Material and chemistry selections are driven by the need to maintain precise profile control while achieving the required etch rates .
In depth
The ILD 4-1 SiCN Etch is a critical patterning step within the BONDPAD module of a nanoscale Backside Illuminated (BSI) CMOS Image Sensor . This process dire
ctly follows the ILD 4-2 Oxide Etch and prepares the wafer for the subsequent ILD 3-2 Oxide Etch . In this integration scheme, the bondpad via must penetrate multiple alternating dielectric layers, requiring sequential etching through successive oxide and SiCN films . The primary objective of this specific step is to completely break through the exposed ILD 4-1 SiCN etch-stop layer, enabling the continuous formation of a high-aspect-ratio contact (HARC) structure that will eventually reach the underlying metal pad . The physical removal of the SiCN material relies on fluorinated plasma etching mechanisms, where both active radicals and directionally accelerated ions interact synergistically with the film surface . Within the plasma, feed gases dissociate to generate fluorine atoms and fluorocarbon (CFx) radicals, which chemically react with the silicon, carbon, and nitrogen in the matrix to form volatile byproducts such as SiF4, COx, and NOx . Simultaneously, positively charged ions gain directional kinetic energy across the plasma sheath, bombarding the surface to break chemical bonds and drive the desorption of these reaction products . Because the SiCN material contains carbon, it is naturally prone to forming a fluorinated polymer layer during the etch; therefore, sustained etching requires continuous physical ion bombardment to penetrate this polymer mixing layer and expose the underlying material . Material and chemistry selections are driven by the need to maintain precise profile control while achieving the required etch rates . Fluorocarbon mixtures, such as CF4 combined with O2 or H2, are heavily utilized because they allow fine-tuning of the anisotropic etch profile and selectivity . The introduction of oxygen into the plasma chemistry specifically helps to manage the polymer thickness by volatilizing carbon residues as COx, thereby preventing etch stoppage and accelerating the SiCN removal . Furthermore, RF power and bias voltage parameters are strictly modulated to balance the chemical etching with physical sputtering, effectively suppressing lateral etching to ensure vertical sidewalls . The dielectric nature of SiCN also helps it act as an efficient intermediate barrier, providing controlled etch stop characteristics during the preceding oxide etch before being intentionally punched through in this step . What distinguishes the ILD 4-1 SiCN Etch from similar steps (like the shallower ILD 5-1 or deeper ILD 1-1) is its intermediate position within the deep bondpad stack . As the via depth progresses down to the fourth level, fundamental reactive ion etching challenges such as the RIE-lag effect become increasingly prominent due to constrained gas conductance in the high-aspect-ratio trench . The localized fluorine-to-carbon ratio at the etch front deviates from the bulk plasma, requiring different pressure and gas flow optimizations compared to shallower etches to prevent polymer clogging . Consequently, the ILD 4-1 etch must deliver sufficient ion energy to maintain a steady etch rate at this specific depth while avoiding excessive plasma-induced damage to the surrounding dielectric layers .
Risks & Challenges
[High] Incomplete SiCN Breakthrough (Etch Stop): If the fluorine-to-carbon ratio in the plasma is too low, the carbon-rich polymer mixing layer becomes excessively stable and thick, completely suppressing fluorine penetration and stopping the etch . This prevents access to the underlying ILD 3-2 oxide, resulting in electrically open bondpad connections (Engineering Practice).
[Medium] Aspect Ratio Dependent Etching (RIE Lag): In deep features, the transport of neutral radicals to the bottom and the extraction of volatile reaction products are restricted by Knudsen diffusion dynamics . This causes the SiCN etch rate to drop significantly in narrower regions, leading to non-uniform via breakthrough across the array (Engineering Practice).
[Medium] Sidewall Bowing and CD Loss: An overabundance of oxygen or inadequate CFx radical flux in the plasma chemistry can prematurely consume the protective fluorinated polymer layer on the via sidewalls . Without this passivation, scattered ions induce lateral chemical etching, creating a bowed profile that compromises the dielectric structural integrity .
[Low] Plasma-Induced Charging Damage: The continuous bombardment of the dielectric stack in a plasma environment can lead to localized charge accumulation due to the difference in electron and ion angular distributions . This shading effect can deflect incoming energetic ions to attack the sidewalls (notching) or induce electrical stress in underlying device layers .