Anisotropic etching of the main optical lens material at the bondpad periphery removes overlying layers to enable electrical packaging .
In depth
In the fabrication of a nanoscale Backside Illuminated (BSI) CMOS Image Sensor, the bondpads located at the device periphery must be cleared of all overlying op
tical and dielectric layers to enable final electrical packaging . The Lower OCL (On-Chip Lens) Etch is a critical clearance step within the BONDPAD module, designed to selectively remove the inner micro-lens material—typically a high-refractive-index silicon nitride or hybrid organic-inorganic layer—from the bondpad regions . This step sequentially follows the Lower OCL Coating Etch, which removes the underlying planarization or anti-reflective coatings, and prepares the structure for the subsequent Optical Pad etches that will ultimately expose the metal bondpad . Unlike the "Sacrificial Lower OCL Etch," which is utilized earlier in the flow as a global etch-back to define the lens curvature across the active pixel array, or the "Lower OCL Coating Etch," which targets organic sub-layers, this specific step strictly focuses on anisotropically breaching the main optical lens material at the periphery without damaging the adjacent pixel structures . The physical and chemical mechanisms of this etch rely on high-density plasma Reactive Ion Etching (RIE), which leverages the synergistic effect of chemical reactivity and physical ion bombardment . For silicon nitride-based lens materials, fluorocarbon plasmas (such as CF4 or CHF3) are employed to generate fluorine radicals that react with Si–N bonds, forming volatile byproducts . To prevent excessive lateral etching and maintain vertical sidewalls in the deep bondpad opening, a cooperative deposition-etch mechanism can be implemented . In this regime, gases with a high carbon-to-fluorine (C/F) ratio form a carbon-rich polymer film on the sidewalls, effectively suppressing horizontal chemical attack, while the directional ion flux continuously clears the polymer from the horizontal etch front to sustain downward etching . Alternatively, to strictly control thermal damage to nearby temperature-sensitive materials, the process may utilize a cyclic mechanism consisting of low-temperature fluorination followed by inert gas plasma bombardment to physically remove the chemically weakened surface layer . Material and method selection for this step is heavily dictated by the need for high etch selectivity between the lower OCL material and the underlying optical pad layer (often silicon dioxide) . Achieving this selectivity requires precise tuning of the plasma chemistry, such as the introduction of specific additives like H2, O2, or CH4 to modulate the C/F ratio and the resulting polymer formation rate . Adding polymerizing agents enhances the deposition of a passivation layer on the oxygen-rich underlying pad, significantly reducing its etch rate upon exposure . Furthermore, plasma source power, pressure, and gas residence time must be carefully balanced; adjusting the residence time effectively controls radical dissociation and the competition between etching and polymerization, which is a core factor in simultaneously achieving a practical etch rate and high selectivity . At the 40nm technology node, the stringent geometric constraints and scaling rules demand highly vertical and smooth etch profiles to prevent the bondpad opening from encroaching upon the densely packed active pixel array . The aspect ratio of the peripheral openings is elevated at this node, increasing the risk of ion scattering and micro-trenching at the feature bottom (Engineering Practice). Consequently, minimizing sidewall roughness is critical, as rough interfaces in the optical stack can introduce Rayleigh scattering and degrade the optical isolation between the periphery and the imaging array . The thermal budget is also strictly constrained to prevent degrading the subthreshold slope and leakage characteristics of the scaled peripheral MOSFETs , justifying the use of low-temperature plasma techniques that protect both the active devices and the integrated colloidal or polymeric optical components .
Risks & Challenges
[High] Loss of Etch Selectivity to Underlying Pad: If the plasma becomes over-dissociated due to excessive source power or incorrect gas residence time, the fluorocarbon polymer passivation on the underlying oxide pad may fail, leading to rapid consumption of the pad material .
[High] Sidewall Bowing or Profile Degradation: Insufficient polymer film formation by the primary process gas or an excessively low carbon-to-fluorine ratio will fail to protect the feature sidewalls from radical attack, resulting in an undefined or bowed etch profile .
[Medium] Incomplete Etch or Residue Formation: If the energy of the inert gas plasma is insufficient or the physical removal time is too short during cyclic low-temperature etching, the fluorinated surface layer may not be fully removed, leaving localized residues over the bondpad area .
[Medium] Micro-trenching at Step Corners: High-energy ion bombardment directed at the bottom of the deep bondpad opening can cause localized physical sputtering and micro-trenching at the corners, which may propagate into the underlying grid seal layers and cause reliability issues .
[Low] Thermal Degradation of Adjacent Structures: Overly aggressive plasma conditions or inadequate wafer cooling can raise the substrate temperature beyond acceptable limits, potentially causing physical degradation or refractive index shifts in adjacent temperature-sensitive optical materials .