Statistical Analysis of the Influence of Thinning Processes on the Strength of Silicon
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1112-E03-09
Statistical Analysis of the Influence of Thinning Processes on the Strength of Silicon Yu Yang1,2, Ricardo Cotrin Teixeira1, Philippe Roussel1, Bart Swinnen1, Bert Verlinden2 and Ingrid De Wolf1,2 1 IMEC vzw, Kapeldreef 75, B-3001, Leuven, Belgium 2 K.U. Leuven, Dept. MTM, Kasteelpark Arenberg 44, B-3001, Leuven, Belgium ABSTRACT Silicon wafer thinning is one of the key enabling techniques in 3D-IC structures. However, the mechanical integrity of Si can be degraded by different thinning steps. In this paper, the strength of Si strips thinned by rough grinding (RG), fine grinding (FG), plasma etching (PE), and chemical-mechanical polishing (CMP) was assessed by 4-point bending. The results were plotted in Weibull statistics and studied with fracture analysis. RG strips show a bimodal Weibull distribution, which is associated with the orientation of the grinding lines. Grinding lines perpendicular to the tensile stress initiate cracks propagating along {110} planes, and lines parallel to the stress result in fracture along {111} planes. FG, PE, and CMP all increase the fracture stress of Si compare with RG. The Weibull plot indicates that PE can enhance part of the FG samples to the level of CMP samples. However, for samples with large defects introduced by FG, there is no improvement after PE. INTRODUCTION Three-dimensional (3D) integration of Si electronic circuits is a promising technology due to its unique advantages, providing the possibility of higher circuit density, better performance, and heterogeneous integration. In the latest International Technology Roadmap for Semiconductors (ITRS), 3D integration has been listed as one of the top three challenges for interconnects [1]. New processes, which may be substantially different from the traditional planar processes, should be evaluated to minimize their influence on the circuits. Wafer thinning is an essential step, allowing 3D stacking of thinned Si chips. However, various studies showed that thinning has an impact on the Si fracture strength. Most studies only focused on the average strength, without checking the failure mechanisms in depth [2-6]. This may result in misleading conclusions and the mechanism of fracture remains ambiguous. In this paper, wafers were thinned by different methods and the mechanical strength was assessed by 4-point bending. The experimental results were analyzed with Weibull statistics, together with fracture analysis. A more thorough understanding of the fracture behavior of Si provides guidance to the industry for the wafer thinning strategy. In most of the 3D integration schemes [7-9], wafer thinning is a necessary processing step to decrease the overall package height and to avoid through-silicon-vias (TSVs) with very high aspect ratio. Generally wafers are thinned by a two-step mechanical grinding procedure, which is made up of rough grinding (RG) and fine grinding (FG). Mechanical grinding is the most efficient method of material removal, but it brings a lot of damage [10]. Therefore damage relief steps, such as
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