Focused ion beam process for a formation of metal/insulator/metal double tunnel junctions

A small conductive island which is connected with source and drain via small tunnel junctions is a fundamental for single electron tunneling (SET) devices. An example of such structures consists of metal/insulator/metal double tunnel junctions and a metal oxide can be used as an insulator. Control of SET by the spin of tunneling electrons is possible by using magnetic metals as the island material, source and drain electrodes. However, microfabrication processes of magnetic metals for SET structures are not well established yet. We have proposed a new in situ process to fabricate small double tunnel junctions using a focused ion beam (FIB). This process is made up of following steps: after deposited metal films (Ni) on an insulator substrate are patterned to have a narrow channel (50 /spl mu/m in the present study) by photolithography was formed, a resist layer is spin-coated for the next lift-off step. The resist used is a double layer resist consisting of nitrocellulose and germanium films. Nitrocellulose can be used as a high-sensitive, selfdeveloping ion beam resist. 100 keV Ga FIB is irradiated across the channel to separate the metal channel and to form source and drain electrodes. To minimize the sputtered groove width which determines the island size, the resistance of the metal channel is monitored during the sputtering step and the FIB irradiation is ended just when the resistance become sharply infinite.