Electron-beam lithography damage and contact metallurgy in 1 µm MOSFET technology

This work is motivated by the growing use of electron-beam lithography to fabricate high resolution MOSFET circuits. It deals with the introduction of damage into the gate oxides during resist exposure at the metal level and its subsequent removal by thermal annealing. Radiation damage to the gate oxides is shown to be caused not only by the primary beam electrons but also by electrons which are backscattered from the silicon substrate. This effect causes the threshold voltage of the MOSFET\´s on the chip to depend on the proximity of metal lines to the gate. The annealing cycles required to remove the damage from the gate oxides are described. The dependence of annealing on time, temperature and ambient are discussed. Anneals in pure hydrogen are shown to be superior to those in forming gas for removing damage from the gate oxide. A 400°C hydrogen anneal of a few hours reduces charge to negligible levels. The thermal stability and electrical properties of the Al/Ti metallurgy system are shown to be compatible with annealing cycles required for a 1 m MOSFET technology. Annealing of damage is however impeded when Ti directly overlays the gate area on top of the field oxide. Consequently Ti is used only in the contact holes as a barrier metallurgy.