Low-Temperature Plasma Diagnostics

[IMAGE: A metal probe tip inside a glowing plasma, connected via a shielded cable to a rack-mounted electronics unit with an oscilloscope displaying the I-V characteristic]

Figure 1: RF-compensated Langmuir probe inserted into the helicon plasma plume. The probe tip (tungsten wire, 0.5 mm diameter, 5 mm exposed length) is visible at the centre of the discharge. The automated I-V sweep system and data acquisition computer are in the foreground.

Overview

This programme investigates the fundamental physics of low-temperature, RF-driven plasma sources, with an emphasis on quantitative plasma diagnostics and discharge characterisation. The primary experimental platform is a helicon plasma source operating in argon and argon–hydrogen mixtures, with auxiliary experiments on inductively-coupled and capacitively-coupled configurations.

Helicon Plasma Source

The helicon source operates at 13.56 MHz with RF power up to 3 kW delivered via a water-cooled double-saddle antenna surrounding a 150 mm diameter, 800 mm long quartz tube. An axial magnetic field of up to 0.12 T is provided by a set of four water-cooled electromagnet coils arranged in a Helmholtz-like configuration. The source is evacuated by a 2000 L/s turbomolecular pump backed by a dry scroll pump; base pressure is below 5×10⁻⁷ mbar.

Under optimised conditions (B ≈ 0.08 T, p ≈ 0.5 Pa, P_RF ≈ 1.5 kW), electron densities exceeding 10¹⁹ m⁻³ are achieved with ion energies below 20 eV at the substrate plane, making the source suitable for low-damage materials processing.

Diagnostic Suite

RF-Compensated Langmuir Probes

Single and double Langmuir probes with active RF compensation (tuned inductor and auxiliary electrode) are used for electron energy distribution function (EEDF) measurement, electron density, and electron temperature determination. An automated bias sweep system acquires full I-V characteristics in under 50 ms, permitting time-resolved measurements during pulsed operation. [Volkova, Rev. Sci. Instrum. 2003]

Optical Emission Spectroscopy

A 0.5 m imaging spectrograph with an intensified CCD camera provides spatially- and temporally-resolved optical emission spectra from 200–900 nm. Collisional-radiative modelling of Ar I line ratios yields independent electron temperature measurements for comparison with probe data. Ar II/Ar I line intensity ratios provide a qualitative map of the electron energy distribution across the plasma column.

[IMAGE: Three side-by-side false-colour images showing a cylindrical plasma cross-section with progressively brighter and more sharply-defined emission as the magnetic field increases]

Figure 2: False-colour image of Ar II (488 nm) emission intensity across the helicon plasma column at three different magnetic field strengths, showing the characteristic transition from capacitively-coupled to helicon wave-sustained mode.

Laser-Induced Fluorescence (LIF)

A tunable diode laser (New Focus Velocity, 680 nm) excites the 3p⁵4s → 3p⁵4p transition in neutral argon at 696.5 nm. Fluorescence at 772.4 nm is collected perpendicular to the laser axis to map the spatial distribution of Ar I metastable density with sub-mm spatial resolution. This diagnostic is under active development and expected to be operational by mid-2004.

Applications & Collaborations

Materials processing: The helicon source is used for reactive-ion etching studies of silicon and III-V semiconductors, with in-situ ellipsometry for etch-rate monitoring.
Biomedical: The plasma diagnostic techniques developed here are being transferred to the atmospheric-pressure plasma sterilisation programme in the Biotechnology Division.
External: The MIT Plasma Science and Fusion Center provides advisory support on helicon wave physics through the collaborations programme.

← Plasma & Thermal Sciences Division | ← Research overview