Sustaining mechanism and spatial structure of high-density ring-shaped hollow cuspate magnetized rf plasma for low-pressure plasma processing

The ring-shaped hollow cuspate magnetized rf plasma with the hollow trench and magnets has been developed to improve the low-density of conventional hollow cathode discharge under a low-pressure, around 1 Pa. From the point of view of the sustaining mechanism, magnetic characteristics of plasma particles such as electrons and ions have been investigated. The effective electron-neutral collision frequency in the presence of a magnetic field has the magnitude of THz, which is approximately 100 000 times higher than the collision frequency without the magnetic field. The effective electron mean free path in the presence of the magnetic field is less than a micrometer, so that the electrons can collide with neutral atoms more frequently in the ring-shaped groove. It is found that the plasma density at the distance of 5 mm under the hollow groove attains more than 2.5 × 1011 cm−3 at a power density of 0.14 W cm−2 and an argon pressure of 1 Pa. The sustaining mechanism is based both on the huge effective electron collision frequency and the strong magnetic confinement of electrons across the transverse magnetic field in the exit of the hollow groove. The spatial distribution of the plasma density has become uniform at the distance of 70 mm from the electrode surface and the uniformity and the averaged plasma density are 18% and 6 × 1010cm−3. It is indicated that the uniformity of the plasma density is controlled by gas pressure.