Combinatorial Synthesis of Functional Metal Oxide Thin Films

Abstract

n.\ INTRODUCTION The combinatorial approach to materials is a new wave of research methodology which aims to dramatically increase the rate at which new compounds are discovered and improved. In this approach, up to thousands of different compositions can be synthesized and screened in an individual experiment for desired physical properties.^""^ This methodology initially started in biochemistry, and within the last twenty years, combinatorial chemistry and high-throughput screening for new drugs and biomolecules have already revolutionized the pharmaceutical and DNA-sequencing industries.^ The scope of the combinatorial approach is far-reaching, and it can be used to address materials issues at different levels in a wide spectrum of topics ranging from catalytic powders^ and polymers^ to electronic and bio-functional materials.^'^'^ In solid-state applications, the concept and effectiveness of the approach have been demonstrated in successful discoveries of new compounds in a number of key technological areas including optical materials, dielectric materials and magnetic materials.^" 4,8,9 Various thin film synthesis techniques incorporating spatially varying or selective deposition can be used to create combinatorial libraries and composition spreads. There have been significant advances in this area, and at the most sophisticated level, tools for lattice engineering such as laser 334 molecular beam epitaxy (LMBE), have been used to fabricate compositionally varying samples.^^ Figure 1 is a cartoon depicting the experimental steps of the combinatorial approach to materials using one synthesis technique. In this particular scheme, the library is synthesized using the thin-film precursor technique (discussed below), but the overall picture is representative of combinatorial thin film techniques in general. In the first step, fabrication of a thin film library with a large number of compositionally varying sites is achieved by carrying out a series of thin film deposition in conjunction with precisely positioned shadow masks that allow spatially selective deposition. Using a series of masks in a predetermined sequence, different combinations of amorphous precursor multilayers are deposited at all different sites on the library chip. Following the deposition, the chip is thermally processed in order to diffuse the precursors and to form the desired (crystalline) phases. Figure 1: Cartoon showing experimental steps of a combinatorial experiment. In this scheme, a series of shadow masks are used to deliver different combinations of precursor thin films at different sites. Following the steps, one can take newly found compositions to large scale synthesis.