Active pixels for x-ray applications, implemented in a standard CMOS technology

Abstract

During the past ten years digital technology has entered the world of mammography. It has not taken completely over, however, because there are still disagreements as to whether the digital technology is as good as the analog at detecting breast cancer. The amount of radiation exposed to the patient is another issue, because larger amounts of radiations are used with the digital technology.

To contribute to the improvement of the digital mammography technology, research has been done on the pixels used in the x-ray sensors.

Different pixel architectures have been implemented in a standard CMOS technology and compared with respect to sensitivity to low light exposure. This work has been done in two phases. First some test pixels were implemented on the side of an image sensor belonging to an EU funded project (I-ImaS). Based on the results from this testing and new calculations a new set of test pixels were implemented for further testing.

The first set of test pixels, designed by research scientists at SINTEF, were implemented as 3-transistor pixels (3T). The size of the photo diode is varied, as is the size of the source follower transistor (SF) in the pixel. The pixels were compared with respect to sensitivity. In addition some pixels are covered with metal while their neighbouring pixels are uncovered. This way influence between pixels is measured.

In the second set of test pixels there are 3-transistor pixels just like in the first set. In addition 4-transistor pixels (4T) and photo gate pixels (PG) are implemented on the same chip. The same variations in photo diode and SF size have been made to the 3T pixels to further investigate the results from the first testing. In addition another variation has been made to some of the 3T and 4T pixels: The n-well photo diode has been made with one or more holes, in order to decrease the area and thus the capacitance of the diode without decreasing the reach (radius) and charge collection of the diode.