Metal properties influencing the transistor threshold voltage

Related to the issue of downscaling in nanoelectronics is the constant need to strike a balance between opposing transistor capacities, where an improvement in one direction gives a deterioration in another. In practice, most performance data are linked together by a web of interdependences. Sections 11.1 and 11.3 exposed such problems for the relationship between channel mobility, capacitive gate coupling and effective EOT. In this chapter we will find an additional problem between the threshold voltage of the MOSFET and drain leakage. In order to achieve circuits with as low power consumption as possible, one needs to decrease the supply voltage for the transistors. This leads to a corresponding decrease in the threshold voltage, which in turn puts demands on the choice of work function values of the materials used as gate metals (Lee et al., 2006).

In traditional CMOS technology, including SiO2 dielectrics, the gate electrode is polycrystalline silicon. An advantage of using this material is that its work function, and thus the threshold voltage of the transistors, can be tuned by doping the polycrystalline material: n-type for n-channel and p-type for p-channel transistors. When efficient capacitive coupling between the gate metal and the transistor channel became an issue, the depletion region occurring at the poly-gate/oxide interface gave rise to a series capacitance, which decreased the coupling to the channel and needed to be eliminated (Engström et al., 2010). This motivated a return to metal electrodes, similar to the situation before the end of the 1970s when the gate metal was aluminum. Due to the need for different work functions of gate metals used for n- and p-channel transistors, new materials were needed. Besides the necessity of increased understanding of how novel dielectrics improve the gate capacitance, this required additional studies of metals for tuning the threshold voltage of transistors.