Dependence of etch characteristics on charge particles as measured by Langmuir probe in a multipolar electron cyclotron resonance source

Abstract

A multipolar electron cyclotron resonance (ECR) plasma source was characterized by Langmuir probe measurements and the charged particle energy and density are related to photoresist etch rate. Ion density was found to increase with microwave power but decrease with source distance, and is independent of rf power and flow rate. Among the different gases investigated, Ar plasma was found to have higher ion density compared to O2, N2, and Cl2 discharge. Ion density peaked at 5 mTorr for Ar and 2 mTorr for O2 plasma. For a N2 plasma, the maximum ion density was found to occur at 10 mTorr at 3 cm and 2 mTorr at 23 cm below the ECR source. Electron temperature is ∼3 eV for pressure ranging from 1 to 5 mTorr but decreases to 2 eV at 20 mTorr. Photoresist etch rate follows the same trend as ion density. It increases with microwave power and decreases with source distance. On the other hand, photoresist etch rate increases with rf power and flow rate, even though the ion density remains constant. This suggests that photoresist etching also depends on ion energy and concentrations of neutral species. Ion density uniformity of ±1% for a N2 plasma was measured across the central 24 cm along the stage at 23 cm below the source. For a source distance between 3 and 23 cm, uniformity of ±3% was obtained over a 12 cm diam region on the stage. In an O2 plasma, photoresist etch rate uniformity of ±2% was measured for a 10 cm diam wafer and the corresponding ion density uniformity was ±1%.