Measurements of Enhanced Diffusion of Buried Layers in Silicon Membrane Structures During Oxidation
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MEASUREMENTS OF ENHANCED DIFFUSION OF BURIED LAYERS IN SILICON MEMBRANE STRUCTURES DURING OXIDATION SCOTT T. DUNHAM, ANURADHA M. AGARWAL AND NANSENG JENG Boston University, Electrical, Computer and Systems Engineering Department, 44 Cummington Street, Boston, MA 02215. ABSTRACT Reported calculations of the diffusion coefficient of silicon self interstitials vary over several orders of magnitude at temperatures of interest for integrated circuit fabrication. In this
work, we measure the enhanced diffusion of phosphorus buried layers while interstitials are injected at a wafer surface via thermal oxidation. The starting substrates were either floatzone silicon or Czochralski silicon with a pre-treatment to precipitate excess oxygen. The samples were prepared by implantation of phosphorus followed by the growth of a 40-60/Lm epitaxial layer. They were then etched anisotropically from the frontside or backside to yield membrane structures. Local oxidation was performed at 1100°C on either the frontside or backside of each wafer and buried layer diffusion was monitored, yielding information about interstitial diffusion in silicon and it's dependence on bulk properties. INTRODUCTION There exists considerable disagreement over the correct value of interstitial diffusivity in silicon. Reported values have varied over 2 to 5 orders of magnitude at temperatures of interest for integrated circuit fabrication. The calculated values of interstitial diffusivity appear to fall into two major groups, depending on the type of experiment conducted. Determination of the point defect concentrations at the frontside of a wafer via monitoring of dopant diffusion [1,2] or stacking fault growth [3] during backside oxidation, or by measuring the lateral extent of enhanced diffusion due to frontside oxidation [4,5] gives relatively low diffusivity values (- 10- 9 cm 2 /sec at 1100'C). Much larger values (.-. 10- 7 cm 2 /sec at 11000C) have been reported when monitoring the gettering of gold during during interstitial injection from backside damage [6,7] and also when monitoring the diffusion of buried layers during oxidation [8]. Despite the huge range of reported values, individual experiments or types of experiments generally give consistent and repeatable results. Previously, these differences have been explained by invoking a bulk trapping phenomenon [9]. This theory assumes that the diffusion front of interstitials is slowed down by the need to first fill up bulk trapping sites. The net result is that it takes longer for interstitial supersaturations to propagate, producing a reduced effective diffusion coefficient. The fast diffusion noted in the buried layer experiments is explained by assuming that epitaxial material has a much smaller density of these traps, while the gettering results are accounted for by assuming that the large supersaturation resulting from backside damage quickly fills the traps. An alternative explanation for the observed discrepancy is suggested by the observation that lower values of interstitial diffusivit
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