Characterization of Thin-Film Calibration Elements

The predominate mediums for calibrating microwave on-wafer probes is still Sapphire and alumina. The predominate methods of calibration, Open-Short-Load-Thru (OSL-T) and Line-Reflect-Match (LRM) are still based on lumped calibration elements. These lumped calibration elements have several drawbacks but two of their major limitations are: 1) the impedance standard is a thin-film 50 ohm (at DC) Load, 2) the traditional lumped calibration elements do not have consistent launches and are usually directly under the probe tip. The alternative impedance standard to the Load, precision CoPlanar Waveguide (CPW) via Line-Reflect-Line (LRL) calibration, has been available since 1989. However, the LRL method has not been widely utilized because of misconceptions concerning the dispersion of CPW at higher frequencies. This paper will present data showing that the characteristic impedance, Zo, of the CPW is a much better reference impedance than any available thin-film Load at higher microwave frequencies. We will also show the usefulness of LRL in characterizing OSL-T/LRM calibration elements. And finally, new OSL-T/LRM calibration elements, with consistent launches, will be presented.