Methods and apparatus for semiconductor manufacturing process monitoring and control are provided herein. In some embodiments, apparatus for substrate processing may include a process chamber for processing a substrate in an inner volume of the process chamber; a radiation source disposed outside of the process chamber to provide radiation at a frequency of about 200 GHz to about 2 THz into the inner volume via a dielectric window in a wall of the vacuum process chamber; a detector to detect the signal after having passed through the inner volume; and a controller coupled to the detector and configured to determine the composition of species within the inner volume based upon the detected signal.