A Perspective on Nitrogen in Silicon
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A PERSPECTIVE ON NITROGEN IN SILICON
HERMAN J. STEIN Sandia National Laboratories,
PO Box 5800, Albuquerque,
New Mexico 87185
This is the first symposium to explicitly include nitrogen in discussions on nondopant light impurities in Si. Ordering of the impurity listing (oxygen, carbon, hydrogen and nitrogen) deserves examination. It is obvious that the impurities are not listed alphabetically nor are they listed in the order they appear in the periodic table. Presumably the ordering does not simply reflect a bias of the organizing chairmen. The impurities are, in fact, listed in an order of recognition on a chronological scale of their importance in Si technology. Equilibrium solid solubility for the different impurities, as well as concentrations of the impurities in the processing environment, probably determined the chronology. Based upon published literature, the importance of oxygen was recognized in the mid 50's, carbon in the mid 60's and hydrogen in the mid 70's. Now in the mid 80's nitrogen is receiving increased attention as an impurity in Si. There were studies in the mid 50's to early 60's of hydrogen and nitrogen in Si which showed these impurities have low solid solubilities in Si. So why are we interested now in impurities with low equilibrium solid solubilities? Several changes have been made in Si processing and additional procedures are under exploration. Some processing procedures involve the deliberate introduction of hydrogen or nitrogen, and others involve nonequilibrium processing such as ion implantation, laser annealing or rapid thermal annealing so that equilibrium conditions are not realized and solute trapping can give concentrations orders of magnitude above equilibrium solid solubilities. Formation of silicon-on-insulator(SOI) structures by implantation of nitrogen to produce a buried silicon nitride dielectric layer is an example of nonequilibrium processing. The process of implantation to form a buried silicon nitride layer also introduces large concentrations of nitrogen into the Si layer which remains over the buried nitride. Furthermore, even at low concentrations, nondopant light impurities can play important roles in defect formation and passivation in Si. The electrical passivation characteristics of hydrogen have been widely discussed, but much less attention has been given to the beneficial effects of nitrogen for suppression of swirl defects and for locking dislocations as discussed in this symposium by Abe. Other papers in the symposium discuss the interaction of nitrogen with carbon and oxygen in Si. The impact of nitrogen-defect and nitrogen-impurity interactions on the electrical characteristics of Si are unanswered questions. Likewise, little is know about the effects of direct nitridation of Si or nitridation of oxides on properties of the underlying Si. Much of what is known about the behavior of nitrogen in Si has come from studies of nitrogen-ion-implanted Si. This includes off-center substitutional nitrogen centers, nitrogen pairing and nitrogen aggregation. Th
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