Contact Mask Aligner

A contact mask aligner exposes photoresist by pressing the photomask directly against the wafer surface before switching on the UV lamp, which is the simplest and highest-resolution way to transfer a mask pattern but also the one most likely to damage both the mask and the wafer. With zero gap between mask and resist, diffraction has almost no room to blur the pattern edges, so contact printing can resolve features well below what the same wavelength would allow through an air gap — but every exposure grinds mask chrome against wafer topography, so particles trapped at the interface scratch the mask, and the mask itself slowly accumulates resist residue that prints as defects on every wafer exposed afterward until it is cleaned.

Because contact printing uses no projection lens at all, the mask must be the same size as the final pattern, which is the opposite of a stepper's reduction optics and is exactly why contact aligners are confined to applications where mask wear and defect risk are acceptable trade-offs for resolution and low cost. This makes contact aligners a staple for MEMS, compound-semiconductor prototyping, thick-resist bump and redistribution layers, and teaching/research labs, where feature counts per mask are low enough that gradual mask degradation is tolerable — but it rules the technique out for high-volume CMOS front-end lithography, where a single mask must print millions of wafers without ever touching one of them.

Contact mask aligners sit at one end of the mask-exposure spectrum defined by gap distance — zero gap for contact printing, a controlled air gap for a proximity mask aligner, and a full projection lens for a stepper or scanner — and a double-sided mask aligner extends the same contact/proximity hardware with a second optical path so both wafer faces can be patterned in registration with each other for through-wafer MEMS and interposer work.