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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Front-Side Illumination (FSI): Backside Illumination (BSI):
Light ↓ Light ↓
[Micro-lens] [Micro-lens]
[Color filter] [Color filter]
┌─────────────────────┐ ┌─────────────────────┐
│ Metal 3 │ │ Photodiode (silicon) │ ← Light hits
│ Metal 2 │ ← Light │ Thin silicon (~3 µm) │ directly
│ Metal 1 │ must pass └─────────────────────┘
│ Photodiode (silicon)│ through │ Metal 1 │
└─────────────────────┘ wiring │ Metal 2 │
│ Metal 3 │
│ Carrier wafer │
└─────────────────────┘
FSI: Light blocked/scattered by metal → low QE at small pixels
BSI: Light hits photodiode directly → high QE regardless of pixel size
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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
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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
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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.