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Fig. 5 Flux distribution above a point source, based on cosine deposition distribution equation. (a) Distribution of atoms vaporized from a point source. (b) Thickness distribution of film formed on planar surface above source

In actuality the flux distribution from a free surface may not be cosine but can be modified by source geometry, collisions associated with a high vaporization rate, level of evaporant in the source, and so on. In such cases, the flux distribution must be measured directly (Ref 8). A more complete model for the flux distribution from a Knudsen (orifice) source is given by the Knudsen effusion model proposed by Ruth and Hirth (Ref 9).

Particle Energies. Atoms leave a hot surface with thermal energies given by 3/2 kT, where k is Boltzmann's constant and T is the absolute temperature (Ref 2, 6). The atoms have a Maxwell-Boltzmann distribution in velocities. For example, for a 1500 °C evaporation temperature for copper, the mean energy of the vaporized copper atoms is 0.2 eV, and the mean atom velocity is about 1 km/s.

Shutters. Because the particles from a vaporizing source travel in straight lines in a vacuum, a shutter can be used to intercept vaporized material and prevent it from reaching the substrate. By opening and closing the shutter the deposition time can be precisely controlled.

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