Compression and decompression of structural tantalum films exposed to buffered hydrofluoric acid

Abstract

As a refractory metal with a high melting temperature and large coefficient of thermal expansion, tantalum (Ta) is of great interest as a structural material in MEMS. One of the major issues with any freestanding structure, however, is the control of residual stress. Here, we observe that suspended fixed-fixed beams made from Ta films under tension buckle after a buffered hydrofluoric (BHF) acid release step, implying a large change of stress towards compression. We find that the change in uniaxial stress is proportional to BHF exposure time and reaches −1 GPa after 150 min. Although there are many sources of residual stress in metal thin films, we demonstrate definitively that hydrogen (H) injection due to BHF is the major cause of this change. We show further that the residual stress can be largely recovered by degassing at 500 • C in a high vacuum environment.