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A comparison of the Cu electromigration behavior between Cu lines with 5 m L/S realized by conventional Semi Additive Process (SAP) and an innovative Excimer Laser Damascene Process
Keywords: excimer laser, electromigration, reliability
The technological evolution regarding multi-chip integrated Fan-Out packages and CSPs with high amounts of I/Os demands for even higher routing densities. Conventional used technologies and materials like mask aligner and photosensitive polymers used for semi additive process (SAP) in the BEOL have reached its limits to push the resolution down to two m. New materials and technologies are needed to overcome these limits. As the routing density increases, so does the reliability requirements. The electromigration between Cu lines cannot be neglected and needs to be analyzed as the distance between the Cu lines is decreasing. A new approach for fine-line multi redistribution layers realized by an excimer laser dual damascene process was presented in the past. Trenches and micro-VIA in the polymer were generated within the same process by an excimer laser ablation using a laser stepper with a reticle mask. The ablation depth and by that the depth of embedded lines was controlled by the number of laser pulses. The trenches and micro-VIAs were metallized with copper by a galvanic process and the following Cu-CMP step removed the copper outside the ablated structures. As the Cu-CMP removed only the copper, the metal of the seed-layer, which has the functions of an adhesion and barrier layer, stays intact. The remaining part of the seed-layer, which is usually Ti, was also removed by the excimer laser system. This approach has several advantages regarding the processing as the reduced amount of process steps, the lower consumption of wet chemicals like developers and etch chemicals and the possibility to structure non-photosensitive polymers. One of the most important characteristics of the damascene approach is the improved electromigration behavior. Due to the way of processing are the Cu lines partially cladded by the Ti part of the seed layer. The Ti acts as a barrier layer and inhibits the Cu migration into the surrounding polymer. RDL structures realized by conventional SAP have Ti only under and not between the Cu lines. Different test samples with interdigital structures (interdigital capacitor IDC) with five m lines and space embedded in polymer were realized to analyze the electromigration behavior between the fingers of the IDC. The IDCs were realized by SAP and by the excimer laser damascene process. Two types of polymer were chosen for the preparation of the test samples. One polymer, which is commonly used by the industry and has known good electromigration characteristics and one polymer, which is known to be not Cu compatible as the Cu migration is not inhibit by the polymer itself. All samples were tested by the temperature humidity bias (THB) test. Therefore, the samples were stored under 85 percent relative humidity at 85 degree Celsius (85/85) with an applied voltage of 5 V for 1000 hours. The leakage current of the IDC was measured in situ during the storage. In general, the leakage current is the indicator for this kind of reliability test as the current rises, if the Cu migrates between the copper lines. With the help of this work, we were able to compare the reliability of both process variants and we were able to demonstrate and prove the excellent reliability of embedded copper lines realized by the excimer laser damascene process.
Robert Gernhardt,
Fraunhofer Research Institution for Reliability and Microintegration IZM
Berlin, Berlin

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