[paper] MEMS thermal actuators

Longchang Ni, Ryan M. Pocratsky and Maarten P. de Boer 

Demonstration of tantalum as a structural material for MEMS thermal actuators 

Microsyst Nanoeng 7, 6 (2021) 

DOI: 10.1038/s41378-020-00232-z 

CMU Mechanical Engineering Dept., Pittsburgh, PA, USA

Abstract: This work demonstrates the processing, modeling, and characterization of nanocrystalline refractory metal tantalum (Ta) as a new structural material for microelectromechanical system (MEMS) thermal actuators (TAs). Nanocrystalline Ta films have a coefficient of thermal expansion (CTE) and Young’s modulus comparable to bulk Ta but an approximately ten times greater yield strength. The mechanical properties and grain size remain stable after annealing at temperatures as high as 1000 °C. Ta has a high melting temperature (Tm = 3017 °C) and a low resistivity (ρ = 20 µΩ cm). Compared to TAs made from the dominant MEMS material, polycrystalline silicon (polysilicon, Tm = 1414 °C, ρ = 2000 µΩ cm), Ta TAs theoretically require less than half the power input for the same force and displacement, and their temperature change is half that of polysilicon. Ta TAs operate at a voltage 16 times lower than that of other TAs, making them compatible with complementary metal oxide semiconductors (CMOS). We select α-phase Ta and etch 2.5-μm-thick sputter-deposited films with a 1 μm width while maintaining a vertical sidewall profile to ensure in-plane movement of TA legs. This is 25 times thicker than the thickest reactive-ion-etched α-Ta reported in the technical literature. Residual stress sensitivities to sputter parameters and to hydrogen incorporation are investigated and controlled. Subsequently, a V-shaped TA is fabricated and tested in air. Both conventional actuation by Joule heating and passive self-actuation are as predicted by models.

Fig: Top view of freestanding Ta thermal actuator. In-plane deflection δ ≈ 5µm after hydrogen degas step

Acknowledgements: This work was partially supported by the US National Science Foundation (NSF) grant number CMMI-1635332. We also acknowledge the Kavcic-Moura Endowment Fund for the support. We would like to thank the executive manager, Matthew Moneck, and all the staff members of the CMU Eden Hall Foundation Cleanroom for their guidance and advice on equipment usage and process development. We also acknowledge the use of the Materials Characterization Facility at Carnegie Mellon University under grant # MCF-677785