Image reversal lithography, step by step

1 · Imagewise exposure

Coat, soft-bake and expose the resist as you would for an ordinary positive process, writing your pattern with the mask or — on a maskless tool — directly from the layout. In the exposed regions the resist's photoactive compound decomposes, forming the usual positive latent image: those areas would develop away if you stopped and developed now.

The imagewise dose is set deliberately lower than a straight positive process would use, because the reversal bake and flood exposure do the rest of the work — over-exposing here erodes the eventual undercut. Absolute doses depend on the resist and are calibrated on-tool: the AZ 5214E recipe page is honest that its datasheet publishes no absolute image-reversal dose, only the rule of thumb that the imagewise exposure runs roughly half the dose of a standard positive process. Treat that as a starting point for a dose array, not a specified value, and see the exposure guidelines for the trade-offs.

2 · Reversal bake

This is the step that makes the process "reversal", and it is the most temperature-critical of the four. Baking the exposed film activates a crosslinking agent selectively in the regions that received the imagewise dose: those exposed areas crosslink, become insoluble in developer, and — crucially — lose their remaining photosensitivity. The unexposed field is untouched by the bake and stays fully photoactive.

After this bake the pattern is inverted in potential: the originally-exposed regions are now the ones that will survive, and the originally-unexposed field is still developable. Because the crosslinking reaction is sharply temperature-dependent, the reversal-bake temperature has a narrow window — a few degrees too low and the exposed region never fully locks, a few too high and the whole film crosslinks and the pattern is lost. Use the temperature and time your resist's datasheet or recipe page specifies and hold the hotplate uniform; the post-exposure-bake guidelines explain why bake control matters for any bake-driven step.

3 · Flood (blanket) exposure

Now expose the entire wafer with no mask — a flood, or blanket, exposure — at a generous dose. The originally-exposed regions ignore it: the reversal bake already crosslinked them and stripped their photosensitivity, so more light does nothing. The originally-unexposed field, still photoactive, is now exposed for the first time and becomes soluble in developer, exactly like a normal positive exposure.

The flood step needs no alignment and no pattern — its only job is to sensitise everything the imagewise step left alone, so it is deliberately over-dosed to guarantee the field clears fully in development. After it, the two populations of resist are fully differentiated: crosslinked-and-insoluble where you wrote your pattern, exposed-and-soluble everywhere else.

4 · Develop

Develop in the resist's normal aqueous-alkaline developer. It dissolves the flood-exposed field — the areas you originally left unexposed — and leaves the crosslinked pattern standing. The result is a negative-tone image of your layout: where a straight positive process would have opened holes, image reversal leaves resist, and vice versa. For AZ 5214E, its recipe page lists development in AZ 340 (1:5) or AZ 726 MIF, at a coated thickness in the 1.14–1.98 µm range — the concrete figures for that one resist, quoted from its datasheet.

The prize is not just the inverted tone but the sidewall profile. During the imagewise exposure, the top of the film absorbs more light than the bottom, so after reversal the crosslinked region is narrower at its base than at its top — a negative (re-entrant) slope with an overhang. That overhang is exactly what metal lift-off wants: when you evaporate metal over the pattern, the overhanging edge breaks the deposited film so it doesn't wrap the resist sidewall, and solvent can reach in to lift the resist and its unwanted metal cleanly. Continue with lift-off, step by step.

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