“Wafer bonding” refers to the phenomenon in which mirror-polished, flat, and clean wafers of almost any material—when brought into contact at room temperature—are locally attracted to each other by van der Waals forces and adhere or “bond” to each other. Wafer bonding is alternatively also known as “direct bonding” or “fusion bonding,” or more colloquially as “gluing without glue.” Although this is by no means required, in most cases, the wafers involved in actual applications are typical semiconductor wafers consisting of single-crystalline material used in microelectronics or optoelectronics such as silicon or gallium arsenide.