Silicon-based vertical MOSFETs

Jayanarayanan, Sankaran
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For over three decades, the Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) has successfully undergone scaling to improve performance, and is presently at the sub-100 nm technology node. This has been possible due to several advances in the field, such as the introduction of copper interconnects, low-K dielectrics, silicided contacts, source-drain extensions, etc. Future scaling will require new materials such as strained silicon or silicon-germanium for channel mobility and drive current enhancement, high-K gate dielectrics to reduce the gate leakage current, and novel devices such as vertical MOSFETs or Fin-FETs to suppress short-channel effects. In this work, we have fabricated sub-100 nm silicon-based vertical MOSFETs, such as 70 nm strained and unstrained silicon-germanium vertical MOSFETs, 90 nm vertical MOSFETs with hafnium-oxide gate dielectric deposited by chemical vapor deposition (CVD), and a novel 50 nm Dielectric Pocket Vertical MOSFET (DPV- MOSFET) that shows excellent suppression of short channel effects. All samples were grown with the help of Ultra-high Vacuum Chemical Vapor Deposition (UHVCVD). We have demonstrated improved hole mobility and drive current in the SiGe PMOSFETs with a uniform Ge profile. However, the SiGe devices also had a higher leakage current and lower breakdown voltage due to the smaller bandgap of SiGe as compared to Si. The unstrained SiGe vertical MOSFETs were grown on a relaxed SiGe virtual substrate, and did not show a mobility enhancement, indicating that the improvement in mobility in strained SiGe is due to strain in the crystal lattice and not just Ge content. We observed conformal dielectric deposition and reduced gate leakage currents in the vertical MOSFETs with hafnium-oxide deposited by Rapid Thermal Chemical Vapor Deposition. The SiGe devices with CVD-HfO2 gate dielectric showed improved drive currents. We also fabricated a novel device called the DPV-MOSFET. Introduction of a dielectric pocket at the source-channel junction results in a device with a shallower equivalent source junction depth and hence reduced short-channel effects such as VTrolloff and drain induced barrier lowering (DIBL). Simulation results indicate that the device also a higher ION/IOFF ratio.