NONDESTRUCTIVE DETECTION OF MICROVOIDS AT THE INTERFACE OF DIRECT-BONDED SILICON-WAFERS BY SCANNING INFRARED MICROSCOPY

The density of microvoids with dimensions from several to a few tens o f microns at the interface of bonded silicon wafers formed during anne aling at different temperatures was investigated by using a submicron resolution scanning infrared microscope (SIRM). For low temperature he at-treatment (400 degrees C), the density and size (i.e area) of micro voids have been found to be much larger in the case of bonding with hy drophilic wafers (2.5 x 10(4)/cm(2) and up to 400 mu m(2), respectivel y) 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 densi ty and size of microvoids decreased in both cases, but more significan tly 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 mi crovoids between hydrophilic and hydrophobic samples is believed to be the native oxide forming only on the surface of the hydrophilic wafer s during storage and surface treatment.