Medical Applications of Micro-Electro-Mechanical Systems (MEMS) Technology - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 45

bonding process is performed in a very clean environment. The hydration
of the surfaces makes them highly charged and attractive to particles in the
environment. Any particles attached to the wafer surfaces prior to bonding
will result in voids between the two wafers or the inability of the wafers
to bond. Although the anneal can be performed at lower temperatures, the
bond strengths tend to strengthen as the anneal temperature is increased.
As mentioned, wafer bonding is analogous to welding in the macroscale
world. Wafer bonding is used to attach a thick layer of single crystal silicon
onto another wafer. This can be extremely useful when it is desirable to have
a thick layer of material for applications requiring appreciable mass or in
applications where the material properties of single crystal silicon are advan-
tageous over those of thin-film LPCVD materials. Direct wafer bonding is
also used to fabricate silicon-on-insulator (SOI) wafers having device layers
several microns or more in thickness.
Another popular wafer bonding technique is anodic bonding, which is
illustrated in Figure 3.10. In anodic bonding a silicon wafer is bonded to a
Pyrex ® 7740 wafer using an electric field and elevated temperature [24-26].
The two wafers can be pre-processed prior to bonding and can be aligned
during the bonding procedure. The mechanism by which anodic bonding
works is based on the fact that Pyrex 7740 has a high concentration of Na +
ions; a positive voltage applied to the silicon wafer drives the Na + ions from
the Pyrex glass surface, thereby creating a negative charge at the glass sur-
face. The elevated temperature during the bonding process allows the Na +
ions to migrate in the glass with relative ease. When the Na + ions reach the
interface, a high field results between silicon and glass, and this combined
with the elevated temperatures fuses the two wafers together. As with direct
wafer bonding, it is imperative that the wafers are flat, smooth, and clean
and that the anodic bonding process is performed in a very clean environ-
ment. Any particles on the wafer surfaces will result in voids between the
two wafers. One advantage of this process is that Pyrex 7740 has a thermal
expansion coefficient nearly equal to that of silicon, and therefore there is a
low value of residual stress in the layers. Anodic bonding is a widely used
technique for MEMS packaging.
In addition to direct and anodic bonding there are other wafer bond-
ing techniques that are used in MEMS fabrication. One method is eutectic
bonding, which involves the bonding of a silicon substrate to another silicon

500 to 1000 V

Pyrex 7740

-

Silicon

+

Hot plate - 500 C

FIGuRE 3.10
The setup for anodic wafer bonding.