Frontside and backside illuminated CCDs

CCDs are manufactured as single large-scale integrated devices. They have a front-side, where all the gate structures and surface channel layers are deposited, and a back-side, which is simply bulk silicon generally covered with a thin conductive layer of gold. CCDs used as front-side illuminated devices work as the name implies, that is, illumination occurs on the front of the CCD with the photons being absorbed by the silicon after passing directly through the surface gate structures. The device thickness is of order 300 microns from front to back making these chips relatively high in their susceptibility to detection of cosmic rays. Because the photons must first pass through the gate structures before they can be absorbed by the silicon, front-side illuminated CCDs have lower overall quantum efficiencies than the back-side devices (discussed below). However, front-side devices provide a flatter imaging surface and the actual CCD itself is easier to handle and work with. Figure 2.4 provides a schematic view of a single front-side illuminated pixel.

Back-side illuminated devices, also known as thinned devices, are again justly named. The CCD, after manufacture, is physically thinned to >15 microns by various etching techniques (Lesser, 1994). The device is then mounted on a rigid substrate upside down and illuminated from behind. The incoming photons are now able to be absorbed directly into the bulk silicon pixels without the interference of the gate structures. The advantages in this type of CCD are that the relative quantum efficiency greatly exceeds that of a front-side device and the response of the detector to shorter wavelength

Si02 (THERMALLY GROWN)

ANTIREFLECTION COATING

/¿-n JUNCTION FDGF FRONT CONTACT

ANTIREFLECTION COATING

/¿-n JUNCTION FDGF FRONT CONTACT

n-TYPE SILICON MATERIAL

i+ BACK DIFFUSION ^

n-TYPE SILICON MATERIAL

i+ BACK DIFFUSION ^

BACK METALLIZATION

Fig. 2.4. Schematic view of a single front-side illuminated CCD pixel. The square labeled "front contact" is a representation of part of the overall gate structure. The letters "p" and "n" refer to regions within the pixel consisting of silicon doped with phosphorus and boron respectively.

light is improved since the photons no longer need to pass through the pixel gates. Disadvantages of back-side devices are in the areas of shallower pixel well depths (due to the much smaller amount of material present per pixel), possible nonuniform thinning leading to surface and flat-field nonuniformities, and increased expense incurred by the thinning and mounting process.

In the next chapter, we will explore the idea of quantum efficiency further and provide an example of the large differences present in these two types of CCDs. Back-side devices generally have about twice the quantum efficiency for a given wavelength compared with front-side devices.

Interline transfer CCDs are specially constructed devices in which each column of imaging (active) pixels is paralleled by a light-shielded column of storage (or inactive) pixels. The device is used as a normal CCD but after the exposure ends, each light-sensitive column of data is quickly shifted into its neighboring light-shielded column. The shift occurs in only a few microseconds and so image smear is almost nonexistent and the need for a mechanical shutter is precluded. The shifted image data can then be clocked out of the device while the active columns are again integrating on the source of interest. Interline devices have been used in many of the world's high-speed photon counting array detectors owing to their fast shift operation (Timothy, 1988).

The imaging area in an interline device is not continuous as each active column is paralleled by an inactive one; thus there is an immediate reduction

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