Spin coating

Spin CoatingIntroduction to Spin Coating and

Derivation of a Simple Model Presented By: Katie Weigandt

Agenda Introduction to spin coating

Definition and brief history of spin coating Uses of spin coating technology Common spin coating defects

Physics of spin coating Basic physics behind spin coating The Spinning Disk Problem

Further work on spin coating in relation to my honors college thesis

What is Spin Coating?

A process in which solution is spread evenly over a surface using centripetal force.

Spin coating will result in a relatively uniform thin film of a specific thickness.

Spin coating is an important way of creating thin films in the microelectronics industry.

Brief History of Spin Coating

Spin coating was first used to apply coatings of paint and pitch around seventy years ago.

In 1958 Emslie et. al. developed the first spin coating model.

This model has been used as a basis for future more specific or complicated models.

Lawrence and Zhou: “Spin Coating of Non-Newtonian Fluids”

Spin Coater Schematic

Wafer is held to chuck with vacuum pump.

Lid is placed over spinning basin before spin is initiated.

Vacuum

Wafer

Chuck

Basin

Lid

Basic Physics of Spin Coating

• Centripetal force is responsible for the spread of liquid across the wafer.

• At long times the fluid will flow only negligibly, resulting in a lower limit of the final thickness.

Industrial Uses of Spin Coating

Photoresist for patterning wafers in microcircuit production.

Insulating layers for microcircuit fabrication such as polymers.

Flat screen display coatings. Antireflection coatings and conductive oxide. DVD and CD ROM Television tube antireflection coatings.

Common Spin Coating Defects

•Bubbles on the surface of the coated wafer.

•This occurs when fluid is deposited as the wafer is spinning, and may be caused by a faulty dispense tip.

•A swirling pattern may be observed.

•Causes:

•Fluid deposited off center

•Acceleration too high

•Spin time to short

•Exhaust rate too highhttp://www.cise.columbia.edu/clean/process/spintheory.pdf


•A mark or circle in the center of the wafer could indicate a chuck mark.

•If a chuck mark occurs the type of chuck should be changed.

•Streaks can occur on the wafer for a number of reasons including:

•Acceleration too high

•Fluid deposited off center

•Particles on surface prior to spin

http://www.cise.columbia.edu/clean/process/spintheory.pdf


•Uncoated areas on wafer occur when to little fluid is deposited on the wafer.

•Pinhole defects can be caused by:

•Air bubbles

•Particles in fluid

•Particles on substrate.


Agenda

Introduction to spin coating Definition and brief history of spin coating Uses of spin coating technology Common spin coating defects

Physics of spin coating Basic physics behind spin coating Derivations of common spin coating models


Spin Coating Process

Four main processing steps:

Step 1: Deposit fluid onto substrate.

Step 2: Accelerate wafer to final radial velocity.

http://www.mse.arizona.edu/faculty/birnie/Coatings/

Spin Coating Process

Four main processing steps:

Step 3: The coating thins at a rate that depends on the velocity at which the wafer is spinning and the viscosity of the fluid.

Step 4: Solvent is evaporated from the film, resulting in further thinning.

http://www.mse.arizona.edu/faculty/birnie/Coatings/

The Spinning Disk Problem

Problem: Consider unsteady behavior of liquid film

thickness under centripetal force. Goal:

Develop relationship between film thickness and time.

Middleman, Introduction to Fluid Dynamics


Assumptions Axisymmetric flow of fluid across the wafer Laminar flow of the thinning film Film thickness decreases slowly with time Angular velocity of fluid is equivalent to the angular

velocity of the disk Film is thin and has uniform thickness over the wafer Newtonian and incompressible fluid Liquid is not volatile


0u


r

ru

rz

u

r

ru

rrzr

)(1)(10

Continuity Equation:


Momentum Equation:

2

22

z

u

r

p

r

u

r

uu rrr

2

22

z

ur

r

uu rrr

2r


By assuming the nonlinear term in the momentum equation is small compared to other terms we are able to solve the resulting equation:

2

22

z

ur r

Boundary Conditions:

00

)(0

zatu

rhzatdz

du

r

r



We can now say that the volumetric flow, Q, across the edge of the spinning disk is equal to the rate change of the solution volume on disk:

322

2

03

222

0

2

3

2

;3

2

),(2

HR

dt

dHR

HHHConditionInitialHR

dt

HdR

dt

HdRdzRzurQ

RR

H

r

R



Integrating the previous equation we obtain an expression for film thickness, H, in terms of time, t :

21

20

2

0

2

202

3

41

)(

3

411

tH

H

tH

tHH


Model Limitations

This model is limited by the assumptions used to derive equations so it only applies to: Newtonian and non-volatile liquids Uniform substrates

21

20

2

0 3

41

)(

t

H

H

tH

Development of more general models is significantly more difficult When developing a model for non-Newtonian flow it must

be considered that the viscosity changes with shear force.

Agenda

Introduction to spin coating Definition and brief history of spin coating Uses of spin coating technology Common spin coating defects

Physics of spin coating Basic physics behind spin coating Derivations of common spin coating models


Honors College Thesis Topic

Model flow of spin coated Newtonian fluid using FEMLab, a finite element modeling program.

Extend the FEMLab model to flow of non-Newtonian and viscoelastic fluids on spin coated wafer.

Verify experimentally that the model is valid by spin coating fluids with relevant properties on 6 in silicon wafers and comparing the resultant film thickness with the predicted film thickness.

References

Lawrence, C.J, Zhou, W. “Spin coating of non-Newtonian Fluids”. Journal of Non-Newtonian Fluid Mechanics, 39 (1991) 137-187

Middleman, S. An Introduction To Fluid Dynamics. John Wiley and Sons. New York. 1998


http://www.mse.arizona.edu/faculty/birnie/Coatings

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