Increasing the RF bias power enhances ion directionality and physical sputtering capability, which helps maintain a constant single-pass etch amount at varying depths .
In depth
In advanced backend-of-line (BEOL) and packaging integration, thick dielectrics are often etched in multiple stages (e.g. ,
1-1, 1-2, 2-1) to precisely manage mask erosion, control profile tapering, and mitigate depth-dependent variations . The RIE etch ILD 1-2 Etch step serves as the intermediate bulk removal phase within the BONDPAD module for a 40nm BSI CMOS Image Sensor . Unlike a standard, shallow Oxide Etch or Nitride Etch, this intermediate step operates in a regime where the aspect ratio begins to increase significantly, necessitating strict control over sidewall passivation and ion energy flux to maintain vertical profiles . Furthermore, this segmented approach allows for gas chemistry adjustments at different depth thresholds to compensate for the rapid depletion of deposition precursors and etchants inside the developing trench . Reactive Ion Etching (RIE) operates through the synergistic action of directional ion bombardment and neutral radical chemical reactions . In the ILD 1-2 step, fluorocarbon-based plasmas generate neutral radicals that chemically react with the exposed inter-layer dielectric to form volatile byproducts . Simultaneously, charged ions accelerated by the RF bias provide directional kinetic energy to physically clear the etch front, thereby suppressing lateral etching and forming highly anisotropic structures . As the trench deepens during this second etch stage, the physical mechanism must precisely balance chemical etching with physical sputtering to counteract RIE lag, a well-documented phenomenon where etch rates decrease in higher aspect ratio structures due to restricted reactant and product transport [P1, P3]. The selection of a dry plasma RIE method over traditional wet etching is strictly required because wet chemical etching is entirely isotropic and cannot meet the critical dimension (CD) control requirements for highly scaled geometries . To maintain a straight sidewall and prevent localized bowing, sidewall passivation is carefully regulated by utilizing hydrofluorocarbon or similar polymerizing chemistries to increase selectivity during the etch . Increasing the RF bias power enhances ion directionality and physical sputtering capability, which helps maintain a constant single-pass etch amount at varying depths . However, excessively high bias voltages must be avoided to prevent plasma-induced charging damage to the underlying structures . Conversely, manipulating the chamber pressure alters the mean free path of the incoming ions; maintaining a lower pressure improves ion directionality but simultaneously reduces the overall density of reactive radicals, defining a narrow optimization window (Engineering Practice). At the 40nm node, continuous device scaling imposes severe physical restrictions on the available error margins for interconnect alignment, capacitance matching, and via profiles . A multi-step ILD etch scheme is specifically utilized here to artificially decouple the competing challenges of top-corner rounding, main-trench anisotropic etching, and subsequent etch-stop selectivity into separate, optimized plasma regimes . This deliberate segmentation prevents the severe mask erosion and geometric distortion that would inevitably occur if a single, highly energetic etch step were applied to clear the entire thick bondpad dielectric stack .
Risks & Challenges
[High] RIE Lag and Aspect-Ratio Dependent Etching (ARDE): As the trench deepens during this intermediate step, reactant transport to the bottom becomes physically restricted by the shrinking solid angle, causing the local etch rate to slow down relative to shallower features . If uncompensated by process time or bias adjustments, this lag results in under-etched target structures and varying depths across dense versus isolated layout areas .
[High] Sidewall Bowing or Tapering Distortions: Imbalances between the isotropic chemical etch component and the directional ion bombardment can drastically alter the intended geometric profile . Insufficient sidewall passivation during the active etch phase allows lateral attack by neutral radicals, resulting in a bowed profile, whereas excessive polymer accumulation leads to a positively tapered, V-shaped trench [A2, P4].
[Medium] Plasma-Induced Damage and Charging: Energetic directional ion bombardment and localized charge accumulation at the bottom of the high-aspect-ratio trench create concentrated electric field stress . This localized charge build-up can induce dielectric breakdown or degrade the electrical integrity of underlying devices through subthreshold leakage mechanisms .
[Low] Micro-masking and Etch Stop: Sputtered photoresist or excessive fluorocarbon polymer byproducts from the passivation phase can re-deposit at the bottom of the trench . If the directional ion bombardment is insufficient to remove these localized polymers, they act as micromasks, completely blocking the chemical etch and leading to grass-like defects or premature etch termination .