As semiconductor wafer size increases (from the current 200 to 300 mm), scaling up a process chamber to meet the same or even more stringent requirements becomes difficult due to complexity of the nonequilibrium plasmas. Designing 300 mm etching reactors can be costly and time consuming for developers without an understanding of fundamental physical and chemical processes. To expedite development and reduce cost, plasma modeling and plasma diagnostics are used to gain insight and assist the 300 mm etching reactor development. In this article, it is demonstrated that plasma modeling and Langmuir probe measurement can be used to study various plasma properties including the effects of inductively coupled power, chamber pressure, aspect ratio, and coil configuration, for a planar inductively-coupled plasma. The results from these studies are used to optimize an inductively-coupled plasma R&D chamber capable of etching 300 mm wafers.