RF Voltage and Current (VI) Sensors and Measurement Methods

Abstract

The radio frequency sensor assembly includes a sensor casing disposed about a central hole, the sensor casing including a first conductive cover and a second conductive cover. The assembly includes a cavity disposed about the central hole and includes a dielectric material, the cavity being bounded by first and second outer major surfaces along a radial direction from the center of the central hole, the first conductive cover being electrically coupled to the second conductive cover through a bonding region beyond the second outer major surface of the cavity and electrically insulated from the second conductive cover by the cavity and the central hole. The assembly also includes a current sensor including current pickups electrically insulated from the sensor casing and symmetrically arranged about the central hole, the current pickups being disposed within the cavity and insulated from the sensor casing.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application is related to concurrently pending U.S. Non-provisional Patent Application No. 16 / 913,526, Attorney Docket No. 190907US01, filed June 26, 2020, and U.S. Non-provisional Patent Application No. 16 / 913,548, Attorney Docket No. 190883US01, filed June 26, 2020, and claims priority to U.S. Non-provisional Patent Application No. 16 / 913,545, Attorney Docket No. 200391US01, filed June 26, 2020, which applications 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 greater functionality at lower cost. Higher functionality at lower cost is primarily achieved by increasing component integration density through miniaturization. An IC is a network of electronic components (e.g., transistors, resistors, and 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 processing steps such as chemical vapor deposition (CVD), photolithography, and etching. The integration density of circuit elements has been increased by periodically shrinking minimum feature sizes using innovative techniques such as immersion lithography and multiple patterning. Further miniaturization is achieved by reducing device footprints 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 processing (e.g., alternating cycles of deposition and etching), are routinely used in deposition and patterning steps used in semiconductor IC manufacturing. However, the challenge of providing manufacturable plasma technologies for advanced IC designs has intensified with the advent of shrinking feature sizes to a few nanometers while controlling structural features with atomic-scale dimensions. Manufacturable plasma processes are expected to provide structures with precisely controlled characteristics, both in terms of plasma etching characteristics (e.g., sidewall angle, anisotropy, and etch-stop layer selectivity) and plasma deposition characteristics (e.g., conformality, aspect ratio selectivity, and area selectivity for bottom-up patterning), as well as precise dimensions (e.g., linewidth, etch depth, and film thickness), and uniformity across wide (e.g., 300 mm) wafers. In many 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 processes can require innovative measurements of RF signals that are both unobtrusive and accurate. Summary of the Invention [Means for solving the problem]

[0005] According to one embodiment of the present invention, a radio frequency sensor assembly includes a sensor casing disposed about a central bore, the sensor casing including a first conductive cover and a second conductive cover. The assembly includes a cavity disposed about the central bore and including a dielectric material, the cavity being bounded by first and second outer major surfaces along a radial direction from the center of the central bore, the first conductive cover being electrically coupled to the second conductive cover through a bonding region beyond the second outer major surface of the cavity and electrically insulated from the second conductive cover by the cavity and the central bore. The assembly also includes a current sensor including current pickups electrically insulated from the sensor casing and symmetrically arranged about the central bore, the current pickups being disposed within the cavity and insulated from the sensor casing.

[0006] According to one embodiment of the present invention, a radio frequency (RF) sensor assembly includes a sensor casing symmetrically disposed about a central hole, the sensor casing including a first conductive cover and a second conductive cover. The assembly also includes a cavity symmetrically disposed about the central hole, the cavity being shaped like an annulus and including a first dielectric material, the cavity being bounded by first and second outer major surfaces along a radial direction from the center of the central hole, the first outer major surface including a ring-shaped continuous region in physical contact with the central hole, and the second outer major surface including a closed outer boundary at a radial distance greater than the radius of the first outer major surface. A current sensor includes a current pickup coil symmetrically disposed about the central hole, the current pickup being insulated from the sensor casing and disposed within the cavity. The assembly also includes a ring-shaped conductive bump interposed between the current pickup and the central hole and electrically coupled to the second conductive cover, the conductive bump being covered with a dielectric material.

[0007] According to one embodiment of the present invention, a plasma system includes a process chamber including an electrode, a radio frequency (RF) power supply configured to power the process chamber with an RF signal, an RF pipe coupling the RF power supply to the electrode of the process chamber, a toroid-like shaped mandrel arranged symmetrically about the axis of the RF pipe carrying the RF signal, and a voltage pickup arranged symmetrically about the axis of the RF pipe and surrounded by the mandrel.

[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 explanation of the drawings]

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

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

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

[0012] This disclosure describes a sensor design and method 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. The voltage-current (VI) sensor embodiments described herein have been applied to probing the electric and magnetic fields of RF electromagnetic waves along a coaxial transmission line called an RF pipe.

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

[0014] In various embodiments, this application describes an RF VI sensor for measuring the current and voltage of an RF signal passing through an RF pipe. The term RF pipe, as used herein, refers to a coaxial transmission line that carries RF power from one part of a plasma reactor (referred to as a plasma processing system) to another. The waveforms, I and V, transmitted through the RF pipe 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, j 2 = -1, Re is the real part of the complex function. As mentioned above, I and V each have a magnitude and are separated from each other by a phase angle Φ. In general, waveforms I and V may contain multiple frequency components. The voltage V(x,t) refers to the potential of the inner conductor (or core) of the RF pipe (or coaxial transmission line) with respect to the grounded outer conductor (or shield), where ground refers to the reference potential of the RF system.

[0015] As described in various embodiments, RF VI sensors may provide V and I at the sensor location with high accuracy using geometric symmetry and differential measurement techniques. Several VI sensors may be used at various locations along the RF pipe to probe the RF signal at that location. Because V and I are functions of position x, each VI sensor may be placed as close as possible to its desired measurement location. For example, if it is desirable to monitor and control plasma processing using accurate measurements of the voltage and current of the RF signal provided to the plasma chamber, a VI sensor used for that purpose may be placed near where the RF signal enters the plasma chamber. Various embodiments described herein increase the measurement accuracy and sensitivity of VI sensors without increasing costs, thereby providing a plasma processing system with enhanced capabilities for providing plasma processing with better repeatability and tighter process control at the same cost. Furthermore, the RF probe is designed to be unobtrusive, allowing improved VI sensors to be easily retrofitted to existing plasma processing equipment without lengthy and expensive redevelopment of established plasma processing recipes in the manufacturing 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 one 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 exemplary embodiments of a VI sensor shown in Figures 2A, 2B, 3, 4, and 5.

[0017] As will be described in more detail below with reference to Figure 1B, the voltage sensors in the embodiments of Figures 2A, 2B, 3, 4, and 5 are designed with axial symmetry as conductive rings positioned inside an RF pipe loop about a central longitudinal axis running parallel to the direction of current flow. The advantages offered by the axially symmetric design are explained in the discussion below with reference to Figure 2B.

[0018] In an exemplary embodiment, each current sensor is disposed in a sleeve or gallery around the outer periphery of the RF pipe. The gallery is a cavity inside the sensor casing. The sensor casing has a conductive wall covering the gallery and may be connected to the outer conductor of the RF pipe, thereby connected to ground. As described in more detail below with reference to FIG. 1B , the current sensor is a single conductive loop (called a half-loop) with two open ends, which is completed using, for example, an external VI analyzer component connected to the current sensor by a coaxial cable. The VI analyzer is a measurement system that analyzes the raw signals received from the current sensor and voltage sensor. As described further below, both ends of the current sensor may be connected to the VI analyzer to perform differential measurements for accurate analysis. However, to simplify the system at the expense of accuracy, only one end of the current sensor may be connected to the VI analyzer, and the other end may be terminated to ground by a load impedance (e.g., a 50 ohm load) or shorted to ground. The ground connection may be a direct connection to the sensor casing. 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 alternative RF systems using embodiments of the current sensors described in this disclosure, the entire closed loop may be housed inside 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 containing three conductive elements. The three conductive elements of the current sensor half-loop are two identical vertical branches connected by a horizontal branch oriented parallel to the central axis of the RF pipe. Therefore, the current sensor designs discussed herein are mirror-symmetric about a mirror plane perpendicular to the central axis of the RF pipe and passing midway between the two vertical branches. The advantages of mirror symmetry are discussed below with reference to FIG. 2B. However, because they are located on one side of the RF pipe, single-turn half-loop current sensors lack axial symmetry. Axisymmetric multi-turn half-loop current sensor designs are described with reference to FIGS. 6A-7E, where the embodiments utilize 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 electromagnetic electric and magnetic fields may be used to perform VI measurements with negligible disturbance of the RF signal in the RF pipe. Also, advantageous use of geometric symmetry and differential measurement techniques in the sensor design results in measurements that may be immune to machining errors due to standard tolerances of the tools used to form the components, as well as positioning errors during assembly of the VI sensor. Additionally, several structural reinforcement techniques are utilized. For example, elements of the VI sensor may be duplicated to enhance geometric symmetry, and parts designed to provide additional mechanical support may be positioned to reduce / prevent slight deformations in the shape of critical sensor components caused by mechanical stresses during assembly. Thus, 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 typical 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 pipe 110, to carry RF power to an electrode coupled to the plasma inside 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 pipe 110 may be expressed as a combination of traveling RF electromagnetic waves. Due to an impedance mismatch between the output impedance of the RF power source 10 and the load impedance, a portion of the RF power traveling from the RF power source 10 toward the load will be reflected back to the RF power source 10. To suppress such undesired reflections, a matcher 40 including a matching network may 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 power incident on the matching network may be sensed by the matcher 40 (e.g., using a VI sensor and analyzer) and provided to the programmable controller 20. The programmable controller 20 may adjust the impedance of the matching network using, for example, a feedback control loop (indicated in FIG. 1A by the two arrows between the matcher 40 and the programmable controller 20) to reduce the RF power reflected back to the RF power source 10 from the matching network.

[0024] The plasma may be maintained in the plasma chamber 30 using RF power delivered, for example, by an RF pipe 110 from an RF power source 10 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 may be internal to the chamber wall, e.g., a disk-shaped electrode in a capacitively coupled plasma (CCP) chamber, or may be an antenna external to the chamber wall. For example, in an inductively coupled plasma (ICP) chamber, the antenna may be a conductive planar spiral positioned above a dielectric window or a conductive helix wrapped around a dielectric cylinder. The block shown as the plasma chamber 30 in FIG. 1A includes an antenna and an electrode 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., a top electrode and a bottom electrode, electrically coupled to the plasma therebetween. In some designs it may be advantageous for the bottom electrode to also be the substrate holder.

[0025] 1A shows an RF pipe 110 delivering RF power from a single RF power supply 10 to the plasma chamber 30, there may be two or more RF power supplies providing RF power to two or more electrodes. For example, an RF power supply 10 may provide RF power to an electrode (e.g., a top electrode) of the plasma chamber 30, and a second RF bias power supply may provide RF bias power to another electrode (e.g., a bottom electrode) of the plasma chamber 30 using a respective RF pipe, a matcher, and a VI sensor located near the plasma chamber to sense the voltage and current of the RF signal provided to the bottom electrode.

[0026] In FIG. 1A , a VI sensor 100, which is used to sense and measure the current and voltage of an RF signal near an electrode receiving the RF signal, is connected to a VI analyzer 60. The VI analyzer 60 may receive a raw output waveform from the VI sensor 100, reflecting V(t) and I(t), as indicated by the arrows. The VI analyzer 60 may be a signal processor, such as a digital signal processor, which can extract various RF signal features from the raw waveform. These various RF signal features may include magnitude |V|, |I|, the phase angle (Φ) between V and I, and peak RF power |V||I| cosΦ. Furthermore, harmonic analysis may be performed to extract multiple frequency components. The measured RF signal features may reflect plasma properties and plasma impedance, such as free electron and ion density and ion / radical flux and energy. The VI analyzer 60 may be pre-calibrated, for example, using an RF calibration signal over a range of frequencies (e.g., from about 0.4 MHz to about 1 GHz) and powers (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 features reflective of 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. Furthermore, programmable controller 20 may use the received RF measurements for process control. As known to those skilled in the art, plasma properties may be altered by modifying 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 embodiments using a VI sensor 100 to estimate and control plasma parameters, it may be advantageous to position the VI sensor 100 near the plasma chamber 30. The V and I of the RF signal at an electrode may be estimated from the V and I measured at that location by a VI sensor 100 located at a different location. However, as the distance between the electrode and the VI sensor increases, errors in the measurement of V and I at the electrode may increase. Theoretically, the transfer matrix used to transform the sensor signal between two locations deviates more from a unity matrix as the distance between those two locations increases. Therefore, the V and I estimated for an electrode location become more sensitive to errors in the estimation of the respective transfer matrices.

[0029] 1B , according to one embodiment, the VI sensor 100 is attached to an RF pipe 110 that connects to the plasma chamber 30. The RF pipe 110 may be a coaxial structure with two conductive tubes (e.g., aluminum or copper tubes) concentrically arranged around 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 an arrow pointing left. 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 pipe 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., the coaxial line that may be used to carry a signal 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 within an annular gallery 160 inside a sensor casing 165 having conductive walls (e.g., aluminum, brass, stainless steel, or copper). In the embodiment shown schematically in FIG. 1B , the gallery 160 is a hollow annular region that follows the entire outer circumference of the outer conductor 130 and is axisymmetric about the axis of the RF pipe 110. The axisymmetric design of the gallery 160 provides the advantage of preventing additional reflections and non-axisymmetric wave modes of RF electromagnetic waves propagating within the RF pipe 110. The gallery 160 and conductive sensor casing 165 may either be integrally formed with the RF pipe 110 or may be symmetrically attached around the RF pipe 110 and positioned during assembly to help avoid alignment errors between the longitudinal axis of the RF pipe 110 and the current sensor 140. In either case, the conductive sensor casing 165 and the outer conductor 130 are electrically and physically connected, and therefore the sensor casing 165 can be considered an extension of the outer conductor 130 of the coaxial RF pipe 110.

[0031] Although the embodiment of FIG. 1B includes annular gallery 160, in other embodiments, the gallery may not be annular. In other embodiments, the axial symmetry of RF pipe 110 may be unavoidably broken, for example, by bends in RF pipe 110, and therefore further loss of axial symmetry due to asymmetries in the VI sensor design may be less significant. Therefore, it may be appropriate to relax axial symmetry in the VI sensor design. For example, the gallery may partially follow the circumference of outer conductor 130 and not form a complete circuit of RF pipe 110.

[0032] 1B and 1C, the gallery 160 is shown to be completely surrounded by the conductive surface of the sensor casing 165 and the outer conductor 130, except for a slit 132 connecting the hollow region of the gallery 160 to the RF pipe 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 has three conductive branches arranged as three sides of a rectangle (called a half-loop): two vertical branches 142 and one horizontal branch 143. In one embodiment, each of the two vertical branches 142 is threaded into an opening in the horizontal branch 143.

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

[0034] The slits 132 are designed to allow magnetic flux to penetrate the gallery 160. Current flowing in the inner conductor 120 results in 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 approximately zero because the equal but opposite return current flowing on the inner surface of the outer conductor 130 cancels the circulating magnetic flux due to the current in the inner conductor 120, in accordance with Ampere's law. The slits 132 interrupt the continuity of the cylindrical outer conductor 130, thereby diverting the return current so that it flows along the inner surface of the outer conductor of the sensor casing 165. The hollow region of the gallery 160 containing the half-loop current pickups 141 is thereby located within the region between the current flowing in the inner conductor 120 and the respective return current. Now, by Ampere's law, there exists a magnetic field inside the gallery 160 that passes through the rectangular half-loop of the current pickup 141. In one example, the slits 132 may extend along the entire circumference of the cylindrical outer conductor 130 to help maximize the magnetic flux through the half-loop of the current pickup 141.

[0035] In addition to magnetic flux, there is electric flux emanating 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 the slit 132 formed in the outer conductor 130. The changing magnetic flux through the half-loop of the current pickup 141 induces an electrical 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 contaminate the signal generated by the magnetic flux. Therefore, 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 pipe 110.

[0036] It should be understood that while the slit design used in the VI sensor embodiment described with reference to Figures 1B-5 is shaped like a ring around the circumference of the outer conductor, 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 casing 165 by an air gap (or other insulator) and by insulating components used for mechanical support, such as the insulating portion 162 of FIG. 1b (and other similar portions shown in FIGS. 2A-5).

[0038] The current pickup 141 is geometrically a half-loop (a loop with two open ends) that forms one turn around an area with a rectangular cross section with its two vertical branches 142 and horizontal branch 143. The single-turn half-loop current pickup 141 may be placed in the presence of a time-varying magnetic field resulting from RF electromagnetic waves traveling along the RF pipe 110. According to Faraday's Law, a time-varying voltage difference proportional to the time-varying magnetic flux may be induced between the two ends of the current pickup 141. The two ends of the current pickup 141 may be attached to a pair of symmetrical terminals 144 shown above the sensor casing 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 the current pickup 141. The symmetry is exploited by a measurement system (e.g., the 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 140. For this measurement method, the differential signal from the current sensor 140 is its output signal and may be detected, for example, using a differential amplifier.

[0040] The half loop of the current pickup 141 is completed externally 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 requirement to match the detector impedance to the cable impedance 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. To avoid reflections from the measurement system due to impedance mismatch, it is advantageous to terminate the coaxial signal line with a matched impedance. It is also advantageous to connect the two end terminals 144 of the current sensor 140 to symmetrical coaxial signal lines terminated in a symmetrical manner, thereby preserving 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 provides greater accuracy in measuring I. To detect the 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 can 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 (instead of detecting a differential signal), the second terminal of the pair of terminals 144 may be connected to an impedance that mirrors the impedance at the first terminal as closely as possible. For example, the first terminal may be connected to a first 50 ohm coaxial cable to transmit the 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 in place of the detector. Note that other suitable impedances may be used for the terminations. The impedance need not be 50 ohms. Additionally, the second coaxial cable may be omitted and an appropriate impedance termination may be attached directly to the second terminal of the pair of terminals 144 .

[0042] The design of a 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 electronic components. Due to the frequency dependence of parasitic capacitive impedance, the impedance of a component at RF frequencies can be significantly different from the impedance of the component at low frequencies or DC (zero frequency). For example, as the frequency of an electrical signal is increased into the RF range, the impedance of a resistor component at DC can decrease due to the parasitic capacitance to ground associated with the resistor structure. The impedance of a resistor with a higher resistance value is more sensitive to the frequency of the RF signal. Because 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 unit-to-unit impedance variation when the resistor resistance is high. Therefore, to maintain measurement accuracy, it is advantageous to limit the design of the initial detection system to use resistors whose resistance is less than 0.1 of the parasitic RF reactance, even when the initial detection system is located at the sensor location.

[0043] The voltage pickup 151 of the voltage sensor 150 comprises 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 each other by an insulating ring 152, as shown in FIG. 1B . The insulating ring 152 may comprise Teflon or other plastic material or other suitable dielectric. In one embodiment, the voltage pickup 151, such as a 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., the conductive ring) may be the same as the inner diameter of the outer conductor 130. In other embodiments, the inner diameter of the voltage pickup 151 (e.g., the conductive ring) may be different (smaller or larger than the inner diameter of the outer conductor 130). The disturbance to the electric and magnetic fields within the RF pipe 110 caused by inserting the voltage pickup 151 is relatively minimal when the inner diameter of the voltage pickup 151 (e.g., the conductive ring) and the inner diameter of the outer conductor 130 are equal. As explained in more detail below, the output signal from the voltage pickup 151 increases as the inner diameter of the voltage pickup 151 (e.g., the conductive ring) decreases. Contacts to the voltage pickup 151 extend outside the outer conductor 130 and terminate in a third terminal 153 (e.g., a third coaxial cable connector) mounted above the sensor casing 165. The current pickup 141, voltage pickup 151, and contacts to the respective terminals 144 and 153 may comprise a metal with high electrical conductivity (e.g., copper) and may all be insulated from other conductive elements such as the outer conductor 130 and conductive sensor casing 165.

[0045] The design considerations for the coaxial signal lines and termination impedances connecting the initial detection system to terminal 153 of voltage sensor 150 may be similar to the design considerations for the coaxial signal lines and termination impedances connecting the initial detection system to terminal 144 of current sensor 140, as discussed above. The discussion above with reference to current sensor 140 includes considerations for maintaining symmetry of the differential output signal. However, that portion of the discussion does not apply 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 in other embodiments where two voltage pickup rings are symmetrically positioned and the arithmetic mean 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 pipe 110 lies in the plane P1 of the current pickup 141. The longitudinal axis LA1 is also parallel to the direction of current flow in the RF pipe 110. Furthermore, as can be more easily seen in FIG. 1C, along a direction perpendicular to the longitudinal axis LA1 of the RF pipe 110, the current pickup 141 includes a first mirror plane of symmetry M1 that includes the longitudinal axis LA1 of the RF pipe 110 and a second mirror plane of symmetry M2 that is perpendicular to the first mirror plane of symmetry M1. In one or more embodiments, the first mirror plane of symmetry M1 of the current pickup 141 and the longitudinal axis LA1 of the RF pipe 110 are coplanar.

[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). Unwanted coupling to the electric field is significantly reduced by placing the current pickup 141 outside the outer conductor 130. The inductively coupled oscillating magnetic field induces an electromotive force (emf) within the current pickup 141 (a three-sided half-loop). The induced emf is related to the changing magnetic flux according to Faraday's law. Because the strength of the magnetic field around a conducting conductor reflects the respective current, the current sensor 140 may generate a time-varying electrical signal reflecting the RF current in the RF pipe 110 at each location. One aspect of the current sensor 140 is that the electrical signals at both terminals 144 may be received by a detection system, and the differential voltage between the two terminals 144 may 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 electric potential and electric field magnitude are generally circular and centered on the longitudinal axis LA1 of the RF pipe 110. The circular contours are contained within a set of planes perpendicular to the longitudinal axis LA1. Thus, the electric field lines radiate from the inner conductor 120 perpendicular to the longitudinal axis LA1. A ring-shaped voltage pickup 151 is positioned generally on one of the 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 electric potential of the inner conductor at each location, according to the laws of electromagnetic physics. This oscillating potential may be used as an output signal for the voltage sensor 150. The magnitude of the radial electric field between the inner conductor 120 and the outer conductor 130 decreases with increasing radial distance from the longitudinal axis LA1, according to Gauss's law. Thus, the output signal of the voltage sensor 150 may be increased by placing the voltage pickup 151 closer to the inner conductor 120, for example by reducing the inner diameter of the voltage pickup ring.

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

[0050] The raw output signals (e.g., one pair of signals from current sensor 140 and another signal from voltage sensor 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 VI sensor 200 and the RF pipe outer conductor 230 (outer tube). FIG. 2B shows a cutaway view of the same VI sensor 200 along axis 2B-2B'. The VI sensor 200 of FIGS. 2A and 2B is similar to the VI sensor 100 of FIG. 1B. Current sensor 240 and voltage sensor 250 are positioned within annular gallery 260 of VI sensor 200. In FIGS. 2A and 2B, the RF pipe inner conductor has been removed to better show the voltage pickup ring 251 of voltage sensor 250 within outer conductor 230. The perspective view (FIG. 2A) shows three terminals (coaxial cable connectors in this example) of VI sensor 200. As seen in FIG. 2B, a pair of terminals 244 extending above the top of gallery 260 connect to current pickups 241 of current sensor 240. The third terminal 253 is connected to the voltage pickup 251 of the voltage sensor 250 .

[0052] Referring to FIG. 2B, the current pickup 241 of the current sensor 240 is a half-loop containing three conductors. Two conductive vertical branches 242 of the current pickup 241 are insulated by plastic (or other insulating material) from the metal sensor casing 265. The vertical branches 242 connect to the two ends of a horizontal branch 243, a third conductor positioned horizontally within the gallery 260 above the outer conductor 230. A slit 232 along the circumference of the outer conductor 230 allows a magnetic field to pass through the plane of the half-loop and induce an electromotive force in the conductive branch of the current pickup 241. The horizontal branch 243 of the current pickup 241 may 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 casing 265 and outer conductor 230 by plastic portions 262 on the sides 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 enhanced by an additional plastic part placed in the air gap below the horizontal conductor.

[0053] A current pickup (e.g., current pickup 241 in FIG. 2B ) provides an electrical signal by interacting with an RF electromagnetic field. As explained above, it is the magnetic field (not the electric field) that reflects the RF current. Slit 232 allows the magnetic field to penetrate from the RF pipe into gallery 260 where current pickup 241 is located. Coupling of current pickup 241 with the electric field reduces the accuracy of the magnetic field measurement. Current sensor 240, as described herein, may suppress measurement errors that may result from undesired interaction with the electric field. First, current sensor 240 in VI sensor 200 is positioned outside of grounded outer conductor 230, thereby using outer conductor 230 to shield against electric fields. As discussed above with reference to FIGS. 1B and 1C, the RF electric field is in a radial direction (perpendicular to the coaxial axis LA1 of the RF pipe), 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 entering the gallery 260 due to the gap caused by the slit 232 in the outer conductor 230. Secondly, to further reduce the effect of the portion of the electric field that may penetrate into the cavity despite the outer conductor 230, a differential signal may be used as the output signal. Ideally, the differential voltage between the two terminals 244 of the current sensor is , which, according to electromagnetic theory, is approximately proportional to the oscillating magnetic field. However, due to the presence of the slit 232, a weak electric field inside the gallery 260 may be capacitively coupled to the current pickup 241. However, the current pickup 241, the slit 232, and the gallery 260 may be configured to be mirror symmetric with respect to a plane passing through the center of the slit 232 and oriented perpendicular to the longitudinal axis of the RF pipe. This geometric mirror symmetry of the half-loop current pickup 241 described above ensures that the disturbances to the potential appearing at the two terminals 244 along the vertical branch 242 are approximately equal in magnitude and phase.This symmetrical property can be used to advantage, as it means that the difference signal is not affected by parasitic signals in current pickup 241 due to interaction with the transmitted electric field in gallery 260. In other words, the potential difference between the first and second terminals of terminal pair 244 is undisturbed and remains true to first order. These aspects of the design of current sensor 240 can be advantageously utilized to achieve high accuracy current measurements, particularly in applications such as providing an RF bias signal to an electrostatic substrate holder in a plasma chamber, where the load impedance can be such that the electric field amplitude is relatively high and the magnetic field amplitude is relatively low near the point where the RF signal enters the plasma chamber.

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

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

[0056] The ring-shaped design of voltage pickup 251, as described herein, utilizes axial symmetry to reduce the sensitivity of the output of voltage sensor 250 to some placement and sizing errors. First, circular symmetry may eliminate the need for precise placement of voltage pickup 251 because, to a first order, the potential at the conductive surface of the ring is independent of the offset of the position of the center of the ring from the central axis (axis 2B-2B' in FIG. 2A and LA1 in FIG. 1B). Rather, the potential of voltage pickup 251 depends primarily on the dimensions (e.g., inner diameter, outer diameter, and thickness) of the ring. In contrast, in an asymmetric voltage pickup design, the voltage pickup conductor will acquire a potential that depends primarily on the placement and size 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 conductive surface relative to the inner conductor. In such a design, the distance between the voltage pickup and the longitudinal axis of the RF pipe may need to be precisely adjusted during assembly, sometimes manually using a micrometer screw gauge. Second, the design of voltage pickup 251 may largely cancel out centering errors when assembling voltage sensor 250 to VI sensor 200. The axial symmetry of voltage pickup 251 ensures that, to a first order, the overall electric flux remains unchanged even if the circular center of voltage pickup 251 is slightly offset from the longitudinal axis of the inner conductor. An increase in electric flux in one half of the ring, which may be offset closer to the inner conductor, is balanced by a simultaneous decrease in electric flux in the other half of the conductive ring, which is now farther from the longitudinal axis due to the ring's circular geometry.

[0057] 3 shows a cutaway view of another embodiment of a VI sensor 300 having a current sensor 340 disposed in a gallery 360 inside a sensor casing 365. A horizontal branch 343 is shown supported by an insulating portion 362 and connected to two vertical branches 342 of the current sensor 340. A slit 332, which wraps around in the mirror symmetry plane M2, can be seen above the horizontal branch 343.

[0058] 2A and 2B , a first voltage sensor 350 is shown inside the outer conductor 330. Additionally, the VI sensor 300 includes a second voltage sensor 355 symmetrically positioned opposite the current sensor 340. In this embodiment, the voltage pickups and housings of the first and second voltage sensors 350 and 355 are recessed within the body of the outer conductor 330 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 disturbance to the electromagnetic field caused by the insertion of the voltage sensors 350 and 355. In this embodiment, the VI sensor 300 causes negligible disturbance to the electromagnetic field within the RF pipe.

[0059] Measurements from the first voltage sensor 350 and the current sensor 340 have a relative phase error due to the difference in measurement location between them. In this embodiment, the oppositely located second voltage sensor 355 has an opposite phase error due to its symmetrical location relative 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 relative phase error between the voltage and current in 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 cancel, at least to first order, in the sum of these two sensed voltage signals. Therefore, combining the signals from the first and second voltage sensors 350 and 355 may result in more accurate voltage measurements. For example, by using an arithmetic average of the measurements from the first voltage sensor 350 and the second voltage sensor 355, phase errors may be reduced or even eliminated, resulting in a voltage measurement that reflects the voltage at the plane of mirror symmetry.

[0060] Additionally, the presence of the second voltage sensor 355 helps ensure that the two vertical branches 342 and the left and right halves of the horizontal branch 343 of the current pickup 341 are subjected to the same electric and magnetic fields. As explained above, undesirable coupling between the current pickup and the electric field penetrating into the cavity in the gallery 360 can generate parasitic electrical signals. By improving geometric symmetry, the second voltage sensor 355 helps ensure that potential disturbances seen at the first and second terminals 344 of the current sensor 340 are more accurately canceled out by using differential current measurements, as described above with reference to FIGS. 2A and 2B. In some embodiments, using the output of the second voltage sensor 355 may be optional.

[0061] FIG. 4 shows yet another embodiment of a VI sensor 400 attached to an RF pipe 410 including 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 is improved over that of VI sensor 200 (see FIGS. 2A and 2B) by providing additional mechanical support for the horizontal conductor 443 of the current pickup 441 of current sensor 440.

[0063] 4, the support components (e.g., plastic portions 462 and 470) can more securely secure horizontal branch 443 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 sets of holes into which horizontal branch 243 can be positioned, while in the design of VI sensor 400, plastic portions such as portions 462 and 470 may surround more of horizontal branch 443 and have protrusions that fit closely into corresponding cavities in metal sensor casing 465 and the metal outer surface of outer conductor 430.

[0064] As shown in FIG. 4 , a support structure 470 (e.g., made of plastic or other non-conductive material) positioned in addition to the insulating support 462 supports 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. Furthermore, the support structure 470 may prevent the vertical branch 442 from being over-tightened onto 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, stabilizing the shape of the current pickup 441 reduces variations in the electrical output of the current sensor 440 and improves the accuracy of the current measurements.

[0065] 5 shows a VI sensor 500 mounted on an RF pipe 510 with an inner conductor 520 and an outer conductor 530. A current sensor 540 is shown with a pair of terminals 544 located on the sensor casing 565 and a single-turn, half-loop current pickup 541 located inside the gallery 560. The current pickup 541 includes two vertical branches 542 attached to a horizontal branch 543. Similar to the VI sensor 400 of FIG. 4, a plastic portion 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 machining complexity, thereby reducing manufacturing costs compared to VI sensor 400 (shown in FIG. 4). The design of VI sensor 500 improves upon the design of VI sensor 400 by using a voltage sensor 550, as shown in FIG. 5, in which an insulator piece 555 that centers the inner conductor 520 of RF pipe 510 is also used to support the conductive voltage pickup ring of voltage sensor 550. By using the same plastic piece 555 for multiple purposes, for example, some of the plastic pieces used in VI sensor 400 can be eliminated. This reduces the machining complexity and manufacturing costs of VI sensor 500.

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

[0068] While the VI sensors described above with reference to Figures 1-5 use a single-turn, half-loop current pickup, it should be understood that multiple turns may be utilized in the current sensor current pickup design. For example, the current pickup in the VI sensors illustrated 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 mentioned above, multi-turn current pickups may also be constructed by wrapping a conductor around a mandrel, e.g., a toroidal mandrel. The conductor may be coiled around the circular axis of a doughnut-shaped insulating material that symmetrically surrounds the inner conductor of the RF pipe, which passes vertically through the central hole of the toroid. Multi-turn current pickups using toroidal mandrels are described below with reference to Figures 6A-7E.

[0069] It should be understood that the mandrel may not exactly conform to 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 casing 665 of the VI sensor 600. Although not visible in FIG. 6A , current and voltage pickups are housed within the space enclosed by the conductive sensor casing 665. In FIGS. 6A-6C , the inner conductor passes through a central hole 621. The inner conductor itself is omitted from the various views of the VI sensor 600 in FIGS. 6A-6C for clarity. The RF pipe's outer conductor connects to flanges at the top and bottom of the conductive sensor casing 665. In FIG. 6A , two neck regions 631 are shown adjacent to the flanges at the top and bottom of the sensor casing 665. The shape and dimensions of the neck regions 631 may be designed to be similar to the shape and dimensions of the RF pipe's outer conductor. Thus, the sensor casing 665 may be interpreted as an extension of the outer conductor, extending from the neck region 631 to a wider central portion that includes a top cover 663 and a bottom cover 666 with larger diameter conductive walls. The sensor casing 665 and outer conductor form the outer shield of the coaxial structure and may be connected to ground. The wider central portion of the sensor casing 665 houses an annular dielectric cavity 661 around the inner conductor, which 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 on the outside of the sensor casing 665. These 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 that are connected to the current pickups, and the center coaxial connector connects to the center terminal 654 that 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 in FIG. 6B and the cross-sectional view in FIG. 6C along section plane A-A' (shown in FIG. 6A) show the internal structure of the VI sensor 600. The inner conductor of the RF pipe has been omitted for clarity. The inner surface 638 of the central bore 621, shown within neck region 631 in FIGS. 6B and 6C, forms the inner sidewall of the main coaxial structure, which can also be interpreted as the RF pipe. Neck region 631 bulges into the wider-diameter top and bottom covers 663 and 666 of the sensor casing 665, which enclose a dielectric cavity 661 around the central bore 621 of the VI sensor assembly 600. The inner wall 638 continues as the surfaces 627 of the top and bottom covers 663 and 666 until interrupted by slit 671. As shown in FIG. 6C, the dielectric cavity 661 lies between a first major outer surface 627 and a second major outer surface 628, extending radially from the center of the central bore 621. First outer major surface 627 includes a continuous circular ring-shaped region in physical contact with central hole 621. Vertically, this ring-shaped first outer major surface 627 is separated into two portions by slit region 671. Second outer major surface 628 is located at a radial distance greater than the radius of first outer major surface 627.

[0074] Cavity 661 includes slit region 671. As shown in FIGS. 6B and 6C, slit region 671 is a physical discontinuity in inner surface 627 of the cylindrical wall of central hole 621 that forms a gap at the junction between top cover 663 and bottom cover 666 of sensor casing 665. Surface 627 is an extension of surface 638 of the cylindrical wall of central hole 621. When viewed radially from the center of central hole 621, slit region 671 appears as an insulating ring in physical contact with central hole 621. Further radially outward, slit region 671 has a zigzag shape that bypasses ring-shaped conductive voltage pickup 651, as indicated by the zigzag dashed lines in FIGS. 6B and 6C. The continuous insulating annular region of cavity 661, located radially between the physical contact point with central hole 621 and the inner radius of toroidal current sensor 641, is referred to as slit region 671 of VI sensor 600. As shown in FIGS. 6B and 6C , slit region 671 forms a dielectric barrier between conductive voltage pickup 651 and current sensor 641, as well as sensor casing 665. In the radial region between first outer major surface 627 and second outer major surface 628, cavity 661 (including insulating slit region 671) electrically isolates top cover 663 from bottom cover 666. For a radial distance less than first outer major surface 627, top cover 663 is electrically isolated from bottom cover 666 by central hole 621. The top cover 663 is electrically coupled to the bottom cover 666 via a coupling region 629 that extends radially beyond the second outer major surface 628 of the dielectric cavity 661 .

[0075] In this design, the RF current flowing within the grounded sensor casing 665 may be minimal within the area enclosed by the toroidal current sensor 641. The current flows vertically within the neck region 631 along the inner wall 638, then passes around the current sensor 641 due to a physical break in the inner surface 638 formed by the dielectric slit 671. Due to the slit 671, the current is diverted radially outward around the toroidal current sensor 641, flows laterally along the conductive wall of the annular dielectric cavity 661, returns radially, and then continues vertically along the inner wall 638 of the neck region 631.

[0076] 6B and 6C, current sensor 641 is a toroidal structure inside annular dielectric cavity 661 in the outer portion of the cavity, i.e., the region farther from central bore 621. Current sensor 641 includes a conductive coil 647 and a toroidal mandrel 642. Coil 647 includes multiple turns of continuous wire wound around the central circular axis of toroidal mandrel 642. Two opposite ends of coil 647 may be attached to coaxial connectors 645, as shown in FIG. 6B. The wire of coil 647 may be bare, enameled, or coated with insulation. Toroidal mandrel 642 is described in more detail below with reference to FIG. 6D. Current sensor 641 is electrically insulated from conductive sensor casing 665.

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

[0078] The function of 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 pipe. Typically, the RF pipe's outer conductor and conductive sensor casing 665 are grounded. Therefore, voltage pickup 651 may not function properly if its conductive ring is shielded from the RF pipe's inner conductor, for example, by a grounded metal ring disposed in the annular dielectric region between the inner conductor and voltage pickup 651. Excessive shielding will cause the voltage sensor output to be too weak to be useful. As shown in FIGS. 6B and 6C , voltage pickup 651 extends vertically partially into annular dielectric cavity 661 above and below slit region 671 of cavity 661. The cavity is formed by grooves in upper metal top cover 663 and lower metal bottom cover 666. Although 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 of the inner conductor, the grounded metal does not completely shield the voltage pickup 651. A dielectric slit 671, shown in FIGS. 6B and 6C, separates the top cover 663 from the bottom cover 666. The slit 671 appears as a cylindrical, disk-shaped dielectric region within the mirror plane M (shown by the dashed line in FIG. 6C) because the central hole 621 is free of the inner conductor. When the inner conductor is in place, the dielectric slit 671 appears as an annular, disk-shaped region around the inner conductor. The unshielded radial electric field within 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 then provide a usable electrical signal proportional to the RF voltage on the inner conductor at that 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. Faraday's law induces an oscillating electrical signal in the coil that is proportional to the oscillating magnetic flux in the toroidal mandrel 642 enclosed within the turns of coil 647's wire. Ampere's law states that the strength of the magnetic field passing through the current sensor 641 is proportional to the total current across the area of ​​the plane enclosed within the central bore of the toroidal current sensor (similar to the donut hole in a doughnut). As is true for any coaxial structure, the current flowing through the inner conductor at any location in the RF pipe is exactly equal to the opposite current in the outer conductor. The sensor casing 665 of the VI sensor 600 may be considered the equivalent outer conductor of the RF pipe, with the inner conductor passing through the central bore 621. Therefore, the current sensor 641 may not function properly unless the RF current in the sensor casing 665 is restricted to flow outside the circular, disk-shaped area bounded by the outer periphery of the toroidal mandrel 642. For example, if the top cover 663 and 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 casing may flow through this contact. This current, which opposes the current in the inner conductor, reduces the magnitude of the total current contained by the current sensor 641 and, therefore, reduces the magnetic flux passing through the coil 647. If the total current flowing through the contacts within the area bounded by the toroidal current sensor 641 is too small, the magnetic field may be insufficient to induce a usable electrical signal in the current pickup coil 647. As shown in FIGS. 6B and 6C, the dielectric slit region 671 again prevents electrical contact between the top cover 663 and the bottom cover 666 at radial distances less than the inner radius of the toroidal current sensor 641.

[0080] By designing the voltage pickup 651 to be smaller than the vertical height of the cavity 661, a continuous, uninterrupted dielectric region is achieved that separates the top cover 663 and the bottom cover 666 all the way to the outer circle of the toroidal current sensor 641. The conductive ring of the voltage pickup 651 may be positioned generally symmetrically between the top cover 663 and the bottom cover 666, with supporting portions including insulating material. Thus, in all directions, the immediate vicinity of the conductive voltage pickup 651 is insulating material. As explained above, this insulating material is within 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 the zigzag dashed line in FIG. 6B. Note that, as shown by the dashed lines in FIG. 6C, zigzag dielectric slit regions 671 exist both above and below voltage pickup 651 because the conductive ring of voltage pickup 651 must be electrically isolated from grounded sensor casing 665.

[0081] The current pickup of a current sensor is typically shielded from RF electric fields by a grounded conductive portion. Shielding 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 disposed within a dielectric cavity 661 inside the sensor casing 665. As seen in FIGS. 6B and 6C and described above, the conductive portions encountered while traveling radially inward from the toroidal current sensor 641 to the inner conductor include, first, the conductive voltage pickup 651 and, second, a portion of the inner wall of the conductive sensor casing 665. These intervening conductive portions may help shield the current sensor 641 from radial electric fields. Some of the electric field lines emanating from the inner conductor may terminate on the grounded inner wall of the conductive sensor casing 665. Additionally, conductive voltage pickup 651 serves a dual purpose by partially shielding coil 647 from the RF electric field. Because voltage pickup 651 is not shorted to ground, the electric field reduction provided by the conductive ring depends on the magnitude of the impedance to ground at 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 shared axis that passes through the center of the central bore 621 and is perpendicular to the plane of the central bore 621. Furthermore, both the current sensor 641 and the voltage pickup 651 share the same mirror plane (indicated by dashed line M in FIG. 6C ) that is perpendicular to the longitudinal axis. The symmetry of the structure of the VI sensor 600 helps reduce / eliminate deviations in the measurement of the phase angle (Φ) between voltage and current. Furthermore, the first-order cancellation effect resulting from the axisymmetric nature makes the sensor output signal of the VI sensor 600 less susceptible to machining tolerances and positioning errors during assembly.

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

[0084] The toroidal mandrel 642 may comprise plastic or other insulating material and may be manufactured using, for example, 3D printing techniques. After the coil 647 is attached to the grooved toroidal mandrel 642, the structure may optionally be encapsulated in a resin coating, for example, using potting resin techniques. The resin encapsulation securely holds the coiled multi-turn current pickup 647 in place.

[0085] The integrated assembly of VI sensor 600, including the current sensor 641 and the combination of electric field shield and voltage pickup 651 described above, 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 is not voltage sensitive. Also, the design of the toroidal mandrel 742 used in current sensor assembly 701 is different from grooved toroidal mandrel 642, as described further below.

[0087] FIG. 7A shows a perspective view of a current sensor assembly 701 using a toroidal current sensor 741 disposed within a dielectric cavity between a top cover 782 and a bottom cover 784 of a sensor casing 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. The inner conductor of a coaxial transmission line (e.g., an RF pipe) with which the current sensor assembly 701 may be used passes through the central hole 710. The current sensor assembly 701 is thereby symmetrically disposed 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 casing 765 to show the structure of the lower half of the dielectric cavity 720 and the bottom cover 784. (The upper half of this 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 bottom cover 784. Outside the outer circle of this outermost groove, the metal top cover 782 and the metal bottom cover 784 may be physically and electrically connected together, but inside the outer circle of the outermost groove, no electrical contact may be made between the top cover 782 and the bottom cover 784.

[0089] The zigzag dielectric slit 725 includes two groove-like dielectric regions on either side of a ring-shaped conductive bump 750 that protrudes from the floor of the bottom cover 784. The conductive floor of the dielectric cavity 720, including the conductive bump 750, is electrically and physically separated from the respective conductive roof of the dielectric cavity 720 by an uninterrupted, continuous dielectric region. Thus, the top of the conductive bump 750 may protrude into the respective groove in the top metal cover 782 but may not contact the roof. The combined top and bottom portions of the zigzag dielectric slit 725 thus result in a zigzag dielectric region that extends around and above the conductive bump 750, as indicated by the zigzag dashed line in FIG. 7B .

[0090] 7C shows a portion of an RF system 700 including an RF pipe inner conductor 711 passing through a central hole 710 of a current sensor assembly 701 arranged symmetrically around 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 a sensor casing 765. The sensor casing functions as a grounded outer conductor for the portion of the inner conductor 711 passing through the central hole 710, similar to the sensor casing 665 of the VI sensor 600.

[0091] In FIG. 7C , current sensor assembly 701 is shown with an exploded cutaway view including current sensor 741. Current sensor 741 includes a toroidal mandrel 742 as well as a conductive current pickup coil 747. Toroidal mandrel 742 includes a solid dielectric material with a winding passageway. The winding passageway may be accessed through access holes 749 at various locations on the surface of toroidal mandrel 742. A connection mechanism 743 having two holes is positioned over one of the access holes 749. Two opposite ends of the conductor of coil 747 are shown protruding upward through the holes in connection mechanism 743. A portion of toroidal mandrel 742 has been cut away to show the conductor of coil 747 passing through the winding passageway in the solid dielectric material of toroidal mandrel 742. Coil 747 is fitted inside 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 Figure 6D.

[0092] In Figure 7C, the toroidal mandrel 742 is positioned in respective grooves in the floor of the outer dielectric region 723 and bottom cover 784 (see Figure 7B). An exploded view of the current sensor assembly 701 shows that the top half of the toroidal mandrel 742 can fit into a groove in the top cover 782 within the outer dielectric region 723 of the dielectric cavity 720. Similarly, the conductive ridge 750 can 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 toroidal current sensor 741 and the inner conductor 711, can 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 toroidal current sensor 741 would reduce the strength of the magnetic field passing through the current pickup coil 747 and could excessively weaken the output current signal. Therefore, the top of the conductive bump 750 is electrically isolated from the top cover 782 by the zigzag dielectric slit 725. The zigzag shape of the dielectric region is indicated by the zigzag dashed line in FIG. 7C .

[0094] 7D, a cutaway view of a portion of RF system 700 shows current sensor assembly 701 with top cover 782 attached to 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 pipe 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 opposite ends of the coil 747 pass through holes in a connection mechanism 743, which may be positioned over an opening similar to the access hole 749. A ring-shaped conductive ridge 750 is shown interposed between the inner conductor 711 and the current sensor 741. Dielectric slits 725 are visible 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 a large number of turns as current pickups. The greater the number of turns, the greater the sensitivity of each current sensor. The increased sensitivity also allows each turn to have a smaller cross-section, thus reducing the overall size of the current sensor and allowing the current sensor to be placed in areas that would otherwise be inaccessible.

[0097] It should be understood that while the mandrels described in this disclosure are shaped like a toroid, other shapes may be used, such as a square or a regular polygon having any number of sides, etc. 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 in layers of dielectric and conductive material linked by vias, such as in printed circuit board (PCB) technology.

[0099] The toroidal current sensor embodiments described above offer the advantages of the axisymmetric nature of a torus, the high immunity to noise of multi-turn current pickup, and the ease of use afforded by the compact construction.

[0100] The VI sensors and measurement methods described in this disclosure provide embodiments that can enable very high-precision measurements at low manufacturing costs. High accuracy at low manufacturing costs may be achieved by including design features intended to reduce the sensitivity of VI measurements to machining and assembly errors. Current sensor accuracy depends on machining tolerances, which cause variations in dimensions that determine the geometry of the current pickup (e.g., the area enclosed by a rectangular half-loop). Current measurement accuracy may also be limited by assembly tolerances, such as the precision with which the current pickup can be positioned (including the radial distance from the longitudinal axis and the angle between the plane of the half-loop and the longitudinal axis). Similarly, voltage measurement accuracy depends on machining tolerances (e.g., the precision of the diameter and circumference of the voltage pickup ring) and assembly tolerances (e.g., the angle between the plane of the ring and the longitudinal axis). The inventors conducted detailed computer simulations of the sensitivity of VI sensor signals to variations in the dimensions and placement of the current and voltage pickups and found that accuracies as high as 1% can be achieved with standard machines, with placement tolerances as low as 0.005 inches. Computer simulations were performed using a calibrated 3D finite element solution to Maxwell's equations over a wide range of RF powers, RF frequencies, and load impedances used in plasma processing.

[0101] Illustrative embodiments of the present application are summarized here. Other embodiments can be seen throughout the specification and claims filed herewith. [Example]

[0102] Example 1. A radio frequency (RF) sensor assembly includes a sensor casing disposed about a central hole, the sensor casing including a first conductive cover and a second conductive cover. The assembly includes a cavity disposed about the central hole and including a first dielectric material, the cavity being bounded by first and second outer major surfaces along a radial direction from a center of the central hole, the first outer major surface including a continuous ring-shaped region in physical contact with the central hole, the second outer major surface being at a radial distance greater than a radius of the first outer major surface, the first conductive cover electrically coupled to the second conductive cover via a bonding region beyond the second outer major surface of the cavity, and the first conductive cover being electrically insulated from the second conductive cover by the cavity and the central hole. The assembly includes a current sensor including current pickups electrically insulated from a sensor casing and arranged symmetrically around a central hole, the current pickups being positioned inside a cavity and insulated from the sensor casing.

[0103] Example 2. The sensor assembly of example 1, wherein the current sensor includes a mandrel shaped like a toroid.

[0104] Example 3. A sensor assembly as described in any one of Examples 1 or 2, wherein the mandrel comprises a second dielectric material around a continuous hollow passage, the hollow passage being wound symmetrically about an internal circular axis, and the current pickup comprises a continuous conductor having two opposite ends, the conductor being shaped like a coil passing through the hollow passage.

[0105] Example 4. A sensor assembly as described in any one of Examples 1 to 3, further comprising a plurality of access holes extending from the outer surface of the mandrel to the hollow passage and two electrical terminals insulated from the sensor casing, the two electrical terminals being electrically connected to the respective two opposite ends of electrical leads extending out from the mandrel through the plurality of access holes.

[0106] Example 5. A sensor assembly as described in any one of Examples 1-4, wherein the mandrel comprises a second dielectric material having a groove on an outer major surface of the mandrel, the groove being wound symmetrically around an inner circular axis, and the current pickup comprises a continuous conductor having two opposite ends, the conductor being shaped like a coil disposed within the groove of a toroid.

[0107] Example 6. The sensor assembly of any one of Examples 1-5, further comprising two electrical terminals insulated from the sensor casing, the two electrical terminals being electrically coupled to two opposite ends of the conductor, respectively, outside the mandrel.

[0108] Example 7. The current sensor of any one of Examples 1 to 6, wherein the mandrel and current pickup are encapsulated in resin.

[0109] Example 8. The sensor assembly of any one of Examples 1 to 7, further comprising a voltage sensor, the voltage sensor comprising a voltage pickup and an electrical terminal, the voltage pickup being disposed within a cavity interposed between the current sensor and the central hole, the electrical terminal being electrically connected to the voltage pickup, and the voltage sensor being electrically insulated from the sensor casing.

[0110] Example 9. The sensor assembly of any one of Examples 1 to 8, wherein the voltage pickup is a ring-shaped electrode arranged symmetrically around a central hole, and the voltage pickup is mechanically supported by a third dielectric material within a cavity disposed between the first conductive cover and the second conductive cover.

[0111] Example 10. The sensor assembly of any one of Examples 1-9, wherein the third dielectric material is a solid dielectric material and the first dielectric material is a gas.

[0112] Example 11. A voltage sensor described in any one of Examples 1 to 10, wherein the voltage pickup is electrically isolated from the sensor casing by a slit region, the slit region comprising a continuous zigzag region shaped like a ring, and the slit region is in physical contact with the central hole, the voltage pickup, and the current sensor.

[0113] Example 12. A radio frequency (RF) sensor assembly includes a sensor casing symmetrically disposed about a central hole, the sensor casing including a first conductive cover and a second conductive cover. The assembly includes an annulus-shaped cavity symmetrically disposed about the central hole and including a first dielectric material, the cavity bounded by first and second outer major surfaces along a radial direction from the center of the central hole, the first outer major surface including a ring-shaped continuous region in physical contact with the central hole, and the second outer major surface including a closed outer boundary at a radial distance greater than the radius of the first outer major surface. A current sensor includes a current pickup coil symmetrically disposed about the central hole, the current pickup insulated from the sensor casing, the current pickup disposed within the cavity. The assembly also includes a ring-shaped conductive bump interposed between the current pickup and the central hole and electrically coupled to the second conductive cover, the conductive bump covered with a dielectric material.

[0114] Example 13. A sensor assembly as described in Example 12, wherein the first conductive cover includes, along a radial direction from the center of the central hole, a first groove, the first groove being shaped like a ring and positioned vertically above the conductive ridge, a second groove shaped like a ring and at a greater radial distance from the central hole, and a beam shaped like a ring including a conductive region between the first groove and the second groove; the second conductive cover includes a third groove shaped like a ring and positioned vertically below the beam and the second groove; and the current sensor includes an upper portion positioned within the hollow portion including the second groove and a lower portion positioned within the hollow portion including the third groove.

[0115] Example 14. A sensor assembly described in any one of Examples 12 or 13, wherein the cavity includes a slit region along a radial direction from the center of the central hole, the slit region including a continuous zigzag dielectric region shaped like a ring, and the slit region is in physical contact with the central hole, the conductive ridge, and the current sensor.

[0116] Example 15. A sensor assembly described in any one of Examples 12 to 14, wherein the first conductive cover is electrically coupled to the second conductive cover through an area beyond the second major outer surface of the cavity, and the first conductive cover is electrically insulated from the second conductive cover by the cavity and central hole within an area surrounded by the closed outer boundary.

[0117] Example 16. A plasma system comprising: a processing chamber including an electrode; a radio frequency (RF) power supply configured to power the processing chamber with an RF signal; an RF pipe coupling the RF power supply to the electrode of the processing chamber; a toroid-like shaped mandrel arranged symmetrically about the axis of the RF pipe carrying the RF signal; and a voltage pickup arranged symmetrically about the axis of the RF pipe and surrounded by the mandrel.

[0118] Example 17. The system of Example 16, further comprising a current sensor, the current sensor comprising: a mandrel comprising a dielectric material; a current pickup comprising a continuous conductor having two opposite ends, the conductor coiled symmetrically around the interior circular axis of the mandrel; and two electrical terminals electrically connected to the two opposite ends of the conductor, respectively, outside the mandrel.

[0119] Example 18. The plasma system of any one of Examples 16 or 17, further comprising a sensor assembly comprising: a central hole within which the inner conductor of the RF pipe is disposed; and a sensor casing shaped like an annulus disposed adjacent to and around the central hole, the sensor casing including a first conductive cover and a second conductive cover, the sensor casing being divided into a first annular region and a second annular region, the first annular region being adjacent to the central hole on one side and adjacent to the second annular region on the opposite side, in the first annular region, the first conductive cover being electrically insulated from the second conductive cover by a cavity comprising a dielectric material, the mandrel and voltage pickup being disposed within the cavity, and in the second annular region, the first conductive cover being electrically coupled to the second conductive cover.

[0120] Example 19. A plasma system described in any one of Examples 16 to 18, wherein the first conductive cover and the second conductive cover in the first annular region of the sensor casing are electrically coupled to the outer conductor of the RF pipe, and the first conductive cover and the second conductive cover in the second annular region of the sensor casing are electrically coupled to the outer conductor of the RF pipe.

[0121] Example 20. The plasma system of any one of Examples 16 to 19, further comprising a voltage sensor, the voltage sensor including a voltage pickup and an electrical terminal, the voltage sensor being electrically insulated from the sensor casing, the voltage pickup being a ring-shaped electrode arranged symmetrically around a central hole, the voltage pickup being mechanically supported by a solid dielectric material within a cavity disposed between the first conductive cover and the second conductive cover, and the voltage pickup being electrically coupled to the electrical terminal.

[0122] Example 21. A plasma system described in any one of Examples 16 to 20, wherein the voltage pickup is electrically isolated from the sensor casing by a slit region, the slit region comprising a continuous zigzag dielectric region shaped like a ring, the slit region being in physical contact with the central hole, the voltage pickup, and the mandrel.

[0123] 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 those 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 and all such modifications or embodiments.

Claims

1. 1. A radio frequency (RF) sensor assembly comprising: a sensor casing disposed around the central hole, the sensor casing including a first conductive cover and a second conductive cover; a cavity comprising a first dielectric material disposed about the central hole, the cavity being bounded by first and second outer major surfaces along a radial direction from a center of the central hole, the first outer major surface including a continuous ring-shaped region in physical contact with the central hole, the second outer major surface being at a radial distance greater than a radius of the first outer major surface, the first conductive cover being electrically coupled to the second conductive cover via a bonding region beyond the second outer major surface of the cavity, the first conductive cover being electrically insulated from the second conductive cover by the cavity and the central hole; a current sensor including current pickups electrically insulated from the sensor casing and arranged symmetrically around the central bore, the current pickups being disposed within the cavity and insulated from the sensor casing; The current sensor includes a mandrel shaped like a toroid.

2. 10. The sensor assembly of claim 1, wherein the mandrel comprises a second dielectric material around a continuous hollow passage, the hollow passage being wound symmetrically about an internal circular axis, and the current pickup comprises a continuous conductor having two opposite ends, the conductor being shaped like a coil passing through the hollow passage.

3. a plurality of access holes extending from an outer surface of the mandrel to the hollow passage; 3. The sensor assembly of claim 2, further comprising: two electrical terminals insulated from the sensor casing, the two electrical terminals being electrically coupled to the two opposite ends of the conductors that extend out of the mandrel through the plurality of access holes.

4. 2. The sensor assembly of claim 1, wherein the mandrel comprises a second dielectric material having a groove on an outer major surface of the mandrel, the groove being wound symmetrically about an inner circular axis, and the current pickup comprises a continuous conductor having two opposite ends, the conductor being shaped like a coil disposed within the groove of the toroid.

5. 5. The sensor assembly of claim 4, further comprising two electrical terminals insulated from the sensor casing, the two electrical terminals being electrically coupled to the two opposite ends of the conductor, respectively, outside the mandrel.

6. further comprising a voltage sensor, the voltage sensor including a voltage pickup and an electrical terminal; the voltage pickup is disposed within the cavity interposed between the current sensor and the central hole; the electrical terminal is electrically connected to the voltage pickup; The sensor assembly of claim 1 , wherein the voltage sensor is electrically isolated from the sensor casing.

7. 1. A radio frequency (RF) sensor assembly comprising: a sensor casing disposed around the central hole, the sensor casing including a first conductive cover and a second conductive cover; a cavity comprising a first dielectric material disposed about the central hole, the cavity being bounded by first and second outer major surfaces along a radial direction from a center of the central hole, the first outer major surface including a continuous ring-shaped region in physical contact with the central hole, the second outer major surface being at a radial distance greater than a radius of the first outer major surface, the first conductive cover being electrically coupled to the second conductive cover via a bonding region beyond the second outer major surface of the cavity, the first conductive cover being electrically insulated from the second conductive cover by the cavity and the central hole; a current sensor including current pickups electrically insulated from the sensor casing and arranged symmetrically around the central bore, the current pickups being disposed within the cavity and insulated from the sensor casing; further comprising a voltage sensor, the voltage sensor including a voltage pickup and an electrical terminal; the voltage pickup is disposed within the cavity interposed between the current sensor and the central hole; the electrical terminal is electrically connected to the voltage pickup; the voltage sensor is electrically insulated from the sensor casing; the voltage pickups are ring-shaped electrodes arranged symmetrically around the central hole; The sensor assembly, wherein the voltage pickup is mechanically supported by a third dielectric material within the cavity disposed between the first conductive cover and the second conductive cover.

8. 8. The sensor assembly of claim 7, wherein the third dielectric material is a solid dielectric material and the first dielectric material is a gas.

9. 8. The sensor assembly of claim 7, wherein the voltage pickup is electrically isolated from the sensor casing by a slit area, the slit area comprising a continuous zigzag area shaped like a ring, the slit area being in physical contact with the central hole, the voltage pickup, and the current sensor.

10. 1. A plasma system comprising: a processing chamber including an electrode; a radio frequency power source (RF power source) configured to power the processing chamber with an RF signal; an RF pipe coupling the RF power source to the electrode of the processing chamber; a toroid-shaped mandrel symmetrically disposed about the axis of the RF pipe carrying the RF signal; voltage pickups arranged symmetrically about the axis of the RF pipe and surrounded by the mandrel; A plasma system, comprising the sensor assembly according to any one of claims 1 to 9 disposed on the circumferential surface of the RF pipe.

11. Further comprising a current sensor, the current sensor comprising: the mandrel comprising a dielectric material; a current pickup including a continuous conductor with two opposite ends, the conductor being wound symmetrically around an inner circular axis of the mandrel; and two electrical terminals electrically connected to the two opposite ends of the wire outside the mandrel.

12. further comprising a sensor assembly, the sensor assembly comprising: a central hole, wherein the inner conductor of the RF pipe is disposed within the central hole; a sensor casing shaped like an annulus disposed adjacently around the central hole, the sensor casing including a first conductive cover and a second conductive cover, the sensor casing being divided into a first annular region and a second annular region; the first annular region is adjacent to the central hole on one side and adjacent to the second annular region on an opposite side; In the first annular region, the first conductive cover is electrically insulated from the second conductive cover by a cavity containing a dielectric material, and the mandrel and the voltage pickup are disposed within the cavity; The plasma system of claim 10 , wherein in the second annular region, the first conductive cover is electrically coupled to the second conductive cover.

13. the first conductive cover and the second conductive cover in the first annular region of the sensor casing are electrically coupled to an outer conductor of the RF pipe; 13. The plasma system of claim 12, wherein the first conductive cover and the second conductive cover in the second annular region of the sensor casing are electrically coupled to the outer conductor of the RF pipe.

14. further comprising a voltage sensor, said voltage sensor including said voltage pickup and an electrical terminal; the voltage sensor is electrically insulated from the sensor casing; The voltage pickups are ring-shaped electrodes arranged symmetrically around a central hole; the voltage pickup is mechanically supported by a solid dielectric material within a cavity disposed between the first conductive cover and the second conductive cover; The plasma system of claim 12 , wherein the voltage pickup is electrically coupled to the electrical terminal.

15. 15. The plasma system of claim 14, wherein the voltage pickup is electrically isolated from the sensor casing by a slit region, the slit region comprising a continuous zigzag dielectric region shaped like a ring, the slit region being in physical contact with the central hole, the voltage pickup, and the mandrel.

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