Phase resolved optical emission spectroscopy (PROES) measurements were combined with measurements of the optical emission intensity (OEI) and electrical characteristics (RF current and voltage, power, and DC bias voltage) as a function of time during the re-ignition of Ar plasmas pulsed at 100 Hz and 10 kHz. The OEI exhibits a large overshoot at the 100 Hz pulsing rate even though no such overshoot is present in any of the electrical characteristics. The OEI overshoot occurs at a point in time when the RF power, voltage, DC bias voltage, and electron density are all smaller than they become later in the glow. PROES measurements in combination with the time resolved electrical characteristics indicate that the heating mechanism for the electrons changes during the time of the overshoot in the OEI from stochastic heating to a combination of stochastic and ohmic heating. This combination appears to enable a more efficient transfer of the electrical energy into the electrons.
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.
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