Si nanopatterning by reactive ion etching through an on-chip self-assembled porous anodic alumina mask

Abstract

We report on Si nanopatterning through an on-chip self-assembled porous anodic alumina (PAA) masking layer using reactive ion etching based on fluorine chemistry. Three different gases/gas mixtures were investigated: pure SF₆, SF₆/O₂, and SF₆/CHF₃. For the first time, a systematic investigation of the etch rate and process anisotropy was performed. It was found that in all cases, the etch rate through the PAA mask was 2 to 3 times lower than that on non-masked areas. With SF₆, the etching process is, as expected, isotropic. By the addition of O₂, the etch rate does not significantly change, while anisotropy is slightly improved. The lowest etch rate and the best anisotropy were obtained with the SF₆/CHF₃ gas mixture. The pattern of the hexagonally arranged pores of the alumina film is, in this case, perfectly transferred to the Si surface. This is possible both on large areas and on restricted pre-defined areas on the Si wafer.

PACS

78.67.Rb, 81.07.-b, 61.46.-w     

Influence of surface morphology on oxide growth in porous alumina

The formation of aluminium oxide layers on surfaces with different morphologies prepared by various structuring methods has been analyzed. Different growth rates and thus different oxide thicknesses have been observed under the same reaction parameters on planar surfaces in contrast to convex and concave surfaces with different radii in the curvature. The stronger the curvature of the concave or convex surface, the more the growth rates differ from the growth rates on planar surfaces. The influence of the electrical field strength on those differences is discussed in a simple model.     

Novel method and its mechanism of reconstruction of commercial alumina to homogeneous porous alumina composite material

In this study, a novel method for reconstruction of commercial alumina to a homogeneous porous alumina composite material (HPACM) using an alumina powder, boric acid, and Na₂SO₄ and Na₂CO₃ additives is proposed for the first time and experimentally evaluated in detail. The properties of the as-prepared porous materials and their formation mechanisms are investigated. Unlike the traditional complex methods for manufacturing of porous alumina including electrochemical anodisation of aluminium and chemical etching, the HPACM can be simply prepared through sintering and dissolution. Scanning electron microscopy and N₂ adsorption–desorption isotherm analyses demonstrate the formation of the HPACM with a diameter distribution centred at ∼20 nm after the sintering and decomposition at high temperatures and dissolution at room temperature. Transmission electron microscopy and X-ray diffraction results show that the main components of the as-prepared HPACM are alumina and sodium aluminate. The formation of the HPACM is attributed mainly to the reactions between the alumina powder, boric acid, and Na₂SO₄ and Na₂CO₃ additives at high temperatures and dissolution of the intermediate product of sodium aluminate at room temperature. A NaCl additive does not contribute to the formation of the intermediate product during the sintering process, owing to its considerable vapour loss at low temperatures. Moreover, a nanoindentation test shows that the HPACM has a good mechanical strength.     

Long-term stability of Cu/Pd nanoparticles and their feasibility for electroless copper deposition

Poly-vinylpyrrodione (PVP) protected Cu/Pd nanoparticles synthesized with citric complexing agent were developed as activator for electroless copper deposition in printed-circuit boards (PCBs) industry. Cu/Pd nanoparticles in different molar ratio were employed in electroless copper deposition and found that the catalytic activity was influenced not only by the amount of Pd content or particle size but also by the surface protected PVP layer. From the time-resolved electroless copper deposition, we suggested that the catalytic efficiency should be evaluated by both initial activity and the structure of deposited copper film. As a result, the Cu/Pd nanoparticles of molar ratio = 1/2 was found to have best catalytic activity which was comparable to traditional Pd/Sn or pure Pd activator. In regard to the most concerned stability issue of Cu-content system, a surprising slow galvanic corrosion of Cu was found when Cu/Pd nanoparticles were exposed to the open air. Besides, slightly oxidation of Pd was found after air-exposure. Surprisingly, these Cu/Pd colloids maintain part of activity and well suspension even after 6-month exposure, suggesting well long-term stability as activator. In short, Cu/Pd nanoparticles synthesized with citric complexing agent with high catalytic activity and long-term stability was a promising activator for electroless copper deposition.     

Control of the spatial distribution of porous alumina micro-domes formed during anodic oxidation

We found that micro-domes of porous alumina are self-assembled during anodic oxidation of an aluminum plate. We investigated the effects of the morphology of the initial aluminum surfaces on the formation of these micro-domes and found that the formation of micro-domes depends on the initial surface roughness of the substrate. We have also achieved spatial control over the distribution of these micro-domes through the use of artificial scratches on the initial surface. The origin of this control is the fact that micro-domes are preferentially formed inside hollow areas formed by the scratch. We investigated the inner structure of the micro-dome by separating it from the substrate. Inside the micro-domes, we observed nano-pore arrays similar to a porous alumina membrane, though the regularity of these pores is slightly worse than for the nano-pores around the micro-dome. These results indicate that the porous alumina micro-domes can be used as microscale nanoporous components.     

Through-hole membranes of nanoporous alumina formed by anodizing in oxalic acid and their applications in fabrication of nanowire arrays

The nanopore arrays were fabricated by two-step self-organized anodization of aluminum carried out in 0.3 M oxalic acid at the temperature of 20 °C. This relatively high temperature shortens significantly the anodizing time and allows to fabricate quickly thick through-hole membranes without the additional operating cost of a cooling circuit. The structural features of anodic porous alumina such as pore diameter, interpore distance, porosity, pore density and pore circularity were investigated at various durations of pore opening/widening process carried out in 5% H₃PO₄. An excellent agreement of AAO structural features measured in FE-SEM images of the studied samples with results from software calculations was observed. The pore shape can be monitored qualitatively by fast Fourier transforms (FFTs) and quantitatively by calculation the percentage of pore circularity. Additionally, the regularity of the hexagonal arrangement of nanopores in through-hole AAO membranes was compared for various opening/widening time ranging from 40 to 100 min. It was shown that three-dimensional (3D) representations of FE-SEM images and their surface-height distribution diagrams provide interesting information about the surface roughness evolution during the pore opening/widening process. A template-assisted fabrication of Ag and Sn nanowire arrays by electrochemical deposition into the pores of the prepared AAO templates was also successfully demonstrated.     

Anodic formation of low-aspect-ratio porous alumina films for metal-oxide sensor application

Thin nanoporous anodic alumina films, of low aspect ratio (1:1), with two distinctive pore sizes and morphologies were prepared by two-step constant-current anodising of aluminium layers on SiO₂/Si substrates in 0.4 mol dm⁻³ tartaric (TA) and malonic acid (MA) electrolytes and then modified by open-circuit dissolution. The anodic films were employed as a support material for sputtering-deposition of thin WO₃ layers in view of exploiting their gas sensing properties. The films and deposits were characterized by scanning electron microscopy, X-ray diffraction and electric resistance measurements at fixed temperatures in the range of 100-300 °C upon NH₃ and CO gas exposures. Test sensors prepared from the annealed and stabilized alumina-supported WO₃ active layers were insensitive to CO but showed considerably enhanced responses to NH₃ at 300 °C, the sensitivity depending upon the anodic film nature, the pore size and the surface morphology. The increased sensor sensitivity is due to the substantially enlarged film surface area of the TA-supported WO₃ films and the nanostructured, camomile-like morphology of the MA-supported WO₃ films. Sensing mechanisms in the alumina-supported WO₃ active layers are discussed.     

超细活性氧化铝的制备和表征

在强烈搅拌的条件下，用化学沉淀法制备，得到了纳米级的拟薄水铝石。然后在适当的条件下煅烧得到超细活性氧化铝。讨论了沉淀反应中溶液浓度、气体流量、反应终点pH对前驱体晶型的影响，以及前驱体晶体形态、煅烧温度和煅烧时间对氧化铝形态的影响。用XRD、TEM及BET对产品进行了表征，表明本方法可得到粒径小、比表面积较大的活性氧化铝。     

Freeze gelcasting of hydrogenated vegetable oil-in-aqueous alumina slurry emulsions for the preparation of macroporous ceramics

Freeze gelcasting of hydrogenated vegetable oil-in-aqueous alumina slurry (HVO-in-AAS) emulsions has been studied for the preparation of macroporous ceramics. The emulsions with HVO to AAS volume ratios in the range of 1.34–2.69 prepared from a 30 vol.% AAS containing carrageenan using sodium dodecyl sulphate emulsifying agent at 85 °C undergo gelation on cooling to room temperature due to the solidification of HVO and physical cross-linking of carrageenan. Macroporous ceramics obtained by drying, HVO removal followed by sintering at 1500 °C of the gelled emulsion bodies had porosity in the range of 70.7–84% and contain cells of spherical to polygonal shape. The average cell size (13.24–3.6 μm) decreased and the cell interconnectivity increased with an increase in HVO to AAS volume ratio and mixing speed. The macroporous alumina bodies had high compressive strength (6.5–39.6 MPa) and Young's modulus (350–2352 MPa).     

Electrochemical synthesis of nickel hexacyanoferrate nanoarrays with dots, rods and nanotubes morphology using a porous alumina template

Transition metal hexacyanoferrate (MeHCF) have attracted extensive attention because of their outstanding properties including, electrocatalysis, molecular magnetism, biosensing and ion-exchange. This paper describes an approach for fabrication of ordered nanoarrays of Ni hexacyanoferrate (NiHCF) structures with different morphologies such as dots, rods and tubes in order to advance their properties and applications. The method is based on the conversion of Ni into NiHCF nanostructures by electrochemical oxidation in the presence of hexacyanoferrate ions, using nanoporous anodic alumina oxide (AAO) as a template. The structure and morphology of formed Ni and NiHCF nanoarrays were confirmed by scanning electron microscopy (SEM), showing agreement with the pore structures of the AAO template. The electrocatalytic activity of NiHCF nanorod array electrodes showed high catalytic properties for the detection of hydrogen peroxide and the potential to be used as a platform for direct biosensing applications. The ion-exchange ability of fabricated NiHCF nanostructures (nanorods and nanotubes) toward alkali cations such as Na⁺ has been successfully confirmed.     

Porous anodic alumina formed by anodization of aluminum alloy (AA1050) and high purity aluminum

A two-step anodization process performed at 0 °C was used to prepare highly ordered porous anodic alumina on the AA1050 alloy and high purity aluminum foil. The anodizing of both substrates was carried out in 0.3 M sulfuric acid and 0.3 M oxalic acid baths at 25 V and 40 V, respectively. The effect of the extended duration of the second anodizing step on pore order degree and structural features of AAO membranes was studied. The presence of alloying elements affects not only the rate of oxide growth but also the microstructure of the anodic film. It was found that pore circularity and regularity of pore arrangement in AAO membranes formed on the AA1050 alloy were always worse than those observed on the pure Al substrate. The structural features, such as pore diameter, interpore distance, wall thickness, barrier layer thickness, porosity and pore density of porous anodic alumina formed on AA1050 are a little different from those obtained for high purity Al. The extended time of the second anodizing step, up to 16 h does not affect significantly the regularity of pore order and all structural features of AAO membranes, independently of the anodizing electrolyte.     

Development and application of a holistic model for the steady state growth of porous anodic alumina films

A holistic model was developed and applied to anodic alumina films galvanostatically grown in sulphuric acid solution at different anodising conditions thus characterised by different structural characteristics. The O²⁻ and Al³⁺ species transport numbers near the metal|oxide interface were determined that depended on both temperature and current density. The rate of film thickness growth was found to be proportional to the O²⁻ anionic current through the barrier layer near the metal|oxide interface. The results introduced a new growth mechanism theory embracing the rarefaction of barrier layer oxide lattice towards the metal|oxide interface. The oxide density near the metal|oxide is closely independent of anodising conditions and is related to the transformation of Al lattice to a transient oxide lattice about 37% rarer than that of γ-Al₂O₃ that is further suitably transformed to denser, amorphous or nanocrystalline material as this oxide is shifted to the oxide|electrolyte interface and becomes the pore wall material. This gradual lattice density variability can explain many peculiar properties of anodic alumina films.     

Fabrication and structural control of anodic alumina films with inverted cone porous structure using multi-step anodizing

Porous anodic alumina (PAA) film has recently attracted much attention as a key material for the fabrication of various nanostructures. In this study, a multi-step anodization and leaching process was employed to produce three-dimensional nanometer scale structured film. During the leaching process, the porous alumina film was dipped in phosphoric acid solution for pore widening. Each anodization process was followed by this leaching process. This method produced alumina film with multi-step structure. Meanwhile, with five-step film production, the structure showed inverted cone structure. We produced the low aspect ratio pores of this structure, which would be applicable for fabrications of nanomaterials. In addition, the aspect ratio was controlled by changing the anodization duration.     

Annealing effects on the pore structures and mechanical properties of porous alumina via directional freeze-casting

The effects of the annealing methods and annealing temperatures on the pore structures and mechanical properties of porous alumina were investigated. The amorphisation behavior and solidification behavior of the sucrose solutions during annealing were discussed. The pore morphology of porous alumina changed noticeably after uniform annealing. As annealing temperature increased from ?25 ℃ to ?5 ℃, the pore morphology of porous alumina changed gradually from irregular lamellar channels to circular channels. After directional annealing, the pore morphology of porous alumina was similar to that after uniform annealing; however, the uniformity of pore channels and the density of pore walls were increased. During directional annealing at ?15℃, the compressive strength of porous alumina reached 58.8?MPa, which was 35% higher than that of unannealed porous alumina.     

介孔氧化铝的结构调控及除氟性能研究

以铝酸钠（NaAlO₂）和氯化铝（AlCl₃·6H₂O）为铝源,分别以葡萄糖、壳聚糖、十六烷基三甲基溴化铵（CTAB）为结构导向剂,采用水热反应和高温煅烧技术制备了纳米氧化铝粉体材料。用X射线衍射（XRD）、扫描电镜（SEM）、氮气吸附-脱附和傅里叶变换红外光谱（FT-IR）等手段对产物进行了表征。结果表明,产物均为介孔γ-氧化铝（γ-Al₂O₃）,呈纳米颗粒状形貌,分散较为均匀。其中,葡萄糖调控的γ-氧化铝具有较大的比面积（437 m ²/g）和孔体积（0.60 cm ³/g）。4种γ-氧化铝对氟离子（F ^-）的吸附结果表明,葡萄糖调控的γ-氧化铝除氟效果最好。     

Micromachining porous alumina ceramic for high quality trimming of turbine blade cores via double femtosecond laser scanning

Turbine blade cores are made of porous alumina ceramic and determine the molding accuracy of the cavity of turbine blades, which strongly affect thermal diffusion performance and service life of turbine engines. To get a high quality ceramic core, accurate trimming for a preliminarily processed core is needed and therefore, micromachining porous alumina ceramic, which differs from general alumina substrates, is crucial. This paper dealt with a processing technology for the special material via double femtosecond laser scanning. The materials ablation threshold was firstly determined through parameter fitting and then this material was machined at a combination of different laser processing parameters. Considering the produced debris blocks the lasers further propagation into the material, double femtosecond laser scanning was newly proposed and experimentally verified with the comparison of gas jet assist and underwater laser processing ways. The removal profiles of the machined material were characterized in terms of cutting width, cutting depth, deviation of linearity and surface morphology, which exhibited high dependence on the femtosecond laser processing parameters. The optimal laser operating window was identified and high quality laser cutting of the porous alumina ceramic was demonstrated. The developed processing technology has potential application in trimming for ceramic casting cores. In addition, it might also give a novel view for high quality laser micromachining another materials.     

Synthesis and characterization of nanostructured polypyrroles: Morphology-dependent electrochemical responses and chemical deposition of Au nanoparticles

We report here the preparation of nanostructured polypyrroles (PPys) with different morphologies (nanospherical or nanofibrillar) through a surfactant-assisted oxidative polymerization route. Nanofibrillar PPy has a higher redox current, presumably due to a higher surface area accessible to the electrolytes and a lower charge transfer resistance compared to that of the spherical PPy. The impedance spectrum of spherical PPy at lower frequencies suggests a semi-infinite diffusion process, while nanofibrillar PPy displays barrier diffusion and capacitor characteristics. Electrodeless (chemical) deposition of Au particles from AuCl₄⁻ aqueous solution using nanostructured PPy also shows different morphologies, presumably due to a difference in growth kinetics dominated by the differences in surface area and surface chemistry. Our work demonstrates control over the electrochemical responses and charge transfer mechanisms of these conducting polymers. This control arises from their unique length scale geometries and surface areas that allows for the fabrication of Au nanoparticles with tunable morphologies. Our work in the controlled synthesis of nanostructured conducting polymers and metal nanoparticles opens up new opportunities for nanofiber-based electronic and sensory devices.     

Preparation of ultra-active alumina of designed porous structure by successive hydrothermal and thermal treatments of porous anodic Al2O3 films

A porous anodic alumina film was prepared by the anodic oxidation of Al metal sheet in a thermostated and vigorously stirred bath of H₂SO₄ 15% (w/v) at a temperature of 25°C and a current density of 15 mA cm⁻². It had a geometric surface area of 33 cm², a surface density of pores 1.269×10¹¹ cm⁻² and the maximum limiting thickness and porosity achieved at these conditions which are 50.3 μm and 0.42, respectively. This oxide was tried in the catalytic test reaction of the decomposition of HCOOH at temperatures 270–390°C. Then, the oxide was treated hydrothermally in H₂O at 100°C for 5 h and tried in the same test reaction. The procedure of hydrothermal treatment and catalysis experiment was repeated 40 times. In all cases the oxide showed an almost exclusively dehydrative catalytic effect, 98–100%. Both the total activity of the alumina film with the aforementioned constant geometric surface area and its specific activity referred to the unit of oxide mass gave a maximum in the first and a minimum about the fourth hydrothermal treatment; then, they increased strongly with the order of hydrothermal treatment. Despite the decrease of the oxide mass during hydrothermal treatment, the final promotion of the total catalytic activity of oxide was 13.7–10.6 times that of non-treated oxide for temperatures 330–390°C. The corresponding promotion of specific activity was 31.5–24.5 times that of the non-treated oxide. The results of the present study showed that the successive hydrothermal and thermal treatments of porous anodic Al₂O₃ films produce more and more active alumina catalysts. In this way ultra-active alumina catalysts or supports can be prepared.     

Investigating the effect of porosity level and pore former type on the mechanical and corrosion resistance properties of agro-waste shaped porous alumina ceramics

The strength integrity and chemical stability of porous alumina ceramics operating under extreme service conditions are of major importance in understanding their service behavior if they are to stand the test of time. In the present study, the effect of porosity and different pore former type on the mechanical strength and corrosion resistance properties of porous alumina ceramics have been studied. Given the potential of agricultural wastes as pore-forming agents (PFAs), a series of porous alumina ceramics (Al₂O₃-xPFA; x=5, 10, 15 and 20 wt%) were successfully prepared from rice husk (RH) and sugarcane bagasse (SCB) through the powder metallurgy technique. Experimental results showed that the porosity (44–67%) and the pore size (70–178 µm) of porous alumina samples maintained a linear relationship with the PFA loading. Comprehensive mechanical strength characterization of the porous alumina samples was conducted not just as a function of porosity but also as a function of the different PFA type used. Overall, the mechanical properties showed an inverse relationship with the porosity as the developed porous alumina samples exhibited tensile and compressive strengths of 20.4–1.5 MPa and 179.5–10.9 MPa respectively. Moreover, higher strengths were observed in the SCB shaped samples up to the 15 wt% PFA mark, while beyond this point, the silica peak observed in the XRD pattern of the RH shaped samples favored their relatively high strength. The corrosion resistance characterization of the porous alumina samples in hot 10 wt% NaOH and 20 wt% H₂SO₄ solutions was also investigated by considering sample formulations with 5–15 wt% PFA addition. With increasing porosity, the mass loss range in RH and SCB shaped samples after corrosion in NaOH solution for 8 h were 1.25–3.6% and 0.44–2.9% respectively; on the other hand, after corrosion in H₂SO₄ solution for 8 h, the mass loss range in RH and SCB shaped samples were 0.62–1.5% and 0.68–3.3% respectively.     

Structural tuning of photoluminescence in nanoporous anodic alumina by hard anodization in oxalic and malonic acids

ABSTRACT: We report on an exhaustive and systematic study about the photoluminescent properties of nanoporous anodic alumina membranes fabricated by the one-step anodization process under hard conditions in oxalic and malonic acids. This optical property is analysed as a function of several parameters (i.e. hard anodization voltage, pore diameter, membrane thickness, annealing temperature and acid electrolyte). This analysis makes it possible to tune the photoluminescent behaviour at will simply by modifying the structural characteristics of these membranes. This structural tuning ability is of special interest in such fields as optoelectronics, in which an accurate design of the basic nanostructures (e.g. microcavities, resonators, filters, supports, etc.) yields the control over their optical properties and, thus, upon the performance of the nanodevices derived from them (biosensors, interferometers, selective filters, etc.).