What is the calibration procedure for a millimeter wave wafer probe station?

The calibration procedure for a millimeter-wave wafer probe station establishes the measurement reference plane at the probe tips so that the S-parameters of the on-wafer device under test (DUT) are accurately de-embedded from the measurement system (VNA, cables, probe positioners, and probes). The calibration is critical at mmW frequencies (30-300 GHz) because: the electrical lengths of the probes and cables are many wavelengths (creating large, frequency-dependent phase shifts), the probe-to-pad contact quality varies between touchdowns (creating repeatability challenges), and the on-wafer environment has parasitic coupling paths that are not present in connectorized measurements. The calibration procedure involves: selecting a calibration method (TRL (Thru-Reflect-Line) is the gold standard for on-wafer mmW measurements because it uses simple, well-characterized standards fabricated on the same substrate as the DUT; SOLT (Short-Open-Load-Thru) is used when TRL standards are not available but is less accurate at mmW due to imperfect standard models), landing the probes on the calibration standards on the impedance standard substrate (ISS) or on-wafer calibration standards (the probes must make consistent contact with the same pressure and alignment for each standard), measuring each calibration standard (thru: direct connection between probes; reflect: open or short at each probe; line: a transmission line of known impedance and length), and computing the error correction coefficients (the VNA software computes the 12 error terms that de-embed the probe, cable, and VNA systematic errors from the measurement).