Chaptr 3. Thin-Film Evaporation Procsss 1. Physical Vapor Dposition (PVD) Atoms ar rmovd from th sourc (targt) Controllably transfr atoms from a sourc to a substrat whr film formation and growth procd atomistically - Evaporation by thrmal mans - Sputtring by gasous ions 1
I. 1. Evaporation Rat asic quation for th rat of vaporation from both liquid and solid surfacs, vaporation flux Φ α N A ( ) P π R T P h α : cofficint of vaporation P : quilibrium prssur P h : hydrostatic prssur Φ : # of atoms/ unit ara tim
aximum valu of Φ α 1 and P h 0 Φ 3.513 10 P T ( molculs / cm sc ) mass vaporation rat Γ 5.84 10 T P ( g / cm sc ) P in Torr 3
I. Vapor prssur of th lmtnt Clausius Clapyron quation : connction btwn tmpratur and vapor prssur For both solid-vapor and liquid-vapor quilibrium dp dt H T V ( T ) dp P H ( T ) dt RT Sinc V V V V, v H ( T ) H : th molar hat of vaporation c V V v ( RT P constant ) ln P H R T + I For practic Φ 10-5 g/cm sc 10-3 Torr at P is ndd. 4
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I.3 Evaporation of Compounds tals : vaporat as atoms and occasionally as clustrs of atoms Compounds : most inorganic compounds vaporat with molcular chang stoichiomtry of th film dposit will diffr from that of th sourc. 6
I.4 Evaporation of Alloys - Elctronic, magntic, optical, dcorativ applications Al-Cu, Prmalloy (F-Ni), nichrom (Ni-Cr), Co-Cr, Th constitunt of th alloys vaporat narly indpndnt of ach othr - E A E AA E - Idal solution Raoult s law P ( X X P ( 0) mol fraction of tallic solutions usually ar not idal in solution) P a a P ( 0) activity : ffctiv thrmodymic concntration of 7
γ X a γ activity cofficint - Th ratio of th fluxs of A and atoms in th vapor stram abov th mlt is givn by Φ Φ A γ A γ X X A P P A ( 0) ( 0) A 8
< 예제 > For films (Al-Cu) with wt % Cu, assuming Φ Φ Al Cu 98 / / Al Cu P P Al Cu (0) (0) 3 10 10 4 X X Al cu 98 10 10 3 4 63.7 7 15 It is ncssary to hav 13.6 wt % Cu in sourc alloy at 1350K - lt composition usually changs as vaporation procds 9
I-5. Film Thicknss Uniformity I.5.1 Dposition Gomtry Th Sourc Substrat gomtry influncs th ultimat film uniformity Point sourc 10
: total vaporatd mass da c t 0 da s A Γ cosθ da dt d d da s s s : da c : 4πr cosθ : mass 4πrr dpositd pr unit ara` 11
From kintic thory & xprimnt for surfac sourc d da s s cosφ cosθ π r cosin distribution law or ralistically ; cos n φ vaporation law d s das n ( n + 1)cos φ cosθ ( n πr 0) - n is rlatd to vaporation crucibl qomtry and th scals with th ratio of th mlt dpth to th mlt surfac ara. - whn n is larg, th vapor flux is highly dirctd. 1
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1.5. Thicknss Distribution Film thicknss d d, ρ dnsity of ρda s s dposits For th point sourc ; d d d o cosθ 4πρr {1 + ( l 1 / h) h 3 4πρr } 3/, 4πρ( h h + l ) 3/ thickst dposit d o at l 0 14
For th surfac sourc ; d d d o {1 + cosθ cosφ πρr 1 l ( ) h } πρr h r h r h πρ( h + l ) 15
- lss thicknss uniformity with th surfac sourc -uniformity as h but wast of vaporant Rotating Disk 16
A clvr way to achiv thicknss uniformity - surfac sourc and substrats locat on th surfac of a sphr d da S S cosφ cosθ π r 4π r o 17
Conformal covrag - smiconductor contact - intrconnction mtallization Computr modling of stp covrag sourc of failur in dvic 18
1.6 Film purity From background gas Φ 3.5 10 (atoms cm Φ S b From sourc ρ N A a d sc P 1 b b ) T atoms cm C Impurity concntration in th film d :dposition rat d sc i 1 P b b T ρ a 19
Traditionally vaporation producs clanr film than sputtring bcaus of highr d and lowr Pb Gratly improvd in magntron sputtring ths days and givs similar rsults in cass of vaporation and sputtring of Al 0
3.4 Evaporation Hardwar 3.4.1 Elctrically hatd vaporation sourcs - Tungstn wir sourcs - Rfractory mtal sht sourcs - Sublimation furnacs - Crucibl sourcs - Estimating th tmpratur of rsistanc hatrs n p i R i ρ(0)[ T / T (0)] L / A n 1 c Stfan-oltzmann law for radiatd powr p r εσa ( T T (0) s 4 4 ) Emissivity Stfan s constant 1
3.4. Elctron-bam vaporation
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Disadvantags of rsistivly hatd vaporation sourcs : - Contamination by crucibls, hatrs, and support matrials - Limitation of rlativly low input powr lvls Thrfor, -bam vaportion bcoms th prfrrd vacuum vaporation tchniqu for dpositing films. Th vaporant charg is placd in th dprssion of a watrcoold coppr harth. Acclrat th lctrons by 4KV-0KV, dflctd 70oC by a transvrs magntic fild. 4
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3.5. Pulsd lasr dposition(pld) - Gas xcimr lasrs : ArF(193nm), KrF(48nm), XCl(308nm) - Lasr output powr of ~ 500 mj/puls - Puls rptition rats of svral hundrd Hz - Th absorbd bam nrgy is convrtd into thrmal, chmical, and mchanical nrgy, causing lctronic xcitation of targt atoms, ablation and xfoliation of th surfac, and plasma formation. - Evaporants form a plum abov targt, consiting of a motly collction of nrgtic nutral atoms, molculs, ions, lctrons, atom clustrs, mincron sizd particulats, and moltn droplts. Th plum is highly dirctional, i.., cos n φ, whr 8 < n < 1 30
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Th thicknss of matrial vaporatd pr lasr puls: Γ 5.84 10 T P ( g / cm sc ) 1 d 5.84 ρ 10 T P ( cm sc ) For Al, T 793K, P 760torr, vaporation rat0.0436 m/s for a 10ns lasr puls, vaporation rat is 0.4nm/puls Typical valus ar 1-10 nm/puls 33
3.5.4 Ion am Assistd Dposition(IAD) To improv th film proprtis, - Altrnat substrat tmpratur - Employ ion bombardmnt of th substrat - Us of broad-bam (Kaufman) ion sourc - Unlik plasma, indpndntly control ion flux and nrgy - 1mA/cm 6.5 10 15 ions/cm sc, low nrgy (10v-1KV) nhancmnt of th dnsity and indx of rfraction of optical coating - control strss, hardnss, adhsion, rfractiv indx, stp covrag 34
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