DIRECTIONAL COUPLER

Abstract

An HF plasma process excitation configuration includes an HF generator that is connected to a plasma load through a directional coupler. The directional coupler includes a transmission line, a first coupling line for detecting reflected power from the plasma load, and a second coupling line for detecting forward power from the HF generator, is the first coupling line is spaced apart from the transmission line and is terminated at least at one end with a termination resistance. The second coupling line is spaced apart from the transmission line and is terminated at least at one end with a termination resistance. Each coupling line has a predetermined and adjusted characteristic impedance, and the termination resistances each have a resistance value that corresponds within a tolerance to the characteristic impedance of the associated coupling line with a tolerance.

Description

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of an HF plasma process excitation configuration;

• • FIG. 2 shows a sectional view through a directional coupler of the configuration of FIG. 1;

FIGS. 3 a-3c show top views of the different planes of the directional coupler of FIG. 2; and

FIG. 4 shows a schematic view of the configuration of a directional coupler in a housing of an HF plasma process excitation configuration.

DETAILED DESCRIPTION

FIG. 1 schematically shows an HF plasma process excitation configuration 1. The HF plasma process excitation configuration 1 includes an HF generator 2 that is connected to a plasma load 4 through a directional coupler 3. The directional coupler 3 decouples signals or quantities related to the forward power (Pi), which is the power provided by the HF generator 2, and the reflected power (Pr), which is the power reflected by the plasma load 4. Towards this end, the configuration includes a first measuring device 5 (in the form of an electrical circuit) to measure the forward power, and a second measuring device 6 (in the form of an electrical circuit) to measure the reflected power. The measuring devices 5, 6 are, in turn, connected to an evaluation device 7 that detects the power and can control the HF generator 2 and thus can control the forward power provided by the HF generator 2 based on the measured powers from the measuring devices 5, 6.

FIG. 2 shows a cross-section through the directional coupler 3. The directional coupler 3 includes a first coupling line 13 and a second coupling line 15 that are positioned relative to a transmission line 11 to decouple electrical power (forward power and reflected power, respectively) from the transmission line 11. The transmission line 11, which is electrically insulated, is disposed in the same plane as a ground reference member that is at ground potential (in this implementation, the ground reference member is a first ground surface 10). The forward power is transferred from the HF generator 2 to the plasma load 4 through the transmission line 11. In this implementation, the ground surface 10 and the transmission line 11 are in the same plane and are disposed on an electrical insulator 12 that is designed as a printed circuit board. A first coupling line 13 is disposed in the plane below the electrical insulator 12 for decoupling the reflected power from the plasma load 4. The first coupling line 13 is also disposed on an electrical insulator 14 that is designed as a printed circuit board. The first coupling line 13 is disposed at a predetermined vertical separation and is slightly offset from the transmission line 11. Vertical separation is measured in a direction perpendicular to the plane (and is parallel to the normal of the plane) in which the transmission line 11 is disposed.

The second coupling line 15 for decoupling the forward power is disposed at a larger vertical separation from the transmission line 11. The second coupling line 15 is also disposed on an electrical insulator 16 that is designed as a printed circuit board. Due to the larger separation between the second coupling line 15 and the transmission line 11, power is decoupled at a smaller coupling factor by way of the second coupling line 15. The separation between the second coupling line 15 and the transmission line 11 can be predetermined. The coupling line 15 is offset from the transmission line 11 by the vertical separation, and the coupling line 15 does not overlap the first coupling line 13. This ensures electrical decoupling of the two coupling lines 13, 15.

Each coupling line 13, 15 has a predetermined and adjusted characteristic impedance, and the impedance can be adjusted with precision by selecting or by determining the separations between, the coupling line 13, 15 and the ground surface 10. Because the reference potential of the ground surface 10 is fixed, a fixed characteristic impedance can be predetermined with, precision, reliability, and high repetition accuracy. In some implementations, the characteristic impedance of each coupling line 13, 15 can be set as that commonly used in industry. For example, the characteristic impedance can be 50 ohms or 75 ohms. In some implementations, the characteristic impedance can be adjusted by adjusting the length and/or width of the coupling lines 13, 15.

A second reference member that is at ground potential (in this implementation, the second reference member is a second ground surface 17) is provided in a further plane adjacent the insulator 16. The ground surfaces 10, 17 can be connected to several through-contacts (not shown) formed in the printed circuit boards of the insulators 12, 14, 16 to electrically connect the ground surfaces 10, 17 in order to ensure that the current in the ground surfaces 10, 17 is homogeneous. The coupling lines 13, 15 are disposed at a defined vertical separation from the ground surface 17, thereby precisely determining the characteristic impedance of the coupling lines 13, 15. Additionally, there is no coupling line (either 13 or 15) between the transmission line (11) and a ground potential such as the ground surface 10. The characteristic impedance is determined by the length and the width of the coupling lines 13, 15. The length and width of the coupling lines 13, 15 and the vertical separation of the coupling lines 13, 15 from the ground surface 17 are thereby matched in order to obtain a defined, predetermined characteristic impedance for each coupling line 13, 15.

The separation between the coupling lines 13, 15 and one or more of the ground surfaces 10, 17 can be predetermined. Moreover, the characteristic impedance of each coupling line 13, 15 can be adjusted with some precision based on one ground reference potential and the electrical coupling between the transmission line 11 and the coupling lines 13, 15 can be adjusted using the second ground reference member.

The coupling lines 13, 15 can be embedded between the respective electrical insulators 12, 14 and 14, 16. The transmission line 11 can be insulated from the ground surface 10.

Coupling between the coupling lines 13, 15 and the transmission line 11 can be effected through electrical and magnetic fields. If done in this manner, the electric and magnetic couplings need to be balanced. Magnetic coupling can be done through the progression of the lines of magnetic flux in the area of the section where the coupling lines 13, 15 are guided in the direct vicinity of the transmission line 11. Electric coupling can be produced through the progression of the lines of electric flux between the transmission line 11 and the respective coupling line 13 or 15, and the area of the respective coupling line. Short coupling lines 13, 15 imply little magnetic coupling, and to provide balance, little electric coupling would be required in this case.

The progression of the lines of flux can be deflected by the ground surface 10, which, in the implementation shown in FIG. 2, is on the same plane as the transmission line 11. In this way, the lines of flux can be directed away from the coupling lines by the ground surface 10 to reduce electric coupling between the transmission line 11 and the coupling lines 13, 15. Moreover, the lines of flux of the electric field between the transmission line 11 and the coupling lines 13, 15 can be diverted by the ground surface 10 even if there is no coupling line between the transmission line 11 and the ground potential at the ground surface 10 in order to reduce the electric coupling between the transmission line and the coupling lines 13, 15.

In some implementations, the forward power and the reflected power (or values describing them) can be decoupled from the transmission line 11 with different coupling factors (that is, that portion of power that is coupled out of the transmission line 11 by ground surface 10). Usually, the reflected power is smaller than the forward power. If the reflected power is decoupled with a larger coupling factor than the forward power, the signal-to-noise ratio at the input of the evaluation device 7 increases because the temporal behavior of the evaluation device 7 in reaction to an input signal is advantageously utilized. In this way, the reflected power can be measured more precisely. Different coupling factors can be realized by positioning the coupling lines 13, 15 at different separations from the transmission line 11. The coupling factors are also influenced by the length and width of the coupling lines 13, 15, and the width and the length of the transmission line 11. The coupling factors can be adjusted by disposing the transmission line 11, the coupling line 13, and the coupling line 15 in different planes. Moreover, the coupling lines 13, 15 can be offset from each other to reduce or prevent coupling between the coupling lines and therefore to reduce or prevent errors in the measurement results.

In another implementation, the first ground surface 10 can be disposed in a plane that is distinct from the plane in which the transmission line 11 is disposed. For example, the first ground surface 10 can be disposed in a plane that is above and is parallel with a plane of the transmission line 11. The ground surface 10 influences the electric field in the surroundings of the transmission line 11. With this measure, the electric coupling between the transmission line 11 and the coupling lines 13, 15 can be influenced and adjusted.

FIG. 3 a shows a top view of the ground surface 10 and the transmission line 11. The transmission line 11 is completely embedded in the ground surface 10 and is thereby also shielded by it. The directional coupler 3 can include an input terminal 21 for connecting to the HF generator 2 and an output terminal 20 for connecting to the plasma load 4.

FIG. 3 b shows a top view of the electrical insulator 14 on which the first coupling line 13 is disposed. The coupling line 13 is bent outside of a coupling area 22, and in the coupling area 22, the first coupling line 13 extends parallel to the transmission line 11 such that connections 23, 24 of the first coupling line 13 are remote from the transmission line 11. A resistance 35 is exclusively connected to the connection 23, and the resistance 25 has resistance value that corresponds within a suitable tolerance to the characteristic impedance of the first coupling line 13. The connection 23 electrically couples to the transmission line 11. The connection 24 can be connected to the measuring device 6 to supply a quantity that describes the reflected power P_r.

FIG. 3 c shows a top view of the insulator 16 on which the second coupling line 15 is disposed. The coupling line 15 is bent outside of the coupling area 22, and in the coupling area 22, the second coupling line 15 extends parallel to the transmission line 11 such that connections 26, 27 of the second coupling line 15 are remote from the transmission line 11 and from the connections 23, 24 of the first coupling line 13. A resistance 28 is exclusively connected to the connection 26, and the resistance 28 has a resistance value that corresponds within a suitable tolerance to the characteristic impedance of the second coupling line 15. The connection 26 electrically couples to the transmission line 11. The connection 27 can be connected to the measuring device 5 to supply a quantity that describes the forward power P_i.

In some implementations, the length of the coupling lines 13, 15 in the coupling areas 22 can be less than λ/4. In other implementations, the length of the coupling lines 13, 15 in the coupling areas 22 can be less than λ/8. In other implementations, the length of the coupling lines 13, 15 in the coupling areas 22 can be less than λ/10. In this way, the dimensions of the directional coupler 3 can be kept relatively small. If the coupling lines 13, 15 are relatively short, for example, shorter than λ/4, then coupling between the coupling lines 13, 15 and the transmission line 11 in accordance with line theory can be neglected.

For example, the resistance values of the resistances 25, 38 can match or equal the characteristic impedance of the directional coupler 3. That is, the resistance values of the resistances 25, 28 can correspond (within a tolerance of less than ±10%) to the characteristic impedance of the respective coupling line 13, 15. As another example, the resistance values of the resistances 25, 28 can correspond within a tolerance of less than ±5% to the characteristic impedance of the respective coupling line 13, 15. As a further example, the resistance values of the resistances 25, 28 can correspond within a tolerance of less than ±1% to the characteristic impedance of the respective coupling line 13, 15. Moreover, reflections produced by the measurement can be absorbed at the resistances 25, 28 and therefore no new reflections are produced that can potentially contribute to measurement errors at the other coupling line. Reflections due to mismatch can be reduced or prevented through adjustment of the resistances 25, 28 relative to the characteristic impedances of the coupling lines 13, 15 and the tolerances in the directional coupler 3 can be compensated for by such adjustment.

If the directional coupler 3 is designed for operation at frequencies of less than 200 MHz or at frequencies of less than 40 MHz, it can be well suited for operation in HF plasma process excitation configurations such as the configuration 1.

In FIG. 4, the HF generator 2 and the directional coupler 3 are disposed in a housing 30, and the housing 30 is connected to a ground reference member that is at a ground potential. An output terminal 31 of the HF generator 2 is connected to the transmission line 11 of the directional coupler 3 by way of a line 32. The transmission line 11 of the directional coupler 3 is, in turn, connected to an inner conductor 33 of an output terminal 34 that is designed as a connector, for example, a coaxial connector. An outer conductor 35 of the output terminal 34 is connected on a large surface to the housing 30 using mounting devices 36 such as nuts and screws. In particular, the current returned from the plasma load 4 on the outer conductor 35 reaches the housing 30 at and through the outer conductor 35. In order to ensure that a substantial part of the return current flows through the second ground surface 17 and not through the housing 30, a short connecting line 37 is provided between the second ground surface 17 and the output terminal 34 for example, by connection to the mounting devices 36, which provide a direct, short, and large-surface connection to the ground surface 17. The second ground surface 17 is connected to the HF generator 2, for example, its ground potential, through a short line 38.

The connecting fines 37, 38 can be produced from any electrically conductive material such as copper or silver, which have a high electric conductivity. The length of the connecting lines 37, 38 can be ≦10 mm and the width can be ≧5 mm, in particular ≧10 mm. The flat short design of the connecting lines 37, 38 realizes a low-inductance connection between the outer conductor 35 and the ground of the HF generator 2 through the ground surface 17.

The housing 30 can be modified in order to increase the resistance for the returned HF current to thereby ensure that the return current flows substantially through the ground surface 17. For example, the housing 30 can be modified such that connecting elements (that is, elements that connect the HF generator 2 to the housing 30, for example, the lines 37, 38) between the housing 30 and the ground of the HF generator 2 can be ferrite rings. As another example, the connecting elements can be made of materials with a high relative permeability μ_r, since a high μ_r increases the skin effect, thereby impairing the HF current conduction. With this measure, the electric and magnetic fields are formed, which are required for good coupling between the coupling lines 13, 15 and the transmission line 11.

In this way, the entire return current from the plasma load 4 to the HF generator 2 flows through the ground surface 17. The DC resistance of the housing 30 can be increased by introducing inductances into the current path. Alternatively or additionally, the current path can be built up with little inductance through the ground potential of the directional coupler 3 to the ground of the HF generator 2.

Other Embodiments

It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.

Claims

1. A directional coupler for an HF plasma process excitation configuration, the directional coupler comprising: a transmission line;a first coupling line for detecting reflected power by electrical coupling with the transmission line, the first coupling line being spaced apart horn the transmission line and being terminated at least at one end with a termination resistance; anda second coupling line for detecting forward power by electrical coupling with the transmission line, the second coupling line being spaced apart from the transmission line and being terminated at least at one end with a termination resistance;wherein each coupling line has a predetermined and adjusted characteristic impedance, and the termination resistance of each of the coupling lines has a resistance value that corresponds within a tolerance to the characteristic impedance of the associated coupling line.

2. The directional coupler of claim 1, wherein the tolerance is less than ±10%.

3. The directional coupler of claim 1, wherein the tolerance is less than ±5%.

4. The directional coupler of claim 1, wherein the tolerance is less than ±1%.

5. The directional coupler of claim 1, further comprising a first ground reference member at a ground potential, wherein the first and second coupling lines are disposed at a predetermined separation from the first ground reference member.

6. The directional coupler of claim 1, further comprising two ground reference members at ground potential, wherein the coupling lines are disposed between the ground reference members, and the separation of the coupling lines from at least one ground reference members is predeterminable.

7. The directional coupler of claim 1, further comprising two ground reference members at ground potential, wherein the coupling lines are disposed between the ground reference members, and the separation of the coupling lines from both ground members is predeterminable.

8. The directional coupler of claim 1, further comprising two ground reference members at ground potential, wherein the ground reference members are defined by two ground surfaces and the coupling lines are disposed between the two ground surfaces.

9. The directional coupler of claim 1, further comprising a ground reference member at ground potential and being defined by a ground surface, wherein the transmission line is disposed in the same plane as and is electrically insulated from the ground surface.

10. The directional coupler of claim 1, wherein there is no coupling line between the transmission line and a member at a ground potential.

11. The directional coupler of claim 1, wherein each of the coupling lines has exclusively a termination resistance at one end.

12. The directional coupler of claim 1, wherein parallel sections of the coupling lines and the transmission line have a length of less than λ/4, where λ is the wavelength of the power signal from an HF generator to a plasma load through the transmission line.

13. The directional coupler of claim 1, wherein the forward power and the reflected power, or quantities describing the forward power and the reflected power, are decoupled from the transmission line with different coupling factors.

14. The directional coupler of claim 1, wherein the first and the second coupling lines are disposed at different separations from the transmission line.

15. The directional coupler of claim 1, wherein the transmission line, the first coupling line, and the second coupling line are disposed in different planes.

16. The directional coupler of claim 1, wherein the coupling lines are offset from each other.

17. The directional coupler of claim 1, wherein the first coupling line, the second coupling line, and the transmission line are separated from each other by an electrically insulating material.

18. The directional coupler of claim 17, wherein the electrically insulating material includes a printed circuit board material.

19. The directional coupler of claim 1, wherein the directional coupler is designed for operation at frequencies of less than about 200 MHz.

20. The directional coupler of claim 1, wherein the directional coupler is designed for operation at frequencies of less than about 40 MHz.

21. The directional coupler of claim 1, wherein the transmission line includes an input terminal and an output terminal.

22. The directional coupler of claim 1, wherein the transmission line receives at its input terminal forward power from an HF generator and delivers at its output terminal the forward power to a plasma load.

23. A high frequency (HF) plasma process excitation configuration comprising a directional coupler, wherein the directional coupler comprises: a transmission line including an input terminal and an output terminal;a first coupling line for detecting reflected power by electrical coupling with the transmission line, the first coupling line being spaced apart from the transmission line and being terminated at least at one end with a termination resistance; anda second coupling line for detecting forward power by electrical coupling with the transmission line, the second coupling line being spaced apart from the transmission line and being terminated at least at one end with a termination resistance;wherein each coupling line has a predetermined and adjusted characteristic impedance, and the termination resistances of the coupling lines each have a resistance value that corresponds within a tolerance to the characteristic impedance of the associated coupling line.

24. The HF plasma excitation configuration of claim 23, wherein most of the return current flows from a plasma load to an HF generator through a ground surface of the directional coupler.

25. The HF plasma excitation configuration of claim 24, wherein more than about 90% of the return current flows from the plasma load to the HF generator through the ground surface of the directional coupler.

26. The HF plasma excitation configuration of claim 23, wherein the HF resistance for the return current between an output terminal of the HF plasma excitation configuration and the ground surface of the directional coupler is smaller than the HF resistance of a housing between the output terminal and the ground potential of the housing.

27. A method for measuring power supplied to a plasma load of an HF plasma excitation configuration, the method comprising; transferring a forward power from an HF generator to the plasma load through a transmission line;electrically decoupling reflected power from the plasma load at a first coupling line that is spaced apart from the transmission line and is terminated at least at one end with a termination resistance;electrically decoupling the forward power front the HF generator at a second coupling line that is spaced apart from the transmission line and is terminated at least at one end with a termination resistance;predetermining and adjusting a characteristic impedance of each coupling line; andcorresponding the termination resistances of the coupling lines to the characteristic impedance of the associated coupling line.