Optical Characterization and Defect inspection for 3D stacked IC technology

Abstract

Advanced packaging technologies are rapidly evolving and 3D architectures requires new inspection and metrology techniques. Existing techniques need to be improved but new techniques must be developed to address new challenges induced by the last fabrication processes. To increase the development speed, it is a big advantage that metrology and defect inspection need to be present on the same platform and a flexible tool, with multi sensors, to be more versatile facing the different step of the process will be presented in this paper As 3D IC devices utilize TSVs for direct interconnect, the depth, top and bottom CD (critical diameter) of such TSVs with a diameter as small as 5 μm with a high aspect ratio is characterized. During wafer temporary bounding, which is an handling technique that allows wafer thinning with a thickness of less than 100 μm, by selecting the most sensitive sensor, determination of the thickness of each layer of the stack could be determined at the same time: silicon substrate, thin glue layer of few microns only and carrier which could be silicon or glass. After back-side processing and wafer thinning, the determination of the remaining silicon thickness (RST) below the TSV could be determined. Moreover back side roughness after grinding is also determined. After wafer thinning process, the TSVs are revealed at the back side of the wafer, leaving to appear copper pillars. The pillars height and co-planarity measurements are then addressed. Post CMP process control will be addressed by full field interferometry especially prior Copper to Copper direct bonding. Concerning the defect inspection, the NIR microscopy is used to control die to wafer stacking process, to reveal voids in the glue and cracks on the grinded silicon substrate. In this paper, we will present fast and nondestructive optical sensors based on low coherence infrared and white light interferometry and spectrometry techniques. These different sensors mounted on the same tool allow characterizing specifically and with an excellent sensitivity the different process steps described above. Concerning the defect inspections, techniques based on infrared microscopy and images techniques processing will be detailed and results will be presented to illustrate the possibilities of this inspection by microscopy.