TiO2/SiO2 multilayer as an antireflective and protective coating depositedby microwave assisted magnetron sputtering

M. MAZUR*1, D. WOJCIESZAK1, J. DOMARADZKI1, D. KACZMAREK1, S. SONG2,and F. PLACIDO2

1Faculty of Microsystem Electronics and Photonics, Wroclaw University of Technology,11/17 Janiszewskiego, 50–372 Wrocław, Poland

2University of the West of Scotland, High Street, Paisley, PA1 2BE, and Scottish Universities PhysicsAlliance, University Ave., G128QQ Glasgow, United Kingdom

In this paper designing, preparation and characterization of multifunctional coatings based on TiO2/SiO2 has been de−scribed. TiO2 was used as a high index material, whereas SiO2 was used as a low index material. Multilayers were depositedon microscope slide substrates by microwave assisted reactive magnetron sputtering process. Multilayer design was opti−mized for residual reflection of about 3% in visible spectrum (450–800 nm). As a top layer, TiO2 with a fixed thickness of10 nm as a protective film was deposited. Based on transmittance and reflectance spectra, refractive indexes of TiO2 andSiO2 single layers were calculated. Ultra high vacuum atomic force microscope was used to characterize the surface proper−ties of TiO2/SiO2 multilayer. Surface morphology revealed densely packed structure with grains of about 30 nm in size. Pre−pared samples were also investigated by nanoindentation to evaluate their protective performance against external hazards.Therefore, the hardness of the thin films was measured and it was equal to 9.34 GPa. Additionally, contact angle of preparedcoatings has been measured to assess the wetting properties of the multilayer surface.

Keywords: TiO2, optical coating, hardness, wettability, magnetron sputtering.

1. Introduction

Antireflective coatings (ARCs) are of great interest ofa wide range of industries and applications especiallyincluding ophthalmics, optics, flexible displays, bio−engi−neering, energy generation, decorative and barrier coatingsfor pharmaceutical and food products [1–4]. Coatings madefrom thin films can modify electrical, mechanical and/oroptical properties of a solid base material in a cost−effectiveway. Some common examples are scratch−resistant coatingsfor spectacles, anti−reflective coatings for lenses and win−dows, transparent conducting coatings for flat−panel dis−plays and low−friction coatings for bearings [2,5,6].

Coatings are usually composed of thin films of variety ofmaterials such as metals, insulators and semiconductors. Tobecome potential candidates for multilayered structures,thin films should exhibit certain desired properties such ashigh transparency, homogeneity, good adhesion, high hard−ness and ability to survive in different environmental condi−tions. The most often used for optical coating manufactur−ing are TiO2, SiO2, Nb2O5, Al2O3, Ta2O5, MgF2 [7]. Theyare well transparent in a broad wavelength range and havelow (MgF2, SiO2), moderate (Al2O3, Ta2O5) or high (TiO2,

Nb2O5) refraction index. A single layer as an antireflectivecoating can reduce reflection only at very narrow spectral

range (so called V−coat). This is because reflection can beminimized by the interference effect for which the opticalthickness of the film should be equal to the quarter ofdesigned wavelength. For broadband antireflective coatingmultiple layer structures are more effective.

New applications and new costumers’ demands causedan increasing interest for coatings, that beside their primaryrole to be transparent or reflective in a specified wavelengthrange can also combine additional functionality. Very oftenit is also desirable to join different (sometimes opposite)properties in one coating, that is, preparation of multifunc−tional coatings. Such coatings are more widely used inmany fields of science and technology. This concerns thefunctional optical coatings having, among others, additionalproperties, e.g., increased hardness, hydrophobicity or resis−tance to abrasion.

TiO2 thin films have a wide optical band gap that resultsin an excellent optical transmittance in the visible and nearinfra−red regions. It has been used as a protective layer forvery large scale integrated circuits and for different opticalelements, because of its good insulating properties and highrefractive index [8]. Additionally, TiO2 is frequently used infield of gas sensor [9,10], electrochromic devices [11] andplanar waveguides [12]. In turn, SiO2 thin films are oftenused as layer with low refractive index and very good opti−cal transmittance.

Opto−Electron. Rev., 21, no. 2, 2013 M. Mazur 233

OPTO−ELECTRONICS REVIEW 21(2), 233–238

DOI: 10.2478/s11772−013−0085−7

*e−mail: michal.mazur@pwr.wroc.pl

In the present work antireflective multilayer, consistedof TiO2 and SiO2 thin films, has been prepared by micro−wave assisted magnetron sputtering method. Surface mor−phology, optical and protective properties of depositedmultilayer have been determined.

2. Experimental details

Antireflective multilayer based on TiO2 and SiO2 thin filmswas deposited by using a magnetron sputtering with a mic−rowave source to improve the plasma ionisation. Micro−wave assisted reactive sputtering allows to obtain homoge−neous and high optical quality thin films. In this projectMicroDyn® microwave−assisted reactive magnetron sput−tering system was used. System was equipped with a 120mm � 372 mm high purity (> 99.99%) Ti and Si targets, 10kW dc power supply and a 3 kW plasma source. The tar−get−to−substrate distance was approximately 122 mm [13].The substrate holder was rotated with the speed of 1 cycleper second. The cathode was water−cooled. Mass flow con−trollers were used to control argon and oxygen flow. Stan−dard microscope slides were placed on a large, rotatingdrum. Substrates were wiped with isopropanol. The cham−ber was evacuated using a turbomolecular pump backed bya rotary pump. In the chamber, after reaching the operatingpressure of ca. 10–5 Torr, argon and oxygen were introducedand the microwave plasma was turned on for 300 s to pre−−clean substrates surfaces from organic residues. The drumwas continuously rotating past the microwave position andduring the clean step the magnetrons were not powered.During rotation of the drum substrates pass magnetrons withTi and Si targets. Deposited thin films oxidized in plasmaenhanced by the microwave outside the sputtered target.During the process the deposition rate of TiO2 and SiO2 thinfilms was kept at 1.1 �/s using quartz deposition controller.

Microwave plasma acts as a virtual anode and provides

a source of electrons which reduces arcing. The temperature

inside the chamber during the deposition is less than 100°C,therefore MicroDyn® apparatus allows for deposition of dif−ferent materials on a wide variety of substrates, e.g. glass,polymers. The system was operated with optical monitoringwhich allows for controlling of thin film thickness duringthe deposition process. The deposition conditions are listedin Table 1.

During the deposition of magnetron and microwavepower, argon and oxygen flow were kept at the same levelfor each target. Thickness was fitted to the known n and kvalue of TiO2 and SiO2. In case of processes conducted withthe same deposition parameters the thin film density andstoichiometry are the same. Therefore, the refractive indexand extinction coefficient for sputtered material are equal.After reaching the exact value of thickness the process wasstopped.

X−ray diffraction (XRD) method was applied for struc−tural investigations of the single TiO2 thin films. The pat−terns were recorded at room temperature by using DRON−2powder diffractometer, with Co K� radiation. XRD mea−

surements have revealed, that TiO2 thin films directly afterdeposition were amorphous. Surface morphology of depos−ited samples was examined by atomic force microscopeUHV VT AFM/STM Omicron operating in a contact mode.Cantilever used for measurements was NT−MDT CSG01contact silicon probe with tetrahedral tip shape. For an ana−lysis of experimental data WSxM ver. 5.0 software wasused [14].

Optical properties of antireflective multilayer were de−termined based on transmittance and reflectance results.Optical properties of single TiO2 and SiO2 thin films wereinvestigated by using Aquila NKD−8000 spectrophotometerequipped with Pro−Optix software. Transmittance and ref−lectance spectra were recorded for 10 degrees angle of inci−dent light, for S and P polarisation. Based on recorded spec−tra, refractive index and extinction coefficient of single la−yers were estimated. Fitting of experimental results for TiO2

and SiO2 thin films were made using Drude – Lorentz dis−persion model and Powell’s analysis method. Calculationswere performed by using Pearson’s test – �2 (Chi−square).

Hardness and reduced elastic modulus measurementswere performed with a Hysitron nanoindenter instrumenta−tion equipped with Triboscope head. For nanoindentationtests, 2 mN force was suitably matched to the physicalthickness of the multilayer. Values of hardness and reducedelastic modulus are averaged from 10 indentations. Mea−surements were performed with the aid of Berkovich tip,which has a very flat profile and total included angle of142.3 degrees. This tip geometry has been used as the stan−dard for nanoindentation. In the traditional indentation test,an indenter tip with a known geometry is driven into a spe−cific site of the material surface to be tested, by applying anincreasing normal load up to a preset maximum value. Foreach loading/unloading cycle, the applied load value is plot−ted with respect to the corresponding position of theindenter. The resulting load−displacement curve is then ana−lyzed to determine mechanical properties of the samplematerial such as hardness, Young’s modulus, stress−strainstudies, time dependant creep measurement, fracturetoughness, plastic and elastic energy.

The measurements of contact angle and critical surfacetension were carried out with a computer controlled Theta

TiO2/SiO2 multilayer as an antireflective and protective coating deposited by microwave assisted magnetron sputtering

234 Opto−Electron. Rev., 21, no. 2, 2013 © 2013 SEP, Warsaw

Table 1. Parameters of microwave assisted reactive magnetronsputtering process applied for antireflective multilayer deposition.

Parameter TiO2 SiO2

Current (A) 12.86 8.6

Voltage (V) 312 464

Power (kW) 4 4

Microwave power (kW) 2.7 2.7

O2 partial pressure (Torr) 8.5×10–4 6.5×10–4

O2 flow (sccm) 56 51

Ar partial pressure (Torr) 3×10–3 3×10–3

Ar flow (sccm) 190 190

Deposition rate (�/s) 1.1 1.1

Lite tensiometer system manufactured by Attension. Theliquids used for the contact angle determination were dis−tilled water, ethylene glycol and diiodomethane. The con−tact angle measurements were performed according to thesessile drop method and were used to calculate critical sur−face tension of the prepared samples using a softwareprovided with the equipment [15].

3. Antireflective multilayer design details

Design of antireflective multilayer, based on TiO2 and SiO2thin films, was simulated using TFCalc and Film Star soft−ware. Titanium dioxide and silica were selected due to theirhigh transparency in a visible light range and a suitablevalue of refractive index (high and low, respectively). Themain goal of the design was to obtain transmittance above95% and reflectance lower than 5% in the visible spectralrange from 4 50 nm to 800 nm. Designed multilayer shouldbe composed of as least number of layers as possible.Designed antireflective coating (ARC) consists of five lay−ers and the upper one is TiO2 due to its well known protec−tive properties against external hazards. It is clear that coat−ing with larger number of TiO2 and SiO2 layers wouldimprove its antireflective properties. However, the authorsaim was to create coating with an admissible reflection andwith the minimum possible number of layers. The sche−matic view of antireflective coating is shown in Fig. 1.

Antireflective coating was designed by using a refrac−tive index and extinction coefficient characteristics evalu−ated for single thin films of TiO2 and SiO2 deposited bymicrowave assisted reactive magnetron sputtering process(Fig. 2). Refractive index and extinction coefficient of TiO2

at � = 550 nm were equal to 2.45 and 2.8×10–3, respectively,while in case of SiO2 were equal to 1.46 and 1.3×10–7,respectively. High refractive index of about 2.42 (� = 550nm) in case of TiO2 was found due to high energy deposi−tion process of microwave assisted magnetron sputtering.Similar value has been also obtained by others [2].

In Fig. 3 reflection spectrum of designed antireflectivecoating has been presented. The assumption was to obtainresidual reflection of about 3% in a visible spectral range.

In Table 2 the designed thickness of each layer in anti−reflective multilayer has been collected. The total thicknessof multilayer is 249.4 nm.

Table 2. Parameters of designed antireflective multilayer.

Layer number I II III IV VMaterial TiO2 SiO2 TiO2 SiO2 TiO2

Type of refractive index(H – high, L – low) H L H L H

Thickness (nm) 12.89 38.28 126.18 62.04 10.00

4. Results and discussion

Surface morphology of the antireflective multilayer wasinvestigated by using an ultra−high vacuum atomic forcemicroscope equipped with silicon cantilever, working incontact mode. In Fig. 4 AFM images with different magnifi−cation have been shown. As it can be seen, the structure isdensely packed and the surface is strongly homogenous.Grains of about 30 nm in size are visible. The roughness ofthe surface (RMS) is ca. 1.5 nm according to the resultsobtained from WSxM ver. 5.0 software.

Optical properties were measured by using Aquilankd8000 instrumentation. Transmittance and reflectance ofdeposited multilayer on microscope slides are shown inFig. 5. The antireflective multilayer was deposited at the

Opto−Electron. Rev., 21, no. 2, 2013 M. Mazur 235

Fig. 1. Schematic view of designed antireflective multilayer.

Fig. 2. Refractive index of TiO2 and SiO2 single layers.

Fig. 3. Reflection spectrum for designed antireflective coating.

both sides of the substrates. The measurements results haveshown that the prepared structure exceeds 97% of transmit−tance and the reflectance level is lower than 2.5% in thedesigned wavelength range.

During indentation, the indentation depth vs. time andthe load vs. time were recorded simultaneously. The hard−ness was calculated according to the equation (Oliver’s andPharr’s method [16,17])

HF

AMAX

C

� ,

where: Fmax is the maximum load and Ac is the contact areaof the Berkovich tip.

Images of antireflective multilayer after nanoindentationtest, shape of the indent and 2D profile of contact depth areshown in Fig. 6. On the 3D image of nanoindentation test,the line of profile of contact depth is marked.

As it can be seen from 2D profile of the contact depth,the indent has reached the second layer composed of SiO2.The hardness of antireflective multilayer is 9.34 GPa withthe standard deviation of ±0.13 GPa. This result is about 15% higher value than for a standard, single SiO2 layer [18]and about 30% higher than for a single TiO2 layer [19].What is worth to emphasize, the hardness of the preparedantireflective coating is almost two times higher than fora standard soda−lime glass [20]. The measurements havealso shown high value of reduced elastic modulus, whichequals 120.5 GPa with the standard deviation of ±1.31 GPaand testifies about low elasticity of the coating.

Wettability measurement results revealed hydrophobicbehaviour of antireflective multilayer surface. Contact anglewas equal to 96.5°, 72.4° and 59.7° for distilled water, ethy−lene glycol and diiodomethane, respectively. Droplet ima−ges of different liquids taken during the measurements areshown in Fig. 7.

Wettability of different solid materials can be character−ized by the method proposed by Zisman [15,21]. Using

TiO2/SiO2 multilayer as an antireflective and protective coating deposited by microwave assisted magnetron sputtering

236 Opto−Electron. Rev., 21, no. 2, 2013 © 2013 SEP, Warsaw

Fig. 4. AFM images with different magnification of the TiO2 toplayer of antireflective coating deposited on a microscope slide by

microwave assisted magnetron sputtering method.

Fig. 5. Transmittance and reflectance spectra of antireflective multi−layer deposited on both sides of microscope slides (10 degrees of in−

cident light).

Fig. 6. Images after nanoindentation of TiO2/SiO2 antireflectivemultilayer: (a) indent in multilayer, (b) 2D profile of total depth of

penetration.

a series of homologous liquids with different surface ten−sions, a graph of cos � vs. � is determined. Critical surfacetension equals the surface tension at which the plotted lineintersects 1.0 and it is often interpreted as the highest valueof surface tension of a liquid which will completely wet thesolid surface. Critical surface tension calculated from con−tact angles of measured liquids was 20.8 mN/m. Addition−ally, the surface free energy of the investigated antireflec−tive multilayer was determined using geometric and har−monic approaches. In a geometric mean approach, valueswere determined according to the Fowkes approach usingOwens and Wendt equations, while in a harmonic meanapproach values were estimated based on Wu equations.The results of surface free energy investigation have beenpresented in Table 3. The surface free energy is low, whattestifies about poor wettability of the prepared protectiveand antireflective coating. In case of geometric and har−monic approach, the main part of the total surface freeenergy is dispersive component.

Table 3. Results of critical surface tension and surface free energyinvestigations of TiO2/SiO2 protective and antireflective coating.

Critical surfacetension(mN/m)

Surface free energy (mN/m)

Approach: Dispersivecomponent

Polarcomponent

Total value

20.8 ±0.3Geometric: 27.5 ±0.4 1.0 ±0.1 28.5 ±0.5

Harmonic: 29.9 ±0.4 2.5 ±0.2 32.4 ±0.6

5. Conclusions

Antireflective coating consisting of five layers of TiO2 andSiO2, has been designed, manufactured and characterized.Designed multilayer consisting of five layers of TiO2 and

SiO2 thin films and TiO2 thin film at the top of the coatingwas deposited due to its protective properties. The totalthickness of the coating was 249.39 nm. Microwaveassisted reactive magnetron sputtering method was used todeposit the thin films.

Surface properties of antireflective multilayer wereinvestigated by an ultra−high vacuum atomic force micro−scope. The investigation revealed a densely packed struc−ture with visible grains of 30 nm in size. The surface wasvery homogenous and the RMS roughness was about 1.5nm. The transmittance and reflectance of the preparedmultilayer coating matches the designed spectra at the levelof 97% and 2.5%, respectively. The hardness of preparedmultilayer was 9.4 GPa, which is almost two times higherthan in case of a standard soda−lime glass. Therefore, thedesigned antireflective multilayer, consisting of TiO2 andSiO2 thin films with enhanced hardness is a good candidateto be used as a protective coating in, e.g., opthalmics, auto−motive or flat panel displays’ industry. Additionally, wet−tability measurements revealed hydrophobic behaviour ofthe multilayer what makes it easier to clean from dust anddebris.

Acknowledgement

This work was financed from the sources given by the Min−istry of Science and Higher Education in 2011–2013 asa supervisors’ research project number N N515 4970 40 andfrom the sources granted by the Ministry of Science andHigher Education in 2011 – 2013 as a research project num−ber N N515 4963 40.

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238 Opto−Electron. Rev., 21, no. 2, 2013 © 2013 SEP, Warsaw