Dielectric breakdown of SiO2 thin films deposited by ion beam induced and plasma enhanced CVD

2001 IEEE 7 h In e na ional Con e ence on Solid Dielec ics, June 25-29,2001, Eindho en, he Ne he lands 303 DIELECTRIC BREAKDOWN OF Si02 THIN FILMS DEPOSITED BY ION BEAM INDUCED AND PLASMA ENHANCED CVD. A. Ba anco, bA. Jimknez, bF. F u os, "J. Co ino, 'F. Yube 0:J.P. Espinbs, aA.R. Gonzhlez-Elipe a* "Ins i u e de Ciencia de Ma e iales de Se illa (CSIC-Uni e sidad de Se illa). A da. AmL ico Vespucio s/n. 41092 Se illa, Spain. 'Dp o. de Fisica A dmica, Molecula y Nuclea . Facul ad de Fisica. Uni e sidad de Se illa. A da. Reina Me cedes dn. 41012 Se illa, Spain. bDepa amen o de Fisica Aplicada I. Facul ad de In om' ica. Uni e sidad de Se illa. A da. Reina Me cedes dn. 41012 Se illa, Spain. co esponding au ho : [email p o ec ed] INTRODUCTION Silicon dioxide in he o m o hin ilms is a e y in e es ing ma e ial o elec onic, op ical o passi a ion laye applica ions [l-31. Fo many o hese pu poses, in pa icula mic oelec onics, i is c i ical ha compac laye s o Si02 can be ob ained a low empe a u e. Howe e , mos o he es ablished me hods o p epa a ion a e based ei he on he mal CVD, equi ing in his case e y high empe a u es, o on plasma CVD. When wo king a low empe a u e, he mic os uc u e o he ilm is usually no su icien ly good and he composi ion is no well de ined. In p ac ice, plasma-ac i a ed p ocedu es o p epa a ion o Si02 hin ilms wi h op ical o elec onic applica ions yield op imal mic os uc u es a in e media e empe a u es o he o de o 200-300 "C. [4-51 This pape p esen s a compa a i e s udy o Si02 hin ilms p epa ed a oom empe a u e by ion beam induced chemical apo deposi ion (IBICVD) and plasma enhanced chemical apo deposi ion (PECVD) me hods. The ilms a e cha ac e ized by AFM, TEM,RBS, ERDA, NRA, X- ay e lec ome y and spec oscopic ellipsome y. While he ilms p epa ed by IBICVD a e e y compac and dense and ha e a high e ac i e index (n = 1.48 a 0 =550 nm), hose p epa ed by PECVD exhibi a lowe e ac i e index alue (n = 1.45 a 0 = 550 nm) lowe densi y and ha e a highe su ace oughness. Films p epa ed by IBICVD p esen s a b eakdown elec ic ield alue (49.14 MV/cm), ou imes highe han ha o he ilms p epa ed by PECVD (12.11 M /cm) and o he SiOz ma e ials epo ed in he li e a u e. This highe alue is a consequence o he low oughness and high densi y o he IBICVD ilms. This ype o mic os uc u e is p oduced by he e ec o he highly ene ge ic ion beams (e. g. 400 eV) impinging on he su ace o he g owing ilms du ing hei p epa a ion. Expe imen al Si02 hin ilms ha e been p epa ed a oom empe a u e by wo ac i a ed CVD p ocesses: IBICVD and PECVD. In bo h cases Si(CH3)3 C1 has been used as ola ile p ecu so o Si. The i s me hod consis s o he decomposi ion o he p ecu so on he su ace o he di e en subs a es by he ac ion o a beam o accele a ed 02+ ions. A ull accoun o his me hod has been epo ed p e iously [6]. PECVD deposi ion was ca ied ou by using an ex e nal mic owa e ECR sou ce (SLAN) ope a ed a 400 Wa s. A ull desc ip ion o he cha ac e is ics o his sou ce can be ound in e [7]. Fou ie T ans o m In a ed (FT-IR) spec a we e collec ed in ansmission a no mal geome y in a Nicole 5 10 spec ome e . T ansmission elec on mic oscopy (TEM) analysis was pe o med wi h a Philips CM200 mic oscope wo king a 200 kV. KB subs a es we e used o con en ional TEM examina ion. A omic o ce mic oscopy (AFM) images we e ob ained in ai using a Topome ix Explo e mic oscope wo king in he non-con ac mode. Spec oscopic ellipsome y analysis we e pe o med using a SOPRA comme cial-a ailable sys em Fo he anspa en ilms s udied (ex inc ion coe icien Id), he analysis p ocedu e consis s in a eg ession using a Cauchy law o simula e he e ac i e index n( 0) o he laye . X- ay e lec ome y spec a we e collec ed wi h a special se -up using he angle- esol ed dispe si e mode wi h a posi ion-sensi i e p opo ional coun e . The echnique is able o de e mine he hickness and densi y o hin laye s. Ru he o d Backsca e ing spec oscopy (RBS) and Elas ic Recoil De ec ion Analysis (ERDA) measu emen s o he ilms we e pe o med in he Ion Accele a o ARAMIS (O say, F ance) using 1.5 MeV He+ and 3 MeV He" pa icles, espec i ely. The ene gy esolu ion o he de ec o used in he RBS measu emen s was 15 keV and 45 keV in he ERDA 0-7803-6352-3/01/$10.00 0 2001 IEEE 304 measu emen s. RBS and ERDA spec a we e simula ed using he RUMP code. B eakdown ol ages ha e been ob ained h ough a ol age amp wi h a cons an slope o 20 V/s by using he expe imen al a angemen shown in Fig.1. This ol age was applied sepa a ely o Au con ac s o diame e lmm e apo a ed on he sample su ace. - Figu e 1. Expe imen al a angemen used o measu e b eakdown elec ic ield and cha ge o b eakdown: unc ion gene a o , G, wi h his load esis o , I&-, hin ilm sample, M, and oscilloscope p obes, SI and S2. The unc ion gene a o G is able o gene a e ol ages up o 400 V, wi h 100 mA o maximum cu en , and o pe o m amps o slopes anging om 20 V/S o lo6 V/S wi h 2% o ou pu ipple. The sys em is based on a swi ching DC powe supply ( lyback con e e ), con olled by a compu e (on-boa d mic ocon olle ). Ou pu ol age wa e o m is gene a ed by he mic ocon olle using a s o ed da a pa e n, wi h a sampling in e al o 0.1 ms. S o ed da a pa e n can be modi ied a any ime by compu ing a new pa e n wi h he on boa d mic ocon olle (only amps a e allowed), o by down loading a new pa e n om an ex e nal hos compu e , using he on-boa d RS232 po . In he la e case, any ol age shape complying he es ic ions men ioned abo e can be pe o med. As an added alue, he sys em is able o s o e he eal ou pu ol age which can be downloaded back o he hos compu e o analysis (s udy o dielec ic b eakdown). This makes unnecessa y he p esence o addi ional elemen s, e.g. oscilloscopes. S1 and S2 a e digi al oscilloscope p obes in single igge mode. S2 measu es he applied ol age o he sample, M, and S1 he ol age in a load esis ance o 1wZ in o de o calcula e he cu en h ough he sample a any ime. Bo h signals can be s o ed by means o a HP-VEE so wa e o hei la e s udy. In Figu e 2, we show he cu en a ia ion as well as he applied ol age amp a a pa icula Au con ac o a IBICVD sample. F om bo h we can ob ain he alues o b eakdown elec ic ield and, by nume ical in eg a ion, o he cha ge o b eakdown. A pa icula sui ed dis ibu ion, namely ex eme alue s a is ics, was used o calcula e he in insic b eakdown and cha ge o b eakdown o he sample[8-9]. i............................ 01020304050 Tim (s) Figu e 2. Elec ic signals om a pa icula Au con ac in a IBICVD sample: (1) applied ol age amp; (2) cu en a ia ion. Resul s and Discussion The composi ion o he hin ilms p epa ed by ei he IBICVD o PECVD was de e mined by a combined RBS/NRA analysis. They a e o med only by Si and 0 a oms. The ob ained O/Si s oichiome y we e 2 k 0.02 in all samples. Si02 composi ion a he su ace was also ob ained by XPS. Some ca bon was de ec ed ha was eadily emo ed by mild A + o 0,' bomba dmen . This ca bon is he con amina ion o he samples by exposu e o ai . The con en o H in he samples was de e mined by ERDA measu emen s. The WSi a io was less han 0.25 and 0.35 o ion and plasma ilms, espec i ely. These alues a e simila o lowe han hose epo ed in he li e a u e o Si02 hin ilms p epa ed by he md. o plasma CVD [lo]. I is also shown below by FI'-IR analysis ha he concen a ion o -OH g oups is highe in PECVD-Si02 han in IBICVD-Si02 ilms, a ea u e ha can be explained by he oughe mic os uc u e o hese ilms. Figu e 3 shows opog aphic AFM images o he su l'ace o Si02 ilms app oxima ely 100 nm hick p epa ed by IBICVD and PECVD. The images, p esen ed wi hin he same cu en scale, clea ly indica e ha he IBICVD-Si02 sample has lowe su ace oughness. 'The oo mean squa e (RMS) su ace oughness alues deduced om he AFM images a e 0.21 and 1.5 o he IBICVD and PECVD ilms hus con i ming he p e ious assessmen o su ace oughness o he samples. 305 Plana - iew TEM images ha e been ob ained o he wo ypes o oxide hin ilms s udied he e. The IBICVD-Si02 ilm shows an uni o m con as wi hin he ilm (homogeneous laye ) whe e no oids a e dis inguishable. On he o he hand, he PECVD-Si02 sample, exhibi s a con as wi h g anula s uc u e cha ac e ized by agglome a es o -20 nm ha supe impose o o m he whole ilm hickness. This g anula s uc u e is ela ed o he highe su ace oughness (c . Figu e 3) o hese ilms. samples he in e ace is less de ined, being appa en he supe posi ion o he subs a e poin s wi h some shadowed zones ha indica e he o ma ion o Si02/Si0, (xe2) inclusions wi hin he subs a e. The di e en mic os uc u e o he wo ypes o samples mus yield di e en physical and chemical ilm p ope ies. Re ac i e index n(A), ob ained using spec oscopic ellipsome y in he spec al ange 0.21pm-1.2pm, a e p esen ed in Fig 4. One can clea ly obse e ha , o he en i e spec al ange, he n alue o he IBICVD- Si02 sample is highe han ha measu ed o he PECVD-Si02 laye (n = 0.03). The highe e ac i e index o he IBICVD ilms ag ees wi h he alue o densi y ob ained o his sample by X- ay e lec ome y. This e ac i e index is wi hin he highes alues epo ed in li e a u e o Si02 hin ilms p epa ed by di e en me hods [12]. IBICVD The p e ious esul s ha e shown ha Si02 hin ilms p epa ed by IBICVD a e mo e compac and dense han hose p epa ed by PECVD. Main ac o o his enhanced compac ion o he ilm is he kine ic ene gy o he impinging ions [ 131. Figu e 3. AFM images o he wo samples In o ma ion abou densi y and su ace and in e ace oughness o hin ilms was ob ained by X- ay e lec ome y. The densi y o he ilms was deduced om he o al e lec i i y angle cl,, by his p ocedu e, he densi y o he IBICVD-Si02 and PECVD-Si02 ilms we e 2.46 and 2.08 g ~m-~, espec i ely ( he densi y o bulk silica is 2.20 g cm-3 ). Fo he IBICVD ilm, he absence o an in e e ence pa e n in he e lec ome y spec a (no shown) could be ela ed o a no well de ined in e ace be ween he ilm and he subs a e. This may seem con adic o y wi h he low su ace oughness de e mined by AFM o his sample. Howe e , a ough subs a ehilm in e ace is expec ed owing o he ac ha 02' ions o ela i ely high ene gy (i.e. 400 eV) a e used o he deposi ion o he ilms. Based on TRIM calcula ions [ll], a pene a ion dep h o 1.5-2.0 nm is expec ed o hese oxygen ions. Oxygen di usion h ough he Si subs a e could be an addi ional eason o he absence o in e e ence iinges. The hypo hesis o a poo ly de ined ilmhubs a e in e ace was checked by high esolu ion TEM (HRTEM) c oss-sec ion obse a ion o he in e ace. The ilmhubs a e in e ace in he la e sample is well de ined, ha ing a hickness smalle han 1.2 nm o he whole obse a ion a ea. In ac , he well o de ed s uc u e o he Si single c ys al subs a e, cha ac e ized by an o de ed a angemen o poin s, gi es ise ab up ly o he homogeneous shadowed s uc u e ypical o amo phous Si02 laye . Howe e , o he IBICVD 1.401 1.354 . , . , . , . , . , . , . I 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 X @m) Figu e 4. Re ac i e index s A o IBICVD and PECVD samples Fo he IBICVD sample (70 nm wid h o Si02 on Silicon) and using he ex eme alue s a is ics, a b eakdown ol age o b=344 V was ob ained which means a b eakdown elec ic ield o Eb=49.14 MV/cm. Cha ge o b eakdown is Qb=105 mC. Fo he PECVD sample (140 nm wid h o Si02 on Silicon) he co esponding alues a e Vb=169.6 V, Eb=12.11 W/cm and Qb= 73.13 mc. CONCLUDING REMARKS The p e ious esul s ha e clea ly illus a ed he impo ance o ballis ic in e ac ions o impinging pa icles o an enhanced compac ness o Si02 ilms. This enhanced compac ness leads o a modi ica ion o hei physical p ope ies as o example he e ac i e indices (c . Fig. 2), which is o g ea in e es o mic oelec onic and op ical applica ions. I is also 306 wo h o men ion ha ion beams migh be also used o a selec i e deposi ion o he ilms acco ding o a li hog aphic echnique. In ac , deposi ion by IBICVD occu s a oom empe a u e only in he zones whe e ion beams a e impinging on he subs a e su ace. The use o ocused beams as e ed acco ding o a gi en pa e n o ha o a mask pu be ween a b oad beam and he subs a e should p oduce such pa e ned ilms o a la ge in e es o mic oelec onic applica ions. A p esen , esea ch is being ca ied ou in ou labo a o y o explo e his possibili y o he IBICVD p ocedu e. This pape has also shown ha a b oad in e ace de elops be ween he silicon subs a e and he SiOz laye g own by IBICVD. A way o minimizing such in e laye hickness could be ob ained by deposi ing an ini ial Si02 hin laye , ei he by PECVD o lowe ing he ene gy o he ion beam o educe he mixing. Then, he hin ilm could be g own on he op o his hin laye as i is desc ibed in he pape . BIBLIOGRAPHY P Balk Ad . Ma e . 7 (1995) 703. D.L. G iscom. MRS Bull. Jun-Aug. 20 (1987). W Ke n, R.S. Rosle . J. Vac. Sci. Technol. 14 (1977) 1082. T. Roppel, D.K. Reinha d, J. Asmussen. J. Vac. Sci. Technol. B 4 (1986) 295. K. S a e . RCA Re . Dec. (1968) 61 8. J.P. Espinbs, A. Fe nindez, V. M. Jimknez, J. C. Sinchez-Mpez, L. Con e as, D. Leinen, A.R. Gonzdez-Elipe. A d. Ma . CVD 3 (1997) 219. F. We ne , D. Ko zec, J. Engemann, Plasma Sou ces Sci. Technol., 3 (1994) 473. J.F. Ve weij and J.H. 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