However, thinning the metal intrinsically reduces its total optical density; therefore, the deposition process must achieve near-bulk material density without incorporating voids or high concentrations of impurities .
In depth
Following the LS/Aperture Grid Barrier Deposition, the LS/Aperture Grid Deposi
tion step creates the primary opaque metallic layer on the backside of the thinned BSI CMOS image sensor . In modern sub-micron pixel architectures, backside illumination (BSI) greatly enhances the fill factor by moving the wiring to the front side . However, densely packed pixels are highly susceptible to optical crosstalk, where obliquely incident photons penetrate adjacent pixels, degrading color fidelity and spatial resolution . This deposition provides the bulk light-shielding material that will subsequently be patterned to form the aperture grid, defining the exact optical window for each underlying photodiode . It distinctly differs from the preceding barrier layer, which merely prevents metal diffusion and enhances adhesion, and the subsequent oxide seal layer, which will encapsulate the grid to prevent oxidation and assist in subsequent photolithography . It also differs from the lower vertical grid, which typically involves filling deep trenches for lateral electrical and optical isolation, whereas this step forms the horizontal planar aperture network (Engineering Practice). The deposition of the aperture grid typically utilizes Physical Vapor Deposition (PVD) or Chemical Vapor Deposition (CVD) to form a dense, opaque metallic film (Engineering Practice). For metals like tungsten, CVD relies on the hydrogen reduction of metal halides at the wafer surface, governed by surface adsorption and thermally activated chemical reactions (Engineering Practice). Alternatively, PVD relies on the momentum transfer from energetic plasma ions bombarding a metal target, ejecting atoms that condense onto the substrate (Engineering Practice). The fundamental physical requirement here is maximizing the optical extinction coefficient across the visible and near-infrared spectrum to ensure complete photon blockage in the non-aperture regions . Similar to the use of light-shielding and absorbing layers in display technologies to suppress parasitic light leakage , this grid metallic layer acts as a macroscopic optical barrier. The film's crystalline microstructure and density directly dictate its optical skin depth, ensuring that incident electromagnetic waves are rapidly attenuated via free-carrier absorption in the metal . Tungsten or specialized aluminum alloys are heavily favored for this step due to their exceptional opacity, low optical transmission, and compatibility with subsequent high-resolution dry etching (Engineering Practice). Tungsten, in particular, exhibits a low coefficient of thermal expansion and high thermal stability, minimizing stress-induced wafer bowing on the thinned BSI substrate . During deposition, critical parameters such as chamber pressure, plasma power, or precursor flow rates must be carefully balanced to control film stress and minimize surface roughness (Engineering Practice). High surface roughness can lead to diffuse scattering of incident light, which undermines the directional light-guiding function of the grid and exacerbates crosstalk . Furthermore, residual intrinsic stress in the deposited film must be tuned to prevent delamination from the underlying barrier layer, usually by modulating the ion bombardment energy during PVD or temperature in CVD . In 40nm BSI CIS technology, where pixel pitches scale down to the micrometre-scale regime or below, the aspect ratio of the optical stack becomes exceptionally critical . The thickness of the aperture grid must be minimized to reduce the overall focal depth and improve the acceptance angle for oblique light rays . However, thinning the metal intrinsically reduces its total optical density; therefore, the deposition process must achieve near-bulk material density without incorporating voids or high concentrations of impurities . Stringent control over the deposition uniformity is strictly required because any thickness variation directly translates into non-uniform light-shielding capabilities and varying pixel sensitivities across the sensor array .
Risks & Challenges
[High] Severe Intrinsic Film Stress: PVD or CVD metal films can develop extreme tensile or compressive intrinsic stress due to atomic peening or thermal expansion mismatches (Engineering Practice). Because BSI substrates are drastically thinned, high localized stress can cause catastrophic wafer bowing or delamination of the grid from the underlying barrier layer .
[Medium] Poor Optical Opacity (Inadequate Film Density): If the deposition pressure is too high or precursor decomposition is incomplete, the resulting metallic film may be porous or contain trapped gaseous impurities . This structurally reduces the effective optical extinction coefficient, allowing photons to leak into adjacent pixels and causing severe optical crosstalk .
[Medium] Excessive Surface Roughness: Improper deposition temperatures or nucleation conditions can induce abnormal grain growth, leading to a highly roughened metal surface (Engineering Practice). A rough surface unpredictably scatters incoming light rather than cleanly defining the optical aperture, heavily degrading the efficiency of the microlens light concentration .
[Low] Particle Contamination (Flaking): Excessive film buildup on the internal deposition chamber shields over multiple cycles can crack and flake off, landing onto the wafer surface (Engineering Practice). These macroscopic particles act as unintended hard masks during the subsequent Light Shield / Aperture Grid Photo and Etch steps, resulting in catastrophic pixel-level optical blockage .