Fluxless Bonding Using Vacuum Ultraviolet and Formic Acid for 3D Interconnects
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1249-F09-04
Fluxless Bonding Using Vacuum Ultraviolet and Formic Acid for 3D Interconnects Katsuyuki Sakuma1,2, Naoko Unami1, Shuichi Shoji1, and Jun Mizuno1 1 2
Waseda University, Okubo 3-4-1, Shinjuku, Tokyo, 169-8855, Japan IBM Research – Tokyo, 1623-14, Shimotsuruma, Yamato, Kanagawa, 242-8502, Japan
ABSTRACT We have developed a novel surface treatment process using vacuum ultraviolet (VUV) light with a wavelength of 172 nm and formic acid vapor. A previous study showed that the VUV process can help remove the organic contaminants on the bonding surfaces and improve the shear strength [1]. This new work focuses on studying the effects of VUV/O3 and formic acid treatments. The formic acid (HCOOH) vapor removes the metal oxides from the surfaces before the bonding process. Evaporated Cu/Sn and immersion Au were used for the bonding microbumps and bonding pads in our evaluations. Different cleaning conditions with VUV/O3, formic acid vapor, or both were compared and evaluated. X-ray Photoelectron Spectroscopy (XPS) was used to study the surface elemental composition of the micro-bumps and pad surfaces before and after the cleaning process. The photoelectron spectra of C1s, Sn3d, and Au4f were obtained with XPS. The XPS results showed the atomic carbon concentrations were significantly decreased by the VUV/O3 treatment process, while the Sn and Au concentrations were increased by the VUV/O3 and formic acid treatment because of the removal of the organic contaminants and metal oxides from the surfaces. The bonding strength of the Cu/Sn bumps was evaluated using a shear test tool. The results shows that the combination of VUV/O3 and formic acid treatment obtains the highest average shear strength among the treatments tested, with a shear strength almost 2.5 times stronger than the untreated samples. INTRODUCTION Joints with micro-bump interconnections are widely used to connect the electrode pads between integrated circuits (ICs) and substrates [2]. As the systems become faster, there are increasing demands for smaller packages and finer-pitched interconnections. At the same time, 3D chip integration in which multiple 2D circuits are stacked on top of other 2D circuits and electrically connected by vertical interconnections require smaller and finer-pitch micro-bump technologies [3-7]. The sizes of the vertical interconnections are shrinking and reliable bonding technologies are needed for high-I/O-bandwidth 3D integration, as shown in Figure 1 [8]. Conventional solder joints use liquid flux to remove the solid oxide film during the bonding processes. This calls for cleaning the flux residues after the reflow process to avoid reliability problems. However as the bump pitch shrinks, it becomes more difficult to remove these flux residues. There are growing demands for fluxless soldering and new surface cleaning methods for fine-pitch 3D interconnects.
In flip chip bonding and wire bonding, plasma treatment techniques are generally used to reduce the interface delamination problems caused by organic contamination o
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