• Korean Journal of Materials Research
• /
• v.30 no.2
• /
• pp.99-104
• /
• 2020

Recent discoveries of ferroelectric properties in ultrathin doped hafnium oxide (HfO₂) have led to the expectation that HfO₂ could overcome the shortcomings of perovskite materials and be applied to electron devices such as Fe-Random access memory (RAM), ferroelectric tunnel junction (FTJ) and negative capacitance field effect transistor (NC-FET) device. As research on hafnium oxide ferroelectrics accelerates, several models to analyze the polarization switching characteristics of hafnium oxide ferroelectrics have been proposed from the domain or energy point of view. However, there is still a lack of in-depth consideration of models that can fully express the polarization switching properties of ferroelectrics. In this paper, a Zr-doped HfO₂ thin film based metal-ferroelectric-metal (MFM) capacitor was implemented and the polarization switching dynamics, along with the ferroelectric characteristics, of the device were analyzed. In addition, a study was conducted to propose an applicable model of HfO₂-based MFM capacitors by applying various ferroelectric switching characteristics models.

• Proceedings of the Korean Magnestics Society Conference
• /
• 2011.12a
• /
• pp.29-30
• /
• 2011

We have demonstrated the fabrication of sub 30 nm MTJ pillars with PMA characteristics. The multi-step IBE process performed at $45^{\circ}$ and $30^{\circ}$, using NER resulted in almost vertical side profiles. There deposition on the sidewalls of the NER prevented lateral etching of the resist hard mask allowing vertical MTJ side profile formation without any reduction in the lithographically defined resist lateral dimensions. For the 28nm STT-MTJ pillars, the measured TMR ratio was 13 % with resistance of 1 $k{\Omega}$, which was due to remaining redeposition layers less than 0.1 nm thick. With further optimization in multi-step IBE conditions, it will be possible to fabricate fully operating sub 30 nm perpendicular STT-MTJ structures for application to future non-volatile memories.

• Proceedings of the Korean Vacuum Society Conference
• /
• 2010.02a
• /
• pp.233-233
• /
• 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.

• Proceedings of the Korean Vacuum Society Conference
• /
• 2010.02a
• /
• pp.232-232
• /
• 2010

정보화 산업의 발달은 DRAM, flash memory 등을 포함한 기존의 반도체 메모리 소자를 대체할 수 있는 차세대 메모리 소자에 대한 개발을 요구하고 있다. 특히 magnetic random access memory (MRAM)는 SRAM과 대등한 고속화 그리고 DRAM 보다 높은 기록 밀도가 가능하고 낮은 동작 전압과 소비전력 때문에 대표적인 차세대 비휘발성 메모리로 주목받고 있다. 또한 MRAM소자의 고집적화를 위해서 우수한 프로파일을 갖고 재증착이 없는 나노미터 크기의 magnetic tunnel junction (MTJ) stack의 건식 식각에 대한 연구가 선행되어야 한다. 본 연구에서는 고밀도 반응성 이온 식각법(Inductively coupled plasma reactive ion etching; ICPRIE)을 이용하여 재증착이 없이 우수한 식각 profile을 갖는 CoFeB과 IrMn 박막을 형성하고자 하였다. Photoresist(PR) 및 Ti 박막의 두 가지 마스크를 이용하여 HBr/Ar, HBr/$O_2$/Ar 식각 가스들의 농도를 변화시키면서 CoFeB과 IrMn 박막의 식각 특성들이 조사되었다. 자성 박막과 동일한 조건에 대하여 hard mask로서 Ti가 식각되었다. 좋은 조건을 얻기 위해 HBr/Ar 식각 가스를 이용 식각할 때 pressure, bias voltage, rf power를 변화시켰고 식각조건에서 Ti 하드마스크에 대한 자성 박막들의 selectivity를 조사하고 식각 profile을 관찰하였다. 식각 속도를 구하기 위해 alpha step(Tencor P-1)이 사용되었고 또한 field emission scanning electron microscopy(FESEM)를 이용하여 식각 profile을 관찰함으로써 최적의 식각 가스와 식각 조건을 찾고자 하였다.

• Proceedings of the Korean Vacuum Society Conference
• /
• 2010.02a
• /
• pp.213-213
• /
• 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 the Korean Magnetics Society
• /
• v.12 no.4
• /
• pp.127-131
• /
• 2002

We fabricated MTJ devices that have doubly oxidized tunnel barrier using plasma oxidation method to from oxidized AlO$\sub$x/ tunnel barrier. Doubly oxidation I, which sputtered 10 ${\AA}$-bottom Al layer and oxidized it with oxidation time of 10 s. Subsequent sputtering of 13 ${\AA}$-Al was performed and the metallic layer was oxidized for 50, 80 and 120 s., respectively. Doubly oxidation II, which sputtered 10 ${\AA}$-bottom Al layer and oxidized it varying oxidation time for 30∼120 s. Subsequent sputtering of 13 ${\AA}$-Al was performed and the metallic layer was oxidized for 210 sec. Double oxidation process specimen showed MR ratio of above 27％ in all experiment range. Singly oxidation process. 13 ${\AA}$-Al layer and oxidized up to 210 s, showed less MR ratio and more narrow process window than those of doubly oxidation. Cross-sectional TEM images would that doubly oxidized barrowers were thinner and denser than singly oxidized ones. XPS characterization confirmed that doubly oxidation of Fe with bottom insulating layer. As a result, doubly oxidation could have superior MR ratio in process extent during long oxidation time because of preventing oxidation of bottom magnetic layer than singly oxidation.

• Journal of the Korean Vacuum Society
• /
• v.18 no.5
• /
• pp.352-357
• /
• 2009

The InAs multi-quantum dots (MQDs) solar cell and InGaAs multi-quantum wells (MQWs) solar cell to cover 1.1 eV and 1.3 eV were designed by 1D poisson, respectively. The MQDs and MQWs of 5, 10, 15 layers were grown by molecular beam epitaxy. The photo luminescence results showed that the 5 period stacked MQDs have the highest intensity at around 1.1 eV with 57.6 meV full width at half maximum (FWHM). Also we can observe 10 period stacked MQWs peak position which has highest intensity at 1.31 eV with 12.37 meV FWHM. The density and size of QDs were observed by reflection high energy electron diffraction pattern and atomic force microscope. Futhermore, AlGaAs/GaAs sandwiched tunnel junctions were modified according to the width of GaAs layer on p-type GaAs substrates. The structures with GaAs width of 30 nm and 50 nm have backward diode characteristics. In contrast, tunnel diode characteristics were observed in the 20 nm of that of sample.

• Journal of the Korean Magnetics Society
• /
• v.14 no.3
• /
• pp.99-104
• /
• 2004

We have studied to understand the barrier and interface qualities and structural changes through measuring temperature dependent spin-polarization as functions of plasma oxidation time and annealing time. Magnetic tunnel junctions consisting of SiO2$_2$/Ta 5/CoFe 17/IrMn 7.5/CoFe 5/Al 1.6-Ox/CoFe 5/Ta 5 (numbers in nm) were deposited and annealed when necessary. A 30 s,40 s oxidized sample showed the lowest spin-polarization values. It is presumed that tunneling electrons were depolarized and scattered by residual paramagnetic Al due to under-oxidation. On the contrary, a 60s, 70 s oxidized sample might have experienced over-oxidation, where partially oxidized magnetic dead layer was formed on top of the bottom CoFe electrode. The magnetic dead layer is known to increase the probability of spin-flip scattering. Therefore it showed a higher temperature dependence than that of the optimum sample (50 s oxidation). temperature dependence of 450 K annealed samples was improved when the as-deposited one compared. But the sample underwent 475 K and 500 K annealing exhibits inferior temperature dependence of spin-polarization, indicating that the over-annealed sample became microstucturally degraded.

• Journal of the Microelectronics and Packaging Society
• /
• v.24 no.3
• /
• pp.7-11
• /
• 2017

In this paper, we have studied the characteristics of NAND Flash memory in SONOS Poly-Si Thin Film Transistor (Poly-Si TFT) device. Source/drain junctions(S/D) of cells were not implanted and selective transistors were located in the end of cells. We found the optimum conditions of process by means of the estimation for the doping concentration of channel and source/drain of selective transistor. As the doping concentration was increased, the channel current was increased and the characteristic of erase was improved. It was believed that the improvement of erase characteristic was probably due to the higher channel potential induced by GIDL current at the abrupt junction. In the condition of process optimum, program windows of threshold voltages were about 2.5V after writing and erasing. In addition, it was obtained that the swing value of poly Si TFT and the reliability by bake were enhanced by increasing process temperature of tunnel oxide.

• Journal of the Korean Society of Propulsion Engineers
• /
• v.11 no.1
• /
• pp.51-56
• /
• 2007

Flow characteristics in a liquid fuel ramjet combustor were investigated using the PIV method. The combustor has two rectangular inlets that form a $90^{\circ}$ angle each other. Three cases of test combustors are made in which those inlet angles are $30^{\circ},\;45^{\circ}\;and\;60^{\circ}$. The experiments were performed in a water tunnel test with the same Reynolds number as Mach 0.3 at the inlet. PIV software was developed to measure the characteristics of the flow field in the combustor. A large and complex recirculating flow was measured in the dome area with 4 different dome size. Experimental results shows that 1/3 dome size of combustor diameter is suitable and smaller inlet angle provide large recirculation flow at the dome of combustor as a frame holder in this experimental ranges but need to consider secondary recirculation flow in a junction region to optimize the configuration of ramjet combustor.