• Electronics and Telecommunications Trends
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• v.20 no.6 s.96
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• pp.62-69
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• 2005

세계 최고 권위의 반도체 시장 조사기관인 Gartner Dataquest는 2004년 세계 메모리시장 규모는 480억 달러로 2003년의 335억 달러 대비 43% 성장하였다고 보고하고있다[1]. 또한 DRAM은 55%, 플래시 메모리는 35%를 차지하고 있으며, 이들 두 메모리가 전체 메모리 시장을 양분하고 있다[1]. DRAM은 cost 및 random access가 가능하다는 장점을 가지고 있지만 휘발성이라는 단점을 가지고 있으며, 플래시 메모리는cost 및 비휘발성의 장점을 보유하고 있으나 random access가 불가능하다는 단점을 보유하고 있다. 하지만, PRAM은 DRAM과 플래시 메모리의 장점만을 융합한 통합형메모리로서, 현재 가장 각광받고 있으며 양산화에 가장 근접한 메모리이다. 본 고에서는 PRAM의 구조 및 동작특성, 개발동향 및 향후 전망에 대해 논의하고자 한다.

• Journal of Advanced Marine Engineering and Technology
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• v.32 no.1
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• pp.162-168
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• 2008

In this paper, we analyzed the heat transfer phenomenon and the reset current variation of PRAM device with thickness of phase change material using the 3-D finite element analysis tool. From the simulation, Joule's heat was generated at the contact surface of phase change material and bottom electrode of PRAM. As the thickness of phase change material was decreased, the reset current was highly increased. In case thickness of phase change material thin film was $200\;{\AA}$, heat increased through top electrode and reset current caused by phase transition highly increased. And as thermal conductivity of top electrode decreased, temperature of unit memory cell was increased.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.384-384
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• 2012

PRAM switching device by using current pulse modulation was investigated to verify its possibility for 3D architecture. In this work, two phase change materials connected in series having a different crystallization temperature are used. Its structural for different phase change material was evaluated by electrical resistance. We confirmed that Germanium-Antimony-Tellurium (GST) alloy and Germanium- Copper-Tellurium (GCT) alloy material were selected according to crystallization temperature, ${\sim}180^{\circ}C$ for switching and ${\sim}240^{\circ}C$ for memory devices, respectively. From this research, it is expected that phase change switching device could have advantages of process in terms of material similarity and structural simplification.

• Proceedings of the KIEE Conference
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• 2007.07a
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• pp.1335-1336
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• 2007

Among the emerging non-volatile memory technologies, phase change memories are the most attractive in terms of both performance and scalability perspectives. Phase-change random access memory(PRAM), compare with flash memory technologies, has advantages of high density, low cost, low consumption energy and fast response speed. However, PRAM device has disadvantages of set operation speed and reset operation power consumption. In this paper, we investigated scalability of $Ge_{1}Se_{1}Te_{2}$ chalcogenide material to improve its properties. As a result, reduction of phase change region have improved electrical properties of PRAM device.

• Korean Journal of Optics and Photonics
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• v.18 no.5
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• pp.351-361
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• 2007

The samples composed of a GST thin film and the protective layers of $ZnS-SiO_2$ or $Al_2O_3$ coated on c-Si substrate were prepared by using the magnetron sputtering method. Samples of three different structures were prepared, that is, i) the GST single film on c-Si substrate, ii) the GST film sandwiched by the protective $ZnS-SiO_2$ layers on c-Si substrate, and iii) the GST film sandwiched by $Al_2O_3$ protective layers on c-Si substrate. The ellipsometric constants in the temperature range from room temperature to $700^{\circ}C$ were obtained by using the in-situ ellipsometer equipped with a conventional heating chamber. The measured ellipsometric constants show strong variations versus temperature. The variation of ellipsometric constants at the temperature region higher than $300^{\circ}C$ shows different behaviors as the ambient medium is changed from in air to in vacuum or the protective layers are changed from $ZnS-SiO_2$ to $Al_2O_3$. Since the long heating time of 1-2 hours is believed to be the origin of the high temperature variation of ellipsometric constants upon the heating environment and the protective layers, a PRAM (Phase-Change Random Access Memory) recorder is introduced to reduce the heating time drastically. By using the PRAM recorder, the GST samples are heated up to $700^{\circ}C$ decomposed preventing its partial evaporation or chemical reactions with adjacent protective layers. The surface image obtained by SEM and the surface micro-roughness verified by AFM also confirmed that samples prepared by the PRAM recorder have smoother surface than the samples prepared by using the conventional heater.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2009.06a
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• pp.52-52
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• 2009

To date, chalcogenide alloy such as $Ge_2Sb_2Te_5$(GST) have not only been rigorously studied for use in Phase Change Random Access Memory(PRAM) applications, but also temperature gap to make different states is not enough to apply to device between amorphous and crystalline state. In this study, we have investigated a new system of phase change media based on the In-Sb-Te(IST) ternary alloys for PRAM. IST chalcogenide thin films were prepared in trench structure (aspect ratio 5:1 of length=500nm, width=100nm) using Tri methyl Indium $(In(CH_3)_4$), $Sb(iPr)_3$ $(Sb(C_3H_7)_3)$ and $Te(iPr)_2(Te(C_3H_7)_2)$ precursors. MOCVD process is very powerful system to deposit in ultra integrated device like 100nm scaled trench structure. And IST materials for PRAM can be grown at low deposition temperature below $200^{\circ}C$ in comparison with GST materials. Although Melting temperature of 1ST materials was $\sim 630^{\circ}C$ like GST, Crystalline temperature of them was ~$290^{\circ}C$; one of GST were $130^{\circ}C$. In-Sb-Te materials will be good candidate materials for PRAM applications. And MOCVD system is powerful for applying ultra scale integration cell.

• Journal of the Korean Ceramic Society
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• v.45 no.5
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• pp.276-279
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• 2008

In this research, properties of $N_2$-doped $Sb_2Te_3$ thin film were evaluated using 4-point probe, XRD and AFM. $Sb_2Te_3$ material has faster crystallization rate than $Ge_2Sb_2Te_5$, but sheet resistance difference between amorphous and crystallization state is very low. This low sheet resistance difference decreases sensing margin in reading operation at PRAM device operation. Therefore, in order to overcome this weak point, $N_2$ gas was doped on $Sb_2Te_3$ thin film. Sheet resistance difference between amorphous and crystallized state of $N_2$-doped $Sb_2Te_3$ thin film showed about $10^4$ times higher than Un-doped $Sb_2Te_3$ thin film because of the grain boundary scattering.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.24 no.12
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• pp.982-987
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• 2011

In this study, $Ge_8Se_{(2+x)}Te_{(6-x)}$ thin film amorphous-to-crystalline phase-change rate was evaluated in using a nano-pulse scanner. The focused laser beam with a diameter <10 ${\mu}m$ was illuminated in the power (P) and pulse duration (t) ranges of 1-31 mW and 10-460 ns, respectively, with subsequent detection of the responsive signals reflected from the film surface. We also evaluated the material characteristics, such as optical absorption and energy gap, crystalline phases, and sheet resistance of as-deposited and annealed films. The result of experiments showed that the thermal stability of the Ge-Se-Te film is largely improved by adding Se.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2010.06a
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• pp.52-52
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• 2010

상변화 재료로 가장 널리 이용되고 있는 $Ge^2Sb^2Te^5$ 박막에 전기전도성이 높은 Au를 첨가하여 상변환 특성을 연구하였다. ($Au)_x(Ge^2Sb^2Te^5)_{1-x}$ (X = 0, 0.05, 0.1) 박막은 Si 와 Glass 위에 Au 타켓과 $Ge^2Sb^2Te^5$ 타겟을 Co Sputtering 하여 만들었다. 증착된 박막은 Nanopulse Scanner 를 사용하여 결정화 속도를 측정하였다. 또한 $100^{\circ}C{\sim}400^{\circ}C$까지 $25^{\circ}C$간격으로 열처리 후 4 point prove를 이용하여 열처리 온도에 따른 저항의 차이를 측정하였으며 비정질 - 결정질 천이의 구조를 확인하기 위하여 XRD를 측정하였다. UV-VIS/IR 장비를 사용하여 비정질 박막과 결정화된 박막의 물성과 전기적 특성을 분석하였다.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.23 no.4
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• pp.261-266
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• 2010

In this paper, we report the changes of electrical, structural and optical characteristics in $Ge_2Sb_2Te_5$ thin films according to an increase of Si content. The Si-doped $Ge_2Sb_2Te_5$ thin films were prepared by rf-magnetron co-sputtering method. Isothermal annealing was carried out at $N_2$ atmosphere. The crystallization speed (v) of amorphous thin films was evaluated by detecting the reflection response signals using a nano-pulse scanner (wavelength = 658 nm) with illumination power of 1~17 mW and pulse duration of 10~460 ns. Structural phase changes were evaluated by XRD, and the optical transmittance was measured in the wavelength range of 300~3000 nm using UV-vis-NIR spectrophotometer. The sheet resistance (RS) of the thin films was measured using 4 point probe. Conclusivlely, the v-value decreased with an increase of Si content, while the RS-values of both crystalline and amorphous phases were increased. In particular, fcc-to-hexagonal transition was suppressed by the added Si atoms.