Generation of a Pristine Sapphire Surface by Oxidation and Solid State Conversion of a Sputtered Al Coating

Lehigh University, Bethlehem PA

Investigators

Abstract

This project consists of an in-depth investigation into the mechanisms behind a novel method for generating a pristine sapphire surface. The process is named AGOG, representing the three processing steps: Al thin film deposition on sapphire, Growth of an aluminum Oxide layer, and grain Growth to produce a single crystal surface. Processing conditions at each step in the AGOG process will be varied systematically and correlated to the resulting changes in surface structure. The effect of Al film microstructure and thickness on oxidation behavior will be determined. Sapphire of (0001), (11-20), and (01-12) orientation will be used to identify the effect of interface conditions on the Al to alumina phase transformation. Studies of phase transition behavior will be carried out concurrently with oxidation/heating rate experiments. Once a polycrystalline alumina layer has been created, solid-state conversion processing conditions will be examined. Finally, strategies for eliminating hillock growth during the initial oxidation step will be explored. Sapphire, the single crystal form of alumina, is utilized in many important commercial applications where the quality of the final surface finish is critical. One example is the recent use as a substrate material for high-power blue light emitting diodes (LED's) and laser diodes. These devices are required for a large number of applications including full color electroluminescent displays, and read-write laser sources for high-density information storage on magnetic and optical media. The successful commercial production of blue LED's also offers the potential for bright, white light sources derived from solid state devices. Advantages over conventional incandescent and fluorescent sources include far higher energy efficiency and increased reliability. Sapphire is also widely used for visible and midwave infrared (IR) airborne windows and radomes for guidance systems. Currently, the surfaces of sapphire components are finished by a combination of mechanical and chemical polishing. The final polishing steps are time-consuming, and typically make up a significant fraction of the total cost. The AGOG process is a potentially more cost-effective method for achieving a high quality surface finish in sapphire.