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To reach a high fill factor, a new CMOS-compatible thermopile was designed and fabricated. The floating membrane of the thermopile that we designed was formed by a T-shape anisotropic etching window with a minimum etching area. The design and fabrication of thermopile sensors are realized by using 1.2-mum CMOS IC technology combined with a subsequent anisotropic front-side etching. The proposed T-shape etching windows are designed at four quadrants of a membrane to form the extended undercut etching area of opened windows of overlap. The floating membrane has a larger area of 1100 times 1100 mum2 and is 2 mum thick. The area of the proposed membrane is increased by about 21.5%, which absorbs more infrared radiation than the conventional design and enhances responsivity very well, as shown in the measurement. A surface morphology measurement of the thermopile is implemented to evaluate the influence of residual stress and practically characterize the geometric shape of the membrane. More careful analysis of the surface morphology shows that the bending of suspension parts has a deviation of responsivity of less than 0.167%. In this paper, the T-shape structure of the thermopile with large absorption area and high performance by using a CMOS-compatible process is proven to be very successful and is easily fabricated.