Etching through thick optical stacks to a pad can induce micro-trenching or profile bowing if the ion angular distribution is not tightly controlled .
In depth
In a nanoscale Backside Illuminated (BSI) CMOS Image Sensor, the sensing active area is illuminated from the backside, requiring the metal interc
onnects and bond pads to be accessed through a complex stack of backside optical layers . The "Optical Pad 3 Etch" is a critical intermediate step in a sequential etch cascade designed to progressively breach the upper grid seal layer and various optical confinement layers to expose the underlying connection pad . It immediately follows the Upper Grid Seal Layer Etch and prepares the integration scheme for the subsequent Lower Optical Clear Layer (OCL) etches (Engineering Practice). Unlike "Optical Pad 1" or "2" which may target different material strata or perform the final metal pad clearance, this specific step tackles the intermediate optical dielectric or planarization films, ensuring a continuous vertical via profile without prematurely punching through the underlying soft OCL material (Engineering Practice). Ultimately, the complete multi-step pad opening is mandatory to form an external metallic connection with minimal macroscopic specific contact resistance, which is essential for device input/output operations . The etching process primarily relies on Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE) to provide decoupled control over plasma density and ion bombardment energy . High-energy ions in the plasma bombard the material surface under substrate bias, breaking chemical bonds, while reactive neutral radicals form volatile byproducts that are continuously pumped away . If the targeted optical layer contains organic polymers, a cooperative action of reactive oxygen neutrals and a small amount of fluorine-containing gas is utilized . The introduction of fluorine lowers the reaction activation energy and creates radical sites on aromatic rings, allowing atomic oxygen to effectively oxidize the organic backbone without leaving silicon-rich residues from adhesion promoters . Conversely, if this layer comprises inorganic oxides or nitrides, fluorocarbon gas mixtures (e.g. (Engineering Practice), CF4 or C4F8) provide the necessary chemical driving force alongside physical sputtering . The process inherently exhibits Aspect Ratio Dependent Etching (ARDE), where the etch rate locally varies with the depth and micro-loading of the pad opening . The selection of a dry ICP-RIE process over wet etching is driven by the strict requirement for steep, smooth sidewalls to accommodate subsequent packaging metallization and barrier deposition . Purely isotropic etching would aggressively undercut the photoresist mask, whereas a highly anisotropic, ion-bombardment-enhanced directional reaction produces the necessary steep sidewalls . Gas composition determines the primary chemical reaction pathways, while the bias power controls the ion energy and resultant structural anisotropy . Etching through thick optical stacks to a pad can induce micro-trenching or profile bowing if the ion angular distribution is not tightly controlled . Furthermore, maintaining high selectivity to the photoresist and the underlying optical layers is paramount; inadequate selectivity leads to mask erosion, which translates directly into additional sidewall roughness and geometric distortion . At the 40nm node, the optical stack dimensions and pad pitches are aggressively scaled, demanding extraordinarily tight critical dimension (CD) control . Advanced lithographic patterning for these pads employs Optical Proximity Correction (OPC) to mitigate interference and systematic spatial variations . The pad etching must be carefully timed and profiled because any dimensional shift or sidewall tapering directly impacts the step coverage of subsequent metal routing layers, which can lead to localized high resistance or long-term reliability failures . Furthermore, the dense multi-layer stack characteristic of advanced BSI technology means the pad structure must endure significant thermo-mechanical stress, making precise profile control critical to prevent interconnect cracking during final packaging .
Risks & Challenges
[High] Polymer Residue Formation: If the fluorine-to-oxygen ratio is unoptimized during the etching of organic-containing optical layers, atomic oxygen may fail to clear silicon-containing adhesion promoters, leaving a thick, silicon-rich residual layer . This residue blocks subsequent etch steps and severely degrades the final ohmic contact resistance .
[High] Severe Aspect Ratio Dependent Etching (ARDE) / RIE Lag: As the pad via deepens, the depletion of reactive radicals at the trench bottom causes a pattern-size-dependent etch rate reduction . This mechanism leads to incomplete etching of the optical pad layer, preventing proper progression into the subsequent Lower OCL etch step .
[Medium] Mask Erosion and Sidewall Roughness: If the plasma bias power is excessively high, aggressive physical sputtering will prematurely erode the photoresist mask . This erosion transfers edge roughness into the pad sidewall, potentially causing structural voids or poor step coverage during subsequent metal filling operations .
[Medium] Profile Bowing or Undercutting: An imbalance toward chemically driven isotropic etching, rather than ion-driven anisotropic etching, results in sloped or bowed via sidewalls . This geometry weakens the mechanical stability of the pad structure and complicates the continuous metallization required for reliable backside interconnects .