Microstructural Control in Microelectronics
From cellular phones and compact discs to jet engines large enough to contain a standing person, heterolayer technology is a vital part of engineered products. The growth and properties of heterolayers are thus a field of increasingly large importance. Heterolayers are films that differ in structure of chemistry from their underlying substrate. Many of the most important of these are epitaxial, having some crystal/crystal relation to an oriented substrate. Ideally, this could be a perfect matching, as found between a few combinations of semiconductors.
For most crystal combinations, control of epitaxy is an active research area. For example, ferroelectric and superconducting epitaxial films must have only a single, uniform orientation to be amenable to nanotechnology. To make these single-oriented films, substrate surface orientation and lattice parameter become controllable parameters for the optimization process. This is a distinct shift away from the common techniques of using only high-symmetry orientations, and particular substrates.
Another aspect of epitaxial films is that of defect introduction and control. Dislocations, stacking faults, twins, etc., are often introduced to the new layer , either to accommodate the crystal mismatch or simply as growth irregularities. In some cases, such as superconductors, these defects appear to improve the desirable properties. In other cases, such as semiconductors, they can destroy the desired function of the layer. The introduction of these defects is often determined by the attachment of the initial atoms of the new layer onto the substrate. MSE researchers are interested in modifying and controlling the growth of even the thinnest layers.
Microstructural analysis gives the most important clues to what parts of the microstructure need to be modified for property improvement, and how the growth parameters must be altered to effect required changes. Systematic variations are then made to see whether this has the predicted influences on the microstructure, and on the end properties.