Dr. Aleksej M. Rodin, Via Team Leader, XSiL Ltd.

Through-Silicon Vias (TSVs) for 3D Interconnects are one of the hottest topics of 2008. Two technologies can be used today to produce these vias: DRIE and laser drilling. In the past, laser drilling processes suffered from throughput and size limitations. Laser technology was then directed towards applications with low density requirements such as flash memory. Advances in laser process development and throughput achieved at XSiL will enable the usage of laser-drilled TSVs for a wider range of applications.

Recent advances in the processing of blind vias drilled in silicon by UV DPSS lasers exhibit high throughput, clean top surface and excellent sidewall quality along with negligible heat-affected zone. The previously accepted via size limitation of ~30µm diameter is no longer valid.

With the X300V-Ultra via drilling tool launched by XSiL in 2008, drilling rates exceeding 2000-2500vias/s have been achieved, with no impact on quality. An efficient cleaning process is incorporated into production tools, eliminating residual silicon debris. Via depth uniformity of +/-4% or better is achievable over a 300mm wafer.

The taper angle of the scallop-free via sidewalls approaches the desirable 85⁰ value for void free copper plating and can in turn eliminate the need for ion assistance during the PVD process. Being maskless, the laser-based processes eliminate the need for various expensive lithographic steps such as coating, exposure and photoresist stripping.

Laser-produced vias are safe for active devices and suitable even for structured wafers comprising Cu and Al pads and oxide layers. Via diameters of 10-80µm can be drilled. This range of sizes satisfies the majority of current requirements for industrial interconnect applications. Recently, XSiL obtained its first R&D results for via diameters of 4-6ìm, which addresses targeted roadmaps of DRAM manufacturers till 2012.

It already appears that future advances in NAND flash and DRAM memory applications are difficult to envisage without the advantages of laser technology, and the same could probably be said for CMOS sensors. Applications development in laser machining is ongoing with the majority of customers in the semiconductor market. Companies from Asia have been especially pro-active in these areas over the last few years.