What it is
Epoxy negative resists are chemically amplified photoresists built on a multifunctional epoxy resin that crosslinks where it is exposed, leaving a tall, mechanically and chemically robust structure after the unexposed resist is developed away.
How it patterns
Cationic ring-opening crosslinking
On exposure, a photoacid generator (PAG) in the resist releases a strong acid. During the post-exposure bake (PEB) that acid catalytically opens the strained epoxide rings of the resin and links them into a dense three-dimensional network — a cationic ring-opening polymerization. Because one acid molecule drives many crosslinking events before it is consumed, the chemistry is amplified: modest doses cure thick films. The exposed, crosslinked region becomes insoluble, so the resist is negative-tone.
Develop and the negative tone
Development dissolves only the unexposed resist, in an organic solvent developer (PGMEA for SU-8-type resists) rather than an aqueous base. What remains is the crosslinked pattern. Because the cured epoxy network is stiff and solvent-resistant, these resists hold near-vertical sidewalls at high aspect ratios in a single thick coat, which is why they are the default choice for tall MEMS moulds, microfluidic channels and electroplating forms.
Why the process is bake-sensitive
The same amplification that makes epoxy resists sensitive also makes soft-bake and PEB temperature and time the parameters that most affect the result: under-baking leaves solvent and residual stress that cracks thick films, over-baking or an aggressive PEB ramp builds stress that delaminates them. Thick coats are ramped and cooled slowly for exactly this reason, and the datasheet bake schedule scales with target thickness.
Laminated dry-film variant (SUEX)
The same epoxy chemistry is also sold as a solid dry film — SUEX and similar sheets — that is hot-roll laminated onto the substrate instead of being spin-coated. Lamination gives a thickness set by the sheet (tens to hundreds of micrometres) that is independent of substrate topography, so it coats over cavities and coarse features a spun film would pool into, at the cost of the finest resolution. Exposure, PEB, develop and the negative tone are otherwise the same epoxy process.
Where it fits
- For tall MEMS moulds and electroplating forms, epoxy resists hold near-vertical sidewalls at aspect ratios of 10:1 and beyond in a single coat.
- For permanent structural layers and microfluidic channel walls, the crosslinked epoxy network is chemically and thermally robust enough to stay in the finished device rather than being stripped.
- For thick single-coat films, SU-8-type resists span a few micrometres to several hundred, and the laminated dry-film variant reaches the millimetre scale over uneven topography.
- For fine-pitch etch masking, epoxy resists are a poor fit: they are difficult to strip cleanly once fully crosslinked, so they suit permanent structures rather than a sacrificial mask.
- For low-stress thick coats, the bake schedule must be ramped and cooled slowly — an aggressive PEB builds film stress that cracks or delaminates the layer.
- For aqueous-developer process lines, note these resists develop in an organic solvent (PGMEA), not the TMAH base used by novolak resists.
Datasheet-cited recipes
Every value on these pages is cited to the manufacturer’s datasheet — spin curve, bake schedule, exposure dose and developer.
Further reading
- H. Lorenz, M. Despont, N. Fahrni, N. LaBianca, P. Renaud, P. Vettiger. SU-8: a low-cost negative resist for MEMS. Journal of Micromechanics and Microengineering (1997). doi:10.1088/0960-1317/7/3/010
- A. del Campo, C. Greiner. SU-8: a photoresist for high-aspect-ratio and 3D submicron lithography. Journal of Micromechanics and Microengineering (2007). doi:10.1088/0960-1317/17/6/R01
- S. Keller, G. Blagoi, M. Lillemose, D. Haefliger, A. Boisen. Processing of thin SU-8 films. Journal of Micromechanics and Microengineering (2008). doi:10.1088/0960-1317/18/12/125020
Common questions
Why is SU-8 a negative resist?
Exposure crosslinks the epoxy resin where light lands, making that region insoluble; the developer then removes only the unexposed resist, so the pattern that survives is the exposed pattern — the definition of a negative tone.
What develops an epoxy resist like SU-8?
An organic solvent developer — PGMEA (propylene glycol monomethyl ether acetate) for SU-8-type resists — not the aqueous TMAH base used for novolak resists. Incomplete development leaves residue at the base of tall features, so develop time scales with film thickness.
What makes epoxy resists suited to high-aspect-ratio structures?
The post-exposure bake crosslinks the exposed epoxy into a stiff, solvent-resistant network that holds near-vertical sidewalls after development, so a single thick coat can form tall moulds, channels and plating forms that would collapse in a softer resist.
Expose it at 365 and 405 nm
NANYTE BEAM is a desktop maskless lithography system with software-selectable dual-wavelength exposure and 16-bit grayscale — no photomask, no mask cost, same-day iteration.
General photolithography reference material, not a specification of any particular NANYTE BEAM configuration. Product names and trademarks belong to their respective owners; NANYTE is not affiliated with the manufacturers mentioned.