What it is
Image-reversal resists are ordinary positive photoresists processed with an extra reversal bake and an unmasked flood exposure so the developed pattern comes out tone-inverted from a standard positive exposure, with a re-entrant, undercut sidewall profile built for metal lift-off.
How it patterns
Same first exposure as any positive resist
The image-wise exposure step is identical to a standard positive-tone process: light through the mask converts the resist's photoactive compound where it lands, leaving a latent image in an otherwise unchanged film. Nothing about this step is negative-tone yet — the reversal happens entirely in what comes next. (The dissolution chemistry this latent image is built on is covered on the DNQ-novolak resist page, not repeated here.)
The reversal bake locks in the exposed areas
A post-exposure "image reversal bake" — driven by a crosslinking agent or amine catalyst built into the resist — converts the exposed regions into an insoluble, crosslinked state and, critically, renders them no longer photoactive. The previously exposed pattern is now chemically fixed and blind to further light, while the areas that were never exposed remain an ordinary, still-photoactive positive resist.
Flood exposure sensitizes everything else
A second, unmasked flood exposure blankets the whole wafer. Because the reversal bake already deactivated the originally exposed regions, only the areas that were untouched by the first exposure respond to the flood — they become soluble in exactly the way an ordinary positive exposure would make them. Development then dissolves those newly sensitized areas, so what remains is the resist that was NOT exposed the first time: a negative, tone-inverted copy of the original mask pattern.
Why the profile suits lift-off
The reversal process leaves a distinctive re-entrant (undercut) sidewall — the resist opening is narrower at the top than at the base. Deposited metal in a lift-off process breaks cleanly at that overhang instead of forming a continuous film down the sidewall, so the resist strips away cleanly with the unwanted metal, leaving sharp-edged features. It is this undercut, not a specific dose or bake number, that makes image reversal the standard choice for lift-off patterning.
Where it fits
- For metal lift-off, the reversal bake and flood exposure leave a re-entrant, undercut sidewall that breaks the deposited film cleanly at the resist edge, without a separate release/undercut layer.
- For a dark-field pattern (small isolated features surrounded by a large exposed field), image reversal inverts a positive-tone mask's contrast without designing or ordering a separate negative mask.
- For a process line already built around a positive resist and an aqueous TMAH developer, image reversal adds the negative-tone option on the same coating and develop chemistry, rather than switching to a different resist family.
- For a simple etch mask with no lift-off requirement, the added reversal-bake and flood-exposure steps cost process time with no benefit over a single positive exposure and develop.
- For dose and bake control, the reversal step narrows the process window: the image-wise dose and the reversal-bake temperature have to be tuned together to hit a target undercut without eroding the finest features.
- For very thick single-coat films — tens of micrometres and up — image-reversal resists are a poor fit: they coat in the same one-to-few-micrometre range as the parent positive resist they are built from.
Datasheet-cited recipes
Every value on these pages is cited to the manufacturer’s datasheet — spin curve, bake schedule, exposure dose and developer.
Further reading
- H. Klose, R. Sigush, W. Arden. Image reversal of positive photoresist: Characterization and modeling. IEEE Transactions on Electron Devices (1985). doi:10.1109/t-ed.1985.22175
- K. Chiong, H. Hiraoka. Sub-Halfmicron Negative Resist Systems By Image Reversal. SPIE Proceedings, Advances in Resist Technology and Processing IV (1987). doi:10.1117/12.940327
- Hua-yu Liu, E.D. Liu. Diffusion in AZ-5214 image reversal process and its application to e-beam proximity effect correction. Microelectronic Engineering (1987). doi:10.1016/0167-9317(87)90071-2
Common questions
Why use image reversal for lift-off?
Because the reversal bake and flood exposure leave a re-entrant, undercut sidewall instead of the vertical or slightly sloped wall of a standard positive exposure. Deposited metal breaks at that overhang rather than coating the sidewall in a continuous film, so lift-off gives cleaner, sharper edges with fewer torn or stringy features.
What does the reversal bake actually do?
It crosslinks the just-exposed areas of the resist — via a crosslinking agent or amine catalyst built into the formulation — and, in the same step, deactivates their photoactive compound so they no longer respond to light. That is what lets a second, unmasked flood exposure sensitize only the areas the first exposure missed.
Is an image-reversal resist positive-tone or negative-tone?
Both, depending on how it is processed. The same coated film can be developed right after the first exposure as an ordinary positive resist, or carried through the reversal bake and flood exposure to develop as a negative-tone, undercut image — the final tone is a processing choice on one resist, not a fixed property of the material.
Expose it at 365 and 405 nm
NANYTE BEAM is a desktop maskless lithography system with software-selectable dual-wavelength exposure and 16-bit grayscale — no photomask, no mask cost, same-day iteration.
General photolithography reference material, not a specification of any particular NANYTE BEAM configuration. Product names and trademarks belong to their respective owners; NANYTE is not affiliated with the manufacturers mentioned.