Main parameters of the DCMS and HiPIMS discharges described in this work and characterized by resonant optical absorption spectroscopy.
Abstract
The determination of the absolute number density of species in gaseous discharge is one of the most important plasma diagnostics tasks. This information is especially demanded in the case of low-temperature sputtering discharges since the time- and space-resolved behavior of the sputtered particles in the ground state determines the plasma kinetics and plasma chemistry in this case. Historically, magnetron sputtering is often implied when talking about sputtering discharges due to the popularity and the numerous advantages this technique provides for coating applications. The determination of the absolute density of various atomic and molecular species in magnetron sputtering discharges along with its time and space evolution may be important from several points of view, since it may help to estimate the total flux of particles to a virtual surface in the plasma reactor, to compare the throughputs of two different sputtering systems, to use the absolute particle concentrations as an input data for discharge modeling, etc. This chapter is intended to provide an overview on the advantages and main principles of resonant absorption spectroscopy technique as a reliable tool for in situ diagnostics of the particle density, as well as on the recent progress in characterization of magnetron sputtering discharges using this technique, when the role of reference source is played by another low-temperature discharge. Both continuous and pulsed magnetron sputtering discharges are overviewed. Along with the introduction covering the main principles of magnetron sputtering, the description of the basics of resonant absorption technique, and the selected results related to the particle density determination in direct current and high-power pulsed magnetron sputtering discharges are given, covering both space- and time-resolved density evolutions.
Keywords
- Magnetron sputtering
- resonant optical absorption spectroscopy
- ROAS
- atomic absorption
- absolute number density
1. Introduction: Magnetron sputtering discharges
Among the families of plasma-related methods involved in thin films growth, those belonging to physical vapor deposition (PVD) are among the most commonly utilized. The PVD-related techniques cover wide range of the deposition methods, including evaporation, vacuum arc deposition, laser ablation, and sputtering. These methods are different from another class referred to as chemical vapor deposition (CVD) in the sense that the source of material is solid or liquid as opposed to a gaseous one in the case of CVD [1]. Thermal evaporation has been the most used PVD process for many years because of the easy handling and relatively high deposition rate comparing to the first known sputtering process, namely, the diode sputtering [2]. The latter process has been known since its first description in 1852 by W. R. Grove, who had performed sputtering using a massive inductive coil [3]. In diode sputtering, ions of the sputter gas, commonly argon, hit a negatively biased cathode (also known as target) with energy up to several hundred electron volts (eV). This energy is high enough to induce ejection of the superficial atoms, which is followed by their condensation on the chamber surfaces, including a potential substrate [4]. In diode sputtering, however, due to the relatively high process pressure needed to ensure the discharge stability (nearly 0.1 Torr), the mean free path of the ejected atoms is in the mm range, which is much lower than the typical substrate-target distance (cm range) resulting in relatively poor quality of the deposited coating.
The limitations of diode sputtering have been overcome in 196× with the works of E. Kay and W. Gill [5,6], who have proposed to utilize magnetic field to create efficient electron trapping above the cathode (target) in order to accelerate the ionization and sputtering itself. The electron trapping region is created due to the fact that the electrons propagate mainly along the magnetic field lines as a result of well-known gyration effect (with gyration radius being in the µm range). The implementation of magnetic field for sputtering, naturally producing the name “magnetron,” has been shortly followed by the introduction of planar magnetron sources in 1974 [2,7], for which the presence of permanent magnets beneath the cathode was a distinctive feature, as schematically illustrated in Figure 1. As a result of the electron trapping, these innovations led to essential increase in the ionization degree of the sputter (bulk) gas in the cathode vicinity. As a result, the pressure necessary to maintain the discharge current in this case could be reduced by about one order of magnitude, i.e., down to the mTorr range [5]. Another critical point related to efficiency of magnetron sputtering is the topology of magnetic field. Indeed, the arrangement of magnets beneath the cathode affects significantly the degree of electron confinement above the target. In the so-called balanced magnetron sources, the dense plasma region above the target is roughly comparable to its radius (assuming a planar circular target) [8]. If the substrate is located outside this region, the bombardment of growing film by the plasma ions is essentially reduced (the resulting ion current is <1 mA/cm2), limiting the benefit of the sputtering process. This effect is different in the so-called unbalanced sources, where the topology of the magnetic field near cathode is different, letting some magnetic field lines reach the substrate. In this case, plasma is not confined completely, and the ion current densities of about 2–10 mA/cm2 can be reached, which is typically one order of magnitude higher than in the case of balanced magnetrons [9].
In addition to magnetic field configuration, the voltage waveform and its repetition rate also play significant role in magnetron sputtering. The development of the plasma sources utilizing this effect has been mainly driven by the efficient deposition of the insulating compounds using reactive sputtering (see below). Thus, in addition to the well-known direct current magnetron sputtering (DCMS) devices, the radio frequency (RF) magnetron sources mainly working at 13.56 MHz have been introduced. Following the same trend, in the early 1990s, the idea of using pulsed-DCMS (also known as P-DCMS, or pulsed-DC) has been proposed (see, e.g., [10,11]). In most cases, the pulsed-DC power supplies operate successively alternating the negative (sputtering phase) and positive (charge dissipation phase) voltage cycles. The power supply in this case might be unipolar or bipolar, depending on polarity of the utilized pulses. Nowadays, most of magnetron sputtering processes for the synthesis of insulating compound coatings use pulsed magnetron sputtering approach [9].
Further development of the pulsed magnetron sputtering technology has triggered the new family of magnetron sputtering processes, so-called ionized physical vapor deposition (IPVD) techniques [12]. In a typical IPVD discharge, significant fraction of the sputtered atoms is ionized, reaching up to 100% in certain cases. The main idea behind the IPVD techniques is to generate denser plasmas than those appear in conventional magnetron sputtering in order to ionize the sputtered atoms more efficiently (up to the level of number densities ~1013–1014 cm–3 vs. 108–1011 cm–3 in the DCMS case). Among the different approaches targeted to increase the ionization degree, such as using an inductive coil [13], or hollow cathodes [14], the high-power impulse magnetron sputtering technique, or HiPIMS (also known as high-power pulsed magnetron sputtering—HPPMS), is the most notorious IPVD example. HiPIMS discharge uses the pulse duration ranged from few µs to few hundreds µs, while the pulse repetition frequency typically varies from ~10 Hz to ~10 kHz. Under these conditions, the peak current density may reach values of up to several A/cm–2 compared to a few mA/cm–2 in DCMS, but only during a short time, typically 1–2% of the repetition period [15]. As a result, a dense plasma is generated during the plasma on time enabling not only efficient target sputtering but also high ionization degree of metallic vapor [16], at the same time keeping the average applied power comparable to that of DCMS [15–17]. The schematic comparison of the DCMS, P-DCMS, and HiPIMS techniques in terms of the applied power is shown in Figure 2. Note that the level of time-averaged power applied to the sputtered cathode in each case is roughly the same, which is first of all due to the fact that the permanent magnets beneath the target should not be overheated and reach the Curie temperature (which is as low as about 580 K for widely used neodymium-based magnets). The actual applied power level is typically equal to several hundred Watt and often defined by the magnetron source cooling system (i.e., water temperature) used beneath the target. The interest to the pulsed magnetron discharges and, namely, the HiPIMS discharges from both scientific and application points of view has been continuously increasing since the introduction these techniques [18,19]. The main advantages of the HiPIMS technique, along with the achievements in characterization of these discharges, are described in the numerous works [15,17,20–23].
Why is the knowledge of the absolute density in plasma and particularly in magnetron sputtering discharges important? There are few main reasons making this parameter critical. First,
In the domain of magnetron sputtering discharges, the determination of number density of the discharge particles by ROAS has been undertaken in the variety of systems, including time-resolved [33] and space-resolved [34] characterization. In particular, among the recent achievements, the works of various research groups devoted to absolute density of sputtered atoms in the DCMS discharges [31,34,35], including those amplified by an RF coil [30], as well as the works of devoted to flux measurements in DCMS [36], should be mentioned. What is related to the utilization of ROAS in the HiPIMS domain, the works dealing with the determination of time-resolved absorption line profile [33], which have been realized using tunable diode laser absorption spectroscopy (TD-LAS), should be emphasized. In addition, a detailed time evolution of the Ar metastable 1s5 states (Armet) has been investigated in a long-pulsed (200 μs) HiPIMS discharge by Vitelaru et al. [26], where the Armet density growth, followed by its rarefication after about 50 μs, and a gradual refill afterward have been clearly demonstrated. These results are generally in a good agreement with the recent studies in HiPIMS involving LIF [37] and ROAS [38] techniques, which are partially overviewed here. The last two works ([37,38]) are devoted to the short-pulsed HiPIMS discharges studying the propagation of the ground state and metastable sputtered particles, being mainly targeted to the systematic time-resolved characterization of HIPIMS discharges in terms of absolute density of species, 2-D imaging of the main atomic states corresponding to the studied discharge species (LIF+ROAS), as well as to the study of several optically available energy states of the discharge species and their sublevels.
Unifying the recent achievements in the domain of the absolute density measurements for two “extreme” magnetron sputtering cases, namely, DCMS and HiPIMS, this chapter is mainly focused on the particularities of the mentioned sputtering discharges in terms of the absolute density of species where resonant absorption is used as a main diagnostics technique. The resonant absorption method overviewed in this chapter is considered as a reliable tool for the determination of the absolute density of species in the discharge volume and for understanding the sputtering processes at the atomic level.
2. Particularities of resonant absorption
The main principles of the resonant absorption are described in details in the work of Mitchell and Zemansky [24]. This method is based on resonant absorption of radiation emitted by a reference source and absorbed (by atoms or molecules) in an optically thin gaseous discharge. At a definite spectral line the absolute density of states corresponding to a lower level of a chosen spectral transition can be determined by measuring so-called line absorption, i.e., an integral under spectral absorption line of interest, if the effective absorption lengths as well as the line width of both plasma and source spectral lines are known. This approach can be applied for the case when the spectral lines in the discharge are Doppler-limited [31], as well as can be generalized for more general cases [39]. The biggest advantage of ROAS technique is its nonintrusiveness. In this regard, this technique is different from the other methods used for density determination, which are based on the introduction of additional gases to the discharge (such as titration [40]), which may potentially change the electron energy distribution and the discharge kinetics. In addition, the fact ROAS uses external source of radiation and does not depend on the discharge emission itself is critical for characterization of the pulsed discharges, such as HiPIMS, especially during the afterglow time when optical emission spectroscopy (OES), cannot be applied.
As mentioned above, ROAS can be implemented in two different ways, namely, using a gaseous discharge emitting the spectral lines of interest, as well as using a monochromatic laser (e.g., a continuous tunable laser diode). Despite the fact that the laser-based diagnostics has several advantages [21,33], the discharge-based ROAS should be implemented. Among the main reasons for this are the following: (i) discharge-based ROAS setup is normally less expensive than the one based on a tunable laser; (ii) discharge-based ROAS depends on the line width ratio between the plasma and the reference source, which can be easily taken into account [24]; (iii) several spectral transitions can be studied in parallel in this case, as far as the emitters in the reference source coincide with the absorbers of interest, which is not possible with a monochromatic laser; and (iv) in case of a large spectral separation (few nm) between the lines of interest, laser-based absorption setup requires separate laser diodes for each transition, which involves additional calibration procedures. Due to the mentioned advantages, for example, the absolute density at the energy sublevels corresponding to a certain electronic state can be probed in a straightforward way, by simply following the changes in the emission intensity of the corresponding peaks from the reference source, as performed in the numerous studies [30,31,34,35,39]. Let us consider the main relations necessary to understand the ROAS method.
2.1. The basics of resonant absorption method
This chapter deals with the classical ROAS method described by Mitchell and Zemansky [24]. In the case of temperature-limited emission (absorption) lines in the reference source (and in the studied discharge), the application of the resonant absorption method is rather straightforward, and the details of its implementation in magnetron discharges can be found elsewhere [35,38]. In this section, only the essential relations important for understanding the absolute density determination using ROAS method are given. The number density of the absorbing species in the lower (often ground) state can be determined using the following relation:
where
where
The absorption coefficient
where
where
After the experimental determination of line absorption
2.2. Role of spectral line broadening
Several important remarks on the line broadening of plasma and reference source spectral lines should be made for proper interpretation of ROAS data. As mentioned above, this chapter deals with discharge-based ROAS when an HCL is primarily used as reference source. Since the gas pressure in HCL normally exceeds few Torr, and the mean free path of the gas particles is in the µm-range, the total particle thermalization can be assumed. The temperature in HCLs under the reviewed conditions, additionally measured by a high-resolution planar Fabry–Perot interferometer [42], is in the range of about 700–1100 K (see Table 1). Despite much lower pressure (~20–30 mTorr) in the magnetron discharges considered here (Table 1), similar considerations are also valid for the
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Target | 99.99% Ti | With water cooling |
Target diameter/thickness | 5 cm/0.5 cm | 10 cm/1 cm for Figure 9 |
Working gas | 99.999% Ar | |
Working pressure | 20 mTorr | 30 mTorr for Figure 9 |
Base pressure | ~10–6 Torr or less | |
Time-averaged power | ~200 W | 500 W for Figure 9 |
Monocromator used | Princeton Instrument SpectraPro-500i | Jobin-Yvon HR-460 for Figure 9 |
Monocromator slit width | 10 µm | |
Spectral resolution | 0.05 nm | |
Optical detector used | Princeton Instrument PI-MAX ICCD | Hamamatsu R928 photomultiplier tube |
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Target | 99.99% Ti | With water cooling |
Target diameter/thickness | 10 cm/1 cm | |
Working gas | 99.999% Ar | |
Working pressure | 20 mTorr | |
Base pressure | <10–6 Torr | |
pulse duration/frequency | 20 μs/1 kHz | Unless stated otherwise |
Supplied energy per pulse(at 20 μs pulse) | ≈0.26 J | Corresponding to ~260 W of time-averaged power |
Monocromator used | Andor Shamrock 750 | |
Monocromator slit width | 20–100 µm | |
Spectral resolution | 0.04–0.1 nm typically | |
Optical detector used | Andor iStar DH740-18F ICCD | With ~103 pulses averaged |
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Beam diameter | ≈1 cm | |
Distance above the target, |
≈5 cm |
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Effective absorption length, L | 30 cm | Variable for Figure 8, L=25 cm for Figure 9. |
Reference source type | Ar-Ba and Ne-Ti hollow cathode lamps | |
Reference source pulse duration | 40–100 µs | Continuous for Figure 8, 150 µs for Figure 9. |
Reference source temperature | ≈1100 K (in pulsed mode [38]) | 950 K for Figure 8 [34], 630 K for Figure 9 [54]. |
DCMS plasma temperature | 580 K (Figure 8), 300 K (Figure 9) | |
HiPIMS plasma temperature (Ar) | ≈500 K | Based on OES measurements [43] |
HiPIMS plasma line width (Ti) | ≈5.5 GHz | Based on LIF measurements [44] |
The broadening of spectral lines corresponding to the
The situation is yet different for HiPIMS discharges, which are far from thermalization during the plasma on time, especially in the target vicinity [37,43]. The distribution of the velocity component parallel to the target (
2.3. Experimental setup for resonant absorption
The typical experimental arrangement for characterization of magnetron sputtering discharge by resonant spectroscopy is shown in Figure 5. The main parameters related to the discharges overviewed in this work are summarized in Table 1. Both in DCMS and HiPIMS cases, a cylindrical vacuum chamber with either horizontally or vertically placed balanced magnetron sputtering source holding a planar circular magnetron target has been used. The Ti magnetron targets, 5 or 10 cm in diameter, attached to a magnet-supporting water-cooled copper base, have been utilized. The commercial power supplies have been used to sustain the DCMS discharges, as described elsewhere [30,34]. In the HiPIMS case, the discharge current and voltage along with the pulse parameters have been controlled by the Lab.-made power supply, described by Ganciu et al. [46]. The HiPIMS discharge typically had 20 µs of the pulse duration and 1 kHz of the repetition frequency (thus having 980 µs of the plasma off time). The typical current–voltage waveforms can be found elsewhere [43,44,47]. The base pressure in the magnetron reactor has been kept <10–6 Torr, whereas the bulk gas (99.999% Ar) pressure was normally fixed at 20 mTorr during all the ROAS measurements, unless stated otherwise. In order to avoid deposition on the quartz window surface, the special collimators (Ni tubes) with high length-to-radius ratio have been installed in front of each viewport inside the reactor.
The Perkin Elmer hollow cathode lamps (one with Ti cathode filled by Ne, and another one with Ba cathode filled by Ar) have been used as the reference sources. The HCLs have been running either in the continuous or pulsed regime (mainly in the HiPIMS case) in order to increase the emission intensity and make it comparable with the HiPIMS plasma emission, aiming at reducing the measurements error during the plasma on time [43,48]. Apart from this, the pulse regime of HCL source favors the ionization of the cathode material inside the lamp, which is critical for analysis of the Ti+ states [30]. During the measurements, the HCLs have a pulse duration typically ranging from 40 to 100 µs. About 103 pulses from the reference source have been averaged by the detector during the measurements of the absorption coefficient. The HCL pulse sequence was triggered by the HiPIMS power supply using the external transistor-transistor logic (TTL) trigger and was time shifted relatively to the HiPIMS plasma pulse using an additional analog TGP-110 pulse generator.
In some cases, when the density of absorbers in the studied discharge is too low, an improved detection scheme, including a triple optical fiber and allowing simultaneous acquisition of the
A monochromator equipped with an intensified charge-coupled device (ICCD) camera and connected to the discharge reactor by an optical fiber has been used for spectral acquisition in most cases. The spectral resolution of the monochromator was typically ≈0.05–0.1 nm. The typical emission spectra acquired in Ar-Ti HiPIMS discharge at the end of plasma on time are presented in Figure 7. From the general emission spectrum (Figure 7(a)), the relevant spectral regions containing Ti and Ti+ emission lines are visible (Figure 7(b–d)). The representative emission lines used in this work for resonant absorption are marked by arrows (see also Table 2). In most cases, the studied emission lines are well-resolved from the neighboring peaks. Due to this fast, rather low spectral resolution is normally sufficient for ROAS measurements. The total procedure for the number density determination consisted of the emission peaks intensities measurements (
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Ground | 0.000 0.021 0.048 |
363.55 364.27 365.35 |
3 3 3 |
5 7 9 |
0.20 0.16 0.14 |
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Metastable | 0.813 0.818 0.826 0.836 0.848 |
501.43 500.72 499.95 499.11 498.17 |
3 3 3 3 3 |
3 5 7 9 11 |
0.38 0.26 0.22 0.17 0.22 |
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Ground | 0.000 0.012 0.028 0.049 |
338.38 337.28 336.12 334.94 |
3 3 3 3 |
4 6 8 10 |
0.28 0.25 0.26 0.27 |
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Metastable | 0.574 0.607 |
376.13 375.93 |
3 3 |
6 8 |
0.22 0.21 |
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Metastable | 11.548 11.723 |
811.53 794.82 |
3 3 |
5 1 |
0.51 0.56 |
3. Particle density behavior in DCMS discharges
3.1. Spatial characterization of the DCMS discharge
Even though the main plasma parameters of the DCMS discharges do not alter in time, the plasma density as well as the density of the sputtered species may vary significantly as a function of the spatial position in the discharge. The first parameter in this case defines mainly the excitation level of the bulk and sputtered species as well as their ionization degree, whereas the (ground state) density of the sputtered species is mainly affected by the applied power, sputtering yield, target size, diffusion processes in the discharge volume, as well as by the other processes. The effects of ground state density depletion also may take place, as recently shown for DCMS discharges by LIF [49], but these effects are rather minor, especially comparing with HiPIMS case [37,38], since the ionization degree in the DCMS discharges generally remains at the level of few percent [15].
The optical diagnostics applied to a DCMS discharge clearly visualizes the plasma region corresponding to the confined electrons, which is represented by the bright area near the target surface observable by a naked eye, as shown in Figure 8. This region clearly corresponds to a spatial segment formed by the magnetic lines in the target vicinity, as shown in Figure 8(b), where the simulated magnetron magnetic field lines are drawn. The target diameter in this case is equal to 5 cm, but nevertheless the obtained data can be compared to the typical absolute density measured with larger targets (see Table 1), as soon as the target materials are the same and the current density are known, due to the scalability of the magnetron discharges [15]. In addition to this, as shown by Britun et al. [34], the emission lines in the plasma region decay exponentially along
In addition, the ground state density of the sputtered Ti as well as Ti metastables, measured by ROAS, follows the trend found for the emission lines by Britun et al. [34], as shown in Figure 8(c). As one can observe, the number density of Ti neutrals reaches nearly 1012 cm–3 near the target at the examined conditions (see Table 1), dropping about 5 times already at
3.2. Absolute density in reactive DCMS discharge
Another interesting possibility that ROAS provides is visualization and control of the physical processes during the so-called reactive sputtering. This process is mainly known for synthesis of oxide, nitride, and oxynitride thin films [21,53]. Reactive sputtering normally involves standard magnetron sputtering sources, where a certain percentage of, e.g., molecular oxygen or nitrogen, is added to the bulk gas (typically Ar) in order to finally synthesize oxide or nitride compounds. Physically, during the reactive sputtering, oxidation (nitriding, etc.) of the cathode superficial layer takes place, which is often referred to as “poisoned” regime. This process is characterized by a significant drop of the density of metallic species in the discharge volume [54]. Apart from this, a hysteresis effect (in terms of discharge voltage or metal atom density) depending on the direction of change of the reactive gas content is often considered to be among the main characteristics of reactive sputtering [15,55–57].
The example of measurements of the absolute number density of Ti atoms sputtered in a DCMS discharge at
4. Particle density dynamics in HiPIMS discharges
HiPIMS discharges, possessing very short plasma on time, as illustrated in Figure 2, demonstrate different physics from that known for the DCMS discharges. The main reason for this is significantly elevated plasma density as well as the ionization degree related to both the bulk gas and the sputtered particles in HiPIMS, as mentioned in the Introduction and analyzed in the numerous works. Despite the fact that DCMS and HiPIMS sputtering processes can be realized using the same magnetron source, the direct comparison between these discharges is rather difficult, especially at the high values of the peak power applied to HiPIMS (or cathode current density), even though the time-averaged level of the applied power might be the same. In this section, for the sake of illustration of the dynamics of HiPIMS discharge in terms of the absolute density of species, the time-resolved evolution of the main discharge species in an Ar-Ti short-pulsed HiPIMS discharge analyzed by ROAS is presented. The experimental details on the HiPIMS system used for diagnostics are listed in Table 1, whereas the corresponding spectroscopic data are available in Table 2, or can be found elsewhere [31,38].
4.1. Time-resolved particle density evolution in HiPIMS
The time-resolved evolution of the absolute densities of Ti and Ti+ atomic species sputtered in a 20-µs pulse Ar-Ti HiPIMS discharge measured at
The evolution of the Ti ion density shown in Figure 10(b) reveals similar tendencies, especially during the plasma off time, where the density has a maximum at 100–200 µs, reaching ~3 × 1011 cm–3. During the plasma on time, two phenomena can be observed: the first one is the beginning of density depletion interval, which is similar to that found for Ti neutrals (called interval A), and the second one which characterized by the strong instabilities of the measured Ti+ density at the end of the plasma on time, resembling the stochastic density behavior (called interval B). Note that six independent density measurements made during the plasma on time always lead to the different Ti+ density evolution. Since Ti ionization is likely defined by the electrons presented in the discharge volume during this time (
(where
The summary of the time evolution of several atomic species studied in a 20-µs Ar-Ti HiPIMS discharge at
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Electrons | Generation | ||
Ar | Ionization + acceleration toward the cathode | |
Related works [15] | |
Ti | Have their “background” density level, remaining from previous plasma pulse. | |||
Timet | ||||
Ti+ | ||||
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Electrons |
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LIF imaging data [37], Langmuir probe measurements [77] |
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Armet | Generation by |
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LIF imaging data [37] | |
Ti | VDF broadening and strong ionization, resulting in density depletion | LIF imaging data [37], measured VDF width [44] | ||
Timet | Generation by |
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Presence of Armet during this interval [37] | |
Ti+ | Generation by |
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Not visible, probably due to the discharge current instabilities [59,60] | |
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Electrons |
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LIF imaging data [37], inversion of sublevel populations [67] (Figures 13 and 14). | |
Armet | 1. Saturation, rarefication 2. Quenching by incoming Ti neutrals |
2. |
1. Decrease in [Armet], Related studies [15,26] 2. LIF imaging data [37] |
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Ti | 1. Propagation of sputtered Ti 2. Ti VDF relaxation |
1. LIF imaging data [37]2. Related studies [44,45] | ||
Timet | Follow the growth of [Ti] | |
Same growth rate for Ti and Timet (Figure 12) | |
Ti+ | Generation (following Ti wave) | 1. |
1. Faster growth of [Ti+] and slower growth of [Ti] at the same time 2. Negligible, due to very small drop in [Armet] (Figure 12) |
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Electrons | Thermalization | where |
Loss of the inversion of sublevel populations (Figure 14), Langmuir probe data [64,65], modeling [78] |
Armet |
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Figure 12 | ||
Ti | Diffusion | Gradual decrease of [Ti |
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Timet | Quenching, diffusion | where |
Permanent density drop, as well as [Timet]/[Ti] ratio drop (Figure 12) | |
Ti+ | Density saturation (production-loss balance) | Figure 12 | ||
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Electrons | Thermalized | Related works [64,65,78] | |
Armet |
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Increase in [Armet], refill time estimations [26,66] | ||
Ti | Continuing diffusion | Gradual decrease of [Ti |
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Timet | Reaching background [Timet] level ~1010 cm–3. | Figure 12 | ||
Ti+ | Slight drop after 600 µs, might be due to: | 1. 2. where |
1. [Ti+] drop + [Armet] increase 2. [Ti+] drop (Figure 12) |
4.2. Inversion of the energy sublevel populations
An interesting effect can be observed if the measured density is represented separately for each energy sublevel at the studied electronic state, as shown in Figure 13. As one can observe, a clear inversion among the studied energy states is achieved for five Ti sublevels corresponding to the metastable Ti a5FJ states (see Table 2). An inversion of the considered energy sublevels (i.e., when the population of the state corresponding to higher energy is found to be higher) takes place in Figure 13(a) during the 20- to 100-µs time interval. The maximum of the mentioned inversion for Timet case is located around Δ
Despite the fact that the inversion effect is rather new for HiPIMS, it is well known for the other low-temperature discharges, such as Ti hollow cathode plasma [67], and RF-amplified DCMS discharge [30]. In the former case, it is explained by the (partial) equilibrium of the heavy discharge species with the energetic electrons during the short plasma on time interval. Besides this, the inversion of the sublevel populations might be also responsible for the apparent density depletion in the magnetron target vicinity detected in the DCMS discharges [30,49,68,69]. The sublevel inversion can be explained based on the Boltzmann distribution of the corresponding energy sublevel populations. Assuming the presence of the dense flux of hot electrons during certain time in the discharge, the heavy particles should be intensively excited by the electrons during this time. Under these conditions, for the close energy sublevels, the factor
where
which, after normalization to
The sublevel inversion is even more clear if a “stabilized” modification of the resonant absorption method is implemented, when a pulsed reference source is synchronized with the time gate of the ICCD detector, as discussed above. The relative error of the ROAS measurements can be significantly reduced in this case, and fine spectral effects in the discharge, such as inversion of the energy sublevels, can be easily visualized. The example of these measurements using three Ti ground state energy sublevels is given in Figure 14. In Figure 14(a), the experimental ROAS results, clearly showing the presence of the Ti ground state sublevel inversion (roughly between 20 and 300 µs), are shown. At the same time, a sketch of the same level populations drawn in accordance to the inversion model described by Eqs. (6) and (7) is given in Figure 14(b) for the sake of clarification of the experimental data. We have to note that before the inversion interval, the studied Ti sublevels do not perfectly follow the Boltzmann distribution, as the middle level is mostly populated in this case. This fact might be due to the complicated HiPIMS discharge kinetics, which is not well studied yet. Let us also note that the inversion interval might be significantly shifted in time depending on position of the ROAS line of sight, and the inversion should happen much earlier if the measurements are performed closer to the target surface.
Fitting the experimental data corresponding to Ti sublevels, the Ti
4.3. Density dynamics in reactive HiPIMS
It is also interesting to consider some selected phenomena in the case of reactive HiPIMS discharge. The behavior of the number density of the sputtered species in reactive HiPIMS should follow the same tendency as in the reactive DCMS case (shown in Figure 9), i.e., a density drop by several times in the poisoned regime of sputtering should be expected. Let us consider the behavior of the absolute density of
Analyzing the Omet atoms in any discharge, one should bear in mind that these are excited species which keep the dynamics of the plasma electrons as well as the dynamics of the ground state O2 and O. The time evolution of Omet atoms measured by ROAS at
Since in the case of Ar–20% O2 gas mixture magnetron target is supposed to be completely oxidized [47], the sputtering of oxide material from (i.e., M
5. Summary and conclusions
5.1. DCMS discharges
Resonant absorption analysis of the direct current sputtering discharges shows that the absolute density of the sputtered particles is ranged in a relatively narrow interval, being always in the range of 1010–1012 cm–3 in the Ti case (assuming the average power level to be several hundred Watt). These values may be somewhat different varying the cathode materials with essentially different sputtering yield and/or in the sputtering systems, which use significantly higher power levels. The spatial distribution of the sputtered particles is mainly determined by their diffusion (at high pressure), by the angular directivity of sputtering (low pressure), as well as by the magnetic field topology (critical mainly for the sputtered ions), which lead to nearly exponential decay of the observed density of species as a function of the distance from cathode, so the density might decay at least by one order of magnitude already at few cm away from the target surface (for 5 cm diameter round target).
One of the advantages of using ROAS technique for diagnostics of sputtering processes is its ability to probe the species in a wide range of absolute density, starting from roughly 107–108 cm–3, where the signal-to-noise ratio of the used detector is a limiting factor, and finishing by the values several orders of magnitude higher, when the increase of the optical thickness of the volume of interest becomes the limiting factor. The bottom sensitivity level may be additionally affected by the type of the absorption scheme used (e.g., single-pass, multipass, with or without simultaneous signal acquisition, etc.). The variety of the ROAS implementation schemes allow real time monitoring of the sputtered species also during
5.2. HiPIMS discharges
The physical phenomena in the HiPIMS discharges are rather different from those observed in DCMS and P-DCMS cases, first of all in terms of the plasma density and much shorter timing (µs scale). As a result, numerous dynamic effects should be taken into account explaining the properties of the HiPIMS discharges. Among these effects, the following time-dependent phenomena should be mentioned: the propagation of the fast electron wave the dynamic gas rarefaction and refill, the ionization and excitation of various discharge particles, quenching of the excited states as a result of collisions, the anomalous electron transport, etc. ROAS technique, being essentially limited by only the time-resolution of the used optical detector, suits very well for systematic characterization of these discharges in terms of the absolute density. In addition, the modern ICCD detectors typically provide ns time resolution, which surpasses the typical HiPIMS time scale by few orders of magnitude.
In terms of the time-resolved evolution of the discharge particles density, there are several processes in HiPIMS discharge, which can be visualized using ROAS technique. The time-resolved summary of these processes is given in Table 3, considering the main discharge species studied in the Ar-Ti nonreactive HiPIMS case. The presence of such dynamic effects as the density depletion, gas rarefaction, gas refill, quenching of the metastable states, ionization, as well as the propagation of the electron wave away from the target surface can be concluded based on the undertaken ROAS analysis. Despite a rather high HiPIMS repetition rate considered in this work (1 kHz), the gradual relaxation for the majority of the listed processes till the end of the plasma off time can be observed. Even though the total range of the measured absolute density of the discharge species reaches nearly three orders of magnitude in the considered case, the detected densities are still well above the detection threshold (which is estimated to be roughly 107–108 cm–3 with the detection scheme used). The sensitivity of the used ROAS method can be additionally enhanced using the lock-in detection technique, dynamic triggering of reference source, and related approaches.
One of these approaches, namely, the simultaneous acquisition of the emission signals using a triple optical fiber, has been utilized for time-resolved detection of the oxygen metastable atoms during the reactive HiPIMS process. The results reveal the presence of the same discharge phases (such as gas rarefaction and refill, as well as the density depletion in some cases) as detected in the nonreactive HiPIMS case. The absolute density of the O metastable atoms detected in this case is still above the mentioned ROAS sensitivity threshold, being in the range of 108–109 cm–3. The optical diagnostics of the reactive HiPIMS discharges, however, is a relatively new area and the obtained results require additional verification by the other methods, such as LIF, in order to be understood completely.
5.3. General remarks
The resonant absorption technique represents a powerful diagnostic tool suitable for the determination of the absolute density of the relevant species in sputtering discharges, such as DCMS, P-DCMS, HiPIMS, as well as the other types of magnetron discharges. This technique has several modifications allowing its optimization depending on the discharge geometry etc. For example, using a plasma discharge as a reference source, ROAS setup can be rather simple, requiring only the additional optics for reference source beam collimation, and (sometimes) additional synchronization between the source and the discharge of interest. Both direct current and pulsed reference sources can be used for resonant absorption purposes. In addition to this, in case a broad range spectral detector, such as ICCD, is utilized, a parallel detection of several spectral transitions as well as the corresponding absolute densities becomes possible. These advantages open new possibilities to study the basic effects in the discharge volume. Among these effects are the population of the metastable levels of the various discharge species, the population of the atomic energy sublevels of the atoms and their time-resolved dynamics, the study of the excitation temperature across the discharge volume, etc. These possibilities are important for studying magnetron sputtering processes at the fundamental level, whereas rather basic configuration of the ROAS setup may be helpful in industry, e.g., for real-time density monitoring of the relevant species during a certain process.
The sensitivity of the absorption method can also be significantly improved by varying the detection schemes, such as lock-in amplification (improving the sensitivity by 1–2 orders of magnitude typically), multipass methods, as well as by simultaneous signal acquisition, dynamic source triggering, etc. Using the simultaneous signal acquisition for example, the sensitivity below 108 cm–3 has been achieved for O metastable atoms in a reactive HiPIMS discharge case, as illustrated in this chapter. This value is far below the typical concentrations of the sputtered atoms in the discharge (1010–1012 cm–3) achieved under the laboratory conditions, which is enough for most applications. For very weakly populated energy states, like those appearing in molecular discharges, the multipass absorption, including the ring-down spectroscopy method, could be suggested, which are, being out of the scope of this chapter, still based on the same fundamental principles.
Acknowledgments
This work is supported by the Belgian Government through the “Pôle d’Attraction Interuniversitaire” (PAI, P7/34, “Plasma-Surface Interaction,” Ψ). N. Britun is a postdoctoral researcher; S. Konstantinidis is a research associate of the Fonds National de la Recherche Scientifique (FNRS), Belgium.
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