Quantitative secondary ion mass spectrometry analysis of oxygen isotopes and other light elements in silicon oxide films

Secondary Ion Mass Spectrometry (SIMS) has been proved to be a useful technique for quantitative profiling of ¹⁶O, ¹⁸O and various impurities such as fluorine and boron in SiO₂ films. The analytical depth resolution and the sensitivity of the analysis have been determined. Quantitative calibration has been made by using ion implantation of well-known doses of boron, fluorine and comparison to the SIMS signal; in the case of ¹⁶O, ¹⁸O and fluorine measurements, nuclear microanalysis was used as a standard. As typical applications silicon anodic oxidation mechanisms have been studied by ¹⁶O/¹⁸O duplex oxidations and by analysis of fluorine and boron movements in the film during its growth. The results demonstrate the oxygen mobility in the layer with no reversal of the oxygen order. The incorporation of fluorine in anodic SiO₂ grown in KF comtaminated baths is complex; a fluorine rich region at the SiSiO₂ interface is created at the beginning of the oxidation process and remains unaffected by further oxidation demonstrating that this layer is not due to fluorine accumulation during oxide growth or during analysis. The fluorine concentration in the bulk of the oxide is approximately uniform and depends on fluorine contamination of the electrolytic bath.     

The development of silicon-containing oxides during the oxidation of iron-chromium-base alloys

The influence of silicon on the oxidation of Fe-14% Cr and Fe-28% Cr has been studied at high temperature, with particular emphasis on the development and nature of the healing SiO₂ layer. In general, silicon is a less effective addition than aluminium to these alloys in improving oxidation resistance because SiO₂ grows at a lower rate than α-Al₂O₃. Hence, silicon is a less successful oxygen secondary getter and development of a complete healing layer of SiO₂ is less rapid than that of α-Al₂O₃ on a corresponding aluminium-containing alloy. Nonetheless, the addition of only 1% Si to Fe-28% Cr causes a marked reduction in the overall oxidation rate, particularly by facilitating development of the Cr₂O₃ scale. Precipitates of SiO₂ form at the alloy/scale interface. These grow inwards and laterally until they eventually link up to establish a continuous healing layer at the interface after several hundred hours exposure at 1000°C. Similar features are observed for Fe-14% Cr-3% Si but the healing SiO₂ layer develops after a much shorter time for Fe-14% Cr-10% Si, due to the high silicon availability. In every case, the healing layer has been shown to be amorphous SiO₂. Although this phase is very protective during isothermal oxidation, it is a site of weakness during cooling and scale spallation is very extensive from specimens where the SiO₂ is continuous, with failure occurring cohesively within that layer. Ion implantation of silicon into Fe-14% Cr and Fe-28% Cr gives a reduced oxidation rate due to facilitation of a more rapid establishment of a Cr₂O₃ scale. Similar implantation of yttrium into the ternary alloys assists in development of the silicon-containing oxide layer, possibly associated with an influence on the nucleation of the oxide precipitates in the early stages of exposure.     

The influence of silicon on the high-temperature oxidation of nickel

An investigation of the oxidation of nickel-silicon alloys has been carried out in order to ascertain the mode of development of partially or fully protective SiO₂ layers. The addition of 1% Si has little effect on the oxidation rate of nickel at 1000°C but is sufficient for partial-healing layers of amorphous SiO₂ to be established. These layers are incorporated into the inner part of the duplex NiO scale but do not react with the oxide to form a double oxide. Increasing the silicon concentration to 4% or 7% facilitates the development of apparently continuous amorphous SiO₂ layers at the base of the NiO scale, resulting in reduced rates of oxidation. However, these layers develop imperfections, possibly microcracks resulting from oxide growth stresses, and are unable to prevent some continued transport of Ni²⁺ ions into the NiO scale and oxygen into the alloy, particularly for Ni-4% Si. Although the formation of SiO₂-healing layers can reduce the rate of oxidation of nickel, they provide planes of weakness that result in considerable damage under the differential thermal contraction stresses during cooling. In particular, severe scale spalling occurs for Ni-4% Si and Ni-7% Si as failure occurs coherently within the SiO₂ layer.     

Evolution of the structural and magnetotransport properties of magnetite films depending on the temperature of their synthesis on the SiO<Subscript>2</Subscript>/Si(001) surface

The methods of transmission and reflection electron diffraction have been used to investigate the structure of Fe₃O₄ films depending on the temperature of their synthesis on an Si substrate coated with an ultrathin layer of SiO₂. The thus-grown polycrystalline films of magnetite had a texture, the axis of which was perpendicular to the surface of the SiO₂ film. It has been revealed that, with an increase in the growth temperature, a structural rearrangement occurs which is characterized by an increase in the volume fraction of grains with the preferred (311) orientation. A study of the magnetotransport properties of the films has shown that the magnitude of their magnetoresistance increases with an increase in the temperature of their synthesis. It has been established that in the Fe₃O₄/SiO₂/Si system with a tunneling-thin layer of SiO₂ the magnetoresistance decreases as a result of the flow of an electric current through the silicon substrate.     

Silicon oxynitride gas barrier coatings on poly(ether sulfone) by plasma-enhanced chemical vapor deposition

Thin silicon oxynitride (SiO_xN_y) has been deposited for a gas barrier layer on the surface of poly(ether sulfone) film using plasma-enhanced chemical vapor deposition (PECVD) of a mixture of hexamethyldisiloxane (HMDSO) and ammonia. The chemical structure of the deposited layer varied from organic to inorganic structures depending on RF plasma input power applied to the reaction system. A silicon-based undercoat layer, which has an organic/inorganic hybrid structure, was used as an interfacial buffer layer between the organic PES and inorganic SiO_xN_y layer. With the help of the undercoat layer, the dense inorganic SiO_xN_y layer gave a superior oxygen barrier property of 0.2 cm³/m² day at a critical coating thickness of ca. 20 nm. In a highly stressed SiO_xN_y film, the effect of the undercoat layer was remarkable in preventing crack formation during bending tests.     

Oxidation of Ti3SiC2 at 1000°C in Air

Oxidation of polycrystalline Ti₃SiC₂ was investigated at 1000°C by XRD and SEM. Results show that a thin SiO₂-rich scale forms after short-term oxidation, but it cannot prevent further oxidation effectively. The TiO₂ content increased with increasing time and covered the whole surface eventually. For the long-term oxidation, the oxide scales are layered, the outer layer is comprised of pure TiO₂, while the inner layer is a mixture of TiO₂ and SiO₂. The position of the pre-formed SiO₂ layer indicates that it can serve as an initial marker for the subsequent oxidation.     

Stress induced delamination methods for the study of adhesion of Pt thin films to Si

Adhesion of Pt films to Si substrates with a native oxide has been investigated using two methods of quantitative adhesion characterization. The nanoindentation induced delamination method uses an impression to store compressive strain in an overlayer film to induce delamination at the Pt/SiO₂ interface. Likewise, the telephone cord delamination method involves sputtering a thick compressively stressed overlayer onto the Pt/SiO₂ films to induce telephone cord delamination patterns in the Pt film. Crack extension forces and interface toughnesses are calculated from the dimensions of the circular blister or the telephone cords using currently available models. Focused ion beam (FIB) observations show that the nanoindentation method is difficult to implement because of extensive crack formation in the substrate beneath the indentation, causing interface toughnesses from this test to be gross overestimates. The telephone cord measurements, by comparison, give realistic interface toughnesses, allowing us to show that decreasing the argon pressure during Pt sputtering significantly increases the adhesion of the films to the substrate.     

Further considerations on the high-cycle fatigue of micron-scale polycrystalline silicon

Bulk silicon is not susceptible to high-cycle fatigue but micron-scale silicon films are. Using polysilicon resonators to determine stress-lifetime fatigue behavior in several environments, oxide layers are found to show up to four-fold thickening after cycling, which is not seen after monotonic loading or after cycling in vacuo. We believe that the mechanism of thin-film silicon fatigue is “reaction-layer fatigue”, involving cyclic stress-induced thickening of the oxide and moisture-assisted cracking within this layer.     

Design and fabrication of compositionally graded inorganic oxide thin films: Mechanical,optical and permeation characteristics

Different types of inorganic oxide films composed of a chemical composition gradient single layer were designed, fabricated and characterized. Compositionally graded thin films were created by power-controlled co-sputtering of alumina (Al₂O₃) and silica (SiO₂) at room temperature, allowing the structural design of the film to be tailored at the nanometer scale. Two distinct graded thin films were fabricated: one with a compositionally asymmetric structure consisting of a SiO₂-rich bottom interface and a Al₂O₃-rich top surface, and the other with a compositionally balanced sandwich structure consisting of both the top surface and bottom interface rich in SiO₂ and a core rich in Al₂O₃ (referred to as SGS for ‘sandwich graded structure’). Smoothly graded thin films without interfacial boundaries were verified by Auger electron spectroscopy profiles. X-ray photoelectron spectroscopy demonstrated that the Al₂O₃/SiO₂ graded structures consisted of Si–O and Al–O bonds, as well as Al–O–Si bonds in the transition layer. Neat Al₂O₃ or SiO₂ and their graded ones were all investigated for their mechanical, optical and permeation properties. A SGS thin film presented the best mechanical stability (i.e., about three times improved film toughness of a neat Al₂O₃ single layer), demonstrating that balanced internal stresses and alternating bonding structures, achieved via a graded structure without interfaces, are crucial for enhancing mechanical stability. Furthermore, neat and graded thin films exhibited the similar level of optical transmittance and the permeation properties for the graded films were well matched with the behaviors of mechanical stability.     

Modification of the oxidation behavior of high-purity austenitic Fe-14Cr-14Ni by the addition of silicon

The oxidation behavior of Fe-14Cr-14Ni (wt.%) and of the same alloy with additions of 1 and 4% silicon was studied in air over the range of 900-1100° C. The presence of silicon completely changed the nature of the oxide scale formed during oxidation. The base alloy (no silicon) formed a thick outer scale of all three iron oxides and an internally oxidized zone of (Fe,Cr,Ni) spinels. The alloy containing 4% silicon formed an outer layer of Cr₂O₃ and an inner layer of either (or possibly both) SiO₂ and Fe₂SiO₄. The formation of the iron oxides was completely suppressed. The oxidation rate of the 4% silicon alloy was about 200 times less than that of the base alloy, whereas the 1% silicon alloy exhibited a rate intermediate to the other two alloys. The actual ratio of the oxidation rates may be less than 200 due to possible weight losses by the oxidation of Cr₂O₃ to the gaseous phase CrO₃. The lower oxidation rate of the 4% silicon alloy was attributed to the suppression of iron-oxide formation and the presence of Cr₂O₃, which is a much more protective scale.     

Passivation and corrosion of microelectrode arrays

The application of silicon based microsensors in aqueous environments is hindered by unsatisfactory barrier properties and poor corrosion resistance of common passivation layers which give insufficient protection to electronic microstructures. This paper reports on investigations of the protective effect of various types of layers (compatible to silicon planar technology) against 1 M NaCl at pH 2 to 10. Failures of the passivation layers were detected by leak current and conductivity measurements with subsequent investigations of failure mechanisms by scanning electron microscopy (SEM). Both organic and inorganic films were tested with chips which were completely covered with the passivation layer. Organic films had a time to failure of at best 500 h, achieved by Probimer® and plasma treated polyimide. The poor barrier properties of PECVD‐SiO₂ and Si₃N₄ monolayers (only a few hours) were clearly surpassed by combining the monolayers to SiO₂ / Si₃N₄‐ duplex and SiO₂ / Si₃N₄ / SiO₂ triplex (ONO) layers. The most promising barrier properties were achieved by the triplex (ONO) layer which yielded a time to failure of 1200 h compared to 500 h for the duplex layer on non‐buried conducting tracks. Burying the conducting tracks into the thermal SiO₂ layer significantly improved the performance of the duplex (2000 h) and the SiC layer (1000 h compared to 700 h on non‐buried tracks) once again. In the case of open electrodes the Si₃N₄ layer quickly failed, whereas the duplex and the SiC layer revealed better protective properties. Organic films failed due to swelling and the formation of blisters. Intrinsic mechanical stress with chemical interaction resulted in stress corrosion cracking (SCC) and finally lead to the failure of the inorganic PECVD layers.     

Structural, optical and electrical properties of N-doped ZnO thin films prepared by thermal oxidation of pulsed filtered cathodic vacuum arc deposited ZnxNy films

In this study, N-doped ZnO thin films were fabricated by oxidation of Zn_xN_y films. The Zn_xN_y thin films were deposited on glass substrates by pulsed filtered cathodic vacuum arc deposition (PFCVAD) using metallic zinc wire (99.999%) as a cathode target in pure nitrogen plasma. The influence of oxidation temperature, on the electrical, structural and optical properties of N-doped ZnO films was investigated. P-type conduction was achieved for the N-doped ZnO obtained at 450 °C by oxidation of Zn_xN_y, with a resistivity of 16.1 Ω cm, hole concentration of 2.03 × 10¹⁶ cm⁻³ and Hall mobility of 19 cm²/V s. X-ray photoelectron spectroscopy (XPS) analysis confirmed the incorporation of N into the ZnO films. X-ray diffraction (XRD) pattern showed that the films as-deposited and oxidized at 350 °C were amorphous. However, the oxidized films in air atmosphere at 450-550 °C were polycrystalline without preferential orientation. In room temperature photoluminescence (PL) spectra, an ultraviolet (UV) peak was seen for all the samples. In addition, a broad deep level emission was observed.     

Decorative and Corrosion-Protective Effect upon Colouring Anodised Aluminium with Azodyes

The possibilities for decorative colouring of anodic oxide films on aluminium have been studied applying ten types of azodyes, serial products of ‘Bulcolor’ JSC, manufactured for the purpose of colouring leather, cotton and woollen articles. It has been established that four of these dyes—‘Acid Metal Complex Black RSL’, ‘Acid Yellow P’, ‘Acid Bordeaux’ and ‘Acid Black’ are appropriate also for the colouring of anodised oxide films on aluminium, formed potentiostatically in solutions of H₂SO₄. It has also been found in addition to their colouring effect, that the dyes ‘Acid Yellow P', ‘Acid Bordeaux’ and ‘Acid Black’ produce a favourable inhibiting action both on the total corrosion and on the pitting corrosion of the system Al₂O₃/Al. It has been shown that the additional sealing of the oxide films in a solution of Ni(CH₃COO)₂, improves the light resistance of the coloured films. This effect is explained on the basis of an additional complex formation process between the dye molecules and the Ni²⁺ ions, occurring inside the pores of the oxide films during the process of sealing. The investigations carried out provide evidence that the azodyes studied can be used for the preparation of high quality oxide films of a definite colour, whose light resistance and corrosion protective properties are optimised by the introduction of an additional sealing in a solution of nickel acetate.     

Fe-Si合金在600℃不同气氛中的腐蚀

研究了Fe-5Si,Fe-9Si和Fe-13Si合金在600℃下H2-CO2和H2-CO2-H2S两种气氛中的腐蚀行为。结果表明:在两种气氛中,3种合金的腐蚀均遵循近似抛物线规律;并且合金在含S和不含S气氛中的抛物线速率常数数量级分别为10-8和10-10g2·cm-4·s-1。在H2-CO2气氛中合金氧化膜由Fe的氧化物外层、Fe与Si的混合氧化物内层及Si的内氧化区组成;在H2-CO2-H2S气氛中则由层状的FeS外层和FeS+SiO2混合内层组成。两种气氛中合金均没有生成连续的SiO2外氧化膜。气氛中S的加入使合金表面生成了大量的FeS,FeS中较高的离子缺陷浓度为Fe2+向外扩散提供了通道,从而明显增大了合金的腐蚀速率。Si氧化物的生成提高了Fe在两种气氛中的耐腐蚀性。     

Oxidation Behavior of Fe–25Cr–20Ni–2.8Si During Isothermal Oxidation at 1,286 K; Life-time Prediction

The oxidation behavior of an austenitic steel, type 1.4841, with a Cr content of 25 wt% and a high-Si content of 2.8 wt% was studied during isothermal oxidation at 1,286 K in air. A thick, crystalline Cr₂O₃ layer, on top of a much thinner, amorphous SiO₂ layer, developed on the alloy substrate. After formation of a closed Cr₂O₃ scale, parabolic growth kinetics prevailed as long as the associated constant, steady-state Cr concentration in the alloy at the substrate/oxide interface of about 13 ± 1 wt% was maintained. Upon prolonged oxidation, successive cracking and spallation of the thickening oxide scale eventually led to breakaway oxidation, because the “bulk-”Cr concentration in the interior of the alloy dropped below the critical value required to ‘heal’ the protective oxide layer after oxide spallation. Application of a lifetime prediction model of the alloy substrate under isothermal oxidation conditions allowed determination of the breakaway-oxidation time as a function of alloy-sheet thickness, by employing the Cr volume-diffusion coefficient in the alloy and the parabolic growth-rate constant, both determined in the present study by fitting calculated to experimental Cr-depletion profiles for various oxidation times.     

Prevention of hillock formation during micro-machining of silicon by using OTS-SAM and SiO2 coatings

The feasibility of using a dual coating system consisting of SiO₂ and OTS-SAM thin films on the micro-machining characteristics of silicon wafer were investigated with the aim to eliminate the formation of undesirable hillocks. The outermost OTS-SAM coating was used as a sacrificial layer to pattern the SiO₂ film, which in turn served to pattern the silicon substrate. After selectively removing the OTS-SAM coating by micro-machining, HF and KOH chemical etching processes followed to remove the SiO₂ layer and create patterns on the silicon substrate. By this process, groove patterns of about 1 μm width could be successfully fabricated on a silicon wafer without the formation of undesirable hillocks.     

Effect of Ga on the Oxidation Properties of Sn–8.5Zn–0.5Ag–0.1Al–xGa Solders

The oxidation properties of Sn–8.5Zn–0.5Ag–0.1Al–xGa lead-free solders in the liquid state (250?°C) under O₂ atmosphere were investigated using a thermal gravimetric analyzer. The Ga content of the investigated solders was 0.05–2?wt%. The results indicate that Ga enhances the oxidation resistance of Sn–Zn–Ag–Al solder. Cross-sections of the solder surfaces were examined using focus ion beam milling. The thickness of the oxidation layer, which was about 30–100?nm, increased with increasing Ga content. The oxidation layer was found to be nonuniform at low Ga content. The oxide layers on the surface of solders were investigated using Auger electron spectroscopy and thin-film XRD. The results showed that the oxide layer formed was ZnO. Al and Ga tended to segregate on the surface of the solder.     

New environmental barrier coating system on carbon-fiber reinforced silicon carbide composites

Carbon-fiber-reinforced silicon carbide composites (C/SiC) are promising materials for high-temperature, light weight structural components. However, a protective coating and environmental barrier coating (EBC) are necessary to prevent the oxidation of the carbon and the reaction of the formed silica scale with water vapor. Current EBC systems use multiple layers, each serving unique requirements. However, any mismatch in the coefficients of thermal expansion (CTE) creates internal stresses and might lead to crack formation. In this case, oxygen and water vapor penetrate through the EBC, reducing the lifetime of the component. Mullite (Al₆Si₂O₁₃) is used in many known EBC systems on silicon-based ceramics either as an EBC itself or as a bondcoat. Due to its low CTE and its sufficient thermal cycling behavior, mullite was chosen in this investigation as a first layer. As mullite suffers loss of SiO₂ when exposed to water vapor at high temperatures, an additional protective top coat is needed to complete the EBC system. Different oxides were evaluated to serve as top coat, especially high-temperature oxides with low coefficients of thermal expansion (LCTE). An EBC containing mullite as bondcoat and the LCTE oxide La₂Hf₂O₇ as a top coat is proposed. Both layers were applied via atmospheric plasma spraying. In this paper, results of the influence of processing conditions on the microstructure of single mullite and LCTE oxide layers as well as mullite/LCTE oxide systems are presented. Special emphasis was directed toward the crystallinity of the mullite layer and, in the top layer, toward low porosity and reduced crack density.     

High-temperature oxidation of nano-multilayered TiAlCrSiN thin films in air

Nano-multilayered TiAlCrSiN films consisting of alternating, crystalline TiCrN and AlSiN nanolayers were deposited by cathodic arc plasma deposition. Their oxidation characteristics were studied between 600 and 1000 °C for up to 70 h in air. The formed oxides consisted primarily of Cr₂O₃, α-Al₂O₃, SiO₂, and rutile-TiO₂. The TiAlCrSiN films oxidized slower than the TiN films and faster than the CrN or CrAlSiN films, with an apparent activation energy of 36.4 kJ/mol. During their oxidation, an outermost TiO₂ layer was formed by outward diffusion of Ti ions, and the outer Al₂O₃ layer was formed by outward diffusion of Al ions. Simultaneously, an inner (Al₂O₃, Cr₂O₃)-mixed layer and an innermost TiO₂ layer were formed by the inward diffusion of oxygen ions. SiO₂ was present mainly in the lower part of the oxide layer due to its immobility.     

N-channel operation of pentacene thin-film transistors with ultrathin polymer gate buffer layer

N-channel operation of pentacene thin-film transistors with ultrathin poly(methyl methacrylate) (PMMA) gate buffer layer and gold source–drain electrode was observed. We prepared pentacene thin-film transistors with an 8-nm thick PMMA buffer layer on SiO₂ gate insulators and obtained electron and hole field-effect mobilities of 5.3 × 10^?2 cm²/(V s) and 0.21 cm²/(V s), respectively, in a vacuum of 0.1 Pa. In spite of using gold electrodes with a high work function, the electron mobility was considerably improved in comparison with previous studies, because the ultrathin PMMA film could decrease electron traps on SiO₂ surfaces, and enhance the electron accumulation by applied gate voltages.