Measurement, optimization and multiscale modeling of silicon wafer bonding interface fracture resistance
Wafer bonding is a process by which two or more mirror-polished flat surfaces are
joined together. This process is increasingly used in microelectronics and microsystems
industries as a key fabrication technique for various applications: production
of SOI wafers, pressure sensors, accelerometers and all sorts of advanced MEMS.
Unfortunately, the lack of reliability of these systems does not allow them to enter
the production market. This lack of reliability is often related to the lack of
understanding and control of the thermo-mechanical properties of materials used
for the fabrication of MEMS (indeed, at this small scale, properties of materials
are sometimes quite different than at large scale) but it is also due to the limited
knowledge of the different phenomena occurring during the working of these devices,
the most detrimental of them being fracture. Among all of these fracture
processes, the integrity of the interfaces and, particularly, the interfaces created by
wafer bonding is a generic problem with significant technological relevance.
In order to understand the bonding behavior of silicon wafers, the interface chemistry
occurring during the different steps of the bonding process has been detailed.
The formation of strong covalent bonds across the two surfaces is responsible of
the high fracture resistance of gwafer bondingh interfaces after appropriate surface
treatments and annealing. The bonding process (surface treatments and annealing
step) has been optimized toward reaching the best combination of interface toughness
and bonding uniformity.
The fracture resistance of gwafer bondingh interfaces or interface toughness has
been determined using a steady-state method developed in the framework of this
The high sensitivity to geometrical and environmental factors of gwafer bondingh interfaces has been quantified and related to the interface chemistry.
A new technique involving the insertion of a dissipative ductile interlayer between
the silicon substrate and the top silicon oxide has been proposed in order to increase
the overall fracture resistance. A multiscale modeling strategy which involves the
description of the interface fracture at the atomic scale, of the plasticity in the thin
interlayer at the microscopic scale, and of the macroscopic structure of specimen
has been used to guide the optimization of this technique. Numerical simulations
have shown the influence of the ductile interlayer parameters (yield strength, workhardening
exponent and thickness) and the critical strength of the interface on the
overall toughness of such assemblies.
A first set of experimental data has allowed increasing the interface toughness by
The critical strength of the interface is finally determined by inverse identification
and turns out to be in the expected range of theoretical strength.
The knowledge of the strength and the fracture toughness of gwafer bondingh interfaces
is of practical importance because these two values can be used in a simple
fracture model (e.g. cohesive-zone model) in order to observe the behavior of such
interfaces under complex loading using finite element simulations.
School:Université catholique de Louvain
Source Type:Master's Thesis
Keywords:wafer bonding interface toughness multiscale modeling
Date of Publication:10/20/2006