backside illumination sensor
**Backside Illumination (BSI) Image Sensors** are the **CMOS image sensor architecture where light enters from the back of the silicon wafer (opposite the metal wiring)** — eliminating the optical obstruction caused by metal interconnect layers above the photodiodes, increasing quantum efficiency by 30-90% compared to front-side illumination (FSI), and enabling smaller pixel sizes (down to 0.56 µm pitch) that are essential for the high-resolution cameras in modern smartphones, automotive, and surveillance systems.
**FSI vs. BSI Architecture**
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**BSI Performance Advantage**
| Metric | FSI | BSI | Improvement |
|--------|-----|-----|------------|
| Quantum efficiency (green) | 40-55% | 70-85% | +50-90% |
| Quantum efficiency (blue) | 25-40% | 60-80% | +100-140% |
| Angular response | Poor at edges | Uniform | Significant |
| Minimum pixel pitch | ~1.4 µm | 0.56 µm | Much smaller |
| Crosstalk | Medium | Low (with DTI) | Better color |
**BSI Fabrication Process**
```
Step 1: Standard CMOS process on bulk wafer (front-side)
- Photodiodes, transfer gates, readout transistors
- Full BEOL metal stack (M1-M5+)
Step 2: Wafer bonding
- Bond CMOS wafer (face-down) to carrier wafer or logic wafer
- Oxide-oxide or hybrid bonding
Step 3: Wafer thinning
- Grind and CMP the original substrate
- Thin silicon to ~3-5 µm (need photodiode but not more)
Step 4: Backside processing
- Anti-reflection coating (ARC)
- Color filter array (Bayer pattern RGB)
- Micro-lens array (one lens per pixel)
- Deep trench isolation (DTI) between pixels
Step 5: Backside pad opening and interconnect
- TSV or bond pad connections to front-side circuits
```
**Key Technologies in Modern BSI Sensors**
| Technology | What It Does | Impact |
|-----------|-------------|--------|
| Deep Trench Isolation (DTI) | Oxide-filled trench between pixels | Prevents optical/electrical crosstalk |
| Stacked BSI | Pixel array wafer bonded to logic wafer | Pixel + CPU in one package |
| 2-layer stacked | Pixel + ISP logic | Faster readout, HDR |
| 3-layer stacked | Pixel + DRAM + logic | Global shutter, extreme speed |
| Phase detection AF | Split photodiodes for autofocus | DSLR-like AF in phones |
**Pixel Size Evolution**
| Year | Pixel Pitch | Resolution (phone) | Sensor |
|------|-----------|--------------------|---------|
| 2010 | 1.75 µm | 5 MP | FSI |
| 2015 | 1.12 µm | 13 MP | BSI |
| 2020 | 0.8 µm | 48-108 MP | BSI stacked |
| 2023 | 0.56 µm | 200 MP | BSI stacked + DTI |
**Major Manufacturers**
| Company | Market Share (2024) | Key Products |
|---------|--------------------|--------------|
| Sony | ~45% | IMX series (iPhone, Sony cameras) |
| Samsung | ~25% | ISOCELL (Galaxy, HP2) |
| OmniVision | ~10% | OV series (automotive, security) |
| ON Semiconductor | ~8% | Automotive image sensors |
BSI image sensors are **the enabling technology behind the smartphone camera revolution** — by solving the fundamental optical limitation of front-side illumination where metal wiring blocked light from reaching photodiodes, BSI architecture made sub-micron pixels practical, enabling 200-megapixel sensors in devices thin enough to fit in a pocket while capturing images that rival dedicated cameras.