Achieving a vertical profile relies heavily on directional ion bombardment to continuously remove bottom passivation layers while leaving the sidewalls protected by polymeric inhibitors .
In depth
In the nanoscale Backside Illuminated (BSI) CMOS Image Sensor process flow, the Upper Grid Seal Layer Etch is
a critical step within the BONDPAD module . Positioned immediately after the Bond Pad Opening 2 photoresist patterning, this step selectively removes the protective upper grid seal material overlying the peripheral bond pads and optical pads . Unlike earlier oxide grid seal layer etches (such as step #323) which define the high-resolution pixel-level isolation grid itself, this specific step operates at the packaging integration level to expose the underlying connection interfaces . By clearing the seal layer strictly within the lithographically defined areas, it prepares the device topography for the subsequent Optical Pad 3 Etch and Lower OCL (Optical Clear Resin) Coating Etch, ensuring unobstructed mechanical and electrical access for final chip packaging . The physical removal of the seal layer is achieved through reactive ion etching (RIE), which is fundamentally governed by the coupling between ion-neutral cooperative reactions and transport-limited neutral reactant delivery . In this plasma environment, directional ion bombardment breaks surface chemical bonds while neutral radicals adsorb onto the surface to form volatile byproducts (Engineering Practice). The efficiency of this process is heavily influenced by geometric factors, as the transport of neutral reactants to the bottom of the etch feature is restricted by geometric shadowing as the depth increases . Consequently, the steady-state surface coverage of reactive species decreases at the feature bottom, leading to aspect-ratio-dependent etching (ARDE) where deeper or narrower features etch more slowly than wide open areas . Achieving a vertical profile relies heavily on directional ion bombardment to continuously remove bottom passivation layers while leaving the sidewalls protected by polymeric inhibitors . The selection of an anisotropic dry etch over wet etching is driven by the need to maintain strict dimensional fidelity without undercutting the photoresist mask . By carefully modulating process parameters such as bias power, chamber pressure, and gas ratios, process engineers can control the delicate balance between the physical sputtering component and the chemical etching component . For example, adjusting the bias power directly influences the kinetic energy of the incident ions, which is necessary to overcome the activation energy for surface reactions and to efficiently clear etch-inhibiting polymer layers at the bottom of the pad opening . Meanwhile, tuning the chamber pressure modifies the mean free path of the plasma species, directly impacting both the neutral radical transport efficiency and the vertical directionality of the ion flux . In 40nm BSI technology, the extreme miniaturization of pixel pitches dictates that peripheral structures, including bond pads and upper grid seals, must be fabricated with highly constrained spatial tolerances . As critical dimensions shrink, microloading and pattern factor effects become highly pronounced, meaning that etch rates become heavily dependent on local layout and design rules . Over-etch times must be precisely calculated to completely clear the slowly etched, dense features while ensuring that previously cleared, wider features are not excessively exposed to the damaging effects of the plasma environment . If these transport-limited mechanisms are not properly compensated through dynamic parameter control, the resulting non-uniformities can severely degrade the electrical contact resistance and the overall reliability of the final packaged sensor .
Risks & Challenges
[High] Aspect-Ratio Dependent Etching (ARDE) and RIE Lag: As the depth-to-width ratio of the pad openings increases, the neutral reactant flux drops significantly due to geometric shadowing . This transport limitation causes narrower features to etch slower than wider ones, leading to incomplete clearing of the seal layer in constrained areas unless prolonged over-etching is applied .
[Medium] Etch Termination / Polymer Etch Stop: During plasma etching, an imbalance between the reactive etch chemistry and the passivation regime can lead to excessive polymer deposition at the feature bottom . This dense etch-inhibiting layer blocks further ion bombardment and neutral adsorption, prematurely halting the downward etch progress .
[Medium] Sidewall Undercutting: If the chamber pressure is too high or the bias power is insufficient, the vertical directionality of the ion flux is compromised . This allows isotropic chemical reactions to dominate, causing the etchant to laterally erode the seal layer beneath the photoresist mask and degrade the spatial definition of the pad opening .
[Low] Underlying Layer Plasma Damage: Prolonged over-etching, which is often required to completely clear slowly etched features affected by ARDE, exposes the underlying structures to excessive ion bombardment . This prolonged exposure can physically sputter or chemically alter the underlying optical pad or dielectric interfaces, degrading their intended functional properties (Engineering Practice).