• Proceedings of the Korean Magnestics Society Conference
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• 2002.12a
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• pp.144-145
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• 2002

Al$_2$O$_3$층을 터널 장벽으로 사용하는 Magnetic Tunnel Junction(MTJ)시료의 특성에 가장 크게 작용하는 요인 중에 한 가지는 양질의 Al산화막 형성에 있다. Al산화막이 터널 장벽으로 제대로 된 역할을 하기 위해서는 Al층에 인접한 자성층에 영향을 미치지 않으면서 Al층을 균일하게 산화시킬 수 있는 조건이 만족되어야 하며, 이러한 $Al_2$O$_3$층의 제작에 가장 적합한 실험적 조건은 Al층의 산화에 Low power plasma를 사용하며, 산화 Chamber내부를 되도록 높은 분압의 산소 분위기로 유지시켜서 조금씩 장시간 동안 Al을 산화시키는 것이다. (중략)

• Journal of the Korean Magnetics Society
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• v.15 no.4
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• pp.231-235
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• 2005

We studied the microstructural evolution of ZrTM-Al (TM=Nb and Ti) alloy films, MR and electrical properties of the MTJ with $ZrTM-AlO_x$ barrier as a function of Zr/TM ratio. We observed that the ternary-oxide barrier reduced the TMR ratio due mainly to the structural defects such as the surface roughness. The change in TMR ratio and $V_h$ with Zr/TM ratio exactly corresponds to the systematic changes in the microstructural variation. Although the MTJ with ternary oxide reduced the TMR and the electrical stabilities, the junction resistances decreased as the Ti and Nb concentration increased due to the band-gap reduction caused by the formation of extra bands

• Journal of the Institute of Electronics Engineers of Korea SD
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• v.44 no.9
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• pp.10-17
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• 2007

It opens a new horizon on spintronics for the potential application of MTJ as a universal logic element, to employ the magneto-logic in substitution for the transistor-based logic device. The magneto-logic based on the MTJ element shows many potential advantages, such as high density, and nonvolatility. Moreover, the MTJ element has programmability and can therefore realize the full logic functions just by changing the input signals. This magneto-logic using MTJ elements can embody the reconfigurable circuit to overcome the rigid architecture. The established magneto-logic element has been designed and fabricated on a triple-layer MTJ. We present a novel magneto-logic structure that consists of a single layer MTJ and a current driver, which requires less processing steps with enhanced functional flexibility and uniformity. A 4-bit gray counter is designed to verify the magneto-logic functionality of the proposed single-layer MTJ and the simulation results are presented with the HSPICE macro-model of MTJ that we have developed.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.408-408
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• 2012

It is well known that magnetic random access memory (MRAM) is nonvolatile memory devices using ferromagnetic materials. MRAM has the merits such as fast access time, unlimited read/write endurance and nonvolatility. Although DRAM has many advantages containing high storage density, fast access time and low power consumption, it becomes volatile when the power is turned off. Owing to the attractive advantages of MRAM, MRAM is being spotlighted as an alternative device in the future. MRAM consists of magnetic tunnel junction (MTJ) stack and complementary metal- oxide semiconductor (CMOS). MTJ stacks are composed of various magnetic materials. FePt thin films are used as a pinned layer of MTJ stack. Up to date, an inductively coupled plasma reactive ion etching (ICPRIE) method of MTJ stacks showed better results in terms of etch rate and etch profile than any other methods such as ion milling, chemical assisted ion etching (CAIE), reactive ion etching (RIE). In order to improve etch profiles without redepositon, a better etching process of MTJ stack needs to be developed by using different etch gases and etch parameters. In this research, influences of $O_2$ gas on the etching characteristics of FePt thin films were investigated. FePt thin films were etched using ICPRIE in $CH_4/O_2/Ar$ gas mix. The etch rate and the etch selectivity were investigated in various $O_2$ concentrations. The etch profiles were studied in varying etch parameters such as coil rf power, dc-bias voltage, and gas pressure. TiN was employed as a hard mask. For observation etch profiles, field emission scanning electron microscopy (FESEM) was used.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.167-167
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• 2011

Magnetic random access memory (MRAM) is one of the prospective semiconductor memories for next generation. It has the excellent features including nonvolatility, fast access time, unlimited read/write endurance, low operating voltage, and high storage density. MRAM consists of magnetic tunnel junction (MTJ) stack and complementary metal-oxide semiconductor (CMOS). The MTJ stack is composed of various magnetic materials, metals, and a tunneling barrier layer. For the successful realization of high density MRAM, the etching process of magnetic materials should be developed. Among various magnetic materials, FePt has been used for pinned layer of MTJ stack. The previous etch study of FePt magnetic thin films was carried out using $CH_4/O_2/NH_3$. It reported only the etch characteristics with respect to the variation of RF bias powers. In this study, the etch characteristics of FePt thin films have been investigated using an inductively coupled plasma reactive ion etcher in various etch chemistries containing $CH_4$/Ar and $CH_4/O_2/Ar$ gas mixes. TiN thin film was employed as a hard mask. FePt thin films are etched by varying the gas concentration. The etch characteristics have been investigated in terms of etch rate, etch selectivity and etch profile. Furthermore, x-ray photoelectron spectroscopy is applied to elucidate the etch mechanism of FePt thin films in $CH_4$/Ar and $CH_4/O_2/Ar$ chemistries.

• Proceedings of the Korean Magnestics Society Conference
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• 2002.12a
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• pp.50-51
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• 2002

최근 실온에서 약 40％ 이상의 높은 자기저항(magnetoresistance, MR)을 나타내는 자기 터널 접합(magnetic tunnel junction, MTJ)이 보고되면서 비휘발성 자기메모리로의 응용을 눈앞에 두고 있다.[1]. 이에 본 실험에서는 Substrate / Ta (base electrode) / NiFe / PtMn (AF pinning layer) / CoFe (pinned) / Ru / CoFe (fixed) / Al-O/ CoFe (free) / NiFe (free) / Ta & Ru (Capping Layer)과 같은 MTJ 증착 구조를 사용하여, MTJ의 보다 향상된 특성을 확보하기 위한 노력으로서 Al-O 두께, 어닐링 조건(Field Intensity ＆ Sequence)변화 등을 시도하였다. (중략)

• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.169-169
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• 2011

STT-MRAM (수직자화 자기메모리)는 자화반전 현상을 원리로 구동하는 비휘발성 메모리로 기존의 메모리 장치에 비해 빠른 접근 속도와 높은 저장 밀도를 가지며 영구적인 기록이 가능하다. 이러한 장점들에 더해 적은 소모 전력을 지니므로 기존의 SRAM등의 한계를 극복할 대안으로 각광받고 있으며 차세대 메모리 군의 선두주자로 가장 적합한 후보중 하나이다. STT-MRAM의 건식 식각 방식에 있어 가장 큰 이슈는 소자 구동에 핵심적인 역할을 하는 MTJ(Magnetic Tunnel Junction)의 식각이다. MTJ는 free layer, tunnel barrier, pinned layer 3개의 층으로 구성되어 있으며 양 끝 layer에는 강자성체인 CoFeB가 사용되고 tunnel barrier에는 절연층인 MgO가 사용되고 있다. 이러한 물질들은 기존의 반도체 소자에서는 사용되지 않았던 물질들로 기존 공정에서 사용되던 Cl2 based plasma etching에서는 측벽에 비화발성 반응물과 잔류 Cl2에 의해 부식이 발생하는 문제점이 드러나고 있다. 이러한 문제점을 해결하기 위한 새로운 대안으로 CO/NH3/Ar나 CH4/Ar 같은 새로운 가스 조합을 사용하는 연구가 진행되고 있다. 이러한 연구에 의해 기존의 Cl2 plasma를 이용한 식각에서 나타나는 문제점은 해결이 되었으나 또 다른 문제점들이 보고되고 있다. 본 연구에서는 stack MRAM sample을 사용하여 기존의 사용되는 Cl2/Ar plasma와 대안 gas인 CO/NH3, CH4/Ar plasma에서의 식각을 진행하였으며 실험 조건(gas 비율 변화, Bias power 변화, 식각 시간)에 따른 식각 속도의 변화나 식각 후의 profile에 대하여 관찰하였다. 이에 따라 식각후에 어떠한 차이점이 있는 지를 알아보았으며 CO/NH3나 CH4/Ar plasma에서 식각시 나타나는 문제점에 대하여도 조명해 보았다.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.02a
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• pp.387-387
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• 2013

Currently, the flash memory and the dynamic random access memory (DRAM) have been used in a variety of applications. However, the downsizing of devices and the increasing density of recording medias are now in progress. So there are many demands for development of new semiconductor memory for next generation. Magnetic random access memory (MRAM) is one of the prospective semiconductor memories with excellent features including non-volatility, fast access time, unlimited read/write endurance, low operating voltage, and high storage density. MRAM is composed of magnetic tunnel junction (MTJ) stack and complementary metal-oxide semiconductor (CMOS). The MTJ stack consists of various magnetic materials, metals, and a tunneling barrier layer. Recently, MgO thin films have attracted a great attention as the prominent candidates for a tunneling barrier layer in the MTJ stack instead of the conventional Al2O3 films, because it has low Gibbs energy, low dielectric constant and high tunneling magnetoresistance value. For the successful etching of high density MRAM, the etching characteristics of MgO thin films as a tunneling barrier layer should be developed. In this study, the etch characteristics of MgO thin films have been investigated in various gas mixes using an inductively coupled plasma reactive ion etching (ICPRIE). The Cl2/Ar, CH3OH/Ar, and CH4/Ar gas mix were employed to find an optimized etching gas for MgO thin film etching. TiN thin films were employed as a hard mask to increase the etch selectivity. The etch rates were obtained using surface profilometer and etch profiles were observed by using the field emission scanning electron microscopy (FESEM).

• Journal of Magnetics
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• v.16 no.2
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• pp.140-144
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• 2011

We report on a numerical calculation study of two new functional properties in magnetic tunnel junctions (MTJs), negative dynamic resistance and RF amplification. The magnetic dynamics in a conventional CoFeB/MgO/CoFeB MTJ with in-plane magnetization was investigated using a macro-spin model simulation. To examine the influence of thermal fluctuations, random external magnetic fields were also included. Using a voltage controlled bias circuit, the negative dynamic resistance was obtained from time averaged I-V characteristics at both 0 K and 300 K under appropriate external magnetic fields and bias voltages. Using this negative dynamic resistance property, we demonstrated RF amplification with a 100 MHz high frequency signal. Sizable RF amplification gain was observed without thermal fluctuation. However, at 300 K, the RF signal was not amplified because low frequency magnetization dynamics were dominant.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.213-213
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• 2010

Magnetic random access memory (MRAM), based on magnetic tunnel junction (MTJ) and CMOS, is one of the best semiconductor memories because it can provide nonvolatility, fast access time, unlimited read/write endurance, low operating voltage and high storage density. For the realization of high density MRAM, the etching of MTJ stack with good properties is one of a key process. Recently, there has been great interest in the MTJ stack using MgO as barrier layer for its huge room temperature MR ratio. The use of MgO barrier layer will undoubtedly accelerate the development of MTJ stack for MRAM. In this study, high-density plasma reactive ion etching of MgO films was investigated in an inductively coupled plasma of $Cl_2$/Ar gas mixes. The etch rate, etch selectivity and etch profile of this magnetic film were examined on vary gas concentration. As the $Cl_2$ gas concentration increased, the etch rate of MgO monotonously decreased and etch slop was slanted. The effective of etch parameters including coil rf power, dc-bais voltage, and gas pressure on the etch profile of MgO thin film was explored, At high coil rf power, high dc-bais voltage, low gas pressure, the etching of MgO displayed better etch profiles. Finally, the clean and vertical etch sidewall of MgO films was achieved using $Cl_2$/Ar plasma at the optimized etch conditions.