Multi-threshold CMOS

Multi-threshold CMOS (MTCMOS) is a variation of CMOS chip technology which has transistors with multiple threshold voltages (V_th) in order to optimize delay or power. The V_th of a MOSFET is the gate voltage where an inversion layer forms at the interface between the insulating layer (oxide) and the substrate (body) of the transistor. Low V_th devices switch faster, and are therefore useful on critical delay paths to minimize clock periods. The penalty is that low V_th devices have substantially higher static leakage power. High V_th devices are used on non-critical paths to reduce static leakage power without incurring a delay penalty. Typical high V_th devices reduce static leakage by 10 times compared with low V_th devices.

One method of creating devices with multiple threshold voltages is to apply different bias voltages (Vb) to the base or bulk terminal of the transistors. Other methods involve adjusting the gate oxide thickness, gate oxide dielectric constant (material type), or dopant concentration in the channel region beneath the gate oxide.

A common method of fabricating multi-threshold CMOS involves simply adding additional photolithography and ion implantation steps. For a given fabrication process, the V_th is adjusted by altering the concentration of dopant atoms in the channel region beneath the gate oxide. Typically, the concentration is adjusted by ion implantation method. For example, photolithography methods are applied to cover all devices except the p-MOSFETs with photoresist. Ion implantation is then completed, with ions of the chosen dopant type penetrating the gate oxide in areas where no photoresist is present. The photoresist is then stripped. Photolithography methods are again applied to cover all devices except the n-MOSFETs. Another implantation is then completed using a different dopant type, with ions penetrating the gate oxide. The photoresist is stripped. At some point during the subsequent fabrication process, implanted ions are activated by annealing at an elevated temperature.

In principle, any number of threshold voltage transistors can be produced. For CMOS having two threshold voltages, one additional photomasking and implantation step is required for each of p-MOSFET and n-MOSFET. For fabrication of normal, low, and high V_th CMOS, four additional steps are required relative to conventional single-V_th CMOS.