ICMCTF1999 Session B6: Laser Assisted Deposition

Laser ablation of graphite is one of the most versatile techniques for the synthesis of novel carbon coatings. Ablation in vacuum can be optimized to produce hard amorphous diamond coatings, while low pressures of nitrogen can be introduced to reactively produce CNx films. Higher pressures of inert gas can be used during laser ablation to induce clustering for fullerene and carbon nanotube production.

In this paper, in situ plasma diagnostics are employed to understand and control the fundamental processes responsible for the optimized deposition of novel carbon coatings by the laser ablation of graphite. Several techniques (including intensified CCD-array photography, fast ion probe measurements, optical absorption spectroscopy, optical emission spectroscopy, Rayleigh-scattering, and laser-induced luminescence) are combined to form a coherent picture of the processes occurring during laser ablation, transport to the substrate, and deposition of thin films.

Amorphous diamond film properties vary with the laser energy density at the target and can be conveniently characterized by spectroscopic ellipsometry. Increasing the laser intensity at the target results not only in increased plume ionization and kinetic energies as measured by an ion probe, but in a redistribution of the species comprising the multicomponent plume (C3, C2, and C) arriving at the substrate. Gated imaging is utilized to reveal the interactions of the ablation laser pulse with the initial ejecta at different energy-density thresholds for 248-nm and 193-nm excimer laser wavelengths, and how gas-dynamic processes affect the transport of these species to the substrate. These fundamental studies are correlated with the film properties to understand the optimal conditions for pulsed laser deposition of amorphous diamond and carbon nitride thin films.

Background-gas interactions, particulates ejected from the target, clustering, and plasma manipulation techniques will also be discussed. This research was sponsored by the Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp., for the U.S. Department of Energy, under contract DE-AC05-96OR22464.

Magnetoresistive La_{0.7√sub 0.3}MnO₃ polycrystalline films have been deposited by pulsed laser deposition that exhibit an extreme variability in the temperature of the resistance maximum as a function of the deposition conditions. Films have been deposited that exhibit the resistance maximum at temperatures ranging from 155 to 320 K as a function of laser pulse energy and shadowing conditions. An excimer laser using λ = 248 nm with pulse energies from 500 to 1250 mJ and λ = 193 nm with pulse energies from 400 to 650 mJ has been used to deposit mirror-like fine grained La_{0.7√sub 0.3}MnO₃ polycrystalline films. Films directly crystallized onto polycrystalline alumina substrates at 750 C and subsequently annealed at 750 C in 200 Torr oxygen were very highly (220) textured. Curie point measurements were deduced from hysteresis loops measured at different constant temperatures in a vibrating sample magnetometer with a high temperature oven. The temperature was sufficiently regulated that the loops could be repeatedly retraced at a given temperature. Ferromagnetic Curie points up to 398 K have been measured. This is 34 K higher than the value reported in Ref. 1 for bulk samples produced from powders. The magnetoresistance exhibits both low field and high field components. Suitable circuits have been constructed that give a temperature independent sensitivity of 60 mV/T for applied magnetic fields less than 0.015 T in the vicinity of room temperature.

This research is supported by DARPA and the Office of Naval Research under contract ONR-N00014-96-1-0767 and in part by PSC-CUNY FRAP.

^1. Y. Moritomo, A. Asamitsu, and Y. Tokura, Phys. Rev. B 51, 16491 (1998).

The use of the laser-induced vacuum arc technique (laser arc) to deposit thin films in a large scale industrial process is described.

The deposition method is based on a pulsed arc discharge ignited by a laser pulse between an anode and a rotating cylindrical cathode consisting of the material to be deposited. By using a pulsed laser, the motion of the arc spot on the cathode can be controlled in position, velocity and time. To make the deposition method efficient for industrial applications, the deposition window of a small scale research coating unit¹ with a size of about 100cm² has been scaled up to around 1000cm². The industrial version allows the coating of larger batches of up to several 100 components (few 10cm²) with independent two- or three-fold rotation.

The mechanical and tribological properties of the films obtained from the laboratory unit have already been described¹. The industrial equipment allows a wider variation of the coating parameters in order to optimize the film properties for a particular application. The laser arc technology described allows the deposition of thin films at very low temperatures, so that the coatings of temperature sensitive materials and of engine components with high quality films is now possible.

Examples of the application of laser arc deposited films will be presented.

¹ Scheibe, Drescher, Schultrich, Falz, Leonhardt and Wilberg in Surface and Coatings Technology 85(1996)209-214