The ion energy distributions and ion flux composition from a high power impulse magnetron sputtering discharge

The ion energy distributions and ion flux composition from a high power impulse magnetron sputtering discharge

Thin Solid Films 515 (2006) 1522 – 1526 www.elsevier.com/locate/tsf The ion energy distributions and ion flux composition from a high power impulse m...

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Thin Solid Films 515 (2006) 1522 – 1526 www.elsevier.com/locate/tsf

The ion energy distributions and ion flux composition from a high power impulse magnetron sputtering discharge J. Bohlmark a,b , M. Lattemann a , J.T. Gudmundsson c,d , A.P. Ehiasarian e , Y. Aranda Gonzalvo f , N. Brenning g , U. Helmersson a,⁎ a IFM Material Physics, Linköping University, SE-581 83, Sweden Chemfilt Ionsputtering AB, Diskettgatan 11C, 583 35 Linköping, Sweden Department of Electrical and Computer Engineering, University of Iceland, Hjardarhaga 2-6, IS-107 Reykjavik, Iceland d Science Institute, University of Iceland, Dunhaga 3, IS-107 Reykjavik, Iceland e Materials and Engineering Research Institute, Sheffield Hallam University, Howard st., Sheffield, S1 1WB, UK f Hiden Analytical Ltd., 420 Europa Boulevard, Warrington, WA5 7UN, UK g Division of Plasma Physics, Alfvén Laboratory, Royal Institute of Technology, SE-100 44 Stockholm, Sweden b

c

Received 9 February 2006; received in revised form 18 April 2006; accepted 21 April 2006 Available online 9 June 2006

Abstract The energy distribution of sputtered and ionized metal atoms as well as ions from the sputtering gas is reported for a high power impulse magnetron sputtering (HIPIMS) discharge. High power pulses were applied to a conventional planar circular magnetron Ti target. The peak power on the target surface was 1–2 kW/cm2 with a duty factor of about 0.5%. Time resolved, and time averaged ion energy distributions were recorded with an energy resolving quadrupole mass spectrometer. The ion energy distributions recorded for the HIPIMS discharge are broader with maximum detected energy of 100 eV and contain a larger fraction of highly energetic ions (about 50% with Ei N 20 eV) as compared to a conventional direct current magnetron sputtering discharge. The composition of the ion flux was also determined, and reveals a high metal fraction. During the most intense moment of the discharge, the ionic flux consisted of approximately 50% Ti1+, 24% Ti2+, 23% Ar1+, and 3% Ar2+ ions. © 2006 Elsevier B.V. All rights reserved. Keywords: Sputtering; HIPIMS; Mass spectometry; Plasma characterization

1. Introduction High power impulse magnetron sputtering (HIPIMS) is a technique that lately has drawn attention from both industry and academia. The method was introduced by Kouznetsov et al. [1,2] and is an alternative technique for ion assisted film growth and surface engineering. The technique relies on the creation of a high-density plasma in front of the sputtering source, ionizing a large fraction of the sputtered atoms. The increase in plasma density is achieved by increasing the applied power. In conventional direct current (DC) magnetron sputtering, the upper limit of the plasma density is limited by the thermal load on the target, and is usually of the order of 1015–1017 m− 3 [3]. The ⁎ Corresponding author. E-mail address: [email protected] (U. Helmersson). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.04.051

fraction of ionized species of the target material is therefore low, typically less than 10% [4]. Thus the majority of particles reaching the substrate surface are electrically neutral and not affected by a substrate bias. In HIPIMS, this problem is solved by applying high power in pulses with a low duty factor. Since the thermal load of the target is limited by the average power rather than the peak power, the peak power during the active discharge can be very high. Typical applied peak voltage can be 1–2 kV, resulting in peak discharge currents of the order of a few A cm− 2 and a peak power density of several kW cm− 2 on the target surface. The high power leads to peak electron densities exceeding 1019 m− 3 in the close vicinity of the magnetron [5,6]. The high density of electrons increases the probability for ionizing collisions between the sputtered atoms and energetic electrons, and results in a high degree of ionization of the sputtered material. Values of 5–70% are reported [1,7,8],

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and optical emission studies suggest even higher peak values [9]. The observed degree of ionization do vary over a wide range [1,7–9], which is likely to be due to different experimental conditions, such as the magnetic field configuration, or different analytical techniques. The high degree of ionization opens new opportunities, since the ions may be controlled by the use of electric or magnetic fields. For example, the energy of ions arriving at a surface can be controlled by the use of a substrate bias. Momentum transfer to the growing film by ion bombardment is an efficient way of cleaning substrates or affecting the microstructure of the film [10]. An example of this is work done by Ehiasarian et al. [11,12] where HIPIMS was used for both pretreatment of the steel substrates by Cr etching and for CrN film deposition. It resulted in films with excellent adhesion, high density, and high corrosion and wear protection capabilities. In this paper a mass-spectrometry study of the composition and energy distributions of ionized flux from the HIPIMS plasma are presented and compared to a continuous DC discharge. Both time-resolved and time-averaged measurements are reported. 2. Experiment Experiments were performed in a cylindrical vacuum chamber 44 cm in diameter and 75 cm in height. The magnetron was mounted on the top circular flange facing downwards. The magnetron was a standard “weakly unbalanced” type, with about 35 mT magnetic field strength at the target surface. A 150 mm in diameter Ti disc was mounted on the magnetron. The chamber was evacuated to a base pressure of about 0.1 mPa. Argon of purity 99.9997% was introduced through a leak valve to start and maintain the discharge. The cathode was operated by a pulsed power supply manufactured by Chemfilt Ionsputtering AB, Sweden, capable to deliver 2000 V and 1200 A peak values at 50 Hz repetition frequency and duty cycle of ∼ 0.5%. The pulse length was about 100 μs. The ion energy distributions were measured using an energy resolving mass spectrometer (PSM003, Hiden Analytical, UK). The instrument was inserted from the side of the chamber, parallel to the target surface. The spectrometer orifice was positioned 5.5 cm from the target surface, and 10 cm from the center axis. A schematic of the experimental setup can be seen in Fig. 1. The mass spectrometer consists of a series of electrostatic lenses as extraction together with, ionization source, energy

Fig. 2. The ion energy distributions taken from a conventional DC magnetron discharge. The Ar pressure was 0.4 Pa, the applied power 1 kW, and the target material was Ti. The recorded counts have been adjusted with the corresponding isotope abundance.

analyzer (Bessel Box), a quadrupole mass filter, and a secondary electron multiplier (SEM) detector. The instrument has a front end driven electrode with a sampling orifice of 300 μm in diameter. For this study the driven electrode was grounded so ions entering the mass spectrometer gained energy as accelerating from the plasma potential into the spectrometer. The ions are then passed to the Bessel Box allowing ions in a narrow energy range to pass. Ions emerging from the energy filter pass through the quadrupole mass filter that consists of two pairs of parallel rods. The potential applied to each electrode has a DC and radio frequency component. The range of ion masses that can pass through the quadrupole are determined by the applied electrode potentials. The quadrupole can therefore be adjusted to sample plasma ions of a chosen mass/charge ratio. The ions passing through the filters are detected by the SEM. The instrument also has the possibility to receive a trigger signal to perform time resolved measurements. Here the switch-on of the magnetron voltage was used as trigger reference via an oscilloscope. A direct reading of time resolved data at any given time, would represent the plasma composition at the detector, and not at the front end as desired. Therefore in the present work the time-of-flight (TOF) of ions through the instrument has been calculated. TOF through the instrument is a function of the ion mass, the initial kinetic energy of the ions, and the various potentials applied to the spectrometer. The TOF, is then the sum of the flight time through each section and can be written as pffiffiffiffiffiffi 2m TOF ¼ dext pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi eðKion −Vext Þ þ jeVendcap j rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi m þ den j2eðVendcap −Vcylinder −Kion Þj sffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffi m 2m þ dmass þ ddet j2eVte j jeVdyn j

Fig. 1. The experimental setup with the target of the magnetron and the column of the mass-spectrometer indicated.

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ð1Þ

where dext, den, dmass, and ddet is the length of the extractor, the energy filter, the mass filter, and the detector, respectively and

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the quantified measurements we assume that the number of ions in the flux from the plasma, at a certain energy, is proportional to the ions recorded. The difference in the transmission function through the instrument is small in the range between 36 and 49 amu. To reduce the risk of detector saturation, the ion energy distributions were in some cases recorded for less abundant isotopes. When recording data from the DC discharge, the 36Ar and 49Ti isotopes were used. For the time-averaged measurements from the HIPIMS discharge, 36Ar and 48Ti were used. The time resolved studies were performed on 40Ar and 48Ti. The abundance numbers for the 40Ar, 36Ar, 48Ti, and 49Ti isotopes are 99.600, 0.337, 73.8, and 5.5% respectively [13]. All data presented in this paper has been compensated with the corresponding isotope abundance number to better represent the total number of ions in the plasma. Fig. 3. The ion energy distributions for Ar1+ and Ti1+ ions measured from a HIPIMS discharge. The Ar pressure was 0.4 Pa, the pulse energy 3 and 10 J, and the target was made of Ti. The recorded counts have been adjusted with the corresponding isotope abundance.

Vext, and Vdyn is the potentials applied to the extraction section, and to the dynode. Vte is the transit energy. Kion is the initial ion energy and m is the ion mass. For multiply charged ions the ion mass in Eq. (1) should be divided by the charge number. The set of parameters that was used in this work was Vext = − 15 V, Vendcap = − 20 V, Vcylinder = 0 V, Vte = 3 V, and Vdyn = − 1200 V. The distances, dext, den, dmass, ddet, are 4, 3.765, 20, and 1.6 cm respectively. An evaluation of Eq. (1) gives TOF for Ti1+ , Ar1+ , Ti2+ , Ar 2+ in the range 62–68, 57–62, 44–48, 40–44 μs respectively, depending on their initial kinetic energy (1– 100 eV), and an average TOF of 64, 58, 45, 41 μs respectively. The ions spend most time in the mass filter, which is the longest part. In the mass filter, they all have the same energy (they have been accelerated by spectrometer potentials), and therefore the contribution of their initial kinetic energy to the flight time is small. In comparison to the time resolution (20 μs), the small discrepancy in TOF generated by the fact that the ions enter the spectrometer with different kinetic energies can be neglected, and the average value of the TOF for each ion is used to adjust the measured values. However, it should be noted that the times used for correcting for the TOF are only approximate. The mass spectrometer is capable of measuring ion energies up to 100 eV with a resolution of 0.05 eV and masses up to 300 atomic mass units (amu) with a resolution of 0.01 amu. For the time average measurements, the active detector time was set to be 100 ms, allowing 5 pulses for each energy data point. For the time resolved measurements, the active detector time was set to be 20 μs. Data was then collected using a built-in loop function with a delay set to increase in 20 μs steps. The measurement was aborted at a time delay of 300 μs. Consequently data was collected in 20 μs steps up to 300 μs from pulse start. For each energy data point data was collected from 50 pulses. The ion energy distributions presented in this paper were collected under constant spectrometer parameter settings. First, the electrostatic lenses where tuned to achieve the maximum sensitivity (for Ar36). In order to have a consistent comparison with the study of another species in the plasma the same tuning parameters were used. For

3. Results and discussion As a reference for the measurement on the HIPIMS plasma, the ion energy distributions were recorded from a conventional DC magnetron discharge, which are presented in Fig. 2. The Ar pressure was 0.40 Pa and the power was 1 kW (400 V and 2.5 A). The ion energy distributions from the DC discharge, shown in Fig. 2, peak at an ion energy of about 2 eV and contains a high-energy tail that can be followed up to around 20 and 40 eV for Ar1+ and Ti1+, respectively. The low energy peak is generally related to low energy (or thermalized) ions accelerated by the potential drop between the plasma body and the grounded spectrometer. The high-energy tail structure is, in the case of Ti, related to the original energy distribution of the sputtered neutrals at the cathode [14,15]. The ion energy distribution of Ar ions shows similarly a low energy peak and a highenergy tail, due to momentum transfer from sputtered species. The data is in good agreement with previous reports [14–18]. Time average ion energy distributions for two different HIPIMS pulse energies are shown in Fig. 3. When collecting these

Fig. 4. Ion energy distributions recorded from a HIPIMS discharge at various time periods during the discharge for Ar1+ ions and Ti1+ ions. (a), (b), and (c) represents the distributions at 0 b t b 20 μs, 40 b t b 60 μs, and 140 b t b 160 μs respectively. The Ar pressure was 0.4 Pa, the pulse energy 9 J, and the target was made of Ti. The recorded counts have been adjusted with the corresponding isotope abundance.

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can be seen in Fig. 5. In the first time interval (0–20 μs) mainly Ar ions are recorded, but thereafter a highly metallic plasma was measured. The plasma evolution observed supports earlier findings by optical emission spectroscopy, but with a higher time resolution [8]. During the most intense moment of the discharge, the ion flux consisted of approximately 50% Ti1+, 24% Ti2+, 23% Ar1+, and 3% Ar2+ ions. Thus, the peak Ti1+/Ar1+ ratio is over two. Higher order of ionization could not be observed. 4. Conclusions

Fig. 5. The relative flux composition of singly and doubly ionized Ar and Ti from the HIPIMS plasma as a function of time after pulse ignition. The Ar pressure was 0.4 Pa, the pulse energy 9 J, and the target was made of Ti.

data, the detector was left open during several pulsing cycles. These measurements were performed in 0.4 Pa Ar pressures using 3 and 10 J pulses. Since the pulses are generated by discharging a capacitor bank, a well defined cathode voltage is not obtained. For the 3 J pulses, the cathode voltage starts at 800 V and drops to around 400 V at maximum discharge current. For the 10 J pulses, the initial voltage peak is about 1.7 kV, but drops quickly to about 800 V as current starts to flow. It is noted that the ion energy distributions are similar for the two different pulse energies used but quite different compared to the conventional DC magnetron discharge shown in Fig. 2. This is especially apparent for Ti. Also for HIPIMS the ion energies peaks at a low energy around 1 eV, however, the distributions contain a more intense high-energy tail, extending to the measurement limit of 100 eV. About 50% of the Ti ions have an energy of N20 eV. Also the Ar distribution is slightly more energetic. The small change when the applied power is changed demonstrates that the increased ion energies observed in HIPIMS is probably not purely connected to a higher cathode power. To investigate the time dependence of the ion energy distributions, time resolved studies were performed. Examples of ion energy distributions collected at different times in the pulse are shown in Fig. 4. These data were recorded using 9 J pulses and 0.4 Pa Ar pressure. The distributions are broad when the high power is applied to the cathode, to quickly narrow and peak at a low energy of 1–2 eV as the applied power is removed. Both the Ar1+ distribution and the Ti1+ distributions are broad during the power on phase. The broad feature of the distributions cannot directly be related to acceleration between the plasma and the spectrometer, for two reasons. First, the plasma potential is expected to be less than our recorded ion energies [5]. Second, accelerating ions over a potential would give rise to a distinct peak around this potential, with some broadening due to gas scattering and their initial kinetic energy. At the present time there is no explanation for this increased energy. In-between the pulses a weak plasma is present dominated by low energy ions, giving rise to a distinct peak in the recorded distributions around 1–2 eV. To obtain the time dependent plasma composition, the energy distributions were integrated for each 20 μs window. The result

The ion energy distributions of the HIPIMS plasma has been measured and showed to be broad and significantly more energetic as compared to a continuous DC discharge. About 50% of the Ti ions have an energy of N 20 eV. Time resolved studies of the ion energy distributions show broad distributions of ion energies in the early stage of the discharge, which quickly narrows as the pulse is switched off. The ion flux from the HIPIMS plasma is highly metallic during the active phase of the discharge. During the most intense moment of the discharge an ion flux composition of 50% Ti1+, 24% Ti2+, 23% Ar1+, and 3% Ar2+ was recorded. The fact that the HIPIMS plasma contains a large fraction of energetic ions is important knowledge when understanding the properties of films grown by HIPIMS. The energetic ions can be used for momentum transfer and film densification during film growth. The highly metallic plasma conditions produced by HIPIMS can serve as an effective adhesion promoter for various substrates. Acknowledgements This work was partially supported by the Swedish Foundation for Strategic Research, the Swedish Research Council, the Icelandic Research Council, and the European Commission (Integrated Project: InnovaTiAl). References [1] V. Kouznetsov, K. Macák, J.M. Schneider, U. Helmersson, I. Petrov, Surf. Coat. Technol. 122 (1999) 290. [2] K. Macák, V. Kouznetsov, J. Schneider, U. Helmersson, I. Petrov, J. Vac. Sci. Technol., A 18 (2000) 1533. [3] J.W. Bradley, S. Thompson, Y. Aranda Gonzalvo, Plasma Sources Sci. Technol. 10 (2001) 490. [4] C. Christou, Z.H. Barber, J. Vac. Sci. Technol., A 18 (2000) 2897. [5] J.T. Gudmundsson, J. Alami, U. Helmersson, Surf. Coat. Technol. 161 (2002) 249. [6] J. Bohlmark, J.T. Gudmundsson, J. Alami, M. Lattemann, U. Helmersson, IEEE Trans. Plasma Sci. 33 (2005) 346. [7] B.M. DeKoven, P.R. Ward, R.E. Weiss, D.J. Christie, R.A. Scholl, W.D. Sproul, F. Tomasel, A. Anders, Proceedings of the 46th Annual Technical Conference Proceedings of the Society of Vacuum Coaters, San Francisco, CA, USA, May 3–8 2003, p. 158. [8] A.P. Ehiasarian, R. New, W.-D. Münz, L. Hultman, U. Helmersson, V. Kouznetsov, Vacuum 65 (2002) 147. [9] J. Bohlmark, J. Alami, C. Christou, A.P. Ehiasarian, U. Helmersson, J. Vac. Sci. Technol., A 23 (2005) 18. [10] I. Petrov, P.B. Barna, L. Hultman, J.E. Greene, J. Vac. Sci. Technol., A 21 (2003) 117. [11] A.P. Ehiasarian, W.-D. Münz, L. Hultman, U. Helmersson, I. Petrov, Surf. Coat. Technol. 163–164 (2003) 267.

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