Local Laser Oxidation of Thin Metal Films: Ultra-resolution in Theory and in Practice

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1 Locl Lsr Oxidtion of Thin Mtl Films: Ultr-rsolution in Thory nd in Prctic V.P. VEYKO *1, E.A.SHAKHNO *1, A.G. POLESHCHUK *, V.P. KOROLKOV *, V. MATYZHONOK *1 *1 Sint-Ptrsurg stt tchnicl univrsity of informtion tchnologis, mchnics nd optics E-mil: * Institut of utomtics nd lctromtry, Novosiirsk Lsr inducd oxidtion on thin mtllic films with following slctiv tching is powrful mthod for micropttrning of diffrnt structurs lik photomsks, diffrctiv opticl lmnts tc.nowdys th most intrsting qustion is how smll could structurs producd y this tchnology, wht r thorticl limittions nd whr is prcticl limit for tht? Undr this rsrch will shown tht Arrhnius dpndnc of oxidtion spd from tmprtur riss this limit on som ordrs of mgnitud highr thn th rsolution of thrml img. Corrsponding clcultion will don nd will vrifid xprimntlly. From th othr hnd régims of lsr irrdition (powr dnsity, tim of irrdition tc) s wll s régims of slctiv tching (typ of tchnt nd its concntrtion), mtril of thin film nd its thicknss hv lso ig influnc on th finl rsults. Exprimntl nd prcticl rsults for friction of mny spcific componnts nd dvics s diffrnt diffrction lnss, diffrction ttnutors, synthsizd hologrms will dmonstrtd. Kywords: lsr inducd oxidtion, micropttrning, film, tching 1. Introduction In optics scinc thr is clss of opticl lmnts tht hv vry complx topology structurs, sd on thin mtl films (TMF) dpositd on dilctric (commonly glss) sustrt. First of ll w cn nm in this clss diffrctiv opticl lmnts (DOE) thos r usd for gomtricl nd wv trnsformtion of light ms diffrcting on thirs structur [1]. At th prsnt momnt DOE r widly implmntd in vrious pplictions lik printrs, csh rgistrs, dvics for rding r cod, lsr CD or DVD plyrs, tc. Nw typs of DOE r implmntd in tchnologicl systms, lsr sclpls, rtificil crystllin lnss, nd so on. In rcnt yrs ccurcy of DOE incrsd vry much tht llows to solv vry difficult prolms construction of wvfront corrctors (zro lns) tht cn form rfrnc wvfronts usd for control of sphric optics min mirrors of modrn tlscops. Sptilly structurd thin film lmnts on trnsprnt sustrts undrli opticl mmory dvics, sound nd vido-rcording (OR) []. Finlly, prcision tchnologicl instrumnt in microlctronics photomsk (PM) cn rfrrd to this clss. In ll of ths css w tlk out finly structurd nontrnsprnt TMF on trnsprnt sustrt (in most css TMF is chrom). Typ of solut contrst structur nnulr (DOE), spirl (opticl dt rcording), rctngulrrndom (PM) dfins th purpos nd ppliction fild of ths lmnts. Tchnologicl principls of forming lmnts r lying in thir foundtion, nd sic rquirmnt for thm is high rsolution. Incrs of rsolution cpility nsur dvlopmnt of microlctronics nd optics, nd llow us to find nd solv nw clsss of prolms []. Lsr lithogrphy in our dys is th st wy of DOE, OR, nd PM topology formtion. In most css it is sd on trditionl photolithogrphy (PL) with lsr xposition of photorsist [5]. But cus of its complxity nd timconsuming ntur vryody wnts to find nothr mthod of topology formtion. On of ths mthods is lsr vportion of TMF, ut it hs som limittions in ccurcy nd qulity of topology structurs du to thrml nd hydrodynmic rrtions in img [6]. On of th most prspctiv mthods is lsr thrmochmicl mthod (LTM) sd on locl lsr oxidtion (LO) of TMF with susqunt tching of non-irrditd rs [7]. By th mns of this mthod rcntly vry intrsting rsults wr otind [1]. But till this tim no on cn point its limits in rsolution cpility, nd this ostcl rstricts th rng of pplictions for this mthod. Aim of this rticl is to mk thorticl nlysis of rsolution cpilitis of LTM with LO nd its comprison with xprimntlly otind rsults. Mking this rsrch w sttd tht th most dvlopd fild of LTM prcticl implmnttion is production of DOE [1]. Annulr structurs r topologicl ftur of DOE tht is why ll xprimnts wr conductd on lsr gnrtor of nnulr structurs, dvlopd in Institut of utomtics nd lctromtry, Sirin dprtmnt of RAS, [1]. In ccordnc with this for ttr nd spcific comprison with xprimnt thr ws dvlopd physico-mthmticl modl of locl surfc oxidtion of chromium thin film y th mns of lsr scnning m with continuous lsr rdition tht hs smll sizd cross-sction nd Gussin distriution of intnsity. It is shown tht shrp dpndnc twn thicknss of oxid lyr nd tmprtur rsults in lyr thicknss grdints xcding th tmprtur grdints. Possiilitis of producing divrs sumicron structurs with siz d smllr thn dimtr of lsr m d, 1

2 nd rdition wvlngth λ, on thin films r shown. Sitution whn d is smllr thn λ w cll ultr-rsolution.. Prolm dfinition Lt us considr oxidtion of chromium thin film on th K-8 glss sustrt. Oxidtion procss is inducd y focusd on th surfc of th thin film scnning m of continuous lsr rdition. Som prt of lsr rdition tht rchs th surfc is ing rflctd from it nd som prt is ing sord y th mtril. Influnc on vry point lsts for som priod of tim. This priod of tim dpnds on dimtr of lsr m nd scnning spd. Th tim puls shp tht ffct vry dfinit point on surfc dpnds on sptil intnsity distriution of lsr m. Asorption of rdition rsults in hting of th thin film dirctly undr th focusd lsr m nd in th nighorhood. Hting of th film ctivts svrl physicochmicl procsss, lik surfc dsorption of oxygn, diffusion nd lctron trnsport of mtl nd oxygn ions, chmicl oxidtion of mtl with oxid lyr formtion, nd othrs [8]. Dfinit rltion twn oxid lyr thicknss H nd tmprtur T is dtrmind y th rtio of th ov mntiond procsss rts. Th most frquntly ncountrd lw is Wgnr oxidtion lw, ccording to which: dh B T xp dt H T (1) whr В prolic oxidtion constnt, T - ctivtion nrgy of diffusion procsss, in Klvin, nd t currnt momnt of tim. For vlution of oxid lyr thicknss in th cs of nonisothrml oxidtion lt s us mthod, dvlopd y Linson M.N. [3]: s fr s th tmprtur of thin film is sustntilly smllr thn T, thn xponntil curv of ctivtion shrply incrss with th growth of tmprtur, nd min contriution in incrsing of oxid film thicknss is md y thos momnts, whn tmprtur Т is clos to its mximum vlu T mx. Considring this w cn vlut thicknss of oxid lyr undr lsr (nonisothrml) hting s quivlnt to thicknss of oxid lyr undr isothrml hting during som dfinit quivlnt tim priod (shortr thn tim of lsr rdition influnc) : T H Bxp t () Tmx Vlu of t is dtrmind y th pttrn of tmprtur chng with tim undr lsr trtmnt. Prticulrly if tmprtur mximum is ing rchd in som momnt of tim during lsr trtmnt, thn [] t t π T T T tt mx ( t ) Thus, dfinition of rltion twn tmprtur of film nd tim in vry point of film mk it possil to dtrmin th structur of gnrtd oxid. Susqunt tching forms th topology y dlting prts of film tht r not covrd with oxid lyr of sufficint thicknss. Criticl vlu of this thicknss is not dtrmind xctly yt. In diffrnt sourcs this vlu is stimtd in rng from to 8 nm. In vlution procdur of t (3) uppr limit of miniml lmnt thicknss in film lyr, otind y th us of lsr oxidtion, w hv usd vlu 8 nm. 3. Clcultion of oxid lyr thicknss undr scnning m of continuous lsr rdition Lt us considr hting of mtllic film with thicknss h on dilctric sustrt. Hting procss is inducd y scnning m of continuous lsr rdition. Lt us considr qusi sttionry hting rgim. W hv md prliminry ssssmnt clcultion of rltion twn ht flows from irrditd rgion to th film P 1 nd to th sustrt : P P1 h () P r s whr r - rdius of lsr m, film thrml diffusivity, s - sustrt thrml diffusivity. In th mjority of css importnt in prctic: P 1 P > 1, prticulrly, for chrom film with thicknss 1 nm on K-8 glss sustrt, nd rdius of irrditd r.5 μm, P1 P. Considring this w will rgrd film, htd y th lsr rdition, s diticlly isoltd film, nd nglct th ht sink to th sustrt. Ht conduction qution for th film is: ( ) ( ) T T T P 1 R x Vt + y + xp (5) x y πr ρch r whr Р lsr output powr, Т film tmprtur, ρ - film dnsity, с ht cpcity of film, V scnning spd, R rflction cofficint of film, r - chrctristic vlu of Gussin intnsity distriution in m, t currnt momnt of tim. Axs x nd y ly on th pln of th film surfc, nd x-xis coincid with dirction of scnning nd crosss th cntr of irrditd r, y-xis is prpndiculr to x-xis. Lt us pss ovr to moving coordint systm connctd with irrditd r. Ht conduction qution is: T T V T P( 1 R) z + y xp 6) z y z π rkh r whr z x Vt, k film ht conduction. Chnging th vrils Vz Vr T f ( y, z), r z + y Vr x Vt + y w will trnsform qution (6) to: ( 1 R) Vr r d f 1 df V P r f dr r dr π r kh (7) + + (8) Using zro-ordr Hnkl trnsform to solv qution (8) w ll gt finl xprssion for tmprtur of film: ξ ξ r T G J ξ dξ (9) ξ + r

3 ( ) P 1 R Vr whr G,, J ( u ) - first-typ Bssl π kh function of zro-ordr, vlu of tmprtur is countd from its initil vlu. Vlu of drivtiv of tmprtur with rspct to tim T T r is r, whr r is clcultd from qution (7). T Clcultions show mximum tmprtur in points r y. In th sm wy w cn find th scond drivtiv: T T r r. For r y y J 1 ξ ξ T GV ξ r dξ t r (1) ξ + y whr J ( ) 1 u - first-typ Bssl function of first-ordr. For simplifiction of susqunt clcultions for dtrmintion of tmprtur nd its scond tim drivtiv w hv usd xpnsion of Bssl function into sris: n n G ( 1) y T xp E1 + n 1 nn! r (11) n n G ( 1) y γ ln + n 1 nn! r whr E1 ( u) - intgrl xponntil function, γ Eulr constnt. n n T GV ( 1) y (1) t r n ( n+ 1! ) r. Clcultion rsults Clcultions show tht lsr powr ndd for thrmochmicl mchining of 1 nm chrom film lis in th rng from 1 to mw for 1-1 cm/s scnning spd. Ths rsults r in good grmnt with xprimntl rsults: 1-3 mw. Powr of lsr rdition must chngd dpnding on scnning spd in such rng, whr from on sid thicknss of th film xcd th criticl vlu, nd from nothr sid thr is no mlting. Admittd rgion of lsr powr output dpnding on scnning spd thorticlly clcultd nd in xprimnt is dmonstrtd on Fig. 1. It is xprimntlly shown, tht for diffrnt vlus of lsr powr nd scnning spd thr r thr following rgims: I is rgion of film mlting, II is working rgims rgion, III is rgion of insufficint lsr hting of th film for oxid lyr formtion. Rdcolord curv is thorticlly clcultd thrshold (oundry of th II nd III rgions). Clcultions showd tht rduction of oxid lyr thicknss from th cntr of th lsr spot (prpndiculr to th dirction of scnning m) occurs much fstr thn rduction of mximum tmprtur (Fig. ). This fct is prmis for otining miniml sizs of lmnts nd, thrfor, high rsolution Fig. 1. Lsr oxidtion of 1nm chromium thin film: frgmnt of th sris of tst lins for nd ftr tching, - dmittd rgion of lsr powr output dpnding on scnning spd. Fig.. Clcultd dpndnc twn oxid lyr thicknss, mximum tmprtur nd distnc from th cntr of th lsr spot (Р 16.5 W, V.5 m/s, h 1 nm) Clcultd vlus of miniml width of otind pttrn (w ssumd tht criticl oxid lyr thicknss, tht corrsponds to th dg of pttrn, is 6 nm) lis in th rng from. μm t minimum lsr powr or mximum scnning spd (lowr curv on Fig. 1) nd to.5 μm t th mlting thrshold (uppr curv on Fig. 1), whn dimtr of lsr m on th surfc is.5 μm. If vlu of lsr powr is slctd in th right wy ccording to scnning spd thn miniml width of otind pttrn prcticlly don t dpnd on scnning spd. Thrftr vlu of rsolving ility lis in th rng twn 5 nd mm Exprimntl rsults Th invstigtion of thrmochmicl ultr-rsolution ws md using th lsr writing systm uilt t th Institut of Automtion nd Elctromtry []. Th sustrt with thin chromium film is fixd on th top of prcision ir-ring spindl. Lsr powr of 1-W rgon lsr is controlld 3

4 with 1-it rsolution. Th lsr m is focusd to.6 µm spot (t ½ lvl) nd th focus is hld y srvo control. Th focusing opticl systm is movd rdilly using n ir ring stg, linr motor, nd intrfromtric position fdck. Th rng of stg displcmnt is 5 mm, th positioning prcision is nr nm rms, nd th positioning rsolution is lss 1 nm. At th xprimnts w usd sputtrd chromium films with out 7 nm thick vportd films on fusd silic sustrts. Th rcording spot vlocity is vrid from to 18 cm/s. It corrsponds to rdii of.5 mm to 3 mm t rottion spd out 1 cycls/sc. Th xposd films wr dvlopd (tim is out 3 min) in th slctiv tchnt [1] consisting of 6 prts of 5% solution of K 3 F(CN) 6 nd 1 prt of 5 % solution of NOH. Invstigtion of xposd pttrns ws md using opticl microscop (INTERFAC) nd AFM (SOLVER PRO). Fig. 3 shows th photomicrogrphy (~x μm) of tst pttrn rcordd on th chromium films (mm rdius, 15 cm/s vlocity). Th lin spcing (or trcks) is 1.5 µm. Th gp twn groups is 1 μm in rdil dirction. Th lsr powr dcrss top-down. Th width of rcordd trcks is sn to dpnd linrly on th lsr powr s shown in Fig.. Lsr m powr ws normlizd to mximum rquird powr (P c criticl powr or powr of chromium film mlting). Trck profils (rcivd with AFM) with.9μm nd. μm width (t ½ lvl) r shown in Fig. 5. On cn s tht trck hs pkdnss (width of nm t.7 lvl) shp t minimum writing powr whrs writing spot hs nr th Gussin distriution with lrgr width. This xprimntl rsult is confirmd th thorticl rsult prsntd in Fig... P c Dirction of lsr spot scnning Fig. 3. Photomicrogrphy of tst pttrn. 1. Trck width., μm 1..6 Fig.5. Chromium trck profils rcivd with AFM. Trck width is.9 μm () nd. μm () t.7 hight lvl Writing powr, P/P c Fig.. Th width of rcordd trcks s function of writing powr. 6. Conclusions 1. Rsolution of thrmochmicl img is shown to much highr thn tht of thrml img du to rvrs xponntil (Arrnius) dpndnc of oxid grow spd nd thicknss on tmprtur. In this cs rsolution dpnds

5 on rltion twn th first drivtions 1 H H x nd 1 T. T x. Rl (xprimntl) rsolution of thrmochmicl img is vn highr du to photo thrmo chmicl fdck twn sord powr dnsity qa, oxid thicknss H (T), nd tmprtur T(qA). 3. Thorticl nd xprimntl limittion on rsolution dpnds on miniml thicknss H min of th oxid rsistnt to tching, th lss r H min, d min, t min, nd th highr is th rsolution δ. Actully H min hs n ordr of mgnitud out 1 1 nm which is much lss tht wvlngth λ. Acknowldgmnts W thnk Dmitry Shchglov for ssisting in invstigtion of chromium films with AFM nd Anloty Mlyshv for chromium film prprtion. Th work ws supportd y RFBR Grnts ( а, 7 89 ), Stt Contrct RNP , prtilly y NATO CBP. EAP.CLG 9878 nd prtilly y intrdisciplinry projct of SB RAS No 15. Rfrncs [1] P. E. Tvrdohl, V. P. Koronkvich, A. G. Polschuk t l.: 3D lsr informtion tchnologis, (Pulishr, IAE, Novosiirsk, 3) p.55. (Books) (in Russin) [] S. Ogt, M. Td, M. Yond: Appl. Opt., 33, (199) 3. (Journls) [3] V. P. Viko, S. M. Mtv: Lsr Assistd Microtchnology (Springr Vrlg, NY Hidlrg, 1998) p.391. (Books) [] A. G. Polshchuk, E. G. Churin, V. P. Koronkvich, V. P. Korolkov t l.: Appl. Opt., 38, (1999) 195. (Journls) [5] Hrun t l.: Appl. Opt., 9, (199) 51. (Journls) [6] V. P. Viko: Lsr trtmnt of thin films, (Pulishr, Lningrd, Mshinostroni, 1986) p.18. (Books) (in Russin) [7] V. P. Viko, G. A. Kotov, M. N. Linson nd M. N. Nikitin: Sov Plys. Dokldy, 18, N1, (1973) 83. (Journls) (in Russin) [8] M. N. Linson: Lsr inducd opticl nd thrml procsss in condnsd mdiums nd thr intrfrnc, (Pulishr, Sint Ptrsurg, Nuk, 7) p.3. (Books) (in Russin) [9] G. A. Kotov, M. N. Linson: Elctronic nginring. Sris 6,, N (1973) 56. (Journls) (in Russin) [1] V.P. Koronkvich, A.G. Polshchuk, E.G. Churin, Y.I. Yurlov. Lsr thrmochmicl thchnology of synthsis of diffrctiv opticl lmnts in chromium films. Quntum Elctronics, , No. pp (Rcivd: July 9, 8, Accptd: Dcmr 3, 8) 5