Thinning and Singulation of Silicon: Root Causes of the Damage in Thin Chips

Abstract

Let us have a look at the main spheres for silicon chips: The processors you find in your PCs, memory-chips used in nearly every electronic module, the power-devices your car is full of and the smart-labels found in access-cards, tickets and labeling applications which make our daily life easier. For each of these applications the chips are going thin. What is the driving force of this development, why are we going thin? For processors the main advantage of thin substrates is improved heat dissipation. Stacking of memory chips needs also thin silicon. For power devices it is reduction in electrical resistance. For smart-cards and related applications the main feature is the flexibility of thin silicon, which makes the IC-chip capable of surviving daily use. The question is: what are the mechanical properties after thinning and chip-singulation? What are their root causes? We have done investigations to find and evaluate causes of the mechanical damage in thin chips. We begin with a search for the stability necessary for a safe and reliable handling of the chips. We demonstrate the resulting threshold for chip-stability and some processes to reach this limit. In a next step we show the influence of chip-separation technologies. Influences to the chip side wall can result in mechanical damage or change stability. Our paper shows results regarding back side hardness and also the differences in mechanical properties between blank chips and processed devices. We present results of our investigations regarding dicing quality and influences to the product chips. Especially the resulting side wall surface of the chips is characteristic for the separating technology applied. Looking at silicon from a material point of view it is hard, stable and brittle. Thus the wafers made of silicon show certain fragility. Especially during wafer thinning, the wafers are stressed mechanically. The reduced stability on wafer level is one of the reasons for wafer breakage. But for back-end processes and product-applications the stability on chip level is of more importance. Pre-Assembly is showing the major influence on mechanical properties of silicon chips. The two main process groups here are wafer-thinning and chip-separation. Also in final applications stresses, coming from e.g. temperature changes, have to be balanced by excellent mechanical performance. Besides the features needed for the chip-applications, chip-stability is also necessary for the process chain from chip-separation to packaging. We did investigations in dicing quality looking at kerf-width, residues, side wall damage and chip-stability. As a result we can judge the different root causes influencing chip damage and stability.