Pre Litho Cleaning removes particulates and chemically conditions the surface to ensure robust adhesion of the dye-doped polymer color filter .
In depth
The Pre Litho Cleaning step in the LS_GRID module serves as the critical surface preparation interface between the deposition of the Upper Grid Seal Laye
r and the subsequent Green Color Filter (CFA) Coat and Develop steps [P1, A3]. Unlike standard front-end pre-lithography cleans that prepare the substrate for sacrificial photoresists used in plasma etching (such as earlier flow steps #7 or #13), this specific step prepares an optical seal layer to receive a permanent, dye-doped polymer color filter . The upper grid seal layer protects the underlying metal light shield grid and requires a pristine, defect-free surface to ensure optimal optical transmission (Engineering Practice). If organic or inorganic contaminations remain on this surface, they will degrade the adhesion of the color filter and act as optical scattering centers, which exacerbates color cross-talk in dense pixel arrays [P1, P2]. Therefore, this cleaning step must comprehensively remove particulates while chemically conditioning the surface for polymeric adhesion . The physical and chemical mechanism of this cleaning step typically begins with a wet chemical treatment to remove contaminants, utilizing solutions such as Standard Clean 1 (SC1), which contains ammonium hydroxide and hydrogen peroxide, or SC2, which contains hydrochloric acid and hydrogen peroxide . These oxidizing chemistries convert organic residues into soluble species and complex metallic ions to prevent their re-adsorption onto the wafer surface [A1, A3]. Following the wet clean, the wafer is subjected to a strict thermal dehydration bake, commonly at temperatures around 150°C, to drive off any interfacial moisture . To further guarantee the robust adhesion of the color filter layer, a gaseous adhesion promoter such as Bis(trimethylsilyl)amine (HMDS) is applied . The HMDS reacts with the surface hydroxyl groups on the seal layer to form a tri-methylated silicon surface . This converts the previously hydrophilic dielectric surface into a highly water-repellent layer, which is essential to prevent aqueous developers from penetrating the polymer-dielectric interface and causing pattern lifting during the subsequent CFA lithography steps . The selection of these specific wet chemistries over dry plasma cleaning is dictated by the need to preserve the precise optical thickness and surface roughness of the upper grid seal layer . Plasma treatments run the risk of inducing surface damage or altering the refractive index of sensitive dielectrics . The chemical formulation of the wet clean must be rigorously optimized; for instance, the concentration of the oxidizer must be sufficient to achieve high cleaning efficiency but tightly bounded to prevent undesirable shifts in the dielectric constant or structural integrity of the seal layer . Furthermore, the interaction between the HMDS priming time and the subsequent polymer spin-coating dictates the final fluid mechanical spreading of the color filter resin, directly impacting the coating uniformity . At the nanoscale Backside Illuminated (BSI) node, pixel dimensions are aggressively scaled into the sub-micrometre-scale range, making the image sensor highly susceptible to optical cross-talk and transmission loss . Any nanoscale residue or inadequate surface passivation prior to the green color filter application can disrupt the local refractive index matching between the seal layer and the CFA . Consequently, this Pre Litho Clean is not merely a defect-reduction step, but a foundational optomechanical integration procedure that ensures the fidelity of the incident light path through the subwavelength structures of the CMOS image sensor .
Risks & Challenges
[High] Color Filter Pattern Lifting: If the dehydration bake is incomplete or the HMDS vapor prime is insufficient, the seal layer surface remains hydrophilic . This allows the aqueous developer used in the subsequent CFA lithography step to penetrate the interface between the color filter polymer and the substrate, resulting in localized peeling or total lifting of the fine color filter structures .
[Medium] Optical Scattering from Residual Particulates: Failure of the SC1/SC2 wet cleaning solutions to completely oxidize and remove nanoscale contaminants leaves particles trapped beneath the color filter . In sub-micrometre-scale pixel architectures, these particles act as localized optical scattering centers, significantly increasing color cross-talk and degrading the quantum efficiency of the sensor .
[Medium] Seal Layer Dielectric Degradation: Excessive exposure time or excessively high oxidizer concentrations in the alkaline/acidic cleaning baths can chemically attack the upper grid seal layer . This interaction can induce porosity or shift the refractive index and dielectric constant of the material, compromising its function as an optical matching and protective layer .
[Low] Watermark Defect Formation: Incomplete drying following the final deionized water rinse can leave microscopic water droplets on the surface (Engineering Practice). These droplets can absorb atmospheric carbon dioxide or dissolve trace surface silica, leaving behind solid watermark residues upon evaporation that degrade the optical transparency of the specific pixel they cover (Engineering Practice).