Atmospheric Stability of E-Beam Deposited Optical Thin Film Stacks
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Atmospheric Stability of E-Beam Deposited Optical Thin Film Stacks Ping Hou∗1; Lianchao Sun2 and Fei Luo3 1 Nortel Networks Corp., Wilmington, MA 01887 2 Sun International (USA), Acton, MA 01720 3 Electrical and Computer Engineering, Boston University, Boston, MA 02215 ABSTRACT Optical thin films with SiO2-TiO2 stack were prepared by the technology of electron beam (ebeam) evaporation with ion beam assistant deposition (IBAD). The mechanical (stress) and optical properties of as-deposited thin films were studied as a function of exposure time in the atmospheric environment. Exposing to the air at the ambient temperature causes incremental compressive stress and spectrum profile changes, which is related to the absorption of water moisture into the films. Making a dense film is, therefore, a practical approach to improve structural stability of thin films and then the performance of optical devices. INTRODUCTION The fabrication of micro-optical-electro-mechanical system (MOEMS) structures, similar to the integrated circuit manufacturing, is mainly developed via thin film deposition technology and photolithography process. In the past decades, significant progress in the thin film fabrication techniques has been made to build functional MOEMS structures. However, preventing undesirable changes in mechanical, micro-structural and optical properties for functional thin films still remains a challenge. Stress control of as-deposited thin films is one of the critical requirements for achieving optical, electronic, magnetic, and mechanical performances in the state-of-art MOEMS devices. Stress of thin films can be harmful or useful, depending on the applications. Undesired in most of cases, residual stress in thin films may cause structural deformation, cracking, buckling or even delamination. Yet stress may be beneficial to optimize performance in some of optical devices, such as the top mirror with a dome shape formed by precisely controlled residual stress in the tunable vertical cavity surface emission laser (TVCSEL) devices. Controlling and stabilizing the film stress in a high-performance MOEMS device can present challenges during the fabrication process and in its application environments. In general, the residual stress (σ) of a thin film can include thermal stress (σT), intrinsic stress (σI), and epitaxial stress (σE), which can be expressed as σ=σT+σI+σE. Thermal stress arises from the temperature changes due to differential thermal expansion between different materials. This kind of stress is almost inevitable in reality when different materials are used. The intrinsic stress is more related to the film growth techniques and can be tuned to certain content in process[1-3]. The effect of epitaxial stress caused by the lattice mismatch of different materials is more obvious when the film is sufficient thin so that it has a perfect coherent interface with its substrate.
∗
Ping Hou, the contact author, is currently with SuperPower Inc., 450 Duane Ave., Schenectady, NY 12304, USA (Tel: 646-239-5331, email:
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