Si interfaces

Si interfaces

Nuclear Instruments and Methods in Physics Research B 268 (2010) 1594–1600 Contents lists available at ScienceDirect Nuclear Instruments and Methods...

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Nuclear Instruments and Methods in Physics Research B 268 (2010) 1594–1600

Contents lists available at ScienceDirect

Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb

Ion energy dependence of interface parameters of ion beam sputter deposited W/Si interfaces A. Biswas, D. Bhattacharyya * Spectroscopy Division, Bhabha Atomic Research Centre, Mumbai 400 085, Maharastra, India

a r t i c l e

i n f o

Article history: Received 7 December 2009 Received in revised form 15 February 2010 Available online 24 February 2010 Keywords: Ion beam sputtering (IBS) Grazing incidence X-ray reflectivity (GIXR)

a b s t r a c t W thin films and W/Si/W tri-layer samples have been deposited on c-Si substrates in a home-made ion beam sputtering system at 1.5  103 Torr Ar working pressure, 10 mA grid current and at different Ar+ ion energies between 600 and 1200 eV. Grazing incidence X-ray reflectivity (GIXR) measurements in specular and diffused (detector scan) geometry have been carried out on the above samples. The measured GIXR spectra were fitted with theoretically simulated spectra and the different interface parameters viz., interface width, interface roughness and interface diffusion have been estimated for both Si-onW and W-on-Si interfaces in the above samples. The variation of the above interface parameters as a function of ion energy used for W sputtering has been studied. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction In the present communication, W thin films and W/Si/W tri layer structures have been deposited in a home-made ion beam sputtering system. Ion beam sputtering (IBS) technique has a certain advantage over vacuum evaporation since energies of sputtered adatoms in IBS process are in the range of few eVs which allow adatoms to rearrange themselves upon arriving on the substrate; hence prohibits columnar growth and leads to smoother morphology of the films [1]. It has also got advantage over magnetron sputtering since in the later case, an inert gas (generally, Ar) plasma has to be generated and sustained in the sputtering zone, at a relatively higher Ar partial pressure (typically, 5  103  5  102 Torr) leading to contamination and Ar trapping inside the growing film. In case of IBS on the other hand, ions are generated inside an independent source away from the sputtering zone and are made incident on the target by using a proper extraction and focusing mechanism. Hence, the IBS process can take place at a working pressure of at least one order of magnitude lower than that in magnetron sputtering and hence in an IBS process, the sputtered atoms reach the substrate almost with similar kinetic energy with which they are emitted from the target [2]. Also since the ion sources generally have independent control over energy and current density of the bombarding ions, deposition by IBS technique has better reproducibility and control [3] and hence yields films with better morphology, packing density and physical properties. The morphological properties of the films and interfaces of multilayer systems deposited by IBS process depend mostly on the en* Corresponding author. Tel.: +91 2225590343. E-mail address: [email protected] (D. Bhattacharyya). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.02.010

ergy dissipated at the growing surface by the sputtered atoms which in turn depends on the energy of the primary ions used for sputtering. For example, Ji et al. [4] have found that penetration and diffusion across the interfaces in their IBS-deposited Co/Pt multilayer increase with the increase in primary ion energy. Apart from the kinetic energy of the sputtered atoms, the energy of the adatoms would also be enhanced by the ions reflected or backscattered from the target surface and this also would have an effect on the morphology of the growing film. This effect is similar to that of an ion beam directed onto the substrate in an ion-assisted e-beam evaporation process which is known to change the surface morphology and interface properties in a multilayer sample significantly [5]. The effect of back-scattered ions have been observed by Cervo and Carter [6] in case of their IBS-deposited TiO2 films, where the authors have found Ar+ ions upto a density of 0.4% get trapped inside the growing films. Effect of reflected ions has also been observed and discussed by Balu et al. [7] on the properties of their IBS-deposited Cr thin films. However, there are discrepancies in the literature regarding the dependence of energy of sputtered particles and back-scattered ions on the energy of the projectile ions. Duchemin [8] has simulated the IBS process using the TRIM Monte-Carlo code and showed that both the energy of the sputtered atoms and the backscattered/ re-sputtered ions increase with the increase in the projectile energy for ion beam sputtering of Mo by Xe ions. However, Franke et al. [9] by putting an energy-selective mass spectrometer inside an IBS chamber, have measured the energies of different ions and neutrals involved in a typical IBS process and have found that the energy of the re-sputtered Ar+ ions from the target surface follows a distribution whose maxima shifts towards lower energy as the energy of the projectile species increases, while the maxima of

A. Biswas, D. Bhattacharyya / Nuclear Instruments and Methods in Physics Research B 268 (2010) 1594–1600

energy distribution of Ti neutrals sputtered from a Ti target almost remains constant. However, they have also observed that the energy distribution of the species strongly depends on the projectile-target combination. Thus determination of the optimum energy of primary ions is important for establishing the process parameters for deposition of a particular multilayer system by IBS technique. In the present communication, we describe deposition and characterization of single-layer W films and W/Si/W tri-layer samples by IBS technique, where the W layers have been deposited with different Ar+ ion energies in the range of 600–1200 eV. The tri-layer samples have been chosen so that both the interfaces, viz., W on Si interface and Si on W interface can be studied. Films described here have been characterized by specular and diffused grazing incidence X-ray reflectivity (GIXR) measurements and the variations of different interface parameters obtained from the GIXR analysis have been studied as a function of the energy of the incident Ar+ ions of the IBS process. The results of the above studies on W/Si deposition would be useful in the preparation of W/Si multilayer soft X-ray mirrors [5,10–12]. 2. Experimental methods In the home-built IBS system used here [13], the ion gun and the substrate holder are mounted from two 63 CF ports fixed at an angle of 45° with each other. An ECR microwave based filament-less plasma ion source [14] is used in the system which uses microwave power of 250 W @2.45 GHz to produce Ar plasma within a hemi-spherical alumina plasma chamber. High purity Ar gas at a working pressure of 1.5  103 Torr is fed into the ion gun through a mass flow controller to generate the plasma and ions are then extracted out by molybdenum acceleration grids. The energy of the ions is defined by the voltage applied on the grids. Two 300 diameter sputter targets are mounted on a horizontal tray which can be rotated by a stepper motor and substrates are continuously rotated during deposition by a d.c. motor mounted on the shaft outside vacuum. Base pressure of 5  107 Torr is regularly achieved in the system by a turbo-molecular pump. Rate of deposition and thickness of the films were monitored in-situ using a quartz crystal thickness monitor. For the present study, single-layer W films (each having in-situ measured thickness of 40 Å) have been deposited on c-Si substrates at different Ar+ ion energies in the range of 600–1200 eV. For the W/Si/W tri-layer samples 30 Å Si layers were grown at a rate of 0.15 Å s1 with a fixed ion energy for all the samples, while 20 Å W layers have been deposited at different ion energies for different samples in the range of 600–1200 eV. GIXR spectra of the samples have been recorded both in specular and diffused geometry. For specular reflectivity measurement, a standard h  2h scan has been carried out at a grazing angle of incidence around the critical angle of W, while for diffused reflectivity measurement, a detector scan has been carried out where, the angle of incidence is kept fixed at a peak position of the specular reflectivity spectrum and the scattering angle is varied. The configuration of the above two scans in the reciprocal space is shown in Fig. 1, where z is the direction perpendicular to the sample surface, hi and hf are the grazing angles of incidence and reflection measured from the sample surface and the two components of the momentum transfer vector q ¼ ðkf  ki Þ, are given by

  ki  ; qx ¼ ki cos hf  cos hi   2 hi  hf U

ð1Þ

and

  qz ¼ ki sin hf þ sin hi  ki U;

ð2Þ

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Fig. 1. Schematic diagram showing the different configurations for GIXR measurements.

where

U ¼ hi þ hf ;

ð3Þ

is the scattering angle. Thus as has been explained in Ref. [15], a specular scan (hi ¼ hf ) means a qz scan with qx ¼ 0, while a detector scan where hi is fixed and hf (and hence the scattering angle U) is scanned, both the qx and qz components of the momentum transfer vector are varied. 3. Theoretical formulation The basic theory of X-ray reflectivity is derived from classical optics and Frensel theory, modified slightly for X-ray. The complex refractive index (g) of an element in the X-ray region is given by [16]:

g ¼ 1  d  ib;

ð4Þ

where,

( )   r 0 k2 qZ NA ; d¼ Aw 2p

ð5Þ

and

 b¼



lk ; 4p

ð6Þ

where, r0 is the classical electron radius (2.82  1013 cm), N A is the Avogadro number, q is the density, Z is the atomic number, Aw is the atomic weight of the element and l is the linear absorption coefficient of the material at the particular wavelength k. The quantity ½N A ðAqwZ Þ actually gives the number of electrons per cm3 of the particular element. Under the well-known Parrat formalism [17], the reflectivity of X-rays from a plane boundary between two media can be obtained using Fresnel’s boundary conditions of continuity of the tangential components of the electric field vector and its derivative at the interface. The reflectivity of the multilayer stack is then obtained by the successive application of the Fresnel’s equation at each interface [18]. Since refractive index of all materials in X-ray region is <1, X-ray reflectivity at very low grazing angle of incidence is 1 (total external reflection regime) and for grazing angle of incidence above a critical angle value defined by

hc ¼

pffiffiffiffiffiffi 2d:

ð7Þ

X-rays start penetrating inside the layer and the reflectivity falls off at a rate defined by the absorption of the stack. The reflectivity tail is also characterized by oscillations defined by the thickness of the film in case of a single-layer sample and also by bi-layer thick-

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ness in case of a multilayer sample. Hence analysis of specular Xray reflectivity spectrum can yield information regarding density and thickness of a thin film/multilayer accurately. However, the Fresnel’s reflectivity gets modified for a rough surface by a ‘Debye–Wallar -like’ factor as follows:

 2 2 q r R0 ¼ Rp exp  ; 2

ð8Þ

where, q is the momentum transfer factor (= 4p sin h=k), R0 is the reflectivity of the rough surface and Rp is the reflectivity of an otherwise identical smooth surface and r is the r.m.s. roughness of the surface. In case of an interface, the above quantity (r) is called the interface width which includes the contribution of both interfacial roughness and interfacial diffusion. These two contributions cannot be distinguished by specular reflectivity measurement and for that diffused reflectivity has been measured on the samples in the ‘‘detector scan” geometry, as described above. The diffused reflectivity is analysed by defining the Power Spectral density (PSD) function of each interface of the structure. The PSD function is given by [19]:

     4p exp  nz? r2r n2k ); PSD qk ¼ (   2 ð1þhÞ n2k 1 þ abs qk

ð9Þ

where, qk is the component of the momentum transfer vector on the plane of the sample and each interface is characterized by three parameters viz., the interface roughness (rr), the in-plane or parallel correlation length (nk ) and the so called jaggedness factor (h). In addition, the vertical correlation function between two interfaces i and j (j < i) of a stack is defined by a vertical or perpendicular correlation length parameter (n? ) as follows:

!

C ji ¼ exp 

i1 X dn ; n? n¼j

ð10Þ

dn being the thickness of the nth layer. The interface diffusion (rd) on the other hand modifies the Fresnel coefficients at each interface, the detailed expressions for which have been derived by Stearns et al. [20] and the diffused reflectivity of the whole structure is then computed using first order Distorted Wave Born Approximation (DWBA) [21]. The diffused reflectivity is presented here in terms of the quantity I10 ðddIXÞ, where dI is the intensity of X-rays scattered from the sample within a solid angle dX made by the illuminated portion of the detector at the point of incidence on the sample surface (3  105 Sr in the present case as defined by the slit width) and I0 is the incident intensity. In each case, the measured data were fitted with theoretical diffused reflectivity spectra using the five fitting parameters: rd, rr, nk ; n? and h. The interface width r defined in Eq (8), which is obtained from the specular reflectivity measurement, is related to the interface roughness (rr), and interface diffusion (rd), by the following expression:



qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r2r þ r2d :

ð11Þ

It should be mentioned here that theoretical simulation and fitting of both specular and diffused GIXR spectra have been carried out using the ‘‘IMD” code under the ‘‘XOP” software package [22] and the best fits are achieved by minimizing the v2 value. 4. Results and discussion Fig. 2(a) and (b) show the specular and diffused GIXR spectra of a representative single-layer W film deposited on c-Si substrate by the IBS technique with an ion energy of 1200 eV. The diffused X-

Fig. 2. (a) Specular and (b) diffused grazing incidence X-ray reflectivity spectra with best-fit theoretical curves for a representative single-layer W film deposited on c-Si substrate with Ar+ ion energy of 1200 eV.

ray reflectivity spectrum has been measured at the angle of incidence corresponding to the maxima of the 1st oscillation in the specular X-ray reflectivity spectrum. The experimental spectra have been fitted with theoretically simulated spectra using the formalism discussed above and the best-fit theoretical simulations have also been shown in Fig. 2(a) and (b). Similar measurements and theoretical fittings have been carried out on all the W films deposited at different ion energies. The thickness measured by the in-situ thickness monitor (40Å) during deposition has been taken as the starting value for the theoretical fitting of the specular GIXR spectrum. The thicknesses of the samples as obtained from the GIXR analysis are found to be 41, 39, 37 and 39 Å for films deposited at ion energies of 600, 800, 1000 and 1200 eV respectively which shows good calibration of the in-situ monitoring system. The other two parameters obtained from the specular GIXR spectra of the samples are roughness and densities of the films. The interface width (r) at the W/c-Si substrate interface and the surface roughness at the vacuum/W interface as obtained from the fitting of the specular GIXR data have been shown in Fig. 3(a) as a function of the ion energy used during deposition of the W films. It should be mentioned here that, considering strong correlation between density and roughness in the GIXR spectrum near the critical angle, roughness values obtained independently from Atomic Force Microscopy (AFM) measurements on the films have been taken as starting guess values for the fitting. It has been found from Fig. 3(a) that interface width of W/c-Si interface reduces as the ion energy for W sputtering increases. This may be due to the fact that at lower ion energy of 600 and 800 eV, adatoms reaching the substrate surface do not obtain sufficient energy to redistribute themselves resulting in the three-dimensional island-like structure on the surface leading to higher interface roughness. As the ion energy increases, mobility of adatoms also increases leading to two-dimensional smooth interface. The best-fit parameters of the diffused reflectivity measurement are shown in Fig. 2(b),

A. Biswas, D. Bhattacharyya / Nuclear Instruments and Methods in Physics Research B 268 (2010) 1594–1600

Fig. 3. (a) Variation of interface width of W/c-Si interface and surface roughness of single-layer W films as a function of ion energy used for W sputtering. (b) Variation of density of the single-layer W films as a function of ion energy used for W sputtering.

from where it is found that the roughness component (rr) is the major contributor to the width of the W/c-Si interface and contribution of interface diffusion (rd) is insignificant. From Fig. 3(a) it is also found that the roughness of the top vacuum/W interface increases continuously as the ion energy increases from 600 to 1200 eV because of the increase in grain sizes at the surface of the W layer with increase in ion energy. The densities of the W layers have also been estimated from the specular GIXR fittings and are shown in Fig. 3(b) as a function of ion energy of sputtering. It shows that density of the films initially increases as the ion energy increases upto 1000 eV above which the density goes down. To obtain an insight into the above phenomenon, we have carried out simulation of the IBS process by using the Monte-Carlo programme ‘‘TRIM”, which works on the principle of binary collision dynamics, to estimate the yield and energy of the sputtered atom and back-scattered ions [23]. The results are shown in Fig. 4(a) and (b) for the sputtered atoms and back-scattered ions respectively. It shows that with increase in energy of the incident particles, the positions of the maxima in the energy distributions of the sputtered atoms and back-scattered ions remain almost constant though the distributions change. In case of sputtered atoms, the number of W atoms emitted with maximum energy increases with increase in the energy of the primary ions, while the reverse trend is observed for the backscattered Ar+2 ions. However, if total energy of the sputtered atoms and the back-scattered ions emitted for a particular flux of incident Ar+2 ions are calculated from the area of the respective curves shown in Fig. 4(a) and (b), it is found that the sum total of the energy of the sputtered atoms and back-scattered ions with which they strike the growing surface of the film increases monotonically as the energy of the primary projectiles increases. The increase in kinetic energy in the sputtered adatoms with increase in energy of the incident ions, results in higher mobility of the adatoms on the growing surface and thus promotes two-dimensional growth and enhanced smoothness of the layers. However, with the increase in the energy of the incident ion beam, sputtering yield of

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Fig. 4. Simulated energy distribution of (a) sputtered W atoms and (b) backscattered Ar+2 ions from a W target under bombardment of Ar+2 ions at different energies.

the target also increases which increases the rate of deposition and results in faster growth of grains, enhanced surface roughness and low density of the films. The simulated sputtering yields from W target under Ar+2 ion bombardment at different energies are shown in Fig. 5, which follows similar trend as observed in case of Ti sputtering by Ar+2 ions [9]. Thus the above two competitive

Fig. 5. Simulated sputtering yield of a W target under bombardment of Ar+2 ions at different energies.

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processes, viz., the faster rate of deposition and movement/readjustment of adatoms on the growing surface, result in the optimised value of 1000 eV Ar+2 ion energy to obtain highest density in the W layers. The specular and diffused GIXR measurements have also been carried out on the tri-layer samples. Fig. 6(a) and (b) respectively show the specular and diffused X-ray reflectivity spectra of a representative tri-layer W/Si/W sample where the W layers have been deposited at an ion energy of 1200 eV. It should be mentioned here that considering higher number of unknown parameters for a trilayer sample, diffused reflectivity spectra have been measured at two angles of incidences corresponding to the maxima of 1st and 2nd oscillations of the specular GIXR spectrum. The best-fit theoretical plots for the measured specular and diffused spectra are also shown in the above figures. Since it had been observed in case of single-layer samples that the thickness estimations by quartz crystal monitors and GIXR measurements agree quite well with each other, in case of tri-layer samples we have assumed the in-situ measured thicknesses as the true thicknesses of the layers and have kept them invariant during fitting. Densities of the layers have also been taken from the GIXR measurements of respective single-layer films. Also the h values of the different layers have been taken as 1 for all the samples during fitting of the diffused reflectivity spectra. The above exercise has been carried out for all the four tri-layer samples and the interface width (r) values as obtained from the fitting of specular GIXR data and interface roughness (rr) and interface diffusivity (rd) values as obtained from the fitting of diffused GIXR spectra are shown in Figs. 7 and 8. The other parameters viz., in-plane (nk ) and vertical (n? ) correlation lengths of the layers derived from the fittings and surface roughness of the top W layer (r)top have been given in Table 1. It should be mentioned here that, for a particular sample, the values

Fig. 6. (a) Specular and (b) and (c) grazing incidence X-ray reflectivity spectra with best-fit theoretical curves for a representative tri-layer W/Si/W sample deposited on c-Si substrate where the W films have been deposited with Ar+ ion energy of 1200 eV.

Fig. 7. Variation of the interface parameters viz., interface width (r), interface diffusion (rd) and interface roughness (rr) of the Si-on-W interface of the W/Si/W tri-layer samples as a function of ion energy used for W sputtering.

of the interface parameters (rr, rd, nk and n? ) required to fit the measured diffused reflectivity data at two angles of incidences are found to be almost same and hence the average values of the parameters have been presented in Table 1 and Figs. 7 and 8. The variation of interface parameters (r, rd and rr) for the Si/ W1 interface where the Si film is deposited on the underlying 1st

Fig. 8. Variation of the interface parameters viz., interface width (r), interface diffusion (rd) and interface roughness (rr) of the W-on-Si interface of the W/Si/W tri-layer samples as a function of ion energy used for W sputtering.

A. Biswas, D. Bhattacharyya / Nuclear Instruments and Methods in Physics Research B 268 (2010) 1594–1600 Table 1 Results of diffused reflectivity measurements on W/Si/W tri-layer films. Sample No.

Ar+ Ion energy of W sputtering (eV)

(nk )Si-on-W (Å)

(nk )W-on-Si (Å)

n? (Å)

(rr)top (Å)

1 2 3 4

600 800 1000 1200

245 170 520 132

251 255 287 60

55 33 32 30

2.67 2.89 4.20 4.95

W layer (viz., W1), is shown in Fig. 7 as a function of the ion energy used during deposition for the W layers. It is found that the interface width (r) decreases as the ion energy increases and it is minimum when the W film is deposited at 1000 eV after which it again increases. It is also observed that the major contributor to the width of the Si/W1 interface is the diffusivity (rd) and the diffusivity parameter also decreases as the ion energy used during W sputtering increases and reaches a minimum at 1000 eV. This is consistent with the fact that Si being lighter element can only penetrate into the voids present in the underlying W layer and as obtained from our analysis on the single-layer films and shown in Fig. 3(b), the density and compactness of the W layer is maximum when the film is deposited with an ion energy of 1000 eV. However, the interface roughness parameter (rr) increases as the ion energy for W sputtering increases, which is consistent with the fact that roughness on the W layer increases with increase in the ion energy of deposition. The interface parameters for the W2/Si interface of the tri-layer samples, where the 2nd W layer (W2) is deposited on the underlying a-Si layer, have been plotted in Fig. 8 as a function of energy of Ar+ ions used during deposition of the W layers. From the above figure, it is found that interface width (r) of the W2/Si interface increases as primary ion energy increases up to 1000 eV, after which it decreases. The interface width of the W2/Si interface is found to be much higher than that of Si/W1 interface which is consistent with our earlier findings on similar systems deposited by r.f. magnetron sputtering [24]. Kessel et al. [5] have also observed that the W/Si interface is generally asymmetric with interface width of W on Si is more than Si on W. Other workers have also reported the fact that in case of sputter deposited multilayers, the interface width depends on the order of deposition, e.g., in case of Mo/Si multilayers the interface is sharper when Si is deposited on Mo than when Mo is deposited on Si [25,26]. It can also be seen from Fig. 8 that the major contributor to the interface width of the W2/Si interface is interface diffusion (rd) i.e., diffusion of W atoms in underlying Si layer which is much higher than the diffusion of Si atoms in underlying W layer as shown in Fig. 7. The above findings is consistent with the fact that the average atomic volumes of W atoms are smaller and W atoms can penetrate through a larger distance in the underlying disordered amorphous Si layer [27] than the Si atoms inside the W film. However, it also shows that the energy of the W atoms and also of the backscattered Ar+ ions may be much higher in case of W sputtering compared to that of Si sputtering. Teyssier et al. [28] have carried out simulation of IBS process of Mo/Si multilayers by TRIM MonteCarlo code and observed that the peak of the energy distribution of sputtered Mo atoms is 5 eV while that of Si atoms is 2 eV. Also the energy distribution of the re-sputtered or back-scattered Ar+ ions spreads upto very high energy (450 eV) in case of sputtering from a Mo target. In case of r.f. sputtered W/Si samples also, we had found earlier from dynamic Secondary Ion Mass Spectrometry (SIMS) study that the width of inter-diffusion layer at W/Si interface is larger for W-on-Si interface than on Si-on-W interface [24]. From Fig. 8 it is also found that the interface roughness parameter (rr) of the W2/Si interface does not increase significantly with increase in ion energy for W sputtering, while the interface diffu-

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sivity (rd) increases considerably with sputtering ion energy up to 1000 eV, beyond which the diffusivity decreases again. This might be because of the increase in sputtering yield and rate of deposition of the W2 layer at sputtering ion energy above 1000 eV. This agrees well with the results reported by Largeron et al. [29] on simulation by Cellular Automation (CA) and measurements on IBS grown Mo/Si multilayers. Largeron et al. [29] have observed that there is a competition between deposition of Mo atoms to form a Mo layer and growth of Mo/Si interface zone by inter-diffusion. It is found that the interface layer is decreased with increase in sputtering rate of Mo since the higher rate of deposition enhances formation of Mo clusters or nano-crystalline grains which prohibits inter-diffusion across the interface. 5. Conclusion A set of single-layer W films and W/Si/W tri-layer films have been deposited on c-Si substrates by ion beam sputtering technique, where the W layers of the different samples have been deposited at different ion energies in the range of 600–1200 eV. Along with the thickness and density of the single-layer W films, interface roughness and interface diffusion for both the Si-on-W interface and W-on-Si interface have been measured, in case of the tri-layer samples, by specular and diffused Grazing Incidence X-ray reflectivity (GIXR) measurements. It has been found that the density of W layer increases as the sputtering ion energy increases from 600 eV because of enhancement in the mobility of the adatoms on the growing surface and is maximum for 1000 eV beyond which the enhanced rate of deposition does not allow proper rearrangement of the adatoms resulting in lower density of the films. In case of the tri-layer samples, the interface width and inter-diffusion at W-on-Si interface are found to be much higher than that at Si-on-W interface. The interface diffusion decreases as ion energy of W deposition increases for Si-on-W interface and is minimum when W layer is deposited with 1000 eV Ar+ ions since the density of W layer is maximum at this energy. The interface diffusion at the W-on-Si interface however, increases as ion energy for W deposition increases and is maximum for 1000 eV, beyond which it decreases since the rate of deposition increases which enhances growth of atomic clusters and prohibits inter-diffusion. Acknowledgement Technical support of Shri D.D. Kandalkar in the maintenance and operation of the ion beam sputtering system is gratefully acknowledged. References [1] A. Paul, J. Wingbermühle, Appl. Surf. Sci. 252 (2006) 8151. [2] S.M. Feng, G.L. Zhu, J.D. Shao, K. Yi, Z.X. Fan, X.M. Dou, Appl. Phys. A74 (2002) 553. [3] M. Gupta, A. Gupta, D.M. Phase, S.M. Chaudhari, B.A. Dasannacharya, Appl. Surf. Sci. 205 (2003) 309. [4] X. Ji, H. Ju, D.E. McCready, K.M. Krishnan, J. Appl. Phys. 98 (2005) 116101. [5] M.J.H. Kessels, J. Verhoeven, F.D. Tichelaar, F. Bijerk, Surf. Sci. 582 (2005) 227. [6] M. Cervo, G. Carter, J. Phys. D: Appl. Phys. 28 (1995) 1962. [7] R. Balu, A.R. Raju, V. Lakshminarayanan, S. Mohan, Mater. Sci. Eng. B123 (2005) 7. [8] Olivier B. Duchemin, An Investigation of Ion Engine Erosion by Low Energy Sputtering, Ph.D. Thesis, California Institute of Technology Pasadena, California, 2001. [9] E. Franke, H. Neumann, M. Zeuner, W. Frank, F. Bigl, Surf. Coat. Technol. 97 (1997) 90. [10] M.J.H. Kessels, F. Bijerk, F.D. Tichelaar, J. Verhoeven, J. Appl. Phys. 97 (2005) 093513. [11] C. Montcalm, S. Bajt, J.F. Seely, Opt. Lett. 26 (2001) 125. [12] Y. Wang, H. Zhou, L. Zhou, R.L. Headrick, A.T. Macrander, A.S. Ozcan, J. Appl. Phys. 101 (2007) 023503.

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