L. Mariucci

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.

Contact effects in high performance fully printed p-channel organic thin film transistors

Contact effects have been investigated in fully printed p-channel organic thin film transistors with field effect mobility up to 2 cm2/Vs. Electrical characteristics of the organic thin film transistors, with channel length <200 μm, are seriously influenced by contact effects with an anomalous increase of the contact resistance for increasing source-drain voltage. Assuming that contact effects are negligible in long channel transistors and using gradual channel approximation, we evaluated the current-voltage characteristics of the injection contact, showing that I-V characteristics can be modeled as a reverse biased Schottky diode, including barrier lowering induced by the Schottky effect.

Surface-scattering effects in polycrystalline silicon thin-film transistors

Mobility reduction, induced at high gate fields by scattering with surface acoustic phonons and surface roughness, has been investigated in self-aligned polycrystalline silicon (polysilicon) thin-film transistors (TFTs). The analysis of this effect can be influenced by the presence of parasitic resistance effects, and a precise evaluation of this effect has been obtained by measuring the transfer characteristics in devices with different channel lengths. In this way, we could reliably determine the mobility reduction effect, which was then analyzed by using two-dimensional numerical simulations. The mobility reduction in polysilicon TFTs can be accurately described by Lombardi’s model, originally proposed for c-Si metal-oxide-semiconductor field-effect transistors.

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. >

Kink effect in short-channel polycrystalline silicon thin-film transistors

Excess current, induced by impact ionization (kink effect) has been investigated in short-channel polysilicon thin-film transistors (TFTs). We have shown, both experimentally and by using two-dimensional (2-D) numerical simulations, that the output characteristics are substantially degraded by the kink effect as the channel length is reduced. In particular, we have shown that the excess current, triggered by the impact ionization and enhanced by the parasitic bipolar transistor action, scales nearly as L−2, thus making very difficult the downscaling of polysilicon TFTs. Such L dependence has been clarified through a detailed analysis of the current components obtained from 2-D numerical simulations. The analysis demonstrates that there are fundamental issues with the output characteristics, and it appears that the introduction of appropriate drain field relief structures will be necessary for the fabrication of short-channel polysilicon TFTs with high output impedance.

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.

Ultra-shallow junction formation by excimer laser annealing and low energy (<1 keV) B implantation: A two-dimensional analysis

Formation of shallow junctions has been investigated by using excimer laser annealing in combination with two implantation schemes: BF2-ions at 20 keV and B-ions at low energies (<1 keV). The latter approach was shown to produce best results, with ultra-shallow profiles extending to a depth as low as 35 nm. The lateral distribution of the implanted B following laser annealing has been studied with two-dimensional measurements using selective etching and cross-section transmission electron microscopy (TEM) on samples where the implanted dopant was confined within an oxide mask. The results show that there is substantial lateral diffusion of B under the oxide mask when melting occurs in this region while, if melting under the oxide mask is prevented, the implanted B close to the oxide mask edge was not activated by laser annealing. The results have been explained by numerical heat-flow calculations and it is concluded that the melting of the Si under the masked region and, therefore, the lateral diffusion, can be controlled by the oxide mask thickness.

Two-dimensional delineation of ultrashallow junctions obtained by ion implantation and excimer laser annealing

Junctions shallower than 100 nm, obtained by ion implantation and excimer laser annealing, have been characterized in two dimensions by transmission electron microscopy ~TEM! on chemically treated samples. The chemical treatment selectively removes silicon as a function of the B concentration, making thinner the regions where B is present in the cross section of the sample, with respect to the n-type substrate. Both secondary ion mass spectrometry and spreading resistance profiling measurements have been performed, in order to quantify the contour line obtained by TEM in terms of B concentration. The results achieved by the two-dimensional technique show interesting features, related to the particular redistribution of B occurring when silicon is melted by excimer laser annealing irradiation. In particular, a rectangular shape of the doped region obtained by laser annealing could be evidenced, caused by the fast diffusion in the melted material, completely different from the typical half-moon-shaped, thermally annealed, two-dimensional B profile. The feasibility of ultrashallow junctions by laser annealing, with depths below 100 nm and high electrical activation, is demonstrated. However, a huge lateral diffusion in the melted silicon is also to be taken into account when considering excimer laser treatments as an alternative to standard rapid thermal annealing.

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.

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.