Introduction
Spin coating is a lab procedure for creating a thin film on a small flat surface. This procedure is useful for preparing samples of a material, most commonly a polymer, for characterization by UV-Vis, Ellipsometry, X-Ray Scattering, and many other common techniques. Spin coating is also used in preparing semiconductors, solar cells, and other electronic device samples on a small scale. Typical flat surface substrates include silica, glass, or quartz. Substrates are rarely larger than a few square inches. Some examples are shown in Figure 1. Thin film thickness can range from 1 nm to 1 µm. Spin coating is carried out primarily by people conducting materials research and is simple enough that those with minimal experience can do it effectively. There are two main steps for spin coating: preparing the solution to be cast, and running the spin coater. Running the spin coater requires choosing the proper settings, properly securing the substrate, and depositing the solution. Once the solution has been cast, the spin coater will fling excess solution away, leaving behind a thin film. Figure 2 illustrates this concept. ⚠Warning: Many solvents are toxic, and their use requires safety goggles, long pants, and closed toe shoes. Always consult an MSDS for more information about hazardous materials. ⚠ Preparing the Spin Coating Solution To get a thin film of a desired material, that material must first be dissolved in a volatile solvent. Solution concentration is proportional to film thickness, so always record the amounts of solvent and material used so that experiments can be repeated. It is difficult to predict exact film thickness ahead of time, so some trial and error may be necessary to achieve specific film thicknesses. For more information, please visit the "How to Choose Solvents" page. 1. Weigh out a desired amount of material to dissolve, and place it in a small container or vial. 2. Add the desired volume of solvent to the material. ⚠Warning: Many solvents are toxic, and their use requires safety goggles, long pants, and closed toe shoes. Always consult an MSDS for more information about hazardous materials. ⚠ 3. Stir and/or gently heat solution until solute is completely dissolved. ⚠Warning: Many solvents are flammable, or have low boiling points such that heat can cause an unsafe buildup in pressure in closed containers. Always consult an MSDS for more information about hazardous materials. ⚠ Running the Spin Coater A spin coater, shown in Figure 3, is a relatively small piece of lab equipment that can rapidly spin small substrates. When a solution is deposited on top of a substrate and spun, the solvent will fly off of the substrate, leaving behind a uniform thin film. It usually takes about five minutes per substrate to perform this technique. The substrate will be held down by a vacuum seal. For more information on substrates, please see the "How to Choose Substrates" page. Spin coaters have a variety of settings that can affect the properties of the final film; please see the "How to Choose Spin Coating Conditions" page for more information. Typical conditions are spinning at 1000 RPM for 60 seconds. Before running a substrate, make sure it is secure as described below. Spin coaters will not run unless the vacuum seal is in effect. ⚠Warning: Keep spin coaters inside of fume hoods or glove boxes to limit exposer to dangerous solvents. Wear safety goggles, long pants, and closed toe shoes when working with chemicals. ⚠ 1. Clean off the desired amount of substrates with organic solvents (acetone, isopropanol, etc...) and any other cleaning techniques necessary. ⚠Warning: Many solvents are toxic, and their use requires safety goggles, long pants, and closed toe shoes. Always consult an MSDS for more information about hazardous materials. ⚠ 2. Select the desired spin coating conditions (time, speed, etc...) from the spin coater menu, which will look similar to Figure 4. 3. Place a blank substrate in the center of the spin coater, such that the vacuum hole is completely covered as shown in Figure 5. 4. Press start, and if the substrate begins spinning, press "stop" and continue. If it does not spin, clean the vacuum hole area with a q-tip, readjust the substrate, and repeat this step until the substrate spins. 5. Deposit approximately 250 mL of solution onto the still substrate with a pipet, avoiding air bubbles so that the substrate appears evenly coated in solution as shown in Figure 6. Immediately press start, and wait until the spin coater stops. 6. Remove the still substrate from the spin coater with tweezers. ⚠Warning: Do not touch the film with you bare hands. Materials in the film and leftover solvent may be toxic or severe irritants. Touching a thin film may also ruin the film. ⚠ Spin coating is complete after removing the substrate. Note that films and film quality may not be observable to the naked eye. It may require further analysis to confirm the presence of a satisfactory film. Conclusion Spin coating creates thin films that can be characterized or play an important role in a device. The desired material is first dissolved in solvent. A recipe is then selected for the spinning, which includes picking a spin speed and duration. Once the substrate is secure, the solution can be deposited and the spinning initiated. Centripetal force throws excess solvent and material off of the substrate, leaving behind a uniform thin film. This process is easy, quick, and produces results that can be very repeatable with some practice. |