Electrical Measurements of Capacitively Coupled Pulsed Power Plasmas

Date

2020-05

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Abstract

Pulsing the power to radio frequency (rf) driven plasmas creates transition periods which exhibit plasma conditions not reachable in continuous wave plasmas. One advantageous effect, is etch feature charge neutralization in integrated circuit manufacturing. Charge neutralization is usually attributed to low energy positive and negative ions interacting with the etch surface during the pulse afterglow; however, the sheath electric fields can be momentarily reversed under some pulsed conditions and accelerate electrons to the surface rather than repel them. This can allow electrons to perform at least some of the positive charge neutralization needed at the surface. Electrons engaging in surface charge neutralization can negate the need for relatively long power off times which allow negative ions to neutralize the surface. Although pulsed power plasmas have processing benefits, there are still engineering challenges which need to be addressed. A major challenge is ultra-fast impedance matching as the plasma impedance varies over a pulse cycle. One method to reduce the unmatched period is to quickly adjust the rf driving (fundamental) frequency; however, a similar reduction in the mismatch period can be achieved more simply by increasing the rf amplifier output power at the beginning of the pulse. This allows the electric fields interacting with the plasma, and therefore the plasma impedance, to reach their steady state values more quickly. Thus, the unmatched period can be reduced through a change in power magnitude rather than frequency. Electrical measurements of the plasma can be used to show evidence for both sheath reversal and decreased unmatched period. This requires high time resolution, which is obtained though performing an FFT on a short sampling period of measured electrode rf current and voltage (RFIV). Further, detailed corrections to the RFIV measurements must be made to account for parasitic impedances and propagation delay in the chamber-electrode circuits which include propagation direction of the fundamental frequency and its harmonics. Lastly, Langmuir probes are an often used plasma diagnostic tool as they can measure both the plasma density as well as the electron temperature or energy distribution function. Unfortunately, the fast fluctuations in the plasma potential which occur at plasma re-ignition can induce significant errors in calculated plasma parameters and even produce IV curves which are unphysical. Diagnosing plasmas with quickly varying plasma potentials requires an understanding of these Langmuir probe-plasma interactions. This dissertation provides four contributions to plasma science; a method to accurately measure time resolved plasma RFIVs, a novel method for impedance mismatch reduction in a pulse cycle, evidence for sheath reversal which allows an anisotropic electron distribution to interact with surfaces, and demonstration of measurement issues when using Langmuir probes in pulsed power plasmas.

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Radio frequency, Plasma engineering, Langmuir probes, Plasma diagnostics

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©2020 Alex F. Press. All rights reserved.

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