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The first CMOS approach was to carry the TDI functionality using digital summation. This approach quickly demonstrated limitations in terms of line rate and power consumption as the entire sensor has to be read for every line on the ground that is sampled. More recently CMOS technology has matured the charge domain CCD approach with comparable electro-optical performance to CCDs while offering higher speed, smaller pixel pitch and high level of integration.
This latest technology step has also considerably eased the integration of the sensor into the satellite, opening new opportunities to produce focal planes at significantly lower cost with much reduced power dissipation, size and weight. The challenge has been to establish a CCD on CMOS technology that can obtain a similar full well capacity and charge transfer efficiency (CTE) performance to CCDs. This CCD on CMOS technology has now reached the point where the performance is comparable to CCDs but with very much lower operating voltages.
This paper will present the evolution of earth scanning image sensors with a focus on the latest TDI CMOS technology including the recent results obtained with the latest CMOS technology using TDI in charge domain approach. These results will include FWC, CTE, radiation performance as well as results from very high speed, up to 3.6Gbps output stream, and highly integrated readout circuitry.
Finally we will provide details of new devices that will provide performance that would not have been possible with CCDs.
In this paper we describe our innovative systems approach to the design of the CCD cameras for two of SDO’s remote sensing instruments, the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI). Both instruments share use of a custom-designed 16 million pixel science-grade CCD and common camera readout electronics. A prime requirement was for the CCD to operate with significantly lower drive voltages than before, motivated by our wish to simplify the design of the camera readout electronics. Here, the challenge lies in the design of circuitry to drive the CCD's highly capacitive electrodes and to digitize its analogue video output signal with low noise and to high precision. The challenge is greatly exacerbated when forced to work with only fully space-qualified, radiation-tolerant components. We describe our systems approach to the design of the AIA and HMI CCD and camera electronics, and the engineering solutions that enabled us to comply with both mission and instrument science requirements.
There are two main advantages with CMOS sensors: First a hyperspectral image consists of spectral lines with a large difference in intensity; in a frame transfer CCD the faint spectral lines have to be transferred through the part of the imager illuminated by intense lines. This can lead to cross-talk and whilst this problem can be reduced by the use of split frame transfer and faster line rates CMOS sensors do not require a frame transfer and hence inherently will not suffer from this problem. Second, with a CMOS sensor the intense spectral lines can be read multiple times within a frame to give a significant increase in dynamic range.
We will describe the design, and initial test of a CMOS sensor for use in hyperspectral applications. This device has been designed to give as high a dynamic range as possible with minimum cross-talk. The sensor has been manufactured on high resistivity epitaxial silicon wafers and is be back-thinned and left relatively thick in order to obtain the maximum quantum efficiency across the entire spectral range
Two generations of the CMOS Imager are planned: a) a smaller ‘pioneering’ device of ⪆ 800x800 pixels capable of meeting first light needs of the E-ELT. The NGSD, the topic of this paper, is the first iteration of this device; b) the larger full sized device called LGSD. The NGSD has come out of production, it has been thinned to 12μm, backside processed and packaged in a custom 370pin Ceramic PGA (Pin Grid Array). Results of comprehensive tests performed both at e2v and ESO are presented that validate the choice of CMOS Imager as the correct technology for the E-ELT Large Visible WFS Detector. These results along with plans for a second iteration to improve two issues of hot pixels and cross-talk are presented.
New CCD designs and technology trends for hyperspectral applications such as Sentinel 4, Sentinel 5, Sentinel 5 precursor (TropOMI), Flex and 3MI are described. In these the sensor design has been optimized to provide highest possible signal levels with lowest possible noise in combination with higher frame rates and reduced image smear.
CMOS sensors for MTG (Meteosat Third Generation) and METImage are then described. Both use extremely large pixels, up to 250μm square, at high line rates. Radiation test data and key performance measurements are shown for MTG and for a test device that has been made for METImage. Finally, newer developments including back-illumination and means for achieving a TDI function in standard-processed CMOS are briefly described.
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