Argon plasma modification of SU-8 for very high aspect ratio and dense copper electroforming

This paper presents a simple but effective method of increasing the wettability of hydrophobic SU-8 grating for use as a copper electroforming template. Untreated SU-8 is hydrophobic; unpatterned SU-8 has a water contact angle of 73.1 ±2.8° and surface energy of 45.5 ±0.3 mJ/m ². Argon plasma treatment oxidizes and roughens the surface leading to increased surface energy. For treatment times of 60 s or more with plasma power of 200 W, the surface energy, roughness, and degree of oxidation plateaus; the water contact angle decreases to 29.3 ±0.2°; and the true surface energy increases to 54.7 ±0.3 mJ/m² after 180 s Ar-plasma treatment. Copper electrolyte cannot fill the very high aspect ratio and spatially dense microchannels in untreated SU-8 grating (with water contact angle of 124 ±4.6°) so that the electroforming circuit remains open. Using a 180 s Ar-plasma treatment, the SU-8 grating becomes hydrophilic (with water contact angle of 7.9 ±1.4°); this enables a dense copper grating with alternate microwalls and microchannels of aspect ratios of 15.3 and 9.2, respectively, over the entire 100-mm-diam area to be successfully fabricated from it. This copper mold was used as a mold for UV embossing, demonstrating the feasibility of this technique for the creation of durable molds for mass replication.