Page 158 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

138 SILICON MICROMACHINING: BULK

5.6 SILICON FUSION BONDING

The construction of any complicated mechanical device requires not only the machining
of individual components but also the assembly of the components to form a complete
set. In micromachining, bonding techniques are used to assemble individually microma-
chined parts to form a complete structure. Wafer bonding, when used in conjunction with
micromachining techniques, allows the fabrication of three-dimensional structures that are
thicker than a single wafer. Several processes have been developed for bonding silicon
wafers. The most common bonding process is fusion bonding.
In the last decade, several groups (Lasky 1986; Ohashi et al. 1986; Apel et al. 1991)
have demonstrated that the fusion of hydrophilic silicon wafers is possible for obtaining
SOI materials. Since then, wafer-bonding techniques have found different applications
in the field of microelectronics; several static random access memory (SRAM), comple-
mentary metal oxide semiconductor (CMOS), and power devices have been fabricated on
bonded SOI material. For micromechanical applications, fusion bonding rendered possible
the fabrication of complex structures by combining two or more patterned wafers. This
section describes the principles of wafer fusion bonding and presents fusion-bonding
processes for MEMS device fabrication.

5.6.1 Wafer Fusion

In its simplest form, the process of wafer fusion bonding is the mating together of a pair
of wafers at room temperature, followed by thermal annealing at temperatures between
700 and 1100°C. At room temperature, the wafers adhere via hydrogen bridge bonds of
chemisorbed water molecules that subsequently react during the annealing process to form
Si—O—Si bonds. Consequently, wafer pretreatment procedures that include hydrophilisa-
tion steps (wet cleaning processes and plasma hydrophilisation) support the process.
A major concern of all bonding processes is the presence of noncontacting areas,
which are generally called voids. Voids are mainly caused by particles, organic residues,
surface defects, and inadequate mating. Therefore, both the surfaces that are fusion-bonded
have to be perfectly smooth and clean because the smallest of particles could cause
large voids. Optimised processing includes wafer surface inspection, surface pretreatment
(hydrophilisation and cleaning), and mechanically controlled, aligned mating in a particle-
free environment.

5.6.2 Annealing Treatment

As discussed in the previous subsection, wafer fusion bonding involves a high-temperature
annealing step that is to be performed after the room-temperature contacting of the
surfaces. This annealing step is necessary to increase the strength of the bond. However,
the high-temperature annealing step (usually at a temperature above 800 °C) may intro-
duce problems, such as doping profile broadening, thermal stresses, defect generation,
and contamination. Annealing also prevents the use of bonding technology for compound
semiconductor materials because their dissociation temperature is often low. In addition,
postmetallisation bonding also requires bonding temperatures that are less than 450 °C as