Formation of Porous Aluminum Films with Isolated Columnar Structure Using Physical Vapor Deposition for Medium-Voltage and High-voltage Capacitors

Abstract

167 substrate with cellular texture were employed to develop aluminum films with an isolated columnar structure. The surface roughness has been evaluated by voltage-time response during anodizing of the deposited films at a constant current density and by capacitance measurements of the anodic films formed. Slight alkaline etching of the deposited films has also been carried out to assist the enhancement of the surface roughness. 2. Experimental Aluminum films were prepared by magnetron sputtering using a 99.999% purity aluminum target of 100 mm in diameter. The chamber was first evacuated to less than 5×10-5 Pa and then high purity Ar gas was introduced to the chamber. During the deposition at a constant current of 0.5 A, the Ar pressure was maintained at 3.3 Pa or 0.3 Pa. The deposition time was 3.6 ks. The films were deposited on flat and textured aluminum substrates. The flat substrate was prepared by electropolishing 99.99% purity aluminum sheet at 20 V in a mixture of ethanol and 60 mass% perchloric acid (80:20 volume ratio) below 283 K for 300 s. The textured aluminum substrate was prepared by anodizing aluminum sheet in 0.1 mol dm-3 phosphoric acid at a constant current density of 100 A m-2 for 3.6 Ks, and then, immersing the anodized specimen in a mixture of chromic acid and phosphoric acid at 343 K to dissolve the anodic film developed. The resultant surface has a cellular morphology as shown in Fig. 2. The cell size was 300-400 nm. Chemical etching of the deposited films was carried out in 0.1 mol dm-3 KOH solution at 293 K to increase the surface area further. The as-deposited and chemically etched specimens were observed by a JEOL JSM-6500F field emission scanning electron microscope (SEM). Cross-sections of the specimens were prepared using an ultramicrotomy technique. The increased surface areas of the as-deposited and chemically etched specimens were evaluated by voltage-time response of the specimens during anodizing at a constant current density of 10 A m-2 in 0.1 mol dm-3 ammonium pentaborate electrolyte at 293 K. The current density with respect to the geometrical surface area was applied during anodizing. Thus, the rate of voltage increase was reduced as the surface area increased. In addition, capacitances of the specimens were measured by AC impedance technique in 0.1 mol dm-3 ammonium pentaborate electrolyte at 293 K after anod-izing at several voltages in the same electrolyte. 3. Results and Discussion Fig. 3 shows the surfaces and cross-sections of the aluminum films magnetron-sputtered under three different conditions. The surface observations reveal that all the films have a grain size of approximately 1 μm. The largest surface roughness is obtained on the textured aluminum substrate at an Ar pressure of 3.3 Pa, as seen from the cross-sectional images. In addition, a number of pores are present between columns of the deposited aluminum film under the condition, while almost no pores are visible in the film deposited on the flat substrate. Even on the textured sub-strate, it is obvious from the cross-sectional images that high Ar pressure is needed to increase the porosity of the deposited film. The findings are in agreement with our expectation. At low Ar pressure of 0.3 Pa, incident angle of aluminum atoms to the substrate should be nearly normal to the substrate surface, since the mean free path of aluminum atoms, assuming the substrate temperature of 373 K, is 41.5 mm, which is almost the same as the distance between the target and substrate (~50 mm). In con-Fig. 2 Scanning electron micrograph of the surface of the textured aluminum substrate. Fig. 3 Scanning electron micrographs of surfaces and cross-sections of the aluminum films magnetron-sputtered under three different conditions: at an Ar pressure of 3.3 Pa on flat surface and at Ar pressures of 0.3 and 3.3 Pa on textured substrate.