40nm BSI CMOS Image SensorPreview

er OCL Reflow step is necessary to transform these steep-sided polymer blocks into functional, continuous hemispherical convex lenses [P1]. This transformation maximizes the light-gathering capability of the pixel by directing incident rays toward the underlying photodiode (Engineering Practice). By transitioning the patterned UOCL into an optimal optical profile, this step sets the foundation for the subsequent UV/Hard bake, which freezes the lens morphology into a permanent, highly crosslinked state before the protective oxide coating is applied [A1]. The fundamental physical mechanism governing this step is the minimization of surface free energy via viscous flow [T1]. When the wafer is heated above the glass transition temperature of the Upper OCL polymer, the material transitions into a viscoelastic fluid state [P1]. To reduce surface energy caused by the sharp, unsatisfied geometrical edges from lithography, surface tension drives the fluid outward and upward, minimizing surface curvature into a naturally spherical or hyperbolic profile [T1]. The base of the lens acts as a capillary pinning boundary, preventing indefinite lateral spreading while the bulk of the resist undergoes volume rearrangement and slight shrinkage [P1]. As described by optical focusing principles, the final lens focal length and curvature are directly governed by the pinned base radius and the volume of the reflowed material [P1]. Thermal reflow is selected as the shaping method because it provides a self-assembling, thermodynamically stable process to produce uniformly smooth micro-optics without requiring complex grayscale lithography or mechanical polishing [P1]. The final lens morphology depends on the interplay between reflow temperature, time, and the underlying surface energy [P2]. Higher temperatures drastically lower the polymer's viscosity, accelerating the reorganization driven by surface tension [P3], but excessive thermal budgets can lead to premature polymer carbonization or irreversible structural degradation [P2]. The substrate's surface energy discontinuities must be optimized to ensure proper adhesion while suppressing unconstrained boundary migration that would cause neighboring lenses to merge [P2]. In a 40nm BSI CMOS image sensor, the pixel pitch approaches the sub-micron regime, imposing exceptionally stringent requirements on the gap between adjacent microlenses [P1]. At these aggressive dimensions, standard reflow physics faces the challenge of merging, where capillary forces may easily pull adjacent fluid droplets together, destroying spatial resolution (Engineering Practice). Consequently, the lithographic patterning and the glass transition temperature behavior—potentially modulated by polymer molecular weight and photo-induced chain scission—must be perfectly co-optimized to maintain an ultra-tight process window that maximizes the light-collecting fill factor while guaranteeing zero gap merging [A1].