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Observation of fast hydrogen atoms formed by ion bombarding of surfaces T. Gans, V. Schulz-von der Gathen, U. Czarnetzki and H.F. Döbele 2 Experimental setup University Essen, Institute for Laser- and Plasma Physics, 45117 Essen, Germany Measurements are performed in an asymmetric capacitively coupled RF discharge at 13.56 MHz in hydrogen with a 1 % argon admixture. The setup is described in detail elsewhere [1]. Two plane circular electrodes of 100 mm diameter are separated by a discharge gap of 25 mm. The RFAbstract power ranges between 10 W and 100 W and the gas pressure between 10 Pa and 150 Pa. The discharge axis is imaged We report on the observation of fast hydrogen atoms in a onto the entrance slit of a 2 m grating spectrograph (Jenopcapacitively coupled RF reactor by optical emission spec- tik PGS). A special gateable CCD-camera (Picostar , LaVtroscopy. For the analysis we use the prominent H emission ision GmbH, 576 x 384 pixels, 13.2 x 8.8 mm ) samples line of atomic hydrogen in combination with other lines from spectral intervals of about 4.5 nm with a spectral resolution molecular hydrogen and argon. Several characteristic emis- of 0.04 nm and a spatial resolution of about 0.5 mm. The sion structures can be identified. One of these structures is optical lines-of-sight are parallel to the electrode surfaces. related to fast hydrogen atoms traveling from the surface of At each of these positions a ’classical’ emission spectrum the powered electrode to the plasma bulk. From the appear- is recorded. The temporal resolution is 6 ns. The camera ance time within the RF period we conclude that this feature gate can be fixed at a defined phase position of the RF-cycle. is originated from ion bombardment of the electrode surface. From a series of such measurements at different phase posiMeasured pressure dependencies and a simple model for the tions the temporal development of specific emission lines at ion dynamics support this assumption. the selected wavelength can be reconstructed. The lifetime of the excited states causes a blurring of the temporal evolution and a delay of the emission maximum with respect to the excitation. In case of the H -line the delay is of the order 1 Introduction of 5 ns. Atomic hydrogen is an essential element in many low- and high temperature plasmas. In fusion plasmas as well as in technical plasmas surface interactions of the atomic or ionic component are of great importance. Open questions are e.g. the mechanisms leading to the generation of fast hydrogen atoms or highly vibrationally excited hydrogen molecules which are of great importance for technical discharges and for the production of negative ions, respectively. Here the interaction of ions with surfaces is observed at the powered electrode in a capacitively coupled RF-discharge. Typical ion energies at the cathode are of the order of several hundred eV. Sheath expansion heating of the electrons, secondary electron emission and ion bombardment of the cathode are modulated temporally. This leads to complicated temporal and spatial emission structures. These structures are observed by a fast gated CCD camera which allows the measurement even of faint molecular lines. The emission structures of various atomic and molecular lines are compared. By this comparison the excitation channels can be identified. 3 Experimental observations Figure 1 displays a space-time evolution of the H line emission. The abscissae comprises 2 periods of 74 ns of the excitation. The transverse axis gives the distance from the powered electrode located at the bottom of the figure. The voltages across the plasma sheath in front of this electrode have been measured by Czarnetzki et al. [2]. These measurements show that there is a temporal reversal of the sheath electric field and potential. During this short phase of positive voltage, electrons are attracted towards the electrode. The time behaviour of the sheath potential is sketched in the lower part of fig. 1. In the asymmetric discharge the discharge chamber is on the same potential as the grounded electrode. The different areas of the powered and the effective grounded electrodes lead to a higher current density close to the surface of the powered electrode. Therefore, also the plasma emission exhibits an asymmetric spatial structure, especially at high 1 15 II III 10 5 IV ABE656 I 0 0 20 40 60 80 100 120 140 25 20 15 10 5 9NC624 Distance from the powered electrode [mm] 20 SH EA T H Distance from the powered electrode [mm] 25 0 0 Time [ns] 20 40 60 80 100 120 140 Time [ns] Usheath Figure 2: Space and time resolved emission profile of a Fulcher- (v=2, Q=0) emission line; discharge conditions as in Fig. 1 Figure 1: Space and time resolved emission profile of H within a hydrogen discharge at 113 Pa and a transmitted power of 100 W. Sketched is the time development of the sheath voltage as measured by Czarnetzki et al. [2] sion in the region of structure III, expanding from the sheath edge to the grounded electrode (fig. 2). A comparison with argon transitions with similar excitation cross sections as H exhibits the same characteristic. Neither the Fulcher nor the argon lines exhibit structure IV of the H emission. Petrovic et al. [4] observed a similar phenomenon in a hydrogen DC discharge and attributed it to fast neutral hydrogen atoms created at the surface by the impact of hydrogen ions. These fast atoms are excited by collisions with hydrogen molecules of the background gas. The cross sections for collisional interaction between fast hydrogen atoms and molecules indicate, however, that the main channel for collisional losses is vibrational excitation of molecules [7]. The dissociation cross section changes by orders of magnitude between the different vibrational levels. Therefore, the production of these vibrationally excited hydrogen molecules can be of great importance for the discharge behaviour. The Doppler-shift of the H radiation is a direct measure of the particle velocity. However, here observation is parallel to the surface while the fast atoms move perpendicular to the surface. In the collision process which causes excitation the fast atom can be scattered with a component in the direction of observation. Therefore, two conditions for the observation of fast atoms by the Doppler-shift have to be met: Firstly, the atom must have enough energy so that the excitation cross section is sufficiently large. This requires energies in excess of about 75 eV. Secondly, the atom has to be scattered with a component in the direction of observation. In general, scattering can occur in all directions and the velocity distribution is blurred. The initial velocity distribution of the fast atoms can be reconstructed from the measure- RF-powers. Emission maxima appear close to the powered electrode. Here the evolution of several structures (I-IV) can be distinguished. Structures I and II can be explained from E-field measurements done by Czarnetzki et al. [2]. Structure I is caused by the field reversal across the space charge sheath, typical for RF-discharges in hydrogen. Electrons are accelerated towards the electrode and the discharge current is at its maximum. When the potential becomes negative again, electrons are pushed out of the sheath towards the bulk. The second emission structure (II) is related to this sheath expansion heating. Both maxima are delayed with respect to the maximum excitation by about 5 ns due to the lifetime of the excited state. Structure III results from fast secondary electrons created by ion impact at the time of maximum ion bombardment of the electrode. The emission maximum is delayed substantially by more than 5 ns with respect to the maximum negative voltage across the sheath due to the ion inertia [8]. The secondary electrons are accelerated to energies beyond the maximum of the excitation cross section already within a distance shorter than the mean free path for collisions. They can contribute to the excitation of H only after loosing part of their energy in collisions with the background gas in the plasma bulk where no further acceleration occurs. This hypothesis is supported by comparison with emission lines from the molecular hydrogen triplet system (Fulcher lines). The excitation cross section of the Fulcher lines is comparable to H at low energies between the threshold and about 25 eV but falls off much more rapidly at higher energies. The Fulcher lines show no emis2 600 20 kinetic energy [eV] Distance from the powered electrode [mm] Kinetic energy of f ast atoms 25 15 10 5 0 0 20 40 60 80 100 120 140 Time [ns] 500 400 300 200 100 0 0 25 50 75 100 125 150 pressure [Pa] Figure 3: Intrusion of fast exciting neutrals into the plasma; observation near H ( nm); p= 28 Pa; P=100 W Figure 4: Pressure dependence of the kinetic energy of those particles which result in excitation of fast hydrogen atoms ment only, if the angle dependent scattering cross section is known. In this work no attempt was made to determine the velocity distribution. With the CCD-camera the entire spectrum of the Doppler-broadened H -line and its spatial distribution at a fixed phase of the RF is recorded simultaneously. Each wavelength within the spectrum is related to a specific Doppler-shifted velocity in the direction of observation. Figure 3 shows the emission profile of atoms at a Doppler nm corresponding to an energy of about shift of 28 eV. The pressure was 28 Pa at an RF-power of 100 W. Most remarkably is the propagation of the maximum of the emission intensity into the plasma. This shows that the emission of fast neutrals from the surface is strongly temporally modulated and occurs basically as a burst around t=20 ns in the first period shown in fig. 3. From the slope a velocity of or an energy of 220 eV results. This propagation velocity is believed to be close to the average velocity of the emitted atoms perpendicular to the surface. The energy of the fast atoms compares well with the expected energy of the hydrogen ions at the applied peak-to-peak voltage of 950 V and the relatively large mean free path at the pressure of 28 Pa. Measurements at other Doppler-shifts under identical conditions give the same result. With increasing Dopplershift, the signal amplitude decreases rapidly and measurements can be performed with a reasonable signal-to-noise ratio up to shift corresponding to an energy of about 120 eV. The propagation velocity was measured at several pressures between 10 Pa and 150 Pa and constant RF-power of 100 W. The result is shown in fig. 4.With increasing pressure, also the number of collisions of ions with the background gas increases and the mean ion energy decreases. The same trend should be expected for the energy of the fast neutrals. Qualitatively, fig. 4 supports this assumption. However, at higher pressures, the kinetic energy approaches a constant value of about 80 eV. The cross section for collisional excitation of H rises steeply by two orders of magnitude around this energy [7]. Therefore, only those atoms within the velocity distribution with an energy above this effective threshold can be observed. If the mean energy is below this effective threshold at high pressures, not the average velocity but part of the high energy tail of the distribution function is observed. 4 Simple Model The emission measurements show that the flux of fast neutral atoms has a strong temporal variation. The maximum has a phase shift of about 10 ns with respect to the maximum negative sheath potential. This phase shift is essentially the same as for the secondary electrons which have a similar temporal variation. These results suggest that also the ion bombardment at the electrode surface is modulated temporally. In RF-discharges it is believed generally that ions due to their inertia are drifting mainly in a phase averaged potential and show only weak temporal variations. However, hydrogen ions are relatively light and the temporal variations can be much stronger. In order to investigate whether the ion bombardment at the cathode can be modulated temporally for light ions we use a simple analytic model which takes into account only charge exchange collisions with a velocity independent cross section. This simplification ignores completely the complex ion chemistry in the sheath and the details of the scattering processes. Therefore, the results can be expected to give only a qualitative picture. The temporal variation of the mean energy is shown in fig. 5. The ion energy is measured in units of " # ! $%'& 3 Further, the dissociation cross section increases by orders of magnitude at higher vibrational levels. Therefore, fast neutral atoms can influence the discharge behaviour strongly. 3.5 average ion energy /0 E 3.0 K =∞ 2.5 2.0 K=1 K = 0.1 References K=0 1.5 [1] T.Gans, V.Schulz-von der Gathen, and H.F.Döbele, Plasma Sources Sci. Technol. 10 (2001) 17-23 1.0 0.5 0 1 2 3 4 RF phaseφ 5 [2] U.Czarnetzki, D.Luggenhölscher and H.F.Döbele, Plasma Sources Sci. Technol. 8 (1999) 230-248 000628a.opj 0.0 6 [3] S.Djurovic and J.R.Roberts, J.Appl.Phys., 74 (1993) 6558 ) ( ( '# Figure 5: Average ion energy as a function of the RF phase. The sinusoidal sheath voltage is zero at and . The energy is measured in units of . K is a dimensionless parameter depending on sheath voltage, mean free path, and ion mass. Details are given in the text. [4] Z.Lj.Petrovic, B.NM.Jelenkovic, Phys.Rev.Lett. 68 (1992) 325 The parameter K is $ * "'# , + '% & $ & and A.V.Phelps, [5] S.B.Radovanov, K. Dzierzega, J.R.Roberts, J.K.Olthoff, Appl.Phys.Lett. 66 (1995) 2637-2639 and [6] S.B.Radovanov, J.K.Olthoff, R.J.VanBrunt, S.Djurovic, J.Appl.Phys. 78 (1995) 746-757 and [7] A.V.Phelps, J.Phys.Chem.Ref.Data 19 (1990) 653-675 [8] U.Czarnetzki, D.Luggenhölscher and H.F.Döbele, Appl. Phys. A (2001) accepted for publication Here is the maximum sheath width, the mean free path, and the time averaged sheath potential. With decreasing K the temporal variation becomes less pronounced but the phase shift increases. For our conditions, K varies typically in the range between 0.1 and 1. Therefore, phase shifts of 1 (corresponding to 10 ns) or more can be expected. This is in good agreement with the observed phase shift in fig 1. According to the model, the phase shift should increase at lower pressures. This is also in agreement with the experimental observations. For ions with higher mass, e.g. argon, K would be substantially smaller and the temporal variation would vanish practically. *.-0/1 2 23 5 Conclusions Temporally and spatially resolved emission spectroscopic investigations of a hydrogen RF discharge have been performed. The emission characteristics of several atomic and molecular lines have been compared. From this comparison the excitation mechanisms of various emission structures in the discharge can be identified. Most remarkable is the observation of fast neutral hydrogen atoms created by ion bombardment of the cathode. The energy of these atoms is comparable to the energy of the impinging ions. It can be expected that the neutral flux is of the same order as the incoming ion flux [4]. An inspection of the cross sections for the interaction of fast hydrogen atoms with molecular hydrogen shows that the dominant channel for energy loss is vibrational excitation[7]. Vibrationally excited hydrogen is regarded as one of the precursors of negative ion formation. 4