Wafer bonding is the process of fusing mechanical or electrical devices to a small wafer of semiconductive material, resulting in a finished chip. The process requires a clean substrate surface, precise temperature, and exact pressure. Different bonding techniques include direct bonding, plasma activated bonding, and thermocompression bonding. Testing methods are used to ensure the finished product is free from defects.
Wafer bonding is the process of creating a device for a microelectromechanical system (MEMS), nanoelectromechanical system (NEMS), or opto or microelectronic object. A “wafer” is a small wafer of semiconductive material, such as silicon, used to make circuit boards and other electronic devices. During the bonding process, mechanical or electrical devices are fused to the wafer itself, resulting in the creation of the finished chip. Wafer bonding is environmentally dependent, which means it can only occur under a rigorous set of carefully controlled conditions.
In order for one to realize the wafer bonding process, three things are required. The first is that the substrate surface – the wafer itself – must be free from problems; this means that it must be flat, smooth and clean for the bonding to be successful. In addition to this, the electrical or mechanical materials to be bonded must also be free from faults and defects. Secondly, the ambient temperature must be set precisely, depending on the specific gluing method used. Third, the pressure and applied force used during bonding must be exact, allowing for bonding without the possibility of breaking or otherwise damaging vital electronic or mechanical parts.
There are different wafer bonding techniques, depending on the specific situation and the types of materials to be bonded. Direct bonding is bonding without the use of intermediate 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 together with a force and a thermal stimulus, essentially “gluing” them together. Other bonding methods include adhesive bonding, reactive bonding, and glass frit bonding.
Once the wafers have been bonded, the bonded surface must be tested to see if the process was successful. 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 any cracks or anomalies that may have appeared during the bonding process, helping to ensure that the finished product is free from defects.
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