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II. Thin Film Deposition

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II. Thin Film Deposition II. Thin Film Deposition Physical Vapor Deposition (PVD) - Film is formed by atoms directly transported from source to the substrate through gas phase • Evaporation • Thermal evaporation « • E-beam evaporation « • Sputtering • DC sputtering « • DC Magnetron sputtering « • RF sputtering « • Reactive PVD Chemical Vapor Deposition (CVD) - Film is formed by chemical reaction on the surface of substrate • Low-Pressure CVD (LPCVD) « • Plasma-Enhanced CVD (PECVD) « • Atmosphere-Pressure CVD (APCVD) • Metal-Organic CVD (MOCVD) Oxidation Spin Coating Platting Applied Physics 298r 1 E. Chen (4-12-2004) General Characteristics of Thin Film Deposition • Deposition Rate • Film Uniformity • Across wafer uniformity • Run-to-run uniformity • Materials that can be deposited •Metal • Dielectric •Polymer • Quality of Film – Physical and Chemical Properties •Stress • Adhesion • Stoichiometry • Film density, pinhole density • Grain size, boundary property, and orientation • Breakdown voltage • Impurity level • Deposition Directionality • Directional: good for lift-off, trench filling • Non-directional: good for step coverage • Cost of ownership and operation Applied Physics 298r 2 E. Chen (4-12-2004) Evaporation ¨ Load the source material-to-be- deposited (evaporant) into the container (crucible) Substrate Film ¨ Heat the source to high temperature ¨ Source material evaporates Evaporant Vapor ¨ Evaporant vapor transports to and Impinges on the surface of the Current substrate Crucible (energy source) ¨ Evaporant condenses on and is adsorbed by the surface Applied Physics 298r 3 E. Chen (4-12-2004) Langmuire-Knudsen Relation Mass Deposition Rate per unit area of source surface: Substrate 1 M 2 1 Rm = Cm cosθ cosϕ ()Pe (T ) − P r T r 2 θ ϕ -2 Cm = 1.85x10 r: source-substrate distance (cm) P T: source temperature (K) Pe: evaporant vapor pressure (torr), function of T P: chamber pressure (torr) Pe M: evaporant gram-molecular mass (g) ¬ Maximum deposition rate reaches at high Source (K-Cell) chamber vacuum (P ~ 0) Applied Physics 298r 4 E. Chen (4-12-2004) Uniform Coating Spherical surface with source on its edge: Spherical Surface r cosθ = cosϕ = 2r0 ϕ 1 r 2 r M Pe 0 θ Rm = Cm 2 T 4r0 P P ¨ Angle Independent – uniform coating! e Source (K-Cell) ¬ Used to coat instruments with spherical surfaces Applied Physics 298r 5 E. Chen (4-12-2004) Uniformity on a Flat Surface Consider the deposition rate difference between wafer center and edge: W / 2 1 Wafer R1 ∝ 2 r1 2 ϕ 1 2 r1 R ∝ cos θ = r1 2 r 2 r 4 2 2 θ r2 Define Uniformity: P R − R σ ()% = 1 2 ()% R1 Pe 2 −2 W W 2 W σ =1− 1+ ≈ or = 2σ Source (K-Cell) 2r 2r 2 r 1 1 1 Applied Physics 298r 6 E. Chen (4-12-2004) Uniformity Requirement on a Flat Surface Source-substrate distance requirement: 160 ) W r r > 140 1% 2σ ce ( 120 2% an 5% st In practice, it is typical to double this i 100 10% number to give some process margin: e D l 80 p am 60 2 S r > W σ ce- 40 ur o 20 S Larger r Means: 0 ¬ bigger chamber 0246810 ¬ higher capacity vacuum pump Sample Size (W) ¬ lower deposition rate Another Common Solution: ¬ higher evaporant waste off-axis rotation of the sample Applied Physics 298r 7 E. Chen (4-12-2004) Thickness Deposition Rate vs. Source Vapor Pressure Thickness deposition rate dh Rm Substrate = Ae dt ρ Film 1 dh 2 dh Ae M 1 = Cm cosθ cosϕ Pe ()T r dt ρ T r 2 θ T: source temperature (K) ϕ 2 Ae: source surface area (cm ) 3 ρ: evaporant density (g/cm ) Ae P Pe is function of source Temperature! Pe Example: Al T -2 2 M ~ 27, ρ ~ 2.7, Ae ~ 10 cm , T ~ 900 K R ~ 50 cm (uniformity requirement) Source (K-Cell) dh = 50P (A/s) ¬ The higher the vapor pressure, the higher the material’s dt e deposition rate Applied Physics 298r 8 E. Chen (4-12-2004) Deposition Rate vs. Source Temperature Typically for different material: dh = (10 ~ 100)P (T ) (A/ s) dt e • For deposition rate > 1 A/s: Pe > ~ 100 mtorr •Pe depends on: 1) materila and 2) temperature • Deposition rates are significantly different for different materials • Hard to deposit multi- Example: for Pe > 100 mtoor component (alloy) film T(Al) > 1400K, T(Ta) > 2500K without losing stoichiometry Applied Physics 298r 9 E. Chen (4-12-2004) Heating Method – Thermal (Resist Heater) Source Material Resistive Current Wire Foil Dimple Boat Crucible Alumina Coated Foil Dimple Boat Contamination Problem with Thermal Evaporation Container material also evaporates, which contaminates the deposited film Cr Coated Tungsten Rod Applied Physics 298r 10 E. Chen (4-12-2004) CIMS’ Sharon Thermal Evaporator Applied Physics 298r 11 E. Chen (4-12-2004) Heating Method – e-Beam Heater e- Electron Beam Crucible Magnetic Field Focusing Aperture Evaporant Evaporant (beam focusing & positioning) Cathode Filament Water Cooled Rotary Copper Hearth Advantage of E-Beam Evaporation: (Sequential Deposition) Very low container contamination Applied Physics 298r 12 E. Chen (4-12-2004) CIMS’ Sharon E-Beam Evaporator Applied Physics 298r 13 E. Chen (4-12-2004) Comparison Typical Deposition Temperature Deposition Material Impurity Cost Evaporant Rate Range Au, Ag, Al, Cr, Sn, Sb, Ge, In, Mg, Metal or low Ga Thermal melt-point High 1 ~ 20 A/s ~ 1800 ºC Low materials CdS, PbS, CdSe, NaCl, KCl, AgCl, MgF2, CaF2, PbCl2 Everything above, plus: Both metal Ni, Pt, Ir, Rh, Ti, 10 ~ 100 E-Beam and Low ~ 3000 ºC High V, Zr, W, Ta, Mo A/s dielectrics Al2O3, SiO, SiO2, SnO2, TiO2, ZrO2 Stoichiometrical Problem of Evaporation • Compound material breaks down at high temperature • Each component has different vapor pressure, therefore different deposition rate, resulting in a film with different stoichiometry compared to the source Applied Physics 298r 14 E. Chen (4-12-2004) Typical Boat/Crucible Material Refractory Metals Temperature for 10-mtorr Melting Point Material Vapor Pressure (Pe) (ºC) (ºC) Tungsten (W) 3380 3230 Tantalum (Ta) 3000 3060 Molybdenum (Mo) 2620 2530 Refractory Ceramics Graphitic Carbon (C) 3799 2600 Alumina (Al2O3) 2030 1900 Boron Nitride (BN) 2500 1600 Applied Physics 298r 15 E. Chen (4-12-2004) DC Diode Sputtering Deposition • Target (source) and substrate are placed 2 – 5kV on two parallel electrodes (diode) • They are placed inside a chamber filled with inert gas (Ar) • DC voltage (~ kV) is applied to the diode Target (Cathode) • Free electron in the chamber are e- e- accelerated by the e-field γ • These energetic free electrons inelastically Ar e- ArAr+ collide with Ar atoms excitation of Ar ¨ gas glows Substrate (Anode) ionization of Ar ¨ Ar+ + 2nd electron •2nd electrons repeat above process ¨ “gas breakdown” ¨ discharge glow (plasma) Applied Physics 298r 16 E. Chen (4-12-2004) Self-Sustained Discharge • Near the cathode, electrons move much faster than ions because of smaller mass 2 – 5kV ¬ positive charge build up near the cathode, raising the potential of plasma ¬ less electrons collide with Ar ¬ few collision with these high energetic electrons results in mostly ionization, rather than excitation ¬ dark zone (Crookes Dark Space) Target (Cathode) • Discharge causes voltage between the electrodes +++++Crookes reduced from ~103 V to ~102V, mainly across the dark Dark Space Ar+ Ar+ space t t • Electrical field in other area is significantly reduced by screening effect of the position charge in front of cathode • Positive ions entering the dark space are accelerated Substrate (Anode) toward the cathode (target), bombarding (sputtering) the target ¬ atoms locked out from the target transport to the substrate (momentum transfer, not evaporation!) ¬ generate 2nd electrons that sustains the discharge (plasma) Applied Physics 298r 17 E. Chen (4-12-2004) Requirement for Self-Sustained Discharge • If the cathode-anode space (L) is less than the dark space length ¬ ionization, few excitation ¬ cannot sustain discharge • On the other hand, if the Ar pressure in the chamber is too low ¬ Large electron mean-free path ¬ 2nd electrons reach anode before colliding with Ar atoms ¬ cannot sustain discharge either Condition for Sustain Plasma: L ⋅ P > 0.5 (cm⋅torr) L: electrode spacing, P: chamber pressure For example: Typical target-substrate spacing: L ~ 10cm ¨ P > 50 mtorr Applied Physics 298r 18 E. Chen (4-12-2004) Deposition Rate vs. Chamber Pressure High chamber pressure results in low deposition rate Mean-free path of an atom in a gas ambient: In fact, sputtering deposition rate R: 5×10−3 λ ~ (cm) P(torr) 1 R ∝ L ⋅ P Use previous example: L = 10 cm, P = 50 mtorr Large LP to sustain plasma ¨ λ ~ 0.1 cm ¨ sputtered atoms have to go through small LP to maintain good hundreds of collisions before reaching the deposition rate and reduce substrate ¨ significantly reduces deposition rate random scattering ¨ also causes source to deposit on chamber ? wall and redeposit back to the target Applied Physics 298r 19 E. Chen (4-12-2004) DC Magnetron Sputtering • Using low chamber pressure to maintain high deposition rate • Using magnetic field to confine electrons near the target to sustain plasma E e- + B + Cathode (Target) SSN Apply magnetic field parallel to the cathode surface ¨ electrons will hope (cycloid) near the Target S N S surface (trapped) Applied Physics 298r 20 E. Chen (4-12-2004) Impact of Magnetic Field on Ions Hoping radius r: 1 2m r ~ Vd B e Ar+ Vd – voltage drop across dark space e- (~ 100 V) E B – Magnetic field (~ 100 G) r + B Cathode (Target) For electron r ~ 0.3 cm For Ar+ ion: r ~ 81 cm Applied Physics 298r 21 E. Chen (4-12-2004) As A Result … ¬ current density (proportional to ionization rate) increases by 100 times ¬ required discharge pressure drops 100 times ¬ deposition rate increases 100 times Magnetron Non-Magnetron n Rate (R) o ~ 1mT Depositi ~ 100mT Chamber Pressure (P) Applied Physics 298r 22 E.
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