What do color filters and microlenses each do?
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Form the green filter pattern
Reading from bottom to top above the pixel: color filters, non-photosensitive planar base, then curved microlenses. The first view shows the green filter pattern formed first.
Silicon photodiodes cannot distinguish wavelength on their own, and inter-pixel grids reduce effective fill factor. Follow green color filter array (CFA) patterning, upper non-photosensitive planar base coating, and microlens thermal reflow to separate wavelength filtering, focal leveling, and refractive light focusing.
After grid pre-clean, green color-filter resist is coated, exposed, developed, and hard-baked to form the green filter pattern that selects its spectral band. Red and blue colors have their own processing steps; the full filter array is not complete yet.
- Mechanism
- Because green pixels occupy 50% of a Bayer array and dominate luminance perception, patterning green first establishes the baseline topography and physical boundary; UV exposure cross-links the pigmented negative resist, development clears unexposed regions, and hard baking fully cross-links the film against chemical intermixing during later red and blue coats.
- Process integration
- Color filters select wavelength via pigment absorption bands; thickness deviations shift transmission spectra, while UV diffraction off underlying metal grids can leave resist scumming in adjacent red/blue cavities that causes cross-contamination and optical crosstalk.
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Form a planar base
Once all green, blue, and red filters are complete and hard-baked, the Upper OCL planarization (base) layer is spin-coated and baked over the full filter array to provide a flat foundation and controlled focal distance for microlens processing.
- Mechanism
- Sequential deposition and patterning of three color resists inevitably leave step-height variations and gaps; a low-viscosity, non-photosensitive transparent resin levels this underlying relief via spin-coating hydrodynamics before thermally cross-linking into a planar optical film.
- Process integration
- This planar base neither filters wavelengths nor focuses light by curvature; instead, it isolates upper microlens formation from underlying CFA relief and sets the exact optical focal distance to the photodiode—insufficient leveling or thermal yellowing directly degrades lens profile and sensitivity.
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Reflow the lens surface
Microlens material is first patterned into steep polymer islands on the planar base and then thermally reflowed into a continuous convex surface that redirects incident light. Actual focusing is assessed through optical design and testing.
- Mechanism
- Heating the wafer above the polymer glass transition temperature (T_g) transitions the patterned islands into a viscoelastic fluid; surface tension minimizes surface free energy while pinned at the footprint boundary, relaxing sharp lithographic edges into a smooth convex dome before UV/hard baking locks the cross-linked shape.
- Process integration
- The curved microlens surface refracts incident rays into the underlying active photodiode to compensate fill-factor loss; if reflow temperature or time exceeds the capillary pinning window, adjacent lenses merge (inter-lens merging) and lose their focusing profile.
Key ideas
The backside pixel optical stack is a precision micro-optical system combining spectral filtering (CFA), topological leveling and focal spacing (Planar Base), and refractive focusing (Microlens): because R, G, and B pigmented resists cannot be patterned in a single exposure, step 368 patterns only the green islands; after separate red and blue passes, step-height relief across the completed CFA is leveled at step 376 by a transparent planar base that seals the pigments and establishes a uniform depth-of-focus plane; microlens resist is then patterned and heated above its glass-transition temperature (T_g) at step 378 so surface tension reflows discrete blocks into a low-gap convex microlens array.
View the process comparison table
| Process dimension | Core mechanism and selectivity | Window mismatch risk and verification boundary |
|---|---|---|
| Green filter pattern (step 368) | Pigment-dispersed negative resist cross-links under UV exposure, retaining green filter islands after development while leaving R/B spaces open. | Insufficient thermal cure causes solvent intermixing during subsequent R/B coating; only the green channel is present at this stage, not the full Bayer array. |
| Planar base layer (step 376) | Low-viscosity transparent resin levels R/G/B CFA relief during spin and bake while setting the vertical optical path to the silicon photodiode. | Residual surface relief causes microlens lithography defocus and radius distortion; a flat transparent plane lacks curvature and cannot focus light. |
| Microlens thermal reflow (step 378) | Above T_g, surface-energy minimization (Laplace pressure balance) drives patterned polymer islands to round into spherical-cap microlenses. | Under-reflow leaves flat tops with poor collection, while over-reflow bridges adjacent lenses; focusing efficiency requires optical testing. |
About this schematic
Simplifies the view to a single three-pixel row rather than a 2D Bayer checkerboard. Separate blue and red filter coat/expose/develop/bake passes between steps 368 and 376, microlens coating and patterning between steps 376 and 378, and post-reflow hardening are omitted; no spectral transmittance curves or 3D ray traces are shown.