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High-resolution imaging of buried metal interconnect structures in advanced microelectronic products with full-field X-ray microscopy is demonstrated in the hard X-ray regime, i.e., at photon energies > 10 keV. The combination of two multilayer optics—a side-by-side Montel (or nested Kirkpatrick–Baez) condenser optic and a high aspect-ratio multilayer Laue lens—results in an asymmetric optical path in the transmission X-ray microscope. This optics arrangement allows the imaging of 3D nanostructures in opaque objects at a photon energy of 24.2 keV (In-Kα X-ray line). Using a Siemens star test pattern with a minimal feature size of 150 nm, it was proven that features < 150 nm can be resolved. In-Kα radiation is generated from a Ga-In alloy target using a laboratory X-ray source that employs the liquid-metal-jet technology. Since the penetration depth of X-rays into the samples is significantly larger compared to 8 keV photons used in state-of-the-art laboratory X-ray microscopes (Cu-Kα radiation), 3D-nanopattered materials and structures can be imaged nondestructively in mm to cm thick samples. This means that destructive de-processing, thinning or cross-sectioning of the samples are not needed for the visualization of interconnect structures in microelectronic products manufactured using advanced packaging technologies. The application of laboratory transmission X-ray microscopy in the hard X-ray regime is demonstrated for Cu/Cu6Sn5/Cu microbump interconnects fabricated using solid–liquid interdiffusion (SLID) bonding.
In-situ study of crack propagation in patterned structures of microchips using X-ray microscopy
(2023)
The motivation of this thesis was to control crack steering into regions of engineered 3D-nanopatterned structures with high fracture toughness and to determine the local critical energy release rate for crack propagation in 3D-nanopatterned systems. On-chip copper interconnect structures of advanced microchips, insulated by organosilicate glasses, were chosen as an example system to study fracture on small scale, since this is a well-defined 3D- nanopatterned system and since a high mechanical robustness is requested for microchips. An experiment for in-situ high-resolution 3D imaging of the fracture behavior of 3D-nanopatterned structures and of the kinetics of microcrack propagation in solids was designed and applied, combining a miniaturized micromechanical test and high-resolution X-ray imaging. Particularly, a miniaturized piezo-driven double cantilever beam test set-up (micro- DCB) was integrated in a laboratory X-ray microscope, and nano X-ray computed tomography was applied for high-resolution 3D imaging of the microcrack evolution in the on-chip interconnect stack of microchips manufactured in the 14 nm technology node. The measured geometry of the microcrack at several loading steps during the micro-DCB test and the subsequent data analysis based on linear elastic fracture mechanics and the Euler-Bernoulli beam model were the basis for the development and application of a new methodology to determine the critical energy release rate for crack propagation in sub- 100 nm regions of a processed wafer quantitatively. It was experimentally proven that specially designed metallic guard ring structures at the rim of the microchips dissipate energy in such a way that the microcrack propagation is efficiently slowed down and eventually stopped, i.e. they are effective to prevent mechanical damage of microchips. It was demonstrated that it is possible to steer the microcrack in a controlled way by tuning the fracture mode mixity locally at the crack tip. The established concept for a controlled crack propagation provides the basis for further fundamental studies of the fracture behavior of nanoscale materials and structures. The results have significant effects for the understanding of fracture mechanics at small scales, e.g. in microchips, but also in other nanopatterned materials, e.g. in bio-inspired, hierarchically structured engineered materials. The experimental results gathered at realistic microelectronic products provide valuable information to control the crack path in on-chip interconnect stacks for design-for-reliability in semiconductor industry and to manufacture mechanically robust microchips in leading-edge technology nodes. The experimental study of controlled microcrack steering into regions with high fracture toughness provides knowledge for the design of guard ring structures in microchips to stop the propagation of microcracks, e.g. generated during the wafer dicing process.
The method of microscopic imaging using X-rays and diffractive lenses was developed at synchrotron radiation facilities and it was recently transferred to systems with laboratory X-ray sources. The first part of this thesis focuses on instrumentation, in particular on the fabrication, characterization, and application of multilayer Laue lenses (MLL). The second part describes a micromechanical in-situ test that is used to study crack propagation with X-ray microscopy in microchips in a dedicated fracture mechanics experiment called micro double cantilever beam test (MicroDCB).
MLLs were fabricated from WSi2/Si multilayer coatings using mechanical preparation and focused ion beam milling. Initial characterization of the obtained lenses using scanning electron microscopy and X-ray microscopy was used to evaluate the quality of the multilayer stack and particularly to identify geometrical imperfections of individual lens elements. Crossed partial MLLs were assembled as a compact lens device for two-dimensional operation, i.e. point focusing of synchrotron radiation or full-field transmission imaging. The optical properties were simulated using a geometrical optics approximation and a physical optics model. Experimental results verify full-field imaging using crossed partial MLLs with a focal length of 8.0 mm for Cu-Ka radiation in a laboratory X-ray microscope. Sub-100 nm resolution is shown and remaining aberrations are discussed. So-called wedged MLLs employ dynamic diffraction to increase the diffraction efficiency. A fabrication process is presented that allows a subsequent geometrical modification of the lens element using a stress layer. Thus, the wedged geometry is realized independently of the multilayer coating. The resulting layer tilt is measured using a laboratory X-ray microscope. First investigations of such wedged MLLs with synchrotron radiation at a photon energy E=15.25 keV show an enhancement of the diffraction efficiency of 57 % in comparison to a tilted MLL with the same dimensions.
The long working distance of the X-ray microscope facilitates the integration of customized equipment to perform in-situ experiments. The MicroDCB tester was designed and built to drive a crack in an appropriately prepared specimen. It is compatible with the X-ray microscope and it allows tomographic studies under load. In particular, the method was applied to investigate crack propagation in the on-chip interconnect stack of advanced microelectronics products. Stable crack propagation at this location was achieved. Subsequent tomographies were acquired at several load steps. The reconstructed datasets show no critical distortions. This test is assumed to provide valuable information about crack propagation such heterogeneous structures, what is of interest to address reliability issues.