Monte-Carlo simulation of the deposition process in PVD-technoIogy
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Introduction
The PVD (physical vapour deposition) technology has been applied in various modem technical fields owing to the wide range of possible coating materials. Besides electronic, optical and decorative applications for PVD-films, hard coatings, in particular, have been widely applied for tool coatings which play an important role for the resistance against wear and corrosion [11. Such coatings are already being developed or are in industrial use. Although individual effects and phenomena that bring about the development of the PVD process, have long be known [2], the theoretical description of this process is not completely matured. Up to now the development of films and the optimization of process parameters depend mainly on empiricism and experiments. In the PVD process many random movements of particles occur simultaneously. Therefore it is impossible or difficult to describe the process by some simple mathematical formulas. But with computer simulation it is possible to investigate the relations between the properties of the film and the conditions of the experiment. The Monte-Carlo method provides an effective tool with few assumptions to simulate the process that includes a large number of stochastic events. The present paper describes a Monte-Carlo model for the d.c. Magnetron Sputter Ion Plating (MSIP) process. This method enables the modelling of the process on the basis of physical laws. 2.
Physical foundation and mathematical model
2.1
Principle of the process
The Magnetron Sputter Ion Plating (MSIP) coating process is one of the PVD processes. High kinetic energy is imparted to electrons in the electrical field between the anode and the cathode, ionizing the electrically neutral inert gas atoms. The positively charged ions are accelerated 359 Mat. Res. Soc. Symp. Proc. Vol. 389 0 1995 Materials Research Society
towards the cathode in the electrical field and impinge with high energy on the target, where an energy and impetus exchange occurs, ejecting atoms and molecules from the target surface. In the case of metal target 95% of these ejected particles are uncharged atoms [3]. The emitted particles have various emission energies and angles which obey certain specific distributions. They are obtained through the investigation of sputtering. 2.2
Movements of the particles from the target to the substrate
In MSIP process the distributions of energy and angle of sputtered particles are characterized not only by the sputtering voltage V and the binding energy Eb of a particle in the bulk, but also by geometrical features of the target erosion zone.
Leaving target surface, an ejected particle travels a distance Ai and collides with a gas molecule. This distance is called current free path. The collision changes the energy and direction of the moving particle and it flies further until next collision (Fig. 1). The length of every current free path is random and characterized by the mean free path 4 and a random number I.: k = -Ap ln ,
0< &I
I
(1)
calculated through the following formula:
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