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ION PROCESSES, NEUTRAL CHEMISTRY, AND THERMOCHEMICAL DATA 47 6 Ion Processes, Neutral Chemistry, And Thermochemical Data INTRODUCTION In this chapter, the panel assesses the needs and status of cross sections and rate coefficients for ion processes and neutral chemistry in plasma processing reactors. It also assesses the availability of thermochemical data. Requirements for ion transport and cross section data are particularly stringent, because low-pressure plasma etching reactors typically operate at high plasma densities, producing ionization fractions of 10-4 to 10-2. Therefore ion collision processes (ion-ion neutralization and scattering, ion-molecule reactions, electron-ion recombination) are proportionally more important compared with neutral chemistry. The range of ion energies for which these cross sections and rate coefficients are required is large. Conventional reactive ion etching (RIE) reactors use capacitively coupled power for ion generation and acceleration and have applied potentials of hundreds of volts. This results in ions having directed energies of tens to hundreds of eV in the sheaths, while their random thermal temperatures in the bulk plasma tend to be small (<< 0.05 eV). Inductively coupled plasma (ICP) and microwave- excited electron cyclotron resonance (ECR) reactors differ from conventional RIE reactors in that they typically operate at lower pressures (<< 10 mTorr), higher power deposition, and larger ionization fraction. These conditions allow bulk ion temperatures to climb significantly above the gas temperature (0.1-0.5 eV). The Knudsen number (Kn = mean free path / characteristic dimension) of ICP and ECR reactors may exceed 0.01 to 0.1. These long mean free paths complicate modeling in that noncontinuum algorithms must be employed as transport approaches the molecular flow regime. Large Knudsen numbers also imply that hot atom transport is more prevalent. Hot atoms have translational energies that significantly exceed their random thermal temperature. (The term "hot atom transport" is used here to refer to all hot neutral species: atoms, molecules, and radicals.) Hot atoms are generated by reactions of energetic ions with neutrals (charge exchange) or other ions (ion-ion neutralization); by energetic ions reflecting from surfaces and returning to the plasma as neutrals; and by dissociative electron collisions of molecules. The latter category includes electron impact dissociation of neutral molecules and dissociative recombination of molecular ions. Hot atoms are important for at least two reasons. First, they may impact on the wafer, thereby modifying the etching or deposition. Second, by virtue of their large translational energy, they may participate in reactions having activation energies that are otherwise energetically disallowed. The availability of neutral chemistry cross sections and rate coefficients varies greatly depending on the specific reaction chemistry being used. Depending on the specific chemistry, rate coefficients may be available from the literature on combustion, upper atmospheric chemistry, or chemical vapor deposition (CVD). An important difference between the database needs in neutral chemistry for plasma etching reactors, as opposed to CVD, is the contributions of excited states. Significant fractions of the neutral species in a plasma processing reactor will have stored internal energy (vibrational or electronic) resulting from collisions with electrons or ions. Reactions that have activation energy barriers for reactants in their ground states may be energetically allowed (or accelerated) if the reactants are either vibrationally or electronically excited, or they may have different branching ratios.
