Limitations on MEMS design resulting from random stress gradient variations in sputtered thin films

Abstract

Residual stress gradients often negatively affect the performance of MEMS devices, causing film curvature and changing the designed gaps of released structures. In this work, we built folded beams designed to compensate for the film curvature and keep the actuator gaps of sensitive resonant switches constant. While the average stress gradient is cancelled by our designs, we find that random variations in the stress gradient (rather than random variations in device dimensions) cause the majority of the observed variation in actuator gap. To our knowledge, this has not previously been reported, and represents an important limitation on MEMS designs using sputtered films. The standard deviation of the 400 nm contact gap for a folded beam of total length 152 µm and width 108 µm was measured to be about 134 nm. Using parameters measured from test cantilevers, our simulations predict that about 98% of the variation in contact gap is due to stress gradient variation, rather than variations in device geometry.

Compensation of the Stress Gradient in Physical Vapor Deposited Al1−xScxN Films for Microelectromechanical Systems with Low Out-of-Plane Bending

“…The stress gradient was reduced with a laser annealing technique which reduced stress from 125 MPa to 25 MPa and deflections from 118 µm to 5 µm [ 17 ]. Zhu determined the stress was caused by variations in film growth resulting in uneven grains and nonuniform boundaries [ 18 ]. When Aluminum Nitride (AlN) and Ruthenium (Ru) is sputtered to fabricate MEMS resonators, a folded beam design was used to limit deflection [ 18 ].…”

“…Zhu determined the stress was caused by variations in film growth resulting in uneven grains and nonuniform boundaries [ 18 ]. When Aluminum Nitride (AlN) and Ruthenium (Ru) is sputtered to fabricate MEMS resonators, a folded beam design was used to limit deflection [ 18 ]. Mulloni et al used similar clamped-clamped and single-clamped cantilever designs for Gold microstructure using electrodepositing [ 19 ].…”