Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2000-09-28
2001-09-04
Dang, Trung (Department: 2823)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C438S003000
Reexamination Certificate
active
06285049
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention (Technical Field)
The invention generally relates to dielectric thin-films belonging to the Ba
x
SryCa
1−x−y
TiO
3
class of perovskite oxide and fabrication methods thereof.
2. Background Art
Thin-film ferroelectric materials have application for various electronic devices, such as dynamic and ferroelectric random access memories (“DRAM” and “FRAM”). Thin-film ferroelectric materials are also widely used in the development of new microwave devices such as frequency agile filters, phase shifters, and tunable high-Q resonators as taught generally in L. A. Knauss, et.al.,
Appl. Phys. Lett.
69:25-27 (1996) and P. Bhattacharya, et.al.,
Jpn. J. Appl. Phys.,
32:4103-4106 (1993).
The dielectric currently utilized in DRAMS and for microwave applications is SiO
2
or a silicon oxide
itride composite layer (“ONO”) with a relative dielectric constant of 6. However, as integrated circuit devices move toward higher and higher integration densities, severe demands are placed on the device design, particularly with respect to squeezing storing capacity into a smaller cell space. A capacitance of about 9 fF/&mgr;m
2
appears to be the maximum achievable value for ONO type of materials. As a result, since the mid-1980's, there has been an increasing effort to replace the ONO dielectric with an alternative dielectric having a substantially higher capacitance per unit area.
Most attention has been focused on (Ba
x
Sr
1−x
)TiO
3
(“BST”), as these materials possess high dielectric constants (∈
r
) and low loss (tan &dgr;). At room temperature, single crystal SrTiO
3
has a very low loss (tan &dgr;<10
−4
) but also a low dielectric constant. On the other hand, BaTiO
3
has very high dielectric constants but high loss. Mixing Sr and Ba has resulted in BST materials with high dielectric constants and improved tan &dgr; over BaTiO
3
. BST is a ferroelectric with the perovskite structure. The BST solid solution also shifts the Curie point of BaTi0
3
at 130° C. to around room temperature for Ba
0.7
Sr
0.3
TiO
3
, thus achieving the maximum permittivity around the operating temperature.
A method of making various elemental compositions of BST type materials is taught by Azuma, et.al., in U.S. Pat. No. 5,723,361. Azuma uses molecular precursors, preferably metal carboxylates or metal alkoxides dissolved in an organic solvent such as xylene. The thoroughly mixed solution is then coated on a substrate by a “spin-on” deposition process. Following each spin coat the solvent is removed by a low temperature drying process. The desired thickness of the final film thus depends upon the number of spin-dry cycles in the process. Although Azuma suggests the combination of the metal calcium with the core BST material the method of Alzuma requires the spin coating of a liquid metal precursor, preferably in the form of a metal carboxylate or alkoxide. Also, the traditional co-deposition method of Alzuma permits only one specific material composition to be made per deposition process. That is, the final stoichiometry of the BST type material is uniform throughout the deposited material, and is predetermined by the solution of mixed metal concentrations. In the present invention, the unique deposition process results in a BST type material that is non-uniform throughout the deposited material, and thus allows the testing of various metal compositions from a single deposition process.
SUMMARY OF THE INVENTION (DISCLOSURE OF THE INVENTION)
The present invention is of a dielectric thin film comprising barium, strontium, calcium, titanium and oxygen of general formula (Ba
x
Sr
y
Ca
1−x−y
)TiO
3
. In the preferred embodiment the thin film comprises the following mole fraction of Group IIA metals; barium is approximately about 0.12 to 0.25, strontium is approximately about 0.35 to 0.47, and calcium is approximately about 0.32 to 53. A dielectric thin film with such an elemental composition would typically have a dielectric constant between approximately 130 and 160 and tan &dgr; of less than 0.02.
The present invention is also of a method of forming a dielectric thin film of general formula (Ba
x
Sr
y
Ca
1−x−y
)TiO
3
comprising: providing a substrate, depositing titanium dioxide, barium, strontium, and calcium on the substrate, and heating the substrate containing the deposited titanium dioxide, barium, strontium, and calcium. The depositing of titanium dioxide, barium, strontium, and calcium can be accomplished in a sequentially random order or concurrently. In the preferred embodiment, the titanium dioxide, barium, strontium, and calcium are deposited by pulse laser deposition.
The present invention is also of a method of forming a dielectric thin film of general formula (Ba
x
Sr
y
Ca
1−x−y
)TiO
3
comprising: providing a substrate, depositing TiO
2
, BaCO
3
, SrCO
3
and CaCO
3
on the substrate, and heating the substrate containing the deposited TiO
2
, BaCO
3
, SrCO
3
and CaCO
3
. The depositing of TiO
2
, BaCO
3
, SrCO
3
and CaCO
3
can be accomplished in a sequentially random order or concurrently. In the preferred embodiment, the TiO
2
, BaCO
3
, SrCO
3
and CaCO
3
are deposited by pulse laser deposition. Following the deposition of the various carbonates the deposited film is heated between approximately about 200 and 500° C., preferably 400° C., for at least approximately 24 hours, followed by a heating step of approximately 500 and 800° C. for at least approximately 12 hours, preferably at 500, 600, 700 and 800° C. for at least approximately 3 hours at each temperature, and finally heating between approximately 900 and 950° C. for at least approximately four hours, preferably at 900 and 950° C. for at least approximately 2 hours at each temperature.
The present invention is also of a method of forming a dielectric thin film of general formula (Ba
x
Sr
y
Ca
1−x−y
)TiO
3
comprising: providing a triangular shaped substrate, depositing titanium dioxide, barium, strontium, and calcium, preferably TiO
2
, BaCO
3
, SrCO
3
and CaCO
3
, on the triangular shaped substrate in a sequentially random order with a computer controlled shutter system, rotating the triangular shaped substrate 120° after each subsequent deposition, and heating the triangular shaped substrate containing the deposited titanium dioxide, barium, strontium, and calcium. In the preferred embodiment, the titanium dioxide, barium, strontium, and calcium, preferably TiO
2
, BaCO
3
, SrCO
3
and CaCO
3
, are deposited by pulse laser deposition. Following the deposition of the various carbonates the deposited film is heated between approximately about 200 and 500° C., preferably 400° C., for at least approximately 24 hours, followed by a heating step of approximately 500 and 800° C. for at least approximately 12 hours, preferably at 500, 600, 700 and 800° C. for at least approximately 3 hours at each temperature, and finally heating between approximately 900 and 950° C. for at least approximately four hours, preferably at 900 and 950° C. for at least approximately 2 hours at each temperature.
The present invention is also of a dielectric thin film manufactured by the methods described above.
The present invention solves the problem of having to make numerous individually distinct compositional specimens by generating a compositional spread of compounds through a single deposition process. Thus, the invention provides the best way to rapidly survey a large range of select material compositions with optimal electronic or dielectric properties. To generate such a compositional spread, the desired metal precursors are deposited sequentially using a high precision computer controlled shutter. Following an annealing step this technique generates a precisely controlled stoichiometric profile within a very small area compared to the traditional co-deposition processes.
A primary object of the present invention is the method of making a thin-film, ferroelectric material of non-uniform composition, Ba
x
Sr
y
Ca
1−x&m
Chang Hauyee
Takeuchi Ichiro
Xiang Xiao-Dong
Caress Virginia B.
Dang Trung
Daubenspeck William C.
Gottlieb Paul A.
Kebede Brook
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