coatings - MDPI

coatings Article

Target Voltage Hysteresis Behavior and Control Point in the Preparation of Aluminum Oxide Thin Films by Medium Frequency Reactive Magnetron Sputtering Qingfu Wang *,† , Liping Fang *,† and Hong Xiao

ID

, Qinghe Liu, Lin Chen, Qinguo Wang, Xiandong Meng

Institute of Materials, China Academy of Engineering Physics, Mianyang 621700, China; [email protected] (Q.L.); [email protected] (L.C.); [email protected] (Q.W.); [email protected] (X.M.); [email protected] (H.X.) * Correspondence: [email protected] (Q.W.); [email protected] (L.F.) † These authors contribute equally to this work. Received: 16 March 2018; Accepted: 16 April 2018; Published: 18 April 2018

Abstract: Aluminum oxide thin films were prepared by medium frequency reactive magnetron sputtering. The target voltage hysteresis behavior under different argon partial pressure and target power conditions were studied. The results indicate that the target voltage hysteresis loop of aluminum oxide thin film preparation has typical behavior of that for reactive sputtering deposition of compound films. The target voltage feedback control approach was applied to circumvent the hysteresis problem. The microstructure and chemical composition of the aluminum oxide thin films prepared at different target voltage control points were investigated by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and Auger electron spectroscopy. The results indicated that the prepared aluminum oxide thin films, which are compact and mostly amorphous, can be obtained with target voltage control point in the range of 25~35%. Keywords: aluminum oxide; thin film; medium frequency; magnetron sputtering; target voltage; hysteresis loop

1. Introduction Aluminum oxide is a widely used functional thin film in the fields of optics, microelectronics and mechanisms, due to its excellent chemical stability, mechanical strength, high hardness, anti-corrosion and anti-wear properties and optical properties [1–5]. As a surface protective coating for highly active metals, aluminum oxide film bears substantial potential to enhance the wear and corrosion resistance of metal surfaces by forming multilayered aluminum/aluminum oxide coatings, with the possible merits of higher density, better adhesion properties, and lower residual stress than the two single layer coatings alone [1,2,6,7]. In this work we report our investigations on the preparation of aluminum oxide thin films deposited by reactive magnetron sputtering, which is a common technique in the preparation of compound thin films and coatings. Reactive magnetron sputtering is realized in magnetron sputtering with a reactive gas (such as oxygen or nitrogen) introduced into the vacuum chamber, in addition to an inert working gas [8–12]. There are several reactive magnetron sputtering techniques, i.e., direct current reactive magnetron sputtering (DCRMS) [10], pulsed reactive magnetron sputtering (PMS) [11], radio frequency reactive magnetron sputtering (RFRMS) [12] and medium frequency reactive magnetron sputtering (MFRMS) [9], which are ordered according to the frequency of the power supply of the sputtering target. The disadvantages of DCRMS are target poisoning and arc problems, and the prepared films

Coatings 2018, 8, 146; doi:10.3390/coatings8040146

www.mdpi.com/journal/coatings

Coatings 2018, 8, 146

2 of 11

often have large embedded particles, which renders the deposition processes hard to control and reproduce [13]; the demerits of RFRMS are the high-cost of equipment and low deposition rates [14]; thus neither of these two techniques is suitable for the preparation of high quality thin films and coatings at an industrial level. High power impulse magnetron sputtering (HiPIMS) is an emerging reactive magnetron sputtering technique, which could produce high density films [15–17]. However, this technique still has several aspects which need to be improved: the high cost of the power supply, the instability of the plasma, low deposition rate, and self-sputtering, etc. MFRMS uses a twin sputtering targets, which alternatively act as the anode and cathode. It is therefore able to effectively circumvent arc discharge and bypass the phenomenon of the disappearing anode [18]. Thus, stable deposition processes could be achieved with high reproducibility and high deposition rate. In this work, MFRMS is utilized to prepare aluminum oxide films to ensure the stability and reproducibility. There are several parameters that could affect the processes of reactive magnetron sputtering, such as argon partial pressure, target power, target voltage, etc. Although the argon partial pressure and the target power could be easily controlled, the surface condition of the sputtering target varies in sputtering processes. Furthermore, we have noticed that under identical argon partial pressure and target power conditions, the target voltage falls as the thickness of the sputtering target decreases; this is possibly due to the enhanced magnetic field strength near the target surface [19]. During reactive magnetron sputtering, the target voltage changes as the variation of the flow rate of the reactive gas. The variation curves of the target voltage do not overlap between gradually increasing and decreasing the flow rate of the reactive gas, i.e., target voltage hysteresis behavior exists [18]. Three sputtering modes exist under different target voltages, i.e., the metallic mode, the mixed (or transition) mode, and the compound mode. The structure and composition of the films obtained under these three modes are quite different. In order to obtain thin films with high deposition rates and optimum properties, several approaches have been proposed to circumvent the hysteresis problem, such as: increasing the pumping speed [20], using the optical emission spectroscopic signal from the sputtered material, using the target voltage or direct mass spectrometry measurements as the feedback signals to directly control the partial pressure of the reactive gas during reactive sputtering deposition [18], etc. Among these approaches, the target voltage control seems probably to be the most effective approach, since the flow rate of the reactive gas could be stabilized at a desired level via the feedback control of the target voltage. Many studies exist for the hysteresis behavior of the target voltage, but most of them use manual control of the reactive gas flow (see [21] and references therein). Therefore, not enough data points could be obtained, and the effective voltage control points are not optimized. The obtained thin films therefore show significant fluctuations of the film structure and chemical composition. In this work, a closed-loop feedback controller is used in the reactive sputtering deposition processes of aluminum oxide, which could measure the voltage hysteresis loops with high accuracy. Based on these hysteresis loops, as well as the structure and composition properties of the obtained aluminum oxide films, the optimal range of the target voltage control point was obtained. 2. Materials and Methods 2.1. Sample Preparation A home-made magnetron sputtering system equipped with two circular planar Al (99.99% purity) sputtering targets was used to prepare the aluminum oxide films on Si (111) substrates. The two sputtering targets were powered by a 40 kHz medium frequency power source (Advanced Energy PEII, Fort Collins, CO, USA). The vacuum chamber was made of 304 stainless steel and sealed by fluororubber. The chamber was evacuated by two turbomolecular pumps backed by a roots blower and a mechanical pump. The setup of the deposition system is shown in Figure 1. The Si substrates were cleaned with 1% hydrofluoric acid solution for 5 min to remove surface oxides, and then rinsed

Coatings 2018, 8, 146

3 of 11

in de-ionized water and ethanol in an ultrasonic bath for 10 min each. Before deposition, the Al targets were pre-sputtered for 15 min to remove the surface oxide and contaminations. The base pressure of the sputtering chamber is lower than 5.0 × 10−4 Pa, and the working gases are argon (99.99% Coatings 2018, 8, x FOR PEER REVIEW 3 ofpurity) 11 and oxygen (99.99% purity). The partial pressure of argon was maintained at 0.3 Pa during all the film Coatings 2018, 8, x FOR PEER REVIEW of 11 (99.99% purity) and oxygen (99.99% purity). The partial pressure of argon was maintained at3a0.3 Pa deposition processes, unless otherwise indicated. The flow of oxygen was controlled via reactive during all the film deposition processes, unless otherwise indicated. The flow of oxygen was sputtering controller (Speedflo mini, Gencoa Ltd., Liverpool, UK), equipped with a digital mass flow (99.99% purity) oxygen (99.99%controller purity). The partial mini, pressure of argon maintained at 0.3 Pa controlled via aand reactive sputtering (Speedflo Gencoa Ltd., was Liverpool, UK), equipped controller (EL-FLOW, Bronkhorst High-Tech B.V., Ruurlo, The Netherlands). The sputtering target during all the film deposition processes, unless otherwise indicated. The flow of oxygen was was with a digital mass flow controller (EL-FLOW, Bronkhorst High-Tech B.V., Ruurlo, The Netherlands). fixed controlled at 1.0~1.5via kWa and the sputtering target-substrate distance was kept at 180Ltd., mm.Liverpool, The deposition time for all reactive mini, Gencoa UK),at equipped The sputtering target was fixed at controller 1.0~1.5 kW(Speedflo and the target-substrate distance was kept 180 mm. of thewith samples was 60 min. a digital mass flow controller (EL-FLOW, Bronkhorst High-Tech B.V., Ruurlo, The Netherlands). The deposition time for all of the samples was 60 min. The sputtering target was fixed at 1.0~1.5 kW and the target-substrate distance was kept at 180 mm. The deposition time for all of the samples was 60 min. MF Power Reactive Sputter MF Power Reactive Controller Sputter Target Target Controller Target

Target

Substrate

MFC

O2

MFC MFC

OAr 2

Ar MFC Figure 1. Schematic of the MFRMS system for aluminum oxide film preparation. Substrate

Figure 1. Schematic of the MFRMS system for aluminum oxide film preparation.

Figure 1. Schematic of Measurements the MFRMS system for aluminum oxide film preparation. 2.2. Target Voltage Hysteresis Loop

2.2. Target Voltage Hysteresis Loop Measurements

The flow of oxygen in Loop reactive sputtering was controlled by the reactive sputtering controller. 2.2. Target Voltage Hysteresis Measurements

For flow targetofvoltage hysteresis loopsputtering measurements, flow curve wassputtering set to follow the The oxygen in reactive was the controlled by of theoxygen reactive controller. Theshown flow ofinoxygen reactive sputtering was first controlled by the reactive controller. pattern Figure in 2. loop The flow rate of oxygen increased, 0 to 50sputtering sccm cubic the For target voltage hysteresis measurements, the flow curvefrom of oxygen was(standard set to follow For target voltage hysteresis loop measurements, theand flow curve of oxygen setattosimilar follow rate. the centimeters minute) at the rate of of 25 oxygen sccm/min, then decreased, 0was sccm pattern shown inper Figure 2. The flow rate first increased, from 0 to to 50 sccm (standard cubic pattern shown in Figure 2. The flow rate of oxygen first increased, from 0 to 50 sccm (standard cubic The target voltage was recorded automatically as the variation of the oxygen flow rate. centimeters per minute) at the rate of 25 sccm/min, and then decreased, to 0 sccm at similar centimeters per minute) at the rate of 25 sccm/min, and then decreased, to 0 sccm at similar rate. rate. The target voltage waswas recorded automatically ofthe theoxygen oxygen flow rate. The target voltage recorded automaticallyas asthe the variation variation of flow rate. 50

O2 flowO(sccm) flow (sccm)

40 50 30 40

2

20 30 10 20 0 10 0

0 0

1 1

2 3 Time (min) 2

3

4 4

Figure 2. Flow curve of oxygen during Time target voltage hysteresis loop measurements. (min) Figure 2. Flow curve oxygenduring during target target voltage loop measurements. 2.3. Film Characterization Figure 2. Flow curve of of oxygen voltagehysteresis hysteresis loop measurements.

TheCharacterization structure of the prepared aluminum oxide films were characterized by grazing incidence 2.3. Film

2.3. Film Characterization X-ray diffraction (XRD, Phillips X’Pert Pro, New York City, NY, USA) with Cu Kα radiation and

The structure of the prepared aluminum oxide films were characterized by grazing incidence incident angle of 2° in the θ–2θ mode. The surface morphology and chemical composition were

The structure of the prepared were by grazing incidence X-ray diffraction Phillipsaluminum X’Pert Pro,oxide Newfilms York City, characterized NY,(FESEM, USA) with Kα radiation andX-ray characterized by (XRD, a field-emission scanning electron microscope FEICu Helios Nanofab 600i, diffraction (XRD, Phillips X’Pert Pro, New York City, NY, USA) with Cu Kα radiation and incident incident angle of 2° in theequipped θ–2θ mode. and chemical composition were Lausanne, Switzerland) withThe an surface energy morphology dispersive X-ray spectroscope (EDS, Bruker ◦ in the θ–2θ anglecharacterized of 2 mode. The surface morphology and chemical composition were characterized by a field-emission scanning electron microscope (FESEM, FEI Helios Nanofab 600i, QUANTAX 100, Berlin, Germany). The surface chemical composition was characterized by scanning Switzerland) equipped with an energy dispersive X-ray spectroscope (EDS, Bruker by a Lausanne, field-emission scanning electron microscope (FESEM, FEI Helios Nanofab 600i, Lausanne, Auger electron spectrometry (AES, Physical Electronics PHI-650, Chanhassen, MN, USA). The film QUANTAX 100, Berlin, Germany). surface chemical was characterized by scanning 100, Switzerland) with anaenergy dispersive X-ray composition spectroscope (EDS, Bruker QUANTAX thicknessequipped was measured by stylusThe profiler (KLA-Tencor P-7, Milpitas, CA, USA). Auger electron spectrometry (AES, Physical Electronics PHI-650, Chanhassen, MN, USA). The film Berlin, Germany). The surface chemical composition was characterized by scanning Auger electron thickness was measured by a stylus profiler (KLA-Tencor P-7, Milpitas, CA, USA). spectrometry (AES, Physical Electronics PHI-650, Chanhassen, MN, USA). The film thickness was measured by a stylus profiler (KLA-Tencor P-7, Milpitas, CA, USA).

Coatings 2018, 8, 146

4 of 11

Coatings 2018, 8, x FOR PEER REVIEW

4 of 11 4 of 11

Coatings 2018, 8, x FOR PEER REVIEW 3. Results

3. Results 3. Results 3.1. Target Voltage Hysteresis Loop 3.1. Target Voltage Hysteresis Loop 3.1. Target Voltage Hysteresis Loop deposition process, the variation of target voltage according to During the reactive sputtering During the reactive sputtering deposition process, the variation of target voltage according to changes in the oxygen flow rate demonstrates hysteresis behavior; a of typical hysteresis loop is shown During theoxygen reactive sputtering deposition process, the variation target voltageloop according to changes in the flow rate demonstrates hysteresis behavior; a typical hysteresis is shown in Figure 3 [21]. When the reactive gas starts to be added to the vacuum chamber, the metallic atoms changes in the oxygen flow rate demonstrates hysteresis behavior; a typical hysteresis loop is shown in Figure 3 [21]. When the reactive gas starts to be added to the vacuum chamber, the metallic atoms sputtered away fromWhen the target, consuming thetoadded reactive molecules; a compound isatoms therefore in Figure 3away [21]. the gas starts be toreactive thegas vacuum thea metallic sputtered from thereactive target, consuming theadded added gas chamber, molecules; compound is sputtered away from the target, consuming the added reactive gas molecules; a compound is level not formed on the target surface. At this stage, the sputtering mode is metallic and an increase therefore not formed on the target surface. At this stage, the sputtering mode is metallic and in an formed on the target surface. At change this stage, thetarget sputtering mode metallic and anstage. of thetherefore reactiveinnot gas yields little change the target voltage. ejected species areis metallic at this increase level of the reactive gasin yields little in The the voltage. The ejected species are increase in level of the reactive gas yields little change in the target voltage. The ejected species are metallic at flow this stage. When the of the reactive gas reaches a critical rate, the compound starts to partially cover the metallic at this stage. theand flowthe of the reactive gas decreases reaches a critical rate, the At compound starts partially cover sputteringWhen target, target voltage significantly. this stage, thetosputtering mode is When thetarget, flow ofand thethe reactive gas reaches a critical rate, the compound starts tosputtering partially mode cover the sputtering target voltage decreases significantly. At this stage, the the mixed mode of metal and compound, and the ejected species are in the form of a mixture of metal the sputtering target, theand target voltage decreases Atare thisinstage, the sputtering mode is the mixed mode ofand metal compound, and the significantly. ejected species the form of a mixture of and compound. is the mixed mode of metal and compound, and the ejected species are in the form of a mixture of metal and compound. Further increasing the flow rate of reactive gas, compound films would fully cover the sputtering metalFurther and compound. increasing the flow rate of reactive gas, compound films would fully cover the sputtering target. AtFurther this stage, sputtering israte mode, andfurther further increasing the reactive gas flow increasing the flow ofcompound reactive gas, compound films increasing would fullythe cover the sputtering target. At this stage, sputtering isinincompound mode, and reactive gas flow yieldstarget. little variation on the target voltage. The ejected species are compounds at this stage. At this stage, sputtering is in compound mode, and further increasing the reactive gas flow yields little variation on the target voltage. The ejected species are compounds at this stage. yields littlewhen variation on the target voltage. The ejected species are compounds atvoltage this stage. However, gradually decreasing the reactive gas flow rate, thethe target variation However, when gradually decreasing the reactive gas flow rate, target voltage variationcurve However, when gradually decreasing the reactive gas flow rate, the target voltage variation will not follow the follow curve the while increasing the gas flow, i.e., hysteresis behavior, as shown in Figure 3, curve will not curve while increasing the gas flow, i.e., hysteresis behavior, as shown in will not follow curve while increasing the gas flow, significantly i.e., hysteresis behavior, as when shown in Figure 3, target exists for thethe target [18]. The target voltage increases only the existscurve for the voltage [18].voltage The target voltage significantly increases only when the reactive Figure 3, exists the target voltage [18]. The target voltage increases only when the reactive flowfor rate drops significantly. The sputtering mode significantly changes compound to the mixed gas flow rategas drops significantly. The sputtering mode changes from from compound to the mixed mode, reactive gas flow rate drops significantly. The sputtering mode changes from compound to the mixed mode, goes and finally goes to the mode metallic mode with a decrease the reactive gas flow. and finally back to theback metallic with a decrease in the in reactive gas flow. mode, and finally goes back to the metallic mode with a decrease in the reactive gas flow.

TARGET VOLTAGE TARGET VOLTAGE

METAL METAL METAL + METAL COMPOUND + COMPOUND COMPOUND COMPOUND REACTIVE GAS FLOW REACTIVE GAS FLOW

Figure 3. 3.AAtypical hysteresisloop. loop. Figure typicaltarget target voltage voltage hysteresis Figure 3. A typical target voltage hysteresis loop.

The hysteresis behavior of target voltage has direct connection with the sputtering power and

650 650 600 600 550 550 500 500 450 450 400 400 350 350 300 300 250 250

(a) p =0.2 Pa 1.5 kW (a) pAr=0.2 Pa Ar 1.25 kW 1.5 kW 1.25 kW 1.0 kW 1.0 kW

0 0

10 20 30 40 flow (sccm) 10 O220 30 40 O2 flow (sccm)

50 50

Target voltage (V)(V) Target voltage

Target voltage (V)(V) Target voltage

The hysteresis behavior of of target voltage has direct connectionwith with the sputtering power and hysteresis behavior target voltage hasunder directdifferent connection the sputtering power argonThe partial pressure [22]. The hysteresis loops sputtering power (1.0~1.5 kW) and and argonargon partial pressure [22]. The hysteresis loops under different sputtering power (1.0~1.5 kW) and pressure (0.2~0.5 [22]. ThePa) hysteresis loops under different (1.0~1.5 argon partial partial pressure were measured and the resultssputtering are shownpower in Figure 4. kW) and argonargon partial pressure (0.2~0.5 Pa) were theresults resultsare areshown shown Figure partial pressure (0.2~0.5 Pa) weremeasured measured and and the in in Figure 4. 4. 550 550 500 500 450 450 400 400 350 350 300 300 250 250

Figure 4. Cont.

(b) p =0.3 Pa 1.5 kW (b) pAr=0.3 Pa Ar 1.25 1.5 kW 1.0 kW 1.25 kW 1.0 kW

0 0

10 20 30 40 flow (sccm) 10 O220 30 40 O2 flow (sccm)

50 50

Coatings2018, 2018,8, 8,x146 Coatings FOR PEER REVIEW

11 5 5ofof11


Target voltage (V)

Target voltage (V)

450

(c) p =0.4 Pa 1.5 kW (c)Ar pAr=0.4 Pa 1.5 kW 1.25 kW 1.25 kW 1.0 kW 1.0 kW

500 450

400

400

350

350

300

300

250

250

0

1.5 kW

450

1.5 kW

450

(d) p =0.5 Pa

Ar (d) pAr=0.5 Pa

1.25 kW 1.25 kW 1.0 kW 1.0 kW

Target volatge(V) (V) Target volatge

500

5 of 11

400 400 350 350 300 300

0

10

10

20

30

20O flow 30(sccm) 40 2 O2 flow (sccm)

40

250 250

50

50

0

0

10

20

30

40

10 O flow 20 (sccm) 30 40 2 flow (sccm) O 2

50

50

Figure 4. Target voltage hysteresis loopsatatdifferent different argon and target power. Figure Target voltage hysteresis loops argonpartial partialpressure pressure and target power. Figure 4.4.Target voltage hysteresis loops at different argon partial pressure and target power.

The results shown in Figure 4 have similar features as the typical hysteresis loop shown in Figure 3.

Target voltage (V)

Target voltage (V)

The in in Figure 4 have similar features aspressure the as typical hysteresis loopvoltage shown inshown Figure 3. The resultsshown shown Figure 4 have features the typical loop in It isresults illustrated in Figure 4a that under the similar argon partial of 0.2 Pa, thehysteresis target increases ItFigure is illustrated in Figure 4a that under the argon partial pressure of 0.2 Pa, the target voltage increases 3. Itapproximately is illustrated in 4a Vthat under the argon partial of 0.2 the target voltage from 530Figure V to 600 as the sputtering power risespressure from 1.0 kW to Pa, 1.5 kW. When the oxygen flow increases 30Vsccm, the target startsrises to drop substantially an increase from approximately 530 Vabove to 600 sputtering power from 1.0 kW to with 1.5 When increases from approximately 530 Vastothe 600 V as voltage the sputtering power rises from 1.0kW. kW to 1.5 the kW. oxygen flow. This is mainly due the to 30 oxide formation on the target surface. the secondary oxygen flow increases above 30 sccm, target voltage starts to drop substantially with an increase Wheninthe oxygen flow increases above sccm, the target voltage starts toBecause drop substantially with electron of oxide is larger thantarget thaton of metallic aluminum (see, for the in flow. This iscoefficient mainly due to oxidedue formation on the surface. Because theBecause secondary anoxygen increase inemission oxygen flow. This is aluminum mainly to oxide formation the target surface. example, [23]), the formation of oxide enhances electron density in the glow discharge plasma near electron emission aluminum oxide is larger that of metallic (see, for secondary electroncoefficient emission of coefficient of aluminum oxidethan is larger than that ofaluminum metallic aluminum the target surface. The collision probabilities of electrons with argon atoms increases; therefore, the example, [23]), the formation of oxide enhances electron density in the glow discharge plasma near (see, for example, [23]), the formation of oxide enhances electron density in the glow discharge plasma resistance of the plasma near the target surface drops, finally leading to a significant decrease in the the target surface. The collision probabilities of electrons with with argonargon atoms increases; therefore, the near the target surface. The collision probabilities of electrons atoms increases; therefore, target voltage. The target voltage becomes stable when the oxygen flow reaches 45 sccm. At this stage, resistance of the plasma near the target surface drops, finally leading to a significant decrease in the the resistance of the plasma near the target surface drops, finally leading to a significant decrease the sputtering target is in the mode with overflow of oxygen, the target surface is fully covered by in target voltage. target voltage becomes stablestable whenwhen the oxygen flow reaches 45 sccm. thisAt stage, the target voltage. The target voltage becomes the oxygen flow reaches 45 At sccm. this oxide, andThe sputtering is then in the compound mode. the sputtering target is in the mode with overflow of target surface is fully covered by stage, the As sputtering target isgradually in the mode with from overflow of oxygen, the target surface is fully covered the oxygen flow decreases 50 oxygen, sccm, thethe target voltage remains almost stable until 10 sccm, at which thethe target voltage starts to soar with a further decrease in the oxygen oxide, andand sputtering is then in the compound mode. by oxide, sputtering is point then in compound mode. flow. hysteresis behaviors have beenfrom observed whenthe thetarget argon voltage partial pressure was fixed stable at As the oxygen flow decreases 50 remains almost As theSimilar oxygen flow gradually gradually decreases from 50 sccm, sccm, the target voltage remains almost stable 0.3, 0.4 and 0.5 Pa, respectively. until until 10 10 sccm, sccm, at at which which point point the the target target voltage voltage starts starts to to soar soar with with aa further further decrease decrease in in the the oxygen oxygen Based on the results shownhave in Figure the influence of argon partialpartial pressure on the hysteresis flow. hysteresis behaviors been4, observed when theargon argon pressure was fixed at flow. Similar Similar hysteresis behaviors have been observed when the partial pressure was fixed at 0.3, loops at similar sputtering power of 1.5 kW is plotted in Figure 5. It is depicted that the target voltage 0.3, 0.4 and 0.5 Pa, respectively. 0.4 and 0.5 Pa, respectively. for the metallic mode increases with a decrease in the argon partial pressure. This is mainly due to Based on the results shown in Figure 4, of argon pressure oninthe hysteresis Based on the results Figure 4,the theinfluence influence argonpartial partial pressure the the reduction of the shown amount,inand the resultant collisionofprobability, of argon ionson thehysteresis glow loops at similar sputtering power of 1.5 kW is plotted in Figure 5. It is depicted that the target voltage loops discharging at similar sputtering powerenhancing of 1.5 kWthe is plotted in Figure is depicted the target voltage plasma, thereby sputtering strength5.ofIt the oxide on that the target surface. for metallic mode increases with decreaseintarget inthe theargon argon partial pressure. This is mainly to for the theLow metallic increases aadecrease partial pressure. This is mainly duedue to the levels mode of oxide coveragewith of the sputtering favor an increase in the target voltage. the reduction ofamount, the amount, and the resultant probability, of argon the glow reduction of the and the resultant collisioncollision probability, of argon ions in theions glowindischarging 650 thestrength discharging plasma, therebythe enhancing sputtering strength of the oxide on the target surface. plasma, thereby enhancing sputtering of the oxide on the target surface. Low levels of PTarget=1.5 kW 600 0.2 favor Pa Low of oxide coverage of the sputtering in the target voltage. oxidelevels coverage of the sputtering target favor antarget increase inan theincrease target voltage. 550 500 650450 600400 550350 500300 450250

0.3 Pa 0.4 Pa 0.5 Pa

0.2 Pa

PTarget=1.5 kW

0.3 Pa turning point 0.4 Pa

0.5 Pa 0 10 20 30 40 50 400 O2 flow (sccm) 350 turningon point 300 Figure 5. Influence of argon partial pressure the target voltage hysteresis loop. 250

In the state of overflow of oxygen, fully 50 covered by oxide. At lower argon 0 the10target 20surface 30 is 40 O2 flow (sccm) partial pressure, less argon ions take part in sputtering the oxide away. Therefore, a longer time is needed for the target surface to move back to the metallic mode. This is evidenced by the fact that the Figure of partial pressure on target hysteresis Figure 5.Influence Influence ofargon argon partial pressure onthe the target voltage hysteresis loop. oxygen flow, at5. the lower turning point of the compound mode to voltage the metallic mode,loop. decreases with the drop in the argon partial pressure, as highlighted in Figure 5.

In the state of overflow of oxygen, the target surface is fully covered by oxide. At lower argon In the state of overflow of oxygen, the target surface is fully covered by oxide. At lower argon partial pressure, less argon ions take part in sputtering the oxide away. Therefore, a longer time is partial pressure, less argon ions take part in sputtering the oxide away. Therefore, a longer time is needed for the target surface to move back to the metallic mode. This is evidenced by the fact that the needed for the target surface to move back to the metallic mode. This is evidenced by the fact that the oxygen flow, at the lower turning point of the compound mode to the metallic mode, decreases with the drop in the argon partial pressure, as highlighted in Figure 5.

Coatings 2018, 8, 146

6 of 11

oxygen flow, at the lower turning point of the compound mode to the metallic mode, decreases with the drop in the argon partial pressure, as highlighted in Figure 5. The result illustrated in Figure 5 also indicates that under fixed target power, the argon partial pressure has little influence on the characteristics of the hysteresis curve between the compound mode and the metallic mode when decreasing the flow rate of oxygen. Note that the target voltages at this state are all roughly the same, i.e., approximately 260 V, regardless of the argon partial pressure. One can conclude that under these experimental conditions, the target voltage is hardly affected by the reactive gas pressure and the plasma composition. A similar phenomenon has been observed by Depla et al. via pumping the vacuum chamber at different rates [22]. 3.2. Target Voltage Control Point The results shown in Figures 4 and 5 demonstrate that the measured target voltage hysteresis loops have typical characteristics of that for reactive magnetron sputtering deposition of compound films. The result also indicates that a small change in the oxygen flow would result in a switch from the metallic mode to the compound mode. In this work, the target voltage control approach is used to maintain the flow rate of the reactive gas at a desired flow rate. The aiming voltage of the sputtering target during the sputtering deposition of aluminum oxide is calculated according to the following formula: VA = VC + (VM − VC ) × η

(1)

where VA is the aiming target voltage, VM and VC the target voltage in the metallic mode and compound mode, respectively, and η the target voltage control point. The deposition parameters of aluminum oxide films are shown in Table 1. It is obvious that under identical target power of 1.5 kW, the target voltage and the deposition rate of the obtained film increase with a rise in the control point. This is due to the fact that the rise of the control point drives sputtering to the metallic mode, which has higher target voltage and higher deposition rate. Table 1. Deposition parameters of aluminum oxide films. Sample No.

Control Point (η)

1 2 3 4

25% 30% 35% 40%

Target Power (W)

Target Voltage (V)

1500

412 451 488 528

Deposition Time (min)

Film Thickness (nm)

Estimated Average Grain Size (nm)

60

506 591 724 907

84 89 87 174

The surface morphology of the aluminum oxide films obtained under different voltage control points is shown in Figure 6. It is illustrated that the surface of the deposited films are made of fine grains with compact arrangements when the control point is between 25~35%. A similar result was obtained with a control point of 40%, but the grain sizes are relatively larger, which is probably due to higher deposition rate at 40%. For anti-corrosion purpose, the compactness property of the prepared film is of paramount importance, and large grain sizes would provide larger penetration channels for corrosive chemicals to reach the surface of the highly active metal substrate. Therefore, it is better to choose the control point between 25~35% for practical anti-corrosion applications. The XRD results of the aluminum oxide films prepared under different voltage control points are shown in Figure 7. It is shown that these results are quite different from the XRD pattern of sputter deposited Al films, as shown in Figure 8. These results indicate that the prepared films are aluminum oxide with low level of crystallization. Therefore, the diffraction peaks are not evident and the obtained aluminum oxide films are mainly in the amorphous form.

points is shown in Figure 6. It is illustrated that the surface of the deposited films are made of fine grains with compact arrangements when the control point is between 25~35%. A similar result was obtained with a control point of 40%, but the grain sizes are relatively larger, which is probably due to higher deposition rate at 40%. For anti-corrosion purpose, the compactness property of the prepared film is of paramount importance, and large grain sizes would provide larger penetration Coatings 2018, 8, 146for corrosive chemicals to reach the surface of the highly active metal substrate. Therefore, channels it is better to choose the control point between 25~35% for practical anti-corrosion applications. Coatings 2018, 2018, 8, 8, xx FOR FOR PEER PEER REVIEW REVIEW Coatings


7 of 11

of 11 11 77 of

7 of 11

Figure 6. 6. Surface Surface morphologies morphologies of of aluminum aluminum oxide oxide film film prepared prepared at at different different target target voltage voltage control control Figure point. For all the images, the magnifications are 100,000, and the actual sizes are around 4.15 × 2.78 µm22.. point. For all the images, the magnifications are 100,000, and the actual sizes are around 4.15 × 2.78 µm

The XRD XRD results results of of the the aluminum aluminum oxide oxide films films prepared prepared under under different different voltage voltage control control points points The are shown shown in in Figure Figure 7. 7. It It is is shown shown that that these these results results are are quite quite different different from from the the XRD XRD pattern pattern of of sputter sputter are deposited Al films, as shown in Figure 8. These results indicate that the prepared films are aluminum deposited Al films, as shown in Figure 8. Theseoxide results that the prepared films are aluminum Figure 6. Surface morphologies of aluminum film indicate prepared at different target voltage control Figure Surface aluminum oxide filmthe prepared at different target voltage control oxide with6. low level of crystallization. Therefore, the diffraction peaks are not evident and the the point. For allmorphologies theof images, the of magnifications are 100,000, and the actual sizes are around 4.15 × 2.78evident µm2. point. oxide with low level crystallization. Therefore, diffraction peaks are not and 2. For all the images, the magnifications are 100,000, and the actual sizes are around 4.15 × 2.78 µm obtained aluminum aluminum oxide oxide films films are are mainly mainly in in the the amorphous amorphous form. form. obtained

3000 5000 3000 Counts (a.u.)

2000 4000 2000 1000 1000

3000 2000

00 1000 20 20 0

30 30 20

40

Al Al22OO33(113) (113)

Al2O3 (113)

Counts (a.u.)

20 20

2000

30 30

40 50 50 60 60 40 (degree) 22 (degree)

1000 0

20 5000 5000

30

40 50 60 2 (degree)

4000 6000 4000

50

60

70 70 70

70 70

80 80

6000 6000

 = 35%

40 50 50 60 60 40 (degree) 22 (degree)

30

3000 0 0

35% == 35% 40 50 60 70 80 2 (degree) Al Al2OO3 (113) (024)

4000 6000 4000

80 80

Al Al22OO33(113) (113)

30

70 70

30%

5000 3000 3000

80 80

4000 2000

2000

3000

1000 1000

2000 1000 0

80

Al2O3 (113)

20

40 50 50 60 60 40 (degree) 22 (degree)

1000 4000 1000

Counts Counts(a.u.) (a.u.)

0


Al Al22OO33(006) (006) Al Al22OO33(113) (113)

1000

5000 5000 Counts Counts(a.u.) (a.u.)

30 30

Counts (a.u.)

6000 6000

2000

Al22O33 (024)

3000 00 20 20

2000 2000 5000

Al2O3 (024)

Counts (a.u.)

1000 4000 1000

3000

3000 6000

= 25%

Al2O3 (006) Al2O3 (113)

3000 3000 6000 2000 5000 2000

Al Al22OO33(113) (113)


The XRD results of the aluminum oxide films prepared under different voltage control points are shown in Figure 7. It is shown that these results are quite different from the XRD pattern of sputter 6000 6000 6000 6000 deposited Al films, as shown in Figure 8.These are aluminum 25% results indicate that the prepared films  30% 30% == 25%  5000 5000 5000 oxide with low level of crystallization. Therefore, the 5000 diffraction peaks are not evident and the obtained aluminum oxide films are mainly in the amorphous 4000 form. 4000 4000 4000

00

20 20

20

30

30 30 40

70

 = 40%

40 50 50 60 60 40 (degree) 22 (degree) 50

60

40% 80  == 40%

70

70 70

80 80

80

2 (degree) 2 (degree) Figure 7. 7. XRD XRD patterns patterns of of aluminum aluminum oxide film film prepared preparedat atdifferent different targetvoltage voltagecontrol controlpoints. points. Figure 7. XRD patterns of aluminum oxide film prepared at different target voltage control points. Figure oxide target

Figure 7. XRD patterns of aluminum oxide film prepared at different target voltage control points. 1800 1800 1800 1600 1600

Al coating coating Al

Al coating

Al (220)

Al Al(220) (220)

200

Al Al (111) Al(111) (111)

Al Al(200) (200)

600600 600 400400 400

Al (200)

1200

1000 1000 1000 800800 800

Counts (a.u.)


1600

1400 1400 1400 1200 1200

200 200 0 00 25 30 35 40 45 50 55 60 65 70 75 25 30 30 35 35 40 40 45 50 55 55 60 60 65 65 70 70 25 50 245 (degree) (degree) (degree) 22

75 75

Figure 8. XRD pattern of sputter deposited Al film.

Figure8. 8.XRD XRDpattern patternof ofsputter sputterdeposited depositedAl Alfilm. film. Figure 8. XRD pattern of sputter deposited Al film. Figure

Coatings 2018, 8, 146 Coatings 2018, 8, x FOR PEER REVIEW Coatings 2018, 8, x FOR PEER REVIEW

8 of 11 8 of 11 8 of 11

A typical typical EDS EDS pattern pattern of of the the prepared prepared aluminum aluminum oxide oxide films films isis shown shown in in Figure Figure9.9. This This data data A A typical EDS pattern of the prepared aluminum oxide films is shown in Figure 9. This data showsthat that traces of argon exist the prepared oxide films. Two points contribute to this shows argon existexist in theinin prepared oxide films. points contribute this phenomenon: shows that traces tracesof of argon the prepared oxideTwo films. Two points to contribute to this phenomenon: argon atoms are adsorbed by aluminum oxide particles, which has high melting point, argon atoms are adsorbed oxide particles, which has high melting in the reactive phenomenon: argon atoms by arealuminum adsorbed by aluminum oxide particles, which haspoint, high melting point, in the reactive sputtering process, and therefore it is difficult for argon atoms to escape from these sputtering process, and therefore is difficult foritargon atomsfor to argon escapeatoms from these particles the in the reactive sputtering process, itand therefore is difficult to escape fromvia these particles melting-reconstruction via the localized melting-reconstruction process; the reactive sputtering products are in the localized process; the reactive sputtering products are in the form of clusters of particles via the localized melting-reconstruction process; the reactive sputtering products are in the form of clusters of molecules and atoms with low diffusion mobility, and thereby it is unfavorable molecules and atoms with low diffusion mobility, and thereby it is unfavorable foritthe argon atoms form of clusters of molecules and atoms with low diffusion mobility, and thereby is unfavorable forbe the argon atoms to be repelledNote and de-adsorbed. Note that the signal intensifies of the Al element intensifies to repelled and de-adsorbed. that the signal of the Al element as the rise of the for the argon atoms to be repelled and de-adsorbed. Note that the signal of the Al element intensifies as the rise of the voltage control point. This is because that higher voltage control point leads to higher voltage control This is because voltage leads to higher rates; as the rise of thepoint. voltage control point.that Thishigher is because thatcontrol higherpoint voltage control pointdeposition leads to higher depositionthere rates; therefore, there are more Al atomsoxide in thefilms. deposited oxide films. therefore, are more Al atoms in the deposited deposition rates; therefore, there are more Al atoms in the deposited oxide films.  

10 10 5 5

0 0 0 0

1 1

2 2

ArAr K K

15 15

AlAl K K

20 20

OO K K11

Counts (a.u.) Counts (a.u.)

25 25

3 4 5 6 7 8 3 4 5 6 7 8 Energy (keV) Energy (keV) Figure 9. 9. A A typical typical EDS EDS pattern pattern of of the the deposited deposited aluminum aluminum oxide oxide film. film. Figure Figure 9. A typical EDS pattern of the deposited aluminum oxide film.

The atomic ratio of the prepared aluminum oxide films revealed by EDS quantitative analysis is Theatomic atomicratio ratioof ofthe theprepared preparedaluminum aluminumoxide oxidefilms filmsrevealed revealedby byEDS EDSquantitative quantitativeanalysis analysisisis The shown in Figure 10. It is shown that all the films exhibit larger O:Al atomic ratio than the shown in It isItshown that all theall films O:Al atomic ratioatomic than theratio stoichiometric shown in Figure Figure10.10. is shown that theexhibit filmslarger exhibit larger O:Al than the stoichiometric value of 3:2, which is due to the high oxygen flow rate under low target voltage control value of 3:2, which is 3:2, duewhich to theis high flow rate flow under low target controlcontrol points. stoichiometric value of due tooxygen the high oxygen rate under lowvoltage target voltage points. This implies that there are free O atoms, or that physically adsorbed O exists in the prepared This implies that there freeare O free atoms, or thator physically adsorbed O exists in thein prepared oxide points. This implies thatare there O atoms, that physically adsorbed O exists the prepared oxide films. With a rise in the control point, the oxygen flow rate drops and the O:Al ratio becomes films.films. With a rise ainrise thein control point,point, the oxygen flow rate and the O:Al closer oxide With the control the oxygen flowdrops rate drops and the ratio O:Albecomes ratio becomes closer to stoichiometry. to stoichiometry. closer to stoichiometry.

Atomic concentration (%) Atomic concentration (%)

70 70

65.32 65.32

63.89 63.89

63.16 63.16

33.61 33.61

34.59 34.59 1.52 1.52

35.75 35.75

60 60 50 50 40 40

30 30 1.5 1.5 0 0

1.07 1.07 25 25

1.09 1.09

59.56 59.56 O O 39.35 39.35 Al Al 1.09 1.09 Ar Ar 40 40

30 35 30 35 Voltage control point  (%) Voltage control point  (%) Figure 10. Atomic concentration of aluminum oxide films prepared under different control points Figure 10. Atomic concentration of aluminum oxide films prepared under different control points Figure 10.by Atomic revealed EDS. concentration of aluminum oxide films prepared under different control points revealed by EDS. revealed by EDS.

A typical AES pattern of the prepared aluminum oxide films is shown in Figure 11. It is apparent A typical AES pattern of the prepared aluminum oxide films is shown in Figure 11. It is apparent that the Al L23VV, KVV and Al KL 23L23 Auger electronoxide linesfilms are identified [24]. on these A typical AESOpattern of the prepared aluminum is shown clearly in Figure 11.Based It is apparent that the Al L23VV, O KVV and Al KL23L23 Auger electron lines are identified clearly [24]. Based on these results, been performed and the atomic concentrations of O and Al are that the quantitative Al L23 VV, O analyses KVV andhave Al KL L Auger electron lines are identified clearly [24]. Based on 23 23 results, quantitative analyses have been performed and the atomic concentrations of O and Al are obtained and shown in Figure 12. these results, quantitative analyses have been performed and the atomic concentrations of O and Al obtained and shown in Figure 12. are obtained and shown in Figure 12.

Coatings Coatings2018, 2018,8,8,146 x FOR PEER REVIEW Coatings 2018, 8, x FOR PEER REVIEW

99 of of1111 9 of 11

10 10

Counts (a.u.) Counts (a.u.)

5 0

 5 

0

-5 AlL23VV -5 AlL23VV

-10 -10 -15 -15 0 0

OKVV OKVV

AlKL L AlKL23L2323 23

400 800 1200 400 Kinetic800 1200 energy (eV) Kinetic energy (eV)

1600 1600

Figure11. 11.AAtypical typicalAES AESpattern patternofofthe thedeposited depositedaluminum aluminumoxide oxidefilm. film. Figure Figure 11. A typical AES pattern of the deposited aluminum oxide film.

The atomic concentration result of O and Al shown in Figure 12 is consistent with the result The atomic atomic concentration concentration result result of of O O and and Al Al shown shown in in Figure Figure 12 12 is is consistent consistent with with the the result result The revealed by EDS measurements, as shown in Figure 10. Nonetheless, all the atomic ratios of O and revealed EDS measurements, measurements,as asshown shownininFigure Figure10. 10.Nonetheless, Nonetheless,allallthe the atomic ratios O and revealed by by EDS atomic ratios of of O and Al Al are larger than the stoichiometric value, which again signifies that there are free O atoms in the areare larger than the the stoichiometric value, which again signifies that that therethere are free atoms in theinbulk, Al larger than stoichiometric value, which again signifies are O free O atoms the bulk, or that are physically adsorbed into the surface of the aluminum oxide films. bulk, orare that are physically adsorbed into the surface the aluminum or that physically adsorbed into the surface of theof aluminum oxide oxide films. films. Atomic concentration (%) Atomic concentration (%)

70 70 65 65

66.3 66.3

65.6 65.6

65.2 65.2 61.3 61.3 O O Al Al 38.7 38.7

60 60 35 35 30 30

33.7 33.7

34.4 34.4

34.8 34.8

25 30 35 40 25 30 control point 35  (%) 40 Voltage Voltage control point  (%)

Figure 12. Atomic concentration of aluminum oxide films prepared under different control points Figure Figure 12. 12. Atomic Atomic concentration concentration of of aluminum aluminum oxide oxide films films prepared prepared under under different different control control points points revealed by AES. revealed by by AES. AES. revealed

4. Conclusions 4. 4. Conclusions Conclusions The target voltage hysteresis phenomenon of medium frequency reactive sputtering deposition The voltage hysteresis hysteresisphenomenon phenomenonofofmedium mediumfrequency frequencyreactive reactive sputtering deposition The target target oxide voltage deposition of of aluminum films has been investigated in this work. The obtained sputtering target voltage hysteresis of aluminum oxide films has been investigated in this work. The obtained target voltage hysteresis aluminum oxide films hasdifferent been investigated in this work. The voltage hysteresis loops, obtained under argon partial pressure (0.2,obtained 0.3, 0.4 target and 0.5 Pa) and target loops, power loops, obtained under different argonpressure partial (0.2, pressure (0.2, 0.3, 0.4 and 0.5 Pa)power and target power obtained under different argon partial 0.3, 0.4 and 0.5 Pa) and target (1.0, and (1.0, 1.25 and 1.5 kW) conditions, are similar and have typical behaviors to those for1.25 reactive (1.0, 1.25conditions, and 1.5 kW) conditions, are typical similar behaviors and havetotypical behaviors to those for reactive 1.5 kW) are similar and have those for reactive sputtering deposition sputtering deposition of compound films. sputtering deposition of compound films. of compound films. The surface morphology, crystalline structure, and chemical composition of the aluminum oxide The surface morphology, crystalline structure, and composition of oxide surface morphology, crystalline structure, and chemical chemical composition of the the aluminum aluminum thinThe films prepared at different target voltage control points, have been investigated by SEM,oxide XRD, thin films prepared at different target voltage control points, have been investigated by SEM, XRD, thin films prepared at different target voltage control points, have been investigated by SEM, XRD, EDS and AES. The results show that the prepared films are aluminum oxide, with low levels of EDS and results show that the films are oxide, with low of EDS and AES. AES. The The show that the prepared prepared films areaaluminum aluminum oxide, with point low levels levels of crystallization and results compact films, could be obtained with target voltage control between crystallization and compact films, could be obtained with aa target voltage control point between crystallization and compact films, could be obtained with target voltage control point between 25~35%. The atomic ratio of O and Al of the prepared oxide films are all slightly larger than 25~35%. The ratio Al ofof the films are all slightly larger than 25~35%. Theatomic atomic ratioof ofO Oand and the prepared prepared oxide oxide arethat all there slightly larger than stoichiometry, as revealed by EDS and Al AES measurements, which films denotes are free O atoms stoichiometry, as revealed by EDS and AES measurements, which denotes that there are free O atoms stoichiometry, as revealed by EDS and AES measurements, which denotes that there are free O exist in the bulk or physically adsorbed into the surface of the aluminum oxide films. exist inexist the bulk orbulk physically adsorbed into theinto surface of the aluminum oxide films. atoms in the or physically adsorbed the surface of the aluminum oxide films. Based on the results of this work, optimal preparation conditions of aluminum oxide films could Based results of work, optimal preparation conditions of oxide films could Based on on the the results of this thiscould work,be optimal preparation conditions of aluminum aluminum oxide could be developed. Similar process applied for the deposition of other compound thinfilms films, such be developed. Similar process could couldbe beapplied appliedfor forthe thedeposition depositionofofother othercompound compound thin films, such be developed. Similar process thin films, such as as oxides, nitrides and carbides. Further research on the corrosion resistance of aluminum/aluminum as oxides, nitrides and carbides.Further Furtherresearch researchon onthe thecorrosion corrosionresistance resistanceof ofaluminum/aluminum aluminum/aluminum oxides, nitrides and carbides. oxide multilayered coating is in progress. oxide oxide multilayered multilayered coating coating is is in in progress. progress.

Coatings 2018, 8, 146

10 of 11

Acknowledgments: This work is partially supported by the National Science Foundation of China (No. 61604138), and the Director’s Funding of the Institute of Materials, China Academy of Engineering Physics (No. SJYB201509). Author Contributions: Qingfu Wang and Liping Fang conceived and designed the experiments; Qinghe Liu and Lin Chen performed the sputtering experiments; Qinguo Wang contributed XRD; Xiandong Meng contributed SEM and EDS; Hong Xiao contributed AES; Qingfu Wang and Liping Fang analyzed the data and wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3. 4. 5. 6. 7.

8.

9.

10. 11. 12. 13. 14. 15. 16. 17.

18. 19. 20.

Xin, Y.; Huo, K.; Hu, T.; Tang, G.; Chu, P.K. Mechanical properties of Al2 O3 /Al bi-layer coated AZ91 magnesium alloy. Thin Solid Films 2009, 517, 5357–5360. [CrossRef] Li, H.Q.; Wang, Q.M.; Gong, J.; Sun, C. Interfacial reactions and oxidation behavior of Al2 O3 and Al2 O3 /Al coatings on an orthorhombic Ti2 ALNb alloy. Appl. Surf. Sci. 2011, 257, 4105–4112. [CrossRef] Matikainen, V.; Niemi, K.; Koivuluoto, H.; Vuoristo, P. Abrasion, erosion and cavitation erosion wear properties of thermally sprayed alumina based coatings. Coatings 2014, 4, 18–36. [CrossRef] Ali, R.; Saleem, M.; Pääkkönen, P.; Honkanen, S. Thermo-optical properties of thin-film TiO2 –Al2 O3 bilayers fabricated by atomic layer deposition. Nanomaterials 2015, 5, 792–803. [CrossRef] Pfeiffer, K.; Schulz, U.; Tünnermann, A.; Szeghalmi, A. Antireflection coatings for strongly curved glass lenses by atomic layer deposition. Coatings 2017, 7, 118. [CrossRef] Lackner, J.; Waldhauser, W.; Major, L.; Kot, M. Tribology and micromechanics of chromium nitride based multilayer coatings on soft and hard substrates. Coatings 2014, 4, 121–138. [CrossRef] Yin, A.; Yan, J.; Chen, L.; Zhu, S.; Long, Z.; Fang, L.; Liu, T. Experimental and numerical investigation of buckling and delamination of Ti/TiN coatings on depleted uranium under compression. Appl. Surf. Sci. 2017, 422, 997–1006. Matthias, W.; Hoang Tung, D.; Hartmut, S.; Rainer, H. Aluminium atom density and temperature in a dc magnetron discharge determined by means of blue diode laser absorption spectroscopy. J. Phys. D Appl. Phys. 2005, 38, 2390–2395. Kulczyk-Malecka, J.; Kelly, P.; West, G.; Clarke, G.; Ridealgh, J. Characterisation studies of the structure and properties of as-deposited and annealed pulsed magnetron sputtered titania coatings. Coatings 2013, 3, 166–176. [CrossRef] Stefanov, B.; Österlund, L. Tuning the photocatalytic activity of anatase TiO2 thin films by modifying the preferred grain orientation with reactive dc magnetron sputtering. Coatings 2014, 4, 587–601. [CrossRef] Navabpour, P.; Cooke, K.; Sun, H. Photocatalytic properties of doped TiO2 coatings deposited using reactive magnetron sputtering. Coatings 2017, 7, 10. [CrossRef] Gao, W.; Li, Y.; Zhang, Y.; Yin, H. Exploration of growth window for phase-pure cubic boron nitride films prepared in a pure N2 plasma. Coatings 2018, 8, 82. [CrossRef] Safi, I. Recent aspects concerning dc reactive magnetron sputtering of thin films: A review. Surf. Coat. Technol. 2000, 127, 203–218. [CrossRef] Bräuer, G.; Szyszka, B.; Vergöhl, M.; Bandorf, R. Magnetron sputtering—Milestones of 30 years. Vacuum 2010, 84, 1354–1359. [CrossRef] Kouznetsov, V.; Macák, K.; Schneider, J.M.; Helmersson, U.; Petrov, I. A novel pulsed magnetron sputter technique utilizing very high target power densities. Surf. Coat. Technol. 1999, 122, 290–293. [CrossRef] Anders, A. Discharge physics of high power impulse magnetron sputtering. Surf. Coat. Technol. 2011, 205, S1–S9. [CrossRef] Stranak, V.; Herrendorf, A.-P.; Drache, S.; Cada, M.; Hubicka, Z.; Bogdanowicz, R.; Tichy, M.; Hippler, R. Plasma diagnostics of low pressure high power impulse magnetron sputtering assisted by electron cyclotron wave resonance plasma. J. Appl. Phys. 2012, 112, 093305. [CrossRef] Sproul, W.D.; Christie, D.J.; Carter, D.C. Control of reactive sputtering processes. Thin Solid Films 2005, 491, 1–17. [CrossRef] Depla, D.; Buyle, G.; Haemers, J.; De Gryse, R. Discharge voltage measurements during magnetron sputtering. Surf. Coat. Technol. 2006, 200, 4329–4338. [CrossRef] Berg, S.; Nyberg, T. Fundamental understanding and modeling of reactive sputtering processes. Thin Solid Films 2005, 476, 215–230. [CrossRef]

Coatings 2018, 8, 146

21. 22.

23. 24.

11 of 11

Martin, P.M. Handbook of Deposition Technologies for Films and Coatings: Science, Applications and Technology; William Andrew: New York, NY, USA, 2009. Depla, D.; Mahieu, S.; De Gryse, R. Depositing aluminium oxide: A case study of reactive magnetron sputtering. In Reactive Sputter Deposition; Depla, D., Mahieu, S., Eds.; Springer: Berlin/Heidelberg, Germany, 2008; pp. 153–197. Smith, P.C.; Hu, B.; Ruzic, D.N. Low-energy ion-induced electron emission from gas-covered surfaces. J. Vac. Sci. Technol. A Vac. Surf. Films 1994, 12, 2692–2700. [CrossRef] Briggs, D.; Seah, M. Practical Surface Analysis: By Auger and X-ray Photoelectron Spectroscopy; Wiley: Hoboken, NJ, USA, 2003. © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).