Polystyrene as a zwitter resist in electron beam lithography based electroless patterning of gold

The resist action of polystyrene (M _w, 2,600,000) towards electroless deposition of gold on Si(100) surface following cross-linking by exposing to a 10 kV electron beam, has been investigated employing a scanning electron microscope equipped with electron beam lithography tool. With a low dose of electrons (21 μC/cm²), the exposed regions inhibited the metal deposition from the plating solution due to cross-linking—typical of the negative resist behaviour of polystyrene, with metal depositing only on the developed Si surface. Upon increased electron dosage (160 μC/cm²), however, Au deposition took place even in the exposed regions of the resist, thus turning it into a positive resist. Raman measurement revealed amorphous carbon present in the exposed region that promotes metal deposition. Further increase in dosage led successively to negative (220 μC/cm²) and positive (13,500 μC/cm²) resist states. The zwitter action of polystyrene resist has been exploited to create line gratings with pitch as low as 200 nm and gap electrodes down to 80 nm.