Test particle simulation of the role of ballistic electrons in hybrid dc/rf capacitively coupled CF4 plasmas

Abstract

Hybrid dc/rf plasma sources are an emerging equipment technology in plasma etching for semiconductor manufacturing. In this type of plasma source, ballistic electrons originate as secondary electrons on a dc/rf (i.e., VHF, 60MHz) biased electrode and are then accelerated in the sheath toward an opposite non-dc biased lower electrode. For electropositive (argon) plasmas it has been shown that the primary contribution of ballistic electrons is ionization in the dc sheath and modulation of the discharge properties. Whether applied dc power net increases or decreases the plasma electron density depends on the rf power environment of the opposing substrate electrode. When rf power is applied to a substrate electrode, the rf self-bias creates a potential well that traps the ballistic electrons, the result being that the electron density increases with dc power. In most cases the fraction of high energy electrons that reach the electrode is small. In this article, the authors describe the use of test particle Monte Carlo simulations to describe the behavior of hybrid dc/rf electronegative (CF4) plasmas. In contrast to the behavior in argon, process experiments with electronegative gases such as CF4 indicate that the electron density is independent of dc bias power when no rf power is applied to the wafer. Test particle simulations show that CF4 provides for a “self-confinement” effect caused by large cross sections for vibrational excitation at intermediate to low electron energies, which results in weak dependence of electron density on dc bias voltage when low frequency bias is not applied to the wafer. This emphasizes the important role of gas composition and cross-section structure in the control of dc/rf plasmas. Consistent with experiment, test particle Monte Carlo simulations also show that when rf is applied to the substrate, overall the CF4 plasma’s macroscopic properties are similar manner to argon plasmas. Even so, differences between the argon and CF4 plasmas occur related to the scaling of the fraction of ballistic electrons that reach the wafer as a function of dc and rf power. A regime is identified in which CF4 and argon plasmas have the equivalent ratio of ballistic to thermal electron density adjacent to the surface. The authors explain that the differences are related to different thermalization and ionization mechanisms in these plasmas. In addition, they find the electron attachment constant is zero in the dc sheath and nearly constant value in the bulk as the transit time in the sheath is far less than the mean attachment time. The same simple function to express ionization rate constant as a function of electric field in the dc sheath, developed for argon, can be also applied to CF4.