We have studied the mode transition in low pressure capacitive RF discharges in nitroge

Dvořák Pavel

Kudrle Vít

Zajíčková Lenka

Šmíd Radek

Univerzitní repozitář Masarykovy univerzity

Study of mode transition in low pressurecapacitive RF discharges in nitrogenL. Zaj´ıcˇkova´, P. Dvorˇa´k, V. Kudrle, R. Sˇm´ıdDepartment of Physical Electronics, Masaryk UniversityKotla´rˇska´ 2, 61137 Brno, Czech RepublicReceived 2 May 2002We observed two types of a capacitively coupled rf discharge in nitrogen – unconfinedand confined mode. The modes differed significantly not only in a visual appearance butalso in the dc self–bias voltage at the driven electrode, the plasma potential and theaverage electron energy. The ion energy distribution measured in the unconfined modeshowed structure typical for the grounded electrode and lower pressure. The ion energyagreed relatively well with the results on the plasma potential.PACS : 52.80.PiKey words: RF discharge, nitrogen, probe measurements, ion energy distribution1 IntroductionCapacitively coupled rf discharges are used in numerous applications. Often theattention is focused on phenomena near the powered electrode where samples fornitridation or deposition are placed. However, for plasma treatment of polymers,there is a need for lower ion energies [1], e. g. obtained at the grounded electrode.We observed a transition of the discharge into the mode where the sheath at thegrounded electrode expands by one order of magnitude and correspondingly thesheath voltage rises enormously. This effect can be very unpleasant in some treat-ment techniques. Therefore we decided to study this phenomenon in finer details.2 ExperimentalRf capacitively coupled nitrogen discharges were studied in a stainless steelreactor of a spherical shape (i. d. 250 mm) with horizontally mounted two parallel–plate stainless steel electrodes separated by 57 mm. The bottom electrode (diameter100 mm) was grounded. The upper electrode (diameter 80 mm) driven at 13.56 MHzwas surrounded by a grounded shielding ring with the outer diameter of 100 mm.We used a constant nitrogen flow of 25.3 sccm which corresponded to the pressureof 2.5 Pa. The rf power delivered by the generator varied from 10 to 45 W. Wemeasured rf voltage, rf current and a dc self–bias voltage on the driven electrode.For diagnostic purposes we used a Langmuir probe (Scientific Systems Ltd.) andthe Plasma Processes Monitor PPM 421 (Balzers). The tip of the probe was placedat the discharge axis, 31 mm from the upper electrode. The horizontal distance ofPPM’s extraction hood was 5 mm from the electrode edges, the vertical distancewas 9 mm from the bottom electrode. The extraction hood was grounded.Czechoslovak Journal of Physics, Vol. 52 (2002), Suppl. D D427L. Zaj´ıcˇkova´ et al.3 ResultsWe observed two discharge modes distinguished at the first glance from thevisual point of view. The first mode was characterized by diffusive glow spreadoutside the interelectrode distance and a very thin dark sheath at the groundedelectrode (see Fig. 1 left). We will refer to this case as to “unconfined mode”. Aswe increased the power delivered by the rf generator up to maximum of 45 W thedischarge abruptly changed its character. In this case, the discharge was evidentlymore confined between the electrodes and the dark sheath at the grounded electrodewas comparable in the thickness with that at the powered one (see Fig. 1 right).We will further speak about this regime as about “confined mode”. If the confinedmode was once achieved the discharge stayed in this mode even after decreasingthe power down to 10 W. Once ignited the confined discharge was sustained by rfvoltage about 0.7 times lower than the unconfined mode. The difference betweenboth modes can be seen also in the current–voltage characteristics of the dischargesshown in Fig. 2. In both cases the current amplitude was a linear function of thevoltage but the current for the same voltage was higher in the confined mode.Two other features distinguished unambiguously the transition from the un-confined to the confined mode: (a) a significant decrease of a dc self–bias voltagemeasured on the powered electrode with respect to ground and (b) a significant in-crease of the plasma potential as well as the floating potential as measured by theLangmuir probe. As a result of these two opposite changes a sheath potential dropat the driven electrode, U1, was similar in both modes for the same rf voltage asshowed in Fig. 3. On the other hand, the potential drop at the grounded electrode,U2, that is equal to the plasma potential Up, increased by the factor of 5–10 due themode transition (see Fig. 4). This fact resulted in the larger sheath thickness (com-pare two CCD camera images in Fig. 1). Theoretical considerations summarized byLieberman [2] gives proportionality between the ratio of the sheath potentials andthe ratio of their areas as U1/U2 = (A2/A1)q where A1 and A2 are the areas of theFig. 1. CCD camera images of both modes of the discharge in nitrogen at similar appliedrf voltage of 125 V: left side – “unconfined”mode, right side – “confined”mode.D428 Czech. J. Phys. 52 (2002)Study of mode transition . . .100 110 120 130 140 150 1604.55.05.56.06.57.0  I rf [A]Urf [V] unconfined confinedFig. 2. Current voltage characteristics ofrf nitrogen discharge in two modes.50 100 150 200100150200250300  U1 [V]Urf [V] unconfined confinedFig. 3. Voltage drop in the sheath at thedriven electrode for two discharge modes.50 100 150 200102080100120  Up [A]Urf [V] unconfined confinedFig. 4. Plasma potential as a function ofapplied rf voltage amplitude.50 100 150 2000510152025  U1/U2Urf [V] unconfined confinedFig. 5. The ratio of sheath voltages fortwo discharge modes.driven and grounded electrodes, respectively and q depends on the model of thedischarge, separately for the sheath and the glow. The sheath model depends onthe pressure, i. e. on the assumption of the ion current density as a function of thesheath voltage and thickness. For the intermediate pressure used we could assumea collisional sheath with the constant mean free path for ions [2]. Then dependingon the glow physics the exponent q varies as 1.25–2.5. In the Fig. 5 there is theratio of the sheath voltages as a function of the applied rf voltage. In fact, this ratioshould be constant. However, we have to take into account an effective area of thegrounded surfaces in touch with the plasma that need not coincide with the area ofthe grounded electrode. Therefore the observed increase of the voltage ratio indi-cates a spread of the discharge, i. e. increasing the area of the grounded electrode.For the unconfined mode even at the lowest rf amplitude the effective area of thegrounded electrode was higher than for the confined mode and depended linearlyon the applied rf voltage (see Fig. 5). The variation of the area was about 12 %of the average value. In the case of the confined mode the variation was only 2 %although we can see again a linear dependence on the rf voltage.The measured electron energy distribution function (EEDF) could not be fittedby one temperature distribution as was found by other authors too [3, 4]. ThereforeCzech. J. Phys. 52 (2002) D429L. Zaj´ıcˇkova´ et al.80 100 120 140 160 1802345678  ne [1014 m-3 ]Urf [V] unconfined confinedFig. 6. The electron density calculatedfrom the integral of the EEDF.80 100 120 140 160 1802.02.53.03.54.0  <ε> [eV]Urf [V] unconfined confinedFig. 7. The average electron energy calcu-lated using integral of the EEDF.we evaluated the electron density and the average electron energy directly integrat-ing the EEDF obtained from the second derivative of the probe characteristics. Theelectron densities are given in Fig. 6. The density compared at the same rf voltagesis higher in the case of the confined mode. We think that it is due to an effect ofsecondary electrons gained at the grounded electrode because of a higher potentialdrop in the case of the confined discharge (see Fig. 4). It is interesting to notethat if we plot the density as a function of the rf current both the modes wouldbehave similarly. The changes in the average electron energy are shown in Fig. 7.In the unconfined mode the average energy decreases from 2.7 to 2.1 eV with therf voltage, i. e. with discharge spreading out. The energy in the confined mode ishigher (approx. 3.4 eV) and does not change with the rf voltage.The measurement of the ion energy distribution (IED) was in fact successfulonly in the unconfined mode. The reason for that is probably the position of thePPM. It was oriented horizontally as in Ref. [5] and not vertically through theelectrode as usual in many other experiments. Olthoff et al. [6] showed that thehorizontal position might not have a big influence on the measured IEDs. We hopethis was the case for the unconfined mode in which the PPM head was in a contactwith the glow and served like the grounded electrode. For the confined mode (Fig. 1right) we can see a large dark space left from the grounded electrode near the PPMhead. Moreover, due to the large sheath thickness at the grounded electrode theextraction hood was inside the sheath. These two facts probably caused a very lowintensity of the ion current in the confined mode.Shown in Fig. 8 are IEDs for N+2 ions sampled from the unconfined discharge asa function of the rf voltage. The ions had IEDs with relatively narrow asymmetricpeak broadened at lower energies. The IEDs for N+ ions were again peaked at thehigher energies but the peak was even narrower and symmetric in the shape. Thecomparison of IEDs for both ionic species is made in Fig. 9 for the rf voltage of144 V. Unlike at the driven electrode [8, 9] the IEDs did not contain any saddleshape or multiple peak structures. This is caused by a much lower sheath voltagemodulation at the grounded electrode. The rf amplitude of the sheath voltagesUrf1,2 can be estimated from the time average voltages U1,2 given in Fig. 4 asD430 Czech. J. Phys. 52 (2002)Study of mode transition . . .7 8 9 10 11 12 1301234  intensity [105 cps]ε [eV]   Urf = 111 V   Urf = 128 V   Urf = 144 VFig. 8. IED of N+2 ions in the unconfinedmode for different rf voltages.7 8 9 10 11 12 130123410 x  intensity [105 cps]ε [eV]   N2+   N+Fig. 9. Comparison of IED for N+2 and N+ions (unconfined mode, Urf = 144 V).80 100 120 140 160 180121314151617  ε [eV]Urf [V] Up N2+ max. energy  N+  max. energyFig. 10. Plasma potential compared withthe maximum energy of N+2 and N+ ions.80 100 120 140 160 18002468 exp. point lin fit <221,400 V>  total area [arb. u.]Urf [V]Fig. 11. Relative changes of ion density inthe unconfined mode.Urf1,2 = a U1,2 where a is 1.20 and 1.28 for collisionless and collisional sheaths,respectively [10]. The shape of the IEDs is in agreement with the results obtainedby other authors at the low pressure [6, 9]. The peak at high energy is consistentwith the expectation that the majority of ions are formed by electron impact inthe bulk plasma, near the sheath–plasma interface, where the density of energeticelectrons is highest [7]. Ions that experience energy loss by collisions contribute tothe low–energy portion of the IEDs. The sheath at the grounded electrode is verythin for the unconfined mode – we estimated about 1 mm. The mean free pathsfor N+2 and N+ ions with the energy of 13 eV calculated from the cross sectionsin Ref. [11] are 1.4 and 12.7 mm, respectively. It means that the sheath at thegrounded electrode is collisionless for N+ ions but there may be some effect ofcollisions for N+2 . This is evidenced by different types of IEDs as shown in Fig. 9.The transit time required for an ion to traverse the sheath region is at thefrequency of 13.56 MHz much higher than the period of the rf voltage. This meansthat the maximum energy of the ions arrived at the surface is determined by thetime average of Urf2, i. e. by Up. The ion maximum energy is compared for bothionic species, N+2 and N+, with the plasma potential Up in Fig. 10. Both, themaximum energy and the energy of the peak maximum exponentially decreases withCzech. J. Phys. 52 (2002) D431L. Zaj´ıcˇkova´ et al.: Study of mode . . .the rf voltage similar as the plasma potential. For lower rf voltages the maximumenergy of ions is about 2 eV lower than the plasma potential but then it coincideswell, especially for N+2 ions. The error of the ion energy was estimated about 0.3 eV(0.1 eV of a standard measurement error and 0.2 eV of a systematic error in theenergy scale calibration) and the error of plasma potential measurements was about0.3 V. The relative ion density calculated as a sum of IEDs’ integrals for both, N+2and N+, ions is given in Fig. 11 in dependence on the rf voltage. Except the firstpoint at the lowest rf voltage it exhibits a linear dependence on the rf voltage similarto the electron density measured by the probe (see Fig. 6). In the range of the rfvoltages the ion density increased by factor of seven although the electron densityonly by factor of three. It may be caused by the changes in the acceptance anglefor spreading discharge but this fact has to be studied further in details.4 ConclusionsTwo observed discharge modes, confined and unconfined, differed significantlyin the dc self–bias voltage and the plasma potential. As a result the sheath voltageat the driven electrode was similar in both modes but the sheath voltage at thegrounded electrode raised from 13 V up to 130 V. The IEDs of N+2 and N+ ionsin the unconfined mode showed effect of different mean free path of both ions. Themaximum ion energy agreed relatively well with the results on the plasma potential.The present work was supported by the Grant Agency of the Czech Republic, contract202/00/P037 and the Ministry of Education of the Czech Republic, contract J07/98:143100003.References[1] C. Jama et al.: Surf. Sci. 490 (1996) 352–354.[2] M. A. Lieberman: J. Appl. Phys. 66(7) (1989) 2926.[3] M. M. Turner, M. B. Hopkins: Phys. Rev. Letters 69(24) (1992) 3511.[4] E. Tatarova, E. Stoykova, K. Bachev, I. Zhelyazkov: IEEE Transactions on PlasmaSci. 26(2) (1998) 167.[5] J. K. Olthoff, R. J. Van Brunt, S. B. Radovanov: J. Appl. Phys. 72(10) (1992) 4566.[6] J. K. Olthoff, R. J. Van Brunt, S. B. Radovanov, J. A. Rees, R. Surowiec: J. Appl.Phys. 75(1) (1994) 115.[7] J. P. Boeuf, Ph. Belenguer: in Nonequilibrium Processes in Partially Ionnized Gases(Eds. M. Capitelli and J. N. Bardsley), Plenum, New York, 1990, 155.[8] D. Field, D. F. Klemperer, P.W. May, Y. P. Song: J. Appl. Phys. 70(1) (1991) 82.[9] M. Zeuner, H. Neumann, J. Meichsner: J. Appl. Phys. 81(7) (1997) 2985.[10] M. A. Lieberman, A. J. Lichtenberg: Principles of Plasma Discharges and MaterialsProcessing, John Wiley, 1994, New York, 344 and 351.[11] A. V. Phelps: J. Phys. Chem. Ref. Data 20(3) (1991) 557.D432 Czech. J. Phys. 52 (2002)

Study of mode transition in low pressure capacitive RF discharges in nitrogen

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