RF voltage and current (VI) sensor and measurement method

Abstract

The radio frequency (RF) system includes an RF power supply configured to power a load with an RF signal, an RF tube including an outer conductor and an inner conductor connected to ground that couples the RF power supply to the load, and a current sensor aligned with a central axis of the RF tube carrying the RF signal. A sensor housing is disposed around the RF tube, the sensor housing including a conductive material connected to the RF tube's outer conductor. A gallery is disposed within the sensor housing and outside the RF tube's outer conductor, and the current sensor is disposed within the gallery. A slit in the RF tube's outer conductor exposes the current sensor to a magnetic field due to the current of the RF signal in the RF tube's inner conductor.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application is related to co-pending U.S. patent application Ser. No. 16 / 913,545, filed June 26, 2020 (Attorney Docket No. 200391US01), and U.S. patent application Ser. No. 16 / 913,548, filed June 26, 2020 (Attorney Docket No. 190883US01), and claims priority to U.S. patent application Ser. No. 16 / 913,526, filed June 26, 2020 (Attorney Docket No. 190907US01), which are incorporated herein by reference.

[0002] The present invention relates generally to plasma processing systems and methods, and in particular embodiments to radio frequency (RF) voltage and current sensors and measurement methods. [Background technology]

[0003] Generally, advances in semiconductor integrated circuits (ICs) are driven by the demand for increased functionality at low cost. Increased functionality at low cost is achieved primarily by increasing the packing density of components through miniaturization. An IC is a network of electronic components (e.g., transistors, resistors, capacitors) interconnected by a multilevel system of conductive lines, contacts, and vias. The elements of the network are integrated together by sequentially depositing and patterning layers of dielectric, conductive, and semiconducting materials on a semiconductor substrate using a manufacturing flow that includes process steps such as chemical vapor deposition (CVD), photolithography, and etching. The packing density of circuit elements has been increased by periodically reducing the minimum feature size through innovations such as immersion lithography and multiple patterning. Further miniaturization is achieved by reducing the footprint of the device using three-dimensional (3D) device structures (e.g., FINFET and stacked capacitor memory cells).

[0004] Plasma processes such as reactive ion etching (RIE), plasma enhanced chemical vapor deposition (PECVD), plasma enhanced atomic layer etching and deposition (PEALE and PEALD), and cyclic plasma processes (e.g., alternating cycles of deposition and etching) are commonly used in deposition and patterning steps used in semiconductor IC manufacturing. However, with the advent of feature sizes shrinking to a few nanometers for structural features controlled with atomic scale dimensions, there is an increasing demand to achieve manufacturing-friendly plasma technologies for advanced IC designs. Manufacturing-friendly plasma processes are expected to provide precisely controlled features for both plasma etching (e.g., sidewall angle, anisotropy, and selectivity to etch stop layers) and plasma deposition (e.g., conformality, aspect ratio selectivity, and area selectivity for bottom-up patterning), and structures with precise dimensions (e.g., line width, etch depth, film thickness) along with uniformity across wide (e.g., 300 mm) wafers. In many of the plasma processes used in IC manufacturing, the plasma is sustained by RF power. Because plasma characteristics are affected by the RF power delivered to the processing chamber, precise control of plasma processing can require innovative measurements of unobtrusive and precise RF signals. Summary of the Invention [Means for solving the problem]

[0005] In accordance with an embodiment of the present invention, a radio frequency (RF) system includes a radio frequency (RF) power supply configured to power a load with an RF signal, an RF tube including an outer conductor and an inner conductor connected to ground coupling the RF power supply to the load, and a current sensor aligned with a central axis of the RF tube carrying the RF signal. The current sensor is configured to monitor the current of the RF signal and includes a conductive half-loop disposed proximate to the RF tube, the conductive half-loop including a first end and an opposite second end. The current sensor is configured to output an output signal between the first end and the second end. A sensor housing is disposed around the RF tube, the sensor housing including a conductive material connected to the outer conductor of the RF tube. A gallery is disposed within the sensor housing and outside the outer conductor of the RF tube, and the current sensor is disposed within the gallery. A slit in the outer conductor of the RF tube exposes the current sensor to a magnetic field due to the current of the RF signal in the inner conductor of the RF tube.

[0006] In accordance with an embodiment of the present invention, a radio frequency (RF) system includes a radio frequency (RF) power supply configured to power a load with an RF signal, an RF tube including an outer conductor and an inner conductor connected to a reference potential node coupling the RF power supply to the load, and a first voltage sensor arranged axially symmetrically about an axis of the RF tube carrying the RF signal, the first voltage sensor being configured to monitor a voltage of the RF signal.

[0007] In accordance with an embodiment of the present invention, a method for measuring a radio frequency (RF) signal includes having a current sensor aligned with an axis of an RF tube carrying the RF signal, the current sensor being disposed within a sensor housing and within a gallery disposed outside an outer conductor of the RF tube. The sensor housing is disposed around the RF tube. The current sensor includes a conductive half-loop, the conductive half-loop including a first end and an opposing second end. The method includes determining a current of the RF signal based on measuring an output signal between the first end and the second end.

[0008] For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions which should be read in conjunction with the accompanying drawings, in which: [Brief description of the drawings]

[0009] [Figure 1A] 1 shows a block diagram of a general-purpose plasma processing system for semiconductor IC manufacturing. [Figure 1B] 1 illustrates a cross-sectional view of a VI sensor for an RF tube, according to an embodiment. [Figure 1C] 1 illustrates a top cross-sectional view of a current sensor of a VI sensor for an RF tube, according to an embodiment. [Figure 2A] FIG. 1 illustrates a perspective view of a VI sensor for an RF tube, according to an embodiment. [Figure 2B] 2B shows a cutaway view of the VI sensor shown in FIG. 2A. [Diagram 3] 1 illustrates a cutaway view of a VI sensor for an RF tube according to some embodiments. [Figure 4] 1 illustrates a cutaway view of a VI sensor for an RF tube according to some embodiments. [Diagram 5] 1 illustrates a cutaway view of a VI sensor for an RF tube according to some embodiments. [Figure 6A] FIG. 1 illustrates a perspective view of a VI sensor for an RF tube according to some embodiments. [Figure 6B] 6B illustrates a cutaway view of the VI sensor shown in FIG. 6A. [Figure 6C] 6B shows a cross-sectional view of the VI sensor shown in FIG. 6A. [Figure 6D] 6B illustrates a perspective view of a current sensor element of the VI sensor shown in FIG. 6A according to one embodiment. [Figure 7A] FIG. 1 illustrates a perspective view of a current sensor assembly for an RF tube according to some embodiments. [Figure 7B] FIG. 7B illustrates an exploded view of the current sensor assembly shown in FIG. 7A. [Figure 7C] 7B shows a cutaway exploded view of the current sensor assembly of FIG. 7A along with an RF conductor for the RF tube. [Figure 7D]7D illustrates a cutaway view of a current sensor assembly having an RF conductor for the RF tube shown in FIG. 7C according to some embodiments. [Figure 7E] FIG. 7D shows a plan view of the bottom portion of the current sensor assembly with the RF conductor shown in FIG. 7C. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The last two digits of all three-digit reference numbers in FIGS. 1A-7E always refer to the same components.

[0011] The making and using of the embodiments of the present disclosure are described in detail below, however, it should be recognized that the concepts disclosed herein can be embodied in a variety of specific contexts, and the specific embodiments described herein are merely illustrative and do not serve to limit the scope of the claims.

[0012] This disclosure describes a sensor design and methodology used to accurately measure the voltage (V), current (I), and phase angle (Φ) between the voltage and current of a radio frequency (RF) electrical signal. An embodiment of a voltage-current (VI) sensor described herein has been applied to probing the electric and magnetic fields of an RF electromagnetic wave along a coaxial transmission line, referred to as an RF tube.

[0013] In plasma processes in semiconductor manufacturing (e.g., plasma etching and deposition processes), RF power is often used to maintain the plasma. As known to those skilled in the art, the RF signal that maintains the plasma in the plasma chamber affects the plasma characteristics, which in turn affect the etching and / or deposition characteristics of the plasma process (e.g., electron density, thickness of the plasma sheath, ratio of ion flux to radical flux, etc.).

[0014] In various embodiments, the present application describes an RF VI sensor for measuring the current and voltage of an RF signal passing through an RF tube. The term RF tube herein refers to a coaxial transmission line that carries RF power from one part of a plasma reactor (called a plasma processing system) to another. The waveforms I and V transmitted through the RF tube are functions of position (x) and time (t), I(x,t) and V(x,t). When a single frequency component f is present, the current and voltage are expressed as Re(I(x)e jωt ) and Re(V(x)e j(ωt+Φ )), where ω=2πf and j 2 =-1, and Re is the real part of the complex function. As mentioned above, I and V each have a magnitude, and each of I and V is distinguished from the other by a phase angle Φ. In general, the waveforms, I and V, may include multiple frequency components. The voltage V(x,t) refers to the potential of the inner conductor (or core) of the RF tube (or coaxial transmission line) with respect to the grounded outer conductor (or shield), with ground representing the reference potential of the RF system.

[0015] RF VI sensors, as described in various embodiments, can provide V and I with high accuracy at the sensor location using geometric symmetry and differential measurement techniques. Several VI sensors can be used at various locations along the RF tube to probe the RF signal there. Since V and I are functions of location x, each VI sensor can be positioned as close as possible to its desired measurement location. For example, if it is desired to monitor and control a plasma process using accurate measurements of the voltage and current of an RF signal provided to a plasma chamber, a VI sensor used for that purpose can be positioned close to where the RF signal enters the plasma chamber. Various embodiments described herein increase the measurement accuracy and sensitivity of the VI sensor without increasing costs, thereby providing plasma processing systems with improved capabilities to provide plasma processing with better repeatability and tighter process control at the same cost. Additionally, the RF probe is designed to be unobtrusive so that the improved VI sensor can be easily retrofitted to existing plasma processing equipment without time-consuming and costly re-creation of established recipes for plasma processing in the production flow.

[0016] In this disclosure, the use of a VI sensor in a plasma processing system is first described with reference to the block diagram shown in Figure 1A. Next, the basic structure and operating principles of a VI sensor in a plasma processing system are described with reference to the schematic diagram shown in Figure 1B according to an embodiment. Then, some innovative aspects of the VI sensor design (similar to the basic structure in the schematic diagram of Figure 1B) are described with reference to the exemplary embodiments of the VI sensor shown in Figures 2A, 2B, 3, 4, and 5.

[0017] As will be explained in more detail below with reference to Figure 1B, the voltage sensor in the embodiments of Figures 2A, 2B, 3, 4 and 5 has axial symmetry and is designed as a conductive ring placed inside the RF tube and looped around a central longitudinal axis extending parallel to the direction of current flow, advantages of which are provided by the axial symmetry design are explained below with reference to Figure 2B.

[0018] Each current sensor in the exemplary embodiment is located in a sleeve or gallery around the outside of the RF tube. The gallery is a cavity inside the sensor housing. The sensor housing has a conductive wall covering the gallery and can be connected to the outer conductor of the RF tube and thereby connected to ground. As described in detail below with reference to FIG. 1B, the current sensor is a single conductive loop (called a half loop) with two open ends, and the loop is completed using an external VI analyzer component connected to the current sensor by, for example, a coaxial cable. The VI analyzer is a measurement system that analyzes the raw signals received from the current and voltage sensors. As described further below, both ends of the current sensor can be connected to the VI analyzer to make differential measurements for accurate analysis. However, to simplify the system at the expense of accuracy, only one end of the current sensor is connected to the VI analyzer, and the other end is terminated by a load impedance to ground (e.g., by a 50 ohm load) or shorted to ground. The ground connection can be a direct connection to the sensor housing. In this configuration, the outer ground cover of the gallery is in circuit between the two ends of the half loop and contributes significantly to completing the loop. In another RF system using the current sensor embodiments described in this disclosure, the entire closed loop may be contained within the gallery with appropriate impedance matching and one or more external signal connections.

[0019] In the exemplary embodiment described with reference to FIGS. 1B-5, the current loop of the half-loop current sensor has one conductive turn including three conductive elements. The three conductive elements of the half-loop of the current sensor are two identical vertical branches connected by a horizontal branch oriented parallel to the central axis of the RF tube. Thus, the current sensor design described herein has mirror symmetry with respect to a mirror plane perpendicular to the central axis of the RF tube and passing through the middle of the two vertical branches. The advantages of having mirror symmetry are described below with reference to FIG. 2B. However, the single turn half-loop current sensor is located on one side of the RF tube and lacks axial symmetry. An axially symmetric multi-turn half-loop current sensor design is described with reference to FIGS. 6A-7E, where the embodiment utilizes a toroidal mandrel to mechanically support the multi-turn current pickup.

[0020] The innovative aspects of the VI sensor design described in this disclosure may provide several advantages. For example, non-intrusive probing of the electric and magnetic fields of electromagnetic waves is used to enable VI measurements to be made with negligible disturbance of the RF signal in the RF tube. Also, geometric symmetry and differential measurement techniques may be advantageously utilized in the sensor design to provide measurements that may be less sensitive to machining tolerances due to standard tolerances of the tools used to form the components and to positioning errors during assembly of the VI sensor. In addition, several structural reinforcement techniques may be utilized, for example, dual placement of elements of the VI sensor may be performed to enhance geometric symmetry, and parts designed to provide additional mechanical support may be placed to reduce / suppress even minor shape deformations of critical sensor components caused by mechanical stresses during assembly. Thus, by using the embodiments described in this disclosure, improved accuracy of RF VI measurements may be achieved without the increased cost of tighter machining tolerances.

[0021] FIG. 1A is a block diagram of a general-purpose plasma processing system that may be used in semiconductor IC manufacturing.

[0022] 1A, in a plasma processing system, an RF signal may be generated by a high power RF power supply 10, e.g., an RF oscillator coupled to an RF power amplifier. The RF signal waveform (e.g., frequency, amplitude, pulsed / continuous, etc.) may be adjusted by a programmable controller 20 and associated electronic circuitry. The RF signal may be transmitted through a conduit, e.g., an RF tube 110, to provide RF power to an electrode coupled to the plasma in a block shown in FIG. 1A as plasma chamber 30.

[0023] As known to those skilled in the art, the RF signal in the RF tube 110 can be represented as a combination of traveling RF electromagnetic waves. An impedance mismatch between the output impedance of the RF power source 10 and the load impedance results in a portion of the RF power traveling from the RF power source 10 toward the load being reflected back to the RF power source 10. To suppress such undesired reflections, a matcher 40 including a matching network can be inserted in the RF signal path between the RF power source 10 and the plasma chamber 30 as shown in FIG. 1A. The ratio of the reflected power to the incident power of the matching network can be sensed by the matcher 40 (e.g., using a VI sensor and an analyzer) and provided to the programmable controller 20. The programmable controller 20 can reduce the RF power reflected back from the matching network to the RF power source 10, for example, by adjusting the impedance of the matching network using a feedback control loop (illustrated in FIG. 1A by the two arrows between the matcher 40 and the programmable controller 20).

[0024] The plasma can be maintained in the plasma chamber 30 using, for example, RF power delivered from an RF power source 10 by an RF tube 110 to an electrode of the plasma chamber 30. As shown in FIG. 1A, a VI sensor 100 can be used to sense the current and voltage of an RF signal provided to the electrode. In various plasma chamber designs, the electrode can be located inside the chamber wall, e.g., a disk-shaped electrode in a capacitively coupled plasma (CCP) chamber or an antenna outside the chamber wall. For example, in an inductively coupled plasma (ICP) chamber, the antenna can be a conductive planar spiral positioned above a dielectric window or a conductive spiral wire wrapped around a dielectric cylinder. The block shown as the plasma chamber 30 in FIG. 1A includes the electrode and antenna coupled to the plasma. For simplicity, in this disclosure, the term electrode refers to the electrode and / or antenna. The plasma chamber 30 includes at least two electrodes, e.g., an upper electrode and a lower electrode electrically coupled to the plasma therebetween. In some designs, it may be advantageous for the lower electrode to also be the substrate holder.

[0025] 1A shows an RF tube 110 delivering RF power from a single RF power source 10 to the plasma chamber 30, there may be more than one RF power source providing RF power to more than one electrode. For example, the RF power source 10 may provide RF power to an electrode (e.g., an upper electrode) of the plasma chamber 30 and a second RF bias power source may provide RF bias power to another electrode (e.g., a lower electrode) of the plasma chamber 30 using respective RF tubes, a matcher, and a VI sensor positioned near the plasma chamber to sense the voltage and current of the RF signal provided to the lower electrode.

[0026] In FIG. 1A, a VI sensor 100 used to sense and measure the current and voltage of an RF signal in the vicinity of an electrode receiving the RF signal is connected to a VI analyzer 60. The VI analyzer 60 may receive raw output waveforms reflecting V(t) and I(t) from the VI sensor 100 as indicated by the arrows. The VI analyzer 60 may be a signal processor, for example a digital signal processor, that may extract various RF signal characteristics from the raw waveforms. The various RF signal characteristics may include magnitude |V|, |I|, phase angle (Φ) of V and I, and peak RF power |V||I|cosΦ. In addition, harmonic analysis may be performed to extract multiple frequency components. The measured RF signal characteristics may reflect plasma properties such as free electron and ion density and ion / radical flux and energy, and plasma impedance. The VI analyzer 60 can be recalibrated using, for example, an RF calibration signal over a range of frequencies (e.g., from about 0.4 MHz to about 1 GHz) and powers (e.g., from about 0.015 kW to about 30 kW), standard load impedances (e.g., short circuit, open circuit, 50 ohms, etc.), and a vector network analyzer (VNA).

[0027] As indicated by the arrows in FIG. 1A, various RF signal characteristics reflecting plasma properties are provided by VI sensor 100 and VI analyzer 60 to programmable controller 20 and may be used, for example, for process monitoring or endpoint detection. In addition, programmable controller 20 may use the received RF measurements for process control. As known to those skilled in the art, plasma properties may be changed by varying the RF signal coupled to the plasma. Programmable controller 20 may use information from the RF measurements to control the plasma process, for example, by adjusting the settings of RF power supply 10 or by adjusting the impedance of the matching network of matcher 40.

[0028] In an embodiment in which the VI sensor 100 is used to estimate and control parameters of the plasma, it may be advantageous to place the VI sensor 100 in the vicinity of the plasma chamber 30. The V and I of the RF signal at the electrode position may be estimated from the V and I measured by the VI sensor 100 located there at a different position. However, the measurement error of V and I at the electrode may increase with increasing distance between the electrode and the VI sensor. Theoretically, the transfer matrix used to convert the sensor signal between two positions deviates further from the identity matrix as the distance between the two positions increases. Thus, the V and I estimated for the electrode position become increasingly sensitive to errors in estimating the respective transfer matrices.

[0029] 1B, the VI sensor 100 is attached to an RF tube 110 that connects to the plasma chamber 30 according to one embodiment. The RF tube 110 may be a coaxial structure including two conductive tubes (e.g., aluminum or copper tubes) arranged concentrically about a shared longitudinal axis. The inner conductive tube, referred to as the inner conductor 120, may be electrically connected to the output terminal of the matcher 40, indicated by the left-pointing arrow. The outer conductive tube, referred to as the outer conductor 130, may be a ground sheath connected to a reference potential, commonly referred to as ground. The RF tube 110 may be referred to as the main coaxial line because it carries RF power from the matcher 40 to the plasma chamber 30. Other coaxial lines in this disclosure are referred to as coaxial signal lines (e.g., coaxial lines that may be used to carry signals from the VI sensor 100 to the VI analyzer 60).

[0030] The VI sensor 100 includes two main components: a current sensor 140 and a voltage sensor 150. The current sensor 140 may be disposed in an annular gallery 160 inside a sensor housing 165 with conductive walls (e.g., aluminum, brass, stainless steel, or copper). In the embodiment shown in FIG. 1B, the gallery 160 is a hollow annular region along the entire circumference of the outside of the outer conductor 130 and axisymmetric about the axis of the RF tube 110. The axisymmetric design of the gallery 160 provides the advantage of preventing further reflections and non-axisymmetric wave modes of the RF electromagnetic waves propagating within the RF tube 110. The gallery 160 and the conductive sensor housing 165 may be integrally formed with the RF tube 110 or may be symmetrically attached to the RF tube 110 and may be positioned during assembly to help prevent alignment errors between the longitudinal axis of the RF tube 110 and the current sensor 140. In either case, the conductive sensor housing 165 and the outer conductor 130 are electrically and physically connected such that the sensor housing 165 can be considered an extension of the outer conductor 130 of the coaxial RF tube 110.

[0031] 1B has an annular gallery 160, in some other embodiments the gallery may not be annular. In some other embodiments, the axial symmetry of the RF tube 110 may be unavoidably compromised, for example, by bending of the RF tube 110 and further degradation of the axial symmetry due to asymmetry in the VI sensor design, and may not be critical. It may then be reasonable to relax the axial symmetry in the VI sensor design. For example, the gallery may partially follow the circumference of the outer conductor 130 and not form a complete circuit of the RF tube 110.

[0032] 1B and 1C, the gallery 160 is shown completely surrounded by the conductive surface of the sensor housing 165 and the outer conductor 130, except for a slit 132 that connects the hollow region of the gallery 160 with the RF tube 110. The current pickup 141 of the current sensor 140 is shown positioned within the gallery 160 directly above the slit 132. In the embodiment shown in FIG. 1B, the current pickup 141 includes three conductive branches arranged as three sides of a rectangle (called a half-loop), namely two vertical branches 142 and a horizontal branch 143. In one embodiment, the two vertical branches 142 are each screwed into an opening in the horizontal branch 143.

[0033] 1B, the vertical and horizontal branches of the current pickup 141 are formed using three separate sections. In some other embodiments, a different number of sections (fewer / more) may be used.

[0034] The slits 132 are designed to allow magnetic flux to enter the gallery 160. The current flowing through the inner conductor 120 results in a magnetic flux circulating around the inner conductor 120 about the longitudinal axis LA1 in the region between the inner conductor 120 and the outer conductor 130. Without the slits 132, the magnetic flux outside the outer conductor 130 would be nearly zero because an equal but opposite return current flowing through the inner surface of the outer conductor 130 cancels the circulating magnetic flux due to the current in the inner conductor 120 according to Ampere's law. The slits 132 interrupt the continuity of the cylindrical outer conductor 130, thereby diverting the return current to flow along the inner surface of the outer conductor of the sensor housing 165. The hollow region of the gallery 160 housing the half-loop current pickups 141 thereby falls within the region between the current flowing through the inner conductor 120 and the respective return current. Ampere's law creates a magnetic field within the gallery 160 that passes through the rectangular half-loop of the current pickup 141. In one example, the slits 132 may extend around the entire circumference of the cylindrical outer conductor 130 to help maximize the magnetic flux passing through the half-loop of the current pickup 141.

[0035] In addition to magnetic flux, electric flux is generated from the inner conductor 120 due to the voltage difference between the grounded outer conductor 130 and the inner conductor 120. Unwanted electric flux can leak into the gallery 160 through a gap in the ground sheath provided by a slit 132 made in the outer conductor 130. The changing magnetic flux passing through the half loop of the current pickup 141 induces an electric signal that is a measure of I(t) at that location. However, the electric flux entering the gallery 160 can couple with the current pickup 141 and adversely affect the signal caused by the magnetic flux. Thus, as shown in FIG. 1C, the slit 132 is designed to have a width (dimension parallel to LA1) of about 1 mm to about 5 mm. The width of the slit 132 can be kept narrow to help reduce the electric flux entering the gallery 160 from inside the RF tube 110.

[0036] The slit design used in the VI sensor embodiment described with reference to Figures 1B-5 is shaped as a ring around the circumference of the outer conductor, although it will be appreciated that various other designs are possible, for example, a zigzag slit design is used in the current sensor assembly described with reference to Figures 7A-7E.

[0037] The conductive portions of the current sensor 140 may be insulated from the conductive surfaces of the outer conductor 130 and the sensor housing 165 by air gaps (or other insulators) and by insulating parts used for mechanical support, such as insulating portion 162 of FIG. 1b (and other similar portions shown in FIGS. 2A-5).

[0038] The current pickup 141 is in the form of a half-loop (a loop with two open ends) going around an area with a rectangular cross section with two vertical branches 142 and a horizontal branch 143. The single turn half-loop current pickup 141 may be positioned in the presence of a time-varying magnetic field resulting from an RF electromagnetic wave traveling along the RF tube 110. By Faraday's law, a time-varying voltage difference may be induced between the two ends of the current pickup 141 and may be proportional to the time-varying magnetic flux. The two ends of the current pickup 141 may be attached to a pair of symmetrical terminals 144 shown on the sensor housing 165 in FIG. 1B. In one embodiment, the terminals 144 may be coaxial cable connectors used to connect coaxial signal lines.

[0039] As will be explained in more detail below, it is advantageous to use a symmetrical design for current pickup 141. This symmetry is exploited by a measurement system (e.g., VI analyzer 60 of FIGS. 1A and 1B) to cancel parasitic signals in the two vertical branches 142, for example, by measuring the differential voltage between the two terminals 144 of the current sensor. In this measurement method, the differential signal from current sensor 140 is the output signal of current sensor 140 and can be detected, for example, using a differential amplifier.

[0040] The half loop of the current pickup 141 is completed external to the VI sensor by a combination of the termination impedance, the input impedance of the initial detection system, and the impedance of the cable (if a cable is used to transmit the output signal of the current sensor 140 to the initial detection system of the measurement system). If the initial detection system is located on the current sensor 140 itself, the need to match the impedance of the detector to the impedance of the cable may be eliminated. If the measurement system is remote from the current sensor 140, the terminal 144 may be connected to the initial detection system of the measurement system using a coaxial signal line, for example a coaxial cable. Coaxial cables typically have an impedance in the range of about 20 ohms to about 300 ohms. It is advantageous to terminate the coaxial signal line with a matching impedance to prevent reflections from the measurement system due to impedance mismatch. It is also advantageous to connect the two end terminals 144 of the current sensor 140 to symmetrical coaxial signal lines that are symmetrically terminated, thereby maintaining the symmetry of the output signal of the current sensor 140. For example, in one embodiment, a pair of identical 50 ohm coaxial cables with 50 ohm terminations may be used.

[0041] As mentioned above, using a differential signal as the output signal of the current sensor results in greater accuracy in measuring I. To detect a differential signal, a pair of signals from the pair of terminals 144 must be provided to the initial detection system, for example using a pair of coaxial cables. However, with some loss of accuracy, the current sensor may also be used in conjunction with a measurement system that detects a signal at one of the pair of terminals 144. In a system that detects a signal at a first terminal of the pair of terminals 144 (rather than detecting a differential signal), the second terminal of the pair of terminals 144 may be connected to an impedance that reflects as closely as possible the impedance at the first terminal. For example, the first terminal may be connected to a first 50 ohm coaxial cable to transmit a signal to a 50 ohm input port of the initial detection system, and the second terminal may be connected to a second identical 50 ohm coaxial cable with a 50 ohm termination at the end of the cable instead of the detector. It should be noted that any other suitable impedance may be used, i.e. the impedance does not have to be 50 ohms. Additionally, the second coaxial cable may be omitted and an appropriate impedance termination may be attached directly to the second of the pair of terminals 144 .

[0042] The design of the measurement system, including the elements used to connect the current sensor 140, also takes into account the impedance to ground due to the parasitic capacitance of the electronic components. Due to the frequency dependence of the parasitic capacitance impedance, the impedance of the components at RF frequencies may be significantly different than the impedance of the components at low frequencies or at DC (zero frequency). For example, due to the parasitic capacitance to ground associated with the resistive structure, the impedance of the resistive components at DC may be reduced as the frequency of the electrical signal increases to the RF range. The impedance of resistors with higher resistance values ​​is more sensitive to the frequency of the RF signal. Since the parasitic capacitance to ground depends on the geometry and the geometric environment in which the resistor is placed, it is difficult to control the impedance variation unit by unit when the resistor has a high resistance value. Therefore, in order to maintain the measurement accuracy, it is advantageous to limit the design of the initial detection system to using resistors whose resistance value is less than 0.1 of their parasitic RF reactance, even when the initial detection system is placed at the sensor position.

[0043] The voltage pickup 151 of the voltage sensor 150 includes a conductive ring that may be disposed along the inner surface of the outer conductor 130. The outer conductor 130 and the conductive voltage pickup 151 may be insulated from one another by an insulating ring 152, as shown in FIG. 1B. The insulating ring 152 may include Teflon, or other plastic material, or other suitable dielectric. In one embodiment, the voltage pickup 151, such as the conductive ring, may be exposed to air (or other insulator) between the inner conductor 120 and the outer conductor 130. In another embodiment, the voltage pickup 151 may be embedded in an insulating housing. In all embodiments, the voltage pickup 151 (e.g., the conductive ring) may be electrically insulated from the outer conductor 130 and mechanically supported by an insulating structure.

[0044] In one embodiment, the inner diameter of the voltage pickup 151 (e.g., conductive ring) may be the same as the inner diameter of the outer conductor 130. In some other embodiments, the inner diameter of the voltage pickup 151 (e.g., conductive ring) may be different (smaller or larger than the inner diameter of the outer conductor 130). The perturbation of the electric and magnetic fields in the RF tube 110 caused by inserting the voltage pickup 151 is relatively smallest when the inner diameter of the voltage pickup 151 (e.g., conductive ring) and the inner diameter of the outer conductor 130 are equal. The output signal from the voltage pickup 151 increases as the inner diameter of the voltage pickup 151 (e.g., conductive ring) decreases, as described in more detail below. The contact to the voltage pickup 151 extends outside the outer conductor 130 and terminates in a third terminal 153 (e.g., a third coaxial cable connector) mounted above the sensor housing 165. The current pickup 141, the voltage pickup 151, and the contacts to the respective terminals 144 and 153 may comprise a metal having high electrical conductivity (e.g., copper), all of which may be insulated from other conductive elements such as the outer conductor 130 and the conductive sensor housing 165.

[0045] Considerations regarding the design of the termination impedance and coaxial signal line connecting the initial detection system to terminal 153 of voltage sensor 150 may be similar to those regarding the design of the termination impedance and coaxial signal line connecting the initial detection system to terminal 144 of current sensor 140, as described above. The above regarding current sensor 140 includes considerations regarding maintaining symmetry of the differential output signal. However, that portion of the above is not applicable to voltage sensors because in the VI sensor 100 embodiment, voltage sensor 150 has only one ring-shaped voltage pickup 151 and one terminal 153, while current sensor 140 has a pair of terminals 144. Symmetry considerations may be applicable to alternative embodiments in which two voltage pickup rings are symmetrically arranged and an arithmetic average of the two signals may be used, for example, in VI sensor 300, described with reference to FIG. 3.

[0046] 1B and 1C, the longitudinal axis LA1 of the RF tube 110 is in the plane P1 of the current pickup 141. The longitudinal axis LA1 is also parallel to the direction of the current in the RF tube 110. In addition, as can be more easily seen in FIG. 1C, along a direction perpendicular to the longitudinal axis LA1 of the RF tube 110, the current pickup 141 includes a first mirror symmetry plane M1 that includes the longitudinal axis LA1 of the RF tube 110 and a second mirror symmetry plane M2 that is perpendicular to the first mirror symmetry plane M1. The first mirror symmetry plane M1 of the current pickup 141 and the longitudinal axis LA1 of the RF tube 110 are coplanar in one or more embodiments.

[0047] The magnetic field lines are approximately concentric about the longitudinal axis LA1 and pass perpendicularly through the plane P1 of the half loop. In this configuration, the magnetic field is inductively coupled to the current pickup 141 (as desired). By placing the current pickup 141 outside the outer conductor 130, the undesired coupling to the electric field is significantly weakened. An electromotive force (emf) is induced in the current pickup 141 (a three-sided half loop) by the inductively coupled oscillating magnetic field. The induced emf is related to the change in magnetic flux according to Faraday's law. Since the magnetic field strength around a current carrying conductor reflects the respective current, the current sensor 140 can generate a time-varying electrical signal that reflects the RF current in the RF tube 110 at the respective location. An aspect of the current sensor 140 is that the electrical signals at both terminals 144 can be received by a detection system and the differential voltage between the two terminals 144 can be used as the output signal of the current sensor 140. The advantages provided by the differential output technique are described in more detail below with reference to FIG. 2B.

[0048] The contours of the potential and electric field magnitude are approximately circular with a center on the longitudinal axis LA1 of the RF tube 110. The circular contours are contained in a set of planes perpendicular to the longitudinal axis LA1. Thus, the electric field lines are led radially from the inner conductor 120 perpendicular to the longitudinal axis LA1. A ring-shaped voltage pickup 151 is located on one of the approximately circular contours. In this configuration, the oscillating electric field in the space outside the inner conductor 120 is capacitively coupled to the voltage pickup 151, and the conductive ring acquires an oscillating potential that is approximately proportional to the potential of the inner conductor at the respective location according to the physical laws of electromagnetism. This oscillating potential can be used as the output signal of the voltage sensor 150. The magnitude of the radial electric field between the inner conductor 120 and the outer conductor 130 decreases as the radial distance from the longitudinal axis LA1 increases according to Gauss's law. Thus, by positioning the voltage pickup 151 of the voltage sensor 150 closer to the inner conductor 120, for example by reducing the inner diameter of the voltage pickup ring, the output signal of the voltage sensor 150 may be increased.

[0049] The voltage pickup 151 is capacitively coupled to the electric field, but there is little coupling to the magnetic field because the magnetic flux perpendicular to the plane of the ring-shaped voltage pickup 151 is negligible for this geometry. Since the electric field strength around the conductive tube (inner conductor 120 in this example) reflects the electric potential of the conductor, the voltage sensor 150 can generate a time-varying electrical signal that reflects the RF voltage on the RF tube 110 at each location.

[0050] The raw output signals (eg, one pair from current sensors 140 and another pair from voltage sensors 150) may be transmitted to VI analyzer 60, as indicated by the arrows (see also FIG. 1A).

[0051] FIG. 2A shows a perspective view of the VI sensor 200 and the outer conductor 230 (outer tube) of the RF tube. FIG. 2B shows a cutaway view of the same VI sensor 200 along axis 2B-2B'. The VI sensor 200 of FIG. 2A and FIG. 2B is similar to the VI sensor 100 of FIG. 1B. The current sensor 240 and the voltage sensor 250 are disposed in an annular gallery 260 of the VI sensor 200. In FIG. 2A and FIG. 2B, the inner conductor of the RF tube has been removed to better show the voltage pickup ring 251 of the voltage sensor 250 inside the outer conductor 230. The perspective view (FIG. 2A) shows three terminals of the VI sensor 200 (coaxial cable connectors in this example). A pair of terminals 244 extending above the top of the gallery 260 connect to the current pickups 241 of the current sensor 240, as seen in FIG. 2B. The third terminal 253 connects to a voltage pickup 251 of the voltage sensor 250 .

[0052] 2B, the current pickup 241 of the current sensor 240 is a half-loop that includes three conductors. The two conductive vertical branches 242 of the current pickup 241 are insulated from the metal sensor housing 265 by plastic (or other insulating material). The vertical branches 242 connect to the two ends of the horizontal branch 243, a third conductor that is horizontally placed inside the gallery 260 above the outer conductor 230. Slits 232 along the circumference of the outer conductor 230 allow the magnetic field to pass through the plane of the half-loop and induce an electromotive force in the conductive branches of the current pickup 241. The horizontal branch 243 of the current pickup 241 can be attached to a horizontal non-conductive (e.g., plastic) portion 262 along the side of the conductor. 2A and 2B, the horizontal branch 243 is insulated from the grounded metal sensor housing 265 and the outer conductor 230 by plastic sections 262 on either side and by air in the gap between the bottom of the horizontal branch 243 and the top of the outer conductor 230. In another embodiment, described in more detail below, mechanical support for the horizontal branch 243 may be supplemented by an additional plastic section placed in the gap below the horizontal conductor.

[0053] A current pickup (e.g., current pickup 240 in FIG. 2B) provides an electrical signal through interaction with an RF electromagnetic field. As explained above, it is a magnetic field (not an electric field) that reflects the RF current. The slit 232 allows the magnetic field to penetrate from the RF tube into the gallery 260 where the current pickup 241 is located. Coupling of the current pickup 241 to the electric field reduces the accuracy of the measurement of the magnetic field. The current sensor 240, as described herein, can suppress measurement errors that may result from undesired interaction with the electric field. First, the current sensor 240 in the VI sensor 200 is positioned outside the grounded outer conductor 230, thereby using the outer conductor 230 to shield the electric field. As described above with reference to FIGS. 1B and 1C, the RF electric field is in a radial direction (perpendicular to the coaxial line LA1 of the RF tube), and therefore the electric flux leaking into the gallery is approximately directly proportional to the slit width, defined above as the dimension parallel to LA1. The width of the slit 232 may be selected to be relatively small to reduce the amount of electric flux that enters the gallery 260 due to the gap created by the slit 232 in the outer conductor 230. Secondly, a differential signal may be used as the output signal to further reduce the effect of a portion of the electric field that may penetrate into the cavity despite the outer conductor 230. Ideally, the differential voltage between the two terminals 244 of the current sensor is approximately proportional to the oscillating magnetic field, according to the theory of electromagnetics. However, due to the presence of the slit 232, the electric flux leaking into the gallery 260 may be approximately proportional to the slit width. A weak electric field may be capacitively coupled to the current pickup 241. However, the current pickup 241, the slit 232 and the gallery 260 may be constructed with mirror symmetry with respect to a plane passing through the center of the slit 232 and oriented perpendicular to the longitudinal axis of the RF tube. Due to this geometric mirror symmetry of the half-loop current pickup 241 described above, the perturbations along the vertical branch 242 and the potentials appearing at the two terminals 244 are approximately equal in magnitude and phase. This symmetry can be advantageously used because it means that the differential signal is not affected by parasitic signals in the current pickup 241 due to interaction with the intruding electric field in the gallery 260. In other words, the potential difference between the first and second terminals of the terminal pair 244 is not perturbed and remains correct to first order.These aspects of the current sensor 240 design may be advantageously used to achieve highly accurate measurements of current, particularly in applications such as providing an RF bias signal to an electrostatic substrate holder in a plasma chamber, where the load impedance may be such that the electric field amplitude is relatively high and the magnetic field amplitude is relatively low near where the RF signal enters the plasma chamber.

[0054] 2B, a ring-shaped conductor disposed inside the outer conductor 230 near the inner surface of the outer conductor 230 is the voltage pickup 251 of the voltage sensor 250. The strength of the signal generated by the voltage pickup 251 may be determined by the dimensions of the voltage pickup 251. Its diameter may be approximately determined by the diameter of the outer conductor 230, while the width and thickness are adjustable design parameters. The conductive voltage pickup 251 in this embodiment is a ring electrically connected at one point to the third terminal 253 (e.g., a coaxial cable connector) of the VI sensor 200. The conductive voltage pickup 251 is insulated from the conductive outer conductor 230 by a ring-shaped dielectric piece 252 attached to the voltage pickup 251.

[0055] As explained above, voltage pickup 251 provides an electrical signal resulting from charge polarization induced by the RF electromagnetic field to a third terminal 253 of VI sensor 200. The potential at third terminal 253 reflects the oscillating voltage of the inner conductor (not shown in FIG. 2B for clarity). The RF electric field is capacitively coupled to voltage pickup 251. However, as explained above with reference to FIG. 1B, the longitudinal axis is perpendicular to the plane of the ring, so interaction with the magnetic field is negligible.

[0056] The ring-shaped design of the voltage pickup 251, as described herein, uses axial symmetry to reduce the sensitivity of the output of the voltage sensor 250 to some of the placement and sizing errors. First, the circular symmetry may make precise placement of the voltage pickup 251 unnecessary, since the potential at the conductive surface of the ring is primarily independent of the deviation of the center position of the ring from the central axis (axis 2B-2B' in FIG. 2A and axis LA1 in FIG. 1B). Instead, the potential of the voltage pickup 251 is determined primarily by the dimensions of the ring (e.g., inner diameter, outer diameter, and thickness). In contrast, in an asymmetric voltage pickup design, the voltage pickup conductor acquires a potential that is primarily dependent on the size and placement of the asymmetric voltage pickup. For example, a mushroom-shaped voltage pickup may be sensitive not only to the dimensions of the conductive surface of the mushroom head, but also to the position of the mushroom head relative to the inner conductor. In such a design, the distance between the voltage pickup and the longitudinal axis of the RF tube must be precisely adjusted during assembly, possibly manually using a micrometer screw gauge. Second, the design of voltage pickup 251 can nearly cancel centering errors when assembling voltage sensor 250 to VI sensor 200. The axial symmetry of voltage pickup 251 ensures that, to the first order, the total electric flux remains unchanged if the circular center of voltage pickup 251 is slightly offset from the longitudinal axis of the inner conductor. The increase in electric flux in one half of the ring, which may be located closer to the inner conductor, is now balanced by a concomitant decrease in electric flux in the other half of the conductive ring, which is away from the longitudinal axis, due to the circular shape of the ring.

[0057] 3 shows a cutaway view of another embodiment of a VI sensor 300 having a current sensor 340 located in a gallery 360 inside a sensor housing 365. A horizontal branch 343 is shown supported by insulators 362 and connected to two vertical branches 342 of the current sensor 340. A circumferential slit 332 in the mirror symmetry plane M2 can be seen above the horizontal branch 343.

[0058] The first voltage sensor 350 is shown inside the outer conductor 330, similar to the VI sensor 200 described with reference to Figures 2A and 2B. In addition, the VI sensor 300 has a second voltage sensor 355 located symmetrically opposite the current sensor 340. In this embodiment, the voltage pickups and housings of the first voltage sensor 350 and the second voltage sensor 355 are embedded in the body of the outer conductor 330 in order to keep the inner surface of the outer conductor 330 as smooth as possible. The smooth inner surface of the outer conductor 330 provides the advantage of reducing the perturbation of the electromagnetic field caused by the insertion of the voltage sensors 350 and 355. In this embodiment, the VI sensor 300 causes negligible perturbation to the electromagnetic field in the RF tube.

[0059] The measurements from the first voltage sensor 350 and the current sensor 340 have a relative phase error due to the difference in the measurement positions of the first voltage sensor 350 and the current sensor 340. In this embodiment, the oppositely located second voltage sensor 355 has an opposite phase error due to its symmetrical position with respect to the current sensor 340 (i.e., the mirror symmetry plane M2 of the current sensor 340 is equidistant from the first voltage sensor 350 and the second voltage sensor 355). Due to the symmetry, the error in the relative phase of the voltage and current of the RF signal waveform sensed by the first voltage sensor 350 and the respective errors in the RF signal waveform sensed by the second voltage sensor 355 are at least to first order cancelled by the sum of the two sensed voltage signals. Thus, by combining the signals from the first voltage sensor 350 and the second voltage sensor 355, a more accurate voltage measurement may be provided. For example, by using an arithmetic average of measurements from the first voltage sensor 350 and the second voltage sensor 355, phase errors can be reduced or even eliminated to obtain a voltage measurement that reflects the voltage at the mirror symmetry plane.

[0060] In addition, the presence of the second voltage sensor 355 helps to ensure that the left and right halves of the horizontal branch 343 of the current pickup 341 as well as the two vertical branches 342 are subjected to the same electric and magnetic fields. As explained above, parasitic electrical signals can be generated by undesired coupling of the current pickup to the electric fields penetrating the cavity in the gallery 360. By improving the geometric symmetry, the second voltage sensor 351 helps to ensure that the perturbations in the potential seen at the first and second terminals 344 of the current sensor 340 are more accurately countered by using differential current measurements, as explained above with reference to Figures 2A and 2B. In some embodiments, using the output of the second voltage sensor 355 can be optional.

[0061] FIG. 4 illustrates yet another embodiment of a VI sensor 400 attached to an RF tube 410 that includes an inner conductor 420 and an outer conductor 430 .

[0062] 2A and 2B, VI sensor 400 includes a current sensor 440 and a voltage sensor 450 disposed within a gallery 460. The design of VI sensor 400 improves upon the design of VI sensor 200 (see FIGS. 2A and 2B) by providing additional mechanical support to the horizontal conductor 443 of the current pickup 441 of current sensor 440.

[0063] 4, the support parts (e.g., plastic portions 462 and 470) may secure horizontal branch 443 more securely than the respective portions (e.g., plastic portion 262) in VI sensor 200 shown in FIG. 2B. For example, in one embodiment, plastic portions 262 at the two ends of horizontal branch 243 in VI sensor 200 are rings with a set of holes into which horizontal branch 243 may be positioned, while in the design of VI sensor 400, plastic portions such as portions 462 and 470 may have bosses that surround more of horizontal branch 443 and fit snugly into corresponding cavities in the metallic outer surface of metal sensor housing 465 and outer conductor 430.

[0064] As shown in FIG. 4, a support structure 470 (e.g., made of plastic or other non-conductive material) arranged in addition to the insulating support 462 holds the conductive horizontal branch 443 of the current pickup 441 from all sides. The support structure 470 includes a first portion for supporting the first portion of the horizontal branch and a second portion for supporting the second portion of the horizontal branch, separated by a gap. In contrast, as shown in FIG. 2B, the plastic portion 262 (similar to the support 462) does not support the horizontal branch from below. In FIG. 2B, there is an empty space between the horizontal branch 243 of the current pickup 241 and the outer conductor 230 of the current sensor 240. The additional support prevents the horizontal conductor 443 from bending when the vertical branch 442 of the current pickup 441 is placed in contact with the horizontal branch 443. In addition, the support structure 470 may prevent over-tightening of the vertical branch 442 against the horizontal branch 443. Variations in the magnetic flux coupled to the current pickup 441 are affected by variations in the shape and area of ​​the half-loop geometry of the current pickup 441. Therefore, by keeping the shape of the current pickup 441 constant, variations in the electrical output of the current sensor 440 are reduced and the accuracy of the current measurement is improved.

[0065] 5 shows a VI sensor 500 mounted in an RF tube 510 including an inner conductor 520 and an outer conductor 530. A current sensor 540 is shown having a pair of terminals 544 disposed on a sensor housing 565 and a one-turn, half-loop current pickup 541 disposed within a gallery 560. The current pickup 541 includes two vertical branches 542 attached to a horizontal branch 543. As with the VI sensor 400 of FIG. 4, a plastic section 570 is used to prevent over-tightening of the vertical branches 542 and bending of the horizontal branch 543 of the current pickup 541 during assembly of the current sensor 540.

[0066] VI sensor 500 includes improvements that reduce processing complexity, and therefore manufacturing costs, relative to VI sensor 400 (shown in FIG. 4). The design of VI sensor 500 improves upon that of VI sensor 400 by using a voltage sensor 550 in which an insulator piece 555 that centers the inner conductor 520 of RF tube 510 is also used to support the conductive voltage pickup ring of voltage sensor 550, as shown in FIG. 5. Using the same plastic part 555 for multiple purposes allows for the elimination of some of the plastic parts used in, for example, VI sensor 400. This reduces processing complexity and manufacturing costs for VI sensor 500.

[0067] The conductive voltage pick-up ring of the voltage sensor 550 of Figure 5 is positioned closer to the inner conductor 520 by designing the diameter of the voltage pick-up ring to be smaller than the diameter of the outer conductor 530. The smaller diameter of the voltage pick-up ring increases the output signal strength of the voltage sensor 550, as explained above with reference to Figures 1B and 1C.

[0068] Although a single turn half loop current pickup is used in the VI sensor described above with reference to Figures 1-5, it is understood that multiple turns may also be utilized in the design of the current sensor current pickup. For example, the current pickup in the VI sensor shown in Figures 1-5 may include multiple rectangular turns between the two ends of the current pickup connected to the two terminals of the current sensor. As discussed above, a multi-turn current pickup may be constructed by winding a conductor around a mandrel, for example, a toroidal mandrel. The conductor may be coiled around a circular axis of a doughnut-shaped insulating material that forms a circle symmetric about the inner conductor of the RF tube that passes vertically through the central hole of the toroid. A multi-turn current pickup using a toroidal mandrel is described below with reference to Figures 6A-7E.

[0069] It is understood that the mandrel may not exactly fit the mathematical definition of a toroid, but is generally shaped like a toroid with structures for mounting the coil, connecting to terminals, etc.

[0070] 6A shows a perspective view of VI sensor 600, FIG. 6B shows a cutaway view, and FIG. 6C shows a cross-sectional view of VI sensor 600 along axis A-A'.

[0071] FIG. 6A shows the conductive sensor housing 665 of the VI sensor 600. The current and voltage pickups, not visible in FIG. 6A, are housed within the space enclosed by the conductive sensor housing 665. In FIGS. 6A-6C, the inner conductor passes through a central hole 621. The inner conductor itself has been omitted from the various views of the VI sensor 600 in FIGS. 6A-6C for clarity. The RF tube's outer conductor connects to flanges at the top and bottom of the conductive sensor housing 665. Two neck regions 631 are shown adjacent to the flanges at the top and bottom portions of the sensor housing 665 in FIG. 6A. The shape and dimensions of the neck regions 631 may be designed to be similar to the shape and dimensions of the RF tube's outer conductor. Thus, the sensor housing 665 may be considered an extension of the outer conductor, extending from the neck region 631 to a wider central portion that includes the top cover 663 and the bottom cover 666, which have larger diameter conductive walls. The sensor housing 665 and the outer conductor form an outer shield of a coaxial structure and may be connected to ground. The wider central portion of the sensor housing 665 contains an annular dielectric cavity 661 around the inner conductor that passes through a central hole 621, as described below with reference to Figures 6B and 6C.

[0072] 6A also shows three coaxial cable connectors assembled to the outside of the sensor housing 665. The three coaxial cable connectors are the three terminals of the VI sensor 600. The outer pair of coaxial connectors 645 connect to the terminals of the current sensor 641, which are connected to the current pickups, and the center coaxial connector connects to the center terminal 654, which is connected to the voltage pickup 651 of the voltage sensor. The current and voltage pickups are located between the top cover 663 and the bottom cover 666.

[0073] The cutaway view of FIG. 6B and the cross-sectional view of FIG. 6C along the cutting plane A-A′ (shown in FIG. 6A ) show the internal structure of the VI sensor 600. The inner conductor of the RF tube has been omitted for clarity. The inner surface 638 of the central hole 621 forms a sidewall inside the main coaxial structure, which can be considered as an RF tube, as shown inside the neck region 631 in FIG. 6B and FIG. 6C . The neck region 631 extends into the larger diameter top cover 663 and bottom cover 666 of the sensor housing 665, which encloses the dielectric cavity 661, in the section around the central hole 621 of the VI sensor assembly 600. The inner wall 638 is continuous as the surface 627 of the top cover 663 and bottom cover 666 until it is interrupted by the slit 671. As shown in FIG. 6C , the dielectric cavity 661 is between the first outer major surface 627 and the second outer major surface 628 along a radial direction from the center of the central hole 621. The first outer major surface 627 includes a continuous circular ring-shaped region in physical contact with the central hole 621. The ring-shaped first outer major surface 627 is divided vertically into two portions by a slit region 671. The second outer major surface 628 is located at a radial distance greater than the radius of the first outer major surface 627.

[0074] The cavity 661 includes a slit region 671. As shown in Figures 6B and 6C, the slit region 671 includes a physical discontinuity in the inner surface 627 of the cylindrical wall of the central hole 621 that creates a gap at the junction of the top cover 663 and the bottom cover 666 of the sensor housing 665. The surface 627 is an extension of the surface 638 of the cylindrical wall of the central hole 621. When viewed radially from the center of the central hole 621, the slit region 671 has the appearance of an insulating ring that is in physical contact with the central hole 621. Further radially outward, the slit region 671 takes on a zigzag shape that bypasses the ring-shaped conductive voltage pickup 651, as shown by the zigzag dashed line in Figures 6B and 6C. The continuous insulating annular region of the cavity 661 located radially between the physical contact with the central hole 621 and the inner diameter of the annular current sensor 641 is referred to as the slit region 671 of the VI sensor 600. As shown in Figures 6B and 6C, the slit area 671 forms a dielectric barrier between the conductive voltage pickup 651 and current sensor 641 and the sensor housing 665. In the radial region between the first outer major surface 627 and the second outer major surface 628, the cavity 661 (including the insulating slit area 671) electrically insulates the top cover 663 from the bottom cover 666. For a radial distance smaller than the first outer major surface 627, the top cover 663 is electrically insulated from the bottom cover 666 by the central hole 621. The top cover 663 is electrically coupled to the bottom cover 666 via a coupling area 629 located radially beyond the second outer major surface 628 of the dielectric cavity 661.

[0075] With this design, little to no RF current flowing within the grounded sensor housing 665 may flow in the area surrounded by the annular current sensor 641. The current flows vertically within the neck region 631 along the inner wall 638 and then is routed around the current sensor 641 due to a physical break in the inner surface 638 caused by the dielectric slit 671. Due to the slit 671, the current is diverted radially outwardly around the annular current sensor 641, flows laterally along the conductive walls of the annular dielectric cavity 661, returns radially, and then continues to flow vertically along the inner wall 638 of the neck region 631.

[0076] 6B and 6C, the current sensor 641 is an annular structure in the annular dielectric cavity 661 in the outer portion of the cavity, i.e., the region away from the central hole 621. The current sensor 641 includes a conductive coil 647 and a toroidal mandrel 642. The coil 647 includes multiple turns of a continuous conductor wrapped around the central circular axis of the toroidal mandrel 642. Two opposing ends of the coil 647 may be attached to a coaxial connector 645, as shown in FIG. 6B. The conductor of the coil 647 may be a bare conductor, an enameled conductor, or a conductor coated with an insulator. The toroidal mandrel 642 will be described in more detail below with reference to FIG. 6D. The current sensor 641 is electrically insulated from the conductive sensor housing 665.

[0077] As shown in Figures 6B and 6C, the conductive voltage pickup 651 of the voltage sensor is shaped as a conductive ring. The voltage pickup 651 is shown disposed in the region of the annular dielectric cavity 661 between the annular current sensor 641 and the central hole 621. A solid dielectric material (e.g., plastic) may be used for the portion used to mechanically support the voltage pickup 651. The slit region 671 of the cavity 661 and the dielectric support electrically insulate the conductive voltage pickup 651 from the conductive sensor housing 665. The connection between the voltage pickup 651 and the central terminal 654 is shown in Figure 6B (the coaxial connector 645 and the central terminal 654 are not included in the cut plane A-A' and therefore are not visible in the cross-sectional view shown in Figure 6C).

[0078] The function of the voltage pickup 651 is to sense the RF voltage of the inner conductor at the center of the central hole by sensing the radial electric field between the inner and outer conductors of the RF tube. In general, the outer conductor of the RF tube and the conductive sensor housing 665 are grounded. Thus, if the conductive ring of the voltage pickup 651 is shielded from the inner conductor of the RF tube, for example, by a grounded metal ring placed in the annular dielectric region between the inner conductor and the voltage pickup 651, the voltage pickup 651 may not function properly. With excessive shielding, the output of the voltage sensor becomes too weak to be used. As shown in Figures 6B and 6C, the voltage pickup 651 extends vertically partially into the annular dielectric cavity 661 above and below the slit region 671 of the cavity 661. The cavity is formed by a groove in the upper metallic top cover 663 and a respective groove in the lower metallic bottom cover 666. If the grounded conductive inner walls of these grooves are interposed between the conductive ring of the voltage pickup 651 and the central axis of the central hole 621 for the inner conductor, the grounded metal will not completely shield the voltage pickup 651. There is a dielectric slit 671, shown in Figures 6B and 6C, that separates the top cover 663 from the bottom cover 666. The slit 671 is considered a dielectric region shaped like a cylindrical disk in the mirror surface M (shown in dashed lines in Figure 6C) because there is no inner conductor in the central hole 621. When the inner conductor is in place, the dielectric slit 671 is shaped like an annular disk around the inner conductor. The unshielded radial electric field in the dielectric slit 671 is sensed by capacitive coupling between the inner conductor and the central portion of the ring-shaped voltage pickup 651. The voltage pickup 651 can now provide a usable electrical signal proportional to the RF voltage of the inner conductor at its location.

[0079] The function of the current pickup coil 647 is to sense the RF current in the inner conductor at the center of the central bore by sensing the circulating magnetic field passing through the coil in a direction parallel to the circular central axis of the toroidal mandrel 642. By Faraday's law, an oscillating electrical signal is induced in the coil proportional to the oscillating magnetic flux in the toroidal mandrel 642 surrounded by the turns of wire of the coil 647. By Ampere's law, the strength of the magnetic field passing through the current sensor 641 is proportional to the total current traversing the area of ​​the plane enclosed by the central hole of the toroidal current sensor (similar to the donut hole of a donut). As is true for any coaxial structure, the current flowing through the inner conductor at any location in the RF tube is exactly equal to the opposite current flowing in the outer conductor. The sensor housing 665 of the VI sensor 600 may be considered the equivalent outer conductor of the RF tube, with the inner conductor passing through the central bore 621. Thus, the current sensor 641 may not function properly unless the RF current in the sensor housing 665 is restricted to flow outside the disk-shaped area bounded by the outer circumference of the toroidal mandrel 642. For example, if the top cover 663 and the bottom cover 666 make electrical contact at a radial distance from the central axis that is less than the inner radius of the toroidal mandrel 642, some of the current in the conductive housing may flow through the contact. This current, which is in the opposite direction to the current in the inner conductor, reduces the magnitude of the total current bounded by the current sensor 641 and therefore reduces the magnetic flux passing through the coil 647. If the total current through the contacts in the area bounded by the toroidal current sensor 641 is too small, there may be insufficient magnetic field to induce a usable electrical signal in the current pickup coil 647. Again, the dielectric slit area 671 prevents electrical contact between the top cover 663 and the bottom cover 666 at a radial distance less than the inner radius of the toroidal current sensor 641, as shown in Figures 6B and 6C.

[0080] A continuous unbroken dielectric region separating the top cover 663 and the bottom cover 666 all the way to the outer circle of the annular current sensor 641 is obtained by designing the voltage pickup 651 to be smaller than the vertical height of the cavity 661. The conductive ring of the voltage pickup 651 can be positioned approximately symmetrically between the top cover 663 and the bottom cover 666 by supports that include insulating material. Thus, in all directions, there is insulating material in the immediate vicinity of the conductive voltage pickup 651. As explained above, this insulating material is in the slit region 671 of the cavity 661. The shape of the dielectric above the conductive ring of the voltage pickup 651 is depicted by a zigzag dashed line in FIG. 6B. It can be noted that the zigzag dielectric slit region 671 exists both above and below the voltage pickup 651 because the conductive ring of the voltage pickup 651 must be electrically insulated from the grounded sensor housing 665, as shown by the dashed line in FIG. 6C.

[0081] The current sensor current pickup is generally shielded from RF electric fields by a grounded conductive part. Shielding of the current pickup is advantageous in applications where the electric field is strong and the magnetic field is weak, such as near a high impedance load. In the VI sensor 600, the current pickup coil 647 is located in a dielectric cavity 661 inside the sensor housing 665. The conductive parts encountered while moving radially inward from the annular current sensor 641 to the inner conductor include the first conductive voltage pickup 651 and a portion of the inner wall of the second conductive sensor housing 665, as seen in Figures 6B and 6C and described above. These intervening conductive parts may help shield the current sensor 641 from the radial electric field. Some of the electric field lines originating from the inner conductor may terminate at the grounded inner wall of the conductive sensor housing 665. In addition, the conductive voltage pickup 651 serves two purposes by partially shielding the coil 647 from the RF electric field. Because the voltage pickup 651 is not shorted to ground, the reduction in the electric field provided by the conductive ring depends on the magnitude of the impedance to ground at the center terminal 654 .

[0082] The structures of both the current sensor 641 and the voltage pickup 651 of the VI sensor 600 are axisymmetric about a common axis that passes through the center and is perpendicular to the plane of the central bore 621. Additionally, both the current sensor 641 and the voltage pickup 651 share the same mirror plane (shown in FIG. 6C by dashed line M) that is perpendicular to the longitudinal axis. The symmetry of the structure of the VI sensor 600 helps to reduce / eliminate any discrepancies in the measurement of the phase angle (Φ) between the voltage and current. Additionally, the first order cancellation effect due to the axisymmetric nature makes the sensor output signal of the VI sensor 600 less sensitive to processing tolerances and positioning errors during assembly.

[0083] FIG 6D illustrates an exemplary toroidal structure that may be used as the mandrel 642 of the toroidal current sensor 641 shown in FIG 6B. The toroidal mandrel 642 has a continuous groove on its outer surface into which a conductor wire may be placed to form a coil 647 (shown in FIG 6B). Two opposing ends of the coil 647 may be threaded through two openings 643, shown in FIG 6D, and subsequently attached to a pair of coaxial connectors 645 (see FIG 6B). A voltage pickup 651 (see FIG 6B) may be connected by a conductive element threaded through the hole and opening 653 in the toroidal mandrel 642 for attachment to a central terminal 654.

[0084] The toroidal mandrel 642 may comprise plastics and other insulating materials and may be manufactured, for example, using 3D printing techniques. After the coil 647 is attached to the grooved toroidal mandrel 642, the structure may optionally be encapsulated with a coating of resin, for example, using an encapsulation resin technique. The resin encapsulation provides a rigid fixation for the coiled multi-turn current pickup 647.

[0085] The integrated assembly of VI sensor 600 described above, including current sensor 641 and combined electric field shield and voltage pickup 651, provides the advantage of a compact VI sensor design.

[0086] 7A-7E show a current sensor assembly 701, which is similar in design to that of VI sensor 600. Unlike VI sensor 600, current sensor assembly 701 does not sense voltage. Also, the design of the toroidal mandrel 742 used in current sensor assembly 701 differs from grooved toroidal mandrel 642, as described further below.

[0087] 7A shows a perspective view of a current sensor assembly 701 using a toroidal current sensor 741 disposed in a dielectric cavity between a top cover 782 and a bottom cover 784 of a sensor housing 765. The current sensor 741 is further described below with reference to FIGS. 7C-7E. The top cover 782 and the bottom cover 784 may comprise a metal (e.g., copper or aluminum). The current sensor assembly 701 has a central hole 710. An inner conductor of a coaxial transmission line (e.g., an RF tube) in which the current sensor assembly 701 may be used passes through the central hole 710. The current sensor assembly 701 is thereby positioned symmetrically about the longitudinal axis of the coaxial transmission line.

[0088] FIG. 7B shows an exploded view of the current sensor assembly 701. In FIG. 7B, the current sensor 741 has been removed from the sensor housing 765 to show the structure of the lower half of the dielectric cavity 720 and the lower cover 784 (the upper half of the structure is further described below with reference to FIG. 7C). The dielectric cavity 720 may be divided into an outer dielectric region 723 and an inner dielectric region, referred to as the zigzag dielectric slit 725. The outer dielectric region is the region above the outermost groove in the floor of the lower cover 784. Outside the outer circle of this outermost groove, the metallic upper cover 782 and the metallic lower cover 784 may be physically and electrically connected to each other, but no electrical contact between the upper cover 782 and the lower cover 784 is made inside the outer circle of the outermost groove.

[0089] The zigzag dielectric slit 725 includes a dielectric region above two grooves on either side of a conductive ridge 750 shaped like a ring that protrudes from the floor of the bottom cover 784. The conductive floor of the dielectric cavity 720, including the conductive ridge 750, is electrically and physically separated from the respective conductive ceiling of the dielectric cavity 720 by an uninterrupted continuous dielectric region. Thus, the top of the conductive ridge 750 may protrude into the respective groove of the metallic top cover 782, but does not contact the ceiling. The combined top and bottom of the zigzag dielectric slit 725 thus forms a zigzag shaped dielectric region that goes around the conductive ridge 750, as shown by the zigzag dashed line in FIG. 7B.

[0090] 7C shows a portion of an RF system 700 including an RF tube inner conductor 711 passing through a central hole 710 of a current sensor assembly 701 positioned symmetrically about the inner conductor 711. A grounded outer conductor is physically and electrically attached to a top cover 782 from above and a bottom cover 784 from below, thereby grounding the sensor housing 765. The sensor housing serves as a grounded outer conductor for the portion of the inner conductor 711 passing through the central hole 710, similar to the sensor housing 665 of the VI sensor 600.

[0091] In FIG. 7C, the current sensor assembly 701 is shown in a cutaway exploded view including a current sensor 741. The current sensor 741 includes a toroidal mandrel 742 and a conductive current pickup coil 747. The toroidal mandrel 742 includes a solid dielectric material having a winding path. The winding path may be accessed through access holes 749 at various locations on the surface of the toroidal mandrel 742. A fixture 743 having two holes is positioned over one of the access holes 749. Two opposing ends of the conductor of the coil 747 are shown protruding upward through the holes in the fixture 743. A portion of the toroidal mandrel 742 is cut away to show the conductor of the coil 747 passing through a winding path in the solid dielectric material of the toroidal mandrel 742. The coil 747 is fitted inside the mandrel 742. The design of mandrel 742 provides greater mechanical support than the grooved design of mandrel 642, thereby eliminating the resin encapsulation step described above with reference to FIG. 6D.

[0092] In Fig. 7C, the annular mandrel 742 is disposed within the outer dielectric region 723 and within respective grooves in the floor of the bottom cover 784 (see Fig. 7B). An exploded view of the current sensor assembly 701 shows that the top half of the annular mandrel 742 may fit into a groove in the top cover 782 in the outer dielectric region 723 of the dielectric cavity 720. Similarly, the conductive ridge 750 may extend into an adjacent groove in the top cover 782 within the zigzag dielectric slit 725 of the dielectric cavity 720. The conductive ridge 750, which is a continuous ring interposed between the annular current sensor 741 and the inner conductor 711, may effectively shield the current sensor 741 from RF electric fields.

[0093] As explained above, electrical contact between the grounded top cover 782 and the grounded bottom cover 784 within the area enclosed by the annular current sensor 741 would reduce the strength of the magnetic field through the current pickup coil 747 and would excessively weaken the current signal output signal. Therefore, the top of the conductive ridge 750 is electrically isolated from the top cover 782 by the zigzag dielectric slit 725. The zigzag shape of the dielectric area is shown in Figure 7C by the zigzag dashed line.

[0094] 7D, a cutaway view of a portion of RF system 700 shows current sensor assembly 701 with top cover 782 mounted over bottom cover 784. The zigzag shape of dielectric slit 725 is indicated by the zigzag dashed lines in FIG.

[0095] 7E shows a top view of the bottom portion of the current sensor assembly 701 and the inner conductor 711 of the RF tube passing through the central hole 710 of the current sensor assembly. A current sensor 741 including a toroidal mandrel 742 and a current pickup coil 747 is shown on top of the bottom cover 784. Two opposing ends of the coil 747 pass through holes in a fitting 743. The fitting 743 may be placed over an opening similar to the access hole 749. A ring-shaped conductive ridge 750 is seen interposed between the inner conductor 711 and the current sensor 741. Dielectric slits 725 are seen on either side of the conductive ridge 750.

[0096] The use of mandrels, such as mandrels 642 and 742, allows current sensor designs to use coils with many turns as current pick-ups. The more turns, the more sensitive the respective current sensors become. The increased sensitivity allows each turn to have a smaller cross-section, thus miniaturizing the overall current sensor and may allow the current sensor to be placed in areas that would otherwise be inaccessible.

[0097] Although the mandrels described in this disclosure are toric in shape, it will be understood that other shapes may be used, such as a square or a regular polygon having any number of sides. Additionally, pickups of various shapes may be implemented without the use of a mandrel.

[0098] Various aspects of the embodiments described in this disclosure may be applied to fabricate VI sensors using a variety of other fabrication techniques, for example, current pickups may be fabricated with layers of dielectric and conductive material coupled through vias, such as with printed circuit board (PCB) technology.

[0099] The toroidal current sensor embodiments described above offer the advantages of axial symmetry of the torus, the higher immunity to noise of multi-turn current pickup, and the convenience of a compact construction.

[0100] The VI sensor and measurement method described in this disclosure provide embodiments that may enable extremely high accuracy measurements at low manufacturing costs. High accuracy at low manufacturing costs may be achieved by including design features intended to reduce the sensitivity of the VI measurement to processing and assembly errors. The accuracy of the current sensor is determined by the processing tolerances that cause variations in the dimensions that determine the geometry of the current pickup (e.g., the area enclosed by the rectangular half-loop). The accuracy of the measurement of current may also be limited by the accuracy with which the current pickup may be positioned, including assembly tolerances, e.g., the radial distance from the longitudinal axis and the angle between the plane of the half-loop and the longitudinal axis. Similarly, the accuracy of the measurement of voltage is determined by the processing tolerances (e.g., the accuracy of the diameter and circumference of the voltage pickup ring) and the assembly tolerances (e.g., the angle between the plane of the ring and the longitudinal axis). The inventors have performed detailed computer simulations of the sensitivity of the VI sensor signal to variations in the dimensions and placement of the current and voltage pickups and found that a high accuracy of 1% may be achieved for standard machines and placement tolerance values ​​of 0.005 inches. The computer simulations are performed using a calibrated 3D finite element solver of Maxwell's equations over a wide range of RF powers, RF frequencies, and load impedances used in plasma processing.

[0101] Exemplary embodiments of this application are summarized here, and other embodiments can be seen throughout the specification and claims filed herewith.

[0102] Example 1. A radio frequency (RF) system includes a radio frequency (RF) power supply configured to power a load with an RF signal, an RF tube including an outer conductor and an inner conductor connected to ground coupling the RF power supply to the load, and a current sensor aligned with a central axis of the RF tube carrying the RF signal. The current sensor is configured to monitor the current of the RF signal and includes a conductive half-loop disposed proximate to the RF tube, the conductive half-loop including a first end and an opposite second end. The current sensor is configured to output an output signal between the first end and the second end. A sensor housing is disposed around the RF tube, the sensor housing including a conductive material connected to the outer conductor of the RF tube. A gallery is disposed within the sensor housing and outside the outer conductor of the RF tube, and the current sensor is disposed within the gallery. A slit in the outer conductor of the RF tube exposes the current sensor to a magnetic field due to the current of the RF signal in the inner conductor of the RF tube.

[0103] Example 2. The system of Example 1, wherein the slit has a length along the inner circumference of the outer conductor and a width parallel to the central axis of the RF tube, the width being between 0.5 mm and 5 mm.

[0104] Example 3. The system of Example 1 or 2, wherein along a direction orthogonal to the central axis of the RF tube, the conductive half-loop includes a first mirror plane of symmetry that includes the central axis of the RF tube and a second mirror plane of symmetry that is orthogonal to the first mirror plane of symmetry, and the first mirror plane of symmetry of the conductive half-loop and the central axis of the RF tube are coplanar.

[0105] Example 4. A conductive half loop includes a branch aligned parallel to the axis of the RF tube, a second branch coupled to a first end of the first branch, the second branch being orthogonal to the first branch, and a third branch coupled to a second end of the first branch, the third branch being orthogonal to the first branch and parallel to the second branch. The system of any one of Examples 1 to 3, comprising:

[0106] Example 5. The system of any one of Examples 1-4, further comprising insulating support structures for supporting various branches of the conductive half-loop.

[0107] Example 6. The system of any one of Examples 1-5, wherein the RF tube includes an inner conductor electrically coupled to the RF power source and the load, and an outer conductor electrically coupled to a reference potential node.

[0108] Example 7. The system of any one of Examples 1-6, further comprising a first voltage sensor for monitoring a voltage of the RF signal, the voltage sensor being axially symmetrically disposed about the RF tube.

[0109] Example 8. The system of any one of Examples 1-7, wherein the first voltage sensor includes a conductive ring disposed along an inner surface of the outer conductor of the RF tube, and an insulating ring disposed between the conductive ring and the outer conductor of the RF tube, where the insulating ring electrically insulates the conductive ring from the RF tube.

[0110] Example 9. The system of any one of Examples 1-8, further comprising a second voltage sensor arranged symmetrically around the RF tube, the first voltage sensor being located at a first position on the axis of the RF tube and the second voltage sensor being located at a second position on the axis of the RF tube, and a first distance between the first position and a mirror symmetry plane of the current sensor being approximately the same as a second distance between the second position and the mirror symmetry plane.

[0111] Example 10. A radio frequency (RF) system comprising: a radio frequency (RF) power supply configured to power a load with an RF signal; an RF tube including an outer conductor and an inner conductor connected to a reference potential node coupling the RF power supply to the load; and a first voltage sensor disposed axially symmetrically about an axis of the RF tube carrying the RF signal, the first voltage sensor configured to monitor a voltage of the RF signal. Radio frequency (RF) systems, including:

[0112] Example 11. The system of Example 10, further comprising a second voltage sensor arranged symmetrically about the RF tube, the first voltage sensor being located at a first position on the axis of the RF tube and the second voltage sensor being located at a second position on the axis of the RF tube.

[0113] Example 12. The system of Example 10 or 11, further comprising a current sensor disposed around the RF tube at a third position, the current sensor aligned with an axis of the RF tube carrying the RF signal, the current sensor configured to monitor a current of the RF signal.

[0114] Example 13. The system of any one of Examples 10-12, wherein the third position is disposed between the first position and the second position.

[0115] Example 14. The system of any one of Examples 10-13, wherein the current sensor includes a conductive half-loop including a first end and an opposing second end, and along a direction perpendicular to the axis of the RF tube, the conductive half-loop includes a first mirror symmetry plane that includes the axis of the RF tube and a second mirror symmetry plane that is perpendicular to the first mirror symmetry plane, and the first mirror symmetry plane of the conductive half-loop and the axis of the RF tube are coplanar.

[0116] Example 15. The system of any one of Examples 10-14, wherein a first distance between the first position and the mirror symmetry plane is approximately the same as a second distance between the second position and the mirror symmetry plane.

[0117] Example 16. A method of measuring a radio frequency (RF) signal, comprising: having a current sensor aligned with an axis of an RF tube carrying the RF signal, the current sensor being disposed within a sensor housing and within a gallery disposed outside an outer conductor of the RF tube, the sensor housing being disposed around the RF tube, the current sensor including a conductive half-loop, the conductive half-loop including a first end and an opposing second end; determining a current of the RF signal based on measuring an output signal between the first end and the second end; A method comprising:

[0118] Example 17. The method of example 16, wherein the RF tube includes an inner conductor electrically coupled to an RF power source and a load, and an outer conductor, the method further including grounding the outer conductor.

[0119] Example 18. The method of Example 16 or 17, further comprising having a first voltage sensor axially symmetrically disposed about the RF tube, and determining a voltage of the RF signal based on measuring an electrical signal at a terminal of the first voltage sensor.

[0120] Example 19. The method of any one of Examples 16-18, further comprising having second voltage sensors arranged symmetrically around the RF tube, where the first voltage sensor is located at a first position on the axis of the RF tube and the second voltage sensor is located at a second position on the axis of the RF tube, and measuring another electrical signal at a terminal of the second voltage sensor, where a voltage of the RF signal is determined based on the electrical signal and the another electrical signal.

[0121] Example 20. The method of any one of Examples 16-19, wherein the conductive half-loop includes a first plane of mirror symmetry that includes the axis of the RF tube and a second plane of mirror symmetry that is orthogonal to the first plane of mirror symmetry, and the first plane of mirror symmetry of the conductive half-loop and the axis of the RF tube are coplanar.

[0122] While the present invention has been described with reference to exemplary embodiments, this specification is not intended to be construed in a limiting sense. Various modifications and combinations of the exemplary embodiments, as well as other embodiments of the present invention, will become apparent to those skilled in the art upon reference to this specification. It is therefore intended that the appended claims cover any such modifications or embodiments.

Claims

1. 1. A radio frequency (RF) system comprising: an RF power source configured to power a load with an RF signal; an RF tube including an outer conductor and an inner conductor connected to ground coupling the RF power source to the load; a current sensor aligned with a central axis of the RF tube carrying the RF signal, the current sensor configured to monitor a current of the RF signal, the current sensor including a conductive half-loop disposed proximate to the RF tube, the conductive half-loop including a first end and an opposing second end, the current sensor configured to output an output signal between the first end and the second end; a sensor housing disposed about the RF tube, the sensor housing including a conductive material connected to the outer conductor of the RF tube; a gallery disposed within the sensor housing and outside the outer conductor of the RF tube, the current sensor being disposed within the gallery; a slit in the outer conductor of the RF tube for exposing the current sensor to a magnetic field due to the current of the RF signal in the inner conductor of the RF tube; Including, the system.

2. 2. The system of claim 1, wherein the slit has a length along an inner circumference of the outer conductor and a width parallel to the central axis of the RF tube, the width being between 0.5 mm and 5 mm.

3. 2. The system of claim 1, wherein along a direction perpendicular to the central axis of the RF tube, the conductive half-loop includes a first plane of mirror symmetry that includes the central axis of the RF tube and a second plane of mirror symmetry that is perpendicular to the first plane of mirror symmetry, and wherein the first plane of mirror symmetry of the conductive half-loop and the central axis of the RF tube are coplanar.

4. The conductive half loop comprises: a first branch aligned parallel to the central axis of the RF tube; a second branch coupled to a first end of the first branch, the second branch being orthogonal to the first branch; a third branch coupled to a second end of the first branch, the third branch being orthogonal to the first branch and parallel to the second branch; The system of claim 1 , comprising:

5. The system of claim 4 , further comprising insulating support structures for supporting the first, second, and third branches of the conductive half-loop.

6. the inner conductor is electrically coupled to the RF power source and to the load; The system of claim 1 , wherein the outer conductor is electrically coupled to a reference potential node.

7. 2. The system of claim 1, further comprising a first voltage sensor for monitoring a voltage of the RF signal, the first voltage sensor being axially symmetrically disposed about the RF tube.

8. The first voltage sensor includes: a conductive ring disposed along an inner surface of the outer conductor of the RF tube; an insulating ring disposed between the conductive ring and the outer conductor of the RF tube, the insulating ring electrically insulating the conductive ring from the RF tube; The system of claim 7 , comprising:

9. 9. The system of claim 8, further comprising a second voltage sensor disposed symmetrically about the RF tube, the first voltage sensor located at a first position on the central axis of the RF tube and the second voltage sensor located at a second position on the central axis of the RF tube, a first distance between the first position and a mirror symmetry plane of the current sensor being approximately the same as a second distance between the second position and the mirror symmetry plane.

10. 1. A radio frequency (RF) system comprising: an RF power source configured to power a load with an RF signal; an RF tube including an outer conductor and an inner conductor connected to a reference potential node coupling the RF power source to the load; a first voltage sensor arranged axially symmetrically about an axis of the RF tube carrying the RF signal, the first voltage sensor configured to monitor a voltage of the RF signal, the first voltage sensor being located at a first position on the axis of the RF tube; a current sensor disposed around the RF tube at a second location on the axis of the RF tube, the current sensor aligned with the axis of the RF tube carrying the RF signal, the current sensor configured to monitor a current of the RF signal; and Including, the system.

11. The system described in claim 10, further comprising a second voltage sensor arranged symmetrically around the RF tube, the second voltage sensor being located at a third position on the axis of the RF tube.

12. The system of claim 11 , wherein the second location is located between the first location and the third location.

13. 12. The system of claim 11, wherein the current sensor includes a conductive half-loop including a first end and an opposing second end, and along a direction perpendicular to the axis of the RF tube, the conductive half-loop includes a first plane of mirror symmetry that includes the axis of the RF tube and a second plane of mirror symmetry that is perpendicular to the first plane of mirror symmetry, and the first plane of mirror symmetry of the conductive half-loop and the axis of the RF tube are coplanar.

14. 14. The system of claim 13, wherein a first distance between the first position and the second plane of mirror symmetry is approximately the same as a second distance between the third position and the second plane of mirror symmetry.

15. 1. A method for measuring a radio frequency (RF) signal, comprising: having a current sensor aligned with an axis of an RF tube carrying an RF signal, the current sensor disposed within a sensor housing and within a gallery disposed outside an outer conductor of the RF tube, the sensor housing being disposed around the RF tube, the current sensor including a conductive half-loop, the conductive half-loop including a first end and an opposite second end; determining a current of the RF signal based on measuring an output signal between the first end and the second end; A method comprising:

16. the RF tube includes an inner conductor electrically coupled to an RF power source and a load, and an outer conductor; The method of claim 15 , further comprising grounding the outer conductor.

17. a first voltage sensor arranged axially symmetrically around the RF tube; determining a voltage of the RF signal based on measuring an electrical signal at a terminal of the first voltage sensor; The method of claim 15 further comprising:

18. having a second voltage sensor symmetrically disposed about the RF tube, the first voltage sensor being located at a first position on the axis of the RF tube and the second voltage sensor being located at a second position on the axis of the RF tube; measuring a further electrical signal at a terminal of the second voltage sensor, the voltage of the RF signal being determined based on the further electrical signal and the second electrical signal; 20. The method of claim 17, further comprising:

19. the conductive half-loop includes a first plane of mirror symmetry that includes the axis of the RF tube and a second plane of mirror symmetry that is orthogonal to the first plane of mirror symmetry; 20. The method of claim 18, wherein the first plane of mirror symmetry of the conductive half-loop and the axis of the RF tube are coplanar.

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