Influence of operating frequency and coupling coefficient on the efficiency of microfabricated inductively coupled plasma sources

Microfabricated inductively coupled plasma (mICP) generators, operating at 690 and 818 MHz, have been constructed and characterized. The mICP consists of a single-turn coil that is 5 mm in diameter and a microfabricated matching network. Ion densities of similar to9 x 10(10) cm(-3) in argon at 400 mTorr consuming only I W were obtained. This ion density is three times larger than previous MICP Sources under the same conditions. The influence of the frequency of operation and the coupling coefficient on the power efficiency has also been studied. Contrary to what was observed in former generations of mICP sources operating at lower frequencies, the efficiency of the new mICP sources decreases as the frequency increases. A model that incorporates the electron inertia, the power dependence of the plasma resistance and the frequency dependence of the coil resistance agrees with the new experimental results as well as with the results of previous mICP sources. It was also observed that bringing the coil closer to the plasma increases the coupling coefficient of the ICP sources and thereby improves the efficiency of the device. The improvement in efficiency, however, is limited by the non-scalable plasma sheath width near the coil.