A Simple and Robust Fabrication Process for SU-8 In-Plane MEMS Structures

Abstract

In this paper, a simple fabrication process for SU-8 in-plane micro electro-mechanical systems (MEMS) structures, called "border-bulk micromachining", is introduced. It aims to enhance the potential of SU-8 MEMS structures for applications such as low-cost/disposable microsystems and wearable MEMS. The fabrication process is robust and uses only four processing steps to fabricate SU-8 in-plane MEMS structures, simplifying the fabrication flow in comparison with other reported attempts. The whole fabrication process has been implemented on copper-polyimide composites. A new processing method enables the direct, laser-based micromachining of polyimide in a practical way, bringing in extra processing safety and simplicity. After forming the polymeric in-plane MEMS structures through SU-8 lithography, a copper wet etching masked by the SU-8 structure layers is carried out. After the wet etching, fabricated in-plane MEMS structures are suspended within an open window on the substrate, similar to the final status of in-plane MEMS devices made from industrial silicon micromachining methods (such as SOIMUMPS). The last step of the fabrication flow is a magnetron sputtering of aluminum. The border-bulk micromachining process has been experimentally evaluated through the fabrication and the characterization of simple in-plane electrically actuated MEMS test structures. The characterization results of these simple test structures have verified the following process qualities: controllability, reproducibility, predictability and general robustness.Micromachines 2020, 11, 317 2 of 16 process to form microstructures, with its simplicity leading to a low fabrication cost, while SU-8 has better mechanical properties and chemical stability, in comparison with other photoresists [3,4]. For the fabrication of both out-of-plane and in-plane SU-8 electrostatic MEMS structures, the most commonly used fabrication flow is layer-by-layer surface micromachining [5][6][7][8][9][10][11][12]. The generic schematic of a layer-by-layer SU-8 micromachining process is illustrated in Figure 1.Micromachines 2020, 11, x 2 of 17The fabrication cost, speed and process simplicity are vital to polymeric MEMS structures and their applications. This explains why SU-8 series negative photoresist has been advantageous and popular as structural layers for polymeric MEMS devices. Such processes require only a single lithography process to form microstructures, with its simplicity leading to a low fabrication cost, while SU-8 has better mechanical properties and chemical stability, in comparison with other photoresists [3,4]. For the fabrication of both out-of-plane and in-plane SU-8 electrostatic MEMS structures, the most commonly used fabrication flow is layer-by-layer surface micromachining [5][6][7][8][9][10][11][12]. The generic schematic of a layer-by-layer SU-8 micromachining process is illustrated in Figure 1.