How CMOS SPADs are Revolutionizing Our View of the Quantum World
Imagine a camera so sensitive it can detect a single particle of light—a photon—flying through darkness.
This isn't science fiction; it's the reality enabled by Single-Photon Avalanche Diodes (SPADs). What makes this revolutionary? Scientists have now harnessed standard, unmodified CMOS processes—the same technology making your smartphone's camera chip—to create ultra-sensitive SPADs. This breakthrough slashes costs while enabling mass production, bringing what was once confined to physics labs into medical scanners, quantum computers, and even your next car's LiDAR system 1 9 .
At their core, SPADs are "light traps." When a single photon strikes their silicon core, it ignites a chain reaction—an "avalanche"—of electrons, generating a measurable electrical pulse. Unlike conventional sensors needing thousands of photons, SPADs register the faintest flickers: neurons firing, entangled photons dancing, or tumors hiding deep within tissue 3 8 .
When a SPAD's p-n junction is reverse-biased beyond its "breakdown voltage," it enters a hair-trigger state called Geiger mode. A single photon striking it liberates an electron, accelerated by the high-voltage field. This electron collides with atoms, freeing more electrons—an avalanche amplifying one photon into billions of electrons. A "quenching" circuit then resets the diode, readying it for the next photon 3 7 .
Traditionally, SPADs required exotic materials and costly custom fabrication. The game-changer? Building SPADs in standard 180 nm CMOS—a process optimized for cheap, high-volume chips. Researchers engineered structures like Shallow Trench Isolation (STI) guard rings to prevent premature edge breakdown and deep N-wells to create uniform electric fields. The result: SPADs with 10.75 V operating voltage, 286 Hz/μm² dark count rates, and 21.48% photon detection probability—all on mass-production lines 1 9 .
| Parameter | Value | Impact |
|---|---|---|
| Operating Voltage | 10.75 V | Enables portable, low-power devices |
| Dark Count Rate (DCR) | 286.3 Hz/μm² | Reduces false signals in low-light conditions |
| Photon Detection Prob. | 21.48% (@405 nm) | Boosts sensitivity for blue/UV applications |
| Fill Factor | >80% | Maximizes light-collecting area per pixel |
Operation above breakdown voltage enables single-photon sensitivity
Prevents premature edge breakdown in CMOS implementation
Creates uniform electric fields for consistent performance
Silicon SPADs struggle with infrared light—critical for medical imaging. Enter nano-engineered metasurfaces: microscopic structures etched atop SPADs. Like funnels for light, they bend photons into longer paths within the detector. Inverse pyramids, cones, or pyramids—some as small as 850 nm wide—trap light through diffraction and anti-reflection. The payoff: broadband 30–50% efficiency gains, especially in near-infrared, without sacrificing speed or noise performance 2 .
SPADs' nemesis is "dark counts"—false alarms from thermal electrons. Researchers cooled SPAD arrays to –15°C using tiny Peltier coolers. Dark counts plunged tenfold, allowing three times lower laser power in live-cell imaging. This is vital for observing delicate biological processes without damaging samples 5 6 .
| Temperature (°C) | Dark Count Rate | Photon Detection Efficiency |
|---|---|---|
| 25 | 1000 Hz/μm² | 21.5% |
| 0 | 200 Hz/μm² | 22.1% |
| -15 | 100 Hz/μm² | 22.3% |
In a landmark 2025 experiment, scientists fused two chips: a reconfigurable photonic circuit (PIC) and a CMOS SPAD array . The goal? Create a self-contained quantum sensor needing no bulky fiber optics.
| Misalignment | PDE Drop | Practical Implication |
|---|---|---|
| ±15 μm (lateral) | <10% | Vibration-resistant in field use |
| 50 μm (gap increase) | ~12% | Tolerates thermal expansion/contraction |
| Angular tilt (1 degree) | ~15% | Relaxes precision in assembly lines |
| Overvoltage | Photon Detection Efficiency | Dark Count Rate |
|---|---|---|
| 5 V | 45.3% | 16.6 kcounts/s |
| 6 V | 48.1% | 21.0 kcounts/s |
| 7 V | 50.2% | 25.4 kcounts/s |
The system hit 41% total detection efficiency at 561 nm—the highest ever for integrated photon counters. Even when "misaligned" by ±15 μm laterally or pulled back by 50 μm, efficiency dropped only 10%. This resilience proves such systems are viable outside vibration-free labs.
Building cutting-edge SPAD systems relies on key components:
| Component | Function | Example/Value |
|---|---|---|
| CMOS SPAD Array | Converts photons to digital pulses | 93-pixel hex array (SPAD93G) 6 |
| STI Guard Ring | Prevents edge breakdown in CMOS | 180 nm process integration 1 |
| Metasurface Layers | Enhances light trapping/absorption | Inverse pyramids (850 nm period) 2 |
| Peltier Cooler | Reduces dark counts via cooling | –15°C operation 5 |
| Time-Correlated Counter | Timestamps photons with picosecond precision | 20 ps resolution 6 |
| Femtosecond Laser Writer | Directly patterns waveguides for PICs | Low-loss glass circuits |
CMOS SPAD arrays are already transforming fields:
Endoscopes with FLIM (fluorescence lifetime imaging) spot precancerous cells by their metabolic "glow" timing 3 .
"Ghost imaging" techniques reconstruct objects using entangled photons, enabled by SPADs' timing precision 8 .
The future shines brighter still. With metasurfaces pushing efficiency toward 60% and 3D-stacked designs marrying SPADs to processors on a single chip, these unassuming CMOS marvels will soon make quantum sensing as commonplace as a camera lens. As light's quantum secrets unfold, SPADs—born from the humblest silicon—will be our eyes.