Maskless lithography

In maskless lithography, the radiation that is used to expose a photosensitive emulsion (or photoresist) is not projected from, or transmitted through, a photomask. Instead, most commonly, the radiation is focused to a narrow beam. The beam is then used to directly write the image into the photoresist, one or more pixels at a time. An alternative method, developed by Micronic Laser Systems (today MYCRONIC) or Heidelberg Instruments Mikrotechnik, is to scan a programmable reflective photomask, which is then imaged onto the photoresist. This has the advantage of higher throughput and flexibility. Both methods are used to define patterns on photomasks.

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.

Currently, the main forms of maskless lithography are electron beam and optical. In addition, focused ion beam systems have established an important niche role in failure analysis and defect repair. Finally, systems based on arrays of probe tips have recently been announced.

The most commonly used form of maskless lithography today is electron beam lithography. Its widespread use is due to the wide range of electron beam systems available accessing an equally wide range of electron beam energies (~10 eV to ~100 keV). This is already being used in wafer-level production at eASIC, which uses conventional direct-write electron beam lithography to customize a single via layer for low-cost production of ASICs.

Most maskless lithography systems currently being developed are based on the use of multiple electron beams. The goal is to use the parallel scanning of the beams to speed up the patterning of large areas. However, a fundamental consideration here is to what degree electrons from neighboring beams can disturb one another (from Coulomb repulsion). Since the electrons in parallel beams are traveling equally fast, they will persistently repel one another, while the electron lenses act over only a portion of the electrons' trajectories.

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