Laser Photochemical Synthesis of Si.sub.3 N.sub.4

The mechanical strength of optical fibers in excess of 2.times.10.sup.5 psi is a desirable feature for certain specialized application (e.g., optical waveguides employing lengths of fiber).

Lack of mechanical strength is due to submicron flaws in the surface attributed, mainly, to chemical attack by atmospheric contaminants (e.g., moisture) during and after fiber drawing. Attempts to solve these problems have been studied by applying organic coatings to the fiber following the drawing of the fiber. Failure resulted because those organic coatings are not impervious to moisture or hydroxyl penetration. The penetration by moisture or hydroxyl resulted in reduced strength of the coated fiber during periods of storage and/or use.

Silicon nitride (Si.sub.3 N.sub.4) is an appropriate coating since it is resistant to moisture penetration and bonds securely to the silicon substrate fiber. Previous attemps to clad silicon fibers with Si.sub.3 N.sub.4 uses thermal (pyrolysis) techniques.

Thermal (pyrolysis) methods of influencing chemical processes lead, mainly, to the excitation of all degrees of freedom of the molecule. Both external (translational) and internal (electronic, vibrational and rotational) degrees of freedom are usually in thermodynamic equilibrium. In addition to there being an unproductive waste of energy, reactions with equilibrium excited molecules characteristically proceed in the direction of breaking the weakest bond, have a considerable percent of back reaction, many side reactions, and produce polymers.

Advantageous would be a method which employs laser photochemical reactions (LPR) to produce silicon nitride to clad freshly-drawn silicon fibers in an atmospheric controlled chamber. A particular advantage is recognized over the prior art vapor deposition method since the LPR method can be effected at room temperature.

An object of this invention to provide a method of coating optical fibers with Si.sub.3 N.sub.4 produced by laser photochemical reactions.

Another object of this invention is to deposit Si.sub.3 N.sub.4 on freshly-drawn silicon optical fibers in an atmospheric controlled chamber in a continuous operation which employs SiF.sub.4 and NH.sub.3 as the reaction gases.

A further object of this invention is to deposit Si.sub.3 N.sub.4 on freshly-drawn silicon optical fibers in an atmospheric controlled chamber in a continuous operation which employs SiH.sub.4 and NF.sub.3 as the reaction gases.

Si.sub.3 N.sub.4 is produced at room temperature in an atmospheric controlled chamber are coated with Si.sub.3 N.sub.4 by laser photochemical reactions which are initiated in accordance with the following equation: ##EQU1## wherein X is H and/or F when reaction is initiated by a CO.sub.2 laser.

A silicon optical fiber is hermetically sealed by cladding with Si.sub.3 N.sub.4 synthesized by laser photochemical reactions initiated on the gases SiX.sub.4 and NX.sub.3 in a controlled atmosphere.

The silicon optical fiber is prepared in a standard fiber drawing furnace from a preform rod of fused silica which is brought to its yield point at about 2000.degree. C. The silicon fiber is drawn from the preform and through a chamber with controlled atmosphere (SiX.sub.4 and NX.sub.3 gases, wherein X is hydrogen and/or fluorine) at room temperature. The laser is used to irradiate this atmosphere to form Si.sub.3 N.sub.4 which is coated on the silicon optical fiber to a thickness of about 0.02-0.20 micrometer.

The laser photochemical reaction initiated by the process of this invention when conducted in the presence of other substrates; e.g., other than optical fibers such as rocket motor nozzles or other components is effective in providing a coating of Si.sub.3 N.sub.4 on the substrates at room temperature (about 25.degree. C.). This method permits coating other materials which could not withstand high temperature conditions employed by vapor deposition method. A pressure between about 10 and 200 torr of the reactive gases is satisfactory for coating the silicon optical fiber with the reaction product, Si.sub.3 N.sub.4. The thickness of the coating is proportionate to the flow rate of the reactants through the reaction chamber.

The laser photochemical reaction initiated by the process of this invention is an effective method for producing the compound Si.sub.3 N.sub.4 at room temperature. For this particular method the laser photochemical synthesis can be carried out by employing a pulsed laser or a continuous wave laser. The laser radiation is directed through a salt window into a controlled atmosphere of SiX.sub.4 and NX.sub.3, wherein X is selected from hydrogen and/or fluorine. For example Si.sub.3 N.sub.4 is synthesized from SiF.sub.4 +NH.sub.3 and/or SiH.sub.4 +NF.sub.3 at room temperature in either a batch process or in a continuous flowing process. The product is separated as a solid powder in high yield by using a cold finger or scraping the solid powder from reaction chamber surfaces. The laser photochemical synthesis is illustrated as follows: ##EQU2##