Evaluation on Stress and Optical Property of Thin Films Used in Optical MEMS Device
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U5.17.1
Evaluation on Stress and Optical Property of Thin Films Used in Optical MEMS Device Lianchao Sun SOPRA INC, Westford, MA
Ping Hou∗ Nortel Networks Inc
Abstract Control of the film stress and optical property has long been considered as an issue in the tunable optical MEMS (Micro-Electro-Mechanical Systems) devices. In this paper, the atmospheric evolution of Titanium Dioxide (TiO2) and Silicon Dioxide (SiO2) thin films for the optical MEMS devices were studied. These films were prepared by ion-assisted e-beam evaporation. It is found that as-deposited SiO2 films exhibit compressive stress; whereas, it is tensile in the TiO2 films under present processing conditions. When annealed at 150 oC, both SiO2 and TiO2 films show slight changes in stress with annealing time. However, increasing the anneal temperature to 250oC caused an apparent change of film stresses with time, in which SiO2 film turns into less compressive and TiO2 film appears to be more tensile. The optical properties after annealing were also investigated by measuring the thickness and the refractive index changes using the spectroscopic ellipsometry technique. At both experimental temperatures, the film thickness increases slightly and the refractive index at 1550 nm decreases a little at the initial annealing stage for SiO2 films. For TiO2 films, it is found that the refractive index increases after annealing at 250oC. This might be caused by the TiO2 film densification process during amorphous-to-crystalline phase transformation. Because most of the significant film evolutions occur during the initial 12 hours of annealing, a practical way of stabilizing the film properties in a MEMS device is to pre-anneal the as-deposited thin films. 1. INTRODUCTION Oxides such as silicon dioxide, titanium dioxide and tantalum oxide have been widely used as the interference coatings1-8. For an optical MEMS tunable vertical surface emission laser, the wavelength is tuned through changing the cavity length between the top and the bottom mirrors, which have the TiO2/SiO2 multi-stack structures. Due to the multi-structure nature of a typical MEMS device, the stress of thin film layers should be carefully controlled so that they can function properly as well as to be compatible to their adjacent configurations inside the device. In addition, the reliability of a MEMS devoice strongly depends on the stability of its substructures. Therefore, the evaluation of as-deposited films becomes very crucial. In this paper, the effects of annealing at 150 °C and 250 °C on the stress, thickness and refractive index of asdeposited TiO2 and SiO2 thin films had been studied.
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The author is now working at SuperPower Inc. , 450 Duane Ave., Schenectady, NY 12304
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2. EXPERIMENTAL DETAIL 2.1 Film Deposition TiO2 and SiO2 films were deposited by the technique of ion-assisted e-beam evaporation. Substrates used for the experiments were 2-inch (100) silicon wafers. To minimize the temperature gradient during the evaporation process, the vacuum chamber was preheated to 1
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