The selection of cleaning chemicals is driven by the need to balance residue removal efficiency with the preservation of the polished interconnect geometry .
In depth
The Post CMP Cleaning step in the MET2 module immediately follows the chemical mechanical planarization of the Copper (Cu) overburden and t
he Ta-based liner . Its primary function is to remove slurry abrasives, organic corrosion inhibitors, and metallic residues left on the wafer surface before the subsequent ILD 2-1 dielectric deposition . Unlike the STI CMP Post Cleaning which primarily deals with oxide and nitride surfaces, this back-end-of-line (BEOL) cleaning step must handle exposed, highly reactive copper and a dissimilar barrier metal simultaneously . If these contaminants are not removed, they will severely degrade the adhesion of the subsequent dielectric layer and introduce leakage pathways between adjacent metal lines . Furthermore, leftover micro-particles can act as masking defects in subsequent lithography steps, directly impacting the process yield of the CMOS Image Sensor . During the preceding Cu and barrier CMP, the surface is continuously passivated by organic corrosion inhibitors, such as benzotriazole (BTA), to prevent uncontrolled isotropic wet etching of the copper . BTA coordinates with surface copper atoms to form a Cu(I)-BTA polymeric passivation film . While this film is critical for achieving global planarization without localized corrosion during CMP, it leaves behind thick, hydrophobic organic residues that are difficult to remove . Additionally, colloidal silica abrasives used in the barrier CMP step tend to agglomerate due to van der Waals attraction, generating large particles that strongly adhere to the wafer . The post-CMP cleaning process relies on a combination of mechanical brush scrubbing to overcome particle adhesion forces and specialized chemical mixtures to dissolve the Cu-BTA complexes and suspend the particles . The cleaning chemistry typically contains complexing agents and pH adjusters designed to lift off the organic layer while slightly oxidizing the Cu surface to form a controlled, removable native oxide . The selection of cleaning chemicals is driven by the need to balance residue removal efficiency with the preservation of the polished interconnect geometry . Highly aggressive chemical cleans could exacerbate metal dishing, which directly reduces the conductive cross-sectional area and increases interconnect resistance . To mitigate organic residue challenges, advanced CMP formulations may replace traditional BTA with 5-methyl-benzotriazole (MBTA), which provides effective corrosion inhibition at lower concentrations and leaves a passivation layer that is significantly easier to remove during this subsequent cleaning step . Furthermore, the introduction of nonionic dispersants in the cleaning fluids or preceding slurries can provide steric hindrance around silica particles, preventing their re-agglomeration and reducing scratch defects on the soft copper surface . The cleaning system also prevents cross-contamination by ensuring that removed metallic ions are fully chelated, preventing them from precipitating back onto the dielectric regions . For a 40nm BSI CMOS Image Sensor, the scaling of the BEOL dimensions makes the process extremely sensitive to both particle defects and geometrical variations like dishing and erosion . As the line width and spacing shrink to nanoscale, even nanoscale conductive residues or unremoved slurry agglomerates can bridge adjacent interconnects, causing catastrophic short circuits . Therefore, the post-CMP clean must achieve near-perfect defect removal while strictly limiting any galvanic corrosion between the Cu interconnects and the Ta-based barrier, ensuring high-fidelity signal routing and low RC delay for the image sensor array .
Risks & Challenges
[High] Organic Residue Defectivity: High concentrations of traditional corrosion inhibitors (like BTA) from the CMP slurry form thick polymeric Cu(I)-BTA films that resist dissolution in standard cleaning chemicals, leaving organic defects that degrade dielectric adhesion .
[High] Micro-scratch Generation: Colloidal silica abrasives agglomerate into large particles under van der Waals forces if sufficient steric stabilization is lacking; these hard particles can be dragged across the wafer during brush cleaning, scratching the soft copper surface .
[Medium] Exacerbated Metal Dishing: Overly aggressive complexing agents in the cleaning chemistry may chemically attack the exposed copper at a faster rate than the surrounding dielectric, deepening the cylindrical dishing profile formed during CMP and negatively impacting interconnect electrical performance .
[Medium] Galvanic Corrosion: The electrochemical potential difference between the exposed Cu interconnects and the Ta-based barrier layer can drive anodic dissolution of the copper in the presence of a conductive cleaning fluid, leading to localized voiding at the metal-barrier interface .