40nm BSI CMOS Image SensorPreview

quent deposition of a planarizing base layer, the Upper On-Chip Lens (UOCL) Coat/Expose/Develop/bake step is executed [A2]. The primary function of the Upper OCL is to gather and focus incident light directly into the underlying pinned photodiode, thereby maximizing the pixel's quantum efficiency and minimizing signal loss [P2]. Unlike the preceding CFA patterning steps, which utilize dye-doped polymers designed to absorb specific spectral bands to establish color [P3, T2], the UOCL process strictly employs highly transparent materials to ensure broad-spectrum photon transmission [A1]. This step uses photolithography to define the pre-reflow footprint and volume of the lens material, creating discrete resin pillars that will subsequently be shaped into convex lenses during the reflow process [A2]. This step is distinct from the sacrificial Lower OCL, as the Upper OCL forms the permanent, final light-gathering interface at the top of the backside-illuminated (BSI) optical stack [A1]. The physical mechanism of this step relies on the precise application and photochemical transformation of a photosensitive polymer resin [A2]. First, spin-coating distributes a uniform layer of the liquid resin across the wafer, where the final coated thickness fundamentally dictates the volume of the lens [A2]. During ultraviolet exposure, the mask defines the lateral dimensions (critical dimension, CD) and the gaps between adjacent pixels, initiating photochemical crosslinking or chain scission depending on the resist tone [A2]. The development process then selectively removes the highly soluble regions to isolate the individual resin blocks (Engineering Practice). Finally, a soft bake drives out residual developer solvents and stabilizes the polymer matrix, providing the structural integrity required to prevent premature deformation before the high-temperature reflow stage [A2]. The selection of the microlens resin is driven by its optical properties, specifically requiring high transmittance across the visible spectrum and a tailored refractive index—typically in the range of 1.6 to 1.8—to achieve sufficient refractive power at the lens-air or lens-protective-layer interface [A1]. The interaction between the coated film thickness and the exposed pillar CD is tightly controlled, as these parameters jointly determine the eventual radius of curvature of the reflowed lens [P4]. Optical metrology, utilizing scatterometry and diffraction models, is heavily employed at this stage to measure the pre-reflow profile and monitor process variations, ensuring sub-nanometer precision in volume control [P4]. If the gap separating adjacent pillars is patterned too narrowly, the subsequent reflow will cause the lenses to merge; conversely, a gap that is too wide diminishes the pixel fill factor and introduces optical dead zones that degrade sensitivity [A1]. In nanoscale Backside-Illuminated (BSI) CMOS image sensor technology, the heavily scaled pixel pitch places stringent demands on the UOCL patterning step [P1]. As feature sizes shrink into the deep sub-micron regime, the alignment overlay of the microlens to the underlying photodiode and color filter becomes highly critical to prevent incident light from spilling into adjacent pixels [P3, P4]. The BSI architecture inherently places the microlens physically closer to the photodetector by removing the frontside metallization stack, permitting a wider optical acceptance angle [P2]. However, to fully leverage this architectural advantage, the UOCL Coat/Expose/Develop/bake step must produce exceptionally uniform resin pillars to ensure perfectly symmetric focal profiles across the entire high-density pixel array [A2].