Measurements, Testing, and Calibration Advanced Measurement Topics Informational

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

Q: What probes are used for mmW measurements?

GSG (Ground-Signal-Ground) probes with coplanar waveguide (CPW) contact tips. The probe pitch (spacing between ground and signal contacts) is typically 100-150 um for frequencies up to 110 GHz, and 50-75 um for frequencies above 110 GHz. Probe manufacturers: FormFactor (Cascade Microtech) Infinity probes, GGB Industries picoprobes, and MPI TITAN probes. The probes connect to the VNA via waveguide (WR-10 for 75-110 GHz, WR-6 for 110-170 GHz, WR-3 for 220-330 GHz) or 1 mm coaxial connectors (DC-110 GHz).

Q: How do I verify the calibration quality?

After calibration, measure a verification standard (a device not used in the calibration): a known-length transmission line (the measured loss and phase should match the calculated values), a known resistor (the measured impedance should match the nominal value), and a redundant thru or line. Also check the calibration residuals: the VNA software reports the residual error for each standard. Good calibration: residual < 0.1 dB magnitude, < 1 degree phase at 110 GHz.

Q: What are the common sources of error?

Probe placement repeatability (landing on a slightly different spot on the pad changes the contact impedance). Probe skate (the probe tip sliding during contact). Substrate mode coupling (the calibration substrate supports surface waves that couple between probes). Radiation from the calibration structures (at high frequencies, the open standard radiates, invalidating the ideal model). Cable movement (bending or moving the cables between calibration and measurement changes the phase). Temperature drift (the VNA and cables change phase with temperature; stabilize for 30-60 minutes before calibrating).

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).

Category: Measurements, Testing, and Calibration

Updated: April 2026

Product Tie-In: VNAs, Probes, Chambers, Signal Generators

Millimeter-Wave Wafer Probe Calibration

Calibration accuracy at millimeter-wave frequencies is the difference between useful measurements and meaningless data. The measurement uncertainty at 110 GHz can easily exceed the DUT characteristics if the calibration is not performed carefully.

Calibration Standards

ISS (Impedance Standard Substrate): A ceramic substrate (alumina or quartz) with precisely fabricated calibration structures: thin-film resistor loads (50 ohms), short circuits (metallized pads), open circuits (probe tip in air or defined open pad), and thru lines. ISS substrates from Cascade Microtech or FormFactor are the standard for frequencies up to 110 GHz
On-wafer TRL standards: Fabricated on the same wafer as the DUT using the same process. The thru is a direct probe-to-probe connection or a short CPW line. The line is a CPW line of known length chosen so that the electrical length difference from the thru is 20-160 degrees at the calibration frequency. Multiple lines may be needed to cover a wide bandwidth
Above 110 GHz: ISS standards become less accurate due to radiation and higher-order mode excitation. On-wafer TRL using CPW lines fabricated in the DUT process is preferred. The line dimensions must be tightly controlled at these frequencies (1 um of variation at 200 GHz causes measurable phase error)

Wafer Probe Calibration Parameters

TRL line length: L = lambda/(4) at the center frequency (90° electrical length)
At 60 GHz on alumina (Er=9.9): lambda_g ≈ 2.4 mm, L ≈ 0.6 mm
TRL bandwidth: approximately 8:1 frequency range per line
Probe contact force: 20-50 mN (enough for consistent contact without pad damage)
Measurement repeatability target: < ±0.1 dB, < ±1° at 110 GHz

Common Questions

Frequently Asked Questions

What probes are used for mmW measurements?

GSG (Ground-Signal-Ground) probes with coplanar waveguide (CPW) contact tips. The probe pitch (spacing between ground and signal contacts) is typically 100-150 um for frequencies up to 110 GHz, and 50-75 um for frequencies above 110 GHz. Probe manufacturers: FormFactor (Cascade Microtech) Infinity probes, GGB Industries picoprobes, and MPI TITAN probes. The probes connect to the VNA via waveguide (WR-10 for 75-110 GHz, WR-6 for 110-170 GHz, WR-3 for 220-330 GHz) or 1 mm coaxial connectors (DC-110 GHz).

How do I verify the calibration quality?

After calibration, measure a verification standard (a device not used in the calibration): a known-length transmission line (the measured loss and phase should match the calculated values), a known resistor (the measured impedance should match the nominal value), and a redundant thru or line. Also check the calibration residuals: the VNA software reports the residual error for each standard. Good calibration: residual < 0.1 dB magnitude, < 1 degree phase at 110 GHz.

What are the common sources of error?

Probe placement repeatability (landing on a slightly different spot on the pad changes the contact impedance). Probe skate (the probe tip sliding during contact). Substrate mode coupling (the calibration substrate supports surface waves that couple between probes). Radiation from the calibration structures (at high frequencies, the open standard radiates, invalidating the ideal model). Cable movement (bending or moving the cables between calibration and measurement changes the phase). Temperature drift (the VNA and cables change phase with temperature; stabilize for 30-60 minutes before calibrating).

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