11 00 Dhr Dijkstra
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Overlay als gevolg van ‘wafer heating’
in wafer steppers
March
2010Willem Dijkstra
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Company profile
MECAL BVLocation: Enschede, Veldhoven, GroningenConsultancy & product development# employees: 90
Customers: ASML, Zeiss, Océ, Philips, ICOS, Nedinsco, BESI
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MECAL
Semiconductor industrySimulationProduct developmentTurn-key solutionsOptronics and Vision
(mainly Veldhoven)
Wind energyProduct developmentTurn-key solutions
(mainly Enschede)
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Competencies semiconductor industry
•
Statics ⇒ stress, stiffness, tolerances, deformation, force path
•
Dynamics ⇒ vibration, damping, mass, stick-slip, mode shapes, eigen frequencies
•
Kinematics ⇒ DOF, rigid body systems, acceleration, inertia, set point, friction
•
Thermal ⇒ conductivity, convection, radiation, thermo-mechanics
•
Fluid dynamics ⇒ Air-bearing stiffness and loads, low vacuum, contaminations, flow induced vibrations
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Analysis
FEM simulation
problem identification
output: performance parameters
validation
design improvement
hand calculations
measurements
At MECAL: FEM simulation is a tool, not the goal
understanding the physics=
design optimization
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Production of chips: lithographic processFor 175 wafers/hour: huge power required heatDissipated heat can lead to errors in chips:
Lithographic process
Process chips: features of O(45 nm)
Total allowable error: O(15 nm)
Specific allowable error: O(1 nm)
problem identification
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reticle
lens
wafer
table
chuck
Exposure
mirror
interfero- meter
inte
rfer
o-m
eter
problem identification
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Wafer is divided in fieldsFields are exposed one after anotherSeveral exposures95% of light is transformed to heat and absorbed in waferHeat transfer to tableChuck: very low conductivity
Heat dissipation
first field
last field
problem identification
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Deformations
Wafer + table deform due to thermal expansionWafer pressed onto chuckChuck deformsPositioning is affected
Questions:o
Wafer deformations?o
Design improvements?
Keep in mind: mirrors, needed for positioning, are also deformed
overlay
problem identification
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Output
Outline model
Temperature profile changing with time
Deformation chuck
Overlay at wafer
Input Power
Heat dissipation
Long path between input and output!
problem identification
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Boundary
conditionsHeat load at wafer surface in [mJ/cm2]Convection to environment [22 oC] (air shower at wafer)
Chuck statically fixed no reaction forcesVacuum pressure to push wafer + table onto chuck
Finite Element Model
air shower
FEM simulation
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Thermal results
00.0980.1960.2930.3910.4890.5870.6850.7820.880
00.0020.0040.0060.0080.0100.0120.0150.0170.019
00.0290.0590.0880.1180.1470.1760.2060.2350.265
Temperatures after exposure first
field Wafer
Table
Chuck
FEM simulation
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Thermal results
0.0170.1260.2360.3450.4540.5630.6720.7820.8911.000
00.0120.0250.0370.0490.0620.0740.0860.0980.111
0.0180.0660.1140.1620.2100.2580.3060.3540.4020.450
Temperatures after exposure last
field Wafer
Table
Chuck
FEM simulation
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Mechanical results
-0.019-0.0080.0030.0140.0250.0360.0470.0580.0690.080
-0.049-0.034-0.019-0.0040.0110.0260.0410.0560.0710.086
-0.047-0.031-0.0150.0010.0170.0330.0490.0660.0820.098
Deformations after exposure first
field z-dir
y-dir
x-dir
y
x
z
FEM simulation
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Mechanical results
-0.306-0.162-0.0170.1290.2750.4190.5650.7090.8531.000
-0.250-0.196-0.140-0.084-0.0290.0270.0820.1370.1930.248
-0.285-0.236-0.187-0.139-0.090-0.0420.0070.0550.1040.153
Deformations after exposure last
field z-dir
y-dir
x-dir
y
x
z
FEM simulation
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Output: performance parameters
−150 −100 −50 0 50 100 150
−100
−50
0
50
100Displacement plotsFor each field, displacements are plotted directly after exposure of dieCorrection for chuck deformations
Overlay = O(1 nm)?
Performance parameters
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Validation
Row averaged displacements
uxuy
row number
row number
row number
row number
model
model
measurements
measurements
Difference in amplitude:
ux: 32%uy: 12%
Validation
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25 −20 −15 −10 −5 0 5 10 15 20 250
5
0
5
0
5
0
5
0
Validation
Exposure of one die in the center of the waferDifference in magnitude: 14%
model measurements
Validation
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Current materials:-
wafer: silicium: high conductivity, high CTE-
table: glass/ceramics: low conductivity, low CTE
Conductivity low
/ high ΔT high / lowExpansion = CTE * ΔT
Design improvementMaterials
ΔT CTE expansion
wafer high high high2
table high low low
wafer + table high low FEM?
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Other materials:-
wafer: silicium: high conductivity, high CTE-
table: material X : high conductivity, high CTE-
water cooling in table
Design improvementMaterials
ΔT CTE expansion
wafer low high moderate
table low high moderate
wafer + table low high FEM?
best material: machine dependent
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FEM model to predict overlay caused by wafer heatingGood agreement with measurements Model can be used for design improvements:-
add water cooling-
materials-
feed forward corrections
Conclusions