Fabrication and Evaluation of Nanopillar-Shaped Phase-Change Memory Devices

Sung-Hoon Hong, Ju-Hyeon Shin, Byeong-Ju Bae, and Heon Lee�

Department of Materials Science and Engineering, Korea University, Seoul 136-713, Korea

Received September 26, 2009; accepted December 22, 2010; published online March 22, 2011

In this study, nanopillar-shaped phase-change memory devices of various sizes were simply fabricated by nanosphere lithography, and their

electrical characteristics were evaluated by conductive atomic force microscopy (AFM). As nanosphere materials, 180-nm diameter polystyrene

balls were used for a size-controllable mask, silica balls with a diameter of 200 nm for a high etching-resistance mask, and sub-50 nm Ag

nanoparticles were used for sub-50-nm-scale fabrication. Using the polystyrene balls, silica balls, and Ag nanoparticles, nanopillar-shaped phase-

change memory devices with various diameters, heights as large as 1 �m, and sizes as small as less than 50 nm were successfully fabricated. The

electrical properties of the nanopillar-shaped Ge2Sb2Te5 devices were evaluated by conductive AFM with an electrical measurement system.

# 2011 The Japan Society of Applied Physics

1. Introduction

Phase-change memory (PCM) has been intensively studiedfor next-generation non volatile memory devices owing toits low operation voltage, high-speed operation, good dataretention, multi bit potential, and high scalability.1–4) Theoperation principle of PCM is the reversible phase transitionbetween a low-resistance crystalline phase and a high-resistance amorphous phase through Joule heating inducedby an electrical pulse. The phase transition volume of PCMdevices can be reduced to the nanoscale owing to thehigh scalability of PCM as compared with that of othercompetitive non volatile memories. However, the volumereduction seriously affects the reset current, switching speed,thermal cross talk, and power consumption. Therefore, it isnecessary to study PCM at the nano scale.

Various studies investigating the device structure, phase-change material, heating electrode material, and fabricationprocess must be conducted at the nanoscale in order toreduce these factors in PCM devices. Many researchershave investigated nano size PCM fabricated by electronbeam lithography. However, the fabrication tools are verycomplicated and expensive.

Also, some bottom-up lithography techniques such asnanowire-, nanoparticle-, ordered-nanosphere-, and block-copolymer-based techniques have been reported for the fab-rication of materials at a sub-lithography scale.5–10) However,the material properties of fabrication of PCM devices cannotbe studied because of the difficulty of fabrication devicesarises using these bottom-up processes. Hence, alternativefabrication and evaluation methods should be developed.

In this study, we developed a fabrication and evaluationmethod for nanopillar-shaped PCM using nanospherelithography, which can be used to simply fabricate patternsthat are as small as 10 nm, and the conductive atomic forcemicroscopy (c-AFM), respectively. Nanopillar-shaped PCMwas fabricated with various sizes and heights by nanospherelithography using polystyrene (PS) balls, silica balls, and Agnanoparticles. The fabricated nanopillar-shaped PCM wassuccessfully evaluated by c-AFM with a pulse generator anda voltage source.

2. Experimental Procedure

Figure 1 shows a schematic diagram of the fabrication

procedure for nanopillar-shaped PCM devices by nano-sphere lithography using PS balls or silica balls and Agnanoparticles. First, a 100-nm-thick TiN layer was depositedonto a SiO2/Si substrate for use as a bottom electrode. Then,the phase-change material Ge2Sb2Te5 (GST) was depositedby RF sputtering. The RF power was set to 50W, and thecorresponding deposition rate was about 3.8 nm/min. Thethickness of the GST layer was set to 100 nm or 1 �m. Afterthe deposition of GST, the film was heated to 200 �C for10min under a nitrogen atmosphere in a rapid thermalannealing (RTA) system in order to induce its crystal-lization. Then, a solution containing 180-nm-diameter PSballs were spin-coated onto the substrate and dried. ThePS balls were prepared through the reaction of styreneand a potassium persulfate initiator.11) The size of the PSballs can be adjusted to control the pattern size by oxygenplasma treatment. The O2 flow rate was 40 sccm, and theplasma power was 100W. After the size of the PS balls wasreduced, the GST film was etched using the PS balls as anetching mask. The GST etching was carried out in an OxfordICP 100 plasma lab system.12,13) A mixture of 45 sccm Arand 5 sccm Cl2 was introduced into the chamber. Thepressure was set to 5mTorr and the bias power and plasmapower were 100 and 500W, respectively. The etching rate ofthe GST layer was about 7 nm/s. The residual PS balls wereremoved by dipping the device into acetone solvent.

The silica balls were synthesized using a sol–gel methodbased on hydrolysis and condensation. The size of the silica

Fig. 1. (Color online) Schematic diagram of GST nanopillar device

fabrication by nanosphere lithography using (a) polystyrene balls and

(b) silica balls and Ag nanoparticles.

�E-mail address: heonlee@korea.ac.kr

Japanese Journal of Applied Physics 50 (2011) 036501

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REGULAR PAPERDOI: 10.1143/JJAP.50.036501

balls was controlled by regulating the concentrations ofammonia and tetraethylorthosilicate (TEOS).14) The silicaballs were also mixed in deionized water and spin-coatedonto the GST/TiN layer. Then, the GST layer was etchedusing the silica balls as the etching mask. The etchingconditions were identical to those for the PS balls. After theGST layer was etched, the silica balls on the GST layer wereremoved by sonication in deionized water.

A 5wt% solution of 50 nm Ag nanoparticles was spin-coated on the GST/TiN-deposited substrate. Following this,the GST layer was etched using the Ag nanoparticles as anetching mask. The etching conditions were identical to thoseused for the PS balls.

The fabricated GST nanopillar-shaped PCM was char-acterized by c-AFM. A Pt-coated AFM tip was used as thetop electrode to measure the electrical characteristics of theGST nanopillar-shaped PCM. The measurement systemconsisted of a voltage source (Keithley 2612), a pulsegenerator (HP 81150A), and a c-AFM system (XE-100).

3. Results and Discussion

In order to fabricate the nano size PCM device, PS balls,silica balls, and Ag nanoparticles were used. First, the GSTnanopillar-shaped PCM was fabricated by nanospherelithography using PS balls owing to there size controllabil-ity. Figure 2(a) shows a scanning electron microscopy(SEM) image of the 180-nm-diameter PS ball array on theGST/TiN/SiO2/Si substrate. Subsequently, the diameterof PS balls was reduced during an oxygen reactive ionetching (RIE) process. The lateral etching rate of the PSballs was about 1.26 nm/s during the RIE process.Figure 2(b) shows that the diameter of the PS balls wasreduced from 180 to 130 nm by the oxygen plasma process.The GST film was etched using the reduced PS balls as theetching mask. After the residual PS balls were removedusing an acetone solvent, a GST nanopillar device with adiameter of 130 nm and a height of 100 nm was formedon the bottom electrode layer as shown in Figs. 2(c) and2(d). In the case of the 130 nm GST nanopillar shown in

Fig. 2(d), it can be seen that the reduced PS balls withstoodthe etching of the 100-nm-thick GST thin film. However, PSballs of less than 100 nm in diameter could not this etchingprocess, and the top layer of the GST nanopillar was erodedinto a cone shape.

Next, a GST nanopillar device was fabricated by nano-sphere lithography using silica balls owing to there highetching resistance. After the GST etching process, a verticalslope was observed for the GST nanopillars, as shown inFig. 3, because of the high etching resistance of the silicaballs. In Figs. 3(a) and 3(b), the silica balls were etched verylittle during the etching of the 100-nm-thick GST thin film,and a 200-nm-diameter GST nanopillar device was success-fully fabricated after removing the silica balls. A 1-�m-thickGST thin film was also etched using silica balls as an etchingmask. As shown in Figs. 4(a) and 4(b), the 200 nm silicaballs withstood the etching of the 1-�m-hick GST film.Using this process, a GST nanopillar device with a diameterof 200 nm and a height of 1 �m was formed on the bottomelectrode layer as shown in Fig. 4(c).

A sub-50 nm GST nanopillar-shaped PCM was fabricatedby nanosphere lithography using Ag nanoparticles to make asmaller structure. Figure 5(a) shows the spin-coated Agnanoparticles on the GST/TiN/SiO2/Si substrate. The sub-50 nm Ag nanoparticles on the GST layer can be clearlyobserved. Using Ag nanoparticles as an etching mask, a 100-nm-thick GST thin film was etched, and the Ag layer wasremoved during the GST etching process. As shown inFigs. 5(b) and 5(c), a sub-50 nm GST nanopillar device wassuccessfully fabricated.

The switching characteristics of the GST nanopillar-shaped PCM were successfully evaluated using the c-AFMsystem. Figure 6(a) shows a schematic diagram of the systemused to evaluate the c-AFM based measurement system. TheAFM was set to the contact mode, and a Pt-coated conductive40-nm-diameter AFM tip was used as the top electrode tomeasure the electrical characteristics of the GST nanopillars.The resistance of the as-fabricated GST nanopillar-shapedPCM was on the order of 104 �. The reset characteristics ofthe GST nanopillar devices were investigated by applying avoltage pulse with a pulse width of 50 ns. As shown inFig. 6(b), the resistance of the 180-nm-diameter GSTnanopillar-shaped device was successfully changed from5:9� 104 to 5:35� 106 � through the formation of theamorphous phase at 5.5V. The set voltage of the 180-nm-diameter GST nanopillar-shaped device was measured at2.5V, with a 200 ns pulse width and a 200 ns fall time.

Fig. 2. SEM images of the (a) as-coated PS balls on the GST/TiN layer,

(b) reduced PS balls after the oxygen plasma treatment, (c) GST nanopillar

device after GST etching and PS ball removal, and (d) cross-section image

of the GST nanopillar device.

Fig. 3. SEM images of the GST nanopillar device fabricated using silica

balls after (a) GST etching, and (b) silica ball removal.

S.-H. Hong et al.Jpn. J. Appl. Phys. 50 (2011) 036501

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Fig. 5. SEM images of the sub-50-nm-diameter GST nanopillar device fabricated using Ag nanoparticles (a) after spin-coating, (b) after GST layer etching,

and (c) cross-sectional view.

Fig. 4. SEM images of the 1-�m-high GST nanopillar device fabricated using silica balls (a) after GST etching (b) magnified view, and (c) after silica ball

removal.

Fig. 6. (Color online) (a) Schematic diagram of the c-AFM-based system used to evaluate the GST nanopillar device, (b) reset characteristics of the 130-

and 180-nm-diameter GST nanopillar devices, (c) repeated set-reset switching of the 180-nm-diameter GST nanopillar device, and (d) reset characteristics of

the sub-50-nm-diameter GST nanopillar device (pulse width 50 ns, fall time 2.5 ns).

S.-H. Hong et al.Jpn. J. Appl. Phys. 50 (2011) 036501

036501-3 # 2011 The Japan Society of Applied Physics

Also, the resistance of the 130-nm-diameter GST nano-pillar-shaped device was successfully changed from 1:3�105 to 1:8� 107 � through the formation of the amorphousphase at 4.5V.

Figure 6(c) shows the repeated set-reset switching of the180 nm GST nanopillar device described above. Using thismeasurement system, the GST nanopillar was switchedbetween the amorphous and crystalline phases more than tentimes.

In the case of the sub-50 nm GST nanopillar-shapeddevice, which was fabricated by nanosphere lithographyusing Ag nanoparticles, the resistance of the device wassuccessfully changed from 5:4� 106 to 2:86� 109 �

through the formation of the amorphous phase at 5V witha 50 ns pulse, as shown in Fig. 6(d). The reset behaviorof sub-50 nm GST nanopillar was more stable becausethe tip radius was similar to the GST pillar diameter. Thereset current of the sub-50 nm-diameter GST nanopillar-shaped device was measured to be 0.297mA, which ismuch less than that of the 180-nm-diameter GST nanopillar(0.5mA).

4. Conclusions

In this work, a nano size GST nanopillar-shaped PCMdevice was simply fabricated by nanosphere lithographyusing PS balls, silica balls, and Ag nanoparticles. In the caseof nanofabrication using PS balls, the size of the device wasfurther reduced to 130 nm by adjusting the oxygen plasmaprocess time. In the case of nanofabrication using silicaballs, a GST nanopillar device with a height of 1 �m and adiameter of 200 nm was fabricated owing to the high etchingresistance of the silica balls. In addition, a GST nanopillardevice was fabricated with a diameter of less than 50 nm bynanosphere lithography using Ag nanoparticles to realize a asmaller structure. The programming characteristics of theGST nanosize pillars with diameters of 180, 130, and less

than 50 nm were successfully evaluated using a c-AFMsystem.

Therefore, GST nanopillar fabrication by nanospherelithography and the AFM-based measurement methodeffectively characterized the nanoscale PCM device.

Acknowledgments

This work was financially supported by Samsung Electro-nics and a support program for the advancement of nationalresearch facilities and equipment, supported by the Ministryof Education, Science, and Technology, Korea. The authorsthank Dr. M. S. Hyun of National Nanofab Center for theoperation of AFM (NFEC-2007-11-047855).

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