An Introduction to mems (Micro-electromechanical Systems)

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stiction.  Stiction refers to the sticking of structural elements either to the substrate or the 

adjacent elements.  Capillary forces from rinsing liquids, as well as electrostatic and van der 

Waals forces can also produce permanent adhesion after the system has dried. 

 

3.4.1  Fusion Bonding 

In order to form more complex and larger MEMS structures, micromachined silicon wafers 

can be bonded to other materials in a process known as fusion bonding.  It is a technique that 

enables virtually seamless integration of multiple layers and relies on the creation of atomic 

bonds between each layer either directly (with heating and pressure in the case of glass to 

wafer bonding), or through a thin film of silicon dioxide (Figure 23).  The resulting composite 

has very low residual stress due to matching coefficients of thermal expansion from each 

layer.  In addition, the mechanical strength of the bond is comparable to that of the adjoining 

layers resulting in a very strong composite fabrication technique for enclosed cavities and 

channels.  High-aspect ratio structural layers can also be bonded to silicon substrates in a 

similar manner.  Photoresist and polymethylmethacrylate (PMMA) are used as MEMS fusion 

bonding media and have proved very successful for the bonding of polyimide [4].