Gas phase hydrophobisation of MEMS silicon structures with self-assembling monolayers for avoiding in-use sticking

During MEMS fabrication several techniques like dry etching with HF-gas, sublimation or supercritical drying, etc., for avoiding the liquid/air phase transition below released structures are employed. However, these procedures do not prevent them from sticking during exposition in humid environments.For minimizing chemical and time consumption, a gas phase and industry-suited batch process has been developed for hydrophobisation instead of coating in liquid solutions. In this two step procedure the silicon-contenting surfaces are terminated with OH-groups in water plasma or in a heavy oxidizing liquid solution containing H2O2. Afterwards a saturated atmosphere consisting of silane molecules (FDTS, FOTS) is created where they form a self-assembling monolayer on the substrate with a low energy surface.The SAM's have been characterized by contact angle (θ) measurements, where also surface energies (γ) have been determined. Here FDTS and FOTS produced best results with θ = 115.5°, γ = 11.0 mJ/m2 and θ = 113.3°, γ = 12.4 mJ/m2 on mono-Si, respectively. A batch coating of 13 samples produced comparable results.Moreover, thermal stabilities have been analyzed. Here also FDTS and FOTS are the best choice since they are stable up to 400–450 °C, which is suited for common MEMS packaging processes. XPS measurements showed at most very weak Cl signals indicating that the coating reaction was complete. In addition beam arrays made of different materials have been hydrophobized, resulting in a dramatic improvement of anti-sticking qualities. Even a dip into water did not cause any sticking.