Aligned Nano-porous Alumina with High-Strength and High-Thermal Conductivity Prepared by Electrochemical Process

Abstract

Anodization technology is one of the surface treatment technologies that have been developed to prevent corrosion of aluminum surfaces and improve mechanical strength. Anodic aluminum oxide(AAO) is a ceramic material directly synthesized on the surface of an aluminum metal using the anodized technology. Due to the regular nanopore characteristics of the surface, AAO is used in a wide range of fields such as semiconductors, batteries, and bio-industry. However, since the existing AAO was applied as an amorphous material, hardness, thermal conductivity, and chemical stability were lacking compared to crystalline materials. Therefore, this paper suggests a synthesis method of AAO with improved hardness, thermal conductivity, and chemical stability through a heat treatment process. Then, the possibility of the synthesized new crystalline AAO as a next generation semiconductor post-process interposer material. First, the effect of anodization temperature on the chemical composition, mechanical strength, dielectric constant and thermal conductivity of AAO was investigated. The invasion of impurities such as oxalate ion and hydroxyl group was relatively low at 5℃-anodization, with a low porosity of 35% compared with 20℃-anodization. And, about 2 times, 13% and 18% higher for Vickers hardness, dielectric constant and thermal conductivity, respectively. Next, the cause of stress generated in the material during the annealing process was analyzed through COMSOL simulation, and a heat treatment method for synthesizing crystalline AAO was introduced. In simulation part, the stress concentration in the heat treatment process was investigated according to the pore characteristics of front and back sides of AAO. About simulation data, a structure capable of reducing the compressive and tensile strength from 506,637 MPa to 10 and 3 MPa, respectively. Then, pore-widening process conditions for synthesizing non-crack AAO were optimized. Thereafter, the characteristics of the synthesized crystalline AAO were studied. Specifically, 850 and 1300℃-AAO were investigated. Information on the crystal phase of the material was investigated through crystallographic analysis according to the heat treatment temperature. Based on these results, the chemical composition was studied, and the impurity removal and phase change mechanism inside the material were identified. After that, the mechanical, electrical, and thermal characteristics of the AAO template according to the heat treatment temperature were analyzed by Vickers hardness, dielectric constant, and thermal conductivity, respectively. As a result, compared to amorphous-AAO, alpha-AAO increased hardness and thermal conductivity by 3.3 and 6.7 times, respectively, and the dielectric constant decreased by about 50%. In addition, the chemical and electrochemical corrosion behavior of AAO was investigated through weight measurement and electrochemical impedance spectroscopy(EIS). Finally, and additional mechanical mechanical strength improvement method through ALD process was studied. The correlation between the change in porosity of the AAO surface and the mechanical strength according to the number of ALD cycles was investigated. For alpha-AAO and ALD 500 cycles-AAO, the mechanical strength in the fracture strength, micro-region, and macroscopic region was improved by 27%, 100%, and 30%, respectively.