Polymeric nanolayers are introduced here as active, thermal-stress mediating structures facilitating extremely sensitive thermal detection based upon the thermomechanical response of a bimaterial polymer-silicon microcantilever. To maximize the bimaterial bending effect, the microcantilever bimorph is composed of stiff polysilicon, with a strongly adhered polymer deposited via plasma-enhanced chemical vapor deposition. The polymer layers with thickness ranging from 20 to 200 nm possess a rapid and pronounced response to temperature fluctuations due to intrinsic sensitive thermal behavior. We show that by taking advantage of the thermal stresses generated by the huge mismatch of material properties in the polymer-silicon bimorph, unprecedented thermal sensitivities can be achieved. In fact, the temperature resolution of our bimaterial microcantilevers approaches 0.2 mK with thermal sensitivity reaching 2 nm/mK; both parameters are more than an order of magnitude better than the current metal-ceramic design. This new hybrid platform overcomes the inherently limited sensitivity of current sensor designs and provides the basis to develop the ultimate uncooled IR microsensor with unsurpassable sensitivity.