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Protection circuit preventing electrostatic discharge damage in a-si array x-ray sensors Disclosure Number: IPCOM000029218D
Publication Date: 2004-Jun-18
Document File: 5 page(s) / 494K

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The Prior Art Database



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Protection circuit preventing electrostatic discharge damage in a-si array x-ray sensors

                    Background of the invention

ElectroStatic Discharge (ESD) poses a risk to flat panel a-Si:H based X-ray sensors during manufacturing of the plate itself or during the production of the assembly of plate, electronics and scintillator.

Especially sensors based on photosensitive a-Si diodes that are switched using a-Si Thin Film Transistors (TFT) are vulnerable. Unprotected lines that are used to switch the gates of the TFTs on and off can be irreversibly damaged at voltage levels of 100V and below. The root cause for this low damage threshold level lies in the insulating nature of the gate dielectric of the TFTs and the small thickness of it, usually between 100 and 500 nm. An electric field of 2MV/cm and higher across the TFT gate dielectric causes a large threshold voltage shift that leads either to a TFT that is always on (positive voltage ESD event) or off (negative voltage ESD event). At electric fields above 10 MV/cm the gate dielectric electrically breaks down, leading to visible damage. For a description of the problem see also WQR-29-03-00/099.

Problems or disadvantages overcome by the invention

In array X-ray sensors based on a-Si TFT and a-Si diodes there is no effective protection circuit being used that is known to us.

In TFT-LCDs, ESD protection circuits on the glass plates with TFTs are already used for many years. There, TFTs switched as diodes are being used as protection devices. However, these TFTs itself are vulnerable (as well as the pixel TFTs). Figures 1 and 2 show embodiments of ESD protection circuits used in TFT-LCDs.

At present, our customer tries to keep the static voltage levels as low as possible, but it is known that keeping static voltages below 1000V is very difficult in practice.

The essential feature(s) of the invention

The invention relies on the effect of a voltage limiting circuit positioned at either the beginning or the end of the gate switching lines, or both.

Usually, an ESD event is introduced at the periphery of the sensor, because handling occurs usually close to this periphery. This explains the position of the ESD protection circuit.

Since virtually no power is needed to damage the TFTs connected to the gate switching lines, a voltage limiting circuit provides optim...