1. Field
Example embodiments relate to a non-volatile variable resistance memory device and a method of fabricating the same. Other example embodiments relate to a non-volatile variable resistance memory device employing a buffer layer on a lower electrode of the memory device including a transition metal oxide having variable resistance characteristics, thereby to result in a decrease of a reset current and a method of fabricating the same.
2. Description of the Related Art
Much effort has been made to develop semiconductor devices that have an increased number of memory cells per unit area, for example, an increased integration density and may be operated at relatively high speed and with relatively low power consumption. A semiconductor memory device may include a large number of memory cells connected by circuits. In a dynamic random access memory (DRAM) used as a typical semiconductor memory device, a unit memory cell normally may include one switch and one capacitor. The DRAM may have advantages of relatively high integration density and relatively high operating speed, but when the power supply is shut down, it may lose all its stored data.
In a non-volatile memory device (e.g., a flash memory device), all stored data may be maintained even when the power is cut off. Unlike a volatile memory, the flash memory may have characteristics of non-volatility, but it may have a relatively low integration density and a relatively low operating speed compared to those of the DRAM. Non-volatile memory devices, which are currently the subject of much study, may include a magnetic random access memory (MRAM), a ferroelectric random access memory (FRAM), a phase-change random access memory (PRAM) and/or a resistance random access memory (RRAM). The non-volatile memory device (e.g., the RRAM described above) may use the resistance of a transition metal oxide that may vary in accordance with a voltage applied to the transition metal oxide (variable resistance characteristics).
FIG. 1A illustrates a typical structure of a variable resistance RAM (RRAM). An RRAM using a transition metal oxide (TMO) as a variable resistance material may have switching characteristics that enable it to be used as a memory device.
Referring to FIG. 1A, a lower electrode 10, an oxide layer 12, and an upper electrode 14 may be formed. The lower electrode 10 and the upper electrode 14 may be composed of a conductive material, for example, metal, and the oxide layer 12 may be composed of a transition metal oxide having variable resistance characteristics. Particular examples of transition metal oxide may include ZnO, TiO2, Nb2O5, ZrO2 and/or NiO.
FIG. 1B is a graphical representation illustrating operating characteristics of the conventional non-volatile variable resistance memory device shown in FIG. 1A. A lower electrode may be composed of Ru with a thickness of about 20 nm, and an oxide layer may be formed on the lower electrode. The oxide layer may be composed of NiO with a thickness of about 50 nm. An upper electrode may be formed on the oxide layer and the upper electrode may be composed of Ru with a thickness of about 20 nm. A voltage may be applied and the current may be measured.
Referring to FIG. 1B, when a voltage of about 0.7 V is applied in a first switching cycle, a reset current may be about 3 mA. When about 50 cycles of a switching operation have been performed, a reset current may be increased to about 50 mA. When the switching operation is repeatedly performed, the resistance state of the oxide layer 12 may be continuously changed. Operation voltage may be increased and reset voltage may be increased, which causes the reliability of the memory device to decrease. A memory device with a structure having relatively stable operating characteristics may need to be developed.