nonionic surfactants are introduced to lower the dynamic and static surface tension of the fluid, enabling capillary ingress and intimate contact with the residues .
In depth
Following the W and TiN etch of the Light Shield Grid (LS_GRID), the wafer surface is left with a hardened photoresist (PR) crust a
nd halogen-containing organometallic post-etch residues (PER) . The metal grid structure physically isolates adjacent pixels to minimize optical and electrical crosstalk in the BSI CMOS Image Sensor (Engineering Practice). Leaving etch residues behind would severely impede the subsequent uniform deposition of the Upper Grid Seal Layer, potentially leading to adhesion failures or trapped contaminants . Furthermore, leftover conductive residues (e.g. (Engineering Practice), Ti- or W-containing polymeric byproducts) can cause electrical leakage across the pixel array, which fundamentally alters the localized carrier transport and parasitic resistance characteristics of the device . Thus, complete removal of PR and PER without attacking the sensitive W/TiN grid structure is a mandatory preparation step . Traditional high-power oxygen plasma ashing is often unsuitable for this step because it can heavily oxidize the exposed W and TiN sidewalls, increasing the grid's sheet resistance and degrading its precise vertical profile . Instead, advanced node processing frequently relies on a plasma-less or mild chemical activation route combined with specialized wet cleaning . The PR crust, highly cross-linked by ion bombardment during the preceding plasma etch, strongly resists pure solvent dissolution . To overcome this, ultraviolet (UV) irradiation can be employed to induce photochemical chain scission of C-C and C-F bonds within the polymer backbone . This targeted UV photon energy reduces the polymer's molecular weight and crosslink density, increasing surface polarity without subjecting the underlying dielectric to damaging ion bombardment . Subsequently, an all-wet cleaning solution utilizes selective dissolution and complexation mechanisms to strip the weakened organic network . The formulation of the wet clean must be precisely balanced to remove residues while maintaining perfect selectivity against W and TiN . To achieve this, specific corrosion inhibitors, such as long-chain primary alkylamines, are often added to selectively adsorb onto the W/TiN surfaces, passivating them against the oxidizing components of the bath . Because the nanoscale grid trenches present a high aspect ratio and the post-etch residues possess inherently low surface energy, pure aqueous solutions cannot easily penetrate the structures . Therefore, nonionic surfactants are introduced to lower the dynamic and static surface tension of the fluid, enabling capillary ingress and intimate contact with the residues . To compensate for mass transport limitations in these tight geometries, megasonic acoustic energy is applied to generate cavitation microjets, which physically delaminate the chemically weakened PR/BARC interfaces . What distinguishes this specific Ashing & Strip/Clean step from other routine cleans in the flow is its position immediately after a complex bi-layer (W/TiN) metal etch in the optical path . Unlike front-end cleans that handle pristine silicon, this step must simultaneously manage exposed multi-metal stacks, suppress galvanic corrosion, and navigate dense nanoscale pixel array topologies . Finally, the process requires a carefully selected rinsing agent, such as isopropyl alcohol (IPA), to fully dissolve and carry away any residual surfactants from the trench bottoms, preventing organic contamination that could otherwise compromise the integrity of the upcoming seal layer .
Risks & Challenges
[High] W/TiN Sidewall Corrosion: If the wet cleaning formulation contains excessive oxidizer concentrations or insufficient amine-based corrosion inhibitors, the solution will chemically attack the exposed tungsten and titanium nitride sidewalls . This uncontrolled etching degrades the grid's physical profile, reducing its optical blocking efficiency and altering the series resistance pathways of the device .
[High] Incomplete PR/Residue Removal: If the UV dose during pretreatment is insufficient to cause adequate polymer chain scission , or if megasonic cavitation fails to penetrate the high-aspect-ratio trenches , the highly cross-linked PR crust formed during the TiN etch will not fully dissolve . Leftover organic or organometallic debris will physically block the subsequent Upper Grid Seal Layer deposition, creating voids (Engineering Practice).
[Medium] Surfactant Trapping and Contamination: The use of nonionic surfactants to lower surface tension for trench penetration can lead to surfactant molecules remaining strongly adsorbed on the feature surfaces . If the final rinse step (e.g. (Engineering Practice), using IPA) is inadequate, these trapped organic molecules will cause localized adhesion failures for the next deposition step and potentially induce leakage currents .
[Medium] Dielectric Surface Densification: If conventional reactive plasma ashing is used too aggressively instead of mild UV/wet modification, the energetic oxygen or fluorine radicals will break bonds in the exposed underlying dielectric layers . This bombardment causes surface densification, carbon depletion, and moisture absorption, leading to an unwanted increase in the interfacial dielectric constant .