Front-illuminated Charge-coupled Device Research Articles

The Effect of the CCD Gate Structure in the Determination of the Undersampled Point Spread Function

Using numerical simulations, we evaluated the gate structure effect of the front-illuminated (FI) charge-coupled device (CCD) on determining the point spread function (PSF) from the analysis of undersampled experimental data. The PSF of the Soft X-ray Telescope onboard Yohkoh was studied as a model case. Its full-width at half-maximum is about one pixel size, and the FI CCD was equipped in the telescope as a detector. Of the three emission lines used during the pre-launch experiment, the data from the lowest-energy line (carbon K-line, 0.28 keV) were significantly influenced by the gate structure. The results from previous studies regarding the analysis of pre-launch data were examined and compared with the results from our simulation, and the expected error ranges of the gate structure effect are discussed. We found that the error caused by the effect of the gate structure is significant; at an energy of 0.28 keV, the error may lead to a too sharp PSF by about a few tens of percent.

Absolute determination of charge-coupled device quantum detection efficiency using Si K-edge x-ray absorption fine structure

We report a method to determine the quantum detection efficiency and the absorbing layers on a front-illuminated charge-coupled device (CCD). The CCD under study, as part of a crystal spectrometer, measures intense continuum x-ray emission from a picosecond laser-produced plasma and spectrally resolves the Si K-edge x-ray absorption fine structure due to the electrode structure of the device. The CCD response across the Si K-edge shows a large discontinuity as well as a number of oscillations that are identified individually and uniquely from Si, SiO(2), and Si(3)N(4) layers. From the spectral analysis of the structure and K-edge discontinuity, the active layer thickness and the different absorbing layers thickness can be determined precisely. A precise CCD detection model from 0.2 to 10 keV can be deduced from this highly sensitive technique.

Detailed analysis of events from high-energy X-ray photons impinging on a two-phase front-illuminated CCD

A study of the single-photon events generated by the interaction of X-rays up to 60keV with a true two-phase charge coupled device (CCD) is reported. In particular, a relevant classification of the events is carried out according to their size and collected charge. This classification shows the occurrence of two main groups, characterized by a quite large difference in the ADU values that has been observed between events having different sizes but coming from photons with the same energy. Based upon 2D numerical calculations accounting for the charge cloud dynamics, diffusion and recombination, an explanation is suggested for this difference, arising from the difference in the electric field strength in the point of initial interaction. Moreover, the relative abundance of these two groups was found to be energy dependent. A model accounting for the true two-phase pixel structure was found to be a valid tool for a correct prediction of this abundance and an enhanced reconstruction of the spectra of the impinging photons.

Relative detection efficiency of back- and front-illuminated charge-coupled device cameras for x-rays between 1keV and 18keV

High-resolution x-ray measurements were performed with a von Hamos-type bent crystal spectrometer using for the detection of the diffracted photons either a back-illuminated charge-coupled device (CCD) camera or a front-illuminated one. For each CCD the main x-ray emission lines (e.g., Kalpha, Kbeta, Lalpha, and Lbeta) of a variety of elements were measured in order to probe the performances of the two detectors between 1 and 18 keV. From the observed x-ray lines the linearity of the energy response, the noise level, the energy resolution, and the quantum efficiency ratio of the two CCDs were determined.

Anomalous annealing of a high-resistivity CCD irradiated at low temperature

The front-illuminated charge-coupled device (CCD) detectors in the Chandra X-ray Observatory's ACIS instrument suffered radiation damage from soft protons focused by the telescope mirror early in the mission. In the course of assessing this damage, the focal plane was temporarily warmed from its normal operating temperature (then -100/spl deg/C) to +30/spl deg/C. Following this "bakeout", the radiation-damaged CCDs exhibited significantly greater charge transfer inefficiency (CTI). We performed a laboratory experiment with a sibling of the flight detectors in an attempt to reproduce and better understand this phenomenon. The test CCD was cooled to -100/spl deg/C, irradiated by 120 keV protons and then warmed to +30/spl deg/C for 8 hours. As expected, after the initial irradiation, but before detector warmup, a substantial CTI increase was observed. The subsequent warmup itself then produced an additional factor /spl ap/2.5 increase in CTI. Following smaller subsequent irradiations with the detector cold, a "bakeout" for 8 hours at -60/spl deg/C produced no observable increase in CTI. However, a subsequent bakeout to +30/spl deg/C for another 8 hours resulted in an additional increase in CTI of roughly 15%. The CTI changes produced by the room temperature bakeout are accompanied by dramatic changes in the de-trapping times of electron traps responsible for the CTI. The distributions of signal amplitudes in the pixels trailing X-ray events indicate that annealing at room temperature can cause large changes of the trap emission times, from which we infer that conversion of trapping defects takes place. The observed phenomena can be explained by the previously suggested mechanism of carbon-related defect transformation. Specifically, the room-temperature annealing may allow carbon interstitials to form metastable complexes with phosphorus and/or carbon substitutional atoms.

Degradation Behavior and Damage Mechanisms of CCD Image Sensor With Deep-UV Laser Radiation

As the deep-ultraviolet (DUV) laser technology continues to mature, an increasing number of industrial applications is shifting to intense DUV radiation sources. This trend necessitates the development of DUV sensitive charge-coupled device (CCD) cameras to provide imaging capability for process control and inspection purposes. In this paper, we examine the effects of DUV laser radiation on CCD image sensor characteristics and the mechanisms responsible for DUV laser damage in CCDs. When samples of thinned front-illuminated linescan CCD sensors are exposed to F/sub 2/(/spl lambda/=157 nm) excimer laser radiation, fluctuation in the extrinsic quantum efficiency (QE) and a substantial upsurge in the dark current density are observed as a function of exposure dose. The visible QE, dark current, and charge conversion efficiency (CCE) are also permanently altered by the DUV irradiation. These instabilities can be attributed to a variety of UV-induced effects that modify the optical and electrical properties of the SiO/sub 2/ layer and Si-SiO/sub 2/ interface, resulting in temporary and permanent shifts in CCD performance. Optimization of the overlying oxide thickness and the Si-SiO/sub 2/ interface quality are necessary in order to realize CCD sensors with the desired performance, radiation tolerance and stability at DUV wavelengths.

Modeling of proton-induced CCD degradation in the Chandra X-ray Observatory

Modeling results are presented for proton-induced degradation of charge-coupled devices (CCDs) used in the Advanced CCD Imaging Spectrometer instrument on the Chandra X-Ray Observatory. A methodology is described that provides insights regarding degradation mechanisms and on-orbit performance for front-illuminated and back-illuminated CCDs. Proton-induced changes in charge transfer inefficiency are modeled. The observed amount of on-orbit degradation can be accounted for using a proton spectrum at the CCD location that has the same shape as the spectrum incident on the spacecraft but has a fluence that is reduced in magnitude by a factor of /spl sim/10/sup 5/.

Application of the Mesh Experiment for the Back-Illuminated Charge-Coupled Device: I. Experiment and the Charge Cloud Shape

We have employed a mesh experiment for back-illuminated (BI) charge-coupled devices (CCDs). BI CCDs possess the same structure as front-illuminated (FI) CCDs. Since X-ray photons enter from the back surface of the CCD, a primary charge cloud is formed far from the electrodes. The primary charge cloud expands through a diffusion process until it reaches the potential well that is just below the electrodes. Therefore, the diffusion time for the charge cloud produced is longer than that in the FI CCD, yielding a larger charge cloud size than expected. The mesh experiment enables us to specify the X-ray point of interaction with subpixel resolution. We measured a charge cloud shape produced in the BI CCD. We found that there are two components of the charge cloud shape having different sizes: a narrow component and a broad component. The size of the narrow component is 2.8–5.7 µm in units of standard deviation and strongly depends on the attenuation length of incident X-rays in Si. The shorter the attenuation length of the X-rays, the larger the charge cloud. This result is qualitatively consistent with a diffusion model inside the CCD. On the other hand, the size of the broad component is roughly constant at ≃13 µm and does not depend on X-ray energies. Judging from the design value of the CCD and the fraction of each component, we conclude that the narrow component has its origin in the depletion region whereas the broad component originates in the field-free region.

Influence of nonuniform charge-coupled device pixel response on aperture photometry

The extensive use of charge-coupled devices (CCDs) in astronomical imaging and spectroscopic applications has resulted in techniques being developed to correct for sensitivity variations attributable to the focal plane array detector. One normally overlooked effect is that these sensitivity variations also include intrapixel spatial effects that can influence measurements made with CCDs in certain circumstances. We demonstrate the influence of CCD pixel response nonuniformity on photometric measurements. An investigation is presented of how the relation between pixel size and the image size affects the photometric accuracy. Results are presented for a front-illuminated CCD array using an experimentally measured pixel sensitivity functions.

Measurement of the response characteristics around K absorption edges of dead-layer materials of a charge-coupled device

We measure various spectral response characteristics around the oxygen, silicon, and nitrogen K absorption edges of a front-illuminated Charge-Coupled Device (CCD) X-ray detector used in the X-ray Imaging Spectrometer (XIS) developed for the ASTRO-E mission. We have evaluated X-ray Adsorption Fine Structure (XAFS) around oxygen K edge in detail. A strong absorption peak of 45% is confirmed just above the oxygen K edge and an oscillatory structure follows whose amplitude decreases from 20% at the edge to less than 1% at 0.9 keV. We also show XAFS and discuss on a change of the response function around the silicon K edge. The discontinuity of the signal-pulse–height at the silicon K edge is less than 2.9 eV. We determine the thickness of silicon, silicon dioxide, and silicon nitride in the dead-layer using the depth of the absorption edge.

Observer performance comparison of digital radiograph systems for stereotactic breast needle biopsy

Two digital radiograph systems for stereotactic mammography, one using a lens to couple a Lanex Regular screen to a back-illuminated charge-coupled device (CCD) and one using a fiber-optic taper to couple a Min-R Regular-type screen to a front-illuminated CCD, were evaluated with respect to observer performance. A contrast-detail phantom was imaged in a variety of equivalent exposure conditions on both systems. Six observers viewed images on a video monitor and recorded which objects were detected. Performance (percent correct detections) with the lens-coupled system using the Lanex Regular screen was significantly higher than with the fiber-optic-coupled system using the Min-R Regular-type screen. Differences in absorption efficiencies of phosphors used, as well as differences in design of the two cameras, can explain differences in observer detection performance.

Transmittance of air/SiO/SUB 2//polysilicon/SiO/SUB 2//Si structures

Polycrystalline silicon films are presently used as the semitransparent gate electrodes in front-illuminated charge-coupled device (CCD) and charge-injection device (CID) image sensors. The transmittance of air/SiO/SUB 2//polysilicon/SiO/SUB 2//Si structures is calculated in spectral region between 0.4 and 1.0 /spl mu/. A proper choice of the thicknesses of the oxide films can substantially increase the transmittance over a narrow wavelength band or over the entire wavelength region of interest.