Although research into oxide thin films has continued since the 1960s, it was the discovery of high temperature superconductivity in 1986¹, which provided a major impetus to the research in the field of multi-component complex oxide thin films. Consequently, many areas of interest have emerged such as ferroelectric devices, optoelectronic devices, CMR devices etc. (see Table 9.1). These oxides are the subject of scientific studies because they represent immense promise for the 21st century solid-state devices. Although in the past these materials have been used as bulk materials for many applications, it is the thin film form of these oxides which makes them more attractive for various applications. For example, integration of semiconductor technology with epitaxial metal oxide thin films such as superconducting oxides is a very promising field of research for many device applications. Superconducting oxides such as YBCO (yttrium barium copper oxide) for SQUIDs² or ferroelectric oxide such as PZT (lead zirconium titanate) for non-volatile memory applications³ show excellent potential in device applications.

In recent years, an enormous amount of emphasis has been paid towards developing epitaxial metal oxide thin films, in order to develop devices, which show better performance over an extended life span as compared to the devices based on polycrystalline films. Significant progress has been made towards studying the growth mechanisms of the epitaxial thin films by various techniques such as RHEED, LEED, and STM.^4,5,6,7. But there are some issues which are yet to be tackled effectively. One of them is the processing temperature, which needs to be of the order of 500°C or less, for processing to be compatible with the Si integrated circuit processing. Other issues to be dealt with are reproducibility of composition in complex oxide deposition, phase stability, high quality of epitaxy and atomic level flatness of the films whilst maintaining the commercial viability of the device.

Table 9.1    Some important metal oxides and their applications

Over the years various processes have been developed for the deposition of metal oxide thin films. Almost all of these deposition techniques can be broadly divided into two categories, namely physical vapour deposition processes (PVD) and chemical processes. PVD processes include laser ablation, sputtering, evaporation whilst chemical processes are chemical vapour deposition techniques (CVD), liquid phase epitaxy, sol-gel and metal organic deposition (MOD) and spin coating. There have been many extensive reviews on the deposition of epitaxial oxide films.^8,9,10 The growth of epitaxial metal oxides on single crystal substrates usually requires temperatures higher than those in the processing of semiconductor devices. It still remains a major challenge for most of the thin film processing groups to bring the deposition temperature down to acceptable limits. Ideally, a manufacturing process for fabricating metal oxide thin films should have:

• the capability to produce highly oriented films in a reproducible manner at lowest possible deposition temperatures,

• the ability to produce stoichiometric films of complex compositions,

• compatibility with the integrated Si circuit with respect to the deposition temperature,

• ability to produce uniform thickness with good conformal coverage,

• the ability to produce patterned and layered heterostructure

• should operate at low cost.