WO2005124820A1

WO2005124820A1 - Device for coating optical glass by means of plasma-supported chemical vapour deposition (cvd)

Info

Publication number: WO2005124820A1
Application number: PCT/EP2005/006245
Authority: WO
Inventors: Andreas Neuffer; Lothar Holz
Original assignee: Carl Zeiss Vision Gmbh
Priority date: 2004-06-18
Filing date: 2005-06-10
Publication date: 2005-12-29
Also published as: DE102004030344A1; EP1761942A1; DE102004030344B4

Abstract

The invention relates to a device (10) which is used to coat optical glass by means of plasma-supported chemical vapour deposition (CVD). A cylindrical reactor (12) is used in order to receive the glass which is to be coated. At least one microwave generator (24a, 24b) is provided in order to couple a microwave signal having a predetermined microwave frequency to the reactor (12). The at least one microwave generator (24a, 24b) comprises a microwave source (26a, 26b) which is coupled to the reactor (12) via a modem interference filter (30a, 30b).

Description

Device for coating optical glasses by means of plasma-assisted chemical vapor deposition (CVD)

The invention relates to a device for coating optical glasses by means of plasma-assisted chemical vapor deposition (CVD), with a cylindrical reactor for receiving the glasses to be coated, and with at least one microwave generator for coupling a microwave signal of a predetermined microwave frequency into the reactor, the at least one microwave generator has a microwave source. From EP 0 718 418 A it is known to provide plastic lenses in the plasma CVD process with a transparent coating, for example a scratch protection layer made of TiO ₂ . For this purpose, the spectacle lenses to be coated are exposed in a reactor at a substrate temperature of 50 ° C. to a coating gas mixture with a pressure of 0.1 to 2 mbar. To generate the plasma, microwave energy is supplied to the reactor, for example at a frequency of 2.45 GHz. The microwave energy is sampled, with pulse lengths of 0.01 to 10 msec and pulse pauses of 1 to 1,000 msec with a pulse power of 10 to 100,000 W.

In this connection cylindrical reactors have also been used. In these cylindrical reactors, the most homogeneous possible distribution of the microwave field is advantageous in order to achieve a coating that is as uniform as possible. This applies in particular to the application of interest here, because spectacle lenses have convex or concave curved surfaces. For this reason, configurations are preferred for this application in which an oscillation mode of the microwave, which is symmetrical to the cylinder axis, propagates in the reactor.

The reactors are fed by pulsed microwave sources, for example magnetrons. Magnetrons usually have an output that is designed as a standard rectangular waveguide. The microwave energy must therefore be injected from the rectangular waveguide into the cylindrical reactor. However, it is inevitable that, in addition to the desired, undesirable vibration modes will also arise. These are spreading asymmetrical in the reactor and lead to an uneven thickness of the coating.

Another problem that arises with the plasma CVD method is that the plasma, which is at a comparatively high pressure, itself causes the coupled-in microwave energy to be reflected back into the microwave generator. Care must therefore be taken to ensure that asymmetrical components do not return to the reactor as a result of this reflection.

In another field of coating technology, namely electron cyclotron resonance (ECR), which works at much lower pressures and under the simultaneous action of a static magnetic field, microwave sources (magnetrons) with a rectangular waveguide output are also used, which have a cylindrical one Are coupled reactor and trigger the resonance processes there.

For an ECR coating system, US Pat. No. 5,172,083 proposes arranging a mode converter between the microwave source and the reactor in order to couple the microwave signal coming from the rectangular waveguide into the cylindrical reactor. This is to prevent undesirable vibration modes from occurring. For this purpose, mechanically very complex and large rectangular / cylinder waveguide transitions are proposed. The occurrence of undesirable vibration modes should therefore be prevented from the outset, while accepting the aforementioned effort. US Pat. No. 5,433,789 and US Pat. No. 5,646,489, also for ECR applications, state that conductive plates should be provided in a certain spatial arrangement in the transition between the microwave source and the reactor in order to prevent the occurrence of undesired oscillation modes due to electrical short circuits in the area of their electrical Prevent field lines. This measure is also extremely complex and prone to failure, because the sheets are positioned extremely precisely and must remain in this position even in long-term operation, since they would otherwise impair the symmetry of the desired vibration mode. The approach is therefore the same as described above, namely that the undesired vibration modes cannot be allowed to occur. In this case, too, this is bought through considerable expenditure on equipment.

The invention is therefore based on the object of developing a plasma CVD coating device of the type mentioned at the outset such that the aforementioned disadvantages are avoided.

This object is achieved according to the invention in a device of the type mentioned at the outset in that the microwave source is coupled to the reactor via a mode interference filter.

The object underlying the invention is completely achieved in this way. By using an interference filter, the approach in the invention is different from that in the arrangements known from the field of ECR. While the undesired modes cannot occur in the first place, this is the case with the device according to the invention. deliberately allowed and the suppression of the undesired vibration mode takes place by causing it to interfere with itself and thus being extinguished. In practice, this enables much simpler constructions with considerably less space.

In experiments, the arrangements according to the invention have also proven to be particularly suitable for the special application in the plasma CVD method, in which - to the extent that, unlike in the ECR method - the plasma, which is under a higher pressure, reflects the coupled-in microwave energy to a considerable extent. The same applies to a much greater extent if, in order to further improve the homogeneity of the coating, work is carried out on a reactor with two opposing microwave generators which are directed towards one another, so that inevitably mutually determined microwave energy components are radiated into the other microwave generator.

In the case of arrangements known from the field of ECR, this would lead to the fact that these reflected or irradiated components in degenerate form nevertheless get back into the reactor because these arrangements are not set up for the actual presence of these undesired oscillation modes. These arrangements may therefore meet the requirements of the ECR process, but they are unsuitable for applications in the plasma CVD process.

In a preferred development of the invention, it is accordingly provided that two microwave generators are mutually opposed Above end walls are arranged symmetrically to a radial central plane of the reactor.

This measure has the advantage already mentioned that the homogeneity of the coating can be further improved without fear of a deterioration in the symmetry of the coupled desired vibration mode.

In further preferred exemplary embodiments, the microwave source is a magnetron, the microwave frequency being in the S band and preferably being approximately 2.45 GHz.

A particularly preferred exemplary embodiment of the invention is characterized in that the microwave source is connected to the mode interference filter via a rectangular waveguide, and that the mode interference filter only couples a first oscillation mode into the reactor, which is capable of symmetrical propagation in the reactor and a second oscillation mode suppressed, which would be asymmetrically spreadable in the reactor. The first vibration mode of type TM ₀₁ and the second vibration mode of type TE _{n are} preferred.

According to the invention, this can be achieved in a practical embodiment in that the mode interference filter has at least two coaxial circular waveguide sections which adjoin one another in a radial plane, a first of which is coupled to the reactor, and that the round waveguide sections are dimensioned such that the first oscillation mode is only capable of propagation in the first circular waveguide section and the second oscillation mode in the first and in the second circular waveguide section. This measure has the advantage that a particularly simple and space-saving structure is created which retains the desired properties even in long-term use. In this way, the separation of the vibration modes or the extinction of the second vibration mode is achieved in a simple manner.

In a practical embodiment of the aforementioned exemplary embodiment, the second circular waveguide section has a second diameter which is dimensioned such that the critical wavelength for the second oscillation mode at this diameter is smaller than the wavelength of the microwave signal.

This measure also contributes to a simple structure.

The length of the second circular waveguide section is preferably equal to an integer multiple of half the waveguide wavelength of the second oscillation mode in the second circular waveguide section.

It is further preferred if the length of the first circular waveguide section is not equal to an integer multiple of a quarter of the wavelength of the first oscillation mode in the first circular waveguide section.

This measure has the advantage that the first circular waveguide section is neither a resonator nor an anti-resonator for the desired first oscillation mode. Therefore, there can be no fluctuations in the intensity of the microwave energy coupled into the reactor if there is a higher level in a resonator Goodness fluctuations in the structural condition would have a strong impact, for example a covering on a surface. On the other hand, avoiding anti-resonance means that no reflections occur.

In addition, it is also preferred if the length of the first circular waveguide section is equal to an odd integer multiple of a quarter of the wavelength of the second oscillation mode in the first circular waveguide section.

This measure has the advantage that remaining portions of the undesired, second oscillation mode in the first waveguide section are extinguished.

A particularly good effect is achieved when a section of the rectangular waveguide is coupled to the first circular waveguide section, with a narrow side of the section lying in the radial plane.

This measure has the advantage that there are defined conditions at the transition from the rectangular to the circular waveguide.

In further preferred embodiments of the invention, the rectangular waveguide is connected to the mode interference filter via a choke joint, which is preferably designed as a screwed-on ring.

This measure has the advantage that it is possible to work with very high microwave powers in the kW range, without the fear of arcing, and that the coupling can be easily changed. It is further preferred if the first circular waveguide section is provided with a vacuum-tight window in the transition to the reactor.

This measure has the advantage that the interior of the mode interference filter is decoupled from the negative pressure side and is therefore not exposed to the coating gas mixture.

Furthermore, in embodiments of the invention it is provided that the first circular waveguide section is provided with a metallic screen in the transition to the reactor, the screen in particular on the side of the window facing away from the first circular waveguide section, i.e. is arranged in the vacuum region of the reactor.

This measure has the advantage that concave surfaces of the glasses can be coated particularly well due to the limitation of the plasma expansion caused by the aperture.

Finally, an embodiment of the invention is preferred, in which the mode interference filter has an outer body connected to the reactor and an inner body which can be inserted axially into the outer body, the circular waveguide sections being formed in the inner body and the rectangular waveguide section in the outer body and furthermore the inner body on Outer body is releasably attached.

This measure has the advantage that the inner body with the round waveguide sections as well as the quartz window and the cover, if provided, can be removed in a simple manner, in particular for cleaning purposes. Further advantages result from the drawing and the attached drawing.

It goes without saying that the features mentioned above and those yet to be explained below can be used not only in the combination indicated in each case, but also in other combinations or on their own without departing from the scope of the present invention.

Embodiments of the invention are shown in the drawing and are explained in more detail in the following description. Show it:

Figure 1 is an extremely schematic side view of an embodiment of a device according to the invention; and

FIG. 2 shows, on an enlarged scale, a sectional illustration of a microwave generator as used in the device according to FIG. 1.

In FIG. 1, 10 as a whole designates an exemplary embodiment of a device according to the invention, namely a plasma CVD coating system. The system 10 contains a cylindrical reactor 12, the longitudinal axis of which is designated 14. The reactor is designed to be symmetrical about a radial center plane 15. In the interior 16 of the reactor 12, as already stated at the beginning of the state of the art according to EP 0 718 418 A, an atmosphere is set which consists of a coating gas mixture, a predetermined negative pressure prevailing. For this purpose, supply lines 18 for the coating tion gas mixture and a vacuum connection 20 are provided. Carriers for the glasses to be coated, in particular spectacle lenses made of plastic or glass, are provided in the interior 16. These are known to the person skilled in the art and are therefore not shown for the sake of clarity.

Microwave generators 24a, 24b are arranged on opposite end walls 22a and 22b so as to couple microwave energy symmetrically from both sides into interior space 16, namely in longitudinal axis 14. Microwave generators 24a, 24b each contain a microwave source 26a, 26b, for example a magnetron, which preferably operates in the S band, in particular at approximately 2.45 GHz, and delivers pulse powers in the kW range. The microwave sources 26a, 26b are connected via rectangular waveguides 28a, 28b to a mode interference filter 30a, 30b, from which the microwave energy is coupled into the interior 16.

Details of the mode interference filter are shown in Figure 2, namely in the position of the mode interference filter 30b of Figure 1, hereinafter referred to as "30" for the sake of simplicity.

Each mode interference filter 30 has an inner body 32 and an outer body 33, which, of course, can also each be multi-part in practical embodiments. The bodies 32, 33 are preferably metallic. The inner body 32 contains a front part 34 and a rear part 35. The inner body 32 and the outer body 33 are arranged coaxially to the longitudinal axis 14, the inner body 32 in FIG. 2 can be inserted from the right into the outer body 33 and fastened to it in the inserted state. For this purpose, the inner body 32 is provided with a flange 36, which can be fixed to a radial end face of the outer body 33 with screws 37.

The front part 34 encloses a first circular waveguide section 38 and the rear part 35 a second circular waveguide section 40. The circular waveguide sections 38, 40 are arranged coaxially with one another and with the axis 14. However, they have different dimensions, namely a diameter d _x and a length l _x for the first circular waveguide section 38 or a diameter d ₂ and a length 1 ₂ for the second circular waveguide section 40. The dimensions of the lengths l _λ and 1 ₂ and the diameter d _λ and d ₂ will be discussed further below.

The round waveguide sections 38, 40 are provided in the inner body 32 with an electrically highly conductive surface, for example a gold coating. The first circular waveguide section 38 is electrically open at its left end in FIG. 2. The second circular waveguide section 40 is closed at its right end in FIG. 2 with an electrically conductive bottom 41. The base 41 can be designed as an axially movable piston for adjustment purposes.

The circular waveguide sections 38, 40 merge into one another in a radial plane 42. There, a rectangular waveguide section 44 is also provided in the inner body 32, the narrow side 45 of which lies in the radial plane 42 and which is open to the first circular waveguide section 38. To the rectangular waveguide section 44 2, a flange 46 of the rectangular waveguide section 28 is screwed on in FIG.

A so-called “choke joint” 48 or a throttle connection or coupling is provided in the transition from the rectangular waveguide section 44 to the first circular waveguide section 38. The choke joint 48 has a pocket slot of defined depth on the broad side of the rectangular waveguide section 28, which is like an interference filter The microwave penetrates there into the sack slot, is reflected at its end and interferes with half of the sack slot in such a way that the microwave intensity there becomes zero, thus creating an area of the smallest wall current at the point of transition Joint 48 not present, the maximum current would flow in the middle of the broad side of the rectangular waveguide section 44 with the permitted basic mode, and this would lead to arcing if there was a poor line transition.

In a practical exemplary embodiment (not shown), the choke joint 48 can be implemented as a screwed-on ring.

The outer body 33 has at its left end in FIG. 2 a generator flange 50 for fastening the microwave generator 24 to the reactor 12. For this purpose, the generator flange 50 is screwed to a reactor flange 52 of the reactor 12. The reactor flange 52 sits tightly in the end wall 22 of the reactor 12. In order to connect the inner body 32 to the interior 16 of the reactor 12 in a pressure-tight manner, there is a conical fit 54 between them the inner body 32 and the reactor flange 52, which is provided with a vacuum seal 56.

As can be seen from FIG. 2, it is thus possible to pull the inner body 32 to the right after loosening the screws 37 from the outer body 33, for example for cleaning purposes, the conical fit 54 with the vacuum seal 56 then being free lies.

A window 58 is inserted into the end of the inner body 32 or the first circular waveguide section 38 that is to be directed toward the interior 16 of the reactor 12. The window 58 is preferably designed as a quartz window that is transparent to microwaves and glued into a conical fit 60 by means of a silicone adhesive.

A metallic screen 62 is preferably arranged in front of the window 58 and is provided with a defined opening 64. The aperture 62 restricts the plasma expansion in the interior 16, which is useful for the coating of concavely curved surfaces.

The device according to the invention works as follows:

The microwave signal is emitted from the microwave sources 26a, 26b via the rectangular waveguides 28a, 28b. From these, the signal is coupled into the first circular waveguide section 38 via the rectangular waveguide section 44 (FIG. 2). In the interior 16 of the reactor 12, only the symmetrically propagating vibration mode TM _{01 is to be} excited, but the asymmetrically propagating vibration mode TE _n is not. For this purpose, the diameter d _{j of} the first circular waveguide section 38 is first dimensioned such that the two vibration modes TM ₀₁ and TE _{U can be} propagated in it. The diameter d _{2 of} the second circular waveguide section 40, on the other hand, is dimensioned such that only the undesired vibration mode TE _n , but not the desired vibration mode TM _01, can propagate in it.

For the TE _n mode, the critical wavelength λ _{c of} the condition applies:

enough. The following applies accordingly to the vibration mode TM ₀₁ : ^λ _{o (M} oi ₎ ⁼ 2.613 xd / 2

For example, if one dimensioned d ₁ = 100 mm and d ₂ = 80 mm, then the critical wavelengths λ _{c {TE11)} and λ _{c (TM01)} for the first circular waveguide section 38 result in: ^λ ₍ τEii) 38 ⁼ 170.6 mm ^λ _{c (TM} 0i) 38 = 130.7 mm

and for the second circular conductor section 40 to: ^λ c ₍ τEii) 0 ⁼ 136.5 mm

In the case of the microwave signal with a frequency of 2.45 GHz considered for the exemplary embodiment in FIG. 2, the wavelength in the vacuum is λ _v = 122.4 mm. This is below the above-mentioned values for the critical wavelengths, except λ _{c (TM01) 40} . Consequently, only the oscillation mode TE _{n can be} propagated in the second circular waveguide section 40.

In the first circular waveguide section 38, both vibration modes, ie TM ₀₁ and TE _n , are capable of propagation. The desired vibration mode TM ₀₁ thus only propagates in the left, first circular waveguide section 38 and is coupled from there through the window 58 into the interior 16 of the reactor. The undesirable asymmetrical vibration mode TE _n , however, spreads in both round waveguide sections.

The length of the right, second waveguide section 40 is dimensioned such that it forms a λ / 2 line for the asymmetrical vibration mode TE _n , so that the short circuit of the preferably axially displaceable base 41 is transformed into the radial plane 42. It must be tuned to the waveguide wavelength λ _{H of} the vibration mode, which is always greater than the wavelength λ ^ in a vacuum. The following applies to the waveguide wavelength λ _H :

For the asymmetrical vibration mode TE _n in the right, second circular waveguide section 40, λ _{H (TEU) 40} = 277 mm is calculated therefrom. It follows for 1 ₂ = λ _{H (TE11) 40/2} = 139 mm. As a result of the condition 1 ₂ = λ _{H (TE11) 40/2} , the undesired asymmetrical oscillation mode TE _{n is} _{automatically canceled out} by interference at the transition point between the round waveguide sections 38 and 40, that is to say at the location of the rectangular waveguide coupling.

For the left, first waveguide section 38, it initially applies that it must not represent a resonator for the symmetrical vibration mode TM ₀₁ . If this were the case, then, due to the high quality of the resonator thus formed, even the smallest fluctuations in the structure, for example a coating on the quartz window 58, would result in strong changes in intensity of the microwave signal coupled into the reactor 12. The length l _{x of} the left, first waveguide section 38 must therefore not be equal to nx λ _{H (TM01) 38/2} .

Furthermore, there should be no anti-resonance nx λ _{H (TM01) 38/4} (for odd-numbered n). This would lead to clear reflections of the overall performance. Therefore you choose a value between 1/2 and 3/4, e.g. 0.67. Then l _j = 0.67 x λ _{H (TM01) 38} or with λ _{H (TM01) 38} = 350 mm, 1 ₁ = 235 mm. By suitable selection of the factor and taking into account the unavoidable structural tolerances, an anti-resonance can also be set for the asymmetrical vibration mode TE11 in the first, left circular waveguide section 38, in this case l _x = 5/4 x λ _{H (TE11) 38} . Then, any remaining TE _n components present in the first circular waveguide section 38 are also extinguished.

This dimensioning of d _lf d ₂ , 1 ₁ and 1 _{2 means} , at a given frequency, for the aforementioned vibration modes TE _U and TM ₀₎ on the one hand causes interference and thus cancellation of the undesired asymmetrical oscillation mode TE _n . On the other hand, the reflection of microwave signals is minimized, namely within the mode interference filter 30 and from the interior 16 of the reactor 12 back through the window 58 into the mode interference filter 30. The latter applies regardless of whether these reflected portions originated from the own microwave generator 24b itself and were reflected on the plasma which is under relatively high pressure or are interspersed by the other, opposite microwave generator 24a.

Claims

claims

1. Device for coating optical glasses by means of plasma-assisted chemical vapor deposition (CVD), with a cylindrical reactor (12) for receiving the glasses to be coated, and with at least one microwave generator (24a, 24b) for coupling a microwave signal of a predetermined microwave frequency into the reactor (12 ), the at least one microwave generator (24a, 24b) having a microwave source (26a, 26b), characterized in that the microwave source (26a, 26b) is coupled to the reactor (12) via a mode interference filter (30a, 30b) ,

2. Device according to claim 1, characterized in that two microwave generators (24a, 24b) are arranged on mutually opposite end walls (22a, 22b) in a folding symmetry with respect to a radial central plane (15) of the reactor.

3. Device according to claim 1 or 2, characterized in that the microwave source (26a, 26b) is a magnetron and that the microwave frequency is in the S-band and is preferably about 2.45 GHz.

4. Device according to one or more of claims 1 to 3, characterized in that the microwave source (26a, 26b) is connected to the mode interference filter (30a, 30b) via a rectangular waveguide (28a, 28b, 44), and that the mode interference filter (30a, 30b) only couples a first oscillation mode into the reactor (12), which is capable of symmetrical propagation in the reactor (12) and suppresses a second oscillation mode that would be asymmetrically propagatable in the reactor (12).

Device according to claim 4, characterized in that the first vibration mode is of type TM ₀₁ and the second vibration mode is of type TE _n .

Apparatus according to claim 4 or 5, characterized in that the mode interference filter (30a, 30b) has at least two coaxial circular waveguide sections (38, 40) adjoining each other in a radial plane (42), a first one (38) of the reactor (12) is coupled, and that the round waveguide sections (38, 40) are dimensioned such that the first oscillation mode is propagatable only in the first round waveguide section (38) and the second oscillation mode in the first (38) and in the second (40) round waveguide section is.

Apparatus according to claim 6, characterized in that the second circular waveguide section (40) has a second diameter (d ₂ ) which is dimensioned such that the critical wavelength (λ _c ) for the second oscillation mode is smaller at this diameter (d ₂ ) than the wavelength (λ) of the microwave signal.

Apparatus according to claim 6 or 7, characterized in that the length (1 ₂ ) of the second circular waveguide Section (40) is an integer multiple of half the waveguide wavelength (λ _{H (TE11)} ) of the second oscillation mode in the second waveguide section (40).

9. The device according to one or more of claims 6 to 8, characterized in that the length (1 _{:) of} the first circular waveguide section (38) is not equal to an integer multiple of a quarter of the wavelength (λ _{H (TM01)} ) of the first oscillation mode in the first Round waveguide section (38).

10. The device according to one or more of claims 6 to 9, characterized in that the length (1 ₁ ) of the first circular waveguide section (38) is equal to an odd integer multiple of a quarter of the wavelength (λ _{H (TE11)} ) of the second vibration mode in the first circular waveguide section (38).

11. The device according to one or more of claims 6 to 10, characterized in that a section (44) of the rectangular waveguide is coupled to the first circular waveguide section (38), a narrow side (45) of the section (44) in the radial plane (42 ) lies.

12. The device according to one or more of claims 4 to 11, characterized in that the rectangular waveguide (28a, 28b) is connected via a choke joint (48) to the mode interference filter (30a, 30b), which is preferably designed as a screwed-on ring is.

13. The device according to one or more of claims 6 to 12, characterized in that the first circular waveguide section (38) in the transition to the reactor (12) is provided with a vacuum-tight window (58).

14. The device according to one or more of claims 6 to 13, characterized in that the first circular waveguide section (38) in the transition to the reactor (12) is provided with a metallic screen (62).

15. The apparatus according to claim 14, characterized in that the diaphragm (62) on the side of the window (58) facing away from the first circular waveguide section (38), i.e. is arranged in the vacuum region of the reactor (12).

16. The device according to one or more of claims 8 to 15, characterized in that the mode interference filter (30) has an outer body (33) connected to the reactor (12) and an inner body (32) which can be inserted axially into the outer body (33). The circular waveguide sections (38, 40) are formed in the inner body (32) and the rectangular waveguide section (44) in the outer body (33) and the inner body (32) is detachably (37) attached to the outer body (33).