An effective bitline technique for high density DRAMs is studies. The open-type bitline structure where the bitlines are activated alternately can decrease the bitline noises and the current dissipation in memory cell arrays. In spite of several disadvantages inherent to the open-type bitline structure, this technique is found to get the larger read-out signal than the conventional bitline configurations for the DRAMs of 64 Mb and beyond. The effectiveness is confirmed with the measurement of the test-chips. This technique is expected to be more efficient for DRAMs of higher density, where the contribution of the inter-bitline capacitance is increased.

Thickness dependence of breakdown properties in control and N2O-Oxynitrided oxides was investigated. Nitrogen atoms piled up at the Si/SiO2 interface increase charge-to-breakdown (QBD) under substrate injection conditions for oxide thickness below 10 nm, while no meaningful improvement is observed above 10 nm. This thickness dependence is explained by the fact that N2O-oxynitridation reduces oxide defects near the Si/SiO2 interface. N2O-oxynitridation of the oxides reduces the number of neutral electron traps due to the chemical reaction of oxide defect with nitrogen atoms. Electron trapping of N2O-oxynitrided oxides is significantly suppressed; the reduction of electron trapping events into neutral electron traps increases QBD under substrate injection. On the other hand, under gate injection, N2O-oxynitrided oxides show low rate of hole trapping during the initial stress period. However, in heavily injected condition, electron trapping is not suppressed, resulting in little improvement of QBD. In addition, the control and N2O-oxynitrided oxides show quite similar dependence of QBD on stress current density, which is related primarily to the carrier transport phenomena (tunneling, traveling, impact ionization and hole injection).