May 10, 2011
HV-SSRM profiles carriers in nanowire-based transistors
Researchers from the nanoelectronics research centre Imec in Belgium have successfully developed a methodology to map quantitatively the distribution of active dopants in confined 3D volumes. The method is based on high-vacuum scanning spreading resistance microscopy (HV-SSRM). This research is an important step towards in-depth understanding of nanowire-based transistors.
The incorporation of dopants and their activation and diffusion behaviour in 3D nanostructures for nanowire-based transistors can be significantly different from planar devices and are not yet fully understood. Recently, researchers from Imec have extended the applicability of HV-SSRM towards fully integrated nanowire-based transistors. The new procedure was applied to Si-nanowire-based tunnel-FETs and enabled the team to identify a diameter-dependent dopant-deactivation mechanism, which only occurs in small 3D structures and cannot be predicted using standard process simulation tools. By linking the actual device characteristics to the experimentally observed carrier distributions, the researchers could further emphasize the significance of their findings reported in the journal Nanotechnology.
Scanning spreading resistance microscopy (SSRM) is an atomic force microscopy (AFM)-based technique, which was invented by W Vandervorst et al. at Imec in 1994. Due to its unique spatial resolution (1.3 nm) and high sensitivity, the technique has evolved into the method of choice for carrier profiling in planar MOS transistors during the last decade.
In the current work, the group has used this methodology to demonstrate a diameter dependence of the carrier distribution within nanowire-based tunnel-FETs that can be shown to directly impact on the device performance.
A diameter dependence of the carrier distribution in the nanowire top section ("dual bump" structure for larger diameters vs flat profile for narrow nanowires) resulting from a tilted ion implantation step can be predicted using process simulation tools and thus proves the validity of the team's technique.
In contrast, the researchers observe a dopant deactivation mechanism at the nanowire bottom, which is also diameter dependent and is not accounted for in the current process simulations. It could be shown experimentally and by device simulations that this directly impacts the device characteristics. The group's results prove that HV-SSRM is capable of revealing physical phenomena, which are only present in small, 3D structures, and which cannot be predicted by blanket experiments. Such information is essential for the process development of future nanowire-based transistors.
More details can be found in the journal Nanotechnology.
About the author
Andreas Schulze is a PhD student in the department of physics and astronomy at KU Leuven (Belgium) and conducts his research on dopant profiling of advanced nanostructures at Imec (Leuven, Belgium) in the group of Prof. Wilfried Vandervorst, head of Imec's material analysis group and a professor at KU Leuven.