Laser Drilling of Through Silicon Vias TSV
Laser Drilling of Through Silicon Vias (TSV)
Figure 1. SEM cross section view of laser drilled through vias in silicon ~35 µm entrance diameters by 65 µm depth show consistency in both diameter and depth, with little thermal damage to the surrounding area
The demand for small, hand-held electronics with more features is driving demand for smaller, more dense electronic packages. Due to size limitations, virtually all next generation microprocessors and memory manufacturing are based on three dimensional (3D) chip or wafer stacking technologies, where smaller footprint devices are built by stacking silicon wafers and/or dies and interconnecting them vertically so that they behave as a single device.
Laser drilled through silicon vias (TSV) as interconnects between the stacked wafers have a number of advantages over conventional chip technologies, including greater packaging density for same footprint, much shorter wire lengths, lower manufacturing costs, higher performance, less power consumption and smaller devices. TSV packages can also be flatter than edge-wired or wire bonded 3D packages.
Today, vias with diameters of 10-80 µm are possible using lasers, typically with diameters <50 µm. In laser drilling, there is a trade off between throughput, cost, and damage to the product. When the pulse width is too long, excessive heat from laser drilling can damage the surrounding crystalline silicon. A shorter pulse width laser helps reduce the damage to surrounding silicon.
The Pulseo 355-20 laser is the perfect tool for through silicon via drilling. The Pulseo laser’s short pulse width, high peak power, and high repetition rate translate to clean processing, round holes, faster throughput, and less thermal damage to the parts. Figure 1 is a SEM cross section of laser-drilled vias in 675 µm thick silicon with ~35 µm entrance diameter and ~65 µm depth showing consistency in both diameter and depth with minimal thermal damage to the surrounding area. The application demonstrated in Figure 2 is drilling ~50 µm size hole in 200 µm thick multi-crystalline silicon (mc-Si) wafer with 18.6 W power at 100 kHz PRF.
TSV Entrance (left) & Exit (right) Details
Figure 2. 50 µm holes drilled in silicon using Pulseo 355-20 laser at 100 kHz PRF with a drill rate of 500 holes/sec