What it is
Lift-off underlayers such as LOR and PMGI are non-photoimageable polymer coatings spun beneath a conventional imaging resist; they are never exposed themselves, but develop faster and sideways than the resist above them, carving a re-entrant undercut that breaks an evaporated metal film cleanly for lift-off.
How it patterns
A non-photoimageable layer beneath the pattern
LOR and PMGI are polydimethylglutarimide-based polymer coatings, spin-coated onto the substrate and soft-baked first. A conventional imaging resist is then spin-coated, exposed and developed on top of them. Neither LOR nor PMGI is itself exposed in the standard bilayer lift-off flow — the underlayer plays no photochemical role; it exists only to sit beneath the pattern and control what happens to it during develop.
Isotropic dissolution builds the undercut
The same develop step that defines the top resist's pattern also reaches the underlayer beneath it and dissolves it isotropically — sideways as well as downward — at a rate governed chiefly by the underlayer's own soft-bake temperature, not by the top resist's exposure dose or develop time. Because the underlayer clears laterally faster than the imaging resist's edge retreats, development leaves a re-entrant opening: wider at its base than at its top.
Why the overhang enables lift-off
When metal is evaporated over this profile, the overhanging lip of imaging resist breaks the film's continuity: metal lands on top of the resist stack and, separately, on the substrate inside the opening, with no continuous sidewall coating connecting the two. Soaking the stack in a dedicated stripper then dissolves the underlayer and imaging resist together and lifts away the unwanted metal cleanly, leaving only the metal that landed directly on the substrate.
Bake temperature, not exposure, is the process lever
Because the underlayer is never exposed, the undercut is tuned by adjusting its soft-bake temperature and time — an independent lever from the imaging resist's dose or develop time. One PMGI datasheet also documents an optional deep-UV flood "Cap-On" exposure of the underlayer for straighter sidewalls, offered as a PMGI-generic option rather than confirmed for every individual grade.
Where it fits
- For lift-off patterning of evaporated metal, an LOR/PMGI underlayer beneath a conventional imaging resist builds the re-entrant undercut that a straight-walled single-layer resist profile does not reliably give.
- For process simplicity, these underlayers develop in the same TMAH-based developer as the imaging resist coated on top, so the bilayer clears in one develop step rather than a separate chemistry.
- For tuning the undercut, the underlayer's own soft-bake temperature is a dedicated process lever, adjustable independently of the imaging resist's exposure dose or develop time.
- For direct patterning, LOR and PMGI are not designed to be exposed and developed on their own — they carry no photospeed or resolution spec, since the pattern is defined entirely by the imaging resist coated above them.
- For a single-layer process, adding an underlayer means an extra coat, an extra bake, and a dedicated stripper step versus a resist that lifts off on its own.
- For dialing in a specific undercut geometry, published bake-temperature-vs-undercut-rate curves are not available for every LOR/PMGI grade, so datasheets recommend a small bake matrix per grade rather than trusting one stated condition.
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
- B. Cord, C. Dames, K. K. Berggren, J. Aumentado. Robust shadow-mask evaporation via lithographically controlled undercut. Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena (2006). doi:10.1116/1.2375090
- Y. Chen. A lift-off process for high resolution patterns using PMMA/LOR resist stack. Microelectronic Engineering (2004). doi:10.1016/s0167-9317(04)00111-x
Common questions
Why do I need an underlayer for lift-off?
A single, straight-walled imaging resist coats the sidewall of a deposited film continuously, leaving no break for a stripper to attack. An underlayer beneath the imaging resist develops faster and isotropically, carving an overhanging profile that breaks the metal film's continuity so lift-off works cleanly instead of tearing.
LOR vs PMGI — what's the difference?
Both are polydimethylglutarimide-based underlayers from the same manufacturer datasheet family, used the same way: spin-coated beneath an imaging resist, never exposed in the standard bilayer flow, and dissolved isotropically in develop at a rate set mainly by soft-bake temperature. They differ mainly in product-line grading; PMGI is separately documented with an optional deep-UV flood "Cap-On" exposure for straighter sidewalls, an option not documented for LOR in the same datasheet.
What controls the size of the undercut?
The underlayer's own soft-bake temperature and time — not the top resist's exposure dose or develop time, since the underlayer is never itself exposed. Because bake-vs-undercut-rate curves aren't published for every LOR/PMGI grade, a small bake matrix per grade is the recommended way to dial in a target profile.
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.