UNIT 14 • THE PHOTORESIST PROCESS

Spin on a film thinner than a hair's width

A small puddle of resist is dropped at the wafer's center, and the wafer spins at thousands of rpm. Centrifugal force spreads the resist into a uniform film. Faster spinning gives a thinner film: from about 50 nm for EUV to a few micrometers for thick MEMS and packaging resists.

Spin speed and resist thickness trade off: faster spinning makes thinner films.

PREPARING THE WAFER

Prime, coat, then bake

Dehydration bake Drives off surface moisture so the resist can stick

Adhesion promoter A vapor of HMDS (hexamethyldisilazane) makes the surface hold resist better

Coat Spin on the resist and let the excess fly off the edge

Soft bake About 90 to 110 °C drives out most of the solvent and leaves a solid film

Resist that will not stick or is still wet will not print cleanly.

EXPOSE AND BAKE

Light starts a chemical reaction

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Expose

The scanner aligns the wafer and flashes the pattern onto the resist

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Photoacid

Modern resists contain a photoacid generator that makes acid where light lands

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Post-exposure bake

Heat lets that acid spread and change many resist molecules, amplifying the effect

Such chemically amplified resists need only a small dose of light, which speeds up printing.

One photon can trigger many chemical changes, so the resist responds to a small dose.

DEVELOP

Wash away the soft part

The developer A dilute water-based base, most often TMAH (tetramethylammonium hydroxide)

Positive tone The exposed, now-soluble resist dissolves in the developer

Rinse and dry Deionized water stops development and removes dissolved resist

Hard bake An optional extra bake toughens the resist for a harsh etch or implant

Development turns a chemical image into a physical stencil.

THE TRACK

Coater, scanner, developer: one connected line

track + scanner

In a modern fab, a robotic 'track' coats and bakes the wafer, hands it to the scanner for exposure, then takes it back for post-exposure baking and development. Scanners handle roughly 100 to 300 wafers per hour, so the track has to keep pace.

The scanner is the star, but the track makes a wafer ready for it.

STRIPPING

Remove the stencil and clean up

After the etch or implant The resist has done its job and must go completely

Oxygen plasma ashing Burns the organic resist away as gas

Wet strip and clean Solvents or acid mixtures lift off any residue

Then The clean wafer is ready for the next layer's coat

Resist is temporary: it is removed once its pattern has been copied.

UNIT 14 STUDY COMPLETE

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Ready for the Fab Challenge?

You can follow a wafer through coat, bake, expose, develop, and strip.