Fluorocarbon-based gas chemistries are typically selected for oxide ILD etching because they provide excellent selectivity and control over the etch profile through continuous polymer passivation .
In depth
The "RIE etch ILD 3-2 Etch" step in the BONDPAD module of a 40nm BSI CMOS Image Sensor is a critic
al process used to expose the underlying metal pad for external electrical connection . Following the preliminary removal of upper dielectric layers in "ILD 3-1 Etch", this specific step completes the penetration through the remaining thick interlayer dielectric (ILD) to land precisely on the Bond Pad barrier . Unlike general oxide or nitride etches used in early device formation, this bond pad etch requires managing a large exposed open area and deep topologies without excessively sputtering the underlying metal . The overarching goal is to establish a low-resistance, high-reliability pathway for packaging, analogous to the requirement for high-quality metal contacts essential for overall integrated circuit performance . The process relies on Reactive Ion Etching (RIE), which leverages the synergistic interaction between plasma-generated chemical radicals and directionally accelerated ions . In a radio-frequency (RF) glow discharge, the process gas is dissociated into neutral radicals, positive ions, and electrons . The neutral radicals diffuse to the wafer surface to spontaneously react with the ILD material, forming volatile byproducts that are pumped away . Simultaneously, positive ions are accelerated by the DC self-bias across the plasma sheath, bombarding the substrate vertically . This ion bombardment breaks chemical bonds and removes surface passivation layers, driving the reaction in the vertical direction to achieve the highly anisotropic profile required for deep pad openings . Without this directional energy flux, the etch would be isotropic and suffer from severe lateral undercutting that compromises the pad structure . Fluorocarbon-based gas chemistries are typically selected for oxide ILD etching because they provide excellent selectivity and control over the etch profile through continuous polymer passivation . During the process, heavy fluorocarbon precursors deposit a protective polymeric film on the sidewalls, while directional ion bombardment continuously clears this polymer from the horizontal bottom . Controlling the ratio of carbon to fluorine in the plasma mixture determines the balance between polymer deposition and chemical etching, directly dictating the selectivity to the underlying metallic barrier layer . Process parameters such as RF power and chamber pressure interact non-linearly: higher bias power increases ion kinetic energy, which enhances the physical sputter rate and maintains verticality but risks inducing physical damage in the underlying materials . Conversely, lower pressures increase the mean free path of ions, reducing ion scattering and promoting tighter profile control, though this can exacerbate localized etch rate variations . For a 40nm BSI (Backside Illuminated) Image Sensor, the bond pad etch is uniquely challenged by the integration scheme, where precise dielectric removal must occur without compromising the surrounding optical structures or inducing plasma charging damage . As the etch lands on the barrier metal, it prepares the structure for the subsequent "Bond Pad Metal 7 Barrier Etch" step (Engineering Practice). Terminating the etch precisely is critical to prevent metal sputtering, which could cause micromasking or redeposition contamination across the sensitive image sensor array .
Risks & Challenges
[High] Over-etch and Metal Sputtering: Excessive ion bombardment energy or prolonged etch time can punch through the ILD and physically sputter the underlying metal barrier . Sputtered metallic particles can redeposit onto the pad sidewalls, potentially leading to leakage or poor adhesion during subsequent wire bonding .
[Medium] RIE Lag and Incomplete Etch: Due to variations in pad opening sizes or local pattern density, differential transport of neutral species and ions can cause larger areas to etch faster than smaller ones . This RIE lag results in residual ILD remaining on some pads, directly increasing contact resistance or causing open circuits during packaging (Engineering Practice).
[Medium] Plasma-Induced Charging Damage: As the insulating ILD is etched away and the metal pad is exposed, localized charge accumulation from the directional ion flux can build up high potentials . This charge can be conducted down through the interconnect stack, causing electrical breakdown or threshold voltage shifts in the underlying MOSFET devices .
[Low] Sidewall Polymer Residue: An overly rich fluorocarbon gas chemistry intended to improve selectivity can lead to excessive polymer deposition on the pad sidewalls . If not fully removed in the subsequent ashing and wet clean steps, this fluorocarbon residue degrades the mechanical reliability of the final bond pad (Engineering Practice).