201 (2006) 1109–1116www.elsevier.com/locate/surfcoat

Surface & Coatings Technology

Structure-property correlations in aluminum oxide thin filmsgrown by reactive AC magnetron sputtering

Atul Khanna a,1, Deepak G. Bhat a,⁎, Adrian Harris b, Ben D. Beake b

a Department of Mechanical Engineering, University of Arkansas, Fayetteville, AR-72701, USAb Micro Materials Limited, Unit 3, Wrexham Technology Park, Wrexham, LL13 7YP, UK

Received 1 October 2005; accepted in revised form 12 January 2006Available online 23 February 2006

Abstract

Crystalline alumina thin films were deposited on glass, silicon and WC-Co substrates by reactive inverted cylindrical AC magnetron sputteringtechnique both with and without aluminum precoating at two working pressures. Coatings prepared on Si wafers with Al precoating were found tobe γ-phase of aluminum oxide. However alumina coatings prepared on Si wafer without Al precoating showed evidence for the presence of δ andθ-phases besides the γ-phase. Two coating samples grown on Si substrates with Al prelayer were annealed at 400 and 900°C for 8h under flowingoxygen conditions. Annealing at 400°C increased the grain size for γ-alumina phase while annealing to 900°C formed δ and κ-phases in thecoatings. Scanning electron microscopy and energy dispersive X-ray microanalysis were carried out to investigate the thin film surfacemorphology and composition. While γ-alumina coatings were ultrafine grained, coatings containing δ- and κ-phases showed large size grains.Nanoindentation and nanoscratch tests were performed on all samples to determine the thin film hardness and scratch resistant properties. Higherworking pressure during film deposition seemed to lower film hardness values. Significant differences were observed in the mechanical propertiesof alumina thin films grown with Al precoating on silicon substrates. While the Al precoating improves thin film adhesion and leads to less severefailure, it significantly decreased the thin film hardness as measured by nanoindentation technique.© 2006 Elsevier B.V. All rights reserved.

Keywords: γ-Alumina thin films; Inverted cylindrical magnetrons; AC reactive sputtering; X-ray diffraction; SEM and EDX; Nanoindentation and nanoscratch tests

1. Introduction

Aluminum oxide is an outstanding ceramic which, due to itsseveral excellent physical and chemical properties like highmelting temperature, high hardness, abrasion and oxidation resis-tance, finds application on cutting tools in conjunction with otherhard coating materials like TiN, TiCN and TiC [1–3]. Due to itshigh electrical resistivity, alumina coatings are also of interest tomicroelectronics industry. A very large number of physical andchemical vapour deposition techniques have been reported for thedeposition of amorphous and crystalline alumina coatings in theliterature [4–20]. We recently reported the X-ray diffractionstudies on γ-alumina coatings prepared by a novel inverted

⁎ Corresponding author.E-mail addresses: akphysics@yahoo.com (A. Khanna),

dgbhat@engr.uark.edu (D.G. Bhat).1 On leave from Department of Applied Physics, Guru Nanak Dev University,

Amritsar-143005, India.

0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2006.01.033

cylindrical magnetron AC sputtering technique. These films wereprepared by carrying out reactive sputtering in the poisoned modeof two hollow cylindrical Al targets which gives stoichiometricalumina coatings but at low deposition rates of 0.09nm s−1 [14].In this paper, we report the growth and characterization ofcrystalline alumina films on silicon, glass andWC-Co (cementedcarbide) substrates by reactive magnetron sputtering techniqueboth with and without Al precoating at two working pressures.

2. Experimental

2.1. Thin film deposition by inverted cylindrical magnetronsputtering

We used an Isoflux Model ICM-10 sputtering system todeposit thin films of aluminum oxide on silicon, glass and ce-mented carbide substrates. Our sputtering system consists of twohollow cylindrical aluminum targets of diameter about 33cm andheight 9.8cm. The two targets were powered by an Advanced

Table 1Thin film samples and their growth conditions (deposition time was 4 h for allsamples)

Sampleno.

Substrate Aluminumprecoating

Gasflowrates(sccm)

Chamberpressure(Pa)

Sputteringpower(kW)

Post-depositionannealingtemperature(°C)

Ar O2

1 Silicon Yes 20 30 1.06 4.5 No2 Silicon Yes 20 30 1.06 4.5 4003 Silicon Yes 20 30 1.06 4.5 9004 Silicon No 68 48 0.29 5.0 No5 Glass No 68 48 0.29 5.0 No6 WC-Co No 68 48 0.29 5.0 No

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Energy model PE II AC power supply operated at a frequency of(41±2) kHz. The schematic of the dual cylindrical magnetrons isshown in Fig. 1. The deposition chamber was evacuated with aturbomolecular pump to a base vacuum of 7.9×10−4Pa.Ultrahigh purity argon and oxygen gases were then introducedinto the chamber using mass flow controllers and the PE II powersupply was switched on to create the plasma discharge. Allsubstrates were placed on a horizontal steel plate completelyenclosed by the two hollow targets. The substrate to targetdistance was approximately 3cm. No deliberate heating or ionbombardment (ion plating) of the substrates was done during theentire deposition run.However, higher ion current densities inACplasma aid in the growth of crystalline alumina coatings [14,21].

We prepared two sets of aluminum oxide thin films atsputtering power of 4.5 and 5.0kW both with and without Alprecoating on the Si, glass andWC-Co substrates at pressures of1.06 and 0.29Pa, respectively. This small difference in dischargepower used during two sets of deposition runs is not expected toproduce any differences in final thin film properties but onlyaffect thin film growth rate by an insignificant amount. In thefirst deposition run, carried out at a pressure of 1.06Pa anddischarge power of 4.5kW, we used the throttle valve in the pathof high vacuum line connecting the chamber to the turbomo-lecular pump. This allowed us to increase the chamber pressureeven with lower gas flow rates (see Table 1). In both depositionruns the oxygen/argon gas flow rate ratios were kept sufficientlyhigh to completely poison the targets. The advantage ofpoisoning the targets is that it produces stoichiometric aluminumoxide coatings but at a very low deposition rates in the range of0.05 to 0.09nm s−1 [11,14]. Two aluminum oxide coatingsdeposited with metallic Al precoating on the silicon substrateswere further given post-deposition heat treatment at 400 and900°C under flowing oxygen conditions for 8h in a tube furnace.Table 1 gives details of all thin film samples along with theirgrowth and post-deposition heat treatment conditions.

2.2. UV–visible spectroscopic studies

We recorded the UV–visible transmission spectra for threeidentical alumina thin film samples without Al metal precoating

Fig. 1. Schematic of dual targets in the ICM-10 inverted cylindrical magnetronsputtering system.

on glass substrates (sample No. 5) using a Shimadzu Model2010PC double beam UV–Visible spectrophotometer. Theoptical spectra of three such alumina coated glass samples areshown in Fig. 2. We used the interference effects in thetransmittance spectra to calculate the thin film thickness, t,using the following well known relation [22,23]:

t ¼ k2k12nðk2−k1Þ ð1Þ

where, n is the refractive index of alumina coating and λ1 and λ2are the wavelength positions of two successive interferenceminima.

2.3. X-ray diffraction studies

The crystallinity of thin film samples prepared on glass andsilicon substrates was studied by powder X-ray diffractometer(Philips model PW 1830) using Cu Kα X-rays and “2θ”geometry in which the sample was fixed at a glancing incidenceangle, θ of 2.5° while the detector, 2θ, was scanned from 30° to

Fig. 2. UV–visible spectra of three alumina coated glass samples (sample no. 5)showing interference effects in the transmitted light.

Fig. 4. XRD pattern for the alumina coating on a silicon wafer without Alprecoating (sample 4). The peaks are attributed to the γ, δ and θ phases ofalumina.

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90°. Fig. 3 shows the X-ray diffractograms for alumina coatingsdeposited on silicon substrates with Al precoating. Fig. 4 showsthe XRD pattern for alumina coatings deposited on Si waferwithout Al precoating.

2.4. SEM and EDX microanalysis

The surface morphology of alumina thin film samples wasstudied using FEI-Philips XL-30 Environment ScanningElectron Microscope (ESEM). This microscope can also beoperated in the low vacuum mode (maximum pressure of197.3Pa) to reduce the charging effects on insulating sampleslike alumina, and thus enabled SEM studies on insulatingmaterials without the deposition of conducting layers of gold orcarbon. The surface morphologies of alumina coatings onsilicon, glass and WC-Co substrates are shown in Figs. 5–9.The compositional analysis of samples was carried out by theenergy dispersive X-ray analysis (EDX) technique on at least8 different points on each sample surface. Table 2 presents theaverage values of Al and O atomic % determined from thesemeasurements.

2.5. Nanoindentation and nanoscratch tests

Nanoindentation testing was performed using a MicroMaterials NanoTest system. The NanoTest is a modular systemfor nanoindentation, nano-scratch and nano-impact testing[24,25]. It is a pendulum-based depth-sensing system, with

Fig. 3. XRD patterns for thin film samples 1–3 (peaks marked “x” are due to Al precoalumina).

the sample mounted vertically and the load applied electro-magnetically. Current in the coil causes the pendulum to rotateon its frictionless pivot so that the diamond probe penetrates thefilm surface. Test probe displacement is measured with aparallel plate capacitor achieving sub-nanometer resolution.Transverse sample stage motion enables nano-scratch testing,wear and profilometry to be performed as required. The areafunction for the Berkovich diamond indenter used for the

ating while those labeled “γ”, “δ” and “κ” are due to various transition phases of

Fig. 5. Surface morphology of the alumina thin film (sample 1) deposited on silicon substrate by reactive magnetron sputtering at ×1500 (left) and ×10000 (right).

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nanoindentation testing was determined by indentations intofused silica from 0.5–200mN. Indentations were load-con-trolled experiments to 2mN maximum load that correspond-ed to a maximum penetration depth of 50–70nm. Otherexperimental conditions were: loading rate = unloadingrate=0.05mN s−1, 30s hold at peak load for creep, 30s holdingperiod at 90% unload for thermal drift correction. The data wereanalyzed with the Oliver and Pharr method [26] and 20 repeatexperiments were performed on each sample. The hardness (H)is determined from the peak load (Pmax) and the projected areaof contact, A:

H ¼ Pmax=A ð2ÞTo obtain the elastic modulus, the unloading portion of thedepth-load curve is analysed according to a relationship thatdepends on the contact area:

C ¼ p0:5=ð2ErA0:5Þ ð3Þ

where C is the contact compliance and Er is the reducedmodulus defined by

1=Er ¼ ð1−m2s Þ=Es þ ð1−m2i Þ=E ð4Þwhere νs=Poisson's ratio for the sample, νi =Poisson's ratio forthe diamond indenter (0.07), Es=Young's modulus for thesample and Ei =Young's modulus for the indenter (1141GPa).In this paper Er values are reported.

Fig. 6. SEM of alumina thin film annealed at 400°C (s

Nano-scratch testing was performed with a Micro MaterialsNanoTest using a nominally 5μm tip radius diamond probe[27,28]. Indentation testing of fused silica was performed todetermine the exact probe tip radius. An excellent fit with aradius of (3.0±0.1) μm was found. The scratch procedureinvolved three sequential scans at 2μm/s over a 250μm track.An initial topography (0.05mN constant load) scan wasfollowed by a second (scratch) scan, where the applied loadwas constant until 50μm and then ramped at a constant rate of5mN/s to the maximum load of 500mN, and then a subsequenttopography scan at 0.05mN. At least three repeat tests wereperformed on each coating. The residual scratch tracks wereimaged to confirm critical load values with an integrated opticalmicroscope and digital capture system.

3. Results and discussion

We calculated the average value of thin film thickness of790nm from the interference effects in the UV–visibletransmission spectra of 3 alumina coatings deposited on glasssubstrates without any precoating of metallic aluminum layer(Fig. 2). We used the refractive index value of n=1.65 forcalculating the thin film thickness [29]. The deposition rate isvery low at about 0.05nm s−1 due to the fact that the coatingswere produced in the poisoned mode of the dual targets.The poisoned mode of sputtering has an advantage that itproduces alumina films which are close to the required Al–O

ample no. 2) at ×10,000 (left) and ×50,000 (right).

Fig. 7. SEM of an alumina thin film annealed at 900°C (sample no. 3). Fig. 9. SEM of alumina thin film on a glass substrate (sample no. 5).

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stoichiometric ratio of 2 :3. This was confirmed by our com-positional analysis studies done on all samples by the EDXtechnique (Table 2). It is clear from the values reported inTable 2 that alumina coatings have a small excess of Al (about1.5at.%), and on annealing in flowing oxygen, the excess of Aldecreases, as expected. For instance, sample 3 is the nearest tostoichiometric Al2O3 as it was annealed at the highest tem-perature of 900°C. Alumina films deposited on glass andsilicon substrates without Al prelayers have nearly the samecomposition as for the thin films deposited on silicon substratewith the underlying Al coating. There seems to be little or noeffect of the chemical or physical nature of the substrates onthe composition of deposited alumina thin films.

Fig. 3 shows the X-ray diffraction patterns for the threealumina coatings deposited on silicon substrates with Alprelayers. While sample 1 is an as-deposited thin film, samples2 and 3 were given post-deposition annealing at 400 and 900°C,respectively. We note sharp diffraction peaks due to metallic Alprelayer in samples 1 and 2. The intensity of these peaks due tocrystalline Al bottom layer decreases with annealing at 400°Cand these peaks are completely absent in sample 3, showing thatall of the underlying Al layer gets oxidized by annealing at900°C for 8h. In samples 1 and 2 we observe two broad peaks

Fig. 8. Surface morphology of alumina coating on a cemented carbide substrate(sample 6).

at 2θ angles of about 46° and 67°. They are attributed to theγ-phase of alumina. The exact phase identification is difficult asthe peaks are broad and are overlapping with the sharp peaksdue to Al bottom layer. It may be mentioned here that XRDpatterns of the metastable transient phases, namely, γ, η, δ andθ-alumina are very similar and it is not easy to identify aparticular phase by the XRD studies alone. However, it isknown that γ and η-alumina coatings are generally ultrafinegrained, and therefore, show rather broad peaks centered atapproximately 46° and 67° with a peak width of about 2°. Onthe other hand, powder XRD patterns of δ and θ-aluminaconsist of fairly sharp peaks very close to these two angles [30].These two and other peaks of γ-alumina sharpen and becomemore intense for the sample 2, which was annealed to 400°C. Incase of sample 2 we note an additional peak at angle of 88.1°which is not present in the XRD spectra for sample 1. This peakis also attributed to the γ-alumina. In case of sample 3 (annealedto 900°C) the Al peaks are completely absent and we seenumber of diffraction peaks which match well with those forδ-alumina phase. We also note that peaks centered at approxi-mately 46° and 67° sharpen which is a characteristic of theformation of the δ-phase [30]. Heat treatment of the gammaalumina coatings at 900°C produces γ→δ structural transitionand also a small amount of κ-phase as evidenced by the XRDpeaks at 42.3°, 52.4° and 62.4°. It is reported in the literaturethat these transitions occurs between 750 and 1000°C [30–32].Fig. 4 shows the XRD pattern for the alumina coating preparedon a silicon substrate without Al precoating (sample 4). Thepeaks centered at 2θ angles of about 46° and 67° are sharper as

Table 2Compositional analysis of thin film samples as determined by EDX technique

Sampleno.

Composition (atomic %)

Al O

1 41.3 58.72 41.2 58.83 40.6 59.44 41.5 58.55 41.8 58.26 41.0 59.0

Fig. 10. Hardness of alumina thin films both with and without Al precoating ondifferent substrates.

Fig. 12. Elastic recovery parameter (ERP) of alumina thin films.

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compared to the peaks observed for samples 1–3. These andother peaks are attributed to γ, δ and θ-phases of alumina. Wehave been successful in producing crystalline alumina coatingswithout any substrate heating as normally reported by otherauthors. This is primarily due to the unique design of dualinverted cylindrical magnetrons which greatly enhances theplasma ion current densities and aids in the growth of crystallinecoatings for compounds [21].

We carried out Scanning Electron Microscopy (SEM) studieson alumina films deposited on the silicon and glass substrates.Fig. 5 shows the surface morphology of the sample 1 at twomagnifications. We see the existence of droplets or sphericalclusters on the sample surface. These spherical clusters areagglomerates of ultrafine grains or nanocrystals of γ-alumina.Further, it appears that smaller clusters have agglomerated toform bigger ones. On annealing to 400°C, these bigger clustersbegin to disappear but the grain size seems to grow as shown bythe XRD studies (Fig. 3). Sample 2 has a “cauliflower” or afractal surface microstructure as can been seen clearly in Fig. 6.Drastic differences can be observed in the surface morphologyof the alumina coatings annealed to 900°C (Fig. 7). Thespherical agglomerations of ultrafine particles have completelydisappeared and instead, we see a large number of randomlyoriented crystals/grains of lenticular and faceted morphology inthe size range of 150 to 300nm. Spherical agglomerates couldalso be seen on the surface of the alumina coating deposited onthe WC-Co substrate (Fig. 8). Interestingly, while the surfacemorphology of the alumina coatings deposited on the glasssubstrate (sample 5) again shows some kind of agglomeration ofultrafine grains, these agglomerates or clusters are delocalizedand form a nearly continuous surface film (Fig. 9).

Fig. 11. Reduced elastic modulus of alumina thin films.

Nanomechanical properties of all thin film samples weredetermined by nanoindentation methods as described in Section2.5. Figs. 10–12 show the values of hardness, elastic modulusand elastic recovery parameters for all samples. The elasticrecovery parameter is a useful dimensionless index that isclosely related to the ratio of hardness to modulus and is ofinterest in tribological applications. It is defined as the elasticrecovery parameter :

ðERPÞ ¼ ðhmax−hpÞ=hp ð5Þwhere, hmax=maximum depth and hp = plastic depth. Sample 1,which is an alumina coating on Al prelayer on a siliconsubstrate, has hardness of about 7GPa, which is considerablylower than the hardness value of 22GPa for the alumina coatingprepared on a Si substrate without any Al precoating. Thisdifference could be due to much higher sputtering pressures(1.06Pa) used during the preparation of samples 1–3 ascompared to the pressure of 0.29Pa used during the preparationof samples 4–6. A higher gas pressure during sputtering reducesthe energy of depositing atoms and makes the alumina thin filmsless dense or more porous. Also, a softer underlying Al layer forsample 1 can also contribute to its lower measured hardness.Furthermore, values of the measured thin film hardness andelastic moduli increase with annealing temperature. This is dueto two reasons: (1) the conversion of the underlying, soft layerof Al metal to a harder aluminum oxide and (2) the densificationof the film on its heat treatment. While comparison between theresults on two sets of coatings with and without Al precoatingwould have been more definite if depositions had been carriedout at the same working pressure, some clear conclusions canstill be drawn from this study. The differences in the measuredmechanical properties of the two sets of coatings are not onlydue to Al precoating but also due to different working pressures

Table 3Critical load of failure for alumina thin film samples

Sample no. Critical load Lc (mN)

1 103±32 108±183 170±514 157±315 73±316 93±3

Fig. 13. Optical micrographs showing the results of nanoscratch tests on alumina coatings on silicon substrates.

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used. This is clear from the measured hardness values ofsamples 3 and 4. Sample 3 initially had Al metal precoating,which was then removed by oxidation at 900°C. However, itsmeasured hardness value is less than that of sample 4 which wasprepared without Al precoating but at a lower working pressure.Thus the higher pressure used during the preparation of samples1–3 deteriorates their final film mechanical properties irrespec-tive of the presence or absence of Al prelayers.

Earlier, Chou et al. prepared thin films of amorphous andγ-alumina (with film thickness of 1200nm) by reactive RFsputtering on Si and NaCl substrates and measured theirhardness and elastic moduli by nanoindentation technique.These authors reported nanohardness of 9.0GPa for amorphousalumina and 7.2GPa for γ-alumina films [33]. Xu andRowcliffe also carried out nanoindentation studies on 920nmthick PVD grown alumina coatings and reported a low hardnessof 6.07GPa [34]. We note that the nanohardness value of ourγ-alumina coatings on silicon substrate without Al precoating

Fig. 14. Optical micrographs showing the results of nanoscratch tests on alumin

(sample 4) is significantly higher than those of samples 1–3 andclose to the value of 25GPa recently reported by Schütze andQuinto for γ-alumina coatings deposited by pulsed DC mag-netron sputtering technique at a substrate temperature of 550°C[35]. However, the hardness values of our γ-alumina coatingsare lower than the reported values for bulk and thin films of αand κ-alumina. Murphy et al. carried out nanoindentation studieson bulk biomedical grade α-alumina and found its hardness to bein the range of 27 to 31GPa. They observed that the measuredhardness decreased somewhat with increase in the penetrationdepth [36]. Söderlund et al. reported ultralow load indentationhardness ofα and κ-alumina coated cemented carbide cutting toolinserts to be in the range of 21 to 61GPa as a function of theapplied load [37]. Stollberg et al. reported hardness of combustionCVD grown α-alumina thin films (thickness 360nm) and singlecrystals of alumina (sapphire) to be 28.6 and 30.9GPa, re-spectively [38]. γ-alumina is supposed to be softer than α-alumina due to its more ionic bond character [39,40].

a coatings on glass (sample 5) and cemented carbide substrates (sample 6).

1116 A. Khanna et al. / Surface & Coatings Technology 201 (2006) 1109–1116

The results of nanoscratch tests are shown in Table 3 andFigs. 13 and 14. Fig. 13 shows the differences in scratchbehavior of alumina coatings due to Al precoating. We see thatfailure in case of sample 1, which has maximum thickness ofunderlying Al layer, is less severe although it has a lowercritical load for failure, Lc, due to its lower hardness. Sample 4shows a more dramatic failure due to the absence of Al bottomlayer but has a higher critical load value due to higherhardness. Alumina film on a glass substrate without Alprelayer (sample 5) has the lowest critical load for failure as itappears to be the most stressed coating which is indicated byits highest value for ERP and H /E ratios. As expected, thedamage is less severe after failure on the harder WC-Cosubstrate (Fig. 14).

Our nanoindentation studies show that, although Al prelayerimproves the alumina thin film adhesion and leads to lessdramatic failure under nanoscratch testing, it deteriorates thecoatings' hardness values. It may be noted that our aluminacoatings are 790nm thick. These thickness values are low,causing a large effect of Al prelayer on the top alumina coatings.Cutting tool applications typically require alumina layerthickness in the range of 2 to 5μm at which Al prelayer effectswould be negligible. Also, it should be noted that aluminacoatings on cutting tools are typically deposited on top ofanother hard coating layers, such as TiN, TiCN, TiAlN and HfN[1–3, 20, 41–44]. Thus, it is of considerable interest to study theeffect of suitable intermediate coatings on the properties ofalumina, including its hardness, crystal structure and depositionrate.

4. Conclusions

We conclude that hard, crystalline alumina thin films canbe prepared on silicon substrates without any deliberatesubstrate heating by the novel inverted cylindrical AC mag-netron sputtering technique at moderate discharge power of5kW. Al metal precoating on silicon substrates seems to de-teriorate the crystallinity of alumina coatings grown on top of it.Annealing at 400°C increases the grain size of the γ-phasewhile annealing at 900°C produces δ and κ-phases in thecoatings.

Further, both higher working pressure during sputtering andthe deposition of a strain-compliant Al precoating on thesubstrates seem to lower the measured hardness and elasticmodulus of alumina films but significantly improve theiradhesion or scratch resistant properties.

Acknowledgement

The authors would like to acknowledge the support providedby Arkansas Analytical Laboratory (AAL), University ofArkansas, Fayetteville, AR, USA for the use of XRD andSEM equipment. The research was funded by National ScienceFoundation (NSF) under the NSF GOALI project grant # DMI-00400167.

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