Effects of bonding pressure on quality of SLID interconnects

Abstract

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 str ength 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

Wafer-level Cu–Sn micro-joints with high mechanical strength and low Sn overflow

“…Our experiments have shown that the transition to Cu 3 Sn is incomplete when a shorter annealing time is used, for example 15 min, which will result in a bond line with both Cu 5 Sn 6 and Cu 3 Sn present. The Sn overflow region is smaller than 5 μm, where the Sn rich alloy Cu 6 Sn 5 is the major remaining [1,5,7]. Another benefit of using the two-step temperature bonding profile is the formation of void-free bonding interfaces, since it could prevent the fast diffusion between Cu and Sn.…”

“…The IR transmission microscopy presents a relatively small Sn overflow in the metallic structure of the sealed lines (see figure 1(b). Comparing this with one-step or even three step bonding processes, the type II process leads to the formation of micro-joints with controllable Sn overflows [6][7][8][9][10].…”

“…Solid liquid inter-diffusion occurs between the two metals, resulting in a phase transformation of the liquid component to an intermetallic phase with a higher melting point, which should be strong enough to serve as a bond and withstand exposure to elevated temperatures without re-melting. However, a bonding force is needed for the wafer-level co-planarity of the alloying reaction as well as to break the oxide interface on low melting point metals during the IMC formation [7]. The applied bonding force induces an uncontrollable thickness and line width of the micro-joints due to the overflow of liquidation of low melting metals [1].…”