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Applied Surface Science 351 (2015) 880–888

Contents lists available at ScienceDirect

Applied Surface Science

journa l h om epa ge: www.elsev ier .com/ locate /apsusc

haracteristics of laser textured silicon surface and effect of muddhesion on hydrophobicity

.S. Yilbasa,∗, H. Alia, M. Khaledb, N. Al-Aqeeli a, N. Abu-Dheira, K.K. Varanasi c

ME Department, King Fahd University of Petroleum & Minerals, Kfupm box 1913, Dhahran 31261, Saudi ArabiaCHEM Department, King Fahd University of Petroleum & Minerals, Dhahran, Saudi ArabiaMechanical Engineering, Massachusetts Institute of Technology, Boston, MA, USA

r t i c l e i n f o

rticle history:eceived 21 February 2015eceived in revised form 1 May 2015ccepted 7 June 2015vailable online 14 June 2015

eywords:aser texturingilicon wafer

a b s t r a c t

Laser gas assisted texturing of silicon wafer surface is carried out. Morphological and metallurgicalchanges in the treated layer are examined using the analytical tools. Microhardness and fracture tough-ness of the laser treated surface are measured using the indentation technique while residual stressformed is determined from the X-ray diffraction data. The hydrophobicity of the textured surfaces areassessed incorporating the contact angle data and compared with those of as received workpiece sur-faces. Environmental dust accumulation and mud formation, due to air humidity, at the laser treatedand as received workpiece surfaces are simulated and the effect of the mud residues on the propertiesof the laser treated surface are studied. The adhesion work due to the presence of the mud on the laser

ydrophobicityust

treated surface is also measured. It is found that laser textured surface composes of micro/nano poles andfibers, which in turn improves the surface hydrophobicity significantly. In addition, formation of nitridespecies contributes to microhardness increase and enhancement of surface hydrophobicity due to theirlow surface energy. The mud residues do not influence the fracture toughness and microhardness of thelaser textured surface; however, they reduced the surface hydrophobicity significantly.

© 2015 Elsevier B.V. All rights reserved.

. Introduction

Silicon finds applications in medical and electronic devices1–3]. One of the main users of silicon is the photovoltaic (PV) indus-ry. Low surface reflectance of solar PV cells is required to maximizehe amount of incident solar radiation absorbed by the cells, whichonvert light to electrical energy. Surface texturing is one of theethods to increase photon capturing at the PV cells surface [4,5].

n addition, nano/micro scale surface texturing alters the hydropho-ic characteristics of the surface, which enhances the self-cleaningroperties of the surface [6]. In harsh environments, the dust accu-ulation and the mud formation from the dust particles in the

umid air ambient on PV surfaces are the important challenges toeet for the efficient operations. To improve the surface hydropho-

icity, surface should have low energy, which requires chemicalodification of the surface. Surface energy of silicon can be reduced

hrough a nitride formation at the surface [7]. Although many tech-iques are available to form nitride species at the silicon surface,ome of these techniques face processing difficulties [8]. One of the

∗ Corresponding author Tel.: +966 38604481; fax: +966 38602949.E-mail address: bsyilbas@kfupm.edu.sa (B.S. Yilbas).

ttp://dx.doi.org/10.1016/j.apsusc.2015.06.036169-4332/© 2015 Elsevier B.V. All rights reserved.

effective techniques to produce silicon nitride at silicon wafer sur-face is to use a high temperature gas-assisted nitriding process.Since the laser beam can be used effectively to generate a hightemperature environment at the surface, gas assisted processingtogether with a laser beam heating can produce nitride species atthe irradiated surface [9]. The combination of control evaporationand melting results in textures consisting of micro/nano poles at thelaser treated surface while further improving surface hydropho-bicity [10]. However, high temperature gradients occur within thevicinity of the laser treated layer causing excessive stress levels inthis region, which in turn limits the practical applications of thetreated surface. In addition, surface hardening, due to high coolingrates, and nitride compounds formation reduces the fracture tough-ness of the laser treated surface. Consequently, the cooling ratesand the formation of the nitride compounds need to be controlledduring the laser processing. On the other hand, when the texturedsilicon surface is exposed to the harsh environments, such as dustand humid environments, the dust accumulation and mud forma-tion, due to condensation of water vapor, become unavoidable at

the surface. Dust particles contain alkaline (Na) and earth alkinemetals (CaCO3) [11], which dissolve into the condensate (water)while changing pH of water toward the basic state. The porousstructure of the mud allows sedimentation of the dust solution

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B.S. Yilbas et al. / Applied Su

water with dissolved ionic compounds) at the interface betweenhe treated surface and the mud. The mud and the mud solutiont the laser treated silicon surface dries out by time. The driedud solution results in the formation of the covalent bonds at the

aser treated surface while modifying the surface chemistry andhe adhesion force required to remove the mud from the treatedurface becomes considerably large. Consequently, investigation ofhe properties of the laser treated layer including microhardness,racture toughness, residual stress, friction coefficient, hydropho-icity, and the adhesion work required to remove the mud from theurface becomes essential.

Considerable research studies were carried out to examine laserreatment of silicon surfaces. Laser short-pulse irradiation of siliconurface and the assessment of the microstructures were examinedy Ma et al. [12]. They characterize the surface morphology andhemical compositions of the microstructures using the analyticalools. The modification of silicon surface using a nanosecond laserrradiation pulse was carried out by Wang et al. [13]. They indicatedhat increasing of laser fluences resulted in melt and evaporationhile texturing the surface. In addition, the properly selection of

he process parameters was necessary for a well-defined gratingnd dot structures. The laser texturing of silicon surface using theanosecond and sub-nano second pulses was carried out by Totht al. [14]. They observed that for the nanosecond laser pulse irra-iation, the thickening of the oxide layer and the appearance of aodified, less crystalline silicon layer, took place at the substrate

urface. The study on the fabrication of luminescent a-Si:SiO2 struc-ures due to direct irradiation of high power laser on silicon surfaceas carried out by Dey and Khare [15]. They showed that the micro

luster was formed and amorphous silicon embedded in siliconioxide took place after laser irradiation at the surface. The morpho-

ogical changes and surface hydrophobicity due to laser texturingf silicon surface and was investigated by Li et al. [16]. They indi-ated that the grating microstructures fabricated by femtosecondaser were smoother with small groove width; however, improved

icro-nanoscale hierarchical structures could be achieved by fem-osecond laser overlapped etching for many times. Laser texturingn silicon surface was studied by Crawford and Haugen [17]. Theybserved that surface structures had the spatial periods substan-ially less than the incident laser wavelengths. Laser texturing onilicon and glassy carbon surfaces was carried out by Abramov et al.18]. Their findings revealed that surface structures on silicon (1 1 1)ad deeper pores than on silicon (1 0 0) at equal conditions of laserction. The similar nanostructures were also found on the surfacef glassy carbon after interaction with femtosecond laser radia-ion. The influence of laser pulse intensity on the nanostructuresormed at the silicon surface was examined by Varlamova et al. [19].hey showed that the textured surface had features composed of arater with periodically structured pillar-like walls. A CO2 laser sur-ace texturing of silicon was carried out by Trtica and Gakovic [20].hey demonstrated that the surface texture composed of wave-likeeriodical microstructures mixed with the spherules like configu-ations. The wetting properties of laser-textured surfaces throughepositing triboelectrically charged Teflon particles were investi-ated by Bayer et al. [21]. The findings revealed that when theverage substrate roughness was between 15 and 60 �m, super-ydrophobic surfaces could be easily formed by Teflon depositionith a water contact angle hysteresis less than 8◦.

Although laser texturing of silicon surface was studied pre-iously [9], the main emphases was to examine the surfaceorphology and microstructure. The surface characteristics includ-

ng hydrophobicity and laser treated surface exposed to a mud,

hich is formed from the dust due to vapor condensation, were

eft for the future study. Therefore, in the present study the laserexturing of silicon wafer surface is carried out. The surface char-cteristics including hydrophobicity, fracture toughness, residual

cience 351 (2015) 880–888 881

stress, and scratch resistance are investigated using the analyticaltools. In addition, the influence of mud formed from the environ-mental dust on the surface characteristics of the laser treated siliconsurface is examined.

2. Experimental

The CO2 laser (LC-ALPHAIII) delivering nominal output powerof 2 kW was used to irradiate the workpiece surface. The nomi-nal focal length of the focusing lens was 127 mm while resultingin 200 �m of the irradiated spot diameter at the workpiece sur-face. Nitrogen assisting gas emerging from the conical nozzle andco-axially with the laser beam was used during the laser treat-ment process. The frequency of the laser pulses was 1500 Hz, whichresulted in 72% overlapping ratio of the irradiated spots at the work-piece surface. The laser treatment tests were repeated several timesby incorporating different laser parameters; in which case, laserparameters resulting in the minimum surface defects, such as verysmall cavities and no crack networks, are selected in the light of theprevious study [9]. Increasing laser power at the workpiece by 10%,while keeping the laser scanning speed same as selected, causedcavity formation at the surface, which in turned increase the sur-face roughness considerably. However, similar behavior was alsoobserved when laser scanning speed was reduced by 10% whilekeeping the laser output power the same as selected. On the otherhand, increasing laser scanning speed or lowering laser put powerdid not result in the texture giving rise sufficiently high contactangles. Laser treatment conditions are given in Table 1.

Silicon wafer with 1 mm thickness is used in the experiments.Material characterization of the laser treated surfaces was carriedout using SEM and XRD. Jeol 6460 electron microscopy was used forSEM examinations and Bruker D8 Advanced having CuK� radiationwas used for XRD analysis. A typical setting of XRD was 40 kV and30 mA and scanning angle (2�) was ranged 20–80◦.

Microphotonics digital microhardness tester (MP-100TC) wasused for the surface microhardness measurements. The standardtest method for Vickers indentation hardness of advanced ceramics(ASTM C1327-99) was adopted. The measurements were repeatedfive times at each location for the consistency of the results.

The residual stress measurement relies on the stresses in finegrained polycrystalline structure; in which case, shifting of thediffraction peak occurs as the specimen is tilted by an angle . Themagnitude of the shift is related to the magnitude of the residualstress. The relationship between the peak shift and the residualstress (�) is given [22]:

� = E

(1 + �) sin2

(dn − do)do

(1)

where E is Young’s modulus, � is Poisson’s ratio, is the tiltangle, and dn are the d spacing measured at each tilt angle. Ifthere are no shear strains present in the specimen, the d spacingchanges linearly with sin2 . The �-phase silicon nitride formed atthe surface was hexagonal with the lattice parameters determinedas ao = 7.681 A and co = 2.874 A, which agreed with the previouslyreported data for the bulk silicon nitride [23]. The XRD data wereobtained for the X-ray radiation diffracted from the (3 2 3) �-phasecrystallographic planes. The elastic constant (E/ (1 + �)) of 265 GPawas employed for the residual stress calculations. In this case, theYoung’s modulus of 317 GPa and Poisson’s ratio of 0.27 were used[24]. Fig. 1 shows the d-spacing with sin2 used for the resid-ual stress measurement; in which case, the slope of the curve

is −1.5386 × 10−13 nm and the residual stress estimated is in theorder of 50 ± 5 MPa.

The fracture toughness of the surface was measured using theindenter test data for microhardness (Vickers) and crack inhibiting.

882 B.S. Yilbas et al. / Applied Surface Science 351 (2015) 880–888

Table 1Laser heating conditions used in the experiment.

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Scanning speed (cm/s) (mm/min) Power (W) Frequency (Hz)

10 90 1500

he length of the cracks generated due to indentation at the sur-ace was measured. The crack lengths were individually summedo obtain c =

∑l as described in the previous study [25]. Here, the

ength (l) measured corresponded to the distance from the crackip to the intend. In this case, c = a +

∑l where 2a was the indent

iagonal length [26]. However, depending upon the ratio of c/a, var-ous equations were developed to estimate the fracture toughnessK). The equation proposed by Evans and Wilshaw [27] was usedo determine the fracture toughness (K), which was applicable for0.6 ≤ c

a ≤ 4.5 [28]), i.e.:

c = 0.079(P

a

) 32 × log

(4.5P

a

c

)(2)

here P is the applied load on indenter, c is the total crack length,nd a is the half indent diagonal length. The data used for the cal-ulations are given in Table 2

A linear micro-scratch tester (MCTX-S/N: 01-04300) was used toetermine the friction coefficient of the laser treated and untreatedurfaces. The equipment was set at the contact load of 0.03 N andnd load of 5 N. The scanning speed was 5 mm/min and loading rateas 1 N/s. The total length for the scratch tests was 1 mm.

The wetting experiment was performed using Kyowamodel—DM 501) contact angle goniometer. A static sessilerop method was considered for the contact angle measurement.he water contact angle between the water droplet and the heatreated surface was measured with the fluid medium as de-ionizedater. Droplet volume was controlled with an automatic dispens-

ng system having a volume step resolution of 0.1 �l. Still imagesere captured, and contact angle measurements were performed

fter one second of deposition of water droplet on the surface.In order to simulate the effect of mud on the characteristics of

he laser treated and as received workpiece surfaces, an experi-ent was carried out to resemble the actual mud formation, due to

ondensation of water vapor on the accumulated dust particles, athe surfaces of workpieces. In this case, a layer of 300 �m thicknessomposing of dust particles, which were collected from the local

nvironment, was formed at the laser treated and as received work-iece surfaces. The desalinated water with same volume of dust wasispensed gradually on to the dust layer. It should be noted thathe initially condensation tests carried out in the local humid air

Fig. 1. Linear dependence of d-spacing with sin2 ( ).

Nozzle gap (mm) Focus setting (mm) N2 pressure (kPa)

1.5 127 600

to estimate the amount of condensate accumulated over the time.It was found that the amount of condensate had same volume ofdust over six hours. In order to resemble the water condensation inthe humid air, dispensed water was left on the surface of the dustlayer without mechanical mixing. This results in natural formationof mud at the workpiece surfaces. The workpieces were kept in alocal normal ambient air for three days to dry. The laser treatedand as received workpiece surfaces with the presence of dry mudwere tested for adhesion work measurements. Later, the dry mudwas removed from the surface with a desalinated water jet hav-ing 2 mm diameter and 1.5 m/s velocity. The cleaning process wascontinued for 15 min for each workpiece surface. Finally, the mor-phology, microhardness, fracture toughness, and hydrophobicity ofthe cleaned surfaces were analyzed using the analytical tools.

3. Results and discussions

Laser evaporation and control melting of silicon wafer surfacesis carried out and the surface texture characteristics are exam-ined. The residual stress and the fracture toughness of the lasertreated surface are measured using the XRD technique and theindentation tests, respectively. It should be noted that the laserpower intensity distribution at the irradiated surface is Gaussianand the peak power intensity occurs at the irradiated spot cen-ter. Consequently, temperature at the irradiated center reachesthe evaporation temperature of the substrate material while at asmall distance away from the irradiated spot center melting takesplace due to relatively lower power intensity as compared to thatof at the irradiated spot center. Since the surface melting is con-trolled, the excessive melt flow does not take place across thelaser scanning tracks at the surface. However, the molten mate-rial in the vicinity of the irradiated spot center flows into thesmall cavity formed in the neighborhood of the irradiated spotduring the repetitive pulsing. This in turn reduces the cavity sizein terms of diameter and the depth. Consequently, through con-trolling the power settings and the beam intensity distributiontogether with the pulse repetition rate, the spot size and the scan-ning speed, desired texture could be achieved at the workpiecesurface.

3.1. Morphological, metallurgical and hydrophobicity of treatedsurface

Fig. 2 shows SEM micrograph of laser treated surface. In gen-eral, regular scanning tracks are evident at the surface (Fig. 2a).Since the overlapping ratio of the repetitive laser irradiated sotsis 72%, melting and evaporation result in regular laser irradiatedtracks during the laser scanning at the surface (Fig. 2b). The lasertreated surface is free from large asperities including large sizecracks and voids. However, surface evaporation results in fine sizecavities and micro/nano fiber like structures at the surface (Fig. 2c).Although temperature gradients remain high in the evaporatedregion due to high cooling rates and small area of evaporation,no thermally induced cracks are observed at the surface. This isassociated with the self-annealing effect of the recently formed

laser scanning tracks on the initially formed tracks. In this case,heat conduction from the recently formed tracks toward the earlyheated region modifies the cooling rates in the treated region. Con-sequently, the laser pulse repetition results in crack free with small

B.S. Yilbas et al. / Applied Surface Science 351 (2015) 880–888 883

Table 2Data obtained after indentation tests for microhardness and fracture toughness, and residual stress determined from XRD technique. c Represents the total crack lengtharound the indentation mark, and a is the half indent diagonal length. The error estimated based on the repeats of the indentation test is 7%.

Microhardness (HV) Residual stress (MPa) Fracture toughness (MPa m1/2) P (N) c ×10−6 (m) a ×10−6 (m)

As received surface 710 – 1.81 7 250 90Laser treated surface 840 50 6.05 7 80 45Laser treated surface with mud residues 825 52 6.19 7 75 45

cks, (b

cisrttcctf[tiransvttiAslltt

of nitride species occurs at the treated surface.Fig. 5 shows SEM micrographs of the laser treated workpiece

cross-section. The depth of laser treated layer extends almost

Fig. 2. SEM micrographs of laser treated surface: (a) Regular laser scanning tra

avities at the surface. Since the high pressure nitrogen gas is usedn the irradiated region, nitride formation is unavoidable at theurface. This also causes formation of fine grains in the surfaceegion [29]. The formation of nitride compound is evident fromhe XRD diffractogram. Fig. 3 shows X-ray diffraction patterns forhe laser treated workpiece. It is evident that the surface region isomposed of mainly �-phase silicon nitride with the presence of-phase (crystalline) silicon at the laser treated surface. Althoughhe untreated silicon wafer composes of a-phase silicon only, theormation of c-phase silicon is associated with the thermal effect30]. The formation of nitride phase is associated with the highemperature processing at the surface during the laser scanning;n which case, high pressure nitrogen gas is used in the irradiatedegion. The distribution of the nano-poles appears to be randomnd does not conform a regular pattern. This is associated with theon-uniform cooling and solidification of the molten material at theurface. In addition, nitrogen assisting gas contributes to the con-ection heat transfer from the irradiated surface during the laserreatment process. Consequently, micro-turbulence generated athe surface alters the heat transfer rates at the surface while result-ng in non-uniform cooling rates across the heated surface [31].tomic force microscopy images of the laser treated surface arehown in Fig. 4. The irregular structures are also evident at the

aser treated surface. The roughness is in the order of 0.8 �m at theaser treated surface. The EDS data are given in Table 3a on the laserreated surface. It should be noted that the precise quantification ofhe light elements, such as nitrogen, is difficult in the measurement

) overlapping of irradiated laser spots at the surface, (c) laser textured surface.

system; however, nitrogen in the EDS data indicates the formation

Fig. 3. X-ray diffractogram for laser treated surface and laser treated and mudremoved surface.

884 B.S. Yilbas et al. / Applied Surface Science 351 (2015) 880–888

Fig. 4. AFM images of laser treated surface: (a) 3D image of texture, (b) textureprofile.

Table 3aEDS data after laser surface texturing and mud removal from the surface.

O N Mg Ca K Si

As received surface 5.7 – – – – BalanceLaser treated surface 6.2 15.3 – – – Balance

4pgrbd

Laser treated surfacewith mud residues

23.2 4.2 3.2 8.4 2.1 Balance

0 �m below the surface. In general, the laser treated layer com-oses of two distinct regions. The dense layer consisting of finerains forms the first region in the surface vicinity. The high cooling

ates and the formation of �-phase silicon nitride are responsi-le for the volume shrinkage at the surface while resulting inense layer in this region. Although the cooling rates are high, no

Fig. 6. Optical images of water droplets on the workpiece surfaces: (a) laser treate

Fig. 5. SEM micrographs of cross-section of laser treated layer.

thermally induced crack is observed in the dense layer. This isassociated with the self-annealing effect of the lately formed laserscanning tracks on the initially formed tracks via heat conductionbelow the surface. In the second region, grain size increases becauseof relatively lower cooling rates as compared to those at the surface.

Fig. 6 and shows the optical images of droplets used for thecontact angle measurement on the laser treated and untreatedsurfaces. The Young’s equation provides the relation for the con-tact angle of a liquid droplet on a perfectly smooth and chemicallyhomogenous solid surface, which is [32]:

cos � = (�sv − �sl)�lv

(3)

where � is the contact angle, �sv is the interfacial tensions ofsolid–vapor, �sl is the interfacial tensions of solid–liquid, � lv isthe interfacial tensions of liquid–vapor. In the case of the rough

d surface, (b) laser treated and mud removed surface, (c) as received surface.

rface Science 351 (2015) 880–888 885

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B.S. Yilbas et al. / Applied Su

urfaces, the Young’s equation is modified to include the surfaceoughness (r), which is [33,34]:

os �w = r (�sv − �sl)�lv

(4)

here �w is the rough surface contact angle, r is the surface rough-ess factor, which is defined as the ratio between the actual androjected surface areas,; in which case, r = 1 corresponds to theerfectly smooth surface and r > 1 is the rough surface. The liq-id surface interface has the components of a liquid–solid and a

iquid–vapor interfaces. In this case, the equation for the contactngle yields [35]:

os �c = f1 cos �1 + f2 cos �2 (5)

here �c is the apparent contact angle, f1 is the surface fractionf liquid–solid interface, f2 is the surface fraction of liquid–vapornterface, �1 is the contact angle for liquid–solid interface, and �1s the contact angle for liquid–vapor interface. However, for their–liquid interface, f1 can be presented in terms of the solid frac-ion (f) and air fraction (f2), which is (1 − f). It should be noted that fanges within 0 to 1. Hence, f = 0 corresponds to the liquid droplet,hich is not contacting at the surface; however, f = 1 represents

he surface, which is completely wetted. It was demonstrated thathe small contact area between the liquid droplet and solid surfacellowed the droplet to roll easily at the surface, which correspondedo the Cassie–Baxter state [35]. In the actual situation, dependingn the surface energy, the surface texture, and impact of the droplett the surface, the contact mode changes from Cassie–Baxter stateo Wenzel state [36]. Two states can co-exist on a nano-pillared sur-aces [37]; however, when a liquid–air interface can remain pinnedt the pillars tops, transition to the Wenzel state is possible. Sincehe laser treated surface consists of fine textures of micro/nanoillars and low surface roughness, the air pockets can occupy theaps in between the pillars (Fig. 2b). This results in Cassie and Bax-er state [34,38] while generating a hydrophobic response of theaser treated surface. The contact angle measurements are given inable 4 In addition, the presence of silicon nitride at the surfaceFig. 3) contributes to hydrophobicity of the laser treated surface. Ithould be noted that the surface energy of silicon nitride is less thanilicon [39]. Moreover, the contact angles measured vary over theaser treated surface. This behavior is related to the non-uniformistribution of micro/nano pillars and locally presence of closelypaced fine piles at the surface, which occurs randomly. Never-heless, the laser treated surface is hydrophobic and it becomesuperhydrophobic at some regions of the surface, provided that theuperhydrophobic region covers only 12% of the laser treated sur-ace. In the case of as received surface Wenzel state occurs and theontact angle is in the order of 63 ± 5◦. This is associated with theonditions of the workpiece surface; in which case, the roughnessnd the texture of the surface correspond to the smooth surface. Inddition, untreated workpiece has relatively higher surface energys compared to the laser treated surface, which contributes to theurface hydrophilicity of the as received workpiece.

.2. Microhardness, absorption spectrum, residual stress, fractureoughness, and friction coefficient of laser treated surface

Microhardness data are given in Table 2 for laser treated and aseceived workpiece surfaces. The surface microhardness increasesignificantly after the laser treatment; in which case, it remainsigher than the hardness of the as received workpiece surface.icrohardness enhancement is associated with; (i) dense layer for-

ation consisting of fine grains in the surface region, and (ii) nitride

hase formation at the surface because of high pressure nitrogenssisting gas was used during laser processing. Fig. 7 shows FTIRata obtained for the laser treated workpiece surface. The vibration

Fig. 7. FTIR spectrum for laser treated surface.

of Si Si bond (610 cm−1) and Si O Si bond (1105 cm−1) are visiblein the spectrum. The absorption peaks of 1105 cm−1 and 816 cm−1

are related to asymmetric stretching and symmetry stretching ofSi O Si, respectively. Some weak vibration peaks are also observedsuch as Si OH bending vibration occurring at 905 cm−1 and Si Hbond deformation at 738 cm−1. The absorption peak of 1105 cm−1

appears to be small and broaden, which indicates the presence oflow SiOx; however, large peak of SiOx is observed at 2300 cm−1. Abroad Si N stretching takes place at 734–904 cm−1 and main peakoccurs at 840 cm−1, which agrees well with the data reported inthe previous study for the stoichiometric silicon nitride [40,41]. Thepresence of small oxygen alters the composition from silicon nitrideto silicon oxynitride. This is associated with the induction effect onthe Si N bond by the oxygen atoms; in which case, the shift towardthe higher wavenumbers takes place due to electronegative chargeof oxygen atoms. However, silicon oxynitride peak is not observedfrom the X-ray diffractogram (Fig. 3); therefore, this effect willbe considerably small. The absorption characteristics of the lasertreated surface indicate that absorption increases at certain wave-lengths. This can be explained in terms of the crystallinity inductiontaking place at the surface, which causes restriction group vibrationwhile resulting in band intensity increase in the absorption spec-trum. Table 2 gives the residual stress data measured at the lasertreated surface using the XRD technique. It is evident that the resid-ual stress formed in the surface vicinity of the laser treated layeris in the order of −50 ± 5 MPa, which is compressive. The forma-tion of residual stress is associated with the thermal effect due tohigh cooling rates, nitride phase formation, and crystallization atthe laser treated surface. The molecular structures are modified inthe crystal form while causing the stress formation after comple-tion of the crystallization process [42]. This finding is also supportedfrom the FTIR data; in which case, the crystallinity induction causesrestriction group vibration while contributing to the residual stressformation. Table 2 gives the fracture toughness obtained from theindentation tests. It should be noted that the tests were repeatedfive times at different locations at the laser treated surface and theerror was estimated as 7%. Since the hardness increases at the lasertreated surface, the fracture toughness reduces. This is because ofthe attainment of the high cooling rates and formation of nitridecompounds, which increases the hardness and reduces the frac-ture toughness at the laser treated surface. Fig. 8a shows frictioncoefficient of the laser treated and as received workpiece surfaceswhile Fig. 8b shows the surface scratch marks resulted after thescratch tests. The enhancement of surface microhardness results

in attainment of lower friction coefficient at the laser treated sur-face than that of as received surface. The depth and the width ofthe scratch mark at the laser treated surface are shallow becauseof the increased hardness at the surface. In addition, no crack or

886 B.S. Yilbas et al. / Applied Surface Science 351 (2015) 880–888

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TE

Table 4Contact angles measurement results prior to and after the laser surface texturing,and textured surface with mud residues. Water is used in the measurements.

Contact angle (degrees)

As received 67.2 (+5/−5)

of the presence of mud at the as received workpiece surface is alsogiven for the comparison reason. It is evident that the friction workis considerably smaller than that corresponding to the adhesion

ig. 8. Friction coefficient at the workpiece surfaces: (a) friction coefficient for laserreated, as received, and laser treated and mud removed surface, (b) scratch marksn laser treated and laser treated and mud removed surfaces.

rack network is observed around the scratch marks for both laserreated and untreated surfaces.

.3. Effect of dry mud on characteristics of laser treated surface

Fig. 9 shows optical image (Fig. 9a) and SEM micrographs ofhe laser treated surface covered with the mud. Since the mud isormed from the dust and the desalinated water, resembling theondensation of water in the humid air, the mud surface consistsf closely adhered fine dust particles (Fig. 9b). The dust particleize varies within 20 �m–0.01 �m and the average dust particleseasured is in the order of 1.2 �m. The EDS data for the dust parti-

les at two locations are given in Table 3b. Since the dust particlesossess alkali and alkali earth metals (Na, Ca), they dissolve in theater and the solution penetrates through the mud cross-sectionhile reaching at the laser treated surface. It should be noted that

he mud cross-section composes of a porous structure, which cane observed from SEM micrograph as shown in Fig. 9c. The liq-id solution accumulated at the interface of the mud and the laserreated surface forms a dense crystal layer upon drying the solu-ion, which acts like adhering layer at the interface. However, oncehe mud is removed with a jet of desalinated water, the residue of

ud particles remains at the laser treated surface, which can bebserved from SEM image as shown in Fig. 9d. The mud residuesorm a strong bond at the laser treated surface despite the pres-urized desalinated water jet was used to remove them. The totalrea covered by mud residue is large at the surface, which is esti-

ated as 9%. This indicates the strong adhesion of the mud residues

emaining at the laser treated surface. The XRD diffractogram of theaser treated surface with the present of dust residues reveals that

able 3bDS data at the dust surface.

O Mg Al Ca Fe Na Si

Location 1 43.2 4.4 5.9 14.7 4.7 1.7 BalanceLocation 2 43.7 4.5 3.4 26.2 2.6 1.2 Balance

Laser textured 131.6 (+5/−5)Laser textured with mud residues 78.5 (+5/−5)

CaCO3, NaCl, MgO peaks are present (Fig. 3), which correspondsto the mud residues at the surface. Microhardness and fracturetoughness measurements are repeated at the laser treated sur-face with the presence of the mud residues. The findings are givenin Table 2. Microhardness and fracture toughness of the surfaceremains almost same for the laser treated only and laser treatedwith the presence mud residues. This indicates that the chemi-cal effect of the mud residues, due to the covalent bonding of themud residues at the surface, is not significant on the mechani-cal properties such as microhardness and fracture toughness ofthe surface. Since the mud residues cover the laser treated sur-face partially, the texture of the laser treated surface changes. Thisin turn modifies the surface roughness and surface energy. Thecontact angle data for the laser treated surface with presence ofthe mud residues are given in Table 4. It is evident that the con-tact angle reduces significantly for the laser treated surface withthe presence of the mud residues. This behavior is attributed tothe modified surface texture of the laser treated surface; in whichcase, some of micro/nano size pillars are covered with the mudresidues.

Since the adhesion of the dry mud at the laser treated surfaceis strong and leaves behind mud residues when cleaned with thejet of desalinated water, the adhesion work required to remove themud from the laser treated surface is determined from the tangen-tial force analysis. However, several forces can contribute to theadhesion of the mud on the laser treated surfaces; which include,van der Waals forces, electrostatic forces, and etc [43]. In the case ofthe dry mud-laser treated surface, the dried solution due to the dis-solved alkaline, earth alkaline metals, and other ions, forms a layerat the interface of the mud and the laser treated surface. Therefore,the analysis based on the van der Walls force consideration may notbe appropriate to use estimating the adhesion force. Consequently,micro-tribometer is used to measure the tangential force duringremoving the dry mud from the laser treated surface. The tangen-tial force for the laser treated surface corresponds to the frictionalforce while the tangential force due to laser treated surface withpresence of mud composes of frictional and adhesion force. Conse-quently, integration of the tangential force over the distance givesthe work done against the friction for the laser treated surface whileit gives the frictional and adhesion work for the laser treated withthe presence of the mud. Consequently, subtracting the frictionalwork from the work done due to friction as well as adhesion resultsin the adhesion work. Table 5 gives the work done against the fric-tion and the adhesion work. Moreover, the adhesion work because

work. In addition, adhesion work because of laser treated surface

Table 5Friction and adhesion work obtained from the tangential force.

Friction work (mJ) Adhesion work (mJ)

As received 0.000403 –As received with mud – 1.0843Laser textured 0.000303Laser textured with mud – 0.8381Laser textured with mud residue 0.0303 –

B.S. Yilbas et al. / Applied Surface Science 351 (2015) 880–888 887

Fig. 9. Optical and SEM micrographs of mud at the laser treated surface and mud cross-section: (a) Optical image of mud covering laser treated surface, (b) SEM micrographo ud-wos

wplcwnfaaoatfi6

4

Mlfisbattit

f mud surface, (c) SEM micrograph of cross-section of mud (dried solution at the murface after mud removal.

ith presence of mud is smaller than that corresponding to theresence of mud on the as received surface. This is attributed to the

ow friction coefficient of the laser treated surface (Fig. 8a) and thehemical covalent bonding of the mud at the as received surface,hich is higher than that of the laser treated surface. It should beoted that the nitride compounds formed at the laser treated sur-

ace have lower surface energy than the as received surface [39]. Inddition, microhardness enhancement of the laser treated surfacelso contributes to the attainment of the low friction coefficientf the laser treated surface. Consequently, the low surface energynd small friction coefficient reduce the adhesion force at the laserreated surface. The tangential force measurements are repeatedve times and the experimental error is estimated in the order of%.

. Conclusion

Laser gas assisted surface treatment of silicon wafer is carried.orphological and metallurgical changes in the laser treated

ayer are examined using the analytical tools. Microhardness andracture toughness of the laser treated surface are measured usingndentation test methods while residual stress formed in theurface region are determined from the XRD data. The hydropho-icity of the laser treated surface is evaluated and compared withs received workpiece surface. In order to assess the effect of

he environmental dust accumulation and humidity on the laserreated surface, the mud from the environmental dust is prepared,n line with the environmental conditions, and located at the laserreated and as received workpiece surfaces. The adhesion work

rkpiece surface interface), (d) SEM micrograph of mud residues at the laser treated

of the dry mud on the surfaces is determined from the tangentialforces measured from the microtribometer. The influence ofmud residues on the state of the surface hydrophobicity is alsoinvestigated. It is found that laser treated surfaces compose ofregular laser scanning tracks and they are free from major defectsites including large cavities and cracks. A dense layer consistingof fine grains are formed in the vicinity of the laser treated layer;however, the grain size increases as the depth below the surfaceincreases. Laser treatment significantly increases Microhardness ofthe surface, which is associated with the dense layer formation dueto high cooling rates and nitride compounds formed at the surface.Microhardness enhancement lowers surface fracture toughness;however, this reduction in the order of 12%. The residual stressformed in the surface region is compressive, which is in the orderof −50 MPa. The attainment of low residual stress is related to theself-annealing effect of the laser scanning tracks which are formedvia overlapping of the laser irradiated spots at the surface. The com-bination of melting and evaporation at the laser irradiated surfaceresults in micro/nano texturing. This, in turn, enhances the surfacehydrophobicity significantly. In addition, nitride phase formedat the surface contributes to hydrophobicity enhancement at thesurface due to its low surface energy. The adhesion work becauseof the dust mud at the laser treated surface is lower than thatcorresponding to the untreated surface. This behavior is attributedto the nitride phase formed at the laser treated surface, which has

low surface energy, and surface microhardness enhancement. Themud residues do not have notable effect on the fracture toughnessand microhardness of the surface. However, it changes the surfacetexture while lowering the surface hydrophobicity.

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cknowledgements

The authors acknowledge the financial support of King Fahdniversity of Petroleum and Minerals (KFUPM) through Project#IT11111-11112, and King Abdulaziz City for Science and Tech-

ology (KACST) through project# 11-ADV2134-04 to accomplishhis work.

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