Assessment Of Annealing Treatment For Wrinckle-Less Sio₂ Membrane

Large-aperture (larger than 100 × 100 μm 2 ) membrane structure is one of the essential structures for MEMS and optical devices. We have fabricated 400nm-thick, 400-to700 K 700 μm 2 free-standing SiO 2 membranes by Deep Reactive Ion Etching and optional wet etching. We observed that the CVD-deposited membrane wrinkled according to the elapsed time after deposition; Wrinkling did not occur SiO 2 on Si substrates left for 1 month in a standard cleanroom environment (23 °C, 20-40% relative humidity), but did occur on those left for 2 months. The deflection was as large as 12 μm for 420 μm of initial length, which is equivalent to over 1600 micro strain (0.16%) of elongation. Annealing at high temperature was confirmed to be effective; 400 °C 30min annealing with nitrogen put the membrane back to flat; however the membrane reversibly wrinkled again in a couple of days. These phenomena confirm that CVD SiO 2 does absorb the humidity and thereby elongates considerably. Accordantly, the decrease of initial tensile stress of the SiO 2 thin film on Si substrate was confirmed. Higher temperature annealing such as at 600 and 800 °C were also confirmed to be more efficient and resilient. Long-term (over 3 month) observation has been effectuated and is shared in the conference.