Plasma Method for Coating the Inside Surface of a Glass Tube

The present invention relates to a plasma method for coating the inside surface of a glass tube by reactive deposition from a gas flowing through the tube and having a pressure ranging from 1 to 100 mbars, the plasma zone being moved relative to the tube in the axial direction. Obviously there is a pressure drop in the axial direction.

Such methods are needed for the fabrication of optical fibers for communication systems. Quartz tubes whose inside walls are coated with doped quartz glass are processed into rods which are subsequently drawn into fibers.

Coating methods using an axially moving plasma zone are in the prior art (DE-OS No. 23 28 930, German Pat. No. 24 44 100, German Pat. No. 26 42 949, DE-OS No. 27 12 993, DE-OS No. 28 04 125, DE-OS No. 29 29 166). These methods suffer from the disadvantage that the plasma state changes during the axial movement of the plasma zone. Electron temperature, degree of ionization, gas temperature and absorbed electric power change with gas density and flow rate along the tube. To achieve a coating of as uniform a quality as possible, either the rate of gas flow is kept low or a high gas density is chosen. Both measures result in a reduction of the relative pressure difference between the beginning and the end of the tube.

A low rate of gas flow means a low deposition rate; a high gas pressure results in undesirable particle formation in the gas volume (J. Irven, A. Robinson: "Optical fibres produced by plasma augmented vapour deposition", Physics & Chem. of Glasses, Vol. 21 (1980)).

The object of the invention is to maintain the plasma state during the axial movement constant.

The invention is characterized in that, with gas flow maintained constant, the gas density in the plasma zone or an electrical variable dependent thereon is maintained constant by controlling the rate of evacuation.

At gas pressures>1 mbar and with tube radii on the order of 1 cm, electron density decreases an electron collision frequency increases with increasing gas pressure (gas density). Both effects result in a reduction in plasma conductivity, i.e., an increase in plasma impedance.

To eliminate the need for additional electric apparatus, the impedance of the plasma-producing device can be used to control the rate of evacuation. In most cases, a voltage or current value dependent on the impedance will suffice for the control.

The advantage offered by the invention is that a highly uniform coating along the tube is achieved with low gas densities or high gas flows. With a predetermined gas density, compared with conventional methods, the rate of gas flow and, thus, the deposition rate can be increased.

An embodiment of the invention as claimed in claim 3 is illustrated in the accompanying drawing. A vessel 1 holds a gas mixture of O.sub.2 and SiCl.sub.4 in the molar ratio of 4:1; the gas pressure is maintained constant at 200 mbars. The gas mixture flows through the nozzle 2 into the quartz tube 3 to be coated. The smallest diameter of the nozzle 2 is 1 mm. In this embodiment the gas pressure at the beginning of the tube is always lower than 20 mbars, so that, due to the decrease of pressure caused by the nozzle 2, the rate of gas flow is 40 mbars l/s and is independent of the position of the valve 4, which is more or less open. The suction capacity of the vacuum pump 5 is 100 m.sup.3 /h. The plasma zone 6 is produced by means of microwave power (2 kW at 2.45 GHz). The entire microwave apparatus, consisting of a magnetron 7, a unidirectional transmission line 8 (circulator with water load), and a plasma-producing device 9, is moved to and fro over a tube length of 1 m four times per minute. The circulator 8 has at its water load a probe with a diode detector whose output voltage is proportional to the reflected microwave power, which amounts to 5 to 10% of the downstream power. With the aid of an electronic comparator 10, the electromagnetic valve 4 is controlled in such a way that the output voltage of the diode detector remains constant. The comparator is adjusted so that the valve 4 is wide open when the plasma zone is at the beginning of the tube. As the plasma zone moves toward the end of the tube, the rate of evacuation is automatically reduced by means of the valve 4; as the plasma zone moves back, the valve opens again. In the present example, the gas pressure at the end of the tube varies between 5 and 7 mbars. The SiO.sub.2 coating is deposited at a rate of 1.2 g/min.