Physics and Engineering of Radiation Detection

7.3. Imaging Devices 453

D.1 PropertiesofaDirectImagingCCD

The performance of a CCD is characterized by the following properties.

Spatial Resolution:The spatial resolution of a direct imaging device is equal

to or better than the dimensions of its pixels. Hence, the smaller the pixel the

better the resolution.

Quantum Efficiency: The quantum efficiency of a CCD determines the ef-

ficiency ofuseful absorptionof incident particles in the depletion region. By

useful absorption we mean the absorption that leads to generation of at least

one electron-hole pair inside the depletion region. In a conventional direct

imaging device, the incident particles have to first pass through the readout

electrodes and the oxide layer before reaching the depletion region. These layers

can significantly attenuate the particle beam especially at lower energies, thus

decreasing the overall quantum efficiency of the detector. At higher energies,

however, the particles may pass through the depletion region without deposit-

ing any or very little of their energy, again resulting in a decreased quantum

efficiency. The adverse effect of absorption in front layers can be minimized

if the CCD is illuminated from the back side, though it has its own drawback

of parasitic absorption in the thick diffusion region. This absorption can be

minimized by making this region as thin as possible (see Fig.7.3.2). In such

a configuration the back-illumination technique is quite effective in increasing

the quantum efficiency at low energy end and is therefore commonly employed

in practical systems.

~20μm

Readout Electrodes Oxide Layer

−+ +−

Pixels Depletion Region

Bulk Semiconductor

Photons

Figure 7.3.2: Schematic of a back-illuminated CCD work-

ing in direct detection mode. The semiconductor (generally

silicon) layer is made as thin as 10μmto minimize the low

energy photon absorption in the undepleted region of the de-

tector. The acid etching technique that is generally used for

this purpose ensures uniformity of the layer.

A point to note here is that there are two different parameters that are some-

times mistakenly used in place of each other. The QE as defined here refers to

the ratio of the number of incident photons usefully absorbedNγ,absinside the

depletion region to the total number of incident photonsNγ,inc,thatis

QE=

Nγ,abs

Nγ,inc

. (7.3.1)