Plasma chemical vapor deposition apparatus

Abstract

The invention relates to a plasma chemical vapor deposition device for depositing one or more layers of silicon dioxide onto the inner wall of a long hollow glass base tube, the device comprising a microwave generator, a plasma generator which in use receives microwaves from the generator, a cylindrical cavity extending through the generator, and a cylindrical bushing positioned in the cavity, wherein the base tube in use passes through the bushing, wherein over a part of the length of the bushing the bushing has at least one section with a reduced inner diameter, which at least one section provides a contact zone for the base tube, wherein the microwave generator is configured to generate microwaves with a wavelength Lw in the range of 40 mm to 400 mm, wherein the length of the at least one section with the reduced inner diameter is at most 0.1 x Lw and at least 1 mm.

Description

Technical Field

[0001] This invention relates to a plasma chemical vapor deposition apparatus. This apparatus is used to deposit one or more layers of silica onto the inner wall of a long hollow glass substrate tube. Deposition is achieved by flushing the substrate tube with gas in conjunction with the generation of plasma. This substrate tube with the deposited silica layer is used to produce optical fibers. After deposition, the tube collapses in a heating device to form a core for the optical fiber. Background Technology

[0002] EP 1988065A1 relates to such a deposition apparatus. The apparatus has a resonator and an inner tube extending through the resonator. An inner ring is provided at its end of the inner tube. This inner ring maintains proper radial positioning of the base tube by preventing excessive tube bending, while reducing the unwanted amount of contact, i.e., the size of the contact area, between the inner walls of the base tube and the bushing when they come into contact. Such contact could damage both the base tube and the inner tube.

[0003] In this regard, it was also observed that the radial positioning or centering of the base tube is related to the homogeneity of the deposited layer. Eccentric base tubes may negatively affect plasma generation. Summary of the Invention

[0004] The object of this invention is to provide an improved plasma chemical vapor deposition apparatus. Another object of this invention is to provide a plasma chemical vapor deposition apparatus that enables higher quality deposition processes.

[0005] One or more of the above objectives are achieved by a plasma chemical vapor deposition apparatus according to the invention for depositing one or more layers of silica onto the inner wall of a long hollow glass substrate. The apparatus includes a microwave generator, a plasma generator that receives microwaves from the microwave generator in use, a cylindrical cavity extending through the plasma generator, and a cylindrical liner that is transparent to microwave radiation and positioned within the cavity, the central axis of the liner being coaxial with the central axis of the cavity. The substrate passes through the liner in use, wherein, along a portion of the liner's length, the liner has at least one segment with a reduced inner diameter relative to the remaining portion of the liner. This at least one segment provides a contact area for the substrate. The microwave generator is configured to generate microwaves with wavelengths Lw in the range of 40 mm to 400 mm, wherein the length of the at least one segment with the reduced inner diameter is at most 0.1 × Lw and at least 1 mm.

[0006] The specific limitation of the length of the at least one section with a reduced inner diameter results in greater temperature stability of the bushing, particularly the at least one section of the bushing. In particular, the inventors have found that a length that is too large relative to the wavelength of the microwaves generated during the use of the device negatively impacts the temperature stability of the section, because the temperature of the section will rise too much due to absorption of microwave radiation, which may occur due to contamination and damage to the inner surface of the section, such as from contact with the base tube. In the case of contact between the base tube and the section of the bushing, this undesirable temperature rise will further affect the base tube temperature in an undesirable localized manner, thereby reducing the quality of the deposition. In particular, the bonding efficiency of germanium dioxide will be significantly affected by this undesirable contact between the overheated sections of the base tube and the bushing, resulting in increased variability in the refractive index of the deposited layer between the individual base tubes with the silicon dioxide deposition layer, which is undesirable given the very narrow product specifications required. On the other hand, choosing the length too small will lead to higher wear of the section and thus reduced durability.

[0007] In this embodiment, the bushing is made of quartz glass or fused silica. Preferably, the bushing material has a transparency of at least 98% to microwave radiation, and more preferably at least 99%.

[0008] In embodiments, the length of the at least one segment having a reduced inner diameter is in the range of 1 mm to 20 mm, preferably in the range of 2 mm to 10 mm. A bushing having the at least one segment within this range is also advantageously suitable for devices without a microwave generator. Therefore, the invention also relates to a plasma chemical vapor deposition apparatus for depositing one or more layers of silica onto the inner wall of a long hollow glass substrate tube, the apparatus comprising a plasma generator, a cylindrical cavity extending through the plasma generator, and a cylindrical bushing positioned within the cavity, the central axis of the bushing being coaxial with the central axis of the cavity, wherein the substrate tube passes through the bushing in use, wherein over a portion of the bushing's length, the bushing has at least one segment having a reduced inner diameter relative to the remaining portion of the bushing, the at least one segment providing a contact area for the substrate tube, wherein the length of the at least one segment having a reduced inner diameter is in the range of 1 mm to 20 mm, preferably in the range of 2 mm to 10 mm. In one embodiment, the device further includes a microwave generator, wherein the plasma generator receives microwaves from the microwave generator during use, and wherein the bushing is transparent to microwave radiation.

[0009] The effect of a device whose absolute length measurement is in the range of 1 mm to 20 mm, preferably 2 mm to 10 mm, is similar to the effect described above associated with embodiments of the invention having segment lengths relative to wavelength. That is, in these embodiments of the invention, the aforementioned contamination and damage may also negatively affect temperature stability. The following embodiments apply both to devices where the bushing segment has a relative length related to wavelength and to devices with a length defined in an absolute sense.

[0010] Throughout this specification and claims, the term "inner diameter" as used with respect to a section having a reduced inner diameter shall be interpreted as the inner diameter of the cylindrical inner surface covering the section, and the diameter of the inscribed circle covering the inner surface of the section. Attached Figure Description

[0011] The invention is described below with reference to the accompanying highly schematic drawings, in which embodiments of the invention are shown, and in which the same reference numerals indicate the same or similar elements.

[0012] Figure 1 An embodiment of the plasma chemical vapor deposition apparatus according to the present invention is shown in a vertical cross-section; and

[0013] Figure 2 It shows Figure 1 The cross section of the device is II-II. Detailed Implementation

[0014] Figure 1 A plasma chemical vapor deposition apparatus 1 is shown for depositing one or more layers of silica onto the inner wall 3 of a long hollow glass substrate 2. The apparatus 1 includes a microwave generator 4, which is configured, for example, to generate microwaves with wavelengths Lw in the range of 40 mm to 400 mm, such as approximately 120 mm.

[0015] In an embodiment, the length of the at least one segment having a reduced inner diameter is at most 0.05 × Lw.

[0016] In an embodiment where the wavelength Lw is approximately 120 mm, the length of the at least one segment will then be at most approximately 6 mm (and at least 1 mm). Preferably, the microwave generator generates microwaves at a frequency of approximately 2.45 GHz corresponding to the wavelength. Alternatively, a frequency of approximately 5.6 GHz can be used, or even more alternatively, a frequency of approximately 890 MHz can be used.

[0017] The device 1 shown in the figure also includes a plasma generator 15, which receives microwaves from a microwave generator 4 during use. Device 1 has a cylindrical cavity 6 extending through the plasma generator and a cylindrical bushing 8 made of quartz glass, which is transparent to microwave radiation and positioned within the cavity 6. The central axis 10 of the bushing 8 is coaxial with the central axis of the cavity 6. A base tube 2 passes through the bushing 8 during use. Figure 2 The base tube 2 is not shown in the diagram. In use, the device 1 and the base tube 2 can be moved axially relative to each other while a suitable gas is flushed through the base tube 2 and plasma is generated to deposit a silicon dioxide layer along the length of the base tube. On a portion of the length L of the bushing 8, the bushing 8 has at least one segment 12 with a reduced inner diameter d relative to the inner diameter D of the remaining portion of the bushing, which provides a contact area for the base tube 2. According to the invention, the length l of said at least one segment with the reduced inner diameter d is at most 0.1 × Lw, i.e., 0.1 times the wavelength Lw, and at least 1 mm. In an example where Lw is approximately 120 mm, the length of said at least one segment would then be at most approximately 0.1 × 120 = 12 mm and at least 1 mm. In the example, the length l of said at least one segment 12 with the reduced inner diameter can be in the range of 2 mm to 3 mm.

[0018] In one embodiment, the plasma generator includes a microwave resonant cavity surrounding a cylindrical cavity, wherein the microwave generator is connected to the resonant cavity.

[0019] An example of such a microwave resonant cavity is microwave resonant cavity 14, as shown in the figure, which is formed within a plasma generator 15. This particular cavity 14 rotates around a cavity 6, leaving a slit 16 defined by an inner wall 18 through which microwaves can enter the cavity 6. Figure 1 As shown, slit 16 is axially offset relative to the inlet hole 20 that connects cavity 14 to microwave generator 4 (meaning it is offset in a direction parallel to the central axis 10). Figure 1 The cross-sectional view shown indicates that the slot 16 is positioned further to the right relative to the inlet aperture 20 in the plasma generator 15. The exact axial position of the slot 16 relative to the aperture 20 may depend on the characteristics of the inlet aperture and associated microwave generator components.

[0020] The microwave generator 4 may have a waveguide connected to the microwave generating element. It should be noted that the term "waveguide" as used herein has a broad meaning and should be interpreted as referring to all means used to efficiently transfer microwave energy from the generating element (e.g., a klystron or magnetron) to the resonant cavity. More specifically, the term includes specific devices such as antennas, coaxial conduits, waveguides such as rectangular waveguides, etc.

[0021] In an embodiment, the length of the at least one segment having a reduced inner diameter is in the range of 1 mm to 20 mm, preferably in the range of 2 mm to 10 mm.

[0022] In this embodiment, the reduced inner diameter of the bushing is in the range of 25 mm to 100 mm, preferably in the range of 35 mm to 65 mm.

[0023] In the example shown in the figure, the reduced inner diameter d of section 12 of bushing 8 can be approximately 46 mm, while the inner diameter D of the remaining portion of bushing 8 can be approximately 50 mm.

[0024] In use of the device, the inner diameter can be approximately 0.2 mm to 0.8 mm larger than the outer diameter of the base tube. Alternatively, in other words, the device is preferably used with a base tube having an outer diameter approximately 0.2 mm to 0.8 mm smaller than the inner diameter d of the at least one section having a reduced inner diameter.

[0025] In one embodiment, the inner diameter of the remaining portion of the bushing is at least 2% larger than the inner diameter of the at least one section. This diameter difference is sufficient to avoid contact between the base tube and the inner wall of the remaining portion of the bushing.

[0026] In one embodiment, the at least one segment is formed by attaching an inner ring to the inner wall of the bushing, wherein the reduced inner diameter of the segment is defined by the inner diameter of the ring. Thus, the inner ring and the cylindrical remainder of the bushing can be manufactured independently of each other, and then the inner ring is attached to the cylindrical remainder to obtain a bushing with a segment having a reduced inner diameter. The inner ring can be melted onto the inner wall of the bushing.

[0027] As shown in the figure, bushing 8 is manufactured by first separately manufacturing the cylindrical portion of the bushing and two inner rings, and then melting the inner rings 12 onto the inner wall 9 of the remaining portion of the bushing.

[0028] In this implementation, the bushing and the inner ring are made of the same material.

[0029] In one embodiment, the at least one section is formed by partially collapsing the bushing. Alternatively, or as another manufacturing step of partial collapse, the at least one section can be formed by machining (such as turning) the interior of the bushing.

[0030] The inner diameter of the segment defined by a cylindrical inner ring or by another such cylindrical portion (such as a section resulting from the machining of the inside of a bushing) is the inner diameter of the inner ring, i.e., the diameter of the inner surface of the inner ring, or the inner diameter of the machined inner surface of the machined segment. However, the inner surface of the segment itself does not need to be circular. The segment may have multiple inwardly projecting support blocks, such as three such blocks, spaced circumferentially and defining an inscribed circle that defines the inner diameter of the segment.

[0031] In an embodiment, the bushing comprises up to three, preferably one or two, more preferably two, segments with a reduced inner diameter, spaced apart along the length of the bushing. Each segment has a length within the range defined above. In the case of two such segments, they may be located, for example, at or near corresponding opposite ends of the bushing. Providing up to three, preferably one or two, more preferably two such segments is sufficient to support the base tube in use, meaning contact with the base tube to avoid unwanted bending of the tube.

[0032] In the embodiment shown in the figure, the bushing 8 has two such segments 12 spaced apart, such that the segments 12 are close to opposite ends of the bushing 8.

[0033] In embodiments of the invention that do not have a microwave generator, the plasma generator may include a coil positioned to surround a cylindrical cavity, the coil being connected to or at least connectable to an RF signal source, such that an electromagnetic flux is generated within the base tube in use.

[0034] By studying the accompanying drawings, the disclosure, and the appended claims, those skilled in the art can understand and implement other variations of the disclosed embodiments in practicing the claimed invention. The foregoing description provides embodiments of the invention by way of example only. The scope of the invention is defined by the appended claims. One or more objects of the invention are achieved by the appended claims.

Claims

1. A plasma chemical vapor deposition apparatus for depositing one or more layers of silicon dioxide onto the inner wall of a long hollow glass base tube, the apparatus comprising a microwave generator, a plasma generator that in use receives microwaves from the microwave generator, a cylindrical cavity extending through the plasma generator, and a cylindrical bushing that is transparent to microwave radiation and is positioned in the cavity, the central axis of the bushing being coaxial with the central axis of the cavity, wherein the base tube in use passes through the bushing, wherein over a portion of the length of the bushing, the bushing has at least one section of reduced inner diameter relative to the inner diameter of the remaining portion of the bushing, the at least one section providing a contact zone for the base tube, wherein the microwave generator is configured to generate microwaves having a wavelength Lw in the range of 40 mm to 400 mm, wherein the length of the at least one section of reduced inner diameter is at most 0.1 x Lw and at least 1 mm.

2. The apparatus of claim 1, wherein, the length of the at least one section of reduced inner diameter is at most 0.05 x Lw.

3. The apparatus of claim 1 or 2, wherein, the wavelength Lw is 120 mm.

4. The apparatus of claim 1 or 2, wherein, the plasma generator comprises a microwave resonance cavity that encloses the cylindrical cavity, wherein the microwave generator is connected to the resonance cavity.

5. The apparatus of claim 1 or 2, wherein, the length of the at least one section of reduced inner diameter is in the range of 1 mm to 20 mm.

6. The apparatus of claim 5, wherein, the length of the at least one section of reduced inner diameter is in the range of 2 mm to 10 mm.

7. The apparatus of claim 1 or 2, wherein, the reduced inner diameter of the bushing is in the range of 25 mm to 100 mm.

8. The apparatus of claim 7, wherein, the reduced inner diameter of the bushing is in the range of 35 mm to 65 mm.

9. The apparatus of claim 1 or 2, wherein, the inner diameter of the remaining portion of the bushing is at least 2% larger than the inner diameter of the at least one section.

10. The apparatus of claim 1 or 2, wherein, the bushing is made of quartz glass.

11. The apparatus of claim 1 or 2, wherein, the at least one section is formed by attaching an inner ring to the inner wall of the bushing, wherein the reduced inner diameter of the section is defined by the inner diameter of the ring.

12. The apparatus of claim 11, wherein, the inner ring is fused to the inner wall of the bushing.

13. The apparatus of claim 11, wherein, the bushing and the inner ring are made of the same material.

14. The apparatus of claim 1 or 2, wherein, the at least one section is formed by locally collapsing the bushing.

15. The apparatus of claim 1 or 2, wherein, the at least one section is formed by machining the bushing.

16. The apparatus of claim 1 or 2, wherein, the bushing comprises at most three such sections of reduced inner diameter, the at most three sections being spaced apart along the length of the bushing.

17. The apparatus of claim 16, wherein, the bushing comprises one or two such sections of reduced inner diameter.

18. The apparatus of claim 1 or 2, wherein, the bushing is made of fused silica.

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