Spin-on resins and UV-curable epoxies are heavily preferred over chemical vapor deposition (CVD) films because spin-on dielectrics inherently planarize topography and prevent void formation in deep, narrow trenches .
Lower OCL (On-Chip Lens) Coating Deposition is a critical module in the Backside
-Illuminated (BSI) CMOS image sensor flow for establishing the pixel-level optical transmission path . Following the sacrificial lower OCL etch and ashing steps, the wafer surface exhibits complex topography composed of sub-micron pixel isolation grids . This deposition step fills these cavities with an optically transmissive material, preparing a uniform, filled base for the subsequent Optical Pad 3 Deposition and CMP planarization steps (Engineering Practice). Unlike structural barrier layers such as the Lower Vertical Grid Deposition that define physical walls, this coating selectively fills the optical cavity to manage light transmission and establish a required refractive index contrast . The physical mechanism of this deposition primarily relies on the fluid dynamics of spin-coating a liquid-phase optical polymer, resin, or spin-on glass onto the highly patterned surface . Driven by centripetal force and capillary action, the liquid material flows into the high-aspect-ratio pixel cavities to minimize overall surface energy, which is physically analogous to the viscous flow planarization used in high-confinement damascene processes . Subsequent thermal or ultraviolet curing cross-links the polymer chains, drives out residual solvents, and densifies the organic or hybrid film (Engineering Practice). This curing process permanently sets the macroscopic refractive index of the coating, which strictly governs the refraction and internal reflection of incident photons directed toward the pinned photodiode . Material selection prioritizes extremely high optical transparency across the visible spectrum and a precisely engineered refractive index to maximize quantum efficiency . Spin-on resins and UV-curable epoxies are heavily preferred over chemical vapor deposition (CVD) films because spin-on dielectrics inherently planarize topography and prevent void formation in deep, narrow trenches . If a conventional CVD process were used, conformal film growth would rapidly pinch off at the top of narrow grid structures, leaving highly detrimental light-scattering voids in the optical path . The primary process parameters include fluid viscosity, dispense rate, and spin acceleration, which interact thermodynamically with the curing temperature profile to determine the final film thickness, degree of gap-fill, and structural integrity before the next CMP step (Engineering Practice). In 40nm BSI architectures, the heavily scaled sub-micron pixel pitch heightens the risk of optical crosstalk and necessitates flawless optical isolation . The Lower OCL coating must seamlessly integrate with surrounding low-index grids or air gaps to form a sharp, abrupt refractive index gradient, confining light within individual pixels to suppress the checkerboard effect and improve the modulation transfer function . Precise physical control of this coating layer's optical density directly dictates the efficiency of photon absorption and subsequent carrier collection in the underlying silicon substrate .
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