Plasma Processing of Materials and Atomic, Molecular, and Optical Physics. An Introduction

Hiroshi Tanaka; Mitio Inokuti    Department of Physics,Faculty of Science and Technology, Sophia University, Tokyo, Japan
Physics Division, Argonne National Laboratory, Argonne, Illinois

I Introduction

Low-temperature plasmas are generated by glow discharges of low-pressure gases and are widely used in industry for chemical-vapor deposition (CVD), plasma etching, and other treatments of solid surfaces for the manufacture of useful materials such as amorphous silicon (a-Si: H) for solar cells and of ultra large-scale integrated (LSI) circuits. These examples may be viewed as a new application of atomic, molecular, and optical physics to materials science and technology (Lieberman and Lichtenberg, 1994; Bruno et al., 1995; Fujishiro et al., 1998).

In the manufacture of ultra LSI circuits, for instance, it is desirable rapidly to create prescribed patterns with scales less than 0.1 μm and fine structures with an aspect ratio of over 10 on a silicon wafer with a diameter of more than 30 cm. To this end, one needs an ion beam that has a high current, is well collimated, and is spatially uniform, and a source of such a beam is a high-density plasma. Much research is being carried out to design optimal plasma sources for use under different conditions. Problems to be solved arise from the presence of a magnetic field, from the enormous variety of reacting species present, and from their interactions with vessel walls.

The densities of electrons or ions in plasmas used for material processing are 109 to 1011 cm− 3. The electrons are characterized by an energy distribution with a mean energy of several electron volts (corresponding to a temperature Te of some 10,000 K). Most of the electrons have kinetic energies below 20 eV. Yet a modest number of them with relatively high kinetic energies are responsible for electronic excitation, dissociation, and ionization of molecules. Discharges by radio-frequency (rf) waves or microwaves are usually generated in gas at pressures of 10 to 10− 2 torr, at which two-body collisions among electrons, ions, and molecules are decisive for the structure of discharges and for transport of particles. Thus, characteristics of a resulting plasma reflect the properties of atomic and molecular species, including the cross sections for the two-body collisions with electrons, ions, radicals, and other species and transition probabilities governing interactions with photons. The densities of neutral molecules (in the ground electronic state) are higher by orders of magnitude than the densities of electrons or ions; therefore, one often characterizes such a plasma as “weakly ionized.” The kinetic energies of ions are much lower than the kinetic energies of electrons, and correspond to a temperature Ti not much higher than the gas temperature TN; therefore, one often describes such a plasma with the modifiers “low-temperature” and “thermally non-equilibrium.”

In a weakly ionized low-temperature plasma, various molecular species are abundantly generated either directly or indirectly as a consequence of electron collisions with molecules, and many of the molecular species readily react with other species. For this reason, one sometimes calls such a plasma “chemically reactive.” Applications of chemically reactive plasmas are widespread over organic and inorganic materials, in part because of the relatively low cost of generating of such plasmas. The large variety of chemically active species generated in a plasma is sometimes a disadvantage because they may initiate many reaction pathways, which may be difficult to analyze and to control.

It is thus clearly important to further develop the technology of plasma processing of materials toward advanced goals, including the identification and characterization of useful reactions, the optimization of physical parameters for the generation of a plasma best suited for a given purpose, the easiest and surest control of plasma properties, the best economics, and the minimization of any potentially adverse impact of the technology on the human environment and health (ASET, 1998). A rational and reliable approach to these goals must be based on a full understanding of the fundamentals of plasma chemistry at the molecular level, the scope of which largely belongs to atomic, molecular, and optical physics. The fundamentals are illustrated (though not exhaustively represented) by the following articles in the present volume.

The crucial importance of the fundamentals of atomic, molecular, and optical physics has been seen earlier in fusion-plasma research, astrophysics, and radiation physics. Plasma chemistry is a relatively recent addition to the list of fruitful applications of atomic, molecular, and optical physics.

II Plasma Formation and Collision Processes

A PLASMA STRUCTURE AND MOLECULAR SPECIES

Figure 1 (Japan Society of Applied Physics, 1993) shows schematically the basic structure of a glow-discharge reactor using coupling with a high-frequency capacitor, and also shows the electric potential between the electrodes in the presence of a plasma. A high-frequency (13.56 MHz) electric field is applied to a gas at a pressure of 10− 2 to 1 torr between the electrode S1 on the power-source side and the electrode S2 on the earth side; this causes excitation, dissociation, and ionization of molecules by electron collisions with molecules, and leads to the formation of a self-sustaining glow-discharge plasma. The central part of positive glow, called a bulk plasma, has an electric potential Vp, which is always positive and is comparable to the first ionization potential of a molecule of the major constituent of the gas. The electrode connected to earth (or the anode) has a lower potential than the bulk plasma, and is subjected to impacts of positive ions, as discussed by Hippler (1999). The electrode on the power-source side (or the cathode) also has a lower potential than the bulk plasma, because electrons follow the high-frequency electric field, but ions do not, so that a negative load is applied to the blocking capacitor. As a consequence, a negative self-bias potential VB is established in a region near the cathode; this region does not glow and is called the plasma sheath. The glow is most intense in a region of the bulk plasma close to the sheath, showing the presence of many electrons of relatively high energies.

In addition to the capacitor-coupled reactor shown in Fig. 1, there are two other classes of reactors, namely, those coupled with an inductor and those excited by microwaves. Various gases are used depending on the materials treated, as Table 1 (Samukawa, 1999) summarizes.

B BEHAVIOR OF PARTICLES IN A PLASMA

Figure 2 illustrates collisions and reactions of particles in a low-temperature plasma used for plasma CVD (for instance, the formation of a-Si: H films by SiH4, on the left-hand side) and for plasma etching (for instance, microprocessing of Si surfaces, on the right-hand side). It is useful to distinguish three temporal stages of the numerous and in general complex atomic and molecular processes occurring in such a plasma. The first stage may be called physical or initial, and includes excitation, dissociation, and ionization of molecules by electron collisions (Basner et al., 1999; Morgan, 1999). The second stage may be called physicochemical or secondary, and includes reactions of reactive species such as subexcitation electrons (electrons with kinetic energies below the first electronic-excitation threshold of the major constituent molecule), photons emitted by excited molecules, positive and negative ions (Armentrout, 1999; Lindinger, et al., 1999), excited atoms or molecules, and free radicals (Hatano, 1999) with other molecules. The third stage may be called chemical or thermal, and includes further reactions of the products of the second stage, which occur under nearly thermal-equilibrium...