The Benefits of Bringing BSI to High-Speed Applications

Our new high-speed image sensor employs back side illumination (BSI) to capture light more effectively than CMOS imaging sensors. Until now, the CMOS sensors have been based on front side illuminated (FSI) architectures, in which the sensor’s metal circuitry sits above the pixels’ photodiodes that face the light source. This metal circuitry prevents some incident light from reaching the pixels, which in turn affects the fill factor and reduces the sensor’s sensitivity. 
BSI sensors are set apart because they’re designed with a thick carrier wafer at the top of the metal stack — an arrangement that allows the bulk silicon to be thinned and flipped, exposing the diodes facing the light source and the metal surface behind them. 

This design brings two significant advantages to BSI sensors in high-speed applications:

Improved fill factor. The effectiveness at capturing incident light is expressed as the percentage of the pixel surface area that can capture photons. With its metal circuitry blocking or reflecting some of the light, a typical FSI sensor has a fill factor between 50 and 60%, partially compensated for by a micro-lens. By relocating the circuitry BSI sensors have a fill factor close to 100%.

Increased processing speed. The basic speed of the pixel array is limited by resistor-capacitor (RC) time constants, and adding metal reduces the resistance and increases the speed. In FSI sensors, the amount of metal on the sensor front is limited to allow light to reach the photodiodes. This constraint leads to overhead in the processing speed. As frame rates increase and resolutions decrease, the camera can’t provide maximum gigapixel/second (Gpx/sec) throughput because of losses to overhead. Because BSI sensors don’t have this constraint, they can have significantly increased metal circuitry, reducing or even eliminating overhead. 

This capability allows a BSI sensor to maintain its maximum Gpx/sec throughput even at very high frame rate/low resolution combinations. 

To learn more about how BSI sensors are changing high-speed imaging please read our whitepaper at You can also the TMX camera page for technical information about this revolutionary research tool at