Paper PS1-MoM6
Effect of Dual Frequency Bias on Ion Energy and Angular Distribution and Feature Profile in a Capacitively Coupled Plasma Reactor

Monday, October 15, 2007, 9:40 am, Room 606

Capacitively coupled plasma reactors are commonly used in dry etch processes. Control of ion energy and angular distribution function (IEADF) is key to achieving desired etch profile in a plasma reactor. Dual frequency bias combining medium frequency (MF) and high frequency (HF) can be used to modulate the ion energy distribution. Understanding the effect of MF and HF mixing on IEADF is crucial for etch profile control. MF and HF bias are mixed in this investigation, and the mixing effect is characterized for IEADF and etch profile. Plasma simulations have been conducted using the Hybrid Plasma Equipment Model, while a string-based model is used for feature profile simulations. Interaction between ions and electric field modulation across the sheath determined the ion energy and angular distribution. A mono-energetic peak is observed using HF bias while ion energy distribution is broader using MF bias. DC bias is comparable at HF and MF bias for a given RF voltage. Plasma density with MF bias is smaller than that with HF bias. Mixing of MF and HF leads to lower DC bias and lower ion energy. The plasma density distribution and sheath structure are observed to change as the MF voltage is increased relative to HF bias for a given total MF + HF voltage. Ion flux at the wafer is determined by the plasma profile while the local sheath dynamics control the ion energy and angular distribution. Ion flux and IEADF are calculated near the center, middle and edge of the wafer, which are then input into a feature profile model. The feature profile model includes mechanisms for Si and mask etching. The etch depth and selectivity are calculated at the above three locations on the wafer for various mixtures of MF and HF biases. The etch profile is tuned using the mixing of MF with HF bias. Based on etch depth variation across the wafer, etch profile uniformity is calculated. Profile tilting is observed at locations where sheath is non-planar and the ion angular distribution is asymmetric. Various process-kits are evaluated to minimize profile tilting.