Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
2001-03-01
2003-03-18
VerSteeg, Steven H. (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S192120
Reexamination Certificate
active
06533906
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing an oxide epitaxially-strained lattice film, and more particularly to a method of manufacturing an oxide dielectric device using an epitaxial dielectric film made of a dielectric material having a perovskite crystalline structure, or the like.
2. Related Art
Recently, storage devices using ferroelectric thin films (ferroelectric memory devices) as storage mediums have been developed, and some of them have been brought into practice. Ferroelectric memory devices have the advantages that they are nonvolatile, therefore maintain the storage even after removal of power, quick in spontaneous polarization reversal, when having a sufficiently thin film thickness, and are therefore available for quick write and read equivalent to those of DRAM. Additionally, since each one-bit memory cell can be made of a single transistor and a single ferroelectric capacitor, they are suitable for realization of larger capacities.
Ferroelectric thin films for use in ferroelectric memory devices are required to have properties: remanent polarization being large, dependency of remanent polarization upon temperature being small, remanent polarization being maintained for a long time (retention), among others.
Currently, lead zirconate titanate (PZT) is mainly used as a ferroelectric material. However, regardless of its high Curie temperature (300° C. or higher) and large spontaneous polarization, its major component, Pb, is liable to disperse and vaporize at relatively low temperatures (for example, 500° C.), and this material is estimated to be difficult to cope with miniaturization.
Under the circumstances, the Inventors found and now recognize that the c-axis length of BST can be artificially controlled by selecting strontium titanate (SrTiO
3
, which may be called STO hereinbelow) single crystal as the substrate, strontium ruthenate (SrRuO
3
, which may be called SRO hereinbelow), for example, as the lower electrode, and a material having a slightly larger lattice constant than that of SRO, such as barium strontium titanate (Ba
x
Sr
1-x
TiO
3
, which may be called BST hereinbelow) as the dielectric material, and by epitaxially growing all, employing a deposition method that is more effective in preventing misfit-dislocation in the process of deposition of film by RF magnetron sputtering such that the epitaxial effect maintains BST as a strained lattice even when the film is relatively thick as large as 200 nm or more (see Japanese Patent Publication No. 2878986). As a result, it has been confirmed that, by using BST having a Ba-rich composition, it is possible to realize a ferroelectric thin film quite desirable as ferroelectric memory (which may be called FRAM hereinbelow), which can shift the ferroelectric Curie temperature toward a higher side, exhibit a large remanent polarization at room temperatures and maintain a sufficiently large remanent polarization even when the temperature is raised to about 85° C. Thus, FRAM can be made by using a thin-film capacitor having an epitaxially grown, ferroelectric thin film, and its practical availability is expected.
However, through various researches about methods of making epitaxially-strained lattice ferroelectric thin films, the Inventors have come to the realization that serious difficulty still exists in uniformly making a large dimension of epitaxial ferroelectric film having good crystalline property and ferroelectric property.
The “strain” in the term “strained lattice” used herein has a meaning different from the “strain” naturally introduced into the lattice as a result of appearance of ferroelectricity. For example, barium titanate (BaTiO
3
) has a cubic structure of a paraelectric substance at temperatures higher than the Curie temperature (approximately 130° C.), and all of its a-axis, b-axis and c-axis are 4.01 Å. At the Curie temperature, however, that structure changes to a tetragonal structure due to ferroelectric phase transformation, which results in contraction of the a-axis and the b-axis by approximately 0.005 Å and expansion of the c-axis by 0.01 Å. This type of change is often called a distortion caused by ferroelectricity, but it is different from the strain in the context of the present invention. The strain used in the present invention means a rather artificial strain caused by a restriction that a crystal lattice having such a naturally introduced distortion receives from the substrate while epitaxial growth progresses.
[Prior Art 1]
FIG. 1
is a layout diagram of a well-known parallel-flat RF sputtering apparatus. Numeral
101
denotes a substrate holder,
102
refers to a substrate, and
103
to a cathode. The cathode
103
is made up of a target
105
, backing plate
106
, magnet
107
and yoke
108
. A magnetic field as shown by lines of magnetic force
109
is generated by the magnet
107
. Using that apparatus, using BaTiO
3
ceramic as the target for a ferroelectric film and SrRuO
3
ceramic as the target for an upper or lower electrode, using a single-crystal SrTiO
3
substrate as the substrate
102
, setting the substrate temperature at 600° C., supplying Ar and O
2
by the ratio of 4:1 to adjust the total pressure at 0.25 Pa, the SrRuO
3
lower electrode, BaTiO
3
ferroelectric film and SrRuO
3
upper electrode were stacked in this order to the thicknesses of 30 nm, 40 nm and 30 nm, respectively, on the SrTiO
3
substrate. RF power supplied was 300 W for all of the targets. With the BaTiO
3
ferroelectric film obtained, its lattice constant was measured, and the relations between c-axis lengths and substrate positions were collected as shown in FIG.
2
. Substrate positions were shown by angles &thgr; from the region of the target to be sputtered, which is shown in FIG.
1
and called an erosion region
104
. As shown in
FIG. 2
, epitaxial growth did not occur at the substrate position opposed to the erosion region, i.e., the position where &thgr; is near 0, and its c-axis value could not be measured. On the other hand, at positions distant from the position opposed to the erosion region, where &thgr; is larger than about 15 degrees, epitaxial growth occurs, and the c-axis value increases from the original bulk c-axis value. That is, lattice mismatch with the substrate is maintained without being relaxed by crystal defects, and a strained lattice is made. As shown in
FIG. 2
, the c-axis exhibited the maximum value, and a very strong ferroelectricity was observed near the portion where the angle &thgr; from the erosion region was about 20 degrees.
As reviewed above, if a typical parallel-flat RF sputtering apparatus is used, then the crystalline property is damaged at the portion opposed to the erosion region, and strained lattices are made merely in offset regions distant by a certain value from the position opposed to the erosion region because of improvement of the crystalline property. Therefore, ferroelectric capacitors having good ferroelectric characteristics cannot be made uniformly all over a substrate with the existing apparatus alone.
That phenomenon is known as damaging effect of oxygen negative ions, which occurs during sputtering of oxides (see, for example, D. J. Kester and R. Messier; J. Vac. Sci. Technol., A4-3(1986), 496 or K. Tominaga, N. Ueshiba, Y. Shintani and O. Tada; Jap. J. Appl. Phys., 20-3(1981), 519). This is shown in a schematic diagram of FIG.
3
. When RF power 306 is supplied to the target 301, the target is negatively charged with respect to the plasma potential, and a strong electric field 307 is generated in the plasma sheath portion. Ar positive ions
304
in the plasma
303
are accelerated by the electric field
307
of the plasma sheath portion, hit the surface of the target
301
, thereby bash out atoms forming the target by a sputtering action and can stack them on the substrate
302
located in confrontation with the target. However, in case the target is an oxide like BaTiO
3
, sputtered oxygen is liable to become negative ions
305
,
Abe Kazuhide
Kawakubo Takashi
Yanase Naoko
Yasumoto Taka-aki
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
VerSteeg Steven H.
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