Tunable and switchable multiple-cavity thin film filters

A family of thin film interference filters is described that incorporates amorphous silicon layers for wide thermo-optic tunability and the potential for multiple cavity designs. Plasma enhanced chemical vapor deposition (PECVD) of hydrogenated amorphous silicon (a-Si:H) onto fused silica or crystalline silicon wafer substrates produces films with high index (n=3.71), low loss at 1500 nm (k<4×10^-6), and thermo-optic index coefficients approximately ten times larger than those of the dielectric materials typically used in wavelength division multiplexing (WDM) filters. Pairing amorphous silicon with low index (n=1.77, k=1×10^-6) silicon nitride companion layers followed by post-process annealing leads to multilayer film stacks which may be cycled up to 400 C without delamination or failure, resulting in thermal index modulations as large as 4% and enabling a wide variety of dynamic thin film device designs. In this paper we survey the range of device applications and show experimental demonstrations for two quite different classes of functionality. One class of filters is tunable in wavelength in the conventional sense. Single and dual-cavity narrowband filters are described with temperature coefficients of center wavelength 85-172 pm/°C in the 1550 nm WDM band and tuning ranges as large as 60 nm, an order of magnitude larger thermal tunability than for conventional thin film narrowband filters. A second class of filters is fixed in wavelength but switchable in transmission, based on hybrid structures which combine tunable semiconductor films and cavities with static dielectric films and cavities in an integrated coating design. To demonstrate the principle of a switchable add/drop filter, we fabricated a five-cavity, 117 layer, 200 GHz filter by combining a single cavity of a-Si:H/a-Si_xN_y films deposited by PECVD with four cavities of dielectrics Ta₂O₅/SiO₂ deposited by ion assisted e-beam evaporation. By varying the temperature, this device can be switched thermo-optically between transmission and reflection states at a fixed channel 1548.3 nm with a contrast ratio 18.4 dB. Stable devices can be obtained even with large internal temperature changes in microscopic volumes provided that layer- -to-layer and layer-to-substrate adhesion is robust, film stresses are well controlled through coefficients of thermal expansion matching, and devices are annealed at or above maximum operational temperatures. "Hitless" filters can be obtained by structuring thermo-optic filters in two independently heated portions. Thermo-optically actuated thin film semiconductor devices are manufacturable and testable on a wafer scale and may be packaged by methods adapted from those for conventional thin film filters.