PLASMA PHYSICS VIII. PROCESSING PLASMAS Introduction Plasmas ar usd to manufactur smiconductors, to modify th surfacs of matrials, to trat missions and wasts bfor thy ntr th nvironmnt, tc. Th plasma is a sourc of ions. Industry wants plasma dvics that ar simpl and compact that nabl procssing at high rats with high fficincis procssing to b uniform ovr larg aras In ordr to produc th bst plasma for th procss in qustion, w can control siz and shap of plasma dvic, gas mixtur, ps, V, i, B,ω which dtrmin ion and lctron dnsitis and tmpraturs, ion fluxs and nrgis. Bsids th plasma physics, thr atomic and molcular procsss within th volum of th gas and on th surfac to b undrstood as wll. Applications Dposition by sputtring targt is sourc of coating matrial DC sputtring: mtals, th targt is th cathod RF sputtring: non-conducting matrials ion-bam sputtring: ractiv sputtring:.g., TiN coatings for war rsistanc, Ti targt and N gas substrat may b biassd so ion bombardmnt modifis th growing film Dposition by Plasma-assistd CVD CVD (chmical vapour dposition) is a thrmal procss - th raction btwn gas and hot surfac. rquirs high tmpraturs Plasma-assistd (or plasma-nhancd) CVD Elctron bombardmnt of atoms and molculs in th plasma volum rsults in xcitation, ionization and dissociation thrby producing a varity of chmically ractiv spcis with vastly diffrnt proprtis from thir parnt gas. rquirs lowr tmpraturs 1
.g., TiN for war rsistanc. A gas mixtur of TiCl 4, N and H. thrmal CVD 900-1100 C (abov th softning tmpratur for stl), plasma CVD 500 C.g., Si 3 N 4 for passivation layr in smiconductor manufactur. A gas mixtur of SiH 4, N, NH 3. thrmal CVD 900 C, plasma CVD 300 C..g., diamond thin films. A diamond thin film is xcptionally hard, low lctrical conductivity, high thrmal conductivity. Our xprimnt uss a.45 GHz magntron sourc, no magntic fild. Th procss uss a 99% H, 1% CH 4 gas mixtur at a prssur of 10 s of torr. Th film is grown on a silicon wafr which is subsquntly tchd away. Th individual diamonds ar nm to µm in siz dpnding on th dtail of th dposition procss. Etching sputtr tching ractiv ion tching.g., in smiconductor manufactur, ractiv F radicals ract with Si to form volatil spcis that can b pumpd out. A gnric plasma ractor for dposition and tching DC dischargs Cathod shath
At th cathod, ions acclratd across th shath strik th cathod and caus (i) scondary mission of lctrons (ssntial to th maintnanc of th discharg - s Chaptr I) and (ii) sputtring of matrial from th cathod. This matrial coats th substrat. T >> T i in th plasma. Howvr w will suppos th lctron dnsity in th shath is small nough to ignor and th kt ions hav a small vlocity us = as thy ntr th shath. Small mans that th kintic mi nrgy of th ions as thy ntr th shath is much lss than th kintic nrgy thy gain as thy ar acclratd across th shath towards th cathod. If low prssur, thr ar no ion collisions. Th quation of continuity is nxvx Th quation of consrvation of nrgy is ( ) ( ) 1 = nu (1). ( x) + ( x) = 0 m i v φ (). Eliminat v(x) from (1) and () and obtain an xprssion for n(x). Substitut this into Poisson s quation d φ( x) n( x) = dx ε 0 and solv. Find that th potntial across th shath follows th th Child-Langmuir law and th shath thicknss φ( x) s x s 4 3, s s D φ λ, kt so th thicknss of this shath could b hundrds of λ D s. If high prssur, thr ar ion-nutral charg xchang collisions and both ions and nutrals strik th cathod. d Instad of consrvation of nrgy us v = µ E = µ φ whr µ is th mobility. dx Th ion nrgy distribution function looks lik: 3 4 Th low nrgy continuum is a rsult of ion collisions. 3
Shath nar a floating lctrod or wall. Thr ar both rapidly-moving lctrons and slowly-moving ions in th shath. Th ky quation xprsss th fact that th nt currnt (du to both ions and lctrons) to th floating lctrod is zro (S Chaptr VII, Exrcis 1). Th shath thicknss is λ D and th p.d. across it is V insufficint to acclrat th ions for sputtring. F kt VS which is Magntron Th figur shows th planar magntron. (Th figurs do not show gas inlts, vacuum pumping ports or matching ntworks.) Th magntron is usd for sputtr coating and mtallization. It is capabl of high currnt dnsitis - and fast procssing. Th magntic fild (typically 0.0 T) confins th scondary lctrons so a bright plasma ring sits abov th cathod. Ions howvr can rach th cathod and bombard it. 4
RF dischargs Capacitivly-coupld RF discharg also calld RF diod Undr th applid RF voltag th plasma-shath boundary at ach lctrod oscillats up and down. Th bulk of th plasma rmains uniform. This is th most common plasma sourc for matrials procssing. Low prssur dischargs can provid high ion nrgis for tching and high prssurs dischargs can provid low ion nrgis for dposition. Th main plasma hating mchanism ar: Ohmic hating. P = I R. In this cas th RF currnt is capacitivly-coupld across th shaths. Stochastic hating. Th shath dgs ar oscillating up and down. Elctrons striking th shath dg hav thir vlocitis changd and ovr 1 priod, thr is a nt gain in nrgy. Th word stochastic rfrs to th probabilistic natur of th lctron collisions with th shath. Advantags: simpl construction, no magntic fild rquird Disadvantags: Low ion flux and high ion nrgy (typically 100 s of V), ths cannot b varid indpndntly. If damag to th substrat is likly, procssing must b carrid out slowly. Voltag drop at shath is snsitiv to gomtry (Th total ara of th groundd surfacs is much gratr than th ara of th powrd lctrod. Th shath at th powrd lctrod thrfor has a smallr capacitanc and hnc a largr voltag drop.) Inductivly-coupld RF discharg Th spiral coil is th primary, th plasma is th scondary. 5
A multipol magntic fild can b usd to nhanc th confinmnt of th plasma. Advantags: Ion nrgy can b controlld indpndntly by applying capacitivly-coupld rf to bias th substrat. Th ion nrgis ar much lss, 10 s of V and hav a much mildr action on th substrat. Disadvantags: diamtr/hight is larg making cooling and pumping difficult non-uniform dnsity profil (ring-shapd) RF or microwav hatd dischargs Hr is an outlin of how w might calcuat th powr absorbd by th plasma. Start with th lctron momntum quation ρ v + ρv v = n ( E+ v B) p ρυv t In th last trm w hav assumd that vlocitis v i and v n ar << v. 6
and v This tim, considr oscillations, not wavs, so E = E 1 jω t x $, writ n = n + n j t = v 1 ω and considr only th first-ordr trms. (i) No magntic fild. 0 1 j t ω 1 1 1 vx = E. m jω + ν Powr absorbd pr unit volum p = j E. Now jx = n 0 v 1 x. Som car is rquird hr. To calculat th powr you must tak th ral part of j x and multiply it by th ral part of E. This givs th instantanous powr. Th tim-avragd powr absorbd (ovr on cycl) is 0 1 1 n E ν p = m ν ν + ω Not that if thr ar no collisions, thr is no powr absorbd. (ii) magntic fild p = 1 n m E 1 ν ν ν + ω ω 0 1 1 + ν ( ) ν + ( ω + ω ) c c ECR discharg (lctron cyclotron rsonanc) Not that in cas (ii) abov th powr absorbd is larg if th frquncy is nar th lctron cyclotron rsonanc frquncy. ECR is an improvmnt ovr th non-rsonant cas. Th ECR discharg uss inxpnsiv.45 GHz magntron microwav sourcs that can dlivr 0.3 to 6 kw. Th microwavs ar launchd as RHCP wavs into th high-fild rgion and ar absorbd in th rsonanc rgion whr ω = ω c. Whn f c =.45 GHz, th rsonanc magntic fild B = 0.0875 T. Th plasma cratd in th rsonanc rgion flows into th main chambr. You do. Why dos th ECR discharg us RHCP wavs travlling from th high fild rgion? Rfr to th CMA diagram in Chaptr V. 7
Hlicon plasma RF drivn antnna xcits a hlicon wav (s Chaptr VI) and a rsonant wav-particl intraction transfrs nrgy to th plasma. Th plasma flows into th main chambr. This rsults in a high dnsity plasma. Vacuum arc plasma is fully ionizd high dposition rat, low substrat tmpratur gratst drawback is th formation of macroparticls. Us magntic fild filtr. PI 3 Plasma immrsion ion implantation Ions from th plasma ar acclratd by mans of a sris of ngativ high-voltag pulss. Th bam injctd into th surfac changs th atomic composition and th structur nar th surfac. smiconductor maunufactur; now routin mtallurgy; an mrging tchnology in for cration of nw surfac alloys - not rstricd to planar surfacs. 8