Design of a CMOS readout circuit for dental X-ray imaging

Presents the design of an integrating CMOS readout circuit for bump bonding to a GaAs or CdTe pixel detector. The target application is in dental X-ray imaging, where such a chip could serve as a readout circuit for an intra oral X-ray imaging system. A prototype 64×80 element CMOS active pixel array has been designed, fabricated and successfully tested to show that this technology is suitable for the development of such circuits. The chip was implemented in a 0.8 μm double poly n-well process with a pixel pitch of 50 μm. A special layout technique for the pixel design was employed to achieve a maximum charge storage area. The charge storage element is a poly/poly capacitor occupying 80% of the pixel area. A large charge storage capacity is important to achieve high dynamic range in the system. Segmentation of the clock lines using buffers, to reduce the RC distributed effect, resulted in a very high measured clocking speed (100 MHz). This is important due to the large size of the final chip