In a conventional CMOS process, polycrystalline-silicon doped heavily by ion implantation is used as the gate material. Thermal annealing (either in a furnace or rapid thermal annealer) is carried out to activate the dopants. However, due to the non-uniform implanted dopant profile and thermal budget constraints, it is difficult to achieve very high active dopant concentration (>10E20 cm-3) at the gate/dielectric interface. As a result, the lower portion of the gate electrode is depleted when the MOSFET is turned on, effectively increasing the thickness of the gate dielectric and thereby degrading the transistor drive current. The gate-depletion problem becomes significant as the equivalent oxide thickness (EOT) is scaled below 2 nm and the power-supply voltage is reduced to 1 V and below, and hence is a serious problem for sub-90 nm CMOS technologies.
In this project, pulsed (~30 ns) excimer laser (248 nm) annealing (ELA) is being investigated as a means to achieve high active dopant concentration with minimal thermal budget. The gate material is deposited in amorphous form (to provide a low melting temperature) and then implanted with dopants. An excimer laser pulse is then applied to momentarily melt the gate layer. In the melt, the dopants redistribute rapidly, resulting in a box-shaped concentration profile. The rapid cooling and crystallization process yields an active dopant concentration higher than the solid solubility limit. Thus, the gate depletion problem can be greatly alleviated.
Both n-channel and p-channel MOS devices will be fabricated using Si(1-x)Ge(x) (x=0, 0.2, or 0.4) as the gate material. The purpose of using a silicon-germanium alloy is to lower the gate melting temperature. (For example, the melting point of amorphous Si0.8Ge0.2 is estimated to be ~1000°C.) This may be needed in order for the ELA process to be used in conjunction with a high-permittivity gate dielectric material. The effect of ELA on gate sheet resistance, gate depletion, gate leakage, and gate-dielectric reliability will be monitored as a function of laser fluence for the various gate materials.