W CMP - 40nm BSI CMOS Image Sensor 40nm

W CMP removes thick tungsten overburden to enable subsequent interconnect formation by leaving isolated, planarized tungsten plugs in contact vias .

The primary objective of the W CMP (Tungsten Chemical Mechanical Planarization) step is to remove the thick tungsten overburden deposited in the pre

vious step, leaving behind isolated, planarized tungsten plugs in the contact vias . In the context of a 40nm BSI CMOS Image Sensor, these precise contacts are critical for routing electrical signals from the active pixel transistors to the Metal 0 routing layers (Engineering Practice). This step is typically designed to halt selectively on the underlying Ti/TiN barrier layer, preparing the wafer for the subsequent dedicated TiN/Ti CMP process . Achieving global planarization at this stage is absolutely essential to prevent depth of focus (DOF) limitations in the photolithography of subsequent interconnect levels . The physical operation of W CMP relies on a highly coupled "chemical passivation–mechanical removal" mechanism . In the slurry, strong oxidizers such as H2O2, often accelerated by catalysts like Fe(NO3)3, chemically react with the exposed tungsten metal to form a softer, self-passivating tungsten oxide (WOx) surface layer . Simultaneously, silica-based abrasive particles suspended in the slurry are pressed against the wafer by the polishing pad, mechanically shearing away this softened oxide film . Once the surface oxide is removed, fresh bulk tungsten is exposed to the slurry, re-oxidizing and thereby sustaining a continuous, cyclic material removal process . While the macroscopic removal rate follows the Preston equation—scaling with applied mechanical pressure and relative sliding velocity —the fundamental upper limit of material removal is governed by the chemical oxidation kinetics and the electrochemical potential of the metal surface . Slurry formulation is precisely engineered to balance this chemical-mechanical synergy while preventing topographical defects such as dishing and erosion . Because contact arrays vary in pattern density, recessed features can suffer from uncontrolled chemical dissolution or localized pad deformation, leading to metal loss (dishing) or dielectric thinning (erosion) . To counter this, advanced slurries incorporate specific surface-active inhibitors, such as positively charged bicyclic amidines, which electrostatically adsorb onto the negatively charged WOx surface . These molecules establish a dynamic passivation layer that locally suppresses oxidation kinetics in recessed areas, maintaining planarization . Additionally, the chemical oxidation of tungsten intrinsically generates acidic byproducts that can lower the local pH, which in turn accelerates the etching of the surrounding dielectric oxide . To mitigate this secondary erosion mechanism, specific pH-stabilizing buffering agents are often added to neutralize acidic byproducts in situ . At the 40nm technology node, the tolerance for surface defects is exceptionally narrow, making microscratches a dominant yield-killing issue during W contact CMP . Research indicates that these microscratches do not originate from the nominal nanoscale abrasives, but rather from a minute population of abnormally large abrasive particles or secondary aggregates . These oversized particles induce locally extreme contact stresses that easily exceed the plastic deformation threshold of the tungsten or adjacent dielectric films . Because the bulk tungsten removal rate is largely independent of abrasive size, process optimization must focus strictly on monitoring the abrasive size distribution tail and preventing shear- or temperature-induced agglomeration in the slurry delivery system .

[High] Microscratches from Abrasive Agglomeration: Abnormally large abrasive particles or agglomerates in the bulk slurry can become trapped at the pad-wafer interface, generating locally extreme contact stresses . These stresses exceed the plastic limit of the tungsten and surrounding dielectric, creating deep microscratches that severely degrade device yield .
[High] Tungsten Dishing and Oxide Erosion: In arrays with varying pattern densities, an imbalance between chemical oxidation and mechanical removal can cause the softer tungsten to over-polish relative to the dielectric (dishing), or wear down the dielectric itself (erosion) . This is driven by local variations in pad conformality and unsuppressed chemical dissolution, necessitating the use of surface-passivating inhibitors .
[Medium] Dielectric Erosion via Localized pH Drift: The chemical oxidation of bulk tungsten produces acidic byproducts that can cause the localized slurry pH to drop . In heavily patterned areas, this rapid acidification accelerates the chemical dissolution of the surrounding inter-layer dielectric, causing severe oxide erosion if the slurry buffering capacity is insufficient .
[Medium] Galvanic Corrosion and Contact Recess: Tungsten and the underlying Ti/TiN barrier layers possess different electrochemical corrosion potentials . When exposed simultaneously to an electrically conductive, oxidizer-rich slurry during the clearing phase, they can form a local galvanic cell, driving accelerated anodic dissolution of the more active metal and resulting in unpredictable contact recess .

W CMP removes thick tungsten overburden to enable subsequent interconnect formation by leaving isolated, planarized tungsten plugs in contact vias .

The primary objective of the W CMP (Tungsten Chemical Mechanical Planarization) step is to remove the thick tungsten overburden deposited in the pre

[High] Microscratches from Abrasive Agglomeration: Abnormally large abrasive particles or agglomerates in the bulk slurry can become trapped at the pad-wafer interface, generating locally extreme contact stresses . These stresses exceed the plastic limit of the tungsten and surrounding dielectric, creating deep microscratches that severely degrade device yield .
[High] Tungsten Dishing and Oxide Erosion: In arrays with varying pattern densities, an imbalance between chemical oxidation and mechanical removal can cause the softer tungsten to over-polish relative to the dielectric (dishing), or wear down the dielectric itself (erosion) . This is driven by local variations in pad conformality and unsuppressed chemical dissolution, necessitating the use of surface-passivating inhibitors .
[Medium] Dielectric Erosion via Localized pH Drift: The chemical oxidation of bulk tungsten produces acidic byproducts that can cause the localized slurry pH to drop . In heavily patterned areas, this rapid acidification accelerates the chemical dissolution of the surrounding inter-layer dielectric, causing severe oxide erosion if the slurry buffering capacity is insufficient .
[Medium] Galvanic Corrosion and Contact Recess: Tungsten and the underlying Ti/TiN barrier layers possess different electrochemical corrosion potentials . When exposed simultaneously to an electrically conductive, oxidizer-rich slurry during the clearing phase, they can form a local galvanic cell, driving accelerated anodic dissolution of the more active metal and resulting in unpredictable contact recess .

40nm BSI CMOS Image SensorPreview

Process Cross-Section

Step highlight

In depth

Risks & Challenges

40nm BSI CMOS Image SensorPreview

Process Cross-Section

Step highlight

In depth

Risks & Challenges