How photoresist is made

Photoresist is a liquid that hardens or dissolves where light hits it, and that lets chips be drawn with light.

stations: 6
things upstream: 14
raw sources: 5
steps from the ground: 7

The short version

Conventional photoresist has three parts: a resin binder, a photoactive sensitizer and a solvent that keeps it liquid. Makers prepare and refine the resin, dissolve each component in solvent, and combine the solutions in a mixing vessel. The raw liquid then passes through fine plastic filters, sometimes twice, because stray particles and by-products leave defects in the printed pattern.

Prepare the resin

Resists for g-line and i-line lithography use novolak, a resin made by condensing phenols or cresols with formaldehyde. Far-ultraviolet resists use polymers with groups that an acid can cleave off.

Goes in
Phenolic resin
Refine the crude polymer

Unwanted short and overlong chains left over from polymerization cause scum and bridging in the final pattern. One patent dissolves the crude resin at a lower strength than the finished resist uses, then filters it to remove them.
Dissolve each component

Resin goes into one portion of solvent and the light-sensitive agent into another. Far-ultraviolet types carry an acid generator and a quencher instead. Solvents named in one patent include ethyl lactate and 2-heptanone.
Combine in a mixing vessel

The separate solutions run through their own valves into one vessel and are mixed until uniform. A patent diagram shows this as the first stage of the production apparatus.
Filter out fine particles

Filters of fluorine-based resin or polyolefin catch the particles, with pores of roughly 0.01 to 1 µm. Some lines hold the liquid in a tank between two filter stages. Clogging is the running cost, so one patent back-washes filters with solvent.
Wash down between batches

Using the same equipment for the next batch risks carrying over components and raising particle counts. One patent rinses with the resist solvent, a different solvent able to dissolve the residue, then the resist solvent again.

End of the linePhotoresist Next it goes into microchip, memory chip, printed circuit board and 3 more.

Worth repeating

A positive photoresist is weakened by light and a negative one is toughened by it.
Photoresist is used in photoengraving as well as chip photolithography.