Plasma enhanced chemical vapor deposition of nanocrystalline graphene and device fabrication development

摘要

Large area growth of high quality graphene remains a challenge, and is currently dominated by chemical vapor deposition (CVD) on metal catalyst films. This method requires a transfer of the graphene onto an insulating substrate for electronic applications, and the graphene film quality and performance can vary with the transfer. A more attractive approach is plasma enhanced chemical vapor deposition (PECVD) of graphene and nanocrystalline graphene (NCG) directly on insulating substrates. The aim of this project was to explore the deposition process and microfabrication processes based on these NCG films. A deposition process for nanocrystalline graphene was developed in this work based on parallel-plate PECVD. NCG with thicknesses between 3 and 35nm were deposited directly on wet thermal oxidized silicon wafers with diameter of 150 mm, quartz glass and sapphire glass. High NCG thickness uniformities of 87% over full wafer were achieved. Surface roughness was measured by atomic force microscopy and shows root mean square (RMS) values of less than 0.23nm for 3nm thin films. NCG films deposited on quartz and sapphire show promising performance as transparent conductor with 13kΩ/X sheet resistance at 85% transparency. Furthermore, the suitability of the developed PECVD NCG films for microfabrication was demonstrated. Microfabrication process development was focused on four device types. NCG membranes were fabricated based on through-wafer inductively coupled plasma etching from the back, and consecutive membrane release by HF vapor etching. The fabrication of suspended NCG strips, based on HF vapor release, shows promising results, but was not entirely successful due to insufficient thickness of the sacrificial oxide. Top gated NCG strips are successfully fabricated, and the increased modulation by the top gate is demonstrated. Finally, NCG nanowire fabrication is performed on 150mm wafers. Experiments yielded an increased back gate modulation effect by a reduced NCG thickness, although no nanowire formation was observed. A highly accurate focused ion beam (FIB) prototyping technique was developed and applied to exfoliated graphene in this work. This technique systematically avoids any exposure of the graphene to Ga+-ions through the use of an alignment marker system, achieving alignment accuracies better than 250 nm. Contacts were deposited by FIB- or e-beam-assisted tungsten deposition, and FIB trench milling was used to confine conduction to a narrow channel. A channel passivation method based on e-beam-assisted insulator deposition has been demonstrated, and showed a reduction of ion damage to the graphene. Three fabricated transistor structures were electrically characterized.