Charles DeJoseph

On the Decomposition of Silane in Plasma Deposition Reactors

In a low-pressure discharge, plasma-enhanced decomposition of silane proceeds by various channels including electron-impact, ion- and radical-induced, and heterogeneous reactions. The results of several experiments are presented to clarify the relative importance of the processes. The conclusions of these studies and associated analysis are that the dominant processes are strongly influenced by the gas residence time, the power density input, and the electronegative characteristics of the silane discharge.

Nonlocal effects in a bounded afterglow plasma with fast electrons

Effects connected with nonlocality of the electron energy distribution function (EEDF) in a bounded, afterglow plasma with fast electrons can lead to a significant (many times of Te/e) increase in the near-wall potential drop, even if the density of this fast group is only a small fraction of the total electron density. This can substantially change the near-wall sheath thickness and electric field. Nonlocal fast electrons which are partially trapped in the plasma volume can increase the rate of stepwise excitation, supply additional heating to slow electrons and reduce their diffusion cooling rate. Altering the source terms of these fast electrons, to change their production rate will, therefore, alter the near-wall sheath and, through modification of the EEDF, a number of plasma parameters. Another possibility of modifying the EEDF is by application of a negative potential to a portion of the plasma boundary. This can allow modification of the fast part of the EEDF. The above effects and methods can be used in various research and technical applications

Influence of an additional ballast volume on a pulsed ICP discharge

A spatial and temporal numerical simulation has been carried out of a pulsed (100% modulated), rf inductively coupled plasma discharge in argon, connected to an additional (ballast) diffusion chamber of much larger volume. It is demonstrated that during the active phase, the presence of the large ballast volume has a small impact on the parameters of the plasma in the smaller discharge chamber. In this case the plasma parameters in the discharge chamber can be estimated separately from the diffusion chamber by a standard method using the characteristic ambipolar diffusion time (for example, using a global model). However, during the afterglow phase, the situation is changed significantly. In the afterglow, the densities of charged particles in the discharge chamber become lower than in the large ballast chamber due to more rapid diffusion loss. As a result, the reverse of the active phase situation occurs, namely, the plasma does not flow from the small to the large chamber, but in the opposite direction, from diffusive to discharge volume, and both the plasma density gradient and the self-consistent ambipolar electric field in the small chamber change directions. This phenomenon leads to new effects in the discharge volume, in particular a decreasing rate of decay of densities of charged particles and electron temperature. Thus, in the afterglow the presence of a large additional ballast volume has a significant impact on the plasma transport. In this case, a simple treatment of the plasma in the discharge chamber in the framework of a spatially averaged model (for example, the global model) is inadequate.

Gas-phase ion chemistries in perfluoromethylcyclohexane

Electron impact ionization and ion–molecule reactions of perfluoromethylcyclohexane (C7F14) using Fourier-transfer mass spectrometry are reported. The electron impact ionization produces dominant ions and throughout most of the energy range of 10–200 eV, with the total ionization cross section peaking at ~100 eV with a value of 1.8 × 10−15 cm2. Numerous ions are observed at energies within a few electron volts of the lowest ionization threshold: , , , , , and . The lighter ions CF+, and are found to react with C7F14 forming and other product ions that we believe to result from further fragmentation of the intermediate ion . The charge transfer reaction between Ar+ and C4F14 yields similar product ions.

Comparison of helium two-step plasma emission with that predicted from measured cross sections

Plasma emission from the afterglow of a low-pressure, 100% modulated, radio frequency (rf) excited discharge can originate from collisions between metastable atoms and fast electrons. The fast electrons are generated by collisions between pairs of metastables (Penning ionization of one metastable by another) and collisions of metastables with slower electrons (superelastic collisions). Using time-resolved Langmuir probe data, measurements were made of the electron energy distribution function (EEDF) containing these fast electrons in a helium afterglow. The EEDF data were used, along with measured optical cross sections out of the 2s3S and 2s1S metastable levels, to predict intensities of various He emission lines seen in the afterglow. A comparison between the measured and predicted emission is presented.

Electron Collision Processes in Nitrogen Trifluoride

Nitrogen trifluoride is used extensively in several aspects of semiconductor processing and manufacture and was also employed as an atomic fluorine source in pulsed electrical-chemical lasers. The electron collision database is of interest for modeling and simulation of plasma enhanced etching of materials. We have recently made comprehensive measurements of the absolute dissociative ionization cross-sections of nitrogen trifluoride and also of its dissociative charge transfer from argon ions. These results are reviewed and compared with previous data in the literature. We also compile, where available, the results for electron attachment, momentum transfer, vibrational excitation, and dissociative excitation. This data set is compared with the results from swarm experiments for mixtures of NF3-argon and NF3-nitrogen. The needs and opportunities for additional experimental studies are outlined.