Symmetrical Antidirectional Metallization for Stress Compensation of Transfer-Bonded Monocrystalline Silicon Membranes

This paper demonstrates a very robust and fabrication-parameter insensitive concept of full stress compensation in metallized monocrystalline silicon membranes by symmetrical antidirectional metal deposition on both sides of a transfer-bonded silicon membrane. This concept results in previously unmatched near-perfectly flat, temperature-compensated, and high-reliability metal-coated membranes, independent on the thickness, residual stress, and material of the metal layers. Application examples are high-performance optical mirror devices and quasi-optical tunable microwave surfaces, the latter being presented in this paper. The influence of the thickness ratio of the metal films on the membrane curvature is investigated, demonstrating a controllable curvature range from -0.3 to 0.1 mm^-1 for the investigated devices by varying the top-to-bottom metal thickness ratio from 0.38 to 3.5 using metal thicknesses from 200 to 800 nm. Near-zero curvature down to 0.004 mm^-1 is also demonstrated. Theoretical analysis of the stress-compensated multilayer structures and characterization results of fabricated test devices are included in this paper, as well as the influence of unsymmetrically etched structures in the two metallization layers on the stress-induced curvature. Reliability tests up to 100 million cycles showed no detectable change in curvature or plastic deformation, proving the robustness and repeatability of this new design concept of zero-curvature temperature-compensated monocrystalline silicon-core membranes with thick metal coating.