Selective etching of silicon dioxide over silicon is a frequently used process in the manufacture of semiconductor devices; Limited diagnostic capabilities have forced plasma etch systems to rely on traditional statistical process control and recipes. With the addition of in situ measurements, however, automatic feedback control could be employed for control of the process variables. This paper focuses on the implementation and installation of a radio frequency power sensor suitable for advanced process control of plasma etching. The sensor measured information about the direct current bias voltage, radio fequency voltage, current, and phase angle at three locations in the power delivery system : before the matching network, after the matching network, and at the lower electrode. Matching network efficiency and transmission line analysis were used to transform between each measurement. This information showed the importance of accurate characterization of stray capacitance and inductance in the power delivery system. Plasma parameters of impedance, delivered power, sheath thickness, and sheath capacitance were computed using simple equivalent circuit models for the plasma discharge. Measurement of the fundamental and harmonic components of the voltage, current, and phase showed that the power generated in the plasma at the harmonic frequencies was approximately 3% of the generator power. Amplitudes of harmonic voltage matched analytical predictions.