Influence of Processing Parameters on Microstructure of Pulsed Laser Deposited Au Thin Films
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Influence of Processing Parameters on Microstructure of Pulsed Laser Deposited Au Thin Films Andreas Kulovits, John Leonard, and Jorg Wiezorek Dept. of Mechanical Engineering and Materials Science, University of Pittsburgh, 848 Benedum Hall, Pittsburgh, PA, 15261 ABSTRACT It has long been recognized that thin film polycrystalline microstructures are determined by the thermodynamics and kinetics associated with physical vapor deposition, but it is quite process dependent and not easily quantified. We have examined the microstructure in polycrystalline Au films obtained by pulse laser deposition (PLD) under various conditions and interpret the results in terms of three fundamental parameters common to all physical vapor deposition: Flux kinetic energy, substrate temperature, and deposition rate. With this model, it is predicted that nanocrystalline films are formed in the limits of low temperature, flux, and high deposition rate. The deposited films are analyzed with X-ray diffraction and SEM to determine texture and grain morphology, which are found to fit well within in the process maps. INTRODUCTION Physical vapor deposition (PVD) of pure gold on SiO2 is an ideal system for exploring kinetic and thermodynamic processes underlying microstructure formation. Pure gold is nonreactive in most atmospheres, and is particularly difficult to oxidize. Amorphous SiO2 is easily prepared as a substrate using thermal oxidation of standard silicon wafers, resulting in a high purity, very smooth surface without crystalline order that would affect orientation selection in the early stages of film growth. It is possible that nanocrystalline Au films on SiO2 may provide an ideal system for mechanical measurements such as nanoindentation or compression. RESULTS Gold films have been deposited by pulsed laser deposition (PLD) on SiO2 substrates under different deposition conditions. A polycrystalline gold target of 99.999% purity was ablated with 248nm KrF excimer laser irradiation at 25 ns fwhm, 10 Hz, target rotation, a spot size approximately 1 x 3 mm, and fluence 8.37 to 9.56 J/cm2. A high vacuum chamber with base pressure 1 x 10-7 Torr was backfilled with high purity Argon gas, and distance between the target and the substrate was 58 mm. Background pressure and the substrate temperature were varied for each deposition. Tsubstrate 25º C 25º C 220º C
Pargon 0 mtorr 500 mtorr 500 mtorr
Deposition rate 3.3Å/s 4.4Å/s 3.6Å/s
Film thickness 300nm 400nm 325nm
Table 1. Deposition conditions for each film, with thickness measured by profilometer, and rate calculated for the 15 minute deposition time.
Figure 1 shows overview backscatter electron SEM and BSE images of the different resulting microstructures obtained under these deposition conditions. The film that has been deposited at room temperature with no background gas pressure is continuous. Grains with an apparent size on the order of 150 – 200 nm can be seen as well as grains that appear to be almost an order of magnitude smaller. It seems that the grain size distr
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