Slide 1 Applied Physics 298r 1 E Chen (4 12 2004) II Thin Film Deposition Physical Vapor Deposition (PVD) Film is formed by atoms directly transported from source to the substrate through gas phase •[.]
II Thin Film Deposition Physical Vapor Deposition (PVD) - Film is formed by atoms directly transported from source to the substrate through gas phase ã Evaporation ã Thermal evaporation ô • 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) ô ã Plasma-Enhanced CVD (PECVD) ô ã Atmosphere-Pressure CVD (APCVD) • Metal-Organic CVD (MOCVD) Oxidation Spin Coating Platting Applied Physics 298r 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 E Chen (4-12-2004) Evaporation ă Load the source material-to-bedeposited (evaporant) into the container (crucible) ă Heat the source to high temperature ă Source material evaporates ă Evaporant vapor transports to and Impinges on the surface of the substrate ă Evaporant condenses on and is adsorbed by the surface Applied Physics 298r Substrate Film Evaporant Vapor Current Crucible (energy source) E Chen (4-12-2004) Langmuire-Knudsen Relation Mass Deposition Rate per unit area of source surface: Substrate M Rm = Cm cos θ cos ϕ (Pe (T ) − P ) r T r θ Cm = 1.85x10-2 r: source-substrate distance (cm) T: source temperature (K) Pe: evaporant vapor pressure (torr), function of T P: chamber pressure (torr) M: evaporant gram-molecular mass (g) ¬ Maximum deposition rate reaches at high chamber vacuum (P ~ 0) Applied Physics 298r ϕ P Pe Source (K-Cell) E Chen (4-12-2004) Uniform Coating Spherical surface with source on its edge: Spherical Surface r cos θ = cos ϕ = 2r0 ϕ M Pe Rm = Cm T 4r0 r0 P ă Angle Independent – uniform coating! ¬ Used to coat instruments with spherical surfaces Applied Physics 298r r Pe Source (K-Cell) E Chen (4-12-2004) Uniformity on a Flat Surface Consider the deposition rate difference between wafer center and edge: R1 ∝ W /2 r1 r R2 ∝ cos θ = r2 r2 ϕ r1 θ r2 Define Uniformity: σ (% ) = P R1 − R2 (% ) R1 Pe −2 W 2 W σ = − 1 + ≈ 2r1 2r1 Applied Physics 298r or W = 2σ r1 Source (K-Cell) E Chen (4-12-2004) Wafer Uniformity Requirement on a Flat Surface Source-substrate distance requirement: W 2σ In practice, it is typical to double this number to give some process margin: r >W σ Source-Sample Distance (r) r> 160 Larger r Means: ¬ bigger chamber ¬ higher capacity vacuum pump ¬ lower deposition rate ¬ higher evaporant waste Applied Physics 298r 140 1% 2% 120 5% 100 10% 80 60 40 20 0 Sample Size (W) Another Common Solution: off-axis rotation of the sample E Chen (4-12-2004) 10 Thickness Deposition Rate vs Source Vapor Pressure dh Rm = Ae dt ρ Thickness deposition rate Substrate Film dh dh Ae M = Cm cos θ cos ϕ Pe (T ) dt ρ r T T: Ae: ρ: θ source temperature (K) source surface area (cm2) evaporant density (g/cm3) Ae Pe is function of source Temperature! (A/s) Applied Physics 298r ¬ ϕ P Pe T Example: Al M ~ 27, ρ ~ 2.7, Ae ~ 10-2 cm2, T ~ 900 K R ~ 50 cm (uniformity requirement) dh = 50 Pe dt r Source (K-Cell) The higher the vapor pressure, the higher the material’s deposition rate E Chen (4-12-2004) Deposition Rate vs Source Temperature Typically for different material: dh = (10 ~ 100) Pe (T ) dt • • • • ( A / s) For deposition rate > A/s: Pe > ~ 100 mtorr Pe depends on: 1) materila and 2) temperature Deposition rates are significantly different for different materials Hard to deposit multicomponent (alloy) film without losing stoichiometry Applied Physics 298r Example: for Pe > 100 mtoor T(Al) > 1400K, T(Ta) > 2500K E Chen (4-12-2004) Heating Method – Thermal (Resist Heater) Source Material Resistive Wire Current 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) 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 + S Cathode (Target) N S Apply magnetic field parallel to the cathode surface Target ă electrons will hope (cycloid) near the S surface (trapped) Applied Physics 298r 20 N E Chen (4-12-2004) S ... 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. .. rotation of the sample E Chen (4-12-2004) 10 Thickness Deposition Rate vs Source Vapor Pressure dh Rm = Ae dt ρ Thickness deposition rate Substrate Film dh dh Ae M = Cm cos θ cos ϕ Pe (T ) dt... higher the material’s deposition rate E Chen (4-12-2004) Deposition Rate vs Source Temperature Typically for different material: dh = (10 ~ 100) Pe (T ) dt • • • • ( A / s) For deposition rate >