1866 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 28, NO. 17, SEPTEMBER 1, 2016 A Prism-Based Optical Readout Method for MEMS Bimaterial Infrared Sensors Ulas Adiyan, Student Member, IEEE , Fehmi Civitci, Onur Ferhanoglu, Hamdi Torun, Member, IEEE, and Hakan Urey, Senior Member, IEEE Abstract— This letter demonstrates a novel prism-based optical-readout, which uses a single prism to detect the incom- ing TM polarized wave just below the critical angle. The method is used with a 35-μm-pixel pitch MEMS thermal sensor, whose inclination angle changes with the absorbed infrared (IR) radiation that results in an increase in the reflectivity at the prism’s glass–air interface. We compared this approach with the conventional knife-edge method. Noise equivalent temperature difference for a single sensor was measured as 200 mK for knife-edge method, and 154 mK for the proposed critical angle approach. Our approach shows a significant improvement for the sensitivity of the IR sensor. Both methods utilize an AC-coupled readout method for a single MEMS pixel using a photodetector, which responds only to changes in the scene. This method can be scaled to achieve smart pixel cameras for read sensor arrays with low-noise and high-dynamic range. Index Terms— Thermo-mechanical MEMS, IR detection, optical readout, AC coupling, critical angle. I. I NTRODUCTION U NCOOLED bimaterial sensors convert absorbed IR radi- ation to mechanical motion owing to the mismatch in the thermal expansion coefficients of the bimaterial legs. For a one-end fixed cantilever-type device (Fig.1), this motion induces a small tilt and can be detected via optical methods. Commonly used optical readout methods for bimaterial sen- sors are optical lever readout [1]–[3], knife edge filter- ing [4], [5] and optical interferometry [6], [7]. Generally, the performance of these methods is limited by the reduced dynamic range due to a DC bias on the photodetectors or CCDs of the detection system. Prior work demonstrated a performance improvement for a 35-μm pitch single MEMS sensor element using an AC-coupled technique [8], show- casing Noise Equivalent Temperature Difference (NETD) of 216 mK for diffraction grating-based readout [8], and 198 mK for knife-edge method (KEM) [9]. The AC-coupled optical readout is implemented by the elimination of the DC bias, Manuscript received February 17, 2016; revised May 3, 2016; accepted May 24, 2016. Date of publication May 30, 2016; date of current version June 30, 2016. This work was supported by the Scientific and Technological Research Council of Turkey (TÜB ˙ ITAK) under Grant 114E882. U. Adiyan and H. Urey are with the Electrical and Electronics Engineering Department, Koç University, Istanbul 34450, Turkey (e-mail: uladiyan@ku.edu.tr; hurey@ku.edu.tr). F. Civitci and O. Ferhanoglu are with the Electronics and Communication Engineering Department, Istanbul Technical University, Istanbul 34469, Turkey (e-mail: civitci@itu.edu.tr; ferhanoglu@itu.edu.tr). H. Torun is with the Electrical and Electronics Engineering Department, Bo ˘ gaziçi University, Istanbul 34342, Turkey (e-mail: hamdi.torun@boun.edu.tr). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2016.2574123 Fig. 1. (a) The geometry of single sensor element. (b) Scanning Electron Microscope (SEM) Image of a part of the array. which mitigates the DC related noise term and improves the sensitivity of the sensor. Fig.1a shows the designed and fabricated MEMS sensor geometry, which is suitable for optical lever based readout. The pixel is 35 μm in pitch size and made of 150 nm-thick alu- minum and 150 nm-thick silicon nitride forming the bimaterial layer. The lengths and thicknesses of the legs are optimized considering thermal isolation, and induced displacements due to the temperature change in the sensor. The 40 μm-long isolation legs provide 1.2 × 10 -7 W/K thermal conductivity, resulting in a thermal time constant of 1.5 ms that is suitable for real-time imaging. The 20 μm-long bimaterial legs provide > 40 nm/°C responsivity (Fig.2). The fabrication process of the cantilevers follows the recipe described previously [8]. This design does not require diffrac- tion gratings, which simplifies the fabrication steps, improving effective absorption area since the area of the top reflector can be reduced, and eliminates the need for pixel surface planarization to avoid the tilt errors. Fig.1b shows a SEM image of a portion of the MEMS focal plane array. We use Finite Element Method (FEM) to design the sen- sor structure. IR induced temperature change on the sensor creates a thermal gradient along the pixel, as illustrated in Fig.2a that induces a tilt over the sensor with respect to its substrate (Fig.2b). The thermo-mechanical responsivity was also verified with heating experiments under a white light interferometer (WLI) device. We directly heated up our MEMS device using a thermo-electric cooler (TEC). Measurements under WLI revealed a tilt angle vs. temperature change of 0.08°/1°C as opposed to the simulation result of 0.11°/1°C (corresponding to thermo-mechanical displacement gradient over the MEMS reflector), extracted from FEM (Fig.2c). We attribute the deviation between the measured values and the simulated ones to the deviation of the geometry and material properties from the theoretical values. With an aim of further improving the response of the sensor, here we developed an optical readout method using a glass prism so that the tilt angle of the sensor results in a change in the reflectivity of a glass-air interface, utilizing the 1041-1135 © 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.