Rachel Lau

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.

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.

Nano-spring arrays for high density interconnect

A new type of compliant interconnect derived from a thin metal film fabricated with a controlled stress profile is being developed for flip- flop interconnects and probing devices. Interconnections have been demonstrated on lateral pitches as tight as 6 microns. The interconnect is highly elastic and can provide up to hundreds of microns of vertical compliance.

Achieving high-resolution in flat-panel imagers for digital radiography

Amorphous silicon (a-Si:H) matrix-addressed imager sensors are the leading new technology for digital medical x-ray imaging. Large-area systems are now commercially available with good resolution and large dynamic range. These systems image x-rays either by detecting light emission from a phosphor screen onto an a-Si:H photodiode, or by collecting ionization charge in a thick x-ray absorbing photoconductor with as selenium, and both approaches have been widely discussed in the literature. While these systems meet the performance needs for general radiographic imaging, further improvements in sensitivity, noise and resolution are needed to fully satisfy the requirements for fluoroscopy and mammography. The approach taken for this paper uses indirect detection, with a phosphor layer for x-ray conversion. The thin a-Si:H photodiode layer for detects the scintillation light. In contrast with the present generation of devices, which have a mesa-isolated sensor at each pixel, these imagers use a continuous sensor covering the entire front surface of the array. The p+ and i layers of a-Si:H are continuous, while the n+ contact has been patterned to isolate adjacent pixels. The continuous photodiode layer maximizes light absorption from the phosphor and provides high x-ray conversion efficiency.

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.