# A Thermopile Device with Subwavelength Structure by CMOS-MEMS Technology
## 1 Introduction  
Based on the photonic crystal (PhC), it is proved to successful in the efficient photon coupling into photodetectors when the PhC structure is employed [1]. Besides the visible range, thermopiles as the infrared sensor converting thermal energy into electrical energy can sense heat radiation and generate an output voltage proportional to a local temperature difference or temperature gradient [2]. Thermopiles working as infrared sensors can be used in many applications, such as remote temperature sensing [2; 3; 4; 5] and non-dispersive infrared sensing (NDIR) gas detection [6]. In recent decades, with the development of micro-electromechanical systems (MEMS) technology, the problem of mass production of thermopile IR sensors has been effectively solved, and the manufacturing cost has been greatly reduced [3; 4]. MEMS technology can be a goal of continuous integration, miniaturization, extended functionality, lowering cost, and improved performance and reliability. For the requirements of integration, the monolithic complementary-metal-oxide-semiconductor (CMOS) MEMS integration that integrates MEMS structures with integrated circuits (IC) on a single CMOS substrate is developed [7]. The digital micro-mirror device (DMD) manufactured by Texas Instruments (TI) is a well-known commercial monolithic CMOS-MEMS device [8; 9].  
Some researchers focus on the enhancement of infrared emitters or absorbers by adding specifically artificial structures. A unique thermally stimulated mid-infrared source emitting radiation within a narrow range of wavelengths is experimentally and theoretically investigated [10]. Here, the periodicity of a metal coated silicon-air photonic crystal etched into the emitter surface determines the emission wavelength. Wavelength-selective uncooled IR sensors using square lattice two-dimensional plasmonic absorbers are investigated [11]. The results show that selective enhancement of the responsivityis achieved, and for the detection wavelength the triangular lattice case is shorter than the square lattice case. A 2D photonic crystal is first proposed to enhance the infrared absorption efficiency of thermopiles [12]. The experimental results show that the thermopiles with 2D microstructures have a significantly higher response than those without the photonic crystal. To meet the commercial batch production, the thermopile with photonic crystal is fabricated by the CMOS MEMS process.  
Beyond the development of optimized structured absorbers for CMOS infrared detectors empirically, it was firstly proposed with theoretical analysis and investigation with experiments thoroughly. In this study, various CMOS compatible thermopiles with subwavelength structures are fabricated in the standard 0.35 m CMOS-MEMS, which are numerically and experimentally investigated and reach agreement with response curves. The various subwavelength structures (SWSs) of photonic devices are analyzed by the finite-difference time-domain (FDTD) [13; 14] method. Then, the thermopile with subwavelength structure (SWS) [15] is fabricated by the 0.35 \(\upmu\)m 2P4M CMOS-MEMS process. Finally, we discuss the results of structural measurement and summarize in the conclusion. The measurement and simulation results show that the CMOS compatible thermopiles with SWS are higher than the one without SWS.