Comparative study of positive and negative ion flows extracted from downstream plasmas beyond magnetic and electrostatic electron filters

In the present paper we compare the positive and negative ion flows created using a recently developed electrostatic grid-type filter with the flows formed using a magnetic filter. Langmuir probe measurements show electron cooling with both filters, allowing effective formation of negative ions via electron dissociative attachment in the region of low electron temperature. The energy distribution functions of positive and negative ions extracted from the filtered plasmas are measured in both systems showing an almost monoenergetic nature of the ions with the energy corresponding to the imposed extraction potential. It is shown that in both cases strongly electronegative plasmas where the negative ion density is much larger than the electron density can be formed downstream of the filter. Biasing an internal electrode or the electrostatic filter grid allows control of the plasma potential. In the case of the electrostatic filter the plasma could be biased negatively compared to ground and effective extraction of negative ion was achieved.В данной работе проведено сравнение потоков положительных и отрицательных ионов, генерируемых с использованием недавно разработанного электростатического сеточного фильтра с потоками, формируемыми с использованием магнитного фильтра. Измерения ленгмюровскими зондами показали эффективное “охлаждение” электронов при использовании обоих фильтров, обеспечивающее условия для эффективного образования отрицательных ионов в области с низкой электронной температурой в результате диссоциативного прилипания. Функции распределения по энергии положительных и отрицательных ионов, извлекаемых из вторичной плазмы, измеренные в обеих системах, показали моноэнергетичность генерируемых потоков ионов с энергией, соответствующей приложенному извлекающему потенциалу. Показано, что в обоих случаях возможно формирование сильно электроотрицательной плазмы на выходе фильтра с плотностью отрицательных ионов значительно превышающей плотность электронов. Смещение внутреннего электрода либо сетки фильтра позволило достичь управления потенциалом плазмы. В случае электростатического фильтра потенциал плазмы может принимать отрицательные значения по отношению к заземленному электроду, благодаря чему было достигнуто эффективное извлечение отрицательных ионов.У даній роботі проведено порівняння потоків позитивних і негативних іонів, що генеруються з використанням недавно розробленого електростатичного сіткового фільтру з потоками, що формуються з використанням магнітного фільтру. Вимірювання ленгмюрівськими зондами показали ефективне “охолоджування” електронів при використанні обох фільтрів, що забезпечує умови для ефективного утворення негативних іонів в області з низькою електронною температурою в результаті диссоціативного прилипання. Функції розподілу по енергії позитивних і негативних іонів, витягуваних з вторинної плазми, виміряні в обох системах, показали моноенергетичність потоків іонів, що генеруються, з енергією, відповідною до прикладеного витягуючого потенціалу. Показано, що в обох випадках можливе формування сильно електронегативної плазми на виході фільтру з щільністю негативних іонів що значно перевищує щільність електронів. Зсув потенціалу внутрішнього електроду або сітки фільтр дозволив досягти керування потенціалом плазми. У разі електростатичного фільтру потенціал плазми може приймати негативні значення по відношенню до заземленого електроду, завдяки чому було досягнуте ефективне витягання негативних іонів

The role of thermal energy accommodation and atomic recombination probabilities in low pressure oxygen plasmas

International audienceSurface interaction probabilities are critical parameters that determine the behaviour of low pressure plasmas and so are crucial input parameters for plasma simulations that play a key role in determining their accuracy. However, these parameters are difficult to estimate without in situ measurements. In this work, the role of two prominent surface interaction probabilities, the atomic oxygen recombination coefficient ? O and the thermal energy accommodation coefficient ? E in determining the plasma properties of low pressure inductively coupled oxygen plasmas are investigated using two-dimensional fluid-kinetic simulations. These plasmas are the type used for semiconductor processing. It was found that ? E plays a crucial role in determining the neutral gas temperature and neutral gas density. Through this dependency, the value of ? E also determines a range of other plasma properties such as the atomic oxygen density, the plasma potential, the electron temperature, and ion bombardment energy and neutral-to-ion flux ratio at the wafer holder. The main role of ? O is in determining the atomic oxygen density and flux to the wafer holder along with the neutral-to-ion flux ratio. It was found that the plasma properties are most sensitive to each coefficient when the value of the coefficient is small causing the losses of atomic oxygen and thermal energy to be surface interaction limited rather than transport limited

Enhancing surface production of negative ions using nitrogen doped diamond in a deuterium plasma

The production of negative ions is of significant interest for applications including mass spectrometry, particle acceleration, material surface processing, and neutral beam injection for magnetic confinement fusion. Methods to improve the efficiency of the surface production of negative ions, without the use of low work function metals, are of interest for mitigating the complex engineering challenges these materials introduce. In this study we investigate the production of negative ions by doping diamond with nitrogen. Negatively biased ($-20$ V or $-130$ V), nitrogen doped micro-crystalline diamond films are introduced to a low pressure deuterium plasma (helicon source operated in capacitive mode, 2 Pa, 26 W) and negative ion energy distribution functions (NIEDFs) are measured via mass spectrometry with respect to the surface temperature (30$^{\circ}$C to 750$^{\circ}$C) and dopant concentration. The results suggest that nitrogen doping has little influence on the yield when the sample is biased at $-130$ V, but when a relatively small bias voltage of $-20$ V is applied the yield is increased by a factor of 2 above that of un-doped diamond when its temperature reaches 550$^{\circ}$C. The doping of diamond with nitrogen is a new method for controlling the surface production of negative ions, which continues to be of significant interest for a wide variety of practical applications

On the simultaneous extraction of positive ions and electrons from single-grid ICP source

The results of investigations on the simultaneous extraction of positive ions and electrons from a single-grid ICP source are reported. It is shown that the single-grid ion source is capable of generating coincident ion and electron flows in contrast to the more common two- or three-grid sources. Electron and ion emission characteristics of the source are presented for the cases of DC and RF acceleration bias. A method of ion/electron current ratio control is proposed allowing to change the current compensation state of the ion beam from non-compensated to highly over-compensated. The researches were conducted in the ion energy range 10–250 eV with a high ion current density of 5 mA/cm2, so the presented results may be useful for modern technology applications

Principles of low energy ion beam formation in single-grid ion optical system

The experimental investigations of principles of low energy ion beam formation in single-grid ion optical system with ICP discharge are reported. Two modes of ions extraction are discovered: beam regime and plasma regime with transition at some critical potential of inductively coupled plasma. Numerical estimations of critical potential for different conditions are reported. It is shown that the single-grid low energy ion source is capable of electron flow generation. The results obtained in this paper can be useful for the ion/electron flow ratio control

Secondary electron emission due to multi-species iodine ion bombardment of different target materials

International audienceIon-induced Secondary Electron Emission (SEE) is a fundamental surface interaction that strongly influences many plasma discharges. Recently, interest in iodine plasmas is growing due to new material processing and space propulsion applications, but data for SEE yields due to iodine ion bombardment remains scarce. Additionally, due to the formation of multiple ion species in typical iodine plasmas, and surface chemical reactions leading to iodide layer formation, the effective SEE yield is expected to differ from that for individual ion species on clean surfaces. In this work, we measure the SEE yield of multi-species iodine ion beams bombarding different target materials (Mo, W, Al, Ti, Cu, carbon-carbon, and steel), in the energy range 0.6-1.4 keV. An ion beam is produced by extracting and accelerating ions from a gridded ion source based on an Inductively Coupled Plasma (ICP). SEE yields of downstream targets are measured using a conventional electrostatic probe technique, and the ion beam composition is determined using time-of-flight spectrometry. The beam is composed predominately of atomic (I +) and molecular (I + 2) ions whose ratio changes depending on the ICP power. Yields depend strongly on the target material and beam composition, and vary between 0.05-0.4 depending on whether potential or kinetic emission processes dominate

A new grid-type electron filter for volume-production negative-ion source

A new system providing continuous generation of broad negative-ion beam is described and characterized. The key element of the system is the grid-type electron filter allowing the formation without magnetic field of a stationary highly electronegative plasma which can be biased negatively with respect to a relatively high-area electrode that allows to extract the negative ions. Measurements of negative-ion energy distribution functions conducted using a magnetically filtered energy analyzer show broad (250 mm) negative-ion beam formation with controllable energy starting from several eV. A conclusion is made about the possibility of practical application of the grid-type electron filter in negative ion sources for electric propulsion and etching technologies

Plasma plume expansion with pulsed electron neutralization

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RF broad-beam low-energy ion source with electron compensation

Characteristics of single-grid RF ion source with 250 mm beam diameter and 1A beam current have been studied. Energy distribution functions of electrons and ions emitted by the source have been measured. It is shown that the emitted electron current is sufficient for full ion beam current compensation. The technique of ion to electron current ratio control allowing to change this ratio in wide range is proposed. Using the ICP in the source allows to rich high current density in the low ion energy range with the possibility of independent control of ion energy and current density