Nanometer-scale flatness and reliability investigation of stress-compensated symmetrically-metallized monocrystalline-silicon multi-layer membranes

This paper demonstrates a very robust and fabrication-parameter insensitive concept of full stress compensation in metallized monocrystalline silicon membranes, by symmetrical metal deposition on both sides of a transfer-bonded silicon membrane, resulting in previously unmatched near-perfectly flat and high-reliability metal-coated membranes. Application examples are high-performance optical mirror devices and quasi-optical tuneable microwave surfaces. The influence of the thickness ratio between the metal films on the two membrane sides are investigated, demonstrating a controllable curvature range from -0.3 mm^-1 to 0.1 mm^-1 by varying the top to bottom metal thickness ratio from 0.38 to 3.5, using metal thicknesses from 200 nm to 800 nm, and achieving near-zero curvature down to 0.004 mm^-1. Extensive reliability tests, up to 100 million cycles, showed no detectable change in curvature, no plastic deformation and good repeatability in analog-mode deflection (within 2.5 %), proving the robustness of this concept of metallized monocrystalline membranes.