Magnetic Tunnel Junction Stack Research Articles

Area Efficient High Through-put Dual Heavy Metal Multi-Level Cell SOT-MRAM

This paper proposes a novel multi-level cell spin-orbit torque magnetic random-access memory (MLC SOT-MRAM) cell structure and validates its functionality and performance through simulation. The proposed memory cell comprises two uniform magnetic tunnel junctions (MTJs) that can be programmed separately by the energy efficient SOT technology through two different heavy metal electrodes. This permit employing both single and dual port architectures. The uniform cross-sectional area of the in-series MTJs stack simplifies the fabrication process. In addition, the cell structure requires only four terminals to successfully read and write the two bits. This allows accessing the two bits with only three access transistors, which achieves 25% smaller 1-bit effective area compared to the conventional single level cell (SLC) SOT-MRAM that requires four transistors. Simulation results based on an approximated dynamic model shows it offers nearly similar write energy consumption compared to the conventional SOT-MRAM.

Effect of capping layer material on thermal tolerance of magnetic tunnel junctions with MgO/CoFeB-based free layer/MgO/capping layers

We investigate the effect of the capping layer on the thermal tolerance of magnetic tunnel junctions (MTJs) with free layer of MgO/CoFeB/spacer layer/CoFeB/MgO/capping layers (CoFeB, Ru, or Ta). We observe the largest perpendicular magnetic anisotropy energy density for the free layer with CoFeB capping layer using Ta spacer after annealing at 400°C for 1 h. Energy-dispersive X-ray (EDX) line analysis along film normal direction reveals the absorption of oxygen in MgO by Ta in the stack with Ta capping layer and Ru diffusion into CoFeB free layer in the stack with Ru capping layer, which could cause the reduction of perpendicular magnetic anisotropy. We also evaluate annealing temperature dependence of magnetic properties for the MTJ stacks with different spacer layer. We again observe the largest perpendicular magnetic anisotropy energy density for the MTJ stack using the CoFeB capping layer. The present study reveals that CoFeB capping layer is effective for achieving improved robustness against annealing.

Integration of Tb/Co multilayers within optically switchable perpendicular magnetic tunnel junctions

This work reports the development of perpendicular magnetic tunnel junctions, based on a multilayered stack of [Tb/Co] nanolayers, in which the magnetization can be reliably toggled using a single optical pulse. The helicity-independent single-shot switching of the magnetization in the Tb/Co multilayered stack was achieved using either 60 fs-long or 5 ps laser pulses with incident fluences down to 4.7 mJ/cm2. The magnetic switching was achieved for a Co-rich composition window of the multilayer corresponding to layer thickness ratios tCo/tTb between 1.3-1.5. This was confirmed for the multilayer alone as well as for the multilayer coupled to aCoFeB electrode, with a structure consisting of CoFeB/Ta/[Tb/Co]N. The optical switching is preserved even after annealing at 250°C in magnetic tunnel junctions (MTJ) electrodes, exhibiting a tunnel magnetoresistance (TMR) of 41% and RxA value of 150 Ωμm after its integration, measured on unpatterned MTJ stacks. These results represent the first step towards the development of hybrid spintronic photonic systems with fJ switching energies.

Predictive Materials Design of Magnetic Random-Access Memory Based on Nanoscale Atomic Structure and Element Distribution.

Magnetic tunnel junctions (MTJs) capable of electrical read and write operations have emerged as a canonical building block for nonvolatile memory and logic. However, the cause of the widespread device properties found experimentally in various MTJ stacks, including tunneling magnetoresistance (TMR), perpendicular magnetic anisotropy (PMA), and voltage-controlled magnetic anisotropy (VCMA), remains elusive. Here, using high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy, we found that the MTJ crystallization quality, boron diffusion out of the CoFeB fixed layer, and minimaloxidation of the fixed layercorrelate with the TMR. As with the CoFeB free layer, seed layer diffusion into the free layer/MgO interface is negatively correlated with the interfacial PMA, whereas the metal-oxides concentrations in the free layer correlate with the VCMA. Combined with formation enthalpy and thermal diffusion analysisthat can explain the evolution of element distribution from MTJ stack designs and annealing temperatures, we further established a predictive materials design framework to guide the complex design space explorations for high-performance MTJs. On the basis of this framework, we demonstrate experimentally high PMA and VCMA values of 1.74 mJ/m2 and 115 fJ/V·m-1 with annealing stability above 400 °C.

Eliminating etch stop in high-density magnetic tunnel junction patterning using high-temperature CO/NH3 plasma etching

Increasing the process temperature in CO/NH3 plasma etching has been investigated to suppress an etch stop in high-density magnetic tunnel junction stack (MTJ stack) patterning with a Ta mask. In a previous study, the occurrence of an etch stop was observed when using a 100 nm space pattern; specifically, the etching depth was unchanged with the 100 nm space pattern when the etching time was increased, although when wider spaces were used, the etching depth increased proportionally. In this study, differences in the etch stop depth with 100 nm space patterns were examined by changing the electrode temperature from 120 to 300 °C. The etch stop depth became deeper as the electrode temperature was increased and it was found that a high-temperature CO/NH3 process was an effective way to prevent the etch stop effect. However, at 300 °C, the MTJ stack's thickness was observed to expand, which may induce a deterioration of the junction's magnetic properties. A scanning transmission electron microscope image and an energy dispersive x-ray spectroscopy image of the MTJ stack revealed that nitridation of the MTJ stack was the reason of its expansion in thickness. Moreover, pure N2 plasma irradiation of the MTJ stack clearly indicated that this thickness expansion occurred for an electrode temperature of over 265 °C. Finally, the results demonstrate that, for a CO/NH3 process with an electrode temperature of 250 °C, it is possible to etch a 52-nm-thick MTJ stack without either etch stop or film thickness expansion occurring.

Study on the effect of re-deposition induced by ion beam etching on MTJ performances

Magnetic tunnel junction (MTJ) as a key spintronics device can be used for the high-sensitivity magnetic field sensor and high-density non-volatile magnetic random access memory (MRAM). To obtain a high tunneling magnetoresistance (TMR), precise control of the etching process for MTJs is an essential step. In order to investigate the effect of the etching angle on the performance of MTJ devices, a series of MTJ pillars are fabricated by etching with the incidence angle of 10°, 20° and 30°. The prepared samples are characterized by the optical microscopy and SEM, and the R-H curves are also measured and then statistically analyzed. The results reveal that the performance of the MTJ is strongly affected by the IBE process, displaying the uniformity of the pillars edge and MTJ performance will improve as well with the increase of the etching angle. Then, a simplified model based on the re-deposition effect of the etching process is established to explain the experimental phenomena. Furthermore, a newly defined material parameter is introduced in this model and obtained with fitting the experimental results. This proves a valuable way to evaluate the quality of the MTJ stack film without the interference of the device fabrication process.

LAS-NCS: A Laser-Assisted Spintronic Neuromorphic Computing System

In spintronic-based neuromorphic computing systems (NCSs), the switching of magnetic moment in a magnetic tunnel junction (MTJ) is used to mimic biological neuron firing. However, the high currents required for MTJ switching will lead to a large energy consumption and low speed operation of the MTJ-based NCSs. In this brief, the energy consumption and the delay of the proposed MTJ-based NCS are reduced by thermally assisting the MTJ switching through a microwatt nanosecond laser pulse. Simulation results show 85.7%, 84%, and 97.8% improvements in energy consumption, delay, and energy-delay product, respectively, for the laser-assisted spintronic NCS (LAS-NCS) by heating the MTJs up to 127 °C. The energy consumption of the laser pulse for heating up the MTJ stack is $1.2 {\times }$ – $2.4 {\times }$ lower than Joule heating method. Moreover, in contrast with Joule heating method, the LAS-NCS does not require MTJ with modified structure.

Eddy Current–Tunneling Magneto-Resistive Sensor for Micromotion Detection of a Tibial Orthopaedic Implant

In this paper, an eddy current loop coupled with the tunneling magneto resistor (EC-TMR) is used as a displacement sensor to detect the micromotion of an orthopaedic implant. The high sensitivity and signal to noise ratio of the TMR sensor are used to achieve high resolution at a large standoff distance. First, a small three-turn rectangular eddy current loop of dimension 2.5 mm × 10 mm is designed and simulated inside the human body using a full-wave EM simulator. Then, it is fabricated and tested using vector network analyzer. The magnetic tunnel junction stack is optimized and a six-element TMR sensor is fabricated and characterized. The eddy current and tunnelling magneto resistive sensor are integrated and heterodyne detection technique is used to obtain the high-resolution micromotion detection at an extended standoff range. This technique can be used for in-vivo detection of the micromotion of the orthopaedic implant which will be useful in reducing the revision surgeries due to the mechanical failures of the implant.

Critical Role of W Insertion Layer Sputtering Condition for Reference Layer on Magnetic and Transport Properties of Perpendicular-Anisotropy Magnetic Tunnel Junction

We investigated an effect of sputtering gas species Ar, Kr, and Xe for deposition of a W insertion layer W(Ar, Kr, and Xe) in the synthetic ferrimagnetic reference layer consisting of [Co/Pt] multilayer/Ru/[Co/Pt] multilayer/W/CoFeB on magnetic properties of the reference layer and tunnel magnetoresistance ratio (TMR ratio) of magnetic tunnel junction (MTJ) stacks with the reference layer. In all the cases, the TMR ratio increased with the increase of the W insertion layer thickness $t_{\mathrm {W}}$ and showed a maximum at certain $t_{W}$ . Although the maximum TMR ratio is almost the same for all the cases, $t_{\mathrm {W}}$ range in which we observed the maximum TMR ratio is dependent on the gas species; $t_{W} = 0.2$ nm for Ar, $t_{\mathrm {W}} = 0.2$ –0.4 nm for Kr, and $t_{\mathrm {W}} = 0.2$ –0.5 nm for Xe, indicating that $t_{\mathrm {W}}$ margin giving high-TMR ratio becomes wider with increasing atomic number of the gas species. The lower TMR ratio for W (Ar) at larger $t_{\mathrm {W}}$ region is due to the degradation of ferromagnetic coupling between the CoFeB and Co/Pt multilayer sandwiching W, resulting in canting of magnetization in the CoFeB layer. We found that an intermixing of Pt atoms in the Co/Pt multilayer occurred in the MTJ stack with W (Ar), whereas it did not intermix with W (Kr), indicating that the Pt interdiffusion caused the degradation of ferromagnetic coupling through W layer.

Area Efficient Shared Diode Multi-Level Cell SOT-MRAM

This paper proposes a diode-based multi-level cell spin–orbit torque magnetic random-access memory (SOT-MRAM) for high density memory applications. By stacking a shared diode over two in-parallel magnetic tunnel junctions (MTJs) that share a common heavy metal electrode, our proposed cell requires only one silicon-based transistor to access 2 bits. Having just one transistor to access 2 bits results in at least $2\times $ higher density when compared to various designs in the literature. Moreover, utilizing a high-density MRAM process, a $4F^{2}$ effective 1 bit area can be achieved. Furthermore, our proposed design maintains the high energy efficiency of the conventional SOT-MRAM. In addition, an improvement in figure of merit of at least 30% defined as energy area product is obtained compared to various designs. This paper also discusses the essential requirements to realize our proposed SOT-MRAM, such as the realization of diode MTJ stack and SOT-MTJs, with different critical currents and electrical resistances.

Top-Pinned STT-MRAM Devices With High Thermal Stability Hybrid Free Layers for High-Density Memory Applications

The hybrid free layer (HFL) design consisting of a single layer of CoFeB coupled via a spacer to [Co/Ni] multilayers was proposed previously to increase perpendicular magnetic anisotropy in the free layer of top-pinned magnetic tunnel junction stacks. It is thus expected that a high thermal stability factor ( $\Delta$ ) can be achieved in devices with small dimensions, which is required for data retention in high-density memory applications. Here, we report on the electrical and magnetic properties of the HFL in devices with various critical dimensions (CD), in which case the [Co/Ni] multilayers in the HFL have only one Co/Ni bilayer on Pt seed. It is shown first that the HFL can provide larger coercive field ( $H_{c}$ ) than dual-MgO FL counterparts. $\mu _{0}H_{c}$ around 125 mT is achieved in sub-20 nm devices with the HFL. To derive $\Delta$ , the field sweep method was applied. The fitting of the switching probability versus applied field was compared for a macrospin reversal and a domain wall mediated reversal (DWMR). The DWMR model is valid when the CD is larger than around 40 nm in both dual-MgO FL and HFL cases, where the switching is realized through domain wall motion as shown via micromagnetic simulations. In both regimes, the HFL provides a higher $\Delta$ than the dual-MgO FL.

High‐Performance Flexible Magnetic Tunnel Junctions for Smart Miniaturized Instruments

Flexible electronics is an emerging field in many applications ranging from in vivo biomedical devices to wearable smart systems. The capability of conforming to curved surfaces opens the door to add electronic components to miniaturized instruments, where size and weight are critical parameters. Given their prevalence on the sensors market, flexible magnetic sensors play a major role in this progress. For many high‐performance applications, magnetic tunnel junctions (MTJs) have become the first choice, due to their high sensitivity, low power consumption etc. MTJs are also promising candidates for non‐volatile next‐generation data storage media and, hence, could become central components of wearable electronic devices. In this work, a generic low‐cost regenerative batch fabrication process is utilized to transform rigid MTJs on a 500 µm silicon wafer substrate into 5 µm thin, mechanically flexible silicon devices, and ensuring optimal utilization of the whole substrate. This method maintains the outstanding magnetic properties, which are only obtained by deposition of the MTJ on smooth high‐quality silicon wafers. The flexible MTJs are highly reliable and resistive to mechanical stress. Bending of the MTJ stacks with a diameter as small as 500 µm is possible without compromising their performance and an endurance of over 1000 cycles without fatigue has been demonstrated. The flexible MTJs are mounted onto the tip of a cardiac catheter with 2 mm in diameter without compromising their performance. This enables the detection of magnetic fields and the angle which they are applied at with a high sensitivity of 4.93%/Oe and a low power consumption of 0.15 μW, while adding only 8 and 5 μm to the weight and diameter of the catheter, respectively.

Modeling of Breakdown-Limited Endurance in Spin-Transfer Torque Magnetic Memory Under Pulsed Cycling Regime

Perpendicular spin-transfer torque (p-STT) magnetic memory is gaining increasing interest as a candidate for storage-class memory, embedded memory, and possible replacement of static/dynamic memory. All these applications require extended cycling endurance, which should be based on a solid understanding and accurate modeling of the endurance failure mechanisms in the p-STT device. This paper addresses cycling endurance of p-STT memory under pulsed electrical switching. We show that endurance is limited by the dielectric breakdown of the magnetic tunnel junction stack, and we model endurance lifetime by the physical mechanisms leading to dielectric breakdown. The model predicts STT endurance as a function of applied voltage, pulsewidth, pulse polarity, and delay time between applied pulses. The dependence of the endurance on sample area is finally discussed.

High performance perpendicular magnetic tunnel junction with Co/Ir interfacial anisotropy for embedded and standalone STT-MRAM applications

High volume spin transfer torque magnetoresistance random access memory (STT-MRAM) for standalone and embedded applications requires a thin perpendicular magnetic tunnel junction (pMTJ) stack (∼10 nm) with a tunnel magnetoresistance (TMR) ratio over 200% after high temperature back-end-of-line (BEOL) processing up to 400 °C. A thin reference layer with low magnetic moment and strong perpendicular magnetic anisotropy (PMA) is key to reduce the total thickness of the full pMTJ stack. We demonstrated strong interfacial PMA and a perpendicular Ruderman-Kittel-Kasuya-Yosida exchange interaction in the Co/Ir system. Owing to the additional high PMA at the Ir/Co interface in combination with a conventional CoFeB/MgO interface in the Ir/Co/Mo/CoFeB/MgO reference layer, the full film pMTJ showed a TMR ratio over 210% after annealing at 400 °C for 150 min. The high TMR ratio can be attributed to the thin stack design by combining a thin reference layer with the efficient compensation by a thin pinned layer. The annealing stability may be explained by the absence of solid solution in the Co-Ir system and the low oxygen affinity of Mo in the reference layer and the free layer. High device performance with a TMR ratio over 210% was also confirmed after subjecting the patterned devices to BEOL processing temperatures of up to 400 °C. This proposed pMTJ design is suitable for both standalone and embedded STT-MRAM applications.

Modeling and Analysis of Magnetic Field Induced Coupling on Embedded STT-MRAM Arrays

Spin transfer torque magnetic random access memory (STT-MRAM) is an emerging memory technology which exhibits nonvolatility, high density, high endurance, and nano-second read and write times. These characteristics make STT-MRAM suitable for last-level cache and other embedded applications. The STT-MRAM bit-cell consists of a magnetic tunnel junction (MTJ) which is composed of two ferromagnetic layers (free and fixed layers) and one insulating layer in between. As STT-MRAM arrays become denser to meet cost and requirements of high performance computing, the distance between adjacent MTJ bits reduces. This aggravates the magnetic coupling from free and fixed layer of one MTJ bit to its neighbors. Even though magnetic coupling is expected to become stronger as MTJ scales down, its impacts on static and dynamic properties on MTJ is relatively unexplored. In this paper, we present a model of the magnetic field coupling in high-density MTJ arrays for three different types of MTJ stacks and evaluate the effect of magnetic field induced coupling on static and dynamic properties. Lastly, we show how process induced variations in MTJ characteristics affect the magnitude of magnetic coupling and their effect on the electrical characteristics of the STT-RAM arrays.

Impact of Tungsten Sputtering Condition on Magnetic and Transport Properties of Double-MgO Magnetic Tunneling Junction With CoFeB/W/CoFeB Free Layer

We investigated an effect of sputtering gas species (Ar, Kr, and Xe) for deposition of a W insertion layer in the CoFeB/W/CoFeB free layer on magnetic properties of the free layer and tunnel magnetoresistance (TMR) ratio of magnetic tunnel junctions (MTJs) stacks using the free layer annealed at 400 °C for 1 h. As the W insertion layer thickness tW increased, we found the degradation of perpendicular anisotropy and larger reduction of saturation magnetic moment per unit area mS in the free layer using Ar compared to those using Kr and Xe. We also found a smaller TMR ratio for the MTJ stack using Ar compared to those using Kr and Xe. Energy-dispersive X-ray spectrometry line analysis revealed more significant interdiffusion between W and CoFeB layers in the free layer using Ar than those using Kr and Xe, that could result in the smaller mS and perpendicular anisotropy in the free layer and smaller TMR ratio for the MTJ stack using Ar than those using Kr and Xe. We also investigated concentration of Ar, Kr, and Xe in W layers deposited using Ar, Kr, and Xe, respectively, by high-resolution Rutherford backscattering spectrometry, revealing that 0.2 at% Ar was detected in the W layer using Ar, while Kr and Xe were not detected in W layers using Kr and Xe. Such a difference in concentration of inert gas atoms in the W layer could be one possible reason for the difference about degree of interdiffusion between W and CoFeB layers.

Breakthrough in current-in-plane tunneling measurement precision by application of multi-variable fitting algorithm.

We present a breakthrough in micro-four-point probe (M4PP) metrology to substantially improve precision of transmission line (transfer length) type measurements by application of advanced electrode position correction. In particular, we demonstrate this methodology for the M4PP current-in-plane tunneling (CIPT) technique. The CIPT method has been a crucial tool in the development of magnetic tunnel junction (MTJ) stacks suitable for magnetic random-access memories for more than a decade. On two MTJ stacks, the measurement precision of resistance-area product and tunneling magnetoresistance was improved by up to a factor of 3.5 and the measurement reproducibility by up to a factor of 17, thanks to our improved position correction technique.

(Invited) Interface Control of Advanced Electronic Devices - High-k/Metal Gate System and Magnetic Tunneling Junction

With the scaling of electronic devices, interface control of the constituent materials has become more and more important. Several key issues and related topics are taken up and reviewed with respect to logic and memory devices. The key issues are interface control in high-k/metal gate system for logic application and magnetic tunneling junction for memory application. High-k/Metal Gate System [1] In accordance with the requirement of CMOS scaling, replacement of thermally grown SiO2 gate dielectric with other dielectric materials with higher dielectric constant (k) has become imperative to reduce the equivalent oxide thickness (EOT) to achieve higher drivability of FET. Key factors on Hf-based high-k/metal gate (HK/MG) CMOS technology based on conventional gate first process for low operational power application are overviewed. Work-function (WF) tuning derived from ionic property of high-k material, process cost, EOT scaling with low gate leakage current are issues to be overcome. Among them, the key issue is how to control the atomic distribution in the Hf-based HK/MG system, which is absolutely requisite for WF tuning and EOT scaling. Interpretation of effective work-function (WF): For conventional gate first process, due to the interfacial reaction, the Fermi level is pinned and the WF increases with decreasing the EOT. In addition to this, due to the fixed charge in the high-k dielectric, the WF is also modulated, hence the effective WF (eWF) changes. The effect of the fixed charge is rather complicated due to an oxygen vacancy behavior in HK. Consequently, the eWF results in the “roll-off” behavior caused by the (+) charge induced by the oxygen vacancy. When the oxygen increases, i.e., (-) charge increases in the HK, the obtained WF tends to roll-up. In contrast, it tends to roll-off for more (+) charge induced by oxygen vacancy, Vo 2+in the HK. Formation process of Hf-based high-k (HK): Generally, HK is deposited by metal-organic chemical vapor deposition (MOCVD) or atomic layer deposition (ALD), however, HK dielectric formation by oxidizing a preliminarily sputtered metallic-Hf is reasonable approach for fabricating the HK gate stack in a very pure form with less water content across the wafer. And in a very important thing, by controlling the oxidation time, this process can provide precise thickness control of the interfacial layer (IL) including “zero IL” with excellent uniformity. Magnetic Tunneling Junction A magnetic tunnel junction (MTJ) for spin-transfer-torque (STT) switching is a promising building block of non-volatile working memory embedded in very large scale integrated circuits (VLSIs) because of its potential to reduce a power consumption by replacing the conventional volatile cache memories. Among many materials, a CoFeB/MgO/CoFeB stack structure has attracted much attention as a candidate of MTJ embedded in VLSIs because of its high tunnel magnetoresistance (TMR) ratio caused by a perpendicular magnetic easy axis. And what is better, this MTJ has a potential for high integration into VLSIs down to a junction size of 1X nm regime. To improve the MTJ performance, crystal structure of the CoFeB/MgO interface is a crucial issue as well as B behavior during the process treatment. Redox reaction: As an example of the interfacial reaction, a redox reaction was observed at the surface of Ta/CoFeB/MgO/CoFeB MTJ stack after annealing up to 400℃ for 1 h in vacuum, by angle-resolved X-ray photoelectron spectroscopy (XPS). For 1nm-thick tantalum capping layer of the as-deposited stack sample, both the tantalum layer and the surface of the CoFeB layer underneath the tantalum layer were naturally oxidized. By comparison of the Co 2p and Fe 2p spectra with respect to as-deposited and annealed samples, reduction of the naturally oxidized cobalt and iron atoms was found to occur at the surface of the CoFeB layer. This reduction reaction was more significant at higher annealing temperature. Oxidized cobalt and iron were found to be reduced by boron atoms that diffused toward the surface of the top CoFeB layer. By means of naturally oxidized surface of the CoFeB layer formed on SiO2, the redox reaction behavior after annealing was confirmed by angle-resolved XPS analysis[2]. This redox reaction is thermodynamically reasonable according to the Ellingham diagram. (This work is supported by Industrial Affiliation on the STT-MRAM program in CIES Consortium and ACCEL project under JST.) [1] M. Niwa (2013), Chapter 4, High Permittivity Gate Dielectric Materials, Edited by S. Karr, Springer Series in Advanced Microelectronics 43 , pp. 191-234. [2] S. Sato et al., Appl. Phys. Lett. 196, 142407 (2015).

Enhanced annealing stability and perpendicular magnetic anisotropy in perpendicular magnetic tunnel junctions using W layer

The magnetic properties of the perpendicular storage electrode (buffer/MgO/FeCoB/Cap) were studied as a function of annealing temperature by replacing Ta with W and W/Ta cap layers with variable thicknesses. W in the cap boosts up the annealing stability and increases the effective perpendicular anisotropy by 30% compared to the Ta cap. Correspondingly, an increase in the FeCoB critical thickness characterizing the transition from perpendicular to in-plane anisotropy was observed. Thicker W layer in the W(t)/Ta 1 nm cap layer makes the storage electrode highly robust against annealing up to 570 °C. The stiffening of the overall stack resulting from the W insertion due to its very high melting temperature seems to be the key mechanism behind the extremely high thermal robustness. The Gilbert damping constant of FeCoB with the W/Ta cap was found to be lower when compared with the Ta cap and stable with annealing. The evolution of the magnetic properties of bottom pinned perpendicular magnetic tunnel junctions (p-MTJ) stack with the W2/Ta1 nm cap layer shows back-end-of-line compatibility with increasing tunnel magnetoresistance up to the annealing temperature of 425 °C. The pMTJ thermal budget is limited by the synthetic antiferromagnetic hard layer which is stable up to 425 °C annealing temperature while the storage layer is stable up to 455 °C.

Nanoscale Spin-Transfer Torque MRAM Etching Using Various Gases

As the volatile dynamic random access memory (DRAM) device faces a physical limitation, new memory devices are actively investigated to solve this problem. Among them, spin transfer torque magnetic random access memory (STT-MRAM) has received a lot of attention because of its very high speed of device, endurance, low power consumption of device, and high density memory cell in addition to the non-volatility of information. Especially, the MTJ memory elements which consist of CoxFeyBz/MgO/CoxFeyBz multilayer structure are the critical building blocks for high density magnetic random access memory with high tunneling magnetoresistance(TMR). However, the dry etching of MTJ stack with sub 70 nm is a challenging step in the fabrication of STT-MRAM. The etching of nanoscale MTJ induces oxidation in magnetic layer, pattern degradation and damage or sidewall redeposition at patterned MTJ sidewall especially. In this study, for successful etching of MTJ and MTJ related materials, RIB (reactive ion beam) etching using NF3, CH3OH, and CO/NH3 gas mixture and ICP etching using Ar, He, CO/NH3 and so on were conducted. Etch characteristics of magnetic tunnel junction (MTJ) stacks masked with W were investigated precisely using field emission scanning electron microscopy (FE-SEM) and TEM (Transmission electron microscopy). Simulation of MTJ material and hardmask material was conducted with SRIM simulation to observe tendency with etch process of blank unit thin film samples. In addition, the magnetic hysteresis curves were analyzed by a vibrating sample magnetometer (VSM). The results showed that, among the various gas combinations, the use of He gas improved etch selectivity of MTJ materials over W and also improved the etch profile of MTJ features. Also the He etch gas could be beneficial in reducing damage. The detailed etch mechanism and results on the etching of MTJ materials will be presented.