Session IF3: Probe Diagnostics

A novel approach for calculating the plasma resonance behavior excited by wall-integrated planar diagnostic probes with arbitrary geometry

Active Plasma Resonance Spectroscopy (APRS) is a diagnostic technology that makes use of the natural ability of plasmas to resonate on or near the plasma frequency. Different from the passive plasma resonance spectroscopy, which observes the excitation already existing in the plasma, APRS could obtain important plasma parameters such as electron density or temperature by establishing reliable mathematical relations to the recorded spectral response from the dynamic interaction between plasma and the radio frequent signal (usually in the GHz range) by the probe. Among all the types of probe designs, wall-integrated planar diagnostic probes provide the optimal condition for monitoring plasmas by introducing the minimum undesired disturbance. In this work, a novel approach is presented which allows to determine the dynamic interaction by separating the calculation of the dissipated energy inside the wall-integrated planar diagnostic probe with an arbitrary geometry from the rest part of the computation regions, where the cold plasma model is applied. The advantage of this approach is shown for different available designs and can be utilized by more sophisticated plasma models and even more complicated probe configurations.   

Second harmonic currents in rf-biased, inductively coupled plasmas

Measurements at the rf-biased electrode of an inductively coupled plasma (icp) have detected a current at the second harmonic of the icp source frequency. The origin of this harmonic has been investigated by modeling and experiments in argon at rf bias voltages of 50 V to 1.4 kV.  In this voltage range, nonlinear inductive coupling [1] and nonlinear sheath impedance contribute only small fractions of the second harmonic current, which is instead predominantly produced by two other phenomena.  One is second harmonic modulation of the vacuum ultraviolet photon flux in the plasma, which in turn modulates the photocurrent emitted at the rf-biased electrode.  Another is deflection of emitted and reflected electrons by the rf electric and magnetic fields induced in the plasma.  Models and literature data explain the size and behavior of the observed second harmonic current.  The harmonic measurements, when analyzed by the models, provide accurate values or useful estimates for conditions at the dielectric window beneath the icp source, including the electron elastic reflection coefficient, electron-induced emission yield, electric and magnetic fields, and sheath voltage.

[1] V. A. Godyak, R. B. Piejak, and B. M. Alexandrovich, Phys. Rev. Lett. 83, 1610 (1999).   

The performance of the pulse bias hairpin resonator probe for negative ion diagnostic

The pulsed bias hairpin resonator probe (HP) is a novel concept which has been applied to measure negative ion density and temperature in electronegative discharges. The underlying concept relies on the response of negative ions and electrons to the rapidly changing sheath width around the cylindrical pins of the hairpin resonator. Pulsed bias HP has advantages of spatial resolution and less sophistication over the conventional pulsed laser photo-detachment (LPD) technique, where the spatial resolution is limited to in-line laser beam measurements. In this presentation, pulsed bias HP has been characterised in detail for a range of positive values of pulse bias and working pressures in a 13.56 MHz CCRF oxygen discharge. It is seen that with the increase in positive bias, negative ion density also increases, and it attains saturation when biased above the plasma potential. On the other hand, the negative ion diffusion time scale reduces when biased above the plasma potential. Furthermore, the results have been validated with the LPD technique.   

Power law parametrization of the ion collecting area for a planar Langmuir probe diagnostic

Experimentally measured I-V characteristic of a Langmuir probe exhibits a trend of increasing ion current as an absolute value of the probe bias increases below the plasma potential. The increasing ion current is qualitatively explained by an expansion of the ion collecting area which can be defined as the surface where the ions pass through with the Bohm speed towards the probe, i.e., the surface at the sheath edge. To estimate the ion saturation current, the expanding ion collecting area is commonly compensated by employing various fitting models such as a linear model, the Child-Langmuir sheath model, or Sheridan’s numerically proposed heuristic model [T E Sheridan, Phys. Plasmas (2000)]. We have experimentally investigated the expanded ion collecting area of a double-sided planar Langmuir probe through a systematic scan of plasma parameters for unmagnetized Ar plasmas with plasma density ranging from 3×10⁸ to 10¹⁰ cm^-3 and an electron temperature of ~1 eV. The effective ion collecting area is found to follow a power law with respect to the probe bias for a sufficiently negative bias voltage. The power law constants are parametrized as a function of the normalized probe radius where we identified qualitatively comparable features with the numerical results. We have further investigated the role of ion-neutral collisions in determining the expanding ion collecting area by analyzing the behavior of the power law constants. We also provide a novel scheme for analyzing an I-V characteristic of a planar Langmuir probe based on a power law fitting method. We suggest that a power law fit to the effective ion collecting area must be performed solely based on experimentally obtained data rather than using models from simulation results since a simulation code cannot contain every realistic physical aspect.   

Analysis of temperature dependency of the resonant frequency for electron density measurement with curling probe

We developed a curling probe, one of the plasma diagnostic probes, to find electron density using electrical resonance in the microwave range.  The probe utilizes a slot antenna to get the resonance, and the antenna forms a spiral shape on a top surface plane.   Because the probe is operated in the high-frequency range, the probe realizes the density measurement even when a film deposition occurs. Furthermore, the probe treats plasma as a dielectric material for measuring principle so that the probe can be utilized at any pressure.  Even with the versatility of the probe, we have noticed that a resonant frequency depends on the probe temperature.  Unfortunately, we do not know the main cause of the frequency shift due to the temperature. Still, the dependency is observed even with the probe temperature rising by 10 degrees C. This presentation will show our recent analysis of the temperature dependence of the resonant frequency from the curling probe by conducting in-situ monitoring of the temperature and the resonant frequency during plasma exposure.  After a careful analysis, we found that the shift has a certain regularity, and we finally have developed a technique to calibrate the temperature-dependent frequency.

    

Langmuir probe and Laser Photodetachment Study of Afterglow Phase in Dual RF Frequency Pulsed Plasma Etching Processes Operated with Synchronized DC Bias

The study of the fundamental physical properties that govern the plasmas becomes more crucial for the control of etch profile and etch rate for the development of the high aspect ratio etching (HARE) process. However, despite the importance of radio frequency (RF) discharges using fluorocarbon gases for plasma etching in microelectronics, the connection between fundamental plasma properties, such as the electron temperature (T_e), electron density (n_e), and the kinetics in such plasmas is not yet fully understood. We combinedly used RF compensated Langmuir probe (LP) and laser photodetachment techniques to study the dynamics of plasma parameters and negative ions, respectively, in (40.68 MHz/3 MHz) frequency pulsed RF O₂/C₄F₈/Ar plasmas with synchronized DC bias. Result show that during RF on phase, the plasma remains electropositive with a characteristic high n_e and T_e. The electronegativity (α) enhanced significantly in the afterglow phase with a very small T_e and n_e due to electron attachment processes with a very high decay rate of n_e. The enhanced electronegativity and a high DC bias condition associated with secondary electron emissions are attributed to generating a separate warm electron population (T_w) with a low T_e in the afterglow phase in DC synchronized condition.