Oxygen radicals chemically attack photoresist carbon backbones to form volatile byproducts, enabling their removal from the exposed metallic stack .
In depth
Following the Ta-Barrier etch in the bond pad module, the wafer surface contains residual photoresist, hardened etch polymers, and reactive halogen
species . This Ashing and Strip/Clean step is mandatory to completely remove these organic and inorganic contaminants before the subsequent ILD 6-3 deposition (Engineering Practice). Unlike intermediate via cleans (such as steps #11, #17, or #32) which typically clear homogeneous dielectric bottoms, this specific step must handle a fully exposed metallic stack where both the bulk aluminum and underlying tantalum barrier have been subjected to aggressive reactive ion etching . Failure to achieve a pristine surface at this stage prevents adequate adhesion of the subsequent interlayer dielectric (ILD) films and allows trapped halogens to initiate long-term metal corrosion failures . The primary mechanism of the ashing phase relies on an oxygen-based plasma chemistry to volatilize organic materials . In the plasma, energetic electrons dissociate O2 gas into reactive oxygen radicals, which chemically attack the carbon backbone of the photoresist to form volatile byproducts like CO and CO2 . However, during the preceding metal and barrier etch steps, the resist is heavily bombarded by ions and mixed with sputtered metal and halogens, forming a complex fluorocarbon or chlorocarbon polymeric crust . Conventional high-temperature ashing (e.g. , at 275°C) exacerbates this issue by driving thermal crosslinking and hardening of the resist, creating carbon-rich residues that are highly resistant to removal . To mitigate this, an in-situ low-temperature O2 plasma ashing process is typically employed, which balances the radical reaction rate while strictly avoiding the thermal degradation and hardening of the polymer . Following the dry ash, a wet strip step is executed to dissolve the remaining metallo-organic residues and neutralize trapped halogen atoms (Engineering Practice). The selection of a two-step dry-ash and wet-clean sequence is driven by the dual nature of the post-etch defects . While the O2 plasma efficiently removes bulk organics, it cannot volatilize metal-containing residues originating from the Al and Ta layers . Furthermore, residual chlorine from the main etch can chemically attack the exposed aluminum sidewalls, leading to post-metal etch residues composed of Al, Cl, and other trace elements like F or Cu . The wet clean solvent, often an amine-based chemistry or a highly selective proprietary blend, is chosen specifically for its ability to complex with these metallic residues and safely flush them from the exposed features (Engineering Practice). Process parameters such as RF power, chamber pressure, and gas ratios must be carefully optimized; for instance, excessive fluorocarbon addition during etching or ashing can inadvertently passivate the metal surface, fundamentally inhibiting the subsequent wet clean's capability to dissolve and remove the residues . In a nanoscale Back-Side Illuminated (BSI) CMOS Image Sensor, the physical dimensions and structural integrity of the bond pad interconnects are critical for high-frequency signal extraction . The dense pitch of these interconnects magnifies the impact of micro-masking or incomplete residue removal, which act as unyielding series resistances that degrade device performance . Ensuring an atomically clean interface before depositing the ILD 6-3 layer is fundamentally vital to prevent moisture ingress and mechanical stress accumulation, which would otherwise compromise the structural stability required for ultimate packaging and hybridization .
Risks & Challenges
[High] Polymeric Residue Hardening: If the ashing temperature is set too high (e.g. , near 275°C), the photoresist undergoes severe thermal crosslinking and structural hardening . This hardened, carbon-rich polymeric crust becomes chemically inert to standard wet stripping solvents, leaving massive residues on the wafer that physically block subsequent ILD deposition .
[High] Post-Etch Metal Corrosion: Residual chlorine atoms from the upstream Ta-barrier and Al main etches chemically attack the exposed Al sidewalls during the transfer to the strip tool or inside the wet bath . This aggressive corrosion produces Al-, Cl-, and F-containing residues that systematically degrade the structural integrity and electrical conductivity of the bond pad .
[Medium] Surface Over-Passivation: The presence of excessive fluorocarbon species (such as CHF3 or CF4) from upstream etching steps forms a dense, chemically resilient passivation layer on the metal surfaces . This highly passivated surface reduces the chemical access of the subsequent wet strip solution, resulting in a significantly diminished capability to dissolve and remove the underlying Al-containing residues .