• 한국진공학회:학술대회논문집
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• 한국진공학회 2009년도 제38회 동계학술대회 초록집
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• pp.233-233
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• 2010

Magnetic random access memory (MRAM), based on magnetic tunnel junction (MTJ) and CMOS, is a prominent candidate among prospective semiconductor memories because it can provide nonvolatility, fast access time, unlimited read/write endurance, low operating voltage and high storage density. The etching of MTJ stack with good properties is one of a key process for the realization of high density MRAM. In order to achieve high quality MTJ stack, the use of CoFeB and IrMn magnetic films as free layers was proposed. In this study, inductively coupled plasma reactive ion etching of CoFeB and IrMn thin films masked with Ti hard mask was investigated in a $Cl_2$/Ar gas mix. The etch rate of CoFeB and IrMn films were examined on varying $Cl_2$ gas concentration. As the $Cl_2$ gas increased, the etch rate monotonously decreased. The effective of etch parameters including coil rf power, dc-bais voltage, and gas pressure on the etch profile of CoFeB and IrMn thin film was explored, At high coil rf power, high dc-bais voltage, low gas pressure, the etching of CoFeB and IrMn displayed better etch profiles. Finally, the clean and vertical etch sidewall of CoFeB and IrMn free layers can be achieved by means of thin Ti hard mask in a $Cl_2$/Ar plasma at the optimized condition.

• Journal of Magnetics
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• 제9권1호
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• pp.17-22
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• 2004

We studied the magnetotransport properties of tunnel junctions with AlO and AlN barriers fabricated using microwave-excited plasma. The plasma nitridation process provided wider controllability than the plasma oxidization for the formation of MTJs with ultra-thin insulating layer, because of the slow nitriding rate of metal Al layers, comparing with the oxidizing rate of them. High tunnel magnetoresistance (TMR) ratios of 49 and 44% with respective resistance-area product $(R{\times}A) of 3 {\times} 10^4 and 6 {\times} 10^3 {\Omega}{\mu}m^2$ were obtained in the Co-Fe/Al-N/Co-Fe MTJs. We conclude that AlN is a hopeful barrier material to realize MTJs with high TMR ratio and low $R{\times}A$ for high performance MRAM cells. In addition, in order to clarify the annealing temperature dependence of TMR, the local transport properties were measured for Ta $50{\AA} /Cu 200 {\AA}/Ta 50 {\AA}/Ni_{76}Fe_{24} 20 {\AA}/Cu 50 {\AA}/Mn_{75}Ir_{25} 100 {\AA}/Co_{71}Fe_{29} 40 {\AA}/Al-O$ junction with $d_{Al}= 8 {\AA} and P_{O2}{\times}t_{0X}/ = 8.4 {\times} 10^4$ at various temperatures. The current histogram statistically calculated from the electrical current image was well in accord with the fitting result considering the Gaussian distribution and Fowler-Nordheim equation. After annealing at $340^{\circ}C$, where the TMR ratio of the corresponding MTJ had the maximum value of 44%, the average barrier height increased to 1.12 eV and its standard deviation decreased to 0.1 eV. The increase of TMR ratio after annealing could be well explained by the enhancement of the average barrier height and the reduction of its fluctuation.

• 한국자기학회지
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• 제17권4호
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• pp.162-165
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• 2007

본 연구에서는 CoFeB/MgO/CoFeB 구조를 가지는 자기터널접합에서 capping층 재료의 종류와 열처리 시간에 따른 비정질 top CoFeB 자성층의 결정화 상태 및 자기터널접합의 자기적 특성 변화에 대한 연구결과를 비교 분석 하였다. Hcp(Hexagonal close-packed)의 결정구조를 가지는 Ru(002)를 capping층 재료로 사용한 자기터널접합 박막의 경우에는 열처리 이후 Ru과 인접한 부분의 top CoFeB이 bcc-CoFe(110)로 성장하는 반면, TiAl과 ZrAl을 capping층 재료로 사용한 자기터널접합의 경우는 열처리 이후 top CoFeB이 MgO와 epitaxial하게 bcc-CoFe(002)로 결정성장 하였다. 이로 인해 Ru을 사용한 자기터널접합의 터널자기 저항비(46.7%)보다 약 1.5배 높은 터널자기저항비(TiAl: 71.8%, ZrAl: 72.7%)를 나타내었다.

• 한국자기학회지
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• 제17권3호
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• pp.120-123
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• 2007

이 연구에서는 Ta 45/Ru 9.5/IrMn 10/CoFe $3/AlO_x$/자유층/$AlO_x$/CoFe 7/IrMn 10/Ru 60(nm) 구조를 갖는 이중장벽 자기터널접합(double-barrier magnetic tunnel junction: DMTJ)를 다루었다. 자유층은 $Ni_{16}Fe_{62}Si_8B_{14}\;7nm$, $Co_{90}Fe_{10}(fcc)$ 7 nm 및 $CoFet_1$/NiFeSiB $t_2$/CoFe $t_1$으로 구성하였으며 두께 $t_1,\;t_2$는 변화시켰다. 즉 TMR비와 RA를 개선하기 위하여 부분적으로 CoFe층을 대체할 수 있는 비정질 NiFeSiB층이 혼합된 자유층 CoFe/NiFeSiB/CoFe을 갖는 DMTJ를 연구하였다. NiFeSiB($t_1=0,\;t_2=7$)만의 자유층을 갖는 DMTJ는 터널자기저항(TMR)비 28%, 면적-저항곱(RA) $86k{\Omega}{\mu}m^2$, 보자력($H_c$) 11 Oe 및 층간 결합장($H_i$) 20 Oe를 나타내었다. $t_1=1.5,\;t_2=4$인 경우의 하이브리드 DMTJ는 TMR비 30%, RA $68k{\Omega}{\mu}m^2$ 및 $H_c\;11\;Oe$를 가졌으나 $H_i$는 37 Oe로 증가하였다. 원자현미경(AFM)과 투과전자현미경(TEM)측정을 통하여 NiFeSiB층 두께가 감소하면 $H_i$가 증가하는 것을 확인하였다. 비정질 NiFeSiB층이 두꺼워지면 보통 계면의 기복을 유도하는 원주형성장(columnar growth)를 지연시키는데 유효하였다. 그러나 NiFeSiB층이 얇으면 표면거칠기는 증가하고 전자기적 Neel 결합 때문에 Hi는 커졌다.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2009년도 제38회 동계학술대회 초록집
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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.

• Journal of Magnetics
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• 제11권1호
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• pp.25-29
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• 2006

Recent experiments have demonstrated high TMR ratios in MTJs with the MgO barrier [1,2]. The CoFeB/MgO/CoFeB junctions showed better properties than the CoFe/MgO/CoFe junctions because the MgO layer had a good crystalline structure with (001) texture and smooth and sharp interface between CoFeB/MgO [3]. The amorphous CoFeB with 20 at%B starts the crystallization at $340^{\circ}C$ [4] and this crystallization of the CoFeB helps obtaining the high TMR ratio. In this work, the compositional changes in the MgO barrier and at the interface of CoFeB/MgO/CoFeB after the CoFeB crystallization were studied in annealed MTJs. XPS depth profiles were utilized. TEM analyses showed that the MgO barrier had (100) texture on CoFeB in the junctions. B in the bottom CoFeB layer diffused into the MgO barrier and B-oxide was formed at the interface of CoFeB/MgO/CoFeB after the CoFeB crystallization.

• 한국자기학회지
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• 제19권6호
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• pp.197-202
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• 2009

코닝유리(Corning glass) 기판 위에 이온빔 증착법으로 제작한 버퍼(Ta)/코네틱(Conetic; NiFeCuMo) 박막에 대해 버퍼층에 의존하는 결정성장과 열처리 효과를 조사하였다. 또한 코네틱층을 증착할 때에 인가한 자기장 방향으로 용이축과 곤란축의 자기저항 곡선으로부터 얻은 보자력과 포화자기장값을 버퍼층 유무에 따라 서로 비교하였다. Ta 박막의 두께가 5 nm이고 코네틱 박막의 두께가 50 nm일 때에 보자력은 0.12 Oe으로 작았으며, MH 히스테리시스 곡선에서 얻은 자화율($\chi$)은 1.2 ${\times}\;10^4$으로 우수한 연자성의 특성을 가졌다. 저자기장에 민감한 거대자기저항 스핀밸브(GMR-SV; giant magneoresistance-spin valve)나 자기터널링접합(MTJ; megnetic tunnel junction) 박막구조에서 자유층으로 연자성의 특성이 우수한 코네틱 박막을 사용할 수 있는 가능성을 확인하였다.

• 한국자기학회지
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• 제16권6호
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• pp.279-282
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• 2006

비정질 강자성 NiFeSiB 자유층을 갖는 자기터널접합 (MTJ)에 대하여 연구하였다. 비정질 자유층이 MTJ의 스위칭 특성에 미치는 영향을 알아보는데 역점을 두어 기존의 CoFe와 NiFe층 대신에 NiFeSiB 자성층을 사용하였다. $Ni_{16}Fe_{62}Si_{8}B_{14}$는 $Co_{90}Fe_{10}$보다 더 낮은 포화자화도 ($M_{s}:\;800\;emu/cm^{3}$) 그리고 $Ni_{80}Fe_{20}$보다 더 높은 이방성 상수 ($K_{u}:\;2700\;erg/cm^{3}$)를 갖는다. $Si/SiO_{2}/Ta$ 45/Ru 9.5/IrMn 10/CoFe $7/AlO_{x}/CoFeSiB\;(t)/Ru\;60\;(nm)$ 구조는 그 자체의 낮은 포화자화도와 높은 일축 이방성을 가짐으로 인하여 보자력($H_{c}$)을 감소시키고 스위칭 각형을 증가시키게 함으로서 MTJ의 스위칭 특성에 유리한 것으로 조사되었다. 더욱이 미소두께(1 nm)의 CoFe층을 터널장벽/NiFeSiB 경계면에 삽입하면 TMR비와 스위칭 각형이 증가하고 개선되었다.

• 한국자기학회:학술대회 개요집
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• 한국자기학회 2000년도 International Symposium on Magnetics The 2000 Fall Conference
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• pp.33-59
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• 2000

Demonstrated uniform MR and resistance across 6 inch wafer, Demonstrated successful integration of MTJ and CMOS, Measured address access time of 8ns and read cycle time of 18ns for 256${\times}$2 arrays at 3.0V using a single transistor and MTJ for a cell

• JSTS:Journal of Semiconductor Technology and Science
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• 제2권3호
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• pp.185-196
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• 2002

MRAM(magnetic random access memory) is a promising candidate for a universal memory with non-volatile, fast operation speed and low power consumption. The simplest architecture of MRAM cell is a combination of MTJ(magnetic tunnel junction) as a data storage part and MOS transistor as a data selection part. This article will review the general development status of MRAM and discuss the issues. The key issues of MRAM technology as a future memory candidate are resistance control and low current operation for small enough device size. Switching issues are controllable with a choice of appropriate shape and fine patterning process. The control of fabrication is rather important to realize an actual memory device for MRAM technology.