CST – Computer Simulation Technology

Microwave Plasma Sources

Low-temperature, non-equilibrium plasmas form the basis of a growing variety of plasma related processes such as surface modifications, sterilization, etching and thin film deposition. Plasma sources are a crucial tool in many industrial process steps, for example during the production of integrated circuits. The demand for high-density plasmas over a wide pressure range has stimulated the development and use of microwave plasma sources over the last few years. The Finite Integration Technique (FIT) used in the 3D ECAD simulation package MAFIA has proven to be a powerful tool for the modelling of those complex microwave systems like the SLAN family. CST MICROWAVE STUDIO® is also able to consider the plasma as a dispersive material.

The SLAN family and other plasma sources have been developed at the Microstructure Research Centre (fmt) and are marketed by JE PlasmaConsult GmbH. ...

Figure 1: Cross section of the microwave plasma source mSLAN, the smallest member of the SLAN family

The SLAN family consists of four different plasma sources: Mikro-SLAN (4 cm quartz tube diameter), SLAN I (16 cm quartz tube diameter), SLAN II (60 cm quartz tube diameter), and a linear SLAN (approx. 1 m long). The sources are used for the modification of surfaces (scratch resistant layers, improvement of the biocompatibility, improvement of color adhesion, etc.), creation of hard diamond like carbon surfaces or for the cracking of harmful gases.

Figure 2: Electrical field distribution in the microwave plasma source Mikro-SLAN, the smallest member of the SLAN family

The left picture shows the distribution of excited oxygen radicals (O*), measured by Laser Induced Fluorescence Spectrometry (LIF) in the inner cylinder of a SLAN I. The electric field distribution calculated in Frequency Domain, for a lossy plasma with similar parameters to those used in the experiment, is very close to the measured ion distribution (right picture).

Figure 3

Left picture: Simulated electrical field distribution in the inner cylinder of the SLAN I for an O2 plasma with a real part of the permittivity of -1.7 and a conductivity of 0.017 S/m (assumed plasma parameters: ni=2x10e11 cm-3, p=60 Pa, Te=2eV, n_en=7.15x10e8s-1).

Right picture: CCD picture of O2 plasma at 20 Pa, 2000 W microwave power, f=2.45 GHz. Spatial distribution of O*, 845 nm line.

Figure 4: By shifting the shorting plunger of the Micro-SLAN...

The impedance of SLAN plasma sources can be matched by moving the coupling antenna and the shorting plunger. The latter causes a rotation of the field in the ring shaped resonator. The rotation has been simulated using the transient solver. The fields behave in exactly the same way as in the experiment (Figure 4 and 5).

Figure 5: Field rotation in the circular resonator can be achieved and hence the impedance of the plasma source matched to the impedance of the feeding system

By using the Eigenmode Solver, the homogeneity of the field distribution in the ring shaped resonator of the SLAN I has been improved (Figure 6). This is reflected in the measurement of the plasma density (Figure 7).

Figure 6: Electric field strength (f=2.45 GHz) for a cross section of the inner cylinder in the basic SLAN I (left picture) and the optimized SLAN I (right picture)

Figure 7

Relative azimuthal electron density distribution in the basic SLAN I (yellow stars) and in the optimized SLAN I (green stars). Plasma parameters: Argon, 50 Pa, 550 W microwave power at 2.45 GHz. Measured 6 cm downstream, 2 cm from the inner quartz wall with a double Langmuir probe. The maximum measured absolute density amounts to ne=6x10e11 cm-3.

The simulations allowed:

• a better understanding of the microwave coupling into the plasma chamber
• an optimisation of the plasma homogeneity
• an impedance matching of the source.

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