Title: Electronic device formed from a thin film with vertically oriented columns with an insulating filler material
Abstract: A thin film device comprises: a substrate and a thin film having a thickness formed on the substrate, wherein the thickness of the thin film is at least 1 micrometer, a crystal structure having crystals with a grain size formed within the thin film, wherein the grain size of a majority of the crystals includes a height to width ratio greater than three to two.
Patent Number: 6,979,938 Issued on 12/27/2005 to Solberg
| Inventors:
|
Solberg; Scott E. (Mountain View, CA)
|
| Assignee:
|
Xerox Corporation (Stamford, CT)
|
| Appl. No.:
|
464080 |
| Filed:
|
June 18, 2003 |
| Current U.S. Class: |
310/358; 257/347; 117/902; 117/35 |
| Intern'l Class: |
H02N 002/00 |
| Field of Search: |
310/357,358
257/347
117/35,902
|
References Cited [Referenced By]
U.S. Patent Documents
Other References
Shimomura, et al.; Preparation of Lead Zirconate Titanate Thin Film by Hydrothermal
Method; Japanese Journal of Applied Physics; vol. 30, No. 9B, Sep. 1991, pp. 2174-2177.
Balaraman, et al.; Novel Hydrothermal Processing (<100 deg. C) of Ceramic-Polymer
Composites for Integral Capacitor Applications; 2002 Electronic Components and
technology Conference; pp. 1699-1703.
Vayssieres, et al.; Purpose-Built Anisotropic Metal Oxide Material: 3D Highly
Oriented Microrod Array of ZnO; 2001 American Chemical Society; Pub'd on web Apr.
5, 2001.
|
Primary Examiner: Schuberg; Darren
Assistant Examiner: Aguirrechea; J.
Attorney, Agent or Firm: Fay Sharpe Fagan Minnich & McKee, LLP
Claims
1. A thin film device comprising:
a substrate;
a thin film having a thickness formed on said substrate, wherein said thickness
of said thin film is at least 1 micrometer; and,
a crystal structure having crystals with a grain size formed within said thin
film, wherein said grain size of a majority of said crystals includes a height
to width ratio that is greater than three to two.
2. The thin film device according to claim 1, wherein said crystals are synthesized
using a hydrothermal method.
3. The thin film device according to claim 1, wherein said crystals have a rod
configuration oriented predominantly in the (001) plane.
4. The thin film device according to claim 2, wherein said crystals have a rod
configuration oriented predominantly in the (001) plane.
5. The thin film device according to claim 3, wherein said crystal structure
of said rods is tetragonal.
6. The thin film device according to claim 1, wherein said thin film is at least
one of piezoelectric, electrostrictive, ferroelectric, or anti-ferroelectric material.
7. The piezoelectric thin film device according to claim 6, further comprising:
upper and lower electrode portions provided on respective upper and lower surfaces
of said thin film for applying an electric field thereto, wherein said crystals
extend from said lower surface to said upper surface.
8. The thin film device according to claim 6, wherein said substrate includes
a metal sheet.
9. The thin film device according to claim 6, wherein said substrate includes
a metal-coated sheet.
10. The thin film device according to claim 6, wherein said thin film includes
a seed layer deposited on said substrate.
11. The thin film device according to claim 10, wherein a thickness of said seed
layer is less than 500 nm.
12. The thin film device according to claim 10, wherein said crystals of said
structure of said thin film are grown generally perpendicular to said seed layer.
13. The thin film device according to claim 12, wherein said thin film has a
crystal growth structure oriented such that the extent of growth direction <001>
is greater than extent of growth directions <100>, <010>, and <111>.
14. The thin film device according to claim 12, wherein said thin film includes
gaps between said crystals, said gaps include an insulating filler material therein.
15. The thin film device according to claim 14, wherein said filler material
is in the form of a liquid or a gel to fill said gaps between said crystals, said
liquid or said gel being curable into a solid.
16. The thin film device according to claim 12, wherein said thin film includes
zirconium, titanium, and lead.
17. A thin film device comprising:
a substrate;
a thin film having a thickness formed on said substrate, wherein said thickness
of said thin film is at least 1 micrometer;
a crystal structure having crystals with a grain size formed within said thin
film, wherein said grain size of a majority of said crystals includes a height
to width ratio that is greater than three to two; and,
said crystals oriented predominantly in the (001) plane.
18. A thin film device comprising:
a substrate;
a thin film having a thickness formed on said substrate, wherein said thickness
of said thin film is at least 1 micrometer;
a crystal structure having crystals with a grain size formed within said thin
film, wherein said grain size of a majority of said crystals includes a height
to width ratio that is greater than one; and,
said thin film has a crystal growth structure oriented such that the extent of
growth direction <001> is greater than extent of growth directions <100>,
<010>, and <110>.
Description
BACKGROUND
The present invention relates in general to a thin film device for use as a high
specific energy electronic device, such as a capacitor, and a process for its manufacture.
Specifically, the electronic device and method for manufacturing the electronic
device involves hydrothermal deposition of a predominantly vertically oriented
columnar (crystal) structured high dielectric constant film including an insulating
filler material.
Useful inorganic materials with high dielectric constants are usually piezoelectric,
but certain electrostrictive, ferroelectric, or anti-ferroelectric materials may
be used for some applications. A common material with a high relative dielectric
constant of much greater than 100, depending on composition, is lead zirconium
titanate (hereinafter sometimes abbreviated as PZT). PZT is also strongly piezoelectric,
and thus is also used in many electromechanical applications. Thin films of PZT
are formed by various methods including physical vapor deposition (PVD) techniques
such as sputtering, chemical vapor deposition (CVD) techniques, and chemical solution
methods including sol-gel deposition. The chemical solutions may be applied for
example by spin coating which is followed by a typical heat treatment (sintering)
at a high temperature of 500-1000° C. to evaporate any solvent and to convert
metal-organic precursors to inorganic materials. "Thick" film deposition methods,
which are best used for films greater than about 10 microns thick, although thinner
films of poorer quality have been used in commercial products, involve applying
a mixture of powdered ceramic in an organic vehicle to a substrate and firing at
very high temperature, at least 800° C., but preferably at least 1100°
C. to obtain films with dielectric constants closer to bulk values. For reference,
"bulk" material refers to the best available macroscopic sample with the same or
similar material chemistry. Typically, because of the extremely high sintering
temperatures used in the heat treatment, expensive electrode alloys of palladium
or platinum are usually needed for best results.
The above-mentioned conventional piezoelectric thin film deposition methods are
typically not economical for film thicknesses greater than one to two microns (also
known as micrometers), and furthermore the thickest of such films can suffer from
defects such as stress cracking. The "thick" film deposition methods produce relatively
poor quality films, and furthermore require relatively expensive electrode materials.
Another approach for increasing the thickness of piezoelectric films is based
on the use of hydrothermal synthesis which permits the intended reaction to proceed
at a relatively low temperature (for example less than about 250° C.). Additionally,
using the hydrothermal synthesis technique and low deposition temperatures a reduction
in the electrode cost can be realized by using less expensive electrode materials.
Previously reported hydrothermal synthesis techniques involve growing crystal of
a piezoelectric material such as PZT on a compatible seed layer, for example titanium
oxide, in a reactor with reagents containing for example Pb, Zr, and Ti, and a
mineralizer such as potassium hydroxide, and heated to moderate temperatures of
typically 120 degrees to 160 degrees C. Thick films can be formed at low temperatures
by the hydrothermal synthesis technique, but the crystal grains produced are dependent
on the orientation of the seed crystals, so that nearly randomly oriented seed
crystals will produce a relatively low density film.
Accordingly, it is considered desirable to develop thin film devices
(capacitors) with high specific energy, comparable to that of other capacitors
such as aluminum electrolytic, or multi-layer ceramic capacitors, yet with lower
energy loss than the aluminum electrolytic and lower manufacturing costs than the
multi-layer ceramic capacitors. Current multilayer ceramic capacitors are manufactured
using "thick" film methods such as screen printing or tape casting, thus such ceramic
capacitors suffer from poor performance relative to bulk ceramics because the films
are not fully dense, so that the resulting dielectric constant is typically less
than one-half that of bulk.
SUMMARY
In accordance with the present invention, there is disclosed a thin film device
and method for producing the device. One aspect of the present invention relates
to a thin film device comprising a substrate and a thin film having a thickness
formed on the substrate, wherein the thickness of the thin film is at least 1 micrometer.
Additionally, the device comprises a crystal structure having crystals with a grain
size formed within the thin film wherein the grain size of a majority of the crystals
includes a height to width ratio that is greater than three to two.
In accordance with another aspect of the present invention, a method is provided
for producing a piezoelectric thin film device, within a reactor vessel, having
crystals vertically oriented therein, the method comprises the steps of preparing
a substrate compatible with a hydrothermal growth process, depositing a seed layer
onto the substrate, placing the substrate and at least one reagent into the vessel,
closing the vessel and hydrothermally synthesizing the crystal structure, removing
the substrate from the vessel, filling gaps between the crystals with a filler
material, and applying a top electrode.
It is an object of the present invention to increase the breakdown voltage of
the capacitor. Filling in the pores or gaps of hydrothermally deposited films with
an insulator, for example, a polymer (or sol-gel ceramic) can increase the breakdown
voltage of the capacitor. The energy stored within a capacitor increases with the
voltage squared, thus filled films provide dramatically improved specific energies.
Filling the gaps between vertically oriented crystal grains of, for example, ferroelectric
with a polymer is useful because the polymer increases the breakdown voltage of
the device relative to having ambient (humid) air in the crevices.
Additionally, it is another object of the present invention to provide
a thin film vertical columnar structure which allows most of the high dielectric
constant material to extend between the top and bottom electrodes, so that the
insulator which fills the crevices does not sandwich between the high dielectric
constant material and the electrode which would reduce the effective dielectric
constant and thus the capacitance of the final device. Thus, it is desirable to
concentrate the insulating filler alongside, not above or below, the columnar structure.
Other benefits and advantages of the subject invention will become apparent
to those skilled in the art upon a reading and understanding of the specification.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention may take physical form in certain parts and steps and arrangements
of parts and steps, the preferred embodiments of which will be described in detail
in the specification and illustrated in the accompanying drawings which form a
part hereof and wherein:
FIG. 1 is a schematic cross-section of the thin film device according to the
present invention;
FIG. 2 is a schematic view of a tetragonal crystal according to the present invention;
FIG. 3 is a partial cross-section of a reactor vessel;
FIG. 4 is a perspective view of crystals exhibiting predominantly highly ordered
vertical growth according to the present invention;
FIG. 5 is a perspective view of crystals exhibiting predominantly highly ordered
vertical growth including an epoxy fill therebetween according to the present invention;
FIG. 6 is a perspective view of crystals exhibiting predominantly highly ordered
vertical growth including an epoxy fill whereby the surface has been cut and polished
according to the present invention;
FIG. 7 is a perspective view of crystals with poorly ordered growth;
FIG. 8 is a perspective view of crystals exhibiting ordered growth;
FIG. 9 is an x-ray diffraction spectrum of a sample with crystals exhibiting
predominantly highly ordered vertical growth according to the invention; and,
FIG. 10 is a graph showing electrical measurements of epoxy-filled and polished
hydrothermal PZT according to the present invention.
DETAILED DESCRIPTION
Referring now to the drawings, wherein the showings are for the purposes
of illustrating a preferred embodiment of the invention only and not for purposes
of limiting same. FIG. 1 shows a schematic cross-section of the high dielectric
constant thin film electronic device, such as capacitor
10 with a bottom
or lower electrode
12 embedded in or coated on the surface of the substrate
14. A chemically and structurally suitable seed layer
18 can be deposited,
for example, from a chemical solution using, for example, spin or dip coating.
A hydrothermal deposition of a main, for example, ferroelectric layer-thin film
20 is shown, which in one embodiment is at least 1 micrometer. Using a hydrothermal
synthesis process produces mostly vertically-oriented columnar crystal growth structures
22 (as depicted in FIG. 2). Also shown in FIG. 1 is an insulating filler
material
24, which can be, for example a polymer or sol-gel ceramic, located
in gaps
26 between the ferroelectric columns
22, and a top or upper
electrode
30 formed by, for example, physical vapor deposition. In a preferred
embodiment, the device
10 has both an upper electrode
30 and a lower
electrode
12 for electrically charging the thin film
20. The film
20 composition may be tailored to maximize the amount of charge stored or
to minimize the dielectric loss, so for example various piezoelectric, anti-ferroelectric,
or electrostrictive materials may be used. Filling in the pores or gaps
26
of hydrothermally deposited films
20 with the insulating filler material
24 increases the breakdown voltage of the capacitor
10. Since stored
energy increases with voltage squared, filled films
20 will dramatically
improve specific energies. Filling in the gaps between vertically-oriented <001>
ferroelectric crystal grains
22 with a, for example, polymer
24 (see
FIG. 5) increases the breakdown voltage of the device
10 relative to having
ambient (likely humid) air in the gaps
26. Significantly, the insulating
filler
24 has the additional benefit of even allowing larger gaps
26
due to missing grains (not shown) in the ferroelectric film
20, for example,
from defects that occur in the hydrothermal growth process, because such gaps
26
in the ferroelectric film
20 would have only small effects on the device
capacitance, provided that they constitute a small fraction of the total device
area, but would otherwise undesirably and potentially catastrophically lower the
device breakdown voltage.
The sequence of steps in the manufacture of the piezoelectric thin film device
10 are described below. Initially, the process starts with a substrate
14,
preferably with a uniform crystal texture including, for example, a metal sheet.
The bottom electrode
12 may be the substrate
14 or a thin metal coating
or sheet on the substrate
14. The metal coating or metal sheet can be, for
example, stainless steel, platinum, or nickel. Examples of bottom electrode
12
include, but are not limited to, 1) a randomly textured surface, 2) a predominantly
<111> textured platinum electrode, and 3) a predominantly <100>
textured cubic electrode with compatible structural match to the seed layer and
hydrothermally grown ferroelectric material. Next a chemical solution or other
low-cost method is used to apply the seed layer
18. The seed layer
18
employed may have a thickness of 500 nm (0.5 micrometer) or less. The seed layer
18 is desirably oriented in the (100) plane for subsequent hydrothermal
growth of pseudo-cubic high dielectric constant materials. Next, a film
20
is hydrothermally deposited on one side or both sides of the substrate
14
simultaneously. The substrate
14, seed layer
18, and film
20
is placed in a high temperature, high pressure reactor vessel, for example, a Parr
Instruments floor stand reactor vessel
31 (see FIG. 3). In one embodiment,
the vessel is closed and heated to approximately 160° C. for a period of approximately
14 hours, after which the substrates are removed for subsequent processing. Epitaxial
grain growth occurs during the heating process resulting in a crystal structure
22 having crystals with a grain size formed within the thin film
20.
The grain size of the crystals is predominantly less than about 2 micrometers across
(width) and approximately 12-16 micrometers tall (height). It is to be appreciated
however, that for other crystals the height to width ratio may be different, although
this height to width ratio is preferably greater than three to two. The gaps
26
in the film
20 are then filled with a liquid (or gel) filler material
24,
for example an epoxy, then cured, and then lightly polished (optional step). Polishing
is done to planarize the top surface (FIG. 6) of the composite structure. Sputtering
or another low-cost method is used to apply a top electrode
30 and finally,
the device
10 may be cut, sampled, and packaged. The steps outlined above
will be described in more detail hereinafter.
Hydrothermal processing involves the synthesis of inorganic compounds,
usually oxides, in an aqueous, elevated temperature (typically up to 250°
C.), and elevated pressure environment. One hydrothermal processing recipe used
to produce an embodiment of tetragonal-rod-configured crystal
22 growth
(see FIGS. 2 and 4) involved the following ingredients and methods. A mixture of
1.4 milliliters zirconium propoxide and 1 milliliter titanium isopropoxide, 15
grams lead acetate trihydrate, 500 milliliters of 45 weight percent potassium hydroxide,
and 2.4 liters deionized water was added to a four liter, high temperature, high
pressure, reactor vessel made by Parr Instruments. The vessel
31 was closed
and heated to about 160° C., whereby the pressure was allowed to build to
approximately 6 atmospheres. The reactor
31 was stirred with an impeller
32 at 30 rpms for 14 hours. The resultant thin film
20 was then rinsed
in deionized water. The Parr reactor
31 used in the synthesis was a Model
4551 "1 Gallon Reactor". The crystals
22 that were grown (refer to FIGS.
2 and 4) grew epitaxially from the seed layer
18 and are oriented predominantly
in the (001) plane
33. The grown crystals
22 in this example have
a tetragonal crystal structure and because they are predominantly oriented along
the <001> direction
34, resemble rectangular rods or posts because
they are much taller than wide (FIGS. 2 and 4). It is to be appreciated that the
extent of growth direction <001>
34 is greater than, for example,
the extent of growth directions <100>
35, <010>
36,
and <110>
37. Most of the useful high dielectric constant materials
have slightly distorted cubic structures, for example tetragonal, rhombohedral,
or monoclinic structures. Thus, for example the tall, vertically oriented structures
useful for the present invention grow along the <001> direction
34
for tetragonal materials.
Filling in the pores or gaps
26 of hydrothermally deposited films
(see FIG. 5) with the filler material
24, such as a polymer or sol-gel ceramic,
has the effect of increasing the mechanical strength and breakdown voltage of the
capacitor
10. One way to increase the energy storage capacity of a capacitor
is to increase the voltage across the capacitor, because energy goes up in relation
to the square of the voltage. Filled films increase the voltage stress capability
which allows higher voltages and therefore provides dramatically improved specific
energies of the capacitor
10. The microstructure of the film
20 and
the polymer
24 infiltration allows the synthesis of reliable films
20
with high dielectric constant and low dielectric loss.
An advantage of the vertical columns which predominantly extend from the bottom
to top electrodes, compared to the more common randomly oriented hydrothermally
grown crystals, is that the majority of the lower dielectric constant filler material
is not between an electrode and the high dielectric material, but rather adjacent
to the high dielectric material. Thus in the electrical circuit, with the vertically
oriented columns the low dielectric constant filler material is in parallel with
the high dielectric constant material, so any capacitance reduction is linearly
proportional to the ratio of filler to high dielectric constant material, whereas
if the high dielectric constant material were randomly oriented, then some of the
filler material would be in series, so the device capacitance would be significantly
reduced, typically by at least a factor of ten, depending on the relative dielectric
constants. Typical filler polymers would have relative dielectric constants <10,
whereas useful high dielectric hydrothermally grown materials would have relative
dielectric constants >100. For reference, the formula for calculating the
overall capacitance (Cp) of these capacitors in parallel is:
with the overall capacitor area divided between the area of the high dielectric
constant columns and the filler, whereas the formula for calculating the overall
capacitance (Cs) of capacitors in series (i.e. less desirable configuration) is:
with the thickness in each section of the film divided between the high dielectric
material and the filler.
High voltage power supply applications require capacitors
10 with thick
films to keep electrical fields less than about 50 volts per micron. Currently,
it is expensive to vapor deposit films greater than about 1 micron. Additionally,
it is difficult to get quality films less than 10 microns with "thick film" processes
employing powdered ceramics in an organic binder. Such "thick" films are often
applied by screen printing, and subsequently fired at high temperature, at least
900° C., but even higher temperatures are desired to further densify the films
and thus increase the dielectric constant. Extremely high temperatures place limitations
on the materials used in the "thick" film devices, often requiring expensive noble
metal electrodes for example. The hidden pores in screen printed films cannot be
effectively filled with a liquid or gel, thus screen printed films must rely on
inherent breakdown voltage of the ferroelectric film. In contrast, hydrothermally
deposited films
20 (e.g. vertical type growth) have high quality crystals
22 for maximum dielectric constant when filled with insulator. It is to
be appreciated that the vertical growth is not a 'perfectly' vertical growth, but
rather a predominantly vertical growth. Effective capacitance is proportional to
ferroelectric film coverage.
Various reagent concentrations may result in less than desirable growth morphologies
(grain growth). Specifically, different PZT growth morphologies are displayed in
FIGS. 7 and 8. The growth morphologies can be described as "boulders"
40
and "cubes"
44, respectively. The different growth morphologies
40,
44 result from the fact that there is both growth and etching occurring.
The boulder growth morphology
40 results in a fairly random crystal alignment
42 with less ordered lattices (poorly ordered growth). The growth morphology
44 results in crystals
46 exhibiting ordered cubic growth.
The growth of highly <001> textured crystals
22, may result
from a random textured seed layer
16 under appropriate growth conditions
via a survival-of the-fittest mechanism, because the <001> oriented grains
can grow taller faster than grains of other orientations, however the packing density
of such columns is reduced when disordered seed layers are used.
An x-ray diffraction spectrum of hydrothermal PZT 30/70, i.e. atomic % Zr/(atomic
% Zr+atomic % Ti)=30%, according to the present invention, but without polymer
fill, is shown in FIG. 9. This measurement confirms the predominant <001>
crystal texture versus other textures such as <101> and <110>.
A hysteresis loop (polarization vs. volts) is displayed in FIG. 10 showing electrical
measurements of an epoxy filled and polished hydrothermal PZT 30/70 (zirconium
to titanium) on stainless steel according to the present invention. The measurements
were taken from a sample approximately 14 microns thick and utilized gold in the
top electrode.
While particular embodiments have been described, alternatives, modifications,
variations, improvements, and substantial equivalents that are or may be presently
unforeseen may arise to applicants or others skilled in the art. Accordingly, the
appended claims as filed, and as they may be amended, are intended to embrace all
such alternatives, modifications, variations, improvements, and substantial equivalents.
*
