Optical Run-Out Correction for Improved Lithography Overlay Accuracy for FOWLP Applications

Abstract

Abstract Finer features sizes in advanced Wafer-level-packaging (WLP) processes create increasing demands to improve lithography equipment overlay control. The improvement goes beyond the inherent overlay capability of the equipment (“tool-to-itself”), but requires ways to compensate for mismatches in actual feature size and position on the wafer with respect to the photomask pattern. In Fan-Out wafer level packaging (FOWLP), error contributions from the die placement robot, and especially errors from expanding and shrinking mold compound in which the dies are embedded (magnification error), lead to undesirable errors in layer to layer overlay. Mask-less laser direct imaging (LDI) technologies or die-by-die alignment strategies on steppers can deal with these error contributions, but they cannot provide adequate throughput and have a high cost-of-ownership, and therefore offer a limited solution for the industry. An Active Optical Magnification Correction and Beam-Steering technology option has been developed, available on a full-field UV projection scanner tool. This option allows compensating any type of run-out or run-in up to +/− 200 ppm, or die shifts of +/− 30 μm on a 300 mm wafer, which corresponds to the amount of magnification error caused by the embedding of dies in mold compound nowadays. In addition, an additional optical asymmetrical correction function has been developed to compensate shifts of up to 50ppm in either X or Y. This paper will detail the principle and realization of this optical magnification correction technology. Performance data results for magnification error mitigation will be presented, and its effects on overlay, patterning performance, resolution, and throughput will be studied.