Chapter Nineteen - Strength of Bonded Interfaces

Abstract

In this chapter, commonly used bond test methods are briefly presented together with suggestions of improvement. Integrated circuit technology and microsystem technology make good use of wafer bonding. The bond strength is considered one of the most important characteristics to determine about a bond. Several techniques for the determination of the strength of bonded interfaces coexist. Bond strength measurements contribute to the understanding of chemical reactions behind the bonding process, bonding optimization, and quality assurance. Solid mechanics is the base of the models valid for bond strength measurements. For fracture to occur, it is necessary that local stress at the crack tip reaches the theoretical strength of the material and that the fracture is accompanied by global reduction of energy in the material. The most commonly used method to measure strength of wafer bonded interfaces is the double cantilever beam method. Using this method, the bonded samples are forced apart by inserting a thin blade into the bond. In Tensile test method, bonded samples glued onto pull studs fitting a tensile strain test machine and studs thereafter pulled apart, and the maximum force occurring at separation is recorded. Blister tests used in wafer bonding have regained interest lately. An internal cavity at the bond interface is pressurized pneumatically or hydrostatically until fracture occurs and the fracture pressure is a measure of the bond strength. Introduction of specially formed notch, the chevron into the bond under test has improved measurements. Chevrons have been implemented for various load configurations, such as tensile, three-point bending, and blister tests. Techniques for determination of strength of bonded interfaces coexist; common to these is that good measurements only can be made with holistic approach to experimental setup, where every source of measurement error is addressed.