sputtering transients for some transition elements during high-fluence mevva implantation of si

Sputtering transients for some transition elements during high-¯uence MEVVA implantation of Si

Yanwen Zhang a,b,*, Tonghe Zhang a, Zhisong Xiao a, Harry J. Whitlow c

a Key Laboratory in University for Radiation, Beam Technology and Materials Modi®cation, Beijing Normal University, Institute of Low

Energy Nuclear Physics, Beijing 100875, People's Republic of Chinab Division of Ion Physics, �Angstr�om Laboratory, Uppsala University, Box 534, SE-751 21,Uppsala, Sweden

c Division of Nuclear Physics, Lund Institute of Technology, Box 118, SE-221 00, Lund, Sweden

Received 4 July 2000; received in revised form 31 August 2000

Abstract

The approach to quasi equilibrium sputtering of transition elements Co, Er, V and Ni during high-¯uence im-

plantation of Si(1 1 1) using a metal vapour vacuum arc (MEVVA) source has been studied by time of ¯ight-energy

elastic recoil detection analysis (ToF-E ERDA) and scanning electron microscopy (SEM). The partial sputter yield of

the implanted species was determined from the change in the content of the implanted species with the implanted ion

¯uence. The partial sputter yield of Co exhibits a step-like rise to �0.4 that might be associated with a rapid segregation

of Co to the surface followed by a slow exponential-like increase. Er on the other hand follows an exponential approach

to the quasi-equilibrium partial sputtering yield which is indicative of no strong buildup of Er within the sputter escape

depth. Additional data for Ni and V suggest also an exponential approach to quasi-equilibrium sputtering. Ó 2001

Elsevier Science B.V. All rights reserved.

PACS: 82.80.Yc; 61.82.)d; 81.15.Jj; 81.65.)b; 81.20.)n

Keywords: Sputtering transients; Transition elements; Silicide; MEVVA; ToF-E ERDA

1. Introduction

Partial sputtering yields of ions ejected from atarget during high-¯uence ion bombardment pro-vides unique in situ information about atomictransport in the implanted layer. The ion ¯uence

�H� dependence of the partial sputtering yield ofthe implanted ion species Yion�H� exhibits a tran-sient behaviour [1±4] in the approach from thelow-¯uence limit Yion�0� � 0 to a high-¯uencequasi-equilibrium Yion�1�. Yion�1� < 1 corre-sponds to a situation of implanted species buildupwhilst Yion�1� � 1 corresponds to the case ofbalanced incorporation and sputter ejection lossrates. The transient behaviour is a useful indicatorof the dynamics of atomic transport during im-plantation. This is because the sputtering transient

Nuclear Instruments and Methods in Physics Research B 173 (2001) 427±435

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* Corresponding author. Tel.: +46-18-4713058; fax: +46-18-

555736.

E-mail address: [email protected] (Y.

Zhang).

0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 4 3 0 - 4

is governed by the concentration of implant andsubstrate atoms within the sputter escape depth,their surface binding energies, sputter ejectionprobability, the migration and buildup of im-planted atoms in the surface layer. These resultfrom segregation driving forces, radiation en-hanced di�usion processes, the e�ects of ion beamballistic mixing and atomic transport associatedwith recoil implantation [1,5±7], etc.

The silicides of transition elements Co, Er, Vand Ni formed by implantation of Si are of con-siderable interest for a number of aspects ofmodern technology. CoSi2 has low resistivity, lowmismatch (1.2%) and high temperature stability inSi-based materials. Co-ion implantation of Si andthe formation of silicides have previously beenextensively studied by us [3,8,9]. Er silicides formwell-ordered epitaxial layers on f11 1g Si surfaceswith a lattice mismatch of � 1:22%. NiSi2 grows ina cubic crystal structure with low resistivity�34 lX cm� and has the lowest lattice mismatch��0:4%�. Both Ni- and Er-silicides can form re-markably low Schottky barrier-height (�0:3 eVfor Er-silicides) with n-type f111g Si surfaces [10].Vanadium is also a promising candidate becauseVSi2 is very stable over a large temperature range.

In this work, we report measurements of sput-tering transients of Co, Er, Ni and V ions under``technical'' conditions for formation of thin surfacelayers of metalloid phases using a metal vapourvacuum arc (MEVVA) source. MEVVA ion sourcesare well suited for this application because they canproduce beams with currents suitable for large andmedium scale batch processing. The partial sput-tering yield of the implanted species was measuredby determination of the change in the total contentof the species retained in the target as a function ofthe ion ¯uence [3]. The di�erent sputtering transientbehaviours of various metal ions, their energy andion ¯ux dependence are also discussed.

2. Experimental

2.1. Sample implantation

Czochralski-grown p-type Si(1 1 1) wafers wereimplanted with metal ions at 30° to the surface

normal using a MEVVA-IIA-H ion source [11,12].The ¯uence values quoted in Table 1 were theequivalent ¯uence of ions crossing normally thesurface plane of the target. The ion source wasoperated in 0.4 ms pulses and the average beamcurrent was selected by setting the pulse frequencyin the range of 0±25 Hz. The cathode (feedstock)materials were cut from 99.99% pure metal rods.The beam was not analysed and consisted of1�; 2� and 3� states with mean charge states of1.57, 2.1, 1.7 and 2.4 for Co, V, Ni and Er (Table1), respectively [13]. The accelerating potential, thebeam ¯uence, current and ¯ux, and other im-plantation parameters for implanted samples la-belled from 1 to 22 are listed in Table 1,respectively. The samples were clamped to a mas-sive metal plate within a central �100 mm diam-eter uniform region of the beam. Duringimplantation the pressure in the target chamberwas maintained below 2� 10ÿ6 mbar.

During MEVVA ion implantation, the highbeam power lead to elevated target temperaturesand consequential strong self-annealing. Thetemperature of the Co, Ni and V samples werecalculated using published power±temperaturedata [14] ®tted to a logarithmic law

T �°C� � a loge t �min� � b; �1�

where a and b were interpolated constants for agiven beam power. In the case of the Er implan-tations, the lower beam power (Table 1) impliedthis interpolation procedure was unreliable. Basedon the temperature increase corresponding to theenergy deposited by a 90 W beam a conservativeupper limit of 100°C could be assigned for thesample temperature for the highest ¯uence Er im-plantation. The resulting sample temperatures fordi�erent implantation parameters are given inTable 1, where the uncertainty is estimated to be�30°C.

2.2. Analysis

Time of ¯ight-energy elastic recoil detectionanalysis (ToF-E ERDA) with 48 MeV 81Br8�

(samples 1±3 Co and 11±22 Er in Table 1) and60 MeV 127I10� (samples 4±6 Co, 7±8 V and 9±10

428 Y. Zhang et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 427±435

Ni in Table 1) was used to measure the elementalcontents in the surface region [3,15]. A detaileddescription of the measurement system [16,17]and data analysis [3,18] is given elsewhere. Flightlengths of 0.4375 and 0.738 m between the4 lg cmÿ2 carbon foils of the time detectors wereemployed for the 81Br and 127I projectiles, re-spectively. The incident and exit angles were67:5° to the sample surface normal and the solidangle of the detector telescope was de®ned by a 8mm diameter aperture placed in front of a10� 10 mm ion implanted Si p±i±n detector. Theenergy calibration and e�ciency of the detectortelescope were carried out using the proceduresdescribed by El Bouanani et al. [19] and Zhanget al. [17], respectively. The stopping cross-sections were taken from Ziegler et al.'s SRIMcode [20].

Scanning electron microscopy (SEM) of theCo-implanted samples (1±6, Table 1) was carriedout using a JSM 6400F whilst the Ni-, V- and Er-implanted samples (7±22, Table 1) were analysedusing LE 0440 SEM with LaB6 cathode. Pointresolution down to a few nm was obtainable withthe JSM 6400F and 1nm with LE 0440, accordingto the respective manufacturers speci®cations.

3. Results and discussion

Fig. 1 presents the depth pro®les of Er togetherwith previously published data for Co [3] im-planted into Si(1 1 1) surfaces. Descriptions ofdi�erent samples are listed in Table 1. The errorbar denotes the absolute uncertainty. This hascontributions from the counting statistics, 10%

Table 1

Implantation parameters

Sample

no.

Ion Av.

charge

statea

Accelera-

tion volt-

age (kV)

Fluenceb

(ions

cmÿ2)

Current

(mA)

Ion ¯uxb

lA cmÿ2

Beam

power

(W)

Temp.c

(°C)

1 Co 1.57 40 1� 1016 4 51 160 335

2 Co 1.57 40 1� 1017 4 51 160 442

3 Co 1.57 40 5� 1017 4 51 160 509

4 Co 1.57 40 5� 1017 3 38 120 446

5 Co 1.57 40 5� 1017 4 51 160 509

6 Co 1.57 40 5� 1017 6 76 200 596

7 V 2.1 40 3� 1017 3 38 120 447

8 V 2.1 40 6� 1017 3 38 120 473

9 Ni 1.7 40 3� 1017 3 38 120 447

10 Ni 1.7 40 6� 1017 3 38 120 473

11 Er 2.4 40 5� 1016 0.25 2.63 10 <100

12 Er 2.4 40 5� 1016 0.50 5.26 20 <100

13 Er 2.4 40 5� 1016 0.75 7.89 30 <100

14 Er 2.4 40 5� 1016 1.00 10.52 40 <100

15 Er 2.4 40 5� 1016 1.25 13.15 50 <100

16 Er 2.4 40 5� 1016 1.50 15.78 60 <100

17 Er 2.4 40 1� 1016 0.25 2.63 10 <100

18 Er 2.4 40 3� 1016 0.25 2.63 10 <100

19 Er 2.4 40 8� 1016 0.25 2.63 10 <100

20 Er 2.4 15 5� 1016 0.50 5.26 7.5 <100

21 Er 2.4 30 5� 1016 0.50 5.26 15 <100

22 Er 2.4 50 5� 1016 0.50 5.26 25 <100

a From [13].b Equivalent ¯ux/¯uence of ions crossing the sample surface normally.c See text for details of how this is estimated.

Y. Zhang et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 427±435 429

uncertainty in stopping cross-section, 10 keV ab-solute uncertainty in beam energy and 2% relativeuncertainty in the ToF-E ERDA system detectione�ciency. Smaller uncertainties for Co-implantedsamples are mainly associated with better countingstatistics.

It may be clearly seen from Fig. 1 that both theshapes and penetration of the depth pro®les areclosely similar. For similar ¯uence H up to1� 1017 ions cmÿ2, the maximum implant to Siatomic ratio does not exceed �0:2. The depthpro®le measured by ToF-E ERDA is an indicatorof an average atomic ratio of metal/Si within acertain depth region. It is highly likely that thedepth pro®les are not fully resolved due to the ®-nite depth resolution. In this case, the result in Fig.1 does not preclude that the metal concentrationreaches that for stoichiometric silicides in a thin

layer, or in islands at the surface. The metal con-tent is con®ned to a region less than �80 nm fromthe surface in both Er and Co cases, assuming adensity for the surface layer of �6� 103 kg mÿ3.Comparison with the projected range estimated bySRIM 2000 [20] for 80 keV 166Er2� and 59Co2� inEr0:1Si0:9 and CoSi2, indicates that the implantedions do not di�use signi®cantly to depths deeperthan the implanted layer.

The total content of 12C, 16O, and implantedmetal ions, Er and Co, are shown in Fig. 2. In-spection of Fig. 2(a) shows that the carbon contentcorresponds to a constant coverage of 6� 1015 at.cmÿ2 and the oxygen coverage increases from0:5� 1016 at. cmÿ2 to a constant level of�1:2� 1016 at. cmÿ2 with increasing ¯uence.Similar C and O contents are observed for the Co-implanted samples (Fig. 2(b)) and con®ned to a

Fig. 1. Depth pro®les from ToF-E ERDA for Si(1 1 1) wafers implanted with di�erent ion ¯uences: (a) Er and (b) Co. A depth of 1017

atoms cmÿ2 corresponds to �20 nm, assuming the density to be that of bulk Si. The error bars denote the absolute uncertainties (see

text).


native surface oxide layer [8]. The concentration ofimplanted metal atoms, however, increases morerapidly with ¯uence in the Er case than that forCo, which suggests di�erent sputtering transientbehaviours of these ions under MEVVA implan-tation. Furthermore, the comparable contents of Cand O with the implanted metal species at low¯uences imply that the in¯uence of the contami-nation on the partial sputtering yield may not benegligible in the Er case.

Fig. 3 shows Yion versus equivalent normal ion¯uence for Er and Co as well as additional datapoints for V and Ni ions from previous studies[15]. It is convenient to interpret these data byconsidering the following limiting cases:

(a) Limit of strong implanted species trapping.In this case, no thermal or radiation-enhanceddi�usion takes place. Yi�H� will be governedby the content of the implanted species andsputter ejection probability within the sputterescape depth. If the latter term is independentof the composition and the implant pro®le has

a Gaussian distribution, one may expect aYi�H� / erfH dependence. (This correspondsto the usual assumption in sputter pro®ling.)(b) The limit of large mobility with surface seg-regation of the implanted species. Once the im-plantation has commenced a segregatedsurface layer will be rapidly established. Inthis case, Yi�H� will exhibit a rapid increaseto a value Yi�H > 0� � Rt ni�x� � pi � dx, whereni�x� is the content of the implanted speciesat depth x within the sputter escape depth tand pi is the probability of ejection from thatdepth. A similar argument applies to the case,where the substrate species is segregated;

Fig. 2. Total contents of 12C, 16O and implanted metal ions

remained in the samples. As the beam ¯uence increases, the

data points are taken from (a) Er-implanted samples, 17, 18, 11

and 19; (b) Co-implanted samples, 1±3 (Table 1).

Fig. 3. (a) Partial sputtering yield Yi�H� for Co (diamonds), Er

(squares), V (crosses) and Ni (triangles) in Si(1 1 1) versus av-

erage ion normal ¯uence H. The dashed line indicates the as-

ymptotic quasi equilibrium Yi�1� � 1. The error bars denote

uncertainties as for Fig. 1. (b) The ¯uence dependence±

loge�1ÿ Yi�H�� versus H. The straight-line ®tting of di�erent

ion species are indicated as dotted lines.


however, the sign of the initial transient willbe negative.(c) The case of no segregation but rapid di�u-sion within implanted layer. In this limit,dYi�H�=dH � j�1ÿ Yi�H��, i.e., the rate ofchange of Yi�H� is proportional to the devia-tion from the quasi equilibrium value,Yi�1� � 1. In this case, the transient will fol-low an exponential law Yi�H� � 1ÿexp�ÿjH�.

Fig. 3(b) presents ÿ loge�1ÿ Yi�H�� versus H.Reference to this ®gure shows that the Er transientfollows a straight-line that passes close to the or-igin. This is consistent with an exponential partialsputter yield transient (case c). The Co transient,on the other hand, has a signi®cant positive in-tercept, which is consistent with an initial step suchas expected for surface segregation of Co (case b).The ®tted j values corresponding to the straight-line portion of the slopes shown in Fig. 3(b) are3:05� 10ÿ17 and 7:23� 10ÿ18 normal incidentionsÿ1 cm2 for Er and Co, respectively. The ex-ponential component of the transient is faster in

the case of Er than Co (Fig. 3(a)), however, in theCo case the initial amplitude of the transient ex-ceeds that for Er.

Considering the Co case ®rst, the SEM image ofthe surface (Fig. 4(b)) indicates that at a ¯uence of5� 1017 ions cmÿ2 the surface has morphologythat might be indicative of phase formation [8].However, at smaller ¯uences (H6 1017 Co ionscmÿ2), where the sharp rise in YCo�H� is observedthe surface topography is smooth [9]. The surfaceCo/Si atomic ratio (Fig. 1) is below 0.2, which isconsiderably smaller than the value of 0.5 for themost Si-rich silicide phase. The rapid transient riseof YCo to �0.4 seen at a ¯uence of 1016 ion cmÿ2

implies that Co must be e�ectively transported tothe few uppermost atomic layers, where it can beejected by sputtering. Co atom mobility may resultfrom ion bombardment induced mobility, which isable to maintain a segregated surface layer inmetal alloys during implantation at even liquidnitrogen temperatures [1,21]. The contribution tothe Co mobility from thermal di�usion is unlikely

Fig. 4. SEM images of implanted surfaces. (a) Sample 19 (Table 1) Er-implanted sample with a ¯uence of 8� 1016 ions cmÿ2 (the

structures at the top of the image is a non-representative artefact that was used for focus control). (b) Sample 3 Co-implanted sample

with a ¯uence of 5� 1017 ions cmÿ2.


to contribute much to the transient because thelogarithmic increase of temperature with time (Eq.(1)) implies that thermal di�usion is small duringthe initial phases of implantation. The drivingforce for Co di�usion to the surface may be con-tributed to by, the concentration gradient, Gibb-sian forces [2,21] (which arises from atomic-sizeinduced stress and chemical force) as well as anypreferential sputter ejection of Co. Furthermore,preferential formation of Co silicides at the surfacemay cause an additional driving force. Co incor-porated in silicide phases is also more tightlybound than unreacted Co and hence the formerhas a lower probability for sputter ejection. Theformation of Co silicide phases below the sputterescape depth would on the other hand lead to anunreacted-Co concentration gradient decreasingaway from the surface which would reduce thedriving force. Thus the initial transient could beassociated with di�usion of free Co to the surfacewhilst the slow exponential component is associ-ated with silicide phase formation. The observa-tion that the atomic ratio is not commensuratewith CoSi2 (Fig. 1(b)) and the absence of mor-phology at low ¯uences implies that the phase isformed in small isolated islands at low ion ¯uence.

The behaviour for Er is quite di�erent from Co.The Er transient (Fig. 3(a)) has a behaviour that isclosely similar to the exponential form of case c,where the mobility is large but no surface segre-gation of either substrate or implanted speciestakes place. This implies either that Er does notsegregate to the surface, or that Er does not have alarge sputter ejection probability from the surfacesilicide phase. It may be that the segregation ismodi®ed by the incorporation of considerablequantities of C and O rest gas atoms (Fig. 2(a)),which are located close to the surface. Further-more, the low sample temperature �< 100°C�limits transport of Er to the surface by thermaldi�usion. The absence of a crystalline phase isconsistent with the ¯at topography seen in theSEM data (Fig. 4(a)). At low ¯uences up to3� 1016 Er cmÿ2, the carbon and oxygen contentsare comparable with the amount of Er retained inthe samples (Fig. 2(a)), which may have a signi®-cant in¯uence on the formation of surface phases.The greater mass of Er as compared to Co suggests

that the mobility is greater in the Er case as a resultof the greater energy deposition by nuclear scat-tering. The atomic ratio as function of depthshows no evidence of preferential segregation of Eraway from the surface (Fig. 1).

The data points for Ni and V in Fig. 3(b) de®nelines that intercept the origin suggesting that theseion species have also exponential partial sputteringyield transient behaviours that correspond to casec above. This interpretation is, however, tentativebecause it is based on only two data points foreach element. The sample temperatures are esti-mated to reach the range 470°C (Eq. (1)) and thusthe implantation conditions are similar to the Cocase.

The dependence of metal ion retention on thebeam ¯ux (Fig. 5) and beam energies (Fig. 6) hasalso been investigated. The error bars are attrib-uted to the same uncertainties as indicated in Fig.1. The di�erent beam currents and ¯uence rangesspanned in the Co and Er measurements do notallow direct numerical comparison of the datafrom the two sets of because of the di�erenttarget temperatures (Table 1) and rest-gas incor-poration during implantation (Fig. 2). A numberof interesting general observations may, however,be made. Inspection of Fig. 5 implies that thebeam current is not a dominating parameter forthe retention, the variation of the sputtering yieldis within the error bars. This supports the

Fig. 5. The retention fraction of Er (circles) and Co (diamonds)

versus beam current. The Er data points are measured from

sample 11±16, while the Co data points are from sample 4±6

(Table 1). The error bars are attributed to the same uncer-

tainties as described in Fig. 1.


explanation that the atomic mobility is associatedwith the ion bombardment induced mobilityrather than thermal di�usion. Increasing the ac-celeration potential from 10 to 50 kV has amarked e�ect on the metal ion retention. As theimplantation energy increases, the Er ions pene-trate to a greater depth. This restricts di�usion ofEr to the surface and contributing to the partialsputtering yield YEr. Therefore, the retention in-creases with the increasing beam energy as ex-pected (Fig. 6).

4. Conclusions

The partial sputtering yield for Er ions in Siclosely follows a Yi�H� � 1ÿ exp�ÿjH� depen-dence on ¯uence H. Where j is 3� 10ÿ17

ionsÿ1 cmÿ2 for Er ions from a MEVVA ionsource with 40 kV accelerating potential and meancharge state of 2.4. Although the behaviour of thepartial sputtering yield transient for Er does notrule-out the formation of a surface Er silicidephase it indicates that no strong buildup of Erwithin the sputter escape depth has taken place.The limited data for Ni and V are also consistentwith an exponential transient.

The partial sputtering yield for Co from aMEVVA ion source operated at 40 kV does notfollow a simple exponential behaviour as in thecase of Er but has a initial sharp rise to �0:4 fol-lowed by a slower exponential-like increase with j

value of 7:2� 10ÿ18 ions ÿ1 cm2. This can be as-sociated with rapid transport of implanted Co tothe surface driven by surface segregation.

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sputtering transients for some transition elements during high-fluence mevva implantation of si

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