This is a presentation on Negative Bias Temperature Instability (NBTI), observed in p channel MOSFET devices. Though NBTI has been discovered more than 40 years ago, in the last 10 years it has become a very important reliability concern as the industry moved from thicker SiO2 to thinner SiON gate insulators in order to keep up with Moore's scaling law. The issue is still relevant and also very much exits in the recently introduced HKMG gate stacks.
This presentation is divided into 3 parts.
In the 1st part, I will start with a brief introduction to NBTI and show some important features. This will be followed by the description of fast and ultra-fast NBTI characterization methods that are necessary for proper estimation of device degradation. Information obtained from other characterization methods such as flicker noise, DCIV, charge pumping about process related pre-existing defects and stress generated defects will be covered next.
In the 2nd part, the impact of gate insulator processes on NBTI, especially the role of Hydrogen, Nitrogen and Fluorine will be discussed. We have extensively used the ultra-fast method to study NBTI parametric degradation in FETs having a wide variety of gate insulator processes and this will be covered next. As we shall see, NBTI is strongly gate insulator process dependent and this is a crucial information to understand the underlying physical mechanism of NBTI and its optimization via suitable process changes.
In the 3rd and final part, I will discuss NBTI physical mechanism, and build a simple model to explain measured data obtained from a wide variety of devices during DC stress, recovery following DC stress and during AC stress as a function of frequency and duty cycle. A predictive model for lifetime determination under use condition will also be discussed.
I will conclude the talk with the following take home messages. NBTI stress results in generation of interface traps together with hole trapping in process related pre-existing bulk oxide traps as well as in stress generated bulk oxide traps. Though there is some differences in generated interface and bulk traps, the process related pre-existing defects are primarily responsible for large difference in NBTI magnitude, time and temperature dependence seen in differently processed devices. NBTI can be improved by suitable process modifications that reduces these pre-existing defects. Bulk trap generation is significant at higher stress bias, but due to its stronger voltage acceleration, it becomes relatively negligible, though not zero, at use conditions. The interface trap contribution shows a strong universality in terms of DC and AC degradation, and is primarily responsible for long-time failure at use condition. Finally, once the hole trap contribution is taken into account, the Reaction-Diffusion (RD) model can successfully predict DC and AC NBTI results governed by contribution from interface traps.
Acknowledgement: C. Olsen and K. Ahmed (Applied Materials), H. Aono, E. Murakami (Renesas Electronics), G. Bersuker (SEMATECH), CEN IIT Bombay, NCN Purdue, Applied Materials, Renesas Electronics, SEMATECH, SRC / GRC
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