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Electrochemical Nanolithography on Amorphous WO3 Thin Films Using Scanning Tunneling Microscope in Air

Tungsten oxide (WO3) thin films have shown interesting properties as lithography resist materials. In this study, the scanning tunneling microscope (STM) was used in air for nanometer etching of α-WO3-x thin films, an n-type semiconductor. The current–voltage (I–V) curve was found to be affected by the water layer absorbed on the surface. For low voltage (< 3 V) with long duration (∼ 10 s) or high voltage pulse (> 3 V) with short pulsewidth (∼ 200 ms), holes were formed on the film surface at high humidity (> 70%) by applying a pulse voltage. A threshold voltage existed for hole formation. Higher pulse voltage and negative polarity corresponded to larger modified size. All the structures formed in STM images were topographical in nature by comparison with the AFM images. The hole formation was reasonably attributed to electrochemistry and high dissolution of WO3 in high pH solutions, which was co-manifested by links between the holes and eroded materials on the surface. Alkaline solutions instead of deionized water were chosen to act in the tip-surface gap. Alkaline ions being expelled from the tip due to electric polarity demonstrated the ion movement and their effect on etching. Lines of nanometer width were fabricated.