This project had developed a new method of thin film formation on a substrate. It has solved the following problems, which are very difficult to solve by the standard techniques know today:
1. Columnar structure of the deposited films. This gives rise to surface stress, pin-holes, and insufficient mechanical properties.
2. The need for elvated substrate temperatures for PVD, and even more so for CVD methods.
3. The difficulty of filling via holes or trenches of narrow dimensions.
4. The difficulty of controlling the granularity for films composing of two (or more) different materials, eg the distributions of magnetic clusters in a non-magnetic matrix.
Significant advances have been achieved in all four of the listed areas (and several other ones) by using cluster beams for deposition. Thus the material which will form the thin film does not arrive in atomic of molecular form as in the standard methods of thin film formation, but it is clustered first. For example as Cu1000, that is 1000 copper atoms sticking together. The charged clusters are electrically accelerated and impinge on the surface with variable kinetic energy. The impact leads to a local melting and compactness of the material underneath. The following achievements can be cited:
1. Very compact thin films were grown which show no columnr growth, even at room temperature.
2. It is possible to deposit a golden TiN film at room temperature, while PVD methods need about 400 C and CVD methods 1000 C.
3. Via holes of 0,8um have been filled with Cu at 80 C.
4. By codepositing an atomic Ag beam with a clustered Co beam, the size and density of magnetic clusters could be varied independently.
Additional, fundamental studied of surface diffusion, island formation as well as electronic and structural properties have been performed in order to better understand the underlying mechanisms of thin film formation by clusters.
Definition: A cluster can be defined as aggregate of atoms, e.g.: Si100, Ag500, or Fe1000.
Historical perspective : About 20 years ago the idea to use energetic clusters for thin film formation was developed in Japan. The underlying concept was to use the enhanced surface mobility induced by the impact of a cluster for failored thin film growth.
In conventional deposition methods the substrate was heated for this purpose which has many unwanted side effects. The project failed although a large effort went into it.
It is known today, that the Japanese ICB (Ionised Cluster Beam) sources used hardly contain any clusters. On the other hand significant technological progress has been made since 10 years in Europe in this area. Researchers in Europe are ready today for this challenge in material science. The usual course of events could be reversed in this case: a technology first considered in Japan is starting to emerge today in Europe.
Technological Aspects: The physical properties of thin films formed by cluster impact depend on the size, kinetic energy, and composition of the clusters. As these parameters can be independently controlled today, it can be expected that cluster deposition will be more flexible than the well established evaporation and sputter deposition techniques. The goals of the present proposal are: deposit clusters softly on metals and semiconductors (for band formation studies and mano - Schottky contacts), study energetic cluster impact (for metallisation of semiconductors and for optical mirrors), and characterise size selected magnetic clusters embedded in a matrix (for magnetic recording and magnetoresistive sensors). Deposition of size selected clusters will allow fabrication of specimen of new and/or extremely uniform electronic and magnetic properties. The rich potential of cluster deposition is evident from these many and diverse possibilities.
Funding SchemeCSC - Cost-sharing contracts