As the semiconductor industry continues to shrink critical feature sizes and improve device performance, challenges in etch processing are increasing as a result of smaller features being processed with new device structures. Higher density and higher-aspect ratio features are introducing new challenges that require additional innovation in multiple areas of wafer processing. As a result of their complexity, these innovations are increasingly reliant on comprehensive physical, chemical, and computational models of plasma etch processes.
Plasma etching is a critical process used in semiconductor manufacturing for removing materials from unit surfaces and remains the only commercially viable technology for anisotropic removal of materials from surfaces. Although plasma was introduced into nano-electric fabrication processes in the mid-1980s and transistor size has shrunk by nearly two orders of magnitude, starting at 1.0 μm to ∼0.01 μm today, the progress was mainly driven by trial and error. Unfortunately, detailed mechanisms for plasma etch processes are not well understood yet for a majority of process gasses. Therefore, the development, improvement, and validation of these mechanisms remains a constant endeavor. This would open up more opportunities for innovation in this area.
Every Last, Atomic Detail
The growing costs of etching are threatening to slow the rate of improvement for density and process speed, though manufacturing expenses can be mitigated using simulation tools. Each generation of devices requires more layers, more patterning, and more cycles of patterning that continue to increase overall cost and complexity. Even if component size can be reduced further, this presents manufacturers with additional costs in developing even more precise lithography and etching machines. This highlights the balance between atomic layer processing in high volumes and the need for a renewed approach to miniaturization in order to extend Moore’s Law.
Plasma etching takes place as part of the process of wafer fabrication, which in turn is a main process in the manufacturing procedure for semiconductors. For a wafer to be finalized, cycles must be completed potentially hundreds of times with different chemicals. Each cycle increases the number of layers and features that the desired circuit carries.
Wafers begin as pure, non-conductive, thin silicon discs generally ~6 to 12 inches in diameter. These wafers are made of crystalline silicon, with extreme attention to chemical purity before oxidation and coating can occur. Oxidation is one of the oldest steps in semiconductor manufacturing, and has been used since the 1950s. Silicon has a great affinity for oxygen, and thus it is absorbed readily and transferred across the oxide. Layers of insulation and conductive materials are then coated onto the wafer before a photoresist – a mask for etching into the oxide – can be applied. Photoresist turns into soluble material when exposed to ultraviolet light, so that exposed areas can be dissolved using a solvent. The resulting pattern is what gives engineers control at later stages like etching and doping, when devices are formed. Integrated circuit patterns are first mapped onto a glass or quartz plate, with holes and transparencies that allow light to pass through, with multiple plates masking each layer of the circuit. The aforementioned ultraviolet light is then applied to transfer patterns from photoresist material coatings onto the wafer, with the photoresist chemicals also being removed prior to etching. It is at this point that a feed gas stream – a mixture of gasses with a carrier (like nitrogen) and an etchant (or other reactive gas) – is introduced to create chemical reactions that remove materials from the wafer.
During the etching process, areas left unprotected by the photoresist layer are chemically removed. Etching generally refers to removal of materials, however it requires that photomask layers and underlying materials remain unaffected in the process. In some cases, as with anisotropic etches, materials are removed in specific directions to produce geometric features like sharp edges and flat surfaces, which can also increase etch rates and lower cycle times. Metal deposition and etching includes placing metal links between transistors, and is one of the final steps before a wafer can be completed.
Both physical and chemical attributes are present in the etching process. The active species (atoms, ions, and radicals) are generated in the electron impact dissociation reaction of feed gasses. Feed gas mixtures for plasma etching are usually complex due to the conflicting requirements on etch rate, selectivity to mask and underlayer, and anisotropy. Also, the plasma itself dissociates the feed gas into reactive species which can react with each other in the gas phase and on the surface, leading to a further cascade of species generation in the plasma.
The most common etchant atoms are fluorine (F), chlorine (Cl), bromine (Br), and oxygen (O), which are usually produced by using the mixtures of chemically reactive gasses, such as CF₄, O₂, Cl₂, CCl₄, HBr, and CHCl₃. Inductively coupled as well as capacitively coupled plasma reactors (ICP and CCP, respectively) have found the most widespread use in semiconductor manufacturing. ICP sources allow the generation of relatively dense plasmas (∼10¹⁶–10¹⁷ m⁻³) at relatively low gas pressures (1–10 mTorr). With independent wafer biasing, they also allow independent control of the ion flux and ion energy at the wafer surface. This process can be engineered to be chemically selective in order to remove different materials at different rates.
Molecular Design in Mind
One of the most important applications of plasma etching is the selective, anisotropic removal of patterned silicon or polysilicon films. Halogen atom etchants (F, Cl, Br) bearing precursors’ feedstock gasses are almost always used for this purpose. Common feedstock gasses for F atoms are CₓFᵧ, SF₆, and NF₃. The understanding of physical and chemical processes in reactive plasmas requires reliable elementary finite-rate chemical reaction mechanisms. Tetrafluoromethane (CF₄) is one of the most frequently used gasses for the generation of F atoms. The admixture of a small percentage of oxygen to a CF₄ plasma dramatically increases the etch rates of silicon surfaces, and can also be used to control the lateral etching of silicon.
Distribution of electron temperatures in an ICP reactor modeled using VizGlow™.
Tetrafluoromethane (CF₄) is an important feedgas for plasma etching of silicon. It is relatively easy to handle, non-corrosive, and has low toxicity. CF₄has no stable electronic states which means that the electron energy is spent on the generation of chemically active ions and radicals without electronic excitation losses. While tetrafluoromethane plasmas have been studied since the early development of plasma etching processes, the influence of various gas-phase and surface reactions on the densities of active species is still poorly understood.
VizGlow™is a full-featured, high-fidelity simulation tool for the modeling of chemically reactive plasmas, which are present in half of the steps undertaken in the semiconductor fabrication process described above. The characteristics of gas species and kinetic modeling of their reactions remain an area with yet unexplored potential for further innovation. Radicals created by plasmas are extremely reactive due to unpaired electrons, which is used by semiconductor engineers to speed up the process and cycle times. The same is true for deposition processes, where radicals prevent damage to the chip as it cools from the >1000 °C temperatures produced within etching equipment. Throughout these processes, defects, impurities, and nonuniformities can be detected and diagnosed with help from simulated models. Simulations using VizGlow™can help guide the design iterations to avoid operating conditions that could comprise wafers even after months of processing.
Thanks for reading! If you’re still curious about the topics discussed in this article, check out the following journal papers (and ask us for a free copy!):
Levko, Dmitry, et al. “Computational study of plasma dynamics and reactive chemistry in a low-pressure inductively coupled CF4/O2 plasma.” Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena 39.4 (2021): 042202.
Levko, Dmitry, Chandrasekhar Shukla, and Laxminarayan L. Raja. “Modeling the effect of stochastic heating and surface chemistry in a pure CF4 inductively coupled plasma.” Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena 39.6 (2021): 062204.
Levko, Dmitry, et al. “Plasma kinetics of c-C4F8 inductively coupled plasma revisited.” Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena 40.2 (2022): 022203.
Lee, Chris GN, Keren J. Kanarik, and Richard A. Gottscho. “The grand challenges of plasma etching: a manufacturing perspective.” Journal of Physics D: Applied Physics 47.27 (2014): 273001.
Kanarik, Keren J. “Inside the mysterious world of plasma: A process engineer’s perspective.” Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 38.3 (2020): 031004.
Marchack, N., et al. “Plasma processing for advanced microelectronics beyond CMOS.” Journal of Applied Physics 130.8 (2021): 080901.
Interested in learning more about plasma flow simulations? Click here to take a look at our previous article. Feel free to follow us on Twitter and LinkedIn for more related news, or reach out to us directly at email@example.com. This post’s feature image is by Laura Ockel & Unsplash.
SPIE, the international society for optics and photonics, is holding its annual Advanced Lithography & Patterning Conference in San Jose, California from April 24th-28th. This conference gathers a community of experts in semiconductor design and fabrication to review current research, discuss major breakthroughs, and network with peers.
EsgeeTech’s paper, “VizGlow-MPS: a multi-fidelity process simulator for fast, yet accurate, semiconductor process design and optimization,” is being featured as part of the conference program. The paper discusses how high-fidelity models made with our software, VizGlow™ , provide experimentally validated results for equipment operation that inform reduced-order models which predict results in a few minutes of wall-clock time. The approach constitutes a “digital twin” for process reactors with multiple levels of fidelity that a process engineer can choose from. This approach is demonstrated on c-C4F8 inductively coupled plasma and pulsed CF4/H2 capacitively coupled plasma widely used in etching applications.
Esgee’s presentation is on April 27th from 2:50 PM – 3:10 PM PDT (4:50PM – 5:10PM CST) in Convention Center room 210C.
With renewed investment in the semiconductor industry by both the American government and private investors, opportunities for the next generation of American engineers are set to soar as the United States attempts to reclaim a greater share of global production. Accelerating the increasing demands for semiconductor engineers and technical specialists is a growing shortage of qualified workers. Despite the fact that semiconductors have become a key to supporting global critical infrastructure, injecting billions into the industry alone will not solve the problem. These factors are culminating in a pivotal moment for the trajectory of our shared technological future.
Along with the increase in funding and opportunity is another technical domain that the United States can target with their investments and incentives; plasma. Plasma cleaning, etching, and deposition, which play a heavy role in the early stages of material removal (or addition) from surfaces, have created large intersections between the realms of applied engineering and chemistry. Although plasma techniques are not new to the manufacturing process, expertise in these areas has been outsourced to other stops in the global semiconductor supply chain. Plasma is a key component enabling control and manipulation of physics at the atomic level, at a time where transistors have already pushed into the dimensions of single nanometers. Despite the dearth of expertise surrounding plasma devices and given the ever-increasing expectations from the semiconductor industry, plasma is set to play an even greater role in the semiconductor manufacturing process.
Clairvoyance in the Semiconductor Industry
In an industry that requires such high-level specialization, partnerships with academia and programs that introduce the field to young talent are an important step in fostering interest in these positions. A 2017 survey of US-based semiconductor manufacturers exposed that despite 75% of companies planning increased spending for L&D, there was a critical lack of resources and infrastructure to offer training. Semiconductor manufacturers may offer an unparalleled level of hands-on skill acquisition, but this would also require technical experts to be mentors in the workplace.
For EsgeeTech, a renewed focus on domestic semiconductor production presents the opportunity to serve the needs of the semiconductor industry through enhancing their plasma expertise. Douglas Breden and Anand Karpatne, our in-house plasma experts who have recently trained employees from Applied Materials and Tokyo Electron, made the point that a semiconductor wafer carries thousands of dollars worth of value in an area smaller than the size of a penny. These wafers are processed by complex machines which employ inductively coupled plasma (ICP) or capacitively coupled plasma (CCP) systems. With steep demand for the reliability of such manufacturing systems, root cause analysis of any potential issue would require an in-depth understanding of plasma physics. At Esgee, we equip our customers with customized training to enhance their understanding of plasma systems. As a result of feedback we have received recently, we are planning to expand our training to non-customers later this year. Breden and Karpatne believe that the fundamentals of plasma physics have become crucial for those seeking an understanding of modern semiconductor manufacturing systems.
However, there remains a lack of industry-specific training surrounding plasma theory and application. On the internet, there is still little discussion on this subject, even though the entire internet age has also been enabled by development provided via plasma manufactured systems. This is a core issue that EsgeeTech aims to resolve. The esoteric tag which has been added to plasma manufacturing is unjustified. Although specialization remains an enabling feature of the global supply chain, concentration of critical processes has also created the risk of systematic bottlenecks in the case of geopolitical conflict, natural disasters, or other issues that could affect specialist regions’ contributions to the global chain. Competition in plasma-specialized processes would reinforce vulnerabilities in the chain while alleviating demand.
Such a solution is in the best interest of semiconductor producers as well as the average electronics consumer, whose cell phone, computer, and other similar gadgets rely on plasma to produce. EsgeeTech stands out as a company with an established background in plasma techniques used by the semiconductor industry. Decades of experience through consultation projects with semiconductor manufacturers have contributed to our understanding of the problems faced within the industry, as well as the potential solutions. Our flagship product,VizGlow™, is designed specifically with the semiconductor industry in mind, with the goal of providing an end-to-end software package that enables innovation through robust multiphysics simulations.
VizGlow™ has always been a product developed with a focus on providing and improving its applications for semiconductor engineers. EsgeeTech’s larger clients have engaged with us in verification and validation of experimental data through VizGlow™, with many of these efforts currently available for review in open literature.
“The number of transistors on a microchip doubles every two years, while the cost of computers is halved” is an observation of technological trends known as “Moore’s Law.” Since the concept was first introduced in 1965 by Gordon Earle Moore, it has become a target for the speed of scaling and miniaturization within the semiconductor industry. The result of this desired level of innovation is that designs made today for tomorrow’s devices are done so with the expectation (but not the guarantee) of maintaining this rate. This creates immense pressure within the industry to revise and refine techniques, and is further compounded by the shortage of workers. At this stage of quantum processing and engineering, attracting qualified workers requires a promotion of the fields that semiconductor engineering intersects, as well as improving resources to lower the industry’s entry barrier.
EsgeeTech is a company that also relies on attracting a qualified workforce with interest in providing expertise to serve the semiconductor industry. Our collective backgrounds in fluid flow, electromagnetics, kinetic modeling, computational sciences, and computer science are examples of how diverse teams enable a company to excel amid an ever changing market. It also underscores the nature of plasma as a multidisciplinary subject, and further highlights the difficulties semiconductor manufacturers face in filling vacancies.
Interested in learning more about plasma flow simulations? Click here to take a look at our previous article. Feel free to follow us on Twitter and LinkedIn for more related news, or reach out to us directly at firstname.lastname@example.org. This post’s feature image courtesy of Hal Gatewood & Unsplash.
Recently, consumers have faced rising prices for semiconductor-powered devices, with shortages affecting the availability of products that serve their daily needs. With increased demand for semiconductors in key areas like the automotive industry, healthcare, and within AI-enabled products, manufacturers are vying to remain competitive while making next-generation breakthroughs in order to meet current demand.
So, the question is: how can industrial researchers continue to innovate in order to boost semiconductor production? And how can simulations relieve the pressure for the semiconductor industry to meet ever-growing global demands?
A joint report authored by Boston Consulting Group (BCG) and The Semiconductor Industry Association (SIA) is aimed at combating semiconductor shortages by profiling risks in the current international supply chain and highlighting semiconductors as a central component of shared economic stability across the globe. Central to the report’s findings are a series of statistics that characterize the current issues the world faces in securing a future where semiconductor demands are met as they continue to grow over the next decade.
The Global Semiconductor Supply Chain at a Glance
The current cooperative structure of the global supply chain for semiconductors is as unique as it is complex, with a web of destinations across the globe from the earliest stages of research and design to the final point of sale. Since the 1970s, specialization within these national and regional stages has contributed to the chain’s ability to produce at the speed of demand, while also innovating and improving the capabilities of semiconductors faster than any one country or region could.
Figure 1 below shows the current global semiconductor market share by region as of 2020. South Korea and the United States account for two-thirds of total market production and sales.
In addition to utilizing the specializations offered by each of the six major regions (Europe, Japan, Mainland China, South Korea, Taiwan, and the United States) that contribute to the global supply chain, the roundabout system also makes use of favorable trade conditions among the participating countries to keep production costs and consumer prices affordable. Figure 2 below shows the usage of semiconductors by industry.
Major risks in disruptions to the current supply chain could lead to a sharp rise in the cost of devices to producers and consumers alike. In a hypothetical situation where the global chain is replaced with self-sufficient regions, the report forecasts up to $900 – 1,225B of upfront investment required to maintain current output and meet rising demands, with an overall cost increase of 35% – 65% for consumers if regional and comparative advantages are ignored.
National policies within key regions, most notably Mainland China – which has massive industrial and manufacturing capabilities – have already placed self-sufficiency as a high priority for their future development in semiconductors.
Similar policies in other nations could leave local markets open to unforeseen factors, including greater competition for materials and additional costs in their securing and transportation. Situations like natural disasters and geopolitical conflict could destabilize systems that seek to decouple from the international chain, leading to regional shortages of semiconductors and additional issues with production for critical communications and security sectors.
Researching and Developing Solutions for the Market
In addition to the current issues that the international supply chain faces, SIA’s report highlights the importance of research and development, which is the primary way that producers maintain state-of-the-art techniques and provide security in their devices.
Although the speed of innovation and change in major market devices like consumer electronics is visible from year-to-year, the time for techniques developed at pre-competition research stages to be utilized at a mass scale and included within the global chain can take decades. As a result, original equipment manufacturers (OEMs) and integrated device manufacturers (IDMs) face upfront costs in both R&D and capital expenditure, with years before seeing a return on investment in these areas.
Despite the delayed turnaround for companies investing and participating in pre-competitive and basic research, cooperation at these early stages enables chips to become smaller while increasing performance. Recent innovations like 5G, internet of things (IoT), and autonomous vehicles all began their journey to widespread use at this stage. Figure 3 below illustrates regional spending in R&D among key regions as a percentage of sales.
SIA’s report also cites the need for utilization of emergent technologies in alleviating risks and constraints in the global chain, with modern inventions like augmented and virtual reality (AR/VR) playing a crucial role in enabling operations to continue remotely throughout the pandemic.
Simulation also provides this principle effect of bridging digital and physical worlds by allowing manufacturers to cut material costs and risk of exposure to hazardous materials, all without sacrificing insights that physical experiments and trials offer.
The use of simulation software and digitally based tools to further minimize risks that current global producers face is both economic and modern, and its viability as an industry-wide solution will only become greater as time continues. Simulations offer additional innovation points through applications for commonly used equipment in the semiconductor industry, such as plasma reactors, with details like simulated angular distribution functions deciding process parameters like excitation frequency and excitation voltage.
Industry leaders like Dr. Peter Ventzek and Dr. Alok Ranjan of Tokyo Electron Ltd. – a global supplier of equipment used to fabricate integrated circuits- have already taken advantage of high-fidelity plasma simulation and processing to develop new techniques with a wide array of applications for the semiconductor industry, using the insights offered by numerical simulations using VizGlow™. Here are a few examples of patented methods and techniques using simulations that are contributing to the semiconductors of today and tomorrow:
· Mode-switching plasma systems and methods that allow manufacturers to reduce minimum-required features and the cost of ICs, while also increasing packing density of components. Manufacturers working at the atomic scale are able to continue scaling semiconductor devices with consideration for constraints like equipment configurability, equipment cost, and wafer throughput.
· Techniques that include formation, patterning, and removal of materials in order to achieve physical and electrical specifications for the current and next generation of semiconductor. Plasma etching and deposition are prone to issues with decoupling source power (SP) and bias power (BP) effects, resulting in reduced control and precision. Decoupling these effects helps reduce cross-talk between a source and bias and in turn enhances control while decreasing complexity.
· Utilizing pulsed electron beams to create new plasma processing methods, which enable reduction of feature size while maintaining structural integrity. As device structures continue to densify and develop vertically, these methods which produce atomic-level precision in plasma processes will be useful for profile control, particularly in controlling deposition and etching processes at timescales associated with growth of a single monolayer of film.
Processes in plasma-assisted etching or deposition rely on the accurate determination of the distribution of the ion energy and angle close to the substrate surface. Precise control over these parameters could be used to manipulate the bombardment of the process surface. However, from a process engineer’s perspective, the incremental changes in geometric design, voltage, power, feed gas composition, and flow rates must be correlated with IEADF (Ion Energy and Angular Distribution functions).
The engineering team at Tokyo Electron Ltd. uses our non-equilibrium plasma solver, VizGlow™, and particle solver, VizGrain™, to understand underlying physics and find the best operating conditions for Tokyo Electron Ltd. products. In a paper published in the Journal of Physics D: Applied Physics, Dr. Rochan Upadhyay, and Dr. Kenta Suzuki, Esgee Technologies along with researchers at The University of Texas at Austin validated the VizGlow™ simulations used to obtain IEADF in a capacitively coupled plasma reactor.
Esgee Technologies uses software products, databases, and consulting projects to solve challenges faced by industrial manufacturers. We are dedicated to the development of plasma and physics simulations for manufacturing applications across a wide range of manufacturing industries, including semiconductors, with a legacy of support for analyzing existing equipment, improving processes, and developing new equipment concepts through the use of our software.
Thanks for reading! If you’re still curious about the topics discussed in this article, check out the following journal papers (and ask us for a free copy!):
Upadhyay R., K. Suzuki, L. L. Raja, P.L.G. Ventzek, and A. Ranjan. (2020). Experimentally Validated Computations of Simultaneous Ion and Fast Neutral Energy and Angular Distributions in a Capacitively Coupled Plasma Reactor. Journal of Physics D: Applied Physics. 53. 10.1088/1361-6463/aba068.
Interested in learning more about plasma flow simulations? Click here to take a look at our previous article. Feel free to follow us on Twitter and LinkedIn for more related news, or reach out to us directly at email@example.com.
The inquisitive mind of mankind permitted spaceflight to grow in the last century and exponentiate in recent years. Thanks to the technologies that drive this rapid deployment of spacecrafts. Statistics show that the year 2020 had seen a record breaking 114 launches which placed over 1000 satellites in orbit. This number increased further in 2021 – in a span of 9 months over 1600 satellites were launched.
Launching satellites to orbit is arduous. Maintaining their orbits throughout their lifetime to accomplish their mission is a tough problem, as well. Gravitational effects combined with possible aero-drag can deorbit satellites.
Small orbital thrusters keep the satellites in their desired track by firing periodically to account for gradual changes in orbit, an effect known as orbital decay. With an ever-growing number of satellites orbiting with tight tolerances, maintaining precise orbits and controlled deorbiting upon reaching end-of-life is also critical.
Development of highly efficient and reliable thrusters is carried out by various agencies around the world. Different types of orbital thrusters exist. They are broadly classified as chemical thrusters and non-chemical thrusters such as ion thrusters – which is the topic of this blog.
Same Requirements, two different approaches:
Thrust can be quantified as the amount of force produced by expelling a certain mass flow rate at some exhaust velocity. This gives flexibility to attain the desired thrust at the expense of a large mass flow rate expelled at low velocity or a small mass flow rate at high velocity.
Chemical thrusters eject byproducts at a large mass flow rate but at a relatively low velocity. Powerful thrusters are fed by large fuel tanks during the initial stages of space flight to reach orbit. These thrusters have low specific impulse.
Unlike traditional chemical thrusters, ion thrusters function by Coulombic acceleration ions (typically xenon) in an electrostatic field to high velocity. This results in high specific impulse upwards of 2000 s with electrical efficiencies reaching as high as 80%. Along with fast response times, ion thrusters can be precisely controlled, thus reducing the need for large propellant tanks. Therefore, ion thrusters are highly sought for long-term interplanetary and deep space missions.
Iodine thrusters, a possible alternative to more expensive xenon thrusters:
While xenon-based ion thrusters are great alternatives to chemical thrusters, operational cost is high due to the rarity of xenon. Novel propellants such as water, krypton, bismuth, are being researched. Costly experiments are being conducted worldwide to study these novel propellants. In this blog we explore the possibility of iodine as an alternative. Main advantages of using iodine are: larger ionization cross-section compared to xenon and a small volumetric footprint for storing iodine as it is solid under standard temperature and pressure.
High fidelity numerical models using VizGlow™:
Prototyping ion thrusters for experimentation and iterative design is time consuming and difficult which are associated with large expenses. Replicating operating conditions of these thrusters in laboratories is difficult. Therefore, researchers often rely on numerical models to optimize their designs. Modelling of ion thrusters is a multiple physics problem which encompasses a broad spectrum of physics. Fluid dynamics, reactive flow, plasma, electromagnetics, surface physics have to be accounted for to model ion thrusters at highest fidelity. Coupling between these individual physics is not straightforward in many cases. As a result, approximations have to be made to simplify the mathematics. Such reduced models may not always represent the behavior of ion thrusters. In fact, the literature pertaining to simplified computational studies of ion thrusters is scarce.
VizGlow™ – a non-equilibrium plasma solver that is fully coupled, self-consistent, and allows accurate numerical simulation of plasma thrusters. VizGlow™ allows coupling of multispecies chemically reactive plasma dynamics with Maxwell’s equation for the electromagnetic waves. Our tool also supports hybrid models that combine traditional Navier-Stokes (CFD) equations and particle-in-cell (PIC). This, for example, can be used to describe the behavior of ejected plasma plume into vacuum.
Our team at Esgee Tech along with collaborators have simulated an ion thruster that utilizes iodine.
Simulation setup and discussion:
Here the computational domain consists of a 6 cm x 10 cm plasma chamber within which iodine plasma is generated by a four-turn metal coil antenna that is driven at 13.56 MHz and deposits a power of 100 W. A dielectric quartz layer interfaces between the coil antenna and the plasma chamber. A comparison of the plasma properties at two different pressures, 1.0 Pa and 2.5 Pa is performed in this study.
The simulation has captured non-equilibrium effects such as disparate electron and heavy particle temperatures. We also resolved electrostatic sheaths. The electrostatic Joule heating is negligible compared to the wave power deposited into the plasma. The dielectric quartz wall attains a potential to balance the flux of positive and negative species.
Results from VizGlow™ indicate that for both pressures, almost all of the molecular iodine dissociates into I+ ions which dominate the mixture. The concentration of molecular iodine ion, i.e., I2+ is about one order of magnitude smaller than that of the dominant singly-ionized atomic iodine.
The pressure has an effect on the composition of the generated plasma. The plasma is electropositive with electrons as dominant negatively-charged species for the chamber pressure of 1.0 Pa. This, however, changes with increase in pressure, as plasma formed at 2.5 Pa is electronegative, where atomic iodine anions (I-) are the dominant negatively-charged species.
An interesting feature that VizGlow™ captured is the presence of vortex-like structures surrounding the plasma bulk. This effect is a result of transport of electron and anion species in the electronegative plasma.
Future of iodine thruster modelling:
VizGlow™ is constantly evolving with new features, a growing database of chemistries, fast and accurate solvers. One can perform trade studies to understand the effects of chamber pressure and temperature. Future work in this area could also focus on the reactivity of iodine with the surface of the thruster and spacecraft to estimate the lifetime of the thrusters.
VizGlow™ can simulate actual designs of ion thrusters that are meant to propel next generation efficient spacecraft and expand the horizon of understanding of the cosmos.
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