Research Of Key Techniques In Large-area High-resolution Digital X-ray Detectors

Digital X-ray detectors, which can convert invisible X-ray into digital electronic signal, are important components in X-ray imaging systems. High spatial resolution and large-area digital X-ray detectors are required in "Grating-differential Interference based X-ray Phase-contrast Imaging System". But nowadays X-ray detectors can hardly meet these requirements, because there are two main problems existing. The first problem comes from the fact that the spatial resolution of X-ray converter degrades seriously due to the lateral spreading of fluorescence. The second problem is that the area of light-sensitive part of the image sensor in X-ray detector is much less than the lowest requirement of 75 x 75mm2.To resolve these problems, research works were performed to manufacture a large-area high resolution digital X-ray detector. The works have emphases on two parts:The first part is the manufacture of a pixellated X-ray converter, in order to improve the spatial resolution.Silicon based pore arrays are essential for manufacturing pixellated X-ray converters. Silicon based pore arrays are high aspect ratio microstructure, which are suitable of manufactured by photoelectrochemical etching of silicon. To obtain uniform pore arrays in full 5 inch silicon wafers, a novel large area silicon wafer photoelectrochemical etching setup was established. With the help of a water-cooling system and a shower-head shaped circulator, the novel photoelectrochemical etching setup resolved the problems of electrolyte temperature going high and hydrogen bubbles assembling. There were two kinds of pore arrays manufactured, with side of 5μm and 1.5μm, and aspect ratio of 30 and 100 respectively. After high temperature oxidation and CsI:T1 filling of the pore arrays, the pixellated X-ray converter was finally manufactured. The spatial resolution of the pixellated X-ray converter was 201p/mm under test, which can meet the requirements of initial design. Additionally, by means of increasing the etching current gradually to compensate the influences of lateral etching, two kinds of high aspect ratio wall arrays with good morphology consistency were manufactured for X-ray gratings'fabrication by the novel photoelectrochemical etching setup.The second part is the development of a data acquisition system based on 4 CMOS chips'combination, in order to enlarge the imaging area. Four CMOS chips LUPA-4000 coupled with a 2×2 taper array can form a full image, with the area of 98.5×98.5mm2. Because the size of the taper array was fixed, the data acquisition system can only be developed from bare CMOS chips. The data acquisition system's scheme was that it can be controlled far-distance by Ethernet, and the four CMOS chips'data were temporarily stored each by a DDR2 SDRAM. LUPA-4000's driving timing and DDR2 SDRAM controller were accomplished by Verilog based FPGA design, and ARM was used as MCU, and image data were stored in PC. Every LUPA-4000 has an independent PCB board. There is some space between two boards, so it is convenient for LUPA-4000's coupling to taper's litter end. Besides the basic functions of taking photos and image data's storing, a parameters adjustable data acquisition system was developed, with additional some useful functions, such as photo parameters'adjusting and windowing readout.