Two-dimensional coupled process/device simulations have been used to characterize isolation properties of semi-recessed and fully-recessed LOCOS structures. The simulation results in excellent agreement with measurements demonstrate the sensitivity of the studied devices to the Si/SiO 2 interface shape, and thus outline the necessity of accurate simulation tooLS. INTRODUCTION Device isolation presents critical aspects for circuit packing density in VLSI. LOCal Oxidation of Silicon (LOCOS) is classically limited by the bird's beak extension and lateral diffusion of the channel-stop region into active transistor area. In order to evaluate new isolation processes (Hui, 1982; Sawada, 1935), numerous technological and device physics aspects have to be investigated to clarify important two dimensional (2D) parasitic effects, and so the use of coupled process/device simulations is of prime necessity. The IMPACT3 two dimensional device simulator, originally devoted to MOS transistor optimization, has been extended to isolation structure analysis (IMPACT3.2). This newly developed simulation program is automatically linked to the 2D process simulator IMPACT2 (Collard, 1986). In order to demonstrate the efficiency of this simulation system, a comparison between semi and fully-recessed isolation structures is presented and validated by experimental measurements.

In order to study NCLAD isolation structures, a silicon nitride oxidation model has been studied and implemented in the process simulation program IMPACT-4. Comparing the models proposed by Kamins, Enomoto and Deal and Grove (DG), it appears that the DG model is the best trade-off between accuracy and easiness of implementation. The calibration against one-dimensional experimental data has been performed and reveals an excellent agreement. The use of this new modeling capability shows that the oxidation of silicon nitride cannot be neglected when optimizing a NCLAD structure. As a result, it has been possible to improve the topography of the oxidation stack of a recessed NCLAD structure in order to obtain the minimum bird's-beak punchthrough1, 2.

Local Oxidation of Silicon (LOGOS) remains the common isolation technology for mass-production of integrated circuits, The work reported in this paper contributes to the improvement of the numerical modeling of the LOGOS process, A physical two-dimensional (2-D) modeling of the thermal oxidation of silicon has been developed based on the explicit treatment of the reaction expansion. The originality of this modeling is to propose a general solution taking into account of the silicon deformation, incorporating the viscoelastic behavior of oxide and nitride and, particularly, giving a complete calibration of the stress-dependent parameters. The prediction capabilities are demonstrated by the calculations of oxide shapes and oxidation-induced stresses in silicon substrate for very advanced isolation techniques.

This paper presents a novel approach for solving large sparse linear system arising from 3D finite element process simulation. This new method, also called the domain decomposition technique, is based on a reordering of the unknowns with respect of a partitioning of the structure. One of its major interest is in its easy parallel implementation.

Within the frame of the european PROMPT project in which a multidimensional process simulation system has been developed, a new 3D process simulation program (DIFOX3D) is implemented to simulate the diffusion and oxidation steps. Its integration into the PROMPT system allows to achieve full 3D process simulations of typical 3D problems arising in advanced ULSI devices (e.g. shape of isolation corners in oxidation step). Diffusion/oxidation models and numerical issues are described in this paper with emphasis on the local remeshing strategy which is a key point to solve the oxidation problem. The benefits of these new simulation capabilities are demonstrated by the study of 3D effects in LOCOS structure.

Within the JESSI/ESPRIT project PROMPT, a program system for the multidimensional simulation of semiconductor fabrication steps is being developed. The overall goal is to provide appropriate information on the three-dimensional device geometry and dopant distributions as input to three-dimensional device simulation. For this purpose, both existing one- and two-dimensional process simulation programs and newly developed three-dimensional simulation modules are being used. In addition to the general concept of the PROMPT system, a brief description of the approaches used for the three-dimensional simulation of the main process steps is given.

A mechanical stress analysis of the LDD-MOSFET structure is presented. It includes the cumulative effects from the oxidation, the thermal cycles and the intrinsic stresses, and uses a non-linear viscoelastic model. A calibration of the rheological properties is also proposed for APCVD doped oxides.

A model has been developed for boron diffusion after ion implantation and validated for a very wide range of doses and temperatures. For the first time, it allows the continuous simulation of the transition between amorphizing and non-amorphizing conditions. Transient-enhanced diffusion (TED) and activation are modeled by taking into account the implant damage and precipitation kinetics. It is shown that the initial level of activation is one of the most important parameters in such an analysis.<>

Boron diffusion and activation at high concentrations are key problems in the formation of shallow P+ junctions. Therefore, it is necessary to understand and to predict accurately the dopant behaviour under these conditions. In this paper, the modelling of boron is discussed, by the use of a non-equilibrium point-defect model, including amorphization and a dynamic clustering component. The initial conditions are of major importance, not only for the transient enhanced diffusion, but also for the amount of active dopants. As a result, it is possible to obtain activation levels greater than the solid solubility, as observed experimentally.

This paper addresses the current fields of interest at IEMN/ISSN concerning 3D process simulation. The emphasis is on the diffusion and oxidation steps and the associated issue of 3D mesh generation. For each step, the achievements are presented with special attention to the numerical aspects. In particular, the principles underlying local remeshing are discussed.

A model has been developed for arsenic diffusion after ion implantation and validated for a very wide range of annealing conditions including furnace and rapid thermal annealing (RTA). Transient-enhanced diffusion (TED) and activation are modeled by taking into account the ion implantation damage and clustering kinetics for arsenic. It is shown that for advanced CMOS and BiCMOS technologies the activation of impurities is governed by the rapid annealing steps through the release of charged arsenic defect pairs.

Mechanical stress induced degradations of a MOS technology are investigated. Bidimensional stress simulations, coupled with electrical characterizations reveal the importance of the gate formation and TEOS deposition steps in the generation of mechanical stress. The use of a doped oxide as interlayer material between the polysilicon gate and the TEOS film is shown to significantly reduce the residual stress and associated electrical failures

This paper presents the concepts of a mesh generation technique for 3D process simulation involving structure deformation. One of the main problems is the displacement of boundaries leading to a (complete) remeshing of the structure, large cpu times and complexity of the algorithms. Our approach, based on Delaunay criterion, tetrahedral elements and triangular faces, allows local remeshings of the structure.

A study of high-concentration boron diffusion using a precipitation model is presented in this paper. Recent experimental results on the annealing of boron implanted in preamorphized silicon give the opportunity to analyze, with the help of simulations, the precipitation kinetics and the effect of dislocation loops existing at the amorphous/crystalline interface. A nonequilibrium point-defect diffusion model is used throughout the study, including explicitly the equations describing the kinetics of precipitation. The initial conditions take into account the high level of activation observed experimentally after the solid-phase epitaxy. This influences both the sheet resistance and the doping profile shape at the end of the process. In addition, it is shown that a proper modeling of these diffused profiles includes the effect of the dislocation loops at the amorphous/crystalline interface, acting as a sink term for the interstitials. A more classical formulation of the diffusion equation does not require the modeling of such phenomena since the equilibrium concentration of the point defects is implicitly assumed. Finally, other applications to high-concentration boron diffusion are presented, demonstrating the range of validity of the involved parameters.

Typical 2D effects such as oxide thinning and bird's beak size decrease with nitride masks and windows size reduction are observed in submicron isolation structures. Such phenomena depend on the stresses generated by the oxidation process and consequently on the mechanical properties of the IC-materials. In this paper, an elastovisco-plastic modelling of the silicon oxidation is used for the calibration of the nitride properties and applied to various LOCOS processes.

A physical two-dimensional modeling of the thermal oxidation of silicon has been developed based on the explicit treatment of the reaction expansion. The stress-dependent viscoelastic behavior is fully calibrated. The prediction capabilities are demonstrated by calculations of oxide shapes and oxidation-induced stresses in silicon substrate for very advanced isolation techniques.<>

The numerical modeling of the oxidation of silicon is analyzed from a nonlinear viscoelastic approach. Its mechanical and stress dependent parameters are determined for silicon dioxide and nitride. The study focuses on the rheological behavior of the materials. The two dimensional simulations of silicon cylinders oxidation and local oxidation of silicon processing reveal that at 1000 °C, a nonlinear viscous modeling is equivalent to the nonlinear viscoelastic one. But, for lower temperatures, the discrepancies between these two models, observed in the stress calculation and final oxide shape, demonstrate the necessity for a complete nonlinear viscoelastic formulation. Finally, the calibrated model is used to study the growth of a recessed isolation structure. The investigations quantify the influence of geometrical parameters of the silicon groove on the shape of the final isolation oxide (e.g., parameters such as the silicon overetch under the pad oxide, the depth of silicon etching, the slope of the silicon sidewall and the silicon concave corner rounding).