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e combination of neutral species adsorption and positive ion bombardment results in surface chemical reaction. If the products of the surface reaction are volatile, they leave the surface and etching results. If the products are involatile, a surface film grows. Plasma processes are composed of électrons and ions that "activate" gas phase and surface chemistry, and neutral gases that flow through the reactor. In many ways, plasmaassisted processes are similar to other reacting flows such as combustion, with the différence that the energy "causing" the chemistry is originally electrical and cornes through the plasma.

Complex and parameter-sensitive plasma processes may be better understood by a combination of simulation and equipment. The most important application of plasma processing to date has been in etching, or surface removal of material. Although there are many ways in which etching could be done (for example, liquid acidic solutions hâve traditionally been used), with plasma etching it is possible to etch faster in the direction p e r p e n d i c u l a r to the surface of the material than in the direction parallel to

the surface. This so-called anisotropic etching has allowed finer features to be etched, thus allowing many more transistors and diodes to be manufactured per unit wafer area. Anisotropic etching is the key for making eversmaller (and therefore faster and less expensive) devices. The major disadvantage of plasma processing has been its complexity; one can manipulate such important process characteristics as etch r a t e , spatial uniformity, selectivity and degree of anisotropy by adjusting many operating parameters (flow rates, gas composition and pressure, electrical power, reactor geometry, etc.). Furthermpre, changes in process characteristics due to changes in operating and design parameters are usually non-intuitive. This results in tedious and costly trial-and-error techniques to develop new processes or mpdify existing ones. Our motivation for simulating plasma processes is to understand and eventually directly exploit this parameter sensitivity. Plasma Reactors There are many différent geometries that can be used for plasma-assisted materials processing, but for the purposes of illustration we restrict attention to the parallel plate configuration, shown schematically in Figure 1. Electrical power is supplied to the upper électrode in the form of radiofrequency (rf) or direct current (de) power. Radiofrequency power is the more c o m m o n form of coupling for plasma processing, with frequencies usually between 10 kilohertz and 50 mégahertz. Rf discharges enjoy several advantages compared to the de discharges. For example, in de discharges électrodes must be electrically conducting in order to pass the current that sustains the discharge. In rf discharges this is not necessary because current flows alternately in each direction, so electrical charges built u p at an insulating électrode are neutralized every half cycle. (Insulating électrodes themselves act as capacitors; ac current f