High-resolution imaging of Cu/low-k on-chip interconnect stacks in advanced microelectronic products is demonstrated using full-field transmission X-ray microscopy (TXM). The comparison of two lens-based laboratory X-ray microscopes that are operated at two different photon energies, 8.0 keV and 9.2 keV, shows a contrast enhancement for imaging of copper nanostructures embedded in insulating organosilicate glass of a factor of 5 if 9.2 keV photons are used. Photons with this energy (Ga-Kα radiation) are generated from a Ga-containing target of a laboratory X-ray source applying the liquid-metal-jet technology. The 5 times higher contrast compared to the use of Cu-Kα radiation (8.0 keV photon energy) from a rotating anode X-ray source is caused by the fact that the energy of the Ga-Kα emission line is slightly higher than that of the Cu-K absorption edge (9.0 keV photon energy). The use of Ga-Kα radiation is of particular advantage for imaging of copper interconnects with dimensions from several 100 nm down to several 10 nm in a Cu/SiO2 or Cu/low-k backend-of-line stack. Physical failure analysis and reliability engineering in the semiconductor industry will benefit from high-contrast X-ray images of sub-μm copper structures in microchips.

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.

Visualization of voids and copper extrusion in copper-filled through-silicon vias (TSVs) under different annealing conditions is greatly enhanced using X-ray microscopy. In addition, the dimensions of the TSVs after Cu deposition can also be measured by 3D reconstruction. Surface inspection found mushroom defects on the top of TSVs after deposition of the etch stop layer, forming a small bump on top of the TSV. The similar to 50 nm etch stop layer is part of the dielectric stack used before metal 1 is deposited on top of the TSVs. The TSVs are annealed, then undergo chemical mechanical polishing (CMP) before the etch stop layer is deposited at 350 degrees C. It is assumed that after CMP the etch stop layer on top of the TSVs is planar. However, cross-sectional scanning electron microscopy (XSEM) shows mushroom defects but no particles at the interface of the TSV and the etch stop layer.This study investigates the changes in copper extrusion and induced voiding that result from different annealing temperatures. Round TSVs similar to 25 mu m deep and 4-5 mu m in diameter were filled with copper by electrochemical deposition (ECD), followed by chemical mechanical polishing and pre-annealing to 150 degrees C for one hour. Based upon the material properties of TSVs, the high density copper absorbs photon energy more strongly than silicon or air. This results in good contrast within the copper-filled vias when X-ray micrographs are taken and allows voids within the material to be seen. A lab-based X-ray microscope with 8 Kev X-ray energy allows a penetration of similar to 50 mu m into the silicon. Consequently, sample preparation requires wafers to be backside polished from an initial thickness of 775 mu m to <50 mu m. When scanned with a lab-based X-ray microscope, a set of TSVs is imaged and the 3D X-ray tomography shows voids or seamlines at the bottom of the TSVs. This sample was annealed to 225 degrees C and 300 degrees C, respectively. Upon scanning the same TSVs, the X-ray micrograph shows the seamline changes shape, voids are induced at the center of TSVs, and copper extrudes on top of the TSVs. This suggests the copper extrusion causes the etch stop layer to form a defect at the top of the TSVs. An X-ray microscope offers the potential to inspect voids in multiple vias at the same time without a physical cross section of the TSVs. This technique can be used to advance the study of different stress-induced voids used for 3D interconnects. (C) 2011 Elsevier B.V. All rights reserved.