Ion energies in high power impulse magnetron sputtering with and without localized

Ion energies in high power impulse magnetron sputtering with and without localized ionization zones Yuchen Yang 1,2, Koichi Tanaka 2,3, Jason Liu 2,4, André Anders 2 1 School of Materials Science and Engineering, State Key Lab for Materials Processing and Die & Mold Technology, Huazhong University of Science and Technology, Wuhan 430074, China 2 Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA 3 Central Research Institute, Mitsubishi Materials Corporation, 1002-14 Mukohyama, Naka-shi, Ibaraki 311-0102, Japan 4 Department of Physics, University of California Berkeley, Berkeley, California 94720, USA High speed imaging of high power impulse magnetron sputtering (HiPIMS) discharges has revealed that ionization is localized in moving ionization zones but localization disappears at high currents for high yield targets. This offers an opportunity to study the effect ionization zones have on ion energies. We measure that ions have generally higher energies when ionization zones are present, supporting the concept that these zones are associated with moving potential humps. We propose that the disappearance of ionization zones is caused by an increased supply of atoms from the target which cools electrons and reduces depletion of atoms to be ionized. 1

High power impulse magnetron sputtering (HiPIMS) discharge plasmas have been reported to be azimuthally non-uniform, consisting of localized, drifting plasma regions exhibiting locally enhanced light emission. These features are referred to as spokes, plasma bunches, or ionization zones 1,2,3. These ionization zones have also been associated with plasma jets or flares travelling away from the target 4,5. Localization of ionization is common for all sorts of discharges with crossed electric and magnetic fields ( E B discharges), arising as a consequence of plasma instabilities 6,7,8. Instabilities are generally more pronounced at high power as nonlinear amplification becomes more prominent. However, somewhat surprisingly, recent work has shown that ionization zones in HiPIMS can disappear when the discharge current is high, rendering the discharge plasma azimuthally homogeneous. 9,10 Spectrally selective imaging 11 and ion energy measurements 12,13 have suggested that ionization zones are locations of elevated potential. Traveling potential humps located in ionization zones act like propeller blades, giving ions extra energy primarily in the direction of zone motion, the E B direction. The experimental discovery of ionization zone disappearance therefore offers a test of the potential hump hypothesis, as ion energy distribution functions (IEDFs) should be affected by the presence or absence of ionization localization. Therefore, in this contribution, we combine high speed imaging and IEDFs measurements to study the magnetron from the side-on and end-on directions. With the localization of traveling humps disappearing, we hypothesize that the IEDFs are altered and that the asymmetry caused by zone motion is reduced or perhaps even eliminated. In the experiments presented here, an unbalanced planar magnetron (MeiVac Inc.) was used. The nominal target dimensions are 76 mm (3 in.) in diameter and 6.25 mm (1/4 in.) in thickness. A cylindrical coaxial anode was mounted flush with the target surface, which allows ions and 2

light to escape sideways without being blocked by the anode. The magnetron was mounted in a cryogenically pumped chamber (base pressure approximately 10 4 Pa); argon flow was set through a mass flow controller to obtain the desired process pressure, which was 0.4 Pa unless specified otherwise. The discharge was powered by a high current pulse generator, model SIPP by Melec GmbH, which can provide pulses of approximately constant voltage. The discharge current was recorded using a current transformer (Pearson, model 101), and the discharge voltage was measured at the power feedthrough with a voltage divider probe (Tektronix P5100). All electrical signals were recorded with a Tektronix digital oscilloscope. Intensified charge-coupled device (ICCD) camera images were taken with a Princeton Instruments PIMAX 4 camera equipped with an f =80 mm Nikon lens. The spectral response of the PIMAX photo detector was from 200 to 900 nm. All images presented here make use of the false color scale royal of the image processing software IMAGEJ 14. The setup for IEDF measurements is similar to our previous work 15. IEDFs of singly and doubly charged ions were measured with a particle energy analyzer and mass spectrometer, model EQP300 by HIDEN Ltd. The magnetron was mounted on a movable table within the vacuum chamber, allowing the magnetron to be moved perpendicularly to the ion acceptance direction of the EQP300 analyzer (see Fig. 2 of Ref. 9). IEDFs were measured at x = -23 mm, 0 mm and +23 mm, where the x = -23 mm and +23 mm positions correspond to the E B direction pointing towards or away from the EQP300 entrance aperture, respectively. For side-on and end-on measurements, the target s surface normal was perpendicular and parallel, respectively, to the acceptance direction defined by the EQP300. The distance between target and entrance orifice was 140 mm. IEDFs were recorded in voltage steps of 0.5 V up to a potential of 100 V. 3

The dwell time (the time over which signals were accumulated) was 2000 ms for each energy and mass data point, which means that we effectively integrated each energy and mass measurement over time. Images of the discharge were acquired at different currents by varying the camera delay relative to the increasing current pulse. The pulse width was 200 µs with a repetition rate of 33 Hz. The discharge was operated with a peak current of 400 A for most experiments, set by a pulse voltage (constant during each pulse) of 550 V, 485 V, and 500 V for copper, niobium, and titanium targets, respectively. Within each pulse, when the current is less than 200 A, the ionization zones of Cu target HiPIMS discharges are typically triangularly shaped as shown in Fig. 1. In the ICCD images, one recognizes a wide edge perpendicular to the racetrack at the E B side of the ionization zone, i.e. the end where drifting electrons arrive. As the current approaches 200 A, the zones appear as rhombus-shaped regions with their diagonals perpendicular to the E B direction. However, when the current reaches 300 A, the zones rather suddenly disappear, and the images depict an approximately azimuthally uniform plasma ring (however, we note that the plasma ring is consistently slightly brighter on one side than the other side, presumably due to a slight azimuthal non-uniformity of the magnetron s magnetic field). The Cu target was then replaced by Nb and Ti, and Fig. 1 shows images of HiPIMS discharges on those targets. The ionization zones were typically irregularly shaped with a wider region at the E B end of the zones. As the current was increased, the zones became wider radially and more localized azimuthally. At 400 A, the zones were still localized and there was no indication for zone disappearance at even higher current up to 700 A. Considering many 4

images taken for different pulses but under nominally the same conditions, it is clear that the zones for Nb and Ti targets at 400 A move along the racetrack and undergo strong changes. HiPIMS plasmas have also been observed with the fast camera with a viewing angle of 10 relative to the target surface, as this view allows us to easily correlate ionization zones and plasma flares, if present. We have found that in the regime without zone localization (i.e. copper at high current), the bright plasma emission was always limited to less than 4 mm from the target. In contrast, bright plasma flares can be observed with the Nb target up to 20 mm 4, which suggest that plasma flares are associated with the presence of ionization zones. Additionally, the observation of flares implies that caution must be exercised when describing the shape of ionization zones, as viewed from above: orthorhombic or irregular shapes may be due to the superposition of light near the target and light from the flares atop. Therefore, it is conceivable that the near-target zones are always triangularly shaped. Another interesting observation is that at the very beginning of each pulse, when the discharge current is still low, as shown in Fig. 1 for 0.2 A, the plasma appears as an azimuthally uniform ring, regardless of target material. This is important because DC magnetron discharges 16 and magnetized microplasmas 17 at very low current show azimuthal nonuniformities. This provides evidence for the idea that discharges begin rather uniformly and take > 10 μs to develop zones. After establishing that ionization zones disappear for copper but not for titanium and niobium under our conditions, we focus on IEDF-measurements for copper to elucidate the underlying mechanisms. The corresponding discharge parameters that exhibited localized zones were: 500 μs pulse length, 100 Hz repetition rate, 40 A peak current, and 455 V pulse voltage. Discharge 5

parameters that yielded azimuthally uniform plasma were: 500 μs pulse length, 12.5 or 25 Hz repetition rates, 400 A peak current, and 550 V pulse voltage. The side-on IEDFs of Cu ions, Fig. 2, are very different compared to those reported for Nb ions (Fig. 6 of Ref. 15). The intensity of singly charged ions, Cu +, is always the highest among all the ion species. This is to be expected in light of the very high self-sputter yield of copper (high supply of atoms) combined with its low ionization energy. The first and second ionization energies for copper are 7.73 and 20.29 ev, while for argon the values are 15.76 and 27.63 ev, respectively. The IEDFs show a clear dependence on the measurement position relative to the E B direction when the ionization zones are present, whereas no significant difference can be found at high currents when ionization zones are absent. In the presence of zones, Figs. 2 (a) to (d), at positive x (the E B direction pointing away from the EQP300 orifice), the distribution functions exhibit a low-energy peak corresponding to thermalized ions and another broad, high energy feature. As the measurement position changes to negative x (the E B direction pointing towards the EQP300 orifice), the intensity of the thermal peak is small with the peak shifted to higher energy by approximately 15 ev, and the broad high energy peak rises in intensity and shifts to higher energy. The energy of doubly charged ions is approximately twice that of singly charged ions. In the absence of localized zones, Figs. 2 (e) to (h), the distribution functions exhibit a broad, low-energy peak and some narrow, high energy features. For Cu + and Cu 2+, two high energy peaks are seen for all x-positions. The IEDFs remain approximately the same as the relative 6

position of the EQP300 is changed, and doubly charged ions are now less than twice as energetic as singly charged ions. The side-on IEDF measurements suggest that (i) ions experience a stronger electric field when ionization zones are present, and (ii) doubly charged Ar and Cu ions are produced at similar locations when ionization zones are present and at different locations when zones are absent. The IEDFs measured end-on are shown in Fig. 3 this direction is very relevant for most deposition conditions because the substrate faces the target. Various peaks appear in the data, and we note that these features are not discharge-to-discharge artifacts but are highly reproducible. Fig. 3 shows each IEDF measured twice, proving that the features of the distributions are not noise but highly reproducible. For example, in Fig. 3 (a) the energy distribution of Ar + for 400 A peak current discharges has four distinct peaks. We need to recall that the measurements were done with a long dwell time, effectively time-integrating the signals, meaning that the peaks may correspond to ions at different phases of each pulse. The energy difference between adjacent peaks is approximately 20 ev, therefore one may speculate that plasma potential waves of about 20 V amplitude are involved. For 40 A peak current discharges, when ionization zones are present, the energy distributions of Ar + and Cu + are similar, indicating that the majority of those ions are produced at approximately the same location and experience the same electric field acceleration. This is also applicable for Ar 2+ and Cu 2+. In contrast, for 400 A peak current discharges, when ionization zones are absent, the energy distribution of Ar + has only several low energy peaks while Cu + also has a broad, high energy 7

tail. This indicates that Ar + and Cu + ions experience different electric fields and are thus produced at different locations. Images of ionization zones can be interpreted as approximate distributions of electric potential and related electron energy 18. Combining the information from images (Fig. 1, Cu at 300 and 400 A) with the azimuthal symmetry in energy distributions (Fig. 2) leads us to the conclusion that the potential hump, usually associated with localized zones, is azimuthally smeared out and of smaller amplitude. Additionally, the high energy tail of Cu + suggests Cu + ions are produced closer to the target than Ar + ions, thus experience a greater accelerating potential difference. The intensity of the energy distribution of Ar 2+ ions is very low. This should be expected as gas rarefaction is strong in high current discharges, and the electron temperature is low when the metal density is high (we will elaborate on this point later, as it is central to the question of why the ionization zones disappear). The energy of doubly charged ions is less than twice than that of singly charged ions, indicating that doubly charged ions are produced further away from the highest potential region. As was argued earlier 19, the formation and motion of HiPIMS ionization zones can be associated with localized evacuation of the ionization region: there is a depletion of neutrals by ionization followed by removal of ions by the local electric field. Magnetized electrons thus must drift further to find a sufficient density of neutrals to be ionized, which appears as a displacement of the region of most intense ionization. This situation is changed when the depletion of neutrals is suppressed by conditions that always ensure the supply of copious amounts of neutrals. We propose that the disappearance of the localization of ionization zones is a consequence of the increased supply of neutrals from the target as governed by the discharge conditions. Here we consider both gas neutrals, primarily neutralized argon outgassing from the target, as well as sputtered target atoms. The supply of neutrals from the target is necessarily 8

tied to the discharge current because higher discharge currents imply higher ion currents followed by a higher return flux of (outgassing) argon neutrals. Additionally, higher ion currents also lead to a higher flux of sputtered atoms, which is especially pronounced for targets of low surface binding energy (high sputter yield) like copper. Given the approximately 500 V discharge voltage in our HiPIMS experiment, the self-sputtering yield of Cu is approximately 2.5 atoms/ion. In contrast, the self-sputtering yields of Nb and Ti are approximately 0.5 atoms/ion (higher for multiply charged ions 20 ), and gas return plays a relatively larger role 21. We thus hypothesize that a transition to zone-free HiPIMS discharges occurs when the supply rate of neutrals exceeds the local ionization rate. Simple estimates support this proposition. Since we demonstrated this transition with copper, a target material of very high yield, we consider the flux of copper neutrals as a function of discharge current. The density of sputtered atoms can be estimated using the flux J a and a characteristic velocity v a of sputtered atoms, na Ja va, (1) where J I disch a Ji, (2) eaeff and where A eff is the effective area of discharge on the target, e is the elementary charge. The discharge current is 1 SE I I I since the yield of secondary electron emission is small, disch i SE i 1, which is especially true for the yield generated by singly charged copper ions 22. As characteristic velocity of sputtered atoms one can take the most likely velocity of the Thompson 9

distribution 23, corresponding to ½ of the surface binding energy (1/2 of 3.5 ev, or about 3200 m/s for copper). In our experiment, γ 2.5, I disch 400 A, A eff 2 10 3 m 2, thus equations (1) and (2) give n Cu 10 21 m 3, as the neutral copper density when ionization zones disappear. It can be readily shown that this neutral density exceeds the neutral density that can be supplied from the background gas. We can neglect the role of argon when using the copper target as it is known that the HiPIMS plasma composition is dominated by copper. For targets of lower yield, such as Ti and Nb, one can show that the gas supply is dominated from outgassing of not bonded (noble) atoms coming from the target. The density of outgassed atoms can be estimated using again equations (1) and (2) but this time with the thermal velocity: a 12 target a v kt m (3) as the characteristic velocity because the subplanted ions outgas as neutral atoms with a velocity determined by the temperature of the target surface. For the estimate we use γ = 1, m Ar = 40 1.66 10 27 kg and T target 1000, resulting in a density of outgassed atoms of the order of 10 21 m -3, which is higher than the background gas density. Indeed, de los Arcos and coworkers 9 have found transitions to zone-free HiPIMS for Ta, Mo, and Cr targets. We, however, could not reproduce their results when using transition metals Ti and Nb, which perhaps indicates the great role played by the specifics of the magnetron (magnetic field strength and the degree of field balance). 10

When the supply of neutrals is high, greater than the rate of ionization, the neutral density is not depleted and electrons are always and everywhere able to find enough neutrals for ionization and excitation. The ionization rate Kionnan e has, in its general form, an ionization coefficient 12 2E Kion fe E ea, ion E de m (4) e which contains the electron energy distribution function (EEDF) f section E ea, ion e E and the ionization cross. In the case of copper, there are plenty of low energy excitation levels representing a very large total excitation cross section of order of 10-18 m 2 (ref. 24). As a result, energetic electrons are removed from the EEDF and electrons are effectively cooled by inelastic collisions with metal vapor. This has an important dampening effect on ionization: the more neutrals are present, the more electrons are cooled, and the cooler the electrons, the lower the ionization rate. Reduced ionization, in turn, prevents the depletion of neutrals that is commonly seen with ionization zones. Electrons do not need to drift further to find regions of enhanced neutral and plasma density to cause ionizing collisions. The feedback coupling of neutral density and ionization affects the balance of neutrals and ions that leads to distinct modes of the discharge, i.e. with or without ionization zones. In summary, using high speed imaging we have confirmed that HiPIMS discharges with a copper target can switch to a regime without ionization zones when the discharge current exceeds about 300 A. We did not observe such a transition for HiPIMS discharges with Nb and Ti targets. Ion energy measurements for conditions with and without ionization zones have revealed that ion energies are generally lower and the pronounced azimuthal asymmetry disappears when ionization zones are absent. Estimates of neutral gas and metal atom flux from 11

the target suggest that the depletion of neutrals does not occur when the discharge current is high, thereby providing a criterion for the disappearance of ionization zones. We gratefully acknowledge Changchun Sun of the Advanced Light Source of Berkeley Lab for providing a high speed camera. Yuchen Yang gratefully acknowledges financial support by the China Scholarship Council. Work at LBNL is supported by the U.S. Department of Energy, under Contract No. DE-AC02-05CH11231. 12

Figure captions Fig. 1 End-on images of HiPIMS discharges at different currents, each with 3 ns exposure time. False color indicates intensity of light emitted in the visible spectrum. The images were taken with different intensifier gain settings. Fig. 2 IEDFs of HiPIMS discharges measured along the side of the magnetron at 40 A current for (a) Ar +, (b) Ar 2+, (c) Cu +, (d) Cu 2+, and at 400 A current for (e) Ar +, (f) Ar 2+, (g) Cu +, (h) Cu 2+. Fig. 3 IEDFs of HiPIMS discharges measured in front of the magnetron racetrack for (a) Ar +, (b) Ar 2+ at 40 and 400 A, (c) Cu +, (d) Cu 2+, each for at 40 and 400 A, and each measured twice to show the reproducibility of individual features. 13

Fig. 1 14

Fig. 2 15

Fig. 3 16

References 1 A. Anders, P. Ni, and A. Rauch, J. Appl. Phys. 111, 053304 (2012). 2 A. P. Ehiasarian, A. Hecimovic, T. de los Arcos, R. New, V. Schulz-von der Gathen, M. Böke, and J. Winter, Appl. Phys. Lett. 100, 114101 (2012). 3 N. Brenning, D. Lundin, T. Minea, C. Costin and C. Vitelaru, J. Phys. D: Appl. Phys. 46, 084005 (2013). 4 P. A. Ni, C. Hornschuch, M. Panjan, and A. Anders, Appl. Phys. Lett. 101, 224102 (2012). 5 A. Kozyrev, N. Sochugov, K. Oskomov, A. Zakharov, and A. Odivanova, Plasma Phys. Rep. 37, 621 (2011). 6 N. Brenning, R. L. Merlino, D. Lundin, M. A. Raadu, and U. Helmersson, Phys. Rev. Lett. 103, 225003 (2009). 7 F. Taccogna, S. Longo, M. Capitelli, and R. Schneider, Appl. Phys. Lett. 94 (2009). 8 J. P. Boeuf and B. Chaudhury, Phys. Rev. Lett. 111, 1 (2013) 9 T. de los Arcos, V. Layes, Y. A. Gonzalvo, V. Schulz-von der Gathen, A. Hecimovic, and J. Winter, J. Phys. D, Appl. Phys. 46, 335201, (2013). 10 J. Andersson, P. Ni, and A. Anders, IEEE Trans. Plasma Sci. 42, 2856 (2014). 11 J. Andersson, P. Ni, and A. Anders, Appl. Phys. Lett. 103, 054104 (2013). 12 A. Anders, M. Panjan, R. Franz, J. Andersson, and P. Ni, Appl. Phys. Lett. 103, 144103 (2013). 13 P. Poolcharuansin, B. Liebig and J. W. Bradley, Plasma Sources Sci. Technol. 21, 015001 (2012) 14 W. Rasband, IMAGEJ 1.44p, downloaded from http://imagej.nih.gov/ij (National Institute of Health, 2011). 17

15 M. Panjan, R. Franz, and A. Anders, Plasma Sources Sci. Technol. 23, 025007 (2014). 16 A. Anders, P. Ni, and J. Andersson, IEEE Trans. Plasma Sci. 42, 2578 (2014). 17 T. Ito and M. A. Cappelli, Appl. Phys. Lett. 94, 211501 (2009). 18 A. Anders, Appl. Phys. Lett. 105, 244104 (2014). 19 A. Anders, Appl. Phys. Lett. 100, 224104 (2012). 20 A. Anders and J. Andersson, J. Appl. Phys. 102, 113303 (2007). 21 A. Anders, J. Čapek, M. Hála, and L. Martinu, J. Phys. D: Appl. Phys. 45, 012003 (2012). 22 R. A. Baragiola, E. V. Alonso, J. Ferron, and A. Oliva-Florio, Surf. Sci. 90, 240 (1979). 23 M. W. Thompson, Phil. Mag. 18, 377 (1968). 24 A. Bogaerts, R. Gijbels, and R. J. Carman, Spectrochimica Acta Part B: Atomic Spectroscopy 53, 1679 (1998). 18