Dependence of Trench Charging on the Velocity Distribution of Ions incident on a SiO₂ Wafer

As ULSI device dimensions have been scaled down, a number of issues have been raised during plasma etching. Especially, a charging damage during plasma etching is important issue to be addressed. We focus on the charging caused by the difference of the velocity distribution between electrons and positive ions incident on a SiO₂ wafer. Etching of SiO₂ is performed by using high energy positive ions accelerated in a sheath in a collision dominated region for pressure, 5 mTorr - 50 mTorr in two-frequency capacitively coupled plasma (2f-CCP). The velocity distribution of ions incident on a wafer is, therefore, one of the crucial factors governing the characteristics of the plasma etching. The velocity distribution striking the wafer is strongly affected by the oscillation of the sheath potential formed in front of the wafer, and the distribution depends on the bias frequency, voltage, gas pressure and sheath width etc. A numerical analysis is an attractive approach to predict the time profile of the velocity distribution, because it is difficult to perform a time resolved measurement of the ion velocity distribution.A negative ion injection to the wafer were numerically predicted and designed in a pulsed 2f-CCP driven at VHF(100 MHz) - LF(1 MHz) as one of the practical solutions to the reduction of the charging damage. An experimental evidence of the injection of negative ion to the wafer and of a reduction of charging at the bottom of SiO₂ holes were visualized by the emission selected CT image close to the wafer and the test chip measurement during etching in almost the same system as the numerical. We have, however, few results of ion velocity distribution at the wafer deeply biased by the LF source in a 2f-CCP system.In this work, the time dependence of the velocity distribution of positive and negative ions incident on the wafer in a pulsed 2f-CCP in CF₄(5%)/Ar is first discussed. Quite different behavior of the velocity distribution of ions between ON and OFF period of the VHF plasma source was shown. These velocity distributions with difference in polarity and magnitude have key roles in reducing the topographically local charging inside the trench during plasma etching. The mechanism of the charge reduction is also discussed associated with the ion velocity incident on the wafer with a trench pattern.