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DECEMBER 2018 I DESIGN007 MAGAZINE 61 cuts through the material, a saw "kerf" slightly wider than the blade width is generated. The kerf will add an additional 10–20 μm to the street width. On a 40-μm wide blade, for example, the total material removed would be 60 μm. Saw blade width can impact process throughput as well. A tall, skinny blade will be less stable and prone to wobble and break- age when cutting thick materials. As a rule of thumb, thicker materials will require a wider blade, and in turn, the wider street width must be provided between package elements. With conventional saw blade dicing, a single blade will cut all the way through the material. Edge chipping within the narrow streets, how- ever, is a key quality concern. Chip propaga- tion can be minimized by controlling the feed rate, cut mode, and blade width, concentra- tion, and grit. Single-pass sawing is preferred because it provides greater throughput; how- ever, if the feed rate is not optimized, there will be a greater potential for chipping. To minimize chipping and provide a narrow street width on the active side of the die, the fabricator may adopt a "step-cut" procedure, which uses two blades to sequentially cut at different depths in the wafer (Figure 2). With the active surface facing down, a wide blade is employed to make the first cut from the back- side surface of the wafer (L). A second pass follows using a narrow blade that continues through the silicon, stopping at the surface of the dicing tape (R). Laser Singulation Laser-dicing systems specifically designed for efficient semiconductor backend process- ing are available. The programmable scanner- based cutting technology is fast and flexible and provides extremely precise and small kerf widths (down to 30 μm) and very smooth edges. The systems can provide cutting speeds of up to 200 mm/s for thin wafers with a tol- erance in the range of ±3 μm. Because these systems utilize continuous water jet cooling during ablation, there is no heat impact on the wafer during processing. The examples furnished in Figure 3 compare the finished Figure 2: Two-stage saw singulation. (Source: (L) Spectrum Micro, (R) Photonics Media) Figure 3: Comparison of a saw cut to a laser cut street-edge quality on a silicon wafer.

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