System calibration is critical to getting accurate measurement data from RF wafer devices. During calibration, a set of known standards must be presented to the tip of the probe. This requires multiple RF probe moves to reach each calibration standard; each move requires a precise landing of probes.
Probe positional variation on the calibration standards of an impedance standard causes its electrical behavior to vary from the defined standard parameters, assuming that standard definitions are available. As a result, each move presents risk of a calibration error.
Also, as the number of ports goes up, more standards are measured. For the thru structures, the combinations can be proportional to N2. Therefore, it is highly desirable to minimize the number of structures and electrical definitions used during calibration.
There are several algorithms that can be used for calculating the error terms needed to correct raw data from the VNA. Some of these methods are less useful for on-wafer calibration measurements and less accurate and repeatable for making high-frequency measurements.
For example, for the short-open-load-thru (SOLT) calibration, accuracy is largely affected by the accuracy of the probe placement on the standards, and it requires a precise definition of all seven standards. However, some of these values are dependent upon the probe position and having the correct amount of skate on the standards while calibrating. If there is excessive or insufficient skate, the values entered into the calibration kit are not valid. This leads to errors in the calibration accuracy.
Our advanced 2-port line-reflect-reflect-match (LRRM) calibration technique has been widely adopted for on-wafer use as it corrects for the most common probe-placement errors and minimizes dependence on uncertain knowledge of electrical behavior of calibration standards. But for the 4-port case, with non-deal thrus, more has been needed.
For 4-port calibration, we’ve combined the widely accepted LRRM cal with the short-open-load-reciprocal thru (SOLR) cal, creating a hybrid cal that takes advantage of the strengths of both calibration methods for a superior 4-port calibration. This LRRM-SOLR cal handles the problem of the non-ideal electrical behavior of 4-port standard loop-back thrus as its algorithm only requires that they be electrically reciprocal. The hybrid LRRM-SOLR is provided in our WinCal XE™ calibration and accuracy enhancement software.
With increasing frequency, high-accuracy 4-port on-wafer measurements rely on proper choice of RF probes, 4-port impedance standards and advanced VNA calibration algorithms. We’ve engineered our dual probes, dual-impedance standards, and calibration software to optimize 4-port performance.
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