Cross-linked PMMA as a low-dimensional dielectric sacrificial layer

A surface nanomachining fabrication process using electron beam cross-linked poly(methyl) methacrylate (PMMA) has been developed and characterized. PMMA with different molecular weights (MW 100 K, MW 495 K, MW 950 K) in anisole casting solvent has been crosslinked with different electron beam irradiation levels ranging from 20 C/m² to 240 C/m². This is to investigate the quantifiable relationship between electron dose and its submicrometer remaining thickness after dissolving in acetone. This technique which uses electron beam lithography, offers a high resolution semi-three-dimensional (3-D) nanomachining of the sacrificial layer in a single run. Because of its low Young´s modulus, it has been successfully integrated with nanoelectromechanical systems processing and has the advantage of producing low-stress submicrometer thick structures with lateral dimensions as low as, but not limited to 1 μm. A fast dry release time from 55 to 100 s using oxygen plasma ashing has been demonstrated for a sacrificial layer aspect ratio of 125. This corresponds to an etch rate of about 0.6 μm/s at an average temperature of 40°C. The success of using cross-linked PMMA as a gate dielectric is demonstrated by the fabrication of multilayered gated lateral quantum dot devices. Periodic and continuous conductance oscillations arising from Coulomb charging effects are clearly observed in the transport properties at 50 mK.