The efficient deposition of high frequency microwave energy into the top several microns of a semiconducting material was experimentally demonstrated as a highly effective mechanism for rapid thermal annealing. Simulations show that absorbed power densities of 4 and 32 kW/cm2 produce average Si heating rates of 325 000 and 10 000 000 °C/s up to 1300 °C. Conduction of thermal energy from the absorption region into the bulk substrate yields peak cooling rates that exceed 1 000 000 °C/s after the microwave pulse subsides. At the peak temperature, thermal gradients of 5 and 20 °C/μm exist for the aforementioned power densities of 4 and 32 kW/cm2. The application of a 4.5 ms, 6 kW/cm2 pulse of 110 GHz radiation resulted in an experimentally measured Si heating rate of 275 000 °C/s. Applying this millisecond microwave anneal technology to ultrarapid annealing for shallow implanted dopants resulted in ultrashallow junctions that were 14–16 nm deep with sheet resistances between 500 and 700 Ω/square and an estimated active dopant concentration of 1020/cm3–2×1020/cm3.