Coating processes – Coating by vapor – gas – or smoke – Mixture of vapors or gases utilized
Reexamination Certificate
2001-01-16
2002-11-26
Meeks, Timothy (Department: 1762)
Coating processes
Coating by vapor, gas, or smoke
Mixture of vapors or gases utilized
C427S255320, C556S028000, C556S042000
Reexamination Certificate
active
06485784
ABSTRACT:
The present invention relates to novel precursors for the production of heterometal oxide films by MOCVD, in particular precursors for the growth of strontium tantalum
iobium oxide films.
The ferroelectric metal oxides (strontium bismuth tantalate (SrBi
2
Ta
2
O
9
or SBT) and strontium bismuth niobate (SrBi
2
Nb
2
O
9
or SBN) have a net electric dipole in a certain direction which can be reversed by an applied voltage. These ferroelectric materials retain a remnant polarisation (i.e. charge) even after the power has been switched off which gives them a large potential application in computer technology as capacitor layers in non-volatile ferroelectric random access memories (NVFERAM's). NVFERAM's can also be switched extremely rapidly (in hundredths of a nanosecond) and are particularly suitable for military and space applications as they are radiation hard.
Thin films of the layered perovskite strontium bismuth tantalate SrBi
2
Ta
2
O
9
(SBT), comprising ferro-electric pseudo-perovskite lattices sandwiched between bismuth oxide layers, have a large potential application as capacitor layers in non-volatile ferroelectric computer memories. In contrast to capacitors based on other ferroelectric oxides, such as Pb(Zr,Ti)O
3
, those based on SBT show negligible polarisation fatigue, are fully compatible with conventional Pt-electrode technology, and maintain good electrical properties, even when very thin.
SBT thin films have been deposited by a variety of techniques including solgel, (Y Ito, M Ushikobo, S Yokoyama, H Matsunaga, T Atsuki, T Yonezawa and K Ogi, Int Ferroelectrics, 1997, 14, 23) metalorganic decomposition (T Mihara, H Yoshirnozi, H. Watanabe and C A Pas de Arnujo, Jpn J Appl Phys, 1995, 34, 5233; T Atsuki, N Soyama, T Yonezawaq and K Ogi, Jpn. J. Appl Phys, 1995, 34, 5096), pulsed laser ablation (P X Yang, N S Zhou, L R Zheng, H X Ln and C L Lin, J Phys. D-Appl Phys, 1997, 30, 527) and metalorganic chemical vapour deposition (MOCVD) (T Ami, K Hironaka, C Isobu, N Nagel, M Sugiyama, Y Ikeda, K Watanabe, A Machida, K Miura, and M Tanaka, Mat Res Soc Symp Proc, 1996, 415, 195; T Li, Y Zhu, S Desu C H Peng and M Nagata, Appl. Phys Lett, 1996, 68, 616). MOCVD has a number of advantages over other deposition techniques as it offers the potential for large area growth, good film uniformity and composition control, and excellent conformal step coverage at device dimension <2 &mgr;m. The MOCVD technique is also fully compatible with existing silicon CVD processes.
However for the full potential of MOCVD to be realised, it is essential that precursors with the required physical properties and decomposition characteristics are available. It is important that there is an adequate temperature window between precursor vaporisation and decomposition on the substrate, the precursors need to be compatible and not pre-react, they should decompose to form a pure film of the desired metal oxide at similar substrate temperatures. Ideally, the precursors should also be of low toxicity and be relatively stable under ambient conditions.
The MOCVD of SBT and SBN has thus far been severely restricted by a lack of suitable metalorganic precursors. Conventional precursors include Sr(thd)
2
(where thd=2,2,6,6-tetramethyl-3,5-heptanedionate), Bi(C
6
H
5
)
3
and Ta(OPr
1
)
4
(thd) which are generally not compatible, having widely differing physical properties and/or decomposition characteristics (M de Keijer and G J M Dormans, MRS Bulletin, 1996, 21, 37). In an effort to alleviate this problem the Sr/Ta heterometal alkoxide [Sr{Ta(OPr)
6
}
2
] has been investigated as a precursor to SBT, in combination with Bi(OBu
1
)
3
. A potential advantage of this approach is that the strontium and tantalum ratio in the precursor matches the required ratio in the deposited SBT film, however, there exists the possibility that the strontium and tantalum alkoxide species will partition during precursor evaporation and transport. Another disadvantage is that [Sr{Ta(OR)
5
}
2
] precursors are relatively unsaturated making them susceptible to attack by moisture and reducing their shelf life in solution-based liquid injection MOCVD.
EP-A-0 807 965 (Matsushita Electronics Corporation) Nov. 19, 1997 describes a method of forming a Bi-layered ferroelectric thin film on a substrate, using a mixed composition of a Bi-containing organic compound and a metal polyalkoxide compound.
It is an aim of the present invention to provide new metalorganic precursors for the MOCVD of SBT and SHN which may overcome the above-mentioned drawbacks.
According to one aspect of the present invention there is provided a metalorganic precursor of the formula:
Sr[M(OR
1
)
6-x
L
x
]
z
wherein x is from 1 to 6;
M is Ta or Nb; R
1
is a straight or branched chain alkyl group; and
L is an alkoxide group of the formula:
wherein n=0 or 1; X is N or O; R
2
and R
3
are the same or different and are straight or branched chain alkyl groups, and R
4
is a straight or branched alkyl chain, optionally substituted with an amino, alkylamino or alkoxy group.
According to a second aspect, the present invention provides a method of depositing thin films of or containing strontium metal oxides using metalorganic precursor in a MOCVD technique, wherein the strontium metal oxide precursor has the formula:
Sr[M(OR
1
)
6-x
L
x
]
z
wherein x is from 1 to 6; M is Ta or Nb;
R
1
is a straight or branched chain alkyl group; and L is an alkoxide group of the formula:
wherein n=0 or 1; X is N or O; R
2
and R
3
are the same or different and are straight or branched chain alkyl groups and R
4
is a straight or branched alkyl chain, optionally substituted with an amino, alkylamino or alkoxy group.
Preferably, x is 1 or 2.
The deposition, technique may comprise conventional MOCVD or, more preferably, liquid injection MOCVD. The solvent for deposition of the films by liquid injection MOCVD is preferably tetrahydrofuran.
The alkoxy group OR
1
is preferably an ethoxy group but compounds of the invention where OR
1
is, for example, an iso-propoxy or tertiarybutoxy group may also be useful. Preferred precursors of the invention have the formula:
Sr[M(OR
1
)
5
L]
2
or Sr[M(OR
1
)
4
L
2
]
2
wherein M, R
1
and L are as defined above.
Preferably L is a dimethyl aminoalkoxide group, particularly dimethyl aminoethoxide (OCH
2
CH
2
NMe
2
or DMAE), dimethyl aminopropoxide (OCH(CH
3
)CH
2
NMe
2
or DMAP) or bis-dimethyl aminopropoxide (OCH(CH
2
NMe
2
)CH
2
NMe
2
or bis-DMAP). Alternatively, L may be an alkoxy alkoxide group, particularly —CH
2
CH
2
OMe, —OCH(CH
3
)CH
2
OMe or —OCH(OMe)CH
2
OMe.
The above-mentioned precursors may also be used in combination with a variety of bismuth (Bi) sources to deposit strontium bismuth tantalum and strontium bismuth niobium metal oxides.
Suitable precursors for the source of bismuth include triphenyl bismuth (Bi(C
6
H
5
)
3
, Bi(thd)
3
, Bi(OCH
2
CH
2
NMe
2
)
3
and Bi(OCMe
2
CH
2
OMe)
3
.
The Bi(OCMe
2
CH
2
OMe)
3
precursor is particularly suitable as a co-precursor, being one of the most stable and volatile Bi alkoxide sources available.
The Bi precursors may be evaporated separately or may be combined with the Sr[M(OR
1
)
6-x
L
x
]
2
is a single solution. In the latter case, the bismuth precursor may have the general formula BiL
3
, wherein L is a dialkyl aminoalkoxide or alkoxy alkoxide group as hereinbefore described in relation, to the strontium metal oxide precursor. Preferably, the dialkyl aminoalkoxide or alkoxy alkoxide group of the bismuth precursor is the same as that of the strontium metal oxide precursor. The single solution may be in an organic solvent such as ether or cyclic ether (eg. THF) or a hydrocarbon, such as hexane or heptane.
The precursors of the present invention may be used in a method for depositing a strontium metal oxide ferroelectric film onto a substrate by MOCVD. A suitable substrate is, for example, Si(100). The ferroelectric films may be used, in particular, for the producti
Crosbie Michael J
Jones Anthony C
Leedham Timothy J
Wright Peter J
Meeks Timothy
Nixon & Vanderhye P.C.
Qinetiq Limited
LandOfFree
Precursors for growth of heterometal-oxide films by MOCVD does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Precursors for growth of heterometal-oxide films by MOCVD, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Precursors for growth of heterometal-oxide films by MOCVD will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2994672