Charge Transfer Efficiency Measurements Research Articles

Toward fast, low-noise charge-coupled devices for Lynx

Lynx requires large-format x-ray imaging detectors with performance at least as good as the best current-generation devices but with much higher readout rates. We are investigating an advanced charge-coupled device (CCD) detector architecture under development at MIT Lincoln Laboratory for use in the Lynx high-definition x-ray imager and x-ray grating spectrometer instruments. This architecture features a CMOS-compatible detector integrated with parallel CMOS signal processing chains. Fast, low-noise amplifiers and highly parallel signal processing provide the high frame rates required. CMOS-compatibility of the CCD enables low-power charge transfer and signal processing. We report on the performance of CMOS-compatible test CCDs read at pixel rates up to 5.0 Mpix s − 1 (50 times faster than Chandra ACIS CCDs), with transfer clock swings as low as 1.0-V peak-to-peak (power/gate-area comparable to ACIS CCDs at 100 times the parallel transfer rate). We measure read noise of 4.6 electrons RMS at 2.5 MHz and x-ray spectral resolution better than 150-eV full-width at half maximum at 5.9 keV for single-pixel events. We report charge transfer efficiency measurements and demonstrate that buried channel trough implants as narrow as 0.8 μm are effective in improving charge transfer performance. We find that the charge transfer efficiency of these devices drops significantly as detector temperature is reduced from ∼ − 30 ° C to −60 ° C. We point out the potential of previously demonstrated curved-detector fabrication technology for simplifying the design of the Lynx high-definition imager. We discuss the expected detector radiation tolerance at these relatively high transfer rates. Finally, we note that the high pixel “aspect ratio” (depletion depth: pixel size ≈9 ∶ 1) of our test devices is similar to that expected for Lynx detectors and discuss implications of this geometry for x-ray performance and noise requirements.

Measurements of charge transfer efficiency in a proton-irradiated swept charge device

Charged Coupled Devices (CCDs) have been successfully used in several low energy X-ray astronomical satellites over the past two decades. Their high energy resolution and high spatial resolution make them a perfect tool for low energy astronomy, such as observing the formation of galaxy clusters and the environment around black holes. The Low Energy X-ray Telescope (LE) group is developing a Swept Charge Device (SCD) for the Hard X-ray Modulation Telescope (HXMT) satellite. A SCD is a special low energy X-ray CCD, which can be read out a thousand times faster than traditional CCDs, simultaneously keeping excellent energy resolution. A test method for measuring the charge transfer efficiency (CTE) of a prototype SCD has been set up. Studies of the charge transfer inefficiency (CTI) with a proton-irradiated SCD have been performed at a range of operating temperatures. The SCD is irradiated by 3 × 108cm−2 10 MeV protons.

Studies on the measurement of charge transfer efficiency and photoresponse nonuniformity of linear charge-coupled devices

An accurate method for measuring charge transfer efficiency (η) and photoresponse nonuniformity (PRNU) of linear charge-coupled devices is developed. The apparatus is described. A finely focused light spot is adopted to irradiate each photosensitive element to get its response output signal. Data are collected automatically by scanning peak-searching and processed by linear best fitting. As examples, measurements for some types of devices are reported and factors influencing the results are discussed.

Interface state trapping and dark current generation in buried-channel charge-coupled devices

The effects of interface states on charge transfer efficiency (CTE) are compared in surface-channel and buried-channel charge-coupled devices (CCDs). Although the buried-channel CCD was developed partly to exclude the interaction between signal charge and interface states, this interaction will occur when the amount of signal charge exceeds the buried-channel storage capacity. For this situation, it is shown that CTE measurements may be used to directly and accurately determine the density of interface states in 4-phase buried-channel CCDs, using the same techniques previously developed for surface-channel devices. These measurements have been applied to several different N-buried-channel CCDs, some of which have been fabricated with radiation-hardened oxides, which have been exposed to ionizing radiation. Data from these devices reveal a linear dependence of the interface state electron-hole pair generation rate (i.e., CCD dark current) on the density of interface states. This dependence is the same for all devices studied and does not depend on whether the interface states are intrinsic, created by radiation, or reduced by annealing. From these data, a value for the geometric mean of the interface state electron and hole capture cross sections √(σn σp ) of 5.5×10−16 cm2 is obtained, in excellent agreement with values in the literature obtained by ac conductance and gate-controlled diode measurements.

Charge transfer efficiency in a buried-channel charge-coupled device at very low signal levels

A measurement of charge transfer efficiency (CTE) is described for a 256-element charge-coupled linear imaging device operated below room temperature and at a very low total charge level per charge packet, that is, at a level of approximately 1/20000th of saturation. This measurement was carried out at a register frequency near 15.75 kHz, the standard television-line frequency. CTE was measured by noting the dependence of the size of the principal charge packet upon the number of transfers. It was observed that at a dark current level of 4 electrons per packet and a photosignal level of 15 electrons the signal loss through approximately 250 transfers was 1/spl plusmn/5 electrons at the 50 percent confidence level. A signal-averaging technique was used to obtain this small probable error in the measurement.

Charge-transfer efficiency in a buried-channel charge-coupled device at very low signal levels

A measurement of charge-transfer efficiency (CTE) is described for a 256-element charge-coupled linear imaging device operated below room temperature and at a very low total charge level per charge packet, that is, at a level of approximately 1/20 000th of saturation. This measurement was carried out at a register frequency near 15.75 kHz, the standard television-line frequency. CTE was measured by noting the dependence of the size of the principal charge packet upon the number of transfers. It was observed that at a dark current level of 4 electrons per packet and a photosignal level of 15 electrons the signal loss through approximately 250 transfers was 1 ± 5 electrons at the 50 percent confidence level. A signal-averaging technique was used to obtain this small probable error in the measurement. An analysis has been performed which assumes bulk trapping as the limiting mechanism and from which it may be concluded that to the same probable error the bulk-trap concentration in the CCD channel region is less than 4 × 1012cm-3for trap energies in the range between 0.23 and 0.32 eV below the conduction band. This result demonstrates the possibility of developing a 500 × 500 element charge-coupled imaging device which would have satisfactory CTE for signal levels as low as approximately 10 electrons per picture element.

Effect of Scattering on Charge Transfer Efficiency Measurements: Reply to Peterson's Letter

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Effect of Scattering on Charge Transfer Efficiency Measurements: Comment on a Paper by Mahadevan et al.

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