EP1837946B1 - Directional coupler - Google Patents

Description

The invention relates to a directional coupler, in particular for an RF plasma process excitation arrangement according to the preamble of claim 1.

RF plasma process excitation arrangements include an RF generator that provides RF power to a plasma process. The RF power is typically supplied to the plasma process in a narrow band frequency range, particularly around the industrial frequencies of 13.56 MHz and 27.12 MHz. Measuring devices may be provided to measure the power supplied to the plasma process. For accurate control and / or control of the power, it is necessary to accurately detect the delivered power.

There are various ways to determine the power delivered to the load of an RF plasma process excitation system. One frequently used is the measurement by means of a directional coupler. A directional coupler can determine the forward power P _i and the reflected power P _r . Directional couplers couple out a portion of the power that passes through a through-conductor of the directional coupler.

From the US 5,424,694 a directional coupler with a through-conductor and two coupling lines is known, wherein the two coupling lines are each terminated with a terminating resistor and the terminating resistor corresponds to the impedance of the coupling lines.

From the US 5,767,753 a directional coupler is known in which the through-conductor and the coupling lines are arranged in the same plane. To influence the coupling factor between the lines coupling elements are provided in another level.

To influence the coupling factor between the lines coupling elements are provided in another level.

In the JP 08203692 a directional coupler is disclosed in connection with a plasma application.

The EP 1 014 472 A1 discloses a broadband directional coupler having at least two cascaded coupling sections of different coupling loss. The central coupling section has a line on one side of a substrate and a line on the opposite side of the substrate. On one side of the substrate, a metallization surface is further provided.

The US 2002/0113667 A1 describes a directional coupler in which a coupling line and a transmission line are arranged in the same plane. Furthermore, a ground plane is also arranged in this plane.

Another directional coupler will be in JP 5 014 019 A described.

From the re-published WO 2006/105847 A1 For example, a coupler having four leads on the same side of a substrate is known. Two coupling paths are formed on the substrate on two opposite sides.

There are a variety of applications of directional couplers in stripline technology that are one quarter of the frequency associated with the wavelength λ. Such lengths are not realistic for a system operating in the 10 to 30 MHz range.

The from the prior art, eg DE 10 2004 021 535 A1 . US 2005/0212617 A1 Known directional couplers are designed for much higher frequencies (1 GHz and higher), where line theory plays an essential role. In the known directional couplers, a coupling line runs parallel to the feedthrough line. A small part of the power is decoupled from the transmission line, in which the power of the generator flows to the load, to the coupling line by electrical and magnetic coupling. At one end of the coupling line, a power proportional to the reflected power can be tapped at the other end, and a power proportional to the forward power at the other end. It has also been proposed, one each To use a coupling line for the forward power and a coupling line for the reflected power.

By an additional circuit, which usually consists of a low-pass filter, and a burden, a measured signal can be generated from the tapped services, which can be fed to a controller.

In known directional coupler arrangements, it is disadvantageous that any disturbance that comes back from the circuit in the coupling line, either by a mismatch of the circuit to the directional coupler or by filtering, to Messwertverfälschungen, both in the measurement of forward power and the reflected power leads.

The object of the present invention is to provide a directional coupler in which the electrical coupling between the transmission line and the coupling lines is low and the forward power and reflected power can be coupled out with different coupling factors.

This object is achieved by a directional coupler with the features of claim 1. By accurately matching the terminators to the characteristic impedance, reflections due to mismatch at this location can be avoided. This allows a much more accurate measurement of the services done. The use of two coupling lines has the advantage that the forward power and the reflected power can be measured decoupled from each other. On the coupling lines, the power or a descriptive size can be tapped. If reflections occur in the wiring, for example the filtering of the tapped power, these are absorbed in the terminating resistor of the respective coupling line and do not contribute to a measuring error on the other coupling line. Preferably, the coupling lines extend at least in sections parallel to the passage line.

In a particularly preferred embodiment it can be provided that a first ground reference potential, in particular a first ground plane, is provided and the first and second coupling line in a predetermined Distance from the ground potential are arranged. By selecting or specifying the distances, the characteristic impedance of the coupling lines can be set very accurately. Due to the fixed reference potential, a fixed characteristic impedance can be specified with high accuracy and reliability and with high repeat accuracy. Advantageously, a common wave impedance is set in the industry. For example, a 50 ohm or 75 ohm characteristic impedance is very common. In order to realize the characteristic impedance, in addition the length and / or width of the coupling lines can be suitably specified.

In an advantageous embodiment, two ground reference potentials can be provided and the coupling lines can be arranged between the ground reference potentials, wherein the distance to at least one, preferably two ground reference potentials is predetermined or can be predetermined. With the one ground reference potential, the predetermined characteristic impedance can be set precisely, and with the second ground reference potential, the electrical coupling between the feedthrough line and the coupling lines can be set precisely.

In a preferred development, the coupling lines can be arranged between two ground planes. The coupling lines can be embedded between insulating materials, such as printed circuit boards, which carry the ground planes. As a result, the directional coupler can be produced particularly simply, inexpensively and with high precision.

The through-line is arranged in the same plane as a ground plane, but isolated from it. For relatively short lines, in particular shorter than λ / 4, the coupling according to the line theory is negligible. Rather, there is a coupling by electric and magnetic fields. The electrical and magnetic coupling must be balanced. The magnetic Coupling results from the magnetic field line course in the range of the route in which the coupling lines are guided in the immediate vicinity of the feedthrough line. Short lines mean little magnetic coupling. For the balance, a low electrical coupling is necessary. The electrical coupling results from the electrical field line course between the transmission line and the respective coupling line, as well as from the surface of the respective coupling line. The field line profile can be deflected by a ground plane on the same plane as the transmission line and thus deflected away from the coupling lines. Thus, the electrical coupling of the transmission line to the coupling lines can be reduced.

Even if no coupling line is arranged between the through-line and a ground potential, in particular a ground plane, the field lines of the electric field can be deflected in order to reduce the electrical coupling between the through-line and the coupling lines.

If the coupling lines at one end have only one terminating resistor, which is preferably adapted, the power can be coupled out at the other end. By "matched termination" is meant that the termination resistance is the same as the characteristic impedance of the directional coupler. Reflections resulting from the measurement thus end up in the terminating resistor at the other end of the coupling line, do not lead to any further reflections and do not contribute to a measuring error on the other coupling line. The terminator can be made adjustable, then tolerances can be compensated in the directional coupler.

It is particularly preferred if the parallel sections of the lines have a length <λ / 4, in particular ≦ λ / 8, preferably ≦ λ / 10 exhibit. As a result, the dimensions of the directional coupler can be kept small.

In one embodiment of the invention can be provided that the forward power and the reflected power or these descriptive variables can be coupled out with different coupling factors. The reflected power is usually smaller than the forward power. If it can be coupled out with a larger coupling factor, the signal-to-noise ratio at the input of the evaluation device detecting the power increases because the dynamics of the evaluation device detecting the power are advantageously utilized. The reflected power can be measured more accurately.

In a particularly simple manner, different coupling factors can be realized if the first and second coupling lines are arranged at different distances from the passage line.

Different coupling factors can be realized in a simple, precise and cost-effective production of the directional coupler, when the passage line, the first and the second coupling line are arranged in different planes.

Advantageously, the coupling lines are arranged offset from one another. As a result, a coupling between the coupling lines and thus an impairment of the measurement results can be avoided.

The distance between the lines can be adjusted precisely and reproducibly if the lines are spaced apart by an electrically insulating material, in particular printed circuit board material.

The directional coupler is particularly suitable for operation in RF plasma process excitation arrangements, if it is designed for operation at frequencies <200 MHz, in particular <40 MHz.

The invention also includes an RF plasma process excitation arrangement with a directional coupler as described above.

It is particularly preferred if a large part of the return flow, in particular more than 90% of the return flow, flows from a plasma load to an HF generator via a ground plane of the directional coupler. On the ground surface should a large part, if possible, the entire return flow. This ensures that builds up the electric field, which is necessary for the electrical coupling of the passage line to the coupling lines.

Advantages arise when the HF resistance for the return current between the output terminal of the RF plasma process excitation arrangement and a ground potential of the directional coupler is smaller than the HF resistance of a housing between the output terminal and ground potential of the housing. Preferably, the output is designed as a coaxial plug, on the outer conductor of the return current flows. In known RF plasma process excitation arrangements, the outer conductor is mechanically and electrically usually connected to the ground of the housing. The mass of the HF generator is usually connected at several points or over a large area with the mass of the housing. In general, therefore, even if a ground reference potential is provided on the directional coupler, here not the full flow, but a large part of the current will flow through the housing directly to the ground of the RF generator. The current will be corresponding to the resistances of the divide different current paths to ground. According to the invention, it is now ensured that as far as possible the entire current flows over the ground reference surface of the directional coupler. For example, a very low DC resistance, as a conventional package in any case, can be increased when inductors are introduced into the current path. Alternatively or additionally, the current path via the ground reference potential of the directional coupler to the ground of the RF generator can be constructed particularly low inductance. For this purpose, it can be provided, for example, that the fastening screws of the output terminal have a direct, short and large-area connection to the ground surface of the directional coupler. Additionally or alternatively, the connection of the ground reference surface of the directional coupler to the mass of the generator can be constructed as short, and low inductance.

Further features and advantages of the invention will become apparent from the following description of embodiments of the invention, with reference to the figures of the drawing, which show details essential to the invention, and from the claims. The individual features can be realized individually for themselves or for several in any combination in a variant of the invention.

Fig. 1

eine schematische Darstellung einer HF-Plasmaprozessanregungsanordnung;

Fig. 2

eine Schnittdarstellung durch einen erfindungsgemäßen Richtkoppler;

Fig. 3a - 3c

eine Draufsicht auf die unterschiedlichen Ebenen des Richtkopplers der Fig. 2; und

Fig. 4

eine schematische Darstellung der Anordnung eines Richtkopplers in einem Gehäuse einer HF-Plasmaprozessanregungsanordnung.

Preferred embodiments of the invention are shown schematically in the drawing and are explained below with reference to the figures of the drawing. It shows:

Fig. 1

a schematic representation of an RF plasma process excitation arrangement;

Fig. 2

a sectional view through a directional coupler according to the invention;

Fig. 3a - 3c

a plan view of the different levels of the directional coupler of Fig. 2 ; and

Fig. 4

a schematic representation of the arrangement of a directional coupler in a housing of an RF plasma process excitation arrangement.

In the FIG. 1 an RF plasma process excitation arrangement 1 is shown schematically. The RF plasma process excitation arrangement 1 comprises an HF generator 2 which is connected to a plasma load 4 via a directional coupler 3. The directional coupler 3 is used to decouple signals or quantities that are related to the forward power output by the RF generator 2 and the power reflected by the plasma load 4. For this purpose, a first measuring device 5 (wiring) for measuring the forward power and a second measuring device 6 (wiring) for measuring the reflected power are provided. The measuring devices 5, 6 are in turn connected to an evaluation device 7, which can control the HF generator 2 and thus the forward power output due to the measured powers.

In the FIG. 2 a cross section through the directional coupler 3 is shown. In the same plane as a ground plane 10, a through-line 11 is arranged electrically insulated. Via the through-line 11, the forward power is transmitted from the RF generator to the load. The ground plane 10 and the through-line 11 are in a plane according to this embodiment. They are arranged on an electrical insulator 12 designed as a printed circuit board. In the plane below a first coupling line 13 is arranged for coupling out the reflected power. Also, the first coupling line 13 is on a circuit board trained electrical insulator 14 applied. The first coupling line 13 is arranged at a predetermined vertical distance and slightly offset from the passage line 11.

The second coupling line 15 for coupling the forward power is arranged at a greater distance from the passage line 11. The second coupling line 15 is arranged on an insulator 16 designed as a printed circuit board. Due to the greater distance of the second coupling line 15 to the through-line 11, power is coupled out by the second coupling line 15 with a lower coupling factor. The distance between the second coupling line 15 and the passage line 11 is also predetermined. The coupling line 15 is arranged offset to the passage line 11 and does not overlap the first coupling line 13. This ensures a decoupling of the two coupling lines 13, 15.

In a further plane, a second ground plane 17 is provided. The ground planes 10, 17 may be connected to a plurality of vias (not shown) to ensure the homogeneity of the current in the ground planes 10, 17. The coupling lines 13, 15 have a defined distance from the ground plane 17. In this way, the characteristic impedance of the coupling lines 13, 15 is precisely determined. The characteristic impedance is further determined by the length and width of the coupling lines 13, 15. The length, width of the coupling lines and the distance to the ground surface 17 are thus matched to each other in order to achieve a defined, predetermined characteristic impedance for each coupling line 13, 15.

The coupling factors are also influenced by the length and width of the coupling lines 13, 15. Another influence on the Coupling factor has the position of the coupling lines 13, 15 with respect to the passage line 11 and the width and length of the passage line eleventh

By the first ground surface 10, which could be arranged in another embodiment, not shown, also above the passage line 11, the electric field in the vicinity of the passage line 11 is influenced. By this measure, the electrical coupling between the through-conductor 11 and the coupling lines 13, 15 can be influenced and adjusted.

In the FIG. 3a a plan view of the ground plane 10 and the through-line 11 is shown. Here it is clear that the passage line 11 is completely embedded in the ground plane 10 and thus also shielded from this. Shown are also an input terminal 21 for connection to the RF generator and an output terminal 20 for connection to the plasma load.

The FIG. 3b shows a plan view of the insulator 14, on which the first coupling line 13 is arranged. Outside the coupling region 22, in which the first coupling line 13 extends parallel to the through-line 11, the coupling line 13 is angled, so that the terminals 23, 24 are located away from the through-line 11. At the terminal 23, only a resistor 25 is connected, whose resistance value corresponds to the characteristic impedance of the first coupling line 13. The connection 24 can be connected to a measuring device to which a variable describing the reflected power P _r is output.

The Figure 3c shows a plan view of the insulator 16, on which the second coupling line 15 is arranged. Outside the coupling region 22, in which the second coupling line 15 extends parallel to the through-line 11, is angled the coupling line 15, so that the terminals 26, 27 of the through-line 11 and the terminals 23, 24 of the first coupling line 13 are removed. At the terminal 26, only a resistor 28 is connected, whose resistance value corresponds to the characteristic impedance of the second coupling line 15. The connection 27 can be connected to a measuring device to which a variable describing the forward power P _i is output.

In the FIG. 4 It is shown that the RF generator 2 and the directional coupler 3 are arranged in a housing 30, wherein the housing 30 is connected to a ground potential. An output terminal 31 of the HF generator 2 is connected via a line 32 to the transmission line 11 of the directional coupler 3. The passage line 11 of the directional coupler 3 is in turn connected to the inner conductor 33 of a designed as a plug, in particular coax connector, output terminal 34. The outer conductor 35 of the output terminal 34 is connected over a large area via fastening means 36 to the housing 30. In particular, the current conducted back on the outer conductor 35 from the plasma load passes via the outer conductor 35 to the housing 30. To ensure that a substantial portion of the return current passes over the ground plane 17 and not over the housing 30, there is a short connecting line 37 between the ground plane 17 and the output terminal 34 is provided. The ground surface 17 is further connected via a short line 38 to the RF generator 2, in particular to its ground potential. The connecting lines 37, 38 are preferably made of copper or silver. These metals have a high electrical conductivity. Preferably, the length of the connecting lines 37, 38 ≤ 10 mm and the width ≥ 5 mm, in particular ≥ 10mm. Due to the flat, short design of the connecting lines 37, 38, a low-inductance connection between the outer conductor 35 and the ground of the HF generator 2 via the ground surface 17 is realized.

Furthermore, measures can be taken on the housing 30 in order to increase the resistance for the recirculated HF current and to ensure in this way that the return current flows substantially across the ground surface 17. Such measures can be, for example: connecting elements between housing and ground of the HF generator provided with ferrite rings, or fasteners made of materials with a high μ _r use because a high μ _r increases the skin effect, thus leading to a deteriorated RF power line. By this measure, the electric and magnetic fields can form, which are necessary for a good coupling of the coupling lines 13, 15 with the passage line 11.

Claims (13)

1. Directional coupler (3), in particular, for an HF plasma process excitation configuration (1), comprising:

   a. a transmission line (11) with an input terminal (20) and an output terminal (21);

   b. a first coupling line (13) for detecting reflected power (P_r), which is spaced apart from the transmission line (11) and is terminated at least at one end with a termination resistance (25);

   c. a second coupling line (15) for detecting forward power (P_i), which is spaced apart from the transmission line (11) and is terminated at least at one end with a termination resistance (28), wherein

   d. each coupling line (13, 15) has a predetermined and adjusted characteristic impedance, and the termination resistances (25, 28) have a resistance value which corresponds to the characteristic impedance of the associated coupling line (13, 15) with a tolerance of <±10%, in particular <±5%, preferentially <±1%, characterized in that

   e. the transmission line (11) is disposed in the same plane as a ground surface (10), but insulated therefrom and

   f. the transmission line (11), the first and the second coupling lines (13, 15) are disposed in different planes.
2. Directional coupler according to claim 1, characterized in that a first ground reference potential, in particular, a first ground surface (10, 17) is provided, and the first and second coupling lines (13, 15) are disposed at a predetermined distance from the ground potential.
3. Directional coupler according to any one of the preceding claims, characterized in that two ground surfaces (10, 17) are provided and the coupling lines (13, 15) are disposed between the ground surfaces (10, 17), wherein the distance from at least one, preferably both ground surfaces (10, 17) is or can be predetermined.
4. Directional coupler according to any one of the preceding claims, characterized in that there is no coupling line (13, 15) between the transmission line (11) and a ground potential, in particular a ground surface.
5. Directional coupler according to any one of the preceding claims, characterized in that the coupling lines (13, 15) have exclusively a termination resistance (25, 28) at one end.
6. Directional coupler according to any one of the preceding claims, characterized in that the parallel sections of the lines (11, 13, 15) have a length of <λ/4, in particular ≤λ/8, preferentially ≤λ/10.
7. Directional coupler according to any one of the preceding claims, characterized in that the forward power (P_i) and the reflected power (P_r) or quantities describing them, can be decoupled with different coupling factors in that the first and the second coupling lines (13, 15) are disposed at different distances from the transmission line (11).
8. Directional coupler according to any one of the preceding claims, characterized in that the coupling lines (13, 15) are offset from each other transversely to their direction of extension.
9. Directional coupler according to any one of the preceding claims, characterized in that the lines (11, 13, 15) are separated from each other by an electrically insulating material, in particular, a printed circuit board material.
10. Directional coupler according to any one of the preceding claims, characterized in that it is designed for operation at frequencies of < 200 MHz, in particular < 40 MHz.
11. HF plasma process excitation configuration (1) comprising a directional coupler (3) in accordance with any one of the preceding claims.
12. HF plasma excitation configuration according to claim 11, characterized in that inductances are arranged in the current path between the output terminal (34) of the plasma process excitation configuration (1) and the ground potential of a housing (30).
13. HF plasma excitation configuration according to claim 11 or 12, characterized in that the output terminal (34) of the plasma process excitation configuration (1) is connected to the ground surface (10, 17) of the directional coupler (3) via a connecting line (37), and/or the ground surface (17) of the directional coupler (3) is connected to the ground potential of the HF generator (2) via a line (30).

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