Write/erase cycling endurance of memory cells with SiO2/HfO2 tunnel dielectric

The write/erase cycling endurance of low voltage floating-gate memory cells programmed and erased by tunneling through a SiO2/HfO2 dual layer tunnel dielectric stack is investigated. The use of fixed single pulse program and erase conditions leads to fast shifting (after similar to1000 cycles) of the threshold voltage window, so that only a limited number of write/erase cycles can be achieved. Increasing the write and erase duration quickly leads to an excessive erase time so that a different erase method has to be used. Improvement of the erase behavior and cycling endurance has been obtained by a combination of two methods. Inclusion of soft write pulses between the erase pulses reduces the amount of charge trapped in the tunnel dielectric and therefore limits the increase in erase time. Also, the erase voltage can progressively be raised in order to further limit the erase time, leading to an endurance of 10 000 cycles on the considered cells. When combining the SiO2/HfO2 stack with channel hot electron injection so that tunneling is only required in one direction, 100 000 write/erase cycles are demonstrated with minimal change of the memory window.