1. Overlay als gevolg van ‘wafer heating’ in wafer steppers
March 2010
Willem Dijkstra

2. Company profile
MECAL BV
Location: Enschede, Veldhoven, Groningen
Consultancy & product development
# employees: 90
Customers: ASML, Zeiss, Océ, Philips, ICOS,
Nedinsco, BESI
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3. MECAL
Semiconductor industry
Simulation
Product development
Turn-key solutions
Optronics and Vision
(mainly Veldhoven)
Wind energy
Product development
Turn-key solutions
(mainly Enschede)
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4. 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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5. Analysis
At MECAL: FEM simulation is problem identification
a tool, not the goal
FEM simulation hand calculations
understanding the physics
= validation measurements
design optimization
output: performance
parameters
design improvement
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6. Lithographic process problem identification
Production of chips: lithographic process
For 175 wafers/hour: huge power required heat
Dissipated heat can lead to errors in chips:
Process chips: features of O(45 nm)
Total allowable error: O(15 nm)
Specific allowable error: O(1 nm)
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7. Exposure problem identification
reticle interfero-
meter
mirror
lens
interfero-
meter
wafer
table
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chuck
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8. Heat dissipation problem identification
first field Wafer is divided in fields
Fields are exposed one after another
Several exposures
95% of light is transformed to heat
and absorbed in wafer
Heat transfer to table
Chuck: very low conductivity
last field
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9. Deformations problem identification
Wafer + table deform due to thermal
expansion overlay
Wafer pressed onto chuck
Chuck deforms
Positioning is affected
Questions:
o Wafer deformations?
o Design improvements?
Keep in mind: mirrors, needed for
positioning, are also deformed
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10. Outline model problem identification
Input Power
Heat dissipation
Temperature profile
changing with time
Deformation chuck
Overlay at wafer Output
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Long path between input and output!
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11. Finite Element Model FEM simulation
Boundary conditions
Heat load at wafer surface in [mJ/cm2] air shower
Convection to environment [22 oC] (air shower at wafer)
Chuck statically fixed no reaction forces
Vacuum pressure to push wafer + table onto chuck
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12. Thermal results FEM simulation
Temperatures after exposure first field Wafer
0
0.098
0.196
Table
0 0.293
0.029 0.391
0.059 0.489
0.088 0.587
Chuck
0 0.118 0.685
0.002 0.147 0.782
0.004 0.176 0.880
0.006 0.206
0.008 0.235
0.010 0.265
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0.012
0.015
0.017
0.019
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13. Thermal results FEM simulation
Temperatures after exposure last field Wafer
0.017
0.126
0.236
Table 0.345
0.018 0.454
0.066 0.563
0.114 0.672
0.162 0.782
Chuck
0 0.210 0.891
0.012 0.258 1.000
0.025 0.306
0.037 0.354
0.049 0.402
0.062 0.450
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0.074
0.086
0.098
0.111
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14. Mechanical results FEM simulation
Deformations after exposure first field z-dir
-0.019
-0.008
0.003
y-dir 0.014
-0.047 0.025
z
y -0.031 0.036
-0.015 0.047
0.001 0.058
x-dir x
-0.049 0.017 0.069
-0.034 0.033 0.080
-0.019 0.049
-0.004 0.066
0.011 0.082
0.026 0.098
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0.041
0.056
0.071
0.086
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15. Mechanical results FEM simulation
Deformations after exposure last field z-dir
-0.306
-0.162
-0.017
y-dir 0.129
-0.285 0.275
z
y -0.236 0.419
-0.187 0.565
-0.139 0.709
x-dir x
-0.250 -0.090 0.853
-0.196 -0.042 1.000
-0.140 0.007
-0.084 0.055
-0.029 0.104
0.027 0.153
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0.082
0.137
0.193
0.248
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16. Output: performance parameters Performance parameters
100
Displacement plots
For each field, displacements are
50
plotted directly after exposure of die
0 Correction for chuck deformations
−50
−100
Overlay = O(1 nm)?
−150 −100 −50 0 50 100 150
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17. Validation Validation
model measurements
ux
Difference in
row number row number
amplitude:
model measurements ux: 32%
uy: 12%
uy
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row number row number
Row averaged displacements
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18. Validation Validation
model measurements
0
5
0
5
0
5
0
5
0
25 −20 −15 −10 −5 0 5 10 15 20 25
Exposure of one die in the center of the wafer
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Difference in style
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19. Materials Design improvement
Current materials:
- wafer: silicium: high conductivity, high CTE
- table: glass/ceramics: low conductivity, low CTE
Conductivity low / high ΔT high / low
Expansion = CTE * ΔT
ΔT CTE expansion
wafer high high high2
table high low low
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wafer to edit Master title style
low FEM?
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20. Materials Design improvement
Other materials:
- wafer: silicium: high conductivity, high CTE
- table: material X : high conductivity, high CTE
- water cooling in table
best material:
machine dependent
ΔT CTE expansion
wafer low high moderate
table low high moderate
Click + table low
wafer to edit Master title style
high FEM?
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21. Conclusions
FEM model to predict overlay caused by wafer
heating
Good agreement with measurements
Model can be used for design improvements:
- add water cooling
- materials
- feed forward corrections
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