RF Parameters

Title: RF Parameters in Plasma: Understanding and Optimizing for Efficient Processing 

Introduction: 

Radiofrequency (RF) parameters play a crucial role in controlling plasma processes, which are fundamental in numerous industrial applications, from semiconductor manufacturing to advanced material synthesis. Understanding and optimizing RF parameters are essential for achieving efficient and reliable plasma processing. In this article, we will explore the significance of RF voltage, current, phase, impedance, and power in controlling plasma properties and how accurate measurement and precise control of these parameters are essential for successful plasma processing. 

 Section 1: RF Power Delivery Network 

RF power refers to the alternating current (AC) electrical energy supplied to the plasma during plasma processing. Unlike direct current (DC) power, RF power has a high frequency, typically in the range of 13.56 MHz or higher, and is commonly used to create and sustain the plasma state.  

RF power being delivered into a plasma system can be thought of as a wave. If the impedance (complex resistance) of the plasma system, where power is being delivered into, is vastly different from that of the RF generator (which is chosen to be 50 Ohms for historical reasons), then the wave will be reflected, and less power will be delivered to your plasma system. When the impedance is matched, the maximum amount of RF power is transferred from the generator to the plasma chamber. This maximizes the energy available for plasma generation and maintenance, leading to higher plasma densities and more effective processing. This is the purpose of the RF power delivery network; to match the impedance of your system to the impedance of the RF generator. 

The network consists of the RF generator, a matching network whose primary function is to match the impedance of the generator to that of the plasma load, and the plasma load itself. These components are connected via high quality RF cables which are designed to minimise power loss and signal degradation. Changes to the network, such as the wearing down of cables or the degradation of the matching network, will directly impact the power delivered to the plasma, and therefore impact the plasma process itself. 

Section 2: RF Voltage 

RF voltage refers to the amplitude of the alternating electric field applied to the plasma and therefore affects the overall plasma behaviour. Higher RF voltages provide more energy to the electrons, increasing their mobility and the likelihood of ionizing neutral particles. Consequently, higher RF voltages leads to a higher density of ions in the plasma. Similarly, higher electron energies will lead to higher plasma temperatures which is crucial for the control of plasma chemistry and processes. 

RF voltage influences the energy distribution of ions within the plasma. Higher RF voltages can accelerate ions to higher energies, resulting in a broader energy distribution. This may lead to a wider range of ion energies bombarding a substrate during processes like etching or deposition. An uneven ion energy distribution can cause non-uniform material removal or film deposition rates, impacting the quality and precision of the process. 

RF voltage significantly affects the uniformity of the plasma density across the processing chamber. Uniformity is critical for consistent processing outcomes, as it ensures that all areas of the substrate or sample experience similar plasma effects. 

Section 3: RF Current 

RF current is the alternating current flowing between the powered electrodes in the plasma processing chamber It is an essential parameter in plasma processes as it directly influences the behaviour of charged particles (ions and electrons) within the plasma.

RF current is directly related to the ion density in the plasma. As the RF current increases, more electrons are driven towards the positively charged electrode, leading to an increase in the number of ions formed through ionization. Consequently, higher RF currents result in higher ion densities within the plasma. 

RF current governs the flow of charged particles between the electrodes, leading to an increased ion flux to the electrodes within a plasma. A higher RF current results in a greater number of ions being accelerated towards the substrate or wafer surface. This enhanced ion flux is critical for improving the material removal rate in processes like plasma etching, or increased deposition rate in processes such as plasma-enhanced chemical vapor deposition (PECVD). 

RF current influences the energy of ions within the plasma. As the RF current increases, the ions experience stronger interactions with the electric field, leading to higher energies. Higher ion energies are desirable for certain plasma processes, such as ion implantation or reactive ion etching, as they result in more effective material removal and surface modification. This also influences the charge accumulation on the substrate or wafer surface. When ions bombard the surface, they can cause charge build-up, leading to surface charging effects. Surface charging can affect the performance of processes like plasma etching and deposition, and in extreme cases, it may lead to damage or arcing in the plasma chamber. 

Section 4: RF Phase 

RF phase refers to the relationship between the voltage and current in the RF power delivery to the plasma. It describes the timing difference between the voltage and current waveforms. In an ideal scenario, the voltage and current are perfectly in phase, meaning their waveforms align, resulting in maximum power transfer and efficient plasma generation. However, in practice, due to various factors, the voltage and current may not be perfectly in phase, leading to phase shifts. 

Phase control is essential for generating and sustaining a stable plasma in the processing chamber. A stable plasma is characterized by consistent and predictable plasma properties, which is critical for reliable and repeatable plasma processes. Proper phase control ensures that the RF voltage and current are in sync, enabling efficient power transfer and plasma excitation. 

During the positive half-cycle of the RF voltage waveform, electrons are accelerated, leading to ionization and plasma generation. The generated ions then participate in plasma processes like etching or deposition. In the negative half-cycle of the RF voltage waveform, the RF current maintains the plasma’s conductivity and prevents plasma extinguishment. 

By adjusting the phase angle, researchers can optimize the timing of plasma excitation relative to the voltage waveform. This fine-tuning allows for precise control over the plasma properties, ensuring a stable and sustained plasma discharge. Moreover, phase control enables researchers to adjust the ratio of time spent in the positive and negative half-cycles of the RF waveform, affecting the duty cycle of the plasma. 

Phase synchronization also significantly impacts the uniformity of the plasma density and temperature across the processing chamber. When the RF voltage and current are in sync, the energy transfer to the plasma is efficient and uniform. This leads to a more uniform plasma density distribution, ensuring consistent processing results across the substrate or wafer surface. 

Phase synchronization affects the energy of ions within the plasma. An in-phase condition ensures that ions receive a consistent amount of energy during each half-cycle of the RF voltage waveform. This results in a more uniform ion energy distribution, contributing to better control over ion bombardment during processes like etching. A uniform ion energy distribution can improve material removal rates and enhance etch selectivity. 

Section 5: RF Impedance 

RF impedance refers to the opposition encountered by the RF source (RF generator) when interacting with the plasma. It represents the combined resistance and reactance to the flow of RF current within the plasma chamber. RF impedance is a complex quantity, usually expressed in ohms, and it is influenced by the properties of the plasma, such as its density, temperature, and composition, as well as the geometry of the plasma chamber. 

The interaction between the RF source and the plasma occurs through the matching network. The matching network serves as an intermediary between the RF generator and the plasma load (plasma chamber), and its primary purpose is to achieve impedance matching. Impedance matching ensures that the RF source “sees” an impedance that matches its own internal impedance, resulting in efficient power transfer and optimal plasma performance. 

Matching the RF impedance is of utmost importance for efficient power transfer from the RF generator to the plasma. When the RF impedance is matched, maximum power is transferred to the plasma, leading to higher plasma densities, increased ionization, and overall more effective plasma processing. 

Efficient power transfer is essential for stable plasma operation. Impedance mismatch can lead to reflected power, which not only wastes energy but also destabilizes the plasma discharge. Reflected power can result in standing waves and unwanted voltage fluctuations, leading to plasma instabilities and potential damage to the RF generator and other components in the power delivery network. The plasma instabilities also lead to process variability and inconsistent plasma performance. 

Small changes in the plasma chamber environment can significantly affect the plasma’s impedance, potentially leading to impedance mismatch. Some factors influencing the plasma’s impedance include the plasma density, the gas composition, the pressure and the temperature. 

The instability caused by impedance mismatch can result in non-uniform plasma density, ion energy, and other plasma properties across the processing chamber. As a consequence, the plasma etch or deposition rates become non-uniform, impacting the overall process uniformity and quality of the final product.

Section 6: RF Power 

RF power refers to the electrical power supplied to the plasma during plasma processing. It is a function of the voltage, current and phase of the RF signal given by the equation below, where P is the RF power, V the voltage, I the current and θ the phase between the current and voltage waveforms. RF power is a critical parameter in plasma processes as it directly controls the energy available for ionization and plasma excitation, influencing various plasma properties and processing outcomes. 

P=V*I*Cosθ

Higher RF power leads to a more significant ionization rate, as it provides more energy to electrons, allowing them to collide with neutral atoms and molecules more frequently, resulting in increased ionization. Consequently, higher RF power results in higher plasma density. 

The energy provided by RF power increases the kinetic energy of the particles within the plasma, leading to a rise in plasma temperature. Higher RF power results in higher plasma potentials, which can lead to higher ion energies.

The RF power significantly affects the etch rates in plasma etching processes, as it will increase the ion energies and flux of ions to the substrate surface, resulting in faster material removal rates. Controlling the RF power allows researchers and engineers to tailor the etch rates for specific materials and achieve the desired etch depths. 

In plasma-enhanced chemical vapor deposition (PECVD) processes, RF power is crucial for film growth and quality. Higher RF power can enhance the dissociation of precursor gases, leading to more efficient deposition and better film properties, such as adhesion and uniformity. However, excessive RF power may also cause damage or roughness on the film surface, so optimization is necessary to achieve the desired film quality.

Section 7: Measurement and Control of RF Parameters 

In plasma processes, various techniques and instruments are utilized to measure crucial RF parameters such as voltage, current, phase and impedance etc. RF probes provide localized information about RF voltage and current within the plasma chamber, aiding in diagnosing plasma behaviour. Impedance analysers offer detailed data on complex impedance, allowing researchers to optimize impedance matching for efficient power transfer. Power meters are essential for monitoring the total RF power delivered to the plasma, ensuring stability and performance assessment. Directional couplers separate forward and reflected power components, enabling researchers to detect impedance mismatches and assess power transfer efficiency.

Real-time monitoring and control of RF parameters during plasma processing are of utmost importance. Rapid detection and correction of deviations contribute to process stability and reproducibility. Continuous adjustment of RF parameters optimizes plasma conditions, enhancing power transfer efficiency and overall process performance. Moreover, real-time monitoring ensures safety by identifying potential issues that could lead to plasma instabilities or damage to RF generators. Measuring the RF parameters of a plasma process provides a non-invasive and invaluable way of doing this. By monitoring RF voltage, current, and phase, researchers can identify wafer misplacement, clean and etch endpoints, or other anomalies that may affect processes. From identifying these issues in real time, process engineers can mitigate wafer loses and minimise tool downtime. 

Power meters and directional couplers, two common RF meters, have limitations in their power measurements. Power meters provide average power readings, which may not capture rapid changes during pulsed RF operations, leading to inaccuracies. Their frequency response may also be limited, affecting measurements at certain RF frequencies. Calibration errors and the presence of standing waves can introduce uncertainties in forward and reflected power measurements. Relying solely on these measurements from the power generator may not accurately represent the power delivered to the plasma. 

Section 8: Impedans’ Solution for RF Parameter Measurement 

Impedans Ltd is renowned for its expertise in developing advanced diagnostic solutions for accurate RF parameter characterization. Their Octiv devices utilize both non-invasive and in line current and voltage pickups, ensuring safe and effective measurements. Octiv devices offer precise measurements of RF voltage, current, and phase, enabling exact calculations of both forward and reflected power. From these values, researchers can gain valuable insights into plasma behaviour, optimise processes, and ensure stable power delivery. 

Octiv devices utilise state of the Impedans monitoring software as well as offering API control options for 24/7 monitoring of RF parameters across multiple process recipes or steps. They can be calibrated for a wide range of frequencies, and are able to monitor up to 5 frequencies with multiple harmonics for each frequency at once. Harmonic signals arise due to the non-linear response of plasma to RF power, and as such are very sensitive to small changes within a plasma chamber. Harmonics have been demonstrated to be able to fingerprint specific issues arising within a plasma process. In addition to measure RF parameters to provide insights into the plasma processes, Octiv sensors also can be used to set up alarm system for fault detection and chamber maintenance.

Conclusion: 

RF parameters, including voltage, current, phase, impedance, and power, significantly influence plasma processes and their outcomes. Accurate measurement and control of these parameters are crucial for achieving desired plasma properties and successful plasma processing. Impedans Ltd’s expertise in RF parameter characterization, through their Octiv sensors, empowers researchers and engineers to optimize their plasma processes effectively. By leveraging advanced diagnostic tools and solutions, researchers can pave the way for advancements in plasma-based technologies. 

For more information click here RF Voltage-Current (VI) Probes | Impedans