Characterization of MOS Structures with Ultra-Thin Tunneling Oxynitride
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H. Fujioka, C. Wann, D. Park, and C. Hu Department of Electrical Engineering and Computer Sciences, University of California at Berkeley, Berkeley, CA94720
ABSTRACT Characteristics of ultrathin silicon oxynitride (15-25A) and its interface with Si have been investigated. Oxynitride films with thickness down to 15A can be grown reproducibly in a conventional furnace. The leakage currents through these films can be well explained by the direct tunneling mechanism and can be fit by the same equation as that for pure oxide. This result indicates that incorporation of nitrogen atoms does not seriously affect the basic properties of the film and its interface such as the effective mass and the barrier height. A p-type poly gate MOS structure with 22A oxynitride has also been fabricated successfully without boron penetration even using BF2 รท ion implantation and a conventional furnace. Since the leakage current thorough oxynitride with this thickness is acceptable for circuit operation, thickness of the gate insulator in the dual poly-Si process can be scaled down at least to 22A. INTRODUCTION Thickness of gate insulator for MOS transistors has been scaled down constantly with the reduction in their gate length. In fact, NMOS transistors with gate oxide thickness as thin as 15A were fabricated and found to be promising for future VLSIs because of their high driving current capability.1 However, there exists a certain scaling limit in the thickness of the gate insulator because tunneling currents through such thin films increase drastically with the reduction in their thickness. The other key issue to achieve ultrathin gate insulator is dopant penetration from heavily doped poly-Si gates into substrates, which could 2cause instability in the threshold voltage. This is especially sever for the dual poly-Si process in which highly diffusive boron is
used for a gate dopant to obtain surface channel PMOS transistors. Recently, it has been demonstrated that the use of silicon oxynitride instead of conventional silicon dioxide in MOS transistors results in reduction in the boron diffusion. It is widely believed that nitrogen atoms incorporated in this film block the diffusion of boron atoms.' There are several additional advantages in the use of oxynitride, which include reduced charge trapping and robustness under hot carrier stress.4 In spite of the technological importance of ultra-thin oxynitride as a gate insulator for future MOS devices, characteristics of ultrathin oxynitride has not been investigated in detail yet. In this paper, we reports on characteristics of ultrathin oxynitride with thickness down to 15A and its interface with Si. EXPERIMENT MOS capacitors were fabricated on 102cm n-type (100) silicon substrates. After definition of active regions by etching field oxide, ultra-thin oxynitride films with various thickness were grown in a furnace in an N 20 gaseous atmosphere at temperatures ranging from 700'C to 850'C. Although formation of very thin native oxide could occur prior to the introduction of N20 gas in
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