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.
Highlights
Micro electro-mechanical systems (MEMS) consist of sensors and actuators transferring information and energy between the electrical and mechanical domains
The copper etchant propagates in a gap defined by the bottom surface of the SU-8 layer and the top surface of the polyimide layer, which is similar to the liquid propagation within microfluidic systems [30]
Since the contact angle of water is 90◦ on SU-8 [31] and 80◦ on polyimide [32], the hydrophobic behavior will act towards stopping the advance of the etchant [30], limiting the depth of the overhang
Summary
Micro electro-mechanical systems (MEMS) consist of sensors and actuators transferring information and energy between the electrical and mechanical domains. Capacitive coupling is one of the major coupling principles for both sensing and actuation of MEMS devices that serve as vital front-end devices for many modern microsystems. Based on their mechanical degree-of-freedom with reference to the substrate plane, MEMS devices can be categorized into out-of-plane and in-plane devices. As an example of this perspective, high performance polymeric capacitive micromachined ultrasound transducer arrays (CMUT) have been fabricated and validated for their use in ultrasound imaging [2]. The performance of the polymeric CMUT has been comparable with classic ultrasound transducers, but the fabrication complexity and cost has been significantly lower
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