Diagnostics of a Multicusp-Assisted Inductively-Coupled Radio-Frequency Plasma Source for Plasma Immersion Ion Implantation

TL;DR

This work addresses the challenge of characterizing a multicusp-assisted inductively coupled RF plasma source for Plasma Immersion Ion Implantation (PIII) by applying laser-induced fluorescence (LIF) and RF-compensated Langmuir probes to map ion temperature $T_i$, drift velocity $v_z$, and plasma potential in both the bulk and presheath. The authors demonstrate that magnetic cusps markedly increase plasma density at low pressures and that $T_i$ and $v_z$ exhibit expected dependence on RF power and proximity to an immersed electrode, with ions accelerating to near the Bohm speed $u_B$ within the presheath. Time-averaged LIF measurements show no distortion during short-duty-cycle high-voltage pulses, indicating rapid plasma recovery between pulses and validating steady-state assumptions for characterizing the presheath with LIF. The results support the use of LIF as a quantitative diagnostic tool for bulk and presheath regions in PIII systems, enabling more accurate control of ion energy and fluence for surface processing.

Abstract

In this article, we present a detailed characterisation of a multicusp-assisted inductively coupled RF plasma source for plasma immersion ion implantation (PIII). Using laser-induced fluorescence (LIF) and RF-compensated Langmuir probe diagnostics, we measured ion temperature T i and drift velocity v z in argon plasmas near an immersed electrode. The multicusp configuration enhances plasma density at low pressure, enabling stable operation down to 0.8 mTorr. Timeaveraged measurements show no detectable perturbation near the pulsed electrode, indicating full plasma recovery between high-voltage pulses. LIF-derived potential profiles match Riemann's presheath theory, and ion velocity distributions reveal acceleration consistent with sheath dynamics. These results support the use of LIF for steady-state characterisation of the bulk and presheath regions in PIII systems.

Figures (9)

• Figure 1: (a) Schematic of the experimental setup for Langmuir probe measurements: 1--Langmuir probe controller adjustable DC power supply; 2--Langmuir probe; 3-- operational amplifier; 4--Oscilloscope; 5--High voltage pulser; 6--vacuum vessel; 7--multicusp belt of permanent magnets; 8--Substrate holder. (b) Picureof the Lanmuir probe. (c) Schematic of the probe’s internal circuitry, with four inductors connected in series to reduce the effect of the RF fluctuations on the measured signal.
• Figure 2: (a) Schematic of the experimental setup for LIF measurements. 1--Laser controller; 2--laser head; 3--Fabry-Perot etalon; 4--photodiode; 5--iodine cell; 6-- iodine cell heater; 7--computer; 8--fibre optic head and wavemeter; 9--beamsplitter; 10--chopper blade; 11--chopper controller; 12--plasma chamber; 13--multicusp belt of permanent magnets; 14--pulsing HV electrode; (15) electrode NPHV pulsing system; 16--LIF collection optics; 17--narrow bandpass optical filter; 18--photomultiplier tube; 19--lock-in amplifier; 20--oscilloscope. (b) Theoretical iodine absorption spectrum around the 668.6138 nm argon ion transition Salami_2005. (c) Raw signal from the LIF setup. From top to bottom: Iodine cell absorption spectrum, Fabry-Perrot etalon signal, LIF signal.
• Figure 3: Partial Grotrian diagram for Ar-II transitions.
• Figure 4: Ion density $n_i$ with (black diamonds) and without (magenta circle) magnetic confinement was measured at various pressures at 500 W input power using a Langmuir probe positioned along the discharge axis. For improved visual clarity, error bars for the measurements without magnetic cusps are omitted; however, they are comparable in magnitude to those associated with the measurements taken in the presence of magnetic cusps.
• Figure 5: Langmuir probe measurements as a function of RF power and pressure: (a) plasma potential $V_{\text{pl}}$, (b) electron temperature $T_e$, (c) electron density $n_e$, and (d) ion density $n_i$. The electron density $n_e$ is approximately five times lower than the ion density $n_i$, likely due to probe-induced perturbations that drain the plasma and alter its equilibrium state chen_2003.