Sub-rf period electrical characterization of a pulsed capacitively coupled argon plasma

Transient plasmas (such as pulsed power plasmas) can be of interest to both industry, where they allow for new processing windows, and basic science, where their dynamics are of interest. However, their study requires time resolved diagnostic techniques. One powerful diagnostic is current and voltage (IV) measurements, which along with the power and impedance calculated from them, can be used to characterize a plasma. This is especially true as it is an outside the chamber, noninvasive technique and can be used in systems where a probe or fiber optic bundle/window will affect processing results or fail due to deposition. To obtain accurate IV values, frequency dependent probe calibrations must be performed and frequency dependent parasitic impedances in the system and propagation delay between the forward traveling fundamental frequency and backward traveling harmonic frequencies must be taken into account. To separate the fundamental and harmonic frequencies, a fast Fourier transform (FFT) is traditionally performed in continuous wave plasmas. In transient plasmas, a time resolved FFT is necessary. This article presents a method to perform each of these steps while demonstrating their importance and giving some measurements of a pulsed power, 75 mTorr, capacitively coupled argon plasma.