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. Chen (4-12-2004) RF (Radio Frequency) Sputtering
DC sputtering cannot be used for depositing dielectrics because insulating cathode will cause 13.56 MHz + charge build up during Ar bombarding ~ ¨ reduce the voltage between electrodes ¨ discharge distinguishes
Target Solution: use AC power • at low frequency (< 100 KHz), both electrons and Target Sheath ions can follow the switching of the voltage – e- e- Ar t ¨ DC sputtering Ar+ • at high frequency (> 1 MHz), heave ions cannot no Substrate Sheath long follow the switching ¨ ions are accelerated by dark-space (sheath) Substrate voltage ¨ electron neutralizes the positive charge buildup on both electrodes • However, there are two dark spaces ¨ sputter both target and substrate at different cycle
Applied Physics 298r 23 E. Chen (4-12-2004) RF (Radio Frequency) Sputtering
13.56 MHz n ~ VT AS ∝ ()n ~ 2 V S AT
Target AT VT – voltage across target sheath VT Vs – voltage across substrate sheath
AT – area of target electrode
As – area of substrate electrode VS AS Substrate
Larger dark-space voltage develops at the electrode with smaller area ¨ make target electrode small
Applied Physics 298r 24 E. Chen (4-12-2004) Comparison between Evaporation and Sputtering
Evaporation Sputtering
High energy atoms / ions (1 – 10 eV) Low energy atoms • denser film (~ 0.1 eV) • smaller grain size • better adhesion High Vacuum Low Vacuum • directional, good for lift-off • poor directionality, better step coverage • lower impurity • gas atom implanted in the film
Point Source Parallel Plate Source • poor uniformity • better uniformity
Component Evaporate at Different Rate All Component Sputtered with Similar Rate • poor stoichiometry • maintain stoichiometry
Applied Physics 298r 25 E. Chen (4-12-2004) Chemical Vapor Deposition (CVD)
Deposit film through chemical reaction and surface absorption
• Introduce reactive gases to the chamber • Activate gases (decomposition) B ¬ heat A B A ¬ plasma
• Gas absorption by substrate surface A B A B
• Reaction take place on substrate surface; W W film firmed Substrate • Transport of volatile byproducts away form substrate • Exhaust waste
Applied Physics 298r 26 E. Chen (4-12-2004) Types of CVD Reactions
Pyrolysis (Thermal Decomposition) AB (gas) → A(solid) + B (gas)
Example α-Si deposited at 580 - 650 ºC:
SiH 4 (gas) = Si (solid) + 2H 2 (gas)
Reduction (lower temperature than Pyrolysis)
AB (gas) + H 2 (gas,commonly used) ↔ A(solid) + HB (gas) Example W deposited at 300 ºC:
WF6 (gas) + 3H 2 (gas) = W (solid) + 6HF (gas) Reversible process, can be used for chamber cleaning
Applied Physics 298r 27 E. Chen (4-12-2004) Types of CVD Reactions (Cont.)
Oxidation
AB (gas or solid) + O2 (gas,commonly used) ↔ AO (solid) +[O]B (gas)
Example
Low-temperature SiO2 deposited at 450 ºC:
SiH 4 (gas) + O2 (gas) = SiO2 (solid) + 2H 2 (gas)
Example SiO2 formed through dry oxidation at 900 - 1100 ºC:
Si (Solid) + O2 (gas) = SiO2 (solid)
Applied Physics 298r 28 E. Chen (4-12-2004) Types of CVD Reactions (Cont.)
Compound Formation AB (gas or solid) + XY (gas or solid) ↔ AX (solid) + BY (gas)
Example SiO2 formed through wet oxidation at 900 - 1100 ºC:
Si (Solid) + 2H 2O(vapor) = SiO2 (solid) + 2H 2 Example SiO2 formed through PECVD at 200 - 400 ºC:
Si H 4 (gas) + 2N2O(gas) = SiO2 (solid) + 2N2 + 2H 2
Example
Si3N4 formed through LPCVD at 700 - 800 ºC:
3Si H 2Cl2 (gas) + 4NH 3 (gas) = Si3 N4 (solid) + 6H 2 + 6HCl
Applied Physics 298r 29 E. Chen (4-12-2004) CVD Deposition Condition
Mass-Transport Limited Deposition - At high temperature such that the reaction rate exceeds the gas delivering rate - Gas delivering controls film deposition rate - Film growth rate insensitive to temperature - Film uniformity depends on whether reactant Mass-Transport Limited
can be uniformly delivered across a wafer and n Rate (log) Regime o wafer-to-wafer Reaction-Rate
Depositi Limited Reaction-Rate Limited Deposition Regime - At low temperature or high vacuum such that the reaction rate is below gas arriving rate 1/T (K) - Temperature controls film deposition rate - Film uniformity depends on temperature uniformity across a wafer and wafer-to-wafer
Applied Physics 298r 30 E. Chen (4-12-2004) Low-Pressure CVD (LPCVD)
Heater Heater Heater
Reactant Exhausted Wafer Gas Gas Horizontal Quartz Tube Z-1 Z-2 Z-3
• Thermal energy for reaction activation • System works at vacuum (~ 0.1 – 1.0 torr), resulting in high diffusivity of reactants ¨ reaction-rate limited • Wafer can stacked closely without lose uniformity as long as they have the same temperature • Temperature is controlled around 600 - 900ºC by “flat” temperature zone through using multiple heaters • Low gas pressure reduce gas-phase reaction which causes particle cluster that contaminants the wafer and system
Applied Physics 298r 31 E. Chen (4-12-2004) Plasma-Enhanced CVD (PECVD)
RF • Use rf-induced plasma (as in sputtering Gases ~ case) to transfer energy into the reactant gases, forming radicals (decomposition) • Low temperature process (< 300 ºC) • For depositing film on metals and other B Shaw Heads materials that cannot sustain high e- e- e- A A+ B+ temperature e- • Surface reaction limited deposition; substrate temperature control (typically Substrate cooling) is important to ensure uniformity
Applied Physics 298r 32 E. Chen (4-12-2004) Common CVD Reactants
Material LPCVD PECVD
SiH4 α-Si SiH4 SiH2Cl2
Si(OC2H5)4 (TEOS) SiH4 + N2O SiO2 SiH2Cl2 + N2O SiH4 + O2
SiH4 + NH3 SiH4 + NH3 Si3N4 SH2Cl2 + NH3 SiH4 + N2
Applied Physics 298r 33 E. Chen (4-12-2004) Comparison of Typical Thin Film Deposition Technology
Film Deposition Substrate Process Material Uniformity Impurity Grain Size Directional Cost Density Rate Temperature
Metal or low Thermal 10 ~ 100 melting- Poor High Poor 1 ~ 20 A/s 50 ~ 100 ºC Yes Very low Evaporation nm point materials Both metal E-beam and 10 ~ 100 Poor Low Poor 10 ~ 100 A/s 50 ~ 100 ºC Yes High Evaporation dielectrics nm
Both metal Metal: and ~ 100 A/s Some Sputtering Very good Low ~ 10 nm Good ~ 200 ºC High dielectrics Dielectric: degree ~ 1-10 A/s
Mainly 10 ~ 100 Some PECVD Good Very low Good 10 - 100 A/s 200 ~ 300 ºC Very High Dielectrics nm degree
Mainly LPCVD Very Good Very low 1 ~ 10 nm Excellent 10 - 100 A/s 600 ~ 1200 ºC Isotropic Very High Dielectrics
Applied Physics 298r 34 E. Chen (4-12-2004)