1. Field
This application relates to technology for non-volatile storage.
2. Description of the Related Art
Materials having a detectable level of change in state, such as a resistance, are used to form various types of non-volatile semiconductor based memory cells. It has been proposed that such memory cells might be used for binary data storage in memory arrays by assigning a lower resistance state of a memory cells to a first logical state such as logical ‘0,’ and assigning a higher resistance state of the memory cell to a second logical state such as logical ‘1.’ Other logical data assignments to resistance states may also be used. Some materials can be reset back to a higher resistance state after being set from an initial state to a lower resistance state. These types of materials can be used to form re-writable memory cells. Multiple levels of detectable resistance in materials might be used to form multi-state memory cells which may or may not be re-writable.
One type of memory cell that exhibits switching behavior between at least two resistance states is referred to as ReRAM for “resistive switching RAM”. ReRAM may also be referred to as R-RAM or RRAM. A ReRAM memory cell may include a first electrode, a re-writable switching material (also referred to as a state change element), and a second electrode. The switching material may be metal oxide (MeOx). Switching the memory cell between resistance states may be achieved by applying a voltage across the memory cell. An alternative way of explaining the switching between resistance states is to provide a current to the memory cell.
One theory that is used to explain the switching mechanism for some types of switching materials is that one or more conductive filaments are formed by the application of a voltage (or other signal) to the memory cell. For some types of switching materials (e.g., metal oxides), the conductive path may arise due to oxygen vacancies that are caused by application of the voltage. This path (or paths) may link the first and second electrodes, wherein the conductive filament(s) lowers the resistance of the memory cell. Application of another voltage may rupture the conductive filament(s), thereby increasing the resistance of the memory cell. Application of still another voltage may repair the rupture in the conductive filament(s), thereby decreasing the resistance of the memory cell once again. Note that other theories might be used to explain switching behavior.
The reversible resistivity-switching element may be in the high resistance state when it is first fabricated. The term “FORMING” is sometimes used to describe putting the reversible resistivity-switching element into a lower resistance state for the first time. Thus, the initial formation of the conductive filaments is sometimes referred to as “FORMING.” The rupture of the filaments is sometimes referred to as RESETTING. The repair of the rupture of the filaments is sometimes referred to as SETTING. Note that there may be other explanations for FORMING, RESETTING, and SETTING.
One problem with the process of FORMING is that all of the non-volatile storage elements in a memory array need to be formed, which can be very time consuming. Further note that the FORMING process may need to be applied in a product environment. Therefore, the FORMING process should take place at temperatures that are compatible with the products.
Note that switching behavior might be explained by other theories than those above. Thus, any of the reversible resistivity-switching elements described herein are not limited to the theories for switching behavior described herein.