Temperature Coefficient of Threshold Voltage in High-k Metal Gate Transistors with Various TiN and Capping Layer Thicknesses

The temperature coefficient of V_th (=dV_th/dT), which is commonly utilized for circuit design, was systematically obtained against various TiN and capping layer thicknesses in high-k/metal gate field-effect transistors (FETs). It is known that the magnitude of |dV_th/dT| for such FETs is larger than that of polycrystalline silicon (poly-Si) gate FETs. The origins of the dV_th/dT difference among high-k/metal gate FETs were attributed to differences in the temperature coefficient of flat band voltage (=dV_FB/dT) and the equivalent gate oxide thickness (EOT). Thicker TiN layers reduced dV_FB/dT, which enlarged the magnitude of |dV_th/dT|. The EOT increased as the TiN metal layer or Al₂O₃ capping layer increased in thickness. The large EOT led to an increase in |dV_th/dT|, since dV_th/dT is a function of the inverse of gate capacitance. In contrast, La₂O₃ capping hardly affected dV_th/dT. This is because La₂O₃ capping did not affect EOT differently from Al₂O₃ capping. The relationship between dV_th/dT and EOT implies that EOT scaling relieves the issue of large |dV_th/dT| for high-k/metal gate FETs.