IMC bonding for 3D interconnection

Abstract

We performed stacking experiments on Si dies using annular tungsten TSVs (Through Silicon Vias) and Cu studs with low-volume solder micro-bumps. Unlike standard 100-micron C4 (Controlled Collapse Chip Connection) solder balls, very small solder volumes (< 6 microns in height) form IMC (InterMetallic Compounds) in the junctions during the bonding or reflow processes. The two interconnect metallurgies of Cu/Ni/In and Cu/Sn joints were considered for low-volume lead-free solder micro-bumps for 3D integration. A previous study on these metallurgies [5] showed that the Cu/Sn joints form thermally stable intermetallics while in the Cu/Ni/In joints, some indium solder remains unreacted due to the presence of the Ni barriers. The shear testing on the stacked systems showed that the die stacks with Cu/Sn joints exhibit higher shear strengths than those with Cu/Ni/In joints. However the impact shock testing on the systems revealed that the die stacks with Cu/Sn joints are less resistant to mechanical shocks than the systems with Cu/Ni/In joints. This new work focuses on thermal cycle testing of the die stack systems with the Cu/Ni/In and Cu/Sn interconnections. Preliminary thermal cycle testing on the die stack systems with Cu/Ni/In joints showed that the joints are stable against thermal cycle stresses for thousands of cycles. To quickly compare the systems with two metallurgies, we mounted the Si die stacks onto organic substrates to impose additional stresses on the systems. In addition to standard DTC (Deep Thermal Cycle) tests, we also conducted a HAATS (Highly accelerated Air to Air Thermal Shock) test [23] with a short cycle time to reduce the testing time. The DTC and HAATS tests showed that the stacked systems with Cu/Ni/In joints had fewer failures and smaller increases in the electrical resistances of the joints during the tests than the systems with Cu/Sn joints.

Prediction of damage and fatigue life of high-temperature flip chip assembly interconnections at operations

“…The viscoplastic damage, measured by indicators such as strain energy density and plastic work, accumulates cycle by cycle and eventually culminates in solder joint failure through fatigue cracking mechanism. Metallurgical reactions involving solder alloys, copper in the bond pad and pad metalisation during both reflow assembly process and life operations produce and grow IMC respectively at the interfaces of solder bulk and the bond pads [12,[16][17][18][19][20][21]. The lead-free solder used in FC manufacture contains Sn3.9Ag0.6Cu alloy materials [22].…”

Process integration of fine pitch Cu redistribution wiring and SnCu micro-bumping for power efficient LSI devices with high-bandwidth stacked DRAM

“…On the other hand, low-volume solder joining, such as In solder with UBM Ni, is being investigated from the aspect that the Table 2 EDX analysis results of intermetallic compounds formed at the interface between UBM Ni and SnCu bump in Fig. 11 substantial properties of brittleness and weakness against stress shock with the Cu-Sn intermetallic compound phases would have an significant impact on reliability of fine pitch interconnections [12]. It is worth noting that further investigation on reliability of the low-volume SnCu solder joints implemented in this study will provide a proper solution to stack more chips with fine pitch interconnects while cost rise of chip stacking is avoided.…”

Thermal stress analysis of 3D die stacks with low-volume interconnections

“…The joints between the Si interposer and the organic substrates were formed by solder. The details of fabrication of the Si interposer, the TSV process and the assembly process is described in details elsewhere [3,4]. We show an assembled Si die stack on an organic substrate in Figure 2.…”

“…The die stacking process with low-volume interconnection is one of the potential methods for 3D integration [3,4]. It is a low temperature process with hierarchy to repeated thermal processing due to the formation of intermetallic compounds in the solder joints.…”