CMOS CIDs promise imaging-sensor advances
Next-generation imagers must resolve the inherent charge-coupled-device (CCD) sensor weaknesses, introduce new capabilities, and improve overall performance. Indeed, nearly all the problems associated with CCDs stem from the need to efficiently transfer electrons through a semiconductor.
Seeking imaging-technology advances, Zoran Ninkov of the Center for Imaging Science at the Rochester Institute of Technology (RIT) and the Center for Electronic Imaging Systems (Rochester, NY) and his colleagues are developing alternate focal-plane architectures and devices that do not have charge-transfer-related problems. The experimental devices are using charge-injection-device (CID) techniques in which preamplifiers at the periphery of the array sense changes in charge within a pixel. Ninkov is improving this architecture by placing an active preamplifier within each pixel.
Dubbed the preamp per pixel (PPP) architecture, the prototype device is structured as an active pixel sensor that provides noise reduction, added functions, and improved capabilities over CCDs. The most recent devices provide true random addressing to any pixel or subarray, nondestructive readout of any pixel, and the ability to erase any subarray on the frame.
"Fabricating the PPP pixel was complex and impractical until device fabrication was transitioned to CMOS technology," says Ninkov. Such devices have been designed and fabricated and are now being tested at both a commercial facility and at the CMOS-fabrication facility in the RIT department of microelectronics (Rochester, NY). At RIT, the investigatory work was performed in collaboration with Lynn Fuller, Gerry Lubberts, and George Lungu.
According to Ninkov, PPP devices should find uses in a range of imaging applications, from radiation-hard spacecraft sensors for star tracking to autofocus camcorder systems. In addition, says Ninkov, "new high-performance digital photon-counting systems will need advanced CID arrays. Features such as subarray windowing allow different subarray regions of the sensor to be read out at different frame rates appropriate to the local brightness of the scene." Photon-counting-intensified CIDs are expected to offer advantages in applications such as astronomy, medical imaging, and remote sensing.