Marcelo Mulato

High-resolution direct-detection x-ray imagers

We report on a-Si direct detection x-ray image sensors with polycrystalline PbI2, and more recently with HgI2. The arrays have 100 micron pixel size and, we study those aspects of the detectors that mainly determine the DQE, such as sensitivity, effective fill factor, dark current noise, noise power spectrum, and x-ray absorption. Line spread function data show that in the PbI2 arrays, most of the signal in the gap between pixels is collected, which is important for high,DQE. The leakage current noise agrees with the expected shot noise value with only a small enhancement at high bias voltages. The noise power spectrum under x-ray exposure is reported and compared to the spatial resolution information. The MTF is close to the ideal sinc function, but is reduced by the contribution of K-fluorescence in the PbI2 film for which we provide new experimental evidence. The role of noise power aliasing in the DQE and the effect of slight image spreading are discussed. Initial studies of HgI2 as the photoconductor material show very promising results with high x-ray sensitivity and low leakage current.

Comparative study of Pbl2 and Hgl2 as direct detector materials for high-resolution x-ray image sensors

X-ray imaging properties are reported for HgI2 and PbI2, as candidate materials for future direct detection x- ray image sensors, including the first results from screen- printed HgI2 arrays. The leakage current of PbI2 is reduced by using new deposition conditions, but is still larger than HgI2. Both HgI2 and PbI2 have high spatial resolution but new data shows that the residual image spreading of PbI2 is not due to k-edge fluorescence and its possible origin is discussed. HgI2 has substantially higher sensitivity than PbI2 at comparable bias voltages, and we discuss the various loss mechanisms. Unlike PbI2, HgI2 shows a substantial spatially non-uniform response that is believed to originate from the large grain size, which is comparable to the pixel size. We obtain zero spatial frequency DQE values of 0.7 - 0.8 with PbI(subscript 24/ under low energy exposure conditions. A model for signal generation in terms of the semiconducting properties of the materials is presented.

Micro-electro-mechanical system fabrication technology applied to large area x-ray image sensor arrays

Micromachining has potential applications for large area image sensors and displays, but conventional micro-electro-mechanical system (MEMS) technology based on crystalline silicon wafers cannot be used. Instead, large area devices use deposited films on glass substrates. This presents many challenges for MEMS, both with regard to materials for micromachined structures and to integration with large area electronic devices. We are exploring the novel thick photoresist SU-8 as well as plating techniques for the fabrication of large area MEMS. As an example of its application, we have employed this MEMS technology to improve the performance of an amorphous silicon based x-ray image sensor array. SU-8 is explored as the structural material for the x-ray conversion screen and as a thick interlayer dielectric for the thin film readout electronics of the imager. Plating techniques are employed to metallize the deep contact vias in SU-8. Processing challenges that are particularly important for large area fabrica...

High-resolution x-ray image sensors based on HgI2

Measurements of polycrystalline HgI2 films on active matrix direct detection image sensors are described, for possible application to high sensitivity room temperature x- ray detection. The arrays exhibit low leakage current and very high sensitivity - roughly an order of magnitude better than has been demonstrated with other designs. The uniformity of the response varies randomly from pixel to pixel, for reasons that are not yet understood, but are probably related to the large grain size.

Charge collection and capacitance in continuous-film flat-panel detectors

The performance of the new generation of high fill factor two- dimensional imagers with high spatial resolution and low data line capacitance is described. These arrays have a continuous a-Si:H sensor layer deposited over the whole imager to improve sensitivity. We have studied charge collection and lateral leakage in the gap region in between two neighboring pixels. Experiments demonstrate that a 10 micrometer gap between pixels leads to an effective fill factor of approximately 92% and can be fabricated in a way to reduce the charge leakage between pixels to a very low level. We have also studied the capacitance of the data lines that can lead to increased electronic noise, degrading the imager performance. Experimental determination of the actual capacitance for different insulator materials are compared with numerical simulations, to identify the optimum structure. Based on these results, the new imager generation could be manufactured with a total parasitic capacitance of about 6 fF/pixel. Finally, we report measurements of the high fill factor imager under light and X-ray exposures.

Matrix-addressed x-ray detector arrays

Amorphous silicon (a-Si:H) technology has created a successful manufacturing business for large area active matrix arrays, of which liquid crystal displays (AMLCD) are the best known, and image sensors are an emerging technology for medical x-ray imaging. The large area, flat plate, format is the key feature of the technology that sets it apart from other digital imaging approaches. The principal requirements for medical imaging are sensitivity and high dynamic range. A-Si:H detectors have already proved to perform at least as well as x-ray film for radiographic applications and comparable to image intensifiers for fluoroscopy. There are several approaches to improving the performance of the image sensors is order to achieve higher sensitivity and higher spatial resolution. This paper describes some of these approaches.