IEEE ELECTRON DEVICE LETTERS, VOL. 36, NO. 2, FEBRUARY 2015
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Superior Retention of Low-Resistance State in Conductive Bridge Random Access Memory With Single Filament Formation Xiaoxin Xu, Student Member, IEEE, Hangbing Lv, Hongtao Liu, Tiancheng Gong, Guoming Wang, Meiyun Zhang, Yang Li, Qi Liu, Shibing Long, and Ming Liu, Senior Member, IEEE
Abstract— Data retention is one crucial reliability aspect of resistive random access memory (RRAM). The retention failure mechanism of the low-resistance state (LRS) for conductive bridge RAM is generally originated from the lateral diffusion of metal ions/atoms from the filament to its surrounding medium. In this letter, we proposed an effective method to improve the LRS retention by controlling the formation of the single filament. For a certain LRS, the effective surface area for metal ions/atoms diffusion in single filament is less than that of multi-filament. Thus, better LRS retention characteristics can be achieved by reducing the metal species diffusion. The validity of this method is verified by the superior retention characteristics of the LRS programmed by current mode, in comparison with voltage programming mode. The former tends to generate a single filament, while the later grows multi-filament. This letter provides a possible way to enhance the retention characteristics of RRAM. Index Terms— Resistive random access memory (RRAM), single filament, multi-filament, data retention.
I. I NTRODUCTION
R
ESISTIVE random access memory (RRAM) is a promising candidate to replace the conventional charge-based nonvolatile memories for its ease of fabrication, high-speed operation and high-density integration [1], [2]. For practical application, the retention characteristic (times to retain the resistance states at certain temperatures) plays a crucial role, especially for embedded application, such as encryption, code storage, et al [3]–[5]. For CBRAM, the stability of LRS is even more demanding [5]–[7]. Based on filament theory, the retention loss of LRS stems from the lateral diffusion of the metal ions/atoms into its surrounding medium [3], [5], [8]. In order to enhance the robustness of LRS retention, metal oxide material (i.e. TaOx ) with higher activation energy for
Manuscript received November 9, 2014; revised December 1, 2014 and December 4, 2014; accepted December 6, 2014. Date of publication December 12, 2014; date of current version January 23, 2015. This work was supported in part by the Ministry of Science and Technology, China, under Grant 2011AA010402, Grant 2011CBA00602, Grant 2011CB921804, Grant 2011CB707600, Grant 2011AA010401, and Grant 2011CB70, and in part by the National Natural Science Foundation of China under Grant 61106082, Grant 61376112, Grant 61334007, Grant 61221004, Grant 61274091, and Grant 61106119. The review of this letter was arranged by Editor D. Ha. The authors are with the Laboratory of Nanofabrication and Novel Devices Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences, Beijing 100029, China (e-mail:
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2014.2379961
ions diffusion was employed as the electrolyte [9]. Both of LRS retention and switching voltage were increased in contrast to the fast ionic conducting electrolyte (GeS, Ag2 S, GeSe, et al) [9]. Wei et al. proposed a two-step forming method to improve the LRS retention, owing to the increased ions density in the filament [3]. Moreover, the retention characteristics can be significantly improved by tuning the filament morphology through adjusting the operating conditions [10], [11]. In other aspect, single filament and multi-filament were commonly observed in the literatures [11]–[13], depending on the programming schemes or the device structures. There is no report addressing the influence of filament quantity on the LRS retention so far. In this letter, we evaluated the LRS retention characteristics for single filament and multi-filament in Cu/HfOx /Pt device. For a given LRS, the surface area for copper diffusion in single filament is less than that of multi-filament. Thus, better LRS retention characteristics can be expected in single filament due to the reduced copper diffusion. The retention of LRS programmed by current mode was found superior to that programmed by voltage mode. The former generally prefers to generate a single filament while the later tends to form multi-filament. This result reinforces that the LRS retention could improved by controlling the formation of single filament. II. E XPERIMENT The Cu/HfOx /Pt memory device with 1T1R architecture was fabricated by 0.13 μm logic CMOS technology. The 4-nm HfOx film was deposited on Cu plug by ion beam sputtering. The cell size is about 300 nm × 400 nm. The electrical characterizations were performed by Keithely 4200 semiconductor parameter analyzer. The programmed arrays were then baked in an oven with different temperatures of 110 °C, 130 °C and 150 °C. The resistance of each cell was periodically measured with a read voltage of 0.1 V after cooling down to room temperature. III. R ESULT AND D ISCUSION The resistive switching behavior of CBRAM is widely described by the formation and dissolution of conductive filament(s) composed by metal species [7]. The metal concentration in the filament region is higher than that in electrolyte layer. Thus, lateral diffusion of metal element from
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IEEE ELECTRON DEVICE LETTERS, VOL. 36, NO. 2, FEBRUARY 2015
Fig. 1. Schematic diagrams of the LRS retention loss for (a) single filament and (b) multi-filament. Reduced metal species diffusion can be achieved by smaller surface area (s) for single filament.
the filament to its surrounding could possibly happen. Once the metal elements are exhausted in some regions of the filament (i.e. the narrowest or weakest points), the failure of LRS retention will occur. At an elevated thermal atmosphere, the diffusion process could be accelerated. According to Fick’s first law [7], the diffusion flux of metal species could be described by: dC (1) dx where D is the diffusion coefficient determined by the temperature and activation energy; dC/dx is the concentration gradient. In the case of LRS here, the metal element loss in the filament due to the lateral diffusion can be expressed by: J = −D
N =S×J
(2)
where N is the metal element loss in the filament and S is the surface area of the filament. As can be seen from Eq. 2, N is proportional to S. The filament with less surface area is favorable to achieve better retention. For a given LRS, the effective size or radius of cylinder shaped filament should be the same, regardless the single or multi-filament case. They should meet the following relation: rs2 = r12 + r22 + · · · + rn2
(3)
where rs is the radius of single filament, n is the number of filaments in multi-filament case. Based on Eq. (3), the surface area for the single filament (Ss ) and multi-filament (Sm ) could be expressed by: (4) Ss = 2πrs l = 2πl r12 + r22 + · · · rn2 and Sm = 2πl(r1 + r2 + · · · rn )
(5)
where l is the thickness of electrolyte. Assuming each of the filament in multi-filament case has the same size, Ss would be satisfied by: √ Ss = Sm n (6) According to Eq.6, the surface area for metal element diffusion is obviously smaller in single filament case. Thus, better LRS retention characteristics could be expected by of single filament. The schematic diagrams of LRS retention loss for single filament and multi-filament are displayed in Fig. 1(a) and 1(b), respectively.
Fig. 2. (a) Typical I –V curve by VPM. Inset: Description of multi-filament formation. (b) Typical I –V curve by CPM. Inset: Description of single filament formation.
The formation of single/multi-filament can be controlled by various methods, such as electrode engineering [14] or different programming methods (current program (CPM), voltage program (VPM)). In our previous work [11], we found CPM tended to generate single filament, whereas VPM preferred to form multi-filament. A 1 × 3 cross-point array with common wordline (WL) and separated bitline (BL1, BL2 and BL3) were adopted to imitate the filament growth under CPM and VPM respectively. Three BLs were forced by electric stimulus simultaneously, followed by measuring the resistance individually. By taking these three cells as a unit or one cell, the probability of each cell switching to LRS indicates the quantity of the filament formed. In most of the cases, only one cell switched to LRS under CPM (16 in 20 cycles), indicating the formation of single filament, whereas all of the three cells turned into LRS under VPM (17 in 20 cycles), implying the generation of multiple filaments. This outcome can also be understood by considering the filament growth dynamic based on the evolution of electric field during programming. Fig. 2(a) and 2(b) show the I –V curves of VPM and CPM, respectively. As shown in the insert of Fig. 2(a), the current through the cell firstly increased sharply when the applied voltage reached a critical value, corresponding to the formation of the first filament. After this, a gradual increase of the current could be observed until reaching the compliance current. During this stage, the voltage (above the switching voltage) did not diminish, indicating the driving force for the filament growth on other locations still exists, in favor of the formation of multi-filament. This idea is also in accordance with our previously proposed multi-filament model [13]. In contrast, the I –V curve of CPM presents a different scenario where the voltage across this device is proportional to the cell resistance. Once the switching from HRS to LRS occurs, the voltage will decrease simultaneously. That is to say, as long as one filament is formed, the growth of the additional filaments will be prohibited (as shown in the insert of Fig. 2(b)) because of the significant reduction of the electric field. The data retentions of LRS programming by CPM and VPM were measured in 1T1R array. The retention measurements were performed by baking the programmed cells in the vacuum oven under raised temperature of 110 °C, 130 °C and 150 °C. The LRS resistance (RLRS ) was periodically read by an array test system after the baked cells were cooled down to room temperature, followed by calculating the failure number of the programmed cells.
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by CPM has better retention, which indicates that the single filament formation is more favorable to enhance the LRS retention characteristics of CBRAM. As for HRS retention, we also found the HRS programmed by CPM exhibits better retention compared with that by VPM. That is resulted from the effective elimination of intermediate states by CPM [12]. IV. C ONCLUSION
Fig. 3. Cumulative probability of RLRS depends on the baking time under (a) voltage programming mode and (b) current programming mode at 130 °C.
In this letter, we evaluated the LRS retention characteristics for single filament and multi-filament cases in a Cu/HfOx /Pt based CBRAM devices. Better LRS retention characteristics can be achieved in single filament by reducing the surface area for copper diffusion. This hypothesis is further verified by the retention test of LRS programmed by CPM and VPM. The former prefers to generate single filament while the later tends to grow multi-filament. The superior LRS retention of CPM reinforces that the single filament formation is helpful to improve the LRS retention. The result presented in this letter provides a possible way to enhance the LRS retention of CBRAM. R EFERENCES
Fig. 4. The failure rate of LRS retentions at (a) 110 °C, (b) 130 °C, and (c) 150 °C with baking time.
The criterion to evaluate whether the LRS failed or not was defined as RLRS > 5 k. It should be mentioned that unsuccessful SET or meta-stable LRS, which turned back to HRS quickly after several repeated read operations, were observed in our study. These meta-stable states would mislead the retention investigation and should be excluded in the baking experiment. After performing the SET operation, we firstly checked the resistance states of all the devices by several times of read operation. On this stage, we could find several unsuccessful SET devices. After that, we further screened the LRS device by a pre-baking process, which was a short time baking (about 50 s) at 85 °C to find out the meta-stable devices. After these two actions, we preliminarily thought the devices left in LRS were stable and could be used for statistic analysis. Fig. 3(a) and (b) show the cumulative probability of RLRS as a function of baking time for VPM and CPM, respectively. The initial values of RLRS are about 1 k. With increasing the baking time, the cumulative probability of the failed RLRS (above 5 k) increases. The trend of the retention failure for the two programming modes at 110 °C, 130 °C and 150 °C can be found in Fig. 4(a) to (c). Both of the time and temperature dependent LRS retention tests show the LRS programmed
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