Paper PS2-TuA1
Surface Interaction Mechanisms Enabling Plasma-Enhanced Strongly Time-Dependent Etching Rates

Tuesday, October 30, 2012, 2:00 pm, Room 25

There is great interest in establishing directional etching methods capable of atomic scale resolution during fabrication of highly scaled electronic devices. We report a new concept to achieve controlled, self-limited etching of extremely thin layers of material using a polymer as a special case. The work was performed in a capacitively coupled plasma reactor. The polymer material is a 248nm photoresist. A complete process cycle consists of: O₂ exposure of the polymer material, exhaust of O₂ from the chamber, low energy Ar⁺ ion bombardment of the surface using Ar plasma to remove the oxygen-bonded carbon species, and Ar exhaust. This sequence is repeated up to 20 times to investigate reproducibility of each cycle and time dependent behavior. Controlled etching is based on deposition of a thin reactive layer during Ar⁺ sputtering from a polymer-coated electrode (polyimide-related material) within the chamber. The polyimide-related film deposition balances etching during the Ar⁺ ion bombardment step once the reactive layer has been removed, and enables control of the etching depth. Ar⁺ ion bombardment energies were selected so that once the oxygen-bonded carbon material and physisorbed layer had been removed, net etching ceased. If the ion energy is too small, deposition of sputtered polyimide-related material dominates over etching. When applying a too high selfbias voltage, significant pristine polymer etching takes place and a self-limited process cannot be achieved. Using real-time ellipsometric monitoring, we demonstrate strongly time-dependent etching rates. Starting with a high etch rate of ≈19 nm/min for 2 sec, it decreases within the next 5 sec to 0 nm/min, and therefore shows self-limitation. During the Ar etching step, the O₂ modified deposited reactive layer along with 0.13 nm unmodified polymer can be removed, while concurrent deposition prevents net etching of the unmodified polymer and enables achievement of self-limited etching cycles. This etch is believed to be d irectional enabled by the energetic ion bombardment of the surface. Subsequently, the reactive surface is modified by O₂ adsorption during the O₂ exposure step. Molecular oxygen does not spontaneously react with carbon-based polymers at room temperature, but can be adsorbed on an activated polymer surface to form a very thin layer of oxidized carbon material over unmodified polymer. A thickness increase of 0.6 nm per cycle is observed via real-time ellipsometry. Additional XPS studies allow the investigation of the surface material composition for further insight on the reactive layer deposited during the Ar⁺ etching step and the adsorbed layer during O₂ exposure.