Inge De Preter

3D stacking of Co- and Ni-based microbumps

In this paper,the wettability, quality of joint formation and electrical yield of daisy chains in 3D stacks when using Cobalt and Nickel as UBM with different finish layers such as immersion Au, ELD NiB, ELD Cu and SAM are investigated. The performance of the stacks are characterized by cross-section SEM images, EDS analysis and electrical resistance measurement of the daisy chains.

3D stacking cobalt and nickel microbumps and kinetics of corresponding IMCs at low temperatures

To improve the performance of 3D electronic chips, dense I/O and interconnects are required. Increasing the density of interconnects requires smaller pitch micro-bumps. However, when scaling down microbumps several challenges have to be taken into account. Lithography of dense and high aspect ratio bump, wet etching of seed and barrier layer, solder volume and intermetallics (IMC) formation are some of the challenges that needs to be addressed. With reducing bump dimensions, solder volume decreases as well, converting Sn to complete IMC during the Thermo-Compression-Bonding (TCB) process. Full IMC formation increases stress in the joint, leading to crack formation and a brittle connection. Beside concerns about the IMC layer, the UBM (under bump metallization) consumption by the solder has to be addressed as well. Therefore, it is important to select the right UBM and solder to have enough Sn and UBM left in the joint for the time the product is working at a specific temperature [1].

Growth rate of IMC in the binary sytems of Co/Sn and Cu/Sn

In this study we discuss superiority of Cobalt for using in 3D interconnection as alternative metal to Cu. Specimens composed of pure Sn and Co or Cu is aged under same aging condition varied time and temperature below melting point of Sn. Thickness of IMC (intermetallic compound) formed at the interface is then calculated for extraction of growth rate and kinetics like activation energy and power factor. Compared with each factors extracted from calculation, IMC formation and growth of Co/Sn has stronger time and temperature dependence than IMC of Cu/Sn. Furthermore, no voids was observed at any interface in Co/Sn bonding.

Investigation of Co Thin Film as Buffer Layer Applied to Cu/Sn Eutectic Bonding and UBM With Sn, SnCu, and SAC Solders Joints

The demand of small-feature-size, high-performance, and dense I/O density applications promotes the development of fine-pitch vertical interconnects for 3-D integration where microbumps are fabricated with Cu through-silicon via and under-bump metallization. Small dimension Cu/Sn bonding has to be developed to address the needs of increasing I/O density and shrinking pitch and size for future applications. For fine-pitch microbumps, it is important to select right UBM and solder materials to obtain lower UBM consumption, which means lower intermetallic compound (IMC) thickness. To find the best binary system material for fine-pitch microbumps with a different annealing temperature and time, we investigate the interfacial reaction and intermetallic compound morphologies of Co UBM with Sn, SnCu, and SAC solders. A thin, uniform, and single-phase IMC between solder and UBM facilitates finer pitch and more reliable microbumps development; the higher activation energies imply longer solder lifetime. Co, as an ultrathin buffer layer (UBL), is also used in Cu/Sn bonding. A comparison between Cu–Sn bonding with and without UBL is conducted. From this study, Co as UBL and UBM is explored and could be applied in semiconductor applications.

Impact of ELD layers in mechanical properties of microbumps for 3D stacking

In this paper the influence of adding ELD barrier and capping layers in die shear strength of 3D stacked chips is discussed. Electroless NiB is used as barrier layer to prevent solder or UBM consumption and immersion Au is used as capping layer to improve the solder wettability. In this study UBM layers are Cu, Co and Ni and pure Sn is used as solder. For bonding both reflow and TCB methods are employed. Mechanical properties of bonded 3D stacks with different UBM/solder systems are characterized by die shear test followed by SEM and EDX analysis for fracture location and mechanism identifications.

Thiol‐Based Self‐Assembled Monolayers (SAMs) as an Alternative Surface Finish for 3D Cu Microbumps

With scaling beyond 40um pitch 3D interconnects, cost, performance and reliability become ever more critical. Thiol-based self-assembled monolayers (SAM) were applied before to enable Cu-Cu connection in dual damascene vias [1]. In this study, we are researched a parallel application, for an alternative, low-cost organic surface finish for electroplated Cu pads/pillar/bumps to enable 3D interconnects [2]. The effects of pre-cleaning, deposition times and self-assembled monolayer (SAM) type (C3, C10, C18) on oxidation resistance and electrical continuity were studied with Voltammetry and X-ray Photoelectron Spectroscopy (XPS). Experiments were performed on electroplated Cu flat samples and process conditions were selected for further processing of 3D patterned dies and subsequent stacking and thermo-compression bonding in a face-to-face configuration. Overall, C18 SAM showed better electrical continuity and lower electrical resistance than C3 and C10, a result which is consistent with the longer C chain and higher thermal stability of C18. A second result of this study — consistent in both flat and patterned samples — was that microwave plasma cleaning prior to SAM deposition was more effective than wet cleaning, indicating either better oxide cleanability or better affinity of SAM’s with more pristine Cu.