1	EVALUATION ON STRESS AND OPTICAL PROPERTY OF THIN FILMS USED IN OPTICAL MEMS DEVICE Richard Sun Angstrom Sun Technologies Inc. Ping Hou Nortel Networks
2	"Oxides such as Silicon dioxide" Oxides such as Silicon dioxide, Titanium dioxide and Tantalum oxide have been widely used as interference coatings. In an optical MEMS structure, such as a tunable vertical surface emission laser, the wavelength is tuned through changing the cavity length between the top and bottom mirrors, which are using TiO₂ and SiO₂ multi-stacks typically. Due to the multi-structure nature of a 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 other 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 annealing effects at 150 °C and 250 °C to the TiO₂ and SiO₂ films on the film properties will be studied. In specific, film thickness, optical properties and film stress results will be reported here.
3
4	"1" 1. Film Deposition Technique: Ion-Assisted E-beam evaporation Source Material: Pre-melted Solid Block from Ti2O3 Tablets for TiO2 and SiO2 disc for SiO2 Deposition rate: 2Å/sec for TiO2 and 3 Å/sec for SiO2, controlled by quartz crystal Total chamber pressure: 2.5x10-4 torr Oxygen partial pressure: 0.6-1.0x10-4 torr Initial Chamber temperature: 150oC (monitored at near sample holder) Substrate: 2” single side polished Silicon wafer rotated at a speed of 32 rpm during deposition Annealing was conducted in an oven with temperature control precision ± 1o under atmospheric environment 2. Film Characterization Ellipsometry Measurement UV-Vis-NIR Variable Angles Spectroscopic Ellipsometer Measured wavelength range: 210nm to 2000nm Angle of incidence used: 65, 70 and 75 Degree Measurement points: 120pts at each AOI Stress Curvature was measured with FSM 128 system and stress was calculated with Stoney’s Equation
5
6
7
8
9
10	"In this paper" In this paper, the atmospheric evolution of TiO₂ and SiO₂ thin films for optical MEMS devices were studied. These films were prepared with ion-assisted e-beam evaporation. It is found that as-deposited SiO₂ films exhibit compressive stress; whereas, TiO₂ film is in tensile under present processing conditions. When annealed at 150 ^oC, both SiO₂ and TiO₂ films show little change of stress with annealing time. However, increasing the annealing temperature to 250 ^oC caused an obvious change of film stresses with time, in which SiO₂ film turns into less compressive and TiO₂ film appears to be more tensile. It is believed that the refractive index decrease for SiO₂ film is due to further oxidation. But, for the TiO₂ film, phase transformation which could cause film to densify at 250^oC may be a main reason to the increase in tensile stress after annealing. In addition, most significant changes in the film properties take place in the early stage of annealing, therefore, it could be beneficial from this finding that stability or reliability could be improved through applying such a short period annealing on the film.
11	"www.angstec.com" www.angstec.com info@angstec.com Phone: (978) 263-6678 Fax: (978) 263-6675