Selective Ta-barrier etching exposes the underlying aluminum while preventing halogen penetration that causes corrosion or hillock formation .
In depth
In the fabrication of 40nm BSI CMOS Image Sensors, the bond pad stack commonly consists of a top barrier, a main aluminum (Al) layer, and a bottom barrier
. Following the Metal 7 Bond Pad photolithography step, this specific Ta-Barrier etch acts as the top layer breakthrough process . Its primary function is to accurately transfer the photoresist pattern into the top tantalum (Ta) or tantalum nitride (TaN) anti-reflective and protective capping layer, thereby exposing the underlying Al for the subsequent bulk metal etch . This step is functionally distinct from the later bottom Ta-Barrier etch (such as step #261 or #414); the current step must stop precisely on the highly reactive Al surface, whereas the later step must clear the bottom barrier without over-etching into the underlying interlayer dielectric . Removing this top barrier cleanly is a prerequisite for achieving uniform pattern definition in the thick Al pad during subsequent steps (Engineering Practice). The physical mechanism of this process relies on Reactive Ion Etching (RIE), which utilizes a synergistic interaction between plasma-generated chemical radicals and physical ion bombardment . A radio-frequency glow discharge dissociates the halogen-based process gases into reactive neutral radicals, positive ions, and electrons . The neutral radicals diffuse to the wafer surface and chemically react with the Ta/TaN layer to form volatile byproducts . Simultaneously, positive ions are accelerated by the DC self-bias across the plasma sheath, bombarding the substrate vertically . This highly directional ion energy flux physically removes surface passivation layers, breaks the strong chemical bonds in the barrier, and stimulates the local chemical etch reactions, yielding the anisotropic profiles required for fine structural features . The selection of Ta and TaN in the bond pad stack is due to their high density, low halogen permeability, and robust thermal stability, making them excellent diffusion barriers . To etch these resilient materials effectively, dry plasma etching is chosen over traditional wet etching, because wet etching suffers from isotropic lateral undercutting that severely compromises critical dimension (CD) control in advanced device nodes . During the RIE process, balancing the chemical and physical etch components is critical; increasing the radio-frequency bias enhances the directional ion kinetic energy for steeper sidewalls, but excessively high ion energy can induce physical damage or sputter the masking material . Furthermore, careful modulation of the gas chemistry controls the reaction spontaneity and selectivity to the underlying Al layer, preventing premature halogen penetration that could cause corrosion or hillock formation in the metal . In the context of a 40nm node BSI CMOS Image Sensor, the bond pad pitch is significantly scaled down to support higher I/O densities for integrated packaging . This stringent geometry requires that the initial pattern transfer into the top Ta-barrier be exceptionally precise, as any dimensional bias introduced here will directly propagate through the entire thick Al etch . To achieve this precise profile control and maintain uniformity across the wafer, advanced process control (APC) systems are integrated into the plasma etching reactor to tightly monitor and adjust the plasma parameters in real time .
Risks & Challenges
[High] Etch Stop or Insufficient Breakthrough: The fundamental interaction in RIE requires continuous physical removal of passivation layers by ion bombardment to allow chemical radicals to react with the substrate . If the self-bias or ion flux drops, surface passivation dominates, causing the etch process to halt prematurely and leaving residual Ta that blocks the subsequent Al etch .
[Medium] Halogen-Induced Metal Corrosion: Halogen plasmas exhibit high chemical reactivity and can readily diffuse into metal layers if the protective barrier is compromised . If the top Ta-barrier etch proceeds too far into the underlying Al without switching to a highly selective chemistry, halogens can penetrate the metal, leading to severe chemical corrosion and the stress-induced formation of hillocks .
[Medium] Photoresist Mask Degradation (Sputtering): The directionality of RIE originates from positive ions being accelerated by a self-bias electric field to bombard the substrate vertically . If the applied bias power is too high, this energetic ion flux physically sputters the photoresist mask, leading to mask faceting and subsequent lateral critical dimension (CD) loss during pattern transfer .
[Low] Plasma-Induced Physical Damage: High-energy ions in the plasma, accelerated by the electric field, can transfer sufficient kinetic energy to the lattice to generate defects, dangling bonds, and localized charge accumulation . While the top barrier etch is relatively shallow, excessive ion energies above the damage threshold can alter the electrical and physical properties of the exposed surfaces, complicating subsequent processing and interface reliability .