Characterization of XeF2 Etching for Release of Piezoelectric Micro-Robots

and Introduction: Thin-film piezoelectric micro-robots could potentially help save human lives with their ability to do reconnaissance in hazardous places, or help avoid costly demolition processes during infrastructure maintenance by utilizing images provided from inconvenient and/or unreachable locations. In order to achieve these goals, there should be an optimized fabrication process for these micro-robots using microelectromechanical systems (MEMS) technology. A critical fabrication step in MEMS technology is the safe release of these micro-robots using xenon difluoride (XeF2) etching, which etches only silicon in an isotropic manner. Due to the nature of isotropic etching, the etching results depend on the amount and location of the silicon’s open surface area. Micro-robots can also be designed with trenches such that XeF2 gas flows through the trenches and undercuts piezoelectric actuators, allowing free movement. Thus, the geometry and location of the trenches is important to successfully release micro-robots. In this work, XeF2 etching of several piezoelectric microrobot designs have been characterized, which can be used to modify the design of the micro-robots for a faster and safer release. Additionally, the etching of C4F8 polymer, which was deposited on the side wall of the trenches in deep reactive ion etching (DRIE), was characterized after the polymer was observed to protect silicon from being etched. Experimental Procedure: Characterization of Fluorocarbon Polymer. Polymer etching was tested using modules containing dummy microrobotic devices fabricated from silicon oxide and gold. Images were taken of each module using the scanning electron microscope (SEM). Then, tape was placed on the module to prevent the fluorocarbon polymer from being deposited at certain locations. Next, a fluorocarbon polymer was deposited on the module. Afterword, the tape was removed confirming the polymers presence. Subsequently images were taken using the SEM to further verify the presence of the fluorocarbon polymer. The SEM also provided the thickness of the polymer where the tap was. Later the fluorocarbon polymer was removed using an oxygen based recipe. This recipe was then confirmed to remove the fluorocarbon polymer after SEM images showed no fluorocarbon polymer on the module. Releasing Micro-Robotic Devices. XeF2 etching was completed on silicon wafer modules containing between one and fifteen micro-robotic devices of various shapes. First, the module was mounted with the backside exposed on a sixinch silicon wafer with crystal bond. Then the sample was etched with reactive ion etching (RIE), removing the oxide layer. Next the module was etched using deep reactive ion etching (DRIE) process to etch away the silicon until there was visible etching on the module. Subsequently the module was removed from the silicon wafer and the photoresist (PR) material was removed from the module. The cleaned module was mounted with the topside exposed on a silicon wafer and then placed in the STS Pegasus to remove the residual Teflon® using the oxygen-based cleaning recipe. Later, the module was detached from the silicon wafer and cleaned. Thereafter the module was mounted with the backside exposed on a silicon wafer and the DRIE process was continued until the oxide layer was visible.