Handbook of Silicon Based MEMS Materials and Technologies

37.2.4. Indirect Alignment Methods

Backside Alignment

In this process, one wafer has alignment keys defined on the front-side, while the second has alignment keys defined on the backside. After loading the first wafer its alignment key's position (later to be enclosed in bonded interface) is digitized and stored. The second wafer is aligned to the digitally stored reference image. The final alignment accuracy for this type of process is affected also by errors induced by potential front-to-back lithographic processes used for defining alignment keys.

Front-to-Front (Face-to-Face) alignment:

In this wafer-to-wafer alignment approach the alignment keys are located on the front sides of the two wafers and will be enclosed in the bonded interface. ...