The x-ray imaging system comprises an x-ray source for producing an x-ray beam having an energy of at least 30 kVp and an x-ray detector. The x-ray detector comprises a solid state integrated circuit having a silicon substrate and a plurality of charge storage devices. The detector is responsive to x-rays of at least 30 keV to directly produce free electrons which interact with the charge storage devices.

X-ray imaging system and solid state detector therefor

The charge-coupled device has shown unprecedented performance as a photon detector in the areas of spectral response, charge transfer, and readout noise. Recent experience indicates, however, that the full potential for the CCD's charge-collection efficiency (CCE) lies well beyond that realized in currently available devices. In this paper we present a definition of CCE performance and introduce a standard test tool (the photon-transfer technique) for measuring and optimizing this important CCD parameter. We compare CCE characteristics for different types of CCDs, discuss the primary limitations in achieving high CCE performance, and outline the prospects for future improvement.

Charge-Coupled-Device Charge-Collection Efficiency And The Photon-Transfer Technique

For most thinned silicon CCDs, the photosensitive volume is bounded on top and bottom by layers of silicon dioxide. The frontside oxide is grown to serve as an insulator beneath the conductive gates of the parallel array while the backside oxide forms naturally as the initially bare silicon oxidizes. This paper describes the characteristics of the interface between these oxides and the photo-sensitive silicon and indicates the extent to which CCD performance (e.g. dark current, spectral response, charge collection efficiency, charge transfer efficiency, pixel-nonuniformity read noise full well capacity blooming residual image and vulnerability to ionizing radiation damage) depend* upon these interfacial characteristics. Techniques are described to achieve optimum passivation of these interfaces and to thereby obtain superior performance in the areas just listed. Specifically an implanted structure (the Multi-Pinned-Phase, MPP) is described which provides excellent frontside passivation and several techniques (backside charging, flash gate, the biased flash gate and ion-implantation) are presented for back surface passivation.

Charge-Coupled Device Pinning Technologies

Recent developments of scientific CCDs have produced sensors that achieve ultra low read noise performance (less than 2 electrons rms) and near perfect charge transfer efficiency (09999996) without the addition of a fat-zero This progress has now made it possible to achieve Fano-noise-limited performance in the soft X-ray where the detector's energy resolution is primarily limited by the statistical variation in the charge generated by the interacting X-ray photon In this paper, Fano-noise-limited test data is presented for two different CCD types and a CCD derived estimate of the Fano factor is determined By evaluating ultra low-modulation images (less than 1 electron peak-to-peak) it is shown that the CCD's global CTE is now superior to its read noise floor To capitalize on this capability CCD manufacturers are now focusing their attention on reducing the noise floor below the 1 electron level thereby matching the sensor's CTE performance This improvement, if accomplished, will push Fano-noise-limited performance for the CCD into the extreme ultra-violet

Fano-noise-limited CCDs

The x-ray imaging system comprises an x-ray source for producing an x-ray beam and an x-ray detector. The x-ray detector comprises a solid state integrated circuit having a silicon substrate. The detector is constructed as a PiN diode with the voltage bias direction parallel to the direction of incident radiation. A tissue sample, such as a breast, is positioned between the x-ray source and x-ray detector. The x-ray detector directly receives the x-ray photons transmitted through the tissue and produces electrical signals. The detector signals are processed to provide a digital image of the tissue sample.

Low dose mammography system

Preliminary findings are presented for a new approach that significantly improves the quantum efficiency of the current generation of high-performance, thinned, backside illuminated silicon CCDs. Experiments have shown that the application of a less than 4-micron thick layer of metal with high work function to the backside of the CCD can yield 100-percent internal quantum efficiency in the visible, UV, XUV and soft X-ray regions of the spectrum. Theory and solid state models describing the new technique (the 'CCD flash gate'), and a considerable amount of experimental data, are discussed. Specific recommendations for use of the flash gate in present and future CCDs are also reviewed.

Torn Elliott

Papers

The CCD flash gate