Analysis of laser processing of metal wires used in microelectronics applications

Metal fuses for laser redundant links have been used for years in laser repair applications. However, reliability problems occur for laser metal cut structures, such as the material remaining at the bottom of the cut site or the formation of a lower corner crack. Furthermore, the improvement of laser cut processing remains an unsolved problem due to the variety of materials used for links as well as the complex thermo-mechanical mechanism of laser processing. This work discusses various aspects of the laser metal cut processing and the vertical make linking. Simulation results prove earlier experimental results that a laser-energy window exists for each cut structure under a specified laser pulse.; Experimental observations reveal that the differential between upper corner stress and lower corner stress is temporarily dependent on the passivation breakthrough caused by upper corner cracks as predicted by simulation, and that lower-corner cracks can be formed at lower laser-energy level below passivation-break threshold energy. 3-D finite element modeling shows that upper corner stress in the case of pad cut structure is higher than that of simple line cut structure but the lower corner stress is lower due to the stress-relief effect. This proves that the pad cut structure is an improvement over simple line cut structure by increasing the time difference between upper cracking and lower cracking. In the analysis of copper cut process, upper cracking which is parallel to surface suggests lateral make-linking would be successful for the laser process of copper. In contrast to the metal cut link process, previously developed lase-induced vertical make link, connecting two different levels of metal lines using a laser pulse, is also studied in this work. From the scalability study of make link structure, two limitations are found in the process of scaled-down link structures which are the ratios of the thickness of inter-layer dielectric and the quality of laser beam parameters. A simple equation to estimate the scalability is acquired and 0.5 μm is evaluated to be possible for the metal 1 line with a 0.6 μm thick metal 1 line and a 0.5 μm thick inter layer dielectric. Energy process windows of various make link structures were simulated through finite element modeling and the results show a decreasing trend of relative energy window for smaller links, which is consistent with experimental results. It is also shown that there always exists an acceptable energy process window for any scaled links. A simple equation to evaluate the optimal spot size of laser beam for various link structure is presented. Dense layouts of interconnect were designed and proposed with a 1 μm pitch of 0.5 μm wide metal 1 line. As an application, an LPMCM substrate is designed using the vertical make-link structure.