Study On CMOS Active Pixel Sensor For Direct X-ray Imaging1

The X-ray imager based on digital image sensor, which has ability of real-timedisplay and digital storage, has been widely used in many field, such as medicaldiagnosis, industrial non-destructive testing and traffic safety check. But this type ofimage sensor for X-ray imaging needs a special medium, such as scintillator orfluorescent, to translate X-ray to visible light, thus affecting its temporal and spatialresolution, and making it unable to meet the high-resolution X-ray imaging field, suchas biomedical imaging.With reference to related researches, one type of CMOS active pixel image sensor fordirect X-ray imaging is proposed in this paper supported by NSFC. This new imagesensor for direct X-ray imaging, which direct detects X-ray photons, and uses standardCMOS process technology, need no special medium to translate X-ray to visible light,and can obtain a higher temporal and spatial resolution and100%pixel fill factor.Firstly, it is analyzed and compared several structures of CMOS APS for directdetection of X-ray, then selected the structure of CMOS APS on the non-epitaxialsilicon substrate. The X-ray detecting element array based on non-epitaxial siliconsubstrate has been modeled and simulated using Silvaco TCAD to analyze the affectionof pitch, substrate doping and N-well depth to the crosstalk and collected charges.NMOS transistor and charge collecting diode have been radiation-hardended designed.Shift register for pixels addressing is improved, making the ability of anti-SEU of itincreased by10times. The device-level modeling and simulation of Dog Bone MOSFET is conducted, and a fitting formula of equivalent aspect ratio is obtained. The chipincluding test transistors fabricated on CSMC0.5μm DPTM CMOS process ismeasured.The readout circuit and timing and control circuit of CMOS active pixel image sensorfor direct X-ray imaging are designed, and integrated with pixel array on the same chip.The readout circuit consists of pixel circuit with three transistors structure, correlateddouble sample circuit by column shared, sample and hold circuit, and output buffer. Thesample and hold circuit is a low-power circuit, there is current in source follower onlywhen sample, no current during maintaining. The timing and control circuit isfull-custom designed with Cadence Composer schematic entry tool and Spectre analogemulator. At last, the layouts of chip, are detailedly designed. The chip die size is 4400×3730μm~2, which integrates128×128pixel array with the readout circuit, the rowand column shift registers, and timing and control circuit.