Investigations of strength properties of ultra-thin silicon

Thin silicon offers a variety of new possibilities in microelectronical and micromechanical industries, e.g. for 3D-integration (stacked dice) or optoelectronic components (LED). The thin wafers are fabricated by back thinning technologies like grinding, polishing or etching and diced into single chips. The separation technologies can be coupled with back thinning technologies (dicing-by-thinning) to increase the reliability and strength of dies. In order to characterize and optimize relevant process steps in terms of quality and fabrication yield, also the mechanical properties have to be investigated with respect to defect formation and strength. In this paper three different dicing technologies were characterized by 3-point bending tests. The first technology is a common sawing process. The second and third technology are "dicing-by-thinning" processes, one with sawn grooves and the other with dry-etched trenches. In addition to the experimental investigations, analytical and numerical calculations were performed in order to understand the nonlinear relationship of force and displacement and to calculate fracture stresses from fracture forces. The results were statistical evaluated by the Weibull theory. Using this approach allows a more comprehensive understanding of the influence of the process on strength properties independently of geometric factors. In particular, it forms a base to predict the strength determined from tests for real devices and to quantify the potential of strength increase by using improved technologies, such as Dicing-by-Thinning. It was shown in this paper, that the nonlinear relationship of force and displacement can be described and explained. Thus the fracture stress as parameter of strength could be calculated for all tested samples. Samples, being separated by "dicing-by-thinning", have much higher strength than simply sawed samples. If trenches are made by dry-etched process the strength can be increased tremendously.