The first optical pad layer is deposited using PECVD or ALD to ensure high film density and conformal step coverage, which enables reliable interface with the underlying silicon and metallization .
In depth
In nanoscale Back-Side Illuminated (BSI) CMOS Image Sensor technology, minimizing optical and elect
rical crosstalk is critical, often necessitating complex light shield grid structures . The Optical Pad 1 Deposition step occurs immediately following the tungsten (W) and titanium nitride (TiN) etch and subsequent strip processes in the light shield (LS) grid module . Its primary function is to serve as the foundational layer of a multi-film dielectric stack that will eventually act as a hard mask and optical buffer for the subsequent Lower Vertical Grid Trench patterning . Unlike Optical Pad 2 and Optical Pad 3, which are deposited subsequently to build a graded refractive index profile or provide staggered etch selectivities, Pad 1 directly interfaces with the underlying metallization and silicon . This requires the film to possess excellent adhesion and act as a strain buffer, analogous to how pad oxide buffers stress before silicon nitride deposition in shallow trench isolation schemes . The deposition of this first optical pad layer typically employs plasma-enhanced chemical vapor deposition (PECVD) or atomic layer deposition (ALD) to ensure high film density and conformal step coverage . During PECVD, reactant gases are ionized into a plasma state, lowering the activation energy required for the chemical reaction to proceed at reduced substrate temperatures (Engineering Practice). This low-temperature processing is vital to prevent the thermal degradation of the pre-existing W/TiN grid structures, as excessive thermal budgets can cause structural instability . If ALD is used, the process relies on self-limiting surface reactions to achieve highly conformal growth, ensuring a step coverage exceeding 90% even over complex topologies . The structural integrity of this deposited film determines its resilience during the subsequent photolithography and reactive ion etching steps . Furthermore, managing the intrinsic stress of the deposited film is crucial to prevent wafer bowing, a principle similarly applied when depositing thick oxides over deep trench structures . Material selection for Optical Pad 1 balances optical properties, such as refractive index, with mechanical durability and etch selectivity . Silicon nitride (SiNx) or silicon oxynitride are frequently chosen because they provide robust masking capabilities against deep silicon etching while offering tunable optical transmission . Just as varying deposition parameters in PECVD SiNx affects its passivation qualities , tuning the precursor gas ratios, RF power, and chamber pressure during Pad 1 deposition directly modulates the film's stoichiometry and refractive index (Engineering Practice). An increase in the nitrogen-to-silicon ratio typically alters the band structure of the dielectric, which can influence both optical absorption and electrical leakage characteristics . Additionally, precise control over the deposition rate and temperature prevents the formation of voids or weak seams, which are common failure points when filling or covering high-aspect-ratio structures . At the 40nm node, the physical dimensions of the BSI CIS pixels approach the wavelength of visible light, making the precise thickness and uniformity of the optical pad layers highly sensitive to optical interference effects . As device scaling reduces available dimensional tolerances , the optical pad stack must simultaneously fulfill stringent optical reflection constraints and withstand aggressive high-aspect-ratio trench etching (Engineering Practice). Dividing the optical pad into multiple discrete depositions (Pad 1, Pad 2, and Pad 3) provides the necessary degrees of freedom to independently optimize the interface stress, etch selectivity, and anti-reflective properties across the nanometer-scale grid .
Risks & Challenges
[High] Film Delamination / Peeling: Poor adhesion between the deposited Pad 1 film and the underlying W/TiN or silicon surfaces occurs due to inadequate pre-deposition cleaning or excessive intrinsic film stress . As seen in multi-layer depositions, unmanaged stress gradients can lead to mechanical failure or severe wafer bowing .
[High] Inadequate Step Coverage: If the deposition process fails to achieve highly conformal coating over the existing topography, localized thinning at the corners will occur . This exposes the underlying structures to unintended corner loss during the subsequent reactive ion etching, similar to the corner erosion mechanisms mitigated by targeted selective deposition techniques .
[Medium] Refractive Index Shift: Fluctuations in chamber pressure or precursor gas flow ratios can unexpectedly alter the stoichiometry of the deposited dielectric (Engineering Practice). This compositional shift changes the fundamental band structure and optical properties of the material , leading to optical impedance mismatch and increased light crosstalk in the final image sensor (Engineering Practice).
[Low] Plasma-Induced Damage: Excessive RF power during PECVD can cause plasma-induced charging or energetic ion bombardment damage to the underlying active regions (Engineering Practice). Similar to high-field stress in thin dielectrics, this energetic bombardment can generate interface traps that degrade device performance and increase subthreshold leakage currents .