Stress Evolution Kinetics in Ultra Thin Sputtered Au Films
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QUANMIN SU, CECIL.,.AILLY*, MANFRED WUTrG*, SEAN CORCORAN*/**" and KARL SIERADZKI ... * Dept. of Matls and Nucl. Eng., Univ. of Maryland, College Park, MD 20742-2115 ** Stagidre de l'Ecole Nationale Superieure des Techniques Industrielles et des Mines d'Ales, France *** Code 6170, Naval Research Laboratory, Washington DC 20375-5342 **** Dept. of Mech. and Aerospace Eng., Arizona State University, Tempe, AZ 85287-6106
ABSTRACT The stress and microstructure of a thin film evolve in time if the deposition is interrupted or terminated. To establish the parameters which control the kinetics of both processes, ultra thin Au layers were sputter deposited on Si membranes and the stress evolution was monitored by a vibrating membrane technique. The evolution of the surface morphology was studied by scanning tunnelling microscopy. Aging after the termination of each deposition causes stress evolution towards higher tension which, around ambient temperature, follows an exponential law with a characteristic relaxation time of the order of tenths of seconds. This time was found to depend strongly on the accumulated film thickness as well as the surface morphology. The intrinsic stress of the depositing layer increases with the coverage of the film on the substrate. Scanning Tunnelling Microscopy shows that the film grows in a Volmer-Weber mode and that the average stress reaches a sharp maximum as the film become continuous.
INTRODUCTION The correlation between the intrinsic stress of thin films and it's growth mode, physical properties and microstructure has been a subject of numerous investigations 1' 2' 3 . The growth of ultra thin films on amorphous substrates was studied systematically establishing a close relationship between intrinsic stress and morphology of the films 4 . Further work with different low melting materials, such as Au, Ag and Cu, growing on single crystal mica also shows a strong dependence of the stress evolution on the film growth mode. The dependence of the stress on the thickness yields details of the film growth mechanism 5 . The growth dynamics was also studied by controlling the substrate temperature of mica (001) during deposition of Ag6 . It was concluded that the polycrystalline Volmer-Weber(VW)-type growth involves two major stress sources; 1. tensile sources stemming from recrystallization and grain boundary relaxation; 2. compression sources stemming from capillarity strains. The latter arises due to compression of islands as a result of surface tension. In situ stress measurements during Cu deposition on Si also demonstrated that film deposition is a kinetic process where stress evolves with time immediately after termination of deposition 7 . These measurements proceeded via layer by layer deposition with an average unit layer thickness of 1.6 nm. After each layer deposition, the evolution of stress with time was monitored by a vibrating membrane technique. The characteristic time of the stress evolution was found to be a linear function of the accumulated film thickness, implying that th
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