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Inductively Coupled Plasma
Introduction
Plasma chambers play a pivotal role in diverse plasma-based processes, serving as controlled environments where ionized gases generate reactive species essential for applications in semiconductor manufacturing, materials synthesis, and surface treatments. Among various plasma chamber configurations, Inductively Coupled Plasma (ICP) chambers stand out for their distinctive attributes. ICP chambers utilise radiofrequency induction to create a high-density plasma, characterized by elevated electron temperatures and efficient ionization.
The versatility and precision offered by ICP chambers make them indispensable tools in advancing technologies across multiple industries. In this article we will discuss the working principle behind ICP chambers, as well as some of their advantages and specific applications over other plasma chamber types.
Understanding ICP Chambers
ICP (Inductively Coupled Plasma) chambers work on the principles of radiofrequency (RF) energy coupling to a gas leading to the creation and control of a highly ionized plasma. The coupling of RF energy to the plasma in an ICP chamber occurs using an antenna or coil surrounding the chamber. This antenna can be arranged either cylindrical, planar or immersed in ferrite cores (see figure 1). An RF power source supplies power to the antenna, generating an oscillating magnetic field to ionise the gas in the chamber, forming a plasma. For applications such as etching and material depositions, there is often another powered electrode that a substrate sits on to further control the energy of ions accelerated across the plasma sheath to hit the substrate (see our article on the IEDF plasma parameter for more info). This electrode setup allows for controlled and uniform plasma distribution across the chamber, crucial for consistent material treatments.
Figure 1. Arrangement of antenna in ICP chambers (a) cylindrical, (b) planner and (c) immersed in ferrite cores
In an inductively coupled plasma, power is transferred from the electric fields to the plasma electrons within a skin depth layer of thickness d near the plasma surface by collisional (ohmic) dissipation and by a collisionless heating process in which bulk plasma electrons “collide” with the oscillating inductive electric fields within the layer. This inductive coupling of RF energy ensures a high-density plasma within the chamber, where charged particles and ions are abundant, enabling efficient ionisation and sustaining the plasma state. Typically, an ICP has a higher plasma density than an Capacitively Coupled Plasma (CCP, see article for more information) as the higher electron temperatures promote increase the number of ionised species.
In operation ICP chambers take advantage of different plasma modes—capacitive (E) and inductive (H) modes. At low RF power, it is hard to maintain an inductively coupled plasma, so the ICP plasma is maintained in E mode, through the current passing through the antenna producing a voltage drop and an electrostatic field between the antenna and plasma. Plasma of this kind exhibits low plasma density and not much light emission. As the RF power is increased, the ionisation density will be increased, shortening the skin depth of the plasma. Inductive power is transferred over the skin depth of the plasma, so once that length is comparable to the size of the plasma chamber, the plasma will undergo a sudden transition from E to H mode. The plasma’s density will increase drastically at this point, with significantly more light emission as the H mode provides a more efficient power transfer to the plasma. As the RF power is increased further, the power efficiency reaches a turning point as the skin depth of the plasma is decreased and the power is transferred to a smaller and smaller region in the plasma. A plot of this power transfer efficiency against plasma density can be seen in figure 2.
Figure 2. Calculated power transferred by the capacitive (dotted curve) and inductive couplings (dashed curve), and the total transferred power (solid curve), at 50 mTorr argon pressure and 0.3 A coil current. In regions I and II, power is predominantly transferred by the capacitive coupling. In regions III and IV, power is predominantly transferred by the inductive coupling. (Lee et al., 2006)
The H mode features an increased magnetic field that is induced by the RF antenna and plays a vital role in confining the plasma within the chamber and controlling the plasma spatially. Moreover, the confined plasma allows for better control over its characteristics, such as density and uniformity, enabling precise material treatments and processing across diverse applications.
Advantages of ICP chambers
As previously mentioned, ICP excels in generating high-density plasmas compared to other RF plasma techniques such as Capacitively Coupled Plasma (CCP). High-density plasmas can achieve higher etching or deposition rates due to increased ionization. This makes processes faster and more efficient, which is highly beneficial in industrial applications to make faster product. They also tend to improve the uniformity of plasmas, due to having a more uniform ion flux and energy distribution across substrates.
Another advantage of ICP sources is the decoupling of ion density and ion energy. In CCP systems, the same electric field that creates the plasma also accelerates the ions, making it challenging to control ion energy independently of ion density. However, in ICP systems, the ion density is primarily controlled by the inductive power (RF coil), while the ion energy can be independently controlled using a separate bias power applied to the substrate. This independent control allows for much finer tuning of process parameters, enabling more precise etching profiles, reduced substrate damage, and improved control over thin film properties.
ICP chambers are known for their superior uniformity in plasma generation over large areas. This uniformity is crucial in semiconductor fabrication, where consistent processing across an entire wafer is essential. The ability of ICP systems to maintain uniform plasma conditions is a significant advantage, especially as semiconductor wafers increase in size. Additionally, the design of ICP chambers allows for scalability – the systems can be relatively easily scaled up for larger substrates without losing uniformity or efficiency.
The high-density plasma in ICP chambers often operates at lower pressures. This reduced pressure operation decreases the likelihood of contamination from residual gases and reduces the extent of physical sputtering of the electrode material, which can contaminate the substrate. Moreover, the ability to control ion energies independently helps in minimizing ion-induced damage to the substrates, which is particularly important in sensitive applications like microelectronics manufacturing, where even minor damage or contamination can significantly impact the device performance.
Applications of ICP chambers
In semiconductor fabrication, ICP chambers are pivotal in etching and deposition processes. The high plasma densities achievable in ICPs, often in the order of 10^11 – 10^12 ions/cm^3, enable rapid material removal rates, essential for efficient manufacturing. The ability to independently control ion energy and plasma density facilitates the creation of highly precise etching profiles with minimal substrate damage, critical in fabricating nanometer-scale features on semiconductor wafers. ICP etching is used for advanced photolithography processes, gate etching, and creating structures in integrated circuits, MEMS devices, and photonics, where control over aspect ratios and feature fidelity is paramount.
ICP chambers are extensively used for depositing thin films in electronics, optics, and mechanical applications. The energetic and dense plasma environment, characterized by a high electron temperature (typically 1-10 eV) and low ion energy, enables efficient, uniform deposition across large substrate areas. This method is employed for depositing a diverse array of materials, including metals, insulators, and complex compounds, under controlled conditions to ensure uniform film thickness and compositional integrity. Applications include depositing conductive layers, dielectrics, and barrier films for photovoltaics, flat-panel displays, and protective coatings in high-wear environments.
ICP sources are a cornerstone in ICP Mass Spectrometry (ICP-MS) and ICP Optical Emission Spectroscopy (ICP-OES), thanks to their high-temperature (6000-10000 K) and energetic nature. These conditions are ideal for the efficient ionization of samples, leading to heightened sensitivity and precision in trace element detection. The high plasma density ensures a larger population of ionized species, resulting in enhanced detection capabilities. This is particularly valuable in environmental monitoring for detecting pollutants, in food safety for identifying contaminants, and in clinical research for trace element analysis.
The high-density plasma generated in ICP chambers is exploited in materials science for synthesizing and modifying materials. The plasma’s energetic environment is conducive to forming nanoparticles and complex materials with unique electronic, optical, or mechanical properties. Surface modification techniques leverage ICP to alter surface chemistry, roughness, or other physical properties, which is crucial in developing advanced materials for applications in aerospace (e.g., coating turbine blades), biomedical implants (e.g., enhancing biocompatibility), and nanotechnology (e.g., fabricating carbon nanotubes or graphene).
Process Control and Optimization in ICP
ICP processes necessitate meticulous control over numerous operating parameters to tailor plasma characteristics. External parameters like gas composition, flow rates, RF power, RF substrate biasing techniques, antenna orientations and magnetic field strength profoundly impact plasma processes. Accurate control over these variables ensures desired plasma parameter values which fuel material interactions. Monitoring and adjusting parameters such as the plasma density, electron temperature, ion flux, ion energy and plasma potential are vital to ensure reproducibility and consistency in material treatments.
Advanced diagnostics tools play a pivotal role in monitoring and analyzing these parameters in real-time. Langmuir probes are widely considered the industry standard for plasma diagnostics, due to their simplicity and wide range of uses. They consist of a thin piece of wire inserted into the plasma to provide a localised measurement of bulk plasma parameters such as electron temperature, plasma potential and plasma density. Retarding Field Energy Analyzers (RFEAs) provide substrate level measurement of parameters such as the flux and energy of ions hitting the substrate, as well as a measurement of the deposition rate for thin film deposition applications. VI probes are also critical for the characterisation of RF power being delivered and reflected from the plasma source. Real-time monitoring of plasma parameters allows immediate adjustments to optimise plasma conditions, ensuring optimal process performance and desired material outcomes. These diagnostics aid in understanding the complex plasma behaviours and enable quick response to deviations, enhancing process control and reliability.
Integrating predictive models and simulations aids in pre-emptive adjustments and optimisation of ICP processes. Computational models simulate plasma behaviour, predicting how alterations in parameters would affect plasma characteristics. These predictive models aid in optimising process parameters and anticipate outcomes before actual implementation. This approach streamlines the optimisation process, allowing for efficient adjustments to achieve desired plasma conditions and material treatments.
Conclusion
In inductively coupled plasma, power is transferred to the plasma by the induction methods and no electrode exists inside the plasma. These chambers play a critical role in advancing a wide array of plasma-based applications, thanks to their capacity for generating high-density plasmas with considerable uniformity. This capability is crucial for sectors such as semiconductor processing, materials engineering, and analytical applications, where the precision and control of plasma characteristics are paramount.
Impedans provides a comprehensive portfolio of diagnostic tools that enable precise measurement and analysis of plasma parameters, facilitating the fine-tuning of plasma conditions to achieve desired outcomes. Researchers and practitioners aiming to enhance the efficiency of their ICP operations are encouraged to reach out to one of our expert plasma engineers. By leveraging advanced diagnostic capabilities, stakeholders can gain deeper insights into plasma dynamics, leading to improved process control and innovation in plasma applications.
References
1. Lieberman, M. A. and Lichtenberg, A. J., Principle of Plasma Discharges and Materials Processing, 2nd ed. (Wiley, New York, 2005).
2. Godyak, V. A. (2011). Electrical and plasma parameters of ICP with high coupling efficiency. Plasma Sources Science and Technology, 20(2). https://doi.org/10.1088/0963-0252/20/2/025004
3. Hyo-Chang Lee; Review of inductively coupled plasmas: Nano-applications and bistable hysteresis physics. Appl. Phys. Rev. 1 March 2018; 5 (1): 011108. https://doi.org/10.1063/1.5012001
4. Min-Hyong Lee, Chin-Wook Chung; On the E to H and H to E transition mechanisms in inductively coupled plasma. Phys. Plasmas 1 June 2006; 13 (6): 063510. https://doi.org/10.1063/1.2212387
