Charge-coupled device (CCD) imaging at low light levels

We present the analysis, design, fabrication and evaluation of low light level, anti-blooming, charge-coupled device (CCD) image sensors in line and area arrays. The transfer of low light level charge is determined by the non-linear electric field beneath the gate electrode. The CCD image sensor array is analyzed with a figure-of-merit which is the product of signal-to-noise (S/N) and modulation transfer function (MFT). The noise equivalent signal (NES), S/N = 1, is formulated at the system output and referenced to the readout diode. The CCD arrays use a 2-phase, stepped-oxide, clock system with interline transfer. Front surface imaging is achieved with thin silicon-gate electrodes (1000-2000 Å) which also serve as a "field shield" and the bottom layer of the lower phase line. The lowerphase line is constructed with an aluminum-silicon sandwich to provide low charging time constants (< 5 nsec) and reduce the incidence of phase line-to-line substrate shorts over large areas. The upper phase line is aluminum and a 3rd level aluminum forms a light shield over the interline shift-registers and CMOS readout circuit. The latter in combination with an analog signal processor provides a low-noise, composite video output signal. A unique cell design is used to combine active CCD sensor, interline shift-register, transfer gate and "stopper" diffusions into compact 2 mil centers in the area array. 1 × 128 element line arrays and 75 × 100 element area arrays are discussed in terms of individual elements NES, responsivity and spectral response.