11 00 dhr dijkstra

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