Role of self- and boron-interstitial clusters in transient enhanced diffusion in siliconG. Mannino, N. E. B. Cowern, F. Roozeboom, and J. G. M. van Berkum Citation: Applied Physics Letters 76, 855 (2000); doi: 10.1063/1.125607 View online: http://dx.doi.org/10.1063/1.125607 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/76/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Transient enhanced diffusion of B mediated by self-interstitials in preamorphized Ge Appl. Phys. Lett. 96, 201906 (2010); 10.1063/1.3429084 Electrical activation of B in the presence of boron-interstitials clusters Appl. Phys. Lett. 79, 3764 (2001); 10.1063/1.1423775 Effects of boron-interstitial silicon clusters on interstitial supersaturation during postimplantation annealing Appl. Phys. Lett. 79, 1103 (2001); 10.1063/1.1396310 Boron-interstitial silicon clusters and their effects on transient enhanced diffusion of boron in silicon J. Appl. Phys. 88, 4547 (2000); 10.1063/1.1311826 The effect of reactive plasma etching on the transient enhanced diffusion of boron in silicon Appl. Phys. Lett. 71, 1834 (1997); 10.1063/1.119414

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Role of self- and boron-interstitial clusters in transient enhanceddiffusion in silicon

G. Mannino,a) N. E. B. Cowern, F. Roozeboom, and J. G. M. van BerkumPhilips Research Laboratories, Professor Holstlaan 4, 5656 AA Eindhoven, The Netherlands

~Received 11 October 1999; accepted for publication 16 December 1999!

We investigate the nucleation and evolution of boron-interstitial clusters~BIC!, driven by highinterstitial supersaturations,S(t), during Si implant damage annealing. The BICs are ‘‘fabricated’’in a narrow band by overlapping the Si implant damage tail with a lightly doped B buried layer. TheBIC band is found to be a net sink for interstitials at supersaturationsS(t).104. Our results suggestthat silicon self-interstitial defects are the primary source of interstitials driving transient enhanceddiffusion, and that BICs act as a secondary ‘‘buffer’’ for the interstitial supersaturation. ©2000American Institute of Physics.@S0003-6951~00!03207-1#

Dopant profiles in advanced silicon devices are stronglyinfluenced by nonequilibrium phenomena such as ion im-plantation, oxide growth, silicide formation and dry etching.In the case of ion implantation, the very high supersaturationof interstitials~I! generated by the implant leads to transientenhanced diffusion~TED! of dopants such as B, P, In, andAs, as well as clustering of B atoms at concentrations farbelow the dopant’s equilibrium solubility in silicon. Thesephenomena can significantly degrade device properties, aswell as causing difficulties for computer modeling of thesilicon fabrication process. These problems are most appar-ent in the case of B implantation, the most widely used tech-nique forp-type doping in silicon.

Ion-implanted B typically undergoes clustering at con-centrations one order of magnitude or more below solubility,due to nucleation of B–I clusters~BICs! during the earlystages of TED. This family of defects can be represented inthe form BnBInI wherenB is the number of B atoms in thedefect andnI the number of interstitials. It has not yet beenprecisely established how BICs are involved in TED. Zhanget al. have argued that small interstitial clusters~possiblyBICs! are the main source of interstitials driving TED ofmedium-energy~1–10 keV! B implants.1 At the same time,the BIC family of defects is also known to be responsible forB deactivation which persists long after TED is over. A verywide range of BIC defect energies would be needed to ac-commodate this range of behavior. In the absence of defini-tive experimental data on BIC defect energies, severalgroups have carried outab initio and/or Monte Carlo inves-tigations of the energetics of small BICs and their behaviorduring TED. These results have been controversial as differ-ent research groups predict substantially different defect en-ergies, ripening pathways, and transient activationbehavior.2–5 In the last couple of years, Pelaz and colleagueshave carried out observations of TED and electrical activa-tion following B or Si implantation into B-doped markerstructures.3–6 On the basis of their work they proposed thatBICs nucleate and grow along I-rich pathways (nI>nB) but

subsequently, after TED is completed, relax towards morestable B-rich structures (nB.nI).

6 Up to now, this interest-ing hypothesis has not been confirmed by direct experiments.

In this letter we explore the evolution and stability ofBIC defects by using an approach different to that presentedso far in the literature. The method uses a comparison ofmarker layer diffusion data in wafers with and without a BICdefect band. The setup is shown in Fig. 1. Wafer A containstwo lightly B-doped marker layers, located at depths of 900and 1300 nm, well beyond the reach of the implant damagedistribution. Wafer B contains in addition a lightly B-doped‘‘box’’ profile located in the depth range 200–500 nm. Bothwafers are grown by chemical vapor epitaxy~CVD! using anEpsilon One reactor which is known to produce high-puritydefect-free silicon. After growth, the wafers are implantedwith 40 keV Si ions to a dose of 231013 cm2. The damagefrom this implant will evolve into a band of intrinsic inter-stitial defects—interstitial clusters~ICs! and$113% defects—close to the projected range of the Si implant, during subse-

a!Permanent address: INFM and Dipartimento di Fisica, Universita` di Cata-nia, Corso Italia 57, 95129 Catania, Italy; electronic mail:mannino@ct.infn.it

FIG. 1. SIMS profiles of boron before and after annealing for a range oftimes at 600 °C. Also shown is the implant damage~Frenkel-pair! profilegenerated by the Si implant.

APPLIED PHYSICS LETTERS VOLUME 76, NUMBER 7 14 FEBRUARY 2000

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quent annealing.7 For convenience we will refer to thisdefect band as the ‘‘IC band.’’ In addition, in wafer B, thedeep tail of the Si implant damage overlaps with the B-dopedbox profile. As we will see, this overlap causes nucleation ofa band of BICs at the left-hand box edge, located betweenthe IC band and the diffusion markers. A comparison ofmarker-layer diffusion in wafers A and B will then providedirect information on the relative stability of BICs and ICs.

After implantation, samples from the two wafers are an-nealed together in dry N2 at 600, 700, and 800 °C using rapidthermal annealing or furnace annealing. B profiles from theas-grown and annealed samples are measured by secondaryion mass spectrometry~SIMS!. Figure 1 shows typical SIMSprofiles at 600 °C for a representative subset of annealingtimes. Three major phases of annealing can be observed.

~i! Fast diffusion to the left side of the box profile, in theregion damaged by the Si implant, is evident in the 1s-anneal profile of wafer B. This diffusion evidently takesplace at a very early stage of annealing, before the intersti-tials have had time to diffuse through the doping structure.We suggest it is associated with the displacement of B atomsat low temperature due to capture of free interstitials fromthe ion collision cascade. This diffusion phase is accompa-nied by immobilization~clustering! of a significant fractionof B atoms at the top left corner of the box profile~this canbe seen better if we ‘‘zoom in’’ using a linear concentrationplot as shown in Fig. 2!. Note that BIC nucleation only oc-curs in the region where the implant damage tail overlaps theB profile. The narrow band of BICs is stable enough to per-sist for at least 2 h of further annealing~compare with thesignificant diffusion at the top right corner of the box pro-file!.

~ii ! Rapid marker-layer diffusion occurs in the period 1s–15 min, indicating that interstitials diffuse through the en-tire doping profile within 1 s, and that a very high interstitialsupersaturation is present during this period. After 1 s thebroadening of the shallow and deep marker layers is essen-tially the same, indicating that there is no longer a significantgradient~and thus no significant flux! of interstitials into thebulk of the wafer. This confirms that the density of intersti-tial traps in our CVD-grown material is negligible. The mea-

sured marker-layer diffusion in wafer A therefore reflects theinterstitial supersaturation,SIC(t), in the region of the ICband. During this time period, the TED measured in wafer Bis clearly much lower than in wafer A, but is still extremelyhigh, as we show in later paragraphs.

~iii ! During the period 15 min–2 h the rates of diffusionin wafers A and B are much reduced and appear to haveconverged to approximately the same value.

The presence and location of the BIC band is most evi-dent when one compares SIMS data with a TSUPREM4 dif-fusion simulation where BIC formation is not taken into ac-count. The simulation uses standard models—fully coupleddiffusion model and interstitial clustering—to account forinterstitial injection from the IC band. The simulation usesthe 1 s data as its starting profile, in order to focus on diffu-sion effects during phase~ii ! onwards. The simulation showsexcellent agreement with SIMS data for the right-hand sideof the box, but fails to describe the static region at the topleft corner and the reduced concentration level at which theB diffuses out of the left side of the box. This discrepancyarises from the presence of a BIC band, indicated schemati-cally by the shaded region in Fig. 2. The fact that normalTED takes place at the right-hand side of the box indicatesthat the supersaturation during phases~ii ! and~iii ! is too lowto support further nucleation of BICs, and thus, outside theBIC band, the box profile has become transparent to intersti-tials.

We are now ready to make a quantitative analysis of themarker diffusion in wafers A and B. The SIMS data are fittedfor each anneal time and the differential diffusion betweensuccessive diffusion times is extracted. This procedure issimilar to that described in Ref. 7. We extract values forSm(t)5DB /DB* , whereDB is the measured diffusivity of Bin the marker layers andDB* is the diffusivity under equilib-rium point-defect conditions estimated by Fair.8 Figure 3shows the extractedSm(t) for wafers A and B for all threeannealing temperatures investigated. The supersaturation de-creases as a function of time in both wafers, but there areobvious differences at short times and low temperatures. Forexample, at 600 °C, the supersaturation in wafer A fallsgradually during the period up to 15 min, then drops rapidly.This drop is caused by a ripening transition in the IC band, at

FIG. 2. Comparison of SIMS data with simulated diffusion of the B boxprofile at 600 °C. The mismatch on the left-hand side indicates the presenceof BICs at the near-surface edge of the box. The BIC band is located wherethe implant damage tail overlaps the B profile.

FIG. 3. Interstitial supersaturation at the marker layers,Sm(t), as a functionof time for three annealing temperatures~600, 700, and 800 °C!. Symbols~open for wafer A and filled for wafer B! represent experimental values with2s uncertainties.

856 Appl. Phys. Lett., Vol. 76, No. 7, 14 February 2000 Mannino et al.

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a characteristic timet ~;15 min at 600 °C!, from small ICsto more stable$113% defects.7 In wafer B, Sm(t) starts atlower values, the IC-$113% transition appears to be‘‘blurred,’’ and at long timesSm(t) converges towards thevalue in wafer A. In the following paragraphs we propose anexplanation for the behavior ofSm(t) in wafer B.

In the very early stages of annealing, possibly even dur-ing implantation, free self-interstitials escaping from ion col-lision cascades attach themselves to others to form ICs, mi-grate and annihilate with vacancies or vacancy clusters, ormigrate and react with B atoms in the box. This latter reac-tion can be written I1B⇒BI where BI is a B interstitialdefect~a pair or an interstitial!. This reaction is followed bya chain of further agglomeration reactions leading to thenucleation of smaller BICs. These BICs form close to the leftedge of the box, since this is the only region where the as-implanted concentration of Frenkel-pair interstitials exceedsthe B concentration, and because the trapping length for adefect migrating through the box is very small.

After IC nucleation and initial growth,SIC rapidlyreaches a local equilibrium within the IC band as describedin Ref. 7. Because the intercluster distance within the ICband is orders of magnitude smaller than the distance to theBIC band, this local equilibrium value is unaffected by theexistence of the BIC band. Consequently the values ofSIC inwafers A and B are the same, and the differenceSA

m2SBm as

observed in Fig. 3 can be taken as a direct measure of thedecrease inS(x,t) with depth in wafer B. Since, during thediffusion phases~ii ! and~iii !, the B-doped regions are trans-parent to interstitials, the spatial decrease inS(x,t) can onlyarise from trapping of interstitials in the BIC band. Based onthese arguments we identifySA

m5SIC and SBm5SBIC where

SBIC is the supersaturation at the BIC band. After timet5t, the value ofSIC falls by 1–2 orders of magnitude, to avalue lower than that previously existing in the BIC band.The BIC band no longer acts as an interstitial sink, and mayeven become a weak source. The overall behavior ofS(x,t)is summarized in Fig. 4. The change in BIC behavior as afunction ofSIC shows that the BICs are more stable than theICs which control TED at short times, and are in fact of

comparable stability to$113% defects despite their muchsmaller size.

We now make some approximate estimates of the num-ber of interstitials transported between the IC and BIC bandsduring phase~ii !, and the resulting change in BIC composi-tion. The number of B atoms in the BICs, estimated from theshaded region of Fig. 2, is;131012 cm2. The number ofinterstitials added to the BIC band due to the arrival of in-terstitials from the IC band is given byDNI

5(D ICI* /L)*(SIC2SBIC)dt, whereD ICI* is the interstitialtransport capacity of silicon andL is the distance betweenthe IC and BIC bands. For example, to estimate the numberof I trapped in the BIC band during the IC-driven TED phaseat 600 °C, we takeD ICI* 50.014 cm21 s21 ~unpublished,from the work of Ref. 7!, L'150 nm, and estimate the timeintegral of SIC2SBIC from the data in Fig. 3. The result ofthis calculation is that about 131012/cm2 interstitials areadded to the BIC layer. Thus, the as-nucleated BICs gainabout one extra interstitial per clustered B atom during theIC-driven phase. Since there is no evidence for pile-up of Bin the BIC layer during TED, our results suggest that thenumber of clustered B atoms remains roughly constant andthe BICs simply become enriched in interstitials. This sup-ports Pelaz’s idea that BIC evolution follows an I-rich path-way during TED,6 and shows that ICs are the primary driverfor this process.

In conclusion, the BICs formed in our experiment areconsiderably more stable than silicon interstitial clusters,ICs, of a similar size. If our BICs are of similar stability tothose formed after B implantation, then the rapid TED thatoccurs after medium-energy B implantation cannot be ex-plained in terms of BIC annealing. We suggest that TED inthe absence of$113% defects is primarily driven by the an-nealing of ICs, and that the BICs act as a secondary‘‘buffer’’ for the IC-generated supersaturation.

The authors acknowledge the late Professor S. U. Camp-isano for his encouragement of this cooperation, and dedicatethis letter to his memory. This work is partly supported byESPRIT LTR project RAPID.

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FIG. 4. The figure sketches the depth and time variation ofS(x,t) in wafersA and B after the initial BIC nucleation phase. Sample regions containingboron are shaded in light gray.

857Appl. Phys. Lett., Vol. 76, No. 7, 14 February 2000 Mannino et al.

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