Radio-frequency plasmas are used for a variety of processes including uses in the semiconductor industry for plasma deposition and plasma etching. The plasma can be viewed as the combination of at least three processes: (1) a voltage is applied to one electrode while another one is grounded creating an electric field between the electrodes; (2) the electric field accelerates electrons to very high energies (1 to 20 eV); (3) at those high energies, electrons react with neutrally charged molecules to form more electrons, ions, free radical species, and excited molecules. (4) the free radicals react with each other, other gas phase molecules, or with the molecules on solid surfaces of the reactor; and (5) ions are accelerated through the plasma sheathes to produce high energy bombardment (up to several hundred eV) of the solid surfaces which influence the surface reactions occurring there. At the present time the development and control of most etch and deposition processes is mainly governed by empirical approaches that examine the final effect created (such as etching rate, uniformity across a wafer, etch directionally, etc.) as a function of the process conditions (power, pressure, gas composition, frequency) for a particular reactor. A more fundamental understanding of a particular process would speed up the development process (especially of new orsubstantially different plasma processes) and aid in understanding the control of these processes. To obtain such a fundamental understanding and to ultimately model the whole system, a quantitative understanding and/or measurement of each individual process occurring is needed. The objectives of this research project are to establish and use a new method to determine electron concentrations as function of time in low frequency (40 - 400 Hz) discharges. High frequency discharges produce faster etch and deposition rates while low frequency discharges produce higher ion bombardment energies of solid surfaces which leads to improved surface effect such as better etching directionality. The PI's method involves perturbing the low frequency discharge with high frequency signals, and then measuring and analyzing the high-frequency impedance as a function of time within the low frequency cycle. These impedances will be interpreted in terms of the plasma as an electrical circuit to give the electron concentrations in the discharge as a function of time. The technique will allow experimenters to determine quantitatively the electron concentrations occurring in specific etching or deposition discharges. Electron concentration measurements will help in predicting rates of the electron-molecule reactions occurring in the plasma reactor. These measurements can be combined with other information to determine the ultimate effects on etching or deposition rates and characteristics.
