Rational solvent selection strategies to combat striation formation during spin coating of thin films
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Striation defects in spin-coated thin films are a result of unfavorable capillary forces that develop due to the physical processes commonly involved in the spin-coating technique. Solvent evaporation during spinning causes depletion at the surface of the more volatile solution components while simultaneous viscous out-flow occurs providing the main source of solution thickness reduction during any typical spinning run. The composition changes in the surface layer can either stabilize or destabilize the surface with respect to convective motions within the coating solution. Destabilization (and therefore possible striation formation) happens when the surface composition changes so that a larger surface tension will develop. Thus, a careful cross-referencing of solvent volatility with surface tension effects can help establish solution conditions that will prevent this instability from arising. A plot of solvent vapor pressure (Pv) versus solvent surface tension () is introduced and utilized to help discuss the impact of solvent choice when making coatings via spin coating. One important result is that when desiring to deposit a coating having a surface tension of solid, then it is favorable to use a fully miscible solvent that has a higher surface tension (i.e., liquid > solid ). More complicated solution mixtures were also examined, including dual-solvent systems and water-containing systems.
I. INTRODUCTION
Spin coating is a simple process for rapidly depositing thin coatings onto relatively flat substrates. The substrate to be covered is held by some rotatable fixture (often using vacuum to clamp the substrate in place), and the coating solution is deposited onto the surface; the action of spinning causes the solution to spread out and leave behind a very uniform coating of the chosen material on the surface of the substrate. The spincoating process has been studied extensively in the past, and much is known about factors that control coating deposition and the final thickness of the deposit that results.1–10 For most solutions, a balance is established between viscous outward radial flow of the solution on the surface of the substrate and the evaporation of solvent from the coating solution. Meyerhofer treated this dualaction process by splitting the spin-coating run into two stages—one controlled only by viscous flow and the second controlled only by evaporation.2 With this
a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 4, Apr 2001
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approach he was able to predict the final coating thickness, hf , in terms of several key solution parameters, according to hf = x
冋
e 2共1 − x兲K
册
1Ⲑ3
(1)
,
where e and K are the evaporation and flow constants, defined below, and x is the solids content of the solution. The evaporation and flow constants are defined respectively as e = C公 K⳱
(2)
,
2 , 3
(3)
where is the rotation rate, is the solution’s density, is its viscosity, and C is a
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