Photoresist for microfluidics: master moulds and channels

1 · Two roles for resist, then a clean substrate

Before any coating, it helps to know which of two roles the resist is playing. In the dominant workflow — soft lithography — the resist is a reusable master mould: you pattern a relief of the channels in resist on a flat wafer, cast liquid PDMS (a silicone elastomer) against it, cure and peel, and the PDMS carries the channels as an inverse of the resist. SU-8 is the classic master-mould resist, and maskless direct-write moulds are common — Dupuit et al., for example, lithograph neuron-culture chambers and microchannels in stacked mr-DWL layers, hard-bake the mould, and cast PDMS from it.

In the second role the resist is the device: the imaged film forms permanent channel walls that are capped with a bonded lid rather than replicated in PDMS. Thick epoxies do this well — AZ 125 nXT has been patterned into 90 µm-deep channels and sealed by adhesive bonding as an SU-8 alternative (Knoll et al.), and dry-film resists such as ADEX and SUEX laminate up multi-level channel structures with no spin-coating at all (Cao et al.). A thinner route instead images a resist such as AZ 1518 as an etch mask and wet-etches the channels into glass (Mitra et al.). Browse them in the library filtered to microfluidics.

For a master mould, start from a clean, flat, dehydrated wafer: flatness and cleanliness set the floor of every channel, and a particle becomes a defect in every PDMS part cast off that mould. Prime or dehydration-bake as the resist calls for (see adhesion, in the guidelines).

Sources: Dupuit et al. (2023) — mr-DWL PDMS neural-network moulds; Knoll et al. (2015) — AZ 125 nXT bonded microfluidic chips; Cao et al. (2021) — laminated SUEX dry-film microfluidics; Mitra et al. (2013) — AZ 1518 glass microchip electrophoresis

2 · Coat the mould to the channel height

The coating step sets the channel height. In a PDMS chip the resist relief's thickness becomes the channel's depth, so you pick the resist grade and spin speed that give exactly that height. Deeper channels call for a thicker grade — SU-8 2050 spans tens to over a hundred microns, for instance — while shallow channels use a thinner grade; the target thickness is read off the resist's own spin curve, published on every recipe page, never guessed.

Geometry has limits. Very tall, narrow walls in the mould can bow, peel or collapse during development, and re-entrant or badly undercut sidewalls tear when the PDMS is demoulded — so channel aspect ratio and sidewall verticality are design constraints, not free parameters (see aspect ratio, in the glossary). Multi-height chips are built by coating and patterning more than one resist layer in sequence, as in the stacked mr-DWL mould above.

3 · Soft-bake the film

The soft bake drives casting solvent out of the coated film. A thick channel-mould film holds a lot of it, so the bake is longer and often ramped; under-baking leaves solvent that blisters during exposure or sticks to the cast PDMS, while over-baking can stress or crack a thick epoxy film. Use the temperature and time the recipe gives for that thickness.

4 · Expose the channel pattern

Exposure defines the channels. On a maskless tool you write the layout directly — no photomask — which suits microfluidics, where designs iterate often and feature sizes are forgiving; Sooriyaarachchi et al., for example, pattern a herringbone-mixer microfluidic device by maskless direct-write on a Microwriter ML3. A negative channel-mould resist such as SU-8 cross-links the walls you keep, and a thick film needs enough dose to expose to its full depth. Published dose windows are on each recipe page (see exposure, in the guidelines).

Sources: Sooriyaarachchi et al. (2020) — maskless herringbone microfluidics

5 · Post-exposure bake

Epoxy channel-mould resists are chemically amplified and need a [post-exposure bake](/photoresist-guidelines#peb) to complete cross-linking of the exposed regions before development. This is where a thick SU-8 mould most often cracks if it is ramped too fast, because the cross-linking film and the substrate expand differently — gentle ramps up and down are the usual remedy. Some negative acrylics, such as AZ 125 nXT, are formulated to cross-link fully during exposure and skip the post-exposure bake entirely (Knoll et al.); follow the recipe for the grade you chose.

Sources: Knoll et al. (2015) — AZ 125 nXT bonded microfluidic chips

6 · Develop the mould

Development dissolves the unexposed resist and opens the channels of the relief. For a deep mould this means a long, gently agitated immersion in the resist's own developer, followed by a rinse that does not attack the cross-linked walls; incomplete development leaves residue on the floor that transfers into every PDMS cast. Timings scale with thickness and are per-recipe (see development, in the guidelines).

7 · Hard-bake the master mould

A brief [hard bake](/photoresist-guidelines#hard-bake) after development anneals and toughens the relief so it survives being cast against repeatedly — a good master mould yields many PDMS parts. This is also the point at which a resist used as permanent channel walls is fully hardened before a lid is bonded, rather than being replicated in PDMS.

8 · Silanise the mould, then cast PDMS

Cured PDMS bonds tenaciously to bare SU-8, and pulling it off can tear fine features or shorten the mould's life. So the finished mould is usually surface-treated first: a vapour-phase silanisation with a fluorinated silane lays down an anti-stiction layer, so PDMS releases cleanly and the mould can be reused.

Then cast: pour degassed PDMS pre-polymer over the silanised mould, cure it (typically on a hotplate or in an oven), and peel the cured slab off — it carries the channels as the inverse of the resist relief. Punch the inlet and outlet ports, bond the PDMS to a glass slide or a second PDMS layer (oxygen-plasma or thermal bonding), and the microfluidic device is complete. Permanent-wall devices reach the same end differently, by bonding a lid directly onto the imaged resist walls (Knoll et al.; Cao et al.). Grayscale exposure can even texture a channel wall with a graded relief, as Guastella et al. show by writing wedge-shaped grooves into ma-P 1275G.

Sources: Knoll et al. (2015) — AZ 125 nXT bonded microfluidic chips; Cao et al. (2021) — laminated SUEX dry-film microfluidics; Guastella et al. (2025) — grayscale-textured channel wall

Common questions

Where to next

1. Dupuit et al.. Portrait of intense communications within microfluidic neural networks. Scientific Reports (2023). doi:10.1038/s41598-023-39477-9
2. Knoll et al.. Fabrication of microfluidic chips using lithographic patterning and adhesive bonding of the thick negative photoresist AZ 125 nXT. Proceedings of SPIE (2015). doi:10.1117/12.2178812
3. Cao et al.. Dry Film Resist Laminated Microfluidic System for Electrical Impedance Measurements. Micromachines (2021). doi:10.3390/mi12060632
4. Mitra et al.. Microchip electrophoresis at elevated temperatures and high separation field strengths. ELECTROPHORESIS (2013). doi:10.1002/elps.201300427
5. Sooriyaarachchi et al.. ZnO Nanowire-Anchored Microfluidic Device With Herringbone Structure Fabricated by Maskless Photolithography. Biomedical Engineering and Computational Biology (2020). doi:10.1177/1179597220941431
6. Guastella et al.. Directional Fluidity of Dense Emulsion Activated by Transverse Wedge-Shaped Microroughness. Micromachines (2025). doi:10.3390/mi16030335