Plasma-produced nanoparticles are of interest for many applications. They have very specific properties that can vary greatly from those in the atomic and bulk materials, including thermodynamic properties, such as a reduced melting temperature, and optical properties, such as blue-shifted blackbody radiation. Since a plasma is dominated by free electrons, charge related properties, like a reduced work function for electron attachment and increased work function for photo ionization, are of major importance. In situ detection of nanoparticles inside a plasma is difficult. Hence, indirect methods including changes in optical emission, plasma voltage and currents are used to study particle growth. Here, electron density measurements in the plasma are presented using an ultrafast microwave resonance technique (time resolution below 1 micros). This technique allows studying the charge and charging kinetics of nanoparticles within the initial milliseconds of their growth.
Non-intrusive and accurate voltage, current and phase measurements of fundamental and harmonic radio-frequencies (rf) are compared with the temporal evolution plasma emission and scattered laser light from in situ formed particles in an argon-silane low-pressure rf discharge. The measured parameters show a corresponding behaviour during particle evolution. With increasing reactor temperature, not only is particle nucleation delayed in time but all subsequent phases of particle formation are slowed down as well. A close correspondence between the variations of the fourth harmonic of the current and the Si-H emission line is found. The results show that electrical parameters provide valuable clues to the particle growth starting from the nucleation phase, making them good candidates for process monitoring in industrial devices.
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