The EBCCD achieves nearly 'ideal', noiseless gain through the electron bombarded semiconductor (EBS) cascade process. Electron gain in an EBCCD sensor results when a high energy primary electron dissipates its energy in the silicon of the CCD. Every 3.6 eV of energy lost by the primary electron generates approximately one electron-hole pair. Diffusion in the silicon separates the electron-hole pairs. The substrate connection collects the holes. The potential wells formed by the applied gate voltages collect the electrons, which constitute the amplified signal charge. To the first order, the signal gain in the EBCCD is proportional to the kinetic energy of the photoelectrons prior to their impinging on the CCD back surface. This mechanism provides a convenient mean of controlling the overall gain of the tube by varying the acceleration potential. Figure 4 displays an energy loss profile for the back-illuminated CCD [1].

In order to maximize the EBS gain, the 'active' material must dissipate the energy from the incident electron and the CCD pixel must efficiently collect the electrons. To obtain EBS gain in the active material, it is necessary for the photoelectron to enter the pixel from the back surface of the CCD -- away from the gate structures that dissipate the electron energy. The back surface is typically thinned to a thickness of 10 to 15 microns to optimize signal electron collection efficiency. Because the back surface contains only a thin layer of epitaxial silicon and no device structures, the incident electron is able to enter the active material with sufficient energy to allow high EBS gain.

Critical to the performance and stability of the device is the recombination phenomena at the back surface. Proper back surface passivation (accumulation) is required to increase the collection efficiency and to prevent 'surface trapping'. Figure 4 illustrates that below 2 keV a majority of the incident electron energy is dissipated within a tenth of a micron from the back surface. To prevent recombination from interfering with the gain process, it is critical that the surface be properly accumulated.

At acceleration voltages sufficient to overcome the surface 'dead-layer', the back surface region where a majority of electron-hole pairs recombine, the following equation approximates the EBS gain:

where: V_acc is the accelerating voltage applied to the tube between the photocathode and the CCD; V_dl is the voltage equivalent of the loss due to electron-hole pair recombination in the CCD back surface dead layer; and b is the proportion of back scattered electron energy. The average energy required to create an electron-hole pair in silicon is about 3.6 eV. In practice, 6 kV accelerating voltages obtain gain greater than one thousand. For accelerating energies less than about 3 keV, the dead voltage model breaks down and EBS gain is no longer linear. In fact, non-zero gains are measurable at virtually all accelerating voltage potentials [1].

The EBS multiplication process exhibits low fluctuations. The gain variance, s², is expressed as:

where F, the Fano factor, is 0.12 for silicon [2]. The low noise EBS process allows the EBCCD to obtain much larger STN than a standard Gen-III image intensified CCD. To describe the degradation in signal-to-noise attributable to the gain process, one uses the 'noise figure' coefficient, NFC. The ratio of the input signal STN to the image tube's output STN defines the NFC and quantifies the system signal-to-noise degradation due to the sensor. A perfect sensor has a 1.0 noise figure and introduces no additional noise to the input signal. The EBCCD noise factor is expressed as:

As silicon back scatters approximately 16% of the incident primary electrons, a noise figure of 1.09 is attainable. In comparison, a noise figure of 2.0 or greater is typical for a standard Gen-III image intensifier [3].

The Gen-III filmed MCP electron multiplication noise dominates the noise figure of a Gen-III image intensifier tube. By not using an MCP, the EBCCD takes advantage of the high quantum efficiency of the GaAs photocathode without suffering the degradation in STN due to the MCP. The EBCCD tube has almost double the STN performance of a standard tube. In fact, for EBS gains greater than 10, the noise figure will be almost entirely determined by b, the electron back scatter.

Because the first stage gain inversely reduces the noise factor contribution of subsequent stage, the first amplification stage typically dictates its noise factor. Low noise CCD amplifiers exist with noise performance equivalent to as little as 35 electrons per pixel at RS-170 bandwidths. A first stage gain of 200 will sufficiently eliminate further signal degradation by noise. An EBCCD with 35 electron readout noise and a 200 EBS gain is capable of single photon sensitivity.

Figure 5 shows the EBS gain curve measured on a SITe model SI502AB back-illuminated CCDs using a Hitachi S4000 scanning electron microscope as an electron source. To optimize the back surface accumulation process, CCDs manufactured with two different passivations were tested. To calculate EBS gain, the beam currents on a Faraday cup were compared to the currents measured in the back-illuminated CCDs. Due to the process dependent effects of electron-hole pair recombination at the back surface, one passivation process shown in Figure 5 has significantly higher gain characteristics than the other. The better process approaches theoretical gain performance.

The EBCCD gain performance was verified in an operational mode with two EBCCD sensors fabricated using SI502AB back-illuminated CCDs and GaAs photocathodes. The CCDs' thinned back surfaces were accumulated using the process shown in Figure 5 to result in higher gain characteristics. The EBCCD EBS gain was tested using two methods. First, the ratio of the average signal in the electron bombarded CCD pixel to a calibrated input light signal was used to compute the EBS gain. Second, the EBS gain was calculated from the ratio of the variance to the mean of the signal in the CCD. The two methods used to calculated the EBCCD EBS gain are consistent, and demonstrate agreement with the measurements made using the electron beam of the SEM. Plotted in Figure 6 is the experimental device gain measured at various acceleration voltages.