Plasma density measurements are an essential tool in understanding and controlling processing plasmas across a wide range of applications. Charge collection probes (Langmuir probes) are of limited utility in depositing plasmas, high pressure applications or in processes that require the use of reactive gases, as these environments result in unreliable data acquisition. Plasma frequency probes are an attractive alternative to Langmuir probes in such applications since they do not suffer significant performance degradation in these environments. This work presents frequency probes measurements of plasma density over a range of 10⁹ to 10¹² cm^-3 in a variety of processing plasma chemistries (N₂, CH₄, NH₃, O₂ and SF₆). In addition to electron density measurements frequency probes are also useful for measuring plasma potential, electron temperature, and electron energy distribution functions in the gas chemistries mentioned above.

A new diagnostic tool to measure plasma parameters as well as Electron Energy Distribution Function (EEDF) by a floating-emissive probe has been proposed[1], and a diagnostic system has been newly developed and applied to radio-frequency (RF) plasmas. It is generally difficult for a conventional probe method to measure EEDF in RF plasmas, because of the plasma potential fluctuation, particularly in the capacitive mode. The present method has an advantage that there is no need of an external compensation circuit and all measurements can be made in the floating condition. The method is based on measurement of the functional relationship between the floating potential change ΔV_F and the heating voltage V_H of emissive probe. If the plasma electrons are in Maxwellian, the equation can be obtained for the value of ΔV_F as a practical and useful formula.[1]

It is important to know that the value of ΔV_F contains information of electron energy distribution. In the experiment, the data of V_F and ΔV_F was measured in a 13.56 MHz RF plasma produced by single-loop antenna[2], as a function of V_H. In the conditions of high RF power, the plasma mode was ICP and the measured values of ΔV_F were in agreement with the theoretical value, stating that the plasma electron was in Maxwellian. The electron temperatures thus obtained were very consistent with those measured by Langmuir probe. The electron density was also obtained from the value of ΔV_F near the plasma space potential and they were consistent with Langmuir probe data. Consequently, by using a new diagnostic system one can obtain the electron temperature and density, the plasma space potential and floating potential, as well as the EEDF in the floating condition of the probe. It should be stressed that this is the first success of floating probe to be able to measure all plasma parameters. One can also expect that the present method is applied to plasmas which are produced in insulated vessels.

References:

[1] K.Kusaba and H.Shindo, Review of Scientific Instruments, 78, 123503-1(2007).　

[2] Y.Jinbo and H.Shindo, Applied Physics Express, 2, 016001-1(2009).

Accurate measurements of the plasma potential is a critical challenge especially for complex plasmas such as magnetized and flowing. We compare various emissive probe techniques for measurements of the plasma potential. The measurements were conducted in a low-pressure magnetized discharge of the Hall thruster. The thruster was operated with xenon gas in subkilowatt power range and the discharge voltage range of 200-450 V. The probe was placed at the channel exit where, the electron temperature is in the range of 10 to 60 eV and the plasma potential is in the range of 50 to 250 V. The floating point method is expected to give a value ~Te/e below the plasma potential. The experimental results are consistent with these expectations. Specifically, it is shown that the floating potential of the emissive probe is ~2Te/e below the plasma potential. It is observed that the separation technique varies wildly and does not give a good measure of the plasma potential.

This work was supported by US Department of Energy grants No. DE-AC02-09CH11466, No. DE-FG02-97ER54437, and No. 3001346357 and the Fusion Energy Sciences Fellowship Program administered by Oak Ridge Institute for Science and Education under a contract between the U.S. Department of Energy and the Oak Ridge Associated Universities.

Ion energy distributions (IED) on the substrate electrode were measured in a Faraday-shielded inductively coupled plasma. Narrow distributions with well-controlled ion energy were obtained by pulsing the plasma and applying a synchronous DC bias on a “boundary” electrode during the afterglow. The peak ion energy was controlled by the DC bias, as the plasma potential and the electron temperature decayed drastically in the afterglow. IED measurements were performed in Ar, Kr and Xe plasmas, using a retarding field energy analyzer. A Langmuir probe was also used to measure time- and space-resolved plasma density and electron temperature during a pulse as a function of power and pressure. The quasi-steady electron temperature (late in the active glow) followed the order Ar > Kr > Xe i.e., the gas with the highest ionization potential had the largest electron temperature. The opposite order of T_e (Xe > Kr > Ar) was observed in the afterglow, as the decaying electron temperature was controlled by diffusion cooling, and the diffusivity is lower for heavier gas. The full width at half maximum (FWHM) of the IEDs followed the order Xe>Kr>Ar. Higher electron temperature in the afterglow correlated with larger FWHM. The width of the IED could also be controlled by varying the pulsed plasma frequency and duty cycle, or the time window of the application of the DC bias during the afterglow. Small additions (up to 5% by volume) of chlorine gas resulted in IEDs that were similar to those in the corresponding pure noble gas plasma, except that the peak ion energy was lower by a few eV.

Work supported by the DoE Plasma Science Center and NSF.

With each new generation of integrated circuits and reduction in transistor and interconnect dimensions, plasma etching of fine features in silicon, aluminum and insulating thin films encounters new sets of challenges. Over the past thirty years, our understanding and control of plasma etching processes has greatly improved, due to the advances in diagnostic techniques and basic mechanistic studies, combined with advanced modeling methods. This talk will review studies, mostly from our laboratories, spanning this era with an emphasis on the connections between studies, the influence of other fields, and the interactions between collaborators and colleagues. Some old controversies will be revisited and perhaps revived.