Improvement of laser dicing system performance I: - High-speed, high-quality processing of thick silicon wafers using spatial light modulator -

In the laser wafer dicing technique of stealth dicing (SD), a laser beam that is tightly focused inside a silicon wafer is scanned multiple times at different depths. The focused beam creates multilayered cracks that allow dry, debris-free dicing. To reduce the dicing time, it is desirable to produce longer cracks with each scan. However, when the laser beam is focused in a deep region of the wafer, the beam is blurred, and its power density decreases owing to spherical aberration caused by a refractive index mismatch between air and the wafer. Consequently, the generated cracks become shorter. We present an approach to making longer cracks deep within the wafer by correcting the spherical aberration. This correction is made using an SD machine incorporating a phase-only spatial light modulator to apply aberration correction patterns, which are calculated by a method based on inverse ray tracing. Experimental results using 300-mu m wafers show that, when the aberration was corrected, the cracks formed during multidepth scans became longer even deep within the wafer and that the dicing speed with correction is more than twice that without correction. This is because each scan produced longer cracks, so fewer scans were necessary. We also demonstrated that the quality of dicing was improved.