Color filters, clear layers and microlenses: three different jobs in a BSI pixel

Color filters select the light reaching a pixel, a clear base prepares the surface above the filter array, and a microlens redirects light toward the sensing region. Reading these structures as three interchangeable “sensitivity layers” misses why the process needs all three. [P1]

A filter selects a spectrum; it does not focus the light

A photodiode converts absorbed light into collected charge. In a color image sensor, different filter materials give neighboring pixels different spectral responses. Their wavelength-dependent transmission and absorption help distinguish color; the final image also depends on how the pixel signals are processed.

In this Flow, Step 368 patterns green filter material at selected pixel positions. This is one color pass, not a picture of the completed array. Other color passes follow before the upper optical layers are built. [S368]

The schematic shows where the green filter remains. It does not measure transmission or demonstrate the completed red, green and blue array.

The clear base prepares a surface, but remains in the optical path

Different filter patterns leave height variations and gaps. At Step 376, a clear coating levels the topography before the lens is patterned. Without a suitable base, the underlying relief can influence the shape of the optical structure formed above it. [S376]

Transparent does not mean optically absent. Light still propagates through this layer, so its optical properties and geometry matter together with the layers above and below. Its distinguishing task in this sequence is to provide the starting surface for lens formation, rather than to select one color.

Compare the upper surface with the earlier filter stage. The intervening color passes have been completed; this image is not the immediate next operation after green-filter patterning.

Reflow changes the shape that redirects light

After lens patterning, polymer islands have relatively sharp edges. Step 378 uses reflow to reshape those islands into curved surfaces. Surface tension and viscous flow drive this shape change; the resulting refracting surface redirects incident light toward the intended sensing region. [S378] [P2]

The curve is a visible structural change. The diagram is not a ray-tracing calculation and cannot establish collection efficiency. Alignment among the lens, filter and sensing region remains important: a lens cannot repair every mismatch elsewhere in the stack.

Follow the handoff, not just the layer names

This three-stop comparison skips other color passes and lens patterning. It explains the handoff from color selection to surface preparation to light redirection; it is not the complete manufacturing sequence.

A useful check is to ask: if the top lens looks smooth but the filter beneath it is misplaced, which task has been completed and which remains wrong? Surface shape alone does not establish correct spectral selection or registration.

Source steps

These figures and process positions come from the current approved Flow steps: green-filter patterning, clear-base formation and microlens reflow. Step text follows the reading access shown on the destination page.