Self-assembled monolayer (SAM) of 1-hexanethiol is applied on copper (Cu) surface to retard surface oxidation during exposure in the ambient. This SAM layer can be desorbed effectively with an annealing step in inert N2 ambient to provide a clean Cu surface. Using this passivation method with SAM, wafers covered with thin Cu layer are passivated, stored, desorbed, and bonded at 250 °C. The bonded Cu layer presents clear evidence of substantial interdiffusion and grain growth despite prolonged exposure in the ambient. This method of passivation is proven to be effective and can be further optimized to enable high quality Cu–Cu direct bonding at low temperature for application in three-dimensional integration.

Cu-Cu diffusion bonding enhancement at low temperature by surface passivation using self-assembled monolayer of alkane-thiol

3D integration with through-silicon via (TSV) is a promising candidate to perform system-level integration with smaller package size, higher interconnection density, and better performance. TSV fabrication is the key technology to permit communications between various strata of the 3D integration system. TSV fabrication steps, such as etching, isolation, metallization processes, and related failure modes, as well as other characterizations are discussed in this invited review paper.

/pdf/three-dimensional-integrated-circuit-3d-ic-key-technology-4nsugch9hi.pdf

Three-Dimensional Integrated Circuit (3D IC) Key Technology: Through-Silicon Via (TSV)

The through-silicon via (TSV) approach is essential for 3-D integrated circuit (3-DIC) packaging technology. TSV fabrication process, however, is still facing several challenges. One of the widely known challenges is via protrusion. Annealing a TSV wafer puts the copper (Cu) TSVs under high stress and may form a protrusion where the Cu is forced out of the blind TSV. This phenomenon occurs because the large mismatch in the coefficient of thermal expansion between Cu via and silicon (Si) surrounding it. Cu protrusion can lead to crack or delamination of the back-end-of-line, thus, it is a risky threat to the metal layer interconnect. Experiments are conducted to characterize the protrusion using several techniques. Scanning electron microscopes and atomic force microscopes are used to observe the protrusion shape and measure the height. An electron backscatter diffraction technique is implemented to study the grain size distribution and evolution inside Cu vias. For the experiment, arrays of 5- $\mu{\rm m}$ TSVs are fabricated and annealed in nitrogen gas environment in different temperatures. In this paper, finite element analysis (FEA) is carried out to study the Cu protrusion under different annealing conditions. Correlation between numerical results and experimental data is then carried out. Based on the verified FEA methodology, several parametric studies are then conducted, including the effect of via diameter, depth, pitch, annealing temperature, and duration on Cu protrusion and TSV stress. The simulation results help to understand and solve the key problem in TSV fabrication process and reliability challenge.

Study on Cu Protrusion of Through-Silicon Via

A method and a system for the enhancement of the sensitivity in surface plasmon resonance (SPR) sensors based metallic grating by exploiting the conical configuration is presented. We consider the propagation of surface plasmon polaritons (SPPs) excited by light from the visible to infrared spectrum range, incident on a plasmonic grating at different directions by varying both the zenith and azimuthal angles. For specific azimuthal angles, SPPs propagate in the grating plane perpendicular to the incident light momentum. This is the condition that allows increasing the number of different excited SPPs modes largely. We exploit this effect to increase the sensor sensitivity with the change of refractive index of thin film on the plasmonic grating surface. Polarization effects also contribute to a further modes enhancement and increase the sensitivity. A scheme for a lab-on-chip implementation of a system that allows a parallel detection in microfluidic channels has been shown.

/pdf/sensitivity-enhancement-in-grating-coupled-surface-plasmon-4651qnvtzd.pdf

Sensitivity enhancement in grating coupled surface plasmon resonance by azimuthal control

In this paper, we demonstrated the plasticity mechanism for copper (Cu) extrusion in through-silicon via structures under thermal cycling. The local plasticity was directly observed by synchrotron x-ray micro-diffraction near the top of the via with the amount increasing with the peak temperature. The Cu extrusion was confirmed by Atomic Force Microscopy (AFM) measurements and found to be consistent with the observed Cu plasticity behavior. A simple analytical model elucidated the role of plasticity during thermal cycling, and finite element analyses were carried out to confirm the plasticity mechanism as well as the effect of the via/Si interface. The model predictions were able to account for the via extrusions observed in two types of experiments, with one representing a nearly free sliding interface and the other a strongly bonded interface. Interestingly, the AFM extrusion profiles seemed to contour with the local grain structures near the top of the via, suggesting that the grain structure not only affects the yield strength of the Cu and thus its plasticity but could also be important in controlling the pop-up behavior and the statistics for a large ensemble of vias.

/pdf/plasticity-mechanism-for-copper-extrusion-in-through-silicon-3a55j3kl8r.pdf

Plasticity mechanism for copper extrusion in through-silicon vias for three-dimensional interconnects

Three-dimensional (3D) integrated circuit (IC) technologies are receiving increasing attention due to their capability to enhance microchip function and performance. While current efforts are focused on the 3D process development, adequate reliability of copper (Cu) through-silicon vias (TSVs) is essential for commercial high-volume manufacturing. Annealing a silicon device with copper TSVs causes high stresses in the copper and may cause a “pumping” phenomenon in which copper is forced out of the blind TSV to form a protrusion. This is a potential threat to the back-end interconnect structure, particularly for low-κ materials, since it can lead to cracking or delamination. In this work, we studied the phenomenon of Cu protrusion and microstructural changes during thermal annealing of a TSV wafer. The extruded Cu-TSV was observed using scanning electron microscopy (SEM), 3D profilometry, and atomic force microscopy (AFM). The electron backscatter diffraction (EBSD) technique was employed to study the grain orientation of Cu-TSV and evolution of the grain size as a function of annealing temperature. The elastic modulus and yield stress of copper were characterized using nanoindentation. A model for Cu protrusion is proposed to provide insight into the failure mechanism. The results help to solve a key TSV-related manufacturing yield and reliability challenge by enabling high-throughput TSV fabrication for 3D IC integration.

Effect of Copper TSV Annealing on Via Protrusion for TSV Wafer Fabrication

A reduction in the bonding temperature required for direct gold-gold (Au–Au) thermocompression bonding is observed by coating a monolayer of dodecanethiol on gold surfaces prior to bonding. The results show that Au–Au bonded joints of 26.9g per bump strength can be achieved at a temperature as low as 160°C. The temperature drop becomes more apparent when both coated and blank gold samples are exposed in air over some time before bonding. The authors propose that self-assembled monolayers passivate metal surface by obviating the adsorption of surface contaminants, in particular, carbon and oxygen but get desorbed just before bonding takes place.

/pdf/self-assembled-monolayers-for-reduced-temperature-direct-22uikcvnjz.pdf

Self-assembled monolayers for reduced temperature direct metal thermocompression bonding

A study on temperature dependence in gold-gold (Au–Au) thermocompression bonding was performed. Gold studs were bonded to two kinds of surfaces—cofired gold on alumina and electroless nickel covered with immersion Au on silicon. A critical bonding temperature was observed for both substrates. No bonding occurs when the temperature is below this threshold value, whereas bond strength increases with bonding temperature beyond the threshold. This critical temperature can be related to the activation of organic films on the bonding surfaces. Under similar bonding conditions, the critical temperature is lower for a harder substrate than for a softer substrate, primarily because of larger interfacial shear stresses. This is supported by the observation on the interfacial shear stress distribution at the bonding interface based on finite element simulation models of substrates with different hardness.

/pdf/critical-temperatures-in-thermocompression-gold-stud-bonding-18uq0mpk7o.pdf

Critical temperatures in thermocompression gold stud bonding

The tensile strength of thermocompression gold joints formed with prior surface coatings of alkanethiol self-assembled monolayers (SAMs) depends on the chain length (n) of the SAM. Enhancement of bond strength is most significant at n=6 while no improvement can be achieved using octadecanethiol (n=18). These contrasting behaviors can be interpreted as a consequence of two dominant roles of alkanethiols that govern the bonding phenomenon, namely, the passivation of gold surfaces and the ease of mechanical and/or thermal displacement.

/pdf/effect-of-chain-length-on-low-temperature-gold-gold-bonding-1gaq3la7rh.pdf

Effect of chain length on low temperature gold-gold bonding by self-assembled monolayers

Thermal desorption investigations on self-assembled monolayers (SAMs) had previously been carried out using techniques such as thermal desorption spectroscopy (TDS), scanning tunneling microscopy (STM) and X-ray photo-electron spectroscopy (XPS). In this paper, the thermal dissociation of alkanethiols (CnH(2n + 1)SH) at various chain lengths (n= 6, 12, 18) on sputtered gold layers was monitored in-situ using the Kretschmann surface plasmon resonance configuration on a spectroscopic ellipsometer. We found that the longest alkanethiol (C18) exhibits the greatest thermal stability, manifested by the least amount of angular shift, during heating, in the resonant spectral features. Predictions of desorption temperatures from SPRS for the longer chain thiols are in good agreement with XPS measurements.

X. F. Ang

Papers

Effect of Copper TSV Annealing on Via Protrusion for TSV Wafer Fabrication

Self-assembled monolayers for reduced temperature direct metal thermocompression bonding

Critical temperatures in thermocompression gold stud bonding

Effect of chain length on low temperature gold-gold bonding by self-assembled monolayers

In-situ monitoring of the thermal desorption of alkanethiols with surface plasmon resonance spectroscopy (SPRS).