High-aspect-ratio etching over multiple layers necessitates strict aspect-ratio independent etching (ARIE) characteristics to prevent micro-loading effects as the trench deepens .
In depth
In the packaging module of a 40nm BSI CMOS image sensor, deep bond pad cavities must be excavated through multiple in
terlayer dielectric (ILD) tiers to expose the underlying metal pads . The etching sequence alternates between bulk oxide removal and selective barrier breaching, driven by the stack's structural composition . Following the ILD 6-2 Oxide Etch, which selectively stops on the ILD 6-1 SiCN layer, the current ILD 6-1 SiCN Etch step is deployed to penetrate this specific diffusion barrier . Unlike the subsequent ILD 5-1 through 1-1 SiCN etches that progressively deepen the cavity towards the lower metal tiers, this ILD 6-1 step is the initial barrier breakthrough in the top-down bond pad opening sequence . Because it acts as the geometric template for the entire deep-trench excavation, establishing near-perfect mask fidelity and an anisotropic profile here is critical to prevent cascading pattern degradation in the subsequent layers (Engineering Practice). The etching of SiCN is executed via reactive ion etching (RIE) utilizing a carefully calibrated mixture of fluorinated gases and oxygen . The process relies on a synergistic interplay between chemical volatilization and physical sputtering . Fluorine radicals generated in the plasma primarily react with the silicon component of the film to form volatile SiFx byproducts . Concurrently, because carbon does not spontaneously react with fluorine under these conditions, oxygen is introduced to volatilize the carbon as CO and CO2, mitigating the formation of a carbon-rich inhibiting layer . Furthermore, a threshold DC bias is required to provide positive ions with sufficient kinetic energy to break the strong Si–C and Si–N bonds . By precisely controlling this physical ion bombardment, the etch restricts lateral chemical activity, effectively transferring the pattern vertically . Fluorocarbon-based inductively coupled plasma (ICP) is frequently utilized because it enables the decoupling of plasma density from ion energy . This decoupling allows the system to supply a dense flux of reactive radicals while keeping substrate bias relatively low, thereby limiting plasma-induced damage to the exposed sidewalls . A low-fluorine gas, such as CHF3, is often preferred because it promotes fluorocarbon polymer deposition on the sidewalls during the etch, naturally enhancing anisotropy through a dynamic passivation mechanism . Adjusting the chamber pressure modulates the ion mean free path; lower pressure increases ion directionality at the expense of overall etch rate, optimizing feature verticality . The careful balance of oxygen concentration is also critical: excessive oxygen depletes the protective fluorocarbon polymers, while insufficient oxygen triggers the carbon-blocking mechanism, sharply suppressing the SiCN etch rate . In 40nm technology, the dimensional tolerances for pad openings are stringent due to pitch scaling, while the dielectric stack thickness remains substantial to accommodate multi-tier metallization . High-aspect-ratio etching over multiple layers necessitates strict aspect-ratio independent etching (ARIE) characteristics to prevent micro-loading effects as the trench deepens . If the ILD 6-1 SiCN etch process induces profile bowing or excessive micro-trenching, these geometric defects will propagate and amplify through the subsequent ILD 5 and ILD 4 oxide etch steps . Thus, this specific barrier etch must maintain structural integrity to guarantee stable contact resistance, which is a major parasitic component in scaled device performance .
Risks & Challenges
[High] Etch Rate Suppression (Carbon Blocking): If the oxygen concentration in the fluorocarbon plasma mixture is too low, the process fails to adequately volatilize the carbon constituents of the SiCN film . This leaves a carbon-rich layer on the surface that acts as an etch block, severely dropping the etch rate or halting the barrier penetration entirely .
[High] Substrate Over-etch / Poor Selectivity: If the DC bias or platen power is set too high, the physical sputtering component of the plasma overcomes the chemical self-limiting behavior . This causes the plasma to punch through the thin SiCN layer and gouge the underlying ILD 5 oxide, compromising the precise step-by-step layer removal strategy .
[Medium] Loss of Anisotropy (Sidewall Bowing): If the ratio of polymerizing precursors (e.g. , C4F8 or CHF3) is insufficient relative to the etching radicals, sidewall passivation will be inadequate . The resulting excessive lateral chemical attack widens the upper trench profile, causing bowing that can degrade critical dimensions and reduce spacing to adjacent structures (Engineering Practice).
[Low] Plasma-Induced Sidewall Damage: High-energy ion bombardment, resulting from unoptimized accelerating platen voltages, can introduce trap states and point defects into the dielectric sidewalls . This damage can create leakage pathways, which is highly problematic given the thermodynamic limits of subthreshold leakage in scaled nodes .