By modulating the RF bias voltage, engineers can carefully balance the chemical etching rate and physical sputtering force to prevent the RIE-lag effect and minimize surface roughness .
In depth
The Al Metal Etch step in the PACKAGING module defines the bulk geometry of the top-level bond pads for a 40nm
BSI CMOS Image Sensor . This step sits strategically between two distinct Ta-barrier etch operations, targeting the thick, highly conductive aluminum layer while leaving the thin refractory metal liners for separate, highly selective removal . Forming this bulk Al structure correctly is essential to achieve a low specific contact resistance during final packaging and external system integration . Unlike the preceding barrier etch, which chemically targets Ta/TaN to prevent electromigration and diffusion, this step must bulk-remove aluminum at high rates while maintaining strict critical dimensions . The physical mechanism relies on reactive ion etching (RIE), which utilizes a low-pressure discharge plasma to generate both active neutral radicals and directionally accelerated ions . In an aluminum etch, halogen chemistries are dissociated by RF power to supply the neutral radicals that drive the primary chemical conversion of metallic Al into volatile byproducts . Simultaneously, the electric field in the plasma sheath accelerates ions vertically toward the wafer surface, imparting directional kinetic energy . This continuous ion bombardment clears non-volatile etch inhibitors from horizontal surfaces while leaving sidewall passivation intact, ensuring highly anisotropic pattern transfer . RIE is selected over conventional wet etching because wet processes induce severe lateral undercut, which violates the strict anisotropy required for modern scaled devices . A major challenge in aluminum etching is that the top surface is not pure Al, but rather a multilayered skin of complex oxides, hydroxides, and fluorides . Therefore, the etch sequence must begin with a physically driven breakthrough step, utilizing high-energy ion bombardment to sputter away this resilient nonmetallic layer before bulk chemical etching can proceed . By modulating the RF bias voltage, engineers can carefully balance the chemical etching rate and physical sputtering force to prevent the RIE-lag effect and minimize surface roughness . For 40nm BSI CIS technology, maintaining a pristine pad surface is critical, as any remaining surface contamination directly increases the diffusion barrier and contact resistance during subsequent assembly bonding . During the RIE process, reactive gas species inevitably interact with the metal and photoresist to form complex chlorinated and oxyfluoride residues on the etched surfaces . If left untreated, these halogenated complexes can induce severe reliability issues, including spontaneous localized corrosion of the aluminum structure . Consequently, this etch must be immediately followed by the subsequent plasma ashing and wet stripping steps to volatilize the halogens and safely passivate the freshly etched metal sidewalls .
Risks & Challenges
[High] Post-Etch Aluminum Corrosion: Halogen-based plasma etching leaves reactive oxyfluoride or chlorinated residues on the aluminum surfaces . If these residues react with ambient atmospheric moisture prior to the subsequent ashing/cleaning steps, they form aggressive acids that rapidly consume the aluminum pad, leading to void formation and JEDEC reliability failures .
[High] Incomplete Oxide Breakthrough: The surface of the aluminum layer naturally forms a passivating, multilayered layer of oxides and hydroxides prior to etching . If the initial plasma ion kinetic energy is insufficient to physically sputter this resilient skin, the chemical radicals cannot reach the bulk metal, causing the chemical reaction to stall completely and resulting in large-area electrical shorts .
[Medium] Plasma-Induced Charging Damage: The continuous bombardment of directionally accelerated ions transfers significant electrical charge to the electrically isolated aluminum bond pads . This accumulated charge generates immense local electric fields that can cause dielectric breakdown or subthreshold leakage degradation in the underlying FEOL MOSFET devices .
[Medium] Lateral Etching and Notching: An imbalance between the isotropic chemical radical flux and the protective sidewall polymer passivation can cause uncontrolled lateral etching at the base of the aluminum pad, known as the notch phenomenon . This undercut structural defect severely reduces the mechanical integrity of the pad, making it susceptible to delamination during high-force wire bonding or flip-chip operations .