Plasma Confinement by Photoemission-assisted Discharge

Artificial plasma is easily created in some controlled atmosphere by electric discharge with a high-power source. It is called discharge plasma. The plasma that is generated in dry process methods such as plasma-enhanced chemical vapor deposition (PECVD) and sputtering is essential to the synthesis and processing of materials suitable to electronics or semiconductor engineering. As we can see in a plasma ball, however, the plasma is wandering, and its contour is vague. It is generally difficult to create plasma only at a desired site in the chamber with a couple of reproducible voltage and certain current density.

Our group has been developing the photoemission-assisted plasma that can be created only in a designated area where vacuum ultraviolet (VUV) light is irradiated. In this system, plentiful photoelectrons are generated in the VUV-irradiated area by the photo-electric effect and work as initial electrons. Due to this far different number of electrons in between the VUV-irradiated area and the other, discharge is preferred in the former area, and the plasma is created there only as a result. The VUV-irradiated area making the photoemission plasma there is actually restricted by a designed hole (16 mm in diameter) in a quartz cover. Quartz is insulative enough to avoid discharge under it.

However, the geometrically restricted photoemission-assisted plasma is spontaneously confined by controlling pressure in the atmosphere: from 16 mm to 3 mm in diameter. This is explained by imbalance between α and γ regimes. When the pressure increases, argon ions generated by the plasma is evacuated and is replaced by new argon atoms. However, continuous VUV irradiation keeps supplying photoelectrons over the substrate in parallel. The γ-regime reaction with argon ions survives at the center on the substrate. Because the reaction is accompanied with luminescence, the photoemission-assisted plasma seems to be confined as a result.