J. Mater. Sci. Technol., 2011, 27(1), 51-58.

Influence of Sputtering Gas on Morphological

and Optical Properties of Magnesium Films

Yogendra K. Gautam1,2), Amit K. Chawla2), Vipin Chawla2), R.D. Agrawal1)

and Ramesh Chandra2)†1) Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee,

Roorkee-247667, India2) Nano Science Laboratory, Institute Instrumentation Centre, Indian Institute of Technology Roorkee,

Roorkee-247667, India[Manuscript received March 22, 2010, in revised form July 21, 2010]

The influence of sputtering gas (He & Ar) on the structural properties of Mg thin films has been investigated.The optical property (reflectance) that results from the growth of films at varying substrate temperatures(Tsub) was also studied. The deposited films were characterized by using X-ray diffraction (XRD), field emissionscaning electron microscopy (FE-SEM), atomic force microscopy (AFM) and UV-Vis-NIR spectrophotometer.The smaller crystallite size and lower deposition rate were observed in the presence of Helium atmospherecompared to Argon. Morphology of the films shows 2D hexagonal geometry of grains in the depositiontemperature range (Tsub≈50–150◦C) in both the sputtering gases. The surface roughness of the polycrystallinefilms were found to increase with increase in the deposition temperature of both ambient gases. Opticalreflectance of Mg films was measured in near infrared region and larger reflectance was observed from Mgfilms sputtered in He atmosphere compared to that in argon.

KEY WORDS: X-ray diffraction (XRD); Atomic force microscopy (AFM), Reflectance; Mg films;

Microstructure

1. Introduction

The optical properties of thin films of metals,such as magnesium, yttrium or lanthanum, cappedwith thin palladium can be changed reversibly be-tween reflective and transparent states as a resultof hydrogenation and dehydrogenation[1–5]. There-fore, much attention is being paid worldwidely todevelop these coatings for the applications in opti-cal mirrors. These mirrors control the optical trans-mittance mainly by changing their optical reflectance.They are of great interest as the switchable glazing insmart windows, and thus can contribute in saving en-ergy for air conditioning. Magnesium could be thefuture industrial material, e.g., for automotive andaerospace industries, due to its very low density. Mg

† Corresponding author. Prof., Ph.D.; Tel.: +91 01332285743;Fax: +91 1332 286303; E-mail address: ramesfic@iitr.ernet.in(R. Chandra).

based switchable mirror are also attractive materialsfor regulation of light and heat transfer in buildings,vehicles, satellites and may have applications in sensortechnologies. A window using this technology can beprogrammed to respond to local sunlight and weatherconditions, optimizing the amount of daylight enter-ing a building, while controlling the gain or loss ofinfrared radiation or heat. When the sun is bright,transition metal switchable mirror, switch to a highlyreflective state while in low sun light, window can beswitched to a partially-reflective state[6–10].

In addition to the switchable mirrors, magnesiumfilms are also very useful for hydrogen storage forfuture energy application as magnesium can absorb7.65 wt pct of atomic hydrogen[11–13]. Magnesium-rare earth alloy films were shown by Van der Sluiset al. to offer improved dynamic ranges and color neu-tral transparent states[6]. Recently Richardson et al.reported switchable mirror effects in thin films of Mg

52 Y.K. Gautam et al.: J. Mater. Sci. Technol., 2011, 27(1), 51–58

Table 1 Sputtering parameters for Mg films

Target Base Gas Deposition Deposition Sputtering Substrate Substrate

pressure used power time pressure temperature

Mg 3.33×10−4 Pa Ar/He 50 W 15 min 666.6×10−3 Pa Glass 50–350◦C (varying)

and 3d transition metals (Ni, Mn, Fe, and Co)[8,14].In earlier investigations, physical vapor deposition(PVD) technique was used to prepare pure magne-sium coatings on silicon substrates at various ar-gon pressures and deposition angles for corrosionapplication[15,16]. Cultrera et al. studied pure Mgfilms on Cu substrates, deposited by pulsed laser de-position for photocathode applications[17]. Thus, em-ploying a PVD technique provides a major advantagebecause of the use of target materials with a very highpurity level. However, the morphology and structureof these coatings depend on the deposition conditionsused.

In order to take advantage of these effects Mg coat-ings were prepared on glass substrates under variousdeposition conditions[17]. In this paper, we reportedhow the nature of the sputtering gas (argon and he-lium) and substrate temperature affect the structure,surface morphology and the optical reflectance of thenanocrystalline Mg thin films.

2. Experimental Details

2.1 Deposition of Mg thin films

Mg thin films were deposited on glass substrateusing Mg (99.95% purity) target (2 inch in diameterand 5 mm in thickness) by DC magnetron sputter-ing. In the present case sputtering was carried out ina custom built 12′′-diameter chamber (Excel Instru-ments, India). The chamber was initially evacuated tohigh vacuum (3.33×10−4 Pa) by using a turbo mole-cular pump backed by a rotary pump. Thereafter,high purity (99.99%) inert gas (Ar and He) was usedin sputtering. The substrates were cleaned by rinsingin ultrasonic bath of acetone and methanol. The de-position time and sputtering power were kept fixed at15 min and 50 W, respectively. The substrate tem-perature (Tsub) was varied from 50–350◦C for bothsputtering gases. The target was pre-sputtered for2 min before starting deposition. Substrate to targetdistance of 45 mm was kept constant for all deposi-tions. The sputtering parameters for all samples areshown in Table 1.

2.2 Characterization

An X-ray diffractometer (XRD), a field emis-sion electron microscope (FE-SEM) and an atomicforce microscope (AFM) were used to characterizethe films. The room temperature XRD measure-ments were carried out by using CuKα radiationin an X-ray diffractometer (Bruker AXS, D8 Ad-vance) in (θ-2θ) geometry. The coherently diffract-ing domain size (d) was calculated from the inte-

25 30 35 40 45 50

(102)

(101

)

(100)

(002

)

300oC

250oC

200oC

150oC

100oC

50oC

2 / deg.In

tens

ity

Fig. 1 XRD patterns of the Mg films on glass depositedat different substrate temperatures in Ar sputter-ing gas

25 30 35 40 45 50

(102

)

(101

)

(100

)(002

)

350oC300oC250oC200oC150oC100oC50oC

2 / deg.

Inte

nsity

Fig. 2 XRD patterns of the Mg films deposited at differ-ent substrate temperatures in He sputtering gas

gral width of the diffraction lines using the well knownScherrer′s equation[18]. FE-SEM (FEI, Quanta 200F)was used to characterize the surface morphology andcross sectional view of films. AFM (NT-MDT, Nte-gra) operated in semi contact (tapping) mode wasused to measure surface roughness of the films. Opti-cal reflectance spectra of Mg films were measured inwavelength range from 850–2500 nm by using a UV-Vis-NIR spectrophotometer (Varian, Cary 5000).

3. Results and Discussion

The XRD patterns of the Mg films deposited in Arand He sputtering gas at different Tsub are shown inFigs. 1 and 2, respectively. From Fig. 1, it is observedthat the polycrystalline Mg films show major XRD

Y.K. Gautam et al.: J. Mater. Sci. Technol., 2011, 27(1), 51–58 53

Table 2 Different properties of Mg films with varying substrate temperature inAr sputtering gas atmosphere

Substrate Film thickness Crystallite size Av. roughness Reflectance

Temp./◦C /µm /nm /nm /%

50 10.2 64.2 26 30

100 9.9 64.8 30 20

150 10.8 66.4 45 12

200 11.3 68.3 50 05

250 10.4 74.2 52 04

300 12.9 76.7 73 00

Table 3 Different properties of Mg films with varying substrate temperature inHe sputtering atmosphere

Substrate Film thickness Crystallite size Av. roughness Reflectance

Temp./◦C /µm /nm /nm /%

50 4.0 50.0 15 60

100 3.3 54.1 19 51

150 9.8 56.2 21 47

200 6.9 56.7 35 25

250 8.9 65.3 43 06

300 3.3 70.3 64 02

350 6.6 73.7 72 01

peak corresponding to a hexagonal closed packed(hcp) structure with dominant orientation along (002)plane. In 50–250◦C temperature range, the observedorientations at 32.21, 34.50, 36.65 and 47.83 deg. cor-responds to the (100), (002), (101) and (102) planesof the hcp structure of Mg respectively. The reasonfor this highly intense (002) orientation is the lowenergy configuration corresponding to this plane[19].The increase in substrate temperature provides suf-ficient energy for bulk diffusion of Mg atoms (graingrowth) to attain the lower surface energy configura-tion along (002) orientation of its hcp structure, butfurther increase in substrate temperature up to 300◦Ctransforms (002) orientation into (101) dominant ori-entation. The enhanced mobility of adatoms in thefilm surface with the increase in substrate tempera-ture has favored the formation of (101) orientationof grains[20]. The competition between strain energyand surface free energy affecting the texture of thegrains is heavily dependent on the deposition para-meters such as substrate temperature, power, sput-tering pressure and film thickness[20]. The thermalstress induced in the thin films deposited at highersubstrate temperature may also be in favor of theformation of (101) oriented grains. The Mg film de-posited at higher substrate temperature (350◦C) getspeeled off due to higher stress caused by large differ-ence between coefficient of thermal expansion (α) ofMg films and glass substrate. The value of thermalexpansion coefficient for Mg and glass is 26×10−6/Cand 3.3×10−6/C at 20◦C, respectively[21]. In Fig. 2,orientation (002) presents dominant peak in the tem-perature range of 50–150◦C. With further increase insubstrate temperature, the (002) orientation subsides,while (101) orientation dominates as similar to thatin Ar atmosphere. The enhanced mobility of adatoms

in the film surface with increase in substrate tempera-ture has favored the formation of (101) grains. But in300–350◦C temperature range, multiple orientationsat 32.21, 34.50, 36.65 and 47.83 deg. correspondingto the (100), (002), (101) and (102) planes of the hcpstructure were observed. The lattice constants of thehexagonal Mg films were found to be a=0.3181 nmand c=0.5192 nm, and the c/a-value of the Mg struc-ture comes out as 1.632, which are in agreement withthe bulk values of Mg (a=0.3209 nm, c=0.5210 nmand c/a=1.623)[22]. Scherrer formula was used to cal-culate the crystallite size, d of the samples[18]

d =0.9λ

B cos θB(1)

where, λ, θB and B are the X-ray wavelength of Cu(0.154056 nm), Bragg diffraction angle and full widthat half maximum (FWHM), respectively. The calcu-lated average crystallite sizes of Mg films deposited inAr and He are given in Tables 2 and 3, respectively.It was observed that with increase in the substratetemperature, crystallite size increases in both sputter-ing gas (Fig. 3). The reason can be attributed to thefact that at higher temperatures atoms obtain enoughthermal energy to move and diffuse, thus promotingcrystal growth[23]. From Fig. 3, it was also observedthat the film deposited in He gas has smaller parti-cle size compared to those deposited in Ar gas. Thiscan be explained on the basis of mean free path. Themean free path of the atoms in a gas is given by:

λ =2.33× 10−20

(Pδ2m)

T (2)

where, T is the temperature, P is the pressure and δm

is the atomic (or molecular) diameter of sputtering

54 Y.K. Gautam et al.: J. Mater. Sci. Technol., 2011, 27(1), 51–58

50 100 150 200 250 300 35045

50

55

60

65

70

75

80

Ar He

He

Cry

stal

lite

size

/ nm

Tsub / oC

Ar

Fig. 3 The crystallite size dependence on substrate tem-peratures

gas[24]. Thus as the size of the sputtering gas atomsincreases the mean free path of the inert gas atomsdecreases and hence the collision frequency increases.We can therefore expect that as the diameter of thegas molecule increases the sputtered Mg atoms wouldalso undergo multiple collisions leading to a higherprobability of agglomeration and growth even beforearriving at the substrate surface. In such a case, wewould expect an increase in the particle size with in-crease in atomic mass of the sputtering gas. Themolecular diameter of He gas molecule is smaller thanAr gas molecule; therefore particle size was small inHe as compared to that in Ar[25]. The film thick-ness was measured by taking cross-sectional image ofMg films by FE-SEM. The film thickness was foundnearly constant in all deposition temperature rangefor both sputtering gas and given in Tables 2 and 3,respectively. The cross-sectional image of the films isshown in Fig. 4(a) and 4(b), respectively. From Ta-bles 2 and 3 it was observed that the films depositedin He are thin as compared to that in Ar atmosphere.It is due to the fact that the ionization potential of

He (24.6 eV) is higher than that of Ar (15.8 eV) andthus He atoms are hard to ionize through the directionization of electrons and hence the sputtering yieldof metals by He+ ions is less in comparison to the Ar+

ions. Therefore, deposition rate is higher in Ar thanin He atmosphere[26].

The results from the XRD analysis were in com-plete agreement with the surface morphology of thefilm in both cases, which had been observed by FE-SEM as shown in Figs. 5 and 6. From Fig. 5(a–b)and 6(a–b), it can be observed that up to the sub-strate temperature of 150◦C, 2D hexagonal geometryof grains on the surface reflects hexagonal symmetryperpendicular to the film plane, i.e. the hexagonal c-axis perpendicular to the surface. At higher tempera-tures and the dominant orientation (101), one wouldexpect to lose this apparent symmetry as shown in(Fig. 5(c–d)), while in He sputtering atmosphere athigher temperature, the overlapping of particle wasobserved (Fig. 6(c–d)). The reason of the overlappingis due to the diffusion of the particles. In He sputter-ing atmosphere deposition rate is lower in comparisonto Ar and thus particles get sufficient time to diffuseat higher temperature.

In order to show effect of substrate tempera-ture on surface roughness (δRMS) of films, atomicforce microscopy (AFM) perspective images (scanarea 3 µm×3 µm) are shown in Figs. 7 and 8, forAr and He cases respectively. It was observed that upto the substrate temperature of 150◦C, the grains areof 2D hexagonal shape (Fig. 7(a–b) and Fig. 8(a–b)),similar to what as shown in FE-SEM images. The sur-face roughness (δRMS) of film in both cases Ar andHe are given in Tables 2 and 3, respectively. FromFig. 9, it had been found that the surface roughness(δRMS) increases with increase in substrate temper-atures for both sputtering gas. This might be due tothe random growth of surface particles with increas-ing temperature, resulting into an increase in averagesurface roughness. It was also observed that surface

Fig. 4 FE-SEM cross-sectional views of Mg films deposited at substrate temperature 100◦C in different sputteringgases: (a) Ar sputtering gas and (b) He sputtering gas

Y.K. Gautam et al.: J. Mater. Sci. Technol., 2011, 27(1), 51–58 55

Fig. 5 FE-SEM morphology of the Mg film deposited in Ar sputtering gas at different substrate temperatures:(a) 50◦C, (b) 150◦C, (c) 200◦C and (d) 300◦C

Fig. 6 FE-SEM morphology of the Mg film deposited in He sputtering gas at different substrate temperatures:(a) 50◦C, (b) 150◦C, (c) 200◦C and (d) 300◦C

56 Y.K. Gautam et al.: J. Mater. Sci. Technol., 2011, 27(1), 51–58

Fig. 7 AFM images of the Mg film deposited in Ar sputtering gas at different substrate temperatures: (a) 50◦C,(b) 150◦C, (c) 200◦C and (d) 300◦C

Fig. 8 AFM images of the Mg film deposited in He sputtering gas at different substrate temperatures: (a) 50◦C,(b) 150◦C, (c) 200◦C and (d) 300◦C

Y.K. Gautam et al.: J. Mater. Sci. Technol., 2011, 27(1), 51–58 57

50 100 150 200 250 300 35010

20

30

40

50

60

70

80

Ar He

He

Ar

Av.

sur

face

roug

hnes

s / n

m

Tsub / oC

Fig. 9 Average surface roughness dependence on sub-strate temperatures

1000 1250 1500 1750 2000 2250 2500-5

0

5

10

15

20

25

30

35

250oC

300oC

200oC

150oC

100oC

50oC

Ref

lect

ance

/ %

Wavelength / nm

Fig. 10 Reflectance spectra of the Mg films deposited inAr sputtering gas at different substrate tempera-tures

1000 1250 1500 1750 2000 2250 2500

0

20

40

60

80

50oC

350oC

200oC

250oC

300oC

100oC

150oC

Ref

lect

ance

/ %

Wavelength / nm

Fig. 11 Reflectance spectra of the Mg films deposited inHe sputtering gas at different substrate tempera-tures

roughness (δRMS) was smaller in He as compared tothat in Ar. This was due to the fact that the crys-tallites with smaller size exhibit smooth surface than

crystallites of Mg[27] with bigger size.The measurement of the total reflectance at an in-

cident angle of 45◦ in the near infrared region (λ=850-2500 nm) has been carried out on a UV-Vis-NIR spec-trophotometer. Figures 10 and 11 present the totaloptical reflectance spectra of the films in Ar and He at-mosphere respectively. The measured values of opticalreflectance of Mg films in both cases are given in Table2 and Table 3. The crystallite size and surface rough-ness (δRMS) increase but reflectance decreases withan increase of substrate temperature in both sputter-ing atmosphere. It was observed that the change inreflectance is directly related to surface roughness, i.e.larger surface roughness results in smaller reflectanceand vice versa[28]. From Tables 2 and 3, the smallersurface roughness (δRMS) and larger reflectance wereobserved in He as compared to that in Ar[27].

4. Conclusion

In this study the influence of sputtering gas anddeposition temperature on the structural and opticalproperties of nanocrystalline magnesium thin films onglass substrate deposited by sputtering were reported.The XRD results indicate that the particle size in-creases with increase in the atomic mass of sputteringgas as well as substrate temperature. Lower deposi-tion rate was found in He sputtering gas in compari-son to that of in Ar as observed from the thickness ofthe films. The smaller thickness was observed for thefilms deposited in He sputtering gas in comparison tothat of Ar. It was observed from the microstructureof the films that the grains have hexagonal shape atlow substrate temperature for both sputtering gases(Ar and He). The polycrystalline Mg film gets peeledoff at 350◦C in Ar ambient due to the large differencebetween coefficient of thermal expansion of Mg filmand the glass substrate. It was also observed that theaverage surface roughness of films increases with in-crease in deposition temperature in both Ar and Hesputtering atmosphere. The smaller average surfaceroughness and larger reflectance was observed in Hecompared to that in Ar gas.

AcknowledgementsOne of the authors, Yogendra K. Gautam, would

like to thank MHRD, New Delhi for the award ofSenior Research Fellowship. The financial assistancefor this work by DRDO, Govt. of India (Grant No.ERIP/ER/0800354/M/01/1125) is gratefully acknowl-edged.

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