Apparatus for determining the rate of flow of particles in a vacuum

Abstract

An apparatus for determining the rate of flow of particles in a vacuum deposition device using the relationship that the rate of flow of the particles is equal to the density of the particle current times the mean velocity of the particles comprises establishing a flow or current of particles in the device and the directing of a laser beam through the current of particles in order to measure the attenuation of the beam after it has passed through the particles and thus to determine the vapor density within the current. The absorption wavelength shift of the laser beam provides an indication of the velocity component of the current because the shift is produced by a Doppler effect by the velocity component of the particle current in the direction of flow from the source of the particles to the substrate to be coated. In a preferred form the vacuum deposition device comprises a housing which has a vacuum chamber in which the substrate to be coated is held in a position across the chamber from a source of the coating material which is evaporated or sputtered so that a current of particles moves from the source to the substrate across the chamber. At least one laser beam is directed through the particles and is picked up on the opposite side by a detector which is capable of measuring the absorption wavelength shift. Two laser beams may be directed through the particle stream including one transverse to the current direction and one parallel to the current direction. The second beam may be effected by using a semi-transparent mirror to deflect the original laser beam so that it is passed again through the chamber in a direction at substantially right angles to the first beam direction.