How Custom CMOS Image Sensors Enable Extreme Low-Light Imaging Applications: Part I

While both image intensifier (II) and electron-multiplying charge-coupled device (EMCCD) technologies have historically dominated extreme low-light imaging applications, their performance is limited by noise, dynamic range and single-photon sensitivity. However, advances in low-noise CMOS imaging sensors (CIS) offer alternatives to these legacy technologies.

Developed by Forza and EOTech, a new backside-illuminated CIS (BSI-CIS) technology uses the electron-bombarded semiconductor (EBS) gain process to overcome the challenges faced by II and EMCCD cameras. This new BSI-CIS is incorporated into electron-bombarded active pixel sensor (EBAPS®) cameras. Thanks to EBS, the BSI-CIS allows imaging systems to excel in extreme low-light imaging applications and provides low noise, high frame rates and high dynamic range.

Our new blog series delves into how these low-light EBAPS cameras function. First, let’s discuss the EBS process and how the BSI-CIS meets advanced requirements.

How EBS Minimizes Noise and Increases Gain

Low-light EBAPS image sensor design features a gallium-arsenide (GaAs) photocathode and a high-resolution BSI-CIS anode in close proximity to each other. Applying a voltage of 1 to 2 kilovolts (kV) accelerates emitted photoelectrons to the anode. The EBS process enables low-noise gain by converting high-energy photoelectrons into electron-hole pairs. When applying energy to silicon, electron-hole pairs are generated for every 3.64 electron-volts (eV).

Traditional EBAPS designs operate at voltages that lose photoelectrons due to elastic and inelastic backscatter and incomplete electron cloud collection. These losses generate an excess noise factor of 1.2. Traditional EBAPS designs also feature a temporal noise much less than 1 photoelectron. Because it is deterministic, the EBS process generates little noise and enables low-light EBAPS designs with an excess noise factor of 1.03.

In a low-light EBAPS, the pixel photodiode collects and then reads out the multiplied electrons. The EBS gain is high enough to mitigate noise attributed to pixel readout, dark current and other temporal and fixed pattern sources. By mitigating noise from these sources, EBS enables EBAPS to offer superior performance in low-light conditions compared to standard low-noise CIS or EMCCD cameras. The EBS gain process enables a higher signal-to-noise ratio (SNR) at extremely low light levels, allows signal intensity measurements and is a linear gain mechanism.

BSI-CIS Meets Advanced Low-Light Requirements

EBAPS sensor performance is largely determined by CIS architecture and design. The sensor’s fill factor, temporal duty cycle, dynamic range and frame rate contribute to low-light performance.

Fill factor. To ensure optimal performance, a CIS pixel must have close to 100% fill factor to minimize photoelectron losses. Lost photoelectrons directly contribute to a photocathode’s quantum efficiency (QE) and at extremely low light levels, photon characteristics dictate camera performance. To maximize low-light-level resolution and performance, an imager must detect as many photons as possible.

Because of a lower fill factor compared to BSI-CIS, frontside-illuminated CIS are not used in an electron-bombarded mode. The frontside design also has a metal-dielectric stack that blocks photoelectrons from reaching the silicon at moderate acceleration voltages (1-2 kV).

A properly designed BSI-CIS can achieve close to 100% fill factor. The pixel photodiode collects generated charges regardless of the photoelectron impact position. To reduce carrier recombinations, the silicon surface of a BSI-CIS is passivated.

Temporal duty cycle. CIS architecture must also maximize image photon integration with nearly 100% temporal duty cycle. If a CIS features 100% temporal duty cycle and fill factor, the sensor is optimized for signal collection at low light levels.

High dynamic range. High dynamic range is also a critical CIS requirement because it enables capture of brightly lit nighttime scenes by accommodating intra-scene dynamic ranges. The high dynamic range improves the capability to capture details of dark areas in scenes that contain light sources.

High frame rate. Low-light-level cameras also must have high frame rates at least above 120 hertz (Hz) when used in augmented reality systems. Additional sensor requirements include a low dark current, a megapixel format and a large pixel size of about 10 micrometers (μm).

Because this sensor technology mitigates noise from pixel readout, dark current and other sources, it is extremely useful for night vision or other low-light applications. Compared to low-noise video frame rate CIS or EMCCD cameras, a BSI-CIS with high gain from EBS provides superior performance under starlight illumination and lower light levels.

Stay tuned for our next installment where we discuss low-light EBAPS camera design. In the meantime, check out our white paper or contact us to share your thoughts on the future of CMOS sensors