Marks, Michael Raj
(2016)
Effect Of Nanosecond Laser Dicing On Ultrathin Silicon Die With Copper Stabilization Layer.
Masters thesis, Universiti Sains Malaysia.
Abstract
Die ultra-nipis memerlukan satu lapisan kuprum (Cu) penstabilan di belakangnya untuk menghalang ledingan dan retakan semasa proses pengimpalan die serta penyambungan dawai. Pemotongan wafer silikon (Si) dengan lapisan Cu di belakangnya sangat mencabar. Pemotongan dengan bilah secara mekanikal akan mengakibatkan tersumbatnya bilah tersebut dan kerosakan akan dialami. Hasilannya, kerosakan die akan berlaku. Pemotongan menggunakan plasma berkos tinggi dan memerlukan proses tambahan seperti fotolitografi dan punaran. Pemotongan dengan laser mempuntai prospek yang baik dan sekarang ianya digunakan untuk memotong wafer Si yang nipis. Tetapi, tiada kajian yang melaporkan penggunaan teknik ini untuk memotong wafer ultra-nipis dengan lapis Cu dibelakang. Kajian ini menunjukkan kebolehan untuk menghasilkan wafer Si ultra-nipis setebal 20 μm dengan lapisan Cu depan setebal 5-20 μm dan belakang setebal 10-30 μm. Ketebalan lapisan logam dan Si berada dalam 10% sasaran proses. Tiada pengasingan di antaramuka dapat dikesan. Keupayaan pemotongan atas wafer Si ultra-nipis dengan 10-30 μm lapisan Cu di belakang dengan menggunakan laser nano-saat ultra-ungu telah dibuktikan. Kesan laser nano-saat ke atas kekuatan dinding tepi die telah dinilai berasaskan ujian bengkok tiga titik (3PB).
Ultrathin dies require a Cu stabilization layer, which is essentially a backside Cu layer, to prevent warpage and cracks during solder die attach and wire bonding. The dicing of Si wafers with a backside Cu layer is challenging. Mechanical blade dicing through the Cu layer causes blade clogging and damage, which eventually results in severe die chipping and cracks. Plasma dicing is costly as it requires additional photolithography and etching steps. Laser dicing is promising and is currently used to singulate thin Si wafers. However, there is no reported work on its application for dicing ultrathin wafers with a backside Cu layer. In this work, the feasibility of fabricating 20 μm ultrathin Si wafers with 5-20 μm frontside Cu and 10-30 μm backside Cu has been shown. The thicknesses of the metal and Si layers are within 10% of the process target. No interfacial delamination was detected. The feasibility of dicing through 20 μm ultrathin Si wafers with 10-30 μm backside Cu with nanosecond UV laser have also been demonstrated. The effect of nanosecond laser dicing on the die sidewall strength was evaluated with the three-point bend (3PB) test.
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