Compositions: coating or plastic – Coating or plastic compositions – Heavy metal compound containing
Reexamination Certificate
1998-01-15
2001-03-06
Brunsman, David (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Heavy metal compound containing
C106S287190, C252S06290R, C252S06290R
Reexamination Certificate
active
06197102
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to coating solutions for use in forming Bi-based ferroelectric thin films, and ferroelectric thin films, ferroelectric capacitors and ferroelectric memories formed with said coating solutions, as well as processes for production thereof. More particularly, the invention relates to coating solutions that lead to only a small amount of separation (segregation, precipitation) of excessive metallic elements' particles, little leakage current, are excellent in hydrogen heat treatment resistance as well as voltage resistance, are capable of forming dense Bi-based ferroelectric thin films, and have good keeping quality. The invention also relates to ferroelectric thin films, ferroelectric capacitors and ferroelectric memories formed with such coating solutions, as well as processes for the production thereof.
2. Description of Related Art
Thin films of bismuth layer-structured ferroelectrics (BLSF) represented by the general formula (Bi
2
O
2
)
2+
(A
m−1
B
m
O
3m+1
)
2−
[where A is a mono-, di- or trivalent ion (as of Bi, Pb, Ba, Sr, Ca, Na, K or a rare earth element) or combinations of these ions; B is a tetra-, penta- or hexavalent ion (as of a metallic element like Ti, Nb, Ta, W, Mo, Fe, Co or Cr) or combinations of these ions; and m is an integer of 1-5] have recently been found to feature good characteristics such as requiring small coercive field in remanent polarization P-E hysteresis curves and hence experiencing less fatigue as a result of repeated polarization switching. This has spotlighted the potential use of BLSF thin films as materials for the fabrication of semiconductor memories and sensors (T. Takenaka, “Bismuth Layer-Structured Ferroelectrics and Their Grain Orientation” in Report of the Workshop on Applied Electronics Properties, The Japan Society of Applied Physics, pp. 1-8, Nov. 22, 1994; and “Ceramics”, vol. 30, No. 6, pp. 499-503, 1995). Among the BLSF thin films so far reported, those of an SrBi
2
Ta
2
O
9
system which are represented by the formula (Bi
2
O
2
)
2+
(SrTa
2
O
7
)
2−
are of particular interest since the desired characteristics are conspicuous in them.
Such BLSF thin films can be formed by various methods including sputtering, CVD and coated film formation. However, due to the many metal oxides that have to be incorporated as constituents, sputtering and CVD techniques require costly apparatus and considerable difficulties are involved in controlling the composition of ferroelectric thin films at desired levels; hence, these techniques are not suitable for practical applications, particularly on large-diameter substrates. In contrast, the coated film formation technique does not need expensive apparatus and can deposit films at relatively low cost; in addition, they provide ease in controlling the composition of ferroelectric thin films at desired levels. Therefore, the coated film process for the formation of BLSF thin films holds much promise for commercial use.
While several formulations have been proposed for use as coating solutions in the formation of BLSF thin films by the coated film process, two typical cases are those prepared by dissolving carboxylate (e.g., 2-ethylhexanate) of Sr and Bi and alkoxides of Ta in acetate esters (Proceedings of the 12th Ferroelectric materials and their Applications meeting on May 24-27, 1995, Paper presented by Mitsubishi Materials Corporation, 24-TP-11, pp. 57-58; and “Jpn. J. Appl. Phys.”, vol. 34, pp. 5096-5099, 1995) and those prepared by dissolving 2-ethylhexanate of Sr, Bi, Ta, Nb, Ti, etc. in xylene to form coating solutions of a metallo-organic decomposition (MOD) type (Proceedings of the 12th Ferroelectric Materials and their Applications meeting on May 24-27, 1995, Paper presented by Olympus Optical Co., Ltd. and Symetrix Corporation, 26-TC-10, pp. 139-140). However, these coating solutions have had various problems. First, the 2-ethylhexanate of the respective metal components has a long-chain (C
8
) organic group, so a large portion of the coating solution is occupied by the organic content and there is much loss in the coating weight due to the burning out of the organic content in the process of film formation consisting of the application of the coating solution, baking of the applied coating and crystallization and a porous film will result. In addition, the surface morphology of the coating film is not satisfactory enough to provide for easy application to the fabrication of VLSI devices. Further in addition, in order to form a thin film using the applied coating, the applied coating has to be annealed twice at an elevated temperature of 800° C. to ensure appropriate electrical characteristics thereof, however, this is problematic from the viewpoint of semiconductor fabrication process. It is pointed out that in the conventional BLSF-based coating solutions, high-volatility metals such as Bi are burnt out on forming a thin film, particularly during prebaking and annealing, and therefore electrical characteristics are impaired. Attempt have been made to add in advance high-volatility Bi excessively in a molar amount 1.15-1.3 times as great as the stoichiometric amount to the coating solution in order to compensate the loss of burnout of Bi.
The foregoing “Jpn. J. Appl. Phys.” vol. 34 (1995) reports that thin films which contain Sr in an amount of smaller than stoichiometric amount have improved in their film characteristics such as crystallinity.
Even when using a coating solution having a composition out of the theoretical amount (stoichiometric amount) as mentioned above, it was difficult to form a BLSF film of a composition eventually satisfying the stoichiometry since the amount of burnout of metallic elements differ from one another, depending on annealing conditions. Such a BLSF film out of stoichiometry is apt to suffer defects in the film, and therefore excessive metallic elements sometimes separate among particles (grain boundaries) during the growth of the crystallized particles. Particularly, fine particles rich in Bi may separate at grain boundaries and cause an increase in leakage current. When applying a ferroelectric memory processed by using the film above to an actual semiconductor device memory having a very small area, problems such as film fatigue resulting from repeated polarization switching, increase in leakage current, and decrease in hydrogen heat treatment resistance and voltage resistance are posed under the effect of the aforesaid defects and film non-uniformity caused by precipitates.
On the other hand, a rapid heating method for forming a film known as RTP (Rapid Thermal Processing) is now attracting the general attention because of such favorable merits as the possibility of accomplishing a baking treatment in a very short period of time, and hence of reducing damages to the substrate, and of applying heat uniformly to inside and outside of the substrate. The present inventors have confirmed that the foregoing fine particles rich in Bi apparently separate at grain boundaries on forming a film by RTP.
Although it is desirable to form a film from the very beginning of whose composition is closest to the stoichiometric composition, the amount of burnout of Bi and other metallic elements during a formation of a thin film differ from one another due to the conditions of the thin film. It was therefore difficult to form a film having a composition close to the stoichiometric composition in the conventional BLSF-based coating solutions.
The prior art coating solutions have further problems. The long-chained metal carboxylates (metallic soaps) of monobasic acids which are commonly used in the coating solutions are generally slightly soluble in polar solvents and, hence, aromatic solvents such as xylene and toluene are used to prepare the coating solutions. However, the coating solutions using such aromatic solvents have to be stored in glass or metallic containers in order to ensure that the evaporating solvents will not be lost to t
Hashimoto Akira
Kanehara Takao
Kato Hiroyo
Koiwa Ichiro
Mita Juro
Brunsman David
Tokyo Ohka Kogyo Co. Ltd.
Wenderoth , Lind & Ponack, L.L.P.
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