Session ET2: Diagnostics I

New model for the ion collection by cylindrical probes over a wide range of collisionality

Langmuir probes remain one of the most important diagnostic tools for plasma processing applications. To accurately determine the electron density, it has become customary to rely on the electron current part of the Langmuir probe characteristic using the Druyvesteyn method. However, in cases where no reference electrode for the plasma is available, double probes need to be employed to perform the measurements. Such double probes rely on the ion current collected by the probe to determine the ion density. However, even at low pressures of a few Pa, the ion current is affected by collisions due to the large cross section for charge exchange. Available theories for collisional or collision-enhanced ion currents onto probes are complex, not well validated, and often only valid for a certain range of probe sheath thickness or collisionality.

Thus, in this contribution, we compare available collisional probe theories for the ion collection by a probe immersed in a plasma to particle-in-cell (PIC) simulations. Using these results, we propose a simpler and more intuitive model for the ion current collected by the probe, based on the model of Gatti and Kortshagen (Phys. Rev. E 78, 046402, 2008), developed for the charging of dust particles.   

Optical emission spectroscopy spatial characterization of a silicon based Micro Hollow Cathode Discharge operating in Helium in DC regime

Microplasma sources, with at least one dimension below the millimeter scale, have garnered significant attention due to their potential for a broad range of applications [1]. These non-thermal plasma microreactors can operate at atmospheric pressure in DC excitation due to their high surface area volume ratio. Among the typical configurations of microplasma reactors is the Micro Hollow Cathode Discharge (MHCD) reactor, where two electrodes are separated by a thin dielectric layer. At GREMI, we produce MHCD made of silicon from clean room process technology. Some works have been carried out on closed cavity microdischarge devices [2]. We are now producing and investigating Through Silicon Via (TSV) type MHCD.

Spatially resolved Optical Emission Spectroscopy is used as a method to investigate the plasma generated in this type of reactor [3]. With this technique, we are able to map the emission intensity of specific spectral lines across the microplasma. This mapping reveals interesting behavior close to the microreactor walls. For a TSV microplasma ignited in He at 600 mbar with less than 1% of hydrogen, the electron density of the plasma and the gas temperature are estimated to be around 10¹⁵ cm^-3  and 650K respectively for a current of 1.8mA. In the case of closed cavity MHCD, the line profile is substantially modified exhibiting some wings at its edges. This point will be discussed and a comparison will be made between the two types of microreactor.   

Electrical and Optical Diagnostics to Measure the Electron Plasma Frequency

The electron plasma frequency is a fundamental parameter of plasmas. As the governing equation of the electron plasma frequency (ω_pe) is ω_pe=[(n_ee²)/(ε₀m)]^0.5 , one can obtain the absolute electron density from the measured ω_pe.  In this invited talk, we will introduce electrical and optical methods of measuring the electron plasma frequency for non-invasive, non-perturbing, and precise plasma monitoring. The electrical method is based on the microwave cutoff probe [1], but it is modified to a flat-type sensor that can be imbedded into the substrate chuck or chamber wall. We focus on a bar-type flat-cutoff sensor, called BCP, owing to its simple structure, absence of unwanted resonance signals, and high signal-to-noise ratio, as well as the precise measurement of ω_pe [2-4]. The optical method for obtaining ω_pe is to measure the time interval between multiple electron beams generated during a single phase of sheath expansion at one of the electrodes by phase resolved optical emission spectroscopy. A previous study [5] clearly showed experimental evidence of such multiple electron beam generation at frequencies corresponding to the local electron plasma frequency in a radio-frequency (RF) capacitive discharge, caused by the plasma perturbation through the electron beam propagation. Thus, we propose a new method for measuring the electron plasma frequency by using the time interval between the electron beam optical emission maxima in the presence of an oscillating RF sheath. The electron plasma frequency and density measured by this optical diagnostic coincides with those obtained from the electrical method.   

Plasma-assisted Synthesis of Carbon Nanomaterial Studied by Spatially-resolved Laser-induced Fluorescence and Optical Emission Spectroscopy

Crystalline nanographite and carbon nanodots have been of great interest for their applicability in photocatalysis, energy conversion, optoelectronics, and biosensing. In this work, we report a scalable synthesis of sp² carbon nanomaterial consisting of crystalline carbon nanodots and nanographite. This chemical vapor deposition (CVD) approach takes a nonthermal CH₄/Ar capacitively coupled plasma actuated by a 13.56 MHz source in a flow-through tubular reactor. Raman spectroscopy confirmed sp² hybridization with the signature D and G bands. TEM image analysis indicated the average particle size to be ~6 nm.

 

High-resolution optical emission spectroscopy (OES) was used to estimate a gas temperature of ~1200 K. Spatially-resolved planar laser-induced fluorescence (PLIF) was used to capture C₂ species distribution at different locations along the length of the tubular reactor for discharges from different CH₄/Ar ratios. For higher CH₄/Ar ratios, the yield of the sample (collected at the bottom of the tube) is higher and the corresponding C₂ density in the plasma is lower. C₂ density is also found to decrease from 10¹⁸ m^-3 to 10¹⁶ m^-3 down the tube. This can be due to a higher conversion rate of C₂ to nanographite and hence a lower available amount of C₂ to be measured in the plasma. These diagnostics allow for understanding how C_x species evolve from the discharge to form ~6 nm nanoparticles thereby providing for a comprehensive description of the spatiotemporal evolution of graphite nanoparticle growth.   

Brewster angle-cavity ringdown spectroscopy for low temperature plasma diagnostics

Measuring gas molecules in liquids is one of the challenges in diagnostics of low temperature plasma interacting with a liquid. We report on the development of a Brewster angle-cavity ringdown spectroscopy (BA-CRDS) system for low temperature plasma measurements in multiphases. The system can measure gas species in solutions, with a detection limit (minimum detectable absorbance) of 9.1×10-5, which is equivalent to a detection limit of 0.04 parts per billion for measuring OH radicals in water at 308 nm. With further developments, the detection limit can be potentially up to 10-6 or lower.  In this exploratory study, absorption cross sections of HgBr2 and H2O2 in aqueous phase at 256 nm are measured. Furthermore, temporal profiles of absorbance from distilled water, HgBr2, and H2O2 solutions when interacting with a helium atmospheric plasma jet are individually characterized at different plasma powers, gas flow rates, and/or solute concentrations. The observed linear temporal profiles of absorbance from the plasma-interacted water suggest formation of H2O2 from plasma-generated OH radicals, while the nonlinear temporal profiles from the plasma-treated HgBr2 solutions reveal possible removal of HgBr2 by OH radicals. Our result demonstrates that the new BA-CRDS system is a powerful tool for quantification of reactive plasma species in multiphases or other complex settings.    

Development of hybrid fs/ps CARS for quantitative detection of molecular nonequilibrium and concentration imaging of Cl and O radicals

Hybrid fs/ps CARS has been recently developed for plasma applications as a highly accurate and time-resolved detection method for instantaneous vibrational and rotational temperatures of plasma constituents, particularly N₂.  Here, we report on recent progress in our group for improving detection of molecular nonequilibrium and for extending the hybrid CARS approach for imaging of the time-resolved concentration of Cl and O radicals.  Virtually imaged phased array (VIPA) resolved hybrid CARS measurements are demonstrated as a highly sensitive and widely applicable method for detecting molecular nonequilibrium among rotational and vibrational degrees of freedom.  One of the clear advantages of this approach is the multiplex nature of probing the low-frequency transition region of the Raman spectrum.  Plasma-generated radicals, such as Cl, F, and O, are important for plasma-enhanced chemical vapor deposition and etching processes in the semi-conductor fabrication industry.  We probe the possibility of using such multiplex CARS measurements to drive transitions in electronic spin-orbit split ground states of Cl, F, and O for coherent detection.  Further, we take advantage of CARS imaging approaches developed in our lab to acquire the spatial distribution of target molecules normal to a surface.  Time-resolved data allows determination of spectroscopic constants important to the modeling and quantification of measured species.   

Method to Characterize Experimentally Linear and Nonlinear Growth of Electrostatic Waves in Low Temperature Plasmas

An experimental method is presented for investigating the spatiotemporal properties of electrostatic waves that induce particle and energy transport in low temperature plasmas (LTP).  The technique is based on formulating the evolution of the instabilities in terms of a governing equation for the complex amplitudes of the plasma instabilities.  This equation encodes information about the dispersion, linear growth, and wave-wave coupling of the resonant modes in the plasma.  A technique that leverages a translating, rotating two-point probe diagnostic is then presented for generating the data necessary regress the governing wave equation.  Multiple methods for inferring the wave properties from this data are presented including a technique based on Bayesian regression with conjugate priors. The method is validated against direct numerical simulations of a crossed-field  LTP in which numerical probes are employed to simulate the placement of physical probes.  The extension of the technique to laboratory plasmas is also discussed.