US Patent 7960224 Operation method for multi-level switching of metal-oxide based RRAM

A method for operating a memory device includes applying a sequence of bias arrangements across a selected metal-oxide memory element to change among resistance states. The sequence of bias arrangements includes a first set of one or more pulses to change the resistance state of the selected metal-oxide memory element from the first resistance state to a third resistance state, and a second set of one or more pulses to change the resistance state of the selected metal-oxide memory element from the third resistance state to the second resistance state.