Fullerene solutions to chip manufacture

Materials World magazine
1 Oct 2008

A water-soluble fullerene resist for lithography could make electronics manufacturing safer and more environmentally friendly by removing the need for toxic and flammable solvents.

Using the new resist, a team at the University of Birmingham, UK, has demonstrated a high resolution of about 30nm for the transfer of integrated circuit patterns onto silicon wafers and a strong durability to etching of these substrates.

Conventional light sensitive resists are made from epoxy-based polymers or a mixture of diazonaphthoquinone and novolac resin in a liquid solvent that is deposited onto the substrate, spin coated and baked. On irradiation, specific areas of the resist are removed/ dissolved using solvents, while others remain as a insoluble mask film to shield parts of the silicon while it is etched and processed.

Examples of damaging solvents in current or past use include isopropyl alcohol and ethylene-based glycol ethers, and tetramethylammonium hydroxide.

‘Manufacturers currently have to deal very carefully with all of their waste streams, which is expensive. In addition they are also looking to reduce usage and emissions,’ explains Dr Alex Robinson, Senior Scientist at the Nanoscale Physics Research Laboratory at the University of Birmingham.

The fullerene resist his department has developed uses more water during both phases of lithography – spin coating of the resist and removing unwanted areas during patterning – thus eliminating complex waste management.

The idea originated from the laboratory’s work on molecular resists. Scientists usually avoid molecular materials because they form rough films on crystallisation. ‘The film needs to be high quality with no pinholes and low roughness to ensure fidelity of the mask,’ says Robinson.

However, certain materials with weak intermolecular forces such as fullerenes do not suffer from this problem. He adds, ‘Molecular resists use much smaller molecules than polymeric resists and this confers potential advantages in terms of better resolution [of the pattern transfer], lower line width roughness and so on.’

Forming good quality films from fullerenes was a challenge, requiring highly concentrated solutions of fullerol derivatives in water (around 50g/l), alongside high quality hydrophilic silicon surfaces, to ensure good wetting of the solution to the substrate.

Robinson explains, ‘Typically [in] organic solvents the concentration would be one to 10g/l – for fullerenes in anisole, for instance. So the concentration required [in water] is up to 50 times higher’.

The fullerene resists are still at proof of principle stage, with more work required to improve sensitivity to irradiation – they are not yet fast enough for industrial application.