Plasma Parameters in a New Plasma Source

Summary form only given. Optimization of plasma sources needs knowledge of plasma parameters, first of all, electron temperature (Te) and plasma density (Np). This paper is concerned with experimental study of Te and Np in a new type plasma source for dry etching. It is based on ICP (2 MHz), and has a big advantage over usual ICP etchers: it can produce a directed flux of energetic ions on the wafer without applying the bias voltage. The source consists from two parts: electro-magnetic accelerator (EMA) and main chamber (MC). EMA can produce directed flux of energetic ions into MC. EMA consists from two coaxial quartz cylinders. Plasma is generated in the gap between them with a flat 3-turn coil placed on the EMA top. EMA is connected to the metal MC having larger diameter. The source can generate plasma in a wide range of conditions: pressure 0.5-100 mtorr, absorbed power 100-2000 W. We used axially movable double and single Langmuir probes to measure Te and Np. For pressures >5 mtorr, the plasma parameters show a normal behavior. But at low pressures (about 1 mtorr) and high powers (>800 W), the behavior is rather unusual. Now Te in EMA gap is very high (9-13 eV), while in MC volume it is much lower (4-5 eV). Np is almost constant over the source with a minimum at the boundary between EMA and MC. Te drops rapidly at this boundary, and the drop is accompanied by rapid decrease of the plasma potential. This means existence of strong electric field accelerating ions in the axial direction. Our measurements of ion energy distributions have shown the existence of hot ions with kinetic energies up to 15 eV. Several research groups reported similar results for helicon sources, namely, existence of strong potential gradient and group of hot ions. In our case there is no static magnetic field, so it can be the first observation of such effect in an ICP-type plasma source