FLUOROCARBON ETCHING OF POROUS SILICON DIOXIDE: PLASMA CHEMISTRY AND SURFACE KINETICS

Transcription

1 FLUOROCARON ETCHING OF POROUS SILICON DIOXIDE: PLASMA CHEMISTRY AND SURFACE KINETICS Avnd Sankaan, Alex Vasenkov and Mak J. Kushne Unvesty of Illnos Depatment of Electcal and Compute Engneeng Ubana, IL Octobe 2002 ADVMET_1002_01

2 AGENDA AND ACKNOWLEDGEMENTS Fluoocabon plasma etchng of delectcs Descpton of modelng heachy Scalng laws fo sold and poous delectc etchng Concludng Remaks Acknowledgements: Pof. Gottleb Oehlen Semconducto Reseach Copoaton, Natonal Scence Foundaton, Sematech, Appled Mateals ADVMET_1002_02 Unvesty of Illnos Optcal and Dschage Physcs

3 FLUOROCARON PLASMA ETCHING: DIELECTRICS Fluoocabon plasma etchng of delectcs, and selectvty wth espect to conductos, s one of the fst plasma pocessng technologes. Ealest woks of Cobun and Wntes addessed CF 4 etchng of S and SO 2. In spte of longevty, fluoocabon plasma etchng s stll the foemost pocess fo obtanng selectvty between delectcs (e.g., SO 2, S 3 N 4 ) and undelyng conductos (e.g., S, p-s). Optmzaton of these pocesses s ctcal as delectcs thn and selectvty equements become exteme. The use of low-k delectcs fo nteconnect wng wth new mateals has bought new challenges. ADVMET_1002_03 Unvesty of Illnos Optcal and Dschage Physcs

4 FLUORCARON PLASMA ETCHING: SELECTIVITY Selectvty n fluoocabon etchng eles on polyme deposton. Electon mpact dssocaton of feedstock fluoocabons poduce polymezng adcals and ons, esultng n polyme deposton. e A/C4F8 CFn, M CFn, M COFn, SFn CFn, M SFn CFx Polyme CFx Polyme SO2 S Compound delectcs contan oxdants whch consume the polyme, poducng thnne polyme layes. Thcke polyme on non-delectcs estct delvey of on enegy (lowe etchng ates). ADVMET_1002_04 Unvesty of Illnos Optcal and Dschage Physcs

5 FLUORCARON PLASMA ETCHING: SELECTIVITY Low bas: Deposton Hgh bas: etchng Etch Rate (SO 2 > S) Polyme Thckness (SO 2 < S) G. Oehlen, et al., JVSTA 17, 26 (1999) ADVMET_1002_05 Unvesty of Illnos Optcal and Dschage Physcs

6 PLASMA ETCHING OF LOW-K DIELECTRICS Low delectc constant (low-k) delectcs ae geneally classfed as noganc, oganc o hybd. Inogancs ae etched usng fluoocabon chemstes; ogancs ae etched usng oxygen chemstes. ADVMET_1002_06 Sold State Technology, May 2000 Unvesty of Illnos Optcal and Dschage Physcs

7 POROUS SILICON DIOXIDE Poous SO 2 (xeogels) have low-k popetes due to the lowe mass densty esultng fom (vacuum) poes. Typcal poostes: 30-70% Typcal poe szes: 2-20 nm Poous SO 2 (P-SO 2 ) s, fom a pocess development vewpont, an deal low-k delectc. Extensve knowledge base fo fluoocabon etchng of conventonal non-poous (NP-SO 2 ). No new mateals (though most P-SO 2 contans some esdual ogancs) Few new ntegaton equements ADVMET_1002_07 Unvesty of Illnos Optcal and Dschage Physcs

8 ETCHING OF P-SO 2 : GENERAL TRENDS Etchng of Poous SO 2 typcally poceeds at a hghe ate than NP-SO 2 fo the same condtons due to the lowe mass densty. When coectng fo mass, etch ates ae ethe lage o smalle than NP- SO 2, dependng on poosty, poe sze, polymezaton. ADVMET_1002_08 Standaet et al, JVSTA 18, 2742 (2000). Unvesty of Illnos Optcal and Dschage Physcs

9 MODELING OF FLUOROCARON PLASMA ETCHING Ou eseach goup has developed an ntegated eacto and featue scale modelng heachy to model plasma pocessng systems. HPEM (Hybd Plasma Equpment Model) Reacto scale 2- and 3-dmensonal ICP, CCP, MERIE, ECR Suface chemsty Fst pncples MCFPM (Monte Calo Featue Pofle Model) Featue scale 2- and 3-dmensonal Fluxes fom HPEM Fst pncples ADVMET_1002_09 Unvesty of Illnos Optcal and Dschage Physcs

10 HYRID PLASMA EQUIPMENT MODEL MAGNETO- STATICS MODULE MATCH OX-COIL CIRCUIT MODEL ELECTRO- MAGNETICS FREQUENCY DOMAIN ELECTRO- MAGNETICS FDTD σ(,z) J(,z,φ) ID(cols) (,z) E(,θ,z,φ) (,θ,z,φ) ELECTRON MONTE CARLO SIMULATION ELECTRON EAM MODULE ELECTRON ENERGY EQUATION OLTZMANN MODULE NON- COLLISIONAL HEATING ON-THE-FLY FREQUENCY DOMAIN Es(,z,φ) N(,z) S(,z,φ) Te(,z,φ) µ(,z,φ) CONTINUITY MOMENTUM ENERGY LONG MEAN FREE PATH (MONTE CARLO) SPUTTER MODULE POISSON ELECTRO- STATICS AMIPOLAR ELECTRO- STATICS SIMPLE CIRCUIT MODULE SHEATH MODULE S(,z,φ) Es(,z,φ) Φ(,z,φ) s(,z) Φ(,z,φ) V(f),V(dc) MONTE CARLO FEATURE PROFILE MODEL IAD(,z) IED(,z) PLASMA CHEMISTRY MONTE CARLO SIMULATION MESO-SCALE MODULE SURFACE CHEMISTRY MODULE EXTERNAL CIRCUIT MODULE VPEM: SENSORS, CONTROLLERS, ACTUATORS SNLA_0102_39 Unvesty of Illnos Optcal and Dschage Physcs

11 Unvesty of Illnos Optcal and Dschage Physcs ELECTROMAGNETICS MODEL AVS01_03 The wave equaton s solved n the fequency doman usng spase matx technques (2D,3D): Conductvtes ae tenso quanttes (2D,3D): ( ) ( ) t J E t E E E = σ ε µ µ ))) ( ( )exp( ( ), ( t E t E ω ϕ = ( ) m e 2 o m 2 z 2 z z z 2 2 z z z m o m n q m q E j 1 q m ν σ ν ω α σ α α α α α α α α α α α ν σ σ θ θ θ θ θ θ θ = = = =, / v

12 Contnuum (2D,3D): 3 2 ELECTRON ENERGY TRANSPORT 5 n ekte / t = S 2 ( Te ) L( Te ) ΦkTe κ ( Te ) Te SE whee S(T e ) = Powe deposton fom electc felds L(T e ) = Electon powe loss due to collsons Φ = Electon flux κ(t e ) = Electon themal conductvty tenso S E = Powe souce souce fom beam electons Powe deposton has contbutons fom wave and electostatc heatng. ( t) f ε,, Knetc (2D,3D): A Monte Calo Smulaton s used to deve ncludng electon-electon collsons usng electomagnetc felds fom the EMM and electostatc felds fom the FKM. SNLA_0102_41 Unvesty of Illnos Optcal and Dschage Physcs

13 Unvesty of Illnos Optcal and Dschage Physcs PLASMA CHEMISTRY, TRANSPORT AND ELECTROSTATICS Contnuty, momentum and enegy equatons ae solved fo each speces (wth jump condtons at boundaes) (2D,3D). AVS01_ 05 Implct soluton of Posson s equaton (2D,3D): ( ) ( ) = Φ q t - - s N q t t φ ρ ε S N t N = ) v ( ( ) ( ) ( ) ( ) v E m N q N v v kn T m t N v µ = 1 ( ) j j j j m m j v v N N m j ν ( ) ) ( ) U ( U Q E m N q N P t N ω ν ν ε ε = ± j j j j j j j j j j s T k R N N T T k R N N m m m E m N q 3 ) ( ν

14 MONTE CARLO FEATURE PROFILE MODEL (MCFPM) The MCFPM pedcts tme and spatally dependent pofles usng enegy and angulaly esolved neutal and on fluxes obtaned fom equpment scale models. Abtay chemcal eacton mechansms may be mplemented, ncludng themal and on asssted, sputteng, deposton and suface dffuson. Enegy and angula dependent pocesses ae mplemented usng paametc foms. Mesh centeed dentfy of mateals allows bual, ovelayes and tansmsson of enegy though mateals. SCAVS_1001_08 Unvesty of Illnos Optcal and Dschage Physcs

15 f IASED INDUCTIVELY COUPLED PLASMAS Inductvely Coupled Plasmas (ICPs) wth f basng ae used hee. < 10s mto, 10s MHz, 100s W kw, electon denstes of cm SOLENOID DOME HEIGHT (cm) 0 GAS INJECTORS ULK PLASMA f IASED SUSTRATE WAFER PUMP PORT RADIUS (cm) s COILS s POWER SUPPLY ADVMET_1002_10 POWER SUPPLY Unvesty of Illnos Optcal and Dschage Physcs

16 TYPICAL ICP CONDITIONS: [e] FOR C 4 F 8, 10 mtorr An ICP eacto pattened afte Oehelen, et al. was used fo valdaton. Reacto uses 3-tun col (13.56 MHz) wth f based substate (3 MHz) Electon denstes ae cm -3 fo 1.4 kw. C 4 F 8, 10 mto, 1.4 kw, MHz ADVMET_1002_11 Unvesty of Illnos Optcal and Dschage Physcs

17 POWER, C 4 F 8 DENSITY Lage powe deposton typcally esults n nea total dssocaton of feedstock gases. ADVMET_1002_12 C 4 F 8, 10 mto, 1.4 kw, MHz Unvesty of Illnos Optcal and Dschage Physcs

18 MAJOR POSITIVE IONS: C 4 F 8, 10 mtorr CF 3 CF 3, CF 2, and CF ae domnant ons due to dssocaton of C 4 F 8. C 4 F 8, 10 mto, 1.4 kw, MHz ADVMET_1002_13 Unvesty of Illnos Optcal and Dschage Physcs

19 CF 3 TEMPERATURE The on tempeatue s peaked nea the walls whee ons gan enegy dung acceleaton n the pesheath. C 4 F 8, 10 mto, 1.4 kw, MHz ADVMET_1002_14 Unvesty of Illnos Optcal and Dschage Physcs

20 I P VERSUS ICP POWER fo C 4 F 8 Extensve valdaton of the plasma models ae pefomed wth avalable data fo denstes, tempeatues and fluxes. Ion satuaton cuent deved fom the model ae compaed to expements: Ion denstes ae lage wth modeate statc magnetc felds. I (ma) C 4 F 8, 10 mto, MHz, 100 V pobe bas Expements: G. Oehlen, Pvate Comm Exp. wthout magnets Calc. Exp. wth magnets Calc Powe (W) ADVMET_1002_27 Unvesty of Illnos Optcal and Dschage Physcs

21 ION/NEUTRAL ENERGY/ANGULAR DISTRIUTIONS The end poducts of eacto scale modelng ae enegy and on angula dstbutons to the suface. In complex gas mxtues the IEADs can sgnfcantly vay fom speces to speces. ADVMET_1002_28 A/C 4 F 8, 40 mto, 10b MHz, MERIE Unvesty of Illnos Optcal and Dschage Physcs

22 SURFACE KINETICS DURING S/SO 2 ETCHING Fluoocabon etchng of SO 2 eles on a polymezaton and chemcally enhanced sputteng. C x F y passvaton egulates delvey of pecusos and actvaton enegy. Chemsopton of CF x poduces a complex at the oxde-polyme nteface. 2-step on actvated (though polyme laye) etchng of the complex consumes the polyme. Actvaton scales nvesely wth polyme thckness. Etch pecusos and poducts dffuse though the polyme laye. Plasma C x F y F CF n CF 4 I SF n, CO 2 CF x I, F CO 2 I, F SF n In S etchng, CF x s not consumed, esultng n thcke polyme layes. Passvaton C x F y Laye CF n CF n SO 2 SO 2 SO 2 SF x CO 2 SF x SCAVS_1001_07 Unvesty of Illnos Optcal and Dschage Physcs

23 Etch Rate (nm/mn) ETCH RATES AND POLYMER THICKNESS Etch ates fo S and SO 2 ncease wth nceasng bas due, n pat, to a decease n polyme thckness. The polyme s thnne wth SO 2 due to ts consumpton dung etchng, allowng fo moe effcent enegy tansfe though the laye and moe apd etchng Expement Model SO Self-as Voltage (-V) S Polyme Layes C 2 F 6, 6 mto, 1400 W ICP, 40 sccm λ Polyme SO2 S Self-as Voltage (-V) Tansfe Coeffcent (λ) Exp. Ref: T. Standaet, et al. J. Vac. Sc. Technol. A 16, 239 (1998). ADVMET_1002_15 Unvesty of Illnos Optcal and Dschage Physcs

24 POLYMERIZATION AIDS SELECTIVITY Less consumpton of polyme on S elatve to SO 2 slows and, n some cases, temnates etchng, povdng hgh selectvty S Inteface SO Etch Depth (µm) TIME (mn) ADVMET_1002_16 Unvesty of Illnos Optcal and Dschage Physcs

25 TAPERED AND OWED PROFILES In hgh aspect ato (HAR) etchng of SO 2 the sdewall of tenches ae passvated by neutals (CF x, x 2) due to the boad angula dstbutons of neutal fluxes. PR SO2 Ethe tapeed o bowed pofles can esult fom a non-optmum combnaton of pocessng paametes ncludng: Degee of passvaton Ion enegy dstbuton Radcal/on flux composton. OWED TAPERED ADVMET_1002_17 Unvesty of Illnos Optcal and Dschage Physcs

26 PROFILE TOPOLOGY: NEUTRAL TO ION FLUX RATIO The etch pofle s senstve to the ato of polyme fomng fluxes to enegy actvatng fluxes. Small atos esult n bowng, lage atos tapeng. Photoesst Wt SO2 Wb / Wt Depth Wb / Wt Depth (µm) Wb Φn / Φon Φn/Φon = SCAVS_1001_12 Unvesty of Illnos Optcal and Dschage Physcs

27 PROFILE TOPOLOGY: ENGINEERING SOLUTIONS Knowledge of the fundamental scalng paamete fo contollng sdewall slop enables engneeng solutons and eal-tme-contol optons. Example: A/C 2 F 6 ato contols polymezng/on flux ato, and hence pofle topology. A/C2F6 = 0/100 20/80 40/60 60/40 Photoesst SO2 Φn / Φon Wb / Wt 4 Φn/Φon A Facton Wb / Wt Φn/Φon= 12 SCAVS_1001_ Unvesty of Illnos Optcal and Dschage Physcs

28 ASIDE ON REACTION MECHANISMS Lack of fundamental paametes often eques calbaton of models usng desgn of expement methodologes. In tun, the domnant, ate lmtng pocesses ae detemned. ADVMET_1002_19 Expement (C. Cu, AMAT) Smulaton SO 2 Etch Rate (nm/mn) A/C 2 F 6 = 20/80, 1000 W ICP, 150 V bas σ = Self-as Voltage (-V) Paametezaton of CF 2 stckng pobablty Unvesty of Illnos Optcal and Dschage Physcs

29 ASIDE ON REACTION MECHANISMS A eacton mechansm s smply a set of eactons wth fundamental coeffcents and pobabltes whch should not depend on the chemsty (e.g., CHF 3 vs C 2 F 6 vs C 4 F 8 ) The chemsty meely detemnes the magntude of the fluxes but not the eacton pathways. An etch mechansm vald fo C 2 F 6 plasmas should, wth no change, also be vald fo C 4 F 8 plasmas. Development of eacton mechansms acoss dffeent chemstes should esult n moe elable mechansms. ADVMET_1002_20 Unvesty of Illnos Optcal and Dschage Physcs

30 CALIRATION OF REACTION MECHANISM: I The mechansm was valdated by compason to expements by Oehlen et al usng C 2 F 6 gas chemsty Theshold fo SO 2 etchng was well captued at selfbas 20 V. Fo S the etch ates wee lowe due to thcke polyme. Etch Rate (nm mn -1 ) Model - M Expemental - E SO2-M SO2-E S-M S-E Self as (V) 1 J. Vac. Sc.Technol. A 17, 26 (1999) ADVMET_1002_21 Unvesty of Illnos Optcal and Dschage Physcs

31 Etch Rate (nm mn -1 ) Etch Rate (nm/mn) Model-M Expemental-E Self as (V) Model - M Expemental - E SO2 - E SO2-E S-E SO2-M S-M SO2 - M CALIRATION OF REACTION MECHANISM: II Theshold fo SO 2 and S etchng wee well captued at fo CHF 3. Dffeences between model and expements fo SO 2 ae attbuted to H adcals fomng hydocabon polyme chans. Ths s accounted fo n the model by modfyng sputteng ates to account fo mass dffeences ADVMET_1002_22 Self as (V) Unvesty of Illnos Optcal and Dschage Physcs

32 WHAT CHANGES WITH POROUS SO 2? The openng of poes dung etchng of P-SO 2 esults n the fllng of the vods wth polyme, ceatng thcke layes. Ions whch would have othewse ht at gazng o nomal angle now ntesect wth moe optmum angle. An mpotant paamete s L/a (polyme thckness / poe adus). Adapted: Standaet, JVSTA 18, 2742 (2000) ADVMET_1002_18 Unvesty of Illnos Optcal and Dschage Physcs

33 ETCH PROFILES IN SOLID AND POROUS SO 2 Poous SO 2 s beng nvestgated fo lowpemttvty delectcs fo nteconnect wng. In polymezng envonments wth heavy sdewall passvaton, etch pofles dffe lttle between sold and poous slca. Poston (µm) Sold MJK_RESEARCH_0207 Poston (µm) Poosty = 45 % Poe adus = 10 nm The open sdewall poes quckly fll wth polyme. Unvesty of Illnos Optcal and Dschage Physcs

34 ETCHING OF POROUS SO 2 Etch ates of P-SO 2 ae geneally hghe than fo non-poous (NP). Examples: 2 nm poe, 30% poosty 10 nm poe, 58% poosty Hghe etch ates ae attbuted to lowe mass densty of P-SO 2. CHF 3 10 mto, 1400 W Exp: Oehlen et al. Vac. Sc.Technol. A 18, 2742 (2000) ADVMET_1002_23 Unvesty of Illnos Optcal and Dschage Physcs

35 PORE-DEPENDENT ETCHING To solate the effect of poes on etch ate, coected etch ate s defned as Etch Rate (ER) p = poosty If etchng depended only on mass densty, coected etch ates would equal that of NP- SO 2. 2 nm poes L/a 1 : C-ER > ER(SO 2 ). Favoable yelds due to non-nomal ncdence may ncease ate. 10 nm poes L/a 1 : C-ER < ER(SO 2 ). Fllng of poes wth polyme decease ates. ADVMET_1002_24 coected = ER egula (1- p), Etch Rate (nm/mn) C - Coected C-2 nm Non poous C-10 nm Self as (V) Unvesty of Illnos Optcal and Dschage Physcs

36 EFFECT OF POROSITY ON LANKET ETCH RATES Etch Rate (nm mn -1 ) ~ Regula Coected 2 nm poes Poosty (%) Etch Rate (nm mn -1 ) Regula Coected 10 nm poes Poosty (%) 2 nm poes: Etch ate nceases wth poosty. 10 nm poes: Polyme fllng of poes educes etch ate at lage poostes. ADVMET_1002_25 Unvesty of Illnos Optcal and Dschage Physcs

37 EFFECT OF POROSITY ON HAR TRENCHES Photoesst Wt Regula 0.35 SO2 Wb Depth (µm) Coected Poosty = 15% 35% 45% 10 nm poes. W t = 0.1 µm 0.20 ~ Poosty (%) At hghe poostes, moe oppotunty fo poe fllng poduces thcke aveage polyme layes and lowe etch ates. Coected etch ates fall below SO 2 ates when ctcally thck polyme layes ae fomed. ADVMET_1002_26 Unvesty of Illnos Optcal and Dschage Physcs

38 EFFECT OF φ n / φ on ON HAR TRENCHES φ n = total neutal flux φ on = total on flux Small values of φ n / φ on may be polyme staved, poducng lowe etch ates. Medum and lage φ n / φ on poduces thcke polyme, lowe etch ates. Inceasng φ n / φ on poduces nceasng tape. ADVMET_1002_ Photoesst SO2 φn/φon = Depth (µm) Wb/Wt Depth ~ φn/φon Wb/Wt Unvesty of Illnos Optcal and Dschage Physcs

39 EFFECT OF φ n / φ on ON POROUS HAR TRENCHES 2 nm poes. P- SO 2 s moe senstve to the consequences of vayng φ n / φ on compaed to NP-SO 2. Photoesst SO2 Fo lage values of φ n / φ on pevously enhanced etch ates (fo small poes) become depessed untl etchng fnally stops. Once tapeng begns the L/a nceases dspopotonately quckly. ADVMET_1002_30 φn/φon = Depth (µm) Depth (Nonpoous) Regula Coected Wb/Wt φn/φon Wb/Wt Unvesty of Illnos Optcal and Dschage Physcs

40 EFFECT OF PORE RADIUS ON HAR TRENCHES Photoesst SO2 Depth (µm) Depth Wb/Wt Wb/Wt Poe adus = 2nm 6nm 10nm ~ ~ Poe Radus (nm) Poosty 25%. Fo suffcently low poosty s lttle change n the etch ate o tape wth poe adus. ADVMET_1002_31 Unvesty of Illnos Optcal and Dschage Physcs

41 CONCLUDING REMARKS Etchng of poous slcon-doxde obeys many of the same scalng laws as sold mateals. Net enhancements ae seen wth low poosty; net slowng of the etch ate occus wth lage poosty (o poe sze). The ato of polyme thckness-to-poe sze appeas to detemne much of ths behavo. Thn polyme layes whch fll poes appea to be thcke. Inceased senstvty to small changes n neutal-to-on atos could make mantanng CDs moe poblematc. ADVMET_1002_32 Unvesty of Illnos Optcal and Dschage Physcs