Resistive random access memory (RRAM) is a type of nonvolatile memory. Generally, RRAM memory cells each include a resistive dielectric material layer sandwiched between two conductive electrodes. The dielectric material is normally insulating. However, by applying the proper voltage across the dielectric layer, a conduction path (typically referred to as a filament) can be formed through the dielectric material layer. Once the filament is formed, it can be “reset” (i.e., broken or ruptured, resulting in a high resistance across the RRAM cell) and set (i.e., re-formed, resulting in a lower resistance across the RRAM cell), by applying the appropriate voltages across the dielectric layer. The low and high resistance states can be utilized to indicate a digital signal of “1” or “0” depending upon the resistance state, and thereby provide a reprogrammable non-volatile memory cell that can store a bit of information.
FIG. 1 shows a conventional configuration of an RRAM memory cell 1. The memory cell 1 includes a resistive dielectric material layer 2 sandwiched between two conductive material layers that form top and bottom electrodes 3 and 4, respectively.
FIGS. 2A-2D show the switching mechanism of the dielectric material layer 2. Specifically, FIG. 2A shows the resistive dielectric material layer 2 in its initial state after fabrication, where the layer 2 exhibits a relatively high resistance. FIG. 2B shows the formation of a conductive filament 7 through the layer 2 by applying the appropriate voltage across the layer 2. The filament 7 is a conductive path through the layer 2, such that the layer exhibits a relatively low resistance across it (because of the relatively high conductivity of the filament 7). FIG. 2C shows the formation of a rupture 8 in filament 7 caused by the application of a “reset” voltage across the layer 2. The area of the rupture 8 has a relatively high resistance, so that layer 2 exhibits a relatively high resistance across it. FIG. 2D shows the restoration of the filament 7 in the area of the rupture 8 caused by the application of a “set” voltage across layer 2. The restored filament 7 means the layer 2 exhibits a relatively low resistance across it. The relatively low resistance of layer 2 in the “formation” or “set” states of FIGS. 2B and 2D respectively can represent a digital signal state (e.g. a “1”), and the relatively high resistance of layer 2 in the “reset” state of FIG. 2C can represent a different digital signal state (e.g. a “0”). The reset voltage (which breaks the filament) can have a polarity opposite that of the filament formation and the set voltages, but it can also have the same polarity. The RRAM cell 1 can repeatedly be “reset” and “set,” so it forms an ideal reprogrammable nonvolatile memory cell.
One of the most critical operations involves the initial formation of the filament, as it will define the switching characteristics of the memory cell (e.g. operational power, device-to-device resistance variation, etc.). The voltage needed to form the filament is relatively high (i.e. significantly higher than the voltages needed to set and reset the memory cell). Using a filament forming voltage that is too low will not adequately form the filament. Using an excessive filament forming voltage could cause uncontrolled filament formation which can damage the device and result in inferior resistance switching behaviors, or result in over-forming the filament. Over-forming results in higher set and reset voltage peaks (which many circuit applications cannot accommodate), cycling induced resistance degradation, poor reset and set resistance distributions, and cell performance degradation. Therefore, there is a need for a reliable and effective technique for initially forming the filaments in RRAM devices.