An innovative methodology for monitoring the sacrificial layer removal process in MEMS structures

Abstract

The sacrificial layer is a key component for the fabrication of a released structure in the MEMS sensors and actuators. Wet etching is a practical microfabrication process that minimizes costs compared to dry etching. Since the sacrificial layer exists between the structural layer and the substrate, characterization of the etching process is unavailable to observe and evaluate directly. This research, for the first time, presents a methodology for monitoring sacrificial layer removal. It takes advantage of using a transparent substrate (during process development) to observe the removal process from the backside. This method can be used as a separate test during surface micromachining to monitor and optimize the release process of the MEMS device. To evaluate the efficiency of the method, the copper sacrificial layer was selected. The removal process was investigated for typical structures used in MEMS sensors and actuators including the etch-holes, the cantilever beams, comb fingers, and the pads. The experimental test showed the removal of the sacrificial layer, the non-uniformity of the etching, and all the veritable chemical reactions and phenomena under the structural layer. In addition, the etch-rate were obtained in the order of 0.35–5.5 μm min−1 for various structural features. The procedure developed in this research is an approach to the process monitoring of the sacrificial layer removal. Therefore, it can be used to organize the quality control in the released structures of MEMS and optimization in batch processing. This method can be adopted for non-metallic sacrificial layers and dry etching as well.