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A Guide to Better Vector Network Analyzer
Calibrations for Probe-Tip Measurements
A vector network analyzer (VNA), 6-mil distance 1-mil overlap of the pitched probe heads at both sides
such as the HP 8510, 8753, or between probe probe blade on the ISS (e.g., port-1 probe, 150 pm, port-2
87XX (includes 8719, 8720 and blades element probe, 200 pm).
8722) series, combined with Although the dominant VNA cali-
Cascade Microtech's 50-ohm Probes placed on a bration method, the SOLT calibra-
probes, such as the WPH, ACP, through-connect, or tion is not the best. Its disadvan-
FPC, or RFIC series, provides a line standard tages include:
powerful tool for IC or module l Determining parasitic inductances
characterization, verification, or Probes placed on a and capacitances is difficult, and
screening measurements. Calibrating short circuit standard their values must be consistent
the VNA with standards at the with respect to a single reference
probe tips allows removal of repeat- plane. For this reason, SOLT cali-
Probes placed on a
able errors from the VNA, cable, brations are best used below 20 GHz.
load, or match standard
and probe losses and reflections. l Most standards to contact (typi-
Accurate calibrations require care- cally 6).
ful attention to the calibration LRM Calibration. With LRM,
Probes in the air, for an
method and elements used, probe you need three calibration stan-
open standard
tip physical placement accuracy, dards: a line (or thru) standard, a
and system repeatability. reflect standard, which doesn't
require characterization, and two
GSG probe tips on ISS standards. identical match (load) standards.
Impedance Standard Substrate With an LRM calibration, the
(ISS) elements match (load) determines the refer-
Calibration methods reviewed
An ISS contains impedance stan- ence impedance. Cascade's VNA-
dards with known microwave char- There are numerous calibration Cal software also determines the
acteristics, used to calibrate the sys- methods available. The method you inductance of the match standard
tem at the probe tips. The com- choose affects the calibration. To during the calibration process.
monly used calibration standards compare two-port calibrations, Therefore, you only need to know
are: short-circuit, open-circuit, load, please refer to the table. Here we its resistance (not its reactance), and
and thru. These correspond to com- explore the four most commonly you can accurately measure resis-
monly used coaxial calibration ele- used for wafer probe calibrations: tance at dc.
ments (see Figure). SOLT, LRM, LRRM, and TRL. The LRM method's advantages
Cascade's ISS family includes: SOLT Calibration. With SOLT, include:
Ground-Signal...................(103-726) the four known references are: l Greater S-parameter accuracy
short, open, load, and thru. l Match is the only impedance
LRM Ground-Signal-Ground With SOLT, you must correctly needing self-consistent calibration
. . . . . . . . . . . . . . . . . . . . . . . . . .(101-190) define the reference planes and par- algorithm definition. This helps
asitic inductances and capacitances you avoid errors caused by
Wide-Pitch, GSG & GS for the short, open, and thru. At improperly defined open or short
. . . . . . . . . . . . . . . . . . (106-682, 106-683) higher frequencies, inaccuracies in standards. The reference imped-
W-band...........................(104-783) the parasitic descriptions greatly ance is a well-behaved, trimmed
impact the accuracy of subsequent coplanar resistor and thus provides
General-Purpose..............(005-016) measurements. a good match, enabling broad-
The reference impedance in band, accurate calibrations.
SOLT on-wafer calibrations is a l Since the reflect only needs to be
physically small, trimmed, 50-ohm identical for each port, it's easy to
coplanar resistor. This resistor is realize physically.
best modeled as a constant resis- l Suitable for systems with fixed
tance in series with a small, lumped
CASCADE
TM
probes, since it doesn't use stan-
reactance. dards of varying separation.
SOLT is useful for different
LRRM Calibration. With this that impractical lengths are neces- Cascade's VIVA-calibration software
LRM method variation, the sary at lower frequencies. ( VNACAL and WINCAL).
unknown reflects are undefined l Probe systems with fixed probes Carefully control probe tip place-
shorts and undefined opens. You are unable to accommodate the ment on the calibration elements.
need to measure the match on only additional TRL line standard Typically, these VNAs repeatably
one of the two ports. The match does length. detect phase changes due to probe
not need to be exactly 50 ohms. l Loss in the thin film coplanar displacement as small as
With LRRM, you need these cali- waveguide and microstrip lines 5 pm. All Cascade ISSs (except
bration standards: a line (or thru) causes Z, errors. This directly 005-016) include probe tip align-
standard, two different reflect stan- affects the reference impedance of ment patterns for accurate align-
dards, which do not require charac- the calibration. ments on the calibration elements,
terization, and a match (or load) and are configured to work with
standard. Cascade's automated calibration
The LRRM method has one key
Cascade's recommendations for
software module (VNACAL-10K).
advantage over the LRM method: it
achieving the best calibration
This module achieves the best
avoids discrepancies between the
accuracy
probe placement repeatability and
load reactances seen by the port-I Choose the best calibration automates the whole calibration
and port-2 probes. method for your measurement. process. The operator just places the
TRL Calibration. Also called LRL Except for special cases, the LRM or probe tips on the alignment marks
(Line-Reflect-Line), this method LRRM methods are superior for and starts the routine. (Refer to
uses: thru (or line), reflect, and one most on-wafer or module and inter- Verification of LRRM Calibrations
or more delay lines. connect measurements. The with Load Inductance Compensation
The TRL method uses the charac- firmware of the HP 85 10 B, C sup- for CPW Measurements on GaAs
teristic impedance of the delay lines ports the LRM method. And the Substrates.)
to set the reference impedance (Z,). HP 87XX VNAs support a limited Ensure system repeatability. The
TRL method disadvantages LRM method. The LRRM method VNA, test set, coaxial cables,
include: and LRM / LRRM load inductance probes, and any adapters must all
l You need more than one line corrections provide accuracy maintain very stable microwave
length to cover greater than an 8:1 improvements over the basic LRM characteristics.
frequency band. This also means method, and are only available in
Calibration Expected Typical Accuracies
VNAs Application Notes (available from Cascade) What Cascade Recommends
Methods Results (w/ 8510 ~20 GHz)
SOLT 8510A, B, C good 0.09 On-wafer measurements using the HP 8510 For probes other than GSG
87XX Network Analyzer and Cascade Microtech Only method for 8700 series
waferprobes (HP product note 8510-6) SOL (no T) for single-port cal-
ibrations
LRM 8510 B, C very 0.04 40 GHz On- Wafer Measurements with the 8510 B, C two-port, front-
87xx' good HP 8510 Network Analyzer and Cascade panel calibrations with GSG
Microtech Wafer Probes probes
LRRM 8510 B, C best 0.02 LRM and LRRM Calibrations with Automatic All other measurements
(w/L connection Determination of Load Inductance
TRL 8510 B, C poor 0.15 Applying the HP 851OB TRL Calibration for Not generally recommended,
(LRL) 2, ,87XX1 Non-Coaxial Measurements (HP product although Cascade supports the
note 8510-8) TRL method with its 8510
calibration kit (101-338)
1 implemented through the TRL* calibration kit--not recommended for wafer calibrations
2 not recommended by HP <5 GHz
3 [Sij measured - Sij actual] when measuring a passive two-port
Cascade Microtech, Inc., 2430 NW 206th Avenue, Beaverton, Oregon 97006, USA
Tel: (503) 601-1000 Fax: (503) 601-1002 Email: [email protected]
Japan: 03-5478-6100; UK: 44-1295-81-2828; www.cmicr.com
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Copyright 0 1994 Cascade Microtech, Inc.
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CASLHUC
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