Assessment Of Annealing Treatment For Wrinckle-Less Sio2 Membrane

Large-aperture (larger than $100\times 100\mu \mathrm{m}^{2}$) membrane structure is one of the essential structures for MEMS and optical devices. We have fabricated 400nm-thick, 400-to-$700\times 700\mu \mathrm{m}^{2}$ free-standing $\mathrm{S}\mathrm{i}\mathrm{O}_{2}$ 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 $\mathrm{S}\mathrm{i}\mathrm{O}_{2}$ on Si substrates left for 1 month in a standard cleanroom environment ($23^{\circ}\mathrm{C}$, 20-40% relative humidity), but did occur on those left for 2 months. The deflection was as large as 12 $\mu \mathrm{m}$ for 420 $\mu \mathrm{m}$ 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 $\mathrm{S}\mathrm{i}\mathrm{O}_{2}$ does absorb the humidity and thereby elongates considerably. Accordantly, the decrease of initial tensile stress of the $\mathrm{S}\mathrm{i}\mathrm{O}_{2}$ 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.