Jun-Soo Bae

Improved magnetic tunnel junction with amorphous seed layer, surface treatment,and high-polarization magnetic materials

Dependence of magnetic tunnel junction (MTJ) properties on seed layer, surface treatment and magnetic materials was investigated to improve TMR ratio. Roughness of tunnel oxide layer generated from seed layer roughness reduces TMR ratio as well as RA product owing to locally reduced oxide layer thickness. Crystallinity of seed layer is found to be important to enhance TMR ratio. Surface treatment with ion beam etching on a pinned layer improves tunnel oxide uniformity providing higher TMR ratio. Also, highly polarized magnetic material increases TMR ratio but the effect of surface treatment is quite different depending on pinned layer material.

Microstructural Characterization in Reliability Measurement of Phase Change Random Access Memory

The cell failures after cycling endurance in phase-change random access memory (PRAM) have been classified into three groups, which have been analyzed by transmission electron microscopy (TEM). Both stuck reset of the set state (D0) and stuck set of the reset state (D1) are due to a void created inside GeSbTe (GST) film or thereby lowering density of GST film. The decrease of the both set and reset resistances that leads to the tails from the reset distribution are induced from the Sb increase with cycles.

Investigation on physical origins of endurance failures in PRAM

Endurance failures are classified into three groups, all of which originate from the atomic transport of GST due to electromigration in molten phase. Based on the analysis, cell structure insusceptible to the atomic transport and for better cycling performance was proposed.

Impacts of external magnetic field and high temperature disturbance on MRAM reliability based on FPGA test platform

As the semiconductor manufacturing technology continuously scales down, the working memory, such as SRAM and DRAM, suffers from the issues of dramatic leakage and power consumption. Non-volatile memories are proposed to deal with these problems. Among them, MRAM is one of the most promising candidates for DRAM replacement. However, MRAM is sensitive to external magnetic field and high temperature disturbance theoretically. In this paper, we investigate the impacts of external magnetic field and high temperature disturbance on MRAM and set up a test platform based on FPGA to analyze MRAM reliability under different external environment. Test results show that MRAM has a good thermal reliability but a notable sensitivity to external magnetic field disturbance.

Effect of Ti diffusion on the microstructure of Ge2Sb2Te5 in phase-change memory cell

The dependence of the microstructure of Ge2Sb2Te5 (GST) on Ti diffusion into GST by annealing in GST/Ti/TiN phase-change random access memory stack was studied by various transmission electron microscopy (TEM) techniques. The microstructure and crystal structure of GST were identified with high-resolution TEM (HRTEM) and image simulation technique, and the Ti diffusion into GST was revealed by scanning transmission electron microscope-energy-dispersive X-ray spectroscopy analysis. It was observed that Ti atoms of Ti/TiN thin layers were incorporated into GST cell through several thermal annealing steps and they could retard the phase transition from face-centered cubic (FCC) phase into hexagonal close-packed (HCP) phase partially and restrain the increase in grain size. Thus, it is concluded that Ti diffusion can affect the microstructure of GST including the type of the crystal phase and grain size of GST. It was shown that the insertion of diffusion barrier between TiN and GST could block Ti diffusion into GST and make it possible for FCC phase to completely transform into HCP phase.

Study of ion beam damage in magnetic tunnel junction on FIB prepared samples

Magnetic Random Access Memory (MRAM) has emerged as the leading candidate for future universal memory due to its non-volatility, excellent endurance and read/write performance. The magnetic tunnel junction (MTJ) is a data storage element in MRAM and is basically composed of two ferromagnetic layers separated by the magnesium oxide (MgO) tunnel barrier. MgO between two ferromagnetic layers was adopted to enlarge the resistance difference between two kinds of magnetic arrangements by tunneling current through MgO. Like this, it is important to understand characterization of MgO for developing Mram. Due to thin thickness of MgO, FIB milling should be used for the preparation of TEM specimens in Mram. The major problem in MgO sampling by FIB milling is the transform of MgO between two ferromagnetic due to FIB induced damage, which leads to high tunnel current through MgO and high resistance difference between two kinds of magnetic arrangements. An understanding of FIB generated artifact on MgO is important to analysis Mram and to optimize the sample preparation process. The normal ion beam damage are compared with low-keV FIB ion beam damage on blanket MgO wafer. Experiments were performed using Helios 450 FIB(FEI) and XV-200TBs(SII) with gallium ion sources operated at 30 keV to 2 keV, respectively. As a preliminary, the thicknesses of all specimens were fixed at 100nm for the final ion beam milling currents of 210 pA(30 keV) by Helios 450 FIB(FEI). Specimens of 100nm were transferred to low-keV FIB (Helios 450/XV-200TBs) to do the low-keV ion milling. Then each specimen had a 2 keV cleaned surface and a 30 keV FIB prepared surface. In this paper, we understand the normal ion beam damage on blanket MgO through changing beam current and beam voltage. Then we present the optimized recipe and which equipment is better to analysis.

Thermal stability of MTJ using Zr capping layer

The simple stacking structure such as [Sub/Zr/AlO/sub x//NiFe (or CoFeB)/capping layer] was prepared to investigate the changes of magnetic properties of free layer with annealing temperature. Ta, Ru, or Zr was used as the capping layer. Samples were annealed in a vacuum (below 1/spl times/10/sup -6/ Torr) for 1 hour under a magnetic field of 10 KOe. The magnetic properties were measured using vibrating sample magnetometer in sample with 1/spl times/1 cm/sup 2/ area. The changes of the coupling field between the pinned layers through Ru as a function of annealing temperature for various pinned layer materials were also studied.