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

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

BSI Performance Advantage

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

Pixel Size Evolution

Major Manufacturers

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.