Crystallographic Aspects of Low Energy Boron Implantation Into Silicon
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CRYSTALLOGRAPHIC ASPECTS OF LOW ENERGY BORON IMPLANTATION INTO SILICON R. F. LEVER AND K. W. BRANNON IBM East Fishkill Facility, Hopewell Junction, NY 12533 ABSTRACT The computer program MARLOWE has been used to investigate low energy boron implantation into silicon. When implanted in a "random" direction at 5 keV, the most deeply penetrating ions are seen to have spent an appreciable part of their path travelling in , or - 112> axial directions or in {1 I1} or {220} planar channels. Channeling was then investigated more directly by observing the mean path travelled by the ions in the direction of incidence as a function of both azimuthal and tilt angles between the direction of incidence and the 001 direction. At 5 keV, {(Il planar channeling and , , axial channeling is prominent, with {220} planar and < 310 > and < 114 - axial channeling observed to a lesser degree. Higher order planar channeling such as f400), f331 }, {422} or (5111 was not observed at 5 keV. At 0.75 keV only < 110 -. - 100> and - 112> axial channeling and {( II ) planar channeling remained. INTRODUCTION Ion implantation is a standard method for introducing dopants into silicon semiconductor devices. The need for increasingly shallow structures has led to the use of lower energies, where appreciable channeling tails are observed in the dopant profiles. Implantation into {100} oriented wafers requires the avoidance of both axial and planar channels including, but not confined to the < 100 > axial channel. A large number of relatively high index axial and planar channels may be observed by examining I MeV Helium backscattering [I]. For example, decrease in backscattering yield can be observed for (I 151 and {f135} planar directions and < 1,2,17 > , < 2,3,23 > and < 1,3,20 > axial incidence. However, for low energy boron ions, high index channeling runs into the problem of channel definition. Cho et.al. [2] have suggested excluding those directions where the critical angle for channeling is less than a crystal lattice related reference angle based on the row repetition distance and the interplanar spacing. At very low energies, a single deflection with impact parameter equal to half the planar spacing can exceed this reference angle. RESULTS In this paper, low energy boron implantation into {100} silicon is simulated by using the computer program MARLOWE [3,4]. MARLOWE was developed in the 1960's by M. T. Robinson and co-workers at Oak Ridge National Laboratory to model collision processes in crystalline solids. MARLOWE is a Monte Carlo program in which ion trajectories in solids are modeled as a series of binary collisions with target atoms correctly located in the appropriate crystal lattice. Channeling theory is NOT included in the program; indeed channeling was observed by Robinson and Oen in 1962 as a result of running the program [5]. We have used version 12 of MARLOWE (ORNL 1984) to model 5 keV boron implants into silicon [6]. Fig. 1 shows experimental SIMS profiles measured by Michel et. al. [7] for 2 x 101'#/cm2 ) dose, 5 keV boron implant
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