Sacrificial etch

Lee, J. R. Barber, J. P. George, Z. A. Lee, M. L. Schmidt, H. Hawkins, A. R., Micro channels with different core shapes fabricated using sacrificial etching, J. Micro/Nanolith. MEMS MOEMS 2007, 6, 013010... 

Figure 1.2 A cross-section scanning electron microscope image of a six-level metal backend structure. The insulating layers between the metal layers have been etched away, similar to the sacrificial etch that is used to release structures in the polysilicon surface micromachining process described below. (Reprinted with permission from JOM Journal of the Minerals, Metals and Materials Society.)...

A common mistake made by those who are first learning to use a surface micromachining process is to leave out anchors. Since you must specify where a hole in this layer is to be placed, it is easy to leave out the anchor hole. If no anchor hole is specified, the polysilicon that is deposited over the oxide will be released during the sacrificial etch and float away. A good practice to avoid this common mistake is to take cross sections of the thin-film stack to make sure that all structures are anchored. An example of a bond pad that was not anchored and was released during the sacrificial etch is shown in Figure 1.9. 

Figure 1.9 Bond pads that were not anchored are released after the sacrificial etch. Not only does this make electrical testing difficult, but the bond pads usually end up in places you do not want them. Your neighbors on a multiproject chip will be particularly annoyed if your parts land in their area The parts that do not get stuck to the chip are often observed as floaters in the etch bath that is used for the sacrificial release.

Figure 1.10 Stiction that can occur during the sacrificial release etch, (a) Before sacrificial etching, the sacrificial oxide is below the mechanical layer, (b) After the chip is removed from the etch bath it begins to dry and the remaining fluid forms a bridge between the substrate and the mechanical layer, (c) Capillary forces from the meniscus of the fluid exert a downward force on the cantilever and cause it to come into contact with the substrate, (d) The surface forces, such as Van der Waals attraction, that dominate at the microscale cause the cantilever to become stuck to the substrate. (Reprinted with permission from lOP Publishing Ltd.) [15].

---