W CMP - 40nm BSI CMOS Image Sensor 40nm

Process parameters such as applied pad pressure and rotational speed must be carefully modulated to balance high removal rates against topography degradation, specifically dishing and erosion .

Following the sequential deposition of Ti for adhesion, TiN for a diffusion barrier, and bulk chemical

vapor deposition (CVD) of Tungsten within the contact holes, the resulting structure exhibits significant metallic overburden and surface topography . The Tungsten Chemical Mechanical Planarization (W CMP) step exists to globally planarize the wafer surface by removing this bulk W overburden, physically isolating individual contact plugs prior to the subsequent barrier TiN/Ti CMP and cleaning steps . This inverse metallization scheme relies on CMP to achieve the required planarity for advanced sub-micron interconnects . Isolating these high-aspect-ratio tungsten plugs forms the critical electrical bridge between the underlying silicon source/drain regions and the first metal routing layer (M1) . Establishing this high-quality electrical connection is physically predicated on minimizing the metal-semiconductor contact potential and managing interface states, which are intrinsically governed by the work function and electron affinity differences detailed by semiconductor band theory . Tungsten CMP operates through a highly coupled "chemical oxidation and mechanical removal" mechanism . Chemical oxidizers within the slurry, most commonly hydrogen peroxide catalyzed by ferric nitrate, react continuously with the exposed tungsten surface to form a chemically softer, passivated tungsten oxide or hydrated oxide layer . Concurrently, the polishing pad and suspended nanoscale abrasive particles mechanically shear away this softened passivation layer under an applied downforce and relative rotational velocity . The macroscopic material removal rate depends heavily on this mechanical contact, adhering largely to a first-order relationship modeled by the Preston equation where removal rate is proportional to pressure and velocity . At the microscopic contact level, the transient high-frequency stick-slip interactions between the pad asperities, abrasive particles, and the passivated surface further govern the dynamic removal efficiency and tribological state . Process parameters such as applied pad pressure and rotational speed must be carefully modulated to balance high removal rates against topography degradation, specifically dishing and erosion . Dishing typically occurs in wider tungsten lines where the pad flexes into the softer metal, while erosion happens in dense pattern arrays where the chemically inert oxide dielectric is mechanically worn away . To suppress these failure modes, advanced slurries incorporate surface-active corrosion inhibitors, such as bicyclic amidines, which preferentially adsorb onto the metallic tungsten surface to retard local oxidation and limit chemical over-etching . Furthermore, localized tungsten oxidation dynamically generates acidic byproducts that can depress the slurry pH, a shift that dramatically accelerates the chemical dissolution and erosion of the surrounding oxide dielectric . Consequently, buffering agents with a pKa of 4 or higher are often integrated into the slurry formulation to stabilize the local pH and protect the dielectric support structures during the final over-polish phase . In a highly scaled 40nm CMOS image sensor integration scheme, the allowable defect margin is exceptionally stringent, demanding controls beyond traditional removal-rate optimizations (Engineering Practice). Microscratches have been identified as highly destructive yield-killing defects in middle-of-line (MOL) contact planarization . These microscratches are primarily generated not by the baseline nanoscale abrasives, but by high local contact stresses induced by abnormally large, agglomerated particles penetrating the pad-wafer interface . Therefore, strictly bounding the upper tail of the abrasive size distribution through source control and minimizing temperature- or agitation-induced secondary agglomeration is physically more vital than adjusting baseline pressure parameters to preserve device yield . Following successful planarization, the structure is prepared for post-CMP cleaning, where weakly alkaline solutions can be used to neutralize residual acidic sites and stabilize the exposed interfaces .

[High] Microscratching: Microscratches occur when abnormally large abrasive particles or secondary agglomerates in the bulk slurry enter the polishing interface . These localized large particles concentrate the applied mechanical downforce, exceeding the plastic or brittle deformation thresholds of the tungsten and surrounding dielectric, generating severe surface gouges that disrupt downstream M1 patterning .
[Medium] Contact Plug Dishing: Dishing manifests as a concave depression within the W plug relative to the surrounding oxide layer . It is driven by the sustained chemical etching and mechanical yielding of the relatively soft tungsten oxide passivation layer, particularly in features where the mechanically rigid oxide support is distant . Failure to properly tune adsorption-based inhibitors leads to unconstrained local oxidation and material loss .
[Medium] Dielectric Erosion: In regions with high contact pattern density, the spatial fraction of the structurally robust oxide is minimized, exposing the narrow dielectric regions to enhanced mechanical abrasion . Additionally, localized acidic byproducts from the tungsten oxidation reaction can depress the slurry pH, accelerating the chemical dissolution of the dielectric unless adequately neutralized by pKa-stabilizing buffers .
[Low] Incomplete W Clearing (Underpolishing): Failure to fully clear the tungsten overburden leaves unintended conductive residues across the dielectric surface, potentially shorting adjacent contact structures . This typically results from an inadequate chemical oxidation rate—often due to depleted hydrogen peroxide or insufficient ferric nitrate catalyst—which disrupts the duplex reacting mechanism required for efficient material removal .

Process parameters such as applied pad pressure and rotational speed must be carefully modulated to balance high removal rates against topography degradation, specifically dishing and erosion .

Following the sequential deposition of Ti for adhesion, TiN for a diffusion barrier, and bulk chemical

[High] Microscratching: Microscratches occur when abnormally large abrasive particles or secondary agglomerates in the bulk slurry enter the polishing interface . These localized large particles concentrate the applied mechanical downforce, exceeding the plastic or brittle deformation thresholds of the tungsten and surrounding dielectric, generating severe surface gouges that disrupt downstream M1 patterning .
[Medium] Contact Plug Dishing: Dishing manifests as a concave depression within the W plug relative to the surrounding oxide layer . It is driven by the sustained chemical etching and mechanical yielding of the relatively soft tungsten oxide passivation layer, particularly in features where the mechanically rigid oxide support is distant . Failure to properly tune adsorption-based inhibitors leads to unconstrained local oxidation and material loss .
[Medium] Dielectric Erosion: In regions with high contact pattern density, the spatial fraction of the structurally robust oxide is minimized, exposing the narrow dielectric regions to enhanced mechanical abrasion . Additionally, localized acidic byproducts from the tungsten oxidation reaction can depress the slurry pH, accelerating the chemical dissolution of the dielectric unless adequately neutralized by pKa-stabilizing buffers .
[Low] Incomplete W Clearing (Underpolishing): Failure to fully clear the tungsten overburden leaves unintended conductive residues across the dielectric surface, potentially shorting adjacent contact structures . This typically results from an inadequate chemical oxidation rate—often due to depleted hydrogen peroxide or insufficient ferric nitrate catalyst—which disrupts the duplex reacting mechanism required for efficient material removal .

40nm BSI CMOS Image SensorPreview

Process Cross-Section

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In depth

Risks & Challenges

40nm BSI CMOS Image SensorPreview

Process Cross-Section

Step highlight

In depth

Risks & Challenges