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Investigation of reactive high power impulse magnetron sputtering processes using various target material–reactive gas combinations
Surface & Coatings Technology 205 (2011) 3613–3620
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Investigation of reactive high power impulse magnetron sputtering processes using various target material–reactive gas combinations Martynas Audronis ⁎, Victor Bellido-Gonzalez Gencoa Ltd, Physics Road, Speke, L24 9HP, UK
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Article history: Received 10 October 2010 Accepted in revised form 23 December 2010 Available online 31 December 2010 Keywords: High power impulse magnetron sputtering HIPIMS Reactive sputtering Plasma optical emission monitoring Process control
a b s t r a c t The two most important issues limiting reactive high power impulse magnetron sputtering (HIPIMS) process applicability until recently were the absence of suitable reactive HIPIMS control means and the limited capability of HIPIMS power supplies in terms of arc handling. The significant advancement has been made recently by the development of the optical plasma monitoring (PM)-based process control technology for reactive HIPIMS [Surface & Coatings Technology 204 (2010) 2159–2164]. The initial studies of reactive HIPIMS processes however have only covered Ti–O2 target material–reactive gas system. In this paper the recently developed PM-based active feedback control technology was applied to explore further reactive HIPIMS processes now using a variety of different target material and reactive gas combinations. Data for hysteresis behaviour and process control using either PM or constant gas flow methods for Ti–O2, Ti–CO2, Cr–O2, Cr–C2H4, Al–O2, and Zn:Al–O2 material–gas systems is presented and compared. In all cases the processes were found to exhibit hysteresis behaviour. The magnitude and features of the hysteresis loop were found to depend strongly on a particular metal–reactive gas pair. Similar to AC and DC reactive sputtering processes the hysteresis behaviour in reactive HIPIMS was found to be more pronounced for the gases that have high chemical affinity for a metal sputter target. The PM-based process control technology monitoring either metal or gas plasma emissions was shown to provide accurate control and stable operation of reactive HIPIMS discharges. © 2011 Elsevier B.V. All rights reserved.
1. Introduction HIPIMS is a plasma-based physical vapour deposition (PVD) process that is able to provide a highly ionised flux of sputtered species [1–4]. It is thought to be particularly important for applications where there is a need to coat 3D features (e.g. vias and trenches in semiconductor applications) [5]. HIPIMS may have other added benefits, as compared to DC or medium frequency AC/pulse-DC magnetron sputtering, related to different coating structure–property relationship control through self-species (sputtered metal) plasma/ion assistance. In reactive sputtering [6] the hysteresis1 effect originates from the reaction of a target surface with a reactive gas fed into a sputtering system [7–16]. It is recognised that two processes/mechanisms are at the core of this phenomenon: i) chemisorption — leading to the formation of a compound layer on the target surface and ii) ion implantation — leading to a compound formation within the target ⁎ Corresponding author. Tel.: + 44 151 486 4466; fax: + 44 151 486 4488. E-mail addresses: [email protected] (M. Audronis), [email protected] (V. Bellido-Gonzalez). 1 The definition of the word ‘hysteresis’ is commonly accepted as the lagging of an effect behind its cause. That is hysteresis refers to systems that have memory, where the effects of the current input (or stimulus) to the system are experienced with a certain delay in time. (Source: www.thefreedictionary.com and www.wikipedia.org. Accessed on the 8th of August 2010). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.12.038
surface. Implantation depth of the reactive species (such as oxygen and nitrogen), at typical direct current (DC) or radio frequency (RF) reactive sputtering conditions, reaches several nanometres (e.g. 7 nm in case of Si sputtering in Ar/O2 atmosphere [17] and 2.5 to 4 nm in case of Ti and Si sputtering, respectively, in Ar/N2 atmosphere [17,18]), which significantly exceeds the thickness of the adsorbed monolayer. As the partial pressure (p.p.) of reactive gas is increased, ion implantation surface compound formation mechanism becomes dominant, as for example shown in reference [9]. Chemisorption of the reactive gas during reactive HIPIMS (R HIPIMS) takes place in a similar way as it happens during reactive alternating current (AC) or DC sputtering, while the reactive ion irradiation induced target surface poisoning mechanism is pronounced even more significantly due to very high peak voltages exhibited by HIPIMS [19]. References [19,20] report the observation of the hysteresis effect in R HIPIMS by employing a PM-based technology. It appears that reactive HIPIMS process has all the features of a typical reactive AC or DC sputtering process i.e. it exhibits a sudden transition from ‘metal’ to ‘compound’ target state, arcing and hysteresis effect (Fig. 1). Due to these similarities reactive HIPIMS processes are also unstable and require active feedback process control systems to ensure process stability. Constant reactive gas flow control method does not lead to a stable reactive HIPIMS deposition process [20]. Reference [19] also shows that the target pulsing parameters, such as frequency, magnitude
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Fig. 1. A comparison of the hysteresis curves (sensor signal vs. O2 flow) observed during reactive (a) DC sputtering and (b) HIPIMS of Ti in Ar/O2 [20].
of voltage pulses, and duty cycle influence the overall shape of the hysteresis loop. It was also found that in reactive HIPIMS the sputter target state/oxidation level depends strongly on the history of its use [21]. This effect appears to be pronounced strongly due to the significance of reactive ion irradiation induced sputter target poisoning processes. The target sputter cleaning times (e.g. after a reactive deposition run) can be long (compared to DC sputtering case) because of the same reason. It was also shown that the hysteresis loop can in some cases be artificially suppressed, with the consequential substantial reduction of the reactive deposition process window (can be a number of times; e.g. four times as it was demonstrated for 325 Hz/50 μs pulsing case when sputtering Ti in Ar/O2), if the starting target surface is not clean [21]. Until recently, the two most important issues limiting the use of reactive HIPIMS were the absence of suitable control means and the limited capability of HIPIMS power supplies in terms of arc handling. Significant advancements regarding the process control have been made by the development of the optical PM-based process control technology for reactive HIPIMS [20]. This initial work however only concerned R HIPIMS within the Ti–O2 system. The aim of this work was therefore to use the recently developed PM-based active feedback control technology to explore and shed some light on more R HIPIMS processes. In this paper results on the following target material and reactive gas combinations are reported: Ti–O2, Ti–CO2, Cr–O2, Cr–C2H4, Al–O2 and Zn:Al–O2. These material–reactive gas pairs when used in a deposition system can produce technological thin films that are used in a wide range of markets for various applications. Examples are: TiO2 — self cleaning coatings on glass and high refractive index optical layers in multilayer films, metal oxycarbides — decorative coatings, Cr2O3 – optical and hard wear-resistant coatings, chromium carbide — hard wear-resistant coatings for machining and/or forming tools, Al2O3 — optical and/or hard wear-resistant coatings, and aluminium doped zinc oxide (AZO) — transparent conducting oxide films for semiconductor and photovoltaic applications. Hysteresis behaviour and reactive process control of the abovementioned processes are investigated and discussed. Hysteresis evaluation method is discussed with a new approach to presenting hysteresis effect for reactive sputtering processes proposed. 2. Experimental procedure The GenLab coating deposition system has been used for experiments and is described in detail in [20]. 99.5% pure Ti, Cr, Al (188 mm × 296 mm × 9.5 mm) and Zn:Al (2 wt.% Al) targets were reactively sputtered using an essentially balanced planar rectangular magnetron. Base vacuum, better than 0.002 Pa, was attained prior to reactive sputtering experiments. The working pressure was maintained between 0.2 and 1 Pa during all runs. Gencoa Ltd active
feedback reactive sputtering process controller Speedflo™, the version that is designed for HIPIMS process, was used to control reactive gas flow. PM using 330 nm (Ti+ OES lines), 520 nm (Cr⁎ OES lines), 395 nm (Al⁎ OES lines) and 780 nm (O⁎ OES lines) filters (10 nm bandwidth) was employed for reactive HIPIMS experiments. A Chemfilt Ionsputtering AB and Huettinger Gmbh HIPIMS power supplies were used as target sputter power sources. Power applied to the targets was between 2500 and 3000 W in the cases of Ti, Al and Cr targets and 1000 W in the case of Zn target. The sputtering trials were carried out at target pulsing frequencies of 325 Hz (Ti), 600 Hz (Ti, Cr, and Al) and 60 Hz (Zn) and the pulse-on times were 50, 50 and 200 μs respectively. The reactive HIPIMS hysteresis plots were generated by ramping reactive gas (O2, CO2 and C2H4) flow up at a constant rate to a certain value and then ramping it down back to zero while recording the PM sensor response at the same time. Such a reactive sputtering process characterisation procedure is practised routinely in vacuum coating industry as well as in certain academic departments. 3. Results 3.1. Ti–O2 R HIPIMS process Fig. 2a shows a ramp of O2 gas in an Ar/O2 Ti HIPIMS discharge (at a pulse frequency of 325 Hz and an average power of 2500 W) and a response of the PM sensor that is monitoring Ti+ emissions using a 330 nm filter. The sensor response plot against time contains valuable information on the process characteristics (e.g. dynamics of target depoisoning processes after O2 flow rate had become 0 sccm) in the time domain. This information is not available when the sensor signal is plotted conventionally i.e. against the reactive gas flow rate. Fig. 2b shows a conventional hysteresis loop plot (PM signal vs. O2 flow). In this figure and Fig. 2a the optical emission Ti+ signal correlates to the sputtered flux of Ti+ ions. It becomes smaller (i.e. less intense PM signal) when the target gets oxidised due to lower sputter rate of the oxide material. The same is valid for many other excited and/or ionised metal optical emission lines. The data shown in Fig. 2a and b, in terms of reactive sputtering process features, is very similar to that typically observed for AC and DC reactive sputter deposition processes. More extensive discussion on this can be found in reference [20]. Fig. 2c shows examples of reactive HIPIMS Ti–O2 deposition runs using a PM-based active feedback process control. Ti+ optical plasma emission at 330 nm is monitored and used as a signal for the active feedback control of the process. It can be seen that the method used provides accurate control (i.e. operation anywhere between the ‘metallic’ and ‘poisoned’ sputter target states can be readily obtained and maintained) of this complicated process, also providing the
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Fig. 2. R HIPIMS of Ti–O2 at 325 Hz (50 μs ON time) ~2500 W average power: (a) oxygen flow ramp and associated response of PM sensor signal (monitoring Ti+ using a 330 nm filter); (b) hysteresis plot (Ti+ PM signal vs. O2 flow); (c) process control examples at different set-points (percentages are shown on the right hand side) using the PM-based (monitoring 330 nm Ti+ emission) reactive sputtering control technology; (d) constant flow process control at different oxygen flow set-points (monitoring Ti+ emission using a 330 nm filter).
required process stability. Therefore, it appears that this process control technology will enable deposition of titanium oxide films with controlled chemical composition, structure and properties. Long term (N3 h) performance of controlled R HIPIMS Ti–O deposition has been demonstrated earlier and the data has been made public in reference [20]. Contrary to results shown in Fig. 2c, Fig. 2d shows data obtained from the constant reactive gas flow experiment and demonstrates that fixed flow rate R HIPIMS process control method does not provide a stable deposition process. A continuous drift from the ‘metal’ target state towards the ‘compound’ target state occurs and its rate depends on the oxygen flow rate. Such unstable and unpredictable process could hardly be used for production/research purposes, unless the target is operated in a fully poisoned mode or the hysteresis effect is eliminated by other means, such as very high pumping speed. Reactive HIPIMS processing in a fully poisoned mode is however substantially disadvantageous not only because of lower sputter and deposition rates, poorer film properties and stoichiometry, but also because of severe arcing that usually takes place at these conditions. Influence of negatively charged ions that originate at the target surface [22–24] has to be taken into account too, as the flux of negatively charged species will depend on the operating set-point on the hysteresis loop. 3.2. Ti–CO2 R HIPIMS process Fig. 3a shows a ramp of CO2 gas in an Ar/CO2 Ti HIPIMS discharge (at a pulse frequency of 600 Hz and an average power of 3000 W) and a response of two PM sensors monitoring Ti+ and O⁎ emissions using 330 nm and 780 nm filters, respectively. Fig. 3b shows conventional hysteresis loop plots (PM signals vs. CO2 flow). Again, the hysteresis
behaviour can be clearly observed using both, metal and reactive gas optical emission lines. Both signals are equally good for process monitoring and control purposes [PM O⁎ signal correlates to partial pressure (p.p.) of O2 within the vacuum vessel while PM Ti+ signal correlates to Ti+ flux]. Monitoring a reactive gas optical emission line instead of a metal line is an often practised reactive sputtering process control technique used for oxide thin film deposition. The difference, as compared to monitoring ‘metal’ line, is in the direction of signal change upon introduction of reactive gas to the system. Also, it can be harder, but not impossible, to notice when the target state changes from ‘transition’ to ‘compound’. Essentially, process behaviour during ‘poisoning’ and ‘depoisoning’ phases in Ti–CO2 case appears to be very similar to that observed for Ti–O2. This is due to the fact that CO2 gas is decomposed by the HIPIMS discharge into components one of which is O (as evidenced by an O⁎ optical emission line), which then reacts with Ti preferentially due to higher affinity of O2 to Ti. It is not clear at the moment what other constituents CO2 gas is broken to and will be a subject of further studies. Fig. 3c and d shows examples of the PM-based reactive HIPIMS Ti–CO2 deposition process control. Here Ti+ and O⁎ optical plasma emissions are monitored and used as signals for the active feedback control of the process. It can be seen that accurate control of reactive HIPIMS Ti–CO2 process can be obtained and the deposition process stability is acceptable. 3.3. Cr–O2 R HIPIMS process Fig. 4a shows a ramp of O2 gas in an Ar/O2 Cr HIPIMS discharge (at a pulse frequency of 600 Hz and an average power of 3000 W) and a response of PM sensor monitoring Cr⁎ emission using a 520 nm
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Fig. 3. R HIPIMS of Ti–CO2 at 600 Hz (50 μs ON time) ~3000 W average power: (a) CO2 flow ramp and associated response of PM sensors (monitoring Ti+ using a 330 nm filter and O⁎ using a 780 nm filter); (b) hysteresis plot (Ti+ and O⁎ PM signals vs. CO2 flow); (c) process control at different set-points using a PM-based reactive sputtering control technology (monitoring Ti+ emission using a 330 nm filter); (d) process control at different set-points using a PM-based reactive sputtering control technology (monitoring O⁎ emission using a 780 nm filter).
filter, respectively. Fig. 4b shows a conventional hysteresis loop plot (PM Cr⁎ signal vs. O2 flow). The hysteresis behaviour can be clearly observed. It is interesting to find from Fig. 4a and b that in Cr–O2 system the transition (‘poisoning’ phase) region exhibits at least four regions (marked as A, B, C and D) where target poisoning dynamics, or processes taking place at the target surface, are substantially different. This has not been observed for any other material gas combination investigated in this study (except for maybe Al–O2). No explanation to such behaviour can be offered at present. Yet another
feature that has been observed for Cr–O2 system is severe arcing during the sputter target ‘depoisoning’ phase (close to ‘metal’ state) as can be seen in Fig. 4a. No such arcing has been observed for Ti–O2/ CO2 processes. Fig. 5 shows an example of the PM-based R HIPIMS Cr–O2 deposition process control. Cr⁎ optical plasma emission (at 520 nm) is monitored and used as a signal for the process feedback control. Again, accurate control and good stability of R HIPIMS Cr–O2 process is obtained.
Fig. 4. R HIPIMS of Cr–O2 at 600 Hz (50 μs ON time) ~3000 W average power: (a) oxygen flow ramp and associated response of PM sensor signal (monitoring Cr⁎ using a 520 nm filter); (b) hysteresis plot — Cr⁎ PM signal vs. O2 flow.
M. Audronis, V. Bellido-Gonzalez / Surface & Coatings Technology 205 (2011) 3613–3620
Fig. 5. Process control of R HIPIMS using Cr–O2 at different set-points (monitoring Cr⁎ emission using a 520 nm filter).
3.4. Cr–C2H4 R HIPIMS process Fig. 6a shows another example of a reactive gas ramp and associated PM sensor signal in HIPIMS plasma discharge (monitoring Cr⁎ at 520 nm) obtained for Cr–C2H4 system at a pulse frequency of 600 Hz and an average power of 3000 W (using Chemfilt Ionsputtering AB Sinex-3 power supply). Similar to the Ti–O2 case, as discussed in Section 3.1., the PM signal in this example represents the sputtered flux of Cr atoms. Hence, the signal becomes smaller when the target is poisoned due to lower sputter rate of a compound material formed on
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the target surface. It can be seen from Fig. 6b that in the Cr–C2H4 case the hysteresis effect is relatively less strong as compared to the Ti–O2/CO2 processes. This is due to the fact that ethylene (C2H4) is less reactive than O2. In any case reactive Cr–C2H4 HIPIMS process still exhibits the hysteresis effect and relatively fast transition between the different target states. This means that chemisorption of reactive gas and implantation of its ionised constituents into the target surface is taking place, which also means that implementation of a stable coating production process is very likely to require the use of an active feedback process control system. Fig. 6c shows examples of the PM-based reactive HIPIMS Cr–C2H4 deposition process control. 520 nm Cr⁎ optical plasma emission is monitored and used as a signal for the active feedback control of the process. It can be seen that an accurate coating deposition process control can be readily obtained and maintained and the operation anywhere between the ‘metallic’ and ‘compound’ sputter target states is provided. The process appears to be very stable too. Hence, the R HIPIMS control technology used here will enable the deposition of chromium (or other metal) carbide films with controlled chemical and physical properties. Fig. 6d demonstrates poor process stability when C2H4 flow is constant. It can be seen from this figure that accurate control can be difficult to obtain. 3.5. Al–O2 R HIPIMS process Fig. 7a shows a ramp of O2 gas in an Ar/O2 Al HIPIMS discharge (at a pulse frequency of 600 Hz and an average power of 3000 W) and a response of a PM sensor monitoring Al⁎ emission at 395 nm. It can be seen from this figure that severe arcing took place just below the 60%
Fig. 6. HIPIMS of Cr–C2H4 at 600 Hz (50 μs ON time) ~ 3000 W average power: (a) ethylene flow ramp and associated response of PM sensor signal (monitoring Cr⁎ using a 520 nm filter); (b) hysteresis plot — 520 nm Cr⁎ PM signal vs. C2H4 flow; (c) process control examples at different set-points (percentages are shown on the right hand side) using the PM-based (monitoring 520 nm Cr⁎ emission) reactive sputtering control technology; (d) constant flow process control at different flow set-points (monitoring Cr⁎ emission using a 520 nm filter).
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Fig. 7. HIPIMS of Al–O2 at 600 Hz (50 μs ON time) ~ 3000 W average power: (a) oxygen flow ramp and associated response of PM sensor signal (monitoring Al⁎ using a monochromator tuned to 395 nm); (b) slow O2 flow ramp and associated response of PM sensor signal (Al⁎ at 395 nm); (c) process control at 70% set-point using a PM-based reactive sputtering control technology (monitoring Al⁎ emission at 395 nm); (d) constant flow process control at different flow set-points (monitoring Al⁎ emission at 395 nm).
mark of the PM signal. Hence, the full information on the target ‘poisoning’/‘depoisoning’ processes has not been obtained. Slow oxygen ramp was carried out in order to try to obtain more information (Fig. 7b). It, however, only confirmed the fact that at ~55% PM Al⁎ signal the process becomes unstable due to uncontrolled severe arcing which is followed by a shutdown of the power supply. It was still possible to obtain reactive process control at Al PM signal N60% as demonstrated in Fig. 7c. In contrast, Fig. 7d shows that constant oxygen flow does not yield a stable aluminium oxide deposition
process. This is in good agreement with the data provided in earlier sections of this paper on other R HIPIMS processes. 3.6. Zn:Al–O2 R HIPIMS process Fig. 8a and b shows examples of an oxygen ramp and a hysteresis plot (monitoring O⁎ PM signal at 780 nm and plotting it against O2 flow) obtained for reactive HIPIMS of Zn:Al at a pulse frequency of 60 Hz and an average power of 1000 W (using Huettinger TruPlasma
Fig. 8. HIPIMS of Zn:Al(2 wt.%) in Ar/O2 atmosphere at 60 Hz (200 μs ON time) ~1000 W average power: (a) oxygen flow ramp and associated response of PM sensor signal (monitoring O⁎ using a 780 nm filter); (b) hysteresis plot (PM signal at 780 nm vs. O2 flow).
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Fig. 9. An example of reactive HIPIMS of AZO deposition (metal Zn:Al target) process control at a 30% set-point using the PM-based reactive sputtering control technology.
power supply). Preliminary trials of reactive Zn:Al HIPIMS process have revealed that this process is prone to intense arcing. It was also found that in this particular situation, the gas signal (O⁎) was less disturbed (noisy) by arcing events than that of metal (Zn+) and hence the control was better using the O⁎ 780 nm filter, as described in the section below. Fig. 9 shows an example of the PM-based R HIPIMS AZO deposition process control. 780 nm O2⁎ optical plasma emission is monitored and used as a signal for the active feedback control of the process. It is shown again that the developed method provides accurate control of R HIPIMS coating deposition process, also providing good process stability. Ability to operate anywhere within the hysteresis loop will enable deposition of AZO films by HIPIMS with controlled compositional, optical, electrical and structural properties. The PM signal roughness during control seen in Fig. 9 is resulted by arcing. A better HIPIMS power supply with a good arc handling circuitry would be required in order to result in even smoother deposition process. 4. Discussion Results produced by this work and described in the previous section of this paper demonstrate rather clearly that all the processes studied were found to exhibit hysteresis behaviour. It is also revealed by the results that the magnitude of the hysteresis loop depends on the particular metal target–reactive gas pair (we neglect here other variables that are well known to the reactive sputtering community and affect the hysteresis effect, such as pumping speed [12,14]). Similar to conventional AC and DC reactive sputtering processes the hysteresis loop in R HIPIMS is more pronounced for gases that have high chemical affinity for the metal sputter target. Interestingly, different sputter target material–reactive gas systems can have significantly different target ‘poisoning’–‘depoisoning’ process dynamics, which is likely to be related to different target poisoning mechanisms and target surface chemical, physical and electrical properties. These processes and mechanisms are however yet to be studied in order to gain better understanding of R HIPIMS. The process control examples demonstrate that the PM-based active feedback control technology (monitoring either metal or gas plasma emissions) used in this study is able to provide accurate control and stable operation of R HIPIMS discharges. 4.1. Hysteresis loop plot: ‘sensor vs. flow’ or ‘sensor vs. time’? In this section authors describe a hysteresis plot methodology that is different from the conventionally used. The traditional approach is to plot a sensor signal (or deposition rate) vs. reactive gas flow [10,15], which is perfectly suitable in most reactive AC or DC
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Fig. 10. Hysteresis curves for reactive DC and HIPIMS Ti–O2 processes obtained by plotting sensor signals against time as opposed to flow rate.
sputtering situations. Experimental evidence gathered in this and previous [19–21] studies however suggest that in certain R HIPIMS cases such an approach may conceal the important information about the process that is being investigated. For example Fig. 2b shows a conventional R HIPIMS hysteresis loop plot for the Ti–O2 system. The problem with this plot is that when reactive gas rampup/ramp-down procedure is over and reactive gas flow is 0 sccm, the sensor signal has not fully recovered yet and is still changing. The only direction that the sensor signal can now be plotted is upwards. This unfortunately conceals the full extent of the hysteresis effect. This is due to the fact that target depoisoning during R HIPIMS can be much slower, as discussed earlier [19], hence longer time delay. In certain R HIPIMS cases this is a significant portion of the hysteresis loop. The above is demonstrated clearly in Fig. 10 where the sensor signal is plotted against time during R HIPIMS Ti–O2 process. Data for reactive DC sputtering at otherwise equivalent process conditions is included for comparison. Since the reactive gas flow rate is constant and there is a direct and linear relationship with time, ‘sensor vs. time’ approach is essentially very similar to the conventional method and is therefore perfectly valid. It is also clear from this example that plotting hysteresis loop in the way that authors of this paper propose, the full extent of the R HIPIMS (and likely other processes with long delay time) hysteresis can be revealed and appreciated with no information lost and extra important information on process features gained.
5. Conclusions The recently developed plasma monitoring-based active feedback control technology for reactive HIPIMS has been applied to explore processes using different target materials and gaseous environments. It has been shown that it provides means to characterise and control accurately reactive HIPIMS processes. All the processes studied were found to exhibit hysteresis behaviour. The magnitude of the hysteresis loop depends on the particular metal target–reactive gas pair. Similar to AC and DC reactive sputtering processes the hysteresis loop in R HIPIMS is more pronounced when gases that have high chemical affinity for the metal sputter target are used. Different sputter target material– reactive gas systems can have quite different target ‘poisoning’–‘depoisoning’ process dynamics. This is likely to be related to the different target poisoning mechanisms and target surface chemical, physical and electrical properties. ‘Sensor vs. time’ hysteresis loop presentation approach has been proposed. This approach allows the full extent of the hysteresis revealed and appreciated with extra important information on the process features gained.
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Acknowledgments Help from professors Allan Matthews at the University of Sheffield and Peter Kelly at Manchester Metropolitan University providing access to HIPIMS power supplies is acknowledged with thanks. References [1] S.P. Bugaev, K.V. Oskomov, V.G. Podkovyrov, S.V. Smaikina, N.S. Sochugov, Russ. Phys. J. 42 (1999) 1032. [2] V. Kouznetsov Patent 1998. [3] V. Kouznetsov, K. Macak, J.M. Schneider, U. Helmersson, I. Petrov, Surf. Coat. Technol. 122 (1999) 290. [4] K. Macak, V. Kouznetsov, J. Schneider, U. Helmersson, I. Petrov, J. Vacuum Sci. Technol. A Vacuum Surf. Films 18 (2000) 1533. [5] U. Helmersson, M. Lattemann, J. Bohlmark, A.P. Ehiasarian, J.T. Gudmundsson, Thin Solid Films 513 (2006) 1. [6] E. Hollands, D.S. Campbell, J. Mater. Sci. 3 (1968) 544. [7] D. Depla, A. Colpaert, K. Eufinger, A. Segers, J. Haemers, R. De Gryse, Vacuum 66 (2002) 9.
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Investigation of reactive high power impulse magnetron sputtering processes using various target material–reactive gas combinations Martynas Audronis ⁎, Victor Bellido-Gonzalez Gencoa Ltd, Physics Road, Speke, L24 9HP, UK
a r t i c l e
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Article history: Received 10 October 2010 Accepted in revised form 23 December 2010 Available online 31 December 2010 Keywords: High power impulse magnetron sputtering HIPIMS Reactive sputtering Plasma optical emission monitoring Process control
a b s t r a c t The two most important issues limiting reactive high power impulse magnetron sputtering (HIPIMS) process applicability until recently were the absence of suitable reactive HIPIMS control means and the limited capability of HIPIMS power supplies in terms of arc handling. The significant advancement has been made recently by the development of the optical plasma monitoring (PM)-based process control technology for reactive HIPIMS [Surface & Coatings Technology 204 (2010) 2159–2164]. The initial studies of reactive HIPIMS processes however have only covered Ti–O2 target material–reactive gas system. In this paper the recently developed PM-based active feedback control technology was applied to explore further reactive HIPIMS processes now using a variety of different target material and reactive gas combinations. Data for hysteresis behaviour and process control using either PM or constant gas flow methods for Ti–O2, Ti–CO2, Cr–O2, Cr–C2H4, Al–O2, and Zn:Al–O2 material–gas systems is presented and compared. In all cases the processes were found to exhibit hysteresis behaviour. The magnitude and features of the hysteresis loop were found to depend strongly on a particular metal–reactive gas pair. Similar to AC and DC reactive sputtering processes the hysteresis behaviour in reactive HIPIMS was found to be more pronounced for the gases that have high chemical affinity for a metal sputter target. The PM-based process control technology monitoring either metal or gas plasma emissions was shown to provide accurate control and stable operation of reactive HIPIMS discharges. © 2011 Elsevier B.V. All rights reserved.
1. Introduction HIPIMS is a plasma-based physical vapour deposition (PVD) process that is able to provide a highly ionised flux of sputtered species [1–4]. It is thought to be particularly important for applications where there is a need to coat 3D features (e.g. vias and trenches in semiconductor applications) [5]. HIPIMS may have other added benefits, as compared to DC or medium frequency AC/pulse-DC magnetron sputtering, related to different coating structure–property relationship control through self-species (sputtered metal) plasma/ion assistance. In reactive sputtering [6] the hysteresis1 effect originates from the reaction of a target surface with a reactive gas fed into a sputtering system [7–16]. It is recognised that two processes/mechanisms are at the core of this phenomenon: i) chemisorption — leading to the formation of a compound layer on the target surface and ii) ion implantation — leading to a compound formation within the target ⁎ Corresponding author. Tel.: + 44 151 486 4466; fax: + 44 151 486 4488. E-mail addresses: [email protected] (M. Audronis), [email protected] (V. Bellido-Gonzalez). 1 The definition of the word ‘hysteresis’ is commonly accepted as the lagging of an effect behind its cause. That is hysteresis refers to systems that have memory, where the effects of the current input (or stimulus) to the system are experienced with a certain delay in time. (Source: www.thefreedictionary.com and www.wikipedia.org. Accessed on the 8th of August 2010). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.12.038
surface. Implantation depth of the reactive species (such as oxygen and nitrogen), at typical direct current (DC) or radio frequency (RF) reactive sputtering conditions, reaches several nanometres (e.g. 7 nm in case of Si sputtering in Ar/O2 atmosphere [17] and 2.5 to 4 nm in case of Ti and Si sputtering, respectively, in Ar/N2 atmosphere [17,18]), which significantly exceeds the thickness of the adsorbed monolayer. As the partial pressure (p.p.) of reactive gas is increased, ion implantation surface compound formation mechanism becomes dominant, as for example shown in reference [9]. Chemisorption of the reactive gas during reactive HIPIMS (R HIPIMS) takes place in a similar way as it happens during reactive alternating current (AC) or DC sputtering, while the reactive ion irradiation induced target surface poisoning mechanism is pronounced even more significantly due to very high peak voltages exhibited by HIPIMS [19]. References [19,20] report the observation of the hysteresis effect in R HIPIMS by employing a PM-based technology. It appears that reactive HIPIMS process has all the features of a typical reactive AC or DC sputtering process i.e. it exhibits a sudden transition from ‘metal’ to ‘compound’ target state, arcing and hysteresis effect (Fig. 1). Due to these similarities reactive HIPIMS processes are also unstable and require active feedback process control systems to ensure process stability. Constant reactive gas flow control method does not lead to a stable reactive HIPIMS deposition process [20]. Reference [19] also shows that the target pulsing parameters, such as frequency, magnitude
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Fig. 1. A comparison of the hysteresis curves (sensor signal vs. O2 flow) observed during reactive (a) DC sputtering and (b) HIPIMS of Ti in Ar/O2 [20].
of voltage pulses, and duty cycle influence the overall shape of the hysteresis loop. It was also found that in reactive HIPIMS the sputter target state/oxidation level depends strongly on the history of its use [21]. This effect appears to be pronounced strongly due to the significance of reactive ion irradiation induced sputter target poisoning processes. The target sputter cleaning times (e.g. after a reactive deposition run) can be long (compared to DC sputtering case) because of the same reason. It was also shown that the hysteresis loop can in some cases be artificially suppressed, with the consequential substantial reduction of the reactive deposition process window (can be a number of times; e.g. four times as it was demonstrated for 325 Hz/50 μs pulsing case when sputtering Ti in Ar/O2), if the starting target surface is not clean [21]. Until recently, the two most important issues limiting the use of reactive HIPIMS were the absence of suitable control means and the limited capability of HIPIMS power supplies in terms of arc handling. Significant advancements regarding the process control have been made by the development of the optical PM-based process control technology for reactive HIPIMS [20]. This initial work however only concerned R HIPIMS within the Ti–O2 system. The aim of this work was therefore to use the recently developed PM-based active feedback control technology to explore and shed some light on more R HIPIMS processes. In this paper results on the following target material and reactive gas combinations are reported: Ti–O2, Ti–CO2, Cr–O2, Cr–C2H4, Al–O2 and Zn:Al–O2. These material–reactive gas pairs when used in a deposition system can produce technological thin films that are used in a wide range of markets for various applications. Examples are: TiO2 — self cleaning coatings on glass and high refractive index optical layers in multilayer films, metal oxycarbides — decorative coatings, Cr2O3 – optical and hard wear-resistant coatings, chromium carbide — hard wear-resistant coatings for machining and/or forming tools, Al2O3 — optical and/or hard wear-resistant coatings, and aluminium doped zinc oxide (AZO) — transparent conducting oxide films for semiconductor and photovoltaic applications. Hysteresis behaviour and reactive process control of the abovementioned processes are investigated and discussed. Hysteresis evaluation method is discussed with a new approach to presenting hysteresis effect for reactive sputtering processes proposed. 2. Experimental procedure The GenLab coating deposition system has been used for experiments and is described in detail in [20]. 99.5% pure Ti, Cr, Al (188 mm × 296 mm × 9.5 mm) and Zn:Al (2 wt.% Al) targets were reactively sputtered using an essentially balanced planar rectangular magnetron. Base vacuum, better than 0.002 Pa, was attained prior to reactive sputtering experiments. The working pressure was maintained between 0.2 and 1 Pa during all runs. Gencoa Ltd active
feedback reactive sputtering process controller Speedflo™, the version that is designed for HIPIMS process, was used to control reactive gas flow. PM using 330 nm (Ti+ OES lines), 520 nm (Cr⁎ OES lines), 395 nm (Al⁎ OES lines) and 780 nm (O⁎ OES lines) filters (10 nm bandwidth) was employed for reactive HIPIMS experiments. A Chemfilt Ionsputtering AB and Huettinger Gmbh HIPIMS power supplies were used as target sputter power sources. Power applied to the targets was between 2500 and 3000 W in the cases of Ti, Al and Cr targets and 1000 W in the case of Zn target. The sputtering trials were carried out at target pulsing frequencies of 325 Hz (Ti), 600 Hz (Ti, Cr, and Al) and 60 Hz (Zn) and the pulse-on times were 50, 50 and 200 μs respectively. The reactive HIPIMS hysteresis plots were generated by ramping reactive gas (O2, CO2 and C2H4) flow up at a constant rate to a certain value and then ramping it down back to zero while recording the PM sensor response at the same time. Such a reactive sputtering process characterisation procedure is practised routinely in vacuum coating industry as well as in certain academic departments. 3. Results 3.1. Ti–O2 R HIPIMS process Fig. 2a shows a ramp of O2 gas in an Ar/O2 Ti HIPIMS discharge (at a pulse frequency of 325 Hz and an average power of 2500 W) and a response of the PM sensor that is monitoring Ti+ emissions using a 330 nm filter. The sensor response plot against time contains valuable information on the process characteristics (e.g. dynamics of target depoisoning processes after O2 flow rate had become 0 sccm) in the time domain. This information is not available when the sensor signal is plotted conventionally i.e. against the reactive gas flow rate. Fig. 2b shows a conventional hysteresis loop plot (PM signal vs. O2 flow). In this figure and Fig. 2a the optical emission Ti+ signal correlates to the sputtered flux of Ti+ ions. It becomes smaller (i.e. less intense PM signal) when the target gets oxidised due to lower sputter rate of the oxide material. The same is valid for many other excited and/or ionised metal optical emission lines. The data shown in Fig. 2a and b, in terms of reactive sputtering process features, is very similar to that typically observed for AC and DC reactive sputter deposition processes. More extensive discussion on this can be found in reference [20]. Fig. 2c shows examples of reactive HIPIMS Ti–O2 deposition runs using a PM-based active feedback process control. Ti+ optical plasma emission at 330 nm is monitored and used as a signal for the active feedback control of the process. It can be seen that the method used provides accurate control (i.e. operation anywhere between the ‘metallic’ and ‘poisoned’ sputter target states can be readily obtained and maintained) of this complicated process, also providing the
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Fig. 2. R HIPIMS of Ti–O2 at 325 Hz (50 μs ON time) ~2500 W average power: (a) oxygen flow ramp and associated response of PM sensor signal (monitoring Ti+ using a 330 nm filter); (b) hysteresis plot (Ti+ PM signal vs. O2 flow); (c) process control examples at different set-points (percentages are shown on the right hand side) using the PM-based (monitoring 330 nm Ti+ emission) reactive sputtering control technology; (d) constant flow process control at different oxygen flow set-points (monitoring Ti+ emission using a 330 nm filter).
required process stability. Therefore, it appears that this process control technology will enable deposition of titanium oxide films with controlled chemical composition, structure and properties. Long term (N3 h) performance of controlled R HIPIMS Ti–O deposition has been demonstrated earlier and the data has been made public in reference [20]. Contrary to results shown in Fig. 2c, Fig. 2d shows data obtained from the constant reactive gas flow experiment and demonstrates that fixed flow rate R HIPIMS process control method does not provide a stable deposition process. A continuous drift from the ‘metal’ target state towards the ‘compound’ target state occurs and its rate depends on the oxygen flow rate. Such unstable and unpredictable process could hardly be used for production/research purposes, unless the target is operated in a fully poisoned mode or the hysteresis effect is eliminated by other means, such as very high pumping speed. Reactive HIPIMS processing in a fully poisoned mode is however substantially disadvantageous not only because of lower sputter and deposition rates, poorer film properties and stoichiometry, but also because of severe arcing that usually takes place at these conditions. Influence of negatively charged ions that originate at the target surface [22–24] has to be taken into account too, as the flux of negatively charged species will depend on the operating set-point on the hysteresis loop. 3.2. Ti–CO2 R HIPIMS process Fig. 3a shows a ramp of CO2 gas in an Ar/CO2 Ti HIPIMS discharge (at a pulse frequency of 600 Hz and an average power of 3000 W) and a response of two PM sensors monitoring Ti+ and O⁎ emissions using 330 nm and 780 nm filters, respectively. Fig. 3b shows conventional hysteresis loop plots (PM signals vs. CO2 flow). Again, the hysteresis
behaviour can be clearly observed using both, metal and reactive gas optical emission lines. Both signals are equally good for process monitoring and control purposes [PM O⁎ signal correlates to partial pressure (p.p.) of O2 within the vacuum vessel while PM Ti+ signal correlates to Ti+ flux]. Monitoring a reactive gas optical emission line instead of a metal line is an often practised reactive sputtering process control technique used for oxide thin film deposition. The difference, as compared to monitoring ‘metal’ line, is in the direction of signal change upon introduction of reactive gas to the system. Also, it can be harder, but not impossible, to notice when the target state changes from ‘transition’ to ‘compound’. Essentially, process behaviour during ‘poisoning’ and ‘depoisoning’ phases in Ti–CO2 case appears to be very similar to that observed for Ti–O2. This is due to the fact that CO2 gas is decomposed by the HIPIMS discharge into components one of which is O (as evidenced by an O⁎ optical emission line), which then reacts with Ti preferentially due to higher affinity of O2 to Ti. It is not clear at the moment what other constituents CO2 gas is broken to and will be a subject of further studies. Fig. 3c and d shows examples of the PM-based reactive HIPIMS Ti–CO2 deposition process control. Here Ti+ and O⁎ optical plasma emissions are monitored and used as signals for the active feedback control of the process. It can be seen that accurate control of reactive HIPIMS Ti–CO2 process can be obtained and the deposition process stability is acceptable. 3.3. Cr–O2 R HIPIMS process Fig. 4a shows a ramp of O2 gas in an Ar/O2 Cr HIPIMS discharge (at a pulse frequency of 600 Hz and an average power of 3000 W) and a response of PM sensor monitoring Cr⁎ emission using a 520 nm
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Fig. 3. R HIPIMS of Ti–CO2 at 600 Hz (50 μs ON time) ~3000 W average power: (a) CO2 flow ramp and associated response of PM sensors (monitoring Ti+ using a 330 nm filter and O⁎ using a 780 nm filter); (b) hysteresis plot (Ti+ and O⁎ PM signals vs. CO2 flow); (c) process control at different set-points using a PM-based reactive sputtering control technology (monitoring Ti+ emission using a 330 nm filter); (d) process control at different set-points using a PM-based reactive sputtering control technology (monitoring O⁎ emission using a 780 nm filter).
filter, respectively. Fig. 4b shows a conventional hysteresis loop plot (PM Cr⁎ signal vs. O2 flow). The hysteresis behaviour can be clearly observed. It is interesting to find from Fig. 4a and b that in Cr–O2 system the transition (‘poisoning’ phase) region exhibits at least four regions (marked as A, B, C and D) where target poisoning dynamics, or processes taking place at the target surface, are substantially different. This has not been observed for any other material gas combination investigated in this study (except for maybe Al–O2). No explanation to such behaviour can be offered at present. Yet another
feature that has been observed for Cr–O2 system is severe arcing during the sputter target ‘depoisoning’ phase (close to ‘metal’ state) as can be seen in Fig. 4a. No such arcing has been observed for Ti–O2/ CO2 processes. Fig. 5 shows an example of the PM-based R HIPIMS Cr–O2 deposition process control. Cr⁎ optical plasma emission (at 520 nm) is monitored and used as a signal for the process feedback control. Again, accurate control and good stability of R HIPIMS Cr–O2 process is obtained.
Fig. 4. R HIPIMS of Cr–O2 at 600 Hz (50 μs ON time) ~3000 W average power: (a) oxygen flow ramp and associated response of PM sensor signal (monitoring Cr⁎ using a 520 nm filter); (b) hysteresis plot — Cr⁎ PM signal vs. O2 flow.
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Fig. 5. Process control of R HIPIMS using Cr–O2 at different set-points (monitoring Cr⁎ emission using a 520 nm filter).
3.4. Cr–C2H4 R HIPIMS process Fig. 6a shows another example of a reactive gas ramp and associated PM sensor signal in HIPIMS plasma discharge (monitoring Cr⁎ at 520 nm) obtained for Cr–C2H4 system at a pulse frequency of 600 Hz and an average power of 3000 W (using Chemfilt Ionsputtering AB Sinex-3 power supply). Similar to the Ti–O2 case, as discussed in Section 3.1., the PM signal in this example represents the sputtered flux of Cr atoms. Hence, the signal becomes smaller when the target is poisoned due to lower sputter rate of a compound material formed on
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the target surface. It can be seen from Fig. 6b that in the Cr–C2H4 case the hysteresis effect is relatively less strong as compared to the Ti–O2/CO2 processes. This is due to the fact that ethylene (C2H4) is less reactive than O2. In any case reactive Cr–C2H4 HIPIMS process still exhibits the hysteresis effect and relatively fast transition between the different target states. This means that chemisorption of reactive gas and implantation of its ionised constituents into the target surface is taking place, which also means that implementation of a stable coating production process is very likely to require the use of an active feedback process control system. Fig. 6c shows examples of the PM-based reactive HIPIMS Cr–C2H4 deposition process control. 520 nm Cr⁎ optical plasma emission is monitored and used as a signal for the active feedback control of the process. It can be seen that an accurate coating deposition process control can be readily obtained and maintained and the operation anywhere between the ‘metallic’ and ‘compound’ sputter target states is provided. The process appears to be very stable too. Hence, the R HIPIMS control technology used here will enable the deposition of chromium (or other metal) carbide films with controlled chemical and physical properties. Fig. 6d demonstrates poor process stability when C2H4 flow is constant. It can be seen from this figure that accurate control can be difficult to obtain. 3.5. Al–O2 R HIPIMS process Fig. 7a shows a ramp of O2 gas in an Ar/O2 Al HIPIMS discharge (at a pulse frequency of 600 Hz and an average power of 3000 W) and a response of a PM sensor monitoring Al⁎ emission at 395 nm. It can be seen from this figure that severe arcing took place just below the 60%
Fig. 6. HIPIMS of Cr–C2H4 at 600 Hz (50 μs ON time) ~ 3000 W average power: (a) ethylene flow ramp and associated response of PM sensor signal (monitoring Cr⁎ using a 520 nm filter); (b) hysteresis plot — 520 nm Cr⁎ PM signal vs. C2H4 flow; (c) process control examples at different set-points (percentages are shown on the right hand side) using the PM-based (monitoring 520 nm Cr⁎ emission) reactive sputtering control technology; (d) constant flow process control at different flow set-points (monitoring Cr⁎ emission using a 520 nm filter).
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Fig. 7. HIPIMS of Al–O2 at 600 Hz (50 μs ON time) ~ 3000 W average power: (a) oxygen flow ramp and associated response of PM sensor signal (monitoring Al⁎ using a monochromator tuned to 395 nm); (b) slow O2 flow ramp and associated response of PM sensor signal (Al⁎ at 395 nm); (c) process control at 70% set-point using a PM-based reactive sputtering control technology (monitoring Al⁎ emission at 395 nm); (d) constant flow process control at different flow set-points (monitoring Al⁎ emission at 395 nm).
mark of the PM signal. Hence, the full information on the target ‘poisoning’/‘depoisoning’ processes has not been obtained. Slow oxygen ramp was carried out in order to try to obtain more information (Fig. 7b). It, however, only confirmed the fact that at ~55% PM Al⁎ signal the process becomes unstable due to uncontrolled severe arcing which is followed by a shutdown of the power supply. It was still possible to obtain reactive process control at Al PM signal N60% as demonstrated in Fig. 7c. In contrast, Fig. 7d shows that constant oxygen flow does not yield a stable aluminium oxide deposition
process. This is in good agreement with the data provided in earlier sections of this paper on other R HIPIMS processes. 3.6. Zn:Al–O2 R HIPIMS process Fig. 8a and b shows examples of an oxygen ramp and a hysteresis plot (monitoring O⁎ PM signal at 780 nm and plotting it against O2 flow) obtained for reactive HIPIMS of Zn:Al at a pulse frequency of 60 Hz and an average power of 1000 W (using Huettinger TruPlasma
Fig. 8. HIPIMS of Zn:Al(2 wt.%) in Ar/O2 atmosphere at 60 Hz (200 μs ON time) ~1000 W average power: (a) oxygen flow ramp and associated response of PM sensor signal (monitoring O⁎ using a 780 nm filter); (b) hysteresis plot (PM signal at 780 nm vs. O2 flow).
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Fig. 9. An example of reactive HIPIMS of AZO deposition (metal Zn:Al target) process control at a 30% set-point using the PM-based reactive sputtering control technology.
power supply). Preliminary trials of reactive Zn:Al HIPIMS process have revealed that this process is prone to intense arcing. It was also found that in this particular situation, the gas signal (O⁎) was less disturbed (noisy) by arcing events than that of metal (Zn+) and hence the control was better using the O⁎ 780 nm filter, as described in the section below. Fig. 9 shows an example of the PM-based R HIPIMS AZO deposition process control. 780 nm O2⁎ optical plasma emission is monitored and used as a signal for the active feedback control of the process. It is shown again that the developed method provides accurate control of R HIPIMS coating deposition process, also providing good process stability. Ability to operate anywhere within the hysteresis loop will enable deposition of AZO films by HIPIMS with controlled compositional, optical, electrical and structural properties. The PM signal roughness during control seen in Fig. 9 is resulted by arcing. A better HIPIMS power supply with a good arc handling circuitry would be required in order to result in even smoother deposition process. 4. Discussion Results produced by this work and described in the previous section of this paper demonstrate rather clearly that all the processes studied were found to exhibit hysteresis behaviour. It is also revealed by the results that the magnitude of the hysteresis loop depends on the particular metal target–reactive gas pair (we neglect here other variables that are well known to the reactive sputtering community and affect the hysteresis effect, such as pumping speed [12,14]). Similar to conventional AC and DC reactive sputtering processes the hysteresis loop in R HIPIMS is more pronounced for gases that have high chemical affinity for the metal sputter target. Interestingly, different sputter target material–reactive gas systems can have significantly different target ‘poisoning’–‘depoisoning’ process dynamics, which is likely to be related to different target poisoning mechanisms and target surface chemical, physical and electrical properties. These processes and mechanisms are however yet to be studied in order to gain better understanding of R HIPIMS. The process control examples demonstrate that the PM-based active feedback control technology (monitoring either metal or gas plasma emissions) used in this study is able to provide accurate control and stable operation of R HIPIMS discharges. 4.1. Hysteresis loop plot: ‘sensor vs. flow’ or ‘sensor vs. time’? In this section authors describe a hysteresis plot methodology that is different from the conventionally used. The traditional approach is to plot a sensor signal (or deposition rate) vs. reactive gas flow [10,15], which is perfectly suitable in most reactive AC or DC
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Fig. 10. Hysteresis curves for reactive DC and HIPIMS Ti–O2 processes obtained by plotting sensor signals against time as opposed to flow rate.
sputtering situations. Experimental evidence gathered in this and previous [19–21] studies however suggest that in certain R HIPIMS cases such an approach may conceal the important information about the process that is being investigated. For example Fig. 2b shows a conventional R HIPIMS hysteresis loop plot for the Ti–O2 system. The problem with this plot is that when reactive gas rampup/ramp-down procedure is over and reactive gas flow is 0 sccm, the sensor signal has not fully recovered yet and is still changing. The only direction that the sensor signal can now be plotted is upwards. This unfortunately conceals the full extent of the hysteresis effect. This is due to the fact that target depoisoning during R HIPIMS can be much slower, as discussed earlier [19], hence longer time delay. In certain R HIPIMS cases this is a significant portion of the hysteresis loop. The above is demonstrated clearly in Fig. 10 where the sensor signal is plotted against time during R HIPIMS Ti–O2 process. Data for reactive DC sputtering at otherwise equivalent process conditions is included for comparison. Since the reactive gas flow rate is constant and there is a direct and linear relationship with time, ‘sensor vs. time’ approach is essentially very similar to the conventional method and is therefore perfectly valid. It is also clear from this example that plotting hysteresis loop in the way that authors of this paper propose, the full extent of the R HIPIMS (and likely other processes with long delay time) hysteresis can be revealed and appreciated with no information lost and extra important information on process features gained.
5. Conclusions The recently developed plasma monitoring-based active feedback control technology for reactive HIPIMS has been applied to explore processes using different target materials and gaseous environments. It has been shown that it provides means to characterise and control accurately reactive HIPIMS processes. All the processes studied were found to exhibit hysteresis behaviour. The magnitude of the hysteresis loop depends on the particular metal target–reactive gas pair. Similar to AC and DC reactive sputtering processes the hysteresis loop in R HIPIMS is more pronounced when gases that have high chemical affinity for the metal sputter target are used. Different sputter target material– reactive gas systems can have quite different target ‘poisoning’–‘depoisoning’ process dynamics. This is likely to be related to the different target poisoning mechanisms and target surface chemical, physical and electrical properties. ‘Sensor vs. time’ hysteresis loop presentation approach has been proposed. This approach allows the full extent of the hysteresis revealed and appreciated with extra important information on the process features gained.
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