40nm BSI CMOS Image SensorPreview

tep is engineered to form a temporary topographical template that will eventually define the curvature of the lower on-chip lens (LOCL) (Engineering Practice). Positioned immediately after the Lower OCL Planar Layer Deposition and Pre Litho Cleaning, this process coats, exposes, and develops a photosensitive organic polymer into discrete geometric blocks (typically cylinders) [P1]. These structures prepare the wafer for the subsequent Sacrificial Lower OCL Reflow, where surface tension will reshape them into spherical lens profiles (Engineering Practice). This function strictly distinguishes this step from the similar Color Filter (Green/Blue/Red) Coat/Expose/Develop/Bake steps; whereas color filters form permanent, dye-doped device layers meant to absorb specific optical wavelengths, the sacrificial LOCL resist is entirely consumed during the downstream dry etch step to transfer its 3D profile into the underlying permanent high-index planar layer [P3]. The physical and chemical mechanics of this step rely heavily on chemically amplified photolithography [A2]. During the coat phase, a photosensitive polymer containing photoacid generators (PAGs) is spin-coated, with film thickness governed by fluid viscosity and centripetal acceleration [P1]. Upon lithographic exposure, the PAGs absorb UV photons and generate localized acid molecules, which remain latent until thermal activation [P3]. During the subsequent post-exposure bake (PEB), thermal energy activates acid-catalyzed deprotection or crosslinking reactions within the polymer matrix [P3]. This chemically amplified mechanism ensures that one photogenerated acid molecule catalyzes multiple reaction events, significantly changing the polymer's solubility in the developer solution [A2]. The acid diffusion length and reaction kinetics strictly determine the final critical dimension (CD) of the resist block [A2]. Finally, the development step washes away the highly soluble regions, leaving the isolated pre-lens patterns (Engineering Practice). The material selection for this sacrificial layer balances photolithographic resolution with precise thermomechanical properties [P1]. The polymer must possess a carefully tuned glass transition temperature ($T_g$) so that it softens and reflows into a smooth hemispherical shape during the next thermal step, without melting completely and losing structural integrity (Engineering Practice). Additionally, it must exhibit sufficient plasma etch resistance to act as a reliable transfer mask during the subsequent fluorocarbon-based dry etch [P1]. Process parameters interact competitively: a higher exposure dose increases acid concentration, thereby accelerating network formation or deprotection, while higher bake temperatures drastically increase acid diffusion lengths [P3]. If the soft bake removes too much solvent, the resist may become excessively rigid and fail to reflow properly later, whereas retaining too much solvent can induce dark erosion and unacceptably thin the resist during development [P3]. For nanoscale generation image sensors, the extremely small pixel pitch necessitates highly aggressive control over the lithographic critical dimensions (Engineering Practice). As pixel dimensions shrink, the gap between adjacent resist cylinders prior to reflow must be exceptionally narrow to ensure a high fill-factor for the final lens array, yet wide enough to prevent adjacent lenses from merging during the reflow process (Engineering Practice). Because the final optical crosstalk and quantum efficiency are heavily dependent on the volumetric uniformity of these resist blocks, variations in spin-coating uniformity or PEB temperature gradients across the wafer directly translate to severe optical degradation in the final BSI device [P3].