Kiseok Moon

Spin-transfer torque magnetic random access memory (STT-MRAM)

Spin-transfer torque magnetic random access memory (STT-MRAM) is a novel, magnetic memory technology that leverages the base platform established by an existing 100+nm node memory product called MRAM to enable a scalable nonvolatile memory solution for advanced process nodes. STT-MRAM features fast read and write times, small cell sizes of 6F2 and potentially even smaller, and compatibility with existing DRAM and SRAM architecture with relatively small associated cost added. STT-MRAM is essentially a magnetic multilayer resistive element cell that is fabricated as an additional metal layer on top of conventional CMOS access transistors. In this review we give an overview of the existing STT-MRAM technologies currently in research and development across the world, as well as some specific discussion of results obtained at Grandis and with our foundry partners. We will show that in-plane STT-MRAM technology, particularly the DMTJ design, is a mature technology that meets all conventional requirements for an STT-MRAM cell to be a nonvolatile solution matching DRAM and/or SRAM drive circuitry. Exciting recent developments in perpendicular STT-MRAM also indicate that this type of STT-MRAM technology may reach maturity faster than expected, allowing even smaller cell size and product introduction at smaller nodes.

Study of Lithographically Defined Data Track and Servo Patterns

We report on fabrication of discrete tracks on perpendicular magnetic recording (PMR) media with an e-beam lithographical process. We studied the recording performance of the e-beam media on a spinstand in parallel with conventional PMR media. Discrete track media show significant reduction in adjacent track erasure (ATE). We studied and quantitatively measured the source of the ATE improvement, and developed a triple track geometrical model to calculate achievable track density for both discrete track recording (DTR) and continuous media. From the model, we identify two factors of DTR that contribute to reaching a higher TPI. Using the same fabrication technique, we also studied servo burst design and its playback waveform quality. At 250 ktpi, we compare DTR servo bursts with servo bursts written with a conventional method. DTR servo bursts show better edge definition, which can translate to better position error signal sensitivity and support higher TPI in the future.Discrete tracks are fabricated on conventional PMR media with an e-beam litho graphical process. The recording performance is studied on a spinstand in parallel with conventional PMR media. Discrete track media shows significant reduction in adjacent track erasure (ATE). The source of the ATE improvement is studied and quantitatively measured. A triple track geometrical model is developed to calculate achievable track density for both DTR and continuous media. From the model, we identify two factors of DTR, which contribute to reaching a higher TPI. Using the same fabrication technique, we also study servo burst design and its playback waveform quality. At 250 ktpi, we compare DTR servo bursts and servo bursts written with a conventional method. DTR servo bursts show better edge definition, which can translate to better PES signal sensitivity and support higher TPI in the future.

Fabrication of Flyable Perpendicular Discrete Track Media

Discrete track media offers many potential recording advantages over conventional continuous media in hard disk drives. In this study, we present a novel fabrication process for discrete track perpendicular magnetic media via electron beam lithography, ion milling, and the use of a protective Al sacrificial layer. Physical characterization of the media confirms the process is able to produce patterned tracks with no damage to the media. Spin stand analysis verifies the disks are flyable and capable of recording sharp transitions without any degradation in the magnetic signal

PtMn-based spin-dependent tunneling materials with thin alumina barrier fabricated by two-step natural oxidation

Spin-dependent tunneling (SDT) materials with bottom-pinned structure (substrate/Ta/NiFeCr/PtMn/CoFe/t Al2O3/CoFe/NiFe/Ta) are fabricated by magnetron sputtering in ultrahigh vacuum. In this study, a two-step natural oxidation was used, in which the second Al layer was deposited and naturally oxidized after the natural oxidation of the first Al layer. The top and bottom leads were also patterned into bow-tie shaped structures. The two-step oxidation process results in a perfectly decoupled pinned and free layer in a film with a total as-deposited aluminum thickness of 7 A, whereas, the one-step oxidation process gives rise to strongly coupled magnetic layers in a film with this thickness of aluminum. By using this two-step natural oxidation technique, an optimum tunneling magnetic resistance (TMR) ratio of 29.3% and resistance×area (RA) product of 34 Ω μm2 were achieved in junctions with 8 A barrier (5+3 A). In conclusion, a two-step oxidation process was used to fabricate spin-dependent tunneling devices...

Write Synchronization in Bit-Patterned Media

Writing bit-patterned media has been a challenging problem because it is necessary to synchronize write timing with physical bit-pattern locations on disk. This timing requirement must be addressed not only in disk drives but also in testing environments such as a spin stand. This paper first describes a technique for synchronizing write timing on a spin stand. Testing results show that, with data block length similar to a disk drive sector, the write timing jitter due to synchronization error is much smaller than the magnetic transition jitter. We then discuss the potential timing jitter sources in a disk drive, including disk speed variation, head vibration, and write clock stability. Experimental data show that the head vibration is potentially the most difficult jitter source for the write clock to handle. The possibility and difficulty in applying coding techniques to correct write timing error is also investigated. While it is possible to create codes for this purpose, practical considerations can limit its usefulness.

Recording Performance Study of PMR Media With Patterned Tracks

Discrete track recording technology offers a potential advantage in reducing adjacent track erasure/interference. As nano-imprinting technology advances, fabrication of such media has been demonstrated. In recent years, discrete track recording has generally been viewed as one of the next promising technologies for areal density advancement. In this study, we evaluated the recording performance of PMR media with patterned tracks. To accurately assess the advantage of discrete track performance and compare with current continuous media, both recording performances were measured on one single track. The head flying height is monitored on the patterned and continuous media regions. At 100-nm data track width, patterned tracks show noticeably better signal-to-noise ratio and significantly lower adjacent track erasure compared with continuous media at the same track width. Such measured performance advantages are critical to increase track density beyond 300-400 ktpi

Lithographically patterned servo position error signal patterns in perpendicular disks

For discrete track media to become a viable alternative, it is essential to produce accurate servo patterns in a cost effective manner. This study presents a spin stand analysis and comparison of position error signals generated from various lithographically defined servo patterns and servo bursts written on continuous regions of the same perpendicular recording media. It is demonstrated that the edge variation on the patterned servo elements is much less than that found on conventional servo bursts. In addition, evidence shows that despite some amplitude loss resulting from the removal of magnetic material, patterned servo bursts produce good quality position error signals when compared to servo bursts written on continuous media.

Kinetics of pinhole nucleation and growth in magnetic tunnel junctions

Tunnel junctions with different pinhole probabilities were obtained by varying natural oxidation condition. In pinhole-free junctions, the observed abrupt changes in magnetoresistance and junction resistance with increasing bias current are the signature of dielectric breakdown, and are attributed to the process of pinhole nucleation. On the contrary, low RA junctions show gradual changes in magnetoresistance and junction resistance with voltage, indicating the absence of pinhole nucleation, but only the process of pinhole growth starting from the inherent pinholes. Experimentally, the activation energy of pinhole growth (EG∼0.3 eV) is found to be much lower than that of pinhole nucleation (EN∼0.6eV), which explains the easy breakdown behavior observed in low RA junctions.

Writability in Discrete Track Media

Discrete track recording has emerged as a promising candidate for high storage capacity since it reduces adjacent track erasing (ATE) and alleviates narrow head requirements. In this paper, the writability of discrete lines was studied in discrete track media (DTM) fabricated by e-beam lithography and ion-milling on perpendicular magnetic recording (PMR) media. The writability of discrete lines with finite length and three kinds of line width (50, 100, and 150 nm) is compared with continuous media on the same track. When writing current is small (less than 12 mA), the narrowest discrete lines (50 nm) are not easily magnetized, most likely due to deformed magnetic layer created by a shallow wall angle. Conversely, wider discrete lines ( ges 100 nm) are magnetized like continuous media. No difference in the level of magnetization was observed for the narrowest lines if writing current was large enough (more than 12 mA). This smooth magnetization of the narrowest line at sufficient writing current flow makes it certain that narrow discrete line can be used for high capacity storage.