The density of microvoids with dimensions from several to a few tens of microns at the interface of bonded silicon wafers formed during annealing at different temperatures was investigated by using a submicron resolution scanning infrared microscope (SIRM). For low temperature heat-treatment (400 degrees C), the density and size (i.e area) of microvoids have been found to be much larger in the case of bonding with hydrophilic wafers (2.5 x 10(4)/cm(2) and up to 400 mu m(2), respectively) than for the hydrophobic one (3 x 10(3)/cm(2) and up to 20 mu m(2)). By increasing the annealing temperature to 1150 degrees C, the density and size of microvoids decreased in both cases, but more significantly for hydrophobic wafer bonding (90% and <4 mu m(2) compared to 66% and <100 mu m(2)). The cause of the different annealing behavior df microvoids between hydrophilic and hydrophobic samples is believed to be the native oxide forming only on the surface of the hydrophilic wafers during storage and surface treatment.