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A simple mass producible uncooled thermoelectric infrared microsensor has been designed and fabricated. To improve the cost-efficiency, an advanced micromachining process, which combines wet anisotropic pre-etching and XeF2 dry isotropic post-etching, is adopted for the sensor fabrication. The wet anisotropic pre-etching removes bulk silicon from back-side and forms a thin silicon membrane for device fabrication, the XeF2 dry isotropic post-etching undercuts silicon membrane and releases the microstructure. Experimental results show that the sensor with advanced micromachining exhibits a two times higher responsivity and detectivity than the sensor with only XeF2 front-side etching. In air at room temperature, the sensor with advanced micromachining has a responsivity of 71.57 V W-1, noise equivalent power of 0.64 nW Hz-1/2, detectivity of 6.21×107 cm Hz1/2 W-1 and a time constant of 13.2 ms. The effect of back-side etch window size on sensor performance is also characterized by finite-element method simulation.