A. Pecora

Lateral growth control in excimer laser crystallized polysilicon

Abstract The control of the structural properties of polysilicon obtained by excimer laser crystallization has become of great importance to further develop the polysilicon thin-film transistors technology. The most attractive crystallization regime is the so-called super lateral growth (SLG), characterized, however, by a very narrow energy density window and a strongly non-uniform grain size distribution. In this work we have investigated several approaches to achieve a control of the lateral growth mechanism through lateral thermal gradients, established by the opportune spatial modulation of the heating. To this purpose, three different patterned capping layers have been used: anti-reflective (SiO2), heat-sink (silicon nitride) and reflective (metal) overlayers. For all three types of overlayers, when the conditions to trigger the lateral growth mechanism are achieved, a band of oriented grains (1–2 μm wide) appears at the boundary between capped and uncapped region and extending in the more heated region. Among the different approach the use of reflective overlayers appears promising and further engineering of this process is in progress.

Hot carrier effects in n-channel polycrystalline silicon thin-film transistors: a correlation between off-current and transconductance variations

The application of bias-stresses with high source-drain voltage and different gate voltages in polycrystalline thin-film transistors modifies the transconductance as well as the off current. These effects have been explained in terms of hot-holes injection into the gate insulator causing the formation of trap centers in the oxide and interface states near the drain. >

Advanced excimer laser crystallization techniques

Abstract In high performance polysilicon thin film transistors (TFTs) the uniformity of electrical characteristics remain a major problem. This situation has stimulated a growing activity aiming to control the lateral growth phenomenon. However, most of the techniques require additional processing steps or a rather high shot density. We present a technique based on a two-pass excimer laser crystallization process: during the first irradiation the sample is irradiated through a patterned mask, while the second irradiation, performed without the mask, results in the homogeneous crystallization of the sample. This technique allows the possibility of forming uniform polysilicon layers, with large (∼2 micron) and aligned grains, with a reduced number of shots and a relatively large process energy window. The results of crystallization performed at different laser energy densities, sample thickness and laser pulse duration are analyzed.

Off-current in polycrystalline silicon thin film transistors : an analysis of the thermally generated component

Abstract The thermal generation component of polycrystalline silicon TFTs off-current is analysed experimentally and theoretically. In order to minimize the field-enhanced component of the leakage current, hot-hole injection, obtained by stressing the device at negative gate voltage and high source-drain voltage, has been used to reduce the electric field at the drain junction. After stress, the electrical characteristics in the off-regime are channel length independent and do not depend on gate voltage. This behaviour has been associated with the thermal generation-recombination processes occurring at the drain junction. Two-dimensional numerical simulations have been carried out with the program HFIELD, which has been modified to take into account the presence of gap states in polysilicon, and to incorporate the thermal generation-recombination processes by using the Shockley-Read-Hall statistics. Numerical simulations confirm that the generation occurs in the depletion region of the drain junction. The experimental I d - V ds characteristics measured at negative gate voltage have been compared with the calculated characteristics. The best fit with the experimental data was obtained only by using a rather short carrier lifetime (10 −12 s). The simulations indicate that a decrease of the density of states produces a lower off-current owing to a longer carrier lifetime and to a reduction of the drain junction depletion layer.

Flexible pH sensors based on polysilicon thin film transistors and ZnO nanowalls

A fully flexible pH sensor using nanoporous ZnO on extended gate thin film transistor (EGTFT) fabricated on polymeric substrate is demonstrated. The sensor adopts the Low Temperature Polycrystalline Silicon (LTPS) TFT technology for the active device, since it allows excellent electrical characteristics and good stability and opens the way towards the possibility of exploiting CMOS architectures in the future. The nanoporous ZnO sensitive film, consisting of very thin (20 nm) crystalline ZnO walls with a large surface-to-volume ratio, was chemically deposited at 90 °C, allowing simple process integration with conventional TFT micro-fabrication processes compatible with wide range of polymeric substrates. The pH sensor showed a near-ideal Nernstian response (∼59 mV/pH), indicating an ideality factor α ∼ 1 according to the conventional site binding model. The present results can pave the way to advanced flexible sensing systems, where sensors and local signal conditioning circuits will be integrated on the same flexible substrate.

Polysilicon TFT structures for kink-effect suppression

Experimental results and numerical simulations of asymmetric fingered polysilicon thin-film transistors (AF-TFTs) are analyzed in detail. In the AF-TFTs, the transistor channel region is split into two zones with different lengths separated by a floating n/sup +/ region. This structure allows an effective reduction of the kink effect depending on the relative length of the two subchannels, without introducing any additional series resistance. In addition, an appreciable reduction of the leakage current is also observed. The AF-TFTs characteristics have been analyzed by two-dimensional numerical simulation and by modeling the device with two transistors in series. This model clarifies the mechanisms of kink effect suppression in AF-TFT. On the basis of this analysis, two new modified device structures for kink-effect suppression are also proposed and discussed.

Analysis of electrical characteristics of polycrystalline silicon thin-film transistors under static and dynamic conditions

Abstract Polycrystalline silicon TFT technology is rapidly emerging for large-area electronic applications, because of the relatively large mobility values of charge carriers with respect to the corresponding values in amorphous silicon. In contrast, because of the complex energy distribution of localized states within the energy gap, and the resulting space-charge effects, the TFT electrical characteristics are difficult to model, and a numerical approach is needed in order to better understand the physical effects which influence the device performances. In this article we perform numerical simulations of TFTs at different temperatures under static and dynamic conditions and, by fitting experimental data, extract the energy distribution and the capture cross-section of the grain-boundary traps and the parameters of the impact-ionization model. As opposed to single-crystal silicon SOI devices, we find that the TFT current and transconductance increase as temperature increases.

PEDOT-CNT-Coated Low-Impedance, Ultra-Flexible, and Brain-Conformable Micro-ECoG Arrays

Electrocorticography (ECoG) is becoming a common tool for clinical applications, such as preparing patients for epilepsy surgery or localizing tumor boundaries, as it successfully balances invasiveness and information quality. Clinical ECoG arrays use millimeter-scale electrodes and centimeter-scale pitch and cannot precisely map neural activity. Higher-resolution electrodes are of interest for both current clinical applications, providing access to more precise neural activity localization and novel applications, such as neural prosthetics, where current information density and spatial resolution is insufficient to suitably decode signals for a chronic brain-machine interface. Developing such electrodes is not trivial because their small contact area increases the electrode impedance, which seriously affects the signal-to-noise ratio, and adhering such an electrode to the brain surface becomes critical. The most straightforward approach requires increasing the array conformability with flexible substrates while improving the electrode performance using materials with superior electrochemical properties. In this paper, we propose an ultra-flexible and conformable polyimide-based micro-ECoG array of submillimeter recording sites electrochemically coated with high surface area conductive polymer-carbon nanotube composites to improve their brain-electrical coupling capabilities. We characterized our devices both electrochemically and by recording from rat somatosensory cortex in vivo. The performance of the coated and uncoated electrodes was directly compared by simultaneously recording the same neuronal activity during multiwhisker deflection stimulation. Finally, we assessed the effect of electrode size on the extraction of somatosensory evoked potentials and found that in contrast to the normal high-impedance microelectrodes, the recording capabilities of our low-impedance microelectrodes improved upon reducing their size from 0.2 to 0.1 mm.

Crystallization mechanisms in laser irradiated thin amorphous silicon films

Abstract Structural properties of thin polycrystalline silicon films, crystallized by single shot excimer laser annealing at different laser energy densities, have been investigated. Formation of disk structures has been observed in a wide range of energy densities, from complete melting down to 180 mJ/cm 2 . These structures have been correlated to the lateral growth of grains starting from the small grains present in the central regions of the disks. We propose a new crystallization scenario for energy densities below the complete melting. In this framework, the recalescence effect plays an important role while the super lateral growth-regime is no longer a particular crystallization condition but simply represents the upper energy density limit of partial melt crystallization regime.

On-chip fabrication of ultrasensitive NO2 sensors based on silicon nanowires

We report a very simple, robust, and reliable on-chip fabrication method of a chemoresistive sensor based on silicon nanowires (NWs). Our method permits the use of nanowires without the need of their removal and transfer to a support different from the growth substrate. Our method, completely based on the silicon technology platform, exploits nanowires directly grown onto a selected area, over and between pre-patterned, interdigitated electrodes defined on oxidized silicon. The fabricated sensor is capable to detect NO2 down to a few ppb levels operating at room temperature. The sensor characteristics benefit of the presence of self-welded nanowires.