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.^[1] 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 or Heidelberg Instruments, 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.

Forms of maskless lithography

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.

Electron beam

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.^[2] 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.

Optical

Direct laser writing is a very popular form of optical maskless lithography, which offers flexibility, ease of use, and cost effectiveness in R&D processing. This equipment offers rapid patterning at sub-micrometre resolutions, and offers a compromise between performance and cost when working with feature sizes of approximately 200 nm or greater.

Interference lithography is another form of optical maskless lithography, but is limited to forming periodic patterns only.

For improved image resolution, ultraviolet light, which has a shorter wavelength than visible light, is used to achieve resolution down to around 100 nm. The main optical maskless lithography systems in use today are the ones developed for generating photomasks for the semiconductor and LCD industries.

In 2013, a group at Swinburne University of Technology published their achievement of 9 nm feature size and 52 nm pitch, using a combination of two optical beams of different wavelengths.^[3]

Focused ion beam

Focused ion beam systems are commonly used today for sputtering away defects or uncovering buried features. The use of ion sputtering must take into account the redeposition of sputtered material.

Probe tip contact

IBM has developed an alternative maskless lithography technique based on atomic force microscopy see here.^[4] In addition, Dip Pen Nanolithography is a promising new approach for patterning submicrometre features.

Future of maskless lithography

Maskless lithography is already used for the production of photomasks and in limited wafer-level production. There are some obstacles ahead of its use in high-volume manufacturing. First, there is a wide diversity of maskless techniques. Even within the electron-beam category, there are several vendors (Mapper Lithography, Canon, Advantest) with entirely different architectures and beam energies. Second, throughput targets exceeding 10 wafers per hour still need to be met. Third, the capacity and ability to handle the large data volume (Tb-scale) needs to be developed and demonstrated.

In recent years DARPA and NIST have reduced support for maskless lithography in the U.S.^[5]

There was a European program that would push the insertion of maskless lithography for IC manufacturing at the 32-nm half-pitch node in 2009.^[6] Project name was MAGIC, or “MAskless lithoGraphy for IC manufacturing,” in frame of EC 7th Framework Programme (FP7).^[7]

Due to the increased mask costs for multiple patterning, maskless lithography is once again increasing in visibility.

References

1. R. Menon et al.,, Materials Today, Feb. 2005, pp. 26-33 (2005).
2. T. H. P. Chang et al., Microelectronic Engineering 57-58, pp. 117-135 (2001).
3. Nature Communications DOI: 10.1038/ncomms3061
4. P. Vettiger et al., IBM J. Res. Develop. 44, pp. 323-340 (2000).
5. EETimes.com - Darpa, NIST to end funding for U.S. maskless lithography
6. EU forms new maskless litho group