Nanofabrication using focused ion beam

Focused ion beam (FIB) technique uses a focused beam of ions to scan the surface of a specimen, analogous to the way scanning electron microscope (SEM) utilizes electrons. Recent developments in the FIB technology have led to beam spot size below 10 nm, which makes FIB suitable for nanofabrication. This project investigated the nanofabrication aspect of the FIB technique, with device applications perspective in several directions. Project work included construction of an in-situ FIB electrical measurement system and development of its applications, direct measurements of nanometer scale FIB cuts and fabrication and testing of lateral field emission devices. Research work was performed using a number of materials including Al, Cr, SiO2, Si3N4 and their heterostructures. Measurements performed included in-situ resistometric measurements, which provided milled depth information by monitoring the resistance change of a metal track while ion milling it. The reproducibly of this method was confirmed by repeating experiments and accuracy was proven by atomic force microscopy (AFM). The system accurately monitored the thickness of 50 nm wide and 400 nm thick (high aspect ratio) Nb tracks while ion milling them. Direct measurements of low aspect ratio nanometer scale FIB cuts were performed using AFM on single crystal Si, polycrystalline Nb and an amorphous material. These experiments demonstrated the importance of materials aspects for example the presence of grains for cuts at this scale. A new lateral field emission device (in the plane of the chip) was fabricated, as FIB offers several advantages for these devices such as control over sharpness and decrease in anodeto- cathode spacing. FIB fabrication achieved field emission tip sharpness below 50 nm and anode-to-cathode spacing below 100 nm. For determining the field emission characteristics of the devices, a low current (picoampere) measurement system was constructed and devices operated in ultra high vacuum (10-9 mbar) in picoampere range. One device fabricated using a FIB sharpening process had a turn on voltage of 57 V