Medical Applications of Micro-Electro-Mechanical Systems (MEMS) Technology - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 44

bonding using an intermediate layer. In general, all bonding methods require
substrates that are very flat, smooth, and clean, in order for the wafer bond-
ing to be successful and free of voids. Direct or fusion bonding is typically
used to mate two silicon wafers together, or alternatively, to mate one silicon
wafer to another silicon wafer that has a thin film on the surface such as an
oxide layer. Direct wafer bonding can be performed on other combinations,
such as bare silicon to a silicon wafer with a thin film of silicon nitride on the
surface as well. The basic wafer bonding process has five steps [23]:
1. Hydration and cleaning of the wafer surfaces (RCA clean, piranha
immersion, etc.)
2. Physical contacting and pressing together of wafers (must be done
quickly and in a clean environment since the substrates will have a
surface charge after cleaning that will attract particles and prevent
bonding or cause voids)
3. Inspection (usually infrared inspection if the wafers are made of sili-
con) of pre-anneal bond quality
4. Elevated temperature anneal,
5. Infrared inspection of post-anneal bond quality
Figure 3.9 illustrates some of the steps involved in the direct wafer bond-
ing process. The wafers are initially held together after physical contact as
a result of the hydrogen bonds created by hydration of the surfaces. The
two wafers to be bonded can be pre-processed and then aligned during the
bonding procedure so as to register features on the top wafer to features on
the bottom wafer. After the elevated temperature anneal, the bond strength
between the two wafers can be similar to that of single crystal silicon. There
is relatively low residual stress in the bonded layers after anneal. It is impera-
tive that the wafers are flat, smooth, and clean prior to bonding and that the
Wafer 1
Hydrated Surfaces
Wafer 2
Step 1- Clean and hydration
Wafer 1
Wafer 2
Step 2 - Physical contacting
Wafer 1
Wafer 2
Step 4 - High-temperature anneal
FIGuRE 3.9
Direct or fusion wafer bonding.