CCD back thinned

The implementation of so-called back-thinned CCD removes sensitivity limitations of classical front-illuminated and UV-coated CCD. Using this improved technology, the substrates with extremely thin silica layers are back-illuminated and a large increase in quantum efficiency in the UV range up to 0.9 electrons per photon can be achieved. Specific anti-reflection layers on the detector surface produce further enhancement of the detector efficiency for certain spectral bands. 

A thinned CCD will naturally build up an internal electric held that will pull electrons to the back surface with a subsequent loss of photoelectrons. In order to overcome this natural electric held, a backside passivation process must be used. There are three primary technologies to passivate the backside ... 

Figure 4 Principle of a back-illuminated thinned CCD chip. (Reproduced by permission from EG G Life Sciences, Evry, France.)...

Figure 1.6. Spectra of solid glassy carbon obtained with a state-of-the-art spectrometer in 1985 (Spex 1403 double monochromator with photon counting PMT) and a multichannel/CCD spectrometer of 1996 (Chromex 250 spectrograph, back thinned silicon CCD) 514.5 nm laser at 50 mW in both cases measurement times and signal/noise ratios (SNR) as shown.

A front-illuminated CCD may be back thinned by ion etching on the side of the silicon wafer opposite the circuit mask. If the silicon is thin enough after... 

Most recently, a new detector which combines both deep depletion and back-thinning has been produced [157]. In these devices, a proprietary surface treatment is incorporated into production, which effectively eliminates etaloning in the NIR. The NIR performance is compared to alternative CCD devices in Fig. 17. It can be seen from these curves that the back-thinned, deep depletion CCD is the preferred device for NIR-excited Raman spectroscopy. The only drawback to the deep depletion devices is that they tend to be... 

This class of CCD has a higher quantum yield than a front-illuminated similar CCD. The origin of this improvement is as follows. The silicon substrate for the chip is etched down, on the face opposite the electrodes, to reach a thin layer (—15 pm), and the light illuminates the array from the back, opposite from the usual way (Fig. 4). It was noted in Sec. 5 that electrode gates that are used for the readout process overlap the pixel surface. Since these electrodes have a given thickness and are laid on the front face, photoelectrons have to travel across a... 

Today, CCDs are thinned and back-side illuminated and have quantum efficiencies of greater than 70% and a spectral range from 350 to 1000 nm. The response of these detectors is quite good in the red, and with quantum efficiencies peaking near 700 nm. 

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