D. Mathiot

Ostwald ripening of end-of-range defects in silicon

End-of-range (EOR) defects are interstitial type dislocation loops which nucleate just beneath the crystalline/amorphous (c/a) interface formed by ion implantation in Si, after the preamorphization of the substrate, and during the ramping-up of the anneal. They originate from the presence of a high supersaturation of “excess” Si self-interstitial atoms located just beneath the c/a interface. Upon annealing, the mean radius of the defects increases while their density decreases through the exchange of Si self-interstitial atoms between the loops. The number of interstitials stored in the loops stays constant. For sufficiently high thermal budgets, when the nucleation is finished, and when the local equilibrium between extended and point defects is established, the coarsening of the EOR defects can be modeled through the Ostwald ripening theory applied to the dislocation loops geometry. Indeed, and as expected from the theory, the square of the mean radius of the loop population increases with time while th...

Nucleation, growth and dissolution of extended defects in implanted Si: impact on dopant diffusion

Abstract Transient Enhanced Diffusion (TED) of boron in silicon is driven by the large supersaturations of self-interstitial silicon atoms left after implantation which also often lead to the nucleation and subsequent growth, upon annealing, of extended defects. In this paper we review selected experimental results and concepts concerning boron diffusion and/or defect behavior which have recently emerged with the ion implantation community and briefly indicate how they are, or will be, currently used to improve “predictive simulations” softwares aimed at predicting TED. In a first part, we focuss our attention on TED and on the formation of defects in the case of “direct” implantation of boron in silicon. In a second part, we review our current knowledge of the defects and of the diffusion behavior of boron when annealing preamorphised Si. In a last part, we try to compare these two cases and to find out what are the reasons for some similarities and many differences in defect types and thermal evolution depending on whether boron is implanted in crystalline or amorphous silicon. While rising many more questions, we propose a “thermodynamical” vision of the nucleation and growth of clusters and extended defects and stress the interactions between these defects and the free Si self-interstitial atoms which surround them and are the source for TED in all cases. A pragmatic approach to the simulation of TED for various experimental conditions is proposed.

TRANSIENT ENHANCED DIFFUSION OF BORON IN PRESENCE OF END-OF-RANGE DEFECTS

The presence of a supersaturation of Si self-interstitials in ion implanted silicon has been shown to be the origin of several physical phenomena such as transient enhanced diffusion (TED) of boron, the formation of extended defects at the projected range of implanted atoms at doses below the amorphization threshold, and the formation of end-of-range (EOR) defects in the case of a preamorphization stage. In this article, we discuss the relation between boron anomalous diffusion and end-of-range defects. Modeling of the behavior of these defects upon annealing allows one to understand why and how they affect dopant diffusion. This is possible through the development of the Ostwald ripening theory applied to extrinsic dislocation loops. This theory is shown to give access to the variations of the mean supersaturation of Si self-interstitial atoms between the loops and also to be responsible for anomalous diffusion. This initial supersaturation is, before annealing, at least five decades larger than the equi...

The effect of the boron doping level on the thermal behavior of end-of-range defects in silicon

The effect of the doping level on the formation and growth of end-of-range (EOR) defects is studied. Transmission electron microscopy observations have been performed on boron doped Si wafers (from 1015 to 1020 atom3) preamorphized with germanium and subjected to rapid thermal annealing. When increasing the doping level up to a few 1018 atom3, a delay in the coarsening process encountered by the loops is observed while above this threshold the EOR defects quickly disappear. These results are interpreted by considering three possibly concomitant phenomena: the formation of boron/Si-interstitial clusters, the gettering of boron at the loop periphery, and a change from the intrinsic to the extrinsic regime for self-diffusion. Moreover, an estimate of the effective trapping efficiency of boron is reached and gives about 1±0.3 Si interstitial per boron atom, a value consistent with the one obtained from studies of B transient enhanced diffusion.

Silicon self-diffusivity measurement in thermal SiO2 by 30Si/28Si isotopic exchange

Isotopic exchange is used to measure the silicon self-diffusivity in thermal silicon dioxide. The experiments, using an isotopically enriched 28Si oxide layer, are specially designed to obtain the actual equilibrium diffusivity in the oxide. A simple Arrhenius law, with an activation energy of 5.34 eV, very nicely describes the measured diffusivities as a function of the temperature. Our values are compared with literature data, and we discuss the possible origin of the observed differences.

Ultrathin n+∕p junction in preamorphized silicon by phosphorus and carbon coimplantation engineering: Influence of C location

This paper reports the influence of carbon coimplantation on the redistribution of phosphorus implanted in preamorphized Si. A strong influence of the carbon location, with respect to the P profile, is evidenced. With the help of specific simulations, a model is proposed to explain this dependence. It is shown that, in optimized conditions, it is possible to achieve P profiles suitable for the thin junctions required for future device generations.

Diffusion of low-dose implanted aluminum in silicon in inert and dry O2 ambient

Redistribution of a high-energy (3 MeV) low-dose (5×1013 cm−2) implanted aluminum profile in silicon under inert and dry O2 is investigated in the temperature range (900–1100 °C). The chemical profiles were measured by secondary ion mass spectroscopy and the effective diffusivities were extracted from the experimental data from fitting with calculated profiles obtained by numerical resolution of Fick’s law. It is found that the aluminum diffusion is significantly enhanced during thermal oxidation. The diffusivity enhancement decreases with the temperature. Comparison with boron data suggests that the mechanism of aluminum diffusion in silicon is similar to that of boron.

On the «A Symmetrical» Behavior of Transient Enhanced Diffusion in Pre-Amorphised SI Wafers

We have studied by SIMS the diffusion of boron in Ge-preamorphised silicon over a range of anneal temperatures and times, focusing on the influence of the depth of the boron profile relative to the crystalline-amorphous (c/a) interface. It is shown that, for all durations, transient enhanced diffusion (TED) occurs on both sides of the c/a interface. For short annealing times, the amplitude of TED varies by about two orders of magnitude between the surface and the end-of-range (EOR) defect band, formed just below the original c/a interface. We propose a model in which TED arises from the coupling between a Si-interstitial supersaturated « box », the EOR defect region, whose supersaturation decreases with time as the EOR defects grow, and a surface whose recombination efficiency is close to that of a perfect sink. The model successfully describes the different behavior of deep and shallow boron profiles, without requiring the existence of a diffusion barrier.

Materials Science Issues for the Fabrication of Nanocrystal Memory Devices by Ultra Low Energy Ion Implantation

Nanocrystal memories are attractive candidate for the development of non volatile memory devices for deep submicron technologies. In a nanocrystal memory device, a 2D network of isolated nanocrystals is buried in the gate dielectric of a MOS and replaces the classical polysilicon layer used in floating gate (flash) memories. Recently, we have demonstrated a route to fabricate these devices at low cost by using ultra low energy ion implantation. Obviously, all the electrical characteristics of the device depend on the characteristics of the nanocrystal population (sizes and densities) but also on their exact location with respect to the gate and channel of the MOS transistor. It is the goal of this paper to report on the main materials science aspects of the fabrication of 2D arrays of Si nanocrystals in thin SiO2 layers and at tunable distances from their SiO2/interfaces.

A Fully Characterizable Asynchronous Multiphase Delay Generator

A fully characterizable, 128-stage asynchronous Multiphase Delay Generator (MDG) integrated in standard 0.35 μm CMOS technology is presented. The circuit consists of a mirror Voltage-Controlled Delay Line (VCDL), driven by a Delay-Locked Loop (DLL), and an analog memory block. The master DLL ensures the stability over temperature and the absolute precision of the delay, whereas the mirror VCDL allows an asynchronous operation of the MDG with respect to the DLL reference clock. The memory block carries out a precise stage-to-stage delay and jitter characterization by analog sampling the state of the mirror VCDL upon an external request. Two versions of the circuit differing by their VCDL layout configurations were processed in order to compare their absolute time accuracy and jitter performance. In the first variant the two delay lines were laid out in an interlaced arrangement, whereas in the second, the mirror VCDL was positioned under the master VCDL. A maximal temporal dynamic range of 125 ps-1 ns was achieved. The single-stage delay variation with temperature was less than 1% over the 10-60°C range considered. The mean RMS jitter level per stage remained below 3% of the elementary delay over the entire dynamic range of the MDG for both circuit versions.