A CMOS-compatible approach is presented for the fabrication of a wideband mid-IR metamaterial-based absorber on top of a Si3N4 membrane, which contains poly-Si thermopiles. The application is in IR microspectrometers that are intended for implementation in portable microsystem for use in absorption spectroscopy. Although Au is the conventional material of choice, we demonstrate by simulation that near-perfect absorption can be achieved over a wider band when using the more CMOS-compatible Al. The absorber design is based on Al disk resonators and an Al backplane, which are separated by a SiO2 layer. The fabrication process involves the deposition of Al and SiO2 layers on top of a Si3N4 membrane, lithography and a lift-off process for patterning of the top Al layer.