Characterization of low temperature deposited atomic layer deposition TiO2 for MEMS applications

Abstract

TiO2 is an interesting and promising material for micro-/nanoelectromechanical systems (MEMS/NEMS). For high performance and reliable MEMS/NEMS, optimization of the optical characteristics, mechanical stress, and especially surface smoothness of TiO2 is required. To overcome the roughness issue of the TiO2 films due to crystallization during deposition at high temperatures (above 250 °C), low temperature (80–120 °C) atomic layer deposition (ALD) is investigated. By lowering the deposition temperature, the surface roughness significantly decreases from 3.64 nm for the 300 °C deposited crystalline (anatase phase) TiO2 to 0.24 nm for the 120 °C amorphous TiO2. However, the layers deposited at low temperature present different physical behaviors comparing to the high temperature ones. The refractive index drops from 2.499 to 2.304 (at 633 nm) and the stress sharply decreases from 684 to 133 MPa. Superhydrophilic surface is obtained for the high temperature deposited TiO2 under ultraviolet illumination, while little changes are found for the low temperature TiO2. The authors demonstrate that by suitable postdeposition annealing, all the properties of the low temperature deposited films recover to that of the 300 °C deposited TiO2, while the smooth surface profile (less than 1 nm roughness) is maintained. Finally, micromachining of the low temperature ALD TiO2 by dry etching is also studied.