Jackson Ho

Pentacene thin film transistors on inorganic dielectrics: Morphology, structural properties, and electronic transport

The structural and transport properties of evaporated pentacene organic thin film transistors (TFTs) are reported, and they show the influence of the deposition conditions with different inorganic dielectrics. Dielectrics compatible with large area fabrication were explored to facilitate low cost electronics on glass or flexible plastic substrates. X-ray diffraction and atomic force microscopy show a clear correlation between the morphology and the structure of the highly polycrystalline films for all dielectrics investigated. The roughness of the dielectric has a distinct influence on the morphology and the structural properties, whereas the films on smooth thermal oxide are in general highly ordered and independent of the deposition conditions. The ordered films exhibit a “thin film” and a bulk phase, and the bulk phase volume fraction increases with the deposition temperature and the film thickness. Careful control of the deposition conditions gives virtually identical films on thermal oxide and silico...

Comparison of PbI2 and HgI2 for direct detection active matrix x-ray image sensors

The factors determining the x-ray sensitivity of HgI2 and PbI2 as direct detector materials for large area matrix addressed x-ray image sensors are described, along with a model to explain their different properties. The imaging studies are made on test arrays with 512×512 pixels of size 100 μm. The x-ray sensitivity and spatial resolution are reported, along with measurements of the various mechanisms that influence the sensitivity, such as charge collection, x-ray absorption, fill factor, and image lag. The spatial resolution of PbI2 decreases with increasing film thickness, but this effect is not observed in HgI2. The x-ray response data are used to compare the sensitivity to the theoretical values for the ionization energy and to identify the various loss mechanisms. We find that the sensitivity of HgI2 can be explained by a few small and well characterized loss factors. This material exhibits good spatial resolution, high fill factor, and high charge collection. PbI2 films exhibit lower sensitivity, ...

Amorphous silicon thin-film transistors and arrays fabricated by jet printing

Phase-change wax-based printed masks, in place of conventional photolithography, were used to fabricate hydrogenated amorphous silicon thin-film transistors (TFTs). Wax-mask features with a minimum feature size of ∼20 μm were achieved using an acoustic-ink-printing process. Both discrete and matrix addressing structured bottom-gate TFTs with source–drain contacts overlapping the channel were created using a four-mask process. The TFTs had current–voltage characteristics comparable to photolithographically patterned devices, with mobility of 0.6–0.9 cm2/V s, threshold voltage of 2–3 V, and on/off ratios exceeding 107 for devices with channel lengths below 50 μm.

X-ray imaging using lead iodide as a semiconductor detector

The x-ray imaging performance is reported using polycrystalline lead iodide as a thick semiconductor detector on an active matrix flat panel array. We have developed a test image sensor with 100 micron pixel size in a 512 X 512 format, using amorphous silicon TFTs for matrix addressing. The new 14 bit electronic system allows radiographic and fluoroscopic x-ray imaging. PbI2 has larger x-ray absorption and higher charge generation efficiency than selenium, and has the potential for higher sensitivity imaging. The films are deposited by vacuum sublimation and have been grown thicker than 100 micrometer. Measurements of the carrier transport and charge collection, together with modeling studies show how the x-ray sensitivity depends on the material properties. Imaging measurements find excellent spatial resolution and confirm models of the x-ray sensitivity. Both radiographic and fluoroscopic imaging are demonstrated. While good overall imaging is obtained, the dark leakage current and image lag need further improvement.

Polycrystalline pentacene thin films for large area electronic applications

Abstract The growth of and the electronic transport in pentacene films on organic and inorganic dielectrics were studied. The morphology and structural properties of pentacene films are clearly correlated with the deposition parameters and substrate properties. To study the electronic properties we have formed inverted staggered transistors. The mobility in the transistors is correlated with the structural properties of the films and increases with crystal size. The TFTs exhibit typical mobilities of 0.4 cm 2 / V s and on/off ratios >108 on thermal oxide and smooth silicon nitride. The dielectrics influence on device performance will be discussed.

Image capture array with an organic light sensor

A large-area image capture device using an organic sensor is reported. The 512×512 pixel array, with 100×100 micrometer pixel size, combines amorphous silicon matrix addressing with a continuous organic sensor. The bilayer sensor comprises a tetraphenyldiamine hole transport layer on top of a benzimidazole perylene generator layer. This combination provides high sensitivity across the visible with low dark current. We present imaging properties and in particular show that the lateral charge transport between pixels is small, and that the effective fill factor is ∼90%. X-ray imaging with a phosphor converter is demonstrated.

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.

Flip-chip bonding on 6-/spl mu/m pitch using thin-film microspring technology

Bonding-pad densities on high-performance integrated-circuit chips are beginning to exceed the limits of available interconnect technologies. Also, stresses due to thermal mismatch in flip-chipped packages are reducing time to contact failure. We have addressed both of these problems by microlithographically fabricating highly elastic cantilever springs in linear arrays on pitches down to 6 /spl mu/m. We have soldered test arrays of 52 springs on this pitch to Si chips with 100% contact yield and good solder wetting to every spring. The fine-pitch capability also facilitates off-chip routing; the very high compliance of the springs should avoid thermal fatigue; and the low thermal conductance along the springs should allow fast-cycle soldering of chips to multichip modules as well as replacement of chips subsequently testing faulty.

Flat panel imagers with pixel level amplifiers based on polycrystalline silicon thin-film transistor technology

We report here the realization of a large-area compatible, flat panel imager with pixel level amplifiers. The imager is based on excimer-laser crystallized, polycrystalline silicon (poly-Si) thin-film transistors. By incorporating pixel level amplification, flat panel imagers are expected to be able to achieve unprecedented noise performance, with the hope of achieving single photon detection. We have demonstrated good noise performance of 1300 erms, exceeding the commonly accepted industry standard of 2000 erms. We also briefly discuss the source of the extra noise, as well as the possibility of further reducing the noise level.

LASER PROCESSING OF POLYSILICON THIN-FILM TRANSISTORS : GRAIN GROWTH AND DEVICE FABRICATION

Pulsed excimer-laser processing of amorphous silicon on non-crystalline substrates is an important processing technology for large-area polysilicon electronics, such as flat-panel displays and two-dimensional imaging arrays. It also allows for the integration of amorphous silicon and polysilicon devices on the same glass substrate and provides procedures for the doping of self-aligned thin-film transistors. Materials studies show that laser-crystallized polysilicon exhibits a narrow peak in the average grain size as a function of the excimer laser energy density, with a corresponding peak in the electron mobility. This is of particular significance for devices since large grains imply high electron mobility. On the other hand, the peak in the grain size is very narrow and is also accompanied by a peak in the surface roughness of the film. These relationships force a compromise between large grain size for high mobility and homogeneous size distribution for uniformity of device characteristics. A window exists in process parameter space where good-quality devices with uniform characteristics have been obtained. Also, laser-processing enhancements, such as laser doping and fabrication of self-aligned transistors, provide additional tools to fabricate unique devices.