Magnetic Tunnel Junction Materials Research Articles

Study on etch characteristics of magnetic tunnel junction materials using rf-biased H2/NH3 reactive ion beam

In order to etch CoFeB and MgO constituting the magnetic tunnel junction (MTJ) layer of magnetic random access memory, rf-biased reactive ion beam etching (RIBE) with a H2:NH3 gas mixture was introduced and the etching characteristics were investigated. The H2:NH3 gas mixture of 8:1 was used for rf-biased RIBE because the etch selectivity is increased with H2 percentage in the H2:NH3 gas mixture while the etch rates of MTJ materials are the highest between 2:1 and 1:1 ratio of H2:NH3 due to the synergy effect of H2 and NH3 in the etching of MTJ materials. When a high rf power was applied to the substrate (high rf-biasing) during RIBE, even though etch rates of MTJ materials were increased, the etch selectivities of MTJ materials over hard mask materials were decreased possibly due to the high physical sputtering effect. However, when small rf-biasing was added to the substrate during RIBE, improved etch characteristics such as higher etch selectivity of MTJ over the top electrode (TE) material, improved etch anisotropy of the MTJ pattern masked with TE, and a thinner sidewall residue on the MTJ pattern could be observed. The improvement in etch characteristics by the addition of small rf-biasing during RIBE is believed to be related to the increased chemical etching effect through H2/NH3 gas dissociation and the increased H/NH ratio in the plasma generated just above the substrate without increasing the physical sputtering effect. Specifically, for the substrate located on the insulating substrate holder, the charging of the substrate could be removed without using an electron emitting neutralizer by supplying electrons from the plasma generated by rf-biasing during RIBE.

Etch characteristics of magnetic tunnel junction materials using H2/NH3 reactive ion beam

Magnetic tunneling junction (MTJ) materials such as CoFeB, Co, Pt, MgO, and the hard mask material such as W and TiN were etched with a reactive ion beam etching (RIBE) system using H2/NH3. By using gas mixtures of H2 and NH3, especially with the H2/NH3( 2:1) ratio, higher etch rates of MTJ related materials and higher etch selectivities over mask materials (>30) could be observed compared to those etching using pure H2( no etching) and NH3. In addition, no significant chemical and physical damages were observed on etched magnetic materials surfaces and, for CoPt and MTJ nanoscale patterns etched by the H2/NH3( 2:1) ion beam, highly anisotropic etch profiles >83° with no sidewall redeposition could be observed. The higher etch rates of magnetic materials such as CoFeB by the H2/NH3( 2:1) ion beam compared to those by H2 ion beam or NH3 ion beam are believed to be related to the formation of volatile metal hydrides (MH, M = Co, Fe, etc) through the reduction of M-NHx( x = 1 ∼ 3) formed in the CoFeB surface by the exposure to NH3 ion beam. It is believed that the H2/NH3 RIBE is a suitable technique in the etching of MTJ materials for the next generation nanoscale spin transfer torque magnetic random access memory (STT-MRAM) devices.

Impact of Process Variability on Write Error Rate and Read Disturbance in STT-MRAM Devices

Spin-transfer torque magnetic random-access memory (STT-MRAM) is the most promising next-generation memory technology that combines the advantages of mainstream memory technologies, such as SRAM, DRAM, and flash. In the STT-MRAM, a magnetic tunnel junction (MTJ) is used as a bit-cell to store the data, and its magnetic properties have a critical role in thermal noise-aware STT-switching operations. This work analyzes the impact of MTJ material and geometric parameter variations, such as saturation magnetization (MS), magnetic anisotropy (HK), damping factor (α), spin polarization efficiency factor (η), oxide thickness (tOX), free layer thickness (tF), tunnel magnetoresistance (TMR), and cross-sectional area of free layer (AF) variations on write error rate (WER) and read disturbance rate (RDR) for reliable write and read operations, respectively. To evaluate the scalability of MRAM devices, we investigate both WER and RDR with a wide range of MTJ diameters between 90 and 30 nm, which corresponds to mainstream technology nodes from 40 up to 14 nm advance node. In our work, the Fokker-Planck (FP) numerical approach is mainly utilized for an efficient analysis, which allows for parametric variation and evaluates its impact on switching. Although the impact of material and geometric parameter variations on WER is decreased as MTJ scales down from 90 to 30 nm, the variation effect can be still critical with small MTJ diameter, and the most significant influential variation is η, MS, HK, and α in that order. By contrast, the impact of material and geometric parameter variation on RDR increases in MTJ scaling, and we show that negative variations of HK and MS could be a critical bottleneck in 30 and 40 nm MTJ diameters. Our work finally emphasizes the necessity of the WER and RDR analysis by considering the parameter variation in MTJ scaling for practical STT-MRAM development.

Nanoscale Spin-Transfer Torque MRAM Etching Using Various Gases

The magnetic tunnel junction (MTJ)-related materials were etched using He, Ar, and CO/NH3 in an inductively coupled plasma etching system. When the MTJ materials were etched with He, the etch selectivities and the etch profiles of the etched MTJ patterns were significantly improved. The sputter etch characteristics were also investigated using the Stopping and Range of Ions in Matter (SRIM) simulation program. The results showed higher sputter etch selectivity of CoFeB over W using He than those obtained using Ar. Also He exhibited higher energy and narrower angular probability density function of the sputtered atoms. Also, by using He, similar to Ar, magnetic properties of the MTJ were preserved compare to CO/NH3 due to no oxidation of magnetic materials.

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.

Improved Etch Characteristics of Magnetic Tunneling Junction Materials by Using Helium

The etch characteristics of magnetic tunneling junction (MTJ) materials for spin transfer torque magnetic random access memory (STT-MRAM) using He inductively coupled plasma (ICP) were compared with the etch characteristics of ICP using Ar or CO/NH3 (1:3). The ICP etching using He showed a higher etch selectivity of CoFeB over W and a more anisotropic etch profile compared to the etching using Ar at similar etch rates. The sputter etch characteristics of CoFeB and W were also investigated using the Stopping and Range of Ions in Matter (SRIM) simulation program. The results showed higher sputter etch selectivity of CoFeB over W in addition to narrower angular distribution of the sputtered atoms than those obtained using Ar. The etching of MTJ materials using CO/NH3 (1:3) ICP exhibited good MTJ etch characteristics comparable to He ICP, however, the MTJ etched using CO/NH3 (1:3) showed the lowest saturation magnetization (MS) measured by a vibrating-sample magnetometer (VSM) possibly due to the sidewall oxidation of MTJ while the MS values of the MTJ etched using He and Ar are similar. It is found that, by etching the MTJ materials using He ICP with the ion energy near the hardmask sputter threshold energy, highly anisotropic etch profiles could be obtained without forming the sidewall redeposited materials and without significant degradation of magnetic properties.

Etching of Magnetic Tunnel Junction Materials Using Reactive Ion Beam

Etching of Magnetic Tunnel Junction Materials Using Reactive Ion Beam

Perpendicular magnetic anisotropy of Co\Pt bilayers on ALD HfO2

Perpendicular Magnetic Anisotropy (PMA) is a key requirement for state of the art Magnetic Random Access Memories (MRAM). Currently, PMA has been widely reported in standard Magnetic Tunnel Junction material stacks using MgO as a dielectric. In this contribution, we present the first report of PMA at the interface with a high-κ dielectric grown by Atomic Layer Deposition, HfO2. The PMA appears after annealing a HfO2\Co\Pt\Ru stack in N2 with the Keff of 0.25 mJ/m2 as determined by Vibrating Sample Magnetometry. X-Ray Diffraction and Transmission Electron Microscopy show that the appearance of PMA coincides with interdiffusion and the epitaxial ordering of the Co\Pt bilayer. High-κ dielectrics are especially interesting for Voltage Control of Magnetic Anisotropy applications and are of potential interest for low-power MRAM and spintronics technologies.

Influence of pulsed bias frequency on the etching of magnetic tunneling junction materials

The effect of different bias frequencies during the etching of magnetic tunneling junction materials in an inductively coupled plasma with rf pulse biasing (50% duty percentage) on etch characteristics was investigated. The decrease of rf bias frequency from 13.56 MHz to 400 kHz while keeping the same average dc bias voltage at −300 V increased not only the etch rates of MTJ related materials but also the etch selectivities. We observe improved etch characteristics of CoFeB and CoPt, and also improved etch profiles of CoPt, CoFeB and MTJ (CoPt/MgO/CoFeB) masked with nanoscale W(100 nm)/Ti(20 nm)/Ru(5 nm) at lower bias frequencies. The observed improvements are attributed to the higher sputter yields due to the wider ion energy distribution (with increase high energy component) for the metal carbonyl related compounds formed during the etching of MTJ materials with CO/NH3 etchant gas.

Etch characteristics of magnetic tunnel junction materials using substrate heating in the pulse-biased inductively coupled plasma

Magnetic tunnel junction (MTJ)-related materials such as CoFeB, MgO, and W were etched in a pulse-biased inductively coupled plasma etch system using a CO/NH3 gas combination, and the effects of substrate temperature (room temperature ∼200 °C) in the pulse-biased condition on the etch characteristics of the MTJ-related material were investigated. The etch selectivity of MTJ materials over W was improved by substrate heating possibly due to the easy removal of the compounds from the etched CoFeB surface during the pulse-on time at the elevated substrate temperature. At high substrate temperature, decreased thickness of etch residue was observed not only on the bottom surface but also on the sidewall surface during the etching, which indirectly indicated the increased volatility of the etch compounds at higher substrate temperature. The etching of CoFeB features masked with W also showed a more anisotropic etch profile by heating the substrate up to 200 °C possibly due to the increased the etch selectivity of CoFeB over W and the decreased redeposition of etch products on the sidewall of the CoFeB features.

Effect of RF pulsing biasing on the etching of magnetic tunnel junction materials using CH3OH.

The magnetic tunnel junction (MTJ)-related materials such as CoFeB, CoPt, MgO, and Ru, and W were etched using CH3OH in a pulse-biased inductively coupled plasma system and the effect of bias pulsing (100% 30% duty percentage) on the etch characteristics of the MTJ-related materials was investigated at the substrate temperature of 200 degrees C. The etch selectivity of MTJ-related materials over W was improved by using pulse-biasing possibly due to the formation of more stable and volatile etch products during the pulse-off time and the removal of the compounds more easily on the etched CoFeB surface during the pulse-on time. X-ray photoelectron spectroscopy also showed that the use of lower duty percentage decreases the residue thickness remaining on the etched MTJ materials indirectly indicated the higher volatility of the etch products by the bias pulsing. The etching of nano-patterned CoFeB masked with W also showed more anisotropic etch profile by pulse-biasing probably due to the increased the etch selectivity of CoFeB over W and the decreased redeposition of etch products on the sidewall of the CoFeB features. The most anisotropic CoFeB etch profiles could be observed by using CH3OH gas in the pulse biasing of 30% duty ratio.

Etch characteristics of magnetic tunnel junction materials using bias pulsing in the CH4/N2O inductively coupled plasma.

The etch characteristics of magnetic tunneling junction (MTJ) related materials such as CoFeB, MgO, FePt, Ru, and W as hard mask have been investigated as functions of rf pulse biasing, substrate heating, and CH4/N2O gas combination in an inductively coupled plasma system. When CH4/N2O gas ratio was varied, at CH4/N2O gas ratio of 2:1, not only the highest etch rates but also the highest etch selectivity over W could be obtained. By increasing the substrate temperature, the linear increase of both the etch rates of MTJ materials and the etch selectivity over W could be obtained. The use of the rf pulse biasing improved the etch selectivity of the MTJ materials over hard mask such as W further. The surface roughness and residual thickness remaining on the etched surface of the CoFeB were also decreased by using rf pulse biasing and with the decrease of rf duty percentage. The improvement of etch characteristics by substrate heating and rf pulse biasing was possibly related to the formation of more stable and volatile etch compounds and the removal of chemically reacted compounds more easily on the etched CoFeB surface. Highly selective etching of MTJ materials over the hard mask could be obtained by using the rf pulse biasing of 30% of duty ratio and by increasing the substrate temperature to 200 degrees C in the CH4/N2O (2:1) plasmas.

A study on the etching characteristics of magnetic tunneling junction materials using DC pulse-biased inductively coupled plasmas

The etching properties of magnetic materials composing the magnetic tunnel junction (MTJ) such as CoPt, MgO, CoFeB, and CoPt/MgO/CoFeB were investigated in DC pulse biased CO/NH3 inductively coupled plasmas (ICPs) and their etch characteristics were compared with those etched by RF CW biased ICPs. The use of DC pulse biased ICPs instead of RF CW biased ICPs improved the etch selectivity of the MTJ materials over W and also decreased the residue on the surface of the etched materials possibly due to the more stable and volatile etch product formation during the DC pulse off time and the enhanced removal of the etch products by mono-energetic ions during the DC pulse on time. When MTJ materials masked with W were etched, more anisotropic etch profile could be also observed when the MTJ materials were etched with the DC pulse biasing of 60% duty percentage compared with those etched with RF CW biasing due to the decreased redeposition of etch products on the sidewall of the etched feature in addition to the enhanced etch selectivity over W.

Selective Etching of Magnetic Tunnel Junction Materials Using CO/NH3 Gas Mixture in Radio Frequency Pulse-Biased Inductively Coupled Plasmas

The etch characteristics of magnetic tunneling junction (MTJ) materials and the etch selectivity over W have been investigated using RF pulse-biased conditions in addition to the continuous wave (CW) bias condition with a CO/NH3 gas combination in an inductively coupled plasma system. By using a time-averaged substrate DC bias voltage condition for the RF pulse biasing, the etch rates of MTJ materials for the RF pulse-biased conditions were generally similar to those etched using the CW RF bias condition even though the etch rates were slightly decreased with decreasing the duty percentage of the RF pulse biasing. However, the use of the RF pulse biasing improved the etch selectivity of the MTJ materials over mask materials such as W. When the surface roughness and the residual thickness remaining on the etched surface of the MTJ material such as CoFeB were investigated by using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), respectively, it was clear that the use of the RF pulse biasing instead of CW RF biasing also decreased the residual thickness and the surface roughness. This is believed to be related to the formation of a more uniform chemically reacted layer on the etch CoFeB surface during the RF pulse-biased etching condition.

The effect of substrate temperature on the etching properties and the etched surfaces of magnetic tunnel junction materials in a CH3OH inductively coupled plasma system

The etching characteristics of the magnetic films (PtMn, CoFe) and hard mask materials (W, Ta) forming a magnetic tunnel junction (MTJ) stack in a CH3OH inductively coupled plasma (ICP) system were investigated. We examined the etch rates of the metal films as a function of substrate temperature, and assessed the microstructures of the etched surfaces using high resolution transmission electron microscopy (HR-TEM). We also analyzed the surface states using X-ray photoelectron spectroscopy (XPS) and TEM electron energy loss spectroscopy (TEM-EELS). The PtMn and CoFe etch rates increased as the temperature increased, whereas the etch rates of W and Ta decreased slightly. Therefore the etch selectivity increased linearly with increasing substrate temperature. The CH3OH plasma formed nonvolatile etching byproducts with the magnetic films and hard mask metals. In the case of PtMn and CoFe, the surface composition of the etching byproducts changed with increasing temperature; the relative concentration of pure metal compared with metal oxide or carbide increased as the substrate temperature rose. The etch rate was determined by the sputtering yield of the materials formed on the etched surface; accordingly the etch rates of those magnetic films would increase due to the higher sputtering yield of pure metal.

Effect of Substrate Temperature on the Magnetic Tunnel Junction Material Etching Using Inductively Coupled CH3OH Plasma

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Development of Hard Mask Process on Magnetic Tunnel Junction for a 4-Mbit Magnetic Random Access Memory

To enlarge the read margin of magnetic random access memory (MRAM), we developed a SiO2/Si3N4 hard mask process for magnetic tunnel junction (MTJ) stack patterning. This process can protect MTJ materials from oxidation during the resist removal process and reduces the distribution of MTJ resistance for 0.32-µm-wide bits on an 8-in.-diameter wafer more than the conventional process does. We also developed process integration for 4-Mbit toggle MRAMs: the read margin for 4-Mbit reached nearly 18σ.

MgO-based tunnel junction material for high-speed toggle magnetic random access memory

We report the first demonstration of a magnetoresistive random access memory (MRAM) circuit incorporating MgO-based magnetic tunnel junction (MTJ) material for higher performance. We compare our results to those of AlOx-based devices, and we discuss the MTJ process optimization and material changes that made the demonstration possible.We present data on key MTJ material attributes for different oxidation processes and free-layer alloys, including resistance distributions, bias dependence, free-layer magnetic properties, interlayer coupling, breakdown voltage, and thermal endurance. A tunneling magnetoresistance (TMR) greater than 230% was achieved with CoFeB free layers and greater than 85% with NiFe free layers. Although the TMR with NiFe is at the low end of our MgO comparison, even this MTJ material enables faster access times, since its TMR is almost double that of a similar structure with an AlOx barrier. Bit-to-bit resistance distributions are somewhat wider for MgO barriers, with sigma about 1.5% compared to about 0.9% for AlOx. The read access time of our 4 Mb toggle MRAM circuit was reduced from 21 ns with AlOx to a circuit-limited 17 ns with MgO.

Development of the magnetic tunnel junction MRAM at IBM: From first junctions to a 16-Mb MRAM demonstrator chip

This paper reviews the remarkable developments of the magnetic tunnel junction over the last decade and in particular, work aimed at demonstrating its potential for a dense, fast, and nonvolatile random access memory. The initial focus is on the technological roots of the magnetic tunnel junction, and then on the recent progress made with engineered materials for this device. Following that, we discuss the development of the magnetic random access memory (MRAM) technology, in which the magnetic tunnel junction serves as both the storage device and the storage sensing device. The emphasis is on work at IBM, including demonstrations of basic capabilities of the technology and work on a 16-Mb product demonstrator design in 180-nm node technology, which was targeted to be a realistic test bed for the MRAM technology. Performance and cost are compared with those of competing technologies. The paper also serves as an introduction to more specialized papers in this issue on MRAM device physics, magnetic tunnel junction materials and device characterization, MRAM processing, and MRAM design.

Comparison of oxidation methods for magnetic tunnel junction material

Advances in reducing the resistance and enhancing the magnetoresistance (MR%) of the magnetic tunnel junction (MTJ) material has made it useful for magnetoresistive random access memory as well as magnetic field sensing applications. One of the most important aspects for producing the MTJ material is the method used for forming the tunnel barrier, and its impact on the properties of MTJ such as resistance and area product (RA), MR%, and RA uniformity across a large area. We have explored forming the aluminum oxide tunnel barrier with air; reactive sputtering; plasma oxidation with plasma source; plasma oxidation with power introduced from the target side; and plasma oxidation with power introduced from the substrate side. Our results show that all techniques can be made to work. Plasma oxidation is favored due to its simplicity and manufacturing compatibility. It was also discovered that different oxidation methods used in this study caused little difference in MTJ resistance uniformity. The latter is mainly determined by the Al metal-thickness uniformity. Modeling based on Simmons’ theory supports our experimental finding. This illustrates the importance in improving Al metal-film uniformity for producing MTJ with ultra-uniform resistance.