Anodic bonding is a wafer bonding process to seal glass to either silicon or metal without introducing an intermediate layer; it is commonly used to seal glass to silicon wafers in electronics and microfluidics. This bonding technique, also known as field assisted bonding or electrostatic sealing,[1] is mostly used for connecting silicon/glass and metal/glass through electric fields. The requirements for anodic bonding are clean and even wafer surfaces and atomic contact between the bonding substrates through a sufficiently powerful electrostatic field. Also necessary is the use of borosilicate glass containing a high concentration of alkali ions. The coefficient of thermal expansion (CTE) of the processed glass needs to be similar to those of the bonding partner.[2]
Anodic bonding can be applied with glass wafers at temperatures of 250 to 400 °C or with sputtered glass at 400 °C.[3] Structured borosilicate glass layers may also be deposited by plasma-assisted e-beam evaporation.[4]
This procedure is mostly used for hermetic encapsulation of micro-mechanical silicon elements. The glass substrate encapsulation protects from environmental influences, e.g. humidity or contamination.[2] Further, other materials are used for anodic bonding with silicon, i.e. low-temperature cofired ceramics (LTCC).[5]
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