Arbitrarily profiled 3D polymer MEMS through Si micro-moulding and bulk micromachining

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A technique is described for the fabrication of 3D polymeric features with arbitrary sidewall angle using Si micromoulding and bulk micromachining. Si substrates are machined using deep reactive ion etching (DRIE) to form mould inserts with overhanging profiles. Despite the presence of overhang, complete filling with polymer is observed following spin-coating. Release of moulded parts is achieved via destruction of the mould using either wet or dry bulk micromachining processes. Photopolymer epoxy SU-8 was used in this study due to its wide applicability in MEMS and because it is chemically stable during Si bulk micromachining processes. Successful mould release via wet etching requires a suitable etch masking layer at the Si/SU-8 interface to prevent peeling due to hydrogen bubble formation during Si etching. Surface roughness of moulded features with similar geometries is found to lie in the range 68–88 nm. Oxide machining using a 300 nm thermal oxide can reduce mould insert surface roughness by 33%. An extended fabrication process is provided in which moulded features face upwards after the final step. This involves transferral to a carrier wafer and bonding using two-part epoxy. If the Si mould is removed via wet etching, delamination between adhesive and carrier wafer can result unless mechanical interlocking is present at the interface. The technique described is advantageous compared to other means of forming arbitrary 3D polymeric structures as it relies on the use of standard MEMS processes and can be modified to meet the needs of a range of polymer MEMS applications.