A. Lesniewska

Reliability study on cobalt and ruthenium as alternative metals for advanced interconnects

Cobalt and ruthenium are being proposed to replace copper in BEOL interconnects. Using intrinsic TDDB studies, we show that Co needs a barrier to prevent it from drifting into SiO2, where for Ru no drift into any of the three studied dielectrics is observed. Although our intrinsic EM studies on single damascene lines filled with Co suffered from bondpad delamination and a non-optimized CMP, we could still conclude that the EM-performance is better compared to Cu filled lines, where a much better performance of Ru filled lines is demonstrated (>25x). Via failures on Ru schemes show a >5x higher lifetime compared to Cu schemes.

CVD-Mn/CVD-Ru-based barrier/liner solution for advanced BEOL Cu/Low-k interconnects

Aggressive downscaling of the barrier/liner thickness is the key to meet line and via resistance requirements from 15nm metal half pitch and below interconnects. For this purpose, porous low-k(2.4) dielectric/Mn-based barrier/Ru-liner/Cu system was extensively studied. Mn-silicate (MnSiO3) formation, intrinsic Cu diffusion barrier property and O2 barrier efficiency of the system were demonstrated. A stack of 1nm Mn-based barrier/1nm Ru liner was successfully integrated in tight pitch dual damascene (DD) Cu wires and its extendibility to at least 15nm feature size was confirmed both morphologically and electrically. Although, it was shown that Mn/Ru-based system is intrinsically reliable from electro-migration (EM) perspective, the absence of the flux divergence at the via bottom was also established, which needs to be addressed. Overall, this work shows that the Mn/Ru-based system is a serious barrier/liner solution for future technology nodes.

Thermal compression bonding of 20 μm pitch micro bumps with pre-applied underfill — Process and reliability

Thermal compression bonding (TCB) process in combination with a pre-applied underfill material has been developed and investigated for assembling 20 μm pitch Sn-based micro bumps. It is found bonding force has a profound impact on the joint formation behavior. A low bonding force produces bump joints with heavier underfill entrapment and incompletely reacted solder. A higher bonding force leads to more solder squeezing-out, leaving a thin and completely reacted inter-metallic compound (IMC) layer in the joints. Electrical measurement of the daisy chains on the as-bonded chips does not reveal any significant difference between the samples made with different bonding forces. The reliability of the two types of joints were further studied in two post-bonding tests, namely the resistance measurement of daisy chains at an elevated temperature and stack-level thermo-cycling test. Both tests show a better reliability performance from the bump joints with less underfill entrapment and completely reacted IMC layer.

Cu passivation for integration of gap-filling ultralow-k dielectrics

For Cu/low-k interconnects, the reversed damascene is an alternative integration approach where the metal wires are patterned first and then the spacing filled with a flowable dielectric. In this paper, the replacement of a sacrificial template by gap-filling ultralow-k dielectrics is studied, focusing on yield and transport performance (“replacement dielectric” scheme). On non-passivated copper, the low-k curing processes induce severe damage to the metal lines, leading to the degraded electrical properties. This is confirmed by chemical inspection on the blanket Cu films and morphological inspection on patterned structures. In order to avoid Cu oxidation and out-diffusion at elevated temperature, Cu passivation by plasma-enhanced chemical vapor deposition SiCN is proposed and studied in detail. The inter-metal dielectric properties of replacement low-k are evaluated by resistance-capacitance and IV measurements using a Meander-Fork structure. By tuning the passivation layer thickness and ultraviolet cur...

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.