Spin CoatingIntroduction to Spin Coating and Derivation of a Simple ModelPresented By: Katie Weigandt

Agenda

Introduction to spin coating

Definition and brief history of spin coating Uses of spin coating technology Common spin coating defects Basic physics behind spin coating The Spinning Disk Problem

Physics of spin coating

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 SchematicLid

Wafer Basin Vacuum Chuck

Wafer is held to chuck with vacuum pump. Lid is placed over spinning basin before spin is initiated.

Basic Physics of Spin Coating

Centripetal force is responsible for the spreadof 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 A swirling pattern may be observed. Causes: Fluid deposited off center Acceleration too high Spin time to short Exhaust rate too high

as the wafer is spinning, and may be caused by a faulty dispense tip.

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

Common Spin Coating Defects

A mark or circle in the center of the wafer could indicate a chuck mark. If a chuck mark occurs the type ofchuck 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 spinhttp://www.cise.columbia.edu/clean/process/spintheory.pdf

Common Spin Coating Defects

Uncoated areas on wafer occur Pinhole defects can be caused by: Air bubbles Particles in fluid Particles on substrate.

when to little fluid is deposited on the wafer.

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

Agenda

Introduction to spin coating

Definition and brief history of spin coating Uses of spin coating technology Common spin coating defects Basic physics behind spin coating Derivations of common spin coating models

Physics of spin coating

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

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. Develop relationship between film thickness and time.

Goal:

Middleman, Introduction to Fluid Dynamics

The Spinning Disk Problem

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

xuU !0 xU

Middleman, Introduction to Fluid Dynamics

The Spinning Disk ProblemContinuity Equation:

1 x (ru r ) xu z 1 x (ru r ) 0! } r xr xz r xrMomentum Equation:

uU2 xu r xp x 2u r Vur V ! Q 2 xr xr xz rVr[ 2

xu r x 2u r Vu r V r[ 2 ! Q 2 xr xzMiddleman, Introduction to Fluid Dynamics

The Spinning Disk ProblemBy assuming the nonlinear term in the momentum equation is small compared to other terms we are able to solve the resulting equation:

Boundary Conditions:

x ur Vr[ ! Q 2 xz2 2

du r ! 0 at z ! h(r ) dz ur ! 0 at z ! 0Middleman, Introduction to Fluid Dynamics

The Spinning Disk ProblemWe 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:

Q ! 2Tr

HR

0

dH ur ( z , R)dz ! TR dt2

d H 2TV[ 2 R 2 3 TR 2 ! H R ; Initial Condition H ! H R ! H 0 dt 3Q dH 2TV[ 2 R 2 3 TR 2 ! H dt 3QMiddleman, Introduction to Fluid Dynamics

The Spinning Disk ProblemIntegrating the previous equation we obtain an expression for film thickness, H, in terms of time, t :

1 1 4 V[ 2 2 ! t H2 H0 3Q H (t ) 4 V[ H @ ! 1 t H0 3Q 2 2 0 1 2

Middleman, Introduction to Fluid Dynamics

Model Limitations

This model is limited by the assumptions used to derive equations so it only applies to:

H (t ) 4 V[ H ! 1 @ t H0 3Q 2 2 0

1

2

Newtonian and non-volatile liquids Uniform substrates

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 Basic physics behind spin coating Derivations of common spin coating models

Physics of spin coating

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

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.

ReferencesLawrence, 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.cise.columbia.edu/clean/process/spintheory.pdf http://www.mse.arizona.edu/faculty/birnie/Coatings

Questions?