperature-compensated capacitance device 20 of
the present invention, utilizing an SBTN ferroelectric layer 28 and
a Ba.sub.05Sr.sub.05TiO.sub.3 (BST) paraelectric layer 32. The
temperature-compensated capacitance device 20 exhibits some
temperature dependence of the total capacitance, but substantially
less than that of the uncompensated ferroelectric capacitor. If
only the small signal capacitance is of interest, the total
capacitance of the temperature-compensated capacitor device 20 may
be made to be nearly temperature invariant.
[0033] FIG. 5 illustrates a preferred approach for practicing the
invention to make the preferred embodiment of the
temperature-compensated capacitance device 20 shown in FIG. 2. The
first electrode 38 in the form of the first electrode layer is
provided, step 60. The first electrode 38 may be of any operable
material, and may be provided by any operable approach. The first
electrode 38 is desirably a platinum electrode deposited upon a
substrate by vacuum evaporation of the platinum, and then thermally
annealed at a temperature of about 700.degree. C. to stabilize the
first electrode 38.
[0034] A ferroelectric precursor layer of a ferroelectric precursor
material is deposited on the first electrode layer, step 62. In the
preferred approach, a liquid solution of the metal oxide
ferroelectric precursor material is prepared and then spun onto the
first electrode layer. In the preferred case, the
metal-2-ethylhexanoate salts of strontium, bismuth, tantalum, and
niobium are dissolved in a solvent of xylene and n-butylacetate. In
the preferred case, the atomic ratio of
strontium:bismuth:tantalum:niobium is 0.9:2.18:1.5:0.5. The
resulting ferroelectric precursor solution is spun onto the first
electrode layer in one or more steps to achieve the desired
thickness, with drying between each spin-on step. The ferroelectric
precursor layer is reacted, step 64, by crystallizing in a rapid
thermal processor and then sintering in a tube furnace to form the
ferroelectric material of the ferroelectric layer 28. In this case,
the crystallizing is performed at a temperature of about
725.degree. C., and the sintering is performed at a temperature of
about 700.degree. C.
[0035] A negative-temperature-variable precursor layer of a
negative-temperature-coefficient of capacitance material is
deposited on the ferroelectric layer 28, step 66. In the preferred
approach, the temperature-precursor material is a mixture of the
metal-2-ethylhexanoate salts of strontium, barium, and titanium,
dissolved in the solvent of xylene and n-butylacetate. In the
preferred case, the atomic ratio of strontium:barium:titanium is
0.5:0.5:1.05. The resulting temperature-variable precursor solution
is spun onto the ferroelectric layer 28 in one or more steps to
achieve the desired thickness, with drying between each spin-on
step. The temperature-variable precursor layer is reacted, step 68,
by crystallizing in a rapid thermal processor and thereafter
sintering in a tube furnace to form the ferroelectric material of
the paraelectric layer 32. In this case, the crystallizing is
performed at a temperature of about 725.degree. C., and the
sintering is performed at a temperature of 700.degree. C.
[0036] The second electrode 40 in the form of a second electrode
layer is placed on the paraelectric layer 32, step 70. The second
electrode 40 is preferably deposited in the manner described for
the first electrode 38.
[0037] A temperature-compensated capacitor device 20 as discussed
above in the form illustrated in relation to FIG. 2 was prepared as
described in relation to FIG. 5. The resulting
temperature-compensated capacitor device functioned as described
above.
[0038] Although a particular embodiment of the invention has been
described in detail for purposes of illustration, various
modifications and enhancements may be made without departing from
the spirit and scope of the invention. Accordingly, the invention
is not to be limited except as by the appended claims.
* * * * *