Publisher Summary A critical comparison of optical probes for real-time measurements depends on many factors: the specific application, the current state of development of each probe, competitive nonoptical probes, probe adaptability to tools, and the cost of implementation and operation. Another factor is how the probe would be accepted by industry; although this is a somewhat nebulous consideration, it is vital to assess the likelihood of acceptance. Some of these factors are rapidly changing due to new breakthroughs in technology and newly recognized needs, while others are highly subjective. Most thin film processes involve thermally activated steps. In many of these processes, the rate-limiting step is determined by kinetics rather than mass transport; it is therefore controlled by temperature. Since this dependence is usually exponential, as in the Arrhenius form kr= A exp, small changes in temperaturecan significantly affect the rate of the process. This is particularly true in deposition processes, such as CVD (chemical vapor deposition), and in rapid, high-temperature processes, such as RTP (rapid thermal processing).