Because the underlying grid features are relatively narrow, capillary forces dominate over gravity and viscous resistance, acting as the primary drivers for film flow and topography leveling.
In depth
In the 40nm BSI CIS flow, after the W CMP and Optical Pad 2 Deposition steps, the substrate typically ex
hibits localized topography variations . The Lower OCL (On-Chip Lens) Planar Layer Deposition step is strictly designed to eliminate these nanoscale topographical variations and provide an optically flat baseline for the subsequent sacrificial lens lithography . This objective distinctly separates it from prior steps like the Oxide Grid Seal Layer Deposition, which conformally encapsulates structures to provide physical passivation rather than global planarity . Without absolute planarity, the subsequent lithographic exposure for the sacrificial lower OCL would suffer from depth-of-focus variations, severely distorting the final microlens shape . Decreasing the exposure wavelength or increasing the numerical aperture of the lens to improve resolution inevitably leads to a decrease in depth of focus, making such substrate planarization absolutely necessary as optical lithography is pushed toward its limits. Furthermore, precise control of the focal plane helps mitigate systematic variations and photolithography distortions caused by the interference of neighboring patterns.
The fundamental mechanism of this planarization process relies on the spontaneous lateral flow of a liquid film after spin coating. During the initial spin coating phase, centrifugal forces dominate the film thickness distribution to produce a conformal coverage. However, true planarization occurs after spinning ceases, driven by surface tension tending to reduce interface curvature and forcing fluid from raised regions toward recessed regions. This fluid-mechanical evolution is governed by capillary-driven viscous flow, where Laplace pressure differences generate pressure gradients that gradually fill depressions. Because the underlying grid features are relatively narrow, capillary forces dominate over gravity and viscous resistance, acting as the primary drivers for film flow and topography leveling.
The material selected for this planar layer must possess specific rheological properties, balancing low viscosity with an optimized initial film thickness. A larger initial film thickness provides a greater fluid volume per curvature change, which mathematically accelerates the planarization process . Following deposition, the coated substrates are baked to remove residual spinning solvent and cure the film. It is critical to manage this bake step carefully, as planarization is significantly enhanced during the early stages of baking due to thermal flow that occurs before the hardening or cross-linking of the film. Premature curing would arrest this flow, leading to conformal contours rather than true leveling over wider topographic gaps.
In the 40nm node, continuous scaling creates highly dense grid structures that exacerbate the physical trade-offs in structural modeling and planarization. Because the planar layer remains part of the permanent optical path, its electronic and optical properties must not absorb the incident photons governed by the material's band structure and quantum states. Consequently, achieving surface-tension-driven leveling without introducing defect centers or thermal degradation ensures that the subsequent microlenses focus light accurately into the underlying photodiodes. (Engineering Practice)
Risks & Challenges
[High] Incomplete Topography Planarization: If the film viscosity is excessively high or the initial thickness is insufficient, the capillary-driven viscous flow will be too slow to fully level the underlying grid topography before the film hardens . This leaves residual structural steps that directly consume the limited depth of focus during the subsequent sacrificial lens lithography, causing asymmetric or distorted microlens formation.
[Medium] Thermal Flow Arrest: Excessive ambient temperature during spin-coating or too rapid a temperature ramp during the soft bake can cause the spinning solvent to evaporate too quickly, prematurely hardening the film. This limits the lateral fluid flow distance, preventing the material from filling wider topographic gaps and resulting in conformal rather than planar profiles.
[Medium] Optical Transmittance Degradation: Improper curing conditions or chemical incompatibility can induce phase separation or thermal degradation within the organic layer. (Engineering Practice) Because this layer remains in the optical path, such molecular defects can introduce localized states that alter optical absorption and emission processes, directly degrading the sensor's quantum efficiency.
[Low] Micro-void Formation in Dense Grids: In areas with exceptionally dense sub-micron topography, the rapid multi-step deposition and localized pattern variations can trap air or solvent vapor before the fluid fully wets the trench bottom. This mechanism limits the effective contact area and can cause localized mechanical weakness or optical scattering during subsequent processing.