Ultrahigh resolution electron beam lithography for molecular electronics

As the downscaling of conventional CMOS IC technology approaches its fundamental limits of power dissipation and fabrication difficulties, alternative computing approaches must be exploited. One of the promising candidates is molecular electronics. Significant progress for molecular devices and related theoretical calculations have been achieved during the last decade. Fabrication methods, however, are still a big challenge. In this work, we study ultrahigh resolution electron beam lithography (system, resist capability, and process) for the fabrication of molecular electronics, and molecular quantum-dot cellular automata (QCA) in particular.; The major work contains three parts: the development of an EBL system based on a cold cathode field emission scanning electron microscope (Hitachi S-4500 SEM); the development of an ultrahigh-resolution lithography process with cold development of PMMA; and the application to the fabrication of molecular QCA, which involves the patterning of self-assembled monolayers (SAMs), the patterning of DNA rafts, and the patterning of PMMA tracks for molecular liftoff of QCA molecules. The nanofabrication for molecular QCA is a close collaboration with Dr. Marya Lieberman's group (Dr. Xuejun Wang and Dr. Koshala Sarveswaran) in the Department of Chemistry and Biochemistry of the University of Notre Dame.; The ultrahigh resolution (5 nm) EBL in PMMA was achieved by the use of the SEM converted cold cathode field emission system and a cold development process, which proved to reduce the resist residue in the sub-10 nm lithographic patterns. Such high resolution has been successfully transferred to metal and nanoparticle structures by liftoff. The fabrication of 5-nm-wide metal and Au-nanoparticle lines has been demonstrated. Dense line patterns with sub-30 nm pitches in PMMA have been obtained. In addition, we have developed a novel precision wafer cross-sectioning technique to study EBL in resists. Besides PMMA, we have studied a chemical amplified resist called "XP9947" (Shipley Co.) for its capability for the 90 nm lithography node. 50-nm single-pass lines and 70-nm half-pitch resolution are achieved on the XP9947 resist.; Application to molecular QCA involves the patterning of SAMs and DNA rafts. EBL is performed to cleave the embedded disulfide bonds in the SAMs to define nanometer scale patterns for chemical binding of molecules. Sub-30 nm lines are fabricated, and attachment of fluorescent molecules, N-(1-pyrene)maleimide is demonstrated. Also, a well-controlled molecular liftoff process was developed to pattern 4-tile DNA rafts onto silicon substrates with low level defects and low surface roughness. The goal is to use an appropriate combination of lithography and self assembly technology to fabricate molecular systems.