DE4034211C1 - Coating interior of pipe-glass tube - comprises coupling HF energy to tube using resonator to deliver pulsed microwave discharges - Google Patents

Abstract

In a process for coating the interior of glass tubes, high frequency energy is coupled to the tube via several high frequency resonators arranged adjacent to and spaced from each other in the coating zone. The resonators, which are coaxial with the tube, are independently switched and controlled and pref. operated at 2.45 GH for pulsed microwave discharges in a reaction gas flowing through the tube. ADVANTAGE - Improved axial coating homogeneity.

Description

The present invention relates to a method for Internal coating of tubes made of glass plasma pulse induced chemical Vapor deposition (PICVD process), in which through a pulsed microwave discharge from a flowing through the glass tube to be coated Reaction gas a sequence of layers on the Is deposited inside the glass tube. The The invention also relates to a device for Execution of the procedure.

Process for the interior coating of glass tubes under Use of a plasma (e.g. PCVD, PICVD) general state of the art. In doing so, the Inside of a glass tube from thin layers deposited doped quartz glass. By choosing and the concentration of the dopant individual layers is the desired one radial refractive index curve set.

A method for coating the inside of the pipe PICVD method is for example from the EP-A-00 36 191 and from U. Ackermann et al., "Schott: Possibilities of a special fiber manufacturer ", SPRECHSAAL, Vol. 122, No. 5, pp. 461-466 (1989) known, the latter using the PICVD technology for Manufacturing of optical fibers is applied to to realize different refractive index curves.

In the PICVD process, the plasma is pulsed microwave discharge generated.  

The reaction gases and dopants are through that of a shorter inner and a longer one outer metal pipe to be coated Glass pipe passed. One stops around this coaxial Tubes arranged around the temperature in the Coating area at the required level of approx. 1373 K.

The plasma ignites during a microwave pulse initially at the end of the shorter inner tube. From there the plasma spreads through the glass tube and allows the microwave to spread through this from the plasma column and the metallic Coaxial system formed outer conductor ("Plasma cable") through to the end of the Coating area (see H. Bauch et al., "Chemical Vapor Deposition in Microwave Produced Plasma for Fiber Preforms ", J. Opt. Commun. B, 130-135 (1987)). The maximum length of the plasma column depending on the operating mode, either by the duration of the microwave pulse limited (temporal Limitation) or by damping the microwave when spreading along the plasma cable (spatial limitation), because when falling below a Threshold value of the microwave power at the end of the Plasma column no longer ignited. Such Procedure is also from DE 38 30 089 A1 known to have a large number of thin dielectric layers are created to be planar To manufacture glass substrates.

A disadvantage of this method is that the plasma onto the glass tube to be coated Dependence on the axial coordinate acts differently and strongly because Amplitude and exposure time of the microwave pulse  because of the axial damping of the microwave and the finite rate of propagation of the Reduce the plasma ignition front axially continuously. As a result, pipe interior coatings e.g. B. dopants, the degree of installation of the Layer temperature and the mean Microwave power depends, axially different heavily built. Then in the coating area the properties of the coating are axially unequal, which is generally undesirable.

It also leads towards the Microwave propagation declining Microwave power to a temperature distribution inside the tube, the maximum of which is near the Microwave coupling lies. The maximum allowable mean microwave power is limited that the pipe is exceeded at this point a threshold value Pw of the middle Microwave power due to the additional Oven heat softens and deforms.

An increase in the length of the coating zone by increasing the microwave pulse power is due to of the above. Overheating of the pipe near the Coupling point only possible to a limited extent.

Due to the drop in microwave power in The direction of propagation of the microwave results in given coating length a limitation of maximum possible coating rate from the following Reason. To the plasma at the distal end ignite, the power density of the microwave must be there are above a threshold value Ps. This threshold and also the amount of axial power drop increase when the coating rate goes through Increasing the pressure is increased. Since the  Microwave power at the coupling end However, multiples of Ps are not greater than Pw there is an upper limit for the Microwave power and thus the coating rate. However, one is possible for many coatings high coating rate desired.

Another method for interior coating is that PCVD procedure. Doing so will improve axial layer homogeneity a plasma generator axially moved along the glass tube and thereby the Microwave power continuously the plasma supplied (see P. Bachmann "Review of plasma deposition applications: preparation of optical waveguides ", Pure & Appl. Chem., Vol. 57, 1299-1310 (1985) and H.v.Seefeld "Glass separation from the Gas phase ", PLASMA TECHNOLOGY, surface + JOT, Issue 3, pp. 24/25 (1990) in conjunction. P. Geittner "Low loss optical fibers prepared by plasma-activated chemical vapor deposition (CVD) ", Applied Physics Letters, Vol. 28, No. 11, S 645/646 (1976)). So is from the DE 32 22 189 C2 a plasma process for Inner coating of pipes known in which the Displacement of the separation zone by electrical Means is effected. Through a radio frequency field creates a plasma column, the length of which changes the high-frequency power fed in varies is, the separation zone by means of a Microwave generator along the tube axis electrically is moved. In the PCVD process according to D. Küppers et al., "Codeposition of Glassy Silica and Germania Inside a tube by plasma activated CVD ", J. Electrochemical. Soc., Vol. 123, No. 7, pp. 1079-1083 (1976) become a very large number in this way generated by layers.  

A major disadvantage of this PCVD technology lies however in the fact that through this axial Movement of the microwave generator along the Tube building a coating of very thin Single layers is very time consuming. In particular is used in the manufacture of preforms for Optical fiber such an apparatus very much complex and therefore also expensive, because on the one hand the mechanical movement very precise in this process must be and secondly the electrical Feed lines of the moving plasma generator through the furnace surrounding the coating area must be passed through.

The present invention is therefore the object based on a method and an apparatus of to create the type specified at the beginning, with the the axial layer homogeneity in a simple manner improves and the economics of the process can be increased. Another job of The present invention is as high as possible Coating rate in the coating area too obtained without the pipe to be coated through Softening and deforming becomes unusable.

According to the invention, these tasks with the Method according to claim 1 and the device solved according to claim 10.

According to the invention, several radio frequencies resonators in the coating area on the Glass tube spaced that their axes with the glass tube axis coincide and become theirs High frequency fields can partially overlap. To Transmission of the high-frequency power into the plasma can use these high-frequency resonators can be switched and controlled independently.  

The radio frequency power is transmitted in this way not from a single one Coupling point in the axial direction of the coating glass tube, but azimuthal from each of the resonators, the dimensions of the Resonators are chosen so that only certain Modes can arise in the resonator.

This method has the advantage that both the Temperature load for the thing to be coated Glass tube significantly below the deformation temperature can be held as well as a uniform coating over the entire too coating tube length is achieved.

The spacing of the resonators from one another is thereby chosen so that the superimposition of the High frequency fields from the middle of the one to the Middle of the adjacent resonator an im sets substantially constant plasma in the glass tube. This distance can easily be determined experimentally will.

For this purpose, a distance of z. B. half a resonator width and a coating of z. B. fluorine-doped SiO ₂ . The degree of incorporation of fluorine into the layer and the refractive index difference to undoped SiO ₂ thus obtained decrease with increasing substrate temperature. The substrate temperature in turn increases with the microwave power.

The axial course of the Refractive index difference from layer and substrate tube Quartz glass is now measured. Depending on the size and  Opening diameter at the front of the Resonators can be the difference in refractive index in the range between the resonators (= intermediate area) larger or less than the refractive index difference in Resonator area.

Is the refractive index difference in the intermediate range the microwave power in the Intermediate area too small; about the microwave power the distance between the resonators must be increased be made smaller.

Is the refractive index difference in the intermediate range smaller, then the microwave power in the Intermediate area too large; about the microwave power to reduce, the distance must be increased.

This way it can be iterative with a few tries the distance can be found at which the difference the refractive index in the resonator and in the intermediate range can be brought under an opposite value. The one obtained in this manner described above axially homogeneous plasma is created by superimposition the microwave fields of several, separately controllable individual resonators. This offers the Advantage that even slight deviations from the Homogeneity of the coating due to the possibility to switch the individual resonators independently and to control, can be balanced.

In a preferred embodiment of the inventive method are two neighboring Operated resonators just so that one Pulse the performance of one by just the value degraded by the performance of the neighboring Resonators is increased. At the following pulse become the resonators with reversed power  operated. This has the advantage that the Intermediate area with low thermal load of the resonator area coated more uniformly because the microwaves couple out more.

Such resonators are expedient used whose electric fields are azimuthal are constant because then the coating is azimuthally homogeneous.

The coating can be homogenized according to the inventive method can also be improved in that the Can rotate tube about its longitudinal axis.

In a further embodiment of the invention can through the pipe in the axial direction known means to move back and forth be, with a shift of the glass tube by half a resonator width each is preferred, since then possibly existing Irregularities in the plasma also in the Resonator range can be compensated.

The coating can also be made more uniform through a suitable choice of the opening diameter can be reached in the front sides of the resonators, through which the glass tube to be coated passes is led. Is z. B. the opening diameter enlarged, can have a stronger microwave field emerge so that a larger intermediate area can be equalized. As by this measure but at the same time also the microwave field in the Resonator is affected, depending on the application if a compromise is found.

Due to the possibility of independent control and  The individual resonators can be switched Improvement of the coating process too due to a temporal variation of the Achieve high frequency performance of the resonators. Becomes for example, each in the gas flow direction neighboring resonator for a certain period of time switched later than the previous one, so becomes a defined spread of the plasma ignition front and same burning time of the plasma all over Coating length guaranteed. It can also be a any resonator first and the respective one Neighbors offset by this certain amount of time be ignited. This period should be in Ratio to the pulse duration be small. A preferred value for this period is in the Of the order of 10 microseconds.

The high-frequency resonators are equipped with a Frequency between 800 MHz and 4 GHz, preferably operated at 2.45 GHz.

The device for carrying out the inventive method has along the Coating area of the glass tube several over or statically arranged next to each other at a distance, held by known fasteners Individual resonators on that to be coated Enclose the glass tube so that its axes align with the Glass tube axis collapse.

The distance between the individual resonators is chosen so that by superposition of high frequency fields from the middle of one to to the center of the adjacent resonator sets constant plasma.

Since the entire arrangement in one furnace area  Temperatures between 500 K and 1500 K, preferred 1300 K is operated, both exist Resonators as well as the leads temperature-resistant metal, preferably made of Platinum. The inner conductor of the coaxial pipe system by ceramic body against the cylindrical outer conductor protected. Since the High frequency resonators cannot be moved a relatively simple and therefore cheaper Oven construction can be selected.

The invention is based on Embodiments in connection with the Drawings explained in more detail.

Show it:

Fig. 1 is a schematic cross-sectional drawing of an arrangement according to the invention of high-frequency resonators along the coating region of a glass tube,

Fig. 2 shows a cross section through a single resonator,

Fig. 3 shows a cross section through the glass tube to be coated and a high-frequency resonator with the arrangement of the leads, and

Fig. 4 is a diagram with the achieved according to the invention the constant refractive index profile within the coating area for an arrangement with three resonators.

According to Fig. 1 (not shown) gas generator provides a stream of reaction gases in the glass tube 10 in which the reaction takes place. The gas flow direction 9 is represented by the arrow in FIG. 1. The glass tube 10 is located in an arrangement 12 of high-frequency resonators 14 for carrying out the inner tube coating according to the known PICVD method.

For a coating area of 60 cm in length, eight individual resonators 14 are arranged along the coating area. For a high frequency of 2.45 GHz, the length l of each individual resonator is 14 l = 10 cm (see FIG. 2). With this length, axial upper modes are not capable of spreading. The distance d between the individual resonators 14 is chosen up to 1 cm. This ensures that the edge portions of the area to be coated are also covered.

In order that only the TM _olo mode (azimuthal-symmetric mode) can be propagated, the resonator inner diameter is preferably chosen to be d _r = 10 cm (see FIG. 2).

As can be seen in FIG. 2, each resonator 14 has on its end faces 4 , 5 openings 2 , 3 through which the glass tube 10 is passed. The face opening of each resonator is d _ö = 3 cm. In this case, a sufficiently large part of the microwave energy can couple into adjacent resonators 14 , which leads to a homogenization of the resonator edge regions with regard to the energy.

Each individual resonator 14 is supplied with microwave energy via a coaxial line 16 with an outer diameter of 30 mm. In the jacket 6 of the resonator 14 z. B. introduced two coupling openings 7 , 8 , which are connected to the feed line 16 . The component E _{z of} the microwave fields is identified by the arrow.

As can be seen in FIG. 3, the feed lines 16 are arranged, for example, in a circle at angular intervals of 45 ° around the glass tube 10 and the resonator 14 (only the first resonator is shown in this illustration).

The individual resonators are powered by pulses operated between 500 W and 5 kW, preferably 1 kW, the clock ratio is between 1: 5 and 1:10.

Fig. 4 shows the profile of the refractive index difference between the substrate layer and a structured in the coating area with the coating process of this invention as a function of the glass tube length. For comparison, the course according to the prior art is shown. It can be seen that the coating according to the method according to the invention has a refractive index difference which is essentially constant over the coating length.

Reference symbol list

2 opening
3 opening
4 end face
5 end face
6 outer jacket
7 coupling opening
8 coupling opening
9 direction of gas flow
10 glass tube
11 axis
12 arrangement
14 single resonator
16 coaxial line

Claims (18)

1. A method for the inner coating of glass tubes by plasma pulse-induced chemical vapor deposition (PICVD method), in which a sequence of layers is deposited on the inside of the glass tube by a pulsed microwave discharge from a reaction gas flowing through the glass tube to be coated, characterized in that the high-frequency energy is coupled in via a plurality of high-frequency resonators ( 14 ) arranged around the glass tube ( 10 ) in its coating area and the axes of which coincide with the glass tube axis ( 11 ), the high-frequency resonators ( 14 ) being switched and controlled independently. 2. The method according to claim 1, characterized in that the high-frequency resonators ( 14 ) are operated at a high frequency between 800 MHz and 4 GHz. 3. The method according to claim 2, characterized in that the high-frequency resonators ( 14 ) are operated at a high frequency of 2.45 GHz. 4. The method according to any one of the preceding claims, characterized in that adjacent high-frequency resonators ( 14 ) are each clocked with different powers. 5. The method according to claim 4, characterized in that the power of one high-frequency resonator ( 14 ) is reduced by just the value by one pulse, by which the power of the adjacent high-frequency resonator is increased, and that the high-frequency resonators are operated with interchanged power at the next pulse . 6. The method according to any one of claims 1-5, characterized in that high-frequency resonators ( 14 ) are used, the electric fields of which are azimuthally constant. 7. The method according to any one of claims 1-6, characterized in that the glass tube ( 10 ) is rotated about its axis ( 11 ) during the coating. 8. The method according to any one of the preceding claims, characterized in that the glass tube ( 10 ) to be coated is displaced in the axial direction during the coating process. 9. The method according to claim 8, characterized in that the glass tube to be coated ( 10 ) is shifted by half a resonator width. 10. Apparatus for carrying out the inner coating of glass tubes according to the PICVD method, characterized in that several high-frequency resonators ( 14 ) are arranged in the coating area in such a way around the glass tube ( 10 ) to be coated and along the glass tube ( 10 ) that their Axes coincide with the glass tube axis ( 11 ) and their high-frequency fields partially overlap. 11. The device according to claim 10, characterized in that the high-frequency resonators ( 14 ) can be operated with a high frequency between 800 MHz and 4 GHz. 12. Device according to one of claims 10 or 11, characterized in that adjacent high-frequency resonators ( 14 ) can be clocked with different powers. 13. Device according to one of claims 10-12, characterized in that the glass tube ( 10 ) to be coated is rotatable about its longitudinal axis ( 11 ) and / or displaceable in the axial direction during the coating. 14. The apparatus according to claim 13, characterized in that the glass tube ( 10 ) is displaceable by half a resonator width. 15. The device according to any one of claims 10-14, characterized in that the high-frequency resonators ( 14 ) in their outer jacket ( 6 ) spaced two coupling openings ( 7 , 8 ) through which the microwaves can be coupled by means of a coaxial feed line ( 16 ). 16. The apparatus according to claim 15, characterized in that the supply lines ( 16 ) are arranged coaxially to the glass tube axis ( 11 ) and equidistant from each other around the high-frequency resonators ( 14 ). 17. The device according to one of claims 15 or 16, characterized in that the feed lines ( 16 ) are made of platinum. 18. Device according to one of claims 10-17, characterized in that the distance d of the high-frequency resonators ( 14 ) is selected such that the difference in the refractive index in the high-frequency resonator ( 14 ) and in the area between the high-frequency resonators ( 14 ) falls below a specified value .