A thin, highly transparent protective silicon oxide layer is formed on the curved microlens array to prevent chemical degradation and enable subsequent processing .
Following the formation, reflow, and UV/hard baking of the organic Upper On-Chip Lenses (UOCL), the lenses require a robust encapsul
ation layer to survive subsequent packaging and bonding processes . This step deposits a thin, highly transparent protective silicon oxide coating directly over the curved microlens array to act as a physical and chemical barrier . Unlike the subsequent "Upper OCL Coating Etch" step, which is a subtractive process designed to selectively remove this oxide over the peripheral bond pads, the current step is a purely additive deposition process designed to hermetically seal the active pixel array . By providing a conformal dielectric sealing structure, this layer suppresses moisture ingress, prevents ionic contamination, and protects the soft organic lenses from the aggressive chemical environments of the upcoming pre-lithography cleans and bond pad etch steps, functioning similarly to the encapsulation principles used for top-level high-voltage structures . To prevent thermal degradation or melting of the underlying organic microlens material, the deposition must be performed under strict low-temperature conditions . A radio frequency downstream plasma-enhanced chemical vapor deposition (PECVD) method using tetraethoxysilane (TEOS) and oxygen is typically employed, as it allows film formation to be dominated by surface diffusion and condensation rather than purely thermal gas-phase decomposition . In a downstream reactor configuration, the generation of active oxygen species is physically separated from the substrate, actively avoiding direct high-energy ion bombardment that would physically pit or degrade the underlying organic lenses . During the reaction, active oxygen species cause TEOS to undergo partial cleavage of Si–O bonds, generating surface precursor species rich in Si–OH that retain, on average, one Si–O bond from the original TEOS molecule . These intermediate species diffuse across the curved lens surface before undergoing secondary condensation reactions to form a crosslinked silicate network . This specific reaction pathway ensures high surface mobility of the precursors, which is the fundamental physical mechanism enabling highly conformal step coverage over the dense, undulating topography of the microlens array . Silicon oxide derived from TEOS is selected over standard silane-based oxide because of its inherently superior conformality and its ability to coat complex, high-aspect-ratio topographies at lower temperatures . The process parameters, such as RF power, oxygen-to-precursor ratio, and deposition pressure, must be tightly co-optimized to balance film density, optical transparency, and residual stress . Increasing the deposition temperature or reducing the TEOS flow generally decreases the surface precursor concentration, making the condensation reaction kinetically limited; this increases the effective diffusion mean free path and improves overall step coverage . However, because the maximum temperature is strictly bounded by the glass transition limits of the underlying organic UOCL, the plasma active species concentration must be carefully modulated to drive the crosslinking process without inducing thermal reflow . Strict control of the oxygen flow and plasma density is also required to minimize the retention of unreacted Si-OH (silanol) groups and carbon species, which would otherwise render the film hygroscopic and degrade long-term stability . In a nanoscale Backside Illuminated (BSI) CMOS image sensor, the optical path efficiency is highly sensitive to the thickness, uniformity, and refractive index of every overlying dielectric layer . The protective oxide must be precisely thickness-controlled to avoid acting as an unintended thin-film interference layer that could induce optical cross-talk or spectral quantum efficiency shifts across different color pixels . Furthermore, this conformal dielectric film must maintain a stable intrinsic stress state to prevent micro-cracking over the steep geometric steps formed at the array periphery, aligning with the stress-engineering principles required to maintain mechanical integrity in advanced multilayer structures .
Sign in to continue through all 417 steps