The Effects of Carbon on Czochralski Silicon Used for Dynamic Random Access Memory Production
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THE EFFECTS OF CARBON ON CZOCHRALSKI SILICON USED FOR DYNAMIC RANDOM ACCESS MEMORY PRODUCTION
R. A. CRAVEN*, W. E. BAILEY**, J. W. MOODY*, R. J. FALSTER*, AND L. W. SHIVE* *Monsanto Electronic Materials Company, 800 N. Lindbergh Blvd., St. Louis, MO, 63167 *-Texas Instruments, Semiconductor Process and Design Center, Dallas, TX, 75265
ABSTRACT Czochralski silicon with constant controlled oxygen level of 15+/-0.5 ppma (ASTM F121-80) and varying carbon content intentionally doped at five different levels from 0.1 ppma to 4.1 ppma (ASTM F123-81) was used to fabricate 16K dynamic random access memories, MOS test capacitors with guard rings, and pn junctions. The results of the experiment have been analyzed for relative yield to functional and refresh characteristics, MOS generation and bulk recombination lifetime, pn junction leakage, and both surface and bulk defect densities. Peak performance of the silicon did not occur at the lowest carbon level, but was dominated by the oxygen precipitate defect density and depth of the denuded zone near the active device regions. The results of the capacitor measurements, the DRAM yield measurements, the junction leakage measurements and the bulk and surface lifetime measurements are self-consistent and emphasize the need for control of the oxygen precipitation whether it is nucleated by carbon or other homogeneous and heterogeneous processes. There is no evidence that carbon has any impact on the device performance other than its impact on the precipitation kinetics of the interstitial oxygen.
INTRODUCTION Oxygen and carbon are the dominant impurities in Czochralski grown silicon. Oxygen is typically present from 10 to 20 ppma (ASTM F-121-80) and has a dramatic impact on the utility and yield of silicon slices when they are used in the manufacture of integrated circuits. Although it was first an unwanted by-product, a result of dissolution of the quartz crucibles that contained the molten silicon, an understanding soon developed that oxygen precipitates within the wafer could be beneficial to the device operation. This benefit arises because the precipitate traps impurities which can cause poor parametric, performance in the device. The precipitates themselves can be recombination and generation centers for minority carriers. This is an asset for controlling soft error rates due to alpha particle radiation and injected carriers from transient behavior of driver circuits, but can be a hazard if the generation rate is high enough and the defects are close enough to the active device region. In that case diffusion of minority carriers to the active device area may cause excessive junction leakage. Although the impact of oxygen is fairly well understood, the direct or indirect role of carbon is less clear. Several earlier studies indicated that carbon and oxygen can interact in CZ silicon. The nature of the interaction focuses on the ability of carbon impurities to nucleate oxygen precipitation, to stabilize the early stage oxygen precipitates, and to form chemical bonds with oxy
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