Precursors to lanthanide oxides for microelectronics applications

In order for microelectronics circuits to continue to shrink in size, the size of the fundamental device, the MOSFET must also shrink in size.

Metal Oxide Semiconductor Field Effect Transistor

MOSFET diagram

At the moment SiO2 is used for the gate dielectric layer, but when dimensions of the dielectric layer approach 2-3 nm, then SiO2 becomes unsuitable, and an alternative must be found. Lanthanide oxides are attractive alternatives to SiO2 because they have higher dielectric constants and form a stable interface with Si. The only problem is the preparation of the lanthanide oxide thin film.

MOCVD (Metal Organic Chemical Vapour Deposition) is an attractive technique for this, but requires volatile precursor compounds. In collaboration with Professor A C Jones and Professor P R Chalker we are using Liquid Injection MOCVD which operates at a lower pressure than conventional MOCVD. Even with this technique, lanthanide compounds with sufficient volatility are relatively rare.

Schematic diagram of liquid injection MOCVD reactor

MOCVD reactor

See A. C. Jones et al Chemical Vapor Deposition 1998, 4, 46-49


Precursor compounds

We are investigating the use of the mmp ligand shown below:

Hmmp structure

This ligand has high steric demands due to the two a-Me groups, and is potentially bidentate, forming chelates via the OMe donor. Some complexes with the late lanthanides have already been reported (See R. Anwander et al, Inorg. Chem., 1997, 36, 3545.), but there was no work reported on early Ln.

We have successfully grown oxides of La and Pr using tetraglyme stabilised [La(mmp)3] and [Pr(mmp)3] precursors.

Crystalline derivatives of the early lanthanides with mmp are very difficult to obtain. However we have managed to crystallise [Pr(mmp)3LiCl]2 from solutions of stabilised [Pr(mmp)3] in the presence of LiCl. Its structure is shown below:

click here to see a 3-d interactive picture

The complicated coordination chemistry of the early lanthanides with mmp is gradually being unravelled.

Lanthanium Aluminate LaAlO3

Lanthanum aluminate has attracted a great deal of recent attention both as a buffer layer for the overgrowth of various perovskite films and as an alternative gate dielectric to SiO2.We have recently shown that a heterometallic lanthanum aluminium isopropoxide [LaAl(OPri)6(PriOH)]2 (the structure is shown below) can be used as a single source precursor for liquid injection MOCVD of LaAlO3. Stoichiometric, amorphous LaAlO3 is deposited at 450-500C.

click here to see a 3-d interactive structure


References

H. C. Aspinall, P. A. Williams, J. Gaskell, A. C. Jones, J. L. Roberts, L. M. Smith, P. R. Chalker and G. W. Critchlow, Chem. Vap. Deposition 2003, 9, 7-10. ' Growth of Lanthanum Silicate Thin Films by Liquid Injection MOCVD Using Tris-[bis(trimethylsilyl)amido]-lanthanum'

H. C. Aspinall, J. Gaskell, P. A. Williams, A. C. Jones, P. R. Chalker, P. A. Marshall, J. F. Bickley, L. M. Smith, and G. W. Critchlow, Chemical Vapour Deposition 2003, 9, 235-238. 'Growth of Praseodymium Oxide Thin Films by Liquid Injection MOCVD Using a Novel Praseodymium Alkoxide Precursor'

T. D. Manning, et al. Journal of Materials Chemistry 2005, 15, 3384-3387 'Deposition of LaAlO3 films by liquid injection MOCVD using a new La-Al single source alkoxide precursor'


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