Bumpless Interconnect of 6-$\mu$m-Pitch Cu Electrodes at Room Temperature

Abstract

Bumpless interconnect of 6-m-pitch Cu electrodes was realized at room temperature with the surface activated bonding (SAB) method. In this study, we propose a novel bumpless structure, where the electrodes and a surrounding Cu frame are fabricated with the same height to increase bond strength and demonstrate the feasibility of a sealing interconnection between Cu surfaces. The damascene process, assisted by the reactive ion beam etching (RIE), was used to fabricate the Cu structures. 923 521 electrodes placed inside the frame were arranged into a spiral chain to enable the detection of the positions with insufficient interconnection by electrical resistance measurements. Using the SAB conditions optimized with simple chemo-mechanical polishing (CMP)-Cu film samples, we found that 744 769 electrodes were successfully interconnected, except some specific lines near the frame, which might be due to sample preparation error rather than a bond defect. The mean contact resistance was below 0.08 ; a sealing effect was achieved at the frame structure because there was little increase in the contact resistance in high temperature storage testing performed at 150 C for 1000 h, in ambient air.Index Terms-Bumpless interconnect, Cu frame, room temperature, sealing, surface activated bonding (SAB).

Heterogeneous 2.5D integration on through silicon interposer

“…To date, Au-Au and Cu-Cu are the most studied metals for chip stacking with metal diffusion bonding. 150,151 Gold has no surface oxide thus eliminate surface cleaning issues before bonding. Conversely, Cu is more susceptible for surface oxidation and requires additional processing steps for surface cleaning.…”

“…[151][152][153] Cu-Cu bonding essentially comes with Cu pillars with diameter smaller than 10 lm and aspect ratio below 1. Cu can get deformed during bonding and absorbs minute changes in height variations.…”

“…[147][148][149] The work carried out by Lin et al, wherein solder paste reflow or electroplated Cu is applied with fan-out configuration for Comic-Con International (CCI). However, this approach is more focused on low-cost manufacturing process than scaling down of the interconnect pitch.…”

Plasma assisted bonding of copper and silver substrates

“…To improve these difficulties, many studies and development are performed currently. Therefore, Cu direct bonding requires a chemical mechanical polishing (CMP) to form a smooth and clean surface, and CMP-Cu can be bonded by ion beam activation process [1], or by a hydroxyl bonding [?]. Furthermore, Cu direct bonding was achieved by a bonding method using formic acid for simultaneously bonding and control of the oxide layer on Cu surface at 200 • C [3,4].…”

Modified diffusion bond process for chemical mechanical polishing (CMP)-Cu at 150°C in ambient air

“…In such cases, heating times of several hours were necessary after the oxide bonding process to accelerate the diffusion of Cu atoms across a thick adsorbate (mainly oxide) layer to obtain sufficient bonding strength and electrical conductivity. Meanwhile, we performed room temperature bonding of 6-m-pitch bumpless Cu electrodes fabricated with the applied damascene process [5].…”