Low-cost and high-performance micro-fabrication method using low numerical-aperture optical projection lithography on copper-clad plastic boards

Combination of optical lithography patterning in the case of a thickly coated resist and electroplating using the resist replica is investigated for developing a new easy, low-cost and high-performance micro-fabrication method. In order to form patterns with a high aspect ratio and vertical side walls, low numerical aperture (NA) projection exposure is applied, and acrylic resist SU-8 is used due to its high transparency for the exposure light of near UV and visible light from violet to blue. When a lens with a low NA of 0.063 is used, a large depth-of-focus (DOF) of more than 200 mum is obtained for a 50-mum-wide and 100-mum-thick resist. If the focal position is adjusted suitably, 275-mum-thick patterns with vertical side walls and a high aspect ratio of more than 10 are obtained. These resist patterns are printed on copper clad plastic boards instead of silicon wafers. The copper-clad boards are directly available for electroplating without evaporating base metal layers on them. Thus, fabrication of micro nickel gears is demonstrated successfully. Since the new method uses low-cost optical lithography, it is applicable even for small production lots, and will be useful for fabricating various micro-parts.