Novel MEMS pressure and temperature sensors fabricated on optical fibers

This thesis presents the design, fabrication, and testing of novel MEMS pressure and temperature sensors fabricated on optical fiber end faces. A simple micromachining process compatible with MEMS was developed in fabricating sensors directly on optical fibers. The pressure sensor configuration involves anodic bonding of a piece of an extremely thin silicon wafer onto the fiber end face over a cavity etched in the central portion of the fiber end face. Final device diameter is thus the same as that of the optical fiber. The temperature sensor is based on anodically bonding a thin piece of silicon onto the fiber end face.; The pressure sensors were fabricated on 400 μm diameter fibers while temperature sensors were fabricated on both 200 and 400 μm diameter fibers. Pressure measurements were made over the 14 to 80 psi range while temperature measurements were made over the 23 to 300°C range. Pressure sensor sensitivities of 0.1 mV/psi and 0.2 mV/psi were obtained. The pressure sensors were designed with cavity diameter d = 150 μm, and cavity depth h = 0.640 μm. Diaphragm thickness for the two sensors were t = 7.1, and t = 3.4 μm. Higher sensitivity was achieved by design of a sensor with the thinner diaphragm. A sensor array fabrication effort demonstrated that our micromachining process could be extended to simultaneous processing of an array of fibers. The temperature sensor was fabricated by bonding 3.1 μm thick silicon onto the fiber end face. An oxidant-resistant encapsulation scheme for the temperature sensor was proposed, namely aluminum coated silicon nitride (Al/Si₃N₄). The uncoated side of silicon was bonded to a fiber end face using the anodic bonding method. The measured values of κ_&phis; = λ_m −1dλ_m/dT for capped and uncapped sensors were κ _&phis; = (7.5 ± 0.6) × 10−5/°C, and κ_&phis; = (7.2 ± 0.1) × 10−5 /°C respectively.{09}The measured κ_&phis; value for the uncapped sensor is equal to that which was determined using the published material properties for crystalline silicon (κ_&phis; 7.9 × 10−5/°C) within measurement uncertainty.; The micromachining process developed for micromachining fiber end faces along with the bonding of silicon to fiber end faces can be extended to fabrication of other MEMS based microoptic devices where fiber optic interrogation is advantageous.