Multiple-tunnel-junction-based nonvolatile single-electron-memory devices are promising for fast write/erase operation and long retention time. Fabrication of multiple-tunnel junctions with a predetermined number of barriers and islands is a major problem in realizing such devices. We have fabricated multiple-tunnel-junction-based single-electron devices by an electron-beam direct writing technique in a silicon-on-insulator layer. Using this technique, it is possible to fabricate multiple ultrasmall islands and tunnel barriers at a predetermined position, which is very important for reproducible device characteristics. Single-electron-memory devices based on multiple-tunnel junctions are fabricated. In these devices, a multiple-tunnel junction connects the gate electrode and a storage island. The Coulomb blockade across the multiple-tunnel junction acts as an energy barrier. Single-electron-memory operation is observed at 20 K. Retention time of at least 4 h has been observed.

A simple but potent method for electron-beam (EB) direct writing is introduced. This method is based on the use of negative electron-beam resist RD2000N. The resist offers high sensitivity to EB exposure and high resistance to halide plasma etching conditions, which is ideal for application in Si/SiO2 based nanodevice fabrication. Dot exposure shows that dots of a minimum diameter of 16 nm could be patterned using this resist. Linear arrays of dots, connected to each other by very narrow constrictions, are patterned using this resist. When transferred to a thin silicon-on-insulator layer, by reactive ion etching, this structure forms a multiple tunnel junction. Memory devices based on this multiple tunnel junction are fabricated. Memory operation is observed at 20 K.