System and method for non-invasive detection of radio frequency current spectrum flowing through plasma processing chamber

Abstract

To overcome many of the problems and limitations associated with an induction loop used to detect an RF spectrum from an external location in a plasma chamber.SOLUTION: A system for non-invasive detection of a radio frequency current spectrum includes a plasma process chamber, a plasma generator, and a shunt connector having a resistor. The shunt connector is attached across an opening portion in a ground return path between the chamber and the generator.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] This disclosure relates to the remote sensing and analysis of radio frequency current signals in a plasma system.

Background Art

[0002] Plasma treatment of materials is ubiquitous in modern industrial production. Common examples are the etching and deposition of layers to form transistors during the manufacture of integrated circuits in the semiconductor industry. Plasma treatment is also used, to name just a few, in the manufacture of solar panels, flat panel displays, thin film coatings, and medical devices.

[0003] The processing plasma is generated at low pressure inside a vacuum chamber. The air is evacuated and a gas recipe is added to the chamber at a selected gas pressure. Energy, which is usually electrical energy, is supplied to the vacuum chamber to excite the gas into a plasma state. The plasma generates the ions necessary to change the surface of the product being manufactured during processing.

[0004] Radio-frequency (RF) electrical energy is commonly used to power plasma processes. DC, pulsed DC, and microwave power are also commonly used. The RF range is typically between tens of kilohertz and hundreds of megahertz. In RF plasmas, an RF generator supplies power to the plasma chamber through a matching network to maximize power transfer. Power can be coupled to the plasma in several different ways. In one configuration, an RF-powered electrode can be used to excite the plasma. The electric field (E-field) applied between the powered electrode and the counter electrode, and / or the dielectric breakdown of the process gas by the chamber body, form the plasma. The RF current maintains a discharge flowing from the generator to the powered electrode and through the plasma to the counter electrode and / or the chamber body. The current then recirculates back to the generator through a ground return path. Electrons are stripped from their parent atoms and molecules, oscillating back and forth in the electric field, ionizing the background gas in the process, and thus maintaining the plasma.

[0005] In an alternative configuration, RF power is coupled to the plasma through an RF antenna. The antenna does not need to be in direct contact with the background gas. The RF current flowing through the antenna induces a time-varying magnetic field perpendicular to the direction of the current flow. The magnetic field is generally coupled to the chamber through a dielectric window. When breakdown occurs, the magnetic field induces an E-field in the plasma that drives the RF current. Free electrons oscillate in the RF field, ionizing the background gas and thus maintaining the plasma. Many other plasma reactor configurations and plasma generation mechanisms exist, and it is possible that the present invention may be applied to them as well.

[0006] Regardless of the type of RF coupling mechanism, an interface region is formed between the electrode or antenna and the plasma. This region is called the plasma sheath. The sheath has a nonlinear RF impedance. Inevitably, harmonics of the fundamental drive frequency are generated. As a result, the RF plasma voltage and current can have a varied harmonic spectrum. The RF harmonic characteristics of a plasma process are determined by many variables, including fundamental plasma parameters, plasma chemistry, chamber geometry, chamber surface conditions, and the mechanical properties of the chamber. Therefore, the harmonic spectrum contains important information about the tone and performance of the plasma process. Depending on the plasma reactor geometry, there will be one or more drive frequencies. The harmonic spectrum of each fundamental drive frequency, and the intermodulation frequencies. Non-invasive sensing devices are highly desirable for measuring the RF harmonic spectrum generated in an RF-powered plasma reactor. Accurately measured harmonic spectrum characteristics can be used to monitor process performance in real time. Many methods and sensing devices have been developed to measure RF spectra in RF plasma processes.

[0007] VI sensors are commonly used to measure the RF voltage and current spectra flowing through a transmission line or RF path between an RF generator and a plasma chamber. The VI sensing element is typically placed under grounded shielding, very close to the current carrier conductor, to measure downstream RF voltage and current signals.

[0008] WO2014 / 016357A2 describes a VI sensor device. This sensor is designed to be connected in series with an RF feed line. The sensor includes a section of the transmission line, as is common with sensors of this type. A broadband capacitive pickup (E field probe) is used to determine the voltage signal on the RF line connected to the plasma. An inductive loop (B dot probe) is used to determine the RF current in the RF line connected to the plasma. The voltage and current pickups are embedded in the RF transmission line section of the VI sensor structure. The signals representing current and voltage are transmitted to an analog-to-digital converter (ADC), and the digitized signals are processed by a field-programmable gate array. Linearly positioned VI sensors are very important tools, but installation can be difficult without making significant changes to the plasma system configuration. VI sensors may not be as sensitive to changes in plasma conditions as the device provided by this invention.

[0009] Another group of sensing devices utilizes the fact that areas of a plasma processing system can "leak" into the electromagnetic field associated with the RF voltage applied to the system and the RF current flowing through the system. The time-varying electric and magnetic fields of RF radiate through any unshielded or improperly grounded area.

[0010] Chamber viewports, often used to monitor emission from a plasma, are examples of gaps or openings in a plasma system that allow monitoring of the RF field leaking from the processing chamber. Several prior art inventions use time-varying electric (E) field sensors and / or time-varying magnetic (B) field sensors to detect the RF spectrum radiating from an opening in a plasma processing system, such as a viewport. Capacitive pickups are typically used to detect the E-field spectrum, while induction loops are typically used to detect the B-field spectrum.

[0011] Through the processing and analysis of the detected RF spectrum, the plasma process can be monitored in real time. To avoid the need for detailed calibration of the measured RF spectrum, statistical methods can be used to establish specific processing conditions as a baseline or fingerprint. This is typically done for a known "healthy" process. Subsequent processes can be compared to the baseline to check for statistically significant changes against the same target value. While overall changes may be easily detected in a single frequency channel, other changes may include more subtle differences, such as small air leaks or slight wafer misplacement, which may require multivariate analysis using many data channels from the RF harmonic spectrum to detect.

[0012] Elahi and Ghoranneviss (IEEE Transactions on Plasma Science, vol.38, no.11, November 2010) present a novel technique for determining the plasma position by detecting the plasma current inside the chamber using an induction loop installed on the outside of the chamber vessel of an IR-T1 tokamak.

[0013] WO2018 / 177965A1 describes an apparatus in which a specially designed magnetic loop antenna is used to detect plasma currents flowing near a viewport of a plasma chamber from an external location. The antenna is carefully designed and calibrated. The antenna output is coupled to a spectral analyzer to view the frequency spectrum detected by the antenna. The inventors describe a frequency analysis technique for detecting the resonant characteristics of the plasma. A good match is obtained between this apparatus and linearly positioned VI sensors, as well as an OES detector.

[0014] WO2004 / 006298A2 describes a device for remotely detecting RF radiation from a plasma system using an RF antenna. The antenna can detect harmonic signals and is connected to a processing unit for analysis. The processing unit is connected to a plasma tool controller, where the detected RF signals are used to adjust and maintain the parameters of the plasma process based on the measured signal levels.

[0015] U.S. Patent Application Publication No. 2007 / 0227667 describes a device consisting of two magnetic loop antennas placed inside the plane of a plasma chamber wall. The antennas are positioned near two electrodes of a capacitively coupled plasma reactor. The voltage signals generated by the magnetic flux passing through each loop are coupled to a signal processing unit. The current flowing from the plasma to the chamber wall is calculated in this way.

[0016] Inductive / magnetic loop type detectors used to measure RF fields originating from viewport windows or other areas of a plasma system have several limitations, including the following: a) The glass window attenuates the magnetic field, and this attenuation is frequency-dependent. Therefore, it is difficult to obtain an accurate reflection of the relative intensity of the harmonic signal inside the chamber. b) When the plasma driving frequency is low (e.g., 400 kHz), the wavelength becomes larger, requiring a larger viewport or gap in the system to allow sufficient signal levels to escape for detection. c) Some chambers have mesh shielding grounded inside the window, in particular to block the leakage of RF magnetic fields. d) The orientation, placement, and dimensions of the loop are important for determining the detected signal level, and this makes it difficult to perform repeatable measurements after the loop is returned to its original location following maintenance. e) Since the induction loop is used to detect the magnetic flux passing through the loop, it is susceptible to interference from unintended sources, such as a nearby plasma chamber. f) Temperature fluctuations at the installation site, for example when the chamber is heated, may cause variations in loop dimensions and, consequently, loop signal levels during the plasma process, which may be misinterpreted as a fault condition. [Overview of the project] [Problems that the invention aims to solve]

[0017] This invention overcomes many of the problems and limitations associated with induction loops used to detect RF spectra from locations outside the plasma chamber. [Means for solving the problem]

[0018] In a first independent aspect of the present invention, a system for non-invasive detection of radio frequency current spectra is provided, comprising a plasma processing chamber, a plasma generator, and a shunt connector having a resistor, the shunt connector being mounted across an opening in the ground path between the chamber and the generator.

[0019] The system may be configured to detect only the current in the return path.

[0020] The sensor may further be configured such that a current flowing through the ground return path generates a voltage in the resistor.

[0021] The system can further include an amplifier configured to detect a voltage drop across the resistor and output an RF signal.

[0022] Optionally, the shunt connector is configured to be attached across the viewport of the chamber.

[0023] The system can further include a digitization circuit configured to extract the RF signal from the amplifier and convert the RF signal into a digital signal for processing and analysis.

[0024] The output RF signal may be an alternating current signal in the RF band.

[0025] The system can further include a storage container surrounding the resistor and the amplifier.

[0026] The system may be configured such that the shunt connector and the resistor create a path for current to flow as part of the ground return path.

[0027] Optionally, the path created by the shunt connector and the resistor is oriented in the same direction as the flow of current in the ground return path.

[0028] The shunt connector can include at least one of a ground shunt strap, a cable, a bar, and a rod.

[0029] In a second independent aspect of the present invention, a method for non-invasive detection of a radio frequency current spectrum flowing in a plasma processing chamber is provided, including providing a shunt connector having a resistor and attaching the shunt connector across an aperture in a ground return path between the chamber and a plasma generator.

[0030] The method may further include detecting only the current within the ground return path.

[0031] By choice, the current flowing through the ground return path generates a voltage in the resistor.

[0032] The method may further include mounting a shunt connector across the viewport of the chamber.

[0033] This application will be described from here on with reference to the attached drawings. [Brief explanation of the drawing]

[0034] [Figure 1] This figure shows an overview of a system in which the sensor described in this instruction can be used. [Figure 2] This figure shows the sensor placement locations in the system shown in Figure 1 in more detail. [Figure 3] This figure shows the sensor used in this instruction, mounted in the viewport of the chamber, and the internal configuration of the sensor. [Figure 4] This figure shows the test results achieved using the sensor described in this instruction. [Figure 5] This figure shows further test results achieved using the sensor described in this instruction. [Figure 6] This diagram shows how the sensor described in this instruction can be used to detect faults. [Modes for carrying out the invention]

[0035] The present invention approaches the detection of RF spectra from a novel perspective. The invention utilizes the fact that all sources of RF plasma currents are RF generators, and that these RF currents must return to the generators through a return path to ground. This is illustrated in Figure 1. In particular, Figure 1 shows an RF generator 101 (or plasma generator) connected to ground 102. A match unit 103 is provided in a transmission line or RF path between the RF generator 101 and the plasma chamber 104, as is known in the art. The RF plasma current flows down a path 105 from the RF generator to the chamber 104. The RF plasma current returns from the chamber 104 to the generator via a return path 106.

[0036] Conventional devices detect the RF current spectrum in the downstream path 105 between the source (e.g., RF generator 101) and the starting point of the return path 106. As will be outlined in more detail below, the apparatus according to this teaching detects the RF current in the return section of the path between the starting point of the return path and the generator, i.e., in the return path 106. With respect to an RF plasma reactor, the starting point of the return path is typically the metal chamber vessel body.

[0037] As will be explained in more detail below, the sensor provided in this instruction comprises a shunt connector having an RF current sensing element mounted across an opening in the return path of a plasma system, such as a viewport. The shunt connector may include a ground shunt strap, cable, bar, or rod. Unlike conventional inductive loop sensors, it does not detect the time-varying magnetic field emitted through the viewport. Instead, only the RF current is resistively sensed in the return path to the generator. A typical plasma process may be driven by currents of several tens of amperes. In the down path, these currents are guided through clearly defined and restricted current paths. In the return path, the return current diffuses across the entire surface area of ​​the return element, such as the chamber body. Thus, the sensor will only "see" or detect a small return current, determined by a) the ratio of the sensor's surface area to the total surface area of ​​the return region on which the sensor is installed, and b) the resistance of the sensing resistor. A current considerably smaller than one millionth of the downstream current may flow through the sensor on the return journey to the generator. This can be on the order of microamperes, generating microvolts in the sensing resistor used for detection.

[0038] The output of the RF current sensing resistor may be connected to a signal processing unit. The signal processing unit provides information about the remotely sensed RF spectrum that has not been reported in conventional remote RF plasma sensor designs. The measurements include a) phase between harmonics and fundamental frequencies, b) harmonic spectrum analysis in pulsed RF and frequency-tuned plasma processes, c) RMS detection of arcs, and RMS detection of individual pulsed RF profiles.

[0039] Statistical methods based on spectral fingerprinting of known "healthy" plasma process states may be used. Phase and amplitude variations of spectral components are analyzed, and a fault score is assigned to each new process measurement. Thus, thresholds can be configured to inform the user of detected process faults. Phase measurements are particularly susceptible to small changes in plasma chemistry and plasma impedance. The ability to measure interharmonic phases makes the sensor according to this teaching a very useful diagnostic tool for detecting subtle process changes occurring during low-open-area etching, for example, when standard endpointing techniques are currently insufficient.

[0040] As is known in the art, in a well-shielded RF system, the current flowing through the central conductor is shielded by the current flowing through the surrounding earth shielding. These currents cancel each other out so that the system does not "radiate" RF energy. This invention utilizes the discontinuity of the earth shielding. Unlike the prior art inductive loop sensors discussed previously, this sensor is inserted in the return-to-earth path in a region where the currents do not completely cancel each other out, enabling the measurement of the return-to-earth current.

[0041] Moving on to Figure 2, which shows the sensor placement location according to this teaching in more detail. In modern plasma tools, viewport sizes are miniaturized, and RF shielding is added to minimize RF leakage through the viewport. Photodetectors always require a viewport, but there are many regions in the plasma system where the harmonic spectrum of the RF current can be determined if a gap exists in the ground shielding or if it is added. To demonstrate the capabilities of the present invention, the viewport will be used as an illustrative case. Nevertheless, it will be understood that the sensor according to this teaching may be placed across any suitable opening in the ground path.

[0042] Figure 2 shows two diagrams of the plasma chamber 104. In particular, Figure 2 shows the RF current entering the plasma chamber 104, i.e., the central arrow 201, and the RF current beginning its return path through the chamber wall, i.e., the peripheral arrow 202. Two viewports are shown. The first viewport 203 is fitted with a sensor (not shown) according to this teaching. The second viewport 204 does not have a sensor. The second viewport 204 without a sensor does not have a complete conduction path to allow current to flow through the viewport cavity, i.e., its front is made of glass. Nevertheless, when a sensor is fitted to the first viewport 203, a path 205 is created for the flow of RF current across or through the viewport. This current flow can be measured with the sensor according to this teaching.

[0043] The orientation of the sensor installed in viewport 203 is important for detecting the RF current, as will be explained in more detail with reference to Figure 3. Figure 2 shows that the RF current travels mainly vertically to and through the chamber electrodes and plasma volume in a capacitively connected plasma chamber 104, i.e., the central arrow 201. Therefore, the return current must flow mainly vertically down the chamber wall in the reverse direction, i.e., the peripheral arrow 202. There will also be some current flow from the plasma to the wall in other directions, including horizontally in Figure 2, i.e., arrow 206.

[0044] Moving to Figure 3, which depicts the sensor 301 according to this teaching, mounted across viewport 203, with the RF return current flowing from direction A to B. Direction A to B corresponds to the current flow 202 shown in Figure 2. Theoretically, there should be less RF current flowing from the indicated direction C to D (this direction corresponds to the current flow 206 in Figure 2). As is known to those skilled in the art, in complex plasma processing systems, there may be currents flowing in several directions, so it is possible to orient the sensor to measure the current of interest.

[0045] Figure 3 also depicts the internal configuration of sensor 301. As previously mentioned, sensor 301 includes a sensing resistor R1. The grounding path is constructed from conductive / metallic materials and surfaces with very low resistance. Therefore, the sensing resistor R1 must have a low resistance value to allow sufficient current to flow through the sensor for detection. R1 in Figure 3 has a resistance value of less than 1 Ohm. A voltage difference proportional to the current flow is generated across the resistor R1, and this voltage difference is measured. The circuit converts the RF current into a voltage signal V OUT The circuit further includes an amplifier U1 for conversion. Specifically, Figure 3 shows a measurement circuit comprising a low-value resistor R1 in series with the direction of current flow and a differential amplifier U1 for detecting the voltage drop across R1. It should be understood that the amplifier output is not limited to a voltage signal, but can be any RF output, such as a current signal.

[0046] The resistive sensing element R1 and amplifier U1 are key components of the sensor's analog front end. The analog front end may be housed in a grounded metal enclosure to shield the analog circuitry from electric and magnetic fields that may be emitted through the viewport. However, this is not essential, and a non-metallic enclosure may be used. The output of U1 may be coupled to a digitization circuit for processing and analysis. The major advantages of using a resistive sensing element are a) a flat response with respect to frequency, unlike inductive loop sensors; b) stable resistance over a wide temperature range, unlike inductive loop sensors; and c) easier achievement of common-mode blocking compared to inductive loop sensors.

[0047] It should be understood that any RF current sensing element may be used. The inventors have found the sensing resistor described above to be the most convenient element, but low-impedance capacitors or inductors may also be used instead of resistors. If capacitors or inductors are used, an alternative sensing circuit will be required. To sense the current in the return path, other methods may be applied, namely, Hall effect sensors in the shunt connector or current transformers around the shunt connector.

[0048] Analog voltage output V OUT This is an alternating current (AC) signal in the RF band. A signal processing unit is used to extract the frequency spectrum in a form that can be analyzed and visualized in a useful manner. A coaxial cable carries the AC signal to the signal processing unit. An ADC is used to sample the current waveform. Typically, a data block of 512 samples is recorded as the first step. The block size can be arbitrarily chosen and varied to meet various requirements. The data block is transferred to a field-programmable gate array (FPGA), where a fast Fourier transform (FFT) is performed. The FFT converts the time-domain AC waveform into a frequency spectrum. The frequency spectrum is sent to a microprocessor for further processing, including storage and averaging. Multiple FFTs are averaged together to reduce the signal-to-noise ratio.

[0049] A practical example of the sensor according to this instruction is provided with reference to Figure 4. For comparison, the sensor can incorporate a separate E-field detector to measure the electric field emitted through the viewport. As mentioned above, the sensor does not measure the E-field. The E-field detector provides a reasonable reference point for measuring the sensor current scale. To demonstrate the function of the sensor, it was mounted on a capacitively coupled plasma reactor shown in Figure 1. In this example, the reactor has two parallel plate electrodes, each 300 mm in diameter. RF power is supplied at 13.56 MHz. The background gas used to form the plasma is argon at a flow rate of 100 SCCM and held at a pressure of approximately 2 Pa. Figure 4 shows the amplitude of the basic 13.56 MHz frequency component plotted as a function of time, while the RF power from the generator is increased from 20 W to 150 W. The current profile tracks the voltage profile very well. The data is uncalibrated, and the current scale is approximately 10 8 It is a single data unit.

[0050] Specifically, Figure 4 shows the sensor-measured amplitude of the fundamental (13.56 MHz) component of the RF current spectrum compared to the fundamental component of the RF voltage spectrum emitted from the window. Changes in processing power can be easily identified by the sensor. The sensor orientation is from A to B, as shown in Figure 3.

[0051] Figure 5 shows the basic current amplitude measurements from the sensor compared to the E-field probe measurements, with the sensor positioned in the orientation C to D shown in Figure 3. The voltage amplitude is, as expected, very similar to that shown in Figure 4, since the E-field probe is not affected by orientation. Nevertheless, the current amplitude is low 10 7 The current dropped significantly to this magnitude. This confirms that the sensor is truly susceptible to the effects of ground current, and the expected drop in current is observed in the other orientation of the installed sensor.

[0052] Figure 6 shows how the oscillation of the basic current signal amplitude correlates with the faulty pressure value on the gas supply line, and the pressure value demonstrates the ability of the sensor / probe according to this teaching to monitor the condition and performance of the plasma process.

[0053] Therefore, this teaching provides a sensor for detecting the RF current spectrum in a plasma system from a non-invasive location outside the plasma chamber. The processed signal can be used to determine the condition and stability of the process.

[0054] The sensor comprises a shunt connector with an RF current sensing element mounted across an opening in the return path of the plasma system, such as a viewport. The output of the current sensing resistor is coupled to a measurement system for sampling the RF current waveform, which is then digitized and converted into Fourier space for analysis of the harmonic frequency spectrum. The amplitude of each harmonic component, as well as the phase angle of each harmonic component relative to the fundamental frequency, are measured. Phase measurements are particularly susceptible to changes in the RF impedance of the plasma.

[0055] The present invention is not limited to the embodiments described herein and can be modified or altered without departing from the scope of the invention.

Claims

1. 1. A system for non-invasive sensing of a radio frequency current spectrum, comprising: a plasma processing chamber; a plasma generator; a shunt connector having a resistor; Equipped with The system wherein the shunt connector is mounted across an opening in a return path between the chamber and the generator.

2. The system of claim 1 , wherein the system is configured to detect only current in the ground return path.

3. The system of claim 1 or 2, wherein the system is configured such that current flowing through the ground return path generates a voltage across the resistor.

4. 3. The system of claim 1 or 2, further comprising an amplifier configured to sense the voltage drop across the resistor and output an RF signal.

5. The system of claim 1 or 2, wherein the shunt connector is configured to mount across a viewport of the chamber.

6. The system of claim 4 , further comprising a digitization circuit configured to extract the RF signal from the amplifier and convert the RF signal to a digital signal for processing and analysis.

7. The system of claim 4 , wherein the output RF signal is an alternating current signal in the RF band.

8. The system of claim 4 further comprising a housing enclosing the resistor and amplifier.

9. The system of claim 1 or 2, wherein the system is configured such that the shunt connector and the resistor create a path for current to flow as part of the ground return path.

10. 10. The system of claim 9, wherein the path created by the shunt connector and the resistor is oriented in the same direction as current flow in the ground return path.

11. The system of claim 1 or 2, wherein the shunt connector comprises at least one of a ground shunt strap, a cable, a bar, and a rod.

12. 1. A method for non-invasive sensing of a spectrum of radio frequency current flowing in a plasma processing chamber, comprising: providing a shunt connector having a resistor; Mounting the shunt connector across an opening in a ground return path between the chamber and a plasma generator. A method comprising:

13. The method of claim 12 further comprising detecting only the current in the ground return path.

14. 14. The method of claim 12 or 13, wherein current flowing through the ground return path generates a voltage across the resistor.

15. The method of claim 12 , further comprising mounting the shunt connector across a viewport of the chamber.

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