Pulsed Laser Deposition of Poly(Tetrafluoroethylene) Films
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ABSTRACT Thin films of poly(tetrafluoroethylene) have been deposited on amorphous (7059 Corning Glass) and silicon(100) substrates at various temperatures by the Pulsed Laser Deposition technique. The deposition was carried out at high vacuum (_-10-6 torr) at temperatures ranging from room temperature to 350'C. The mechanical properties of these films at the varying process temperatures have been evaluated by nano-indentation techniques and compositional properties of the films have been characterized by Fourier Transform Infrared spectroscopy. The deposition parameters have been optimized in order to produce good quality films. INTRODUCTION Poly(tetrafluoroethylene) [PTFE] is a familiar bulk polymer used in a variety of tribological applications due to its low coefficient of friction and high durability. Thin film applications of PTFE would likely be for similar purposes. In addition, PTFE has a low dielectric constant, which, coupled with a high dielectric strength, makes it a good material for device separation and packaging. PTFE also enjoys excellent thermal, corrosion, and UV resistance [ 1,2]. The last makes excimer laser ablation difficult, but as the ablation products
consist primarily of the monomer [3,4], the ablated materials have been shown to readily repolymerize at the substrate surface, While PTFE has been successfully grown by Pulsed Laser Deposition [PLD] before [5,6], the mechanical properties of PTFE films have not yet been studied. This work is primarily directed towards the optimization of the mechanical properties of PTFE thin films for potential tribological applications. EXPERIMENT PTFE was deposited by irradiating a PTFE target (-2.5 cm diameter and 1 cm thick, supplied by the Goodfellow Corporation) with 248 nm pulses produced by a LEXTRA 200 excimer laser operating with KrF. These pulses were 30 nanoseconds long and contained -400 mJ of energy. The pulses were redirected towards the target through a 450 dielectric plane mirror (-99% reflectance at 248 nm), focused by a one meter focal length, plano-convex lens (-95% transmission), and entered the vacuum chamber through a fused silica laser window (-90% transmission). The pulses were incident on the target plane at a 450 angle and the substrate was located 5 cm away from the target. Temperature measurements were taken from the substrate surface. During the deposition, the target was continuously rotated and the beam was incrementally scanned vertically up and down the target face. 223
Mat. Res. Soc. Symp. Proc. Vol. 397 01996 Materials Research Society
Deposition occurred in high vacuum (10' Torr) on both glass and silicon (100) substrates, using high laser energies (>400 mJ per pulse, resulting in a fluence of 2 Joules/cm2) at a 30 Hz repetition rate for a total of 72,000 pulses. Deposition on glass was completed for temperatures of 350TC, 300TC, 250TC, 200TC, 175TC, and room temperature. PTFE on silicon depositions were completed for temperatures of 350 0C, 300TC, 250TC, 200TC, and room temperature. The mechanical properties, such a
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