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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Moore’s Law, a traditional benchmark in the semiconductor industry, has led to miniaturization, speed and efficiency improvements in modern microelectronics. The primary approach to keep up with the trend is to shrink the size of various features within an integrated circuit (IC). In recent years, however, engineers have extended fabrication techniques to sub-10 nm scales and layering circuitry on top of one another to develop 3D circuits and thereby adhering to Moore’s Law.
There are several benefits of modern 3D semiconductor architectures including small footprint, improved performance, reduced power, and integration of diverse circuits in a single package. However, disadvantages such as design and manufacturing complexity along with poor fabrication yield which directly affect the cost of these chips. With research accelerating the means of producing cheap mainstream 3D IC structures, atomic layer deposition (ALD) has emerged as a critical fabrication technique.
Atomic Layer Deposition: advantages and disadvantages
Atomic Layer Deposition (ALD) is a technique for sequentially depositing thin films of gaseous reactants (called precursors) onto a substrate. The advantage of ALD over other chemical vapor deposition techniques is that gas-to-surface depositions that occur in ALD processes are intrinsically self-limited. Each ALD cycle starts with the introduction of a specific mixture of precursors followed by deposition which terminates when all the sites on the wafer have reacted. Therefore, ALD can achieve atomic layer precision as deposition occurs one layer at a time. Repeated cycles of different precursors result in the formation of a desired chemical deposition on a substrate which is ready for next stages of fabrication. The self-limiting nature of ALD processes is beneficial for chemically processing high aspect ratio via, trenches, etc. in next generation high-capacity flash memory devices.
The main drawback of depositing one-layer at a time comes with low throughput – ALD processes are slow. Novel reactor designs are needed to achieve faster processing times while maintaining quality of deposition. Processing a batch of 10s – 100s of stacked wafers inside a reactor is one viable method of increasing throughput. Several configurations of batch processors exist – these designs focus on the arrangement, orientation and number of wafers of a stack.
Numerical modelling: a high fidelity and low cost alternative to experiments
Optimization of reactor design and chemistry is critical to achieve high yields and lower costs of manufacturing semiconductors. Experimental prototyping of large reactors with complex geometries is tedious and expensive. Some overheads that are associated with every experiment are high cost of precursor gases, reactor prototyping, long experiment cycle times, and needless human exposure to toxic chemicals. These undesirable factors prompt engineers to resort to design and development using high fidelity numerical software tools such as Esgee Tech’s suite of multiphysics simulators.
Wafer processing typically occurs in high vacuum (~ milli-Torr) regimes. At such low pressures, the mean free path of participating gas species becomes significant and comparable to the size of the reactor, resulting in high Knudsen number slip flows. Results obtained using traditional CFD models raise several uncertainties in these cases. A true physics-based approach such as gas kinetic modeling is required to capture physics across very different length scales – from the meter-scale reactor scale dynamics to nanometer-scale mechanisms. Moreover, modelling ALD processes using numerical methods requires comprehensive coupling of physics: gas dynamics, multispecies reactive mechanisms in gas and surface phases, magnetohydrodynamics, electromagnetics and dust/macro-particle dynamics.
VizGlow™: Simulate Real Physics
Our flagship solver, VizGlow™ has particle simulation capabilities (in addition to a host of other modeling features) that allows fully-coupled 3D particle-based simulations, representing conditions observed in industrial reactors with high accuracy. VizGlow™ is capable of simulating reactive gas dynamics of semiconductor wafer processing devices using both direct-simulation-Monte-Carlo (DSMC), particle-in-cell (PIC), and hybrid coupled plasma-fluid simulations. Example applications of VizGlow™ include: simulation of capacitively coupled plasma (CCP) reactors, inductively coupled plasma (ICP) reactors, feature-scale etching/deposition simulations and magnetron sputtering. With VizGlow™ computational models, researchers gather a complete picture of the mechanisms of a wafer processing reactor occurring in macroscale and microscale for a fraction of cost compared to long, complex and costly experimental setups and analyses.
Our users at The University of Texas at Austin in collaboration with researchers at Samsung Electronics have modelled multispecies gas dynamics of an industrially relevant multi-wafer ALD batch reactor. Their work published recently in the Journal of Vacuum Science and Technology A, has also been featured as an editor’s pick for the journal’s September issue (https://doi.org/10.1116/6.0000993).
An in-depth look inside the ALD batch reactor:
The problem consists of a large industrial thermal ALD batch reactor that houses 25 wafers. A specific mixture of carrier gas N2 and silicon precursor Si2Cl6 (hexachlorodisilane, commonly referred to as HCD) are fed into the reactor at a rate of 5 SLPM through the central and lateral inlets. Three chamber pressure is achieved by controlling inlet flow rates. The walls of the reactor are treated as isothermal boundaries at 873 K.
The mesh contains 1.2 million tetrahedral elements and simulated over 15 million particles using the DSMC solver in VizGlow™.
The kinetic particle model in VizGlow™ is able to accurately simulate the gas dynamics in the flow inlet feed pipes, expansion through micronozzles, across the inter-wafer zones and the outlet. A particular region of interest among design engineers is the geometry of the spray nozzles and vicinity past nozzles. In this study, VizGlow™ captures the viscous dominating effects in the nozzle region due to the low pressure, high temperature, and small dimensions, the flow remains subsonic through the nozzle.
An important outcome of this simulation is significant non-uniformities recorded in the precursor distribution within a particular wafer and across different wafers. This is critical to understanding the process yield in these reactors.
Analyse your reactor designs using VizGlow™:
The studies, designs and resulting improvements surrounding wafer processing reactors are endless. This study paves a path to future studies on a variety of reactor configurations using VizGlow™.
Process engineers can optimise their reactor design by studying the effect of important parameters such as precursor flow rates, gas composition, nozzle designs, layout of the nozzles, orientation of wafers, size of wafers, pressure and thermal conditions – everything about a batch reactor can be accurately described with truly and fully coupled multiphysics with VizGlow™. Research to improve reactor efficiency through rapid trade studies can be performed using results from VizGlow™ at a much faster pace and a much cheaper budget than traditional experimentation.
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If you have reached this far, you must be interested in knowing more about how our tool can help you model your semiconductor reactors. Send us a message at firstname.lastname@example.org. You can follow us at LinkedIn to keep up with our latest posts surrounding the semiconductor industry.
Humans took a big leap forward when we learned to control fire and use it for our daily needs. A similar leap happened when we learned to generate and handle plasmas, the fourth and most abundant state of matter found mostly in stars.
Today, every consumer electronic device and computing hardware relies on plasmas in their core manufacturing technology. The semiconductor industry isn’t the only industry to be revolutionized by plasma and its limitless applications; the metallurgical, aerospace, automotive, chemical and medical industries also rely on plasma technologies.
Plasma can be generated in several ways; one of the common forms of plasma used in industrial applications is Inductively Coupled Plasma (ICP). As its name suggests, the energy for producing and sustaining the plasma is provided by induction coils to which high-frequency alternating current is supplied. This type of plasma can be produced at a wide range of operating pressures, flow rates, and feed-gas compositions. As a result of its versatility, ICP’s are especially favored among semiconductor manufacturers to produce high-density plasmas for etching, deposition, and other processes in the fabrication of invisible electronic components embedded in very large-scale integrated circuits that constitute logic, memory, and other types of chips. A similar class of ICP’s discharges, albeit much bigger in size, is also used in the manufacture of flat panel displays and solar panels.
Plasma discharge, flow, and electromagnetic simulations are challenging on many accounts: transient electromagnetic fields, spatio-temporal variations in electron and ion densities due to chemical reactions, complex circuit configurations for power supply, and a plethora of surface reactions all need to be taken into account. The industrial researchers and product developers desire nothing less than highest fidelity simulations for understanding the plasma behavior at a wide range of length-scales.
Over the past 15 years, at Esgee Tech, we have developed our flagship simulation software VizGlow™ to accomplish the most challenging objectives as desired by the semiconductor industry. VizGlow™ is capable of simulating ICPs at a great fidelity by coupling plasma, electromagnetics, reactive flow, surface chemistry, and circuit dynamics. It supports large-scale 3D unstructured mesh of accurate and complex industrial wafer-processing reactors. In addition, various modifications to the classical ICP are possible such as multiple turn coils (this study), pulsing, multi-frequency input power, RF biasing at select surfaces, application of permanent magnetic fields, and operation in cavity resonant modes. All of these modifications can be simulated using VizGlow™.
In this blog post, we discuss a large-scale multi (dual) coil ICP reactor simulated using VizGlow™.
Large-Scale ICP Reactor: An Industrially-Relevant Problem
An ICP reactor is a workhorse for several semiconductor, flat panel, and solar industrial applications. The advantage of inductive power coupling is that there is low plasma potential drop across sheaths, generation of high-density plasma at lower electron temperature, which leads to higher ion flux and reduced ion bombardment energy at the wafer surfaces. Additionally, multiple spatially distributed inductive coils with multiple turns each helps generate a more uniform plasma covering a wider volume and processing surface area. Using multiple coils also allows for the generation of a complex and deliberately tailored spatial distribution of the electromagnetic fields that in turn enables exploitation of interesting properties of electromagnetic wave interference patterns for generating plasmas. Several parameters such as: position of the coils, coil structure, and current direction, could be altered to optimize the design of an ICP plasma. These are merely a few of the large number of parametric studies that one could explore using VizGlow™.
VizGlow™ enables large-scale reactor simulations such as the reactor discussed here, which measures 2.03m x 1.54m x 0.805m (LxWxH), and is used for processing wide area substrates such as for flat panel displays and solar panels. The computational domain is divided into following regions/sub-domains:
Coils: two spiral excitation coils are operated in-phase at a frequency of 13 MHz and power of 1 kW to generate EM waves.
Coil cage: a Faraday cage which houses two coils
Plasma: the region with an initial pressure of ~1 Torr in which argon plasma is generated
Window: an interface between the coil cage and plasma that isolates plasma and coils
Substrate: the surface that is exposed to the generated plasma for processing
Detailed Insights into Plasma Field
Plasma uniformity across a wide area is essential for generating uniform ion fluxes and radical fluxes at the wafer surface. The figure on the right illustrates that a dense plasma is generated directly underneath the coil and is highest underneath the dielectric window where the electromagnetic waves are introduced. VizGlow™ is able to capture the electromagnetic skin effect, i.e, shielding of the electromagnetic waves by high density plasma. Moreover, power deposition only occurs at a small zone within the skin depth. The active radicals and the ions then diffuse from this generation zone to the substrate usually located at the bottom of the chamber.
The results indicate that this fundamental ICP property is well represented by VizGlow™.
In an electromagnetic field, Poynting Vectors represent the electromagnetic energy flux and its direction. It is particularly important to understand the flow of energy as it is critical to the design and optimization of the plasma devices. Results obtained by VizGlow™ can be viewed in an immersive environment in the figure on top to see how these three-dimensional vectors carry the energy from the coils and deposit it in the plasma field. The color of the vector indicates the strength of the vector. These three-dimensional visualizations allow researchers to comprehend the plasma field in greater detail.
If you are someone as excited about plasma flow simulations as we are, feel free to follow us at LinkedIn or reach out to us at email@example.com.
Meanwhile, if you are curious about the latest developments in plasma flow simulations, feel free to go through the following journal papers (do ask us for a free copy!):
D. Levko, C. Shukla, R.R. Upadhyay, L.L. Raja, Computational study of plasma dynamics and reactive chemistry in a low-pressure inductively coupled CF4/O2 plasma, Journal of Vacuum Science & Technology B 39, 042202 (2021), https://doi.org/10.1116/6.0001028
T. Iwao, P.L.G. Ventzek, R.R. Upadhyay, L. L. Raja, H. Ueda, & K. Ishibashi, “Measurements and modeling of the impact of radical recombination on silicon nitride growth in microwave plasma assisted atomic layer deposition”, Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, vol. 36(1), pp. 03DB01, 2018, https://doi.org/10.1116/1.5003403
L. L. Raja, S. Mahadevan, P.L.G. Ventzek, and J. Yoshikawa, “Computational modeling study of the radial line slot antenna microwave plasma source with comparisons to experiments,” Journal of Vacuum Science &Technology A, vol. 31, no. 3, p. 031304, 2013, https://doi.org/10.1116/1.4798362