Ru Capping Layer Research Articles

Improved oxidation resistance of Ru/Si capping layer for extreme ultraviolet lithography reflector

The authors report on the chemical durability and oxidation resistance of Ru/Si, Ru/B, Ru/C, and Ru capping layers on the extreme ultraviolet (EUV) reflector surface. Surface etching and changes in the oxidation state were probed with x-ray photoelectron spectroscopy. The changes in surface morphology and roughness are characterized using scanning electron microscopy and atomic force microscopy. Out of four different capping layers, Ru/Si layers exhibited the least surface oxidation after oxygen plasma and UV/ozone treatment, indicating a superior oxidation resistance. The authors found that the reflectivity of the Ru/Si capped reflector is similar to that of a bare Ru capped reflector. This study suggests that a Ru/Si layer can be an excellent capping layer for the EUV reflector.

Perpendicular magnetization of CoFeB on single-crystal MgO

CoFeB films deposited on single-crystal MgO(100) exhibit significantly reduced out-of-plane demagnetization fields after magnetic annealing in the film plane, resulting in perpendicular magnetization for a 15 Å CoFeB film. The perpendicular magnetic anisotropy can be enhanced further by inserting a thin Ru capping layer on top of CoFeB, resulting in perpendicular magnetization in even thicker CoFeB films.

Tunnel magnetoresistance properties and annealing stability in perpendicular anisotropy MgO-based magnetic tunnel junctions with different stack structures

We have investigated the effect of stack structures on tunnel magnetoresistance (TMR) properties in perpendicular anisotropy MgO-based magnetic tunnel junctions (p-MTJs) with CoFe/Pd multilayer and CoFeB insertion. By adopting Ta and Ru cap-layers, the TMR ratios of 113 and 106% are obtained at annealing temperature (Ta) of 325 °C, respectively. Particularly, the Ru cap-layer is effective in realizing a TMR ratio of 100% at Ta = 350 °C. By replacing (Co25Fe75)80B20 with (Co25Fe75)85B15, the TMR ratio increased quickly at low Ta, reaching a maximum of 120% at Ta = 300 °C.

Magnetic tunnel junctions with large tunneling magnetoresistance and small saturation fields

There is a continuing need for greater sensitivity in magnetic tunnel junction (MTJ) sensors. We have found a new approach to achieving large tunneling magnetoresistance (TMR) with a very soft free layer. The high TMR is achieved by conventional means of annealing a bottom pinned MTJ that has Ta and Ru capping layers. The soft free layer is achieved by etching almost to the MgO tunnel barrier and depositing a thick soft magnetic film. The results are far superior to annealing the MTJ with the thick soft layer already deposited.

Inter-diffusion study in MgO tunneling magneto-resistive (TMR) system by XPS

In this paper, we investigated the elemental inter-diffusion in MgO TMR system, namely, between MgO barrier and free layer (CoFeB, NiFe or their combination) interface and the oxygen diffusion into the capping layers (Ta, Ru, TaN) at elevated temperatures using simple sheet film stack to simplify the results interpretation. Boron, cobalt, iron, and nickel show various diffusion tendencies into the MgO barrier after annealing the sheet film stack. Oxygen has different penetration depth into single CoFeB free layer upon annealing under N 2 + Ar protective atmosphere for different capping layers. Ru and TaN capping layer provide much better O 2 diffusion barrier, compared with Ta capping layer. This could potentially change the boron segregation tendency at free layer and capping layer interface and thus affect the interface crystallization process and lattice matching between the crystallized CoFeB free layer and the MgO(0 0 1) barrier layer. All these effects will impact the overall TMR performance.

Current-perpendicular-to-the-plane giant magnetoresistance of an all-metal spin valve structure with Co40Fe40B20 magnetic layer

A current-perpendicular-to-the-plane giant magnetoresistance is demonstrated at room temperature by using Co40Fe40B20 alloy with an exchange biased stack of IrMn/CoFeB/Cu/CoFeB. The GMR effect is significantly enhanced by inserting a Co75Fe25 layer between CoFeB and Cu spacer to adjust the interfacial resistance and improve the interfacial asymmetric spin scattering at the interfaces with spacer. Microstructure observation shows that boron of the top CoFeB layer prefers to be enriched at the interface with Ru capping layer after annealing.

Influence of MEEF change on the mask shadowing effect in extreme ultraviolet lithography

The influence of mask error enhancement factor (MEEF) on the mask shadowing effect was investigated for extreme ultraviolet lithography. Imaging properties including horizontal–vertical (H–V) CD (critical dimension) bias and MEEF change through the pitch according to absorber thickness and process condition were studied using aerial image simulation. The mask structure used in this study consisted of tantalum nitride (TaN) absorber and 2 nm ruthenium (Ru) capping layer on the 40 pair of Mo–Si multilayer. As the absorber thickness increased and the pattern pitch decreased, both H–V CD bias and MEEF increased. At the illumination condition of 0.32 numerical aperture (NA), the H–V CD bias variation through the pitch was negligible and slightly increased at 1:1 pitch, while it steeply increased at 1:1.2 and 1:1 pitch for NA of 0.25. The MEEF value was below 1.5 for all calculated absorber thicknesses when the pitch was from 1:1.2 to 1:5, whereas it was 3 with 64 nm thick TaN for 1:1 pitch at vertical pattern. With the increment of absorber thickness, the MEEF difference between the horizontal and vertical pattern increased. We also calculated the H–V overlapping process window (PW) according to TaN thickness using 22 nm 1:1 line and space (L/S) pattern. As absorber thickness decreased, the overlapping zone in the EL of the focus–exposure plots between the horizontal and vertical features increased. Enough image contrast and H–V overlapping PW could be achieved by applying 38 nm thick TaN.

Chemical effect of dry and wet cleaning of the Ru protective layer of the extreme ultraviolet lithography reflector

The authors report the chemical influence of cleaning of the Ru capping layer on the extreme ultraviolet (EUV) reflector surface. The cleaning of EUV reflector to remove the contamination particles has two requirements: to prevent corrosion and etching of the reflector surface and to maintain the reflectivity functionality of the reflector after the corrosive cleaning processes. Two main approaches for EUV reflector cleaning, wet chemical treatments [sulfuric acid and hydrogen peroxide mixture (SPM), ozonated water, and ozonated hydrogen peroxide] and dry cleaning (oxygen plasma and UV/ozone treatment), were tested. The changes in surface morphology and roughness were characterized using scanning electron microscopy and atomic force microscopy, while the surface etching and change of oxidation states were probed with x-ray photoelectron spectroscopy. Significant surface oxidation of the Ru capping layer was observed after oxygen plasma and UV/ozone treatment, while the oxidation is unnoticeable after SPM treatment. Based on these surface studies, the authors found that SPM treatment exhibits the minimal corrosive interactions with Ru capping layer. They address the molecular mechanism of corrosive gas and liquid-phase chemical interaction with the surface of Ru capping layer on the EUV reflector.

Evaluation of distribution of exchange coupling in CoFe/Ru/CoFe synthetic antiferromagnetic structure after annealing

We investigated the exchange-coupling fields of CoFe/Ru (tRu)/CoFe synthetic antiferromagnetic (SyAF) structures with a Ru underlayer and capping layer before and after annealing at 300 and 350 °C. Exchange-coupling field Hex decreased after annealing and distribution increases in Hex were observed in the structure of smaller tRu, especially at tRu≤0.7 nm. Remanence magnetizations increased at smaller tRu. These results indicate that distribution is caused by the existence of the antiferromagnetic coupling of locally various coupling fields due to annealing. A larger exchange-coupling field after high temperature annealing can be obtained by suppressing the distribution of the exchange coupling in the SyAF structure.

Lower-temperature crystallization of CoFeB in MgO magnetic tunnel junctions by using Ti capping layer

Effects of capping materials on magnetoresistance (MR) properties of MgO magnetic tunnel junctions (MTJs) with a CoFeB free layer were investigated. MR ratios of samples with various capping materials showed a difference in annealing temperature dependence. MTJ with a Ti capping layer annealed at 270°C showed a MR ratio 1.4 times greater than that with a conventional Ta or Ru capping layer. Secondary ion mass spectroscopy and high-resolution transmission electron microscopy images revealed that crystallization of CoFeB was remarkably affected by adjacent materials and the Ti capping layer adjoining CoFeB acted as a boron-absorption layer. These results suggest that the crystallization process can be controlled by choosing proper capping materials. Ti is one of the effective materials that accelerate the crystallization of CoFeB layers at lower annealing temperature.

Hydrogen-induced mitigation of O on Ru(101̄0): a density-functional study

The reaction of hydrogen with oxygen adsorbed on an Ru(1010) surface has been studied by density-functional calculations and kinetic Monte Carlo simulations. In agreement with experiment, it has been found that molecular hydrogen does not react with adsorbed O. In contrast, the hydrogenation of oxygen by an atomic H beam occurs spontaneously and results in the formation of adsorbed OH molecules. Subsequent impinging H-atoms can either initiate the formation of water, which readily desorbs at room temperature thus removing the O from the surface, or lead to formation and desorption of H2. It is the latter channel that hinders, at 300 K, a complete removal of O from Ru capping layers on Si/Mo mirrors for extreme ultraviolet radiation. The estimated height of the barrier for the Langmuir-Hinshelwood reaction between adsorbed H and OH, 0.92 eV, and related position of the H2O peak in model desorption spectra (approximately 320 K) are consistent with recent experiments. The H2 desorption peak appears at higher temperature, approximately 350 K, so that in the range from 320 to 330 K adsorbed hydrogen atoms will react predominantly with OH. Hence, the present simulations predict that an efficient removal of the chemisorbed O from Ru capping layers can be achieved by heating the surface to 320-330 K in a molecular hydrogen atmosphere.

The effects of oxygen plasma on the chemical composition and morphology of the Ru capping layer of the extreme ultraviolet mask blanks

Contamination removal from extreme ultraviolet (EUV) mask surfaces is one of the most important aspects to improve reliability for the next generation of EUV lithography. The authors report chemical and morphological changes of the ruthenium (Ru) mask surface after oxygen plasma treatment using surface sensitive analytical methods: x-ray photoelectron spectroscopy (XPS), atomic force microscopy, and transmission electron microscopy (TEM). Chemical analysis of the EUV masks shows an increase in the subsurface oxygen concentration, Ru oxidation, and surface roughness. XPS spectra at various photoelectron takeoff angles suggest that the EUV mask surface was covered with chemisorbed oxygen after oxygen plasma treatment. It is proposed that the Kirkendall effect is the most plausible mechanism that explains the Ru surface oxidation. The etching rate of the Ru capping layer by oxygen plasma was estimated to be 1.5±0.2Å∕min, based on TEM cross sectional analysis.

DIET processes on ruthenium surfaces related to extreme ultraviolet lithography (EUVL)

The aim of this work is to provide insights into desorption induced by electronic transitions (DIET) processes that affect the reflectivity of ruthenium-capped Mo/Si multilayer mirrors working under EUVL (extreme ultraviolet lithography) operating conditions [high vacuum, and 13.5 nm (92 eV) photons]. Critical issues are associated with possible oxidation of the 2 nm thick Ru capping layer due to the inevitable background pressure of H 2O, and carbon build up due to background hydrocarbons. In the present work, we discuss aspects of the radiation-induced surface chemistry of Ru irradiated by 100 eV electrons and 92 eV photons. The cross section for electron-stimulated desorption of oxygen from O-covered Ru is ∼6 × 10 −19 cm 2. Carbon accumulation several nm thick occurs on the Ru surface during electron irradiation in methyl methacrylate (MMA) vapor, a model background impurity hydrocarbon. Radiation damage by low-energy secondary electrons is believed to dominate over direct photoexcitation of adsorbates under EUVL conditions. The secondary electron yield from Ru varies strongly with photon energy, and is ∼0.02 electrons/photon at 92 eV.

Texture development and magnetoresistance properties of CoFeB∕MgO∕CoFeB-based magnetic tunnel junction depending on capping layer crystallinity

We investigated the effects of the crystallinity of the capping layer materials on the crystallization of amorphous top CoFeB (t-CoFeB) and the magnetoresistance properties and temperature sensitivities of the magnetic tunnel junctions (MTJs). When a (002)-textured hcp Ru capping layer was used, the amorphous t-CoFeB was crystallized to bcc-CoFe (110) during annealing. Using the CoFe (110)/Ru (002) texture relation is the best way to reduce the lattice mismatch down to 5.6%. However, when a polycrystalline TiAl (two phase: amorphous and fine polycrystalline) or amorphous ZrAl capping layer was used, the amorphous t-CoFeB was crystallized to bcc-CoFe (002) during annealing due to the MgO (001) template effect. The tunneling magneto resistance ratios of the annealed MTJs capped with Ru, TiAl, and ZrAl were 46.7%, 71.8%, and 72.7%, respectively, which depended on the texture and the epitaxiality of t-CoFeB∕MgO (001). Consequently, the texture evolution of the amorphous t-CoFeB during annealing can be controlled by adjusting the crystallinity of the adjacent capping layer which, in turn, affects the magnetoresistance properties and temperature sensitivities of the MTJs.

保護層材料による層間交換結合型スピンバルブＧＭＲ膜の耐熱性向上

Interlayer-exchange-coupled (IEC) type spin-valve giant magnetoresistive (SVGMR) films in which the pinning layer does not contain an anti-ferromagnetic layer have been investigated. The IEC-type SVGMR films had good SV properties with a 10% MR ratio as deposited. However, the MR ratio decreased with increasing annealing temperatures over 310°C. From cross sectional TEM observation, interfacial roughness between the free and pinned layers was increased by annealing, and such roughness causes the degradation of SVGMR properties. Thermal stability of SVGMR films was improved when the thickness of Ta capping layer was 5 nm. In order to reduce the thickness of capping layer, various materials of Ru, Cu, and NiFeCr were investigated instead of Ta. No degradation in SV properties after 320°C annealing was achieved with the 2 nm of Ru capping layer.

Effect of capping layer on the crystallization of amorphous CoFeB

AbstractWe investigated the effects of capping layer on the crystallization of amorphous top‐CoFeB electrode in the glass/CoFeB/MgO(001)/CoFeB/cap stack. Evolution of texture and crystallization of the top‐CoFeB electrode can be affected by the texture and the crystallinity of the capping layer. When the (002)‐textured Ru capping layer was used, a (110) texture of CoFeB was developed during annealing at 350 °C for 90 min. On the other hand, when the amorphous TiAl capping layer was used, a (002) texture of the CoFeB was developed during crystallization due to MgO (001) texture seeding effect. Consequently, the texture evolution of the amorphous top‐CoFeB electrode during annealing could be controlled by the texture and crystallinity of the adjacent capping layer. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of ruthenium thin films as capping layer for extreme ultraviolet lithography mask blanks

In extreme ultraviolet lithography (EUVL), the multilayer (ML) damage-free mask patterning processes and damage-free usage cycle are the keys in obtaining a successful, functional EUVL mask. A robust ML capping layer design will enable a long mask lifetime. In this article detailed investigation on the viability of ruthenium (Ru) thin films as capping layer for EUVL ML mask blanks is presented. The study is focused on Ru capping layer design for high reflectivity and its properties relevant to EUVL mask applications, such as microstructure, stress, optical properties at EUV wavelength, and chemical durability. The authors found that Ru thin films with a crystalline structure present a very high compressive stress which is insensitive to the primary ion deposition source energy. The Ru∕Si interdiffusion layer, however, presents a much lower stress than the of Ru-only film. Amorphization of the Ru film is via atomic composition modification, which the authors believe could be one of the keys in reducing Ru film stress. The ruthenium cap, under a piranha chemical clean, was found to be more durable than Si capped ML blanks, indicating the advantages of using Ru as the EUVL ML mask blank capping layer.

Capping층 재료에 따른 CoFeB/MgO/CoFeB 자기터널접합의 미세구조와 자기저항 특성

본 연구에서는 CoFeB/MgO/CoFeB 구조를 가지는 자기터널접합에서 capping층 재료의 종류와 열처리 시간에 따른 비정질 top CoFeB 자성층의 결정화 상태 및 자기터널접합의 자기적 특성 변화에 대한 연구결과를 비교 분석 하였다. Hcp(Hexagonal close-packed)의 결정구조를 가지는 Ru(002)를 capping층 재료로 사용한 자기터널접합 박막의 경우에는 열처리 이후 Ru과 인접한 부분의 top CoFeB이 bcc-CoFe(110)로 성장하는 반면, TiAl과 ZrAl을 capping층 재료로 사용한 자기터널접합의 경우는 열처리 이후 top CoFeB이 MgO와 epitaxial하게 bcc-CoFe(002)로 결정성장 하였다. 이로 인해 Ru을 사용한 자기터널접합의 터널자기 저항비(46.7%)보다 약 1.5배 높은 터널자기저항비(TiAl: 71.8%, ZrAl: 72.7%)를 나타내었다. We investigated the effects of the capping layer materials on the crystallization of the amorphous top-CoFeB (t-CoFeB) electrode and the magnetoresistance properties of the magnetic tunnel junctions (MTJs). When the hcp(002)-textured Ru capping layer was used, the amorphous t-CoFeB was crystallized to bcc-CoFe(110). The CoFe(110)/Ru(002) texture relation can be minimized the lattice mismatch down to 5.6%. However, when the fine polycrystalline but almost amorphous TiAl or amorphous ZrAl were used, the amorphous t-CoFeB was crystallized to bcc-CoFe(002). When the amorphous capping materials were used, the evolution of the t-CoFeB texture was affected mainly by the MgO(001) texture. Consequently, the M ratios of the annealed MTJ capped with the ZrAl and TiAl (72.7 and 71.8%) are relatively higher than that of the MTJ with Ru capping layer (46.7%). In conclusions, the texture evolution of the amorphous t-CoFeB during the post deposition annealing could be controlled by the crystallinity of the adjacent capping layer and in turn, it affects the TMR ratio of MTJs.

Oxidation and Reduction of Ultrathin Nanocrystalline Ru Films on Silicon: Model System for Ru-Capped Extreme Ultraviolet Lithography Optics

Ultrathin ruthenium films are promising capping layers protecting extreme ultraviolet lithography optics against carbon growth and oxidation. The structure and reactivity of 2, 5, and 7 nm thick nanocrystalline Ru films on Si (serving as a model system for Ru capping layers) were studied by multiple techniques including scanning electron microscopy, X-ray diffraction, X-ray reflectivity, and X-ray photoelectron spectroscopy. The structural analysis indicated dense and flat Ru films, consisting of preferentially (0001)-oriented grains. High resolution core-level Ru 3d5/2 and O 1s spectroscopy studies have revealed that these Ru films exposed to O2 ambient are resistant to oxidation up to ∼470 K similar to the oxidation of single-crystalline Ru(0001) surfaces. The reduction of the oxide in the H2 ambient is highly effective and proceeds already below 370 K.

Improvement of Transport Properties in Magnetic Tunneling Junctions by Capping Materials

The capping effects on the properties of magnetic tunneling junctions (MTJs) have been investigated to improve the magnetoresistive (MR) ratio [Nagamine , Applied Physics, vol. 99, p. 08K703, 2006] and thermal stability [Fukumoto , Japanese Journal of Applied Physics, vol. 45, p. 3829, 2006. In this paper, MgO/Mg and MgO capping layers for MTJs were used to replace our previously used Ta and Ru capping layer [Yen , IEEE Transactions on Magnetics, vol. 42, no. 10, p. 862, 2004]. The MTJs with NiFe and CoFeB free layers were patterned to ellipses of 0.36 mum times 0.72 mum. The MR ratios of the MTJs with an NiFe free layer were enhanced from 26% to about 33% by capping MgO/Mg bilayers. A great improvement was also found on the MTJs with the CoFeB free layer, where the MR ratio was 50.4% with the MgO cap layer enhanced from 44.7% of Ru-capped MTJs. In addition, the influences of those capping layers included Ta, Ru, MgO, and MgO/Mg on thermal stability were also discussed