Iuliana Panchenko

Effects of bonding pressure on quality of SLID interconnects

The investigation of the bonding pressure change on the different quality aspects of the solid-liquid interdiffusion (SLID) interconnects is presented. The stacks were produced by a flux-assisted bonding of two Si dies with an area array of square Cu/SnAg bumps on the bottom die and Cu bumps on the top die at approx. 250 °C. The bonding pressure was varied between 0 MPa, 0.35 MPa, 0.69 MPa, 1.04 MPa, 1.38 MPa, 1.73 MPa, 2.08 MPa, 2.42 MPa. Cross-sections of the stacks were analyzed by optical microscopy and scanning electron microscopy (SEM). Tilt, standoff height (SOH) variation, void fraction, interlayer thickness and Cu3Sn thickness were measured. It will be shown that increase of the bonding pressure can reduce the void fraction from 35.1 % (0 MPa) to 10.7 % (2.42 MPa) and decrease the interlayer thickness at the same time. Decrease of the interlayer thickness is accompanied by solder squeeze and increase of Cu3Sn thickness. Shear tests revealed an average shear strength of (81.3 ± 21.5) MPa for the produced samples. The analysis of the fracture surfaces with SEM revealed that the weakest interface is located between Cu6Sn5 and Cu3Sn intermetallic compounds (IMCs) close to the initial Cu bump.

Metallographic preparation of the SnAgCu solders for optical microscopy and EBSD Investigations

In order to investigate the microstructure of lead-free SnAgCu solders by optical microscopy and Electron Backscatter Diffraction it is necessary to provide a qualitative metallographic preparation. The difficulties of a preparation of these solders are connected with a high softness and presence of hard intermetallic compounds. They are discussed in this study and the appropriate recipe is proposed. The samples were manufactured of SnAg3Cu0.7 (wt. %) solder in a form of solder spheres with a diameter of 270 μm, cooled at the rate of 1.1 K/s. The metallographic preparation was carried out on a TegraSystem from Struers. The quality of the prepared samples was checked by image quality maps, created by the Orientation Imaging Microscopy software.

Microstructure Characterization Of Lead‐Free Solders Depending On Alloy Composition

Fatigue and crack nucleation in solder joints is basically associated with changes in the microstructure. Therefore the microstructure evolution of SnAgCu solder joints during solidification and subsequent application is an important subject for reliability investigations and physics of failure analysis. The scope of this study is a systematic overview of the as‐cast microstructures in small sized lead‐free SnAgCu solder spheres after solidification. A total of 32 alloy compositions have been investigated with varying Ag content from 0 to 5 wt.% and varying Cu content from 0 to 1.2 wt.%. The solder spheres had a diameter of approx. 270 μm and were all manufactured under the similar conditions. Subsequent cross‐sectioning was carried out in order to analyze the microstructure by optical and electron microscopy as well as Electron Backscatter Diffraction and Energy Dispersive X‐ray Spectroscopy. The results allow a comprehensive overview of the dependence of the as‐cast microstructure on the solder composit...

Cu passivation with self-assembled monolayers for direct metal bonding in 3D integration

Direct metal bonding is a preferred fine-pitch technology for stacking of Si dies in 3D integration. Cu is a metal of choice for direct metal bonding because it is the most common metal for redistribution layer in advanced semiconductor manufacturing, Cu has high conductivity and it is a low cost candidate. However Cu oxidises very fast in air which makes the bonding procedure challenging. In this study we present the novel technique of Cu passivation with temporary protective self-assembled monolayer (SAM). X-ray photoelectron spectroscopy (XPS) analysis was used in order to carry out the chemical analysis of the Cu surface. Contact angle (CA) measurements provided the information about the monolayer formation. The influence of immersion time and storage conditions on the SAM passivation quality was examined. Storage of a coated Cu surface at low-temperature air conditions was found to be a promising technique for a long-term oxidation retarding. We summarize the key substrate parameters that influence SAM protective capability.

Cu-In-Microbumps for Low-Temperature Bonding of Fine-Pitch-Interconnects

The advancing heterogeneous integration of various electronic components into a single package requires in particular further development of bonding technologies towards lower temperatures. In our present study, we show the successful joining of dies consisting of Cu-microbumps with In solder caps. Stable interconnections were achieved at a bonding temperature of 180 °C in 2 min by solid-liquid-interdiffusion (SLID).

Low temperature Cu/In bonding for 3D integration

This study represents the results of Cu/In bonding based on solid-liquid interdiffusion (SLID) principle for 3D integration. Fine-pitch interconnects were successfully fabricated at the bonding temperature of 170 °C. The final microstructure of the interconnects consists of Cu and Cu/In intermetallic compounds (IMCs) and it is described in detail. The influence of the isothermal storage on the microstructure development is investigated as well.

Solidification processes in the Sn-rich part of the SnCu system

In this study SnCu solder spheres (Ø 270 μm, CR ∼ 1 K/s) were investigated in order to verify the solidified microstructure according to the Sn-rich part of the SnCu phase diagram. The investigated alloys are Sn99.9, SnCu0.25, SnCu0.5, SnCu0.7, SnCu0.9, SnCu1.2, SnCu1.5, and SnCu3.0. In order to understand the solidification process, such aspects as morphology, grain structure and undercooling were analysed. The microstructure was investigated by optical microscopy, SEM and EDX. The undercooling was measured by DSC. It will be shown that small SnCu solder spheres solidify not only with commonly known β-Sn dendrites and fine Cu6Sn5 IMCs in the interdendritic spacing, but with specific and systematic changes in morphology, which depend on composition. The successive morphology transitions were found: from 1) fine Cu6Sn5 IMCs in β-Sn to 2) small β-Sn cells to 3) β-Sn cellular/dendritic to 4) fine Cu6Sn5 IMCs in β-Sn or undirected β-Sn cells. The area fraction of these different morphologies and the number of grain orientations were estimated from the cross-sections of about 20 solder spheres per composition. This allows a quantitative description of the microstructure and its compositional dependency. The results also show that the formation of large Cu6Sn5 IMCs provokes more grain orientations compared to SnCu solders solidified without large intermetallic phases.

Characterisation of Cu/Cu bonding using self-assembled monolayer

Purpose Cu/Cu diffusion bonding is characterised by high electrical and thermal conductivity, as well as the mechanical strength of the interconnects. But despite a number of advantages, Cu oxidises readily upon exposure to air. To break through the adsorbed oxide-layer high temperature and pressure, long bonding time and inert gas atmosphere are required during the bonding process. This paper aims to present the implementation of an organic self-assembled monolayer (SAM) as a temporary protective coating that inhibits Cu oxidation. Design/methodology/approach Information concerning elemental composition of the Cu surface has been yielded by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) spectroscopy. Two types of substrates (electroplated and sputtered Cu) are prepared for thermocompression bonding in two different ways. In the first case, Cu is cleaned with dilute sulphuric acid to remove native copper oxide. In the second case, passivation with SAM followed the cleaning step with dilute sulphuric acid. Shear strength, fracture surface, microstructure of the received Cu/Cu interconnects are investigated after the bonding procedure. Findings The XPS method revealed that SAM can retard Cu from oxidation on air for at least 12 h. SAM passivation on the substrates with sputtered Cu appears to have better quality than on the electroplated ones. This derives from the results of the shear strength tests and scanning electron microscopy (SEM) imaging of Cu/Cu interconnects cross sections. SAM passivation improved the bonding quality of the interconnects with sputtered Cu in comparison to the cleaned samples without passivation. Originality/value The Cu/Cu bonding procedure was optimised by a novel preparation method using SAMs which enables storage and bonding of Si-dies with Cu microbumps at air conditions while remaining a good-quality interconnect. The passivation revealed to be advantageous for the smooth surfaces. SEM and shear strength tests showed improved bonding quality for the passivated bottom dies with sputtered Cu in comparison to the samples without SAM.

Influence of flux-assisted isothermal storage on intermetallic compounds in Cu/SnAg microbumps

With decreasing solder volumes and joint sizes, new aspects in electronics packaging occur. Previous publications report porous structures in Cu/Sn microbump interconnects after flux-assisted bonding and storage. The origin and mechanisms of pore formation are still discussed among researchers. In this study, the influence of no-clean flux during isothermal storage is investigated on soldered Cu/SnAg3.5 microbumps with intermetallic compounds. Soldering was carried out on single dies in air atmosphere without cleaning agent at 240 °C for 15 min. Subsequent isothermal storage was performed in air and N2 atmosphere at 240 °C for 1 min, 10 min and 20 min. The microbumps were exposed to flux and flux fumes during isothermal storage. Reference samples were stored separately without any flux contact. The results show pores in samples with flux contact of any kind. Inert atmosphere seems to diminish pore formation. The study reveals different residue appearances on and around the microbumps according to different storage conditions. Reasons for pore formation are also discussed in this study.

Accelerated SLID Bonding for Fine-Pitch Interconnects with Porous Microstructure

Solid-liquid interdiffusion (SLID) interconnectsbased on Cu and Sn-solder are excellent candidates forstacking of Si chips in 3D integration. If the high-temperaturestable intermetallic compound (IMC) Cu3Sn is desired, themanufacturing of the interconnect can be very time consuming(long annealing time for growth of Cu3Sn). In this paper wepropose a method for the accelerated formation of Cu-Cu3Sn-Cu interconnects by selective dissolution of Sn from Cu6Sn5. This leads to a porous network of Cu3Sn. The influence oftemperature, flux and atmosphere onto the pore formation willbe addressed in detail.