Ru Capping Layer Research Articles

Surface-limited deuterium uptake of Ru films under plasma exposure

Blister formation has been an emerging research topic for extreme ultraviolet (EUV) mirrors exposed to hydrogen plasmas. Similar to plasma-facing materials in nuclear fusion reactors, it has been reported that blister formation in EUV mirrors is initiated by hydrogen uptake due to hydrogen ion or atom bombardment. However, the research so far has focused on Mo/Si multilayers exposed to only hydrogen ions or atoms, while the EUV mirror typically has a Ru capping layer facing hydrogen plasmas. We present experimental work to measure plasma-induced hydrogen uptake of Ru films. We bombarded our designed Ru-capped target with a low-temperature deuterium plasma and measured the deuterium retention using elastic recoil detection. Contrary to ion-driven deuterium uptake, the deuterium uptake rate of the Ru film had no dependence on the deuterium ion flux or energy after a period of plasma exposure. A reaction–diffusion model has been built to calculate the time evolution of deuterium retention, which well fits the experimental data. Based on this model, we conclude that the surface composition of the Ru film is the limiting factor for the deuterium uptake, which is seriously weakened when the surface is covered by Ru oxide. After the Ru oxide is reduced by the plasma, the uptake rate is predominantly driven by the deuterium surface coverage on metallic Ru. Our model also indicates that at the deuterium-populated Ru surface, deuterium has a low absorption barrier to penetrate the surface, which is supported by previously reported computational work.

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.

Local Magnetoresistance at Room Temperature in Si <inline-formula> <tex-math notation="LaTeX">$\langle100\rangle$ </tex-math> </inline-formula> Devices

A spin metal-oxide-semiconductor field-effect transistor (spin-MOSFET) is one of the emerging devices for the low power consumption in silicon-based electronics from the viewpoint of logic-in-memory architectures [1]. To realize these kinds of spintronic applications, one of the main issues for realizing the spin-MOSFETs is an observation of the high magnetoresistance (MR) ratio obtained by two-terminal local measurements at room temperature [2]. Up to now, although there are lots of studies of the local MR effect through the silicon (Si) channels, the values of the MR ratio are less than 0.8 % at 100 K [3] and 0.03 % at room temperature [4]. In this paper, we show relatively large MR ratios at room temperature in Si $ $ lateral spin valves (LSVs) with a small size $(0.305 \mu \mathrm {m}^{2})$ cross section in the spin-transport layer. For comparison of the crystal orientation of the Si spin-transport layers, we prepared two kinds of LSVs along Si $ $ and Si $ $ with CoFe/MgO electrodes on phosphorus-doped $(n \sim 1.3 \times 10 ^{19}$ cm $^{-3})(100)$ textured Si on insulator (SOI) ($\sim 61$ nm) layer, as shown in Fig. 1(a). An MgO (1.1 nm) tunnel barrier was deposited on the SOI spin-transport layer at 200 °C by electron beam evaporation. Then, a CoFe (10 nm) and a Ru capping layer were sputtered on top of it under a base pressure less than $5 \times 10 ^{-7}$ Pa. The MgO and CoFe layers were epitaxially grown on the (100) textured SOI, where the (100)-textured MgO layer was grown on Si(100). Device fabrication methods are described in detail elsewhere.[5] We have checked that these resistivity and Hall mobility of the Si spin-transport layer were almost the same by evaluating from longitudinal resistivity and Hall-effect measurements for Si $ $ and Si $ $ Hall-bar devices. Figures 1(b) and 1(c) show four-terminal nonlocal Hanle-effect curves for Si $ $ and Si $ $ LSVs, respectively, at a bias current of 0.5 mA at 20 K. These data mean that we can obtain reliable spin transport in Si layers in our LSVs, as shown in our previous work [5]. It should be noted that the magnitude of the spin signal, $\vert \Delta R_{NL}\vert $, for Si $ $ is approximately twice as large as that for Si $ $. Although the detailed will be published elsewhere [6], it is inferred that this phenomenon is tentatively interpreted by the difference in the spin injection/detection efficiency associated with the valley structures of the conduction band in Si. We hereafter focus on the MR effect that is one of the most important points for realizing the spin-MOSFETs. Figures 2(a) and 2(b) display the two-terminal local-MR signals for Si $ $ and Si $ $ LSVs, respectively, at 20 K. Here the bias current is 0.5 mA. It should be noted that the magnitude of the local-MR signals, $\vert \Delta R_{L}\vert $, for Si $ $ is also larger than that for Si $ $. Irrespective of measurement schemes, we can find the large difference in the spin injection/detection efficiency between Si $ $ and Si $ $ LSVs. We can also observe this effect even at room temperature (303 K), as shown in Fig. 2(c) and 2(d). Thanks to the crystal orientation effect, a relatively large $\vert \Delta R_{L}\vert $ of $2 \Omega $, which is the largest $\vert \Delta R_{L}\vert $ value reported so far, can be obtained. The estimated MR ratio is approximately 0.06 %, twice as large as that observed in the previous work [4]. For a different bias current condition, the MR ratio reached up to approximately 0.2 % at room temperature. Although the obtained value of the MR ratio is still insufficient to realize the spin-MOSFETs, the use of Si $ $ spin-transport channel is more effective to develop the related Si-based devices. This work was partly supported by a Grant-in-Aid for Scientic Research (A) (No. No. 16H02333) from the Japan Society for the Promotion of Science (JSPS), and a Grant-in-Aid for Scientic Research on Innovative Areas “Nano Spin Conversion Science” (No. 26103003) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT).

An effect of capping-layer material on interfacial anisotropy and thermal stability factor of MgO/CoFeB/Ta/CoFeB/MgO/capping-layer structure

We investigate the magnetic properties of a MgO/CoFeB/Ta/CoFeB/MgO/capping-layer (Ru or Ta) structure and properties of a magnetic tunnel junction with the structure as a free layer. By using Ru instead of Ta as the capping layer, interfacial anisotropy Ki increases by a factor of ∼2 and a smaller damping constant is obtained. The increase in Ki results in an enhancement of the thermal stability factor of the free layer with the Ru capping layer compared with that with the Ta capping layer in magnetic tunnel junctions.

Removal of EUV exposed hydrocarbon from Ru capping layer of EUV mask using the mixture of alkaline solutions and organic solvents

Under extreme ultraviolet (EUV) exposure, the surfaces of EUV mask and optics are exposed to hydrocarbon contaminant, which reduces the imaging performance and lifetime of EUV mask. Thus, an efficient method is required to remove hydrocarbon contaminant from the mask surface. In this work, a mixture of alkaline chemicals and organic solvents has been proposed for the removal of EUV exposed hydrocarbon contaminant from Ru (Ruthenium) capping layer of EUV mask. A hydrocarbon film with similar properties to EUV hydrocarbon contaminant was prepared by PECVD (plasma enhanced chemical vapor deposition). The hydrocarbon removal efficiency was evaluated by using four kinds of quaternary ammonium hydroxides as alkaline chemicals, and DMSO (dimethyl sulfoxide) and THF (tetrahydrofuran) as polar aprotic organic solvents. Among the alkaline chemicals, tetrabutyl ammonium hydroxide, together with an organic solvent, produced effective removal of hydrocarbon from Ru. Removal of hydrocarbon film using a higher polar organic DMSO solvent required a higher concentration and longer time than THF. The combination of megasonic power and cleaning chemistry enhanced the hydrocarbon removal efficiency significantly.

A Novel Cleaning Approach to Remove the Carbon Contaminant from Ru Surface of EUV Mask

The surfaces of extreme ultraviolet (EUV) mask and optics are very vulnerable to carbon contamination during EUV lithography process. The top surface of EUV mask is coated with Ru capping layer to protect the mask substrate. Carbon contamination occurs on Ru surface when the EUV light of 13.5 nm interacts with the surface. The carbon contamination on mask surface is known to lose its reflectivity and change the critical dimension (CD). Thus, the removal of carbon contaminant from mask surface is required to maintain the integrity of EUV lithography process. Conventional cleaning methods such as SPM and UV cleaning are widely used for the removal of organic contaminants but can cause surface damages on EUV mask such as surface oxidation and etching of absorber pattern. In this study, a novel cleaning mechanism is proposed to remove carbon contaminant using a quaternary ammonium hydroxide solution and an organic solvent mixture solution. According to the SN2 (nucleophilic substitution) reaction, hydroxyl ions are ready to react with carbon atoms as nucleophiles. Also, polar aprotic solvents such as DMSO solvate cations to actively assist hydroxyl ion reactions. Therefore, a study on the removal of carbon contaminants was carried out using a mixture of quaternary ammonium hydroxide solution and polar aprotic solvent. There have been limited studies reported on organic solvent cleaning in a semiconductor process for removal of photoresist. However, physical cleaning such as megasonic and UV treatment were applied to the organic solvent cleaning process to improve organic removal. Few studies have also been carried out to remove the photoresist by applying an additional cleaning process such as CO2 aerosol, megasonic, and UV pretreatment to the organic solvent. So far no studies have been reported on the removal of carbon contaminants using quaternary ammonium hydroxide solution with an organic solvent. This study investigated the effect of cleaning solution on hydrocarbon film removal by using four kinds of quaternary ammonium hydroxide (TMAH, TEAH, TPAH and TBAH) as alkaline chemicals and DMSO and THF organic solvents as polar aprotic solvents. Instead of EUV carbon contaminant, a hydrocarbon film with similar properties to EUV carbon contaminant was prepared and evaluated using alkaline chemicals and organic solvents. The hydrocarbon film was effectively removed from Ru surface in the mixture of TBAH and THF. This is because of low polarity nature of both TBAH and THF. When DMSO used with quaternary ammonium hydroxide solution, hydrocarbon was also removed with longer process time. The carbon removal efficiency is improved and reduced the cleaning process time for TBAH+DMSO cleaning process with megasonic due to the easy lift-off of hydrocarbon film from Ru surface. The polarity of carbon contaminant and cleaning solution have to be considered for removal carbon contaminant from Ru surface.

Modulation of interlayer exchange coupling strength in magnetic tunnel junctions via strain effect

Interlayer exchange coupling of two ferromagnetic electrodes separated by a thin MgO tunnel barrier is investigated using magneto-optical Kerr effect. We find that the coupling field can be reduced by more than 40% as the thickness of a top Ta capping layer increases from 0.5 to 1.2 nm. In contrast, a similar film stack with an additional 3 nm Ru capping layer displays no such dependence on Ta thickness. Transmission electron microscopy study shows that the oxidation of the exposed Ta capping layer induces changes in the crystalline structures of the underlying films, giving rise to the observed reduction of the interlayer coupling field.

Effects of Ru and Ag cap layers on microstructure and magnetic properties of FePt ultrathin films.

The effects of Ru and Ag cap layers on the microstructure and magnetic properties of the FePt ultrathin films have been investigated. The results indicate that i) The Ag cap layer segregates from the FePt/Ag bilayer, lowers the FePt ordering temperature, promotes the FePt thin films to form island structure, and enhances the coercivity; ii) The Ru cap layer increases the FePt ordering temperature, helps to maintain smooth continuous structure film, and restrains the FePt (001) orientation and perpendicular magnetic anisotropy (PMA). The effects become more pronounced for the 3-nm-thick FePt thin films. The effects can be mainly attributed to the different melting point and thermal expansion stress between the cap layer and FePt thin films.

Characterization of Ru thin films from a novel CVD/atomic layer deposition precursor “Rudense” for capping layer of Cu interconnects

The authors have succeeded in development of a novel Ru precursor, Ru(EtCp)(η5-CH2C(Me)CHC(Me)O) [Rudense], for CVD and atomic layer deposition (ALD) processes under nonoxidative condition. Rudense has sufficient vapor pressure and good thermal stability (decomposition temperature = ca. 230 °C). Ru thin films were grown on Pt, Ru, Si, and SiO2 substrates using Rudense and NH3 as Ru precursor and reactant, respectively. Rudense gave the conformal, low-impurity (<1021 atoms/cc), and low-resistivity (16 μΩ cm) Ru thin films. Moreover, Rudense showed substrate selectivity; therefore, Rudense will be a candidate for area-selective CVD and ALD precursor for Ru capping layers of Cu interconnects.

Magnetic properties of ultrathin Ni81Fe19 films with Ta and Ru capping layers

Magnetic properties of Ni81Fe19 (permalloy) ultrathin films with Ru and Ta capping layers (CLs) were investigated for applications to magnetic random access memory units (MRAM). The sample structure, which simulated an MRAM free layer, is Si- sub./SiO2/Ni81Fe19/Ru(Ta). The Ni81Fe19 thin films less than 3 nm thick with Ru CL show low coercive fields compared with the Ta capping layer. Both systems showed loss of momentum equivalent to magnetically dead layers of thickness (δ) ∼0.6 nm for Ru cap layer and ∼1.4 nm for Ta cap layer, respectively. Moreover, after annealing the thicknesses are slightly increased to an equivalent magnetic dead layer thickness of δ ∼ 0.84 nm and ∼1.80 nm for Ru and Ta CL, respectively. Our calculations showed that the presence of only 11% Ta concentration at the interface reduced the Ni momentum to zero, with the Ni–Ta coupling being anti-ferromagnetic; while 50% Ru intermixing at the interface reduced the Ni momentum to zero with the coupling between Ru and Ni being ferromagnetic. To find out more about the intermixing at the interface, the composition and chemical states were characterized by the x-ray photoelectron spectroscopy and peak decomposition technique. The result showed that the peak positions were different from the pure metallic case at the interface region, mainly because of the intermixing between two layers. In conclusion, the Ru capping layer might be important for MRAM use in terms of low coercive field and small δ layer thickness if compared with the Ta capping layer.

Terahertz spin current pulses controlled by magnetic heterostructures

In spin-based electronics, information is encoded by the spin state of electron bunches. Processing this information requires the controlled transport of spin angular momentum through a solid, preferably at frequencies reaching the so far unexplored terahertz regime. Here, we demonstrate, by experiment and theory, that the temporal shape of femtosecond spin current bursts can be manipulated by using specifically designed magnetic heterostructures. A laser pulse is used to drive spins from a ferromagnetic iron thin film into a non-magnetic cap layer that has either low (ruthenium) or high (gold) electron mobility. The resulting transient spin current is detected by means of an ultrafast, contactless amperemeter based on the inverse spin Hall effect, which converts the spin flow into a terahertz electromagnetic pulse. We find that the ruthenium cap layer yields a considerably longer spin current pulse because electrons are injected into ruthenium d states, which have a much lower mobility than gold sp states. Thus, spin current pulses and the resulting terahertz transients can be shaped by tailoring magnetic heterostructures, which opens the door to engineering high-speed spintronic devices and, potentially, broadband terahertz emitters.

Holographic images reconstructed from GMR-based fringe pattern

We have developed a magneto-optical spatial light modulator (MOSLM) using giant magneto-resistance (GMR) structures for realizing a holographic three-dimensional (3D) display. For practical applications, reconstructed image of hologram consisting of GMR structures should be investigated in order to study the feasibility of the MOSLM. In this study, we fabricated a hologram with GMR based fringe-pattern and demonstrated a reconstructed image. A fringe-pattern convolving a crossshaped image was calculated by a conventional binary computer generated hologram (CGH) technique. The CGH-pattern has 2,048 × 2,048 with 5 μm pixel pitch. The GMR stack consists of a Tb-Fe-Co/CoFe pinned layer, a Ag spacer, a Gd-Fe free layer for light modulation, and a Ru capping layer, was deposited by dc-magnetron sputtering. The GMR hologram was formed using photo-lithography and Krion milling processes, followed by the deposition of a Tb-Fe-Co reference layer with large coercivity and the same Kerr-rotation angle compared to the free layer, and a lift-off process. The reconstructed image of the ON-state was clearly observed and successfully distinguished from the OFF-state by switching the magnetization direction of the free-layer with an external magnetic field. These results indicate the possibility of realizing a holographic 3D display by the MOSLM using the GMR structures.

Thermal conduction properties of Mo/Si multilayers for extreme ultraviolet optics

Extreme ultraviolet (EUV) lithography requires nanostructured optical components, whose reliability can be influenced by radiation absorption and thermal conduction. Thermal conduction analysis is complicated by sub-continuum electron and phonon transport and the lack of thermal property data. This paper measures and interprets thermal property data, and their evolution due to heating exposure, for Mo/Si EUV mirrors with 6.9 nm period and Mo/Si thickness ratios of 0.4/0.6 and 0.6/0.4. We use time-domain thermoreflectance and the 3ω method to estimate the thermal resistance between the Ru capping layer and the Mo/Si multilayers (RRu-Mo/Si = 1.5 m2 K GW−1), as well as the out-of-plane thermal conductivity (kMo/Si 1.1 W m−1 K−1) and thermal anisotropy (η = 13). This work also reports the impact of annealing on thermal conduction in a co-deposited MoSi2 layer, increasing the thermal conductivity from 1.7 W m−1 K−1 in the amorphous phase to 2.8 W m−1 K−1 in the crystalline phase.

Influence of capping layers on CoFeB anisotropy and damping

Magnetic behavior of CoFeB at various thicknesses ranging from 2 nm to 8 nm capped with different materials, such as MgO, Ta, Ru, and V have been studied. The films were sputter-deposited and subsequently characterized by magnetometry and broadband ferromagnetic resonance (FMR). There are magnetically dead layers at the interface observed with Ru and Ta capping layers, while MgO and V have almost no effect on the magnetization of the CoFeB. As the ferromagnetic layer is made thinner, the effective magnetization decreases, indicating an interfacial perpendicular anisotropy. Particularly in the case of MgO, V/Ru, and V/Ta capping layers, interfacial perpendicular anisotropy is induced in CoFeB, and the Gilbert damping parameter is also reduced. The origin of this perpendicular magnetic anisotropy (PMA) is understood to be caused by the interface anisotropy between the free layer and the capping layer. The effect of post-deposition annealing and CoFeB thickness on the anisotropy and damping of V/Ta capped samples are reported. Doping CoFeB with vanadium (V) greatly reduced the 4πMs and 4πMeff values, resulting in an effective increase in the PMA.

Effective Carbon Contaminant Cleaning Condition Using Ozone Dissolved Water and Megasonic for Ru-Capped Extreme Ultraviolet Lithography Mask

A possible candidate for carbon contaminant removal in Ru-capped extreme ultraviolet lithography (EUVL) mask is ozone dissolved water (DIO3). However, the use of DIO3 leaves reflectivity loss and serious surface damages on Ru capping layer caused by its high oxidation potential. In this study, an optimum DIO3 cleaning condition for effective carbon cleaning without surface damage was investigated both theoretically and experimentally. The effect of feed gases such as O2, CO2, and N2 of various concentrations were tried during DIO3 generation for oxidation stability on Ru capping layer and N2 added 15 ppm DIO3 was found to be the best condition. However carbon contaminant was ineffectively removed at this condition. Thus megasonic is irradiated during DIO3 process and the results show that carbon is not only completely removed in a shorter time but also lower reflectivity loss was accomplished with minimal increase in the surface roughness.

Enhanced thermal stability of Ag ohmic reflector for InGaN/GaN light-emitting diode using a Ru capping layer

A Ru capping layer is employed to improve the thermal stability of Ag ohmic reflectors for high-power GaN-based light-emitting diodes (LEDs). The 20-nm-thick Ru capping layer is shown to be fairly effective in suppressing agglomeration by forming RuO2. The Ag/Ru contacts exhibit specific contact resistance of 8.1×10−5Ωcm2 and reflectance of ∼89% at a wavelength of 450nm when annealed at 500°C for 1min, which are much better than that of Ag only contacts. Blue LEDs fabricated with the 500°C-annealed Ag/Ru contacts give a forward voltage of 2.98V at an injection current of 20mA, which is lower than that (3.02V) of LEDs with the 500°C-annealed Ag only contacts. LEDs with the 500°C-annealed Ag/Ru contacts show 25% higher output power (at 20mA) than LEDs with the 500°C-annealed Ag only contacts. X-ray photoemission spectroscopy examinations are performed to describe the improved electrical performance of the Ag/Ru contacts.

Silicide Formation of Atomic Layer Deposition Co Using Ti and Ru Capping Layer

was formed through annealing of atomic layer deposition Co thin films. Co ALD was carried out using bis(N,N`-diisopropylacetamidinato) cobalt () as a precursor and as a reactant; this reaction produced a highly conformal Co film with low resistivity (). To prevent oxygen contamination, sputtered Ti and ALD Ru were used as capping layers, and the silicide formation prepared by rapid thermal annealing (RTA) was used for comparison. Ru ALD was carried out with (Dimethylcyclopendienyl)(Ethylcyclopentadienyl) Ruthenium ((DMPD)(EtCp)Ru) and as a precursor and reactant, respectively; the resulting material has good conformality of as much as 90% in structure of high aspect ratio. X-ray diffraction showed that was in a poly-crystalline state and formed at over of annealing temperature for both cases. To investigate the as-deposited and annealed sample with each capping layer, high resolution scanning transmission electron microscopy (STEM) was employed with electron energy loss spectroscopy (EELS). After annealing, in the case of the Ti capping layer, about 40 nm thick was formed while the interlayer, which is the native oxide, became thinner due to oxygen scavenging property of Ti. Although Si diffusion toward the outside occurred in the Ru capping layer case, and the Ru layer was not as good as the sputtered Ti layer, in terms of the lack of scavenging oxygen, the Ru layer prepared by the ALD process, with high conformality, acted as a capping layer, resulting in the prevention of oxidation and the formation of .

Effect of post-etch cleaning on Ru-capped extreme ultraviolet lithography photomask

Ru-capped extreme ultraviolet lithography photomasks require cleaning after patterning of the absorber layer. In this study, it was confirmed that, during Cl2 dry etching to remove the absorber layer, RuCl3 was formed on the Ru capping layer surface, and the surface roughness thereby deteriorated. Therefore, the changes in RuCl3 formation and surface roughness with various cleaning processes were investigated. Among the treatments used, i.e., sulfuric peroxide mixture, an ammonia peroxide mixture or ozonated water (DIO3), DIO3 exhibited the most effective Cl removal efficiency and surface roughness recovery. DIO3 treatment successfully reduced the Cl-terminated Ru surface to its original state and decreased the surface roughness to the pre-Cl2-etched Ru value.

Optimization of DIO3 with Megasonic Cleaning of Ru Capped EUVL Mask for Effective Carbon Contaminant Removal

A possible candidate for carbon contaminant removal in Ru capped EUVL mask is ozone dissolved water (DIO3). However, the use of DIO3 leaves reflectivity loss and serious surface damages on Ru capping layer caused by its high oxidation potential. In this study, an optimum DIO3 cleaning condition for effective carbon cleaning without surface damage was investigated both theoretically and experimentally. The effect of additive gases such as O2, CO2 and N2 of various concentrations were tried during DIO3 generation for oxidation stability on Ru capping layer and N2 added 15 ppm DIO3 was found to be the best condition. However carbon contaminant was ineffectively removed at this condition. Thus megasonic is irradiated during DIO3 process and the results show that carbon is not only completely removed in a shorter time but also lower reflectivity loss was accomplished with minimal increase in the surface roughness.

Behavior of Ru surfaces after ozonated water treatment

In order for the development of cleaning technology of extreme ultra violet lithography photomask, the behavior of Ru surfaces after treatment with ozonated deionized water (DIO 3) solution was studied using Ru and ruthenium oxide particles and 2 nm-thick Ru capping layers. No significant changes in crystalline structures or chemical states of the Ru surfaces, nor any similarities with the structures or states of ruthenium oxide, were observed after DIO 3 treatment. Oxidation of ruthenium to form RuO 2 or RuO 3 was not observed. Adsorption of H 2O molecules on the Ru layer increased the surface roughness, but the desorption of H 2O molecules recovered it. Local chemisorption of H 2O molecules on the Ru surface may be the reason why rougher Ru surfaces were observed after DIO 3 cleaning.