High and Low Temperature Measurements of the Chromium Diffusivity in Silicon
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HIGH AND LOW TEMPERATURE MEASUREMENTS OF THE CHROMIUM DIFFUSIV1TY IN SILICON J. ZHU AND D. BARBIER Laboratoire de Physique de la Matitre, Institut National des Sciences Appliqu~es de Lyon, 20 Avenue Albert Einstein, F6962l, Vifleurbanne cedex, FRANCE ABSTRACT By grouping high and low temperature diffusivity measurements in boron-doped silicon, a new diffusivity law for chromium in the 20-1050 *C temperature range has been established. High temperature diffusivities were deduced from erfc fits of chromium-boron pair profiles measured by means of Deep Level Transient Spectroscopy in chromium-plated substrates, after annealing for a short time in a lamp furnace. Low temperature diffusivities were derived from the association time constants of the chromium-boron pairing reaction in chromium-contaminated specimens. The whole data points were well fitted using the following expression for the diffusion coefficient: D= 2.6x10- 3 exp(-0.81 ± 0.02 eV/kT). Because of the wide l/T interval available, the migration enthalpy value is more accurate than the previous determinations using only high temperature diffusivity results. INTRODUCTION Transition metals are known as fast diffusers in silicon and therefore constitute a major source of electrically active defects in bulk or layered structures during microelectronic device processing. A detailed analysis of the chemical trends and properties of transition metals in silicon can be found in a review paper by E.R. Weber [1]. The 3d elements preferentially reside in the tetrahedral interstitial sites of the silicon lattice, with very low solid solubilities, and diffusivities at high temperature in the range of those measured for substitutionnal impurities in liquid silicon. Going from Cu to Ti in the 3d row, the solubility and the diffusivity decreases by several orders of magnitude. This trend is likely to be related to the increasing atomic size with the decreasing
nucleus charge. The diffusivity of most of the 3d metals in silicon have already been assessed in
the high temperature range, using sensitive methods such as Neutron Activation Analysis (NAA) [2], the radiotracer method or Deep Level Transient Spectroscopy (DLTS) [3,4,5]. However the scattering observed in the diffusivity values together with the shrinkage of the liT interval at high temperature yield only rough estimates for preexponantial factors and migration enthalpies. The diffusivity of Cr was first investigated in the 900-1250 'C temperature range by Bendik et al. [6], with the p-n junction method.These authors derived the diffusivity law: D=10-z exp(-1.0 eV/kT), which was afterwards confirmed by Wtirker et al. [7] using the same experimental technique. The migration enthalpy value of 1.0 eV suggests a gradual increase from Mn (0.63 eV) [3] to Ti (1.8 eV) [5]. However, as pointed out by Gilles et al.[3], there is only a small change in the migration enthalpy from Cu to Mn, so that the question of a sharp or a gradual increase from Mn to Ti still remains unanswered. In this work we have combined high and low
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