SEM images of bottom Al₂O₃ template film. (a) A compact Al₂O₃ barrier layer cover on Al₂O₃ template bottom, (b) dissolving barrier by 5% H₃PO₄ solution at 25°C for 10 min during which grain boundaries were etched, (c) 30 min during which partial barrier layer was dissolved, (d) 50 min during which barrier layer has become thin film, (e) 70 min during which most of barrier layer was removed, and (f) 90 min during which barrier layer was removed, completely.