UV Projection Scanner Performance in Thick Resists for High Aspect Ratio Cu Pillars

Abstract

Integration in the third dimension is becoming increasingly more common in advanced packaging to overcome limitations in Moore's Law. One popular example for 3D Integration is Package-on-Package (PoP), where memory stacks are mounted above the processor. This approach requires tall, high density Cu pillars for interconnection around the processor. Ever increasing I/O requirements creates a need for Cu pillars with smaller critical dimension (CD) in thick resist. This creates high aspect ratio challenges for materials and equipment. For lithography tools, low NA projection systems are fundamentally well-suited to achieve high aspect ratio patterns, due to the inverse relationship between depth-of-focus (DOF) and Numerical Aperture (NA). However, the NA of low NA steppers is not low enough for thick resists. The full-field projection scanner with lower NA provides superior performance than the stepper counterpart at higher throughput and lower cost. This paper presents the high aspect ratio performance of the full-field UV projection scanner tool in various thick resists ranging from 50um to 300um film thickness. Aspect ratios as high as 20:1 are demonstrated for square vias with 1:1 pitch. Further, as feature resolution is only practical with alignment, the alignment of thick resists is also included.