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
- Substrate
- Resist
- Exposed
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. SU-8, HARE SQ, SUEX and mr-DWL develop in a PGMEA-based solvent developer and ADEX in cyclohexanone; KMPR is the exception, developing in standard 0.26 N TMAH. 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.
What each step does
- 01 Spin coat
Epoxy resist is viscous, so it is spread at low speed first and then ramped to the final speed. Solvent starts leaving the moment the wafer turns, so the film thins and stiffens at the same time; the ramp decides how much of the thinning happens while it is still fluid.
Adjusting it: Final speed sets thickness, read off the spin curve. A film coming out thicker than the curve says often means the ramp was slower than the one the curve was measured at, so use the datasheet's spread and ramp as written. A thick coat builds a large edge bead, so remove it before baking.
- 02 Soft bake
Drives the casting solvent out so the film is dry and firm enough to expose. In a thick coat the solvent has to diffuse up through the whole layer, so the time grows with thickness.
Adjusting it: Too short leaves a soft, tacky film that wrinkles when it is re-heated and lifts in the developer. Heating or cooling too fast locks in stress that cracks the film later, so thick coats are ramped up and cooled down slowly.
- 03 Expose
Light turns the photoacid generator into a strong acid wherever it lands. Nothing else happens yet: at room temperature the film is too rigid for the acid to reach the epoxy groups, so what you have is a hidden pattern of acid.
Adjusting it: Dose sets how much acid forms, and the base of a thick film gets the least. Too little and the bottom never crosslinks, so features peel off in the developer. Too much and stray light crosslinks resist past the edge, widening lines and closing narrow gaps.
- 04 Post-exposure bake
The film heats and molecular mobility rises. The acid diffuses, catalyses the opening of the epoxy rings, and the chains crosslink into a 3D network. Each acid molecule drives many of these reactions, which is the amplification. The pattern normally shows up in the film during this bake.
Adjusting it: Temperature and time together set how far the crosslinking goes. Too little gives a weak network that swells, cracks or lifts in the developer. Too much, or too fast a ramp, adds stress that cracks thick films and lets acid spread into narrow gaps. If no image shows during the bake, the dose or the bake is too low.
- 05 Develop
The developer — PGMEA-based solvent for SU-8, HARE SQ, SUEX and mr-DWL, cyclohexanone for ADEX, standard aqueous TMAH for KMPR — dissolves only the resist that never crosslinked. It has to reach the bottom of every opening, so thick films and deep, narrow trenches take longer and need agitation.
Adjusting it: Develop time scales with thickness. With a solvent developer, a milky white film during the IPA rinse means resist is still left in the openings, so develop longer. If well-exposed features are lifting, the real cause is usually an under-crosslinked base from the dose or PEB, not the developer.
- 06 Hard bake
Optional, for resist that stays in the finished device. Baking above the PEB temperature takes the crosslinking further, so the epoxy becomes harder, more solvent-resistant and more thermally stable, and fine surface cracks can close.
Adjusting it: Use it only on permanent structures, because a fully cured epoxy is very hard to strip. Ramp and cool slowly, as in the PEB, because thermal stress is what cracks a hard-baked film.
When it goes wrong
Why does my patterned epoxy peel off the substrate?
Cause: The substrate wasn't clean and dry when it was coated, the surface is one the resist adheres to poorly, or stress built up during the post-exposure bake pulled the film away.
Fix: Clean and dehydration-bake the substrate immediately before coating, check the resist is rated for that surface, and ramp the post-exposure bake and cool slowly.
Sources: Kayaku, SU-8 2000 processing guidelines, Substrate preparation; DJ MicroLaminates, SUEX/ADEX FAQ, Adhesion loss
Why are there air bubbles under my laminated SUEX or ADEX film?
Cause: The sheet touched the substrate before it reached the nip rollers and trapped air, the substrate carried organic contamination, or the sheet was pressed on a hotplate instead of a laminator.
Fix: Hold the sheet just clear of the substrate until it enters the rollers, clean and dry the substrate right before laminating, and use a laminator with controlled temperature and pressure.
Source: DJ MicroLaminates, SUEX/ADEX FAQ, Bubbles / air entrapment
Why does my SU-8 or KMPR film wrinkle during the soft bake?
Cause: Solvent is still in the film: the bake was too short, or a convection oven formed a skin on the surface that held the solvent in.
Fix: Bake on a level contact hotplate, not in an oven. To check, cool the wafer to room temperature and put it back on the hotplate; if it wrinkles, bake a few minutes more and repeat until it no longer does.
Sources: Kayaku, SU-8 2000 processing guidelines, Soft bake; Kayaku, KMPR 1000 datasheet, Soft bake
Why does the resist stick to the photomask during exposure?
Cause: Solvent left by a short or cool soft bake keeps the film tacky, or particles and bubbles in the film transfer to the mask. Hard contact makes any of these worse.
Fix: Complete the soft bake, keep the mask clean, and use soft or proximity contact, or an anti-stick mask coating, if sticking continues.
Sources: DJ MicroLaminates, thick SUEX datasheet, Exposure, mask sticking; MicroChemicals, Lithography trouble-shooter, Sticking between resist film and mask
Why can I see the pattern in KMPR before the post-exposure bake?
Cause: The dose was too high: the image is forming during the exposure itself, before the bake.
Fix: Lower the dose and run an exposure matrix. The latent image should appear only once the wafer is on the PEB hotplate.
Source: Kayaku, KMPR 1000 datasheet, Exposure
Why doesn't an image appear during the post-exposure bake?
Cause: The dose was too low, the bake isn't getting the film hot enough, or both, so the acid-driven crosslinking that reveals the image hasn't happened.
Fix: Run an exposure-dose matrix, and check the hotplate's actual temperature and the wafer's contact with it. A correctly processed film shows its image early in the bake.
Sources: Kayaku, SU-8 2000 processing guidelines, Post-exposure bake; Kayaku, KMPR 1000 datasheet, Post-exposure bake
Why does my thick epoxy film crack after the post-exposure bake or hard bake?
Cause: The epoxy expands far more with heat than silicon or glass does, so fast heating and cooling build stress in the film, and shrinkage from crosslinking adds to it.
Fix: Ramp the bake in steps and cool slowly, over hours for the thickest films, instead of pulling the wafer straight off the hotplate. A short, controlled hard bake can close fine surface cracks.
Sources: Kayaku, SU-8 2000 processing guidelines, Hard bake; KemLab, HARE SQ datasheet, PEB and hardbake; DJ MicroLaminates, SUEX/ADEX FAQ, Wafer bow / stress delamination
Why did my exposed dry-film wafer crack while it waited for the bake?
Cause: Stress in the exposed regions distorts the still-soft unexposed resin around them while the wafer sits between exposure and bake, and the film cracks or delaminates.
Fix: Go straight from exposure to the post-exposure bake, and don't store exposed, unbaked wafers in a closed container for long.
Source: DJ MicroLaminates, SUEX/ADEX FAQ, Cracking / distortion during storage
Why does my SUEX or ADEX laminate bow or lift after the bake?
Cause: The film shrinks as it cures, and a cooling rate that doesn't suit the substrate pulls it into a bow or lifts it at the edges.
Fix: Bake in an oven rather than on a hotplate, use the manufacturer's lower-temperature, longer bake option, and cool slowly over several hours, ideally overnight.
Sources: DJ MicroLaminates, SUEX/ADEX FAQ, Wafer bow / stress delamination; DJ MicroLaminates, thick SUEX datasheet, Post-exposure bake
Why do small isolated features lift off in the developer?
Cause: The dose or the post-exposure bake was too low for that feature size, so it isn't crosslinked enough to hold on in the developer.
Fix: Raise the dose or the post-exposure bake and re-run the process matrix, watching the smallest features on the layout.
Source: MicroChemicals, Lithography trouble-shooter, Image reversal or negative tone resists?
Why does a white haze appear during the IPA rinse?
Cause: In a solvent-developed resist (SU-8, HARE SQ, SUEX, or KMPR run in SU-8 developer), the film is under-developed. Uncrosslinked resist is still in the openings and precipitates when it meets the IPA.
Fix: Put the wafer back in fresh developer to finish, then rinse again. Tight pitches and vias need stronger agitation, or an ultrasonic or megasonic bath.
Sources: Kayaku, SU-8 2000 processing guidelines, Rinse and dry; Kayaku, KMPR 1000 datasheet, Rinse and dry
Why won't my thick SUEX or ADEX film clear to the substrate?
Cause: A single still bath can't clear a film that thick in the usual time, or the developer isn't reaching the bottom of tall features.
Fix: For SUEX, develop face-down in two PGMEA baths with mild agitation; ADEX develops in cyclohexanone instead. Follow the datasheet's develop time for the film thickness, and refresh the developer partway through for the thickest sheets.
Sources: DJ MicroLaminates, thick SUEX datasheet, Development, table 1; DJ MicroLaminates, SUEX/ADEX FAQ, Incomplete development
Why is cured SU-8 or KMPR so hard to strip?
Cause: Crosslinked epoxy is built to be permanent, and a hard bake or high dose raises the crosslink density past what ordinary strippers can attack.
Fix: If SU-8 may ever need removing, plan a release layer underneath it; fully cured SU-8 comes off only with an oxidising wet etch or plasma. Strip KMPR promptly in heated remover and skip the hard bake when removal is planned.
Sources: Kayaku, SU-8 2000 processing guidelines, Removal; Kayaku, KMPR 1000 datasheet, Plating; removal
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 that most of these resists develop in an organic solvent (PGMEA-based, or cyclohexanone for ADEX), not the TMAH base used by novolak resists. KMPR is the one that fits a TMAH line: it develops in standard 0.26 N TMAH.
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?
SU-8 develops in an organic solvent developer based on PGMEA (propylene glycol monomethyl ether acetate), as do HARE SQ, SUEX and mr-DWL; ADEX uses cyclohexanone. KMPR is the exception and develops in the standard 0.26 N TMAH 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
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