GB2370271A

GB2370271A - Novel precursors for the growth of heterometal oxide films by MOCVD

Info

Publication number: GB2370271A
Application number: GB0110862A
Authority: GB
Inventors: Anthony Copeland Jones; Timothy John Leedham
Original assignee: Inorgtech Ltd
Current assignee: Inorgtech Ltd
Priority date: 2000-05-06
Filing date: 2001-05-03
Publication date: 2002-06-26
Anticipated expiration: 2021-05-03
Also published as: GB2370271B; GB0110862D0; GB0010926D0

Abstract

Novel metalorganic precursors for the growth of heterometal oxide films by MOCVD, the precursors having the general formula:- <CHE>Sr(Ta,Nb) (OR<SB>1</SB>)<SB>10</SB> L2</CHE> wherein; R<SB>1</SB> is a straight or branched chain alkyl group; and L is an alkoxide group of the formula:- <EMI ID=1.1 HE=32 WI=44 LX=593 LY=1351 TI=CF> <PC>wherein n=0 or 1; X is N or O; R<SB>2</SB> and R<SB>3</SB> are the same or different and are straight or branched chain alkyl groups, and R<SB>4</SB> is a straight or branched, optionally substituted, alkyl chain. Preferably, L is a dimethyl aminoalkoxide group or an alkoxy alkoxide group.

Description

TITLE: Novel Precursors for the Growth of Heterometal oxide films by MOCVD DESCRIPTION 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 niobium oxide films.

The ferroelectric metal oxides SrBiz Taz09 (SBT), SrBi2Nb209 (SBN) and SrBi2 (Ta, Nb) 09 (SBTN) have net electric dipoles in a certain direction which can be reversed by an applied voltage. These ferroelectric materials retains a remnant polarisation (i. e. charge) even after the power has been switched off which gives it a large potential application in computer technology as capacity 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 perovskites SBT, SBN or SBTN 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 contract to capacitors based on other ferroelectric oxides, such as Pb (Zr, Ti) 03, those based on SBT and related oxides such as SBN or SBTN show negligible polarisation fatigue, are fully compatible with conventional Pt-electrode technology, and maintain good electrical properties, even when very thin.

SBT, SBN and SBTN thin films have been deposited by a variety of techniques including solgel, metalorganic decomposition, pulsed laser ablation and metalorganic chemical vapour deposition (MOCVD). 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 conforma step coverage at device dimensions < 2am.

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 (C6H5) 3 and Ta (OPrl) 4 (thd) which are generally not compatible, having widely differing physical properties and/or decomposition characteristics. 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 (OBut) 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) 6} 2] precursors are relatively unsaturated making them susceptible to attack by moisture and reducing their shelf life in solution-based liquid injection MOCVD It is an aim of the present invention to provide new metalorganic precursors for the MOCVD of strontium tantalum niobium oxide films, such as SBT, SBN and SBTN 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 (Ta, Nb) (ORI) lo Lz wherein; Ri is a straight or branched chain alkyl group; and L is an alkoxide group of the formula : -

wherein n=O or 1; X is N or 0 ; R2 and R3 are the same or different and are straight or branched chain alkyl groups, and R4 is a straight or branched, optionally substituted, alkyl chain.

According to a second aspect, the present invention provides a method of depositing thin films of or containing strontium tantalum niobate using metalorganic precursors in a MOCVD technique, wherein the strontium tantalum niobate precursor has a formula: Sr (Ta, Nb) (OR1) m L2 wherein Ri 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 0 ; R2 and R3 are the same or different and are straight or branched chain alkyl groups and R4 is a straight or branched, optionally substituted, alkyl chain.

Optional substituents for R4 may include amino, alkylamino or alkoxy groups.

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, is preferably an ethoxy group but compounds of the invention where OR, is, for example, an iso-propoxy or tertiarybutoxy group may also be useful.

Preferably L is a dimethyl aminoalkoxide group, particularly dimethyl aminoethoxide (OCH2CH2NMe2 or DMAE), dimethyl aminopropoxide (OCH (CH3) CH2NMe2 or DMAP) or bis-dimethyl aminopropoxide (OCH (CH2NMe2) CH2 NMe2 or bis-DMAP). Alternatively, L may be an alkoxy alkoxide group, particularly-OCI-CHOMe,-OCH (CH3) CH20Me or - OCH (OMe) CH20Me.

The above-mentioned precursors may also be used in combination with a variety of bismuth (Bi) sources to deposit strontium bismuth tantalum niobium metal oxides.

Suitable precursors for the source of bismuth are of the formula Bi (R4) 3, wherein R4 is a straight or branched chain alkyl group, a phenyl group, a thd (tetramethyl heptanedionate) group, OCH2CH2NMe2 or OCMe2CH2OMe, especially trimethyl bismuth, Bi (CH3) 3 triethyl bismuth, Bi (CH2CH3) 2 triphenyl bismuth (Bi (C6H5) 3), Bi (thd) 3, Bi (OCH2CH2NMe2) 3 and Bi (OCMe2CH2OMe) 3.

The Bi (OCMe2CH2OMe) 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 (Ta, Nb) (ORs) lo L2 in a single solution. In the latter case, the bismuth precursor may have the general formula BiL3, 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 tantalum niobium metal oxide precursor. The single solution may be in an organic solvent such as ether or cyclic ether (eg. tetrahydro furan) or a hydrocarbon, such as hexane or heptane.

The precursors of the present invention may be used in a method for depositing a strontium tantalum niobium metal oxide ferroelectric film onto a substrate by MOCVD. A suitable substrate is, for example, Si (lOO). The ferroelectric films may be used, in particular, for the production of non-volatile ferroelectric random access memories.

This invention will be further described by way means of the following example and with reference to the accompanying drawing which illustrates the molecular structure of the novel precursor, Sr (Ta, Nb) (OEt) lO (dmae) 2 Example Synthesis and characterisation ofSr (Ta, Nb) (OEt) lo (dmae) 2.

Strontium metal (10. 3g, 0.118 moles) was dissolved in dry ethanol (SOOml). To this solution was added Ta (OEt) 5 (47.9g, 0.118 moles) and Nb (OEt) 5 (37. 55g, 0.118 moles) and the pale yellow mixture set to reflux for 1 hour. The ethanol was stripped on the Buchi and

the product held at 80 c (bath temperature) and O. 1mm Hg for 1 hr to yield a pale yellow waxy solid. This solid was then taken back up in 1 Litre of n-hexane (99%) and N, Ndimethylaminoethanol (21. 0g, 0.236 moles) added. The pale yellow mixture was set to reflux for 1 hr and then solvent stripped on the Buchi evaporator. The pale yellow liquid was distilled as one fraction at 140 C/0. 1mm Hg to yield a pale yellow oil.

Elemental microanalysis showed that Sr (Ta, Nb) (OEt) 1O (dmae) 2 has been prepared as a single distillable species, making it suitable for the MOCVD of SrBi2(Ta,Nb)09.

C H N Analysis for SrTaNbC28H7oN20i2 :- Calculated : 34.03% 7.14% 2.84% Found: 34.03% 7.21% 2.87%.

Claims

CLAIMS 1. A metalorganic precursor of the formula : - Sr (Ta, Nb) (ORl) lo L2 wherein: Rl is a straight or branched chain alkyl group; and L is an alkoxide group of the formula : -

wherein : n=O or 1 ; X is N or 0 ; R2 and R3 are the same or different and are straight or branched chain alkyl groups, and R4 is a straight or branched, optionally substituted, alkyl chain.
2. A metalorganic precursor as claimed in claim 1, wherein optional substituents for R4 are selected from amino, alkylamino or alkoxy groups.
3. A metalorganic precursor as claimed in claim 1 or 2, wherein the alkoxy group OR ! is an ethoxy group, an iso-propoxy group or a tertiarybutoxy group.
4. A metalorganic precursor as claimed in claim 1,2 or 3, wherein L is a dimethyl aminoalkoxide group.
5. A metalorganic precursor as claimed in claim 4, wherein the dimethyl aminoalkoxide group is selected from dimethyl aminoethoxide (OCH2CH2NMe2 or DMAE), dimethyl

aminopropoxide (OCH (CH3) CH2NMe2 or DMAP) and bis-dimethyl aminopropoxide (OCH (CH2NMe2) CH2 NMe2 or bis-DMAP) groups.
6. A metalorganic precursor as claimed in claim 1,2, or 3, wherein L is an alkoxy alkoxide group.
7. A metalorganic precursor as claimed in claim 6, wherein the alkoxy alkoxide group is selected from-OCH2CH2OMe,-OCH (CH3) CH20Me or - OCH (OMe) CH20Me.
8. A method of depositing thin films of a containing strontium tantalum niobate using a strontium tantalum niobate precursor according to any one of claims I to 7.
9. The use of metalorganic precursor as claimed in any one of claims 1 to 7 in a method for depositing a strontium tantalum niobium metal oxide ferroelectric film onto a substrate by MOCVD.
10. The use of a metalorganic precursor as claimed in claim 9 wherein the the substrate is Si (lOO).
11. A method of depositing thin films of or containing strontium tantalum niobate using metalorganic precursors in a MOCVD technique, wherein the strontium tantalum niobate precursor has a formula : - Sr (Ta, Nb) (ORt) lo L2 wherein: Rl 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 0 ; R2 and R3 are the same or different and are straight or branched chain alkyl groups and R4 is a straight or branched, optionally substituted, alkyl chain.
12. A method as claimed in claim 11 wherein L is a dimethyl aminoalkoxide group or an alkoxy alkoxide group.
13. A method as claimed in claim 11 or claim 12 wherein the deposition technique is MOCVD.
14. A method as claimed in claim 13 wherein the MOCVD technique is liquid injection MOCVD.
15. A method as claimed in claim 14 wherein the solvent for deposition of the films is tetrahydrofuran.
16. A method as claimed in any one of claims 11 to 15 wherein the strontium tantalum niobate precursor is used in combination with a bismuth (Bi) source to deposit strontium bismuth tantalum niobium metal oxides.
17. A method of depositing thin films of strontium bismuth tantalum niobium metal oxides by MOCVD techniques using a bismuth precursor and a strontium tantalum niobate precursor wherein the strontium tantalum niobate precursor has a formula : - Sr (Ta, Nb) (OR1) lo L2 wherein: Rl 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 0 ; R2 and R3 are the same or different and are straight or branched chain alkyl groups and R4 is a straight or branched, optionally substituted, alkyl chain.
18. A method as claimed in claim 16 or claim 17 wherein the source of bismuth are precursors of the formula Bi (R4) 3, wherein R4 is a straight or branched chain alkyl group, a phenyl group, a thd (tetramethyl heptanedionate) group, OCH2CH2NMe2 or OCMe2CH20me.
19. A method as claimed in claim 18 wherein the bismuth precursor is selected from the group consisting of trimethyl bismuth, triethyl bismuth, triphenyl bismuth, Bi (thd) 3, Bi (OCH2CH2NMe2) 3 and Bi (OCMe2CH20Me) 3.
20. A method as claimed in claim 19 wherein Bi (OCMe2CH2OMe) 3 is the Bismuth co-precursor.
21. A method as claimed in any one of claims 16 to 20 wherein the Bi precursors are evaporated separately to the strontium tantalum niobate precursor.
22. A method as claimed in any one of claims 16 to 20 wherein the Bi precursors are combined with the strontium tantalum niobate precursor in a single solution.
23. A method as claimed in claim 22 wherein the bismuth precursor has the general formula BiL3, wherein L is a dialkyl aminoalkoxide or alkoxy alkoxide group.
24. A method as claimed in claim 23 wherein the dialkyl aminoalkoxide or alkoxy alkoxide group of the bismuth precursor is the same as that of the strontium tantalum niobium metal oxide precursor.
25. A method as claimed in any one of claims 22 to 24 wherein the single solution is in an organic solvent.
26. A method as claimed in claim 25 wherein the organic solvent is selected from the group consisting of an ether, a cyclic ether and a hydrocarbon.