Measurements, Testing, and Calibration Network Analysis Informational

How do I perform an on-wafer measurement of a MMIC using probes and a VNA?

Q: How often do I need to recalibrate during wafer testing?

Recalibrate when: (1) The probe tips are cleaned or replaced (every 100-500 contacts depending on pad material and contamination). (2) Temperature changes by > 2°C (chuck temperature affects the probe and cable phase). (3) Every 2-4 hours during a long measurement session (drift in VNA, cables, and probes). (4) If the verification standard measurement shows degradation. For production testing with automated probe stations: recalibrate every 1-2 hours and verify with a check standard every 15-30 minutes. Some automated systems continuously monitor calibration quality and trigger recalibration when needed.

On-wafer measurement of a MMIC uses microwave probes connected to a VNA to characterize the die before it is packaged. The setup: (1) Equipment: VNA with sufficient frequency range, coplanar waveguide (CPW) probes (ground-signal-ground, GSG, or GSSG for differential), probe station with microscope and micro-positioner, calibration substrate (impedance standard substrate, ISS), DC bias supplies, and thermal chuck (if temperature testing is needed). (2) Probe selection: match the probe pitch to the DUT pad spacing (typical: 50, 100, 150, 200, 250 um). Match the probe frequency range to the measurement (probes are specified to 40, 67, 110, or 220 GHz). Common probe types: RF probes from FormFactor (Cascade), MPI, and GGB Industries. (3) Calibration: calibrate the VNA at the probe tips using on-wafer calibration standards on an ISS (impedance standard substrate). Standard calibration methods: SOLT with on-wafer standards (short, open, load, thru fabricated on alumina or quartz substrate). TRL/LRM/LRRM (preferred for highest accuracy): standards are fabricated on the ISS with known characteristic impedance. The calibration reference plane is at the probe tip contact point. (4) Measurement: lower the probes onto the DUT pads with controlled contact force (typically 20-50 grams). Monitor the DC contact resistance (should be < 0.5 ohms for good contact). Measure S-parameters. Apply DC bias if needed (through the probe bias tee or through separate DC probes). (5) Repeatability: probe placement must be consistent (±5 um) for repeatable measurements. Over-travel (depth of probe contact) should be controlled (typically 25-75 um for GSG probes).

Category: Measurements, Testing, and Calibration

Updated: April 2026

Product Tie-In: VNAs, Calibration Kits, Cables

On-Wafer MMIC Characterization

On-wafer measurement is the primary characterization method for MMICs, transistors, and integrated circuits during development and production. It enables testing of hundreds of die on a single wafer without the time and cost of individual packaging.

Parameter	SOLT Cal	TRL Cal	eCal
Accuracy	Good	Excellent	Good-very good
Standards Needed	4 (S,O,L,T)	3 (T,R,L)	1 (module)
Bandwidth	Broadband	Band-limited	Broadband
Setup Time	5-10 min	10-20 min	1-2 min
Best For	Coaxial, general	On-wafer, waveguide	Production, speed

Calibration Procedure

(1) GSG (Ground-Signal-Ground) probes: the most common type for single-ended RF measurements. Three tips contact the DUT pads in a G-S-G configuration. The ground tips provide the return current path. Pitch options: 50 um (MMIC, high-frequency transistors), 100 um (common for most MMICs), 150-250 um (power devices, lower-frequency ICs). (2) GSSG probes: for differential measurements. Four tips: G-S-S-G, providing balanced signal contacts. (3) DC probes: separate needle probes for DC bias and control signals. Multi-contact DC probes can provide 4-8 DC connections. (4) Probe materials: tip material is typically beryllium copper (BeCu) for flexibility and durability, with thin film metallization (gold or rhodium) for low contact resistance. Spring-loaded tips: absorb over-travel variation and provide consistent contact force. Typical probe lifetime: 50,000-100,000 contacts before tip wear requires replacement.

Performance verification: confirm specifications against the application requirements before finalizing the design
Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture

Error Sources

The impedance standard substrate (ISS) contains precision on-wafer standards: (1) Open: a CPW open-ended line. The fringing capacitance is characterized and provided in the calibration kit data. (2) Short: a CPW short (signal pad shorted to ground pads). The residual inductance is characterized. (3) Load: a thin-film 50-ohm resistor connected between signal and ground pads. Return loss: 30-40 dB to 110 GHz for precision ISS (e.g., FormFactor ISS 101-190). (4) Thru: a CPW line connecting two probe pads. Length: 200-1000 um. (5) Line: for TRL calibration, one or more CPW lines of known length and impedance. The ISS is typically fabricated on alumina (epsilon_r = 9.9) or fused silica (epsilon_r = 3.8). The same ISS can be used from DC to 110+ GHz. Calibration procedure: (a) Place ISS on the probe station chuck. (b) Land the probes on each standard sequentially. (c) Measure the VNA response for each standard. (d) Apply the calibration algorithm (SOLT, TRL, LRM, or LRRM). (e) Verify calibration quality by measuring a known device or the ISS thru standard.

Common Questions

Frequently Asked Questions

How often do I need to recalibrate during wafer testing?

Recalibrate when: (1) The probe tips are cleaned or replaced (every 100-500 contacts depending on pad material and contamination). (2) Temperature changes by > 2°C (chuck temperature affects the probe and cable phase). (3) Every 2-4 hours during a long measurement session (drift in VNA, cables, and probes). (4) If the verification standard measurement shows degradation. For production testing with automated probe stations: recalibrate every 1-2 hours and verify with a check standard every 15-30 minutes. Some automated systems continuously monitor calibration quality and trigger recalibration when needed.

Can I measure power devices on-wafer?

Yes, but with additional considerations: (1) DC power dissipation: high-power devices (GaN HEMTs, LDMOS) can dissipate watts of DC power. The probe station chuck must provide thermal management (water-cooled or thermoelectric chuck). Monitor junction temperature via infrared imaging or DC I-V characteristics. (2) RF power: standard GSG probes handle 0.5-2 W continuous RF power. For higher power: use specialized high-power probes (handling 5-20W) or pulsed measurements to limit average power. (3) Load pull: to characterize the device under varying load impedance, use on-wafer load pull setups with electronically tuned impedance tuners connected to the probe outputs.

What is the maximum frequency for on-wafer probing?

Current state of the art: coplanar probes are available to 750 GHz (specialized THz probes from FormFactor and other vendors). Practical frequency ranges: GSG probes on alumina ISS: routinely calibrated to 110 GHz with standard VNAs and frequency extenders. 110-220 GHz (WR-5): available with waveguide-output probes connected to VNA frequency extenders. 220-500 GHz (WR-2.2, WR-1.5): research-grade probes for THz characterization. Above 500 GHz: experimental probes for advanced research. At these extreme frequencies: the probe and ISS dimensions become very small (pad pitch < 25 um), and the measurement repeatability is limited by sub-micrometer probe placement accuracy.

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