Freestanding and Permeable Nanoporous Gold Membranes for Surface-Enhanced Raman Scattering.

Surface-enhanced Raman spectroscopy (SERS) demands reliable, high-enhancement substrates in order to be used in different fields of application. Here we introduce freestanding porous gold membranes (PAuM) as easy-to-produce, scalable, mechanically stable, and effective SERS substrates. We fabricate large-scale sub-30 nm thick PAuM that form freestanding membranes with varying morphologies depending on the nominal gold thickness. These PAuM are mechanically stable for pressures up to more than 3 bar and exhibit surface-enhanced Raman scattering with local enhancement factors from 104 to 105, which we demonstrate by wavelength-dependent and spatially resolved Raman measurements using graphene as a local Raman probe. Numerical simulations reveal that the enhancement arises from individual, nanoscale pores in the membrane acting as optical slot antennas. Our PAuM are mechanically stable, provide robust SERS enhancement for excitation power densities up to 106 W cm-2, and may find use as a building block in SERS-based sensing applications.

Resonant Optical Antennas with Atomic-Sized Tips and Tunable Gaps Achieved by Mechanical Actuation and Electrical Control.

Enhanced electromagnetic fields in nanometer gaps of plasmonic structures increase the optical interaction with matter, including Raman scattering and optical absorption. Quantum electron tunneling across sub-1 nm gaps, however, lowers these effects again. Understanding these phenomena requires controlled variation of gap sizes. Mechanically actuated plasmonic antennas enable repeatable tuning of gap sizes from the weak-coupling over the quantum-electron-tunneling to the direct-electrical-contact regime. Gap sizes are controlled electrically via leads that only weakly disturb plasmonic modes. Conductance signals show a near-continuous transition from electron tunneling to metallic contact. As the antenna's absorption cross-section is reduced, thermal expansion effects are negligible, in contrast to conventional break-junctions. Optical scattering spectra reveal first continuous red shifts for decreasing gap sizes and then blue shifts below gaps of 0.3 nm. The approach provides pathways to study opto- and electromolecular processes at the limit of plasmonic sensing.

Insights into Laser Ablation Processes of Heterogeneous Samples: Toward Analysis of Through-Silicon-Vias.

State-of-the-art three-dimensional very large-scale integration (3D-VLSI) relies, among other factors, on the purity of high-aspect-ratio Cu interconnects such as through-silicon-vias (TSVs). Accurate spatial chemical analysis of electroplated TSV structures has been proven to be challenging due to their large aspect ratios and their multimaterial composition (Cu and Si) with distinct physical properties. Here, we demonstrate that these structures can be accurately analyzed by femtosecond (fs) laser beam ablation techniques in combination with ionization mass spectrometry (LIMS). We specifically report on novel preparation approaches for the postablation analysis of craters formed upon TSV depth profiling. The novel TSV sample preparation is based on deep and material-selective reactive-ion etching of the Si matrix surrounding the Cu interconnects thus facilitating systematic focused-ion-beam (FIB) investigations of the high-aspect-ratio TSV structures upon ablation. The particular structure of the TSV analyte combined with the ⌀beam > ⌀Cu-TSV condition allowed for an in-depth investigation of fundamental laser ablation processes, particularly focusing on the redeposition of ablated material at the inner side-walls of the LIMS craters. This phenomenon is of imminent importance for the ultimate quantification in any laser ablation-based depth profiling. In addition, we have developed a new method which allows the unambiguous determination of the crossing-point of the Si/Cu||bare Si interface upon Cu-TSV depth profiling which is based on pronounced, depth-dependent changes in the mass-spectrometric detection of those Si xy+ species formed upon the LIMS depth erosion.

Depth Profiling and Cross-Sectional Laser Ablation Ionization Mass Spectrometry Studies of Through-Silicon-Vias.

Through-silicon-via (TSV) technology enables 3D integration of multiple 2D components in advanced microchip architectures. Key in the TSV fabrication is an additive-assisted Cu electroplating process in which the additives employed may get embedded in the TSV body. This incorporation negatively influences the reliability and durability of the Cu interconnects. Here, we present a novel approach toward the chemical analysis of TSVs which is based on femtosecond laser ablation ionization mass spectrometry (fs-LIMS). The conditions for LIMS depth profiling were identified by a systematic variation of the laser pulse energy and the number of laser shots applied. In this contribution, new aspects are addressed related to the analysis of highly heterogeneous specimens having dimensions in the range of the probing beam itself. Particularly challenging were the different chemical and physical properties of which the target specimens were composed. Depth profiling of the TSVs along their main axis (approach 1) revealed a gradient in the carbon (C) content. These differences in the C concentration inside the TSVs could be confirmed and quantified by LIMS analyses of cross-sectionally sliced TSVs (approach 2). Our quantitative analysis revealed a C content that is ∼1.5 times higher at the TSV top surface compared to its bottom. Complementary Scanning Auger Microscopy (SAM) data confirmed a preferential embedment of suppressor additives at the side walls of the TSV. These results demonstrate that the TSV filling concept significantly deviates from common Damascene electroplating processes and will therefore contribute to a more comprehensive, mechanistic understanding of the underlying mechanisms.

Combining Anisotropic Etching and PDMS Casting for Three-Dimensional Analysis of Laser Ablation Processes.

State-of-the-art laser ablation (LA) depth-profiling techniques (e.g. LA-ICP-MS, LIBS, and LIMS) allow for chemical composition analysis of solid materials with high spatial resolution at micro- and nanometer levels. Accurate determination of LA-volume is essential to correlate the recorded chemical information to the specific location inside the sample. In this contribution, we demonstrate two novel approaches towards a better quantitative analysis of LA craters with dimensions at micrometer level formed by femtosecond-LA processes on single-crystalline Si(100) and polycrystalline Cu model substrates. For our parametric crater evolution studies, both the number of applied laser shots and the pulse energy were systematically varied, thus yielding 2D matrices of LA craters which vary in depth, diameter, and crater volume. To access the 3D structure of LA craters formed on Si(100), we applied a combination of standard lithographic and deep reactive-ion etching (DRIE) techniques followed by a HR-SEM inspection of the previously formed crater cross sections. As DRIE is not applicable for other material classes such as metals, an alternative and more versatile preparation technique was developed and applied to the LA craters formed on the Cu substrate. After the initial LA treatment, the Cu surface was subjected to a polydimethylsiloxane (PDMS) casting process yielding a mold being a full 3D replica of the LA craters, which was then analyzed by HR-SEM. Both approaches revealed cone-like shaped craters with depths ranging between 1 and 70 µm and showed a larger ablation depth of Cu that exceed the one of Si by a factor of about 3.