In photolithography, a light-sensitive polymer called photoresist is spun onto a substrate, exposed to UV light through a patterned mask, and developed so only part of it remains. It's the tool behind computer chips and MEMS devices, here used to define electrodes.
Light draws the pattern; the mask holds the design.
CHOOSING A RESIST
Photoresists for building structures
Positive resistExposed areas dissolve in the developer
Negative resistExposed areas cross-link and stay behind
SU-8An epoxy-based negative resist that forms thick, tall, steep-walled structures
Why tall mattersTall pillars and combs add surface area without a bigger footprint
SU-8 is a common starting point for tall polymer structures.
THE FLOW
Pattern first, then bake
1 Ā· CleanPrepare the substrate, such as silicon or oxidized silicon
2 Ā· SpinCoat resist to a set thickness
3 Ā· Soft bakeDrive off the solvent
4 Ā· ExposeShine UV through the mask
5 Ā· DevelopBake after exposure, then wash away the unwanted resist
6 Ā· PyrolyzeHeat in an inert gas so the polymer becomes carbon
Steps 1 to 5 make a polymer structure. Step 6 turns it into carbon.
PYROLYSIS
Turning polymer into carbon
Nā + 900 Ā°C ā carbon
In a tube furnace under nitrogen or forming gas, the structure is heated in stages, often first to a few hundred degrees and then to roughly 900 to 1000 Ā°C. Hydrogen, oxygen, and nitrogen leave as gases, and what remains is glassy carbon in the same pattern.
Same shape, new material: an insulating polymer becomes conductive carbon.
SHRINKAGE
Design for the shrink
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Big change
Pyrolysis removes much of the mass, so features can lose around half their size or more
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Predictable
Shrinkage is fairly uniform, so the mask can be drawn larger to compensate
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Adhesion risk
A shrinking film can crack or peel, so adhesion and gentle heating ramps help
Shrinkage is a design input, not a surprise.
STRENGTHS AND CATCHES
Photolithography with pyrolysis
PrecisionMicrometer-scale features and gaps, set by the mask
3D structuresTall pillars and combs raise area without growing the footprint
ParallelA whole wafer is patterned at once, so devices match
CostNeeds cleanroom tools and a high-temperature furnace
SubstrateIt has to survive the furnace, which rules out most plastics
Best for small, precise, repeatable devices, not cheap large ones.
UNIT 32 STUDY COMPLETE
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Ready for the Fab Challenge?
You've covered patterning polymer with light, then baking it into 3D glassy carbon.