Steady-state Measurement of the Interface Fracture Resistance in Wafer Bonding

A steady-state wedge-opening test has been developed in order to measure the work of separation of bonded silicon wafers. Non-steady-state and steady-state measurements are compared. Significant influence of i) the surface treatment, ii) the annealing time and temperature and iii) the crack velocity on the toughness is observed and related to the interface chemistry. A methodology based on the insertion of a thin plastically deforming layer near the interface is proposed in order to extract the critical strength of the interface as well as to increase the fracture toughness. Introduction Wafer bonding is a key microfabrication technique that allows assembling MEMS parts and packaging of microsystems. Reliability of MEMS requires the development of characterization and modelling tools that allow the assessment of the integrity of bonded interfaces towards fracture and delamination. According to the recent literature [1,2], a minimum of two parameters is necessary to fully characterize the mechanical response of an interface : i) the work of separation which represents the energy per unit area needed for propagating a crack, noted Gc or Γ and ii) the maximum stress, σc, reached in front of the crack tip in the so called fracture process zone. In order to measure Gc and evaluate various bonding techniques and rate effects, a steady-state wedge-opening test has been developed following earlier works in the literature on the static wedge-opening method [3,4]. The second parameter, σc, could, in principle, be determined by simple uniform tensile or shear tests providing a value of these critical stress required for wafer separation. However, such measurements heavily depend on interface defects and do not deliver the intrinsic strength of the bond [3]. An indirect method has been developed in order to extract the critical stress. The idea is to insert a thin ductile interlayer near the interface in samples presenting identical interfaces. The amount of energy dissipated by plastic deformation of the interlayer is very much dependent on the critical strength of the interface. This interlayer method serves as a “strength sensor”. A model has been developed in order to quantify the link between the global toughness taking into account for plastic dissipation and the strength. The samples preparation is presented in a first section. Then, the measurement is described followed by a presentation and discussion of the main experiment results (without * Corresponding author : Pr. T. Pardoen * Address : Unité PCIM, 2 place Ste Barbe, 1348 Louvain-La-Neuve