Current trends in microelectronics focus on three-dimensionally integrating different components to allow for increasing density and functionality of integrated systems. Concepts pursued involve vertical stacking and interconnecting technologies that employ micro bumping, wafer bonding, and through silicon vias (TSVs). Both the increasing complexity and the miniaturization of key elements in three-dimensional (3-D) components lead to new requirements on inspection and metrology tools and techniques as well as for failure analysis methodologies. For metrology and quality assessment in particular, methods operating nondestructively are of major importance. Scanning acoustic microscopy has the ability of illuminating optically opaque materials and, thus, allowing the assessment and imaging of internal structures. Conventional scanning acoustic microscopy (SAM) equipment can be applied to analyze the quality of wafer-bonded interfaces in 3-D integration but may reach its limitations when structures shrink in size and gain complexity. A new concept of acoustic inspection in the gigahertz (GHz) frequency band is explored for its applicability to 3-D integration technologies. Extending the acoustic inspection frequency allows for lateral resolutions in the 1-μm range and also enables the inspection of microbumps and TSVs in addition to wafer bonded interfaces, which exceed the applicability of conventional SAM. Three case studies are presented here ranging from conventional SAM on a full wafer scale to acoustic GHz microscopy on thin films and TSVs.
Waferbonding techniques are frequently used for MEMS/MOEMS fabrication. In this paper, the potential application and methodical limitations of different strength testing approaches including tensile testing and double-cantilever- beam testing for wafer-bonded components are investigated. Special attention is given to the influence of the interfacial atomic bonding strength, the role of interface voids and notches caused by chemical or physical etching steps prior to bonding on the fracture limit. A particular aim of the paper is to discuss the potential of the Micro Chevron-Test for the assessment of the wafer bonding process with particular respect to the quality control during MEMS fabrication. In addition, the methods can also be applied to investigate the lifetime and fatigue properties of wafer- bonded samples exposed to constant or cyclic stresses.
In this paper, the testing principles and different application examples of the Micro-Chevron-Test (MC) are discussed. The chevron pattern required for testing can be fabricated either by wet or reactive ion etching. It is shown that the test has a higher accuracy than common tensile or bend strength tests, allowing also the determination of fracture mechanic parameters, such as fracture toughness. In addition one can characterize the spatial strength distributions for the bonded wafer in order to determine the sources of production yield problems. Furthermore, the sample size can be reduced to the typical size of micro electro mechanical systems (MEMS) devices allowing the MC sample fabrication to be integrated into the MEMS fabrication process. Therefore, the test can be applied as an effective, reliable and precise tool for wafer bond process development and for quality control during the fabrication of micromechanical devices.
The fracture toughness of micromachined polycrystalline silicon samples, pre-cracked with an indenter or notched using a focused ion beam (FIB) machine, were tested using either bending or tensile loading. Fracture mechanics approaches were applied to determine the fracture toughness from these results. For the pre-cracked specimens tested by tensile loading, a fracture toughness value of KI,crit equals 0.86 MP(root)a derived. The FIB notched specimens had higher fracture toughness values, probably due to the influence of the notch tip radius and the FIB process. In addition, fatigue investigations of un-notched tensile specimens were performed using tensile cyclic loading with frequencies of 50, 200 and 1000 Hz. A reduction in the tensile strength from 1.10 GPa to 0.75 GPa after 108 cycles was detected while no influence of the test frequency on the fatigue behavior was observed.
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