Plasma Parameters

Introduction: #

Understanding and characterizing plasma behaviour is essential in various scientific, technological, and industrial applications. Plasma parameters, such as electron density, temperature, and composition, play a crucial role in unravelling the complex dynamics of a plasma. These parameters provide valuable insights into plasma stability, energy transfer, and particle interactions, enabling scientists and engineers to predict and control plasma behaviour with precision. Moreover, accurate measurement and analysis of plasma parameters drive advancements in fields such as semiconductor device manufacture, fusion energy, materials processing, and space science.

Plasma Potential #

The concept of plasma potential refers to the electric potential difference between a plasma and a reference point, such as a surrounding material or a sheath boundary. It represents the average electrostatic energy experienced by charged particles within the plasma, such as electrons and ions. The plasma potential plays a crucial role in determining the energy levels of these particles as they will interact with the electric field generated by this potential. The potential difference affects their motion and distribution of energy. Electrons, being lighter, tend to respond more rapidly to changes in the plasma potential compared to heavier ions. The plasma potential therefore influences the energy exchange between particles, resulting in the formation of energy distributions which in turn influences various plasma phenomena.

The plasma potential will also determine the direction and magnitude of particle flows within the plasma. When the plasma potential is positive relative to the surrounding environment, electrons have higher energy levels than ions, leading to a flow of electrons towards the surrounding surfaces. Conversely, when the plasma potential is negative, ions possess higher energy levels, causing an ion flux towards the surfaces. This charge separation and particle flow affect plasma sheath formation, plasma-surface interactions, and can even lead to the formation of electric fields and instabilities within the plasma. This influences processes such as plasma etching, thin-film deposition, and plasma propulsion.

To delve deeper into the concept of plasma potential and its significance, readers are encouraged to explore Impedans’ Plasma Potential  article which expands on the material touched on in this section.

Debye Length #

The Debye length is a fundamental parameter in plasma physics that characterizes the spatial extent of electric field screening within a plasma. It represents the distance over which the electric field of a charged particle is significantly reduced by the collective response of other charged particles in the plasma. The Debye length, commonly denoted by the symbol λ_D, is inversely proportional to the square root of the electron density and the square root of the temperature. The Debye length determines how far the influence of an individual charged particle extends within the plasma and the extent to which electric fields can penetrate the plasma before being screened.

Quasi-neutrality is a fundamental characteristic of plasmas, wherein the number of positive and negative charges in a plasma are roughly equal, resulting in an overall neutral charge. The maintenance of quasi-neutrality is crucial for the stability and behaviour of a plasma. The Debye length plays a significant role in achieving this. Due to the collective behaviour of charged particles, any charge imbalance within a region of the plasma would cause local electric fields to arise and redistribute the charges to restore quasi-neutrality. The Debye length determines the spatial range over which this charge redistribution occurs, ensuring that the plasma remains electrically neutral on average. If the size of a perturbation or charge imbalance is larger than the Debye length, it will be effectively screened, leading to the restoration of quasi-neutrality and the suppression of these electric fields.

Another key consequence of Debye shielding is the formation of Plasma, or Debye sheaths at the interface to a material surface. This sheath is a layer of charge imbalance in the plasma that will be several Debye lengths thick, and is formed due to the difference in moving speeds between light electrons and heavier ions. The Debye sheath drives plasma-surface interactions that is used in a wide variety of industrial methods such as thin film deposition, etching and surface treatments of materials.

To further explore the concept and significance of the Debye length in plasma physics, readers are encouraged to refer to Impedans’ Debye Length article. which expands further, and delves into more detail on the material in this section.

Plasma Density #

Plasma density is a crucial parameter in determining the number of charged particles present in a plasma. It represents the concentration of charge-contributing electrons and ions within the plasma volume. The plasma density directly influences the behaviour and characteristics of the plasma. A higher plasma density implies a greater number of charged particles, resulting in more frequent collisions and interactions among them. This, in turn, affects various plasma phenomena, such as energy transfer, particle diffusion, and reaction rates.

Reaction rates in a plasma are influenced by the density of reactant particles. A higher plasma density increases the chances of reactant particles colliding and undergoing chemical reactions, leading to enhanced reaction rates. Similarly, ion flux, which represents the flow of ions onto a substrate or surface, is directly proportional to the plasma density. A higher plasma density results in a greater number of ions bombarding the surface, influencing etch and deposition rates. Controlling the plasma density is crucial in plasma processing to achieve the desired reaction rates and control material removal or deposition rates. Accurate measurement of plasma density, using techniques such an Impedans Langmuir probe, enables researchers and industry professionals to optimize plasma processes, control reaction rates, and tailor etch/deposition rates according to specific requirements.

For readers interested in gaining more in-depth information about the importance and measurement of plasma density, Impedans’ Plasma Density article is a valuable resource that describes this property in more detail.

Electron Energy Distribution Function #

The velocity of an electron is directly related to its energy. According to classical mechanics, the kinetic energy of a particle is given by , where E is the kinetic energy, m is the mass of the particle, and v is its velocity. Therefore, the velocity of an electron influences its energy. Higher velocities correspond to higher kinetic energies. In a plasma, electrons with higher energies can exhibit different behaviours, such as increased collision frequency, more frequent energetic reactions, or the ability to overcome energy barriers during interactions with other particles.

Analysing the electron energy distribution in a plasma provides valuable insights into its properties and behaviours. The shape and characteristics of the distribution function reveal information about the temperature, density, and energy transfer mechanisms within the plasma. It helps to identify the dominant processes governing electron behaviour, such as electron-electron collisions, electron-ion collisions, and energy exchange with external fields. Understanding the electron energy distribution allows researchers to optimize plasma conditions for specific applications, control reaction rates, and manipulate plasma properties to achieve desired outcomes.

The electron energy distribution is particularly crucial in driving ionization and the chemistry in a plasma. High-energy electrons in the tail of the distribution possess sufficient energy to ionize neutral species by colliding with them, leading to the production of ions and additional electrons. This ionization process plays a significant role in sustaining the plasma and creating the charged particle population. Additionally, high-energy electrons can also participate in various chemical reactions, leading to the formation of new compounds and species within the plasma. By analysing the electron energy distribution, researchers can gain insights into the ionization mechanisms and the overall chemistry of the plasma, enabling them to control and optimize plasma-based processes.

Impedans has extensive experience in measuring and analysing these electron energy distribution functions in plasmas. Their advanced diagnostic tools, such as Langmuir probes, allow for precise measurement of plasma parameters, including electron energy distributions.

Electron Temperature #

The electron temperature is a crucial parameter in understanding plasma kinetics and electron behaviour. It represents the average kinetic energy of electrons within a plasma, from the Electron Energy Distribution Function in the previous section, providing valuable insights into the energy distribution and dynamics of the electron population.

The electron temperature plays a significant role in plasma kinetics by influencing collision rates and energy transfer processes. Higher electron temperatures lead to increased collision frequencies, as electrons with greater kinetic energies are more likely to collide with other particles such as ions and neutral atoms. These collisions determine the rate of energy exchange and affect the overall plasma energy balance.

Moreover, the electron temperature affects reaction rates in the plasma. Chemical reactions in plasmas often involve electron interactions, and higher-energy electrons are more likely to participate in these reactions. Understanding the electron temperature helps in predicting and controlling the efficiency and selectivity of plasma-based chemical processes.

The electron temperature of a plasma is typically much higher than the ion or neutral temperatures due to the large differences in mass between the species. This is one of the reasons why plasma has such a wide range of uses as one can achieve similar chemistries that would otherwise require very hot materials, at relatively low ion/neutral temperatures.

Accurate determination of electron temperature using Impedans’ measurement techniques, such as their high precision Langmuir probe, has various applications in plasma diagnostics. It enables researchers and industry professionals to optimize plasma processes, understand energy transfer mechanisms, and study the behaviour of electrons within the plasma. The knowledge of electron temperature is essential for the design, control, and optimization of plasma-based technologies such as plasma etching, thin film deposition, and surface modification.

Ion Energy Distribution Function #

The ion energy distribution function (IEDF) is a crucial parameter in studying ion behaviour and plasma-surface interactions. It describes the distribution of kinetic energies of ions within a plasma and provides insights into their energy transfer processes and interactions with surfaces.

The IEDF is essential for understanding and optimizing plasma etching and deposition processes. In plasma etching, ions bombard a surface to remove material selectively. The energy of the ions plays a critical role in determining the etch rate, selectivity, and surface damage. The IEDF provides information about the energy range and distribution of ions that interact with the surface. The shape of the distribution depends on a variety of factors including the biasing of electrode voltages, the pressure of the chamber and the gas speicies used. Controlling the IEDF enables researchers to tailor processes to achieve desired results.

In plasma deposition processes, ions contribute to the growth of thin films or coatings on a substrate. The energy of the ions affects the film’s quality, adhesion, and morphology. By analysing the IEDF, researchers can optimize the ion energy to enhance film properties and control the growth mechanisms.

One possible method of measuring the IEDF in real time is through the Impedans Semion Retarding Field Energy Analyser (RFEA) which measures the energy of ions by applying an electric field that retards their motion. Below are figures of the IEDF of a plasma measured by the Semion RFEA at different plasma powers (left), pressure (middle) and different gas species (right).

Through accurate measurement and analysis of the IEDF, researchers and industry professionals can fine-tune process parameters, optimize material removal or deposition rates, and improve the quality and performance of the resulting films or surfaces.

Ion Flux #

The ion flux is a critical parameter in plasma processing and surface modification, as it plays a significant role in determining the rate and effectiveness of these processes. Ion flux refers to the number of ions per unit area that impinge on a surface over a given time. Understanding and controlling the ion flux is essential for achieving desired material removal, deposition, and surface modification outcomes.

In plasma etching, the ion flux is responsible for material removal from the substrate surface. Higher ion flux results in a faster etch rate, allowing for more efficient and precise material removal. By controlling the ion flux, researchers and industry professionals can achieve desired material removal rates, selectivity, and surface quality.

In plasma-based surface modification techniques, such as plasma immersion ion implantation and surface functionalization, the ion flux is crucial for introducing ions into the surface and altering its properties. The ion flux determines the depth of ion penetration and the density of implanted ions, which directly impact the resulting surface properties and functionalities. Precise control of the ion flux is necessary to achieve desired surface modifications and tailor material properties for specific applications.

Overall, the ion flux is a significant parameter in plasma processing and surface modification. Accurate measurement of the ion flux allows researchers and industry professionals to optimize plasma processes and surface modifications. This capability is particularly beneficial in fields such as semiconductor manufacturing, where precise control of ion flux is essential for device fabrication and surface modification.

Deposition Rate (including Ion/Neutral Fraction) #

Deposition rate and the ion/neutral fraction are crucial parameters in plasma-enhanced deposition processes, especially when it comes to thin film process matching and process transfer between different tools. These parameters play a significant role in controlling the film growth, material properties, and uniformity.

Deposition rate refers to the speed at which material is deposited onto a substrate surface during a plasma-enhanced deposition process. It determines the thickness and growth rate of the deposited film. Whilst a higher deposition rate will result in faster manufacturing times, it may also result in worse depositions with less uniformities. Therefore achieving the desired deposition rate is essential for meeting the requirements of specific applications, as different films may require different growth rates to achieve optimal properties.

The ion/neutral fraction represents the ratio of ionized species to neutral species in the plasma during deposition. It influences the film properties, adhesion, and density. The ion fraction affects the energy of the deposited species and can impact the film’s microstructure and crystallinity. Controlling the ion/neutral fraction is crucial for tailoring film properties and achieving uniform and high-quality films.

When it comes to thin film process matching and process transfer between different tools, for example if a company wanted to scale up or repeat their process on a different tool, maintaining consistent deposition rates and ion/neutral fractions is essential. Process matching ensures that films grown in different deposition systems exhibit similar properties and meet the desired specifications. It enables reliable and repeatable thin film fabrication across multiple tools and manufacturing facilities.

Precise control and understanding of the deposition parameters allow for reliable process matching and seamless transfer between different deposition tools, ensuring consistent film properties and quality.

Ion Angle/Aspect Ratio #

In plasma etching and material removal processes, the concepts of ion angle and aspect ratio play crucial roles in understanding and optimizing the etching performance and the resulting surface features.

Ion angle refers to the angle at which ions are incident on the surface being etched. It determines the direction and momentum transfer of ions during the etching process. The ion angle affects the etch profile, sidewall angle, and surface roughness. A shallow ion angle leads to a more isotropic (more even) etch, resulting in a wider sidewall angle and smoother sidewalls. Conversely, a steep ion angle produces an anisotropic (less even) etch, resulting in a more vertical sidewall profile.

Aspect ratio, on the other hand, is the ratio of etch depth to feature width. It quantifies the vertical extent of the etch in relation to the lateral dimensions as shown in the below figure. Large aspect ratios indicate deep and narrow features, while small aspect ratios represent shallow and wide features. Aspect ratio plays a significant role in determining the etch selectivity, uniformity, and resolution.

Large aspect ratios can have negative effects on plasma etching processes. They can lead to issues such as micro-loading, where the etch rate decreases with increasing feature density, resulting in uneven etch depths across the substrate. Additionally, high aspect ratios can cause sidewall passivation or redeposition, leading to decreased etch selectivity and rougher sidewall surfaces.

On the other hand, small aspect ratios can also be problematic. They can result in low etch rates and reduced etch selectivity, leading to insufficient material removal and poor pattern fidelity.

For these reasons, it is important to monitor and adjust aspect ratios to optimise them for different etch structures. By accurately characterizing ion angle and aspect ratio, one can improve control over etch profiles, sidewall angles, and surface roughness, resulting in enhanced device performance and manufacturing yield.

RF Parameters (Voltage/Current/Phase/Power/Impedance) #

RF (radio frequency) power delivery is a common method of plasma generation and control. RF power is simply AC power of a particular wavelength that is applied to the plasma through electrodes which are either capacitively or inductively coupled with the plasma. The plasma’s behaviour is highly dependent on key RF parameters, including voltage, current, phase power, and impedance of the circuit.

Voltage and current are essential parameters that determine the energy supplied to the plasma. The RF voltage applied to the plasma affects the electron energy and heating, while the RF current influences the ionization and plasma density. Controlling the voltage and current enables researchers and operators to adjust the plasma properties and optimize the process.

The phase refers to the timing and synchronization of the RF power delivery. The phase relationship between the voltage and current affects the power transfer efficiency and the plasma response. Precise control of the phase power ensures optimal power coupling and efficient energy transfer to the plasma.

The Impedance of the RF circuit plays a critical role in determining the power delivered to the plasma. Since the RF power is simply an electromagnetic wave, applying power directly to the plasma chamber will result in a large reflectance of power and the plasma will struggle to ignite and be able to be maintained. This is due to the different impedances of the power generator (typically 50 Ohms) and the plasma chamber (which can have a wide range of impedance values). This is can be thought of as similar to shining light through an interface of two media with largely different refractive indices, and getting a large portion of the wave reflected back. For this reason a matchbox is introduced to the circuit to make the plasma chamber ‘Look’ like 50 Ohms to the generator.  This impedance matching between the power source and the plasma load is crucial for efficient power transfer.

Analysing these RF parameters provides valuable insights for troubleshooting and optimizing plasma processes. By monitoring and analysing the voltage and current waveforms, researchers and operators can identify abnormalities, such as arcing or power loss, and take corrective actions. Additionally, understanding the impedance characteristics allows for effective matching network design, ensuring maximum power transfer and stability in the RF circuit.

Conclusion: #

Understanding and measuring key plasma parameters is of utmost importance for effective control and optimization of plasma processes. These parameters provide critical insights into the behaviour, characteristics, and performance of the plasma, enabling researchers and industry professionals to achieve desired outcomes and enhance process efficiency.

By accurately measuring parameters such as plasma potential, electron temperature, plasma density, ion energy distribution, ion flux, and more, it becomes possible to gain a comprehensive understanding of the plasma’s fundamental properties and behaviour. These parameters directly impact various aspects of plasma processing, including material removal rates, deposition rates, etch selectivity, surface modification, and overall process uniformity.

Impedans offers a comprehensive range of plasma diagnostic solutions and measurement techniques that directly measure these key plasma parameters. Their advanced plasma diagnostics systems, such as Langmuir probe diagnostics, retarding field energy analysers and power diagnostics, enable researchers and industry professionals to obtain accurate and real-time measurements of plasma parameters.

Whether it’s determining plasma potential, electron temperature, ion flux, or any other critical parameter, Impedans’ measurement solutions provide the necessary tools for precise characterization and control of plasma processes. These solutions offer advanced diagnostics capabilities, data analysis, and monitoring functionalities to optimize process parameters, troubleshoot issues, and improve overall process performance.

To explore further information on plasma parameters, measurement techniques, and Impedans’ advanced plasma diagnostics solutions, readers are encouraged to visit the Impedans website. The website offers a wealth of resources, including more technical articles, product information, case studies, and application notes providing valuable insights into plasma diagnostics and the applications of these techniques in various industries.