G. Fortunato

A phase-field approach to the simulation of the excimer laser annealing process in Si

We present a phase-field methodology applied to the simulation of dopant redistribution in Si during an excimer laser annealing process. The kinetic model derived in the framework of the Ginsburg–Landau thermodynamic formalism is made up of three coupled equations that rule the concurrent evolution of the thermal, phase, and impurity fields. The model was solved numerically by considering, as the initial conditions, the generic material modification due to an ion implant process, i.e., the implanted impurity profile in a SiO2/a–Si/c–Si stack. The model is parametrized for the cases of As and B doping, considering the thermal properties of the materials in the stack and the impurity-dependent diffusivity in the solid, liquid, and interfacial regions (the latter is characterized by a finite dimension). Simulated profiles are compared with the experimental results that have been obtained by secondary ion mass spectrometry and spreading resistance profiling. These comparisons demonstrate the reliability of th...

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.

Evidence of carrier number fluctuation as origin of 1/f noise in polycrystalline silicon thin film transistors

A systematic study of the noise performances of polycrystalline silicon thin film transistors is presented. The drain current spectral density of these devices shows an evident 1/ f behavior and scales, when operating in the linear regime, with the square of the mean value of the drain current. The origin of the noise can be ascribed to carrier number fluctuations related to the dynamic trapping and detrapping of the oxide traps.

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.

Microstructure of polycrystalline silicon films obtained by combined furnace and laser annealing

Amorphous Si films deposited by the low pressure chemical vapor deposition from disilane, and subsequently subjected to a combined furnace annealing at 600 °C/12 h and a sequential excimer laser annealing, results to polycrystalline silicon films with very large grains, low in‐grain defect density, and smooth‐free surface. Large but heavily defected grains are produced by the furnace annealing, the in‐grain defects are mainly microtwins, which are eliminated by a combined liquid–solid state process induced by the laser annealing. The two‐step annealing provides a very high quality polycrystalline material suitable for thin‐film transistor application.

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.

Determination of hot-carrier induced interface state density in polycrystalline silicon thin-film transistors

Polysilicon thin-film transistors are of great interest for their application in large area microelectronics and especially for their circuit applications. A successful circuit design requires a proper understanding of the electrical characteristics and in the present work some specific aspects related to the hot-carrier induced electrical instabilities are presented. In particular, generation of interface states near the drain junction occurs when the devices are operated for a prolonged time in the so-called kink regime. In the present work we show both experimentally and by numerical simulations how the presence of such interface states affects the electrical characteristics. Furthermore, a novel simple method is proposed to extract, from the analysis of the sheet conductances, the interface state density. The hot-carrier induced interface state density relative to the present devices shows a featureless continuous distribution. Reduction of the generated interface states is observed if trapped holes a...