Three-dimensional (3D) integration of ICs provides unique opportunities to improve bandwidth, latency, and power dissipation bottlenecks of interconnects (both on- and off-chip). However, while 3D IC integration improves signal interconnection, it also presents new challenges, especially in power delivery and cooling (“thermal interconnects”). The focus of this paper is on some of the key challenges and promising technologies to address power delivery, cooling, and signaling in a 3D stack of logic ICs.

The recent growth in optoelectronics, nanoelectronics, nanooptics, and chemical and biological sensing has been fueled by the ability to fabricate nanostructures with ever smaller features. However, several significant constraints still remain in terms of cost, limited pattern size, processing conditions, pattern flexibility, and so on. Fabrication of features as small as 50 nm at ambient conditions with high pattern flexibility and low cost remains a serious challenge. Here we report a new solid-state electrochemical imprinting process that is carried out at ambient conditions, requires nominal pressure and very low electric potential, eliminates any liquid electrolytes, shows very high reproducibility, and promises the capability to scale up for large area patterning while retaining a significant cost advantage. Through combination of the best merits of nanoimprint lithography, micro forming, and the solid-state electrochemical imprinting technique, S4, (recently introduced by Hsu et al., Nano Lett., 2007, 7, 446; and Schultz et al., J. Vac. Sci. Techol. B, 2007, 25, 2419), we show a very high pattern flexibility to create nano-scale metallic features.

As a first step, we use a micro-forming-like embossing process to engrave nano-scale features onto a solid electrolyte tool surface using an e-beam fabricated Si mold. Silver sulfide, Ag2S, is used as a solid electrolyte because of its favorable mechanical properties for micro forming and its excellent electrochemical properties. This ionic compound is ductile and has a relatively low yield stress at 80MPa. Followed by embossing, the patterned solid electrolyte surface is then used to carry out the S4 process, creating a negative image on a metallic substrate. This process eliminates the costly Focused Ion Beam milling used by Hsu et al. to create features on the electrolyte tool. It is also highly favorable for large-area patterning as well as mass-production of metallic substrates restricted only by the capability to fabricate the mold at first step. The embossed solid-electrolyte tool surface can be easily trimmed off with a microtome; the tool can then be re-used for embossing and patterning metallic substrates.

Using this process we demonstrate the ability to fabricate silver nanostructures with features <15 nm. Such small features are critical in metal nanostructures for field enhancement that finds applications in SERS and other biological and chemical sensing. So far, a line edge roughness of <10 nm is observed which is significant in the sense that silver is highly mobile and has the tendency to granulate. Finally, we show how this methodology has the capability to fabricate large area patterns at low cost and ambient conditions. As a proof of concept, we demonstrate the ability to fabricate areas >30 sq. mm. Such large scale fabrication is highly desired for applications like biomimetics and patterning for superhydrophobic surfaces.

This study focused on the effect of UV irradiation on modification of polymethyl methacrylate-based photoresist, and then on wet photoresist (PR) removal of patterned structure (single damascene structure). Three single-wavelength UV sources were considered for PR treatment, with λ = 172, 222, and 283 nm. Modification of blanket PR was characterized using Fourier-transform infrared spectroscopy (FTIR; chemical change), spectroscopic ellipsometry (SE; thickness change), and dissolution in organic solvent (solubility change). While for patterned samples, scanning electron microscopy (SEM) was used for evaluation of cleaning efficiency. In comparison to 172 nm, the PR film irradiated by 222 nm and 283 nm photons resulted in formation of higher concentration in C=C bond. Immersion tests using pure N-methyl pyrrolidone (NMP) at 60 °C for 2 min showed that some improvement in PR removal was only observed for PR films treated by 283 nm UV for short irradiation times. Irradiation by photons at the other two wavelengths did not result in an enhancement of removal efficiency.

The PR film treated by 222 nm photons was chosen for further study with O3/H2O vapor at 90°C. Experimental results showed a complete PR and BARC removal for UV-treated PR, which can be explained by C=C bond cleavage by the oxidizer.

In the trough silicon via (TSV) structure for 3-dimensional integration (3DI), large thermal-mechanical stress acts in the TSV caused by the mismatch in thermal expansion coefficient (CTE) of the TSV materials. In this study, the stress of multi-stacked thin silicon wafers composed of copper TSV and copper/low-k BEOL structure was analyzed by the finite element method (FEM), aiming to reduce the stress of TSV of 3D-IC. The results of sensitivity analysis using design of experiment (DOE) indicated that the thickness of the silicon and adhesive layer are the key factors for the structural integration of TSV design.

Ultrathin TiN films were grown by reactive dc magnetron sputtering on amorphous SiO2 substrates and single-crystalline MgO substrates at 600°C. The resistance of the films was monitored in-situ during growth to determine the coalescence and continuity thicknesses. TiN films grown on SiO2 are polycrystalline and have coalescence and continuity thicknesses of 8 Å and 19 Å, respectively. TiN films grow epitaxially on the MgO substrates and the coalescence thickness is 2 Å and the thickness where the film becomes continuous cannot be resolved from the coalescence thickness. X-ray reflection measurements indicate a significantly higher density and lower roughness of the epitaxial TiN films.