Generation of singly charged Ar + and Ti +, doubly charged Ar 2 + and Ti 2 +, and of Ar 2 + and Ti 2 + dimer ions in a high power impulse magnetron sputtering (HiPIMS) discharge with a Ti cathode was investigated. Energy-resolved mass spectrometry was employed. The argon gas pressures varied between 0.5 and 2.0 Pa. Energy spectra of monomer ions are composed of low- and high-energy components. The energetic position of the high-energy component is approximately twice as large for doubly charged ions compared to singly charged ions. Intensities of Ar 2 + and Ti 2 + dimer ions are considerably smaller during HiPIMS compared to dc magnetron sputtering.

Plasma-based processes play an important role in both basic research and industrial applications.1 Deposition of thin solid films is frequently carried out with the help of magnetron discharges.2–4 Pulsed magnetron discharges offering an enhanced plasma density have received much attention in the past years.5–11 Electron densities as large as 10 19 / m 3 or 1% of the gas density and even larger ionisation fractions of metal atoms with charge states up to 4+ can be achieved during so-called high power impulse magnetron sputtering (HiPIMS).5,12–15 The large ion fraction impacts the growing film and may enhance film adherence, uniformity, and compactness. Recent observations of moving ionisation zones or spokes have shed new light on the ionisation process in magnetron discharges.16–18 

In this paper, we investigate the formation of singly charged Ar + and Ti + monomer, Ar 2 + and Ti 2 + dimer, and doubly charged Ar 2 + and Ti 2 + ions in a high power impulse magnetron sputtering (HiPIMS) discharge with a titanium target. Energy-resolved mass spectrometry has been utilized to investigate the details of the ion formation process. A comparison with results obtained during direct current (dc) magnetron sputtering is included.19 

The experimental setup has been described in detail elsewhere.19–21 The vacuum chamber with a diameter of 35 cm is pumped by a turbo-molecular pump (pumping speed 220 l/s) to a base pressure of less than 10 5 Pa. Argon gas (purity 99.999%) is introduced into the chamber with the help of a gas flow controller; the argon flow rate is 18 sccm. The operating pressure is set in the range p = 0.5 2.0 Pa with the help of a ultrahigh vacuum gate valve between pump and chamber. A commercial (Lesker Torus) planar unbalanced magnetron is attached to the horizontal flange of the vacuum chamber and inserted about 29 cm into the chamber. The magnetron is equipped with a Ti target (diameter 2 in., purity 99.7%). The magnetron discharge is driven by a dc power supply (Advanced Energy MDX–1K) in combination with a homemade power switch.22–25 A commercial pulse-delay generator (Agilent 33250A) was employed to set the repetition frequency f = 100 Hz and the pulse width T a = 100 μ s. The magnetron discharge is operated in HiPIMS mode with a typical voltage of 600 V and a mean discharge power of 150 W. The same setup was used for the dc magnetron sputtering mode.

Energy-resolved mass spectrometry is performed with a commercial Hiden EQP 1000 mass/energy analyzer (Hiden Analytical Ltd., UK). The analyzer is equipped with a grounded entrance orifice with a diameter of 300 μ m. The magnetron is mounted at a distance of 5 cm from the orifice. The mass spectrometer is differentially pumped by a separate turbomolecular pump (pumping speed 60 l/s) to a residual gas pressure of better than 10 6 Pa inside the analyzer. Measurements are performed in the positive ion mass spectrometry (+SIMS) mode with a dwell time of 0.2 s. Typical ion spectra are averaged over 10 scans.

Recorded mass spectra of positive ions are dominated by singly charged Ar + and Ti + ions. Argon has three stable isotopes at m / z = 36, 38, and 40 with relative abundances of 0.337%, 0.063%, and 99.6%, respectively,26 where m and z are ion mass and ion charge number, respectively. Titanium has five stable isotopes at m / z = 46–50 with relative abundances of 8.3%, 7.4%, 73.7%, 5.4%, and 5.2%.26 Measurements are reported for the most abundant singly and doubly charged charged Ar + ( m / z = 40), Ti + ( m / z = 48), Ar 2 + ( m / z = 20), and Ti 2 + ( m / z = 48) monomer ions and for Ar 2 + ( m / z = 80) and Ti 2 + ( m / z = 96) dimer ions.

Energy spectra of singly charged Ar + and Ti + ions are displayed in Fig. 1 for three different gas pressures p = 0.5 Pa, 1.0 Pa, and 2.0 Pa. The energy distribution of Ar + ions consists of a low-energy part extending up to about 10 eV followed by a high-energy tail extending up to more than 50 eV. The intensity of the high-energy tail strongly decreases with increasing pressure. Similar energy spectra for Ar + ions have been reported before.4,27–30 The energy distribution of Ti + ions also displays a low-energy peak which is most pronounced at the largest pressure of 2.0 Pa followed by a second broad peak at about 12 eV and a pronounced high-energy tail extending beyond 80 eV. Also shown are energy spectra of Ar + and Ti + ions during dc magnetron sputtering measured under otherwise identical conditions at a gas pressure p = 1.0 Pa. In comparison to HiPIMS, the energy distributions obtained during dc magnetron sputtering are less broad and the high-energy component is significantly reduced. A similar observation has been made by Lundin et al.31 

FIG. 1.

Energy distribution of singly charged (a) Ar + ( m / z = 40) and (b) Ti + ( m / z = 48) ions during HiPIMS at gas pressures p = 0.5 Pa ( °), 1.0 Pa ( ), and 2.0 Pa ( ) and during dc magnetron sputtering at p = 1.0 Pa (▲). Mean discharge power P = 150 W.

FIG. 1.

Energy distribution of singly charged (a) Ar + ( m / z = 40) and (b) Ti + ( m / z = 48) ions during HiPIMS at gas pressures p = 0.5 Pa ( °), 1.0 Pa ( ), and 2.0 Pa ( ) and during dc magnetron sputtering at p = 1.0 Pa (▲). Mean discharge power P = 150 W.

Close modal

Energy spectra of doubly ionised Ar 2 + and Ti 2 + ions are displayed in Fig. 2. The energy distributions of Ar 2 + ions show a narrow low-energy peak followed by a pronounced high-energy tails which, for the lowest gas pressure p = 0.5 Pa, extends to more than 80 eV. Similar energy distributions are observed for Ti 2 + ions where, at the lowest pressure p = 0.5 Pa, the high-energy component extends beyond 120 eV and thus much further compared to singly-charged Ti + ions at the same pressure.

FIG. 2.

Energy distribution of doubly charged (a) Ar 2 + ( m / z = 20) and (b) Ti 2 + ( m / z = 24) ions during HiPIMS at gas pressures of 0.5 Pa ( °), 1.0 Pa ( ), and 2.0 Pa ( ). Mean discharge power P = 150 W.

FIG. 2.

Energy distribution of doubly charged (a) Ar 2 + ( m / z = 20) and (b) Ti 2 + ( m / z = 24) ions during HiPIMS at gas pressures of 0.5 Pa ( °), 1.0 Pa ( ), and 2.0 Pa ( ). Mean discharge power P = 150 W.

Close modal

Energy spectra of Ar 2 + dimer ions are displayed in Fig. 3. Ar 2 + ions are produced with very low kinetic energies, indicating that the formation processes differ considerably from that of Ar + ions.

FIG. 3.

Energy distribution of singly charged Ar 2 + ions during HiPIMS at gas pressures of 0.5 Pa ( °), 1.0 Pa ( ), and 2.0 Pa ( ). Mean discharge power P = 150 W.

FIG. 3.

Energy distribution of singly charged Ar 2 + ions during HiPIMS at gas pressures of 0.5 Pa ( °), 1.0 Pa ( ), and 2.0 Pa ( ). Mean discharge power P = 150 W.

Close modal

The pressure dependence of the energy-integrated ion intensities is shown in Fig. 4. The total intensity of Ar + ions shows little variation with pressure. The intensity of Ti + slightly increases with pressure. Intensities of doubly charged Ar 2 + and Ti 2 + ions decrease with pressure as does the Ar 2 + / Ar + and the Ti 2 + / Ti + ratio of singly-to-doubly charged ions. The intensity of Ti 2 + dimer ions also decreases with pressure. It should be noted that due to its small count rate, the statistical accuracy of the Ti 2 + intensity is comparatively poor (about ± 10 %) and no energy distributions are presented here. The pronounced increase of the Ar 2 + ion intensity with pressure is close to a quadratic dependency.

FIG. 4.

Pressure dependency of Ar + ( °, m / z = 40), Ti + ( , m / z = 48), Ar 2 + ( , m / z = 80), Ti 2 + (▲, m / z = 96), Ar 2 + ( , m / z = 20), and Ti 2 + (▼, m / z = 24) intensity during HiPIMS. Mean discharge power P = 150 W. Dashed lines to guide the eye only.

FIG. 4.

Pressure dependency of Ar + ( °, m / z = 40), Ti + ( , m / z = 48), Ar 2 + ( , m / z = 80), Ti 2 + (▲, m / z = 96), Ar 2 + ( , m / z = 20), and Ti 2 + (▼, m / z = 24) intensity during HiPIMS. Mean discharge power P = 150 W. Dashed lines to guide the eye only.

Close modal

The energy distribution of Ti + ions is dominated by a pronounced high-energy component which peaks at about 11 eV and extends up to more than 80 eV. Ti atoms originate from the cathode and are liberated by sputtering. Sputtered Ti atoms are ionised in the plasma region and detected as positively charged ions. The energy distribution of sputtered atoms is described by Thompson’s formula32 

(1)

where E is the kinetic energy of sputtered atoms and p 3. The distribution has a maximum near E b / 2, where E b is the surface binding energy. The measured energy distribution of Ti + ions (Fig. 1) is considerably steeper than predicted by Eq. (1), however. Only the weak high-energy tail at energies larger than 50 eV is in accordance with Thompson’s formula [Eq. (1)]; it resembles the remaining part of the original sputtering contribution as shown in Fig. 5. It means that sputtered Ti + ions have lost much of their initial energy through thermalizing collisions. Attempts have been made to describe the measured ion energy distribution of thermalized ions by a so-called shifted Maxwellian function19,33–35

(2)

where T i is the ion temperature and E S is the energy by which the Maxwellian distribution is shifted. It turns out that a shifted Maxwellian distribution reasonably well describes the energy distribution of Ti + ions above 10 eV (Fig. 5). A second Mawellian distribution is required to describe the low-energy part. A similar approach with two Maxwellian distributions has been utilized by Hecimovic et al. and attributed to different temporal phases of the discharge.36,37

FIG. 5.

Energy distribution of (a) singly charged Ar + ( m / z = 40, °) and Ti + ( m / z = 48, ) and (b) doubly charged Ar 2 + ( m / z = 20, °) and Ti 2 + ( m / z = 24, ) ions during HiPIMS at an argon pressure of 0.5 Pa. Solid lines represent fits composed of two shifted Mawellian, one Gaussian, and one Thompson distribution (see text). Dashed and dash-dotted lines show individual Maxwellian and Thompson distributions, respectively. For clarity, not all experimental data points are shown.

FIG. 5.

Energy distribution of (a) singly charged Ar + ( m / z = 40, °) and Ti + ( m / z = 48, ) and (b) doubly charged Ar 2 + ( m / z = 20, °) and Ti 2 + ( m / z = 24, ) ions during HiPIMS at an argon pressure of 0.5 Pa. Solid lines represent fits composed of two shifted Mawellian, one Gaussian, and one Thompson distribution (see text). Dashed and dash-dotted lines show individual Maxwellian and Thompson distributions, respectively. For clarity, not all experimental data points are shown.

Close modal

The energy spectrum of Ar + is characterised by a pronounced peak at very small energies of about 0 eV followed by a shoulder at 3 eV and a high-energy tail which is strongly pressure dependent and, at the lowest pressure of p = 0.5 Pa, extends beyond 50 eV. At first glance, the Ar + energy distribution appears different compared to that of Ti + ions. A closer look reveals, however, that the high-energy components of Ar + and Ti + are rather similar both in shape and in position (Fig. 5). The origin of the high-energy component is not fully understood yet. Mechanisms which have been mentioned in this respect are related to accelerated ions impinging on the negatively biased cathode. Part of the impinging argon ions are backscattered and, if simultaneously become neutralized, can leave the cathode with a large kinetic energy.27,28,30

The second mechanism is related to argon ions which are implanted in the cathode’s material due to ion bombardment. Typical implantation depths under the present conditions are about 1.2 nm.38 Buried argon atoms are subsequently liberated by the continuing Ar + ion bombardment; the ejected Ar atoms may become ionized again in the plasma region and are detected as positive ions. It was shown that HiPIMS discharges can be dominated by a large fraction of so-called recycled Ar + ions, i.e., Ar atoms which return from the cathode and become ionised again.11 As in the case of Ti + ions, the measured Ar + energy distribution (Fig. 1) is considerably steeper than predicted by Eq. (1). It means that recycled Ar atoms and ions have lost much of their energy through thermalizing collisions. A shifted Maxwellian distribution [Eq. (2)] reasonably well describes the high-energy component above 20 eV but does not comply with the measured distribution at lower energies. Position and width of this component are, particularly at the lowest pressure p = 0.5 Pa, quite similar compared to Ti + ions (Table I). The smaller Ar + ion intensity of the high-energy component compared to Ti + ions is readily explained by the much larger ionisation energy which reduces the ionisation probability. The low-energy component of slow Ar + ions was accounted for by a second Maxwellian function. Based on time-resolved measurements, it has been shown that the high-component occurs during the on time of the discharge, i.e., when the discharge current reaches its maximum, while the low-energy component occurs during the off time, i.e., during the afterglow when the discharge current approaches zero.36,37

TABLE I.

Parameters E S and T i for for the bi-Maxwellian ion energy distribution of Ar +, Ar 2 +, Ti +, and Ti 2 + ions. Gas pressure p = 0.5 Pa.

A r + A r 2 + T i + T i 2 +
E S ( 1 ) (eV)  2.5 
T i ( 1 ) (eV)  2.1  2.8  1.65  0.9 
E S ( 2 ) (eV)  7.25  17.6  6.9  15.6 
T i ( 2 ) (eV)  2.5  2.9  3.05  4.2 
A r + A r 2 + T i + T i 2 +
E S ( 1 ) (eV)  2.5 
T i ( 1 ) (eV)  2.1  2.8  1.65  0.9 
E S ( 2 ) (eV)  7.25  17.6  6.9  15.6 
T i ( 2 ) (eV)  2.5  2.9  3.05  4.2 

In addition, the mean free path of Ar + is largely determined by charge changing reactions with neutral Ar atoms from the buffer gas, e.g., Ar + + Ar Ar + Ar +. The reaction gives rise to the formation of Ar + ions which are in thermal equilibrium with gas atoms. The cross section σ of this resonant process is large39 and the corresponding mean free path of Ar + ions becomes small. The estimated mean free path l ¯ = ( n a σ ) 1, where n a is the density of argon atoms, is of the order of a few centimetre (Table II), i.e., sufficiently short to explain the conversion of a large fraction of fast into slow Ar + ions. The contribution was accounted with a Gaussian distribution; its inclusion improves the agreement with experiment. We thus have three contributions of which two are related to different phases during the HiPIMS pulse, while the part with the lowest kinetic energy is due to charge changing reactions with gas atoms. The width of this contribution is determined by the energy resolution of the analyser. It also explains the observed negative energies which are an artefact of the measuring process.

TABLE II.

Mean free path l ¯ of the one-electron Ar + + Ar Ar + Ar +39 and the two-electron Ar 2 + + Ar Ar + Ar 2 +40 reaction as a function of ion energy E and gas pressure p. Gas temperature T g = 300 K. σ is the cross section.

Ion E σ Mean free path l ¯ (cm)
(eV) ( 10 20 m 2) p = 0.5 P a p = 1.0 P a p = 2.0 P a
  57  1.5  0.73  0.36 
Ar +   10  45  1.9  0.93  0.46 
  100  34  2.4  1.2  0.61 
  1.5  31  2.7  1.3  0.67 
Ar 2 +   10  26  3.2  1.6  0.80 
  120  21  3.9  2.0  0.99 
Ion E σ Mean free path l ¯ (cm)
(eV) ( 10 20 m 2) p = 0.5 P a p = 1.0 P a p = 2.0 P a
  57  1.5  0.73  0.36 
Ar +   10  45  1.9  0.93  0.46 
  100  34  2.4  1.2  0.61 
  1.5  31  2.7  1.3  0.67 
Ar 2 +   10  26  3.2  1.6  0.80 
  120  21  3.9  2.0  0.99 

A remarkable difference between Ar + and Ti + is noted with respect to the relative intensity of the low-energy and the pressure dependence of the high-energy component. The intensity of the Ar + high-energy component strongly decreases with increasing pressure while a much weaker pressure dependence is noted for Ti +. A possible explanation can be found in the different mean free path of Ar + and Ti + ions in the Ar buffer gas. The charge changing reaction Ti + + Ar Ti + Ar + is endothermic and the corresponding cross section is, hence, much smaller compared to that for the resonant charge changing Ar + + Ar reaction.39 It gives rise to a larger mean free path of Ti + ions resulting in a smaller pressure dependency. As a further consequence, the low-energy component of the Ti + ion energy distribution is comparatively weak and, in particular, much smaller compared to Ar +. As in the case of Ar + ions, the agreement between fitted and measured Ti + energy distributions improves if two Maxwellian distributions which are related to the different phases during HiPIMS and a Gaussian distribution to account for fully thermalized Ti + ions are employed.

The energy distribution of Ar 2 + ions displays similar features when compared to Ar +, i.e., a low-energy peak followed by a pronounced high-energy component which peaks at about 18–24 eV (Fig. 2). The high-energy component is most pronounced at the smallest pressure p = 0.5 Pa and strongly decreases at larger gas pressures. The behaviour is similar to that of singly charged Ar + ions. The mean free path of Ar 2 + ions is largely determined by the resonant two-electron transfer reaction Ar 2 + + Ar Ar + Ar 2 +, while the one-electron transfer reaction Ar 2 + + Ar Ar + + Ar + is more than one order of magnitude smaller.40 The two-electron transfer cross section is thus comparable to that of the one-electron reaction Ar + + Ar Ar + Ar +.39 The estimated mean free path of Ar 2 + is of the order of a few centimetres (Table II), which is reasonable to explain the observed pressure dependency.

As for argon ions, the energy distribution of Ti + and Ti 2 + ions are rather similar. The relative intensities of the low-energy and high-energy components show a much weaker pressure dependence compared to Ar + and Ar 2 + ions, however, which is readily explained by the smaller reaction rates for the non-resonant charge transfer process of Ti ions with Ar atoms.

A major difference between singly and doubly charged ions concerns the energetic position of the high-energy component which peaks near 12 eV for singly charged ions and at 24 eV for doubly charged ions. It appears straightforward to attribute this difference to the different charge number.16Figure 6 displays energy distributions for Ar q + and Ti q + ions ( q = 1 , 2) as a function of the specific energy E ~ = E / q, i.e., kinetic energy per charge number q. The scaling results in rather similar energy distributions. We would like to emphasize that the measured kinetic energy of plasma ions is influenced by the plasma potential.13,36,41,42 Our results indicate an electric field reversal by a localised positive plasma potential. A positive plasma potential or potential hump was employed to explain the formation of moving ionisation zones or spokes in magnetron discharges.17,18,43 The extra energy E q = q e 0 V p provided by the plasma potential V p depends on the charge number q and thus will be twice as large for doubly charged ions compared to singly charged ions ( e 0 is the elementary charge). The present observations indicate that the energy E q gained by the plasma potential is identical to the energy shift E S of the Maxwellian distribution [Eq. (2)] used to fit the high-energy component of investigated ion energy distributions. The deduced (time averaged) plasma potential relevant for the high-energy component amounts to 7 V at p = 0.5 Pa; it decreases with increasing pressure.

FIG. 6.

Energy distribution of Ar + ( °, m / z = 40), Ar 2 + ( , m / z = 20), Ti + ( , m / z = 48), and Ti 2 + (▲, m / z = 24) ions versus kinetic energy E divided by charge number q at an argon pressure of 0.5 Pa. Dashed lines are fits of the high-energy component composed of a shifted Mawellian and a Thompson distribution (see text). For clarity, not all experimental data points are shown.

FIG. 6.

Energy distribution of Ar + ( °, m / z = 40), Ar 2 + ( , m / z = 20), Ti + ( , m / z = 48), and Ti 2 + (▲, m / z = 24) ions versus kinetic energy E divided by charge number q at an argon pressure of 0.5 Pa. Dashed lines are fits of the high-energy component composed of a shifted Mawellian and a Thompson distribution (see text). For clarity, not all experimental data points are shown.

Close modal

Ar 2 + and Ti 2 + dimer ions were also recorded. The total intensity of Ar 2 + ions is about four orders of magnitude smaller compared to, e.g., Ar + ions. The total intensity of Ti 2 + is even smaller and amounts to about 3 × 10 6 of the total Ti + intensity at p = 1 Pa (Fig. 4). Due to poor statistics, no energy distributions of Ti 2 + ions are reported here. This is in contrast to dc magnetron sputtering where orders of magnitude larger intensities and Ar 2 + / Ar + and Ti 2 + / Ti + ratios of 5 × 10 3 were observed.19 Other dimer ions like Ar Ti + were not detected.

The energy distribution of Ar 2 + ions is characterized by a pronounced low-energy peak of fully thermalized ions. It indicates that Ar 2 + ions form via gas phase processes in a region where the plasma potential is close to zero, i.e., presumably during the off time of the discharge. No evidence for the ejection of Ar 2 dimers from bursting gas bubbles caused by Ar + ion implantation has been found.44 Three-body reactions of Ar + ions with two neutral argon atoms and associative ionization of highly excited Ar atoms with ground state Ar atoms are the dominant processes.19,45–49 The mentioned processes depend at least linearly or quadratically on the Ar density which explains the strong increase of the Ar 2 + density with gas pressure.

Loss of Ar 2 + molecules takes place via electron impact-induced dissociation processes and thus increases with electron density.45–50 The rate constant for dissociative recombination strongly increases with decreasing electron temperature. Recombination processes thus play an important role also in the afterglow.21,51 It appears likely that the much larger electron density during the HiPIMS pulse together with an enhanced dissociative recombination in the afterglow is responsible for a larger loss rate of Ar 2 + ions and for its much smaller ion density compared to dc magnetron sputtering. Similar arguments may hold for the loss of Ti 2 + ions.

The energy distribution of singly charged Ar + and Ti +, doubly charged Ar 2 + and Ti 2 +, and of Ar 2 + and Ti 2 + dimer ions was investigated during a HiPIMS discharge. Measured ion energy distributions display low- and high-energy components which can be described by a shifted bi-Maxwellian distribution. The high-energy components of singly and doubly charged ions fall together when plotted versus scaled energy E ~ = E / q, where q is the charge number. The observation points to an electric field reversal and supports the existence of a potential hump used to explain the occurrence of moving ionisation zones or spokes. The generation of Ar 2 + dimer ions displays a quadratic pressure dependency. The intensity of Ar 2 + and Ti 2 + dimer ions is 2–3 orders of magnitude smaller during HiPIMS compared to dc magnetron sputtering; it is attributed to loss processes which strongly increase with electron density.

The work was partly supported by Project No. 17-08389S of the Czech Science Foundation, by Operational Programme Research, Development and Education financed by European Structural and Investment Funds, and the Czech Ministry of Education, Youth and Sports (Project Nos. SOLID21—CZ.02.1.01/0.0/0.0/16_019/0000760 and CZ.02.2.69/0.0/0.0/16_027/0008215), and by the German Academic Exchange Service (DAAD).

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