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Applied Surface Science 253 (2006) 1165–1169

SHI induced silicide formation and surface morphology

at Co/Si system

Garima Agarwal, Pratibha Sharma, I.P. Jain *

Material Science Laboratory, Centre for Non-Conventional Energy Resources, 14-Vigyan Bhawan,

University of Rajasthan, Jaipur 302004, India

Received 6 October 2005; received in revised form 25 January 2006; accepted 25 January 2006

Available online 18 April 2006

Abstract

Ion beam mixing is a useful technique to produce modifications at the surface and interface of the solid material. In the present work, ion beam

induced modifications at Co/Si interface using 120 MeVAu-ion irradiation has been studied at ion fluences in the range of 1012 to 1014 ions/cm2 by

secondary ion mass spectroscopy (SIMS) technique and calculated mixing efficiency at the interface. Silicide formation has been discussed on the

basis of swift heavy ion (SHI) irradiation induced effects. Surface morphology and roughness of irradiated system with fluence 5 � 1013 and

1 � 1014 ions/cm2 is studied by scanning tunneling microscopy (STM). Roughness of the surface shows marks of melting process and confirms the

appearance of some pinholes in the reacted Co/Si system. Comparative study was also undertaken on annealed sample at 300 8C and then irradiated

at a dose 1 � 1014 ions/cm2.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Ion beam mixing; Surface; Interface; Silicide; Ion irradiation

1. Introduction

Metal on silicon have been the subject of investigations over

the last five decades because of the importance of silicides for

contact in the field of microelectronics where silicides are the

compound formation at the metal/Si interface. The extensive

study of metal silicides is related to their applications in silicon

technology as low resistivity interconnect, chemical stability,

low fabrication temperature, small lattice mismatch with

silicon and a range of Schottky barrier heights. Techniques

involved in the formation of metal silicides are solid state

reaction, already existing technique and the ion beam mixing,

new emerging one. Ion beam induced metal silicides have

grater advantages over existing technique formation as it

provides a better smooth surface and it can control the

temperature during the formation as the rise in temperature is

not so good for silicon devices.

Ion beam mixing at the interface of different species using keV

is well understood subject [1]. For this energy regime, mixing at

* Corresponding author. Tel.: +91 141 2701602/9726; fax: +91 141 2701880.

E-mail address: ipjain46@sify.com (I.P. Jain).

0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2006.01.059

the interface of either metal/metal or metal/semiconductor

system mainly caused by ballistic effects, called as nuclear-loss

induced ion beam mixing. At higher energies with MeV beams

ions loss energy predominantly to the electronic system of the

material called as electronic loss. In recent years, many effects

involving atomic displacement via electronic energy loss such as

annealing [2], amorphization [3] and ion beam mixing [4] have

attracted considerable interest both for understanding the

phenomena and for technological applications.

Ion beam mixing in thin metal film/semiconductor systems

has been widely used to form both equilibrium and metalstable

phases [5–7]. In addition to involving lower processing

temperature, this technique provides a high degree of spatial

selectivity. Metal/semiconductor system is a good choice to

study swift heavy ion (SHI), the ions having energy in MeV

range, induced mixing from the fundamental ion–solid

interaction point of view as well as their wide applications.

Hence, the reaction of metal on Si under ion irradiation leads

to the formation of metal silicides. Among the various silicides

most of present works have been concentrated on NiSi2, FeSi2,

TiSi2, CoSi2, etc.

We have planned the study of CoSi2 formation as it has been

receiving increasing importance for low resistance, high

G. Agarwal et al. / Applied Surface Science 253 (2006) 1165–11691166

Fig. 1. SIMS depth profile of the pristine sample.

thermal stability and low lattice defects [8–10]. Co/Si surface

and interface investigated in this work has been prepared by ion

beam mixing of cobalt thin film deposition on Si. Interface

depth profiles of pristine and irradiated were characterized by

secondary ion mass spectroscopy (SIMS) technique and

scanning tunneling microscopy (STM) technique was used

for the surface morphological changes of Co/Si system under

ion irradiation.

2. Experimental

2.1. Sample preparation and ion irradiation

The substrates used in this experiment were one sided mirror

polished (1 0 0) n-type Si having an area of 1 cm � 1 cm and

thickness of 500 mm. Substrates were carefully cleaned in

organic solvents, e.g. TCE, acetone, methanol and then dipped

into a diluted HF solution to remove impurities or any oxide

layer present before loading into vacuum chamber. Thin Co

metal film of 30 nm was deposited on Si substrate using

electron beam evaporation technique at RT at NSC, New Delhi,

India. Deposition was carried under the UHV conditions with

the base vacuum of 4 � 10�8 Torr and a deposition rate of about

0.1 nm/s. The Co/Si samples were irradiated by 120 MeV Au

ions using 15 UD Pelletron Accelerator at NSC, New Delhi

under 10�6 Torr vacuum, with fluences of 8 � 1012, 5 � 1013

and 1 � 1014 ions/cm2 at RT keeping the irradiation flux about

1 pna to avoid sample heating.

The range of the projectile ions and the energy deposited by

them were calculated by TRIM [11], according to which the

values of electronic and nuclear energy loss of 120 MeV Au

ions in Co are 24.3 and 0.44 keV/nm, indicating that the

dominant process of energy loss is due to electronic excitation.

Therefore, the observed mixing at the interface can be

envisaged due to the electronic energy deposition.

2.2. SIMS characterization

Un-irradiated and irradiated samples at 8 � 1012, 5 � 1013

and 1 � 1014 ions/cm2 fluences were analyzed using the

secondary ion mass spectrometer. In the present SIMS

technique with Cs+ ions as primary ions for studying the

interface profile to evaluate the irradiation induced interface

mixing. SIMS analysis was performed in a Quadrupole type

CAMECA IMS 4f ion microscope. For sputtering with Cs, a

10 keV Cs primary ion beam with a spot size of approximately

20 mm was employed. The primary ion beam was raster

scanned over a specimen area 150 mm � 150 mm and positive

secondary molecular ions CsY+ were collected only from a

circular area of 35 mm diameter defined by a transfer lens-field

aperture couple, where for depth profile Y was chosen to be

cobalt. This procedure was adopted to avoid crater edge effects.

The base pressure was maintained at 4 � 10�7 Torr during

experiment.

The depth scale of the profile was established by measuring

sputtered SIMS crater depths using a surface profilometer

(Veeco-Dektak 6M). SIMS spectra of as-deposited sample were

fitted with Gaussian profiles for the known thicknesses as

measured with a quartz crystal thickness monitor. Using the

FWHM of the fitted Gaussian profile, the sputtering rate was

calculated as 1.8 nm/s. The SIMS parameters remained same

for all the samples irradiated at different fluences.

2.3. STM characterization

The surface morphology and roughness of Co/Si system was

performed in tapping mode by scanning tunneling microscope

technique. Root mean square (rms) roughness was calculated

from the aquired images.

3. Results and discussion

3.1. SIMS depth profiling

SIMS depth profiles of Si and Co in un-irradiated and

irradiated Co/Si system with Au ions of energy 120 MeV

having different fluences of 8 � 1012, 5 � 1013 and

1 � 1014 ions/cm2, are shown in Figs. 1–4, respectively. For

the pristine one and irradiated sample at 8 � 1012 and

5 � 1013 ions/cm2 a small peak at a place from where the

interface has its strong part, has been observed which may be

due to the presence of some oxygen during the sample

preparation. This effect is almost negligible for interface

irradiated at highest ion fluence of 1 � 1014 ions/cm2. As seen,

we reach the substrate profile at a lower sputtering time in the

irradiated sample at 1 � 1014 ions/cm2 than in the pristine and

irradiated at lower fluences. This could be because of the higher

sputtering rate of the mixed phase after irradiation. The total

thickness of the film is 30 nm, whereas the range of the

120 MeV Gold is more than 5 mm. Therefore, the mixing

observed in the present films is solely due to the electronic

losses. Also the measured diffusion length and calculated rate

of mixing for all the samples are given in Table 1. It is clear

from the table that diffusion length increases on increasing the

ion fluence hence the mixing increases with the fluence. It is

evident from these studies that partly Co has diffused into the Si

due to irradiation, which indicates the mixing at the interface

G. Agarwal et al. / Applied Surface Science 253 (2006) 1165–1169 1167

Fig. 2. SIMS depth profile of irradiated sample at 8 � 1012 ions/cm2.

Fig. 3. SIMS depth profile of irradiated sample at 5 � 1013 ions/cm2.

Fig. 4. SIMS depth profile of irradiated sample at 1 � 1014 ions/cm2.

leading to the formation of different phases of cobalt silicide,

e.g. Co2Si, CoSi and CoSi2. The possible mechanism of the

mixing at the interface of two different species can be explained

with the help of two models named Coulomb explosion and

Thermal spike which account for the atomic motion induce by

electronic excitation. But the formation of CoSi2 which is the

most stable phase in the series of cobalt silicides, can be

attributed due to the Thermal spike model as Co is a metal and

the Coulomb explosion can be ruled out since it works better for

insulator systems. Although the Thermal spike has already well

employed for metal/metal and insulator/insulator systems to

Table 1

Diffusion length, mixing rate and mixing efficiency with fluence

Fluence, F (ions/cm2) Diffusion length, X (nm) Mixin

Unirradiated 16.0 –

8 � 1012 20.0 5000.

5 � 1013 39.0 3042.

1 � 1014 52.7 2777.

show that interface mixing is due to inter-diffusion in the melt

phase. But in insulators the possibility of atomic motion by the

process of Coulomb explosion cannot be ignored. Therefore,

the present Co/Si mixing is better case to consider under

thermal spike effect. For such hypothesis, we plan an

approximate estimate of the diffusivity in the molten state.

To examine this estimation, we calculate the diffusion

coefficient [12,13] for Co as

D ¼ DX2

2tn

where tn is the diffusion time and DX2 is given as:

DX2 ¼ X2ðFÞ � X2ð0Þ

where X is the diffusion length.

On calculation the diffusivity D at the interface turns out to

be of the order of�10�6 m2 s�1, which is clearly in the range of

the inter-diffusion constants known from liquids and hence

supports the thermal spike hypothesis. Thus, the ion beam

induced by swift heavy ions in the present experiments can be

attributed as consequence of inter-diffusion in the melt phase

which is the novelty of these results to understand better the

mechanism behind the whole process.

3.2. Surface morphology

The variation of surface roughness and morphology of Co/Si

system under irradiation of 5 � 1013 and 1 � 1014 ions/cm2

fluences has been investigated by STM. Surface morphology is

greatly influenced by the irradiation fluence as the surface of

g rate, k = DX2/F (nm4/ion) Mixing efficiency, k/Se (nm5/keV)

–

00 205.76

00 125.18

29 114.29

G. Agarwal et al. / Applied Surface Science 253 (2006) 1165–11691168

Fig. 6. Surface morphology of Co/Si system irradiated at 1 � 1014 ions/cm2.

Fig. 8. (a) Surface morphology of Co/Si system irradiated at 1 � 1014 ions/cm2

followed by annealing at 300 8C and (b) surface morphology with pinhole in the

small scan area.

Fig. 5. Surface morphology of Co/Si system irradiated at 5 � 1013 ions/cm2.

Co/Si system irradiated at lower dose of 5 � 1013 ions/cm2 is

found to be disturbed as shown in Fig. 5 and having rms

roughness of 5.6 nm which indicated some kind of regular

atomic arrangement. These aligned structures become larger

and their separation changes as fluence increases to

1 � 1014 ions/cm2. Accordingly the rms surface roughness

increased to 6.2 nm as shown in Fig. 6. Hence from these

calculations, we can say that increase in surface disorder can be

ascribed to the increase in the silicide thickness, which

increases with fluence.

Fig. 7 is the surface morphology of pristine Co/Si system

after annealing at 300 8C and it does not show any kind of

structure formation as surface becomes almost smooth. When

Fig. 7. Surface morphology of Co/Si system annealed at 300 8C.

the system was irradiated at highest fluence followed by

annealing at 300 8C, the rms roughness of the surface increased

to 8.9 nm and some kind of pinholes appears at surface as

shown in Fig. 8(a). These pinholes are having irregular

positions of high density and of different diameters one similar

to found in the solid state mixing also [14]. Fig. 8(b) gives a

clear picture of pinhole in the small scan area of the same

sample above one where the average diameter of these holes

estimated to be of some nanometer. It is clear that pinholes

appear in the reacted CoxSiy layer and their growth leads to a

complicated pinhole induced CoSi2 network. And as CoSi2 thin

films have good epitaxial and electrical properties, they are

often employed in the semiconductor technology and are used

as buffer layers for the growth of new materials such as high

temperature superconductors.

4. Conclusion

In this work, SIMS profiles have given evidence of

electronic loss induced ion beam mixing in Co/Si system.

Mixing was measured in terms of the diffusion length and

mixing rate. A linear dependency of mixing efficiency on ion

fluence is found. It is inferred that the cause of mixing is due to

inter-diffusion across the Co/Si interface during a transient melt

phase. The STM analysis showed that the roughness of the film

(pinholes) shows obvious traces of the melting by the

irradiation at the surface, which is a supporting evidence for

SIMS analysis.

G. Agarwal et al. / Applied Surface Science 253 (2006) 1165–1169 1169

Acknowledgement

The authors are thankful to Nuclear Science Centre, New

Delhi, India, for the sample preparation and irradiation

experiments. Thanks again to the Nuclear Science Centre for

the financial assistance in the form of Junior Research

Fellowship to one of us (GA). Sincere thanks to the director

of IGCAR, Kalpakkam, for providing SIMS and STM facilities.

References

[1] T.Y. Cheng, Mater. Sci. Rep. 5 (1990) 45.

[2] A. Dunlop, D. Lesueur, G. Jaskiero wicz, J. Schildknecht, Nucl. Instrum.

Methods Phys. Res. B 36 (1989) 912.

[3] S. Klaumunzer, G. M-d Hau, Schumacher, Phys. Rev. Lett. 57 (1986) 850.

[4] C. Dufour, P. Bauer, G. Marchal, et al. Europhys. Lett. 21 (6) (1993) 671.

[5] S. Dhar, T. Som, V.N. Kulkarni, J. Appl. Phys. 83 (1998) 2363.

[6] S. Dhar, M. Milosavljevic, N. Bibic, K.P. Lieb, Phys. Rev. B 65 (2001)

24109.

[7] L. Milosavljevic, S. Dhar, P. Schaff, N. Bibic, Y.-L. Huang, M. Seibt, K.P.

Leib, J. Appl. Phys. 90 (2001) 4474.

[8] J.A. Kittle, W.T. Shiau, Q.Z. Hang, D. Miles, Microelectron. Eng. 50

(2000) 87.

[9] Y.Z. Hu, S.P. Tay, J. Vac. Sci. Technol. B 17 (1999) 2284.

[10] J. Pelleg, S. Zalkind, L. Zevin, B.M. Ditchek, Thin Solid Films 249 (1994)

126.

[11] A.J. Moshfegh, S.J. Hashemifar, O. Akhavan, Solid State Commun. 128

(2003) 239.

[12] S. Kraft, B. Schattat, W. Bolse, S. Klaumunzer, F. Harbmeier, A. Kulinska,

A. Loffl, J. Appl. Phys. 91 (2002) 1129.

[13] D.K. Avasthi, W. Assmann, A. Tripathi, S.K. Srivastava, S. Ghosh, F.

Gruner, M. Toulemonde, Phys. Rev. B 68 (2003) 153106.

[14] I. Belousov, A. Grib, S. Silinzen, P. Seidel, Nucl. Instrum. Methods Phys.

Res. B 186 (2002) 61.