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Title: Mobility Degradation of Advanced CMOS Devices
Authors: Lazar, Heather Rebecca
Advisors: Dr. Eric Vogel, Committee Member
Dr. Veena Misra, Committee Chair
Dr. Griff Bilbro, Committee Member
Dr. Jayant Baliga, Committee Member
Dr. Gerry Lucovsky, Committee Member
Keywords: MOSFET
high k dielectrics
metal gate electrodes
bulk trapping
interface traps
charge pumping
Issue Date: 5-Apr-2005
Degree: PhD
Discipline: Electrical Engineering
Abstract: As alternative materials are being pursued for CMOS gate dielectric scaling to below 1 nm, challenges arise that warrant investigation for greater understanding. Mobility values, in particular, of ultra thin SiO2 and high k MOSFET devices have been reported to be low and the reason behind this degradation is still unclear. Furthermore, a study on the effects of implementation of candidate metal gate electrodes on mobility values has not yet been performed. Since many of the properties of these materials are still under scrutiny, accurate determination of their electrical and analytical characteristics is needed. In this work, an investigation of the mobility of several candidate metal gate electrodes on high k dielectrics was performed and compared with that of SiO2 and polysilicon. This work was separated into two components. The first study compared three metal gate electrodes on HfO2 with that of SiO2 in the 2 nm EOT range fabricated at North Carolina State University. Standard and advanced electrical characterization, including Dit and bulk trapping characteristics, was performed including two and three level charge pumping and other pulsed methods. It was found that mobility values were very similar for the metal gate electrodes both on high k and SiO2. The second study analyzed the mobility values for aggressively scaled EOT (< 2nm) devices with a metal and polysilicon gate electrode indicating a severely degraded mobility. Both the interface and the bulk trapping behavior of these devices were also analyzed and used to correct the mobilities. However, corrections to mobility for both trapping and interface trap density did not recover the mobility of these devices. The mechanism responsible for mobility degradation beyond trapping in these aggressively scaled MOSFETs will be discussed. Finally, steps necessary to enhance mobility will be presented.
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