Wafer Bonding  

A wide range of bonding processes using various intermediate layers has been used for MEMS fabrication. Techniques include:

Glass frit bonding
Thermocompression bonding
Adhesive bonding
Solder bonding
Eutectic bonding

All of these processes can be run on AML wafer bonders.

Glass frit bonding
Low melting point glasses have been used in industry for many decades for forming hermetic seals. The process is typically carried out in the temperature range 400 - 650 oC and contact pressures of ~105Pa. The thermal expansion coefficient of the glass is normally chosen to be between the two values for the wafers being bonded and a wide range of sealing glasses is commercially available.
The glass layer can be applied as a preform, spin-on, screen print, sputtered film, etc and patterned to define sealing areas.
The technique has been used for the production of pressure sensors and other |MEMS devices and AML wafer bonders can be used to achieve in-situ, aligned bonds. The process can be carried out in vacuum (eg for creating sealed evacuated cavities) and suitable glass compositions exist for quartz: quartz bonding, and other material combinations, as well as silicon.
In comparison with anodic and direct bonding, the glass frit process relies on glass flow to form a seal and hence suffers poorer dimensional control for micromachined cavities etc. The process also requires reliable control of temperature profiles and applied forces, but these parameters are all controllable using AML wafer bonders.

Thermo-compression bonding
Thermo-compression bonding is simply the joining of two surfaces via the welding of a layer of soft metals on each surface. The most common metal for MEMS applications is gold, with a suitable adhesion layer. Moderate temperatures (~300oC) and pressures (106Pa) are needed and therefore the process is readily compatible with AML wafer bonders. The technique offers very low outgassing and therefore is attractive for the sealing of evacuated cavities.

Solder bonding
The solder bonding process works by reflowing low melting point metals to form a seal. Typical metals are Au-Sn, Cu - Sn and Pb - Sn. The metals can be applied by various thin film deposition techniques. The reflow process means that he method is not recommended where accurate alignment is needed and, as is the case with thermocompression bonding, the metallic nature of the bond makes it incompatible with the inclusion of metal tracks for interfacing with sealed devices. The technique differs from thermocompression bonding in that the metallic intermediate layer needs to be melted for solder bonding. The solder technique is tolerant to particles and is most widely for electrical contacts (eg flip chip bonding).

Adhesive bonding
Various adhesives (epoxies, silicones, photoresists, polyimides, etc.) can be used to form wafer bonds. In-situ alignment can be used with this technique but like other processes that rely on some flow in the intermediate layer, alignment accuracy is compromised. The adhesive can be applied by spinning, spraying etc., and the process normally requires some heat (typically between room temperature and 400oC depending on the adhesive being used) and pressure. The technique is tolerant to particles and is useful when the wafers have a severe temperature limitation.

Eutectic Bonding
The eutectic temperature of a two-component system corresponds to the lowest melting point composition of the two components. This property can be exploited to form bonding between two wafers by coating one of the wafers with one component of the system and the other wafer with the second component. When the wafers are heated and brought into contact, diffusion occurs at the interface and alloys are formed. The eutectic composition alloy at the interface has a lower melting point than the materials either side of it, and hence the melting is restricted to a thin layer. It is this melted eutectic layer that forms the bond.
The most commonly used eutectic in MEMS is the Au-Si system with a eutectic temperature of 363oC, and eutectic composition of 97.1 Wt % Au : 2.85 Wt % Si. However lower melting point options exist such as SN : Pb with a temperature of 183oC. The eutectic bonds are strong and hermetic and the process can be carried out using AML wafer bonders.