COMPARISON OF ADVANCED PLASMA SOURCES FOR ETCHING APPLICATIONS .2. LANGMUIR PROBE STUDIES OF A HELICON AND A MULTIPOLE ELECTRON-CYCLOTRON-RESONANCE SOURCE

Radial profiles of electron temperature, electron density, and ion current density near the wafer surface are presented for a rf-inductively coupled helicon and a multipole electron cyclotron resonance source as a function of applied source power, applied rf-bias power, and reactor pressure. Both sources show similar trends with respect to changing process conditions, with electron densities in the mid 10(11) cm-3 range and electron temperatures between 5.5 and 6.5 eV. An increase in applied source power results in an increase in both electron density and ion current density as expected. An increase in applied rf-bias power results in a 10% change in the measured electron density and ion current density; however, the electron temperature shows a much stronger dependence, indicating that ion flux and ion energy are not completely independent. Radial uniformity is similar in both sources, with the helicon exhibiting better uniformity at the conditions explored. Neither applied source power nor rf-bias power significantly affect radial plasma uniformity above the wafer in both sources, but higher reactor pressure does degrade uniformity in the helicon. We attribute this to the 2:1 geometric expansion between the upper antenna section and the lower plasma confinement section where the measurements are taken. The conditions resulting in optimum radial uniformity of electron temperature, electron density, and ion current density are similar to those for the optimum etching rate uniformity measured in previous studies; yet the uniformity obtained from the Langmuir probe diagnostic is not affected by changing process conditions to the degree that etching uniformity is affected. This can be a result of the difference in chemistry between the two studies since hydrogen bromide was used for etching and argon was used for Langmuir probe measurements. It may also indicate that the correlation between plasma uniformity and etching uniformity is relatively weak or second order compared to other variables affecting uniformity such as rf bias of the wafer.