Back side illumination (BSI) sensors have been available for more than 10 years in a variety of cellphone and standard digital cameras. They’ve offered proven advantages when it comes to improving low light performance and dynamic range of these consumer-focused cameras. So why did it take so long to bring these sensors to high-speed imaging? In a word, size.

The sensors and pixels used in high-speed cameras are much larger than standard cameras to minimize our speed-resolution-sensitivity trade-offs. For instance, while a cell phone camera may have a pixel that measures less than 2 μm per side, our image sensor pixels are typically more than 6 μm and as much as 28 μm per side.

The manufacturing process for BSI sensors is inherently more difficult than comparable front side illuminated (FSI) sensors, requiring additional manufacturing steps. Among them is a wafer back-thinning step to remove the bulk silicon, bringing the photodiodes closer to the light source. There are also additional processing steps on the back side of the wafer to anneal the surface and provide electrical contacts to the front side. The size of high-speed image sensors only exacerbates manufacturing difficulties.

The BSI image sensor has been worth the wait. It sets new standards for:
  • Speed. The first camera using the sensor captures images at 76,000 fps at full 1-megapixel (1280 x 800) resolution, and it can reach speeds more than an order of magnitude faster at reduced resolutions and with binning. For example, the camera tops out at 1.75 million fps with a resolution of 1280 x 32 and 640 x 64-pixel binned. Historically, the resolutions associated with frame rates above 1 million fps were too low for nearly all scientific uses, but 1280 x 32 represents a truly usable resolution in a wide range of applications.
  • Exposure times. The new sensor supports minimum exposure times as fast as 95 ns with an export controlled FAST option, making the camera the fastest of its class. The fast exposure times make it possible to capture ever-faster events without motion blur, which can be a limiting factor in obtaining high-quality images.
  • Pixel size. Historically, to work in light-starved conditions, high-speed cameras have used very large pixel sizes as a means to catch as many photons as possible. Our existing FSI ultrahigh-speed sensor, for example, has a pixel size of 28 μm per side for an area of 784 μm2. The new BSI high-speed image sensor has an 18.5-μm per side pixel, but its proficiency at capturing light makes it about as sensitive at three times the speed as earlier FSI sensors with 28-μm pixels. Smaller pixels also improve sampling frequency (Nyquist), allowing the sensor to resolve higher lp/mm spatial frequencies before aliasing. This capability enhances the imaging system’s performance in flow cytometry, particle image velocimetry (PIV), digital image correlation (DIC) and other high-speed applications limited by the resolving power of the sensor.