Bottom electrode structure for dielectric capacitors

An integrated circuit capacitor (20) includes a bottom electrode structure (24) having an adhesion metal portion (34), a noble metal portion (36), and a second noble metal layer (40). A process of manufacture includes annealing the adhesion metal portion (34) and the noble metal portion (36) prior to the deposition of second noble metal layer (40) for purposes of forming barrier region (38). The electrode (24) preferably contacts metal oxide layer (26), which is made of a perovskite or perovskite-like layered superlattice material. A temporary capping layer (59) is formed and removed in manufacture, which serves to increase polarization potential from the device by at least 40%.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention pertains to the field of wiring layers for integrated 
circuit devices and, more particularly, electrodes including diffusion 
barrier layers, as well as methods of making the same. More specifically, 
the diffusion barrier layers are preferably used in the electrodes of 
dielectric capacitors or ferroelectric capacitors. 
2. Description of the Prior Art 
Integrated circuit devices can fail or suffer performance degradation due 
to materials incompatibility problems. Thin-film layers can be 
contaminated by diffusion from adjacent layers. Additionally, cracking, 
peeling, and surface irregularity problems can derive from different 
thermal coefficients of expansion in the respective layers, as it is often 
necessary to heat the devices in manufacture. These problems are 
intensified by the micro-thin nature of the circuit layers because it is 
impossible to predict the thermal performance of a given layer without 
also considering the substrate on which the layer is formed. Accordingly, 
circuit designers must carefully select the materials that will form the 
respective thin-film layers. 
A common circuit failure mechanism includes shorting that is induced by the 
cracking or peeling of one thin-film layer away from another layer due to 
poor bonding between the adjacent layers. In silicon technology devices, a 
platinum wiring layer or electrode can bond poorly with a silicon dioxide 
or titanium dioxide isolation layer that separates the platinum electrode 
from the silicon wafer. Researchers have successfully reduced the 
incidence of cracking by applying a titanium metal adhesion layer to the 
isolation layer prior to sputtering the platinum electrode; however, the 
application of titanium metal proved to be problematic. The additional 
titanium served to contaminate other layers through titanium diffusion. 
Diffused titanium contamination is particularly problematic in integrated 
circuits because the titanium cations typically present a variety of 
valence states, i.e., +2, +3, and +4, which induce corresponding lattice 
defects. Metal nitride diffusion barrier layers have been constructed to 
isolate the adhesion layer metals. See for example the U.S. Pat. No. to 
Larson, 5,005,102, and Garceau et al, "TiN As A Diffusion Barrier Layer In 
The Ti-Pt-Au Beam Less Metal System", 60 Thin Solid Films, 237-247, No. 2, 
(1979), which both teach the use of a titanium nitride diffusion barrier 
layer. Annealing of metal nitrides can yield surface irregularities, e.g., 
hillocks, that induce shorting of dielectric or ferroelectric capacitors. 
There remains a need for an effective bottom electrode structure that 
adheres well and does not have short-inducing surface irregularities. 
Additionally, in silicon technology devices, prior barrier layers have not 
proven to be effective against the diffusion or blooming of silicon or 
silicon dioxide, which can significantly degrade the performance of 
high-dielectric capacitors. 
SOLUTION TO THE PROBLEM 
The present invention overcomes the problems that are outlined above by 
providing an electrode structure including a diffusion barrier layer and 
electrode which is substantially free of surface irregularities. The 
barrier layer is effective in preventing or reducing adhesion metal 
diffusion and silicon diffusion or blooming even in the absence of a metal 
nitride. The barrier layer is produced according to a special process that 
includes interdiffusing an adhesion metal portion and a noble metal 
portion to provide a particularly stable barrier region lattice, and 
capping the lattice with a temporary metal oxide or spin-on glass ("SOG") 
layer. The temporary layer is removed, and a relatively pure or undiffused 
noble metal is applied to complete the electrode and barrier region. 
Ferroelectric capacitor devices including an electrode having this type of 
bottom electrode structure exhibit up to a 100% or greater improvement in 
polarization as compared to devices that are made of identical ingredients 
which have not been subjected to special processing. The barrier layer 
also imparts notable performance improvements to dielectric capacitors. 
The present invention includes an electrode structure having a diffusion 
barrier region that is formed of interdiffused metals. The 
barrier-containing electrode structure is particularly useful in 
integrated circuit devices, such as thin-film ferroelectric capacitors and 
dielectric capacitors. The barrier region is formed of an adhesion metal 
portion and a noble metal portion that are at least partially 
interdiffused with one another, e.g., by simultaneously annealing the 
respective portions to at least partially combine them. 
The temporary layer formed of SOG or a metal oxide is formed atop the 
barrier region, and may be annealed simultaneously together with the 
barrier region. During the anneal, the adhesion metal diffuses throughout 
the barrier region and the temporary layer. An abrupt change in adhesion 
metal flux, in addition to that which would normally occur at layer 
boundaries, is subsequently caused by the removal of the temporary layer. 
The electrode is completed by covering or capping the barrier layer with a 
noble metal layer. 
The barrier region presents a first average flux of a diffusible moiety 
selected from a group consisting of adhesion metal moieties, substrate 
moieties, and mixtures thereof. The noble metal layer presents a second 
average flux less than the first average flux of the diffusible moiety. 
The change in flux derives from the removal of a temporary layer that 
receives adhesion metal moieties prior to stabilization of the barrier 
region lattice. The electrode is supported by a substrate, and may be 
covered with additional layers such as dielectric or ferroelectric 
materials. 
In preferred forms of the invention, the substrate is a silicon substrate, 
but may also be any other substrate. The noble metal portion of the 
barrier region and the noble metal of the noble metal layer are preferably 
the same type of noble metal, and are most preferably platinum. The 
adhesion metal is preferably titanium or tantalum. The noble metal portion 
of the barrier region is preferably applied in a thickness ranging from 
about three to eight times the thickness of the adhesion metal portion. 
Especially preferred forms of the invention include an integrated circuit 
capacitor device. The capacitor includes a metal oxide layer deposited 
atop the noble metal layer of the completed barrier-electrode. The metal 
oxide layer is preferably formed of a material that is selected from a 
group consisting of ferroelectric metal oxides, high dielectric metal 
oxides having a dielectric constant greater than or equal to that of 
silicon dioxide, and combinations thereof. Ferroelectric metal oxides are 
particularly preferred. A top electrode is deposited over the metal oxide 
layer to complete the capacitor. 
In the case of high dielectric capacitors, the metal oxide material is 
preferably a perovskite having an empirical formula of ABO.sub.3, wherein 
A is an A-site metal cation, B is a B-site metal cation, and O is oxygen. 
The most preferred type of perovskite is barium strontium titanate 
("BST"), which may be either ferroelectric or non-ferroelectric depending 
upon the relative amounts of structurally equivalent A-site barium and 
strontium metals. Even though both types of BST typically have high 
dielectric constants that exceed the dielectric constant of silicon 
dioxide, the use of non-ferroelectric BST is preferred for use as a 
dielectric. The barrier layer serves to keep the BST essentially free from 
diffused moieties of silicon and silicon dioxide. These diffused silicon 
moieties would, otherwise, migrate to the BST layer and degrade the 
dielectric performance. The most preferred BST formulation has an average 
empirical formula of Ba.sub.0.7 Sr.sub.0.3 TiO.sub.3. 
In ferroelectric capacitors, the metal oxide material is preferably a 
perovskite-like layered superlattice material. The term "perovskite-like" 
refers to a lattice that is formed of respective oxygen octahedra layers 
that are separated by superlattice generator layers including a trivalent 
metal such as bismuth. These materials are recognized as a broad class of 
ferroelectric materials, but have not historically been successfully 
applied in integrated circuit devices due to device reliability problems. 
The perovskite-like portion of the layered superlattice material is formed 
in discrete layers. These layers include a primary cell having an oxygen 
octahedral positioned within a cube that is defined by large A-site metals 
at the corners. The oxygen atoms occupy the planar face centers of the 
cube and a small B-site element occupies the center of the cube. In some 
instances, the oxygen octahedral structure may be preserved in the absence 
of A-site elements. 
The most preferred layered superlattice material is strontium bismuth 
tantalate material having an average empirical formula of SrBi.sub.2 
Ta.sub.2 O.sub.9. The superlattice-generator layers are most preferably 
formed of (Bi.sub.2 O.sub.2).sup.2+ materials, but may also contain 
thallium (111) as the metal. The most preferred oxygen octahedra structure 
layers, accordingly, have an average empirical formula of (SrTa.sub.2 
OT).sup.2-. The respective layers spontaneously generate a layered 
superlattice upon annealing of a metal organic precursor solution. The 
oxygen octahedra layers are ferroelectric and have an average empirical 
formula with an ionic charge that is offset by the superlattice generator 
layers to balance the overall crystal charge. The barrier layer serves to 
keep the layered superlattice material essentially free of adhesion 
metals, which can degrade polarization by inducing point defects. 
A preferred method exists for producing the thin-film barrier-containing 
electrode devices. The adhesion metal portion is deposited on a substrate, 
and a first noble metal portion is applied on the adhesion metal portion. 
Both the adhesion metal and the noble metal are preferably deposited by 
sputtering. A temporary metal oxide or SOG coating, e.g., SiO.sub.2 or 
SrBi.sub.2 Ta.sub.2 O.sub.9, is deposited atop the electrode structure. 
The temporary layer is subjected to a first anneal, and removed by etching 
to leave a bottom electrode having a barrier region that includes 
interdiffused adhesion and noble metal moieties. A second noble metal 
layer is preferably sputtered atop the barrier region. A dielectric or 
ferroelectric metal oxide layer is deposited on the second noble metal 
portion, and the combined layers are subjected to a second anneal. 
Especially preferred methods include annealing the barrier region and the 
temporary metal oxide coating together at a temperature ranging from about 
450.degree. to 850.degree. C. The most preferred anneal temperature is 
600.degree. C. A second noble metal layer is deposited after the temporary 
layer or coating has been removed, and a second metal oxide layer is then 
preferably formed on the second noble metal layer. The most preferred 
method of forming the respective metal oxide layers includes depositing a 
liquid precursor to form a precursor film, and heating the precursor film 
to yield the metal oxide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 depicts capacitor 20 including substrate 22, bottom electrode 24, 
metal oxide layer 26, and top electrode 28. Substrate 22 preferably 
includes conventional silicon layer 30, which is capped by isolation layer 
32. Silicon layer 30 may be a single crystal or polycrystalline silicon, 
and is commercially available from a variety of sources as a silicon 
wafer. Layer 30 may also be formed of other known substrate materials, 
such as gallium arsenide, indium antimonide, magnesium oxide, strontium 
titanate, sapphire, quartz and combinations of the forgoing as well as 
other materials. Isolation layer 32 is preferably made of thick silicon 
dioxide, which is formed on layer 32 by well-known processes, e.g., SOG 
deposition or baking of layer 30 under oxygen in a diffusion furnace. As 
used herein, the term "substrate" specifically means a layer that provides 
support for any other layer. Substrate 22 serves to support all other 
layers, but the term substrate can also mean substrate 22 in combination 
with other layers. Accordingly, the combination of substrate 22 and bottom 
electrode 24 provides a substrate or support for metal oxide layer 26, 
which, in turn, provides support for top electrode 28. 
Bottom electrode 24 includes a plurality of respective layers including 
adhesion metal portion 34, first noble metal portion 36, diffusion barrier 
region 38, and second noble metal layer 40. 
Adhesion metal portion 34 is preferably made of titanium or tantalum 
sputtered to a thickness that preferably ranges from about 50 .ANG. to 250 
.ANG., and is most preferably 100 .ANG.. First noble metal portion 36 is 
preferably platinum, but may also be other noble metals such as gold, 
silver, palladium, iridium, rhenium, ruthenium, and osmium, as well as 
conductive oxides of these metals. First noble metal portion 36 is 
preferably deposited by sputtering platinum atop adhesion metal portion 34 
to a thickness ranging from three to fifteen times the thickness of 
adhesion metal portion 34, with the most preferred thickness being about 
1000 .ANG. when adhesion metal portion is 100 .ANG.. Thicknesses outside 
the preferred range of values are still useful, but a thinner first noble 
metal portion 36 increasingly permits diffusion of adhesion metal moieties 
to layers above portion 36. A thicker portion 36 is increasingly wasteful 
of the noble metal material. 
After deposition, portions 34 and 36 are preferably annealed to promote 
their interdiffusion, thereby providing barrier region 38. Region 38 is 
defined as the material between lower dashed line 42 and upper dashed line 
44. Line 42 may be positioned anywhere within adhesion metal layer 34 
including a position at interface 46 between oxide layer 32 and adhesion 
metal portion 34. Similarly, line 44 may be positioned up to interface 48 
between first noble metal portion and second noble metal layer 40, within 
layer 40, or within layer 26. Interface 50 is positioned between adhesion 
metal portion 34 and first noble metal portion 36, and represents an 
interlayer boundary at a time prior to the diffusion that forms barrier 
region 38. Interface 50 is not necessarily observed in the final structure 
of bottom electrode 20, but the interface may be discernable even after 
some diffusible moieties have crossed between portions 34 and 36. In most 
circumstances, barrier region 38 extends fully through first noble metal 
portion 36 to interface 48, but only partially, if at all, into adhesion 
metal layer 34. Diffusion occurs by Fick's Law, but is also influenced by 
gravitational forces as a function of density differences between the 
respective metals. 
FIG. 2 depicts barrier region 38 in more detail. The dark circles, e.g., 
circle 52, represent adhesion metal atoms of adhesion metal portion 34. 
The white circles, e.g., circle 54, are noble metal atoms of first noble 
metal portion 36. Substantially no diffusion of noble metal atoms has 
occurred into layer 34 below line 42. Substantially no diffusion of 
adhesion metal atoms has occurred into layer 36 above line 44. Region 38 
includes a mixture of adhesion metal atoms and noble metal atoms. The 
interdiffusion of portions 34 and 36 serves to increase the stability of 
the lattice, which correspondingly enhances resistance against diffusion 
through or from region 38. It should be understood that region 32, as 
depicted, contains a non-homogenous distribution of adhesion metal atoms 
and noble metal atoms. Longer annealing times and higher annealing 
temperatures will serve to increase the homogeneity of the atomic 
distribution within region 38, and this distribution can be substantially 
homogenous throughout both portions 34 and 36. Other diffused moieties may 
be present, such as the silicon dioxide from layer 32 that is represented 
by silicon atom 56 (dashed lines) and oxygen atoms 58 (dots). 
Moderate-to-low dielectric silicon or silicon dioxide of the type exhibited 
as atoms 56 and 56 may sometimes migrate through the respective 
metalization layers (portions 34 and 36) as a bloom or slug of material. 
If these blooms succeed in migrating through to a high dielectric 
material, e.g., layer 26, the lower dielectric silicon dioxide can 
significantly impair the high-dielectric performance. The lattice that is 
depicted in region 38 is schematic in nature. The precise lattice 
structure of region 38 may vary, and it may be different from the diagram 
as shown. 
Second noble metal layer 40 is preferably made out of the same noble metal 
as first noble metal portion 36. This noble metal is most preferably 
platinum having a thickness of about 1000 .ANG., i.e., a preferred 
thickness equal to the thickness of noble metal portion 36. Upon annealing 
of layer 40, it is possible for the upper boundary of barrier region 38 to 
move upward into layer 40 and across interface 48; however, in this 
circumstance, the concentration of diffusible moieties within layer 40 is 
greatly reduced or even negligible with respect to the concentration in 
portion 36. The formation of second noble metal layer 40 completes the 
structure of bottom electrode 24. 
Adhesion metal portion 34 and noble metal portion 36 are annealed to form 
barrier region 38 prior to the deposition of second noble metal layer 40. 
An explanation of the general theoretical principles involved will 
facilitate an understanding of the invention. 
A coefficient of diffusion is defined phenomenologically by Fick's Law 
EQU J=-D (dn/dx), 
wherein J is the molecular flux along a concentration gradient (dn/dx), and 
D is the diffusion coefficient. Diffusion through solids is normally a 
very slow process; however, in thin-film materials, the intralayer atom 
migration distances are also very small. During the annealing process, 
flux is accelerated as the elevated temperature promotes a corresponding 
increase in random atomic motions. These motions promote diffusion as flux 
in a net direction from a high concentration towards a low concentration 
of a given element. 
Once the interdiffuse moieties of portions 34 and 36 have cooled, the 
resultant lattice has an increased thermodynamic stability because the 
intermixed noble metal and adhesion metal atoms bond together with 
increased force as compared to the respective pure metal layers. The 
increased force presents an energy barrier to atomic or molecular 
diffusion through the stabilized lattice. In this manner, post-anneal 
diffusion is significantly retarded or altogether eliminated. Therefore, 
second noble metal layer 40 has a significantly reduced flux of diffusible 
moieties, as compared to that of barrier region 38, because layer 40 is 
formed in an essentially pure condition or one having a much lower 
concentration of diffusible moieties. Actual results are also influenced 
by gravitational forces and the orientation of the substrate during the 
anneal. The substrate is preferably annealed in a level orientation. 
If second noble metal layer 40 is merely deposited atop first noble metal 
portion 36 without first annealing portions 34 and 36, then no region 38 
lattice exists to retard the diffusion of adhesion metal atoms throughout 
layer 40. The adhesion metal atoms then exist at higher concentrations 
immediately adjacent to metal oxide layer 26, and diffuse into metal oxide 
layer 26 as contaminants at correspondingly higher rates. These higher 
rates are most significant during the annealing process steps. 
Metal oxide layer 26 is a dielectric or ferroelectric material that 
preferably has a perovskite or a perovskite-like layered superlattice 
structure. The preferred dielectric materials include liquid-deposited 
ABO.sub.3 perovskites. Particularly preferred perovskites include BST. The 
most preferred BST has an average empirical formula of Ba.sub.0.7 
Sr.sub.0.3 TiO.sub.3, which is non-ferroelectric and functions as a 
thin-film high dielectric material. In BST, barium and strontium are the 
A-site elements, and titanium is the B-site element. The preferred 
ferroelectric materials include perovskite-like layered superlattice 
materials. Particularly preferred layered superlattice materials include 
strontium bismuth tantalate. The most preferred strontium bismuth 
tantalate has an average empirical formula of SrBi.sub.2 Ta.sub.2, 
O.sub.9. 
The ABO.sub.3 structure may be composed of respective A and B elements 
having different valences. These A and B site elements are coupled as a 
ternary oxide. Known A-B ternary oxides include A.sup.+1 B.sup.+5 
materials (e.g., potassium niobate), A.sup.+2 B.sup.+4 materials (e.g., 
strontium titanate or barium titanate), A.sup.+3 B.sup.+3 materials 
(e.g., gadolinium iron oxide), complex oxides of the type A.sup.+2 
(B.sup.+3.sub.0.7 B.sup.+6.sub.0.3) e.g., Sr(Cr.sub.0.67 Re.sub.0.33)!, 
complex oxides of the type A.sup.+2 (B.sup.+3 .sub.0.5 B.sup.+5.sub.0.5), 
and numerous other complex oxides. 
Many high-dielectric perovskites are ferroelectrics, and many 
perovskite-like layered superlattice ferroelectrics are also high 
dielectrics. Non-ferroelectric dielectrics are preferred for use in 
dielectric applications because the ferroelectric polarization switching 
phenomenon also stores and releases current, which can sometimes produce 
surges that may interfere with the operation of other portions of 
integrated circuits, e.g., control logic circuits. 
The presently claimed invention provides significant polarization 
improvements for ferroelectric materials and, particularly, for layered 
superlattice ferroelectrics. Layered superlattice materials at least 
include all three of the Smolenskii-type ferroelectric layered 
superlattice materials, namely, those having the respective average 
empirical formulae: 
EQU A.sub.m-1 S.sub.2 B.sub.m O.sub.3m+3 ; (1) 
EQU A.sub.m+1 B.sub.m O.sub.3m+1 ; and (2) 
EQU A.sub.m B.sub.m O.sub.3m+2, (3) 
wherein A is an A-site metal in the perovskite-like superlattice, B is a 
B-site metal in the perovskite-like superlattice, S is a trivalent 
superlattice-generator metal such as bismuth or thallium, and m is a 
number sufficient to balance the overall formula charge. Where m is a 
fractional number in the overall formula, the formula typically provides 
for a plurality of different or mixed perovskite-like layers each having a 
different integer value. The A-site metals and B-site metals may include 
mixtures of cations having similar ionic radii. 
In layered superlattice materials according to Formula (1), thermodynamics 
favor the formation of oxygen octahedra structures in layers having a 
thickness of m octahedra according to the formula 
EQU (A.sub.m-1 B.sub.m O.sub.3m+1).sup.2-, (4) 
wherein m is an integer greater than one and the other variables are 
defined above. These layers are separated by bismuth oxide layers having 
the formula 
EQU (Bi.sub.2 O.sub.2).sup.2+, (5) 
wherein Bi is S of Formula (1). 
The superlattice-generator layers, S, include oxides of bismuth (III), and 
may also include other similarly sized trivalent metal cations such as 
thallium (III). Bismuth also functions as an A-site metal in the 
perovskite-like lattice if it is present in excess of the 
stoichiometrically required amount for generating the layered superlattice 
material according to Formula (I). 
Top electrode 28 is preferably a noble metal that is sputtered over metal 
oxide layer 26. The thickness of top electrode 28 preferably ranges from 
about 1000 .ANG., to about 2000 .ANG., but the thickness can be a value 
outside of this range. Electrode 28 is most preferably made of platinum. 
FIG. 3 depicts a flow chart of a process for making capacitor 20. The 
process shall be discussed in terms of the embodiment of FIG. 1, but those 
skilled in the art will understand its applicability to other embodiments. 
In step P60, a silicon wafer is prepared as substrate 22 having silicon 
layer 30 and silicon dioxide layer 32. Silicon layer 30 can be baked under 
oxygen in a diffusion furnace at a temperature ranging from about 
500.degree. C. to about 1100.degree. C. to eliminate surface impurities 
and water, and form oxide coating 32. Generally, depending upon the nature 
of the device sought to be constructed, step P60 may also include 
conventional procedures such as the etching of contact holes (not 
depicted) and the doping of layer 30 (generally, substrate 22) for 
transistor or memory circuits. 
Step P62 includes sputtering titanium adhesion metal portion 34 atop oxide 
layer 32 to a preferred thickness ranging from about 50 .ANG., to 250 
.ANG.according to conventional protocols as are known in the art. Step P64 
includes sputtering a platinum first noble metal portion 38 to a preferred 
thickness ranging from about 1000 .ANG. to 2000 .ANG. atop portion 34. 
Examples of preferred atomic sputtering protocols include radio frequency 
sputtering and DC magnetron sputtering. 
Step P66 includes depositing a metal oxide precursor or SOG precursor atop 
noble metal portion 36. This precursor is preferably a liquid precursor, 
but may also be sputtered from a solid target. Commercially available SOG 
precursor solutions are preferred precursors. The most preferred precursor 
is the same precursor that will be applied in step P73 and, especially, a 
strontium bismuth tantalate layered superlattice material. The SOG 
precursor is preferably deposited to a thickness of about 1000 .ANG. while 
spinning the substrate at about 3000 rpm. Where the precursor of step P73 
is applied, the substrate will preferably be spun at about 1500 rpm to 
obtain a thickness of about 750 .ANG.. The liquid precursor is optionally 
dried at a temperature ranging from about 200.degree. C. to 500.degree. C. 
for a period of time ranging from about five minutes to about thirty 
minutes. 
Step P68 is referred to as the "first anneal" to distinguish it from other 
anneal steps; however, it should be understood that other anneal steps can 
occur prior to this "first anneal." For example, steps P60 and P62 each 
may include numerous annealing steps. In step P68, the substrate including 
portions 34 and 36 is preferably heated in a diffusion furnace under an 
oxygen atmosphere to a temperature ranging from 450.degree. C. to 
1000.degree. C. for a time ranging from 30 minutes to 2 hours. Step P68 is 
more preferably conducted at a temperature ranging from 600.degree. C. to 
800.degree. C., with the most preferred anneal temperature being about 
600.degree. C. for eighty minutes. The first anneal of step P68 preferably 
occurs in a push/pull process including five minutes for the "push" into 
the furnace and five minutes for the "pull" out of the furnace. The 
indicated anneal times include the time that is used to create thermal 
ramps into and out of the furnace. 
In a commercial manufacturing process, it will be advantageous to provide 
careful control of all annealing temperatures and times for purposes of 
providing consistent and reproducible results. Cooling of the substrate to 
room temperature yields barrier region 38 as generally depicted in FIGS. 1 
and 2. 
FIG. 4 depicts the structure of capacitor 20 through step P68. The device 
is capped by metal oxide or SOG layer 59. As depicted in FIG. 4, diffusion 
region boundary 44 (indicating titanium contamination) has crossed 
interface 48 into layer 59. The remaining layers of FIG. 4 are, otherwise, 
as described in FIG. 1. 
Step P70 includes removal of layer 59 by etching. A liquid etching 
technique is preferably employed. When layer 59 is SOG, it is preferably 
removed using a buffered oxide etchant, such as the 10:1 BOE solution that 
is available for this purpose from General Chemical of Parisippane, N.J. 
These solutions typically include a mixture of water and hydrofluoric acid 
with an ammonium fluoride buffer. The etch rate typically ranges from 
about 20 .ANG. to 25 .ANG. per second. Use of BOE etchant is not 
appropriate for layered superlattice materials, such as strontium bismuth 
tantalate. 
When layer 59 is strontium bismuth tantalate, the etchant preferably 
includes a mixture of HNO.sub.3, H.sub.2 O, and HF. These ingredients are 
mixed in the following volumetric proportions: 200 parts of 60% to 70% 
HNO.sub.3 (weight percent mixed with water); 80 parts distilled H.sub.2 O; 
and 3 parts of 46% to 49% HF (weight percent mixed with water). The etch 
rate typically ranges from 240 .ANG. to 300 .ANG. per minute at 20.degree. 
C., but process conditions may vary to provide a rate that ranges from 50 
.ANG. to 500 .ANG. per minute. 
The etchant is quenched by rinsing in a distilled water dump-rinser bath, 
and the wafer is preferably spun dry at about 1500 rpm. 
Step P72 includes the deposition of platinum second noble metal layer 40 to 
a preferred thickness ranging from about 1000 .ANG. to 2000 .ANG. over the 
annealed barrier region 38. This deposition preferably occurs by 
sputtering. Step P72 completes bottom electrode 24 (FIG. 1). FIG. 5 
depicts the structure of FIG. 4 having layer 59 removed by etching. 
Step P73 includes the preparation of a liquid precursor solution having a 
plurality of metal moieties in effective amounts for yielding metal oxide 
layer 26 upon drying and annealing of the precursor solution. Additional 
details pertaining to the preparation of this precursor solution will be 
provided below. 
In step P74, the precursor solution from step P73 is applied to the 
substrate from step P72, which presents the uppermost surface of bottom 
electrode 24 to receive the liquid precursor. This application is 
preferably conducted by dropping the liquid precursor solution at ambient 
temperature and pressure onto the uppermost surface of bottom electrode 24 
then spinning the substrate at from about 1500 RPM to 2000 RPM for about 
30 seconds to remove any excess solution and leave a thin-film liquid 
residue. The most preferred spin velocity is 1500 RPM. Alternatively, the 
liquid precursor may be applied by a misted deposition technique, such as 
the technique described in copending application Ser. No. 07/993,380, 
which is hereby incorporated by reference herein to the same extent as 
though fully disclosed herein. Preferred precursor solutions include those 
having metal moieties in effective amounts for yielding a layered 
superlattice material. The most preferred layered superlattice material is 
strontium bismuth tantalate. 
In step P75, the liquid precursor film from step P74 is preferably dried on 
a hot plate in a dry air atmosphere and at a temperature of from about 
200.degree. C. to 500.degree. C. The drying time and temperature should be 
sufficient to remove substantially all of the organic materials from the 
liquid thin film and leave a dried metal oxide residue. The drying time 
preferably ranges from about one minute to about thirty minutes. For 
single-stage drying, a 400.degree. C. drying temperature over a duration 
of about two to ten minutes in air is most preferred. It is more 
preferred, however, to dry the liquid film in stepped intervals. For 
example, the film can be dried for five minutes at 260.degree. C. and for 
five minutes at 400.degree. C. Additionally, it is preferred to conclude 
the drying cycle with a brief heating interval at a temperature exceeding 
700.degree. C., e.g., using a tungsten-nickel lamp to heat the substrate 
to 725.degree. C. for thirty seconds. The drying step P75 is essential in 
obtaining predictable or repeatable electronic properties in the final 
metal oxide crystal compositions. 
In step P76, if the resultant dried film from step P75 is not of the 
desired thickness, then steps P72, P74, and P75 are repeated until the 
desired thickness is attained. A thickness of about 1800 .ANG. to 2000 
.ANG. typically requires two coats of a 0.130M to 0.200M precursor 
solution under the parameters disclosed herein. 
In step P78, the dried precursor residue from steps P75 and P76 is annealed 
to form the metal oxide of layer 26. This annealing step is referred to as 
the second anneal to distinguish it from a other annealing steps. This 
second anneal is preferably conducted under conditions that are identical 
to the conditions of the first anneal in step P68. 
In step P80, top electrode 28 is preferably deposited by sputtering 
platinum atop metal oxide layer 26. The third anneal of step P82 is 
optional, and preferably occurs under conditions identical to the first 
anneal of step P68. 
The device is then patterned by a conventional photoetching process, e.g., 
including the application of a photoresist followed by ion etching 
lithography in step P84. This patterning preferably occurs before the 
fourth annealing of step P86 so that the fourth anneal will serve to 
remove patterning stresses from capacitor 20 and correct any defects that 
are created by the patterning procedure. 
The fourth annealing step, P86, is preferably conducted in like manner with 
the first anneal in step P68. 
Finally, in step P88 the device is completed and evaluated. The completion 
may entail the deposition of additional layers, ion etching of contact 
holes, and other procedures, as will be understood by those skilled in the 
art. Substrate or wafer 22 may be sawed into separate units to separate a 
plurality of integrated circuit devices that have been simultaneously 
produced thereon. 
A preferred general process for preparing the polyoxyalkylated metal 
precursors of step P73 is provided in application Ser. No. 08/132,744 
filed Oct. 6, 1993, which is hereby incorporated by reference herein to 
the same extent as though fully disclosed herein, and application Ser. No. 
07/965,190, which is hereby incorporated by reference herein to the same 
extent as though fully disclosed herein. The processes preferably include 
reacting a metal with an alkoxide (e.g., 2-methoxyethanol) to form a metal 
alkoxide, and reacting the metal alkoxide with a carboxylate (e.g., 
2-ethylhexanoate) to form a metal alkoxycarboxylate according to one of 
the generalized formulae: 
EQU (R'--COO--).sub.a M(--O--R).sub.n, or (6) 
EQU (R'--C--O--).sub.a M(--O--M'--(O--C--R").sub.b-1).sub.n, (7) 
wherein M is a metal cation having an outer valence of (a+n) and M' is a 
metal cation having an outer valence of b, with M and M' preferably being 
independently selected from the group consisting of tantalum, calcium, 
bismuth, lead, yttrium, scandium, lanthanum, antimony, chromium, thallium, 
hafnium, tungsten, niobium, vanadium, zirconium, manganese, iron, cobalt, 
nickel, magnesium, molybdenum, strontium, barium, titanium, and zinc; R 
and R' are respective alkyl groups preferably having from 4 to 9 carbon 
atoms and R" is an alkyl group preferably having from 3 to 8 carbon atoms. 
The latter formula, which has a central --O--M--O--M'--O-- structure, is 
particularly preferred due to the formation in solution of at least 50% of 
the metal to oxygen bonds that will exist in the final solid metal oxide 
product. 
The liquid precursor is preferably a metal alkoxide or metal carboxylate, 
and is most preferably a metal alkoxycarboxylate diluted with a xylene or 
octane solvent to a desired concentration. The use of an essentially 
anhydrous metal alkoxycarboxylate is particularly preferred due to the 
corresponding avoidance of water-induced polymerization or gelling, which 
can significantly reduce the shelf-life of solutions that contain alkoxide 
ligands. The presence of any hydrolysis-inducing moiety in solution is 
preferably avoided or minimized. Hydrolyzed precursors, such as 
conventional sol-gels, may also be utilized, but the increased solution 
viscosity tends to impair the uniformity of thickness derived from the 
preferred spin-on application process, and the quality of the hydrolyzed 
solution tends to degrade rapidly with time. As a consequence, made-ready 
hydrolyzed gels increasingly yield poor quality metal oxide films of 
inconsistent quality over a period of time. The preferred method permits 
the preparation of precursor solutions well in advance of the time that 
they are needed. 
The precursor solutions may be designed to yield corresponding layered 
superlattice materials or perovskites, with the understanding that the 
formation of oxygen octahedra structures is thermodynamically favored 
where possible. Generally, in terms of either the perovskite-like 
octahedral structure or the perovskite octahedral structure, equivalent 
substitutions may be made between metal cations having substantially 
similar ionic radii, i.e. radii that vary no more than about 20% at the 
respective lattice sites. These substitutions are made by adding the 
alternative metal moieties to the precursor solution. 
The preferred ingredients of the precursor solutions include the preferred 
metals of the desired perovskite or layered superlattice material in a 
stoichiometrically balanced combination according to the empirical 
formula. The A-site portion is preferably formed by reacting with an 
alcohol or carboxylic acid at least one A-site element selected from an 
A-site group consisting of Ba, Bi, Sr, Pb, La, Ca, and mixtures thereof. 
The B-site portion is preferably derived by reacting an alcohol or 
carboxylic acid with at least one B-site element selected from a B-site 
group consisting of Zr, Ta, Mo, W, V, Nb, and mixtures thereof. The use of 
titanium as an equivalent radius B-site element, though possible, is less 
preferred in practice due to problems that derive from titanium diffusion 
into other integrated circuit components and point charge defects that 
arise from different valence states among the titanium ions. Even so, the 
exceptional dielectric performance and longevity of BST materials makes it 
worthwhile to accommodate the possibility of titanium diffusion. In the 
case of layered superlattice materials, there is also added a trivalent 
superlattice-generator metal, which is preferably bismuth. With heating, 
the bismuth content will spontaneously generate bismuth oxide layers in 
the layered superlattice materials, but an excess bismuth portion can also 
provide A-site elements for the perovskite-like lattice. 
FIG. 6 depicts a flow chart of a generalized process according to the 
present invention for providing a liquid precursor solution to be used in 
step P72. The word "precursor" is often used ambiguously in this art. It 
may mean a solution containing one metal that is to be mixed with other 
materials to form a final solution, or it may mean a solution containing 
several metals made-ready for application to a substrate. In this 
discussion we shall refer to the made-ready type of precursor as a 
"precursor," unless a different meaning is clear from the context. In 
intermediate stages the solution may be referred to as the "pre-precursor. 
" 
The preferred generalized reaction chemistry for the formation of liquid 
solutions of metal alkoxides, metal carboxylates, and metal 
alkoxycarboxylates for use in producing the initial metal precursor 
portions is as follows: 
EQU alkoxides--M.sup.+n +n R--OH .fwdarw. M(--O--R).sub.n +n/2 H.sub.2(8) 
EQU carboxylates--M.sup.+n +n (R--COOH) .fwdarw. M(--OOC--R).sub.n +n/2 
H.sub.2(9) 
EQU alkoxycarboxylates--M(--O--R').sub.n +b R--COOH+heat .fwdarw. 
(R'--O--).sub.n-b M(--OOC--R).sub.b +b HOR, (10) 
where M is a metal cation having a charge of n; b is a number of moles of 
carboxylic acid ranging from 0 to n; R' is preferably an alkyl group 
having from 4 to 15 carbon atoms and R is preferably an alkyl group having 
from 3 to 9 carbon atoms. 
In step P90 a first metal, indicated by the term M in the equations above, 
is reacted with an alcohol and a carboxylic acid to form a 
metal-alkoxycarboxylate pre-precursor. The process preferably includes 
reacting a metal with an alcohol (e.g., 2-methoxyethanol) to form a metal 
alkoxide according to Equation (8), and reacting the metal alkoxide with a 
carboxylic acid (e.g., 2-ethylhexanoic acid) to form a metal 
alkoxycarboxylate according to Equation (10). A reaction according to 
Equation (9) is also observed in the preferred mode when the unreacted 
metal is simultaneously combined with the alcohol and the carboxylic acid. 
The simultaneous reactions are preferably conducted in a reflux condenser 
that is heated by a hot plate having a temperature ranging from about 
120.degree. C. to about 200.degree. C. over a period of time ranging from 
one to two days to permit substitution of the alkoxide moieties by 
carboxylate ligands. At the end of the initial one to two day reaction 
period, the reflux condenser is preferably opened to atmosphere, and the 
solution temperature is monitored to observe a fractional distillation 
plateau that indicates the substantial elimination of all water and 
alcohol portions from the solution, i.e., a plateau exceeding at least 
about 100.degree. C., at which time the solution is removed from the heat 
source. Distillation to atmospheric pressure is more preferably is 
conducted to a temperature of at least 115.degree. C., and most preferably 
to a temperature of about 123.degree. C. to 127.degree. C. 
In the above equations, the metal is preferably selected from the group 
consisting of tantalum, calcium, bismuth, lead, yttrium, scandium, 
lanthanum, antimony, chromium, thallium, hafnium, tungsten, vanadium, 
niobium, zirconium, manganese, iron, cobalt, nickel, magnesium, 
molybdenum, strontium, barium, titanium, vanadium, and zinc. Alcohols that 
may be used preferably include 2-methoxyethanol, 1-butanol, 1-pentanol, 
2-pentanol, 1-hexanol, 2-hexanol, 3-hexanol, 2-ethyl-1-butanol, 
2-ethoxyethanol, and 2-methyl-1-pentanol, preferably 2-methoxyethanol. 
Carboxylic acids that may be used preferably include 2-ethylhexanoic acid, 
octanoic acid, and neodecanoic acid, preferably 2-ethylhexanoic acid. 
The reactions of step P90 and subsequent steps are preferably facilitated 
by the use of a compatible solvent. Solvents that may be used include 
xylenes, 2-methoxyethanol, n-butyl acetate, n-dimethylformamide, 
2-methoxyethyl acetate, methyl isobutyl ketone, methyl isoamyl ketone, 
isoamyl alcohol, cyclohexanone, 2-ethoxyethanol, 2-methoxyethyl ether, 
methyl butyl ketone, hexyl alcohol, 2-pentanol, ethyl butyrate, 
nitroethane, pyrimidine, 1, 3, 5 trioxane, isobutyl isobutyrate, isobutyl 
propionate, propyl propionate, ethyl lactate, n-butanol, n-pentanol, 
3-pentanol, toluene, ethylbenzene, and octane, as well as many others. 
These solvents preferably have boiling points exceeding that of water for 
purposes of distilling the precursor to eliminate water therefrom prior to 
application of the precursor to a substrate. The cosolvents should be 
miscible with one another and may be compatibly mixed in differing 
proportions, especially between polar and apolar solvents, as needed to 
fully solubilize the precursor ingredients. Xylenes and octane are 
particularly preferred apolar solvents, and n-butyl acetate is a 
particularly preferred polar cosolvent. 
Portions of step P90 can be skipped in the event that intermediate metal 
reagents can be obtained in research grade purity. For example, where 
tantalum isobutoxide is available, it will only be preferred to substitute 
the isobutoxide moiety with an acceptable carboxylate ligand by reacting 
the metal alkoxide with a carboxylic acid such as 2-ethylhexanoic acid 
according to Equation (10). 
In a typical second step, P92, a metal-carboxylate, a metal-alkoxide or 
both may be added to the metal-alkoxycarboxylate in effective amounts to 
yield an intermediate precursor having a stoichiometrically balanced 
mixture of superlattice-forming metal moieties that is capable of yielding 
a solid metal oxide for layer 26. At this time the mixture will preferably 
exclude bismuth compounds which, if needed, will be added later due to 
their relative thermal instability. Any of the metals listed above may be 
reacted with any of the carboxylic acids listed above to form the metal 
carboxylate, while any of the metals listed above may be reacted with any 
of the alcohols may form the alkoxide. It is particularly preferred to 
conduct this reaction in the presence of a slight excess amount of 
carboxylic acid for purposes of partially substituting alkoxide ligands 
with carboxylate ligands. 
In step P94 the mixture of metal-alkoxycarboxylates, metal-carboxylates 
and/or metal-alkoxides is heated and stirred as necessary to form 
metal-oxygen-metal bonds and boil off any low-boiling point organics that 
are produced by the reaction. According to a generalized reaction theory, 
if a metal-alkoxide is added to the metal-alkoxycarboxylate, and the 
solution is heated, the following reactions occur: 
EQU (R--COO--).sub.x M(--O--C--R').sub.a +M'(--O--R").sub.b .fwdarw. 
(R--COO--).sub.x M--O--M'(--O--C--R").sub.b-1).sub.a +a R'--C--O--C--R"(11 
) 
EQU (R--COO--).sub.x M(--O--C--R').sub.a +x M'(--O--C--R").sub.b .fwdarw. 
(R'--C--O--).sub.a M(--O--M'(--O--C--R").sub.b-1).sub.x +x 
R--COO--C--R"(12) 
where M and M' are metals; R and R' are defined above; R" is an alkyl group 
preferably having from about zero to sixteen carbons; and a, b, and x are 
integers denoting relative quantities of corresponding substituents 
corresponding to the respective valence states of M and M'. Generally the 
reaction of Equation (11) will occur first since metal alkoxides react 
more readily than metal carboxylates. Thus, ethers having low boiling 
points are generally formed. These ethers boil out of the pre-precursor to 
leave a final product having a reduced organic content and the 
metal-oxygen-metal bonds of the final desired metal oxide already 
partially formed. If the heating is sufficient, some of the reaction (12) 
will also occur, creating metal-oxygen-metal bonds and esters. Esters 
generally have higher boiling points and remain in solution. These high 
boiling point organics slow down the drying process after the final 
precursor is applied to a substrate, which tends to reduce cracking and 
defects; thus, in either case, metal-oxygen-metal bonds are formed and the 
final precursor performance is improved. 
Step P94 is in essence a distillation to eliminate volatile moieties from 
solution as the reactions of equations (11) and (12) proceed. The 
elimination of volatile moieties from solution drives the reactions to 
completion, i.e., a high rate of efficiency. The elimination of volatile 
moieties from solution also serves to prevent film cracking and other 
defects that, otherwise, can be associated with the presence of volatile 
moieties in solution. Accordingly, the progress of reactions (11) and (12) 
can be monitored by the rate of solution heating as well as the volume of 
fluid exiting the solution. It is preferred to heat the solution to a 
boiling point plateau of at least 115.degree. C., more preferably to 
120.degree. C., and most preferably from 123.degree. C. to 127.degree. C. 
If a metal-carboxylate is added to the metal-alkoxycarboxylate and the 
mixture is heated, the following reaction occurs: 
EQU (R--COO--).sub.x M(--O--C--R').sub.a +x M'(--OOC--R").sub.b .fwdarw. 
(R'--C--O--).sub.a M(--O--M'(--OOC--R").sub.b-1).sub.x +x R--COOOC--R'(13) 
where R--COOOC--R' is an acid anhydride, and the terms are as defined 
above. This reaction requires considerably more heat than do the reactions 
(11) and (12) above, and proceeds at a much slower rate. 
In addition to the above reactions which produce metal-alkoxycarboxylates, 
reactions occur such as: 
EQU M(--OR).sub.a +a HO.sub.2 C.sub.8 H.sub.15 +heat .fwdarw. M(--O.sub.2 
C.sub.8 H.sub.15).sub.a +a HOR, (14) 
where the terms are as defined above. This reaction, with heating in the 
presence of excess carboxylic acid, substitutes the alkoxide part of the 
intermediate metal-alkoxycarboxylate to form a substantially full 
carboxylate; however, it is now believed that a complete substitution of 
the alkoxides by the carboxylates does not occur with the parameters as 
disclosed herein. Full substitution of the carboxylates requires 
significantly more heating, and even then may not readily occur. 
At the end of step P94, it is preferable to have formed in solution at 
least 50% of the metal to oxygen bonds of the metal oxide layer 26. The 
reactions are preferably conducted in a vessel that is open to atmospheric 
pressure and is heated by a hot plate preferably having a temperature 
ranging from about 120.degree. 0 to about 200.degree. C. until the 
solution temperature is monitored to observe a fractional distillation 
plateau that indicates the substantial elimination of all water, alcohol, 
ether, and other reaction byproduct portions from the solution, i.e., a 
plateau at least exceeding 100.degree. C. At this time, extended refluxing 
can produce a potentially undesirable amount of an ester or acid anhydride 
byproduct that is often difficult to remove from the solution by 
fractional distillation. The potential complication of an excessive acid 
anhydride concentration can be entirely avoided by adding only metal 
carboxylates in step P92 to eliminate a possible need for refluxing the 
solution. 
Step P96 is an optional solvent exchange step. The use of a common solvent 
in a variety of precursor solutions is advantageous due to the 
predictability of fluid parameters such as viscosity and adhesion tension, 
which influence the thickness of the liquid precursor film after it is 
applied to a substrate. These fluid parameters also affect the quality and 
the electrical performance of the corresponding metal oxide film after 
annealing of the dried precursor residue. In step P96, the standard 
solvent, which is preferably xylenes or n-octane, is added in an amount 
that is appropriate to adjust the intermediate precursor to a desired 
molarity of superlattice ingredients. This molarity preferably ranges from 
about 0.100M to about 0.400M in terms of the empirical formula for the 
metal oxide material, and is most preferably about 0.130M to 0.200M in 
terms of moles of metal oxide material that may be formed from a liter of 
solution. After the addition of the standard solvent, the solution is 
heated to a temperature that is sufficient to distill away any 
non-standard solvents and leave a solution having the desired molarity. 
Step P98 is preferably used only in the case of precursors for layered 
superlattice materials that include bismuth. Bismuth (Bi.sup.3+) is the 
most preferred superlattice-generator element, and the bismuth 
preprecursor will most preferably be bismuth tri-2-ethylhexanoate. The 
addition of bismuth pre-precursors subsequent to the heating of step P94 
is preferred due to the relative instability of these pre-precursors, 
i.e., substantial heating could disrupt coordinate bonds with potential 
deleterious effects upon the ability of the solution to yield superior 
thin-film metal oxides. It should be understood that step P98 is optional 
in the sense that bismuth pre-precursors can often be added in any of 
steps P90 and P94 without problems. 
Special problems exist with regard to the potential for bismuth 
volatilization during heating of the precursor solution and, especially, 
during high temperature annealing of the dried precursor residue in 
forming a layered superlattice material of the desired stoichiometric 
proportions. Accordingly, in step P98, it is preferred to add from about 
5% to about 15% excess bismuth for purposes of compensating the precursor 
solution for anticipated bismuth losses. At annealing temperatures ranging 
from about 600.degree. C. to about 850.degree. C. for a period of about 
one hour, this excess bismuth moiety in the precursor solution will 
typically range from 5% to 15% of the amount that is required for a 
stoichiometrically balanced layered superlattice product. In the event 
that the excess bismuth is not fully volatilized during the formation of a 
metal oxide product, the remaining excess bismuth moiety can act as an 
A-site material and, thus, induce point defects in the resulting layered 
superlattice crystal. 
In step P100, the solution is mixed to substantial homogeneity, and is 
preferably stored under an inert atmosphere of desiccated nitrogen or 
argon if the final solution will not be consumed within several days or 
weeks. This precaution in storage serves to assure that the solution is 
kept essentially water-free and avoids the deleterious effects of 
water-induced polymerization, viscous gelling, and precipitation of 
metallic moieties that water can induce in alkoxide ligands. Even so, the 
desiccated inert storage precaution is not strictly necessary when the 
precursor, as is preferred, primarily consists of metals bonded to 
carboxylate ligands and alkoxycarboxylates. 
The exemplary discussion of the reaction process, as given above, above is 
generalized and, therefore, non-limiting. The specific reactions that 
occur depend on the metals, alcohols, and carboxylic acids used, as well 
as the amount of heat that is applied. Detailed examples will be given 
below. 
The following non-limiting examples set forth preferred materials and 
methods for practicing the present invention. 
EXAMPLE 1 
Preparation of a BST Precursor Solution 
A detailed example of the process of preparing the precursors used to 
deposit BST is given below. Referring to FIG. 6, in step P90, the 
compounds shown in Table I were measured. 
TABLE 1 
______________________________________ 
Major Reagents For A Precursor Capable of Producing 
Barium Strontium Titanate (BST) - Ba.sub.0.7 Sr.sub.0.3 TiO.sub.3 
Compound FW g mmole Equiv. 
______________________________________ 
Barium 137.33 9.4255 68.635 
0.69986 
2-ethylhexanoic 
144.21 19.831 137.51 
1.4022 
acid 
Strontium 87.62 2.5790 29.434 
0.30014 
2-ethylhexanoic 
144.21 8.5005 88.945 
0.6010 
acid 
Titanium Isoprop- 
284.26 27.878 98.072 
1.0000 
oxide 
______________________________________ 
In Table 1, "FW" indicates formula weight, "g" indicates grams, "mmoles" 
indicates millimoles, and "Equiv." indicates the equivalent number of 
moles in solution. 
The barium was placed in 100 ml (milliliters) of 2-methoxyethanol. The 
2-ethylhexanoic acid was added and the mixture was allowed to react while 
stirring. This step may also be performed by placing the barium in the 
2-methoxyethanol, allowing it to react, adding the 2-ethylhexanoic acid, 
and stirring while the mixture reacts. Hydrogen gas was a by-product of 
the reaction and escaped from the solution. The exothermic reaction of the 
barium heated the solution. While the solution was still hot, the 
strontium was added and allowed to react. The heat in solution from the 
barium reaction assisted the strontium reaction. When the strontium was 
all reacted, the second measure of 2-ethylhexanoic acid was added, and the 
solution was stirred while heating for several hours at a maximum 
temperature of 115.degree. C. This ensured that any water present was 
distilled out. 
The mixture was then allowed to cool, and in step P92 the titanium 
isopropoxide was added followed by the addition of enough 2-methoxyethanol 
to make 220 ml of total solution. The solution was then heated and 
stirred. Ethers and some esters could be smelled during this reaction. The 
ethers were low boiling point liquids and generally boiled out of solution 
while the esters were higher boiling point liquids, which tended to remain 
in solution. During the heating, the maximum temperature was 116.degree. 
C., which also ensured that substantially all isopropanol and water were 
distilled out. The resultant solution was diluted to 200 ml total solution 
with additional 2-methoxyethanol. The result was a final BST precursor 
having a 0.490 molar concentration with the ratio of barium to strontium 
equalling 0.69986: 0.30014. The solution was then ready for use in a BST 
coating process for thin-film capacitor formation, per step P73 of FIG. 3. 
EXAMPLE 2 
Preparation of a Layered Superlattice Precursor Solution 
The precursor ingredients of Table 1 were obtained from the indicated 
commercial sources and subdivided to obtain the portions shown. 
TABLE 1 
______________________________________ 
Formula 
Weight Molar 
Ingredient (g/mol) Grams mmole Equiv. 
Vendor 
______________________________________ 
tantalum 546.52 43.722 80.001 
2.0000 
Vnipim 
pentabutoxide 
Ta(OC.sub.4 H.sub.9).sub.5 
2-ethylhexanoic acid 
144.21 72.684 504.01 
12.600 
Aldrich 
strontium 87.62 3.5048 40.000 
1.0000 
Strem 
bismuth tri-2-ethyl- 
(765.50) 66.752 87.201 
2.1800 
Strem 
hexanoate (in naptha) 
Bi(O.sub.2 C.sub.6 H.sub.11).sub.5 
______________________________________ 
The tantalum pentabutoxide and 252.85 mmol portion of 2-ethylhexanoic acid 
were placed in a 250 ml Edenmeyer flask with 40 ml of xylenes, i.e., about 
50 ml xylenes for each 100 mmol of tantalum. The flask was covered with a 
50 ml beaker to assist in refluxing and to isolate the contents from 
atmospheric water. The mixture was refluxed with magnetic stirring on a 
160.degree. C. hot plate for 48 hours to form a substantially homogenous 
solution including butanol and tantalum 2-ethylhexanoate. It should be 
understood that the butoxide moiety in solution was almost completely 
substituted by the 2-ethylhexanoic acid, but full substitution did not 
occur within the heating parameters of this example. At the expiration of 
48 hours, the 50 ml beaker was removed and the hot plate temperature was 
then raised to 200.degree. C. for distillation of the butanol fraction and 
water to eliminate the same from solution. The flask was removed from the 
hot plate when the solution first reached a temperature of 124.degree. C., 
as a temperature indicator that substantially all butanol and water had 
exited the solution. The flask and its contents were cooled to room 
temperature. 
The strontium and 50 ml of 2-methoxyethanol solvent were added to the 
cooled mixture for reaction to form strontium di-2-ethylhexanoate. A 100 
ml portion of xylenes was added to the strontium mixture, and the flask 
and its contents were returned to the hot plate at 200.degree. C. and 
refluxed for five hours with the 50 ml beaker again in place for reaction 
to form a predominant tantalum-strontium alkoxycarboxylate product 
according to Formula (6). The beaker was removed and the solution 
temperature was allowed to rise to 125.degree. C. for elimination of the 
2-methoxyethanol solvent from solution, as well as any ethers, alcohols, 
or water in solution. After removal from the heat source, the flask was 
permitted to cool to room temperature. The bismuth tri-2-ethylhexanoate 
was added to the cooled solution, which was further diluted to 200 ml with 
xylenes to form a precursor solution that was capable of forming 0.200 
moles of SrBi.sub.2.18 Ta.sub.2 O.sub.9.27 in the absence of bismuth 
volatilization. 
Accordingly, this example indicates that step P90, i.e., the reaction of 
strontium metal, an alcohol, and a carboxylic acid, can occur in solution 
with the tantalum alkoxycarboxylate derived from tantalum pentabutoxide 
and 2-ethylhexanoic acid. Therefore, steps P90 and P92 can be conducted in 
a single solution, and in reverse of the FIG. 6 sequence. 
The precursor formulation was designed to compensate for bismuth 
volatilization during a process of manufacturing solid metal oxides from 
the liquid precursor. Specifically, the Bi.sub.2.18 moiety included an 
approximate nine percent excess (0.18) bismuth portion. After accounting 
for the anticipated bismuth volatilization during the forthcoming 
annealing steps, the precursor solution would be expected to yield a 
stoichiometric m=2 material according to Formula (3), i.e., 0.2 moles of 
SrBi.sub.2 Ta.sub.2 O.sub.9 per liter of solution. 
EXAMPLE 3 
Formation of Ferroelectric Capacitors Using a Temporary Capping Layer and 
Coating an Annealed Bottom Electrode with Additional Platinum Metal 
A ferroelectric capacitor of the type depicted in FIG. 1 was produced 
according to the general method of FIG. 3. A conventional four inch 
diameter polycrystalline wafer or substrate 30 was prepared to receive the 
SrBi.sub.2 Ta.sub.2 O.sub.9 solution of Example 2. The preparation process 
included diffusion furnace baking at 1100.degree. C. in oxygen according 
to conventional protocols for yielding a thick layer of silicon oxide 32 
(see FIG. 1). 
The substrate including oxide 32 was cooled to room temperature, and 
inserted into a vacuum chamber for conventional DC magnetron sputtering. A 
discharge voltage of 95 volts and a current of 0.53 amperes was utilized 
at a sputter pressure of 0.0081 Torr to sputter a 100 .ANG. thickness of 
titanium metal as adhesion metal portion 34 on oxide layer 32. A discharge 
voltage of 130 volts and a current of 0.53 amperes was then used to 
sputter a 1000 .ANG. thickness of platinum atop the titanium metal. 
A 2 ml volume of a 0.2M SrBi.sub.2 Ta.sub.2 O.sub.9 precursor prepared 
according to Example 2 was adjusted to a 0.13M concentration by the 
addition of 1.08 ml n-butyl acetate and passed through a 0.2 .mu.m filter. 
An eyedropper was used to apply 2 ml of precursor solution to the 
substrate, which was then spun at 1500 rpm in a conventional spin-coater 
machine. These actions completed the process of FIG. 3 through step P66. 
The liquid precursor film was not dried prior to the first anneal of step 
P68. 
The substrate including the liquid thin film was annealed for thirty 
minutes at a temperature of 600.degree. C. in a diffusion furnace under an 
oxygen atmosphere. This time included a five minute push into the furnace 
and a five minute pull out of the furnace. The resultant structure 
included layer 59 as depicted in FIG. 4. These actions completed step P68 
Layer 59 was removed in step P70 by contacting layer 59 with a liquid 
etching solution under an air atmosphere for two minutes at 20.degree. C. 
The etching solution contained a mixture of 70% (w/w) HNO.sub.3, H.sub.2 
O, and 49% (w/w) HF in the volumetric proportions of 200:80:3, as 
described above. The etchant was quenched in a distilled water dump 
rinser, and the substrate was spun dry at 1500 rpm. 
A 1000 .ANG. thickness of platinum was sputtered as layer 40 (FIG. 1) under 
the same sputtering conditions as before. 
A 2 ml volume of the 0.2M SrBi.sub.2 Ta.sub.2 O.sub.9 precursor from 
Example 2 was adjusted to a 0.13M concentration by the addition of 1.08 ml 
n-butyl acetate and passed through a 0.2 .mu.m filter. An eyedropper was 
used to apply 2 ml of precursor solution to the substrate, which was then 
spun at 1500 rpm in a conventional spin-coater machine as before. The 
precursor-coated substrate was removed from the spin-coating machine and 
dried in air for two minutes on a 140.degree. C. hot plate. The substrate 
was dried for an additional four minutes on a second hot plate at 
260.degree. C. The substrate was dried for an additional thirty seconds in 
oxygen at 725.degree. C. using a 1200 W tungsten-halogen lamp (visible 
spectrum; Heatpulse 410 by AG Associates, Inc., using J208V bulbs by Ushio 
of Japan). The spin-coating and drying procedure was repeated a second 
time to increase the overall thickness of layer 26 to about 1800 .ANG.. 
Substrate 22 including the two coats of dried precursor residue was 
annealed in a diffusion furnace under an oxygen atmosphere to a 
temperature of 800.degree. C. for seventy minutes including a five minute 
push into the furnace and a five minute pull out of the furnace. These 
actions completed the process of FIG. 3 through step P78. 
Platinum metal was sputtered as top electrode 28 to a 1000 .ANG. thickness 
of platinum over layer 26. A photoresist was applied and ion etched 
according to conventional protocols including removal of the resist. The 
patterned device was annealed in a diffusion furnace under an oxygen 
atmosphere at 800.degree. C. for thirty minutes including a five minute 
push into the furnace and a five minute pull out of the furnace. These 
actions completed the process of FIG. 3 through step P88, and provided a 
ferroelectric capacitor of the type depicted in FIG. 1 having strontium 
bismuth tantalate as layer 26. 
This process was repeated in an identical manner, except a standard SOG 
solution from General Chemical of Parisippane, N.J., was substituted for 
the precursor of Example 2 in step P66. The liquid SOG was applied while 
spinning the substrate at 3000 rpm in the spin-coater machine. The 
precursor of Example 2 continued to be used in step P74. 
EXAMPLE 4 
Formation of Ferroelectric Capacitors Using a Temporary Capping Layer and 
without Coating an Annealed Bottom Electrode with Additional Platinum 
Metal 
A wafer including strontium bismuth tantalate capacitors of the type 
depicted in FIG. 1 was produced according to a different method than that 
of FIG. 3. The procedure was identical to that of Example 3, with these 
exceptions: (1) a 2000 .ANG. thickness (not 1000 .ANG.) of platinum was 
sputtered as first noble metal portion 36 in step P64; and (2) step P72 
was never performed. Thus, the final product appeared identical to that of 
FIG. 1, but the 2000 .ANG. thicknesses of bottom electrode platinum 
comprising portions 36 and 40 were deposited as a single layer. In the 
first capacitor, layer 59 of FIG. 4 was deposited as strontium bismuth 
tantalate and removed (steps P66-P70), as in Example 3. In a second 
capacitor prepared according to this revised method, layer 59 was 
deposited as SOG and removed, as in Example 3. 
EXAMPLE 5 
Formation of Ferroelectric Capacitors without Using A Temporary Capping 
Layer and without Coating an Annealed Bottom Electrode with Additional 
Platinum Metal 
A wafer including ferroelectric capacitors of the type depicted in FIG. I 
was produced according to a different method than that of FIG. 3. The 
procedure was identical to that of Example 3, with these exceptions: (1) a 
2000 .ANG. thickness (not 1000 .ANG.) of platinum was sputtered as first 
noble metal portion 36 in step P64; and (2) steps P66, P70, and P72 were 
never performed. In this manner, the bottom electrode formed of 100 .ANG. 
Ti and 2000 .ANG. Pt was preannealed at 600.degree. C. for thirty minutes 
in oxygen, but no temporary capping layer 59 (FIG. 4) was deposited. 
An additional wafer was produced by a process that also included skipping 
step P68, i.e., bottom electrode 24 was not preannealed before deposition 
of the strontium bismuth tantalate liquid precursor. 
EXAMPLE 6 
Scanning Electron Microscope Comparison of Bottom Electrode Surface 
Irregularity Features 
Samples that derived from Examples 3, 4, and 5 were partially repeated to 
provide a substrate that presented an exposed bottom electrode 24. Example 
3 was repeated through step P70 using the strontium bismuth tantalate 
precursor solution to form layer 59 (FIG. 4), which was removed in step 
P70. Example 4 was likewise repeated through step P70. Example 5 was 
repeated through step P68 for the preannealed sample, and through step P64 
for the non-preannealed sample. 
A scanning electron microscope ("SEM") was used to view the bottom 
electrode surface structures of the respective samples at a magnification 
of about 40,000X. FIG. 7 schematically depicts the SEM results for the 
preannealed sample of Example 5. Uppermost surface of bottom electrode 24 
was covered with sharp, irregularly spaced, upwardly pointing hillocks, 
e.g., hillocks 104 and 106, which rose above surface 102 for a distance of 
approximately 900 .ANG.. These hillocks were caused by different relative 
rates of thermal contraction between bottom electrode 24 and the 
underlying substrate. The respective layers expanded upon heating. Upon 
cooling, the contraction of layers 32 and 34 was greater than the 
contraction of metalization portions 34 and 36. Portion 34 adhered to 
layer 32, and the resultant interlayer stresses induced the formation of 
hillocks. 
Thin film ferroelectric or dielectric layers are intended to be 
liquid-deposited on surface 102 as layer 26. These films will preferably 
range in thickness from 500 .ANG. to 3000 .ANG.. Hillocks like hillocks 
102 and 104 serve to reduce process yields by shorting across layer 26 
and, further, present long term device reliability problems. 
No sharp hillocks were observed in the other samples. Some surface 
irregularities were observed, i.e., small, rounded features, but these 
features were estimated to rise less than about 50 .ANG. to 80 .ANG. above 
surface 102. These smaller rolling structures are negligible because they 
pose no serious threat to device performance, i.e., the electrodes of 
Examples 3 and 4 are essentially free of hillocks. 
EXAMPLE 6 
Comparative Evaluation of Capacitor Devices 
The wafers from Examples 3, 4, and 5 included a ferroelectric SrBi.sub.2 
Ta.sub.2 O.sub.9 material of approximately 1800 .ANG. in thickness. 
Selected capacitors from each wafer were subjected to polarization 
hysteresis measurements on an uncompensated Sawyer-Tower circuit including 
a Hewlitt Packard 3314A function generator and a Hewlitt Packard 54502A 
digitizing oscilloscope. Measurements were obtained at 20.degree. C. using 
a sine wave function having a frequency of 10,000 Hz and voltage 
amplitudes of 0.25, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, and 7.0V. 
FIG. 8 depicts a bar graph of the 2Pr polarization values (in 
.mu.C/cm.sup.2) that were obtained for 3V switching measurements. Each bar 
represents a three point average of data obtained from selected capacitors 
on a given wafer. Each bar has been labeled with a corresponding letter, 
A, B, C, D, E, and F, as well as descriptive information for easy 
reference to identify the sample. Differences between bars A and B 
indicate that the first anneal in oxygen provided an approximate 60% 
improvement in polarization with respect to bar A. It is believed that the 
polarization of bar B was improved because titanium from the 200 .ANG. 
portion 34 diffused through the overlying platinum metal to form titanium 
oxide on surface 102 (FIG. 4) during the first 600.degree. C. anneal of 
step P68. The titanium oxide on surface 102 acted as a diffusion barrier 
region to improve the polarization the sample that produced bar B. 
The respective polarizations of bars C and D were less than that of bar B, 
but represent an approximate 40% improvement with respect to bar A. It is 
theorized that the samples which produced bars C and D contained no 
titanium oxide surface coating, but the use of temporary layer 59 reduced 
the level of titanium contamination in ferroelectric layer 26 by abruptly 
truncating a diffusion gradient. Additionally, bar D is about 9% greater 
than bar C, which indicates that it is more effective to use strontium 
bismuth tantalate for layer 59. 
Bars E and F represent the greatest overall polarization, and an 
approximate 110% improvement with respect to bar A. 
The foregoing discussion can be utilized to provide less preferred 
variations of the present invention. For example, the adhesion metal can 
include titanium oxide, tantalum, tantalum oxide, or other known adhesion 
metals. Layer 26 may actually be formed of a plurality of different 
layers, and not all such layers must necessarily be metal oxides. 
Furthermore, the geometries and relative thicknesses that are depicted in 
FIGS. 1, 2, 4, and 5 are presented for illustrative purposes only. These 
figures are not intended to reflect scale models of the actual materials 
which may vary considerably in geometry and thickness. 
Those skilled in the art will understand that the preferred embodiments, as 
described above, may be subjected to apparent modifications without 
departing from the true scope and spirit of the invention. Accordingly, 
the inventors hereby state their intention to rely upon the Doctrine of 
Equivalents for purposes of protecting their full rights in the invention.