Image sensors keeping an eye on Earth

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CMOS image sensors have become Humankind’s remote eyes. They equip all our smartphones and cameras and their range of scientific applications extends from the Earth to space and inspections of hostile environments. They comprise a state-of-the-art research field for the observation of the Earth, climatological effects and radiometric measurements.

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CMOS sensor

CMOS sensors are image sensors that can convert near-infrared electromagnetic radiation, in the visible spectrum, into an analog electrical signal. This signal is then amplified, digitized and processed to obtain a visible digital image on a screen or for recording. It is the basic component of photo cameras and digital cameras, the equivalent of film in traditional analog photography.

The Microelectronic Image Sensor research group (Department of Electronics, Optronics and Signal Processing – ISAE-SUPAERO) also designs and produces image sensors, hand-made for customized scientific applications.

The researchers are expanding their expertise in the space and nuclear fields, areas in which radiation can damage sensors and deteriorate the data gathered. The nuclear energy sector needs optical sensors that are resistant to radiation for inspecting their facilities and possibly to view fusion reactions. The team is considered to be the specialists in the field when it comes to producing sensors resistant to extreme radiation levels. The space sector is also a client for the laboratory for research applied to the development of observation satellites over a period of some ten years.

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Design phase of optical image sensors

Sensors must comply with the strict specifications laid down by the industrial client, who defines most of the performances required such as the number of pixels, sensitivity to a given wavelength, sharpness of the image or the capacity to eliminate glare effects (e.g. windshields).

“We do photography, but through the development of image sensors and their measurement methods for observations in the visible and near-infrared spectra,” said Olivier Marcelot, a research engineer working on the material aspects of sensors. He is seeking to improve the sensor’s optical performances, along with its sharpness and sensitivity by changing and optimizing its component materials.

Spotlight on the birth of a sensor in the lab

Each researcher in the group has his/her field of specialization, whether radiation, noise reduction or measurements. They are backed up by development engineers, designers who draw up the approximately one hundred steps necessary to produce an image sensor, with a precision of some ten nanometers. In order to reduce costs and production times, electrical, physical and optical simulation tools will be used to choose and better define the architecture of the sensors to be embedded so as to meet the target objectives. The department has cleanrooms with several darkrooms equipped with optical benches where power and wavelengths can be controlled to test the sensor and verify its measurements. The computer files are then sent to the factory that has been carefully selected for production.

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Design phase of optical image sensors

Research imagining the future

There are still many fields to be explored. Research is also focused on improving sensor sensitivity in the near infrared spectrum for applications in Earth observations. The group is also working on polarizing imagers, an application that is useful for infantry, for example. Soldiers wear helmets equipped with cameras filming in the visible spectrum and possibly in infrared for night vision, with polarization to help identify hostile objects.

Space has always been a field of application for this research. In order to improve observation of the Earth, smart sensors with intra-pixel data processing functions will be embedded in satellites. We will have a sharper view of our world to better foresee our future.

Focus on a PhD

Alexandre Le Roch’s thesis in microelectronics in microelectronics is one of seven being prepared at the Department of Electronics, Optronics and Signal Processing (DEOS) at ISAE-SUPAERO. His work deals with the effects of space and nuclear radiation on CMOS image sensors in order to improve space instruments and plasma diagnostics for nuclear fusion. More specifically, he is carrying out research on silicon defects caused by the radiation responsible for an increase in dark current and its discrete fluctuations.

His research is being supervised by two doctoral advisors, Vincent Goiffon of ISAE-SUPAERO and Cédric Virmontois of the CNES (France’s National Centre for Space Studies). This thesis is being carried out in collaboration with the CNES and the CEA (France’s Alternative Energies and Atomic Energy Commission).

Did you know?

The image sensors in smartphones measure just a few square millimeters and a pixel just a few square microns (µm). The pixels in image sensors for scientific applications can measure up to several tens of µm square. These sensors, measuring several square centimeters can, for example, be embedded in the telescope focal unit on an observation satellite such as Spot Image. The mirror system focuses the image on the sensor to observe the Earth with high precision and very high resolution.

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