Bird Technologies - Calibration Techniques for Precision Power Measurement in Semiconductor Process Applications

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By Tim Holt, Director Systems and Applications Engineering, Bird Technologies


Introduction


Radio frequency power measurement has been an important component of semiconductor processing since the time that plasma was first used for wafer processing. Power measurement was, and continues to be important, as radio frequency energy is used to excite the plasma, and the plasma characteristics are in large part determined by the excitation energy. In this article, we will discuss various means of measuring radio frequency power, along with their limitations. In addition, we will present the best calibration methods for radio frequency power measuring instruments.


Beginnings

As the need for RF power measurement developed in the semiconductor industry, the first instruments used were high power RF wattmeters that had originally been developed for use in communications applications. For the most part, these products were comprised of a section of precision air transmission line, with a lumped element directional coupler / detector system for sampling the energy in the transmission line. The directional characteristics of this instrument could be used to provide a crude indication of the load impedance relative to the characteristic impedance of the transmission line, (usually 50 ohms) the detector used in these power meters was of a single diode design, with the detector operating in square law, transition, and linear regions of its dynamic characteristic curve. The fact that the diode was operated over both square law and linear regions of its characteristic curve led to accuracy specifications as a percentage of the full-scale capability of the instrument. In most cases, the accuracy was specified as +/-5% of full scale. The majority of the error with instruments of this type is related to diode-todiode changes in dynamic characteristics, and the use of a single analog meter scale for all diode detectors...



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