Ultra-thin Gate Oxide Prepared by Nitridation in ND 3 for MOS Device Applications
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Ultra-thin Gate Oxide Prepared by Nitridation in ND3 for MOS Device Applications
Hyungshin Kwon and Hyunsang Hwang Department of Materials Science and Engineering Kwangju Institute of Science and Technology 1, Oryong-dong, Puk-gu, Kwangju, 500-712, KOREA, email: [email protected]
ABSTRACT The electrical and reliability characteristics of ultra-thin gate oxide, annealed in ND3 gas, have been investigated. Compared with a control oxide, which had been annealed in NH3, the ND3-nitrided oxide exhibits a significant reduction in charge trapping and interface state generation. The improvement of electrical and reliability characteristics can be explained by the strong Si-D bond at the Si/SiO2 interface. This nitridation process of gate dielectric using ND3 has considerable potential for future ultra large scaled integration (ULSI) device applications.
INTRODUCTION As device sizes are scaled down in order to achieve ultra large scaled integration (ULSI), the reliability of ultra-thin gate oxide becomes one of the most critical factors in metal-oxide-semiconductor field effect transistor (MOSFET) technology. The proper scaling of gate oxide thickness can improve current driving capability and reduce short channel effects. However, ultra-thin oxide films have numerous reliability problems, including timedependent dielectric breakdown, interface state generation, charge trapping, and threshold voltage shifts as the results of penetration of dopant. Nitridation of the gate oxide in NH3 has been investigated in order to improve gate dielectrics integrity [1]. The incorporation of nitrogen in gate dielectrics leads to reduced boron penetration which is a contributor to threshold voltage instability, especially for ultrathin gate oxide with a thickness of less than 5nm [2]. However, post-oxidation annealing in NH3 requires an additional reoxidation to reduce the degradation of dielectric reliability [1]. Without this reoxidation, the electron trapping characteristics of NH3 nitrided gate oxide is significantly high, because of the high concentration of hydrogen. It is known that the electrical and reliability characteristics of the dielectric are dependent on the concentration of hydrogen-related species in the oxide bulk and the interface [3, 4]. According to Yoshii et al., forming gas annealing at high temperatures significantly enhances both interface and oxide trap generation under electrical stress [5]. According to Lyding et al., a significant improvement in the reliability characteristics under hot carrier stress was observed, after the deuterium annealing of a MOSFET device
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[6-8]. Because of the heavy mass of deuterium, Si-D bonds are more difficult to break than Si-H bonds under hot carrier stress [7]. However, no improvement in MOSFET hot carrier reliability was observed when the annealing was performed after the deposition of a silicon nitride layer [7, 8]. Since the silicon nitride is a strong barrier to the diffusion of deuterium and hydrogen, it is difficult to incorporate a sufficient amount of deute
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