Invited Paper PS-WeA10
The Control of Electron Shading and Plasma EEDf in a DC/RF Parallel-Plate Etcher
Wednesday, November 2, 2011, 5:00 pm, Room 201
There are several principles in the consideration of plasma etcher design. This paper addresses two important areas: (1) the ability of tailoring the electron energy distribution function (EEDf), (2) the ability of adjusting the charging and neutralization of surface features (the electron shading effect). Stochastic heating by high frequency RF (VHF) energizes the Maxwellian bulk into the energetic tail population for efficient ionization. Such energetically bottom-up heating also indiscriminately populates the below-ionization energetic group that drives chemistry such as molecular dissociation and VUV production. A generic DC/RF system has a RF biased wafer-electrode with a high-negative DC superimposed opposing electrode. The DC/RF system dominates its electron heating with an energetically top-down process. The secondary electrons emitted from the high-negative DC surface disseminate the beam-energy into a distribution of energetic-electrons through collisions and more importantly, various beam-wave instabilities. These energetic electrons are trapped between the sheaths of the two parallel plates when the RF sheath field is sufficiently strong, dissipating their energies mainly into ionization. The energetic part of the EEDf reveals (in descending order) a group of ballistic-electron associated with the applied –DC voltage, an energy continuum, and finally stop at a lower middle-energy peak in the range of ~ 40eV to 300eV depending on the process. The data show extremely efficient ionization by these energetic electrons and as a result, the Maxwellian bulk remains relatively unchanged at Te~1.8eV regardless the bias RF power and process pressure. Such energetically decoupled EEDf enables increased ionization without increasing molecular dissociation. In one RF period, the trapping of these energetic electrons diminishes as the RF sheath collapses. By pulse-synchronizing the –DC voltage and the bias RF voltage, ion-bombardment excitation of the wafer surface can alternate with high-energy electron-bombardment neutralization of the wafer surface. Ion-bombardment of the wafer surface occurs when the bias RF pulse is high and the –DC pulse is at a medium level. As the RF and the DC pulses are synchronously altered to a low RF voltage and a high-negative DC voltage, ion-bombardment excitation of the wafer diminishes and the wafer surface sees an increased current of energetic (>25eV) electrons. High aspect ratio SEM data show that the signatures of electron shading (e.g., bending and twisting of the features) are eliminated when synchronous DC/RF pulsing is implemented allowing energetic electrons to reach the bottom of high aspect ratio features.