Lecture06-Thin Film Deposition

EE143 – Fall 2016 Microfabrication Technologies Lecture 6: Thin Film Deposition Reading: Jaeger Chapter 6 Prof. Ming C. Wu <a href="mailto:[email protected]" target="_blank">[email protected] </a> 511 Sutardja Dai Hall (SDH) 1Vacuum Basics • Units – 1 atmosphere = 760 torr = 1.013x105 Pa – 1 bar = 105 Pa = 750 torr – 1 torr = 1 mm Hg – 1 mtorr = 1 micron Hg – 1 Pa = 7.5 mtorr = 1 newton/m2 – 1 torr = 133.3 Pa • Ideal as Law: PV = NkT – k = 1.38E-23 Joules/K = 1.37E-22 atm cm3/K – N = # of molecules (note the typo in your book) – T = absolute temperature in K 21Dalton’s Law of Partial Pressure • For mixture of non-reactive gases in a common vessel, each gas exerts its pressure independent of others • ꢀꢁꢂꢁꢃꢄ = ꢀꢅ + ꢀꢆ + ⋯ . +ꢀꢇ – Total pressure = Sum of partial pressures • ꢇꢁꢂꢁꢃꢄ = ꢇꢅ + ꢇꢆ + ⋯ . +ꢇꢇ – Total number of molecules = sum of individual molecules • Ideal gas law observed by each gas, as well as all gases – ꢀꢅꢈ = ꢇꢅꢉꢊ – ꢀꢆꢈ = ꢇꢆꢉꢊ – ꢀꢇꢈ = ꢇꢇꢉꢊ 3Average Molecular Velocity • Assumes Maxwell- Boltzman Velocity Distribution ꢌꢉꢊ 3ꢋ = ꢍꢎ • where m = molecular weight of gas molecule 42Mean Free Path between collisions ꢉꢊ ꢏ = ꢆꢍꢐꢆꢀ • where – K = Boltzmann constant – T = temperature in Kelvin – d = molecular diameter – P = pressure • For air at 300K <ul style="display: flex;"><li style="flex:1">ꢓ. ꢓ </li><li style="flex:1">ꢔ. ꢔꢕ </li></ul><ul style="display: flex;"><li style="flex:1">ꢏ(ꢑꢒ ꢎꢎ) = </li><li style="flex:1">=</li></ul>ꢀ (ꢑꢒ ꢀꢃ) ꢀ (ꢑꢒ ꢁꢂꢖꢖ) 5Impingement Rate • ꢗ = number of molecules striking a surface per unit area per unit time 1/cm2-sec] ꢀ ꢗ = ꢘ. ꢕ×ꢅꢔꢆꢆ ꢙꢊ • where – P = pressure in torr – M = molecular weight 63Question • How long does it take to form a monolayer of gas on the surface of a substrate? 7Vacuum Basics (Cont.) At 25oC PIM1 µm/min Plasma Processing Residual Vacuum CVD Pressure (Torr) 84Thin Film Deposition Physical Methods Chemical Methods Evaporation Sputtering Chemical Vapor Deposition (CVD) Atomic Layer Deposition (ALD) Film substrate • Applications – Metalization (e.g., Al, TiN, W, Silicide) – Polysilicon – Dielectric layers (SiO2, Si3N4) 9Evaporation wafer wafer Al vapor Al Al vapor deposited Al fill deposited Al fill ecrucible is water cooled heating hot boat (e.g. W) electron source <ul style="display: flex;"><li style="flex:1">Thermal Evaporation </li><li style="flex:1">Electron Beam Evaporation </li></ul>as Pressure: 10-5 Torr 10 5Evaporation: Filament & Electron Beam (a) Filament evaporation with loops of wire hanging from a heated filament (b) Electron beam is focused on metal charge by a magnetic field 11 Sputtering Negative Bias ( kV) IAl target Ar+ Ar+ Al Al Ar plasma Al Deposited Al film wafer heat substrate to ~ 00oC (optional) •as pressure 1 to 10 mTorr • Deposition rate = constant × ꢚ × ꢛ – Where ꢚ = ion current –ꢛ = sputtering yield 12 6Plasma Basics 13 Basic Properties of Plasma • The bulk of plasma contains equal concentrations of ions and electrons. »• Electric potential is constant inside bulk of plasma. The voltage drop is mostly across the sheath regions • Plasma used in IC processing is a “weak” plasma, containing mostly neutral atoms/molecules. – Degree of ionization is »10-3 to 10-6 14 7Outcomes of Plasma bombardment 15 Sputtering Yield Al Ar Al Al Sputtering ieel S # f jej e cj e c rgjc c om S º in e oing i n n <ul style="display: flex;"><li style="flex:1">0.1 </li><li style="flex:1">S</li><li style="flex:1">0</li></ul>16 8Sputtering of Compound Targets AxBy Ar+ Aflux Bflux Target ¹Because SA SB, target surface will acquire a composition Ax’By’ at steady state. 17 Reactive Sputtering Sputtering deposition Ti Target while introducing a reactive gas into the plasma. N2 plasma Example: • Formation of TiN +– Sputter a Ti target with a nitrogen plasma Ti, N2 TiN Substrate 18 9Step Coverage Problem with PVD • Both evaporation and sputtering have directional fluxes “geometrical shadowing” Flux film step film wafer 19 Step Coverage concerns in contacts 20 10 Methods to Minimize Step Coverage Problems • Rotate + Tilt substrate during deposition • Elevate substrate temperature (why?) • Use large-area deposition source Sputtering Target 21 Advantages of Sputtering over Evaporation • For multi-component thin films, sputtering gives better composition control using compound targets. – Evaporation depends on vapor pressure of various vapor components and is difficult to control. • Better lateral thickness uniformity – Superposition of multiple point sources Sputtering Target Superposition of all small-area sources Profile due to one small-area source 22 11 Chemical Vapor Deposition (CVD) source chemical reaction film substrate More conformal deposition than PVD tShown here is 100% tconformal deposition step 23 LPCVD Examples • SiO2 ꢛꢑꢜꢝ + ꢞꢆ ꢘꢕꢔ~ꢕꢔꢔꢟ ꢛꢑꢞꢆ + ꢆꢜꢆ ↑ • PS (phosphosilicate glass): doped glass – (~ ꢕꢠ ꢀꢆꢞꢕ + ꢡꢕꢠ ꢛꢑꢞꢆ) – The film “reflows” at 900ꢟ ꢝꢀꢜꢘ + ꢕꢞꢆ ꢘꢕꢔ~ꢕꢔꢔꢟ ꢆꢀꢆꢞꢕ + ꢓꢜꢆ ↑ ꢛꢑꢜꢝ + ꢞꢆ ꢘꢕꢔ~ꢕꢔꢔꢟ ꢛꢑꢞꢆ + ꢆꢜꢆ ↑ 24 12 LPCVD Examples • TEOS (Tetraethylorthosilicate) Si(OC2H5)4 ꢛꢑ(ꢞꢢꢆꢜꢕ)ꢝ→ ꢛꢑꢞꢆ + ꢢꢣꢜꢤꢞꢥ ↑ – The liquid chemical TEOS is a safer alternative to gases silane or dichlorosilane Molecular structure of TEOS 25 LPCVD Examples • Si3N4 ꢘꢛꢑꢜꢝ + ꢝꢇꢜꢘ → ꢛꢑꢘꢇꢝ + ꢅꢆꢜꢆ ↑ • Polysilicon Vꢦꢦ°Y <ul style="display: flex;"><li style="flex:1">ꢛꢑꢜꢝ </li><li style="flex:1">ꢛꢑ + ꢆꢜꢆ ↑ </li></ul>• Tungsten ꢧꢨꢓ + ꢘꢜꢆ → ꢧ + ꢓꢜꢨ ↑ 26 13 CVD Mechanisms reactant <ul style="display: flex;"><li style="flex:1">5</li><li style="flex:1">stagnant </li></ul>gas layer 12surface diffusion 4Substrate 1 = Diffusion of reactant to surface 2 = Absorption of reactant to surface 3 = Chemical reaction 4 = Desorption of gas by-products 5 = Out-diffusion of by-product gas 27 CVD Deposition Rate [ rove Model] film ꢩ = ꢬꢪ ꢫSi ꢮꢯ ꢉꢊ ꢉꢛ = ꢉꢔꢭb F1 Fd= thickness of stagnant layer dꢢꢪ − ꢢꢛ ꢨꢅ = ꢩ ꢫꢨꢘ = ꢉꢛꢢꢛ At steady state, ꢨꢅ = ꢨꢘ 28 14 rove model of CVD (cont’d) ꢢꢪ − ꢢꢛ <ul style="display: flex;"><li style="flex:1">ꢨꢅ = ꢩ </li><li style="flex:1">= ꢬꢪ ꢢꢪ − ꢢꢛ = ꢉꢛꢢꢛ = ꢨꢘ </li></ul>ꢬꢪ ꢫ ꢢꢛ = ꢢꢪ ꢉꢛ + ꢬꢪ ꢉꢛꢬꢪ ꢅ ꢨꢘ = ꢢꢪ =ꢢꢪ <ul style="display: flex;"><li style="flex:1">ꢅ</li><li style="flex:1">ꢅ</li></ul>ꢉꢛ + ꢬꢪ +ꢬꢪ ꢉꢛ Film growth rate is constant with time: ꢐꢰ ꢐꢁ ꢨꢘ ꢇ =where ꢇ = atomic density of deposited film Note: This result is exactly the same as the Deal- rove model for thermal oxidation with oxide thickness = 0 29 Deposition Rate vs. Temperature ꢘꢆgas transport ꢱ ∝ ꢊ limited surface-reaction limited 1 / T 0<ul style="display: flex;"><li style="flex:1">high T </li><li style="flex:1">low T </li></ul>30 15 rowth Rate Dependence on Flow Velocity 31 CVD Features 1. More conformal deposition if T is uniform Wafer topography 2. Inter-wafer and intra-wafer thickness uniformity less sensitive to gas flow patterns. (i.e. wafer placement). 32 16 Comments about CVD (1) Sensitivity to gas flow pattern Furnace tube in wafers (2) Mass depletion problem <ul style="display: flex;"><li style="flex:1">more </li><li style="flex:1">less </li></ul>in out 33 Plasma Enhanced CVD (PECVD) • Ionized chemical species allows a lower process temperature to be used – Plasma helps dissociate the precursor molecules at lower temperatures). • Film properties (e.g. mechanical stress) can be tailored by controlling ion bombardment with substrate bias voltage. 34 17 Atomic Layer Deposition (ALD) • The process involves two self-limiting half reactions that are repeated in cycles • Unlike CVD, in ALD pulses of precursors are introduced in each cycle • ALD is highly conformal and enables excellent thickness uniformity and control down to nm-scale 35 ALD for High-k ate Dielectric 36 18

Lecture06-Thin Film Deposition

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