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Measurement of IEDF for floating, DC, and RF bias in an ETP System with the Semion™ Ion Energy Analyzer System

In an Expanding Thermal Plasma (ETP) system, the plasma is generated with a cascade arc, which ionises the argon efficiently at near-atmospheric pressure.

The plasma flows through a 4.0 mm diameter and 24 mm long channel, after which it expands supersonically through a nozzle into the low-pressure reactor chamber. The ETP-reactor is described extensively by other authors [1,2].

A schematic drawing of the ETP-reactor is given in figure 1. The ion density is strongly reduced if hydrogen or oxygen is added. Hydrogen is injected together with argon leading to strong dissociation in the discharge. Oxygen is injected downstream through a ring. It reacts with the argon ions in the reactor chamber.

The Impedans Semion™ System was installed on the ETP system (Figure 2) to investigate the Ion Energy Distribution Function of the plasma in the chamber under floating, DC, and RF bias conditions. The RFEA Probe was placed on a custom-built substrate holder.

All experiments were run at fixed arc current of 50A, in Ar, Ar/O2, and Ar/H2 plasmas under various pressures (up to 35 Pa) and gas mixture compositions.

Figure 1. Schematic illustration of the experimental set-up

Installation

The ETP System used in the experiment is shown in Figure 2. The Semion Control Unit and RFEA feed-through assembly are highlighted. The feed-through assembly contains the RF filters for the attenuation of RF in RF biased mode.

Figure 2. ETP System with Semion™ System Installed

Figure 3. shows the RFEA Probe mounted in a customised substrate holder. This substrate holder holds the sensor in place and emulates the conditions that a substrate would experience under normal operating conditions.

Figure 3. Semion™ RFEA Probe mounted in ETP chamber

Floating Potential Measurements

The ETP substrate was operated in self-bias mode and a range of measurements were performed. The results are plotted in Figure 4a,b,& c below.

Figure 4a: Pure Argon, varying flow rates

Figure 4b: Argon/Oxygen mix, varying O2 content

Figure 4c: Fixed Argon/Oxygen mix, varying pressure

As can be seen from these results, ETP plasmas can be easily measured using the Semion RFEA probe at floating potentials, at pressures up to 35Pa.

DC Bias mode

The ETP substrate was operated in self-bias mode and a range of measurements were performed. In the first instance, a pure Argon plasma, the DC bias was varied from the floating potential to -20V as shown in Figure 5a.

Figure 5a: Pure Argon plasma, varying DC bias

Figure 5b: Mixed Argon/Oxygen plasma, varying DC bias

It can be seen from the resulting IEDF plots that there is a nice correspondence of the maxima of IEDFs with applied DC-bias.

RF Bias Mode

The ETP substrate was operated in RF Bias mode and a range of measurements were performed. Again the resultant data plots are shown below (Figure. 6a & b).

Figure 6a: Pure Argon plasma, varying RF bias

Figure 6b: Mixed Argon/Oxygen plasma, varying RF bias

These measurements show clearly that an RF-bias applied to the plasma will exhibit bimodal IEDFs.

References

[1] J.W.A.M. Gielen et al., J. Appl. Phys. 82, 2643 (1997).
[2] M.C.M. van de Sanden et al., J. Appl. Phys. 84, 2426 (1998).

We would like to extend our thanks to the Plasma and Materials Processing Group at the Technisches Universiteit Eindhoven for allowing publication of this article.