In the field of infrared vision, as in microelectronics, miniaturization is a driving force behind innovation. 7.5-micron pixels offer lower costs and energy consumption as well as being lighter and more efficient than their 15-micron predecessors. CEA-Leti's research engineers have overcome unprecedented technical challenges to enable this transfer to industry.
Sharp images, even at 130 K
The key advantage of this technology is image sharpness. In an infrared detector, photons are absorbed by the semiconductor material and generate an electric charge, which is then diffused throughout the material for a few microseconds. This phenomenon is acceptable as long as the diffusion area is smaller than the size of the pixel.
However, if a charge can travel up to 20 µm and the pixel measures only 7.5 µm, the result is reduced image sharpness. In addition, LYNRED wants its detectors to have a high operating temperature (130 K or more as compared to current 110 K operating levels) as this would reduce the size and cost of the cryogenic cooling system. Unfortunately, higher temperatures also mean that charges travel further.
55% MTF*: well beyond expectations
CEA-Leti's research engineers have come up with an innovative solution to this challenge. The 7.5µm pixels are designed in such a way that electrical charges never reach the neighboring pixel. The innovation resides not in the material (a mercury-cadmium-tellurium alloy common in infrared vision solutions), but in the pixel architecture itself.
The resulting performance is remarkable in terms of sharpness, which is measured as a percentage of the "modulation transfer function" or MTF. While a theoretical detector made up of perfect pixels cannot exceed 64 % MTF, CEA-Leti's detectors achieve 55 %.
An innovative method for measuring MTF
"We devised a new method for measuring MTF sharpness," adds Olivier Gravrand, research director at CEA-Leti. "Instead of injecting photons locally to excite the pixels, we use a scanning electron microscope to inject electrons. This 'electron brush' is much finer than the conventional optical beam and allows for a more accurate measurement of MTF."
These high-performance infrared detectors are primarily intended for use by the defense sector as they provide improved visibility at greater distances thanks to their high resolution and record sharpness. To date, no other published technology demonstrates an MTF of 55% or more.
These infrared detectors can also be used on weather satellites to better monitor temperature, humidity and CO2 content in the atmosphere. Finally, they are of interest to astrophysicists to help them observe very distant stars.
*MTF: Modulation Transfer Function