Optimization of AC Electrochemical Etching for Fabricating Tungsten Nanotips with Controlled Tip Profile

AC electrochemical etching in diluted potassium hydroxide (KOH) solution was optimized to fabricate tungsten (W) nanotips with a controllable sharpness and aspect ratio using an additional lift-up step. The final tip profile was dependent on the extent of interaction between the KOH solution and the side of the W surface, and effective bubble shielding effects near the apex region during the lift-up. Lateral etching rate along the W material was affected by parameters such as electrolyte-cathode positioning, etching voltage, and electrode size that influenced the flow or replenishment rate of OH- ions to the W surface submerged in the solution and at the meniscus region. With the lift-up step, the dense layer of bubbles that formed during etching could provide a good shield in minimizing the etch-back effects on the tip apex. Combining the above investigated effects, sharp nanotips with the required aspect ratio could be achieved with the enhanced lateral etching and the protective shield of bubbles.