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Plasma Processing of Materials: Scientific Opportunities and Technological Challenges
Models and diagnostic techniques for plasma generation and plasma transport are rapidly growing in sophistication. These are needed to simulate, understand, and control energetic bombardment of device wafers in plasmas. Because plasma generation and transport are the primary focus of new reactor design, the simulation tools already developed could be developed further and directly implemented in CAD tools. The advent of new computer technology will enable plasma simulators to meet the challenges of calculating particle energy distribution functions for multidimensional, magnetized systems. Numerical simulation will be invaluable in developing an understanding of the instabilities and turbulence in plasma reactors that currently inhibit reproducibility and control in processing. However, progress is currently impeded by lack of a reference model with which algorithms can be tested and evaluated; a lack of basic data on cross sections and rate constants that are required for making quantitative comparisons with the results of laboratory experiments; and a lack of reliable quantitative diagnostic data.
Diagnostic technology is sophisticated, but experiments are loosely focused and performed on a large variety of different reactors under widely varying conditions. A coordinated effort to diagnose a simple, reference reactor has begun to generate the necessary data base for evaluation of simulation results and to test new and old experimental methodology.
The dearth of basic data needed for simulation of plasma generation and transport results directly from insufficient funding. Data that exist are difficult both to find and to disseminate. The methods—both experimental and theoretical—exist for generation of most of the needed data. Lack of coordination between researchers generating basic data and those simulating and diagnosing plasmas also contributes to the problem. The critical basic data needed for simulations and experiments have not been prioritized.
To control plasma processes and make full use of basic surface and plasma science studies, the problems of plasma-surface interactions must be considered. Foremost among these interactions is how plasmas modify surface properties that affect emission of particles and surface conductivity. There is an urgent need for in situ analytical tools that provide information on surface composition, electronic properties, and material properties that relate directly to device yield. Another challenge is to couple plasma generation and transport simulation to surface processes in order to predict surface profile evolution during plasma etching and deposition.
Although the United States is making strong efforts in each of the three critical research areas, and in many cases the best efforts, these efforts are largely uncoordinated with respect to one another and are disconnected from the plasma equipment vendors who develop new reactors and processes. Funding comes from at least 14 different federal agencies as well as from separate industrial sources. Connections between surface processes, plasma generation and transport, basic data, and plasma-surface interaction research are nonexistent. This situation differs markedly from the situation in Japan and France, where research in these areas is closely coordinated between industrial, national, and university laboratories.