Foreign atom incorporation during metal silicide formation by ionbeam synthesis

Yanwen Zhang a,*, Harry J. Whitlow a, Tonghe Zhang b

a Department of Nuclear Physics, Lund Institute of Technology, Box 118, 221 00 Lund, Swedenb Institute of Low Energy Nuclear Physics, Beijing Normal University, Beijing 100875, People's Republic of China

Abstract

Depth distributions were measured of foreign atoms incorporated during the formation of thin silicide surface layers

by ion beam synthesis. Si (111) wafers were implanted with Co and V ions using a pulsed Metal Vapour Vacuum Arc

(MEVVA) ion source operated at 40 kV extraction voltage. Post-implantation analysis was carried out using Time of

Flight-Energy dispersive Elastic Recoil Detection Analysis (ToF-E ERDA) with 60 MeV 127I10�. The results showed

that addition to the metal ions, carbon and oxygen were incorporated at at.% levels, with distributions that were peaked

at the surface and extended into the implanted layer. The ®ndings suggest that incorporation of C and O is signi®cantly

in¯uenced by the degree of silicide formation, with more continuous and stoichiometric silicides corresponding to a low-

er incorporation of foreign atoms. Ó 1998 Elsevier Science B.V.

PACS: 68.55.Ln; 68.55.-a; 68.35.Dv

Keywords: Silicides; Rest-gas atoms; Ion beam synthesis; Sputtering; Recoil spectrometry

1. Introduction

Metal silicides are widely used for metallizationof VLSI devices because they satisfy several re-quirements for a good metallization to Si, namelylow resistivity, good mechanical properties andhigh temperature chemical stability [1±5]. Bom-bardment of silicon substrates with low-energy(� a few tens of keV) metal ions presents an inter-esting route to fabricate thin surface layers of met-al silicides. This method is a one-shot technique

that can be carried out entirely in vacuum and iscompatible with conventional Si processing tech-nology.

Metal Vapour Vacuum Arc (MEVVA) ionsources [6±9] are suited for silicide formation bylow-energy ion implantation because they can pro-vide high average beam currents of metal ions [10±12]. Foreign atoms may however be incorporatedin the surface layer as a result of recoil implanta-tion and ion beam mixing of rest-gas atoms as wellas post irradiation reaction with the atmosphere.Foreign atom inclusion may have signi®cant con-sequences, not only because of their importancein forming electrically active defect centres in semi-

Nuclear Instruments and Methods in Physics Research B 135 (1998) 392±396

* Corresponding author.

E-mail: yanwen.zhang@nuclear.lu.se.

0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved.

PII S 0 1 6 8 - 5 8 3 X ( 9 7 ) 0 0 6 1 6 - 2

conductors, but also because of their in¯uence onthe growth of the silicide phases. In ion beam syn-thesis foreign atoms may also in¯uence the reten-tion of metal atoms in the target by modi®cationof the partial sputter yields for metal and Si atoms[13].

The objective of this paper is to report somepreliminary Time of Flight dispersive Elastic Re-coil Detection Analysis (ToF-E ERDA) investigat-ions on foreign atom incorporation during silicideformation by using a MEVVA ion source undertechnical conditions.

2. Materials and methods

2.1. Sample preparation

330 Xhÿ1 p-type Si (111) wafers were implant-ed with Co and V ions using the MEVVA-IIA-Hion source [9,14] operated in a pulsed mode with40 kV acceleration voltage at Beijing Normal Uni-versity. The pulse length was �0.4 ls and the meancurrent was selected by altering the frequency from0 to 25 Hz. The metal cathodes (source material)were made from 99.9% pure Co and V. The sam-ples were placed on a massive metal plate duringimplantation and the beam was 10 cm in diameter.Two sets of samples were implanted in a direction30° to the surface normal with 5 ´ 1017 Co ionscmÿ2 at a beam current of 38 and 51 lA cmÿ2, res-pectively. Two other sets of samples were implant-ed at the same angle and with a beam current of 38lA cmÿ2 to 3 ´ 1017 and 6 ´ 1017 V ions cmÿ2, res-pectively. The pressure in the target chamber was�2 ´ 10ÿ3 Pa and the average charge state of Coand V is 1.7 and 2.1 [10], respectively.

2.2. ToF-E ERDA measurements

ToF-E ERDA [15] was used to determine theincorporation of metal and foreign rest-gas atomsthat were incorporated into the samples. The mea-surements were carried out in the Tandem Labora-tory in Uppsala, Sweden using 60 MeV 127I10� ionsas projectiles. The carbon foil detectors with 5 lgcmÿ2 foils, were separated by a 738 mm ¯ight path.Subsequently the recoils impinged on a 10 ´ 10

mm2 Si p-i-n detector (Fig. 1 inset). An 8 mm di-ameter collimator placed in front of the detectorprevented recoil-atoms from impinging on theedge of the active area. The energy calibrationwas established following El Bouanani et al. [16].In converting the recoil energy spectra to elementaldepth pro®les, we assumed a Rutherford recoilcross section. The stopping cross section was takenfrom the STOP code of Ziegler et al. [17].

3. Results and discussions

Fig. 1 shows an isometric representation of themass vs. energy histogram for the sample that wasimplanted with 5 ´ 1017 Co ions cmÿ2 at a currentof 38 lA cmÿ2. In all the samples investigated,only signals that could be attributed to C, O, Siand the implanted metal were observed in theToF-E ERDA data.

In Table 1 the parameters for the implantationare collected. Except for the 3 ´ 1017 cmÿ2 V im-plantation, the amount of implanted metal re-tained in the sample is only �28±46%. Thissuggests that the samples are approaching the qua-si-equilibrium state where the arrival rate of metalatoms is balanced by their loss rate from the im-planted layer by sputtering and potentially also

Fig. 1. (a) ToF-E ERDA con®guration. (b) Mass vs. energy his-

togram for the sample implanted with 5 ´ 1017 Co at. cmÿ2 at

38 lA cmÿ2.

Y. Zhang et al. / Nucl. Instr. and Meth. in Phys. Res. B 135 (1998) 392±396 393

by di�usion deep into the bulk [12]. The depthvariation of the M/Si atomic ratio, where M de-notes the metal, is shown in Figs. 2 and 3. Clearly,in Figs. 2(b) and 3, the peak M/Si ratio corre-sponds closely to the MSi2 silicide phase. Devia-tions might be associated with surface roughness[18]. Table 1 reveals that the surface layer of thesesamples has lower resistivity. These two observa-tions suggest that a well-de®ned continuous silicide®lm has formed at the surface. Scanning ElectronMicroscopy revealed that the surface had a ¯at to-pography with 5±15% of the surface area coveredby perforations that extend along the direction ofmetal ion incidence. The exception is the sampleimplanted with Co at 38 lA cmÿ2. This deviationfrom stoichiometry is greater than that associatedwith the stopping power uncertainty. Further-

more, the resistivity in this sample is considerablygreater than other samples (Table 1). This indi-cates that the beam heating may have been insu�-cient to form the silicide phase, or it has notagglomerated to form a continuous ®lm.

The C and O distributions both have the samegeneral shape with a maximum at the surfaceand a tail that extends into the implanted layer.(The exact position of the surface for C and O var-ies slightly in Figs. 2 and 3 because of a small jumpin beam energy during the measurement.) Inspec-tion of Figs. 2 and 3 shows that the peak carbonconcentrations are similar (�4±5 at.%) whilst theoxygen shows considerable variation (5±15 at.%).It is worth noting that both the C and O signalsare con®ned to the implanted region and do notshow evidence of deeper penetration. C and O inthe ion beam would be accelerated to the same en-

Fig. 2. Elemental concentration vs. depth for Co-implanted

samples. (The depth scale is in units of at. cmÿ2 to avoid uncer-

tainties associated with assumptions of the density, �10 nm in

bulk Si corresponds to 5 ´ 1016 at. cmÿ2.)

Fig. 3. Elemental concentration vs. depth for V-implanted sam-

ples. (Depth scale: as for Fig. 2.)

Table 1

Implantation data

Ion Average charge state Dose (ions cmÿ2) Beam current

(lA cm ÿ2)

Fraction of retained metal atoms

(%)

Resistivity (lX cm)

Co 1.7 5 ´ 1017 38 28 >2250

1.7 5 ´ 1017 51 41 32

V 2.1 3 ´ 1017 38 70 260

2.1 6 ´ 1017 38 46 85

394 Y. Zhang et al. / Nucl. Instr. and Meth. in Phys. Res. B 135 (1998) 392±396

ergy as the metal beam and penetrate deeper be-cause of their smaller atomic number. Further-more, di�usion of C and O from the implantedlayer into the bulk would give rise to an enhancedsignal for these elements at depths greater than theimplanted layer. There is no evidence for this inFigs. 2 and 3 suggesting that: (i) the C and O areincorporated by recoil implantation, ion beammixing during implantation and also possiblychemical reactions with the atmosphere after irra-diation. (ii) No signi®cant amount of C and O isintroduced as contaminant species in the ion beam(or the surface/implanted layer acts as a strong get-ter) and (iii) C and O incorporated by (i) is con-®ned to the implanted layer. Note that theimplantation was carried out under favourableconditions for the inclusion of rest-gas atoms be-cause the pressure is so high that the surface canbe completely covered with rest-gas atoms betweenevery ion pulse. Furthermore, the ion dose perpulse is 20±50% of that estimated to sputter awaya monolayer of gas atoms on the surface.

Comparison of Fig. 2(a) and (b) shows that forthe low-current Co implantation, where the silicide®lm is believed to be poorly developed and non-continuous, the C and O signals extend through-out the implanted layer thickness (�8 ´ 1017 at.cmÿ2). This is in contrast to Figs. 2 and 3, wherethe C and O signals are con®ned to a shallow layer(�3 ´ 1017 at. cmÿ2) close to the surface. This sug-gests that the formation of a well-developed con-tinuous silicide layer was accompanied bycon®nement of the C and O to this layer. It is alsonoteworthy that the low dose V implant shows avery high level of oxygen in the surface. This mightbe associated with the formation of silicide underthe surface with a low solid solubility for oxygen,which drove this element out to the surface or be-cause metallic V has a very high a�nity for oxy-gen. This might lead to getter oxygen by V atlow doses where the silicide layer did not formall the way out to the surface. We also cannot ruleout the possibility that O has been incorporated byreaction with the atmosphere after the implanta-tion. Dytlewski et al. [11] present data for Ge im-planted with Ti from a MEVVA ion sourcewhich suggest an enhanced O surface peak as wellas an oxygen tail extending into the implanted

layer. In this context we might expect preferentialO build up at the surface in the case of V implan-tation.

4. Conclusions

1. Analysis of the Co and V implanted samplesusing ToF-E ERDA showed that the samplescontained C, O, Si and the implanted metal.

2. The C and O depth distributions are similar inall samples and are con®ned to the implantedlayer. These do not show any evidence of deepimplantation or di�usion into the bulk of thesubstrate.

3. The C and O distributions are in¯uenced by theextent of silicide formation and have minimalpenetration and incorporation where a well-de-veloped silicide ®lm appears to have formed.

Acknowledgements

The authors would like to express their grati-tude to the sta� at the Tandem Laboratory, Upps-ala, and accelerator section of the Institute of LowEnergy Nuclear Physics, Beijing Normal Universi-ty, for their assistance and loan of equipment. Thiswork has been carried out under the auspices ofthe NFR/NUTEK Nanometer Structure Consor-tium. Yanwen Zhang is grateful for the supportfrom the Swedish Institute (SI).

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