Invited Paper PS-ThM5
High-Density Plasma Generation in Low-Pressure Metamaterial Space

Thursday, November 2, 2017, 9:20 am, Room 22

Generation of high-density plasmas have been one of the main topics in science and technology of low-temperature plasma since high throughputs in material processing such as dry etching and thin-film deposition are achieved by high electron density which enhances chemical and physical processes in weakly-ionized plasma. When we use microwaves in plasma generation, there have been several methods proposed so far for high-density plasma, like electron-cyclotron-resonance plasma and surface-wave plasma.

Here we propose another scheme in which a magnetic metamaterial makes magnetic permeability in discharge space negative. Microwave propagation in simple discharge space with no objects and no external magnetic field is limited by cutoff density where electric permittivity or dielectric constant is down to zero. When magnetic metamaterial who has negative permeability is installed in the space, microwave propagation is possible beyond the cutoff density, with negative refractive index state with negative permittivity that indicates high electron density. Experimental observation confirmed existence of this scheme, and electron density was much higher than the cutoff density (approximately 7x10¹⁰ cm^-3 when microwave frequency is 2.45 GHz) [1]. The value of electron density has no limitation with smooth microwave propagation with large negative values of refractive index.

In addition to these advantages on generation of high-electron-density plasma, recent experimental studies have revealed that this scheme of plasma generation has roles of high energy storage and an efficient energy converter. From the estimations based on monitored electron density and temperature, the existing energy density in the plasma generation space with the metamaterial is by 3 orders of magnitude larger than that in microwave propagation in the free space. Such stored energy is efficiently converted into the second harmonic wave via nonlinear and asymmetry effects between plasma and metamaterial [2], as well as into chemical energy via enhanced dissociation of gas molecules. These facts indicate that this plasma source will contribute to novel functions that can be hardly established using other plasma generation schemes as well as the general function as a high-density plasma source for material processing.

[1] O. Sakai, Y. Nakamura, A. Iwai and S. Iio, Plasma Sources Sci. Technol. 25 (2016) 055019.