A novel optical sensor is used for monitoring the thickness of deposited thin film in real time. The sensor operates on the basic principle of a Fabry-Perot interferometer. A MEMS based design is used to fabricate the optical fiber sensor. Detail analytical results provide the theoretical model based on the Fabry-Perot interferometer, and show that the optical fiber sensor can successfully monitor and measure the thickness of deposited thin-film in real time. Since thin-film will be deposited simultaneously on both surfaces of sensor head and targeted silicon wafer, the sensor can be used in the fabrication of IC and MEMS devices, as well as in proteomics, nano-sensors, and biosensors.