A possible explanation of the anomalous emissive probe behavior in a reactive RF plasma

Abstract

Emissive probe diagnostics in saturated floating potential mode was carried out in RF plasmas of argon (Ar)-methane (CH 4 ) and Ar-CH 4 -hexa methyl disiloxane (HMDSO). These plasmas are used for the deposition of diamond-like carbon (DLC) and SiO x -containing DLC films, respectively. While performing the experiments it was found that the probe characteristics had two saturation regions instead of one. The same measurements when repeated in Ar and Ar-N 2 plasmas showed a single saturation as expected. The first experiments when repeated again showed the same anomaly. The experimental findings question the validity of emissive probe diagnostics in reactive plasmas. A possible model of dust formation inside the reactive plasma is predicted and the first saturation is linked to dust. The second saturation is credited as the actual plasma potential. The concept of dust was invoked after being sure that no effects of RF and reference electrode contamination are responsible for this behavior. The results indicate that we should remain cautious when using emissive probes in reactive plasmas as they may occasionally lead to erroneous results.

Floating potential method using a thermionic emissive probe including an ionizing and collisional presheath

“…As T w increases, φ w,SCL decreases because the emitted electrons have a greater thermal motion that can counteract the action of the electric field in the vicinity of the probe. This allows the formation of a negative charge cloud near the surface that (19). cancels the electric field even for more negative values of φ w .…”

“…We can obtain the floating potential by solving (17) using (19). Figure 8 illustrates the variation in the floating potential with the temperature of the probe T w for P = 50 mTorr and T e = 4 eV.…”

“…In the 1920s, Tonks and Langmuir [6][7][8] proposed the use of this type of probe for plasma diagnosis. There are different emission mechanisms, such as thermionic emission [9] or secondary emission by impact of energetic particle [10,11], and currently, these probes are used in different contexts, such as Hall thrusters [12][13][14], electron beams [1,15], sheaths [16], RF plasmas [17][18][19], and fusion plasmas [20,21].…”

Analysis of the virtual cathode and floating potential of a thermionic emissive probe operating in the space-charge-limited regime

“…From the first works by Tonk and Langmuir in the 1920s [1][2][3], the use of emissive probes to diagnose plasma has played an important role in many areas involved in plasma studies, such as sheaths [4,5], RF plasmas [6,7], Hall thrusters [8][9][10], magnetized plasmas [11,12], electron beams [13,14], and fusion plasmas [15][16][17].…”

Accurate space potential measurements using an emissive probe in high pressure plasma and neutral gas

“…Plasma potential is one of the most important parameters in plasma diagnostics, serving as a principal factor in the analysis of Langmuir probe I-V characteristics for determining other plasma parameters, such as the electron density, the electron temperature and the electron energy distribution function (EEDF) [1,2]. For plasma potential measurements, the emissive probe is considered superior to the Langmuir cold probe for a wide range of application circumstances, such as RF discharge plasmas [3][4][5][6], plasmas with potential fluctuations [6][7][8][9][10], magnetized plasmas [2,11,12], plasma sheaths [2,[13][14][15], tokamaks [9,[16][17][18][19], as well as vacuums [2,[20][21][22]. Of the many existing emissive probe techniques, the inflection point method, improved by us, can obtain the most accurate plasma potentials [3,12,[23][24][25].…”