Maskless lithography

Maskless lithography utilizes methods that directly transfer the information onto the substrate, without utilizing an intermediate static mask, i.e. photomask that is directly replicated. In microlithography typically radiation transfer casts an image of a time constant mask onto a photosensitive emulsion (or photoresist).Traditionally mask aligners, steppers, scanners, but also other non-optical techniques for high speed replication of microstructures are common. The concept takes advantage of high speed or parallel manipulation technologies that have been enabled by large and cheap available computing capacity, which is not an issue with the standard approach that decouples a slow, but precise structuring process for writing a mask from a fast and highly parallel copy process to achieve high replication throughputs as demanded for in industrial microstructuring. Maskless lithography utilizes methods that directly transfer the information onto the substrate, without utilizing an intermediate static mask, i.e. photomask that is directly replicated. In microlithography typically radiation transfer casts an image of a time constant mask onto a photosensitive emulsion (or photoresist).Traditionally mask aligners, steppers, scanners, but also other non-optical techniques for high speed replication of microstructures are common. The concept takes advantage of high speed or parallel manipulation technologies that have been enabled by large and cheap available computing capacity, which is not an issue with the standard approach that decouples a slow, but precise structuring process for writing a mask from a fast and highly parallel copy process to achieve high replication throughputs as demanded for in industrial microstructuring. Maskless lithography follows two main paths. One is a rasterized approach by generation of a time-variant intermittent image on an electronically modifiable (virtual) mask that is projected with known means (also known as Laser Direct Imaging and other synonyms), or by direct writing, where the radiation is focused to a narrow beam that is scanned in vector form across the resist. The beam is then used to directly write the image into the photoresist, one or more pixels at a time. Also combinations of the two approaches are known and it is not limited to optical radiation, but also extends into the UV, includes electron-beams and also mechanical or thermal ablation via MEMS devices. A key advantage of maskless lithography is the ability to change lithography patterns from one run to the next, without incurring the cost of generating a new photomask. This may prove useful for double patterning or compensation of non-linear material beahaviour (e.g. when utilizing cheaper, non-cristalline substrate or to compensate for random placement errors of preceding structures). The main disadvantages are complexity and costs for the replication process, the limitation of rasterization in respect to oversampling causes aliasing artefact, especially with smaller structures, while direct vector writing is limited in throughput. Also the digital throughput of such systems forma a bottleneck for high resolutions, i.e. structuring a 300mm diameter wafer with its area of ~707cm² requires about 10 TiB of data in a rasterized format without oversampling and thus suffers from step-artefacts (aliasing). Oversampling by a factor of 10 to reduce these artefacts adds another two orders of magnitude 1 PiB per single wafer that has to be transferred in ~1 min to the substrate to achieve high volume manufacturing speeds.Industrial maskless lithography is therefore currently only widely found for structuring lower resolution substrates, like in PCB-panel production, where resolutions ~50µm are most common (at ~2000 times lower throughput demand on the components).