Consequently, the cleaning process must not only remove nano-scale contaminants but also maintain the delicate Cu recess engineered during the CMP phase to ensure successful thermo-compression during hybrid bonding .
In depth
The Post CMP Cleaning step in the Direct Bond Interconnect (DBI) module serves t
o prepare a pristine, planarized hybrid surface for subsequent CMOS Image Sensor (CIS) and Image Signal Processor (ISP) wafer bonding . Following the sequential chemical mechanical planarization (CMP) of bulk Cu, a Ta-based liner, and the final oxide layer, the wafer surface is left exposed to slurry abrasives, organic additives, and metallic by-products . This step must completely remove these residues to enable defect-free RF surface activation and direct oxide-to-oxide and Cu-to-Cu bonding . Unlike standard intermediate BEOL cleans, this specific process must preserve the highly controlled Cu and oxide topographies while ensuring the surface is entirely free of particles that could act as macroscopic wedge defects during the wafer pairing process . The cleaning mechanism relies on the synergistic application of mechanical forces and chemical interactions to overcome contaminant adhesion forces . During CMP, residual abrasive particles adhere to the wafer primarily through electrostatic and chemical interactions, which are governed by the isoelectric points (IEP) of the materials and the local pH . To break these bonds, the cleaning process utilizes polyvinyl alcohol (PVA) brushes that apply hydrodynamic shear stress to physically dislodge particles, while chemical solutions modulate the zeta potential of both the wafer surface and the particles to induce strong electrostatic repulsion . Concurrently, complexing or chelating agents in the cleaning chemistry bind to dissolved metallic impurities to prevent their redeposition on the dielectric, and corrosion inhibitors passivate the exposed Cu surfaces to prevent excessive oxidation or galvanic corrosion . The selection of cleaning chemicals must carefully balance the competing needs of the exposed copper interconnects and the surrounding silicon dioxide . Highly acidic or highly alkaline solutions can lead to severe copper dissolution or dielectric etching, respectively, so pH-optimized chemistries are formulated to ensure a wide process window . Furthermore, post-CMP cleaning chambers often incorporate sequential megasonic agitation and brush scrubbing combined with specific cleaning agents such as dilute organic acids or specialized surfactants . Advanced drying techniques, such as Marangoni drying with isopropyl alcohol (IPA), are frequently employed to prevent the formation of watermarks, which are highly detrimental to the subsequent direct bonding step . As the technology scales to the 40nm node for BSI CMOS image sensors, the dimensions of the Cu interconnects shrink, making them more susceptible to localized corrosion and dishing . The directivity and variance of shear forces during the preceding CMP steps strongly influence the final nanoscale topography and defectivity . Consequently, the cleaning process must not only remove nano-scale contaminants but also maintain the delicate Cu recess engineered during the CMP phase to ensure successful thermo-compression during hybrid bonding . Any residual organic films from slurry additives, if left uncleaned, will decompose during the bonding thermal cycle and outgas, causing large delamination voids at the bonding interface .
Risks & Challenges
[High] Particle-Induced Bonding Voids: Residual silica or ceria abrasive particles remain on the surface due to insufficient electrostatic repulsion or inadequate brush shear force during cleaning . These particles act as physical spacers during the subsequent wafer bonding process, preventing van der Waals contact and resulting in localized unbonded regions or voids .
[High] Copper Corrosion and Roughening: Inadequate concentration of corrosion inhibitors or improper pH in the cleaning solution leads to the uncontrolled oxidation and dissolution of exposed Cu . This alters the precise topography required for hybrid bonding and degrades the electrical performance of the Cu-to-Cu interconnects .
[Medium] Organic Residue Contamination: Incomplete removal of CMP slurry additives, such as dispersants or surfactants, leaves a thin organic film on the final oxide surface . This film interferes with the required surface hydration and RF activation prior to bonding, severely degrading the bonding strength .
[Medium] Watermark Formation: Improper drying sequences or failure to effectively utilize surface tension gradients during IPA drying leads to the precipitation of dissolved silica or metallic ions as water droplets evaporate . These watermarks alter the local surface energy and roughness, hindering the intimate oxide-oxide bonding required for DBI (Engineering Practice).