A high fill-factor uncooled infrared (IR) bolometer has been fabricated by using thin-film titanium resistors sandwiched in a surface-micromachined silicon oxinitride membrane (50 mu m x 50 mu m). This bolometer is realized in multilevel electrothermal structures with a fill-factor over 92%. From the multilevel structure, thermal isolation can be independently optimized without sacrificing LR absorbing area. Initial measurements show a thermal time constant of 12 ms, a responsivity of 1600 V/W, and a detectivity (D*) of 5 x 10(8) cm root Hz/W.