Guiding the Growth of a Conductive Filament by Nanoindentation To

Sep 13, 2017 - The nanoindented top electrodes generate an electric field concentration and the resultant precise control of a conductive filament in ...
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Guiding the Growth of a Conductive Filament by Nanoindentation To Improve Resistive Switching Yiming Sun, Cheng Song,* Jun Yin, Xianzhe Chen, Qin Wan, Fei Zeng, and Feng Pan* Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: Redox-based memristor devices, which are considered to have promising nonvolatile memory, mainly operate through the formation/rupture of nanoscale conductive filaments. However, the random growth of conductive filaments is an obstacle for the stability of memory devices and the cell-to-cell uniformity. Here, we investigate the guiding effect of nanoindentation on the growth of conductive filaments in resistive memory devices. The nanoindented top electrodes generate an electric field concentration and the resultant precise control of a conductive filament in two typical memory devices, Ag/SiO2/Pt and W/Ta2O5/Pt. The nanoindented cells possess a much larger ON/OFF ratio, a sharper RESET process, a higher response speed, and better cell-to-cell uniformity compared with the conventional cells. Our finding reflects that the use of large-scale nanotransfer printing might be a unique way to improve the performance of resistive random access memory. KEYWORDS: resistive switching, nanoindentation, conductive filament, precise control, uniformity, nanotransfer printing surface layers at present. We first take advantage of nanoindentation tests on the storage media to obtain inverted tripyramids. This fabrication method is compatible with the standard photolithography and lifting-off process, without any additional microelectronic processes. We select a typical electrochemical metallization (ECM) mechanism system, Ag/ SiO2/Pt,6,11,13 and a valence change memory (VCM) system, W/ Ta2O5/Pt,14,15 to realize our idea. The observation of the sectional sample under TEM proves that the thickness of the flat film and the distance between Ag pyramids and Pt are 80 and 40 nm, respectively. The nanoindented memory cells possess better resistive switching properties including a huge ON/OFF ratio, a high response speed, and concentrated operating voltages and resistances, when compared with conventional ones. Our method is simple and suitable for improving the performance of most RRAM systems. This work may provide guidance to the application of nanotransfer printing in memory industry.

1. INTRODUCTION Resistive random access memory (RRAM) is considered as one of the most promising types of nonvolatile memory due to its simple metal−insulator−metal (MIM) structure, low power consumption, high response speed, high storage density and complementary-metal-oxide-semiconductor (CMOS)-compatibility.1−3 Among various types of resistive memories, redoxbased RRAM operates through the formation and rupture of conductive filaments (CFs). However, the random growth of CFs results in unexpected resistive switching behaviors, unsatisfactory endurance, and cell-to-cell variation, all of which are supported by direct observation through transmission electron microscope (TEM) graphs.4−6 Several methods have been investigated for the precise control of the CF growth, among which manipulating electric fields by device design proves to be a prospective method to address the issue as well as inserting metal nanoparticles,7,8 impurity doping,9 and fabricating tips.10,11 For example, Shin et al.10 utilized anisotropic etching to prepare 2 μm pyramids in Ag/Al2O3/Pt, and You et al.11 sputtered Ag on etched SiO2 patterned with solvent-assisted nanotransfer printing (S-nTP) to acquire Ag nanocones. However, the methods mentioned above require either specific materials systems or a complicated etching process, demanding urgently for more simple and universal ways to fabricate reliable memory devices. In this study, we consider using physically based deformation to attain tips in memory cells. Nanoindentation is a technique developed over the past decade for probing the mechanical properties of materials at very small scales12 and is commonly used in the mechanical characterization of thin films and thin © 2017 American Chemical Society

2. RESULTS AND DISCUSSION Device Design and Fabrication. The nanoindented RRAM schematic is shown in Figure 1a. The special tripyramid-shaped electrodes are designed to provide an electric-field-concentrated region for the CF growth, as shown in Figure 1b. Figure 1c−j shows the fabrication process in both the top and front views. First, media layers (∼80 nm) are sputtered on the cleaned commercial Pt/Ti/SiO2/Si substrates Received: July 5, 2017 Accepted: September 13, 2017 Published: September 13, 2017 34064

DOI: 10.1021/acsami.7b09710 ACS Appl. Mater. Interfaces 2017, 9, 34064−34070

Research Article

ACS Applied Materials & Interfaces

can be observed under the optical microscope, and the nanoindentations (depth ≈ 40 nm) can be fabricated at the selected locations in the pits (Figure 1g,h). Finally, the samples are placed in the vacuum chamber, and the top electrode (TE) metals are deposited, followed by a lift-off process (Figure 1i,j). In this way, we can ensure that the TE metals can enter the indentations thereby forming the designed memory cells. The whole process is carried out at room temperature, which is compatible with the CMOS process. More details of the fabrication process for the tripyramid-shaped devices are described in the Experimental Section. After the fabrication, we use atomic force microscope (AFM) scanning to observe the nanoindentation on the SiO2 media layer before and after sputtering Ag TE, as displayed in Figure 2a,b. The indentation is shaped like an inverted tripyramid with a tip. The SiO2 layer has an evident bump around the indentation just like a microvolcano. The indentation is deep enough that the tip location can be observed under AFM even though TE is deposited on it. In order to ensure that the parameters of the nanoindented memory devices are consistent to our design, we fabricate a TEM sectional sample from one of the highperformance cells. TEM images are shown in Figure 2c. In the expanded scale images, the thickness of the flat SiO2 media layer is ∼80 nm (Figure 2d), and the distance between the tip of nanoindentation and the bottom electrode (BE) is ∼40 nm

Figure 1. (a) Schematic of the nanoindented memory device. (b) Expanded-scale single cell of the device. (c−j) Top and front views of the fabrication steps for the tripyramid-shaped TE.

as presented in Figure 1c,d. Second, a positive photoresist is spincoated onto the media layer. An array of independent circles (φ ≈ 100 μm) is fabricated after ultraviolet exposure and development, as Figure 1e,f shows. Next, the circle-shaped pits

Figure 2. AFM images of (a) nanoindentation on the SiO2 media layer and (b) nanoindentation under the Ag TE. (c) TEM images of the fabricated Ag tripyramid/nanoindented SiO2/Pt memory device. (d) Expanded-scale image of the flat film part. (e) Expanded-scale image of the location of the tripyramid tip. 34065

DOI: 10.1021/acsami.7b09710 ACS Appl. Mater. Interfaces 2017, 9, 34064−34070

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ACS Applied Materials & Interfaces

Figure 3. (a) Typical I−V curves of the conventional Ag/SiO2/Pt memory cells and nanoindented cells with a compliant current of ∼1 mA. (b) Histograms of the SET/RESET switching voltage in the conventional Ag/SiO2/Pt memory cells and nanoindented cells during 50 cycles. (c) Cumulative probability of HRS and LRS values for the conventional cells and nanoindented cells. (d) Retention tests of the nanoindented Ag/SiO2/Pt memory cells. The reading voltage for all resistive states was 0.08 V.

Figure 4. (a) Typical I−V curves of the conventional W/Ta2O5/Pt memory cells and nanoindented cells with a compliant current of ∼1 mA. (b) Histograms of the SET/RESET switching voltage in the conventional W/Ta2O5/Pt memory cells and nanoindented cells during 50 cycles. (c) Cumulative probability of HRS and LRS values for the conventional cells and nanoindented cells. (d) Retention tests of the nanoindented W/Ta2O5/Pt memory cells. The reading voltage for all resistive states was 0.08 V.

Resistive Switching Properties. The direct current (DC) voltage is applied to the TE, and the BE is grounded by the use of

(Figure 2e). In this way, a tip location that has a much shorter distance than the flat film from BE has been created. 34066

DOI: 10.1021/acsami.7b09710 ACS Appl. Mater. Interfaces 2017, 9, 34064−34070

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ACS Applied Materials & Interfaces a probe station, as shown in Figure 1a. The conventional devices are fabricated for a comparison, of which the functional layers are SiO2 (∼40 nm) and Ta2O5 (∼40 nm). Figure 3 displays the electrical performance of the Ag/SiO2/Pt system. First, a positive voltage is first applied to the TE with the compliance current (cc) of 1 mA to prevent thermal-induced breakdown.16−18 And then, a negative voltage is applied to the TE. Both conventional memory cells and nanoindented cells show stable bipolar resistive switching behaviors after the formation process as presented in Figure 3a. For Ag/SiO2/Pt, the conventional cell exhibits a high resistance state (HRS) of ∼105 Ω, a low resistance state (LRS) of ∼102 Ω, and the corresponding ON/OFF ratio is ∼103, while the nanoindented cells exhibits an HRS of ∼1010 Ω, an LRS of ∼102 Ω, and the corresponding ON/OFF ratio is ∼108. The typical I−V (current−voltage) curves reveal that the nanoindented cells have a smaller VRESET, a much larger ON/ OFF ratio, and a much sharper SET and RESET process, compared with the conventional cells. The histograms and statistical charts in Figure 3b show the switching voltage distributions obtained from the I−V curves. The switching voltages of convention cells are distributed in widespread ranges of 0.25 to 4.80 V (SET) and −4.20 to −0.56 V (RESET), while the switching voltages of nanoindented cells are distributed in ranges of 0.36 to 2.53 V (SET) and −1.64 to −0.42 V (RESET). Obviously, the switching voltages of nanoindented cells are much more concentrated than those of conventional ones. Figure 3c presents the HRS/LRS cumulative probability of conventional and nanoindented cells. The resistances of nanoindented cells are distributed around 102 Ω for LRS and 1010−1011 Ω for HRS. In contrast to the nanoindented cells, the resistances of conventional cells are scattered in a scale of around 102 Ω for LRS and 104−106 Ω for HRS. The tripyramid-shaped electrodes improve the uniformity of resistive switching behaviors in the SiO2 media layer. Meanwhile, the retention time is tested under both HRS and LRS, and the nanoindented memory devices can be maintained for longer than 105 s at room temperature without any obvious degradation. An identical experiment was carried out for W/Ta2O5/Pt. Although the ON/OFF ratio of the nanoindented Ag/SiO2/Pt is much higher than W/Ta2O5/Pt and they belong to different mechanisms, the electrical properties show a similar tendency, qualitatively. The conventional device exhibits an HRS of ∼105 Ω, an LRS of ∼102 Ω, and the corresponding ON/OFF ratio is ∼103, while the nanoindented cell exhibits an HRS of ∼106 Ω, an LRS of ∼102 Ω, and the ON/OFF ratio is ∼104. Also, the distribution of the switching voltage and the resistance are much more concentrated, even better than those of Ag/SiO2/Pt, as shown in Figure 4b,c. The retention time is also up to 105 s at room temperature without any obvious degradation (Figure 4d). Response Time Tests. For this experiment, in order to evaluate the response speed of memory devices, we designed a set of pulses (±5 V for SET and RESET) and used a small reading voltage (constant 0.1 V) before and after pulses, as illustrated in Figure 5a,b. We can obtain the response time by the step-decreased pulse width (reduce 10 ns each time from 200 ns) until the devices complete the SET/RESET process. Figure 5c,d shows I−V curves of the SET and RESET process, respectively. For the SET process, the current lies in a low state under the reading voltage. After the positive pulse, the cell is switched from HRS to LRS successfully. Therefore, the current becomes large under the same reading voltage. The RESET process is similar. The shortest pulse width corresponds to the shortest response time. Corresponding data are displayed in Table 1. In contrast,

Figure 5. Switching characteristics of conventional Ag/SiO2/Pt under pulsed voltages. (a,b) The SET/RESET pulse, SET: 5 V/20 ns, RESET: −5 V/20 ns, and the basal voltage is 0.1 V in order to read the HRS/LRS values after pulses. (c,-d) The I−V curves under the tests.

Table 1. Response Time of the Conventional Memory Cells and Nano-Indented Cells conventional

nanoindented

response time

SET

RESET

SET

RESET

Ag/SiO2/Pt W/Ta2O5/Pt

30 ns 100 ns

100 ns 100 ns

30 ns (−) 90 ns (↓)

50 ns (↓) 80 ns (↓)

our nanoindented method does not affect the response speed of the SET process significantly, but it effectively accelerates the speed of RESET process. Possible reasons will be discussed in the next subsection. These memory cells have a much higher response speed than that of the flash memory,19,20 which is one of the most prominent competitors of next-generation nonvolatile memories. Mechanism of Resistive Switching Behavior in Nanoindented Devices. To further understand the resistive switching mechanism of the memory device, we replotted the I−V characteristics in a log−log scale shown in Figure 4a (Ag/ SiO2/Pt) and in 4b (W/Ta2O5/Pt). The I−V relationships in LRS both exhibit an ohmic conductive behavior with a slope of about 1.00, which is attributed to the formation of CFs in the device during the SET process.4,21 As for HRS, the conductive mechanism is more complex. Fitting results show that the charge transport behavior accords with the space charge limited conduction (SCLC),22−24 which is composed of three portions: the ohmic region (I ∝ V), the Child’s law region (I ∝ V2), and a steep current increase region.4,15 As is reported in previous papers, we can estimate that Ag/SiO2/Pt conforms to the ECM mechanism,6,11,13 and W/Ta2O5/Pt conforms to the VCM mechanism.14,15 The former depends on Ag CFs for resistive switching, while the latter depends on oxygen vacancies (VOs). Next, we demonstrate the mechanism of the RS property improvement in the nanoindented Ag/SiO2/Pt memory devices. Figure 6c−h shows the process of resistive switching behaviors in a single cell. As is reported before, the growth direction is from the electrochemically inert counter electrode (CE) to the electrochemically active electrode (AE) in the Ag/SiO2/Pt structure.5 When a positive voltage is applied to the active Ag TE, Ag atoms are oxidized into Ag+ ions (Ag − e− → Ag+) as shown in Figure 6c,f. The positively charged ions migrate onto the Pt BE along the electric field and are reduced back to Ag atoms (Ag+ + e− → Ag). After the accumulation of Ag atoms as Figure 6d,g shows, the Ag CFs are formed, which results in the LRS. 34067

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Figure 6. Log−log I−V curves with local linear fitting of (a) Ag tripyramid/nanoindented SiO2/Pt memory cells and (b) W tripyramid/nanoindented Ta2O5/ Pt memory cells. Schematic diagrams for the mechanism of the resistive switching process in (c−e) Ag/SiO2/Pt and (f−h) Ag tripyramid/ nanoindented SiO2/Pt. (Oxidation is a process in which a chemical substance changes because of the emergence of oxygen. Reduction is a process in which electrons are added to an atom or ion.).

Obviously, the atom-to-atom coupling of conductive filaments displays shape-like Gaussian distribution even at the nanoscale, and this plays an important role in the RS behavior.5,25−27 Moreover, the growth mode depends on the cluster interactions and formation processes, which are strongly affected by kinetic factors according to the electrodynamics.27−29 When a negative voltage is applied to the Ag TE, an electrochemical dissolution takes place somewhere (usually at the thinnest locations) along the CF, resetting the cell back to the HRS (Figure 6e,h).17,30 It should be noted that in the conventional cell, the Ag CFs grow randomly and have a large quantity. As a result, it is hard to diffuse the Ag CFs completely in the RESET process, which may impair the properties of the memory cell. In the nanoindented cell, the tripyramid-shaped electrode creates an electric-field-concentrated region, which can guide the orientated growth of CFs. Due to the ultrasmall size of the nanoindentations (less than 1 μm2), the tip can control the number of CFs and facilitate the RESET process. Therefore, the easier RESET process reasonably leads to smaller RESET switching voltages and a higher response speed, which explains the improvement in response time to some extent. Moreover, although the W/nanoindented Ta2O5/Pt memory cells display similar improvement of RS properties, the mechanism is different from the ECM Ag/SiO2/Pt system. When the TE is positively biased, the VOs will migrate toward the cathode (Pt) and then accumulate in its vicinity. Because VOs can

create an acceptor level near the conduction band, they are the sources of electron carriers in the nuclei of n-type semiconducting CFs. Subsequently, these nuclei would grow toward the anode (W). Once the CF is complete, the cell is SET to LRS. When the TE is negatively biased, most of the Joule heat would be generated at the thinnest part of the CFs, accelerating the mobility of VOs and leading to the rupture of the VO CFs.1,14 The source of Ag CFs is TE, mainly the tip location, while VOs can form anywhere in the Ta2O5 layer, so the distribution of VOs is much more widespread than that of Ag+, and the resultant HRS resistance of W/Ta2O5/Pt is much lower than that of Ag/SiO2/ Pt. The ON/OFF ratio depends on both the LRS and HRS. When the memory cell lies in HRS, the resistance depends on the insulating media layer;31 hence, the thickness and condition of flat media layers are dominant. In the conventional cell, for the RESET process with a negative bias voltage applied to the TE, the filaments near the TE/SiO2 interface with the highest electrical potential are electrochemically dissolved back to the HRS (Figure 6e).32 Due to a large amount of filaments, the ruptured filament segment is still left in the media, resulting in the actual thickness of the media layer being much lower than the nominal one. Differently, in the nanoindented cell, the filaments can be dissolved in the RESET process much more completely due to the concentrated electric field, so the resistance of HRS is much higher than that in conventional cells. For the LRS, the 34068

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conductivity depends on the CFs,33 and thus, the resistances are comparable in both kinds of cells. Therefore, the ON/OFF ratios are much larger after nanoindentation in both memory systems. The cell-to-cell uniformity also shows a remarkable improvement, whether the resistances or the switching voltages. In the conventional cell, as mentioned above, when it is set to LRS, the random growth and the number of CFs (Ag or VO) is uncontrollable, which lead to the dispersive resistances. The unexpected appearance can be improved by the tripyramid electrodes. Only the location near the tip exists redox reactions and CFs. When it is reset to HRS, CFs usually cannot be ruptured neatly in conventional cells due to the random SET process. In comparison, the nanoindented cells can be RESET back to the original state to the maximum extent, which apparently results in better uniformity.

Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.S.) *E-mail: [email protected] (F.P.) ORCID

Cheng Song: 0000-0002-7651-9031 Fei Zeng: 0000-0001-8735-8766 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Ministry of Science and Technology o f t h e P eo p le’ s R ep u bli c o f C h in a (G r a n t No . 2016YFA0203800) and National Natural Science foundation of China (Grant Nos. 51231004). The authors are grateful for the support of Beijing Innovation Center for Future Chip.

3. CONCLUSIONS

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In summary, a simple and effective method, which is compatible with the photolithography and stripping of CMOS, is developed to improve resistive switching properties in both the ECM and VCM mechanisms of RRAM. Two kinds of nanoindented memory devices based on Ag/SiO2/Pt and W/Ta2O5/Pt are developed with an ultrahigh ON/OFF ratio, a sharp RESET process, excellent cell-to-cell uniformity, and a considerable response time and retention time. Moreover, a reasonable physical model is established to explain the better properties in the modified memory devices. We expect this method can provide guidance for fabricating high-performance and highstability memory applications. This work may promote the application of large-scale nanotransfer printing in RRAM.

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4. EXPERIMENTAL SECTION Device Fabrication. The Ag/SiO2/Pt and W/Ta2O5/Pt devices were completely fabricated at room temperature on commercial Pt(∼120 nm)/Ti(∼15 nm)/SiO2/Si substrates. We take the Ag/ SiO2/Pt as an example. First, the substrates were ultrasonically cleaned in acetone, ethanol, and deionized water for a period of 5 min, respectively. Second, the SiO2 films (∼40 nm) were deposited by radio frequency (RF) magnetron sputtering with a ceramic SiO2 target (99.99% purity, 3 in. in diameter). The base vacuum of the chamber was better than 8 × 10−4 Pa. During the deposition, the RF power and pure argon atmosphere were taken to be 200 W and 0.35 Pa, respectively. Third, the photoresist (PR1−500A, Futurrex, Inc.) was spin-coated onto the substrate at 3000 rpm for 40 s and then dried at 115 °C for 40 s. The ultraviolet intensity was 3.5 mW·cm−2, and the exposure time was 23 s. Fourth, nanoindentations were fabricated in the pits made by photoetching. Finally, the Ag top electrodes (∼200 nm) were deposited by DC magnetron sputtering, followed by the lift-off process. In the W/ Ta2O5/Pt system, Ta2O5 was deposited by RF magnetron sputtering with a Ta2O5 target with the power of 100 W and a pure argon atmosphere of 0.35 Pa, and the W electrodes were deposited by DC magnetron sputtering. Device Characterization. All electrical properties of the RRAM devices were characterized at room temperature and in an atmospheric environment. The pulse measurements were conducted on an arbitrary function generator (B1530, Agilent), and the others were all conducted on a semiconductor device analyzer (B1500A, Agilent). All TEM specimens were fabricated by a focused ion beam (FIB) system (LYRA 3, TESCAN). TEM images of the device in LRS were obtained from JEOL 2011 with 200 keV accelerating voltage. TEM images of the virgin device and the device in HRS were obtained from JEOL 2010F (200 keV). 34069

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