Etching Reactivity of Negative Ions Generated in Cl2 Downstream Plasma

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Etching reactivity of negative ions is studied in the downstream region of inductively coupled Cl2 plasma. Negative ions are confirmed by a quadrupole mass and energy analyzer, and Cl- ions are found to be the dominant species and to show a maximum density at 30 mTorr pressure. The Si etch rate is examined in two ways; one is by direct exposure to the plasma with a certain DC bias and the other is by using a specially designed electrode box which enables independent ion irradiation by eliminating electrons magnetically. A higher etch rate is observed at the positive bias, especially at a pressure higher than 30 mTorr, while at the negative bias, the etching closely depends on pressure and is not observed at pressures higher than 10 mTorr. The Si etch rate obtained by the electrode box with the positive DC bias was maximum at 30 mTorr, being consistent with the result of the mass analysis and revealing that the etching is due to negative ions. It is concluded that the higher etch rate at the positive bias is due to the negative Cl- ions whose reactivity is higher than that of the positive Cl$_{2}^{+}$ ions. An in situ X-ray photoelectron spectroscopy (XPS) observation of the etched Si surface showed that the depth of the SiCl layer was thinner at the positive bias than at the negative bias, confirming that the negative ions have the higher etching reactivity.
2003-03-15