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Novel Protruded-Shape Unipolar Resistive Random Access Memory Structure for Improving Switching Uniformity through Excellent Conductive Filament Controllability

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Japanese Journal of Applied Physics 51 (2012) 06FE06 DOI: 10.1143/JJAP.51.06FE06

Novel Protruded-Shape Unipolar Resistive Random Access Memory Structure for Improving Switching Uniformity through Excellent Conductive Filament Controllability Kyung-Chang Ryoo1;2 , Sungjun Kim1 , Jeong-Hoon Oh1;2 , Sunghun Jung1 , Hongsik Jeong2 , and Byung-Gook Park1 1 Inter-University Semiconductor Research Center and School of Electrical Engineering and Computer Science, Seoul National University, Seoul 151-742, Republic of Korea 2 DRAM Process Architecture Team, Memory Division, Semiconductor Business, Samsung Electronics Co., Ltd., Yongin, Gyeonggi 445-701, Republic of Korea

Received November 27, 2011; accepted February 1, 2012; published online June 20, 2012 Resistive random access memory (RRAM) with a new structure which can effectively control switching area and electric field is proposed. It has been verified that the decrease in area of resistive material with the new structure increases electric field of switching area, and that such increased electric field makes initial forming at unipolar switching rather easier, resulting in effective decrease in forming voltage. Also, as the area in switching area is effectively reduced, decrease in reset current and set voltage in a limited area has also been verified. Excellent resistive switching characteristics are possible by decrease of conductive filament (CF) area in our structure. Random circuit breaker (RCB) simulation model which can effectively explain percolation switching similar to unipolar switching verifies such structural effect. # 2012 The Japan Society of Applied Physics

1. Introduction

Resistive random access memory (RRAM) has been intensively studied as an ultra high performance new memory due to its fascinating advantages such as low power and high density feasibility boosting for next generation nonvolatile memory (NVM) application. Especially, unipolar based RRAM is very promising in many respects although some severe problems such as poor reliability issues due to its insufficient conductive filament (CF) controllability still remain.1–12) In addition, it is very difficult to accomplish low power unipolar RRAM with conventional metal–insulator–metal (MIM) structure due to its high current level for reset switching.13–31) In this paper, we present for the first time a novel protruded shape RRAM cell structure which makes it possible to improve switching uniformity. This structure is one in which contact area between electrode and resistive material is minimized, an effort to CF size relating to reset/ set process, and makes it possible to verify the effects of structure and cell area on various switching characteristics. By using this structure, controlling the interface area where resistive switching occurs is expected to improve conductive filament controllability, in turn effectively reduces the irregular switching results in CF distribution improvement. Obtaining lower forming voltage (VFORMING ), set voltage (VSET ) and reset current (IRESET ) distribution are also possible by field enhancement effect of this structure. Numerical simulation methods for unipolar switching based on SILVACO simulation and random circuit breaker (RCB) network model are used to elucidate these effects.4) Figure 1 shows current–voltage (I–V ) and resistance– voltage (R–V ) curves of reset (a) and set (b) characteristics which show irregular stairway-like switching of our fabricated NiO based unipolar RRAM cell. We have reported that in many cases in which cells are at their initial state, irregular set and reset switching can be observed, and such changes can give a reverse impact on the cell uniformity.4) Especially, if initial forming process is defective, such irregular switching fluctuation is observed more frequently. In case of multi filament bundle or large

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Fig. 1. (Color online) I–V and R–V curves of reset (a) and set (b) characteristics which show irregular stairway like switching of reference cell.

diameter CF formation at switching interface, degradation of reset/set uniformity has been accelerated by unexpected additional tiny filament branch in the resistive material/ electrode interface. By controlling the amount or dimension of filament path at switching interface, irregular switching can be decreased significantly. Therefore, interface engineering for stable and localized initial CF formation is very important to avoid these problems. By materializing sub nm

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Fig. 2. (Color online) Schematic drawings of possible scenario of IRESET reduction at protruded shape RRAM (a) and conventional RRAM structure (b). Distribution is improved due to reduction of conductive filament (CF) associate with resistive switching.

Fig. 3. (Color online) Electric field simulation with protruded shape RRAM (a) and reference cell (b).

cell with presented our novel cell structure, effect of CF in small cell size can also be clearly understood. 2. Results and Discussion

Figure 2 shows the schematic drawings of possible scenario of distribution improvement of protruded shape RRAM structure (a) and conventional structure (b). RRAM cell having protruded shape switching interface is built in between two metal lines that are cross-pointed for selecting a resistive cell and for writing/reading data, respectively. Switching interface can be defined by locally protruded area. In Figs. 2(a) and 2(b), the number of CF associated with resistive switching is reduced as switching area is reduced. Moreover, electric field in the cell is enhanced in this structure contributing to the effective initial CF forming. The field concentration effect of our structure with different cell structure is investigated. Electric field concentration is increased at anodic upper interface area as shown in Fig. 3. We used SILVACOÔ simulation tool for electric field simulation. Tested cell size is 50 20 nm2 and length of upper protruded shape interface is 5 nm. Figure 4 shows lateral (a) and vertical (b) electric field distribution of conventional RRAM cell and our proposed protruded shape RRAM structure. Figure 4(a) shows the electric field distribution of lateral direction near the resistive switching interface between top electrode and resistive material. It shows that the electric field at the

defined contact area is stronger than that of at any point of the conventional RRAM cell. It is easy to form initial CF due to higher electric field, the VFORMING can be reduced by using proposed structure. It also is possible to demonstrate forming free RRAM by using this structure. In repetitive switching operation, the electric field intensity is one of the important factors for formation/rupture of CF. As shown in Fig. 4(a), shape field distribution in interface of protruded shape structure helps to improve repetitive switching behaviors due to stronger electric field than that of adjacent regions. Figure 4(b) indicates vertical electric field distribution characteristics between switching interfaces. In case of the conventional RRAM cell, the electric field distribution where resistive switching occurs is same. On the other hand, proposed protruded RRAM cell has not only much stronger electric field than that of conventional RRAM cell at specific switching region but also the maximum electric field point near the interface due to the electric field concentration effect. Therefore it is expected that switching interface is main region occurring resistive switching in the protruded shape RRAM structure. Maximum electric field is generated at upper top electrode (TE) interface where resistive switching occurs as shown here in Fig. 4, thus finding an optimal location is regarded as an important factor in improving resistive switching uniformity by initial CF control enhancement. Fabrication sequence of our proposed RRAM cell is explained. Once lower Al metal line is

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Fig. 4. (Color online) Electric field concentration with different lateral (a) and vertical location (b).

Fig. 5. (Color online) I–V curves of protruded shape RRAM (a) and reference cell structure (b).

formed, Ir bottom electrode (BE) and unipolar based resistive cell material such as NiO are formed. 2-photo/ etch process are performed for defining protruded shape of switching interface which determines the cell/TE contact area for small number of CF path step by step. Switching area can easily be controlled by current photo/etch technology without process difficulty. After mold oxide deposition and CMP process for planarization are performed, Ir TE is deposited on the cell. And then, top electrode contact (TEC) and upper Al metal line are performed. To verify whether this proposed structure contributes to improving the switching characteristics, a numerical simulation is performed using random circuit breaker (RCB) simulation model, which is a dynamic bond percolation model.3) In this simulation model, we assume that the resistance state switches depending on the magnitude of voltage applied across the circuit breaker.2) Circuit breaker can be defined as CF in this simulation model. This simulation model is composed of resistive network where resistance of each link may undergo resistive switching which depends on applied bias or current.2) Resistance value of electrode bond and resistive cell in this simulation model are 1 and 104 , respectively.2) Figure 5 shows the simulated I–V curves of proposed (a) and conventional cell (written as reference) (b). As shown in the figure, no obvious decrease in IRESET or in switching voltage could be verified. However

in the case of a protruded cell, it was verified that VFORMING compared to reference cell decreases under 0.5 V. Conventional and protruded cells’ resistive switching characteristics have been verified through statistical analysis of much more cells in order to verify such cell characteristics more clearly. Figure 6 shows the statistical analysis of VFORMING as a function of split group (a) and initial cell resistance (b). As shown in Fig. 6, proposed protruded shape RRAM shows better forming characteristics. Interestingly, in case of proposed cell, lower VFORMING is possible due to high electric field concentration at cell interface even though effective cell thickness between two electrodes increases. In general, the relationship between initial resistance and forming voltage have been much described in researches explaining the relationship between the thickness of resistive material and forming voltage. Protruded cell alike, the difference in forming voltage depending on the cell’s initial resistance could be verified. By decreasing initial resistance, it was verified that forming voltage can also be decreased. Especially, the results of statistical analysis show VSET distribution is dramatically improved as shown in Fig. 7(a). Various important resistive switching parameters such as forming, set and reset characteristics results also show that protruded shape RRAM cell has better switching uniformity for low power memory application as shown in Fig. 7(b). When using protruded shape RRAM cell, it has been verified that set voltage uniformity is clearly improved more

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(a) (b)

Fig. 6. (Color online) Statistical analysis of VFORMING as a function of split group (a) and initial resistance (b).

(b)

(a)

Fig. 7. (Color online) Statistical analysis of VSET as a function of split group (a) and standard deviation characteristics comparisons with various resistive switching parameters (b).

(a)

(b)

Fig. 8. (Color online) Statistical analysis of IRESET as a function of split group (a) and forming-reset comparisons of various cell structure (b).

effectively than in the case of conventional cell because electric field in interface region which forms initial filament is high therefore making it more advantageous for filament path to be formed locally. However there was not much difference in average value of voltage. Therefore, it can be concluded that distribution is improved because CF location

becomes relatively steady due to the effects of local area. Figure 8 show the statistical analysis of IRESET comparison between conventional and protruded shape RRAM cell (a) and VFORMING and IRESET characteristics (b) as a function of various structures. Lower forming energy and IRESET distribution is possible in case of protruded shape RRAM

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cell due to its structural advantage for electric field concentration and switching area reduction. In case of protruded shape RRAM cell, initial CF path can be controlled by higher electric field concentration, and it can also be minimized by smaller switching area for resistive switching. So, optimal process condition plays an important role of resistive switching and bi layer with different conducting defect ratio can be the solution for resistive switching improvement. This implies that proposed structure has structural advantages for initial CF conditions, therefore improved resistive switching characteristics with uniform distribution can be obtained without cell process difficulty.

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3. Conclusions

We propose a novel protruded-shape RRAM cell structure which makes it possible to improve resistive switching uniformity by using electrical field enhancement and switching area minimization effect. Lower VFORMING , VSET , and IRESET with excellent uniformity are successfully obtained. Numerical simulation is performed using field effect simulation and RCB model to elucidate this effect. Followings strongly support our proposed structure which contributes to resistive switching improvement, especially on the uniform cell distribution by excellent initial CF controllability which results in accelerations of high density and low power RRAM application.

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Acknowledgement 23)

This work is supported by BK21 program. The authors would like to thank Dr. J. S. Lee for valuable discussion.

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