Coating processes – Direct application of electrical – magnetic – wave – or... – Plasma
Reexamination Certificate
2001-03-14
2002-04-16
Mills, Gregory (Department: 1763)
Coating processes
Direct application of electrical, magnetic, wave, or...
Plasma
C427S163200, C118S7230MW, C118S7230ME
Reexamination Certificate
active
06372305
ABSTRACT:
The invention relates to an apparatus for performing Plasma Chemical Vapor Deposition (PCVD), whereby one or more layers of silica can be deposited on an elongated vitreous substrate, the apparatus comprising an elongated microwave guide which emerges into a resonant cavity which is substantially cylindrically symmetric about a cylindrical axis, along which axis the substrate can be positioned.
The term “silica” should here be interpreted as referring to any substance of the form SiO
x
, whether stoichiometric or not, and whether crystalline or amorphous.
Such an apparatus is well known in the art of optical fiber manufacture, for example, and can be used in that context to manufacture a so-called preform rod from which an optical fiber can be drawn. In one known method of manufacturing such a preform rod, a straight vitreous substrate tube (comprised of quartz, for example) is,coated on its inside cylindrical surface with layers of doped silica (e.g. germaniumdoped silica). This can be achieved by positioning the substrate tube along the cylindrical axis of the resonant cavity, and flushing the inside of the tube with a gaseous mixture comprising O
2
, Si Cl
4
and GeCl
2
(for example); a localized plasma is concurrently generated within the cavity, causing the reaction of Si, O and Ge so as to produce direct deposition of Ge-doped SiO
x
, on the inside surface of the substrate tube. Since such deposition only occurs in the vicinity of the localized plasma, the resonant cavity (and thus the plasma) must be swept along the cylindrical axis of the tube in order to uniformly coat its whole length. When coating is completed, the tube is thermally collapsed into a rod having a Ge-doped silica core portion and a surrounding undoped silica cladding portion. Typically, such a rod is of the order of about 1 m long and 2 cm wide. If an extremity of the rod is heated so that it becomes molten, a thin glass fiber (typically of the order of about 125 &mgr;m wide) can be drawn from the rod and wound on a reel; this fiber then has a core and cladding portion corresponding to those of the rod. Because the Ge-doped core has a higher refractive index than the undoped cladding, the fiber can act as a waveguide, e.g. for use in propagating optical telecommunications signals. It should be noted that the gaseous mixture flushed through the substrate tube may also contain other components; e.g. the addition of C
2
F
6
causes a reduction in the refractive index of the doped silica. It should also be noted that the preform rod may be placed in a so-called jacket tube (comprised of undoped silica) prior to the drawing procedure, so as to increase the quantity of undoped silica relative to doped silica in the final fiber.
The use of such a fiber for telecommunications purposes requires the fiber to be substantially free of flaws (e.g. discrepancies in dopant concentration, unwanted cross-section ellipticity, etc.) since, when considered over great lengths of the fiber, such flaws can cause serious attenuation of the carried signal. As a result, it is important that the PCVD process be highly uniform, since the quality of the deposited PCVD layers will ultimately determine the quality of the fiber; accordingly, it is important that the plasma generated in the resonant cavity be highly rotationally symmetric (about the cavity's cylindrical axis). On the other hand, the economy of the production process would be greatly improved if the preform rod could be made thicker, since greater lengths of fiber could then be obtained from a single rod. However, these two goals are difficult to unify, since increasing the diameter of the resonant cavity so as to allow use of a thicker substrate tube generally leads to a plasma with deteriorated rotational symmetry; moreover, such a plasma can only be generated at the expense of a much higher microwave power.
It is an object of the invention to alleviate this dilemma. More specifically, it is an object of the invention to provide a PCVD apparatus with which a plasma having improved rotational symmetry can be generated. In particular, it is an object of the invention that such an apparatus should be compatible with relatively wide substrate tubes. Moreover, it is an object of the invention that the new apparatus should have a relatively low microwave power consumption.
These and other objects are achieved according to the invention in an apparatus as specified in the opening paragraph, characterized in that:
the cavity is substantially annular in form, with an inner cylindrical wall and an outer cylindrical wall;
the inner cylindrical wall comprises a slit which extends in a full circle around the cylindrical axis;
the guide has a longitudinal axis which is substantially perpendicular to the cylindrical axis and which does not intercept the slit.
The apparatus according to the invention exploits a number of insights, which combine to give exceptional results. First, since the cavity is annular instead of just an open-ended cylinder, it is essentially closed, which allows a more efficient standing wave generation; the slit in the inner wall of the annular cavity is then the only means by which a plasma can be generated by such standing waves, via a leakage field. Second, since the longitudinal axis of the microwave guide does not intercept the slit, the microwave energy carried by the guide cannot directly enter the plasma, resulting in more controlled and efficient mode excitation in the cavity. The combination of these factors produces inter alia the following advantageous effects when the apparatus according to the invention is employed to deposit silica on the inside surface of a tube:
1) the coupling of microwave energy into the plasma is observed to be less dependent on various “load” factors such as the flow rate, composition and pressure of the gaseous mixture which is flushed through the tube (and in which the plasma is generated);
2) the generated plasma appears to be more “intense”, in that Si., O. and SiO. radicals are generated in greater profusion than in prior-art apparatus;
3) for a given microwave power level and “load” (see (1)), the number of radicals which reach the internal wall of the tube is higher than in prior-art apparatus.
As a result, a substantially greater silica deposition rate can be achieved at a given microwave power level. In addition, the efficiency with which GeO
2
dopant can be incorporated into the deposited silica is significantly increased. Combined with the fact that the rotational symmetry of the generated plasma is significantly better than in the prior art, even for larger-than-normal tube diameters, these aspects of the invention immediately illustrate the technical and economic advantages of the new apparatus.
It should be noted that the term “microwave guide” is here intended to have a broad scope, and should be interpreted as referring to any means for efficiently transferring microwave energy from a generation device (e.g. a klystron) to the resonant cavity. More specifically, the term encompasses means such as an antenna, coaxial guide, waveguids, etc.
A specific embodiment of the apparatus according to the invention is characterized in that the longitudinal axis of the guide does not bisect the resonant cavity, i.e. the “mouth” of the guide is not located symmetrically (viz. half way) with respect to the extremities of the cavity along its cylindrical axis. As a result of this measure, the coupling of microwave energy into the generated plasma is made even less dependent on the “load”.
A further embodiment of the apparatus according to the invention is characterized in that the length of the resonant cavity parallel to its cylindrical axis is less than &lgr;/2 , in which &lgr;is the vacuum-wavelength of the microwave radiation delivered by the guide. This measure makes allowance for the capacitive coupling which the slit creates between the resonant cavity and the plasma, and generally helps to promote resonance at the desired frequency.
Another embodiment of the apparatus according to the invention is charact
Breuls Antonius Henricus Elisabeth
Van Bergen Andries Heero
Van Stralen Mattheus Jacobus Nicolaas
Hassanzadeh P.
Mills Gregory
Plasma Optical Fibre B.V.
Seed IP Law Group PLLC
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