Online proceedings - EDA Publishing Association

11-13 May 2011, Aix-en-Provence, France
On-Wafer-Packaging of Crystal Quartz Based
Devises Using Low-Temperature Anodic Bonding
Y. Zimin, T. Ueda
Graduate School of Information, Production and Systems, Waseda University
2-7 Hibikino, Wakamatsu-ku, Kitakyushu-shi, Fukuoka 808-0135, Japan,
Email: zimin-yura@fuji.waseda.jp
Abstract- Low-temperature bonding of crystalline quartz and
silicon wafers is described. The bonding has a big potential for
MEMS applications because it could integrate the processing and
packaging in a single high-tech process. In this work, strong
bonding of silicon and crystal quartz wafers close to the
mechanical strength of the initial materials has been achieved.
Tensile test shows a disruptive stress of the samples at about 35
MPa. High bonding strength is associated with minimization of
the residual stresses, optimization of surface activation, and
application of an electric field during annealing. Lowest possible
annealing temperature and the optimum thickness ratio of
silicon and quartz layers have been used in order to minimize the
residual stresses.
I. INTRODUCTION
Wafer bonding is coming into wide use in MEMS
technology. One of the most important candidates for bonding
is a pair of silicon-crystalline quartz. Quartz is widely used for
generators, high frequency filters, gyroscopes and
microbalances because its physical properties are extremely
stable. Conventional fabrication of devices based on quartz
consists of a high tech processing in the very crystal with
electrodes and subsequent manual assembling to the package.
The manual assembling could be eliminated through
integration of the processing and packaging in a single
high-tech process by means of silicon/crystal quartz bonding.
The integration could also provide a miniaturization and
significantly improve parameters and quality of ready-made
devices. High-temperature direct bonding is well known and
provides a strong coupling and low level of residual stresses
for materials with identical thermal expansion coefficients.
High temperature increases a mobility of atoms across the
interface that largely determines a strong bonding. When
bonded structure consists of materials with different thermal
expansion coefficients, excessive internal stresses may arise at
the interface as result of high annealing temperature. Silicon
and quartz are requiring the processing temperature as low as
possible because the thermal expansion coefficient mismatch
is quite large. Moreover, preprocessed wafers should not be
exposed to high temperature in order to avoid the damage of
the structures. The preprocessed structure could be also
sensitive to residual stresses that can lead to subsequent
degradation of the structure. Therefore, the development of a
low-temperature technology is a key requirement of the strong
bonding of dissimilar materials such as silicon and quartz pair.
The thorough preparation of the surfaces for each specific
pair of dissimilar materials can be an alternative to
high-temperature annealing. The most promising results are
achieved when the surface preparation includes a plasma
treatment [1-3]. Even such dissimilar materials as crystalline
silicon and lithium niobat show relatively strong bonding at
room temperature as result of surface activation [1].
Low-temperature technology can essentially reduce the
residual stresses, but does not completely eliminate them for
materials with different thermal expansion coefficients.
Operating conditions of MEMS devices should include a
temperature range as wide as possible. In this connection,
internal stresses distribution must be given proper weight in
designing the bonded structure. This work aims to produce a
strong bonding of silicon-quartz structures with the lowest
possible residual stresses. The experiment was performed by
plasma-assisted activation of silicon and quartz surfaces, with
further annealing in the electric field. Strong bonding, close to
the mechanical strength of the initial materials, has been
achieved.
II. RESIDUAL STRESS IN BILAYER SYSTEM
Stoney’s [4] and Timoshenko’s [5] formulas are often used
to calculate the residual stresses in layered structures. Stoney
analyzed the model of a thin film deposited on thick substrate.
Timoshenko's approach looks the most appropriate for the
bonding because it imposes no restrictions on the thickness of
the layers. This model was originally developed for analysis
of operation of a bimetal strip thermostat and based on the use
of the radius of curvature ρ of a structure which is curved as
result of a difference ∆α of the thermal expansion coefficients
of the layers. The model is also appropriate for description the
residual stresses under bonding of the plates of dissimilar
material at elevated temperature because the bonded wafers
usually have comparable thicknesses in the range between 0.1
mm and 1 mm. In the case of the bonding, ∆T means a
difference between annealing temperature and room
temperature, or more precisely, concrete operating
temperature of the bonded structure.
Let h 1 and h 2 be the thicknesses of bonded plates, E 1 and E 2
are their Young’s modulus, and ∆T is the difference between
annealing temperature and operating c temperature of the
bonded structure. Then the radius of curvature of the strip of
unit width will be [5]
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