Cu–Cu Bonding in Ambient Environment by Ar/N₂ Plasma Surface Activation and Its Characterization

Abstract

Ar/N2 plasma surface activation in ambient environment for Cu-Cu bonding has been performed and characterized. Cu-Cu bonding occurs at room temperature and under normal atmospheric conditions. Post-bonding annealing is performed at 300 °C and below to control the overall thermal budget. Shear strength and hermeticity property of samples with varying annealing temperature are investigated. Samples annealed at 250 °C demonstrates the highest average shear strength of 20.3 MPa and lowest average helium leak rate of 8.7 × 10-9 atm.cc/s. Daisy chain structures are used to examine the electrical property of sample bonded at 300 °C and temperature cycling tests from -40 to 125 ºC for 1,000 cycles have been performed for reliability assessment. It is observed that the dies are well bonded and intact after 1,000 cycles even though the exposed Cu surface is heavily oxidized.

Time-Dependent Evolution Study of Ar/N2 Plasma-Activated Cu Surface for Enabling Two-Step Cu-Cu Direct Bonding in a Non-Vacuum Environment

“…As shown, the bonded plasma-activated dies with 2-hour cleanroom ambient exposure exhibited the largest shear strength (~5 MPa). Although the shear strength is lower than that reported in the previously published work (~20 MPa) due to different annealing facility, 17,18 the results provide the first revelation about the need to optimize post plasma treatment holding duration prior to the bonding process (e.g. largest shear strength).…”

“…The Rq values (~0.85 nm) of both as-deposited and plasma-activated Cu are less than 1 nm, which fulfills the requirements for direct bonding. 18 Figure 4 shows the 3D topographic mapping of the surface roughness Rq of the as-deposited A) and the plasmaactivated B) Cu surfaces, respectively.…”

“…To mitigate this issue, surface treatments such as chemical modifications, surface assembled monolayer (SAM), specialized chemical mechanical polishing (CMP), and plasma treatment, have been studied. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Among them, plasma treatment is the most favorable process as it does not leave behind any residue. Plasma-treated Cu-Cu direct bonding is generally evaluated at specific conditions such as at high temperature (>300°C) and/or under ultra high vacuum (UHV) for an extended period of time.…”

Recent progress on bumpless Cu/SiO2 hybrid bonding for 3D heterogeneous integration

“…With plasma treatment, contamination on the wafer surface is eliminated, surface hydrophilicity is enhanced and the surface energy increases. The plasma-assisted bonding (PAB) method involves plasma surface prebonding treatment, bonding at RT and atmosphere instead of UHV, annealing the bond at a lower temperature, and the schematic image is shown in Figure 4(e) (Chua et al , 2019). Wang et al (2008, 2018b), Kang et al (2020) and Wang et al (2017, 2018a) bonded SiO 2 , SiC, LiNbO 3 and quartz glass by the PAB method, with O 2 , N 2 , Ar and Ar/H 2 plasma used.…”

“…(e)(Chua et al, 2019) Wang et al (2008Wang et al ( , 2018b,Kang et al (2020). andWang et al (2017Wang et al ( , 2018a bonded SiO 2 , SiC, LiNbO 3 and quartz glass by the PAB method, with O 2 , N 2 , Ar and Ar/H 2 plasma used.…”