Image Sensors

Silicon Properties

• Silicon is widely used for image sensors.
• Photon impact on silicon releases an electron, creating an electron-hole pair.
• Silicon crystals with high purity generate electron flux from photon flux.

Image Sensor Overview

• Converts light into electrons; main challenge is reading out electrons and converting them to voltage.
• Modern image sensors (e.g., 18 megapixels, 1.25 microns/pixel) efficiently pack millions of pixels.
• Pixel size limited by wavelength of light, not following Moore's law.

CCD Technology (Charge Coupled Devices)

• Pixels act as potential wells (buckets) converting photons to electrons.
• Electrons transfer from row to row to a bottom row for voltage conversion.
• Analog to digital conversion (A to D) for digital output.
• Transfer process akin to "bucket brigade." Uses electric fields for charge transfer.
• Key challenge: preventing electron loss and avoiding spurious electrons.

CMOS Technology (Complementary Metal-Oxide Semiconductor)

• Each pixel includes a circuit for electron to voltage conversion.
• Can address individual pixels for readout, allowing flexibility and increased frame rate for specific regions.
• Less light-sensitive area per pixel due to added circuitry.
• Dominates consumer cameras due to flexibility and quality.

Color and Microlenses

• Pixels cannot differentiate colors; color filters (red, green, blue) are used above each pixel.
• Color interpolation after image capture.
• Microlenses focus light on the light-sensitive area of each pixel, avoiding light loss to circuitry regions.
• Microlens-layer structure beneath scanning electron microscope.

Future Trends

• Increasing integration of circuitry in silicon layers.
• Integration for image processing, computer vision on one silicon wafer.
• Development toward optics on wafer technology.