Lift-off patterning processes using solid-condensed-gas layers as resists
This technology has numerous advantages over standard polymer resists such as:
a) gentler conditions (no spinning on of slurries, no baking at elevated temperatures, no exposure to liquid surface tension),
b) economic (fewer steps, no expensive polymer resists and no toxic solvents to dispose),
c) broader array of substrates can be patterned from standard 2D planar inorganic substrates to 3D fragile substrates such as biological substrates to 1D substrates such as nanotubes and nanowires,
d) cleaner processing (all steps occur in a single vacuum chamber, removal of unexposed condensed gasses leaves minimal residues).
These inventions (see also case 2310) involve the use of gases directly solidified onto cryogenically cooled substrates to form conformal thin films as resists for lithographically patterning the substrates. Examples of such gases are water vapor and rare gases, such as argon. Water as a resist is placed on the substrate directly from the gas phase, patterned with an energetic beam (electron, ion or light), and then the rest is removed by sublimation.
Electron or ion beams were shown to sublimate water ice from silicon substrates and to allow the patterning of the substrate with sub-20nm metal lines. This excellent resolution was accomplished without any attempts to optimize the process and with 5nm diameter beams and can likely be improved. The use of frozen gaseous resists appears not to suffer from the proximity effects seen with polymer resists which lessen the pattern resolution available with polymer resists.
Another use of the frozen gaseous resists is to hold 1D substrates rigid allowing them to be cut. Carbon nanotubes coated with water ice were easily cut using an ion beam - this is a major advance in the field because previously there was no reliable way of cutting nanotubes at predetermined locations.
A further surprising discovery is that some resists and substrates can chemically react in the presence of the energy released by the beam and leave a thin film of reactant in a pattern determined by the beam. Water as a resist on silicon substrate and then exposed to an electron beam left a thin coating of silicon oxide. This discovery opens up a new way of placing a very thin film of silica dielectric in any predetermined pattern and could be very useful in the microelectronics fabrication.
Intellectual Property Status: Both Cases 2310 and 2440 were filed as regular utility patent applications in the US Patent Office on December 9, 2004.
For further information see "Nanometer patterning with ice" by King et al., 2005. Nano Letters 5:1157-1160.