The Development of Amorphous Gate Metals for Threshold Voltage Variability Reduction in CMOS Devices
Melody E. Grubbs
Department of Materials Science and Engineering
Advisors: Profs. Bruce M. Clemens and Yoshio Nishi
When: Tuesday May 31st 2011 , 2:30 pm (Refreshments at 2:15 pm)
Where: Paul G. Allen Auditorium (CIS-X 101)
http://cis.stanford.edu/misc/directions.html
Melody E. Grubbs
Department of Materials Science and Engineering
Advisors: Profs. Bruce M. Clemens and Yoshio Nishi
When: Tuesday May 31st 2011 , 2:30 pm (Refreshments at 2:15 pm)
Where: Paul G. Allen Auditorium (CIS-X 101)
http://cis.stanford.edu/misc/directions.html
Threshold voltage variability due to the polycrystalline nature of current metal gates has been identified as a problem in future generations of complementary metal oxide semiconductor (CMOS) devices. It has also been shown that the work function of these gates can vary by as much as 1 eV depending on the grain orientation. This means that as the gate dimensions become comparable to the metal grain size, the grain orientation distribution (and hence work function distribution) no longer averages out. This causes the threshold voltage to vary from device to device since the threshold voltage is directly related to the gate work function. In fact, work function differences as small as 0.2 eV have been shown to cause significant threshold voltage variation.
In order to address this variability problem, we have developed amorphous, high temperature-stable, refractory transition metal-metalloid Ta-W-Si-B and Ta-W-Si-C metal gates. The amorphous microstructure of these materials has been shown to be stable at temperatures as high as 1100C. The work functions of these alloys have also been extracted and methods for tuning their work functions will be discussed. Additionally, since Ta-W-Si-C films have been shown to be amorphous and smooth, integrating these alloys into MOS devices may also reduce mobility degradation. Thus, Ta-W-Si-C has been integrated into long channel transistor devices in order to determine whether the effective channel mobility appears to be enhanced with respect to polycrystalline gates. Finally, we will discuss the experiments that have enabled Ta-W-Si-C to be easily integrated into deposition and processing as well as our ongoing collaboration with both Applied Materials and IMEC to integrate Ta-W-Si-C into short channel devices in order to confirm the reduction of threshold variability when compared to conventional polycrystalline metal gates.
In order to address this variability problem, we have developed amorphous, high temperature-stable, refractory transition metal-metalloid Ta-W-Si-B and Ta-W-Si-C metal gates. The amorphous microstructure of these materials has been shown to be stable at temperatures as high as 1100C. The work functions of these alloys have also been extracted and methods for tuning their work functions will be discussed. Additionally, since Ta-W-Si-C films have been shown to be amorphous and smooth, integrating these alloys into MOS devices may also reduce mobility degradation. Thus, Ta-W-Si-C has been integrated into long channel transistor devices in order to determine whether the effective channel mobility appears to be enhanced with respect to polycrystalline gates. Finally, we will discuss the experiments that have enabled Ta-W-Si-C to be easily integrated into deposition and processing as well as our ongoing collaboration with both Applied Materials and IMEC to integrate Ta-W-Si-C into short channel devices in order to confirm the reduction of threshold variability when compared to conventional polycrystalline metal gates.
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