We present a methodology termed in situ design of experiments (ISDOE) which enables rapid response surface design in plasma etch chambers while greatly reducing test wafer necessity. We experimentally demonstrate the ISDOE procedure on two separate systems using different optical real-time monitoring techniques to obtain wafer state data on film thickness and etch-rate estimation during reactive ion etch processes. Using the real-time wafer measurement tools during processing, we quickly obtain input/output (I/O) data to map plasma inputs to wafer outputs at multiple operating points, thereby reducing the number of test wafers necessary to obtain rich data sets for statistical model building and design of experiments. Both measurement techniques utilize iii situ etch-rate estimations to increase the number of rest set points explored per processed wafer and maximize the parameter space observed during each run. Model results and verification using these in situ techniques indicate no loss of accuracy when compared with traditional ex situ measurement methods for response surface design. Though the ISDOE concept is demonstrated here specifically on etch rate data, the general idea can be applied to many other real-time wafer state measurement methods. (C) 2000 The Electrochemical Society. S0013-4651(00)01-046-6. All rights reserved.