Optical lithography begins with the coating of the substrate (e.g. a silicon wafer) with a thin layer of 'resist'. The resist is selectively exposed to ultraviolet light through a photomask on which the desired pattern has been produced. The photomask usually consists of a glass plate with a thin, opaque coating of chromium; the metal is removed from those regions corresponding to the exposed regions of the resist. Electron beam lithography is often used to prepare the photomask.
Those regions of resist which are exposed to ultraviolet light undergo a photochemical change which alters their resistance to the developer process. In wet development processes, a positive resist is normally insoluble in the developer solution used (aqueous base solution), but becomes soluble after exposure to UV. A negative resist becomes insoluble in the developer (an organic solvent) on exposure to UV light, due to cross-linking. Alternatively, a dry development process (plasma etch) may be employed.
After developing, lift-off metallization is the next step in the fabrication of electrode structures.
Feature sizes down to around 1m can be obtained; but there are fundamental limits to the resolution available from optical lithography, due to diffraction of the light around the boundaries of transparent and opaque regions of the photomask.
If an aperture is illuminated by light whose wavelength is greater than the aperture size, then in the immediate vicinity of the aperture, near-field light emerges. The region over which the near-field light appears is approximately the size of the aperture, regardless of the illuminating wavelength. This means that it can be used as a lithographic light source for much smaller feature sizes than the diffraction-limited sizes in standard lithography.
However, because the near field only exists very close to the mask apertures, it is not able to expose a resist layer of normal thickness. One way around this is to use an ordinary resist layer with an additional resist layer on top. The top resist is exposed using near-field light, and developed, producing high-resolution features. The substrate is then subjected to a plasma etch. Provided the top resist is chosen to be unaffected by the plasma etch, the lower resist will become etched only in those regions where the upper resist has been removed in the near-field process.
For further information on the optical lithography facility (which includes standard and near-field techniques) in the Computer Science department at Manchester, go to: http://people.man.ac.uk/~mbhmnpw/nanoscale/index.htm
Conceptually similar to optical lithography, in that a pattern is produced in a resist layer by selectively exposing certain regions to sufficient energy to produce a chemical change in it. However, instead of using a mask and exposing the entire pattern at once, the pattern is written pixel by pixel onto the resist layer using a narrow beam of electrons, typical energy ~100 keV. The beam is focused and scanned over the region to be exposed using electromagnetic lenses and deflection coils, as in an electron microscope. Feature sizes as small as 10 nm are possible.
Resolution is limited by back-scattering of electrons in resist and substrate.
The lithographic processes are well-established, and are fundamental to the production of integrated semiconductor devices. However, several workers are examining the possibility of simpler printing processes, especially if they are able to reproduce feature sizes inaccessible to conventional optical lithographies.
One example of 'soft lithography' starts out with a master in which a raised pattern has been produced by optical or electron beam lithography. A 'rubber stamp' is produced from the master by coating it with a siloxane monomer, which is then polymerized to form a solid. After peeling away the copy from the master, it is 'inked' with a thiol solution, and 'stamped' onto a gold film on a suitable substrate. The thiols, which have a high affinity for gold, adhere to the gold film as a self-assembled monolayer. This can act as a resist for subsequent processing. Despite its apparent simplicity, this technique can reproduce features down to 50 nm.
see G.M. Whitesides and J.C. Love, Scientific American, Sept. 2001, p. 33
Another example of self-assembly occurs when an etched pattern in a (conductive) master is placed within a few hundred nm of a molten resist such as poly(methylmethacrylate) coated onto a conductive substrate and an electric field applied between the master and the substrate. The resist rises up to form a copy of the master pattern, probably as a result of the electrostatic attraction of mobile charges in the resist to the conductive master. In some cases, probably depending on the surfactant used to coat the master, the resist spontaneously forms a periodic array of columns within the pattern regions.
see: S.Y. Chou and L. Zhuang, J. Vac. Sci. Technol. B 17 3197 (1999); S.Y. Chou, L. Zhuang and L. Guo, Appl. Phys. Lett. 75 1004 (1999))
Metallization processes involve deposition of metal from the vapour phase on to a substrate.
In sputter coating, the metal atoms are dislodged from a target into the vapour phase by bombardment with energetic argon ions. This technique has the advantage that material can be transferred from target to substrate without change of stoichiometry - thus it is possible to sputter-coat alloys, metal oxides etc. In evaporation coating, the metal is vaporized by heating in a vacuum, either with a filament, or a powerful electron beam.
The thickness of the metal coating can be controlled by using a film thickness monitor. This is based on a quartz crystal, which is exposed to the same metal deposition as the substrate. As the metal is deposited on its faces, the resonant frequency of the crystal (in the MHz range) decreases. Because frequency can be measured quite easily to within 0.1 ppm, precise measurement and control of the metal layer thickness is possible.
To deposit patterns of metallization onto the substrate, two methods are possible.
1. A shadow mask may be used, in which the desired pattern has been produced as a set of apertures either by laser cutting of stainless steel foil, or photolithography of a beryllium-copper foil. The mask is then interposed between the metal source and the substrate, so that deposition only occurs under the apertures in the mask. Of course, some patterns cannot be produced by this method on topological grounds, because they would require 'islands' floating in the middle of an aperture. Also, the minimum feature size is in practice restricted to around 2 x foil thickness, so that high resolution requires very thin, fragile foils. Feature size is in practice restricted to a few tens of microns.
2. Lift-off metallization. In this technique, optical or electron beam lithography is used to produce a pattern in a resist layer on the substrate. The mask and resist combination must be such that the areas of substrate to be metallized are free of resist after development. The entire substrate is then coated with metal to the desired thickness. Finally, the substrate is placed in a solvent which dissolves the resist layer, and thus lifts-off its metal coating, leaving only the desired pattern of metallization on the substrate.