Image Sensor CMOS Process
CMOS Image Sensor (CIS) Process Technology is the specialized semiconductor manufacturing flow that creates arrays of millions of photodiodes integrated with per-pixel amplifiers, ADCs, and digital processing circuitry on a single die — converting photons into digital image data using process innovations like Backside Illumination (BSI) and 3D wafer stacking that have made CMOS the dominant image sensing technology.
Why CMOS Replaced CCD
Charge-Coupled Devices required dedicated fabs with non-standard process steps and separate companion chips for signal processing. CMOS image sensors are fabricated in standard (or lightly modified) CMOS foundries, integrating all analog and digital processing on-chip. This integration slashed cost, power, and form factor — enabling the camera in every smartphone.
Key Process Innovations
- Backside Illumination (BSI): In front-side illuminated sensors, metal wiring layers sit above the photodiode, blocking and reflecting incoming light. BSI flips the sensor — the wafer is thinned to ~3 um and bonded upside down so light enters through the silicon backside directly into the photodiode. BSI improves quantum efficiency by 30-50%, especially in small pixels (< 1.0 um).
- Deep Trench Isolation (DTI): At sub-1.0 um pixel pitches, photon-generated electrons can diffuse sideways into neighboring pixels (crosstalk), destroying color fidelity. DTI etches narrow, deep trenches between pixels and fills them with oxide, creating physical barriers that block lateral charge migration.
- 3D Stacked Architecture: The photodiode array is fabricated on one wafer, the analog/digital processing circuitry on a second wafer, and (in the latest Sony designs) DRAM on a third wafer. The wafers are bonded face-to-face with copper hybrid bonding, connecting every pixel to its dedicated processing circuit through micro-vias at 3-5 um pitch.
Pixel-Level Engineering
| Generation | Pixel Pitch | Architecture | Typical Application |
|---|---|---|---|
| Legacy | 2.8 um | FSI, 4T Rolling Shutter | Feature phones |
| Mainstream | 1.0-1.4 um | BSI, DTI, Dual Conversion Gain | Smartphone main camera |
| Advanced | 0.6-0.8 um | Stacked BSI, Global Shutter | Automotive, AR/VR |
Challenge: Global Shutter
Rolling shutter sensors read pixels row-by-row, causing motion distortion. Global shutter captures all pixels simultaneously but requires in-pixel charge storage that competes with the photodiode for area. Advanced 3D stacking moves the storage transistors to the bottom wafer, enabling global shutter without sacrificing fill factor.
CMOS Image Sensor Process Technology is the silicon manufacturing innovation that put a high-quality camera in every pocket — and is now extending into automotive LiDAR, medical endoscopy, and event-driven neuromorphic vision.