LDRD Project 52523 final report :Atomic layer deposition of highly conformal tribological coatings.

Abstract

Friction and wear are major concerns in the performance and reliability of micromechanical (MEMS) devices. While a variety of lubricant and wear resistant coatings are known which we might consider for application to MEMS devices, the severe geometric constraints of many micromechanical systems (high aspect ratios, shadowed surfaces) make most deposition methods for friction and wear-resistance coatings impossible. In this program we have produced and evaluate highly conformal, tribological coatings, deposited by atomic layer deposition (ALD), for use on surface micromachined (SMM) and LIGA structures. ALD is a chemical vapor deposition process using sequential exposure of reagents and selflimiting surface chemistry, saturating at a maximum of one monolayer per exposure cycle. The self-limiting chemistry results in conformal coating of high aspect ratio structures, with monolayer precision. ALD of a wide variety of materials is possible, but there have been no studies of structural, mechanical, and tribological properties of these films. We have developed processes for depositing thin (<lo0 nm) conformal coatings of selected hard and lubricious films (A1203, ZnO, WS2, W, and WIA1203 nanolaminates), and measured their chemical, physical, mechanical and tribological properties. A significant challenge in this program was to develop instrumentation and quantitative test procedures, which did not exist, for friction, wear, fildsubstrate adhesion, elastic properties, stress, etc., of extremely thin films and nanolaminates. New scanning probe and nanoindentation techniques have been employed along with detailed mechanicsbased models to evaluate these properties at small loads characteristic of microsystem operation. We emphasize deposition processes and fundamental properties of ALD materials, however we have also evaluated applications and film performance for model SMM and LIGA devices. We have developed a deposition process for WS 2 using WF 6 and H 2 S precursors in a laminar flow ALD reactor. Growth was found to be limited by adsorption and decomposition of WF 6 on most surfaces, and a novel Zn catalyzed deposition method was identified. We propose that WF 6 is partially reduced by Zn or ZnS on the surface, and periodic introduction of (C 2 H 5 ) 2 Zn greatly enhances the growth of WS 2 . Tribologial measurements of WS 2 films show them to have very low friction coefficient, approx. 0.02. Coatings only tens of nm thick can give low friction and increase wear life since sliding is interfacial, i.e., between the transfer film and the wear track on the coating. The hexagonal crystal texture of ALD WS 2 is temperature dependent, but shear-induced reorientation at our high contact stresses assures low friction. This page intentionally left blank 4W and W/Al 2 O 3 films grown at Univ. of Colorado are shown to also be limited by nucleation of W fro WF 6 , and extensive studies of W nucleation have detailed the growth process. Very uniform, smooth films of W and laminates result from proper control of nucleation ...