Bulletin of the American Physical Society
2005 58th Gaseous Electronics Conference
Sunday–Thursday, October 16–20, 2005; San Jose, California
Session EM2: Plasma Diagnostics I |
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Chair: U. Czarnetzki, Ruhr University - Bochum Room: Doubletree Hotel Cedar |
Monday, October 17, 2005 4:00PM - 4:30PM |
EM2.00001: RF Langmuir probes, revisited Invited Speaker: Though probes have been used for $n_{i}$ and $T_{e}$ measurements in rf plasmas for many years and commercial systems are available for automatic scans of $I-V$ curves, accurate results are by no means guaranteed. Strange $I-V$ curves obtained in the past two years have led us to re-examine the problem of Langmuir probes in low-density plasmas in severe rf environments. There are five main problems. First, the Bernstein-Laframboise theory used for small $\lambda _{D}$/$R_{p}$ (Debye length/probe radius) have been shown to be inaccurate$^{1}$. We therefore use thin probes and the OML (orbital-motion-limited) theory for large $\lambda _{D}$/$R_{p} $, but at low $n$ the electron curve is not exponential. Second, we find that the plasma potential drifts with probe voltage $V_{p}$ on a msec or longer timescale. This can be explained, but the effect is larger than expected. Third, the drift can be overcome by rapid sweeping of the $I-V$ curve, but too fast a $V_{p}$ scan would be distorted by the inductance of the choke chain. Fourth, rf compensation is done with a chokes and a large compensation electrode$^{2}$ (CE). We find that CE cannot do most of the job; the chokes have to have large impedance at their self-resonant frequencies. But tuning the choke chain is not easy. Finally, cleaning the probe tip is difficult at $n \quad \approx $ 10$^{9}$ cm$^{-3}$ because not enough current can be drawn to the probe. This is seen in very slow drifts of the ion current, in the order of seconds. A good rf probe requires careful construction of the compensation elements and judicious choice of the scan speed. \newline \newline $^{1}$F.F. Chen, Phys. Plasmas \textbf{8}, 3029 (2001). \newline $^{2}$I.D. Sudit and F.F. Chen, Plasma Sources Sci. Technol. \textbf{3}, 162 (1994). [Preview Abstract] |
Monday, October 17, 2005 4:30PM - 4:45PM |
EM2.00002: Absolute electron density measurement using the cut-off method in magnetized plasmas Ikjin Choe, Chinwook Chung Electron density measurement in magnetized plasmas is very difficult because electron saturation currents to Langmuir probes are greatly distorted. Recently, the cut-off probe measuring the plasma frequency in unmagnetized plasmas was developed [J.H. Kim, et. al., Appl. Phys. Lett., 83, 4725(2003)]. We constructed a cut-off probe and applied this method to measure electron densities in weakly magnetized plasma. The ordinary wave is used because its cutoff frequency is equal to the plasma frequency even in the DC magnetic field. The electron density is compared with the ion densities from the ion currents that are not affected by the magnetic fields. It is found that the measured electron density is proportional to the ion density. This cut-off method is expected to be one of reliable methods to measure electron density in magnetized plasmas. [Preview Abstract] |
Monday, October 17, 2005 4:45PM - 5:00PM |
EM2.00003: Electron density measurements in inductively coupled plasmas using phase resolved optical emission spectroscopy Timo Gans, Deborah O'Connell, Victor Kadetov, Uwe Czarnetzki The optical emission from radio frequency (rf) discharges exhibits temporal variations within the rf-cycle. Neglecting these variations in classical time averaged optical emission spectroscopy (OES), based on balance equations, can result in serious misinterpretation. The effect of neglecting temporal changes is not as pronounced in inductively coupled plasmas (ICPs) as in capacitively coupled plasmas (CCPs). However, even the relatively small modulations in ICPs can be exploited as a novel access for plasma diagnostics. The modulations of the optical emission are caused by temporal changes of the electron energy distribution function (EEDF). These modulations can be described within the two-term approximation of the Boltzmann-equation. This allows us to determine the induced electric field in the discharge. The penetration of the field into the plasma is determined by the Helmholtz-equation. A spatially resolved measurement of the amplitude and phase of the induced electric field, therefore, yields a 2-dimensional spatial map of the electron density. [Preview Abstract] |
Monday, October 17, 2005 5:00PM - 5:15PM |
EM2.00004: Plasma density determination from surface-wave transmission spectra Sebastien Dine, Jean-Paul Booth, Garrett Curley, Cormac Corr, Jacques Jolly, Jean Guillon We have developed a new plasma density measurement technique based on the transmission spectra of surface waves (SW) propagating along a plasma-sheath boundary. Simple theory indicates that the lowest frequency at which SW’s can propagate is equal to $1/\sqrt{2}$ of the plasma frequency, allowing the plasma density to be determined. Our probe (“Plasma Transmission Probe” or PTP) consists of emitting and receiving antennas joined by a dielectric cylinder, all immersed in the plasma. A sheath forms around this device, creating a cylindrical wave-guide between the antennas along the sheath-plasma interface. The transmission spectrum was measured with a network analyser. Experimental spectra were measured in CCP discharges in argon (40-750\,mTorr) and in an ICP, and are compared to the results of an axi-symmetric finite element model. The densities determined by this method were found to be lower by a factor 0.5-0.6 compared to those obtained with Langmuir and hairpin probes. We attribute this to the density gradient in the pre-sheath around the PTP, which determines the sheath-edge density. The PTP is promising for the measurement of low densities ($<10^{10}$\, cm$^{-3}$) at relatively high gas pressure ($>0.5$\,Torr). [Preview Abstract] |
Monday, October 17, 2005 5:15PM - 5:30PM |
EM2.00005: On the floating type Langmuir probe using the harmonic technique in inductively coupled plasmas SungHo Jang, Minhyong Lee, ChinWook Chung A floating type Langmuir probe using the harmonic technique and its driving circuit are developed and applied to measure the electron temperature and the ion density in inductively coupled plasmas(ICP). The reliability of the harmonic technique is checked by varying the amplitude and the frequency of the applied voltage and the current sensing resistance. Comparisons with a single Langmuir probe show that the electron temperature and the ion density from the floating-type probe are in good agreement with those from single Langmuir probes at various pressures and input rf powers. [Preview Abstract] |
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