40nm BSI CMOS Image SensorPreview

flowed sacrificial polymer into the underlying permanent optical layer [P1]. Following the formation of the lens shape via thermal reflow and its structural stabilization through the final UV/Hard bake, an anisotropic dry etch is applied to carve this topology into the intermediate material of the lower on-chip lens (LOCL) [P1]. This step is distinctly different from subsequent standard Lower OCL etching steps because it relies on a carefully tuned 1:1 etch rate selectivity between the organic sacrificial layer and the underlying optical film, acting as a direct 3D pattern transfer mechanism similar to multi-layer hardmask applications [P1]. By precisely matching these etch rates, the physical curvature required to focus light onto the active photodiode area is faithfully replicated in the substrate, preparing the surface for the subsequent ashing of any residual organics and the deposition of conformal optical coatings [A2]. The core mechanism of this pattern transfer relies on reactive ion etching (RIE) using a meticulously balanced fluorocarbon and oxygen plasma chemistry [P2]. In the plasma chamber, the RF source power dissociates the feed gases into reactive radicals and ions, while the RF bias power accelerates these ions perpendicularly toward the wafer surface [P3]. The oxygen radicals primarily volatilize the carbon-rich sacrificial polymer by breaking C-C bonds and forming gaseous byproducts [A2]. Simultaneously, the fluorine-based species chemically react with the underlying optical material to form volatile fluorides, analogous to continuous etch reactions utilized in advanced pattern transfer [P1]. To achieve the crucial 1:1 etch selectivity that maintains the exact lens curvature during the transfer process, the physical sputtering component driven by ion bombardment and the chemical erosion component must be synchronized across both the organic and inorganic materials [P2]. A dry plasma etch is selected over wet etching because only highly directional RIE can faithfully transfer 3D topographies without catastrophic isotropic undercutting [P1]. The process parameters, particularly the gas ratio of oxygen to fluorocarbons, directly dictate the morphological outcome by shifting the relative chemical etch rates [P2]. Increasing the oxygen ratio accelerates the erosion of the sacrificial polymer, which flattens the resulting lens profile if the underlying layer etches more slowly [P2]. Conversely, increasing the bias power enhances physical ion bombardment, which can help overcome chemical etch rate disparities between the two distinct materials but risks inducing surface roughness [P3]. The preceding UV/Hard bake step is critical here because it heavily crosslinks the sacrificial polymer, increasing its density and plasma etch resistance, thereby preventing the polymer profile from rapidly degrading or outgassing under the intense plasma environment [P2]. For a nanoscale Back-Side Illuminated (BSI) CMOS image sensor, the optical path and pixel pitch are aggressively scaled, making the critical dimension and focal length of the LOCL highly sensitive to etch fidelity [P1]. At these dimensions, minor deviations in the etch rate ratio not only distort the geometric curvature of the lens but also induce severe optical crosstalk between adjacent sub-micron pixels [P2]. Consequently, controlling the exact proportion of physical and chemical etch components becomes a fundamental requirement for maximizing the quantum efficiency of the image sensor [P2].