Marco Camalleri

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...

Material modifications induced by laser annealing in two-dimensional structures

The effects of the laser irradiation on metal-oxide-semiconductor structures are investigated by means of a phase-field methodology. We numerically solved the model equations in one- and two-dimensional structures also containing SiO2/amorphous-Si/crystalline-Si stacks. The simulated laser annealing processes are discussed in detail, pointing out the influence of the geometrical constraints on the irradiation effects in the samples. The simulation results are compared with the experimental two-dimensional delineation of dopant profiles. These comparisons show the importance of the joint theoretical and experimental investigations in order to fully understand the phenomena occurring in submicron sized laser irradiated structures.

Low-Temperature Annealing Combined with Laser Crystallization for Polycrystalline Silicon TFTs on Polymeric Substrate

The formation of polycrystalline Si layers on flexible plastic substrates, through plasma enhanced chemical vapor deposition and excimer laser annealing, is investigated. Combining low-temperature (300°C) annealing with laser dehydrogenation/crystallization produces good-quality polycrystalline silicon with a reduced shot density. By using optimal crystallization conditions it is possible to achieve a superlateral growth crystallization regime, with a grain size up to 1 μm, and void-free material, as confirmed by the presented structural analysis. The beneficial effect of the low-temperature thermal annealing has been related to the removal of nonbound hydrogen, as supported by the elastic recoil detection analysis and IR analysis of the samples. To validate the process, we fabricated non-self-aligned polysilicon thin-film transistors (TFTs) directly on spin-coated polyimide substrates, with a maximum processing temperature of 300°C and with a relatively low shot density ( 10 6 , a field-effect mobility up to 65 cm 2 /V s, and a threshold voltage of 7 V. These results confirmed that the developed crystallization process is suitable to fabricate polysilicon TFTs on polymeric substrates, allowing an increased process throughput.

Evidence for the precursors of nitrided silicon in the early stages of silicon oxynitridation in N2:N2O atmosphere

Nitridation of hydrogen-terminated silicon with N2:N2O has been studied by x-ray photoemission spectroscopy. Our analysis has given evidence that the broad N(1s) peak at 398–399 eV, usually reported in the literature, is preceded by the formation of a narrow peak at 397.5 eV, attributed to the moiety Si3N in which silicon is only marginally oxidized, and two other peaks at 400.0 eV and 401.5 eV, attributed to the moieties Si2NOSi and SiNO, respectively.

Charge transport in ultrathin silicon rich oxide/SiO2 multilayers under solar light illumination and in dark conditions

An extensive study on the electrical properties of Si nanocrystals under dark and solar light exposure in AM1.5G conditions is presented. The nanostructures have been obtained through chemical vapor deposition of multilayers of ultrathin silicon rich oxide/SiO2 films and subsequent thermal annealing. The electrical data demonstrate that the current transport in such systems is mediated by tunnel effect, and the lowest effective energy barrier limiting the carrier transport has been found to be 1.7 eV, well below the values of 3.1 eV and 4.7 eV of free electrons and holes, respectively, at the standard Si/silicon dioxide interface. Under AM1.5G solar light illumination the contribution of the photocarriers increases with the voltage and above 60 V shows a trend toward saturation. A quantitative explanation of this saturation is discussed. Moreover, the photocarrier generation rate in the nanocrystals averaged over the solar spectrum region is evaluated.

Nitrogen Bonding Configurations of SiO x N y Thin Films in Power MOSFET Gate Interfaces

bSTMicroelectronics, 95121 Catania, Italy The structural properties of silicon oxynitride films grown in a N2O environment at temperatures higher than 900°C and for use as gate dielectrics in vertically diffused power metal oxide semiconductor field effect transistor PowerVDMOS technologies have been studied by means of X-ray photoelectron spectroscopy. The progressive modifications of the bonding environments upon reaching the oxynitride‐silicon interface have been analyzed as well as of the relation between these modifications and the selected oxynitridation process. The results show that the chemistry of the oxynitride layer is a rather complex one, and it significantly and progressively changes by moving toward the silicon interface, in a way strongly affected by the growth process. In particular, the medium thermal budget processes 950°C, 20‐60 min favor the formation of a relatively uniform distribution of the single oxidized O‐N‐Si2 bonds both at the interface and throughout its immediate backstage. Such findings can help in assessing the role played by the nitridation process in the quality and reliability performances of the final device. In the last few years, silicon oxynitride thin films have been proposed as an alternative to SiO2 as a thin gate dielectric for microelectronics applications. As the scale of integration increases and the thickness of the dielectric is reduced, the SiO2 dielectric layer properties get less and less sufficient to reliably withstand the increasing electric field. As a consequence, the device degradation due to the high leakage currents and even gate rupture shows up at a higher rate. Silicon oxynitrides SiOxNy are materials with a higher dielectric constant and, in thin layer form, exhibit reduced susceptibility to interface state generation with respect to SiO2, higher timeto-breakdown values and reliability, improved I-V and C-V characteristics. 1-3 Therefore, silicon oxynitride could eventually replace thin gate dielectrics characterized by a smaller thickness but an equal capacitance, thus assuring an improved robustness of the device. In this context, several analyses have been performed to characterize SiOxNy film quality in terms of both device performance and processing. 4,5 A clear understanding of the structure and chemical composition of the oxynitride film could be valuable help in assessing its quality as a gate layer in power devices. Nevertheless, in many works found in the literature, the only relevant parameter taken into account is the overall nitrogen content. In the attempt to disclose the role of the oxynitride structure in the electrical defectivity and reliability, it is of fundamental importance to know the nitrogen bonding configurations present at the silicon substrate interface and their relation with the selected oxynitridation process. In this respect, X-ray photoemission spectroscopy XPS is a powerful tool to perform a compositional analysis and also to investigate the relative bonding arrangements of silicon, oxygen, and nitrogen atoms. The aim of the present work is to investigate the progressive modifications of the nitrogen bonding arrangements upon reaching the interface for a set of oxynitride layers grown by means of furnace N2O-based dry processes at different temperatures and times to be used as gate dielectrics in vertically diffused power metal oxide semiconductor field effect transistor PowerVDMOS technologies. Changes of the main bonding configurations, moving toward the Si interface, were found to significantly depend on nitridation process parameters, such as the process duration and the furnace temperature at which the oxynitridation processes were performed. The overall picture of the atomic concentrations evolution was checked carrying out a deconvolution of the XPS photoelectron spectra using a fourband model according to existing literature data. 3,6-9

Carrier trapping in thin N2O-grown oxynitride/oxide di-layer for powerMOSFET devices

Abstract A detailed study of the carrier trapping properties shown by the silicon/oxynitride/oxide gate layers in PowerVDMOS technologies is reported. A quantitative analysis of hole and electron trap densities versus the specific N 2 O based nitridation process, extracted from Fowler–Nordheim constant current stress kinetics, allows a deep understanding of the role played by those defects in the susceptibility of every nitrided layer.

Reoxidization Process Effects on the Nitrogen Bonding Configurations in SiO x N y Power MOSFET Dielectric Gate

The structural properties of silicon oxynitride films used at the gate dielectrics interface in Power vertically diffused metal oxide semiconductor technologies have been studied by means of X-ray photoelectron spectroscopy. An overall picture of the interface chemistry evolution as a function of the growth parameters in relation to the effects of the postgrowth reoxidation process is reported. The films were grown in an N 2 O environment at temperatures higher than 900°C and subsequently reoxidized at 1000°C in a dry oxygen environment. The results show that the chemistry of the oxynitride layer progressively changes by moving toward the silicon interface and, after the reoxidation process, the interface chemical configurations are strongly affected by the initial specific oxynitridation process. In particular, the application of the final reoxidation plays a significative role in determining the distribution of the oxidized O-N-Si 2 bonds near the interface.

Nitrogen bonding configurations near the oxynitride/silicon interface after oxynitridation in N2O ambient of a thin SiO2 gate.

The identification of the bonding environments and their progressive modifications upon reaching the oxynitride/silicon interface, in a SiO 2 /SiO x N y /Si structure, have been investigated by means of X-ray photoemission spectroscopy (XPS). The SiO 2 film was grown at 850 °C by means of a mixed dry-steam process, followed by a 60 min, 950 °C furnace oxynitridation in N 2 O gas. A depth profile analysis was carried out by a progressive chemical etching procedure, reaching a residual oxide thickness of about 1.2 nm. XPS analysis of the Si 2p and N Is photoelectron peaks pointed out that the chemistry of the oxynitride layer is a rather complex one. Four different nitrogen bonding environments were envisaged. Both the overall nitrogen content, which rises up to 2.5%, and its bonding configurations are progressively changing while moving towards the silicon interface.

Interface states and traps in thin N2O-grown oxynitride/oxide di-layer for PowerMOSFET devices

Abstract A detailed study of the interface state properties shown by the silicon/oxynitride/oxide gate layers used in Vertically Diffused PowerMOSFET (PowerVDMOS) technologies is reported. A quantitative analysis of interface states versus the specific N 2 O based nitridation process, extracted from current–voltage characteristics in depletion regime, provided a clear trend and turns to be of great importance for reliability performances of the final device.