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Plasma Processing and Processing Science
Ionization processes. In classical gas discharge theory, the electron-ion pairs forming the plasma are imagined to be produced by electron impact on neutral atoms. These could be fast electrons in the “primary” electron distribution, or they could be electrons so far in the tail of the thermal distribution that their energies exceed the ionization potential. In the dense plasmas of today, however, particularly the high-pressure discharges used in deposition, multistep ionization via metastables can occur. In fact, in high-pressure discharges such as used in the lighting industry, the propagation of ionization energy occurs via transport of resonant photons rather than electrons. This subject requires both theoretical and experimental study, and the results for etchant gases should yield information on the cross-section data that need to be obtained.
Electron runaway in high-pressure rf discharges. In direct current discharges between a cathode and an anode, applying too large an electric field will cause the discharge to break into an arc. In electrodeless rf discharges, however, extremely large electric fields can be applied without arcing. In that case, some electrons can “run away” into a velocity region of decreasing cross section and therefore reach ionizing energies even when the mean free path for a thermal electron is extremely short. This phenomenon would be expected to occur preferentially in gases, such as hydrogen, that do not have a large peak in the cross-section
Landau and cyclotron damping in rf discharges. Production of primary electrons by Landau damping has been postulated for helicon discharges, but there has been no prediction or measurement of the number of electrons accelerated in this manner. More recently, cyclotron damping has been proposed as an additional mechanism that can be important in low magnetic fields. These kinetic absorption mechanisms may be important in low-pressure plasmas used for materials processing.
Particle confinement by multidipole magnetic fields. Large uniform plasmas can be produced by letting plasma stream from a source into a “magnetic bucket” with permanent magnets lining the walls. Previous studies of confinement by such surface fields were concerned with the overall confinement time of the plasma. In etching tools, however, it is the confinement of each velocity class of electrons that matters, since the electron distribution has a large effect on the production of the various molecular species and on the damage incurred on thin oxide layers. This problem should be reexamined in the light of the new requirements.
Expansion of plasma in rapidly diverging magnetic fields. In “remote” plasma sources, the substrate to be processed is exposed to plasma that has come from the ionization source along magnetic field lines that are sharply curved. The usual adiabatic invariants are not preserved in such an environment, but there may be other invariants if the system is axisymmetric. The manner in which the electrons and ions of various energies move will determine the potential and density gradients in the downstream plasma, as well as the ionization occurring there. Though numerical modeling may ultimately be needed to treat this complicated problem, insight into the physics can be gained by considering general principles such as the invariants mentioned above.
Plasma instabilities. All plasmas, particularly magnetized ones, are subject to instabilities. In industrial devices, there can be drift instabilities due to gradients in density or temperature; gravitational instabilities due to curving magnetic field lines; or streaming instabilities due to non-Maxwellian distributions, such as in ECR. No devastating instabilities have yet been seen, but they will no doubt be found someday. In that case, stabilizing measures, such as minimum-B fields, are well in hand because of what has been learned in magnetic fusion.