Effects of Ion Bombardment on the Environmental Durability of Silver Coatings
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EFFECTS OF ION BOMBARDMENT ON THE ENVIRONMENTAL DURABILITY OF SILVER COATINGS G.A. Al-Jumaily, T.A. Mooney, and A. Smajkiewicz Barr Associates, Inc. 2 Lyberty Way Westford, MA 01886 ABSTRACT Effects of ion bombardment during deposition on the environmental durability of silver coatings have been examined. Films deposited under otherwise identical conditions were bombarded at different levels with 300 eV argon ions. Examination of the environmental durability of coatings revealed that films deposited with ion assisted deposition are more durable than evaporated coatings. Adhesion and scratch resistance of coatings were also improved as a result of ion bombardment. Introduction Metal coatings are widely used as wide-band high reflectors in optical systems, and as interconnectors for microelectronic applications. The most widely used of the metal coatings are gold (Au), silver (Ag), and aluminum (Al). Aluminum coatings are durable but have lower overall reflectance in the IR and visible than Ag and Au. Although Au has the highest reflectance in the IR as well as good durability, its poor reflectance in the visible makes it less attractive. Silver has the highest reflectance in the visible, and its IR reflectance is almost as high as that of Au. Silver coatings suffer from very poor stability. Thin films deposited using conventional techniques are unstable when exposed to the environment. The instability of coatings is caused by the columnar microstructure which is typical of evaporated coatings. The presence of voids between the columns allows moisture to penetrate into the coatings and increase the surface area of the coatings which is in contact with the environment. The columnar growth of thin films should be prevented in order to improve the environmental stability of thin films. Several novel deposition techniques have been proposed to improve the stability of optical coatings. Many of these techniques, such as ion assisted deposition (IAD), ion beam sputtering, and ion plating, involve ion bombardment. [1] It has been shown that ion bombardment during deposition causes significant improvements in the optical and mechanical properties of dielectric [1,2] and metal [2,3] coatings. Metal coatings deposited using IAD have been shown to have improved optical constants, and less optical scatter. [2,3] In this paper we examine the effects of IAD on the environmental durability of metal coatings. Sample Preparation Coatings were deposited in a 60 an box 7coater. The system is cryogenically pumped to a base pressure of 3 x 10 Torr. Deposition rate and film thickness were monitored using a crystal monitor. Samples were deposited using thermal evaporation from a resistively heated boat. Some of the samples were bombarded during deposition with 300 eV argon ions from an ion source. The ion source is a 3 cm Kaufmann source. Films were approximately Mat. Res. Soc. Symp. Proc. Vol. 128.
1989 Materials Research Society
476
40 nm thick. The deposition rate for all the coatings was 2 nm/sec. Films were deposited on unheated g
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