George Edward Possin

Performance of a 41×41 cm2 amorphous silicon flat panel x-ray detector designed for angiographic and R&F imaging applications

We measured the physical imaging performance of a 41 x 41 cm2 amorphous silicon flat panel detector designed for angiographic and R&F imaging applications using methods from the emerging IEC standard for the measurement of detective quantum efficiency (DQE) in digital radiographic detectors. Measurements on 12 production detectors demonstrate consistent performance. The mean DQE at the detector center is about 0.77 at zero frequency and 0.27 at the Nyquist frequency (2.5 cycles/mm) when measured with a 7 mm of Al HVL spectrum at about 3.6 microGy. The mean MTF at the center of the detector for this spectrum is 0.24 at the Nyquist frequency. For radiographic operation all 2048 x 2048 detector elements are read out individually. For fluoroscopy, the detector operates in two 30 frame per second modes: either the center 1024 x 1024 detector elements are read out or the entire detector is read out with 2 x 2 pixel binning. A model was developed to predict differences in performance between the modes, and measurements demonstrate agreement with the model. Lag was measured using a quasi-equilibrium exposure method and was found to be 0.044 in the first frame and less than 0.007 after 1 s. We demonstrated that it is possible to use the lag data to correct for temporal correlation in images when measuring DQE with a fluoroscopic imaging technique. Measurements as a function of position on the detector demonstrate a high degree of uniformity. We also characterized dependences on spectrum, exposure level, and direction. Finally, we measured the DQE of a current state of the art image intensifier/CCD system using the same method as for the flat panel. We found the image intensifier system to have lower DQE than the flat panel at high exposure levels and approximately equivalent DQE at fluoroscopic levels.

Measurement of heavy doping parameters in silicon by electron-beam-induced current

Limits on the magnitude of bandgap narrowing and Auger recombination in heavily phosphorus-diffused silicon layers ∼ 10²⁰cm^-3have been measured by electron-beam-induced current. It is determined that the slope of the bandgap narrowing versus doping must be nearly zero above 3 × 10¹⁹cm^-3to be consistent with previous data at lower doping levels. It is also shown that the low-level minority-carrier lifetime in these layers is consistent only with an Auger recombination coefficient<tex>C_{N} < 0.4 \times 10^{-31}</tex>cm⁶/s.

Measurements of the p-n product in heavily doped epitaxial emitters

A new method is described for the measurement of the p-n product in the heavily doped epitaxial emitters of biopolar tansistors. Quantitative electron-beam-induced conductivity is used to determine the diffusion length in the emitter as well as the emitter thickness. I - V characterization and other standard methods are then used to measure the p-n product. The principal advantage of this method is that corrections for recombination in both the emitter and the base can be made based on measurements on identical regions of the same device. A key problem with other methods for determining the p-n product is their inability to separate changes in the p-n product (bandgap narrowing) from recombination, based on measurements taken on the same region of the device. New data for the p-n product at ∼ 1020.cm3free-carrier electron density are uniformly compared to other published data. The importance of the effective bandgap narrowing parameter both for comparing experimental data and for device modeling is stressed. Diffusion length measurements on the more heavily doped emitters yield lifetimes longer than expected based on published lifetimes determined by the decay of optical luminescence in heavily doped silicon. Possible reason for this difference are discussed and attributed to gettering in our samples.

A semiconductor nonvolatile electron beam accessed mass memory

BEAMOS-beam addressed metal-oxide-semiconductor is a new technology for fast auxiliary memories which is expected to find important applications in military and commercial data systems. The concept is based on electron beam accessing, using a matrix lens, of a simple MOS memory chip. It has performance features which include large bit capacity per module (> 30 × 106bits), short access time ( 10 Mbit/s), and low cost. The BEAMOS module is all electronic, rugged, and relatively insensitive to variations in temperature, making it especially attractive for military computer applications. The operating principles of the BEAMOS memory and its present state of development are described.

Performance of advanced a-Si/CsI-based flat-panel x-ray detectors for mammography

The GE Senographe 2000D, the first full field digital mammography system based on amorphous Silicon (a-Si) flat panel arrays and a Cesium-Iodide (CsI) scintillator, has been in clinical use for several years. The purpose of this paper is to demonstrate and quantify improvements in the detective quantum efficiency (DQE) for both typical screening and ultra-low exposure levels for this technology platform. A new figure of merit, the electronic noise factor, is introduced to explicitly quantify the influence of the electronic noise, conversion factor, modulation transfer function (MTF), and pixel pitch towards the reduction of DQE at low exposure levels. Methods to improve the DQE through an optimization of both the flat panel design and the scintillator deposition process are discussed. The results show a substantial improvement in the DQE(f) at all frequencies and demonstrate the potential for DQE(0) to exceed 80%. The combination of high DQE at ultra low exposures and the inherent fast read-out capability makes this technology platform ideal for both current clinical procedures and advanced applications that may use multiple projections (tomosynthesis) or contrast media to enhance digital mammography.

Temporal response of CZT detectors under intense irradiation

The temporal response of CZT detectors is measured under different X-ray flux, spectra, and detector bias conditions. A comprehensive model has been developed to investigate the detector response under these conditions. The calculations have been compared with our measured results. Reasonable qualitative agreement is shown between the model and measurement results. This model provides a powerful tool to understand the detector temporal response, photocurrent dependence on the irradiation intensity, bias voltage, and defect characteristics. Understanding the detector response from a microscopic level can provide a guide to improve material properties and detector device design.

BEAMOS: a new electronic digital memory

BEAMOS, for Beam Addressed Metal Oxide Semiconductor, is a new technology for auxiliary memories based on an electron beam which reads and writes data on a simple unstructured MOS chip. It can store data for months with or without power.

Diode detection of information stored in electron‐beam‐addressed MOS structure

Charge stored in an insulating layer on a semiconductor can influence the field at the surface of the semiconductor and consequently control the surface recombination velocity. This paper describes the use of this effect to control the current in a reverse‐biased p‐n junction in the semiconductor. When an electron beam is used to write and read the stored charge, the effect can be used to produce high‐resolution high‐gain devices that have a variety of potential uses in information‐storage applications. Experiments demonstrating this effect are described.

High Temperature off Current in a-Si TFTS - Effect of Process and Structure

The OFF current in a-Si TFTs is an important parameter, especially for applications such as active matrix liquid crystal displays. In some demanding applications operation at 70°C or higher is required. This paper reports studies of the OFF current limiting mechanisms at room temperature and above. It is shown that the limiting mechanisms are hole injection from the drain junction at room temperature and electron conduction at higher temperatures. The importance of silicon thickness and interface state density at the passivation interface is stressed.

MOSFET's fabricated in laser-recrystallized Silicon on Quartz using selectively absorbing dielectrical layers

MOSFETs were fabricated in laser-recrystallized silicon islands on fused quartz substrates using a standard n-channel self-registered poly-gate process. Selective absorption obtained with patterned dielectric films was used to control the shape of the melt front during recrystallization of patterned LPCVD polysilicon islands. IR imaging of the laser-heated region was used to optimize and monitor the melt front shape. Devices with various channel lengths and widths were fabricated and the dependence of threshold voltage, channel mobility, and subthreshold leakage on recrystallization conditions and device dimensions were studied. Devices with and without back-channel implants were compared on the same wafer and for the same laser annealing conditions. The back-channel implant consistently reduced the subthreshold leakage to less than 1 pA/µm.