What Is Spin Coating?
A: Spin coating is a procedure used to deposit uniform thin films to flat substrates. Usually a small amount of coating material is applied on the center of the substrate, which is either spinning at low speed or not spinning at all. The substrate is then rotated at high speed in order to spread the coating material by centrifugal force.
A machine used for spin coating is called a Spin Coater.
Spin Coating The Basics
Before Coating Your First Wafer
A successful result requires that you pay attention to the following:
- • Environmental conditions including temperature, humidity, airborne particulates, etc.
- • Quality of your consumables — is your chemistry fresh?
- • Cleanliness of your substrate, and equipment. Starting with a contaminated substrate ruins your process before it begins
Correctly Centered Substrate
Centering the substrate accurately on your spin coater allows for consistent coating. Irregular shapes must be centered by eye. Square and rectangular substrates benefit from embedded chucks that automatically center the substrate and reduce edge build up during spin coating.
Static Dispense
Most people begin spin coating using a static dispense method. This is the simplest way to begin spin coating and requires you to dispense chemistry on to a stationary substrate.
Dispense Your Chemistry
Using a pipette or syringe through the hole in the lid of your spin coater, a smooth stream of chemistry is dispensed on to the center of the substrate. In general the material should be dispensed until it covers 50% of the diameter of the substrate.
Start the Rotation
As soon as the correct amount of material is dispensed, begin the coating program. Laurell spin coaters are fully programmable for speed and acceleration at each step of your spin coating recipe.
Spreading
Your spin coater should be programmed to allow for an intermediate step that accelerates the substrate to a moderate speed, and allows the chemistry to spread smoothly across the surface.
Excess Material is Cast Off
As the material reaches the edge of the substrate a further acceleration step will cast excess material from the wafer. The process chamber of a Laurell spin coater is designed to prevent this excess material from splashing back onto the substrate.
Accelerate to Final Spin Speed
The last step requires your spin coater to accelerate rapidly to the final spin speed required to create a coating of the correct thickness.
Solvent evaporation
At the final spin speed, solvent will evaporate from the coating. Controlling the level of exhaust on your spin coater will allow you to adjust the concentration of solvent vapor in the process chamber.
Edge Bead
The spin coating process can leave a build up of material at the edge of the substrate.
Edge Bead Removal
Edge bead removal on round wafers is best done in your spin coater immediately after the coating process. After the coating is set, a stream of solvent is directed at the edge of the substrate while rotating at a low speed. The wafer continues to spin which allows any remaining solvent to dry.
Finished Coating
With the edge bead removed the coating process is complete.
Dynamic Dispense
Dynamic dispense is a more advanced alternative to static dispense. This requires you to start the spin coater prior to dispensing material onto a rotating substrate.
Dispense While Rotating
The chemistry is dispensed while the spin coater is rotating the substrate at around 500RPM. A smooth stream of material will spread evenly for the best coating.
Dispense Up to the Edge of the Substrate
Stop dispensing your chemistry as the puddle reaches the edge of the wafer. The best results are acheived if the spin coater is programmed to move to the next step as the intial dispense is completed.
Accelerate into the Casting Step
As the chemistry reaches the edge, the wafer should be accelerated to remove excess material. From this point on the process is the same as Static Dispense. Dynamic dispense can result in more consistent coatings and help reduce the amount of chemistry used. Reducing the amount of materials used will save money and the time used to clean your Laurell spin coater.