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Ion-Beam Lithography

Ion-beam lithography, when used to expose resist, provides higher resolution than that possible with an electron-beam because of less scattering. Also, resists are more sensitive to ions than to electrons. A unique feature of ion-beam is that there is the possibility of wafer processing without resists if it is used to implant or sputter selected areas of the wafer. The most important application is repair of photomask, a task for which commercial systems are available.

Ion-lithography employs a scanning focussed-beam or a masked-beam. The problems of ion-optics for scanning ion beams are more severe than for electron optics. The source of ionized material is a gas surrounding a pointed tungsten tip or a liquid metal that flows to the tip from a reservoir. Electrostatic lenses rather than magnetic are used for focussing ion beams. If a magnetic lens were used, the field would have to be much larger than in the electron optics case. Electrostatic optical systems generally have higher aberrations, necessitating small aperature and small scan fields.

Comparison of Various Lithographies

It is expected that photolithography will continue to improve with wavelengths approaching 190 nanometers, the limit for silica. The photo wafer stepper will be the lithography system of choice for many years because of its relative simplicity, convenience, and reasonably high throughput. The practical resolution limit in production application will be 0.5 micro meters or slightly lower. The main limitation to higher photolithographic resolution are

  • Optical material
  • Small depth of focus
  • The difficulty of obtaining diffraction-limited imaging over a large field.

The scanning electron-beam systems are being employed in custom ICs for which high throughput is not needed. The custom ICs require fine definition, good overlay, flexibility, and quick turn around. The main limitation of scanning system is complexity and low throughput. The throughput for scanning system is roughly inversely proportional to the square of the linewidth.

The X-ray lithography with storage ring source and masked ion-beam lithography are the main candidates for high-volume production of advanced circuits with dimensions beyond the optical limit.


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