The major increase in discharge duration and plasma energy in a next step DT fusion reactor will give rise to important plasma-material effects that will critically influence its operation, safety and performance. Erosion will increase to a scale of several centimetres from being barely measurable at a micron scale in today's tokamaks. Tritium co-deposited with carbon will strongly affect the operation of machines with carbon plasma facing components. Controlling plasma-wall interactions is critical to achieving high performance in present day tokamaks, and this is likely to continue to be the case in the approach to practical fusion reactors. Recognition of the important consequences of these phenomena stimulated an internationally co-ordinated effort in the field of plasma-surface interactions supporting the Engineering Design Activities of the International Thermonuclear Experimental Reactor project (ITER), and significant progress has been made in better understanding these issues. The paper reviews the underlying physical processes and the existing experimental database of plasma-material interactions both in tokamaks and laboratory simulation facilities for conditions of direct relevance to next step fusion reactors. Two main topical groups of interaction are considered: (i) erosion/redeposition from plasma sputtering and disruptions, including dust and flake generation and (ii) tritium retention and removal. The use of modelling tools to interpret the experimental results and make projections for conditions expected in future devices is explained. Outstanding technical issues and specific recommendations on potential R&D avenues for their resolution are presented.

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Plasma{material interactions in current tokamaks and their implications for next step fusion reactors

The field of plasma etching is reviewed. Plasma etching, a revolutionary extension of the technique of physical sputtering, was introduced to integrated circuit manufacturing as early as the mid 1960s and more widely in the early 1970s, in an effort to reduce liquid waste disposal in manufacturing and achieve selectivities that were difficult to obtain with wet chemistry. Quickly, the ability to anisotropically etch silicon, aluminum, and silicon dioxide in plasmas became the breakthrough that allowed the features in integrated circuits to continue to shrink over the next 40 years. Some of this early history is reviewed, and a discussion of the evolution in plasma reactor design is included. Some basic principles related to plasma etching such as evaporation rates and Langmuir–Hinshelwood adsorption are introduced. Etching mechanisms of selected materials, silicon, silicon dioxide, and low dielectric-constant materials are discussed in detail. A detailed treatment is presented of applications in current silicon integrated circuit fabrication. Finally, some predictions are offered for future needs and advances in plasma etching for silicon and nonsilicon-based devices.

/pdf/plasma-etching-yesterday-today-and-tomorrow-4a8eftp390.pdf

Plasma etching: Yesterday, today, and tomorrow

An apparatus for dissociating gases includes a plasma chamber that may be formed from a metallic material and a transformer having a magnetic core surrounding a portion of the plasma chamber and having a primary winding. The apparatus also includes one or more switching semiconductor devices that are directly coupled to a voltage supply and that have an output coupled to the primary winding of the transformer. The one or more switching semiconductor devices drive current in the primary winding that induces a potential inside the chamber that forms a plasma which completes a secondary circuit of the transformer.

Toroidal low-field reactive gas source

Macromolecular plasma-chemistry: an emerging field of polymer science

Electric probe methods for diagnostics of plasmas are reviewed with emphasis on the link between the appropriate probe theories and the instrumental design. The starting point is an elementary discussion of the working principles and a discussion of the physical quantities that can be measured by the probe method. This is followed by a systematic classification of the various regimes of probe operation and a summary of theories and methods for measurements of charged particle distributions. Application of a single probe and probe clusters for measurements of fluid observables is discussed. Probe clusters permit both instantaneous and time-averaged measurements without sweeping the probe voltage. Two classes of applications are presented as illustrations of the methods reviewed. These are measurements of cross sections and collision frequencies (plasma electron spectroscopy), and measurements of fluctuations and anomalous transport in magnetized plasma.

Electric probes for plasmas: The link between theory and instrument

Comprehensive measurements of charged particles and neutral radicals in an inductively coupled plasma (ICP) are performed to understand and control the etching process using a CF4/H2 gas. The electron density in the ICP reactor decreases exponentially in a downstream region while the most abundant ionic species CF+ increases in proportion to the rf power with the CF+3 density almost constant. The neutral radical diagnostics by appearance mass spectrometry indicate 10 times more F atoms and somewhat fewer CFx radicals (x=1–3) in ICP, compared with a high‐pressure capacitively coupled plasma diode. Such a small ratio of the CFx density to the F density is possibly a cause of the low etch selectivity of SiO2 to Si in ICP etching. Two innovative methods to achieve the high selectivity in ICPs are demonstrated. One is wall heating (100–200 °C), which leads to a drastic increase in CFx densities with the F density almost constant. The other is a pulse modulation of rf power at 30–50 μs durations where the time‐...

Diagnostics and control of radicals in an inductively coupled etching reactor

Wall conditioning with lithium evaporation

Boronization study for application to large helical device

Fluorocarbon ions ( CF+3, CF+2 and CF+) are mass-selected from a CF4 plasma and irradiated onto aluminum surfaces at energies up to 140 eV, to investigate the surface processes relevant to reactive ion etching. The irradiation of a CF+x beam (x=1-3) at impact energies higher than 50-100 eV yields the smaller fragment species CF+y (y<x) on the surface, in addition to the reflected species (y=x). The energy distribution function of each ion species scattered from the surfaces has been measured for the first time; most of the ions have kinetic energies lower than 10 eV while some reflected species have energies comparable to the incidence energy. In contrast to the previous results of hydrocarbon ions ( CH+x ), the low-energy incidence (<50 eV) of fluorocarbon ions gives a low scatter of ions from the surface, except for the case of CF+3 incidence. The possible mechanisms of dissociation of fluorocarbon ions as a result of ion-surface interactions are discussed.

Dissociative ion yields on metal surfaces bombarded with low-energy fluorocarbon ions

A biased optical probe (BOP) method, ie novel technique for measurements of the electron energy distribution function (EEDF) in a range of such high energies as 10-50 eV has been developed This method is based on the optical emission induced by electrons passing through a negatively biased mesh in a plasma The BOP method was used to measure the EEDF in a DC glow discharge in He/Xe mixture while the low-energy part (<10 eV) of the EEDF was measured using the conventional Druyvesteyn method The measured distribution function considerably deviates from an ideal Maxwellian distribution, because of abundant high-energy electrons generated in a cathode fall On the contrary, the EEDF measured in a low-pressure inductive RF discharge was found to follow a single Maxwellian distribution over a wide energy range

https://iopscience.iop.org/article/10.1088/0963-0252/4/3/006/pdf

H. Sugai

Papers

Diagnostics and control of radicals in an inductively coupled etching reactor

Wall conditioning with lithium evaporation

Boronization study for application to large helical device

Dissociative ion yields on metal surfaces bombarded with low-energy fluorocarbon ions

A biased optical probe method for measurements of electron energy distribution in a plasma