Spin coating
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Spin coating is a procedure used to apply uniform thin films to flat substrates. In short, an excess amount of a solution is placed on the substrate, which is then rotated at high speed in order to spread the fluid by centrifugal force. A machine used for spin coating is called a spin coater, or simply spinner.
Rotation is continued while the fluid spins off the edges of the substrate, until the desired thickness of the film is achieved. The applied solvent is usually volatile, and simultaneously evaporates. So, the higher the angular speed of spinning, the thinner the film. The thickness of the film also depends on the concentration of the solution and the solvent.
Spin coating is widely used in microfabrication, where it can be used to create thin films with thicknesses below 10 nm. It is used intensively in photolithography, to deposit layers of photoresist about 1 micrometre thick. Photoresist is typically spun at 20 to 80 Hz for 30 to 60 seconds.
[edit] Stages of spin coating
Although different engineers count things differently, there are four distinct stages to the spin coating process.
- Deposition of the coating fluid onto the wafer or substrate
This can be done by using a nozzle and pouring the coating solution or by spraying it onto the surface. A substantial excess of coating solution is usually applied compared to the amount that is required.
- The substrate is accelerated up to its final, desired, rotation speed
- The substrate is spinning at a constant rate and fluid viscous forces dominate the fluid thinning behavior
- The substrate is spinning at a constant rate and solvent evaporation dominates the coating thinning behavior
[edit] Tutorial & experimental technique
Although many polymers with a wide range of weights my be spin-coated, one of the easiest and most typical polymers to spin-coat is poly(methyl methacrylate), commonly known as PMMA, with a moderate molecular weight (e.g. ~120,000), dissolved in a solvent with some moderate polarity. The solvent 1,2,3-trichloropropane was once a traditional solvent commonly used in spin-coating, but due to toxicity issues cyclohexanone is generally preferred and produces coatings of comparable quality. More polar solvents such as N,N-dimethylformamide (DMF) are also commonly used. Typically between 10 and 30% (w/w) polymer is dissolved in the solvent. Both the choice of solvent and especially the molecular weight of the polymer significantly affect the viscosity of the solution and thus the thickness of the resultant coating.
For many applications the solution is doped with an active dye or compound. In nonlinear optics, typical doping compounds include Disperse Red and DANS with concentrations up to 30% w/w (dye/polymer). The active doping compound is typically referred to as the "guest," while the inert polymer is known as the "host."
Many substrates may be coated, but in many research applications one-inch square glass plates (for many applications often coated with a transparent electrode made of indium tin oxide or ITO) cut from microscope slides are commonly coated for experimental purposes. Prior to spin-coating, the polymer-solvent solution must be filtered to remove dust, typically with a 0.45 micron filter. Although it is preferable to spin-coat in a clean-room environment (Class 10 or 100), for many simple laboratory experiments, spin-coating may be performed in a clean fume-hood. The glass plate is placed upon the spin-coating apparatus, cleaned successively with acetone and then methanol using lint-free swabs, then coated liberally with the polymer-solvent solution by use of syringe or eye-dropper. The plate is spun in at least two stages which may be programmed into most any simple spin-coating apparatus. During the first stage, the plate is spun at a low to moderate speed 500-1000 RPM for 5-10 seconds to evenly spread the solution. The thickness of the coating is then determined and controlled during the second stage by spinning the coating at a higher speed, between 1500-3000 RPM for anywhere between a few seconds and a minute. These conditions will typically produce high quality coatings of thickness between 2 and 10 microns.
Once spin-coating is complete, the plate is typically placed quickly onto a hot-plate (heated to somewhere around 100C) for several seconds or minutes to initially evaporate solvent and solidify the coating. The slide is than baked-out for several hours, or typically over night, in an oven or vacuum oven, at a temperature high enough to sufficiently remove the remaining solvent. Although it is not uncommon to place plates in a petri dish during one or both bake-out steps to protect films from dust, condensation of solvent on the roof of the dish may greatly affect the film smoothness and quality and thus the lid of the dish must be set ajar to allow for the evaporating solvent to escape.
Many other polymers, such as polystyrene or more polar polymers such as polysulfones or polyetherimides, may also be spin-coated. More polar polymers are typically dissolved in N,N-dimethylformamide (DMF). Some polymers are more difficult to coat than others. Most often poor quality, hazy or "orange peel" coatings (coatings which peel away from the plate), are the result of moisture absorption due to environmental humidity. Although strict control over the laboratory environment is the best way to improve coating quality, when this is difficult or infeasible one quick-fix to significantly improve film quality is to gently spray a dry inert gas (such as nitrogen, or preferably argon) over the sample during coating while heating the sample with an infrared heat-lamp (available at most any hardware store). The heat-lamp significantly reduces the local humidity while simultaneously speeding the rate of solvent evaporation so that the sample does not have a chance to absorb moisture.
[edit] References
- S. Middleman and A.K. Hochberg Process Engineering Analysis in Semiconductor Device Fabrication, McGraw-Hill, p. 313 (1993)
- Dirk W. Schubert, Thomas Dunkel; Spin coating from a molecular point of view: its concentration regimes, influence of molar mass and distribution; Materials Research Innovations Vol. 7, p. 314 (2003)
