Temporal behavior of microwave sheath-voltage combination plasma

TL;DR

This study investigates the temporal evolution of microwave sheath-voltage combination plasma (MVP) generated along a biased Ti rod, revealing two distinct plasma modes with different compositions and densities, relevant for plasma processing applications.

Contribution

It provides the first detailed analysis of the temporal behavior and mode transition of MVP, highlighting the plasma's changing composition and density over time.

Findings

Two plasma modes identified: oxidized state (OS) and ionized sputter state (ISS).

Transition from OS to ISS causes a color change and density increase.

High-density plasma (10^12 cm^-3) achieved at low microwave power (~30 W).

Abstract

Microwave sheath-Voltage combination Plasma (MVP) is a high density plasma source and can be used as a suitable plasma processing device (e.g., ionized physical vapor deposition). In the present report, the temporal behavior of an argon MVP sustained along a direct-current biased Ti rod is investigated. Two plasma modes are observed, one is an "oxidized state" (OS) at the early time of the microwave plasma and the other is "ionized sputter state" (ISS) at the later times. Transition of the plasma from OS to ISS, results a prominent change in the visible color of the plasma, resulting from a significant increase in the plasma density, as measured by a Langmuir probe. In the OS, plasma is dominated by Ar ions and the density is order 10^11 cm^-3. In the ISS, metal ions from the Ti rod contribute significantly to the ion composition and higher density plasma (10^12 cm^-3) is produced. Nearly uniform high density plasma along the length of the Ti rod is produced at very low input microwave powers (around 30 W). Optical emission spectroscopy measurements confirm the presence of sputtered Ti ions and Ti neutrals in the ISS.