BORROWED FROM CHIPS

Lithography makes the mould

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From the chip industry

Light patterns a resist, just as in making transistors

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A master

For PDMS chips, the patterned wafer is a mould, not the chip itself

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Reusable

One master can mould many chips

In soft lithography, the cleanroom step happens once; the copies happen at a lab bench.

SU-8

A thick, tough negative resist

SU-8 is an epoxy photoresist. Where UV light hits it, it cross-links and hardens (a negative resist). It can be spun from about 1 µm to several hundred micrometres thick with steep, straight walls.

The raised SU-8 ridges become the channels in every PDMS copy.

THE MASK

Drawing with light

Printed film Cheap and fast, fine for features down to roughly 10 to 20 µm

Chrome on glass Sharp micron-scale features, but costs more and takes longer

Maskless writer A laser or projector draws the pattern straight from a file

Pick the mask that resolves your smallest feature, and no more.

SPIN COATING

Spin speed sets channel height

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Faster spin Thinner layer

Thicker formulation Thicker layer at the same speed

Example A typical SU-8 grade can give about 50 µm at a few thousand rpm

Two layers Coat, expose, coat again and align a second mask for two channel heights

Channel height is set by the resist thickness, which is why a profilometer checks every master.

OTHER MOULDS

Not every master needs a cleanroom

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Micromilled metal

Aluminium or brass moulds, good for larger features and hot embossing

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3D-printed moulds

Quick and cheap, with rougher surfaces

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Dry-film resist

Laminated sheets that skip spin coating

Feature size, surface finish and budget decide how the master is made.

UNIT 7 STUDY COMPLETE

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Ready for the Fab Challenge?

You've followed how light, a mask and SU-8 resist make the master mould that soft lithography copies.