Applied Surface Science 290 (2014) 509– 513

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Applied Surface Science

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ailoring of optical and wetting properties of sputter deposited silica thin films by glancing angle deposition

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eywords:ilica thin filmlancing angle depositionputtering

a b s t r a c t

Synthesis of silica thin films by RF magnetron sputtering using glancing angle deposition (GLAD) hasbeen reported here. Films deposited at four different oblique angles (45◦, 60◦, 75◦, 85◦) show transitionfrom featureless to columnar structure. The silica films have been characterized for their optical andwetting properties and they exhibit potential for use in applications such as antireflection coatings, self-

cleaning coatings, antifogging coatings, etc. The reflectance decreases to a minimum of 3.15% whereastransmittance increases to a maximum of 93.43% when the films have been deposited at an oblique angleof 85◦. Water contact angle measurements were done to show the hydrophilic nature of the films. Thehydrophilicity of the films gets enhanced and a minimum water contact angle of 20◦ is achieved withincrease in the oblique angle of deposition owing to the increase in the values of roughness.

. Introduction

One-dimensional nanostructured films like nanorods oranocolumns have unique properties like anisotropy and largespect ratio (length/diameter) making them nanoscale buildinglocks in nanoelectronics, photonics, etc. and have stimulated greatesearch interest [1]. Oblique angle deposition (OAD) is a very use-ul technique which has enabled researchers to develop uniqueanostructures with tailored morphology [2–7]. Recently it haseen shown that, by combining oblique angle deposition (OAD)ith active control of the substrate rotation relative to the vapor

ource, a technique referred to as glancing angle deposition (GLAD),ne can produce nano-sized columnar films with controlled poros-ty and shapes [8–13].

Glancing angle deposition (GLAD) is an advanced physical vaporeposition technique which is based on impingement of the inci-ent flux onto the substrate from extremely oblique flux incidencengles ̨ > 80◦ (measured relative to the substrate normal) as shownn Fig. 1(a). During the initial growth of the films, the impingingtoms form isolated columns which cast shadow for the arrivingapor flux following line of sight deposition (as shown in Fig. 1(b)).ence the taller islands will receive more impinging atoms as com-ared to the smaller ones. This allows only the taller islands torow and leads to the formation of a columnar structure. The com-etition between limited adatom surface mobility and shadowingffect plays a key role in the evolution of the columnar structure ofhe thin film. One dimensional nanostructures fabricated by OADnd GLAD find application as antireflection coating [14,15], omni-irectional reflectors [16], superhydrophobic coatings [17], solarells [18], sensors [19], photonic band-gap crystals [20], etc.

Although, e-beam evaporation is more extensively used for this

urpose due to its high directionality, sputtering is also being usedor developing nanostructures by GLAD. Sputtering is a physicalapor deposition (PVD) method which is widely used for deposit-ng thin films applicable for a wide range of applications [21–23].

169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2013.11.006

© 2013 Elsevier B.V. All rights reserved.

Sputter deposited films are more uniform and homogenous innature and exhibit better adhesion with the substrate as comparedto the evaporated films. These advantages provide sputtering anedge over evaporation in the deposition of high quality films. Thesputtering process also provides better step coverage as comparedto evaporation. Silica thin films have been extensively grown byGLAD technique using evaporation but much work has not yet beendone using sputtering. Till date, there is only one report of sputterdeposition of silica using GLAD [24]. In this work, GLAD has beenemployed to grow silica thin films with varied morphology usingsputtering. They have been grown in different GLAD conditionsand have been characterized to study their optical and wettingproperties.

2. Experimental

Silica films were deposited on Si(1 0 0) and glass substratesin a RF magnetron sputtering system (AJA International). 99.9%pure silicon disc (2 in. diameter, 3 mm thick) was used as thesputter target along with oxygen and argon as the reactive andsputtering gasses, respectively. Inclined stainless steel substrateholders were fabricated for holding the substrates at obliqueangles. The films were deposited by GLAD mechanism at obliqueangles (angle between the substrate normal and the incident flux)of 45◦, 60◦, 75◦ and 85◦ and a rotation of 30 rpm was applied to thesubstrates. The level of base pressure achieved before depositionwas 8 × 10−4 Pa (6 × 10−6 Torr). Deposition was done at roomtemperature (no heating) at a RF power of 150 W and a workingpressure of 0.27 Pa (2 mTorr). The O2 and Ar flow rates used were5 and 10 sccm, respectively. The substrates were kept at a distance

of 15 cm from the target.

A field emission scanning electron microscope (FESEM) (Supra55, Carl-Zeiss) was used for imaging of the morphology andcross section of the silica films. The reflectance and transmittance

510 Tailoring of optical and wetting properties of sputter deposited silica thin films by glancing angle deposition / Applied Surface Science 290 (2014) 509– 513

Fig. 1. (a) Schematic of GLAD setup and (b) mechanism of GLAD.

Fig. 2. Morphology of the films deposited at angles of (a) 45◦ , (b) 60◦ , (c) 75◦ and (d) 85◦ , and cross-section of the films deposited at angles of (e) 45◦ , (f) 60◦ , (g) 75◦ and (h)85◦ .

lms by glancing angle deposition / Applied Surface Science 290 (2014) 509– 513 511

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Fig. 3. (a) Reflectance, (b) transmittance and (c) dispersion spectra of the silica films.

Tailoring of optical and wetting properties of sputter deposited silica thin fi

pectra of the films were measured using UV-visible spec-rophotometer (UV-1700 Pharmatech, Shimadzu). Refractive indexnd thickness of the films were measured using spectroscopicllipsometer (J.A. Woollam, M-2000V). Contact angle (CA) mea-urements were done using the CA measuring system (Phoenix00, SEO, South Korea). Roughness values and contour images wereaken using atomic force microscopy (AFM) (Dualscope 95-200,enmark).

. Results and discussion

.1. Film morphology

Fig. 2 shows the morphology as well as the cross-section ofhe films. It can be seen from Fig. 2(a)–(d) that the films possesauliflower type morphology. The film deposited at ̨ = 45◦ areeatureless while those deposited at ̨ = 60◦, 75◦ and 85 (showolumnar structure which becomes more prominent when ̨ = 85◦

s seen from Fig. 2(e)–(h), respectively. The films deposited appearo have a submicroscopic structure where the columns developedeem to have an approximate diameter of 50 nm as evident fromig. 2(h).

At extremely oblique flux incidence angles of ̨ > 80◦, the shad-wing mechanism is greatly enhanced and, in a regime of limiteddatom diffusion, results in a columnar microstructure. The filmsre devoid of any porosity as is evident from Fig. 2. But thelms deposited by evaporation method usually posses noticeablemount of porosity. Sobahan et al. [25] have reported the deposi-ion of silica films by e-beam evaporation using GLAD where theyave achieved considerable amount of porosity in their films. Thetructure comprises of an array of nanorods. As compared to this,he films deposited in our case seem to be much denser wherehe structure comprises of columns attached to each other forminggglomerates as can be seen from Fig. 2.

The level of vacuum generally used in evaporation systemss more or less 1000 times higher as compared to the sput-ering systems. At such low pressures of the order of 10−4 Pa1.3 × 10−3 mTorr), the vapor flux atoms get scattered to a lesserxtent which in turn improves the directionality of the materialapor flux. The angular width subtended by the vapor sources inputtering systems was determined to be 8◦ using the Monte Carloputter flux transport simulator SIMSPUD (SIMulation of SPUtteristribution) [26]. This angular deposition spread is therefore sig-ificantly higher than that of evaporated GLAD depositions, forhich the angular width subtended by the vapor sources were typ-

cally less than 2◦ [27]. This shows that physical sputtering typicallyxhibits a wider angular flux distribution as compared to evapora-ion, partially due to the solid angle subtended by the target as seent the substrate, and the rest due to the scattering of the sputteredaterial with the background gas [27]. This results in lower colli-ation in the sputtering system as compared to the evaporation

ystems. Furthermore, evaporated atoms reaching the substratesually posses less than 1 eV of energy as compared to the sputteredtoms which usually posses more than 10 eV of energy. This in turnelps the sputtered atoms to undergo diffusion on the substrateo a greater extent as compared to the evaporated atoms. Hence,omparatively higher pressure and energy enables the sputteredtoms to reach the shadowed regions and increase the nucleationensity on the substrate which in turn makes the films denser.

.2. Optical properties

Fig. 3(a)–(c) show the reflectance spectra, transmittance spec-ra and the dispersion curves for refractive index of the silicalms, respectively. Upon deposition of the silica films, the average

reflectance of glass has been reduced from 8.75% to 4.65% and theaverage transmittance has been increased from 90.53% to 92.14%over the wavelength range of 400 to 800 nm. The films depositedat ̨ = 85◦ exhibit minimum reflectance, maximum transmittanceand minimum refractive index as shown in Fig. 3 and Table 1.Lower refractive index of the films deposited at an oblique angleof 85◦ reduces reflection from the surface and thereby enhances

transmission.

512 Tailoring of optical and wetting properties of sputter deposited silica thin films by glancing angle deposition / Applied Surface Science 290 (2014) 509– 513

Table 1Optical, wetting, roughness and thickness data of the deposited silica films.

Sample Avg. R (%) R minima (%) Avg. T (%) T maxima (%) Refractive index (477 nm) Thickness (nm) Water contact angle RMS roughness (nm)

45◦ film 6.69 5.64 90.55 91.43 1.47 244 49◦ 3.4860◦ film 6.65 5.57 90.64 91.76 1.466 241 27◦ 5.0275◦ film 6.13 5.1 91.21 91.94 1.465 293 22◦ 6.5685◦ film 4.65 3.15 92.14 93.43 1.456 323 20◦ 7.92

Fig. 4. (a) Water contact angle of silicon substrate and the films and (b) AFM images of the morphology of the films.

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.3. Wetting property

Fig. 4(a) shows the water contact angle (WCA) of silicon andhe silica films. The WCA of silicon is 62◦. The wetting of the sil-con surface gets enhanced further on deposition of silica films.he films have hydrophilic nature and the wettability of the filmsncrease, i.e., the WCA successively decreases from 49◦ to 20◦ withncrease in the oblique angle of deposition owing to the increase inhe roughness of the films as confirmed from AFM analysis.

A simple model described by Wenzel [28] characterized thenfluence of surface roughness on the hydrophilicity of the solidurface, and he proposed that a sufficiently rough surface textureould be readily wetted [29].

Increased roughness will result in an increase in true sur-ace area and, thus, lead to an increased nominal surface energy.ccordingly, the contact angle varies as dictated by Young’s equa-

ion [30]. Wenzel’s law demonstrates that the contact angleill increase with increased roughness of a hydrophobic surface,hereas the contact angle will decrease with increased roughness

f a hydrophilic surface [31]. Li et al. [30] have reported a studyn superhydrophobic silica surfaces based on roughness createdy assembling various nanostructured materials. Here they havehown that the contact angle of hydrophobic films increase withncrease in roughness.

In our case, the films deposited in the oblique angle range of5–85◦ portray an increase in the root mean square (RMS) valuesf roughness from 3.48 to 7.92 nm when measured on 20 �m scanreas as is shown in Fig. 4(b) and Table 1. The corresponding waterontact angle of the films decreases from 49◦ to 20◦ with increasen the oblique angle of deposition. Hence the results of our exper-ments are in accordance with the Wenzel’s model. Similar kindf trend has also been reported by Solar et al. where they haveeposited molybdenum and titanium films by magnetron sputter-

ng using GLAD method [32]. The RMS values of roughness of thelms have been shown to increase with increase in the obliquengle of deposition. Such changes in morphology have led to arastic decrease in the values of contact angle of their films.

. Conclusions

The structure of the silica films grown using RF magnetron sput-ering has been tailored by varying the oblique angle of depositionf the material flux. The columnar structure becomes more promi-ent at an oblique angle of 85◦ which comprises of upright silicaolumns. The optical properties of the films have been tailored toxhibit a minimum reflectance of 3.15% and a maximum transmit-ance of 93.43% when deposited on glass at an oblique angle of 85◦

hich shows potential for application in antireflection coatings.urther, the wetting of the films has been varied to exhibit a min-mum water contact angle of 20◦. This portrays the hydrophilicature of the films which gets enhanced with increase in the obliquengle of deposition owing to the increase in the values of rough-ess. These wetting properties can be exploited for self-cleaningnd antifogging applications. Hence, combining these optical andetting properties, the silica films deposited by GLAD method showotential for application as a functional material.

cknowledgements

We would like to thank the Director, IMMT Bhubaneswar foriving permission to publish the results. We are thankful to Mrs.

y glancing angle deposition / Applied Surface Science 290 (2014) 509– 513 513

V. Aishwarya for help in AFM and contact angle measurement. Weare also thankful to Mr. Shubra Bajpai for help in FESEM imaging.Financial support from CSIR, New Delhi is acknowledged.

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Satyajit Sarkar a,b

Siddhartha Kumar Pradhan a,b,∗a CSIR-Institute of Minerals and Materials

Technology, Bhubaneswar 751013, Odisha, Indiab CSIR-Academy of Scientific and Innovative Research,

New Delhi 110001, India

∗ Corresponding author at: CSIR-Institute ofMinerals and Materials Technology, Bhubaneswar

751013, Odisha, India. Tel.: +91 674 237 9218.E-mail address: skpradhan@immt.res.in

(S.K. Pradhan)

4 September 2013Available online 13 November 2013