Micromechanical Calorimetry

Microcantilevers and microfabricated bridges can be heated in a programmed fashion to provide thermal information of sub-nanogram quantities of adsorbed materials. This technique is similar to that of temperature programmed desorption (TPD), but carried out at a higher heating rate (dT/dt) of 104 oC /s which opens up the possibility of investigating the thermal behavior including thermally-induced decomposition of subnanogram quantities of materials in milliseconds. The measured signal as a function of heating time (or temperature) is the rate of change of thermal mass, which is related rate of change of adsorbed mass with temperature (dM/dT). Since the observed signal is with respect to a reference bridge, the mechanical buckling of the bridge under thermal stress does not play a role. During heating the adsorbed molecules desorb as a function of temperature. The mass loss rate (dM/dt) is related to dM/dT and dT/dt. The mass loss due to evaporation resembles a sigmoid function and rate of evaporation resembles a Gaussian peak. Our MEMS structures resembling a suspended bridge are microfabricated with three buried electrically conducting tracks in silicon-on-insulator wafers. Each suspended bridge has a piezoresistive temperature-measuring track placed between two heating tracks with all tracks separated by thin layers of insulating silicon rich nitride.