Grayscale lithography: 3D structures in photoresist

1 · From dose-per-pixel to depth-per-dose

A binary exposure is all-or-nothing: anywhere the dose clears the film, the resist develops away to the substrate; anywhere it doesn't, the full thickness stays. Grayscale exploits the in-between. Below the dose that fully clears a positive resist, a partial dose only partially sensitises the film, so development removes part of the thickness and leaves the rest. Deliver a different dose at each point in the pattern and you get a different residual depth at each point — a continuous surface relief rather than a vertical wall.

So the quantity you are really designing with is depth as a function of local dose. A maskless tool sets that dose pixel by pixel from a grayscale bitmap — the darker or lighter each pixel, the lower or higher its dose — which is why direct-write systems are the natural fit for grayscale; the pillar page explains that per-pixel modulation and how bit depth sets how smooth the relief can be. The resist's job is to translate that dose map faithfully into a depth map, and how well it does that is a property of the film.

2 · What makes a photoresist grayscale-suitable

Not every resist grades cleanly. The property that matters is low contrast — a gentle, near-linear slope in the region where developed thickness falls off with dose. A high-contrast resist is engineered to do the opposite: it switches from fully-retained to fully-cleared across a narrow dose window, which is ideal for crisp vertical binary features and poor for grayscale, because a small dose error swings the depth from almost nothing to the whole film. A grayscale-suitable resist trades that sharp switch for a wide, well-behaved partial-dose region you can land depths inside.

In the recipe library, exactly one resist is flagged grayscale-suitable in its datasheet: the ma-P 1275G recipe records that its manufacturer explicitly positions the resist for greyscale work, with a published film-thickness range reaching up to 60 µm and a stated depth range of roughly 50–60 µm — a thick, low-contrast positive resist built for exactly this. Browse the library filtered to grayscale / 3D to see it in context.

Several other thick positive resists have been driven in grayscale in the research literature even though their datasheets do not address it — and that distinction is worth stating plainly. Grayscale 3D microstructures have been demonstrated in AZ 4562 by calibrating gray value against exposure depth on a laser direct-write system, then transferring the relief into silicon and electroplated metal. But the AZ 4562, AZ 4533 and AZ 9245 recipe pages each note their datasheets never characterise greyscale exposure; the grayscale behaviour comes from third-party work, not a manufacturer endorsement, so treat it as a starting point to calibrate rather than a specified process.

Sources: Lima et al. — Fabrication of 3D microstructures using grayscale lithography

3 · Calibrating a dose-to-depth curve

Because depth-per-dose depends on the resist, its exact film thickness, the develop chemistry and time, and your tool's optics, it cannot be read off a datasheet — you measure it. The standard move is a calibration array: expose a grid of pads at a stepped ladder of grey levels (dose values), develop the wafer exactly as you intend to develop the real part, then measure the residual thickness of each pad with a profilometer or optical profiler. Plotting measured depth against the grey level gives you the transfer curve for that specific stack.

That curve is the design tool. To render a target 3D shape you invert it — for each height the surface should have, read off the grey level that produces it, and paint your grayscale bitmap accordingly. Recalibrate whenever anything in the stack changes: a new resist lot, a different film thickness, a fresh developer bath or a changed develop time all shift the curve. There are no dose numbers here to copy: the dose-to-depth curve is yours to measure on-tool, and quoting someone else's would be worse than quoting none. The processing guidelines cover the exposure and development trade-offs the curve is sensitive to.

4 · Where grayscale is used

The payoff is any structure whose function is a continuous surface profile rather than a set of vertical walls — the shapes a stacked series of binary layers can only approximate in staircase steps.

• —Micro-optics — microlens arrays, blazed and diffractive gratings, and other refractive or diffractive elements whose optical performance follows their exact surface curvature. A firefly-inspired three-dimensional relief written into AZ 9245 by direct-write laser lithography, for instance, raised the light-extraction efficiency of an LED.
• —Ramps, stairs and sinusoidal reliefs — arbitrary height maps encoded as a grayscale bitmap and reproduced directly in resist; encoding a 3D model as a grey map and writing it into ma-P 1275G on a DMD maskless projector reproduces stairs, ramps and sinusoidal profiles in one exposure.
• —Molds and etch masters — a resist relief is rarely the end product. It is commonly transferred into the substrate by an anisotropic etch (a near-1:1 selectivity carries the resist shape into silicon), or replicated into metal by electroplating, or cast into PDMS to make a mold with a smoothly varying depth.

Sources: Bay et al. — Firefly-inspired overlayer for LED light extraction; Borghi et al. — Simplified grayscale lithography encoding using Blender; Lima et al. — Fabrication of 3D microstructures using grayscale lithography

5 · Limits and things that bite

Grayscale trades the robustness of a binary process for analogue control, and the analogue path has its own failure modes worth planning around:

• —The depth range is bounded by the film. You cannot render a relief taller than the coated thickness, and the deepest useful depth is limited by where the resist stops responding gradually — near full clearing the curve flattens and control is lost.
• —Depth resolution follows contrast and bit depth together. A high-contrast resist quantises depth into a few coarse levels no matter how many grey levels the tool can address; a smooth surface needs both a low-contrast film and enough exposure levels to drive it.
• —Calibration drifts. A curve measured last month on a different lot or a tired developer bath will place your depths wrong — recalibrate rather than reuse.
• —Etch transfer changes the shape. If you etch the relief into a substrate, the etch selectivity scales every height by a fixed ratio and can round or distort features; design the resist profile for what the etch will make of it, not for the final target directly.

Common questions

Where to next

1. Lima et al.. Fabrication of 3D microstructures using grayscale lithography. Advanced Optical Technologies (2019). doi:10.1515/aot-2019-0023
2. Bay et al.. Optimal overlayer inspired by Photuris firefly improves light-extraction efficiency of existing light-emitting diodes. Optics Express (2013). doi:10.1364/OE.21.00A179
3. Borghi et al.. Rapid prototyping of 3D microstructures: A simplified grayscale lithography encoding method using blender. Micro and Nano Engineering (2025). doi:10.1016/j.mne.2024.100294