Generation of slowly rotating microwave plasma by amplitude-modulated resonant cavity

Abstract

Slow rotation of microwave plasma at a rotational frequency of Ω/2π = 0.1–1000 Hz is realized to improve plasma uniformity by using a resonant cylindrical cavity and a solid-state microwave generator at a frequency of ω/2π = 2.4–2.5 GHz. The microwave at ω/2π is modulated in amplitude at Ω/2π and injected into the cavity from two orthogonal positions, exciting the TE111 mode. The cavity fields rotate either clockwise or anticlockwise at a frequency of Ω/2π when the phase differences, Δϕ at ω and ΔΦ at Ω, between the input microwaves are properly set as calculated by a theoretical analysis and finite-difference time-domain simulation. Rotating plasmas are experimentally measured in the microwave discharges of argon at 0.1–20 Torr. When the rotational frequency is low (Ω/2π < 30 Hz), a plasma rotation is visible in the optical emission image; the azimuthal rotation of a local ion density is also confirmed by a rotatable Langmuir probe array. Conversely, when Ω/2π > 1000 Hz, the electron density measurement by a curling probe reveals that the plasma rotation disappears in the downstream region. This observation is supported by a simplified analysis based on the diffusion equation, proving a characteristic distance of plasma rotation disappearance to be (Da: ambipolar diffusion coefficient).

Uniformity enhancement of a microwave surface-wave plasma by a field agitation

“…In this work, we present a different approach to achieve uniformity in a high-density, large-area SWP based on the modulation of TM mode microwave field distribution. There are two major differences between the work in the previous studies 11,12) and this work. Firstly, the cylindrical cavity used in this work is designed to resonate in higher-order mode, namely TM .…”

“…The normalized field distribution of the z-axis component for TM mn0 from two magnetrons (MGT 1 and MGT 2) placed perpendicularly on the same plane is given by the previous studies 11,12) ( ) ( ) ( )…”

“…The non-uniformity of the large-area microwave plasma is attributed to the fact that the typical chamber diameter is significantly longer than the wavelength of the microwave (∼12.2 cm for 2.458 GHz). [6][7][8][9][10] Previous studies 11,12) have proposed rotating microwave fields with TE 111 mode to enhance the plasma uniformity by imposing the phase difference between the two orthogonally placed microwave sources or by modulating the field amplitude of the microwave sources.…”

“…Hence our system is anticipated to exhibit distinctive characteristics such as: (1) higher plasma density from a higher power transfer efficiency than the TE 111 mode-based system used in the previous studies. 11,12) (2) Enhanced uniformity in azimuthal direction for a large-area SWP. Such characteristics are observed from the plasma emission pattern image as well as the ion density from the Langmuir probe.…”

Phase-transition time of a commercial magnetron driving a surface-wave plasma load

“…[14][15][16] To improve the uniformity, a rotating field system to excite a surface-wave plasma (SWP) which generates a specific wave mode between the plasma and a dielectric window was proposed in a cylindrical cavity by imparting a phase difference of microwaves. 17,18) Taking the superiorities of PLL-injected MGTs into consideration, a rotating field system for a SWP driven by MGTs in substitution for the solid-state microwave generators is worthy of investigation. And the rotating field system requires a time-modulation on the phase difference between the microwave inputs.…”