In the current market, non-volatile memories such as flash memories, SONOS memories, and so on are widely used. These non-volatile memories employ a technique of varying a threshold voltage of a semiconductor transistor using charges accumulated in an insulating layer disposed over a channel part. In order to implement a large-capacity memory, it is essential to miniaturize the transistor. However, when the insulating layer in which charges are stored and retained is thinned, a leak current is increased to decrease charge retention capability. For this reason, it is difficult to implement a charge accumulation transistor type non-volatile memory having a large capacity.
Studies have been conducted to further increase capacity by allowing a transistor to have only a switching function of selecting a memory cell to and from which data is written and read, separating a memory element such as a DRAM, and miniaturizing the transistor and the memory element separately. As a technique of realizing continuous miniaturization of the non-volatile memory function, a resistance change element using an electronic element in which a value of electrical resistance is exchanged by two levels or more by any electrical stimulus has been developed. When charges are accumulated in a capacity (capacitance) such as that of a DRAM, an accumulated charge amount is reduced due to the miniaturization such that a reduction in signal voltage cannot be avoided. In many cases, an electrical resistance generally has a finite value, even when it is miniaturized. Awareness of a theory of changing a resistance value and existence of a material are advantageous to continuous miniaturization. Such a resistance change element is used for a switch configured to switch between a set (ON) state and a reset (OFF) state. For example, the resistance change element is applied to a converter (selector) of an interconnection constitution in an LSI.
A plurality of conventional techniques of changing electrical resistances using electrical stimuli are known. Among the techniques, the most widely researched technique involves a memory device that uses a pulse current applied to a chalcogenide semiconductor to change crystalline phases (amorphous or crystalline) to generate a 2 to 3 digit difference in electrical resistance between the crystalline phases. In general, the memory device is referred to as a phase change memory. Meanwhile, it is well known that a change in resistance occurs when a large voltage or current is applied even in a metal/metal oxide/metal (hereinafter, referred to as “MIM”) structure in which a metal oxide is disposed between electrodes.
In the 1950s and 1960s, phenomena of resistance values varying depending on the voltage or current had been researched and reported on various materials.
For example, Non-Patent Document 1 discloses a resistance change element using a nickel oxide (NiO).
FIG. 1 is a cross-sectional view of a conventional MIM resistance change element. The resistance change element includes a lower electrode 300, a resistance change material layer 200 formed on the lower electrode 300, and an upper electrode 100 formed on the resistance change material layer 200.
Current/voltage characteristics of the MIM resistance change element are shown in FIGS. 2A and 2B. The resistance change element maintains the characteristics in a high resistance OFF state or a low resistance ON state in a non-volatile manner even when power is turned off. Then, depending on necessity, a predetermined voltage/current stimulus is applied to switch a resistance state.
FIGS. 2A and 2B show examples of current/voltage characteristics in an ON state and an OFF state. When a set voltage of Vt2 is applied to an element in a high resistance OFF state shown in dotted lines of FIGS. 2A and 2B, the element is changed to a low resistance ON state as shown by an arrow D1 of FIG. 2A so that electrical characteristics represented by solid lines of FIGS. 2A and 2B are shown. Next, when a reset voltage of Vt1 is applied to an element in an ON state shown in solid lines of FIGS. 2A and 2B, the element is changed to a high resistance OFF state as shown by an arrow D2 of FIG. 2A so that electrical characteristics represented by dotted lines of FIGS. 2A and 2B are returned.
The electrical characteristics shown by the dotted lines and solid lines of FIGS. 2A and 2B can be repeatedly changed. These characteristics may be used for a non-volatile memory cell for circuit conversion or a non-volatile switch.
As characteristics of a portion R1 of FIG. 2B are shown, a large current is needed to reset the element. In addition, as characteristics of a portion R2 of FIG. 2B are shown, when a reset current is decreased, an ON resistance is increased.
In the phase change memory, in general, a change in volume caused by a change in crystalline phase is great, and local heating for a short time of several tens of nanoseconds at hundreds of ° C. is needed for a change in crystalline phase. For this reason, when the element is used as a memory element or a switch element, it is difficult to control the temperature of the phase change material. Meanwhile, in the MIM resistance change element, it is not necessary to heat the element to hundreds of ° C. For this reason, a resistance change type memory device using oxides of transition metals such as Cu, Ti, Ni, Cu, Mo, and so on, as resistance change materials, which has attracted attention again in recent times, has been proposed. In the transition metal oxide, a current path 400 referred to as a filament of the transition metal oxide shown in FIG. 3 is formed. It has been reported that the current path 400, the upper electrode 100 and the lower electrode 300 are coupled to or separated from each other to cause a change in resistance.
For example, Patent Document 1 and Non-Patent Document 2 disclose a resistance change type memory device using a nickel oxide as a metal oxide layer. In particular, in Non-Patent Document 2, among nickel oxides, a current path 400 referred to as a filament shown in FIG. 3 is formed. In addition, Non-Patent Document 2 discloses that a resistance of the resistance change element is changed by a connection state of the current path 400, the upper electrode 100 and the lower electrode 300.
However, the above techniques have the following problems.
First, the MIM resistance change element requires a forming process of switching an initial OFF state to an ON state. Conventionally, in order to perform the forming process, it is necessary to apply a voltage of 4 V or more to the MIM resistance change element. When these switch elements are mounted together with the LSI, it is necessary to operate the switch elements at 3 V or less, and thus, low voltage operation of the elements is required. The forming voltage can be lowered when a thickness of the resistance change material layer is reduced. However, when the resistance change material is thinned, a leak current in an OFF state is increased, and thus, it is impossible to obtain a high current ratio of the ON state and the OFF state required to the switch element. That is, in the conventional art, it has been difficult to realize both performance of the forming process at a low voltage and the high current ratio of the ON state and the OFF state, because they are in a trade-off relationship.
Second, the transition metal oxide is likely to include oxygen vacancy or metal vacancy. These become causes of a leak current path. That is, if there is a large amount of oxygen vacancy or metal vacancy in a layer, when the element is repeatedly operated, new vacancy occurs in the resistance change material due to the leak current. Moreover, the leak current is increased, which decreases a resistance in an OFF state. As a result, a reduction in resistance ratio of the ON/OFF state of the element and imbalance of the element characteristics cause reliability of the element to deteriorate.
Third, in the MIM resistance change element, it is possible to further increase the current ratio of the ON state and the OFF state by performing miniaturization. A filament which forms a current path in an ON state has a very small diameter. For this reason, a current value in an ON state is scarcely influenced by the area of the element. Meanwhile, a resistance value in an OFF state depends on the area of the element. That is, when miniaturization of the element progresses, the resistance value in the ON state is constant, but the resistance value in the OFF state increases in reverse proportion to the element area. As a result, a resistance ratio of the ON state and the OFF state increases. However, when miniaturization of the MIM resistance change element progresses, the yield of the MIM resistance change element having switching operation characteristics decreases. This is because miniaturization provides a remarkable influence on imbalance of a grain diameter, a composition, and so on of the resistance change material.