Electronic Shutter

Standard commercial imagers introduce light through the front device surface. To produce a shutter function, the charge is moved behind an opaque metal line in the pixel to block further accumulation of charge. The back-illuminated CCD has been developed to greatly improve sensitivity over commercial imagers by bringing light into the device through the back surface, unobstructed by structures on the front surface. By doing this, however, the normal commercial method of producing a shutter function cannot be used in a back-illuminated CCD.

Cross section of electronic shutterFigure 1. Cross section of electronic shutter. (Click on image to see larger version.)

We have developed an electronic shutter specifically designed for back-illuminated devices. To illustrate how the electronic shutter is formed and how it operates, Figure 1 represents a cross section of the physical structures in the silicon device. One feature not found in a normal CCD is the p+ buried layer, implanted with a high-voltage implanter (about 1 MeV boron). This layer forms a potential barrier that separates the illuminated back surface (bottom) from the front surface (top) of the device.

Depletion region of device storage well.Figure 2. Depletion region of device storage well. (Click on image to see larger version.)

Figure 2 represents the depletion region of the device storage well (blue region) when the storage well gate (VIA) is moved to a very high potential (18 V). The depletion region has reached through the p+ buried layer, and so photoelectrons (represented by the – symbols) may move from the back surface of the device, where they are generated, to the storage well under VIA on the CCD front side. This is the "shutter open" condition.

Figure 3 represents the potential configuration of the device when the storage-well gate is moved to an intermediate voltage (<12 V), which is high enough to maintain a storage well (and the charge in it) under the gate (blue region), but not high enough for the depletion region to reach through the deep implant.

Potential configuration of the device when the storage well gate is moved to an intermediate voltage.Figure 3. Potential configuration of the device when the storage-well gate VIA is moved to an intermediate voltage. (Click on image to see larger version.)

The shutter function also uses shutter drains running down the channel-stops (shown in the physical cross section above [Figure 1] as two n+ diodes imbedded in the p+ channel stop region and connected to a voltage VSD). While the storage-well voltage is reduced, the shutter-drain voltages are increased, allowing their depletion regions (red) in the Shutter Closed diagram (Figure 3) to now reach through the deep buried layer and form a way for the photoelectrons to be drained out of the pixel and discarded. Use of the shutter drain prevents photocharge generated during the shutter-closed condition from building up to the point of leaking over the potential barrier and contaminating previously collected charge in the CCD well.

The electronic shutter structure has enabled a number of devices, such as a four-sample high-speed burst imager and a fifty-sample imaging device, by making it possible to shelter previously collected charge from current photocharge when the integration period of the imager has ended. Both of these devices operate with effective sampling rates well above 1 MHz. Once the event is over, the multiple stored images are read out with low noise at manageable data transfer speeds so the noise is low.



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