• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2007.06a
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• pp.166-166
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• 2007

PRAM (phase-change random access memory)은 전류 펄스 인가에 따른 기록매질의 비정질-결정질 간 상변화와 그에 동반되는 저항변화를 이용하는 차세대 비휘발성 메모리 소자로서 연구되어지고 있다. 본 논문에서는 $(Ge_2Sb_2Te_5)_{1-x}Se_x$ (x=0,0.05,0.1,0.15) 조성에 대한 벌크 및 박막시료를 제작하고 각 조성에 대한 상변화 특성을 분석하였다. XRD를 통해 열처리 온도에 따른 구조적 분석을 실시하였고 UV-Vis-IR spectrophotometer를 사용하여 박막의 광학적 특성을 분석하였다. 또한 각 조성의 결정화 속도를 비교하기 위해 static tester를 사용하여 레이저 펄스 시간에 대한 반사도 변화를 측정하였고 DSC를 통해 결정화 온도를 측정하였다.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.23 no.3
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• pp.199-205
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• 2010

The crystallization speed (v) of amorphous (InTe)$_x$(GeTe) (x = 0.1, 0.3 and 0.5) films and their thermal, optical and electrical behaviors have been investigated using nano-pulse scanner (wavelength = 658 nm, laser beam diameter < 2 ${\mu}m$), X-ray diffraction (XRD), 4-point probe and UV-vis-IR spectrophotometer. These results were compared with those of $Ge_2Sb_2Te_5$ (GST) film, comprehensively utilized for phase-change random access memory (PRAM). Both v-value and thermal stability of (InTe)$_{0.1}$(GeTe) and (InTe)$_{0.3}$(GeTe) films could be enhanced in comparison with those of the GST. Contrarily, the v-value in the (InTe)$_{0.5}$(GeTe) film was so drastically deteriorated that we could not quantitatively evaluate it. This deterioration is thought because amorphous (InTe)$_{0.5}$(GeTe) film has relatively high reflectance, resulting in too low absorption to cause the crystallization. Conclusively, it could be thought that a proper compositional (InTe)$_x$(GeTe) films (e.g., x < 0.3) may be good candidates with both high crystallization speed and thermal stability for PRAM application.

• JSTS:Journal of Semiconductor Technology and Science
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• v.8 no.2
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• pp.128-133
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• 2008

Phase-change random access memory (PRAM) chip cell phase of amorphous state is rapidly changed to crystal state above 160 Celsius degree within several seconds during Infrared (IR) reflow. Thus, on-board programming method is considered for PRAM chip programming. We demonstrated the functional 512Mb PRAM with 90nm technology using several novel core circuits, such as metal-2 line based global row decoding scheme, PN-diode cells based BL discharge (BLDIS) scheme, and PMOS switch based column decoding scheme. The reverse-state standby current of each PRAM cell is near 10 pA range. The total leak current of 512Mb PRAM chip in standby mode on discharging state can be more than 5 mA. Thus in the proposed BLDIS control, all bitlines (BLs) are in floating state in standby mode, then in active mode, the activated BLs are discharged to low level in the early timing of the active period by the short pulse BLDIS control timing operation. In the conventional sense amplifier, the simultaneous switching activation timing operation invokes the large coupling noise between the VSAREF node and the inner amplification nodes of the sense amplifiers. The coupling noise at VSAREF degrades the sensing voltage margin of the conventional sense amplifier. The merit of the proposed sense amplifier is almost removing the coupling noise at VSAREF from sharing with other sense amplifiers.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.136-136
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• 2010

$Ge_2Sb_2Te_5$ (GST)는 광학 스토리지 및 PRAM(Phase-change Random Access Memory)에 적용 가능한 대표적인 상변화 물질이며 상변화 거동에 대한 다양한 연구가 진행되고 있다. 차세대 비휘발성 메모리로 각광을 받고 있는 PRAM의 경우 저전력 그러나 향후 고집적, 고성능 PRAM 소자구현을 위해서는 Reset 전류 감소를 통한 소비 전력 감소, 인접 셀간의 'cross talking'을 방지할 수 있는 열적 안정성 개선 등의 문제점들을 해결해야 한다. GST 물질의 전기적, 열적 특성을 조절하여 이러한 문제를 해결하기 위하여 GST 물질에 이종의 원소를 첨가하는 연구가 활발히 진행되고 있으며, 특히 질소 첨가에 의해 결정 성장 억제를 통한 결정화 온도 증가, 결정질의 저항 증가 등의 보고가 있었다. 본 연구에서는 질소를 첨가한 N-doped $Ge_2Sb_2Te_5$ (NGST) 박막의 상변화 거동을 규명하고 GST 박막과 비교하여 첨가된 질소의 영향을 분석하고자 한다. D.C Magnetron sputtering 방법으로 증착된 GST와 NGST 박막을 등온으로 유지하여 각 온도별로 열처리 시간 증가에 따른 비저항을 실시간으로 측정하여 GST와 NGST 박막의 상분율을 계산하고 Kissinger 모델을 이용하여 effective activation energy ($E_a$)를 구하였다. GST와 NGST 박막의 $E_a$는 각각 $2.08\;{\pm}\;0.11\;eV$와 $2.66\;{\pm}\;0.12\;eV$로 계산되었다. 따라서 첨가된 질소에 의해 NGST 박막의 결정화를 위하여 GST 박막의 경우보다 더 큰 활성화 에너지가 필요하다.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.132-132
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• 2011

For the programming volume of PRAM, Ge2Sb2Te5(GST) thin films have been dominantly used and prepared by physical vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer deposition (ALD). Among these methods, ALD is particularly considered as the most promising technique for the integration of PRAM because the ALD offers a superior conformality to PVD and CVD methods and a digital thickness control precisely to the atomic level since the film is deposited one atomic layer at a time. Meanwhile, although the IST has been already known as an optical data storage material, recently, it is known that the IST benefits multistate switching behavior, meaning that the IST-PRAM can be used for mutli-level coding, which is quite different and unique performance compared with the GST-PRAM. Therefore, it is necessary to investigate a possibility of the IST materials for the application of PRAM. So far there are many attempts to deposit the IST with MOCVD and PVD. However, it has not been reported that the IST can be deposited with the ALD method since the ALD reaction mechanism of metal organic precursors and the deposition parameters related with the ALD window are rarely known. Therefore, the main aim of this work is to demonstrate the ALD process for IST films with various precursors and the conformal filling of a nano size programming volume structure with the ALD?IST film for the integration. InSbTe (IST) thin films were deposited by ALD method with different precursors and deposition parameters and demonstrated conformal filling of the nano size programmable volume of cell structure for the integration of phase change random access memory (PRAM). The deposition rate and incubation time are 1.98 A/cycle and 25 cycle, respectively. The complete filling of nano size volume will be useful to fabricate the bottom contact type PRAM.

• 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 Vacuum Society Conference
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• 2016.02a
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• pp.257-257
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• 2016

최근에 메모리의 초고속화, 고집적화 및 초절전화가 요구되면서 resistive random access memory (ReRAM), ferroelectric RAM (FeRAM), phase change RAM (PRAM)등과 같은 차세대 메모리 기술이 활발히 연구되고 있다. 다양한 메모리 중에서 특히 resistive random access memory (ReRAM)는 빠른 동작 속도, 낮은 동작 전압, 대용량화와 비휘발성 등의 장점을 가진다. ReRAM 소자는 절연막의 저항 스위칭(resistance switching) 현상을 이용하여 동작하기 때문에 SiOx, AlOx, TaOx, ZrOx, NiOx, TiOx, 그리고 HfOx 등과 같은 금속 산화물에 대한 연구들이 활발하게 이루어지고 있다. 이와 같이 다양한 산화물 중에서 AlOx는 ReRAM의 절연막으로 적용되었을 때, 우수한 저항변화특성과 안정성을 가진다. 하지만, AlOx 박막을 형성하기 위하여 기존에 많이 사용되어지던 PVD (physical vapour deposition) 또는 CVD (chemical vapour deposition) 방법에서는 두께가 균일하고 막질이 우수한 박막을 얻을 수 있지만 고가의 진공장비 사용 및 대면적 공정이 곤란하다는 문제점이 있다. 한편, 용액 공정 방법은 공정과정이 간단하여 경제적이고 대면적화가 가능하며 저온에서 공정이 이루어지는 장점으로 많은 관심을 받고 있다. 본 연구에서는 sputtering 방법과 용액 공정 방법으로 형성한 AlOx 기반의 ReRAM에서 메모리 특성을 비교 및 평가하였다. 먼저, p-type Si 기판 위에 습식산화를 통하여 SiO2 300 nm를 성장시킨 후, electron beam evaporation으로 하부 전극을 형성하기 위하여 Ti와 Pt를 각각 10 nm와 100 nm의 두께로 증착하였다. 이후, 제작된 AlOx 용액을 spin coating 방법으로 1000 rpm 10 초, 6000 rpm 30 초의 조건으로 증착하였다. Solvent 및 불순물 제거를 위하여 $180^{\circ}C$의 온도에서 10 분 동안 열처리를 진행하였고, 상부 전극을 형성하기 위해 shadow mask를 이용하여 각각 50 nm, 100 nm 두께의 Ti와 Al을 electron beam evaporation 방법으로 증착하였다. 측정 결과, 용액 공정 방법으로 형성한 AlOx 기반의 ReRAM에서는 기존의 sputtering 방법으로 제작된 ReRAM에 비해서 저항 분포가 균일하지는 않았지만, 103 cycle 이상의 우수한 endurance 특성을 나타냈다. 또한, 1 V 내외로 동작 전압이 낮았으며 104 초 동안의 retention 측정에서도 메모리 특성이 일정하게 유지되었다. 결론적으로, 간단한 용액 공정 방법은 ReRAM 소자 제작에 많이 이용될 것으로 기대된다.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.06a
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• pp.33-34
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• 2008

$Ge_1Se_1Te_2$ chalcogenide amorphous materials was prepared by the conventional melt-quenching method. Samples were prepared by e-beam evaporator system and thermal evaporator technique. The thermal properties were investigated in the temperature range 300K-400K and the electrical properties were studied in the voltage range from 0V to 3V below the corresponding glass trasition temperature. The obtained results agree with the electrothermal model for Phase-Change Random Access Memory.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.37 no.1
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• pp.88-93
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• 2024

To evaluate the possibility as a multi-level memory medium for the Ge₂Sb₂Te₅/TiN/W-doped Ge₂Sb₂Te₅ cell structure, the crystallization rate and stabilization characteristics according to voltage (V)- and current (I)- pulse sweeping were investigated. In the cell structures prepared by a magnetron sputtering system on a p-type Si (100) substrate, the Ge₂Sb₂Te₅ and W-doped Ge₂Sb₂Te₅ thin films were separated by a barrier metal, TiN, and the individual thicknesses were varied, but the total thickness was fixed at 200 nm. All cell structures exhibited relatively stable multi-level states of high-middle-low resistance (HR-MR-LR), which guarantee the reliability of the multilevel phase-change random access memory (PRAM). The amorphousto-multilevel crystallization rate was evaluated from a graph of resistance (R) vs. pulse duration (T) obtained by the nanoscaled pulse sweeping at a fixed applied voltage (12 V). For all structures, the phase-change rates of HR→MR and MR→LR were estimated to be approximately t<20 ns and t<40 ns, respectively, and the states were relatively stable. We believe that the doublestack structure of an appropriate Ge-Sb-Te film separated by barrier metal (TiN) can be optimized for high-speed and stable multilevel PRAM.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.02a
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• pp.186-186
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• 2013

Nitrogen doped Ge-Sb-Te (N-GST) thin films for phase change random access memory (PRAM) applications were investigated by synchrotron-radiation-based x-ray photoelectron spectroscopy and absorption spectroscopy. Nitrogen doping in GST resulted in more favorable N atoms' bonding with Ge atoms rather than with Sb and Te atoms [1,2], which explains the higher phase change transition temperature than that of undoped Ge-Sb-Te thin film. Surprisingly, it was noticed that N atoms also existed in the form of molecular nitrogen, $N_2$, which is detrimental to the stability of the GST performance [3]. N-doped GST experimental features were also supported by ab-initio molecular dynamic calculations [2]. References [1] M.-C. Jung, Y. M. Lee, H.-D. Kim, M. G. Kim, and H. J. Shin, K. H. Kim, S. A. Song, H. S. Jeong, C. H. Ko, and M. Han, "Ge nitride formation in N-doped amorphous Ge2Sb2Te5", Appl. Phys. Lett. 91, 083514 (2007). [2] Zhimei Sun, Jian Zhou, Hyun-Joon Shin, Andreas Blomqvist, and Rajeev Ahuja, "Stable nitride complex and molecular nitrogen in N doped amorphous Ge2Sb2Te5", Appl. Phys. Lett. 93, 241908 (2008). [3] Kihong Kim, Ju-Chul Park, Jae-Gwan Chung, and Se Ahn Song, Min-Cherl Jung, Young Mi Lee, Hyun-Joon Shin, Bongjin Kuh, Yongho Ha, Jin-Seo Noh, "Observation of molecular nitrogen in N-doped Ge2Sb2Te5", Appl. Phys. Lett. 89, 243520 (2006).