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

Hybrid dc/rf plasma sources are an emerging equipment technology in plasma etching for semiconductor manufacturing. In this work, test particle simulations are used to describe the nature and role of the ballistic electrons, which originate as secondary electrons on a dc/rf (i.e., VHF, 60 MHz) biased upper electrode and are then accelerated in the sheath toward the opposite non-dc biased lower electrode. This opposite electrode, on which a wafer is placed, may have rf bias power applied to it at low frequencies. 2 MHz is typical. While a hybrid dc/rf plasma source is sustained by the VHF power source, simulations reveal ballistic electrons assist in the production of a plasma characterized by a relatively high ionization rate constant, especially in the dc sheath. The dc source helps to provide a source of high energy electrons that may reach the wafer. However, when the rf bias is applied on the lower electrode, ballistic electrons are confined in the potential well formed between electrodes and easily thermalized in the bulk. The simulated electron density dependencies on the dc bias and lower electrode rf power describe a dual mode source and are consistent with measured data. When no rf power is applied on the electrode opposite to the dc power source, the bulk electron density drops with increasing dc power at constant VHF power; when rf power is applied to the electrode opposite to the dc power source, the bulk electron density increases with dc power at constant VHF power. The fraction of electrons significantly above thermal reaching the wafer is much higher when no rf power is applied at the lower electrode but the total flux lower. Depending on its level, rf power applied to the wafer may effectively suppress the number of ballistic electrons that reach the wafer. Finally, the ionization rate constant due to ballistic electrons can be fitted as a simple function of the electric field in the dc sheath.