Inter pixel capacitance (IPC) is a deterministic electronic coupling resulting in a portion of signal incident on one pixel of a hybridized detector array being measured in adjacent pixels. Data collected by light sensitive HgCdTe arrays which exhibit this coupling typically goes uncorrected or is corrected by treating the coupling as a fixed point spread function. Evidence suggests that this coupling is not uniform across signal and background levels. Sub-arrays of pixels using design parameters based upon HgCdTe indium hybridized arrays akin to those contained in the James Webb Space Telescope's NIRcam have been modeled from first principles using Lumerical DEVICE software. This software simultaneously solves Poisson's Equation and the Drift Diffusion Equations yielding charge distributions and electric fields. Modeling of this sort generates the local point spread function across a range of detector parameters. This results in predictive characterization of IPC across scene and device parameters that would permit proper photometric correction and signal restoration to the data. Additionally, the ability to visualize potential distributions and couplings as generated by the models yields insight that can be used to minimize IPC coupling in the design of future detectors.

No abstract

No abstract

Interpixel capacitance (IPC) is a deterministic electronic coupling that results in a portion of the collected signal incident on one pixel of a hybridized detector array being measured in adjacent pixels. Data collected by light sensitive HgCdTe arrays which exhibit this coupling typically goes uncorrected or is corrected by treating the coupling as a fixed point spread function. Evidence suggests that this IPC coupling is not uniform across different signal and background levels. This variation invalidates assumptions that are key in decoupling techniques such as Wiener Filtering or application of the Lucy-Richardson algorithm. Additionally, the variable IPC results in the point spread function (PSF) depending upon a star's signal level relative to the background level, among other parameters. With an IPC ranging from 0.68% to 1.45% over the full well depth of a sensor, as is a reasonable range for the H2RG arrays, the FWHM of the JWSTs NIRCam 405N band is degraded from 2.080 pix (0.132") as expected from the diffraction pattern to 2.186 pix (0.142") when the star is just breaching the sensitivity limit of the system. For example, When attempting to use a fixed PSF fitting (e.g. assuming the PSF observed from a bright star in the field) to untangle two sources with a flux ratio of 4:1 and a center to center distance of 3 pixels, flux estimation can be off by upwards of 1.5% with a separation error of 50 millipixels. To deal with this issue an iterative non-stationary method for deconvolution is here proposed, implemented, and evaluated that can account for the signal dependent nature of IPC.

Future high-resolution imaging x-ray observatories may require detectors with both fine spatial resolution and high quantum efficiency at relatively high x-ray energies (E ≥ 5 keV). A silicon imaging detector meeting these requirements will have a ratio of detector thickness to pixel size of six or more, roughly twice that of legacy imaging sensors. The larger aspect ratio of such a sensor's detection volume implies greater diffusion of x-ray-produced charge packets. We investigate consequences of this fact for sensor performance, reporting charge diffusion measurements in a fully depleted back-illuminated CCD with a thickness of 50 μm and pixel size of 8 μm. We are able to measure the size distributions of charge packets produced by 5.9 and 1.25 keV x-rays in this device. We find that individual charge packets exhibit a Gaussian spatial distribution and determine the frequency distribution of event widths for a range of detector bias (and thus internal electric field strength) levels. At the largest bias, we find a standard deviation for the largest charge packets (produced by x-ray interactions closest to the entrance surface of the device) of 3.9 μm. We show that the shape of the event width distribution provides a clear indicator of full depletion and use a previously developed technique to infer the relationship between event width and interaction depth. We compare measured width distributions to simulations. Although we can obtain good agreement for a given detector bias, with our current simulation, we are unable to fit the data for the full range of bias levels with a single set of simulation parameters. We compare traditional, "sum-above-threshold" algorithms for individual event amplitude determination to Gaussian fitting of individual events and find that better spectroscopic performance is obtained with the former for 5.9 keV events, whereas the two methods provide comparable results at 1.25 keV. The reasons for this difference are discussed. We point out the importance of read-noise driven charge detection thresholds in degrading spectral resolution, and note that the derived read noise requirements for mission concepts such as AXIS and Lynx are probably too lax to assure that spectral resolution requirements can be met. While the measurements reported here were made with a CCD, we note that they have implications for the performance of high aspect-ratio silicon active pixel sensors as well.