lable at ScienceDirect

Vacuum 83 (2009) 397–400

Contents lists avai

Vacuum

journal homepage: www.elsevier .com/locate/vacuum

Ion beam induced mixing at Co/Si interface

Garima Agarwal a,*, Pratibha Sharma a, Ankur Jain a, Chhagan Lal a, D. Kabiraj b, I.P. Jain a

a Centre for Non-Conventional Energy Resources, University of Rajasthan, Jaipur 302004, Indiab Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi, India

a r t i c l e i n f o

Article history:Received 17 February 2007Received in revised form 12 May 2008Accepted 13 May 2008

Keywords:InterfaceSilicideIon beam mixingElectronic excitationRBS

* Corresponding author. Centre for Non-ConventVigyan Bhavan, University of Rajasthan, Jaipur 3020271 1049.

E-mail address: garima114@gmail.com (G. Agarwa

0042-207X/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.vacuum.2008.05.016

a b s t r a c t

Ion beam mixing has emerged as a technique for understanding reactivity and chemistry at metal/Siinterface and may find its applications in the field of microelectronics. We have investigated ion beammixing at Co/Si interface induced by electronic excitation using 120 MeV Auþ9 ion irradiation at differentfluences, varying from 1012 to 1014 ions/cm2. Mixing was investigated by Rutherford BackscatteringSpectroscopy (RBS) as a function of ion fluence and its mechanism across the interface is explained by thethermal spike model.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

All evolutionary aspects related to the metal/metal and metal/semiconductor contacts leading interfacial reactions play a majorrole in the emerging electronic technology resulting in a number ofrelated studies in this field. Investigations of the interface of Pd/Recontact to p-type GaN shows the formation of Ga–Pd reactionphases which results in the generation of Ga vacancies at the GaNsurface region that plays a role in reducing contact resistivity [1].Schottky barrier is the main contributing factor to the band align-ment at metal–semiconductor interfaces and some studies on liq-uid metal–semiconductor [2] have also been done simultaneouslywith solid metal–semiconductor interactions leading to the com-pound formation in the form of metal silicides.

The use of silicides in electronic technology has been motivatedby the fabrication of contact structures where both the metallur-gical and electrical properties of the contacts were important. Anumber of studies in this field have started by late 1980s. Hewett etal. [3] have investigated the effect of boundary conditions in ionmixing of multilayered Ni–Si samples to investigate the effect ofindividual layer thickness of multilayered samples on ion inducedreactions. It was found that the mixing efficiency increases with thenumber of layers and so layer thickness and amorphous phase. Ionbeam mixing in the Au/poly-Si and Au/Si (100) systems was

ional Energy Resources, 1404, India. Tel./fax: þ91 141

l).

All rights reserved.

conducted by in situ resistance measurements [4]. The relationshipbetween sheet resistance of the irradiated samples has beenestablished in this system, an analytical model was employed toexplain the linear relation between conductance and dose and thegrowth of an amorphous Au5Si2 layer was measured by RutherfordBackscattering. Effects of arsenic irradiation on lateral Ti silicideformation have been studied by Zheng et al. [5] using scanningelectron microscopy and the electron microprobe and it was foundthat the extent of lateral diffusion was greatly reduced andexhibited dose dependence. These observations suggest that thesuppression of lateral diffusion is primarily due to interactions ofimpurities with the matrix.

While obtaining better smooth patterns, structural and elec-trical properties of silicides formation at Metal/Si bilayer systemthrough swift heavy ion beam induced mixing, this technique hasbeen come in extensively use and it could found a well establishedway for the formation of metal silicides [6]. In IBM the local tem-perature rise due to thermal spikes is very high and the associatedcooling rates are several orders of magnitudes larger than thoseachieved in normal quenching experiments. As a result IBM is anideal technique for preparing exotic metastable compounds.

Among many metal silicides Co silicide is a promising candidatefor contacting and interconnects due to high conductivity, whichallows for lowering the sheet resistance of a shallow junction byshunting, thus decreasing the parasitic contribution to the devicecontact resistance [7]. Another reason for choosing Co silicide forthe present study is its low bulk resistivity (14–15 U cm), highthermal stability, less lattice mismatch with Si (about 1.2% mis-match) and compatible processing treatment.

G. Agarwal et al. / Vacuum 83 (2009) 397–400398

Since the ability of ion beams to initiate efficient atomic trans-port process was first demonstrated, intermixing of bi- or multi-layers with well defined sharp interfaces induced by energetic ionshas been extensively studied experimentally [8–11]. For low energyion beam (wkeV/m) region, intermixing was predominantly due todirect atomic displacements caused by nuclear collisions. Also forswift heavy ion beams (�0.1 MeV/m), the kinetic energy of the ionsis mainly deposited to the target electron subsystem (electronicenergy loss, Se) and hence Se could play an important role in theintermixing process [9–11]. It was suggested that several metals e.g.Ti, Ni, Fe, Co, Zr etc. to be sensitive to electronic energy loss Se

beyond certain threshold [12].However, the observations of latent amorphous nuclear tracks

formed by single ion impacts in various predominantly insulatingmaterials clearly demonstrate that a significant part of the elec-tronic excitation energy is transferred to the lattice resulting inatomic displacements and structural changes.

Two theoretical models are invoked to explain such high localexcitation of the lattice by the energy transfer from the highlyexited electronic system to the lattice atoms either by Coulombexplosion [13] or by electron–phonon coupling i.e. Thermal spikemodel [14]. Both mechanisms should result in atomic transportacross the interfaces of a high energy ion bombarded layer system.Qualitatively, the atomic migration across the interface, due to theelectronic energy loss of high energy heavy ions, was explained bythe thermal spike model, according to which, one assumes firstthermalization of the electronic system occur within some 10�14 sand an energy transfer to the lattice via electron–phonon couplingtakes place within a few multiples of 10�12 s. Depending on thematerial and the energy deposited by the ion the temperature inthe surroundings of the ion path may significantly exceed themelting point and a liquid cylinder of some nm in diameter isformed, which is rapidly quenched to the ambient temperaturewithin a few tens to a few hundreds of ps. Due to the high coolingrate, a nonequilibrium material is left in the ion track. Such a tran-sient increase of temperature within a very small local volume isoften called a ‘‘thermal spike’’.

The present work discusses the formation of silicide in the Co/Si(100) system during ion beam mixing at room temperature.

300 400 500 600 700 8000

2000

4000

6000

8000

0 50 100 150 2000

20

40

60

80

100

RBSFit

Co

un

ts

Energy (keV)

30 nm

CoSiOC

Ato

mic fractio

n (%

)

Depth (1015

at. /cm2)

Fig. 1. RBS Spectra and concentration profile of the pristine sample.

2. Experimental

The Co/Si samples were prepared by electron beam evaporationof 30 nm Co thin film on chemically cleaned Si (100) substrates inan Ultra high Vacuum (UHV) chamber at 4�10�8 mbar vacuum.Thickness was measured and monitored by a digital thicknessmonitor keeping deposition rate as 1 nm/s. The samples were ir-radiated with 120 MeV Auþ9 with fluences between 5�1012 and1�1014 ions/cm2 using 15 UD Pelletron Accelerator at Inter Uni-versity Accelerator Centre, New Delhi. The irradiations were carriedout at room temperature (RT) with current densities of typically1 pnA (particle nano-ampere). The irradiation flux was always keptquite low to avoid sample heating during irradiation. Samples wereuniformly irradiated by scanning the beam over 1 cm� 1 cm areain secondary electron suppressed geometry. Energy deposited byprojectile ions and their ranges were calculated by TRIM code [15].For 120 MeV Au ions, the implanted species will pass through thelayered structure and get buried into the substrate as at such highenergy the projected range will be in micron range. The electronicand nuclear energy losses, Se and Sn for 120 MeV Au ions for Co are24.3 keV/nm and 0.44 keV/nm, respectively. The thickness andcomposition of the pristine and irradiated samples were analyzedby Rutherford Backscattering spectroscopy (RBS) employing900 keV a-particle beam at Gottingen, Germany by heavy ion im-planter IONAS. The backscattered particles are counted by two Si

detectors at �165� to the beam. The RBS data was analyzed usingWINDF [16] software.

3. Result and discussion

Experimental RBS spectra and concentration profiles taken frompristine Co/Si sample and from the samples irradiated at fluences5�1012, 8� 1012, 5�1013 and 1�1014 ions/cm2 are presented. It isseen from the RBS spectrum of Fig. 1 of the pristine Co/Si systemthat the interface to Si is very flat and of the order of less than 1 nm.It is clear from the concentration profile that the samples containa lot of oxygen and carbon, probably due to bad vacuum during firstirradiation. In Fig. 2, spectra for the irradiated sample at 5�1012

ions/cm2 show evidence of a little intermixing of Co and Si as thereis slight reduction in the slope of the low energy edge of Co anda major decrease in the Si counts in comparison of pristine one.While from the depth vs atomic fraction graph for the pristine andirradiated sample at 5�1012 ions/cm2 indicating that after irradi-ation Co atoms have increased their depth inside Si leading to in-crease in mixing at the interface. We can observe from Fig. 3 that onincreasing the irradiation fluence to 8� 1012 ions/cm2, the atomicfraction of Co get decreased more with increasing its depth. How-ever, at fluence 5�1013 ions/cm2 there is no more change in thespectra as shown in Fig. 4, while for the highest fluence 1�1014

ions/cm2 the atomic fraction for both Si and Co decreased and thedepth of Si inside the Co and of Co inside the Si increased gradually,which can be seen clearly in Fig. 5. These observations suggest thatheavy ion induced mixing at the interface has taken place and in-creasing order of fluence gives rise to interface broadening in-dicated increased mixing.

The very careful RBS measurements provided precise values ofthe implanted ion fluence F and the ion-induced changes of theinterface variance,

Ds2 ¼ s2ðFÞ � s2ð0Þ;

300 400 500 600 700 8000

2000

4000

6000

8000

0 50 100 150 200 2500

20

40

60

80

100

RBSFit

Co

un

ts

Energy (keV)

CoSiOC

Ato

mic fractio

n (%

)

Depth (1015

at. /cm2)

Fig. 4. RBS Spectra and concentration profile of the sample irradiated at 5�1013 ions/cm2.

300 400 500 600 700 8000

2000

4000

6000

8000

0 50 100 150 200 2500

20

40

60

80

100

RBSFit

Co

un

ts

Energy (keV)

CoSiOC

Ato

mic fractio

n (%

)

Depth (1015

at. /cm2)

Fig. 2. RBS Spectra and concentration profile of the sample irradiated at 5�1012 ions/cm2.

G. Agarwal et al. / Vacuum 83 (2009) 397–400 399

from which mixing rate k¼Ds2/F was deduced where F is themaximum fluence. We are using the adopted definition that thevariance s2(F) denotes the square of the full thickness of the in-terface zone in which the Si concentration changes from 84% to 16%

300 400 500 600 700 8000

2000

4000

6000

8000

0 50 100 150 200 2500

20

40

60

80

100

RBSFit

Co

un

ts

Energy (keV)

CoSiOC

Ato

mic fractio

n (%

)

Depth (1015

at./cm2)

Fig. 3. RBS Spectra and concentration profile of the sample irradiated at 8� 1012 ions/cm2.

of the bulk value. With the help of the experimental values ofmixing rate we can calculate the mixing efficiency (k/Se) also.

It is clear from the Table 1 that the increase of the interfacevariance, Ds2 does not fully scale with ion fluence.

300 400 500 600 700 8000

2000

4000

6000

8000

0 50 100 150 200 250 3000

20

40

60

80

100

RBSFit

Co

un

ts

Energy (keV)

CoSiOC

Ato

mic fractio

n (%

)

Depth (1015

at./cm2)

Fig. 5. RBS Spectra and concentration profile of the sample irradiated at 1�1014 ions/cm2.

Table 1Interface variance of Co, mixing rate and mixing efficiency with fluence

Fluence F

(ions/cm2)Interface varianceDs2 (nm2)

Mixing ratek¼Ds2/F (nm4)

Mixing efficiencyk/Se (nm5/keV)

0 – – –5� 1012 231.04 4620 1908� 1012 368.64 4608 1895� 1013 1474.56 2949 1211� 1014 2683.24 2683 110

0.0 2.0x1013 4.0x1013 6.0x1013 8.0x1013 1.0x1014

0

500

1000

1500

2000

2500

3000

[n

m2]

ion fluence [ions/cm2]

Fig. 6. Variation of Ds2 with ion fluence for 120 MeV Au ions.

G. Agarwal et al. / Vacuum 83 (2009) 397–400400

Assuming mixing due to transient interdiffusion in the moltonion track, an estimation of the interdiffusion coefficient [11,17] isgiven by,

D ¼ Ds2=2tn

where tn¼ diffusion time and can be obtained as

tn ¼ ðF=FcÞ � ts

where F¼maximum fluence, Fc¼ 1/(2r)2 is the fluence for com-plete overlap of the ion tracks, ts¼ duration of melt phase of Co.

Taking the typical track radius of 1.5 nm and ts as w1 ps foranother metal Ni, the data for which is available [18] and using thevalues of r, ts and experimental values of Ds2, Fc and F, the diffu-sivity D at the interface turns out to be of the order of w10�6 m2/s,which is clearly in the range of the interdiffusion constants knownfrom liquids and hence supports the thermal spike hypothesis. Thusthe ion beam induced by swift heavy ions in the present

experiments can be attributed as consequence of interdiffusion inthe melt phase.

An increasing variation is clearly seen in Fig. 6 which shows thedependency of interface variance with ion fluence. It is clear thatthe variance increases linearly with the fluences. The resultingvalues of interface variance are quite supporting and in goodagreement with the SIMS studies [19] done on the same system. Inthis way we can say that ion beam mixing is a modifying tool for themetal/semiconductor devices.

4. Conclusion

The present work demonstrates the mixing of Co/Si interfaceinduced by 120 MeV Au ions at fluences of 1012–1014 ions/cm2 usingRBS. It is observed that the mixing is linearly dependent of irradi-ation fluences. It is inferred that the cause of mixing is due to in-terdiffusion across the interface during a transient melt phaseaccording to the thermal spike model.

Acknowledgement

The authors are thankful to Inter University Accelerator Centre,New Delhi, India for the sample preparation and irradiation part ofthe experiment. One of us (GA) is thankful to Inter University Ac-celerator Centre for the financial assistance in the form of JuniorResearch Fellowship. We are grateful to Prof. Leib for the RBSfacility provided at IONAS, Gottingen, Germany.

References

[1] Reddy VR, Hassan CKR, Reddy PN. Indian J Pure Appl Phys 2004;42:361–5.[2] Modi BP, Mathai AJ, Patel KD, Pathak VM, Srivastava R. Indian J Pure Appl Phys

2005;43:184–7.[3] Hewett CA, Lau SS, Suni I, Pokar DB. Nucl Instrum Meth Phys Res B 1985;7/8:

597–600.[4] Li J, Allen LH, Mayer JW. Nucl Instrum Meth Phys Res B 1989;36:153–6.[5] Zheng LR, Hung LS, Mayer JW, Choi KW. Nucl Instrum Meth Phys Res B 1985;

7/8:413–7.[6] Johnson WL, Cheng YT, van Rossum M, Nicolet MA. Nucl Instrum Meth Phys

Res B 1985;7/8:657–65.[7] Maex K. Mater Sci Eng Rep 1993;11:55.[8] Cheng YT. Mater Sci Rep 1990;5:45.[9] Leguay R, Dunlop A, Dunstetter F, Lorenzelli N, Braslau A, Bridou F, et al. Nucl

Instrum Meth Phys Res B 1997;122:481–502.[10] Avasthi DK, Assmann W, Nolte H, Mieskes HD, Huber H, Subramaniyam ET,

et al. Nucl Instrum Meth Phys Res B 1999;156:143–7.[11] Kraft S, Schattat B, Bolse W, Klaumunzer S, Harbmeier F, Kulinska A, et al. J

Appl Phys 2002;91:1129–34.[12] Wang ZG, Dufour C, Paumier E, Toulemonde M. J Phys Condens Matter 1994;6:

6733–50.[13] Fleisher RL, Price PB, Walker RM. Nuclear tracks in solids. Berkeley; 1975.[14] Seitz F, Kochler JH. Solid State Phys 1956;2:305.[15] Zeigler JF, Biersack JP, Littmark U. The stopping and ranges of ions in matter,

vol. 1. New York: Pergamon; 1985.[16] Barradas NP, Jeynes C, Webb RP. Appl Phys Lett 1997;71:291–3.[17] Avasthi DK, Assmann W, Tripathi A, Srivastava SK, Ghosh S, Gruner F, et al.

Phys Rev B 2003;68:153106.[18] Shi F, Bolse W, Leib KP. J Appl Phys 1995;78:2303–10.[19] Agarwal G, Sharma P, Jain IP. Appl Surf Sci 2006;253:1165–9.