carbon-rich radicals in the plasma precipitate to form a protective polymer film on the feature sidewalls, which suppresses lateral chemical etching and ensures a highly anisotropic, vertical profile .
In depth
In nanoscale Back-Side Illuminated (BSI) CMOS Image Sensors, optical pads are essential for wir
e bonding or electrical probing, requiring deep openings through the backside dielectric stack over the metal pads . This step follows lithography for the grid trench and precedes Lower OCL (On-Chip Lens) coating steps, indicating it is a critical part of the backside light-receiving surface preparation . The distinction between the sequentially numbered Optical Pad 1, 2, and 3 etches typically relates to the step-by-step removal of different layers within a complex passivation or anti-reflective dielectric stack (e.g. , alternating oxide and nitride layers) (Engineering Practice). Specifically, the Optical Pad 3 Etch targets a distinct dielectric layer within this multi-layer stack, often requiring precise stop-on-layer control to avoid damaging the underlying structures while accommodating features with varying critical dimensions . The etching process physically operates by utilizing high-density inductively coupled plasma (ICP) or reactive ion etching (RIE) to achieve highly directional material removal . For etching silicon dioxide or similar dielectric layers, fluorocarbon gases (such as CF4, C2F6, or CHF3) are employed to drive a synergistic chemical-physical etching mechanism . The fundamental mechanism relies on the reaction of fluorine radicals with the dielectric bonds to form volatile products like SiFx, while directed ion bombardment provides the necessary activation energy for the reaction exclusively at the trench bottom . Simultaneously, carbon-rich radicals in the plasma precipitate to form a protective polymer film on the feature sidewalls, which suppresses lateral chemical etching and ensures a highly anisotropic, vertical profile . By carefully tuning the CF-to-CHF gas ratio, the process can dynamically modulate the rate of this polymerization, which is a critical mechanism for controlling aspect ratio dependent etching (ARDE) across pads of different sizes . The selection of a fluorocarbon-based dry plasma etch over wet chemical etching is dictated by the stringent need for dimensional control and vertical sidewall fidelity at the 40nm technology node . To maintain high etch selectivity to the underlying etch-stop layer or the masking material, hydrogen-containing gases (such as CH4 or H2) are typically added to adjust the carbon-to-fluorine (C/F) ratio in the plasma . Increasing this C/F ratio effectively enhances the polymer film formation rate, which preferentially passivates non-oxygen-containing surfaces (like silicon or metal) more robustly than the oxide being etched, thereby significantly boosting selectivity . Furthermore, if a carbon-based mask is utilized, in-situ mask protection strategies—such as promoting reactive surface sites for selective protective layer deposition—can be integrated to prevent severe mask erosion during these extended pad etches . Process control fundamentally relies on balancing source power, bias, and gas residence time; modulating residence time directly affects radical dissociation and dictates the competition between the etching of the dielectric and the deposition of the passivating polymer . At the 40nm BSI node, managing the aspect ratios for optical pad openings becomes highly challenging, as the pad critical dimensions shrink while the backside dielectric thickness remains substantial to accommodate optical isolation grids . This deep etching is fundamentally limited by RIE lag, a phenomenon where limited reactant transport and ion flux attenuation cause smaller features to etch significantly slower than larger ones . Therefore, advanced process modulations, such as cyclical deposition-etch schemes or dynamic etch-stop layer formation (inverse ARDE), are required to compensate for these transport limits and ensure uniform pad opening depths across the entire sensor array .
Risks & Challenges
[High] RIE Lag and Under-etching: In high aspect ratio pad structures, the limited transport of reactive etchants to the bottom of the feature inherently reduces the local etch rate . This aspect ratio dependent etching (ARDE) can result in incomplete clearing of the dielectric in smaller pads, ultimately leading to electrical open circuits during subsequent bonding steps .
[High] Poor Etch Selectivity and Punch-through: If the fluorocarbon plasma's C/F ratio is too low, the system lacks the capacity to form a sufficient polymer passivation layer on the target stop surface . This failure in the polymerization/etch competition can cause the etch to punch through the intended etch-stop layer, damaging underlying metal pads or optical grid materials .
[Medium] Mask Erosion and CD Loss: The high-energy ion bombardment required to drive vertical deep dielectric etching can physically sputter and chemically erode the patterned mask . Without adequate in-situ protective deposition on the mask surface, this continuous erosion leads to mask faceting and lateral recession, causing undesirable enlargement of the pad critical dimensions .
[Low] Etch Stop due to Over-polymerization: An excessive concentration of polymerizing gases (e.g. , adding too much CH4) significantly increases the radical polymerization rate over the physical removal rate . This imbalance can cause a thick fluorocarbon film to build up at the bottom of the pad, prematurely halting the etch process and leaving residual dielectric material .