Recent Developments in High-End CMOS Image Sensors

1 Recent Developments IN High-End cmos Image SENSORS3rd Round Table on Micro/Nano-Technologies for SpaceW. Ogiers, J. Bogaerts*, D. Uwaerts, J. Seijnaeve, D. Scheffer, B. Dierickx, S. Habinc**FillFactory , Kapeldreef 60, B-3001 Leuven, * IMEC, Kapeldreef 75, B-3001 Leuven, **ESA/ESTEC, Keplerlaan 1, NL-2200 AG Noordwijk, NL, imagers are generally tauted as being particularlysuited to the harsh space environment, if only they couldget their performance up to CCD levels. This paperhighlights present-day High-End cmos APS Sensors fromthe commercial and industrial world. In addition, thecurrent state of radiation tolerance is : cmos APS Image Sensors , high -speedimaging, ultra- high -resolution, radiation hardness, startrackers, visual INTRODUCTIONAt the 2nd Round Table on Micro/Nano-Technologies forSpace in 1997 it was suggested that cmos Image Sensors ,while not quite as performant as CCDs, were amenable touse in several low to medium end space imagingapplications, typically applications that could cash-in onCMOS's inherent qualities/potential of system-on-a-chip-integration and simpler and/or lower power requirements(quoting from ).

2 Low-grade earth and planetary imaging lander and rover near imaging robotics (requires high frame rates and windows forthe simultaneous tracking of several objects) visual telemetry spacecraft optical guidance and navigation (requireslow noise, high sensitivity, and high readout rates).At this time we have to exclude high -quality scientificimaging from the potential applications: cmos opticalsensors currently lack the required performance in noise,non-uniformity and dark signal levels and hence can notcompete with High-End high -cost CCDs. The next most-demanding application would be optical Developments indicate that this was a pessimisticview. Today cmos imaging performs better thanexpected, and gets into domains were CCDs are not instance the first-generation IMEC APS sensordeveloped in the ESA contract Attitude sensor Conceptsfor Small Satellites (ASCoSS, ) led to a miniaturestar tracker breadboard with an accuracy and a NoiseEquivalent Angle both better than 1 arc minute (2 ) per5mv star in a 20 x20 field of view, at an update rate of10Hz.

3 The sensor performed with the same signal to noiseratio, for a given optics aperture, as a CCD. In addition,the sensor proved radiation-tolerant at the dose ratesencountered during typical missions ( ), and thiswithout any rad-hardening design measures. IMEC and itsimager spin-off company FillFactory are now cooperatingin the development of the next-generation cmos sensorsfor star and sun 1: ASCoSS APS-based star tracker breadboardIn the field of spacecraft visual telemetry, experience hasbeen amassed on two flights, with two more flightsscheduled for the near future: FUGA15 Sensors weredemonstrated on Ariane V testflight A-502. These arecontinuous-readout APS imagers, with a logarithmicallycompressing opto-electric transfer which allows for a hugeinput dynamic range.

4 As a drawback, this sensorarchitecture has high noise, and even more importantly, ahigh static non-uniformity that has to be removed by look-up-table correction even before attempting imageprocessing and data CCDs and APSs imagers (ideally) have low noise,low non-uniformity, and a linear transfer function. Such asensor was tried in-flight too, alongside the trustyFUGA15, on the XMM mission. The sensor , namedIRIS1, was developed at the end of 1997, as part of anESA contract. Two lessons learned were, one, that it isfairly hard to predict illumination conditions in order to setthe correct exposure time on a linear-transfer camera, andtwo, that the contrast in a typical space telemetry scene( black sky, sun, earth, dark spacecraft, reflections offmetallic parts.)

5 Demands for more than the 60-70dBdynamic range allowed by CCDs or APSs. Figure 2: XMM mission visual telemetry, left IRIS1picture, right FUGA15 picture. Observe limited contrast,exposure problems, and lower noise with high -DYNAMIC RANGE IMAGINGA method has been devised to allow a linear-transfer APSto accept an intra-scene contrast normally exceeding itsdynamic range, and this in one single Image exposure, reverting to combining two or more separateimages each made with a different exposure obtain this the pixel has been modified to collectphotocharges at its highest sensitivity up to a certainthreshold, after which the pixel's transfer reverts to thesensitivity commanded by the imager's electronic yields a two-piece transfer curve with a compressingcharacter.

6 The knee in the curve can be set to a fixedposition, or alternatively it can be controlled moredynamically with an external 3: integrating APS transfer curves for longexposure time, short exposure time, and dual-slopecombination of A 3k x 2k high -RESOLUTION SENSORIn 1999 an APS imager was realised targeted at true-negative size 35mm digital studio-quality photography(Creo-Scitex Leaf C-MOSTTM camera). This sensor notonly has a for cmos unprecedented resolution of 3000by 2000 pixels, it also had to overcome the 19 mm x19mm reticle size limit imposed by the silicon latter problem was side-stepped by using the stitchingtechnology offered by the fab, Tower Semiconductor ofIsrael. Stitching, a technique inherited from the CCD-world, allows a chip to be built of several mask sets thateach individually undergo their own exposure andprocessing steps.

7 This way a chip larger than the reticlecan be built from modules, each smaller than the obtain a total die size of over 38 x 27 mm, six moduleswere used, including one 1k x 1k pixel array block, andupper/lower and side periphery pixel design noise and saturation levels wereoptimised for the dynamic range demanded by theparticular high -resolution APSP hotosensitive area24x36mm (35mm filmnegative format)Number of pixels3150 x 2100 Pixel m (.5 m process)Number of analogueoutputs4 Sensitivity> 9 V/e-Fill factor55%Electronic shutterRolling curtainNoise level< 40e-Saturation level> 140000e-Dynamic range> 70dBDouble SamplingOn-chipNon-uniformity< 1%Readout speed250 msColourOptionalPower< 250mWFigure 4: 35mm cmos APS with safeguard the sensor 's noise performance no logic orADC has been included on the chip: external componentsare to be used.

8 From a systems point od view this is stillacceptable, as quality cameras tend not to be of minimaldimensions anyway. The power requirements are astandard 5 Vdc. Operation with elevated supply voltage ispossible too, in which case the sensor 's dynamic A 1000pps high -SPEED IMAGER ANDCAMERAHigh-speed imaging traditionally has been the domain ofphotochemical pelicule film cameras, a technology thatcan attain frame rates in the tens of thousands per second,at the expense of bulky equipment and even more bulkyreels of film. The application of standard CCDs in thisfield is hindered by an imperfect charge transfer, leadingto ghost images, especially in over-illuminated parts. Thebest high speed CCDs today are limited to 1000frames/sec at a resolution of 512 by 384 pixels ( KodakEKTAPRO series).

9 The very first generation of dedicatedhigh-speed cmos APS already exceeds CCDperformance: the Photobit PB-1024 is a sensor with 1k x1k pixels at 500 images/second, while the devicepresented here performs at 512 x 512 pixels and 1000frames/second, with its technology still enabling aconsiderable growth in sensor format and latter has been designed by FillFactory for UScompany Vision Research, which commercialises it intheir Phantom camera range. The sensor uses a six-transistor active pixel that avoids ghost imaging by meansof its immediate conversion of photocharge to pixel also implements a true synchronous shutter ( shot shutter) that allows exposure times for the wholearray to be instantaneous and as low as 10 CCDs implement synchronous shuttering by theinterline architecture or the frame transfer architecture,this APS device has an analogue memory element locatedin each pixel.

10 Sensitivity does not suffer thanks toFillFactory's patented near-100%-fillfactor technologywhich was pioneered in the Ibis-1 device in 1997 ( ).The pixel's sensitivity, noise, and saturation levels wereoptimised for the highest possible sensitivity, to allow forthe ultra-short exposure times demanded by high -speed APSP hotosensitive area8x8mmNumber of pixels512 x 512 Pixel size16 m (.5 m process)Number of analogueoutputs16 Sensitivity25 V/e-Fill factor50%Electronic shutterSynchronous, true snapshotDynamic range> 58dBDouble Sampling-Readout speed (nominal)1000 frames/secondReadout speed (windowed)32000 frames/secondColourOptionalPower600 mWFigure 5: 2000 frames/s tennis ball sequence filmed withPhantom camera (reduced to dithered black&whitefor reproduction)Vision Research Phantom cameraFrame storageUp to 512 megabyteInterfacingFirewire, sync, videoDimensions8x10x25 cmMass2250gBuildRugged, high -gPower28 Vdc/1 ATypical ground-based applications are crash-tests andsports motion analysis.