Electrochemical nanolithography on amorphous WO3 thin films using scanningtunneling microscope in air

Tungsten oxide (WO3) thin films have shown interesting properties as lithog raphy resist materials. In this study, the scanning tunneling microscope (S TM) was used in air for nanometer etching of alpha -WO3-x thin films, an n- type semiconductor. The current-voltage (I-V) curve was found to be affecte d by the water layer absorbed on the surface. For low voltage (< 3 V) with long duration (similar to 10 s) or high voltage pulse (> 3 V) with short pu lsewidth (similar to 200 ins), holes were formed on the film surface at hig h humidity (> 70%) by applying a pulse voltage, A threshold voltage existed for hole formation. Higher pulse voltage and negative polarity corresponde d to larger modified size. All the structures formed in STM images were top ographical in nature by comparison with the AFM images. The hole formation was reasonably attributed to electrochemistry and high dissolution of WO3 i n high pH solutions, which was co-manifested by links between the holes and eroded materials on the surface. Alkaline solutions instead of deionized w ater were chosen to act in the tip-surface gap. Alkaline ions being expelle d from the tip due to electric polarity demonstrated the ion movement and t heir effect on etching. Lines of nanometer width were fabricated.