Describe the phenomenon of Stiction, and possible ways to avoid it.
Stiction :- Very thin Polythene pockets often do not open till one blows air to separate the two surfaces. A similar phenomenon is also observed during the final separation of individual microstructures often the completion of the fabrication process. The phenomenon of Sticking together of two separate pieces is called Stiction. Stiction is a major problem faced by MEMS device engineers. A typical process sequence for realizing a cantilever beam structure is shown in fig below. Stiction related problems are often faced after the sacrificial layer removal. This may lead to a collapse of the polycrystalline cantilever beam as shown in fig (b).
Mechanical force is required to separate two layers that join together due to stiction since microstructures used in MEMS devices are small and delicate the mechanical force required can often damage the microstructure. Silicon could be either due to Vander Waals forces. or as a consequence of hydrogen bonding of surface as the interface is rinsed after the removal of the phosphosilicate glass sacrificial layer. There are several ways of reducing striction in microstructures. Dimples can be created in the polysilicon layer to reduced the contact area with an associated reduction in stiction released structure are sometimes also coated with self-assembled monolayer that makes the polysilicon and silicon surfacer hydrophobic and thus stiction is avoided. Another technique for avoiding Stiction is to incorporate temporary spacer using polysilicon and Sacrificial polymer columns. These spacers ore ultimately removed by etching with oxygen plasma.