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“reverse Engineering” Of Porous Anodic Alumina — Investigations On Etching

Posted on:2016-09-07Degree:DoctorType:Dissertation
Country:ChinaCandidate:J F LiaoFull Text:PDF
GTID:1108330503453332Subject:Microelectronics and Solid State Electronics
Abstract/Summary:PDF Full Text Request
Anodization is the process of passivation and formation of oxide films on the metal surface through electrochemical method. The investigation on the anodization behavior of aluminum has lasted for more than a century. Since the earlier years, the anodization of aluminum has been utilized widely in the industrial fields such as electrolytic capacitor, protection and decoration of metal surface. In the recent two decades, due to the discovery of the prominent self-organization behavior of porous anodic alumina(PAA), along with its simple fabrication process, high controllability and low cost, PAA gradually became one of the most promising templates for nano-fabrication. Today, PAA has been widely used for fabrication of low-dimensional materials and related devices, and plays important roles in many emerging fields such as biology, new energy, sensors, memories for information, etc. In fundamental research area, on the other hand, there remain many open questions and controversies concerning the formation mechanisms of PAA up to present.Etching is not only a common process in the preparation of PAA templates, but can be also an effective method for mechanism investigation. In this dissertation, PAAs formed in phosphoric acid were etched from the front and bottom sides, respectively, by both wet-chemical method and ion-beam milling. The etching processes were analyzed systematically by electrochemical measurements and scanning electron microscope(SEM) observation, and the revealed information on the growth of PAA was investigated in depth.In the study of the etching process from the front side, the conventional pore-filling method exhibits some evident disadvantages. Pulse polarization method utilizing pulse signal can overcome such problems. On the other hand, pulse polarization can be also an effective method for the investigations of the transient and steady-state high-field ionic conduction processes. As for the present experiment, the variations of polarization voltage and barrier layer thickness of PAA with etching time were obtained by pulse polarization and capacitance measurements as well as SEM characterization. The electric field distribution within the bilayered barrier layer during anodization was estimated to be approximately uniform. By comparison with the electric field within the AlOOH/Al(OH)3 layers formed by hydrothermal treatment, as well as the gel layers formed on the surface of the barrier films anodized in some specific electrolytes, it was confirmed that the bilayer structure of PAA was formed mainly by solid-state processes, while colloidal chemical processes did not play significant roles as suggested by some other researchers. The dissolution rates of both sublayers of PAA were decreased by annealing, while they can be essentially recovered by applied high field after annealing. Both processes can be explained in terms of the reversible change in the microstructure of PAA.During the etching processes of the barrier layer of PAA from the bottom side by both wet-chemical method and ion-beam milling, prominent and consistent azimuthal etching anisotropy was revealed within the cells. The regions around the cell junctions exhibited higher etching rate and depressions appeared; while the regions near the midpoints of the cell boundaries exhibited slower etching rate and ridges were gradually formed. Under the conditions of sufficiently high etching contrast, larger pore diameter and thinner barrier layer, at the depression sites around the center of the cell, symmetric small holes can be clearly observed when the barrier layer was just beached. The widening and merging of the small holes and the collapse of the central residual dome lead to the appearance of the primary circular opening. The new pore-opening mechanism can explain the phenomenon revealed by previous research that most of the initial openings were irregular. The pore diameter dependence of the barrier layer opening diameter and its variation with the etching time were revealed for the first time. The anisotropy of the etching of the barrier layer from the bottom side can be hardly explained in terms of variations in neither chemical composition, surface curvature nor the diffusion rate of etchant, and most probably arises from the anisotropic residual stress distribution. The compressive stress should be largest near the center of the cell; while along the cell boundaries, the minimum and maximum of the compressive stress should be present at the cell junctions and near the midpoints of the cell boundaries, respectively. Such stress distribution could be inherent in the cell with the specific geometry, and can be explained in terms of stress superposition originated from the interaction between neighboring cells and non-uniform stress relief due to variation in material flow rate during PAA growth. The pattern formed by etching revealed the relations between the stress distribution and cell arrangement. Driven directly by such anisotropic stress, the movements of the cell boundaries can lead to the self-organization behavior of PAA, and the driving force is positively correlated with current density.Based on the controlled etching of PAA combained with growth conditions control as well as annealing, PAA templates with specific structures can be fabricated. After the second anodization for proper duration, through chemical etching(pore-widening) for relatively long period and subsequent anodization for very short period, funnel-shaped PAA with an upper layer of large thickness and pore diameter and a lower layer of small thickness and pore diameter can be fabricated. Such structure can be used as masks for etching and deposition in place of ultrathin PAA, so that the strict requirements on the operation during template fabrication and application will be greatly lowered. After the second anodization, the PAA was annealed at 500 °C, and then re-anodized to recover the ionic conductivity of the barrier layer, and finally etched for relatively long period, so that dropper-shaped PAA including a pore-base segment with larger pore diameter and the rest with smaller pore diameter can be formed. Such PAA could have potential application in drug delivery, and could be helpful to the barrier layer thinning and PAA-substrate detachment processes. The junctions of significantly changing pore diameter inside the dropper-shaped PAA also exhibit prominent etching anisotropy. This phenomenon could reveal the anisotropic distribution of electric field and space charge within the cell of the barrier layer, and their rearrangement during annealing and re-anodization. By annealing in place of hydrothermal treatment to pre-crystallize the through-hole PAA, and then etching for sufficiently long period, the regular and complete inner skeleton of PAA can be also obtained. The refined method can simplify the fabrication process, improve stability and reproducibility, and also possesses wider application. In the fabrication of PAAs with special structures by chemical etching, the capacitance method can be used in general to monitor the etching process in situ under various conditions. Such method is simple, facile, effective and helpful to speed up the research.
Keywords/Search Tags:Porous anodic alumina, Etching, Anisotropy, Electric field, Stress
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