Substrate effect on electronic sputtering yield in polycrystalline fluoride(LiF, CaF2 and BaF2) thin films

Manvendra Kumar a,b,*, Parasmani Rajput c, S.A. Khan d, D.K. Avasthi d, A.C. Pandey a

a Nanophosphor Application Centre, University of Allahabad, Allahabad 211 002, Indiab Laboratorio Nazionale TASC-INFM, SS 14 Km 163.5, 34012 Trieste, Italyc UGC-DAE Consortium for Scientific Research, University campus, Indore 452017, Indiad Inter University Accelerator Centre, Post Box-10502, New Delhi 110 067, India

Applied Surface Science 256 (2010) 2199–2204

A R T I C L E I N F O

Article history:

Received 28 August 2009

Received in revised form 21 September 2009

Accepted 21 September 2009

Available online 30 September 2009

PACS:

61.80Jh

68.49.Sf

72.80.Sk

Keywords:

Swift heavy ions

Sputtering

Thin films

ERDA

Fluoride

A B S T R A C T

Influence of substrate on electronic sputtering of fluoride (LiF, CaF2 and BaF2) thin films, 10 and 100 nm

thin, under dense electronic excitation of 120 MeV Ag25+ ions irradiation is investigated. The sputtering

yield of the films deposited on insulating (glass) and semiconducting (Si) substrates are determined by

elastic recoil detection analysis technique. Results revealed that sputtering yield is higher, up to

7.4 � 106 atoms/ion for LiF film on glass substrate, than that is reported for bulk materials/crystals

(�104 atoms/ion), while a lower value of the yield (2.3 � 106 atoms/ion) is observed for film deposited

on Si substrate. The increase in the yield for thin films as compared to bulk material is a combined effect

of the insulator substrate used for deposition and reduced film dimension. The results are explained in

the framework of thermal spike model along with substrate and size effects in thin films. It is also

observed that the material with higher band gap showed higher sputtering yield.

� 2009 Elsevier B.V. All rights reserved.

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1. Introduction

Due to high energy density deposition per incident ion and thecapability of such a violent process to drive the solid far away fromequilibrium, the energetic ions can be considered for synthesis,characterization and modifications of the materials with novelproperties resulting in non-equilibrium events such as atomicbond breaking, latent track formation, ion beam mixing anderosion of materials, etc. The erosion of atoms from the surface dueto impact of energetic ions is termed as sputtering [1]. Dependingon the projectile energy, different scenario of sputtering occurs,such as material ejection due to atomic collision cascade (nuclearsputtering) at keV energies [1], electronic sputtering governed byelectronic energy loss of higher energy ions (>1 MeV/u) [2] andpotential sputtering which occurs due to potential energy of lowenergy highly charged ions [3,4]. The overwhelming majority ofsputtering studies were conducted using low energy ions. Suchkind of sputtering is known as nuclear sputtering and usually the

* Corresponding author at: Laboratorio Nazionale TASC-INFM, SS 14 Km 163.5,

34012 Trieste, Italy.

E-mail address: kmanav@gmail.com (M. Kumar).

0169-4332/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2009.09.073

observed effects could be attributed to the direct atomicdisplacement and the associated sputtering caused within theelastic recoil cascade of impinging ion and is well described bySigmund’s theory [1]. In the electronic energy loss regime,Sigmund’s theory fails to explain the large increase in sputteringyield [5]. Two models (Coulomb explosion [6], thermal spike [7,8])are presently under discussion in order to explain such hugeincrease in yield due to electronic energy loss, but the electronicsputtering process is still to be understood and is, therefore, afundamental research problem. The electronic sputtering processalso acts as a tool to understand the fundamentals of interaction ofswift heavy ion (SHI) with the matter and thus it has been studiedin a number of materials. In case of metals, the yield is not so highbut definitely larger than what predicted by the Sigmund’s theory[8–11]. High sputtering yield has also been observed in differentinsulating materials [8,12–18]. However, a few work exists onelectronic sputtering in thin films of fluorides [16–18]. Sputteringprocess in a thin film due to irradiation is affected by severalparameters like grain size [16–18] and thickness of the film [18],substrate [19], mass and energy of the incident ion. In the presentstudy, we report SHI induced sputtering in fluoride (i.e. LiF, BaF2

and CaF2) thin films deposited on Si and fused silica/glasssubstrates. These materials have numerous applications in the

Fig. 1. GAXRD pattern of 10 and 100 nm LiF, 100 nm CaF2 and 100 nm BaF2 thin

films deposited on Si and glass substrates. All the films are polycrystalline in nature.

M. Kumar et al. / Applied Surface Science 256 (2010) 2199–22042200

fields of opto-electronic devices, tunable color center lasers,radiation dosimeters, optical isolators and can play an importantrole in the study of ion solid interaction processes. To determinethe stoichiometry and mass removal, on-line elastic recoildetection analysis (ERDA) technique was employed, while thecharacterization of pristine films was performed by glancing angleX-ray diffraction (GAXRD) and atomic force microscopy (AFM) toinvestigate their structural and surface properties, respectively.

2. Experimental

2.1. Thin film deposition

LiF, CaF2 and BaF2, thin films were deposited on the Si h1 1 1iand glass/fused silica substrates at room temperature under avacuum of�10�6 Torr using electron beam evaporation technique.The substrates were thoroughly cleaned before the deposition.Fused pieces of material of 99.9% purity, from Sigma–Aldrich, wereused as source material for deposition. The deposition rate was0.2 nm/s and the thickness of deposited film was monitored using aquartz crystal monitor. The deposited film thicknesses were 10 and100 nm for LiF and100 nm for BaF2 and CaF2 films.

2.2. Sputtering experiments

The films were placed on a strip ladder and inserted in highvacuum chamber (�10�6 Torr). The sputtering experiment wasperformed with 120 MeV Ag25+ ions at a glancing angle ofincidence at 208. For SHI induced surface and/or near surfacemodifications of materials, the charge state of the incident ionsshould be equilibrium charge state, i.e. of the order of effectivecharge defined in the electronic energy loss theory. To reachequilibrium charge state, all the projectiles from the acceleratorhaving +9 charge states were passed through a thin carbon foil. Thecharge state +25, being the most probable in charge statedistribution, was selected by dipole magnet for the experiment.The ion beam was collimated to a size of 1 mm � 3 mm by using adouble slit fixed before the chamber. The energy losses of theseions were 15.4, 15.8 and 16.5 keV/nm in LiF, BaF2 and CaF2 crystals,respectively, and contribution of nuclear energy loss is only up to0.4%. All the measurements were performed at room temperatureand with a typical beam current of 0.4 pnA (particle nano Ampere,1 pnA = 6.25 � 109 ions/s) to minimize target heating. Duringirradiation, the layer thickness and stoichiometry of the samplewere continuously monitored using a large area position sensitivegaseous detector telescope (LAPSDT) to perform ERDA in reflectiongeometry [20].

2.3. Offline characterization techniques

Offline characterization techniques, GAXRD and AFM were usedto get information about the crystallinity and surface morphologyof the pristine films. GAXRD studies were performed using a BrukerAXS D8 advanced diffractometer with Cu Ka (1.54 A), while thesurface morphology was examined by Multiview 2000 NanonicsImaging instrument.

3. Results

3.1. Results of characterization of pristine films

GAXRD pattern of LiF, CaF2 and BaF2 thin films deposited on Siand glass substrates are shown in Fig. 1. From GAXRD results, it isclear that the films are polycrystalline in nature. GAXRD pattern ofLiF films shows reflections at 2u angle of 38.58, 45.68 and 66.78corresponding to (1 1 1), (2 0 0) and (2 2 0) planes, respectively and

that of CaF2 shows reflections at 30.38, 49.08and 57.88 correspondingto (1 1 1), (2 2 0) and (3 1 1) planes, respectively, while GAXRDpattern of BaF2 films shows reflections at 24.98, 28.88, 41.28, 48.78and 59.68 corresponding to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (4 0 0)planes, respectively. The average grain sizes, calculated fromScherer’s formula using full width at half maximum of dominantpeak for each film, are 11 and 32 nm for 10 and 100 nm LiF films,whereas it is 35 nm for CaF2 and BaF2 thin films. There is nosignificant difference between the films deposited on Si and silicasubstrates in terms of grain size and polycrystallinity. Fig. 2 showsAFM images of LiF, CaF2 and BaF2 thin films indicating that the filmsare continuous and uniform in nature.

3.2. Results of sputtering studies

Fig. 3 shows typical two-dimensional recoil spectra for CaF2 andBaF2 thin films deposited on Si substrate. The recoil spectrum forLiF is reported elsewhere [18]. In ERDA spectra, well-separatedbands of the different elements, Si, F, O, C, Li, Ca and Ba present indifferent films/Si substrate were observed. The data were collectedin list mode in several steps at the same spot and electronicsputtering yield was estimated in lower fluence regime (up to�1011 ions/cm2) in order to minimize the effect of surfacemodification. The charge deposited by the projectile in the targetsin each fluence step is measured and used to estimate the numberof incident ions in corresponding fluence step. The ion fluence isestimated in each fluence step by dividing the number of ionsby the beam spot area. The charge calibration is well matchedwith the Si recoil spectra in each fluence bin within 10% ofexperimental error. The reduction in the thickness of the films dueto sputtering was determined in terms of areal concentration, Nc, of

Fig. 2. AFM images of 10 and 100 nm LiF, 100 nm CaF2 and 100 nm BaF2 thin films. All the films are continuous and nano-granular in nature.

M. Kumar et al. / Applied Surface Science 256 (2010) 2199–2204 2201

different elements, Li/Ca/Ba and F, present in the film. The arealconcentrations are determined from the counts, y, of the recoilspectra using the relation [17]

Nc ¼y sina

N p

@s@V

� �V (1)

where Np is the number of incident ions, V (=5 � 0.3 msr) is thesolid angle subtended by detector, a (=208 � 0.58) is the anglebetween the sample surface and beam direction and (@s/@V) isthe recoil cross section. The areal concentration of different elementsversus fluence is plotted as shown in Fig. 4 and the rate of removal ofmaterial, i.e. sputtering yield is determined from the difference inareal concentration at two initial fluences divided by the correspond-ing difference in the fluences. A reduction in the areal concentrationwith increase in the ion fluence is observed for each film (Fig. 4), asreported earlier for 10 nm LiF film deposited on Si substrate [18],indicating sputtering of the material from these films. Thestoichiometry of the films is determined from the ratio of arealconcentration of Li/Ca/Ba and F for respective films and it is observedthat the ratios of Li and Ca/Ba with F are 1:1 and 1:2 for LiF and CaF2/BaF2 thin films, respectively. The stoichiometry of all the filmsremains unchanged before and after sputtering (Fig. 5), indicatingstoichiometric sputtering of fluoride materials. The Li/F sputter yields,calculated from ERDA areal concentration versus fluence curves, are

2.3 � 106 and 6.2 � 104 atoms/ion from 10 and 100 nm LiF filmsdeposited on Si substrates, whereas they are 7.4 � 106 and 1.9 � 105

for 10 and 100 nm LiF films deposited on glass substrates. The F andCa sputter yields are 5.3 � 104 and 2.5 � 104 atoms/ion for CaF2

deposited on Si substrates and 1.7 � 104, and 8.1 � 103 atoms/ion forfilm on glass substrate. The F and Ba sputter yields are 2.5 � 104 and1.2 � 104 atoms/ion in BaF2 films deposited on Si substrates, whilethey are 8.3 � 103, 4.1 � 103 atoms/ion for films deposited on glasssubstrates, respectively. No significant difference in the sputteringyield is observed for films on glass and fused silica substrates.

The sputtering yield for film deposited on glass substrate is nearlythree times higher than that on Si substrate for all the materialsas presented in Fig. 6. The sputtering results are summarized inTable 1.

4. Discussion

The sputtering yield from LiF thin films was compared with thesputtering yield from LiF single crystal, previously reported byToulemonde et al. using catcher technique, the yields of Li and Fwere about 1.7 � 104 and 1.6 � 104 atoms/ion, respectively, forions having electronic energy loss of 16.4 keV/nm [12]. Comparingthe observed yield in previous studies with the present one, theyields are higher for all the cases irrespective of substrates. Nowquestion arises what is the cause of the increase in the yield in

Fig. 4. Variation in areal concentration of Li/Ca/Ba and F due to sputtering in 10 and

100 nm LiF and 100 100 nm BaF2 and CaF2 thin films. The sputter yield is

determined from the difference in areal concentration at two initial fluences

divided by the corresponding difference in the fluences.

Fig. 3. Primary ERDA spectra of CaF2 and BaF2 thin films deposited on Si substrate,

showed bands of Si, F, O, C, Ca and Ba recorded with LAPSDT.

M. Kumar et al. / Applied Surface Science 256 (2010) 2199–22042202

thin films. To understand the electronic sputtering phenomenonin different materials, among several theoretical approaches,Coulomb explosion and inelastic thermal spike models are themost commonly used models. According to the Coulombexplosion model, a highly ionized zone of charged particles iscreated along and in the vicinity of the ion path. If the chargeneutrality is not re-established by target electrons, the electro-static repulsion of ionized target atoms will force a rapidexpansion of the material in the charged domain and will giverise to the erosion of material. On the other hand, according to theinelastic thermal spike model, the energy is deposited by theprojectile ions in the electronic subsystem of the target. Thisenergy is shared among the electrons by electron–electroncoupling and transferred subsequently to the lattice atoms viaelectron–lattice interactions. This leads to a transient increase inthe temperature in a nano-cylindrical zone along the ion path.Rapid quenching of thermal energy creates ion damage zone. Thethermal energy gets converted in to kinetic energy of the atomsand when this energy is more than the surface binding energy,ejection of material takes place from surface. In insulators,however, it is always possible that a Coulomb explosion precedesthe thermal spike and is covered by the latter in the course of time.Therefore the possibility of Coulomb explosion for electronicsputtering in insulator cannot be ruled out. But, sputtering resultscan be explained only qualitatively with the help of Coulombexplosion model and no quantitative information can beextracted. As a quantitative approach to explain the highersputtering yield in case of insulator substrate, we applied inelastic

thermal spike model. As the range of ion (>15 mm for all the cases)is more than the film thickness, a thermal spike will be developedin the substrate also. This temperature spike can increase thetemperature generated in the film resulting in higher sputtering.The temperature in Si substrate will smear out more efficientlydue to its higher thermal conductivity (nearly 40 times morethan that of glass/fused silica) and yield will be higher for glass/fused silica substrates. On the other hand, because of amorphous

Fig. 6. Comparison of yield for different materials, LiF, CaF2 and BaF2 deposited on

glass and Si substrate. Higher yield is observed (�3 times more) for insulator

substrate for all the materials.

Fig. 5. Stoichiometric sputtering in fluoride materials. The stoichiometry of the film

is nearly same before and after sputtering for all the films.

M. Kumar et al. / Applied Surface Science 256 (2010) 2199–2204 2203

nature of glass/fused silica, the electron and phonon couplingstrength will be stronger than that in Si, resulting in highertemperature rise in glass and silica substrates supported bythermal spike model. The contribution from temperature devel-oped in substrate will enhance the yield and the sputtering will belower in case of the film deposited on Si substrate than that is forglass/fused silica.

The increase in the yield, as compared to the reported value forbulk material, is not only due to the substrate effect, but is alsoinfluenced by the thickness and grain size of the films [18]. Asreported earlier, the reduced thickness and grain size can enhancethe sputtering yield. Reduction in the film thickness and the grainsize can restrict the motion of the excited electrons because of thescattering from surface and interface and grain boundaries,respectively. So, the size effect can result in reduction of the meandiffusion length, l, of the electrons resulting in increase in thedeposition of energy in confined region, which finally can enhancethe temperature spike in the thinner films/films having smallergrains. On the other hand, in smaller grains and thinner films, theduration of thermal spike will be more because of less out-diffusion.The sputtering yields for LiF films (of different grain size) are

Table 1Results of sputtering experiment. The experiment was performed with 120 MeV Ag25+

Substrate Sputter yield (atoms/ion)

10 nm LiF Li/F 100 nm LiF Li/F

Si 2.3�106 6.2�104

Glass/fused silica 7.4�106 1.9�105

simulated using inelastic thermal spike code. Numerical calculationof the sputtering yield for different films under normal ion incidencehas been performed using (i) the thermal spike code [21] and is sameas reported earlier [18]. For the calculation, the used l values are 1.1and 3.2 nm for 10 and 100 nm thick films having the grain size of 11and 32 nm, respectively. All the other parameters used in thecalculations are the same as used by Toulemonde et al. [12]. Theyield calculated from thermal spike code has been then multipliedby sin�1.85a in order to correct for the grazing angle, a, of the ionincidence [12]. The experimental sputter yields are 2.3 � 106,7.4� 106 and 8.1 � 104, 1.9 � 105 atoms/ion for 10 and 100 nm LiFfilms deposited on Si and glass substrates, respectively, while thecalculated yields are 7.9 � 104 and 1.7 � 104 atoms/ion for thesefilms, respectively, irrespective of substrates. The discrepancyobserved in the experimental and simulated yields may be due tothe following facts. The thermal spike model does not include thepressure pulse that arises due to the large local temperature in theexcited region. Subsequently, the code does not include the effect ofthickness and substrate on the yield in the simulation, which is alsoresponsible for increase in the yield as discussed above.

The observed yields for different materials were compared asa function of band gap of the bulk materials. The band gap of bulkLiF, CaF2 and BaF2 are 14.2 eV [22], 12.1 eV [23] and 9.2 eV [24],respectively. One can clearly observe that as the band gap of thematerials increases, the yield also increases. This can beexplained on the basis of decrease in the mean diffusion lengthof the excited electrons with increase in the band gap [8], whichresults in higher energy deposition in confined region enhancingthe temperature spike. Thus, the yield from LiF will be higherthan that from CaF2/BaF2, while the yield from CaF2 will be morethan BaF2 thin films.

ions at an angle of 208 with respect to the film surface.

100 nm CaF2 100 nm BaF2

Ca F Ba F

8.1�103 1.7�104 4.1�103 8.3�103

2.5�104 5.3�104 1.2�104 2.5�104

M. Kumar et al. / Applied Surface Science 256 (2010) 2199–22042204

5. Conclusion

Stoichiometric electronic sputtering due to impact of 120 MeVAg25+ ions is observed by on-line ERDA technique for polycrystal-line fluoride (LiF, CaF2 and BaF2) thin films deposited on Si andglass/fused silica substrates having 1:1 and 1:2 stoichiometries.The results showed that the sputtering yield is of the order of 103 to106 atoms/ion. The film deposited on insulator substrate showedmore sputtering than the film deposited on Si substrate. On theother hand, material with higher band gap showed highersputtering yield. The experimental results are assessed withinthe framework of thermal spike model along with influence ofsubstrate and size effect in thin films.

Acknowledgments

The authors gratefully acknowledge Dr. M. Toulemonde, CIRIL,France for the helpful discussion and allowing to use thermal spikecode. IUAC, New Delhi and Nanophosphor Application Centre,Allahabad, are thankful to DST/UGC, India to provide funds for XRDand AFM facilities.

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