Wafer bonding is the process of creating a device for a microelectromechanical system (MEMS), a nanoelectromechanical system (NEMS), or an opto or microelectronic object. A "wafer" is a small slice of semiconductive material, such as silicon, used to make circuits and other electronic devices. During the process of bonding, the mechanical or electric devices are fused to the wafer itself, resulting in the creation of the finished chip. Wafer bonding is environmentally dependent, which means that it can only take place under a strict series of carefully controlled conditions.
In order for one to accomplish the process of wafer bonding, three things are required. The first is that the substrate surface — the wafer itself — must be free from issues; this means that it must be flat, smooth, and clean in order for the bonding to successfully take place. In addition to that, the electrical or mechanical materials being bonded must also be free from defects and faults. Second, the temperature of the environment must be set precisely, depending upon the specific bonding method being used. Third, the pressure and applied force used during bonding must be exact, allowing fusion without the possibility of cracking or otherwise damaging any vital electronic or mechanical parts.
There are a number of different wafer bonding techniques, depending on the specific situation and the types of materials being bonded. Direct bonding is bonding without the use of any intermediary layers between the electronics and the substrate. Plasma activated bonding, on the other hand, is a direct bonding process used for materials involving hydrophilic surfaces, materials whose surfaces are attracted to and dissolved by water. Thermocompression bonding involves joining two metals with a force and heat stimulus, essentially "gluing" them together. Other bonding methods include adhesive bonding, reactive bonding, and glass frit bonding.
Once the wafers are bonded together, the bonded surface must be tested to see if the process was a success. Normally, a portion of the yield produced during a batch is set aside for both destructive and non-destructive testing methods. Destructive testing methods are used to test the overall shear strength of the finished product. Non-destructive methods are used to evaluate whether any cracks or abnormalities have appeared during the bonding process, helping to ensure that the finished product is free from defects.