James R. Janesick

Radiation damage in scientific charge-coupled devices

Two important classes of radiation damage to the scientific CCD are discussed, namely bulk and ionization effects. Bulk damage or displacement damage is a process in which silicon atoms are displaced from their normal lattice positions by high energy photons or particles. Single atomic displacements or cluster defect damage in silicon is produced depending on the energy and type of radiation experienced by the detector. Bulk damage creates trapping sites within the CCDs signal channel which in turn degrades CTE performance. Ionization-induced damage induces a buildup of charge in the CCDs gate insulator causing the sensors drive operating windows to shift (i.e. flat-band shift). In addition, ionization damage creates unwanted electronic sites at the gates interface causing the CCDs dark current to increase. >

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

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 CCDs 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.

CCD Charge Collection Efficiency And The Photon Transfer Technique

The charge-coupled device (CCD) has shown unprecendented 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 CCDs charge collection efficiency (CCE) lies well beyond that which is 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 COD parameter. We compare CCE characteristics for different types of CODs, discuss the primary limitations in achieving high CCE performance, and outline the prospects for future improvement.

Charge-Coupled Device Pinning Technologies

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.

Fano-Noise-Limited CCDs

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 (0.9999996) 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 detectors 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 CCDs 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 sensors CTE performance. This improvement, if accomplished, will push Fano-noise-limited performance for the CCD into the extreme ultra-violet.

Evaluation of a virtual phase charged‐coupled device as an imaging x‐ray spectrometer

The x‐ray response of an 800×800 Texas Instruments virtual phase charge‐coupled device (CCD) has been measured in the range 1–8 keV. In the single‐photon counting mode, we find excellent energy resolution (∼250 eV FWHM) for single‐pixel Fe55 x‐ray events at a spatial resolution of 15 μm. The detector quantum efficiency for all events is 65% at 2.3 keV (S K line) and ∼34% at 5.9 keV (Mn K line from Fe55). The CCD response is linear in energy to a few percent over the 1–8 keV energy range. These results demonstrate that virtual phase CCDs are superior imaging x‐ray spectrometers with applications for x‐ray astronomy and laboratory plasma research.

Backside charging of the CCD

Until recently, the usefulness of the charge coupled device (CCD) as an imaging sensor was thought to be restricted to within rather narrow boundaries of the visible and near IR spectrum. However, since the discovery of backside charging the full potential of CCD performance is now realized. Indeed, the technique of backside charging not only allows the CCD to be used directly in the UV, EUV, and soft X-ray regimes, it has opened up new opportunities in optimizing charge collection processes as well. In this paper, we discuss in considerable detail the technique of backside charging, describing its properties, use, and potential in the future as it applies to the CCD.

The Future Scientific CCD

The charge-coupled device (CCD) dominates an ever-increasing variety of scientific imaging and spectroscopy applications. Recent experience indicates, however, that the full potential of CCD performance lies well beyond that which is realized in currently available devices. Test data suggest that major improvements are feasible in spectral response, charge collection, charge transfer, and readout noise. These properties, their measurement in existing CCDs, and their potential for future improvement are discussed in this paper.

Potential of CCDs for UV and x‐ray plasma diagnostics (invited)

A program is under way to develop charge-coupled device (CCD) sensors for space-based X-ray astronomy imaging spectrometers. To date, laboratory line-emission spectra have been acquired throughout the range of 277 to 8000 eV (carbon through copper K-alpha emission) and CCD sensitivity has been demonstrated throughout the range of 1.1 through 8000 eV. Image resolution is excellent, limited almost entirely by the 15 micron pixel size. These results are presented and specialized techniques are described which permit such low energy response, high spectral resolution, and efficient charge collection. Finally, analysis is presented of one particular CCD characteristic which currently limits UV and X-ray performance: charge diffusion.

Large Format, High Resolution Image Sensors

Tektronix is currently fabricating two very large area charge-coupled-device sensors intended for astronomical and other high performance scientific imaging applications, e.g., medical imaging, fluoroscopy, x-ray imaging, spectrometry, particle detectors, etc. In this paper we discuss the performance requirements for scientific-quality CCDs and then focus on the design of the two Tektronix devices and discuss the progress toward achieving the desired performance. These devices are intended for rear-illuminated applications and have 512 X512 and 2048 X2048 pixel formats. The thinned 10 to 20 pm thick Si membrane is fully supported by a unique glass ceramic substrate. Quantum efficiencies of >70% at 700 nm and >40% at wavelengths <400 nm have been measured on a test device. Dark currents as low as 6 pA/cm2 also have been measured recently.