Richard L. Weisfield

Two Dimensional Amorphous Silicon Image Sensor Arrays

Large two dimensional amorphous silicon image sensor arrays offer a new approach to electronic document input and x-ray imaging. The sensor array technology is now capable of image capture at greater than 10 frames/sec and with resolution of 200–400 spi. We describe our new high resolution imaging system, comprising a page-sized sensor array with nearly 3 million pixels, and the accompanying high speed read out and processing electronics. The key technological issues of pixel resolution, sensor fill factor, leakage currents and noise are reviewed. Measurements of a new array architecture are described, in which the sensor is formed as a single continuous film on top of the matrix addressing components.

High resolution X-ray imaging using amorphous silicon flat-panel arrays

Two dimensional amorphous silicon arrays are the emerging technology for digital medical X-ray imaging. This paper demonstrates an improved pixel design compared with the current generation of imagers. The geometry of the pixel sensor has been extended from a mesa isolated structure into a continuous layer above the readout structures of the array. This approach improves sensitivity to visible light, and to X-ray illumination when coupled with a conversion phosphor. Furthermore, this 3-dimensional geometry allows for the fabrication of the finest pitch amorphous silicon array yet manufactured, with a pixel size of 64 /spl mu/m square. A test array (512/spl times/640 pixels) has been fabricated and tested which demonstrates the success of this approach.

Two-dimensional amorphous silicon image sensor arrays

Abstract Large two-dimensional amorphous silicon image sensor arrays offer an advantage for high speed document scanning and medical X-ray imaging. We describe our page sized 200 spot per inch imager and the accompanying high speed readout electronics. The spatial resolution performance for white light and X-ray imaging is illustrated. We discuss how the important issues of noise and resolution depend on the properties of a-Si:H, and show how the material can be used to give improved performance of the imagers.

HIGH-SENSITIVITY READOUT OF 2D A-SI IMAGE SENSORS

A highly sensitive charge-sensitive amplifier IC was used to read out large-area two-dimensional arrays of amorphous silicon (a-Si) p-i-n photodiodes addressed by a-Si thin film transistors (TFTs). At the highest sensitivity mode, the random noise of the sensor was equivalent to an input charge of 1500 electrons rms. Feedthrough charge injection associated with TFT switching had to be avoided at the expense of the signal charge. Image lag caused by charge trapping in a-Si was suppressed by limiting the input light intensity. A simple model was developed for the noise contribution from a pixel and compared with measurements.

Performance analysis of a 127-micron pixel large-area TFT/photodiode array with boosted fill factor

Sensor fill factor is one of the key pixel design requirements for high performance imaging arrays. In our conventional imaging pixel architecture with a TFT and a photodiode deposited in the same plane, the maximum area that the photodiode can occupy is limited by the size of the TFT and the surrounding metal lines. A full fill factor array design was previously proposed using a continuous sensor layer1. Despite the benefits of 100% fill factor, when applied to large-area applications, this array design suffers from high parasitic line capacitances and, thus, high line noise. We have designed and fabricated an alternative pixel structure in which the photodiode is deposited and patterned over the TFT, but does not overlap with the lines underneath. Separating the diode from the TFT plane allows extra space for an additional TFT which can be used for pixel reset and clipping excessive charge in the photodiode developed under high illumination. This reduces memory effect by 250%. The yield and the reliability are expected to improve as well since the TFTs and lines are buried underneath the diode. With the increased fill factor, we collect 56% more electrons per pixel, thereby improving the signal to noise ratio. The maximum signal to noise ratio is achieved when the increased signal and the undesirable parasitic capacitance on the data line are best optimized. Linearity, sensitivity, leakage, and MTF characteristics of a prototype X-ray imager based on this architecture are presented.

An investigation of signal performance enhancements achieved through innovative pixel design across several generations of indirect detection, active matrix, flat-panel arrays

Active matrix flat-panel imager (AMFPI) technology is being employed for an increasing variety of imaging applications. An important element in the adoption of this technology has been significant ongoing improvements in optical signal collection achieved through innovations in indirect detection array pixel design. Such improvements have a particularly beneficial effect on performance in applications involving low exposures and/or high spatial frequencies, where detective quantum efficiency is strongly reduced due to the relatively high level of additive electronic noise compared to signal levels of AMFPI devices. In this article, an examination of various signal properties, as determined through measurements and calculations related to novel array designs, is reported in the context of the evolution of AMFPI pixel design. For these studies, dark, optical, and radiation signal measurements were performed on prototype imagers incorporating a variety of increasingly sophisticated array designs, with pixel pitches ranging from 75 to 127 microm. For each design, detailed measurements of fundamental pixel-level properties conducted under radiographic and fluoroscopic operating conditions are reported and the results are compared. A series of 127 microm pitch arrays employing discrete photodiodes culminated in a novel design providing an optical fill factor of approximately 80% (thereby assuring improved x-ray sensitivity), and demonstrating low dark current, very low charge trapping and charge release, and a large range of linear signal response. In two of the designs having 75 and 90 microm pitches, a novel continuous photodiode structure was found to provide fill factors that approach the theoretical maximum of 100%. Both sets of novel designs achieved large fill factors by employing architectures in which some, or all of the photodiode structure was elevated above the plane of the pixel addressing transistor. Generally, enhancement of the fill factor in either discrete or continuous photodiode arrays was observed to result in no degradation in MTF due to charge sharing between pixels. While the continuous designs exhibited relatively high levels of charge trapping and release, as well as shorter ranges of linearity, it is possible that these behaviors can be addressed through further refinements to pixel design. Both the continuous and the most recent discrete photodiode designs accommodate more sophisticated pixel circuitry than is present on conventional AMFPIs--such as a pixel clamp circuit, which is demonstrated to limit signal saturation under conditions corresponding to high exposures. It is anticipated that photodiode structures such as the ones reported in this study will enable the development of even more complex pixel circuitry, such as pixel-level amplifiers, that will lead to further significant improvements in imager performance.

Two-dimensional amorphous-silicon photoconductor array for optical imaging

A simple array of amorphous-silicon photoconductors is investigated for use as a two-dimensional imaging device. The characteristics of the lateral photoconductive sensors and the behavior of a 128 x 128 element array are described in detail. It is shown that a two-dimensional array of the photoconductive sensors is capable of high-quality optical imaging with reasonable imaging speed. The array is also capable of performing image processing in real time. Furthermore, the two-dimensional array is well suited for use in optical processing and neural-network architectures.

Radiation imaging with 2D a-Si sensor arrays

Radiation imaging with a large-area amorphous silicon (a-Si) sensor array is discussed. X-ray images approaching medical diagnostic quality are obtained by a 256*240 array of 0.45-ram pixel pitch with 40-ms exposure time. Low-flux gamma -ray imaging is demonstrated by a 64*40 sensor array with pixel pitch of 0.9 mm, operated at a low external bias with 20-s integration time. The signal readout process is modeled and compared with experiments. At low external bias, charge collection and retention characteristics are influenced by the additional sensor bias created by the charge injection associated with the thin-film transistor operation. Charge retention in a sensor element is limited by the leakage through the sensor.<<ETX>>

Large area amorphous silicon x-ray imagers

Abstract Large two dimensional amorphous silicon imaging arrays are of interest for electronic document input and x-ray imaging. The device is a matrix-addressed array of light detectors fabricated from hydrogenated amorphous silicon on a glass substrate. Each imaging pixel consists of a light sensor and a thin film transistor (TFT). X-ray imaging is accomplished by placing a phosphor in contact with the image sensing surface, or by direct detection with a thick photoconductor. The imager technology is now capable of 10 in. arrays with image capture at greater than 10 frames/sec and with resolution of 4–6 lp/mm. We describe our new high resolution imaging system, comprising the sensor array with an active area of approximately 8 × 10 in. having nearly 3 million pixels, and the accompanying readout electronics. Key technological issues and alternative array designs are discussed.

Large-area a-Si:H TFT arrays for printing, input scanning and electronic copying applications

Abstract Using large-area photolithography and batch fabrication techniques, large-area a-Si:H TFT and sensor arrays can be economically produced. Applications of this technology to electronic printers, input scanners and electronic copiers will be described.