Low-temperature direct Plasma-Enhanced Chemical Vapor Deposition (PECVD) oxide to thermal oxide bonding is described. The PECVD oxide is densified at 350C and chemical-mechanically polished to obtain reasonably smooth surface for bonding. The PECVD oxide wafer is bonded to the thermal oxide wafer at room temperature after piranha clean that leaves the wafer surfaces hydrophilic. A postbonding anneal at 300C completes the bonding. A void-free bonding interface is observed from infrared imaging and the bonding strength is estimated to be 432 mJ/m. This bonding method can be used in a variety of applications, including threedimensional integration.