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In this work, we present the fabrication of bulk micromachined microbolometers made of amorphous germanium-silicon-oxygen compounds (GexSi1-xOy) grown by reactive sputtering of a Ge0.85Si0.15 target. We describe the complete procedure for fabricating thermally isolated microbolometers consisting of GexSi1-xOy sensing films deposited on sputtered silicon dioxide membranes suspended over a silicon substrate. The electrical properties of the sensitive material are set by controlling the deposition parameters of the sputtering technique. Under optimum deposition conditions, GexSi1-xOy layers with moderate electrical resistivity and thermal coefficient at room temperature as high as -4.2% · K-1 can be obtained. Isolated structures measured at atmospheric pressure in air have a thermal conductance of 3 × 10-6 W · K-1 and a thermal capacitance of 6·10-9 W · s · K-1, yielding a response time of 1.8 ms. Bolometers with an IR responsivity of 380 V · W-1 and a NEDT of 3.85 K at 100 nA bias current are obtained. The use of sputtered films allows designing a fully low-temperature fabrication process, wholly compatible with silicon integrated circuit technologies.