Developing Ni–Al and Ru–Al intermetallic films for use in microelectromechanical systems

Abstract

Ordered intermetallic films have a favorable combination of properties such as high strength, metallic electrical conductivity, good oxidation and corrosion resistance, and a high melting temperature and thermal stability that make them suitable for microelectromechanical systems (MEMS). One potential drawback to intermetallics is a lack of ductility at room temperature; however, the B2 compounds NiAl and RuAl show some ductility at room temperature, which has been shown to increase as the grain size decreases. Additionally, the fracture toughness of both materials is higher than those of Si and SiGe. It is also possible to deposit these materials at temperatures that make them compatible with complementary metal oxide semiconductor processing. The authors have shown that by controlling the Ar pressure during cosputtering, NiAl and RuAl thin films can be deposited near room temperature with stresses ranging from compressive to tensile, possibly eliminating the need for annealing. This article examines Ni–Al and Ru–Al intermetallic thin films with an overall composition of 50% Al and 50% Ni (or Ru) deposited by cosputtering or the annealing of multilayer stacks for use in MEMS by first establishing an appropriate combination of etchant/sacrificial layer that results in crack-free, freestanding, fixed-fixed beams. Finally, two different MEMS devices were successfully fabricated using a XeF2 etch and actuated: electrostatically actuated resonators and bent beam thermal actuators.