Scale-Dependent Mechanical Properties of Advanced Materials for Nanopatterned On-Chip Interconnect Structures

The mechanical robustness of microelectronic products is an increasing challenge, particularly for microchips and chipsets operated in harsh environments, for use cases that require lifetimes much longer than in the past and for safety-critical applications [1,2]. Microcracks in nanopatterned on-chip interconnect stacks, e.g. introduced during dicing of the wafer, are a serious reliability concern since these they can grow and eventually cause catastrophic failure of the microchip. This reliability-limiting degradation process is pronounced by geometrical shrinking of metal interconnects, novel manufacturing technologies and integration schemes as well as new materials used for interconnect stacks of future electronics. In addition, advanced packaging technologies (e.g. hybrid bonding) can cause local thermomechanical stress on wafer or chip level. The risk of fracture is increased for BEoL stacks of microchips if metal interconnects are insulated with so-called low-k materials. These dense or porous CVD-deposited and UV cured organosilicate glass materials are characterized not only by a low dielectric permittivity but also low Young’s modulus and cohesive strength, and consequently low fracture toughness [3]. The understanding of the kinetics of crack propagation provides valuable information for the evaluation of the risk of mechanical failure, however, a quantitative determination of the critical energy release rate for crack propagation is needed for the design of optimized, mechanically robust BEoL structures.<br/><br/>In this talk, the determination of the critical energy release rate in patterned interconnect structures of advanced microchips is described. The implementation of a micro-double cantilever beam (micro-DCB) test in an X-ray microscope allows a 3D imaging of the pathways of microcracks in fully integrated multilevel on-chip interconnect structures with a spatial resolution of about 100 nm [4]. Based on the measured geometric shape of the crack and cantilever bending lines at several loading steps during the micro-DCB test as input data and a data analysis based on the Euler-Bernoulli beam model, the determination of the critical energy release rate G_c for crack propagation was determined in different patterned regions of a wafer manufactured in 14 nm CMOS technology node with a Cu/low-k interconnect stack. The critical energy release rate G_c for crack propagation of a so-called guard ring structure, designed to stop the crack growth, is significantly larger than the respective values in patterned surrounding regions, and about one order of magnitude higher than the G_c values of the respective unpatterned dielectric thin films. These results show that, in addition to the materials properties of the dielectric materials, the geometry of the metal structures is playing an essential role for the fracture behavior of Cu/low-k interconnect stacks [2].<br/><br/>With the methodology described in this talk, conclusions for the robustness of on-chip interconnect stacks can be drawn and input for the design of guard ring structures can be provided. This new approach has important implications for on-chip interconnect technology development and for risk mitigation strategies in semiconductor industry.<br/><br/>The combination of micromechanics and nondestructive high-resolution 3D imaging of opaque materials opens a way for the fundamental study of the structure behavior of nanoscale structures and materials. One century after Griffith’s fracture theory, it will cause a new era of fracture mechanics, namely fracture mechanics in small dimensions.<br/><br/>1. H. Li, M. Kuhn, IEEE Transactions on Device and Materials Reliability 17, 636 - 642 (2017)<br/>2. K. Kutukova, J. Gluch, M. Kraatz, A. Clausner, E. Zschech, Mater. Des. 221, 110946 (2022)<br/>3. A. Grill, S. M. Gates, T. E. Ryan, S. V. Nguyen, D. Priyadarshini, Appl. Phys. Rev. 1, 011306 (2014)<br/>4. K. Kutukova, S. Niese, C. Sander, Y. Standke, J. Gluch, M. Gall, E. Zschech, Appl. Phys. Lett. 113, 091901 (2018)