Cheruvu S. Murthy

Diffusion of implanted nitrogen in silicon

Growth of thinner gate oxides and their thickness control is one of many challenges in scaling technologies today. Nitrogen implantation can be used to control gate oxide thicknesses. This article reports a study on the fundamental behavior of nitrogen diffusion in silicon. Nitrogen was implanted as N2+ at a dose of 5×1013 ions/cm2 at 40 and 200 keV through a 50 A screen oxide into Czochralski silicon wafers. Furnace anneals at a range of temperatures from 650 to 1050 °C have revealed anomalous diffusion behavior. For the 40 keV implants, nitrogen diffuses very rapidly and segregates at the silicon/silicon-oxide interface. Qualitative modeling of this behavior is also discussed in terms of the time taken to create a mobile nitrogen interstitial through the kickout, Frenkel pair, and the dissociative mechanisms.

Effects of Peierls barrier and epithreading dislocation orientation on the critical thickness in heteroepitaxial structures

We have extended the mechanical equilibrium theory of J. W. Matthews and A. E. Blakeslee [J. Cryst. Growth 27, 118 (1974)] (MB) for determining the critical thickness in semiconducting heteroepitaxial films by including the effect of the Peierls barrier. The new formulation allows an evaluation of the dependence of critical thickness on the orientation of epithreading dislocation, and a comparison of theoretical predictions with measurements indicates that a knowledge of the epithreading dislocation orientation is necessary in predicting critical thicknesses in heteroepitaxial structures. In this formulation, the effect of the Peierls barrier is to bring the theoretical critical thicknesses closer to experimental values as compared to the predictions of the MB theory.

Computer simulation studies of ion implantation in crystalline silicon

Results from systematic computer simulation studies of boron, phosphorus, and arsenic implants into silicon that are important to integrated circuit technology are discussed in terms of interatomic potential, electronic energy loss, and target orientation effects. Detailed crystallographic analyses of axial and planar channeling are presented. The time evolution of an interesting collision cascade which depicts accidental channeling and dechanneling for a 5-keV boron implant is also presented. An examination of the binary collision cascade code MARLOWE is made and applied to simulate hyperchanneling of high-energy alpha particles in silicon. >

Thermodynamic and kinetic studies of laser thermal processing of heavily boron-doped amorphous silicon using molecular dynamics

Laser thermal processing (LTP) has been proposed as a means to avoid unwanted transient enhanced diffusion and deactivation of dopants, especially boron and arsenic, during the formation of ultrashallow junctions. Although experimental studies have been carried out to determine the efficacy of LTP for pure Si and lightly B-doped junctions, the effects of high concentrations of dopants (above 2% B) on the thermodynamic and kinetic properties of the regrown film are unknown. In this study, a classical interatomic potential model [Stillinger–Weber (SW)] is used with a nonequilibrium molecular dynamics computer simulation technique to study the laser thermal processing of heavily B-doped Si in the range 2–10 at. % B. We observe only a small effect of boron concentration on the congruent melting temperature of the B:Si alloy, and thus the narrowing of the “process window” for LTP is predicted to be small. No significant tendency for boron to segregate was observed at either the regrowth front or the buried c-Si interface during fast regrowth. The B-doped region regrew as defect-free crystal with full activation of the boron atoms at low boron concentrations (2%), in good agreement with experiments. As the concentration of boron increased, the number of intrinsic Si defects and boron interstitials in the regrown materials increased, with a minor amount of boron atoms in clusters ( 150 ps, compared to 5 ps in tight binding). The importance of adequate system size is discussed.

Electronic excitation and quenching of atoms at insulator surfaces

A semiclassical method is employed for dynamical calculations of electronic transitions in collisions of gas atoms with insulator surfaces. The theory is based upon combining Micha’s self‐consistent eikonal method (SCEM) with a stochastic reduction of the equations of motion for the condensed phase as represented in a generalized Langevin equation (GLE). The merged theory provides a framework that manifests the attractive computational advantages of both the SCEM and GLE modeling methods and can be readily applied to many modern problems involving electronically inelastic gas/surface collisions. The theoretical approach is numerically illustrated for a simple two‐electronic‐state curve crossing problem, where the effects of model parameters, surface temperature, and collision energy upon transition probabilities and energy accommodation are examined. For the model system studied the loss of energy of the gas atom into the surface is appreciable with pronounced effects depending upon the electronic transit...

Nitrogen-induced transient enhanced diffusion of dopants

Studies of both systematic experiments and detailed simulations for examining the effects of N2+ implant on channel dopants are described. Step-by-step monitor wafer experiments have clearly confirmed the nitrogen-induced transient enhanced diffusion (TED) of dopants. Process simulations within the “+1” N2+ profile approach have demonstrated the need to scale down the +1 model parameter for matching the measured depth profiles. The underlying mechanism for the reduced +1 model parameter is that nitrogen which diffuses toward the Si surface becomes a sink for the interstitials. These combined studies also show that nitrogen-induced TED of dopants increases with N2+ dose.

Kinetics of strain relaxation in Si1−xGex thin films on Si(100) substrates: Modeling and comparison with experiments

We report the results of a theoretical analysis for the kinetics of strain relaxation in Si1−xGex thin films grown epitaxially on Si(100) substrates. The analysis is based on a properly parametrized dislocation mean-field theoretical model describing plastic deformation dynamics due to threading dislocation propagation and addresses strain relaxation kinetics during both epitaxial growth and thermal annealing, including post-implantation annealing. Theoretical predictions for strain relaxation as a function of film thickness in Si0.80Ge0.20∕Si(100) samples annealed after epitaxial growth either unimplanted or after He ion implantation are in excellent agreement with experimental measurements [J. Cai et al., J. Appl. Phys. 95, 5347 (2004)].

Effects of Surface Structure and of Embedded-Atom Pair Functionals on Adatom Diffusion on FCC Metallic Surfaces

Abstract Rates of self-diffusion on the (100) and (110) surfaces of nickel have been calculated using variational transition state theory (VTST) and four different interatomic potential energy functions based on the embedded-atom method (EAM). Static properties of a single nickel atom on the (111) surface, as well as on the (100) and (110) surfaces, are also presented. The embedded-atom pair functionals consist of effective pairwise additive and many-body cohesive interactions parameterized to the bulk and a few defect properties of nickel. VTST calculations of surface diffusion provide Arrhenius parameters and diffusion coefficients for comparison with experiment and among the four EAM potentials employed. An analysis of the estimated diffusion rates based on a hopping mechanism and the four pair functionals reveals that diffusion will occur more readily on the (111) surface and that self-diffusion on the (110) surface exhibits directional anisotropy. The diffusion rate variation from one pair functional to another is interpreted in terms of the effective pair potentials.

Threshold voltage roll-up/roll-off characteristic control in sub-0.2-/spl mu/m single workfunction gate CMOS for high-performance DRAM applications

Threshold voltage (V/sub t/) roll-off/roll-up control is a key issue to achieve high-performance sub-0.2-/spl mu/m single workfunction gate CMOS devices for high-speed DRAM applications. It is experimentally confirmed that a combination of well RTA and N/sub 2/ implant prior to gate oxidation is important to reduce V/sub t/ roll-up characteristics both in nFET and pFET. Optimization of RTA conditions after source/drain (S/D) implant is also discussed as a means of improving V/sub t/ roll-off characteristics. Finally, the impact of halo implant on V/sub t/ variation in sub-0.2-/spl mu/m buried channel pFETs is discussed. It is found that halo profile control is necessary for tight V/sub t/ variation in sub-0.2-/spl mu/m single workfunction gate pFET.

Nitrogen Implantation and Diffusion in Silicon

Nitrogen implantation can be used to control gate oxide thicknesses [1,2]. This study aims at studying the fundamental behavior of nitrogen diffusion in silicon. Nitrogen at sub-amorphizing doses has been implanted as N 2 + at 40 keV and 200 keV into Czochralski silicon wafers. Furnace anneals have been performed at a range of temperatures from 650°C through 1050°C. The resulting annealed profiles show anomalous diffusion behavior. For the 40 keV implants, nitrogen diffuses very rapidly and segregates at the silicon/ silicon-oxide interface. Modeling of this behavior is based on the theory that the diffusion is limited by the time to create a mobile nitrogen interstitial.