Control of electron energy distributions in inductively coupled plasmas using tandem sources

Summary form only given. In plasma materials processing, finer control of the electron energy distribution, f(e), can allow for better selectivity of generating reactants produced by electron impact excitation and dissociation. This is of particularly great importance in low pressure, inductively coupled plasmas (ICPs) where dissociation products often react with surfaces before interacting with other gas phase species, and so these fluxes are most directly a function of electron impact rate coefficients. The control of f(e), and so the reaction rate coefficients, can be achieved by varying the pressure, and power, as well as using pulsed power. Externally sustained discharges, such as the electron beam sustained discharges developed for high pressure lasers, are able to control f(e) by augmenting ionization so that f(e) can be better matched to lower threshold processes. In this vein, a tandem (dual) ICP source has been developed. In this device, the plasma produced by the primary source is coupled to the plasma produced by the secondary source through a grid to control the transfer of species between the two sources with the intent of controlling f(e) in the primary source. Results will be discussed from a computational investigation of the control of f(e) in a tandem source ICP at pressures of tens of mTorr. Both the power and the gas chemistry for the primary and secondary ICPs can be controlled independently. A boundary electrode (BE) at the top of the system, along with the grid separating the two sources, can be dc biased to shift the plasma potential and control the energy of charged species passing into the secondary source. The model used in this study is the Hybrid Plasma Equipment Model (HPEM) with which f(e) and ion energy and angular distributions (IEADs) as a function of position and time are obtained using a Monte Carlo simulation. f(e) will be discussed while varying the relative power in the primary and secondary sources, and dc biases (BE and grids) in continuous and pulsed formats. Results from the model will be compared to experimental data of f(e) and IEDs obtained using a Langmuir probe and a gridded retarding field ion energy analyzer.