Study of SU‐8 to make a Ni master‐mold: Adhesion, sidewall profile, and removal

For disposable microfluidic devices, easy and inexpensive fabrication is essential. Consequently, replication of microfluidic devices, using injection molding or hot embossing, from a master‐mold is widely used. However, the conventional master‐mold fabrication technique is unsatisfactory in terms of time and costs. In this regard, direct Ni growth (electroplating) from a back plate is promising when the photoresist is well‐defined. Here, we demonstrate the use of SU‐8 as a photoresist to define the Ni‐growth region. We accomplish this application by focusing on the adhesion, the sidewall profile, and the removal of SU‐8: the adhesion is enhanced by controlling the exposure dose, the soft‐baking time, and by choosing the adhesion‐promoting layer; the sidewall profile is regulated by selecting the intensity of each exposed wavelength, showing an aspect ratio of up to 20.9; and, easy removal is achieved by choosing a proper photoresist‐stripper. Using the master‐mold fabricated by this method, we test the mechanical stability of the features according to the aspect ratio and length; in the hot embossing process, the features are stable in the aspect ratio of up to 5.8 at a length of 200 µm. In addition, the plastic devices fabricated from this method are applied to the passive stop valves, showing a capillary pressure (−0.2 to −7.2 kPa).