Ashing & Strip/Clean - 40nm

HDI-PR crust must be removed to prevent carbon contamination and degrade dopant activation .

Following the Backside Passivation IIP, the patterned photoresist used to mask the ion implantation must be completely removed to prepare the surface for subsequent hard mask etching and vacuum baking [P

1]. During the upstream implant, the photoresist absorbs high-energy dopant ions, transforming its top layer into a highly cross-linked, carbonized crust known as High-Dose Ion-implanted Photoresist (HDI-PR) . If this hardened crust and underlying bulk resist are not thoroughly eliminated, residual carbon contamination will severely degrade the subsequent vacuum bake's ability to effectively activate dopants and repair crystalline defects in the backside epitaxial layer . Furthermore, unlike standard front-end ashing steps, this specific process must operate within the strict thermal budget of the existing front-side metallization while avoiding plasma-induced damage to the heavily implanted, highly defective backside silicon surface . The removal of HDI-PR typically requires a synergistic combination of dry ashing and wet chemical stripping to overcome the mechanical toughness of the carbonized crust . The process begins with a plasma ashing step, which introduces high-energy radicals, ions, and reactive oxygen species to oxidize and break the highly cross-linked polymer chains . Because the implanted crust exhibits significantly higher surface hardness and elastic modulus compared to the underlying unexposed resist, the plasma acts to physically and chemically breach this hardened shell . Following the crust's removal, a wet cleaning process utilizing organic strippers or plasma-activated solutions facilitates the dissolution of the remaining bulk resist and post-ash residues . Similar to how specific photon energies drive photochemical chain scission in dense polymer networks , the plasma-activated species lower the apparent activation energy of the stripping reactions, promoting backbone scission and oxidation of the photoresist polymer chains . The selection of a sequential dry-wet methodology is driven by the need to balance high removal efficiency with substrate protection . Relying solely on dry plasma ashing requires prolonged exposure and high bias voltages, which can induce physical sputtering damage and drive metallic contaminants deeper into the sensitive backside silicon layer . Conversely, standard wet strippers alone cannot easily penetrate the dense, highly cross-linked network of the HDI-PR crust . Therefore, controlling the plasma activation parameters, such as the applied radio frequency power and activation voltage, is critical; increasing the voltage initially raises the concentration of active species and the etch rate, but excessive voltage can lead to activity saturation or undesirable side reactions . Subsequent wet cleans often utilize solvent mixtures that structurally modify the residue surface energy, enabling better solvent wetting and penetration into the loosened polymer network . In a 40nm BSI CMOS image sensor flow, this specific ashing and strip/clean step is uniquely constrained by the fragility of the thinned silicon membrane and the necessity to preserve the extremely shallow, heavily doped accumulation layer created during the preceding passivation implant . Because the implanted layer relies on steep energy band bending near the silicon surface to repel minority carriers and suppress dark current , any consumption of the silicon surface or alteration of the dopant profile by aggressive cleaning chemistries would directly degrade the sensor's signal-to-noise ratio (Engineering Practice). Consequently, highly selective wet chemistries and indirect, low-damage plasma treatments are strictly preferred to ensure that the delicate interface states and passivation fields remain intact before the critical thermal activation phase .

[High] Silicon Surface Loss and Dopant Depletion: If the wet cleaning chemistry is excessively aggressive or the plasma ashing introduces significant physical sputtering, the very shallow backside accumulation layer can be partially consumed . This reduces the effective dopant concentration at the interface, weakening the built-in electric field required to repel minority carriers, ultimately leading to elevated dark current and white pixel defects .
[Medium] Incomplete HDI-PR Crust Removal: When the applied plasma voltage or active species concentration is insufficient to overcome the high surface hardness and elastic modulus of the ion-implanted resist, a residual carbonized shell remains on the wafer . This residue blocks subsequent wet strippers from penetrating the bulk polymer, leaving organic contaminants that disrupt the subsequent oxide hard mask etch and compromise the vacuum bake's dopant activation efficiency .
[Medium] Plasma-Induced Surface Damage: Prolonged exposure to high-energy plasma environments during the dry ashing phase can introduce deeper crystalline defects into the already heavily damaged epitaxial layer . These defects act as additional generation-recombination centers, exacerbating device leakage mechanisms akin to the defect-assisted conduction and tunneling phenomena observed in stressed dielectric materials .
[Low] Cross-Contamination from Stripper Solutions: If the wet stripper chemistry is not properly optimized or lacks sufficient polarity to dissolve the structurally modified polymer chains, suspended photoresist residues can redeposit onto the wafer surface . This localized contamination can mask small areas during the subsequent hard mask etch, creating micromasking defects that propagate through the final device structure .

HDI-PR crust must be removed to prevent carbon contamination and degrade dopant activation .

[High] Silicon Surface Loss and Dopant Depletion: If the wet cleaning chemistry is excessively aggressive or the plasma ashing introduces significant physical sputtering, the very shallow backside accumulation layer can be partially consumed . This reduces the effective dopant concentration at the interface, weakening the built-in electric field required to repel minority carriers, ultimately leading to elevated dark current and white pixel defects .
[Medium] Incomplete HDI-PR Crust Removal: When the applied plasma voltage or active species concentration is insufficient to overcome the high surface hardness and elastic modulus of the ion-implanted resist, a residual carbonized shell remains on the wafer . This residue blocks subsequent wet strippers from penetrating the bulk polymer, leaving organic contaminants that disrupt the subsequent oxide hard mask etch and compromise the vacuum bake's dopant activation efficiency .
[Medium] Plasma-Induced Surface Damage: Prolonged exposure to high-energy plasma environments during the dry ashing phase can introduce deeper crystalline defects into the already heavily damaged epitaxial layer . These defects act as additional generation-recombination centers, exacerbating device leakage mechanisms akin to the defect-assisted conduction and tunneling phenomena observed in stressed dielectric materials .
[Low] Cross-Contamination from Stripper Solutions: If the wet stripper chemistry is not properly optimized or lacks sufficient polarity to dissolve the structurally modified polymer chains, suspended photoresist residues can redeposit onto the wafer surface . This localized contamination can mask small areas during the subsequent hard mask etch, creating micromasking defects that propagate through the final device structure .

40nm BSI CMOS Image SensorPreview

Process Cross-Section

Step highlight

In depth

Risks & Challenges

40nm BSI CMOS Image SensorPreview

Process Cross-Section

Step highlight

In depth

Risks & Challenges