UV exposure acts to quickly "freeze" the outer surface of the lens through rapid photochemical crosslinking, while the subsequent thermal bake ensures uniform bulk curing .
After the Upper On-Chip Lens (UOCL) is patterned and thermally reflowed into a hemispherical shape, it must be permanently
solidified (Engineering Practice). This step prevents any further deformation of the critical optical profile during subsequent backend processing (Engineering Practice). By highly crosslinking the polymer matrix, the lens material achieves the necessary mechanical and thermal stability to withstand the subsequent Upper OCL protective oxide coating and pre-lithography cleaning steps . This distinct combination of UV exposure and thermal baking specifically targets the chemical and physical stabilization of the reflowed optical lens, distinguishing it from later final hard bakes that primarily serve to drive out residual solvents from planarizing or packaging layers . The core mechanism relies on a dual-action photo-induced and thermally driven crosslinking reaction, analogous to the condensation and activation processes used in advanced hardmasks . Initially, ultraviolet (UV) irradiation penetrates the transparent lens material and activates photopolymerization initiators or photoacid generators within the matrix [A2, P2]. This exposure generates reactive species that initiate dense crosslinking within the polymer network . Following UV activation, a high-temperature hard bake provides the thermal energy required to accelerate reaction kinetics, which typically follow an Arrhenius-type temperature dependence . As covalent bonds form between polymer chains, the free volume of the material decreases and its glass transition temperature (Tg) increases, fundamentally altering the polymer's thermal response similar to property modulations seen in specialized underlayers . The resulting dense, crosslinked network exhibits a significantly higher elastic modulus and improved structural rigidity [P2, A2]. The selection of a combined UV and thermal bake method is necessary because thermal baking alone might induce secondary reflow before the material fully crosslinks, destructively distorting the lens's radius of curvature . UV exposure acts to quickly "freeze" the outer surface of the lens through rapid photochemical crosslinking, while the subsequent thermal bake ensures uniform bulk curing . Higher UV exposure doses increase the concentration of reactive species, which raises the final crosslink density and subsequent chemical durability . Concurrently, higher bake temperatures accelerate network formation, but excessive heat must be actively avoided to prevent intrinsic stress buildup or thermally induced film cracking . Precise control over both the UV dose and the thermal budget is required to maximize the optical transparency of the lens by eliminating residual light-absorbing solvents or unreacted monomer species . In a nanoscale Back-Illuminated (BSI) CMOS Image Sensor, the pixel pitch is significantly reduced, necessitating highly precise and stable optical focus onto the underlying photodiode (Engineering Practice). Any micro-deformation, thermal shrinkage, or structural collapse of the nanometer-scale UOCL during the subsequent protective oxide deposition directly degrades the sensor's quantum efficiency and increases optical crosstalk . Therefore, the heavily crosslinked matrix achieved in this specific step is critical to maintaining physical pattern fidelity and providing high resistance to chemical erosion in subsequent environments, mirroring the stringent physical demands of sub-wavelength pattern transfer stabilization .
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