Lower Vertical Grid Deposition - 40nm

Initially, a thin nucleation layer must be formed on the TiN barrier, typically utilizing silane (SiH4) to reduce WF6, which provides fast deposition with minimal incubation time .

In a 40nm BSI CMOS Image Sensor, the lower vertical grid acts as a deep optical and electrical isolation wall to sup

press crosstalk between adjacent photodiode pixels (Engineering Practice). This Lower Vertical Grid Deposition step fills the high-aspect-ratio trenches that were patterned and lined with a barrier layer in the preceding steps (Engineering Practice). It is distinct from the upper LS/Aperture Grid Deposition because the lower grid extends much deeper into the pixel isolation structure, presenting a significantly higher aspect ratio that severely challenges void-free gap filling (Engineering Practice). The complete metal fill is required to provide maximum optical opacity and structural integrity before the subsequent Chemical Mechanical Polishing (CMP) step planarizes the array . Furthermore, the underlying barrier layer serves to prevent the highly reactive deposition precursors from attacking the silicon or oxide during this bulk fill process . The physical deposition mechanism relies on the chemical reduction of tungsten hexafluoride (WF6) in a multi-step nucleation and bulk fill sequence . Initially, a thin nucleation layer must be formed on the TiN barrier, typically utilizing silane (SiH4) to reduce WF6, which provides fast deposition with minimal incubation time . To overcome the poor step coverage typical of continuous SiH4 reduction in deep contacts, a pulsed chemical vapor deposition (CVD) or atomic layer deposition (ALD) mode is often introduced . This involves time-sequenced cycles of WF6/SiH4 reactions separated by inert-gas purges, keeping the reaction surface-controlled rather than mass-transport-controlled . Following nucleation, the bulk tungsten fill is driven by hydrogen (H2) reduction of WF6, a reaction path specifically chosen for its superior conformality in narrow, deep topographies . Tungsten is selected for this lower grid due to its excellent step coverage capabilities and robust refractory properties (Engineering Practice). The macroscopic electrical and mechanical properties of the deposited film are heavily dependent on its microstructural crystal phase . Specifically, the thermodynamically stable alpha-phase (α-W) exhibits a body-centered cubic structure with significantly lower resistivity compared to the metastable beta-phase (β-W) . By strategically applying a silicon-containing material during the initial nucleation phase, the resulting dopant-containing tungsten seed layer can effectively suppress unwanted excessive nucleation, promoting grain growth that favors the low-resistance α-W phase in the bulk fill . Additionally, optimizing the surface pretreatment with SiH4 generates silicon-rich active sites that drastically improve nucleation density, thereby enhancing the interfacial adhesion strength required to survive the severe shear stresses of the upcoming W CMP step .

[High] Post-CMP Tungsten Delamination: Poor adhesion between the deposited tungsten film and the underlying barrier layer can lead to widespread peeling during the aggressive chemical mechanical polishing step . This failure is often rooted in insufficient nucleation density or interfacial weakening caused by impurity diffusion (such as boron from alternative reducing agents) penetrating the barrier stack .
[High] Void or Seam Formation in Trench: In ultra-high aspect ratio structures, standard continuous CVD kinetics can shift from surface-controlled to mass-transport-controlled, leading to premature pinch-off at the top of the trench . This leaves internal voids or center seams that degrade both the optical isolation efficacy and the structural integrity of the grid (Engineering Practice).
[Medium] Elevated Grid Line Resistance: If the nucleation layer deposition parameters are poorly controlled, the subsequent bulk tungsten may grow predominantly in the metastable, high-resistivity β-phase (A15 structure) rather than the desired α-phase (BCC) . This severely degrades the conductive properties of the grid structure, particularly in highly scaled nanoscale geometries where grain boundary scattering dominates .
[Low] Precursor Attack (Volcano Defects): If the barrier layer is inadequate or the nucleation layer is too thin, the highly reactive WF6 precursor used during the bulk fill step can diffuse through weak spots and violently attack underlying silicon or titanium layers . This parasitic reaction generates volatile byproducts like TiF4, leading to massive expansion defects commonly known as "volcanoes" .

Initially, a thin nucleation layer must be formed on the TiN barrier, typically utilizing silane (SiH4) to reduce WF6, which provides fast deposition with minimal incubation time .

In a 40nm BSI CMOS Image Sensor, the lower vertical grid acts as a deep optical and electrical isolation wall to sup

[High] Post-CMP Tungsten Delamination: Poor adhesion between the deposited tungsten film and the underlying barrier layer can lead to widespread peeling during the aggressive chemical mechanical polishing step . This failure is often rooted in insufficient nucleation density or interfacial weakening caused by impurity diffusion (such as boron from alternative reducing agents) penetrating the barrier stack .
[High] Void or Seam Formation in Trench: In ultra-high aspect ratio structures, standard continuous CVD kinetics can shift from surface-controlled to mass-transport-controlled, leading to premature pinch-off at the top of the trench . This leaves internal voids or center seams that degrade both the optical isolation efficacy and the structural integrity of the grid (Engineering Practice).
[Medium] Elevated Grid Line Resistance: If the nucleation layer deposition parameters are poorly controlled, the subsequent bulk tungsten may grow predominantly in the metastable, high-resistivity β-phase (A15 structure) rather than the desired α-phase (BCC) . This severely degrades the conductive properties of the grid structure, particularly in highly scaled nanoscale geometries where grain boundary scattering dominates .
[Low] Precursor Attack (Volcano Defects): If the barrier layer is inadequate or the nucleation layer is too thin, the highly reactive WF6 precursor used during the bulk fill step can diffuse through weak spots and violently attack underlying silicon or titanium layers . This parasitic reaction generates volatile byproducts like TiF4, leading to massive expansion defects commonly known as "volcanoes" .

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