Deposition Achieves 12 nm/cycle

Daniel S. Burgess

A team of scientists at Harvard University in Cambridge, Mass., has demonstrated an atomic layer deposition process that yields conformal layers of amorphous silicon dioxide and aluminum oxide nanolaminates at a rate of 12 nm per 30-second cycle. The process, which is 100 times faster than such techniques previously demonstrated for silica, may have applications in the production of planar waveguides, microelectromechanical systems and optical filters.

The reaction involves alternately introducing trimethylaluminum and tris(tert-butoxy)silanol vapor to a substrate at temperatures between 225 and 250 °C. The thickness of the aluminum-doped silica layers that are deposited in each self-limiting cycle depends on the dose of the vapors and on the temperature of the substrate, with no deposition at temperatures below 200 or above 300 °C.

To test uniformity of the coatings produced in the process, the team deposited the films on silicon wafers into which they had etched holes 0.1 to 0.2 µm in diameter and 7 µm in length. Nine deposition cycles filled the holes, and cross-sectional scanning electron micrographs revealed that the reaction is a nearly ideal self-limiting process.

They attribute the self-limiting nature of the reaction to cross-linking processes involving siloxane polymer chains that gel and solidify into silica. Experiments with lanthanum, zirconium and halfnium silicates indicate that other metals also act like the aluminum to speed the deposition rate by a catalytic mechanism.