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The oxide gate dielectric: do we know all we should?

Silicon’s importance as a semiconductor owes much to its oxide. This oxide passivates, enables key process steps, and has been the gate dielectric of choice for MOSFETs for many years. Experience and know-how in using this oxide makes it hard for radical alternatives to be accepted. Yet it may not be possible for silicon dioxide to meet the stringent demands of the Semiconductor Industry Roadmap as a gate dielectric. This leads to clear technological questions. Is the oxide the best that can be grown? Could a better oxide be obtained by some modification of the growth process? What are the performance criteria that define the ‘best’ oxide? Are evolutionary changes, like incorporating nitrogen, to be preferred to introducing new oxides, such as those of Hf or Zr? Would there be long term problems with the new oxides? As so often in microelectronics, the technology demands new materials and new ideas from condensed matter physics. The critical role of the gate dielectric points to challenges for basic condensed matter theory, and this paper attempts to define these issues. It is certainly not sufficient to predict an equilibrium structure for oxide on silicon. The front and back regions of the oxide differ in measurable ways. Dynamical events like breakdown behaviour are certainly partly controlled by defects. Many experiments show the standard view of growth processes, ‘bulk’ diffusion and some interface reaction, is incomplete at best. How defects evolve as the oxide grows, and how impurities like H affect what happens could be crucial. Any analysis, even if only to create a framework of understanding, should address the various experiments exploiting different oxygen isotopes, the systematics of oxidation kinetics, and those electron microscope observations that show apparent layer-by-layer growth on terraces. The present paper aims to define an appropriate context for other detailed studies in the hope that progress in theory can contribute effectively to microelectronics futures.