This paper investigates the failure causes for slopped through silicon vias (TSV) and presents process improvement for implementing the slopped TSV for 3D wafer level packaging (WLP). IMEC is developing slopped and scaled generic approaches for 3D WLP. Previously we have reported on the integrated process flow for the slopped (TSV) and showed the feasibility of Parylene N as a dielectric material. In the TSV process discussed here, firstly 200mm device wafer is bonded facedown on a carrier using temporary glue layer and thinned by grinding. TSV's are realized by dry etching from the wafer backside, followed by dielectric deposition and patterning. Dielectric patterning is done at the bottom of the via on 100 microns thin silicon device wafer supported by the carrier. Finally, conformal plating is done inside the via to obtain the interconnections.
This paper discusses the yield killer or failure causes in the slopped TSV process. There can be many parameter including silicon etch uniformity, dielectric etching at the bottom of the via and resist residue inside the via that can reduce the yield of the process. We report that one of the main factors contributing to the yield loss is silicon dry etching effects including non-uniformity and notching. Using standard Bosch etching process, notching at the interface between landing oxide and silicon has been observed. The notching cause a discontinuity at the bottom of the via resulting in no plating at the bottom interface.
In this paper we report on a new via shape that is a combination of slopped and straight etching sequence to overcome the notching problem. Different parameters including influence of grinding marks, mask opening, wafer thickness variation, etching rate and etching profile across the wafer were investigated. The optimized design rules for mask opening and effect of individual etching parameters on the etching profile will be presented. In etching, firstly a sloped via with slope of 60 degrees is optimized with changing different etching parameters including different gasses and pressure. Slope via facilitates in subsequent dielectric deposition and sputtering processes. Secondly, a straight wall etching process based on Bosch process and soft landing step with longer passivation steps were investigated to obtain the notch free etching profile. The optimized etching process is notch free, very repeatable and total variation across different wafers is less then 2 percent for 100 micron target opening.
This paper reports the failure analysis of TSV and discuses the processes improvement to obtain higher yielding vias. Different parameters that reduced the yield are discussed with main focus on notching effects during silicon etching. An improved and characterized, notch free uniform silicon etching across the wafer process based on two step etching is presented. An integration flow implementing the above optimized parameters with electrical yield will be detailed in the paper.