Plasma Applications for Material Modification

From Microelectronics to Biological Materials

Francisco L. Tabarés, Francisco L. Tabarés

• Publisher: Jenny Stanford Publishing
• Year: 2021
• ISBN:
  9781000245271
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Physical & Theoretical Chemistry

This book is an up-to-date review of the most important plasma-based techniques for material modification, from microelectronics to biological materials and from fusion plasmas to atmospheric ones. Each its technical chapters is written by long-experienced, internationally recognised researchers. The book provides a deep and comprehensive insight into plasma technology and its associated elemental processes and is illustrated throughout with excellent figures and references to complement each section. Although some of the topics covered can be traced back several decades, care has been taken to emphasize the most recent findings and expected evolution.

The first time the word 'plasma' appeared in print in a scientific text related to the study of electrical discharges in gases was 1928, when Irving Langmuir published his article 'Oscillations in Ionized Gases'. It was the baptism of the predominant state of matter in the known universe (it is estimated that up to 99% of matter is plasma), although not on earth, where the conditions of pressure and temperature make normal the states of matter (solid, liquid, gas) which, in global terms, are exotic. It is enough to add energy to a solid (in the form of heat or electromagnetic radiation) to go into the liquid state, from which gas is obtained through an additional supply of energy. If we continue adding energy to the gas, we will partially or totally ionise it and reach a new state of matter, plasma, made up of free electrons, atoms and molecules (electrically neutral particles) and ions (endowed with a positive or a negative electric charge).

Chapter 1

Introduction: Cold Plasmas and Surface Processing

F. J. Gordillo^a F.L. Tabarés^b

^aInstituto de Astrofísica de Andalucía (IAA - CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain

^bLaboratorio Nacional de Fusion, CIEMAT; Av Cornplutense 40, 28040 Madrid, Spain

[email protected]

The first time the word “plasma” appeared in print in a scientific text related to the study of electrical discharges in gases dates back to 1928. That year Irving Langmuir published his article “Oscillations in Ionized Gases” in Proceedings of the National Academy of Sciences of the States United. It was a baptism of the predominant state of matter in the known universe (it is estimated that up to 99% of matter is plasma), although not on our planet, where the conditions of pressure and temperature make normal the states of matter—solid, liquid, and gas—that in global terms are exotic. It is enough to add energy to the solid (in the form of heat or electromagnetic radiation) for it to turn into a liquid state, from which gas is obtained through an additional supply of energy. If we continue adding energy to the gas, we will partially or totally ionize it, that is, we will remove electrons from the atoms or molecules that constitute it. In this way, we reach a new state of matter, plasma, made up of free electrons, atoms and molecules (electrically neutral particles), and ions (endowed with a positive or negative electric charge). The energy needed to generate a plasma can be supplied in several ways: through heat from a combustion process; through the interaction between laser radiation and a solid, a liquid, or a gas; or through electrical discharges in gases, in which free electrons take energy from the applied electric field and lose it through excitation and ionization processes of the atoms and molecules in the gas.

The light emitted by a plasma, its characteristic emission spectrum, is determined by the type of atoms, molecules, and ions that form it. These components, when de-energized, emit electromagnetic radiation, visible or not. One of the peculiarities of plasmas is that they conduct electricity. On a macroscopic scale, plasmas are, however, electrically neutral, since the number of positive and negative charges is similar. Thus, the flame produced by burning candle wax in combination with oxygen from the air—a typical example of a plasma that is very little ionized—can conduct electricity. A graphical overview of the different kinds of plasmas according to their microscopic parameters (electron density and temperature) is displayed in Fig. 1.1.

Figure 1.1 Overview of plasmas and the wide range of their microscopic parameters (electron energy and electron density).

The study of common natural phenomena in our world has taught us that lightning (Fig. 1.2), auroras, the ionosphere, and the recently discovered electrical discharges in the upper atmosphere (between 40 and 90 km high) are different types of natural plasmas present in the gaseous envelope of our planet. Beyond the Earth, there are plasmas in the Sun and other stars, in the solar wind, in the tails of comets, and in the interstellar space. The first observations related to plasmas go back to the experiments of Georg Christoph Lichtenberg, a professor at the University of Göttingen in the last third of the eighteenth century and today most remembered as a writer. By placing an insulating material between a pointed electrode and a metal plate and subjecting it to high electrostatic stress, he observed beautiful radiant patterns with tree-like shapes. These patterns were due to the dielectric breakdown of the material. The first attempts to explain Lichtenberg’s observations were made by Michael Faraday, who dedicated some years of his life (1816-1819) to the study of the characteristics of matter when its temperature increases, although he did not elucidate the possible existence of a new state of matter beyond the gaseous state. Another English physicist, William Crookes, discovered in 1879 a green “radiant matter” with striated patterns that appeared when applying voltages between electrodes installed inside a glass tube, filled only with the air that remained after it was emptied. In addition, near the cathode, he observed a dark region, the so-called Crookes’ dark zone. These observations led him to postulate the existence of a fourth state of matter. He conjectured that it was made up of gas molecules endowed with an electrical charge, that is, ions. Before these works, in 1857, Werner von Siemens had already patented an industrial process that used plasmas for the production of ozone: oxygen flowed through an annular electrical discharge between two concentric electrodes, one of which had an insulator material attached as a dielectric barrier. Although Siemens ignored the ultimate scientific reasons on which the method was based, it was very efficient and cost effective.

J. J. Thomson’s work on cathode rays in electrical discharges in gases and his discovery of the electron in 1897 earned him the 1906 Nobel Prize in Physics. His was a remarkable contribution to the knowledge of the structure of atoms (composed of a positive nucleus and surrounding negatively charged electrons), and in doing so, he helped to clarify the nature of plasmas. The first attempt to give an overview of the physics of gas discharges was made by Johannes Stark in his book Elektrizität in Gasen, published in Germany in 1902.

Figure 1.2 Lightning, together with flames, is the most conspicuous kind of plasma in nature.

1.1 Types of Plasmas

Classifying the diversity of types of plasmas that exist in nature or that can be generated artificially is not easy, since it is risky to choose isolated parameters that serve as criteria to establish the differences. Despite these difficulties, we can venture into a first classification of the types of plasmas, one that takes into account their thermal equilibrium, that is, whether or not the temperature or the average energy of the particles that make it up is the same for each type of particle.

All particles have the same temperature (thermal equilibrium) for stellar interior plasma or for its terrestrial analogs, deuteriumtritium fusion plasmas and the impurities generated in experimental controlled nuclear fusion devices, like JET^a and ITER^b (see Fig. 1.4). The plasma inside the stellar interior is usually made up of a high proportion of ionized particles: the number of electrons, and of ions, is similar to that of neutral particles. These plasmas are also called hot or thermal plasmas, since the temperature inside them reaches millions of degrees (10⁷°C–10⁹°C), the same for electrons as for heavy species.

There are other types of thermal plasmas, with certain industrial applications, that are generated at high pressures, above 133 mbar, just over a tenth of an atmosphere, although their temperatures (10⁴°C–10⁵°C) are much lower than those of fusion plasmas. Plasma torches for surface treatment or plasma lamps that produce high-intensity discharges for street lighting or headlights of high-end cars are such plasmas.

When the gas pressure is low or the electrical voltage applied in the discharge is high, the electrons in the plasma acquire, in the time between collisions with other plasma particles, kinetic energies higher than the energy associated with the random thermal movement of the neutral particles (atoms and molecules) of the plasma. We can then attribute some degree of thermal equilibrium deviation to plasmas, since electrons, ions, and neutral particles have different “temperatures” or average kinetic energies. Please note that it only makes sense to talk about temperature when the energy distribution of the particles in question is limited to a certain statistical function, the Maxwellian one. This is not usually the case in plasmas produced at a low pressure and with a small degree of ionization between 10⁻⁶ and 10⁻⁴.

Nonthermal plasmas, also known as cold plasmas, are characterized by the fact that the temperature of heavy species (neutral particles and ions) is close to room temperature (25°C–100°C). Instead, the electronic temperature is much higher (between 5000°C and 10⁵°C). Cold plasmas usually occur at a low pressure (p < 133 mbar) in reactors with very different geometries. Such reactors generate plasmas through direct current, radio frequency, microwave, or pulsed discharge systems.

________________

1. The Joint European Torus.
2. Originally, International Thermonuclear Experimental Reactor.

There are special types of cold plasmas, produced in so-called corona and dielectric barrier discharges, that are generated at atmospheric pressure by using pulses between 10⁻⁶ s and 10⁻⁹ s. In these types of discharges, highly energetic electrons are produced that, due to the shortness of the pulses used, have little time to exchange energy with their surroundings. This establishes a strong temperature gradient between the electrons and the heavy species in the plasma.

The values of density and electronic temperature, two of the main parameters that characterize plasmas, cover a wide spectrum (see Fig. 1.1). Thus, the electron density varies between 1 electron/cm³ and 10²⁵ electrons/cm³; that is, it even exceeds the concentration of electrons in metals. On the other hand, the average free path of the particles in a plasma, that is, the average distance covered before a particle collides with another particle in the plasma, can range from tens of millions of kilometers to just a few microns.

1.2 Cold Plasma in the Industry

Cold plasmas are very useful for many technical applications because, since they are not in thermal equilibrium, it is possible to control the temperature of ionic and neutral species on the one hand and of electrons on the other. However, the high energy of the electrons constitutes the genuine determining factor when initiating many chemical reactions that in thermally activated media would be very inefficient, if not impossible.

Industrial applications of cold plasmas make up a very important part of the productive infrastructure of advanced countries. In cold plasmas, a large number of and diverse highly energetic reactive species are generated that activate physical and chemical processes that are difficult to achieve in ordinary chemical environments. These species include photons in the visible and ultraviolet range, charged particles (electrons and ions), highly reactive neutral species (such as free radicals or oxygen), fluorine and chlorine atoms, excited atomic and molecular species, and excimers and monomers (an excimer is an electronically excited molecule that lacks a stable ground state; a monomer is a highly reactive chemical subunit that can bind to other equals to form polymers).

Thanks to cold plasmas, certain industrial processes are carried out more efficiently and cheaply, thereby reducing pollution and the toxic waste generated. The advantages of the industrial use of cold plasmas are perfectly illustrated in the comparison that W. Rakowski published in 1989 between the resources needed to dye cotton fabrics with a current chemical method that uses chlorine and those required for an equally effective procedure that uses cold plasmas at a low pressure (2.5–7 mbar). Modifying 20 tons/year of wool using the second method saved 27,000 m³ of water, 44 tons of sodium hypochlorite, 16 tons of sodium bisulfite, 11 tons of sulfuric acid, and 685 MW of electrical power. Furthermore, the ordinary chemical process produced toxic residues causing different diseases in the workers. Comparing the energy costs of producing 1 kg of dyeable wool fabric gave figures of 7 kW/kg for the traditional chlorination process versus only 0.3 or 0.6 kW/kg when cold plasma treatment, produced at a low pressure, is used.

1.3 Cold Plasma Chemistry

The chemistry of cold plasmas, or cold chemistry, so called because of the low temperature (generally less than 100°C) of heavy plasma species, can be of the homogeneous type or the heterogeneous type. It will be of a homogeneous type when the reactions take place in the gas phase, as in the synthesis of ozone or in the elimination of sulfides and nitrides present in waste gases. It will be of a heterogeneous type when the plasma interacts with a solid or liquid surface.

In the plasma-solid surface interaction processes, three categories are recognized: erosion, deposition, and physicochemical alteration. Erosion is understood as plasma-assisted wear of a surface by simultaneous sputtering. It is of great interest to the microelectronics industry because it erodes the material anisotropically, that is, the material ends up having a different width and height, while typical of ordinary chemical techniques is isotropic carving. In plasma-assisted vapor phase chemical deposition processes the material is added to the surface in the form of a thin layer. Finally, solid surfaces treated with plasmas undergo physicochemical changes as an effect of the radiation processes and of the particles coming from the plasma that act on it.

The scientific and technical interest in plasma-surface interaction processes arose from a work by Jerome Goodman, published in 1960. He argued that a sheet of material deposited from a plasma could be useful and not just an annoying residue. Specifically, Goodman observed that the 1 µm thick deposit of plasma-polymerized styrene exhibited valuable dielectric properties. From that moment on, the synthesis of polymeric materials under the influence of cold plasmas, or plasma polymerization, ceased to be an undesirable by-product, already observed in 1874 by de Wilde and Thenard, to become one of the plasma material treatments with a greater number and dive...