Effect of Atomic Layer Etching on Residual Stress of Al 2 o 3 Ald Ultra-Thin Film Suspended Structures

This paper presents the first study of the effect of thermal atomic layer etching (ALE) on the residual stress in suspended atomic layer deposition (ALD) structures. Optical curvature measurements on silicon wafers with varying thicknesses of Al 2 O 3 ALD on polyimide were collected ex situ to obtain the residual stress evolution of the film. Brownian motion of micromachined resonators fabricated with an ALD-on-polyimide process [1] was measured in a range of 1-10 MHz using an optical interferometer before and after ALE processing. Experimental resonant frequencies were compared with FEM simulations to validate residual stress in suspended structures. Results show good agreements between the residual stress obtained using resonant frequency and wafer curvature measurements. Furthermore, suspended beams with an initial thickness of 30 nm were thinned down to 10 nm using ALE. TEM imaging was used to confirm beam thickness after etching. Experimental data with FEM validation shows that ALE can be used to remove highly stressed nucleation and coalescence regions in free-standing structures.