Photoresist application for the LIGA process

Abstract

Fabrication of high aspect ratio structures requires the use of a photoresist able to form a mold with vertical sidewalls. Thus the photoresist should have a high selectivity between the exposed and the unexposed area in the developer. It should be relatively free from stress when applied in thick layers necessary to make high aspect ratio structures. PMMA (Poly Methyl Methacrylate) is the photoresist of choice in the LIGA process, mainly for its ability to hold vertical sidewalls for tall structures. It is applied to the substrate by a glue-down process in which a pre-cast, high molecular weight, sheet of PMMA is attached to the plating base on a substrate. The applied photoresist is then milled down to the precise height by a fly-cutter prior to pattern transfer by x-ray exposure. The requirement that the applied layer be relatively free from stress dictates the choice of glue-down over casting. The substrate preparation steps, as well as the conditioning of the PMMA sheet prior to the glue-down, are done, in part, to reduce the stress in the glued down sheet of photoresist. The cutting of the PMMA sheet in the fly-cutter requires specific operating conditions as well as particular cutting tools to avoid introducing any stress and the resultant crazing of the photoresist.

Fundamentals of Microfluidics Devices

“…Application to the substrate is achieved by spin coating or bonding of preformed sheets. 135 Formation of microfluidic structures FIGURE 1.8 Fabrication of microfluidic networks in PDMS elastomer based on the soft lithographic process. In some cases, the peeled PDMS template obtained is used as a master mold for further processing steps to obtain the final microfluidic.…”

Resist substrate studies for LIGA microfabrication with application to a new anodized aluminum substrate

“…In these studies, annealed PMMA sheet was bonded to the substrates using either solvent [32] or a PMMA-based glue [20,33] developed at Forschungszentrum Karlsruhe (FZK). For solvent bonding, a 2 µm layer of PMMA (950 kg mol −1 ) was first applied to the substrate by spin casting.…”