A new single-wafer rapid thermal processing (RTP) reactor concept for low pressure chemical vapor deposition and other thermal processes has been developed. Instead of optical heating, microwave energy is used to heat the silicon wafer. This gives a very simple reactor design with a clean and easy to maintain interior. An added feature compared with lamp RTP is an inherent microwave plasma etch capability. The reactor, in the form of a single-mode microwave cavity, allows us to heat uniformly a 100 mm silicon wafer up to 1000-degrees-C with a uniformity of about +/- 5-degrees-C across the wafer. The reactor can easily be scaled for processing wafers up to 200 mm thick by changing the microwave frequency from the present 2.45 GHz to 915 MHz. This paper presents the principle of microwave heating. The temperature modelling shows that the temperature dependence of power absorption and thermal radiation as well as thermal conduction are important factors that redistribute uniformly the microwave power density over a wafer. Thus good temperature uniformity across the wafer can be realized, despite the radial variation in the microwave field.