Session DF1: Diagnostics III

Associative Ionization Processes in Nonequilibrium Plasmas

Associative ionization processes in molecular and atomic collisions are of critical importance for the understanding of the kinetics of low-temperature plasmas and nonequilibrium high-enthalpy flows. The ionization cross section is greatly enhanced by the vibrational and electronic excitation of the collision partners. Predicting the rates of the associative ionization and other electronically nonadiabatic processes in collisions of excited molecules and atoms requires quantitative insight into the collision dynamics. The progress in the predictive capability of the molecular energy transfer theory over the last few decades owes a great deal to the development of accurate potential energy surfaces and efficient computational techniques. On the other hand, approximate analytic models of vibrational and electronic energy transfer, which yield the closed-form expressions for the energy transfer cross sections, have also demonstrated their accuracy and utility. This is illustrated by the good agreement between the analytic models and the “exact” semiclassical trajectory calculations or quantum scattering calculations. One of the essential factors in the development of these models is the emphasis on the dominant features of the collision dynamics, such as the multi-state coupling, effect of molecular rotations, and interference between the incoming and ongoing wavepackets. The objective of this work is the development of a semiclassical analytic model of nonadiabatic energy transfer in atomic collisions, using the generalized Landau-Zener-Stückelberg theory. This theory has been applied to the collisions between two ground state O atoms, to predict the cross section of electronic excitation, O(³P) + O(³P) ⟶ O(³P) + O(¹S). The results are compared with the exact quantum solution, exhibiting excellent agreement. The focus on the ongoing work is the extension of this approach to the associative ionization in collisions of metastable excited atoms, such as N(²P) + O(³P) ⟶ NO⁺ + e^-, one of the dominant ionization processes behind hypersonic shock waves. This model is complementary to the quasiclassical trajectory calculations and can be used for the predictive simulations of nonequilibrium plasmas and hypersonic flows.   

Sampling of Cold Atmospheric Pressure Plasmas into a differential pumping arrangement for Langmuir Probe Diagnostics

Cold atmospheric plasmas (CAP) are non-thermal gaseous discharges that are generated in ambient (atmospheric) pressure conditions. Here we present a novel diagnostic method where a small volume of CAP is taken from atmospheric pressure (10³ mbar) into a low-pressure environment (1 mbar) using a differential pumping arrangement. Once the plasma sample is brought into a low-pressure region, it can be characterized using conventional plasma diagnosis tools such as the Langmuir probe. However, when a differential pumping stage is used to take a sample of atmospheric plasma, the pressure gradient existing in the flow (from 10³ mbar to 1 mbar) will modify the density of the plasma, which in turn will corrupt the probe and analyzer data. So, to solve this issue, the number density of the flow should be estimated at every point inside the vacuum system. Also, the feasible dimensions of the vacuum chamber to achieve the desired vacuum needs to be fixed before actual fabrication. To solve the above-mentioned issues, the gas flow inside the chamber was simulated using COMSOL multi-physics software. High Mach number flow module which uses Navier Stroke’s equations for compressible flow has been adopted. In the experimental part, we present the Langmuir probe measurement of CAPs sampled into the vacuum system. The ion number density was successfully measured using the IV data acquired from the Langmuir probe and was found to be increasing with the power ranges we have considered in our experiments.    

Development of a flat-cutoff sensor for non-invasive plasma density measurement in plasma processing

Real-time plasma diagnostics have garnered significant attention within the semiconductor processing field due to their potential to improve process yield and detecting process abnormality. In this study, we developed a bar-type flat-cutoff sensor (BCS) [1-3], which is embedded into the chamber wall and electrode to measure electron density during plasma processing. Our investigation includes optimizing the sensor's structure and analyzing its measurement characteristics of plasma density through the utilization of a plasma equivalent circuit model considering the structure of BCS, electromagnetic simulation, and experiment. It was confirmed that BCS measures the electron density near the plasma-sheath boundary from the chamber wall or wafer. Furthermore, the BCS enables measuring of electron density, even when various types of wafers are positioned on it, and a shift in cutoff frequency was observed in extreme cases such as thick dielectric and wafers with metal layer. A dielectric layer thicker than 1 mm caused a low-frequency shift of the cutoff frequency. Conversely, wafers with conductive layers induced a high-frequency shift of the cutoff frequency, which is dependent on the plasma density.   

Floating harmonic probe measurement through thick dielectric material for noninvasive plasma diagnostics

 The floating harmonic method is a well-known technique in plasma diagnostics. It is founded on applying an AC voltage to an electric probe immersed within a chamber. This method allows for measuring plasma parameters, such as electron temperature and ion density, by analyzing the ratio of harmonic current, even in cases where the probe is contaminated or coated by the thin dielectric film. However, there has been increasing interest in noninvasive diagnostic methods in recent years. Because when the probe is immersed in the chamber, it can distort the plasma and contribute to the unwanted generation of particles caused by ion bombardment. We propose a plasma diagnostic using an floating probe via thick dielectrin material in the viewport to address this drawback. Despite the presence of thick dielectric material, we were able to measure harmonic currents by applying a high frequency in the MHz range. To compensate for the phase delay caused by the viewport, we analyzed a non-linear equivalent circuit. The measurements obtained from this method exhibited good agreement with those obtained using the traditional immersed probe technique. Our research is expected to provide an effective approach to noninvasive plasma diagnostics.