Evolving Towards The Perfect CCD


Note: This material is from a 1994(!) Talk about CCDs - the physics remains the same, only the detector size has increased since then.

Characteristics of an Ideal Detector:


Why Astronomers Love CCDs:

Early Limitations:

Most of these limitations have now been overcome and have lead to todays 2048x2048 CCD detector. Charge-coupled devices (CCDs) have been moving closer to becoming an ideal detector and are almost there now


HISTORY:


THEORY AND OPERATION:

CCD MANUFACTURING


QUANTUM EFFICIENCY

PERFORMANCE LIMITATIONS:

But for transfer rates of less than 100 kpixels/sec, theory doesn't matter and CTE is limited by four other factors that are all related to electron traps within the device:

  • Design Traps --> narrowing of signal channel which produces a potential barrier in which charge can be trapped. Usually happens at transfer gate between array and the horizontal register as transfer gates are tapered in order to properly transfer charge to the horizontal register. Hence a trap (potential barrier) is created at the front of the gate. This is most noticeable when low-level point sources are imaged. As the packet passes through the transfer gate, the charge is redistributed into a "tail" of trailing pixels. This is fatal for HST detectors. Problem is temporarily solved by a fat-zero which fills the trapping regions thus allowing signal charge to pass through unimpeded. But this increases the read noise of the detector. Note, about 50,000 HST CCDs were fabricated and tested before this problem was noticed and characterized. Now CCDS are designed so that the signal carrying channel is constricted at the end of beginning of the transfer gate as opposed to mid-phase.

  • Process Induced Traps --> Localized Defects which are usually randomly distributed. Okay if the defect is in a vertical column but is fatal if its in the horizontal register. This traditionally has been a main cause of low sensor yield.

  • Bulk Traps --> lattice defects or deep-level metallic impurities in the substrate. If these traps lie in the charge transfer channel then trapping occurs. Ultimately, CTE is bulk state limited. Bulk traps usually become active at low operating temperature when emission time constant of the trap is equal to the charge transfer time from one phase to the next. So, speed up this time either by operating at higher temperature or adjusting clockrates.

  • Radiation Induced Traps: Energetic particles displace silicon atoms from the lattice structure. These then act like bulk traps. This is mostly a problem for space based CCDs.

    Testing CTE --> this is kind of Cool

    READ NOISE:


    DARK CURRENT NOISE:

    Thermal generation of electrons by the finite temperature of the device.

    Surface dark current is 2--3 orders of magnitude higher than the bulk dark current. Surface dark current depends on:

  • After Two Decades of R&D, CCDs aren't quite the perfect detector but they are PFG (e.g. damn close)


    Standard Astronomer Disclaimer

    ___________________________________________________________________
    The Electronic Universe Project
    e-mail: nuts@moo.uoregon.edu