Material and parameter selections are heavily driven by the need for high etch selectivity between the target SiO2 and the underlying SiCN stop layer .
In depth
The ILD 6-2 Oxide Etch step is a critical back-end-of-line (BEOL) process designed to define the upper portion of the bond pad cavity for packag
ing or wafer-to-wafer integration . Following the "Bond Pad Cavity - Photo" step, this process anisotropically removes the thick uppermost silicon dioxide layer (ILD 6-2) and precisely stops on the underlying ILD 6-1 SiCN layer . This initiates a sequence of alternating dielectric and barrier etches (Oxide -> SiCN -> Oxide -> SiCN) necessary to expose the terminal metal interconnects without exposing sidewall metals . This step is distinctly different from front-end "Pad Oxide Etch" steps, which merely remove thin stress-relief oxides under nitride masks, and "Oxide Hard Mask Etch" steps, which define nanoscale interconnect trenches . Instead, this macro-scale etch must rapidly and uniformly remove a thick oxide film to accommodate complex bonding architectures, such as the high-density interconnects required in 3D-SOC and advanced image sensor applications . The physical mechanism of this step relies on the ion-chemical synergistic reactions characteristic of Reactive Ion Etching (RIE) . Fluorocarbon gases, such as CHF3 or CF4, are introduced into the plasma chamber and dissociated by RF power to generate highly reactive fluorine radicals and fluorocarbon ions . The fluorine radicals chemically attack the silicon dioxide matrix, breaking Si-O bonds to form volatile silicon fluoride byproducts that are evacuated from the chamber . Concurrently, the ionized species are accelerated across the plasma sheath by a direct-current bias, bombarding the wafer surface with directional kinetic energy . This physical bombardment clears passivation polymers from horizontal surfaces, ensuring the etch proceeds anisotropically to yield the steep sidewall profiles required for deep bond pad cavities . Material and parameter selections are heavily driven by the need for high etch selectivity between the target SiO2 and the underlying SiCN stop layer . SiCN is widely integrated in advanced hybrid bonding and BEOL stacks due to its high surface reactivity for bonding and its structural robustness as an etch stop . To maximize selectivity, the plasma chemistry must maintain a delicate balance; for instance, while adding oxygen can increase the overall oxide etch rate, it can simultaneously increase the etching of silicon-containing non-oxide layers, thereby severely degrading selectivity . Furthermore, RF power must be carefully optimized, as higher RF power raises the ion energy, which accelerates the etch rate but can induce severe physical damage to the etched surface and compromise the SiCN barrier . In the context of nanoscale Backside Illuminated (BSI) CMOS image sensors, the packaging layout often employs Bond Over Active (BOA) pad structures to minimize wasted die area by placing pads directly over active circuitry and multilayer interconnects . The mechanical reliability of these structures during wire bonding or packaging heavily depends on the progressive dispersion of transient mechanical loads through the multilayer metal and dielectric stack, which share similar elastic moduli . Consequently, the ILD 6-2 Oxide Etch must avoid creating structural defects, severe undercuts, or micro-cracks in the dielectric, as such geometric flaws can act as stress concentration points that ultimately cause mechanical failure at the contact-silicon interface during packaging stresses .
Risks & Challenges
[High] Etch Stop Punchthrough: Excessive RF power or improper gas chemistry can raise the kinetic energy of bombarding ions, leading to physical damage and unintended etching of the underlying SiCN layer .
[Medium] Excessive Polymer Deposition: In fluorocarbon-based RIE, an imbalance toward high polymerizing gas flow or insufficient ion energy can cause thick fluorocarbon polymers to accumulate at the bottom of the pad cavity, blocking further etching and leaving unetched oxide residues .
[Low] Pad Structure Mechanical Weakening: Isotropic etching components or severe sidewall bowing can alter the geometric integrity of the thick dielectric stack, impairing its ability to safely dissipate transient mechanical loads during subsequent wire bonding or hybrid bonding steps .