Abstract

1. Abstract • Langmuir probes in low-density RF plasmas sometimes show peculiar I-V characteristics with no electron saturation. This is due to the space potential Vs changing with probe potential. This occurs on an ion time scale, but it also means that the ion flux to the walls is impeded by an unknown mechanism. To avoid this, the curve must be swept faster than Vs can change. A mysterious ion feature is also observed. The speed at which data are taken matters!

2. 1.9 MHz, 60-100W, 3-10 mTorr Ar Apparatus

3. Probes (1)

4. Probes (2)

5. Choke chain impedances Two samples of the Hiden chokes for 2 MHz. The straight line is the Z of a non-resonant 1 mH choke. Both probes have also a compensation electrode to drive the probe tip. Impedance of the Chen A probe. Chokes to resonate at 2w could not be found.

6. Example of anomalous I - V curve There is no exponential part, and Ie does not saturate.

7. Measurement of Vf To measure changes in floating potential Vf, we used a second probe close to the first. Though the chamber was grounded, a grounding plate could be inserted to put the ground closer.

8. Measurement of I-V curves Although many systems could be used, the simplest, error-free, method was to read off mA directly from a small DVM, taking date point-by-point. We also swept the I-V curves automatically with either the Hiden ESP Mk2 or the newer ESPion units. All combinations of three probe types and three measurement systems were tried.

9. Change of Vf with Vp This shows that Vf (right scale) rides up with probe voltage Vp. By adjusting the right-hand scale, the red curve can be made to resemble the probe characteristic.

10. Corrected I-V curve By correcting for the shift if Vs (as found from Vf), one obtains a more reasonable curve. Saturation cannot be reached. The red curve is taken with a fast sweep, which suffers from only part of the Vs shift.

11. Exact OML equation The exact Tonks-Langmuir Orbital Motion Limited probe theory. Note that a sheath thickness s has to be assumed. This form has to be used for electron saturation. IONS: for ions, a very good approximation independent of KTe can be made. ELECTRONS: This is for the transition (exponential) region.

12. Comparison with experiment First, the density is found by fitting the ion current. The Vf -corrected data then give a short transition region from which KTe can be obtained.

13. Electron saturation Even the corrected data cannot reproduce electron saturation. This is probably due to inadequate RF compensation. Note that the OML theory is independent of s / Rp as long as it is > 5 or 10.

14. Why does Vp affect Vs? e i i e e Normally, the probe current Ie is balanced by a slight adjustment of the electron current to the walls, Iew, via a small change in sheath drop. Since Iew = Iiw, Vs should not change detectably if Ie << Iiw.

15. The numbers don't work out Area of chamber walls = 4400 cm2 Bohm current density: Ii = 0.5 neAwcs ( n = 2 x 1010 cm3, KTe = 1.6 eV) Ion current to walls: Ii(wall) = 1.5A. But suppose n there is 0 for now. Ion current to grounding plate (25 cm2) » 8.5 mA By quasineutrality, this is also equal to the electron current to the plate. Electron saturation current at +100V: 3.2 mA (calc.), 25 mA (measured) Difference is due to sheath expansion (s /a » 8). Thus, Ii to the plate cannot balance Ie to the probe, even if no electrons are lost to the wall. BUT: 1) Why are there no ions lost at the chamber wall? 2) Why is Vs changing well before Vp = +100V? There seems to be a transport barrier at the grounded walls.

16. Speed of ion response If the probe draws excess electrons at the center, an ambipolar field will develop to drive ions faster to the wall. The density profile n(r) will change from essentially uniform to peaked. The diffusion equation is (for a nearly spherical chamber) where D = Da, the ambipolar diffusion coefficient. The solution is The time constant for the lowest radial mode j = 1 is then

17. Change of density profile In theory, a probe drawing a large Ie can cause n(r) to be peaked after about 1 msec. Below are two measurements of the density profile, but not necessarily connected with this effect.

18. Effect of grounding plate

19. Explanation of test of potential pulling effect OUT means that the grounding plate is out, and the chamber wall is the nearest ground. IN means that the grounding plate is in, and is a ground plane only 3 cm away from the probe. It is seen that the difference between the raw curves and those corrected for potential pulling is smaller with a better ground, as expected. Hi-p means 10 mTorr; Lo-p means 3 mTorr. It is seen that at lower pressure, there is better contact with ground, as expected, since the ion diffusion is faster.

20. Conclusions • Electron saturation is not possible because Vs rides up with Vp. • When Ie to the probe exceeds Ii to the wall, the potential and density profiles change to drive ions faster to the wall. • It takes about 1 msec for the profiles to change. A better I-V curve can be obtained if it is swept in a shorter time. • This should not happen unless ion current to ground is somehow blocked. • This effect causes the peculiar I-V curves in lo-n ICPs, but it does not affect the values of n and Te obtained.

21. An anomalous ion feature A very large ion bump often occurs at large negative Vp. These curves were taken using the Hiden ESP Mark2 system. This feature changes on the 5-10 sec time scale between scans, and it depends on the termination resistance R. R is 250 W on the 10 mA scale and 2500 W on the 1 mA scale. The steady state agrees with the 10-mA curve.

22. A veritable mystery! The anomalous ion feature depends on the voltage Vpon another probe nearby. But... the slow change in time can only be caused by probe contamination. Both probes have insulating shafts, and the nearest ground is at the walls