Color Filter - Green, Coat/Expose/Develop/Bake - 40nm

Processing the green layer first establishes the foundational topography of the CFA structure and provides physical boundaries that help minimize alignment-induced cross-talk for the subsequent red and blue filter passes .

In a complementary metal-oxide-semiconductor (CMOS) image sensor utilizing

the classical Bayer pattern, the green color filter array (CFA) is typically processed first because green pixels comprise 50% of the imaging array and dominate the luminance signal for human visual perception [P1, A1]. Following the pre-lithography cleaning of the light shield grid, the green color filter resin is coated to fill the predefined grid cavities (Engineering Practice). Processing the green layer first establishes the foundational topography of the CFA structure and provides physical boundaries that help minimize alignment-induced cross-talk for the subsequent red and blue filter passes . After the develop and bake steps, a final hard bake is required to fully cross-link the green resist, preparing the adjacent pixel cavities for the Blue Color Filter coat without risk of chemical intermixing (Engineering Practice). The Back-Side Illumination (BSI) architecture allows these optical filtering layers to sit extremely close to the silicon photodiode surface, fundamentally reducing the optical focal depth and angular sensitivity compared to older Front-Side Illumination designs [P1, A1]. The physical and chemical mechanism of this step relies on a negative-tone, dye-doped or pigment-loaded photosensitive polymer [P1, P2]. During the spin-coating phase, fluid dynamics and polymer viscosity dictate the initial wet film thickness, which must be precisely controlled to define the effective optical cavity length and ensure proper transmission spectra . Upon ultraviolet (UV) exposure, photoinitiators within the resist generate free radicals, inducing rapid polymer cross-linking in the exposed regions corresponding to the green pixels (Engineering Practice). The unexposed regions remain chemically soluble in the developer solution and are subsequently washed away (Engineering Practice). Because reflection and optical scattering from underlying metallic grid structures can shift the UV light away from the intended path, determining the proper UV exposure dose is essential to achieve vertical sidewalls and prevent residue . Finally, the baking process drives out residual organic solvents and initiates thermal cross-linking, physically hardening the green filter structure so it can withstand the aggressive solvents used during the subsequent blue and red filter deposition steps (Engineering Practice). The selection of a pigment-dispersed negative photoresist offers an optimal balance between targeted spectral transmittance and robust processing stability [P1, P2]. The thickness of the green color filter is a highly critical parameter; if the film is too thick, absolute transmission efficiency drops and optical crosstalk increases due to a higher structural aspect ratio [P1, P2]. Conversely, if the layer is too thin, the spectral filtering becomes incomplete, leading to severe color cross-talk where blue or red light erroneously contributes to the green photodiode signal . Interaction parameters such as spin speed, dispense volume, and polymer viscosity must be tightly coupled to achieve the exact target thickness required for optimal quantum efficiency . Furthermore, the bake temperature and time must be carefully calibrated; insufficient baking leads to solvent swelling during the next lithography step, while excessive baking can cause thermal degradation of the organic pigments, shifting their optical absorption characteristics (Engineering Practice). For 40nm BSI CMOS image sensors with sub-micrometre-scale pixel pitches, the physical aspect ratio of the color filter within the metallic grid becomes a significant integration bottleneck . At these scaled dimensions, traditional thick dye-doped polymer filters inherently suffer from severe optical cross-talk and limited relative illumination, especially at high chief ray angles (CRA) [P1, P2]. Therefore, precise critical dimension (CD) control during development and minimized total color filter thickness are mandatory to maintain a high quantum efficiency (QE) peaking in the green spectrum while preserving a wide angular response .

[High] Optical Cross-talk and Spectral Shift: Inaccurate control of the green photoresist layer thickness shifts the optical transmission spectrum and reduces spectral separation . If the layer is too thin or improperly developed, out-of-band photons reach the photodiode, severely degrading color fidelity and increasing cross-talk in sub-micrometre-scale pixels .
[High] Resist Scumming and Inter-pixel Residue: Reflection and diffraction of UV light at the edges of the underlying metal grid can cause unintended partial exposure in the adjacent red or blue pixel cavities . This leaves a polymerized green resist residue after development, requiring aggressive in-situ cleaning to prevent cross-contamination .
[Medium] Pattern Delamination (Lifting): Insufficient UV exposure dose or inadequate pre-litho surface preparation weakens the adhesion between the green polymer and the underlying substrate . During the fluidic stress of the development or rinse steps, the green filter blocks can detach, leading to dead or defective pixels (Engineering Practice).
[Medium] Topographical Thickness Non-uniformity: Variations in spin-coating speed, resist viscosity, or underlying step heights can lead to center-to-edge thickness variations across the wafer . This directly alters the optical path length, causing spatially dependent color shifts and luminance variations across the sensor array .
[Low] Thermal Degradation of Pigments: Subjecting the green filter array to excessive bake temperatures or prolonged bake times can break down the organic pigment molecules (Engineering Practice). This failure mode reduces the peak light transmission efficiency and alters the resonant wavelength of the filter .

In a complementary metal-oxide-semiconductor (CMOS) image sensor utilizing

[High] Optical Cross-talk and Spectral Shift: Inaccurate control of the green photoresist layer thickness shifts the optical transmission spectrum and reduces spectral separation . If the layer is too thin or improperly developed, out-of-band photons reach the photodiode, severely degrading color fidelity and increasing cross-talk in sub-micrometre-scale pixels .
[High] Resist Scumming and Inter-pixel Residue: Reflection and diffraction of UV light at the edges of the underlying metal grid can cause unintended partial exposure in the adjacent red or blue pixel cavities . This leaves a polymerized green resist residue after development, requiring aggressive in-situ cleaning to prevent cross-contamination .
[Medium] Pattern Delamination (Lifting): Insufficient UV exposure dose or inadequate pre-litho surface preparation weakens the adhesion between the green polymer and the underlying substrate . During the fluidic stress of the development or rinse steps, the green filter blocks can detach, leading to dead or defective pixels (Engineering Practice).
[Medium] Topographical Thickness Non-uniformity: Variations in spin-coating speed, resist viscosity, or underlying step heights can lead to center-to-edge thickness variations across the wafer . This directly alters the optical path length, causing spatially dependent color shifts and luminance variations across the sensor array .
[Low] Thermal Degradation of Pigments: Subjecting the green filter array to excessive bake temperatures or prolonged bake times can break down the organic pigment molecules (Engineering Practice). This failure mode reduces the peak light transmission efficiency and alters the resonant wavelength of the filter .

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