Session MW2: Diagnostics II: Electrical Probe Diagnostics

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It is shown that emissive probes (EPs) measure plasma potential profiles correctly in plasma presheaths, and that Langmuir probes (LPs) do not, in low temperature, low pressure plasma. It is conventional wisdom that LPs do not work in the sheath near material boundaries, but do work in quasineutral plasma. Experiments were performed in unmagnetized argon discharges, $ 0.1\leq P_n \leq 1mTorr$, with $ 1 \leq T_e \leq 5 eV,$ and $1 \times 10^9 \leq n_e \leq 1 \times 10^{10} cm^{-3}$, that compared plasma potential measurements made by partially coated and uncoated LPs, and cylindrical LPs, with measurements made by emissive probes. Presheaths were set up in the plasma using negatively biased electrodes. Results indicated that the EP potential measurements (in the limit of zero emission) were more negative than LP measurements in the presheath. In the sheath, most LP measurements did not go negative but rather became increasingly positive. Only the EP measurements worked in the sheath and presheath. These differences are thought to be caused by inherent, diffuse, ion flow in the presheath region toward the negatively biased electrode, characteristic of sheath formation.   

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Current-Voltage (I-V) characteristics of strongly emitting emissive probes are investigated in a multi-dipole filament discharge in argon. It is found that at sufficiently high neutral pressure and emission current, the variation of the I-V traces and their associated inflection points no longer follow the previous predictions of space charge limited (SCL) models. A new, steep slope region of the I-V trace appears near the plasma potential when the probe is strongly emitting, causing the inflection point and the floating potential to increase toward the plasma potential as the emission current increases, rather than staying constant. It is also found that the double inflection point structure when the probe is biased below the ionization energy of the working gas is highly likely to be an emission retardation effect from enhanced virtual cathode formation due to the increased local electron density. The dip between the virtual cathode and the bulk plasma potential also increases. This suggests effects predicted by Campanell et al's inverse sheath theory are applicable to a region of emissive probe I-V traces. Mechanisms with which virtual cathode effects limit hot cathode electron emission at high heating power are also investigated.   

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In high power impulse magnetron sputtering (HiPIMS), bright plasma spots are observed during the discharge pulses that rotate with velocities in the order of 10\,km/s in front of the target surface. It has proven very difficult to perform any quantitative measurements on these so-called spokes, that emerge stochastically during the build-up of each plasma pulse. In this contribution, we present a method to perform measurements integrating over many discharge pulses, but without phase averaging of the spoke location, thus preserving the information of the spoke structure. This method is then applied to perform Langmuir probe measurements, employing magnetized probe theory to determine the plasma parameters inside the magnetic trap region of the discharge. Spokes are found to cause strong modulations in electron density, electron temperature, and plasma potential. The electron density is modulated by about 40\,\%, fluctuating between $1 \times 10^{20}$\,m$^{-3}$ and $4 \times 10^{19}$\,m$^{-3}$. Furthermore, the plasma potential is observed to fluctuate between -7\,V and 1\,V, indicating the possibility for anomalous electron transport.   

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An algorithm for automated fitting of the effective electron temperature of a planar Langmuir probe I-V trace taken in a plasma with multiple Maxwellian electron populations is developed through MATLAB coding. The code automatically finds a fitting range suitable for analyzing the temperatures of each of the electron populations. The algorithm is used to analyze I-V traces from both the Diagnostic Test Source device in ASIPP, CAS and a similar multi-dipole chamber previously at UW-Madison which is now at USD. I-V traces reconstructed from the parameters fitted by the algorithm not only agreed with the measured I-V traces but also revealed physical properties consistent with those found in previous studies. Application of the algorithm to cylindrical probe I-V traces is also investigated. The major difficulties of such applications, i.e. distortion of the I-V traces by a low signal-to-noise ratio combined with greater sheath expansion, have been identified. It is recommended to use planar probes when signal-to-noise ratio is poor.   

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In this contribution, we study the interaction between low-pressure argon discharge and metallic nanoparticles via a Langmuir probe which is one of the most established techniques to obtain useful information about plasmas. An unexpected broad peak at the energy in the 4-9 eV range has been observed in the electron energy distribution. The peak position varies with the processing parameters, indicating not an electronic transition, but a phenomenon directly associated with the presence of metallic nanoparticles. We tentatively attribute this observation to field-assisted thermionic electron emission from the nanoparticle surfaces. To illustrate this, we developed a zero-dimensional steady state model for a plasma dosed with thermionically emitting nanoparticles. It comprises four modules which iteratively solve for plasma parameters given a known particle density. The particle charge predicted by our self-consistent model differs from the traditional OML theory, and the model demonstrates good fit with our experimental observations.   

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Nonthermal plasmas are attractive sources for nanomaterials synthesis. As the ability to characterize such plasmas through plasma modeling increases, the need for validating models through targeted experiments increases as well. In this presentation, we present measurements of electron temperatures and ion densities in a flowing argon-silane plasma using double Langmuir probes. Due to the elevated pressures, we apply the modified Talbot and Chou probe theory, which we validate by obtaining almost identical ion densities and electron temperatures measured by three different double probes with different probe tip diameters in different collisional regimes for pressures ranging from 200 mTorr to 2 Torr. Furthermore, relative data of spatial concentration profiles of radical densities are derived from the thickness of films deposited on sample targets that are positioned at different locations within the plasma. Data of electron temperatures and plasma densities are compared to results of a global plasma model and solutions of the electron Boltzmann equation. Radical species profiles are compared to results of a two-dimensional plasma model for flowing argon-silane plasmas.   

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h $-abstract-$\backslash $pard Discharge power is one of important parameter characterizing the discharge condition, but in the majority of published experimental works on rf plasma, the discharge power remained undefined. In these works, the power transmitted from the power source to matching network and then to antenna inductor of ICP, or to rf electrodes of CCP is considered as the discharge power. Such approach neglects rf power losses in the matcher network, antenna coil and that due to eddy currents induced in the ICP metal chamber and nearby conductors. As was proven in many experiments, the discharge power (one absorbed by plasma) is less (and sometimes, much less) and is not proportional to the power consumed from the power source. The techniques for correct measurement of discharge power in CCP and ICP, and methods for evaluation of power loss in the matcher, antenna and surrounding conductors will be discussed in this presentation.$\backslash $fs20 $\backslash $pard-/abstract-$\backslash $\tex