Generation and removal of pits during chemical mechanical polishing process for MgO single crystal substrate

Magnesium oxide (MgO) single crystal is an important substrate for high temperature superconductor, ferroelectric and photoelectric applications. The function and reliability of these devices are directly affected by the quality of polished MgO surface because any defect on the substrate, such as pit or scratch, may be propagated onto device level. In this paper, chemical mechanical polishing (CMP) experiments were conducted on MgO (1 0 0) substrate using slurry mainly comprised of 1-hydroxy ethylidene-11-diphosphonic acid (HEDP) and silica or ceria particles. Through monitoring the variations of the pits topography on substrate surface, generation and removal mechanism of the pits were investigated. The experimental results indicate that the pits were first generated by an indentation or scratch caused by particles in the slurry. If the rate of chemical etching in the defect area is higher than the material removal rate, the pits will grow. If chemical reaction in the defect area is slower than the material removal rate, the pits will become smaller and eventually disappear. Consequently, these findings may provide insight into strategies for minimizing pits during CMP process.     

Microstructure of epitaxial SrRuO3 thin films on MgO substrates

SrRuO₃ thin films have been grown on singular (1 0 0) MgO substrates using pulsed laser deposition (PLD) in 30 Pa oxygen ambient and at a temperature of 400-700 °C. Ex situ reflection high-energy electron diffraction (RHEED) as well as X-ray diffraction (XRD) θ/2θ scan indicated that the films deposited above 650 °C were well crystallized though they had a rough surface as shown by atom force microscopy (AFM). XRD Φ scans revealed that these films were composed of all three different types of orientation domains, which was further confirmed by the RHEED patterns. The heteroepitaxial relationship between SrRuO₃ and MgO was found to be [1 1 0] SRO//[1 0 0] MgO and 45°-rotated cube-on-cube [0 0 1] SRO//[1 0 0] MgO. These domain structures and surface morphology are similar to that of ever-reported SrRuO₃ thin films deposited on the (0 0 1) LaAlO₃ substrates, and different from those deposited on (0 0 1) SrTiO₃ substrates that have an atomically flat surface and are composed of only the [1 1 0]-type domains. The reason for this difference was ascribed to the effect of lattice mismatch across the film/substrate interface. The room temperature resistivity of SrRuO₃ films fabricated at 700 °C was 300 μΩ cm. Therefore, epitaxial SrRuO₃ films on MgO substrate could serve as a promising candidate of electrode materials for the fabrication of ferroelectric or dielectric films.     

Correlation between structural and mechanical properties of PbTiO3 thin films grown by pulsed-laser deposition

Tetragonal lead titanate (PbTiO₃, PT) thin films are grown on (1 0 0) MgO substrate by pulsed-laser deposition (PLD) for expected applications in integrated optics. The realisation of outstanding and reliable devices into integrated circuits requires sufficient mechanical resistance despite that the obtained PT films display interesting waveguiding properties associated with low optical losses. Two mechanical properties characteristic of elasticity and hardness of PT films are studied. The elastic modulus (E or Young's modulus) and the hardness (H) are measured by the nanoindentation technique. These mechanical properties are correlated to the crystalline quality of PT/MgO thin films. The films show epitaxial relationship with the MgO substrate and the orientation of crystallites perpendicularly to the surface substrate may be the consequence of a growth process along c-axis, a-axis or both. Differences on curves plotting hardness and elastic modulus as a function of indentation depth are observed as the curves are less dispersed for the films mainly c-axis oriented.     

Thermal and irradiation induced interdiffusion in magnetite thin films grown on magnesium oxide (0 0 1) substrates

Epitaxial Fe₃O₄(0 0 1) thin films (with a thickness in the range of 10-20 nm) grown on MgO substrates were characterized using low-energy electron diffraction (LEED), conversion electron Mössbauer spectroscopy (CEMS) and investigated using Rutherford backscattering spectrometry (RBS), channeling (RBS-C) experiments and X-ray reflectometry (XRR). The Mg out-diffusion from the MgO substrate into the film was observed for the directly-deposited Fe₃O₄/MgO(0 0 1) films. For the Fe₃O₄/Fe/MgO(0 0 1) films, the Mg diffusion was prevented by the Fe layer and the surface layer is always a pure Fe₃O₄ layer. Annealing and ion beam mixing induced a very large interface zone having a spinel and/or wustite formula in the Fe₃O₄-on-Fe film system.     

Local super-saturation dependent synthesis of MgO nanosheets

Well-crystallized MgO nanosheets have been prepared with MgB₂ as a precursor without any catalyst via a simple chemical vapor deposition (CVD) method. The nanosheets are grown parallel to (2 0 0) plane according to the high-resolution transmission electron microscopy profiles. At the same time, MgO nanowires are formed in the different area of substrate, which is the result of the difference in local super-saturation. Consequently, we propose that the growth mechanism depends on the surface energy and the local super-saturation in the system.     

Microstructure and mechanical properties of alumina coatings prepared by double glow plasma technique

Low-temperature growth (600 °C) of α-Al₂O₃ coatings on the stainless steel substrate by double glow plasma technique was achieved. The compositions and microstructures of the coatings prepared at different oxygen flow rates were characterized, respectively, by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectrometry. A phenomenological mechanism for the formation of the Al₂O₃ ceramic coatings during the oxidation process was proposed on the basis of the experimental results. It was obvious that the oxygen flow rates had a great effect on the surface structure of the prepared Al₂O₃ coatings. The dense and smooth Al₂O₃ coatings were prepared at the oxygen flow rate of 15 sccm. In addition, the correlations between the mechanical properties of Al₂O₃ coating and oxygen flow rates were also discussed. The coating prepared at 15 sccm oxygen flow rate exhibited the best mechanical properties with a maximum hardness of 31 GPa and elastic modulus of 321 GPa. The corresponding critical load of scratch adherence for this sample was 47 N.     

Effect of surface roughness on the development of protective Al2O3 on Fe-10Al (at.%) alloys containing 0-10 at.% Cr

The effect of alloy surface roughness, achieved by different degrees of surface polishing, on the development of protective alumina layer on Fe-10 at.% Al alloys containing 0, 5, and 10 at.% Cr was investigated during oxidation at 1000 °C in 0.1 MPa oxygen. For alloys that are not strong Al₂O₃ formers (Fe-10Al and Fe-5Cr-10Al), the rougher surfaces increased Fe incorporation into the overall surface layer. On the Fe-10Al, more iron oxides were formed in a uniform layer of mixed aluminum- and iron-oxides since the layer was thicker. On the Fe-5Cr-10Al, more iron-rich nodules developed on an otherwise thin Al₂O₃ surface layer. These nodules nucleated preferentially along surface scratch marks but not on alloy grain boundaries. For the strong Al₂O₃-forming Fe-10Cr-10Al alloy, protective alumina surface layers were observed regardless of the surface roughness. These results indicate that the formation of a protective Al₂O₃ layer on Fe-Cr-Al surfaces is not dictated by Al diffusion to the surface. More cold-worked surfaces caused an enhanced Fe diffusion, hence produced more Fe-rich oxides during the early stage of oxidation.     

Growth of superconducting FeSe0.5Te0.5 thin films by pulsed-laser deposition

FeSe_0.5Te_0.5 thin films with PbO-type structure are successfully grown on MgO(1 0 0) and LaSrAlO₄(0 0 1) substrates from FeSe_0.5Te_0.5 or FeSe_0.5Te_0.75 polycrystalline targets by pulsed-laser deposition. The film deposited on the MgO substrate (film thickness ∼ 55 nm) shows superconductivity at 10.6 K (onset) and 9.2 K (zero resistivity). On the other hand, the film deposited on the LaSrAlO₄ substrate (film thickness ∼ 250 nm) exhibits superconductivity at 5.4 K (onset) and 2.7 K (zero resistivity). This suggests the strong influence of substrate materials and/or the c-axis length to superconducting properties of FeSe_0.5Te_0.5 thin films.     

Mg diffusion in K(Ta0.65Nb0.35)O3 thin films grown on MgO evidenced by Auger electron spectroscopy investigation

The diffusion of Mg in pulsed laser deposited K(Ta_0.65Nb_0.35)O₃ thin films epitaxially grown on (1 0 0) MgO single crystal substrate were investigated by Auger electron spectroscopy (AES). A diffusion of Mg from the substrate into the whole thickness (400 nm) of the as-deposited K(Ta_0.65Nb_0.35)O₃ films was observed with an accumulation of Mg at the surface. Ex situ post-annealing (750 °C/2 h) has led to a homogeneous distribution of Mg in all the ferroelectric coating. This strong reaction between film and substrate promotes a doping effect, responsible for the reduction of K(Ta_0.65Nb_0.35)O₃ dielectric losses in comparison with films grown on other substrates.     

The effect of plating on magnetron sputtering: Residual stress and scratch behavior of Au/NiCr/Ta multi-layers

Au/NiCr/Ta multi-layers were deposited on Al₂O₃ substrate by magnetron sputtering and plating. The effect of plating technique on magnetron sputtering film in residual stress, crystal orientation and scratch resistance behavior was investigated. The all magnetron sputtering and plating films were highly textured with dominant Au-(1 1 1) orientation or a mixture of Au-(1 1 1) and Au-(2 0 0) orientation and the (1 1 1)/(2 0 0) intensity ratio were increased after plating. The residual stress in magnetron sputtering films at different substrate temperature was tensile stress with 155-400 MPa and it decreased approximately to 50 MPa after plating. The scratch resistance could be affected by the film thickness, and it increased approximately linearly with the increase of the thickness of metallic films after plating.     

Characterization of Ti-Zr-N films deposited by cathodic vacuum arc with different substrate bias

Ti-Zr-N (multi-phase) films were prepared by cathodic vacuum arc technique with different substrate bias (0 to −500 V), using Ti and Zr plasma flows in residual N₂ atmosphere. It was found that the microstructure and mechanical properties of the composite films are strongly dependent on the deposition parameters. All the films studied in this paper are composed of ZrN, TiN, and TiZrN ternary phases. The grains change from equiaxial to columnar and exhibit preferred orientation as a function of substrate bias. With the increase of substrate bias the atomic ratio of Ti to Zr elements keeps almost constant, while the N to (Ti + Zr) ratio increases to about 1.1. The composite films present an enhanced nanohardness compared with the binary TiN and ZrN films deposited under the same condition. The film deposited with bias of −300 V possesses the maximum scratch critical load (L_c).     

Stress analysis at the interface between Ni-P coating and SiCp/Al substrate of space mirror

This paper investigates the mechanical properties at the interface of the coating-substrate system, which comprises the electroless nickel-phosphorus (Ni-P) coating and the aluminum matrix composite substrate reinforced by the silicon carbide particles (SiC_p/Al), and is used for the space mirror. To estimate the adhesion of Ni-P coating on SiC_p/Al substrate, the scratch adhesion testing has been performed by drawing a spherically tipped diamond indenter with a radius of 200 μm over the coated surface. The influence of the coating thickness on the interfacial stress induced by the inertial accelerations, temperature gradients and thermal soaks has been evaluated by simulation analysis based on the finite element method. The results of the scratch testing indicate that the adhesion strength of Ni-P coating to SiC_p/Al composite is more than 3.0 GPa. Compared the maximum value of the interfacial stress obtained by simulation analysis with results of the scratch testing, it is can be seen that the mirror has enough safety margin. Furthermore, the most significant conclusion that can be drawn from this work is that the coating thickness should not exceed 45 μm in order to ensure the performance and reliability of Ni-P coating and SiC_p/Al substrate system for space applications.     

Microstructure and magnetic properties studies of SmCo5 thin films grown on MgO and glass substrates

In this work, SmCo₅ thin films are deposited on single crystal MgO (1 0 0) and amorphous glass substrates with a Cr underlayer at 400 °C by sputtering. A comparison study shows that the microstructures and magnetic properties are different in the two SmCo₅ films on the MgO (1 0 0) and glass substrates, respectively. An epitaxial growth of Cr-(2 0 0)〈1 1 0〉/SmCo₅-(1 1 2¯ 0)〈0 0 0 1〉 is achieved on the MgO (1 0 0) single crystal substrate with an average grain size of 20 nm for SmCo₅. On the amorphous glass substrate, no significant crystallographic texture is found in the Cr underlayer. After the deposition of SmCo₅, a weak texture of (1 1 2¯ 0) is observed with an average grain size of 8 nm. High remanence ratio value in this film is probably due to strong exchange coupling. Both SmCo₅ films show high in-plane coercivity, high in-plane anisotropy and remanence enhancement.     

Adsorption of SO2 on Li atoms deposited on MgO (1 0 0) surface: DFT calculations

The adsorption of sulfur dioxide molecule (SO₂) on Li atom deposited on the surfaces of metal oxide MgO (1 0 0) on both anionic and defect (F_s-center) sites located on various geometrical defects (terrace, edge and corner) has been studied using density functional theory (DFT) in combination with embedded cluster model. The adsorption energy (E_ads) of SO₂ molecule (S-atom down as well as O-atom down) in different positions on both of O⁻² and F_s sites is considered. The spin density (SD) distribution due to the presence of Li atom is discussed. The geometrical optimizations have been done for the additive materials and MgO substrate surfaces (terrace, edge and corner). The oxygen vacancy formation energies have been evaluated for MgO substrate surfaces. The ionization potential (IP) for defect free and defect containing of the MgO surfaces has been calculated. The adsorption properties of SO₂ are analyzed in terms of the E_ads, the electron donation (basicity), the elongation of S-O bond length and the atomic charges on adsorbed materials. The presence of the Li atom increases the catalytic effect of the anionic O⁻² site of MgO substrate surfaces (converted from physisorption to chemisorption). On the other hand, the presence of the Li atom decreases the catalytic effect of the F_s-site of MgO substrate surfaces. Generally, the SO₂ molecule is strongly adsorbed (chemisorption) on the MgO substrate surfaces containing F_s-center.     

Influence of Si substrate preparation on surface chemistry and morphology of L-CVD SnO2 thin films studied by XPS and AFM

Results of experimental studies of the influence of substrate preparation on the surface chemistry and surface morphology of the laser-assisted chemical vapour deposition (L-CVD) SnO₂ thin films are presented in this paper. The native Si(1 0 0) substrate cleaned by UHV thermal annealing (TA) as well as thermally oxidized Si(1 0 0) substrate cleaned by ion bombardment (IBA) have been used as the substrates. X-ray photoemission spectroscopy (XPS) has been used for the control of surface chemistry of the substrates as well as of deposited films. Atomic force microscopy (AFM) has been used to control the surface morphology of the L-CVD SnO₂ thin films deposited on differently prepared substrates. Our XPS shows that the L-CVD SnO₂ thin films deposited on thermally oxidized Si(1 0 0) substrate after cleaning with ion bombardment exhibit the same stoichiometry, i.e. ratio [O]/[Sn] = 1.30 as that of the layers deposited on Si(1 0 0) substrate previously cleaned by UHV prolonged heating. AFM shows that L-CVD SnO₂ thin films deposited on thermally oxidized Si(1 0 0) substrate after cleaning with ion bombardment exhibit evidently increasing rough surface topography with respect to roughness, grain size range and maximum grain height as the L-CVD SnO₂ thin films deposited on atomically clean Si substrate at the same surface chemistry (nonstoichiometry) reflect the higher substrate roughness after cleaning with ion bombardment.     

Stress dependence of growth mode change of epitaxial layered cobaltite γ-Na0.7CoO2

We report stress dependence of growth characteristics of epitaxial γ-Na_0.7CoO₂ films on various substrates deposited by pulsed laser deposition method. On the sapphire substrate, the γ-Na_0.7CoO₂ thin film exhibits spiral surface growth with multi-terraces and highly crystallized texture. For the γ-Na_0.7CoO₂ thin film grown on the (1 1 1) SrTiO₃ substrate, the nano-islands of ∼30 nm diameter on the hexagonal grains are observed. These islands indicate that the growth mode changes from step-flow growth mode to Stranski-Krastanow (SK) growth mode. On the (1 1 1) MgO substrate, the large grains formed by excess adatoms covering an aperture between hexagonal grains are observed. These experimental demonstrations and controllability could provide opportunities of strain effects of Na_xCoO₂, physical properties of thin films, and growth dynamics of heterogeneous epitaxial thin films.     

Structural and magnetoresistive properties of half metallic Co2Mn1−xSi thin films

Polycrystalline Co₂Mn_1−xSi (CMS) thin films with Mn-deficiency can grow on different types of substrates such as MgO (1 0 0) single crystal, α-sapphire (0 0 0 1) and Si coated with SiO₂ either by using V or Ta/Cu as the seed layer. The magnetic property, especially the coercivity of the CMS thin films strongly depends on the crystalline structure and microstructure of the CMS thin film, hence it is affected by the substrate and also the seed layer. Very soft CMS thin film with coercivity of about 20 Oe has been obtained when MgO (1 0 0) is used as the substrate. Magnetic tunnel junctions (with MR ratio of about 9%–18%) by utilizing the CMS as one of ferromagnetic electrodes have been successfully fabricated. The degradation of the magnetoresistive effect of the MTJ after magnetic annealing is attributed to the diffusion of the Mn-atoms into the tunnel barrier during the annealing process.     

Ultrasonic flexural vibration assisted chemical mechanical polishing for sapphire substrate

The sapphire substrates are polished by traditional chemical mechanical polishing (CMP) and ultrasonic flexural vibration (UFV) assisted CMP (UFV-CMP) respectively with different pressures. UFV-CMP combines the functions of traditional CMP and ultrasonic machining (USM) and has special characteristics, which is that ultrasonic vibrations of the rotating polishing head are in both horizontal and vertical directions. The material removal rates (MRRs) and the polished surface morphology of CMP and UFV-CMP are compared. The MRR of UFV-CMP is two times larger than that of traditional CMP. The surface roughness (root mean square, RMS) of the polished sapphire substrate of UFV-CMP is 0.83 Å measured by the atomic force microscopy (AFM), which is much better than 2.12 Å obtained using the traditional CMP. And the surface flatness of UFV-CMP is 0.12 μm, which is also better than 0.23 μm of the traditional CMP. The results show that UFV-CMP is able to improve the MRR and finished surface quality of the sapphire substrates greatly. The material removal and surface polishing mechanisms of sapphire in UFV-CMP are discussed too.     

Effect of microstructure of MgO buffer layer on BaTiO3 grown on silicon substrates

MgO ultrathin films were grown on Si(1 0 0) substrates as buffer layers for the growth of ferroelectric BaTiO₃ thin films by laser molecular beam epitaxy (L-MBE). The deposition process of MgO buffer layers grown on silicon was in situ monitored by reflection high-energy electron diffraction (RHEED). The structure of BaTiO₃ films fabricated on MgO buffers was investigated by X-ray diffraction. Biaxially textured MgO was obtained at high laser energy density, but when the laser energy was lowered, MgO buffer was transformed to the form of texture with angular dispersion with the increase of the film thickness. BaTiO₃ films grown on the former buffer were completely (0 0 1) textured, while those on the latter were (0 0 1) preferred orientated. Furthermore, the fabricated MgO buffers and BaTiO₃ films had atomically smooth surface and interface. All these can reveal that the quality of textured MgO buffer is a key factor for the growth of BaTiO₃ films on silicon.     

Laser synthesis of palladium-alumina composite membranes for production of high purity hydrogen from gasification

This paper describes a special method of laser-based deposition to synthesize palladium-ceramic composite membranes. Thin film Pd was deposited on a ceramic substrate by Nd-YAG laser irradiation of coating precursor PdCl₂ on γ-alumina substrate. The parameters of the laser processing technique were optimized to synthesize metal-ceramic composite membranes. The physical and chemical characteristics of Pd coated γ-alumina membranes were studied and compared with various other alumina membranes referenced in the literature. Hydrogen permeation experiments were performed in a CO + CO₂ + CH₄ + H₂ environment under typical catalytic steam gasifier exit conditions. The Pd-ceramic composite showed good mechanical and thermal stability and resulted in a hydrogen permeability flux of about 0.061 mol/m² s. The activation energy of the Pd membrane was found to be 5.39 kJ/mol in a temperature range of 900-1300 °F.