Lecture 07 CVD and Oxidation

Lecture 07 CVD and Oxidation - 1 Dept. of ECE, Univ. of...

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Unformatted text preview: 1 Dept. of ECE, Univ. of Texas at Austin Evaporation Condensed Phase (solid or liquid) Gas Phase Transport R. B. Darling / EE-527 Condensed Phase (usually solid) Condensation Gas Phase Physical Vapor Deposition Dean P. Neikirk © 2001, last update February 2, 2001 • PVD: physical vapor deposition • CVD: chemical vapor deposition – vapor phase deposition • not standard IC process – liquid phase deposition • not standard IC process – crystalline, poly crystalline, amorphous – electro-deposition • “deposited” films • example: SiO2 formed by oxidation of Si substrate – typically “converted” from original substrate material • “grown” films Thin film processes – Oxidation – LPCVD – PECVD • Chemical Vapor Deposition (CVD) – Evaporation – Sputtering • Physical Vapor Deposition (PVD) – Polyimide (PI), photoresist (PR) – Spin-on glass (SOG) • Spin-on Films Thin-Film Deposition Fundamentals of Micromachining Dr. Bruce K. Gale BIOEN 6421 EL EN 5221 and 6221 ME EN 5960 and 6960 CVD, Oxidation, and Diffusion diffusion masks surface passivation gate insulator (MOSFET) isolation, insulation Dept. of ECE, Univ. of Texas at Austin hydroxyl group network former network modifier silicon non-bridging oxygen bridging oxygen 1 atm , 1000 C – Dry oxidation produces a better (more dense) oxide as compared to wet oxidation. • Si (s) + 2H2O --->SiO2 + 2H2 – Wet Oxidation • Si(s) + O2 --> SiO2 – Dry Oxidation • Formation of the oxide of silicon on the silicon surface is known as oxidation . • Thermal Oxidation is characterized by high temperatures (900 - 1200 C) . • Two main processes : Thermal Oxidation of Silicon 2 – BUT devitrification rate (i.e. crystallization) below 1000Û & QHJOLJLEOH m.p. 1732Û & JODVV LV ³XQVWDEOH´ EHORZ 1710Û & – can also find “crystal quartz” in nature vitreous silica: material is a GLASS under “normal” circumstances • C V D, evaporate, sputter – deposited: • thermal: “highest” quality • anodization – grown / “native” Formation: – – – – Uses: Dean P. Neikirk © 2001, last update February 2, 2001 • • • • Silicon Oxides: SiO2 • • • • R. B. Darling / EE-527 J. N. Stranski and L. Krastanov, Ber. Akad. Wiss. Wien 146, p. 797 (1938). (3) Stranski-Krastanov: (layers + islands): R. B. Darling / EE-527 F. C. Frank and J. H. Van der Merwe, Proc. R. Soc. London, Ser. A 198, p. 205 (1949). (2) Frank-Van der Merwe: (layer growth; ideal epitaxy): M. Volmer and A. Weber, Z. Phys. Chem. 119, p. 277 (1926). (1) Volmer-Weber: (island growth): Modes of Thin Film Growth Island Stage Coalescence Stage Channel Stage Continuous Film Stage Stages of Thin Film Growth j’ gas oxide x N0 N1 moving growth interface silicon 5 Dept. of ECE, Univ. of Texas at Austin j = −D ⋅ ∂ N oxidizer N −N ≈ −D ⋅ − 0 1 ∂x x j ’ = k ⋅ N1 = steady state j ⇒ 6 ‡ j= D ⋅ N0 x + Dk Dept. of ECE, Univ. of Texas at Austin solve for N1, sub back into flux eq N − N1 k ⋅ N1 = − D ⋅ − 0 x – k is the chemical reaction rate constant in steady state, flux in must equal flux consumed Dean P. Neikirk © 2001, last update February 2, 2001 • • silicon N − N1 ∂N =− 0 ∂x x N1 N0 is limited by the solid solubility limit of the oxidizer in the oxide! – N0O2 ~ 5 x 1016 cm-3 @ 1000Û & – N0H2O ~ 3 x 1019 cm-3 @ 1000Û & flux of oxidizer j’ at SiO2 / Si interface consumed to form new oxide concentration Oxidizer concentration gradient and flux Dean P. Neikirk © 2001, last update February 2, 2001 • x oxide • simplest approximation: gas N0 moving growth interface ∂x – supply of oxidizer is limited by diffusion through oxide to growth interface ∂N • Fick’s First Law: flux j = − D oxidizer • basic model is the Grove and Deal Model Oxide growth kinetics concentration d 0.44d SiO2 3 Dept. of ECE, Univ. of Texas at Austin + Si - Si → Si - O - Si Si - O - Si + H2 Dean P. Neikirk © 2001, last update February 2, 2001 – ρwet §    JP  FP3 4 Dept. of ECE, Univ. of Texas at Austin • this results in a more open oxide, with lower density, weaker structure, than dry oxide Si - O H Si - OH – H2O + Si-O-Si → Si-OH + Si-OH – diffusion of hydroxyl complex to SiO2 -Si interface • proposed process – Si + 2 H2O → SiO2 + H2 • overall reaction is “Wet” oxidation of Si Dean P. Neikirk © 2001, last update February 2, 2001 – it is possible to prepare a hydrogen terminated Si surface to retard this “native” oxide formation • “bare” silicon in air is “always” covered with about 15-20 Å of oxide, upper limit of ~ 40 Å original silicon surface – ρSiO2 = 2.25 gm/cm3 , GMW = 60 – ρSi = 2.3 gm/cm3 , GMW = 28 – oxide d thick consumes a layer 0.44d thick of Si • density / formula differences • does the oxygen go “in” or the silicon go “out”? – Si + O2 → SiO2 – once an oxide is formed, how does this chemical reaction continue? • in dry (<< 20 ppm H2O) oxygen Growth of SiO2 from Si A t+τ ⋅ 1+ 2 2 A 4B −1 t+τ << x (t ) ≈ A2 = 2 DN n 0 = N 0 ⋅k n 9 −1 >> x (t ) ≈ A2 2 ⋅ D ⋅ N0 n – B is the “parabolic rate constant” Dean P. Neikirk © 2001, last update February 2, 2001 10 −1 Dept. of ECE, Univ. of Texas at Austin – diffusivity of oxidizer in oxide (D) AND – solid solubility of oxidizer in oxide (N0) – temperature dependence mainly from diffusivity B ⋅ (t + τ ) A t+τ ⋅ 1+ 2 2 A 4B A t+τ = ⋅ 2 A2 4B 4B x (t ) = • characteristic of a diffusion limited process B= −1 Dept. of ECE, Univ. of Texas at Austin – dependence is “parabolic”: (thickness)2 ∝ time A t+τ ⋅ 1+ 2 2 A 4B t+τ • parabolic rate constant depends on x (t ) = • “long times” Limiting behavior of Grove & Deal oxidation model Dean P. Neikirk © 2001, last update February 2, 2001 – reaction rate between oxidizer and silicon (k) AND – solid solubility of oxidizer in oxide (N0) – temperature dependence mainly from reaction rate • linear rate constant depends on B A 2D k • characteristic of a reaction rate limited process – B/A is the “linear rate constant” A t+τ ⋅ 1+ 2 2 A 4B A t+τ B −1 = ⋅ 1 + 1 ⋅ 2 ⋅ (t + τ ) 2 2 A A 4B 4B x (t ) = – thickness is linearly increasing with time x (t ) = • “short times” Limiting behavior of Grove & Deal oxidation model j n = D N0 n x + Dk Dept. of ECE, Univ. of Texas at Austin A t+τ ⋅ 1+ 2 2 A 4B −1 = Dean P. Neikirk © 2001, last update February 2, 2001 τ 8 (xi )2 B + A⋅ xi Dept. of ECE, Univ. of Texas at Austin – where τ represents an “offset” time to account for any oxide present at t = 0 x (t ) = • integration gives – initial condition x (t = 0) = xi • function of what’s diffusing and what it’s diffusing in – 2DN0/n = B • function of what’s diffusing, what it’s diffusing in, and what it reacts with – 2D/k = A • setting 7 Grove and Deal relation Dean P. Neikirk © 2001, last update February 2, 2001 – now integrate with appropriate initial condition dx dt = ρ SiO2 ⋅ NA 2 for H2O 2 for H2O = 2.25 × 10 22 cm−3 ⋅ ⋅ GMW 2 1 for O2 1 for O2 SiO • then relation is just – # cm-3 n= • n: # of oxidizer molecules per unit volume of oxide: – cm sec-1 • rate of change of interface position: dx / dt (interface velocity) – # cm-2 sec-1 • flux: #oxidizer molecules crossing interface per unit area per unit time Relation between flux and interface position grow thick oxides at reduced time / temperature product – use elevated pressures to increase concentration of oxidizer in oxide • for steam, both B and B/A ~ linear with pressure • rule of thumb: constant growth rate, if for each increase of 1 atm pressure, temperature is reduced ~ 30Û C. – pressures up to 25 atm have been used (commercial systems: HiPOx, FOX) 13 0.2 0.1 1 1 (100) 1 atm 5 atm 10 atm 20 atm Dept. of ECE, Univ. of Texas at Austin Time (hour) 10 adapted from Sze, 2nd, p. 122 (111) Pyrogenic steam 900 º C • Atmospheric Pressure CVD (APCVD) • Low Pressure CVD (LPCVD) • Plasma Enhanced CVD (PECVD) Types of CVD Dean P. Neikirk © 2001, last update February 2, 2001 • Oxide Thickness (microns) 10-1 100 1000°C 900°C 1100°C Time (hours) 1200°C (100) silicon dry • dry oxidation Dean P. Neikirk © 2001, last update February 2, 2001 10-2 10-1 100 11 101 oxide silicon Dept. of ECE, Univ. of Texas at Austin 10-2 10-1 100 101 10-1 °C 00 10 °C 0 95 °C 0 90 Time (hours) 100 1100°C 101 Dept. of ECE, Univ. of Texas at Austin C 0° 80 1050°C 1150°C (100) silicon steam – 640 Torr partial pressure is typical (vapor pressure over liquid water @ 95Û& • wet oxidation 12 silicon k < 1, slow SiO2 diffuser oxide k > 1, slow SiO2 diffuser Oxidation thicknesses Dean P. Neikirk © 2001, last update February 2, 2001 • little effect on parabolic rate constant B • increases linear rate constant B/A – again, really only significant for Nphosphorus > ~1020 cm-3 – k = Cox / CSi ~ 0.1 – dopants “pile-up” at silicon surface • phosphorus • little effect on linear rate constant B/A ( = Nok / n) • can increase parabolic rate constant B ( = 2DNo / n ) – really only significant for Nboron > ~1020 cm-3 – k = Cox / CSi ~ 3 – dopants accumulate in oxide • boron Effect of Si doping on oxidation kinetics Oxide thickness (microns) Pressure Effects on Oxidation Oxide thickness (microns) concentration concentration CVD reactions Gases are introduced into a reaction chamber Gas species move to the substrate Reactants are adsorbed on the substrate Film-forming chemical reactions Desorption and removal of gaseous by-products – – – – Undesirable Form gas phase clusters of material Consume reactants Reduce deposition rate • Homogeneous = occur in gas phase – Desirable – Produce good quality films • Heterogeneous = occur at wafer surface – – – – – • CVD = formation of non-volatile solid film on substrate by reaction of vapor phase chemicals • Steps in CVD Chemical Vapor Deposition (CVD) λ ranges from << 1 µm to ~ 1 mm – silicon nitride • PSG, BSG, BPSG 25 Dept. of ECE, Univ. of Texas at Austin – polycrystalline silicon (poly) – silicon dioxide – phosphosilicate, borosilicate, borophosphosilicate glasses • example materials • thermal: temperatures 100Û - 1000Û & – higher temperature processes increase surface migration/mobility • plasma • optical – reactions driven by • – pressures typically atmospheric to 50 mTorr • general characteristic of gas phase chemical reactions Chemical vapor deposition (LTO) SiO2 is formed using three types of CVD Processes. APCVD ( Most commonly used method ), LPCVD and PECVD SiH4 + O2 : ----->SiO2 + 2H2 (240 - 550 C) (200 - 500 nm/min optimal) and (1400 nm/min possible). Deposition rate increases slowly with increased T (310- 450 C) Deposition rate can also be increased by increasing the O2 /SiH4 ratio APCVD : 325 C ratio 3:1 , 475 C ratio 23:1 , 550 C ratio 60: 1 LPCVD : 360 C ratio 1:1 , 450 C ratio 1.45 : 1 Deposition can occur in the APCVD as low as 130 C For LPCVD Window (100 - 330 C ) 2-12 torr and 14 nm/min at 300 C Dean P. Neikirk © 2001, last update February 2, 2001 • • • • • • • • • • Low Temperature Oxidation of Silicon load lock 26 wafers Dept. of ECE, Univ. of Texas at Austin rf in pump chemical scrubbers filters vacuum pumps graphite rf electrode furnace heater mass flow controller gas supply furnace heater wafers gas flow mass flow controller gas supply gas flow furnace heater chamber wall (tube) gas supply load lock mass flow controller gas supply plasma assisted CVD: PECVD – atmospheric: high deposition rates – low pressure (LPCVD): lower rates, good uniformity • avoid unwanted contamination, escape of hazardous materials (the reactants) thermally driven reactions – requires leak-tight, sealed system heat entire system: CVD system design: hot wall reactors Dean P. Neikirk © 2001, last update February 2, 2001 • ADVANTAGES Low temperatures Fast Deposition rates especially APCVD . Good Step Coverage especially PECVD. • DISADVANTAGES • Contamination especially PECVD. • Inferior electrical properties of PECVD films as compared with thermally grown ones. • Less dense films are obtained . • • • • /LPCVD/ PECVD vs. Thermal Oxidation of Silicon Low Temperature oxide formation by APCVD 2 4 2 2 2 2 4 2 2 • SiH + 2N O:→SiO + 2N + 2H (200- 400 C) , RF, 0.1 - 5 torr . • Low ratio of N O /SiH will increase “n” leading to formation of silicon rich films . • Lower deposition temperatures and higher ratios of N O/SiO will lead to less dense films and faster etch rates • HF etch rate is a measure of the film’s density • Densification of films PECVD – Reaction rate < reactant arrival rate – Reaction rate limited • At low temperatures – Reaction rate > reactant arrival rate – Mass-transport limited • Surface reaction rate increases with increasing temperature at very high temperature – where Ea = activation energy (eV) – k = Boltzmann constant – T = temperature (K) • R = R0 exp(-Ea/kT) CVD Reaction Rate (R) graphite susceptor pump gas inlet Dept. of ECE, Univ. of Texas at Austin Dry Oxidation: Si + O2 SiO2 Wet Oxidation: Si + 2H2O SiO2 + 2H2 SiH4 + O2 SiO2 + 2H2 SiH4 + N2O SiO2 + by-products SiCl2H2 + N2O SiO2 + by-products Si(OC2H5)4 SiO2 + byproducts • Silicon Oxide – – – – – – 29 pump gas flow rf electrodes – “pancake” configuration is similar gas inlet wafers gas flow plasma • parallel plate plasma reactor CVD Chemistries Dean P. Neikirk © 2001, last update February 2, 2001 from: http://www.appliedmaterials.com/prod ucts/pdd.html • barrel reactor • single wafer systems wafers gas flow rf excitation coil • horizontal tube reactor Basic configurations Dept. of ECE, Univ. of Texas at Austin substrate x d d= µ ⋅x ρ ⋅v 28 substrate lines gas flow Dept. of ECE, Univ. of Texas at Austin – v: velocity; ρ: density; µ: viscosity • reactant supply limited by diffusion across boundary layer • geometry of wafers relative to gas flow critical for film thickness uniformity – to improve boundary layer uniformity can tilt wafer wrt gas flow gas flow lines • causes formation of stagnant boundary layer – away from surfaces, flow is primarily laminar – friction forces velocity to zero at surfaces interaction of gas flow with surfaces • typical of LPCVD tube furnace design – reactants are well mixed, no “geometric” limitations on supply of reactants to wafer surface purely “turbulent” flow Dean P. Neikirk © 2001, last update February 2, 2001 • • 27 Gas flow in CVD systems Dean P. Neikirk © 2001, last update February 2, 2001 • inherently a non-isothermal system – more complex to achieve temperature uniformity – hard to measure temperature • disadvantages – reduces contamination from hot furnace walls – reduces deposition on chamber walls • advantages – resistive heating (pass current through “susceptor”) – inductive heating (external rf fields create eddy currents in conductive susceptor) – optical heating(lamps generate IR, absorbed by susceptor) • heat substrate “only” using Cold wall reactors • TiN often used to improve adhesion • causes long “initiation” time before W deposition begins WF6 + 3H2 D W + 6HF cold wall systems ~300Û& can be selective adherence to SiO2 problematic Dept. of ECE, Univ. of Texas at Austin – Cu β-diketones, ~100Û-200Û & copper 31 – tri-isobutyl-aluminum (TIBA) – LPCVD – ~200Û-300Û & WHQV QPPLQ GHSRVLWLRQ UDWH aluminum Dept. of ECE, Univ. of Texas at Austin – frequently used to fill deep (“high aspect ratio”) contact vias – – – – – tungsten Dean P. Neikirk © 2001, last update February 2, 2001 • • • 30 Metal CVD Dean P. Neikirk © 2001, last update February 2, 2001 • dope after deposition (implant, diffusion) • can cause substantial decrease in deposition rate – n-type: arsine AsH3, phosphine PH3 : ρ ~ 0.02 Ω-cm • can cause substantial increase in deposition rate – p-type: diborane B2H6: ρ ~ 0.005 Ω-cm (B/Si ~ 2.5x10-3) • in-situ doping • 950Û & SKRVSKRUXV GLIIXVLRQ  PLQ a —P JUDLQ VL]H • 1050Û & R[LGDWLRQ a-3 µm grain size – grain size dependent on growth temperature, subsequent processing • atmospheric, cold wall, 5% silane in hydrogen, ~1/2 µm/min • LPCVD (~1 Torr), hot wall, 20-100% silane, ~hundreds nm/min – silane pyrolysis: 600Û-700Û & 6L+4 ê Si + 2H2 • deposition – gates, high value resistors, “local” interconnects • uses Material examples: polysilicon – 2WF6 + 3SiH4 2W + 3SiF4 +6H2 Polysilicon: SiH4 Si + 2H2 Silicon Carbide Polycrystalline Diamond Parylene (polymerized p-xylylene) Refractory Metals: CVD Chemistries 3SiH4 + 4NH3 Si3N4 + 12H2 SiCl2H2 + NH3 Si3N4+ by-products SiH4 + 4N2O Si3N4 + by products SiH4 + N2 Si3N4 + by products • II-VI compounds (e.g., CdSe) • • • • • – – – – • Silicon Nitride CVD Chemistries Dept. of ECE, Univ. of Texas at Austin – – – – – 14 Dept. of ECE, Univ. of Texas at Austin • with both layer “below” and “above” • at room temperature and under deposition conditions good electrical characteristics free from pin-holes, cracks low stress good adhesion chemical compatibility general requirements – chemical vapor deposition (CVD) • thermal evaporation • sputtering – physical vapor deposition (PVD) deposition methods – both conductors and insulators need to be able to add materials “on top” of silicon Dean P. Neikirk © 2001, last update February 2, 2001 • • • 35 Deposited thin films Dean P. Neikirk © 2001, last update February 2, 2001 – double walled tubing, all welded distribution networks • helps with dispersal problem associated with gases – monitor! – limit maximum flow rate from gas sources • how to deal with this? • toxic, corrosive – ammonia • very toxic, flammable – phosphine • toxic, burns on contact with air – silane, SiH4 • most gases used are toxic, pyrophoric, flammable, explosive, or some combination of these Safety issues in CVD • • • • • 34 aSiH4 + bNH3 ê SixNyHz + cH2 aSiH4 + bN2 ê SixNyHz + cH2 Si/N ratio 0.8-1.2, ~20% H ρ ~ 2.4-2.8 g/cm3 ; n ~ 1.8-2.5; k ~ 6-9 stress: ~2C - 5T Gdyne/cm2 – PECVD: ~250Û& - 350Û& Dept. of ECE, Univ. of Texas at Austin • 3SiH4 + 4NH3 ê Si3N4 + 12H2 ; 3Si2Cl2H2 + 4NH3 ê Si3N4 + 6HCl + 6H2 • Si/N ratio 0.75, 4-8% H • ρ ~ 3 g/cm3 ; n ~ 2.0; k ~ 6-7 • stress: ~10 Gdyne/cm2, tensile – LPCVD: ~700Û& - 900Û& • in practice Si/N ratio varies from 0.7 (N rich) to 1.1 (Si rich) – stoichiometric formulation is Si3N4 deposition • protect against mobile ion contamination – diffusivity of Na also very low • mask against oxidation, protect against water/corrosion – diffusivity of O2, H2O is very low in nitride uses Dean P. Neikirk © 2001, last update February 2, 2001 • • Dept. of ECE, Univ. of Texas at Austin Silicon nitride Si3N4 32 • gate insulators, de-coupling caps – high k dielectrics: k > ~25-100’s • interlevel insulation with lower dielectric constants (k < ~3) – fluorinated oxides, spin-on glasses, organics – low “k” dielectrics new materials • Si(OC2H5)4 ê SiO2 + by-products – tetraethyl orthosilicate (TEOS) high temperature: ~700Û& – low temperature: ~250Û& plasma-enhanced reaction (PECVD) – SiH4 + O2 ê SiO2 + H2 – cold-wall, atmospheric, ~0.1 µm/min – hot-wall, LPCVD, ~0.01 µm/min • “LTO” (low-temp. oxide) T < ~500Û& – mid-temperature: ~ 500Û& thermally driven reaction Dean P. Neikirk © 2001, last update February 2, 2001 • • • • CVD silicon dioxide λ 3.5 x 1013 3.5 x 1010 3.5 x 107 3.5 x 104 10-3 10-6 10-9 10-12 rough vacuum high vacuum 100 % 98 % λ << system & steps isotropic arrival on ALL surfaces • flat surfaces: 180Û • inside corners: 90Û « thinner • outside corners: 270Û « thicker – – randomizing collisions 180Û LQFLGHQFH randomizing collisions 18 Dept. of ECE, Univ. of Texas at Austin assume material does NOT migrate after arrival!! substrate 180Û 270Û LQFLGHQFH source Dept. of ECE, Univ. of Texas at Austin 29 days 42 min 2.5 sec 2.5 msec 3.3 nsec τ material arrival angular distribution – depends on mean free path compared to both size of system and size of wafer “steps” Case I: “atmospheric pressure”: 760 Torr « λ = 0.07 µm Dean P. Neikirk © 2001, last update February 2, 2001 • • 17 50,000 km 50 km 50 m 2x10-7 % ~0 nd=1m Impact of pressure on deposition conditions Dean P. Neikirk © 2001, last update February 2, 2001 very high vacuum 2.7 x 1019 760 5 cm 0.07 µm number density (#/cm3) pressure (Torr) – mean free path: λ ∝ 1/P – “contamination rate” : τ ∝ 1/P • pressure influences Impact of pressure on deposition conditions − d λ λ= Dept. of ECE, Univ. of Texas at Austin c= kT 2πm ideal gas law P ⋅ V = n⋅k⋅T { unit vol P kT 13 2 ⋅c = Dean P. Neikirk © 2001, last update February 2, 2001 16 – # molecules per unit area / bombard rate • monolayer formation time τ • P in Torr, M is gram-molecular mass – j = 3.4 x 1022 (# / cm2 • sec) • P / ¥07 = • rate of surface bombardment (flux) unit j gas (# / unit area ⋅ time ) = n gas volume ⋅ velocity – ~ 900 miles/hour at rm temp – we’ll use 2.6 × 10 −6 sec P (in Torr ) 1015 cm −2 j Dept. of ECE, Univ. of Texas at Austin ≈ τ≈ P 2 π m kT • for ideal gas, velocity distribution is Maxwellian Velocity distribution 15 "area " of molecule temperature } k⋅ T P π2 { ⋅ 1⋅ σ ⋅ 2 2 3 pressure • at room temp λ ~ 0.7 cm / P (in pascals) ~ 5.3 x 10-3 cm / P (in Torr) • at room temp and one atmosphere λ ~ 0.07 µm λ is the mean free path =e Dean P. Neikirk © 2001, last update February 2, 2001 – n gas n no collisions • fraction of molecules traveling distance d without colliding is • one atmosphere = 1.0132 x 105 pascal = 760 Torr (mm Hg) – 1 Pascal = 1/132 Torr ~ 10-5 atms – N ~ 2.7 x 1019 molecules/cm3 – N ∝ pressure • for a gas at STP: Kinetic theory of gases 33 4.3 εr (low freq.) 4.9 adapted from Sze, 2nd, p. 259. Dean P. Neikirk © 2001, last update February 2, 2001 1.44 1.47 index of refraction (632.8 nm) 8 3T 2.1 3-6 3C - 3T dielectric strength (MV/cm) stress (Mdyne/cm2) density (g/cm ) 2.3 loses H thermal stability densifies nonconformal nonconformal step coverage 3 SiO2 (H) SiO1.9 (H) composition ~450°C ~200°C SiH4 + O2 temperature plasma 4.0 1.46 10 1C 2.2 stable 3.9 1.46 11 3C 2.2 stable "conformal" SiO2 ~1000°C thermal Dept. of ECE, Univ. of Texas at Austin conformal SiO2 ~700°C TEOS summary of SiO2 characteristics top flat surface: 180Û “inside” surface: depends on location! shadowing by corners of features “anisotropic” deposition – – – 19 no randomizing collisions source 20 Dept. of ECE, Univ. of Texas at Austin assumes material does NOT migrate after arrival!! substrate – geometric “shadowing” dominates anisotropic arrival at all surfaces! – long compared to almost everything case III: 10-5 Torr « λ = 5 meters anisotropic deposition – “line-of-sight” deposition – very thin on “side walls” – very dependent on source configuration relative to sample surface source Dept. of ECE, Univ. of Texas at Austin “vacuum” conditions: λ > system, λ >> step Dean P. Neikirk © 2001, last update February 2, 2001 • • • 180Û LQFLGHQFH randomizing collisions 180Û LQFLGHQFH assumes material does NOT migrate after arrival!! substrate no randomizing collisions BUT no scattering inside “hole”!! – small compared to system, large compared to wafer features – isotropic arrival at “flat” surface Case II: 10-1 Torr « λ = 0.5 mm Dean P. Neikirk © 2001, last update February 2, 2001 • • • “low” pressure: λ << system, λ > step ...
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This note was uploaded on 11/16/2011 for the course MSE 5960 taught by Professor Douglas during the Fall '04 term at University of Florida.

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