A digitally controlled solid-state microwave generator allowing variable frequency operation and precise phase control is adopted for plasma generation. In this study, a resonant cylindrical cavity is used as a microwave applicator in place of conventional waveguides. In order to improve the plasma uniformity, the TE111 mode is agitated by injecting microwaves into the cavity from two spatially orthogonal directions, with a temporal phase difference ϕ. Theoretical analyses and finite-difference time-domain simulations derive the following effects of the phase control. In the case of ϕ = ±π/2, fast rotation of the cavity field takes place with a rotational frequency of ω/2π (= 2.4–2.5 GHz), where ω denotes the microwave angular frequency. On the other hand, when ϕ is linearly modulated in time with a low frequency of Ω/2π (= 0.1–1000 Hz), slow pulsation takes place, in which the cavity field alternately excites a circular rotation and a standing oscillation at the modulation frequency. These effects are experimentally confirmed in microwave discharges in argon at 0.1–20 Torr with total injection powers from 50 to 800 W. Two-dimensional images of the optical emission from the generated plasma show that both the fast rotation and slow pulsation improve azimuthal plasma uniformity.

Cone-shaped hollow cathode electrode configuration for a damage free remote plasma removal process has been optimized for given pressures based on Paschen characteristic curves, voltage–current characteristics and time-resolved discharge observations as well as oxide film removal performances. Remote plasmas have been generated in two types of cone-shaped electrodes with mixtures of He, NF3, and NH3 for pressure range of 1–30 Torr. Paschen characteristic curves and voltage–current (V–I) characteristics define an operating pressure for low breakdown voltage and the hollow cathode effect to minimize the particles. Sinusoidal voltage waveform and asymmetry electrode configuration alternate the glow discharge and hollow cathode discharge modes in a cycle. The current and infrared emission intensity from the glow discharge increases together for both cone-shaped electrodes with increasing pressure, whereas the hollow cathode discharge plasma emits strong infrared only when pD condition is satisfied. For the wide cone electrode configuration, high voltage operation at higher pressure results in particle contamination on the processed wafer by high energy ion bombardment. Operating at optimum pressure for a given electrode configuration shows faster oxide etch rate with better uniformity over a whole 300 mm wafer.

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).

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