INSTRUMENTACIÓN REVISTA MEXICANA DE FíSICA 46 (S) 48S-489 OCTUBRE 2000

Inftuence of segregation annealing time on the gettering process

P. Peykov' and M. AcevesInstituto Nacional de Astrojfsica Optica)' Electrónica

Apartados postales 51 y 216, 72000 Puebla, Pue., Mexico

T. DiazCIDS. Instituto de Ciencias, Unil'ersidad Autónoma de Puebla

Apartado postal 1651, 72000 Puebla, Pue .. Mexico.

Recibido el 3 de enero de 2000; aceptado el 16 de mayo de 2000

Thc influence of segregation annealing time on the gettering efficiency of phosphorolls ion implantation gettering in MOS structures wasinveslig<lted. Thc gCllering was pcrformcd by a backside P ion implantation with adose of 1016 atomslcm2 and 120 KeV encrgy. To inves-tigate the gettering efficiency. surface generation velocity and 3-dimensional generalion lifetime profilc as a function of the annealing timewcre investigatcd. h was found thatthcre is an optimum annealing time ror maximum gcttering efficiency. Surface generation vclocity andgeneralion lifetime increase for annealing less or equalto the optimllm annealing time. which was 90 min for our case. For longcr anneal-illg times both p:uameters deteriorate. The analysis of the results shows that the increase of generation lifetime and the decrease of surfacegeneration velocity are due to a gettering of metallic impuritics and shrinkage of oxidntion stacking fauhs (OSFs). The dcterioration of theseparameters fm annealing times longer lhan the optimum annealing lime is explained by the combined innuence of OSFs shrinkage. release(lf the caplured mctallic impurities from gettering siles in the damagcd implanted region and by melallic impurities contamination from (heI"urnacc.The concept nf the optimum annealing time is incorporated in thc segregation model 01" the gettering process.

K('ywords: Silicon; gettcring; ion implantation; lifelime

Se investigó la influcncia del tiempo de recocido sobre la eficiencia de Xt'tterir¡g en estmcturas MOS. Sc estudió la eficiencia de gettering porilllplantación de I"()sforopor la parte de atrás de la oblea con dosis de 1016 atlll/cm2 y 120 KeV de energa de implantación. Para determinar lal'li1.:ienciade ge/tl'l"illg. se usó la variación con el tiempo de recocido del tiempo de vida Llegeneración y la velocidad de generación superficial.Se enconlró que existe un tiempo óptimo de recocido. donde se obtiene el máximo tiempo de vida. yen nuestro caso fuc de 90 mino El tiempodt: vida aumenta y la velocidad super/icial disminuye con el tiempo de recocido hasta el tiempo óptimo. Para tiempos mayores al óptimo,ambos parámetros se deterioran. El getlerillg de impurezas melálicas y la reducción de fallas de apilamiento por oxidación (OSF) son losmecanismos responsables del aumento del tiempo dc vida y la reLlucción de la velocidad superficial. La deterioración de los parámetrosmencionados. se explica por los efectos combinados de la liberación de impurezas de los sitios de Xt'tterillg en la región implantada, lareducción de los OSF y la difusión de impurezas metálicas del mismo horno de recocido. Por último. el concepto del tiempo optimo derecocido es incorporado en el modelo de segregación del proceso de gt'ftcri/lg.

lh'scril'tores: Silicio; gcttering; implantación ionica; tiempo de vida

I'ACS, r,X.55.Ln; 72.20.1v; X5.40.llp

1. Introduction

11 is \Vell known that rnetallic impurities and extended de-I'ccts, cspccially whcn dccoratcd with metals. can introducegcncration-recombillation (G.R) ccntcrs. Thc G-R centcrs in-crease lhe leakage current ami reduce rccombinalion and gen-er<llion lifetime nI' semiconductor devices.

Getlering is a pmcess that reduces or el¡minates metallicimpurities in a wafcr by localizing them away fmm the de-vice active rcgions [1,2]. Getlcring of metallie impurities insilicon is widcly lIsed to improvc the clectrical characteris-tics and lO achicve a high fabrication yield 01' integrated cir-cuits (lC). Variolls gettcring tcchniques have been developed:chlorinc oxidatioll [a-5}, diffusion [6). silicon n¡trite deposi-lion [7]. intrinsic geltering [81. mechanical damagc [91. ionimplanlation [10-151. etc. The gettering technique can repre-sent up to I09?, 01' the total le manufacturing cosl.

In spite 01' the t:1ct that lhe desircd circuit propcrtics andproduction yicld has been achieved to sorne eXlcnd by getter-ing tcchniqucs. the exacl mechanism(s) by which getteringenme out are slill nol well underslood [12, 16, 171,

Thc ncco rm improved suhslratc properlies in the com-ing gigascale inlcgration era. retlcctcd in lhe rcquirements ofSemiconductor Induslry Association Roadmap ror the year200 I [181. resullS in an increased inleresl in the geller-ing 11,2[. The 0.18/,m generalion of devices wilJ require Iheimpurity level in lhe active devicc regions to be reduced be-low 1 x 1010 cm-:J and material spccification may be as lowas lO" percm-3118j.

Among all gcttering tcchniqucs the ion implantation gel-tering has rcccivcd attention due lo its ability lo produce ra-diation damagc in a very conlrollable and reproducible way.Different ions have been used to create ion damage [10].However. to rcspond to the new requirel1lents it is neces-

P.PEYt\oV. T. DIAZ. Ar":1>.\1.ACEVES

3. Experimcntal rcsllll~alld disclIssion

PI(iURE l. Quasi ]-dimcnsional rcprcsenlalion of general ion life.time proliJe wilh the segrc~atioll :mnealing lime as pararncter.

whcrc k is Ihe Boltzmann's conslan!. T is the temperature, bis the Burgers vector, e is the vacancy eoncentration in equi-Iihriul11 with Ihe Jefect ami Co is the equilibrium vacancyconcenlration in a derect free crystal. Factors upposing to thechemical driving force are lhe linear dislocation energy andthe energy oflhe stacking faults. During the oxidation there is

(1 )

iMJ (min.)

F = (kT) 1" (£)"2 Co'

30o10' I

Quasi 3-dimensional plot of generation liretime with the seg-regation annealing time as parameler is shown in Fig. l. Theresults show Ihat the generation Iiretime increases with thesegrcgation annealing up to 90 mino For longer annealingtimes it decreases. On the other hand. in all samples the gen-eration Iifelime decreases towards the Si-Si02 interface. An-othcr peeuliarily is that the lower is Ihe gencration liretimelhe larger is its increase, due to the scgregation annealing, ¡.e.,the gettering effkicncy is higher. It appears that lhe diffcrentprocess steps have allccled Ihe general ion Iifetime Illostly inlhe near surface region.

The smaller values (lf Ty ohserved near the Si.Si02 in-tcrlace can he explaincd hy the process of OSFs growth. Itis well known that OSFs grow hy releasing vacancies 01' ah-sorhing Si self-interstitials from the surrounding matrix. Thechemical driving force ror Ihe change of defect size, is givenhy 1.101

2

1O~

64

Aluminum dots werc cvaporaled through a metal mask onthe top oxide. On the had.sidc ol' the wafers. after rernovingthe oxide. aluminl1lll was also evaporated. The warers weresintcred in N2/H:! alllhient at 42jOC ror 30 mino

Thc sinc-voltage C-V melhod [35] was used to measuresuri"ace generation velocity and generation liretime profile.The ITlcasurements were perforllled al I MHz, l1sing BOON-TON 72 Il eapaeitance meler antl WAVETEK 271 funetiongenerator was llsed as a vo1tage souree.

2. Samplcs prcparatilln and measllrements

N-lypc, (HK)) oriented. 2.5-5 ohm.cm. CZ-grown silicon\\'afers wcre lIseJ in this cxperimcnt. The wafcrs were RCAclcaned. An o.xidation at 1000°C in dry O2 + 2% TeA ambi-ent was performcd to ohtain 800 Á oxide thickness, I"ollowedhy an annealing in N2 at the same tcmperature for 30 minoIt has hcen shown Ihat t!lis type 01'annealing reduces the OX.

id;¡lion stacking faults (OSFs) length (37} and improves thegeneration Iifetime 138]. Afler that a hacksidc phosphorus ionimplanlalion with adose 01' 1016 atoms/cm-2 and 120 KeVcnl'rgy was pcrformed. Thc scgregation annealing was per-formed;;il T = !:)oO°Cin N2. Differenl wafers were anncaled1m diflerent time (O. 30, 60, 90, 120, antl 150 min). Fromour prcviolls experiments \Ve llave fuuno that in the case of Pion implantation gcttcring the oplimulll segregation anneal.ing lcmperalure for maXillll1l1lgettering eftkiency is ahullLDOOQC. Similar rcsults were reported in lhe literalure 1351.

sary a cOlllprehensive understanding of defect evolution uponimplantation and annealing. That is asevere scicntific chal.!cnge since il requires the assesslllent of complex phcnomenasuch as hulk ano surl~1ccdefect reeomhination, oefeel eluster-ing ano defccl-impurity inleraclion [191. Moreover, slIch anassessmcnl must he done for dilTerent comhinations 01"suh.strales ano implanlcJ ions.

Nowadays. the cffOrlS are directeo towards bener undcr-standing of lhe gettering process Il!J.20], oplimization ofthe e.xisting gettcring techniques !21-231 and developmcntoi"new ones [2.1-30].

In most of lhe gettering leehniques a high lelllperatureannealing is used to insure impurity relcase, diffusion. amieaplUre al Ihe gettering siles [31-:H]. That is why lhis pa-rameler is of greal importance ror the gettering efliciency.USlIally Ihe annealing is perfonncd in neutral amhient. ThisIllcans thal Ihe annealing ICll1perature and the anncaling timewill he Ihe parameters alTecling the gettering ertleicney. Kangand Schroder [35] investigated the role ofthe annealing tem-perature ano h;1\"cshown that a optimum one cxists. Gaoct al. [28¡ llave invcstigatcd one and t\\'o step annealing.Koveshnikov el {I1. [22] have investigatco the role of anneal.ing tcmperatllre on the gettering ofiron in MeV Si implanlcdinto Si. and ¡hey have heen surprised Ihal release and capture(Ji"impurilies at oiffcrent damaged regions is possihle with theallnealing lime. However, it has heen already shown lhat suchrclcase 01"impuritics is possihle for long annealing time I:W].It is oh\'iollsly lhat the annealing time can not be a paramelerarhilrarily choscn.

111this \\.ork. analyzing the cvolulion of quasi 3-dimen-siollal prollle 01' gencration lifetime and surfacc gcneration\'l'Iocity wilh lhe anncaling time. we have investigatcd itsrole 011Ihe gcttering efficiency 01"phosphorous ion implan.talion gettcring in ~10S slructures. \Ve show in the prcsenLcase tlWl an oplimum annealing time exists. Thc concepl nI'lhe segrcgaLion Illodel 01"the gettering process was extendedto explain thc exislence of an optimum annealing time.

Re¡'. Me .•.. FI.<. 4(, (5) (2tXXJ)485-489

I~FLUENCE 01' SEGREGAT10~ ANNEALlNG TIME ON THE GETTERING I'ROCESS ~87

where j\'Si is lhe nurnher uf Si atoms per unit volume andEII•i is the aetivation encrgy. The (olal soluhilily of metallieilllpurilies in hOlh regions is Ihe sum of Eqs. (2) and (3)

whcre Xd is a preexponenlial factor which is a funclion oflemperature and is relateo lo the concenlralion of metallicimpurities dissolved in the region of h;gh dislocation densi.,¡es ami Ihus depcnds on Ihe dislocalion density. E(j.rl is lheactivation cnergy.

The s(Jluhility 01"rnclallic irnpurilies in inlrinsic or lighllydopcd Si is given hy

(~)

be-

(2)

(3)

(5)

, (-Ea,')+ ASi exp "kT ., (-En"')= 1\" CXI' ----¡¡y-

l' N",¡,T 1 (N".) [(En,; - En,")], = --- = + -- ('xp ------Nmi.i ,V/ji kT

Thc segregation coefficient 01' metaJlic impuriliest,"vecn P implanlCd and lighlly doped Si is dcllned as

01' high disloealion Jensities is givcn by

, , (-En,")'\mi,rl = l\d('XP ----¡¡y- ,

Al ;¡ gi\'cn tempcrature the segregalion eocftkicnt I{ is aconstanl which will delermine the ralio of equilibrium con-centration nI' metallic impurilies in thc region 01' high dis-local ion dellsily to Ih<:ll in the lightly doped silieon. Thismeans thal Ihe higher is A" Ihe higher is the gettering effi-cieney. Thcrcfore. according lo Eq. (5), tlle dislocalion den-sity should he as high as possihle and the segregation an.ncaling IClllpcrature should he as low as possiblc for high A".\Vith Ihe increase 01' anncaling time dislocalion density. Sd.decreases and according lo Eq. (5) K also decreases. This re-quires release and rcdislrihulion of metallk impurities unlil anew equilibriul11 dislrihulion helwcen tlle implanted damagcdrcgion and Ihe rest of lhe water is reached, corresponding loa new value of 11..". Ir so. then the generation ¡iretime must de-crease. which coincides with our experimental results. Thismodel expklins also the physical rneaning of the oplimum an-nealing lime. Howe\'cr. as il will he shown (alter, other factorscan also intluencc on (he oplimum annealing lime.

Anolher factor. rcsponsihle ror Ihe degraJation of T9 inlhis annealing lime inlerval, could he a metallic impurity COI1-

larninalion from Ihe furnace. Indeed, looking al Fig. l. it canhe seen thal Ihe annealing limes longer than 90 min have af-fected Tg• especially in the near surfacc region. Here, it musthe said lhal separale experimcnts have shown thal ror long an-nealing limes Ihere is a melallic impurily contaminalion fromIhe furnace. A decrcase 01' gcneration lifetime due lo metallicimpurity conlaminalion fmm lhe furnace was aIso rcportedby Manchada et al. (-l5]. II is evident Iha[ the intluence 01'the lasl two processes on lhe generation liretime will compete

¡¡ V(I¡lIllle expansiono Pan 01' this volume cxpansion is accolll-lTlodalcd hy gcncration of Si self-interstitials al lile Si-SiO:!interface. Sume of these sel f-intcrslitiais can rccomhinc al lheoxidizing interface and lhe rest diffusc ¡nlo Ihe hulk 141,421.In this case e > en or thefe is undcrsaturation 01' vacan-des. ami lhe chcmical driving force \ViII be grcatcr than Iheforce dctcrmincd by lhe slacking fault cnefgy ami lhe disloca-tion linear cncrgy. This will rcsult in a growth uf OSFs. Thcgrowth of OSFs procccds a!ong Ihe surfacc ano ¡nlo Ihe hulk.Sinec l!le conccntration 01' lhe Si self-intcrstitials dccrcascswith lhe distance fmm Ihe interface Ihe dcnsity antl lhe sizc01"OSFs also dccrcases. Thus. lhe variation 01' T9 near the Si-SiO-) interface can he due to the spatial distrihution 01' OSFslher~. These defects are preferential sites of metallic impuri-tieso

Several factors can he responsible for such behavior ofgeneration lifelime as a function of segregation annealinglime. From a physical point of view the process of gelteringcOllsists 01"Ihree steps. According to the concept 'of the get-Icring proccss defects. especially metallic impurilies. muslhe released fmm Iheir original sites. diffuse to the implanteJt1amageJ region anJ be captured at the gettering si tes. One01' lhe Ihree steps 01' gettering (rclcase. diffusion or capture)will he Ihe rate-limiling stcp for the gcttcring. For an opti-Illum and conslanl tempcraturc, segregation annealing time(diffusion) ,"viII be the rale.limiting step that conlrols gelter-ing efliciency. In OUT case. in concordance with the abovel1lechanism. one of the faclors responsible to Ihe incrcaseof Til with [he annealing time is the geltering 01' impurities.Such hehavior of 7.q wilh the annealing time SUpp0rlS Ihecomhincd segrcgation-extended defects model 01' Ihe gettcr-ing proccss [35J.

On Ihe olhcr hand. during annealing in neutral alllbienl(hcrc is Si self-interstitial undersaturation 1--131. ¡.(', e < enaod aeeordiog lo Eq. (1) Ihe OSFs Ieoglh \ViII deerease.Therefore. another faclor responsiblc for Ihe inerease of gen-eral ion liretime wilh the segregalion annealing could he lhedecrcase of OSFs lenglh.

Several compeling faelors can he responsihle rol' the he-havior 01' Ty wilh the segregalion annealing in the range YU-150 mino It is clear Ihal the shrinkagc 01' lhe OSFs will con-linuc. On Ihe other halllJ. il is known [4-1] that dislocationloops Iying on (J l J) planes edge dislocations and also dipolesIyiog parallel lo (110) direelioo appear afler aooealiog. Thedislocation loops increase in size with anneallemperature uplo 800'-'C. Al (his tcmperalure apparenlly lhe loops stahilizc,¿lnd do 1101 change lheir size al higher annealing lemperatures.However. ror long annealing timc lhe lotal density of disloea-lion deereases. The annihilalion of defects is enhanced whenthey are deeoratcd with impurities [40J.

Using our experimental results and thesc ones fromthe Iileralure [40,411, \Ve \ViII exteod Ihe eooeepl nf thesegrcgation-extended defeCls model nf gettering (35J lo in-elude Ihe process of lllelaJlic impurities release [mm the gCI-lering siles for long anncaling time. According 10 this model,Ihe soluhility 01' melallic irnpurities in the irnplanted region

Rev. Mex. Fú. 46 (5) (2(XX» 4R5-4R9

488 P. PEYKOV. T. OIAZ. ANO M. ACEVES

.t. Conc1usions

Flc;URE 2. Surfacc gcncration vcJocity as a function oC lhe segre.

galioll <lnllcaling time.

wilh thal 01' lhe OSFs shrinkagc. This raet can cxplain lheslow dccrcasc of Ty for scgrcgation anncaling times longcrIhan 90 mino

Surfacc generation vclocity. S. as a function 0'- lhe an-Ilcaling lime is prcscnlcd al Fig. 2. It is sccn from Pig. 2 IhatS dccrcascs with Ihe anncaling time up lo 90 min, and a1'-ter lhat. t1ccrcascs. Il can he supposcd that lhe gcltcring pro.ccss. rcsponsihlc for lhe gcltcring of impuritics in Ihe ncaTsurfacc region. also gctlcrs impuritics from lhe Si-Sinl inter-face. Thc ¡ncrease of S for longcr anncaling times can onlyhe cxplaincd hy a mctallic impurily contamination fmm Ihefumace afl(1 not oy Ihe ion damage anneal.

Ihe generation lirelime. ami oetler undcrstanding Ihe inllu-ence of the differenl proccss sleps.

It has been found that T9 increases with Ihc dislance fromthe Si-Si02 interface. On the other hand. Ihe general ion life-lime incrcases with the segrcgalion annealing time up lo theoptimum annealing lime, that was found to he 90 min in Iheprcscnt casc. For longer annealing time than the optimum onethe T 9 decreases.

The varialion of T9 near the Si-Sin! interface is explainedby lhe model of OSFs growlh. According lo Ihis modei lhesize and the density of OSFs decrcases with lhe distance fromlhe interface anJ, hencc. T9 increases. The inerease of T9 withlhe scgregation annealing time is tlue to Ihe decrease of theOSFs size bul mainly duc to Ihe gcltcring 01" metallic impuri-tieso

To explain Ihe Tf} hehavior with anncaJing times longcrIhan the optimum annealing time the conecpl 01' Ihe segrega-tion model 01' getlering was extended to inelude the anncalingof dislocations ami the suhscquent rclcase nf metallic impu-rities from the gctlering si tes.

Three factors were idenlified to govern Ihe hehavior ofT9 for long annealing timcs. Thc t1rsl factor, aecording lO theahoye model. is the dccrcase in the amount 01' disorder in theimplanled region. Ihe release 01' part orthe metallic impuritiestrapped Ihere ami ils redislrihulion in Ihe wafer. The seeondfaelor is the shrinkage 01" OSPs due lo lhe allnealing in neutralambienl. Thc third t~1etor is mctallic impurily contaminalionfmm the furnace. The firsl and the Ihird factors, which playa dominant rolc, compete with Ihe second factor. These IhreeI"actors determine the oplimum annealing time neeessary formaximum gettcring cfficiency.

The Si~SiO:! interface is also affcclcd by Ihe gcttering.Surface gcneration vclocily deereascs \Vilh Ihe scgrcgationannealing. Howevcr. it deercases ror times longer than theoptimum annealing time. This lasl fael confirms Ihal there isa contamination from the furnacc.

,ao150

/'2090

m,n60

ANNEAlING TIME

301E-C1

O

1E+01

ti) i1E+OO

Surface ami oulk gcneration properties of MOS structures,geltered hy P ion il1lplantation. have hecn investigated. Theparamctcr oí' Ihe gcttcring was Ihe segregation annealingtimc. It has heen shown that lhe rneasurcment of the gcncra-tion ¡¡fetime depth profile and ils quasi 3-dimcnsional repre-sClltation offers easicr itlcntification of the factors controlling

Acknowledgment

Thc authors thank Mr. 1. Puentes for [he samples preparationand CONACyT for Ihe parliai f¡nancial support.

Presently Dr. Pcykov is wilh Inslitulc of Semiconduclor Physics

ami Tcchnology. Solla Univcrsity.

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INFLUENCE OF SEGREGATION ANNEALlNG TIME ON THE GETTERING PROCESS 489

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