Page 76 - An Introduction to Microelectromechanical Systems Engineering

Advanced Process Tools 55

DRIE is a powerful tool for the formation of deep trenches with near-vertical
sidewalls; however, process development is required for each mask pattern and
depth to optimize for low ARDE, good uniformity, high speed, high verticality,
small scalloping, and small footing. In general, all cannot be optimized simultane-
ously. Sequentially running different processes or slowly changing the process as the
etch proceeds may be done for the best result.

Advanced Process Tools

Anodic Bonding

Anodic bonding, also known as field-assisted bonding or Mallory bonding, is a
simple process to join together a silicon wafer and a sodium-containing glass
®
substrate (e.g., Corning Pyrex 7740 and 7070, Schott 8330 and 8329). It is used in
the manufacturing of a variety of sensors, including pressure sensors, because it
provides a rigid support to the silicon that mechanically isolates it from packaging
stress.
The bonding is performed at a temperature between 200° and 500°C in
vacuum, air, or in an inert gas environment. The application of 500 to 1,500V
across the two substrates, with the glass held at the negative potential, causes
+
mobile positive ions (mostly Na ) in the glass to migrate away from the silicon-
glass interface toward the cathode, leaving behind fixed negative charges in the
glass (see Figure 3.16). The bonding is complete when the ion current (measured
externally as an electron current) vanishes, indicating that all mobile ions have
reached the cathode. The electrostatic attraction between the fixed negative charge
in the glass and positive mirror charge induced in the silicon holds the two
substrates together and facilitates the chemical bonding of glass to silicon. Silicon
dioxide on the silicon surface should be removed before bonding, as a thin
(~100 nm) layer is sufficient to disturb the current flow and the bond. A buried
oxide layer, such as on a silicon-on-insulator (SOI) wafer, however, does not pres-
ent a problem, as it conducts sufficiently well at high temperatures to allow the cur-
rent flow needed for bonding.

Current
Cathode

+
Na −
Glass
V
+
Silicon

Chuck
Figure 3.16 Illustration of anodic bonding between glass and silicon. Mobile sodium ions in the
glass migrate to the cathode, leaving behind fixed negative charges. A large electric field at the
silicon-glass interface holds the two substrates together and facilitates the chemical bonding of
glass to silicon.