The cotton candy effect is a common issue in the semiconductor manufacturing process where dried material comes off the substrate in string-like filaments instead of droplets. To address this issue, process parameters and environmental factors should be adjusted. Optimization testing is required to fine-tune the process until the cotton candy effect is eliminated.
The Cotton Candy Effect
The cotton candy effect occurs when the material at the edge of the substrate dries too quickly and, instead of spinning off in droplets, the material comes off in filaments, which can be redeposited back onto the substrate. There are a variety of factors that influence the drying time of the material including evaporation of the solvent, substrate size, spin speed and acceleration, and exhaust parameters.
Addressing the Cotton Candy Effect
To reduce the cotton candy effect, we recommend the following actions:
Reduce spin speed & acceleration
Consider a reduction in spin speed and acceleration during the casting step. Higher spin speed and acceleration introduce air turbulence, more quickly evaporating solvent and prematurely drying the material. If you are unable to reduce your spin speed and acceleration or have already done so and are still experiencing the cotton candy effect, you may need to adjust your exhaust parameters.
Reduce exhaust flow rate
Excessive exhaust parameters pull solvents too quickly, leading to premature drying of the material. Mitigation can be managed in two ways: by reducing facility exhaust flow rate or with the use of Cee®’s proprietary programmable exhaust solution. As opposed to the static adjustment of facility exhaust which may detrimentally affect other processes and violate organizational safety practices, programmable exhaust allows for recipe-driven dynamic adjustment. Variances in material viscosity, solvents, substrate size, etc. can all influence your success in eliminating the cotton candy effect. Experiment with various parameter adjustments to fine-tune your process. For more information on how to effectively use programmable exhaust, see our post on programmable exhaust.
In the event that adjusting the exhaust parameters doesn’t eradicate the formation of filaments, you will need to manufacture a solvent-rich environment using backside rinse (BSR) and/or edge-bead removal (EBR).
Create a solvent-rich environment with BSR & EBR
In this process you will artificially enrich the spin bowl to encourage the formation of droplets instead of strings. BSR and/or EBR are turned on at the beginning of the cast-off process when the cotton candy effect is encountered. When adequate coverage is achieved, turn both off and spin according to your normal process parameters for desired thickness.
*Be sure to consult your material manufacturer for a list of appropriate solvents for BSR & EBR.
Conclusion
The cotton candy effect occurs when the material at the edge of the wafer dries too quickly and comes off in cotton candy-like strings instead of droplets. To prevent this, it’s necessary to fine tune the parameters during the casting step. These parameters include adjusting spin speed & acceleration, experimenting with different exhaust parameters, and creating a solvent-rich environment. By following one or more of these suggestions, the cotton candy effect will be eliminated.
Did you know that Cee® offers process support for the life of your equipment? Our support team is just a call or email away, ready to provide expert guidance to get your process back on track.
