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Lecture 10.0. Photoresists/Coating/Lithography. Semiconductor Fab. Land$0.05 Billion Building$0.15 Billion Tools & Equipment $1 Billion Air/Gas Handling Sys$0.2 Billion Chemical/Electrical Sys$0.1 Billion Total$1.5 Billion 10 year Amortization~$1 Million/day. - PowerPoint PPT Presentation
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Lecture 10.0Lecture 10.0
Photoresists/Coating/Lithography
Semiconductor FabSemiconductor Fab
Land $0.05 BillionBuilding $0.15 BillionTools & Equipment $1 BillionAir/Gas Handling Sys$0.2 BillionChemical/Electrical Sys $0.1 BillionTotal $1.5 Billion10 year Amortization ~$1 Million/day
80nm Line width with 80nm Line width with =193 nm =193 nm LithographyLithography
Photoresist -Photoresist -Sales $1.2 billion/yr. in 2001 Sales $1.2 billion/yr. in 2001 Resins
– phenol-formaldehyde, I-lineSolventsPhotosensitive compounds
– Polymethylmethacrylate or poly acrylic acid = 638 nm RED LIGHT
– diazonaphthoquinone• Hg lamp, = 365 nm, I-line
– o-nitrobenzyl esters – acid generators• Deep UV, = 248 nm, KrF laser
– Cycloolefin-maleic anhydride copolymer– Poly hydroxystyrene
=193 nm gives lines 100 nm = 157 nm F laser
Additives
PhotoresistPhotoresist
Spin Coat waferDry solvent out of filmExpose to LightDevelop Quench developmentDissolve resist (+) or developed
resist (-)
Spin CoatingSpin Coating
Cylindrical Coordinates– Navier-Stokes– Continuity
Navier-StokesNavier-Stokes
Spin Coating DynamicsSpin Coating Dynamics
Newtonianr
V
z
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rg
gz
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rz
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StokesNavierz
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Continuity
zrrz
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)(1
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Newtonian Fluid-Newtonian Fluid-non-evaporatingnon-evaporating
2/1
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solution
hthCB
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If hois a constant film is uniformFor thin films, h -1 t-1/2
Evaporation Model - Evaporation Model - Heuristic ModelHeuristic Model
CN non-volatile, CV volatile
e= evaporationq= flow rate
Spin Coater - Heuristic ModelSpin Coater - Heuristic Model
Flow Rate, h is thickness
Evaporation rate due to Mass Transfer
Spin Coating SolutionSpin Coating Solution
Dimensionless Equations
Viscosity as a function of composition
Viscosity increases with loss of Viscosity increases with loss of solventsolventViscosity of pure
Resin is very high
Viscosity of Solvent is low
0 0.02 0.041 10 5
1 10 4
1 10 3
0.01
Volume Fraction Vapor Component
Vis
cosi
ty(m
^2/s
ec) 1.521 10 3
1 10 5
x( )
x o0 x
Spin CoatingSpin Coating
Thickness RPM-1/2 o1/4
Observed experimentally
ResultsResults
Effect of Mass Transfer = dimensionless
Mass transfer Coefficient
– Increase MT Increase in Film Thickness
– MT increases viscosity and slows flow leading to thicker film
Dimensionless Film Thickness
Dissolve edge of photoresistDissolve edge of photoresist
So that no sticking of wafer to surfaces takes place
So that no dust or debris attaches to wafers
Wafer with Photoresist
LithographyLithography
Light passes thru die mask
Light imaged on wafer
Stepper to new die location
Re-image Wafer with Photoresist
Mask
Light Source
ReductionLens
LithographyLithography
Aspect Ratio (AR)=3.5– AR=Thickness/Critical Dimension
• Critical Dimension=line width• Thickness= photoresist thickness
Lateral Resolution (R)– R=k1 /NA
Numerical Apparature (NA)– NA is a design parameter of lens
Depth of Focus (DOF)– DOF= k2 /NA2
Lithography - PhotoreactionLithography - Photoreaction
– Photo Reaction Kinetics• dC(x,t)/dt = koexp(-EA/RT) C(x,t) I(x,)
– Beer’s Law• I(x, )/Io=exp(- () C(x,t) x)
() = extinction coefficient
– Solution?• dC(x,t)/dt = koexp(-EA/RT) C(x,t) Io exp(- () C(x,t) x)
– C=Co at t=0, 0<x<L
Drying solvent out of LayerDrying solvent out of Layer
Removal of Solvent– Simultaneous Heat and Mass Transfer– In Heated oven– Some shrinkage of layer
PhotoresistPhotoresist
Positive– Light induced reaction
• decomposes polymer into Acid + monomers
– Development• Organic Base (Tri
Methyl ammonium hydroxide) + Water
• neutralizes Acid group
• Dissolves layer – Salt + monomer
Negative– Light induced reaction
• Short polymers crosslink to produce an insoluble polymer layer
– No Development needed
– Dissolution of un-
reacted material
Photoresist DevelopmentPhotoresist Development
Boundary Layer Mass TransferPhotoresist DiffusionChemical ReactionProduct diffusion, etc.
Reaction Plane
Reactant ConcentrationProfileProduct
ConcentrationProfile
Rate Determining StepsRate Determining Steps
X
Dissolution of Uncrosslinked PhotoresistDissolution of Uncrosslinked Photoresist
Wafers in CarriagePlaced in SolventHow Long??Boundary Layer MT
is Rate Determining– Flow over a leading
edge for MT– Derivation & Mathcad
solutionAlso a C for theConcentration profile
Mass transfer correlation Mass transfer correlation - flow over leading edge- flow over leading edge
Sh=Kgx/DAB
Kg= DAB / C
Sc=/DAB
Re=V x/
Global Dissolution Rate/TimeGlobal Dissolution Rate/Time
Depends on–Mass Transfer• Diffusion Coefficient• Velocity along wafer surface• Size of wafer
– Solubility– Density of Photoresist Film
Local Dissolution Rate/TimeLocal Dissolution Rate/Time
Depends on–Mass Transfer• Diffusion Coefficient• Velocity along wafer surface• Size of wafer
– Solubility– Density of Photoresist Film– Position on the wafer