As semiconductor device integration continues to scale, ensuring the reliability of Cu–Cu direct bonding has emerged as a critical requirement for next-generation three-dimensional integrated circuits (3D-ICs) and wafer-level packaging (WLP) technologies. However, dishing defects induced by chemical mechanical polishing (CMP) lead to non-uniform interfacial contact, thereby causing void formation, debonding, and overall degradation of bonding quality. In this work, we combine finite element simulations, XGBoost–SHAP-based machine-learning analysis, and experimental validation to quantitatively evaluate the influence of dishing gap size, bonding temperature, and applied pressure on interfacial recovery. The results revealed that smaller gaps exhibited higher recovery ratios, with temperature identified as the most dominant factor, followed by gap size and pressure as secondary contributors. Experimental validation was carried out using photolithography-based step-height patterns (1000 μm and 2000 μm). 3D profiler measurements indicated increased step height with larger patterns, while Scanning Acoustic Tomography (SAT) demonstrated that bonding yields above 99% could be achieved under high-temperature and high-pressure conditions, whereas significantly reduced bonding ratios were observed under lower conditions. The experimental results were in strong agreement with the simulations, and further analysis confirmed that when the dishing gap was ≤0.15 μm, stable bonding ratios above 80% could be consistently obtained at ~330–360 °C and 6–6.5 MPa. These results indicate that reliable Cu–Cu bonding does not necessarily require complete elimination of dishing; rather, maintaining the effective dishing gap at or below 0.15 μm and prioritizing temperature-driven recovery enables stable bonding above 80% under approximately 330–360 °C and 6–6.5 MPa for the investigated structure.

As artificial intelligence (AI) and high-performance computing demand greater parallel data processing, next-generation memory architectures must also evolve. High bandwidth memory (HBM) supports this with its 3-D-stacked structure and wide I/O interface, but further improvements in power efficiency and thermal management are still required. In addition, microbumps are currently used for chip stacking face scalability limitations. To address these challenges, hybrid copper (Cu) bonding has been proposed as a promising solution. This study presents a surface treatment strategy for low-temperature Cu-to-Cu direct bonding using CH4-based reductive plasma: 1) native Cu oxides were removed and 2) re-oxidation was suppressed by forming a hydride carbon passivation layer. A design of experiments (DOEs) approach was used to optimize inductively coupled plasma (ICP) power, CH4 flow rate, and pressure. Surface analysis was performed using X-ray photoelectron spectroscopy (XPS), TOF-SIMS, transmission electron microscopy (TEM), and sheet resistance measurements, while postbonding evaluation included TEM, SEM, and shear strength analysis. The results demonstrate that the energy of radicals and ions plays a decisive role in Cu oxide reduction, contributing to effective surface activation. The resulting passivation layer was confirmed to be a hydrogenated amorphous carbon (a-C:H) film rich in C, CH, and C2H species. Bonding was conducted at 260 C-degrees and 15 MPa for 1 h, followed by postannealing at 200 degrees C. TEM analysis revealed a void-free and oxygen-free bonding interface with the presence of carbon, suggesting that the carbon layer acted as a passivating layer. These findings confirm that reductuve CH4 plasma treatment is highly effective for enabling low-temperature Cu bonding and highlight the strong potential of this approach for next-generation hybrid bonding technologies.

This study investigates Ar/CH4 plasma pre-treatment on Cu surface as a dual-functional approach that removes Cu oxides via hydrogen radicals/ion-assisted reactions while forming a hydrogenated amorphous carbon (a-C:H) passivation layer. Surface and interfacial evolution were examined by XPS, TOF-SIMS, SEM, and TEM, along with an evaluation of the a-C:H thickness effect. XPS and TEM verify effective oxide removal and time-dependent growth of a-C:H, exhibiting an approximately linear thickness increase with plasma duration. TOF-SIMS reveals a hydrogen concentration gradient across the a-C:H layer. Consistent with this depth-dependent composition, chemical-state analysis indicates that shortening the plasma exposure reduces the a-C:H thickness such that the remaining film is predominantly sp2-rich. After bonding at 260 °C, cross-sectional TEM shows an ~15 nm interfacial carbon layer under longer Ar/CH4 plasma exposure, whereas direct Cu-to-Cu bonding is observed for the shortest treatment. While TEM indicates void-free interfaces irrespective of carbon thickness in the inspected regions, low-magnification SEM reveals voids that are primarily attributable to pre-existing Cu surface roughness prior to plasma treatment. These results demonstrate the potential of Ar/CH4 plasma pre-treatment for enabling low-temperature hybrid Cu bonding through concurrent oxide removal and formation of a controllable a-C:H passivation layer.

As the demand for fine-pitch and high-density interconnects continues to increase, hybrid bonding has emerged as a promising technology to replace conventional 3-D integration techniques. While chemical mechanical polishing (CMP) is widely used for Cu/dielectric structure planarization, it has limitations when applied to polymer materials due to surface damage and material loss caused by polymers' low hardness and high ductility. In this study, we propose fly cutting as an alternative to CMP-a mechanical, slurry-free planarization process-and evaluate its applicability to Cu/polymer structures. We selected rigid polyimide (PI) and soft polydimethylsiloxane (PDMS) as representative polymers to compare the degree of planarization (DoP), surface roughness, and bonding performance. Cu/PI structures achieved a high DoP up to 99% and low Cu surface roughness below 2 nm, regardless of spindle conditions. In contrast, Cu/PDMS structures achieved only 67% planarization due to PDMS deformation during fly cutting, leaving the PDMS surface lower than the Cu layer. This facilitates earlier Cu-Cu contact during bonding. ATR-Fourier transform infrared spectroscopy (ATR-FTIR) analysis elucidated the surface activation mechanism through O-2 plasma treatment, clarifying the polymer-polymer bonding process through functional group changes. Postbonding FE-SEM evaluation confirmed polymer squeeze behavior and changes in upper and lower Cu pad spacing according to temperature and spindle conditions, along with successful polymer bonding. Fly cutting showed promise as a low-cost polymer planarization method, although further optimization of the polymer and process conditions is needed.

This study investigates the electrical performance and interfacial characteristics of direct copper-to-copper (Cu-to-Cu) bonding mediated by Ar/CH4 plasma treatment. The Ar/CH4 plasma concurrently serves a dual function of surface reduction and self-passivation by forming a hydrogenated amorphous carbon (a-C:H) layer. While the thin carbon layer systematically diffuses into the copper bulk during the subsequent bonding process, its comprehensive and correlated impacts on both the electrical and mechanical integrity of the joint interface have not yet been systematically analyzed in literature. This study explicitly elucidates these effects by integrating analytical modeling with experimental daisy chain structures consisting of 96 serial contact links per line. The experimental extractions, based on the representative trimmed average of the lowest ~51% of the data, revealed that the Ar/CH4 plasma-treated specimens achieved an exceptionally low effective specific contact resistivity (ρc) of 7.02 x 10-7 (Ω ∙ cm2), a significant improvement over the 1.40 x 10-6 (Ω ∙ cm2) observed in the non-plasma specimens. This uniform and stable cumulative resistance distribution was achieved because the self-passivating a-C:H layer successfully suppressed interfacial copper oxidation even under the elevated temperature bonding conditions in ambient air. Furthermore, mechanical evaluations demonstrated a highly robust bonding quality, yielding an outstanding average shear strength of 66.52 MPa. These findings underscore the potential of Ar/CH4 plasma-mediated surface engineering as a highly efficient, high-performance solution for next-generation 3D advanced packaging applications.

Hybrid Cu bonding is a promising interconnect technology for fine-pitch 3D integration. However, Cu–Cu bonding typically requires high temperatures because copper has a high melting point and is prone to oxidation, both of which hinder atomic diffusion. This study investigates the surface and bonding characteristics of electroless-plated Ru (ELP-Ru) and Au (ELP-Au) nanolayers used as Cu passivation layers for low-temperature Cu bonding. Atomic force microscopy revealed that the ELP-Au layer exhibited a lower surface roughness (3.675 nm) than the ELP-Ru layer (4.415 nm), indicating a more uniform bonding interface. GIXRD analysis showed that the ELP-Au nanolayer possessed a predominant (111) orientation with a smaller average grain size (5.60 nm), whereas the ELP-Ru layer exhibited a larger grain size (10.5 nm) with a (100) orientation, suggesting a higher diffusion path density in the ELP-Au layer. TEM and EDS analyses confirmed that oxidation was confined to the surface region for both layers, demonstrating effective Cu oxidation suppression. Bonding at 200 °C revealed efficient Cu–Au solid-state diffusion with minimal interfacial oxidation for ELP-Au, while ELP-Ru showed broader interfacial oxides and lower shear strength (3.93 MPa), indicating inferior bonding performance. In addition, selective electroless deposition of Au on Cu pads was evaluated.

As semiconductor scaling reaches its physical limits, Cu hybrid bonding has emerged as a cornerstone of advanced packaging for high-performance computing and AI applications. To achieve reliable low-temperature bonding while preventing Cu re-oxidation, surface passivation is essential. This study proposes a novel single-step Ar-CH4 plasma treatment using an inductively coupled plasma (ICP) system to simultaneously reduce Cu oxides and deposit an amorphous hydrogenated carbon (a-C:H) passivation layer. The plasma process was optimized through a Central Composite Design (CCD), and the treatment time was varied from 15 to 60 seconds to precisely control the a-C:H layer thickness. Surface analysis via XPS and TEM confirmed the complete removal of Cu2+ species and the linear growth of the a-C:H layer. Cross-sectional TEM observations revealed that the a-C:H thickness plays a dual role: at the optimized P15 condition (~2.5 nm), the layer acts as a diffusion-active species, facilitating a synergistic triple-diffusion mechanism that results in a seamless, monolithic Cu interface. In contrast, thicker layers (P60) act as a diffusion barrier, effectively suppressing metallic intermixing. These findings demonstrate that precise nanometer-scale control of the a-C:H layer is critical for enabling robust, void-free Cu-to-Cu bonding, providing a highly scalable solution for next-generation high-density interconnects.

The direct deposition of polymer dielectrics in CMOS fabrication tools is often restricted due to contamination concerns arising from unreacted monomers and process by-products. To enable contamination-free polymer integration, this study employs an external deposition approach in which the organic polymer dielectric pV3D3 is deposited outside the fabrication line and subsequently incorporated into the device structure through a bonding step. This method prevents polymer exposure to process equipment while leveraging the favorable electrical characteristics of pV3D3. The bonded structures were evaluated through electrical and structural analyses. pV3D3 exhibited stable insulating performance in current-voltage (I-V) and capacitance-voltage (C-V) measurements. Scanning acoustic tomography (SAT) and atomic force microscopy (AFM) confirmed void-free bonding interfaces and an atomically smooth surface (similar to 0.3-nm rms) suitable for bonding. Transmission electron microscopy (TEM) and energy-dispersive spectroscopy (EDS) revealed localized Cu penetration into pV3D3 only after high-voltage stress, indicating that the bonding step itself did not induce material degradation. These results demonstrate that externally deposited pV3D3 can be reliably integrated into semiconductor devices via bonding, providing a practical route toward contamination-free dielectric incorporation in advanced packaging applications.

Hybrid Cu bonding is a promising interconnect technology for fine-pitch 3D integration. However, Cu–Cu bonding typically requires high temperatures because copper has a high melting point and is prone to oxidation, both of which hinder atomic diffusion. This study investigates the surface and bonding characteristics of electroless-plated Ru (ELP-Ru), Pd (ELP-Pd), and Au (ELP-Au) nanolayers used as Cu passivation layers for low-temperature Cu bonding. AFM analysis indicated that the ELP-Pd layer had slightly lower surface roughness than the ELP-Au layer, suggesting comparable surface uniformity, whereas the ELP-Ru layer exhibited higher roughness due to differences in reduction kinetics and growth behavior. GIXRD results showed that ELP-Au had a dominant (111) orientation with a smaller grain size (5.60 nm), while ELP-Ru exhibited a larger grain size (10.5 nm) with a preferred (100) orientation. In contrast, ELP-Pd showed only a weak (111) texture, indicating a comparatively less pronounced texture. TEM and EDS analyses showed that oxidation in ELP-Au and ELP-Ru was mainly limited to the surface, indicating effective suppression of Cu oxidation. In contrast, the ELP-Pd layer exhibited oxygen penetration and noticeable Cu–Pd intermixing, suggesting higher diffusion but comparatively lower oxidation resistance. Bonding at 200 °C revealed efficient Cu–Au solid-state diffusion with minimal interfacial oxidation for ELP-Au, while ELP-Ru showed broader interfacial oxides and lower shear strength. The ELP-Pd exhibited extensive Cu–Pd interdiffusion at the bonding interface along with a considerable amount of interfacial oxide.

Direct Cu bonding stands as a pivotal technology for enabling high-density vertical interconnects in advanced 3D heterogeneous integration. However, achieving reliable low-temperature Cu bonding requires a careful balance between suppressing surface oxidation and allowing sufficient atomic diffusion for bonding. The surface must be effectively protected against oxidation while still permitting atomic diffusion across the interface. In this study, we demonstrate that the thickness of the hydrogenated amorphous carbon (a-C:H) passivation layer is the key factor governing both oxidation suppression and atomic diffusion behavior. By systematically tuning the Ar/CH4 plasma treatment time, we identify a decisive transition in film properties from a thin, diffusion-permeable layer to a thick, diffusion-barrier layer. This transition reveals that the thin a-C:H layer possesses a defect-rich structure characterized by sp2 clusters, whereas increased thickness induces a transition to a rigid, sp2-dominant network. This thickness-dependent structural evolution dictates the dominant bonding mechanism. In the case of a thin a-C:H layer, a synergistic triple-diffusion process may be enabled, involving Cu transport through defects associated with sp2-rich clusters, carbon diffusion along Cu grain boundaries, and carbon diffusion into Cu lattice. This carbon-related lattice distortion is consistent with a characteristic local lattice expansion of ~ 6.8% observed by FFT analysis at the interface. Conversely, a thick a-C:H layer appears to act as a robust diffusion barrier, likely inhibiting the atomic intermixing required for effective bonding. This study demonstrates that optimization of the a-C:H thickness is critical for enabling sufficient atomic diffusion and achieving high-yield Cu direct bonding.

Fine-pitch interconnects have become essential for high-density 3D integrated circuits, with Cu/SiO2 hybrid bonding emerging as a key enabling technology for low-resistance, high-density interconnection. However, Cu pad surface planarity following chemical mechanical polishing (CMP) critically influences bonding quality, with Cu dishing and oxide corner rounding representing a major challenge. This study investigates the relationship between Cu pad dimensions, spacing, and their effects on Cu dishing behavior and hybrid bonding performance. Cu pads with widths of 4, 6, 8, and 10 & micro;m were designed with varying inter-pad spacings, patterned using standard photolithography, and fabricated through electroplating. CMP was applied, and dishing depths were quantitatively evaluated using atomic force microscopy (AFM). Prior to bonding, two-step Ar/N-2 plasma surface activation was conducted to remove native oxide and enhance both Cu-Cu and SiO2-SiO2 bonding interfaces. 4 & micro;m pads exhibited minimal dishing and achieved void-free bonding, while 6, 8, and 10 & micro;m pads showed progressively deeper dishing and incomplete Cu bonding. In addition to Cu CMP optimization, Cu pad geometry plays a crucial role in determining reliable hybrid bonding quality.

This study investigates the deposition and characterization of electroless-plated gold (Au) nanolayers for low-temperature copper (Cu) hybrid bonding applications. A 0.7 mu m-thick thermally grown SiO2 layer on 8-in Si wafers was coated with a 50 nm Ti adhesion layer and 1 mu m Cu layer, followed by electroless Au plating to form similar to 7 nm nanolayers. Before plating, samples underwent wet pretreatment, and post-deposition analyses included X-ray photoelectron spectroscopy (XPS), grazing-incidence X-ray diffraction (GIXRD), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The electroless-plated Au exhibited predominantly metallic characteristics with minimal Cu oxidation and conformally covered the underlying Cu, achieving an average surface roughness (R-q) of 3.21 nm. Die bonding at 200 degrees C and 15 MPa demonstrated successful Cu-Au interface formation, and shear strength of 14.9 MPa showed mechanical integrity. Furthermore, selective deposition of Au on patterned Cu pads surrounded by SiO2 was achieved, and its potential as a cost-effective solution for metal nanolayer formation in Cu hybrid bonding technologies.

This work demonstrates a low-temperature heterogeneous integration strategy for alpha-Ga2O3-based functional devices that complies with the stringent thermal-budget constraints (<400 degrees C) of silicon CMOS back-end-of-line (BEOL) processing. Although alpha-Ga2O3 exhibits an excellent solar-blind UV-C photoresponse, its high epitaxial growth temperature (>450 degrees C) has hindered its direct monolithic integration with CMOS circuitry. To address this incompatibility, alpha-Ga2O3 was synthesized at 470 degrees C and subsequently integrated at 350 degrees C using a Cu-Cu thermal-compression-bonding (TCB) platform. A dual-functional Ti interlayer architecture, consisting of a 10-nm passivation layer and a 50-nm adhesion layer, facilitates low-temperature bonding while maintaining interfacial stability between metal and oxide layers. The Cu-Cu TCB performed at 350 degrees C under an applied pressure of 10 MPa resulted in a continuous and void-free metallurgical junction. Atomic-scale interfacial analyses indicate that the 50-nm Ti adhesion layer plays a key role in limiting Cu diffusion and mitigating thermomechanical stress, contributing to the preservation of the alpha-Ga2O3 active layer during bonding. The heterogeneously integrated UV-C photodetector exhibits photo-to-dark current ratios (PDCRs) comparable to those of devices fabricated under high-temperature growth conditions, even under ultralow-intensity UV-C illumination. These results demonstrate that low-temperature Cu-Cu TCB can be extended beyond conventional BEOL-compatible packaging to enable the integration of high-temperature functional oxides within the BEOL thermal constraints. This work highlights a packaging-level heterogeneous integration strategy rather than a device-level performance optimization.

To enhance the density and performance of semiconductor devices, 3D packaging with hybrid Cu bonding is emerging as a critical technology. One of the dielectrics used in hybrid Cu bonding is SiCN, typically deposited using PECVD (plasma-enhanced chemical vapor deposition). In this study, we investigated SiCN deposited at room temperature using PVD (physical vapor deposition). The SiCN, with a thickness of 150 nm, exhibited a surface roughness of 0.3-0.4 nm after the CMP (chemical mechanical polishing) process and a contact angle of about 10 degrees and dielectric constant of 3.9, indicating its potential as a dielectric for hybrid Cu bonding. And, PVD SiCN-SiCN bonding was performed at 260 degrees C, resulting in a uniform and void-free interface.

This study investigates the optimization of O2 plasma treatment conditions to enhance Cu-Cu bonding. The O2 plasma treatment conditions were optimized using Design of Experiments (DOE), adjusting three parameters: O2 flow rate, plasma power, and treatment time, to minimize oxidation while maximizing surface energy. X-ray photoelectron spectroscopy (XPS) was employed to calculate the Cu atomic percentage (at%) at the surface and at a depth of 25 seconds of etching, while water contact angle (WCA) measurements assessed surface energy. The results indicated that decreasing the O2 flow rate reduced oxidation without significantly impacting surface energy. Plasma power and treatment time were optimized through a balanced approach. The identified optimal conditions were an O2 flow rate of 50 sccm, plasma power of 50 W, and a process time of 20 seconds. Subsequent SEM analysis confirmed a wavy bonding interface indicative of strong Cu diffusion bonding, resulting in approximately a 40% increase in shear strength. The findings suggest that controlled O2 plasma treatment effectively enhances bonding strength, providing direction for the optimization of O2 plasma for Cu bonding in advanced packaging technologies and hybrid bonding applications.

3D packaging technology has been rapidly advancing to meet the demands of high-performance computing, and the importance of hybrid bonding is increasingly emphasized. Cu/polymer hybrid bonding offers advantages such as flexibility and simplified processing. This study explores the feasibility of applying fly cutting for co-planarizing Cu/PDMS in hybrid bonding. Experimental results show that fly cutting achieves an extremely low roughness of less than 1 nm on Cu surfaces and a roughness of approximately 6–7 nm on PDMS surfaces. However, achieving optimal planarization for soft polymers like PDMS remains a significant challenge. This study demonstrates the potential and limitations of fly cutting for Cu/polymer hybrid bonding and provides insights into future research directions aimed at process optimization.

The effect of post-bond annealing on the quantitative interfacial adhesion energy of low-temperature Cu–Cu bonding interfaces was systematically investigated using a double cantilever beam test. A two-step Ar/N2 plasma treatment was applied to achieve low-temperature bonding. This treatment protected the Cu surface from oxidation by forming Cu nitrides such as Cu3N and Cu4N, which were confirmed via electron backscatter diffraction and X-ray photoelectron spectroscopy analyses. Cu3N and Cu4N completely decomposed at 200 °C, enabling pure Cu–Cu low-temperature bonding. The interfacial adhesion energy at post-bond annealing temperatures of 250 °C, 300 °C, and 350 °C was 0.65 ± 0.05 J/m2, 3.81 ± 0.61 J/m2, and 4.12 ± 1.12 J/m2, respectively. As the post-bond annealing temperature increased, Cu atomic diffusion was enhanced, leading to the elimination of voids and seams, grain growth, and the disappearance of grain boundaries. Consequently, the improved Cu‒Cu bonding quality resulted in an increase in interfacial adhesion energy.

This study investigates the applicability of fly cutting as a CMP-free planarization technique for Cu/polymer hybrid bonding. Two polymer dielectrics-polyimide (PI) and polydimethylsiloxane (PDMS)-were selected to represent rigid and soft materials, respectively. Surface roughness and global planarization behavior were evaluated under two spindle speed conditions using a single-crystal diamond tool. Fly cutting successfully achieved sub-2 nm Cu surface roughness for both polymers and <25 nm roughness on the polymer surfaces. The degree of planarization (DoP) was maintained above 90% for PI and improved from 52% to 66.7% for PDMS when the spindle speed was reduced, indicating that planarization is influenced by polymer properties and cutting parameters. However, bonding evaluation revealed that Cu-Cu bonding was not achieved in either structure. In Cu/PDMS samples, thermal expansion of the soft polymer during bonding pushed the Cu pads apart despite an initial Cu height advantage. In Cu/PI samples, partial PI-PI bonding was achieved, but Cu-Cu delamination occurred at all temperatures due to insufficient Cu surface activation and expansion. Compared to PI-PI interface, PDMS-PDMS bonding was successfully achieved without voids, suggesting effective surface activation via O-2 plasma. These results demonstrate that while fly cutting is effective for planarizing both soft and rigid polymers, optimization of the CTE mismatch between Cu and polymer and surface activation are essential to realize stable Cu/polymer hybrid bonds.

As semiconductor devices approach miniaturization limits, 3D packaging with hybrid bonding has emerged as a key solution. This study focuses on silicon carbon nitride (SiCN) films deposited by physical vapor deposition (PVD) as a cost-effective alternative to plasma-enhanced chemical vapor deposition (PECVD) for use as a low-k dielectric in Cu hybrid bonding. SiCN films were fabricated via reactive sputtering, with RF power and Ar/N2 gas flow ratio varied systematically. Higher RF power and gas flow ratios led to increased carbon and nitrogen content and improved film density by about 10

Cu/dielectric hybrid bonding has emerged as a promising solution, enabling higher integration density, reduced interconnect distances, and minimized power dissipation. A key dielectric in hybrid bonding is silicon carbon nitride (SiCN), known for its excellent surface quality after chemical mechanical polishing (CMP) and strong bonding under low-temperature annealing. Typically deposited via plasma-enhanced chemical vapor deposition (PECVD), SiCN film fabrication introduces added complexity and cost. In this study, we explore a simplified alternative using physical vapor deposition (PVD) for SiCN thin films, eliminating the need for post-deposition annealing. The influence of PVD deposition parameters on the properties of SiCN films was systematically investigated. The results reveal that increasing RF power during deposition enhances film density, as evidenced by a higher refractive index and an increased content of carbon and nitrogen. This leads to the high potential of PVD SiCN as a viable candidate for Cu/dielectric hybrid bonding applications.