Making Accurate and Reliable 4-Port
On-Wafer Measurements
There is an increasing need for engineers to characterize wafer-level,
high-speed IC devices that use differential or 4-port design architecture. A
calibrated and validated RF measurement system is needed that is capable
of testing on four ports instead of two.
However, the addition of two more measurement ports is not just the simple
addition of 2+2=4. System and calibration-related complexity and potential for
problems expand exponentially. Only recently have wafer-level tools become
available that provide precision electrical measurement for 4-port. This
paper will show how to solve the measurement complexity issues that arise
from 4-port designs and ensure that your system calibrations are accurate,
reliable and repeatable.
Overview
This application note will cover what you need to know to be successful when making on-wafer RF measurements on 4-port devices. It addresses
the most common problems encountered related to measurement and calibration, plus shows how they can be solved using Cascade Microtech’s
dual-wafer probes, calibration impedance standards, and new advanced hybrid calibration methods that minimize undesirable side effects.
Introduction to 4-Port On-Wafer Measurement
When moving from 2-port measurements to 4-port device measurements, the first thing you might ask is, “Can my 2-port methods be directly
applied to 4-port?” The answer is clearly: NO. Several issues arise related to dual-signal wafer probes, dual impedance standards, and
calibration methods used for 4-port.
Specific problems often encountered with 4-port measurements include:
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Measurement variations related to probe placement inconsistency
Electrical crosstalk from dual signal wafer probes
Calibration sensitivity to non-ideal 4-port calibration structures
Incomplete electrical knowledge of calibration elements
Propagation of unwanted modes
Lack of advanced calibrations for 4-port devices
To address these issues, Cascade Microtech has developed RF wafer-probing systems that incorporate low-crosstalk dual probes, calibration
impedance standards that minimize unwanted modes, and calibration software with advanced 4-port calibration algorithms that are less
sensitive to non-ideal cal structures.
Addressing the Challenges of 4-Port On-Wafer Measurements
Precision 4-port measurement relies on proper selection of system elements that will ensure accurate and repeatable calibration and validation
results. When building a 4-port test system, each of the components should be considered individually.
The Vector Network Analyzer
Modern VNAs provide 4-port capability to ever higher frequencies. For the most part, they use multiple 2-port calibrations (short-open-load at
each port and thrus between ports). This requires excitation of a single port at a time; the single-ended data is then mathematically converted to
mixed-mode S-Parameters. A key constraint is that the device-under-test (DUT) must be linear for accurate computation of mixed-mode terms.
As with 2-port, 4-port network analyzers crosstalk models do not address DUT dependent crosstalk inherent in wafer probing fixtures.
Differential Dual Probes
For 4-port work, dual or differential probes are often required. Although dual
probes are not new, only recently has their performance been challenged by highperformance, high-frequency 4-port analyzers. For differential measurement,
probes with low signal-to-signal crosstalk are essential. Through years of experience,
Cascade Microtech has uncovered many of the dual-probe limitations and dramatically
enhanced the technology with the Infinity Probe® line.
GSGSG Versus GSSG
For 4-port structures and corresponding dual-line probes, the ground-signal-groundsignal-ground (GSGSG) tip configuration is ideal for enabling effective calibration and
is available for frequencies up to 67 GHz. The ground connection isolates both signal
lines, reducing crosstalk between lines. Conversely, the disadvantages of groundsignal-signal-ground (GSSG) dual probes are higher signal-to-signal coupling and a
limited frequency specification.
Crosstalk in conventional metal-finger dual probes
limits their bandwidth. Note that the Infinity probe
isolation is > 40 dB up to 67 GHz.
FIG. 1
Whether using GSGSG or GSSG, typical conventional probes are limited by their long
metal tips. These mechanical probe fingers are side-by-side (coplanar waveguide),
which creates a fringing field effect between the fingers. These field patterns are hard
to control and cause coupling. Coupling between the signal lines limits the bandwidth
performance for 4-port. As a rule of thumb, 40 dB of signal-to-signal isolation is
required for best dual-calibration results. Only the Infinity probe, with its microstrip
architecture, can provide this performance. (See figure 1)
Reducing Crosstalk and Parasitics
Today’s 4-port VNAs are not able to mathematically correct for imperfect isolation
between the adjacent signal lines of on-wafer dual probes. Thus, the extent to
which the physical architecture of the probes is able to reduce this isolation is a very
important limiting factor of 4-port calibrations. One study of the magnitude of this is
shown in figure 2, which demonstrates that a high degree of isolation is required to
reduce the crosstalk impact on overall error mechanisms.
Cascade Microtech’s dual-signal Infinity probe uses microstrip transmission lines on
the probe’s thin-film tips which confine fringing fields more tightly than conventional
probes’ flexible coplanar tips. (See figure 3) The resulting improved field confinement
reduces unwanted coupling to nearby devices and other probe tips, thus increasing
RF-measurement accuracy. In addition to preventing the field lines from interacting
with the DUT, the microstrip minimizes crosstalk, making it possible to configure
dense, fine-pitch, multi-tip probes that simultaneously handle higher frequencies.
Two calibrations mathematically compared with
differences plotted as error bounds. One calibration
had the effects of the imperfect isolation artificially
removed, while the other calibration ignored these
effects (as is the typical situation in the lab). Point A and
B show the results from using the Infinity GSSG and
GSGSG probe respectively. Point C shows the isolation
results of two opposing GSG probes. Only the Infinity
GSGSG probe is able to reduce the inherent isolation to
the point that its effects approach the repeatability of the
PNA shown in orange.
FIG. 2
SOURCE: L. Hayden: “VNA Error Model Conversion for N-Port
Calibration Comparison,” Proc. ARFTG Conf., Honolulu, June 2007.
Impact of Coupling
Figure 2 shows the relatively poor isolation provided by a metal tip probe and shows
how the Infinity GSGSG probe is able to reduce the inherent isolation to the point that
its effects approach the repeatability of a VNA.
The Infinity probe’s thin film tip design, with proprietary non-oxidizing nickelalloy metal tips, allows contact on aluminum pads (typical with MOS devices)
with low and stable contact resistance. It is designed to minimize skating and pad
damage, while reducing corrosion that can cause particles to stick to the tips of
the probes. The tip design allows the probe to pierce the aluminum oxide layer on
the probe pads and hold a low and constant contact resistance of approximately
0.03 Ω. Infinity probe tips are small compared to conventional RF probes,
12 µm x 12 µm, and therefore able to contact smaller pads. Smaller pads consume
less wafer space and result in lower parasitics.
APP NOTE
::
Making Accurate and Reliable 4-port On-wafer Measurements
::
Cascade Microtech, Inc. FIG. 3 The Infinity probe’s microstrip transmission lines
on the probe’s thin-film tips confine fringing fields thus
reducing coupling.
2
Problems Posed by Impedance Standard Substrates
Impedance standard substrates are used as references for calibration to the probe
tip reference plane. For the 4-port case, they present potential problems:
• Physically, large dual standards are more susceptible to undesired modes
• The dual loop-back thrus exhibit non-ideal electrical behavior
• Limited availability of suitable dual structures
Undesirable micro-strip modes can occur when using coplanar waveguide over
a ground plane structure such as a metal chuck. These unwanted modes are
suppressed by the use of microwave absorbing material under the Impedance
Standard Substrate (ISS). Cascade Microtech’s auxiliary ISS chucks are built of this
material, thus eliminating the possibility of these modes. (See figure 5)
Cascade Microtech has also made significant improvements in small-structure
probing by optimizing calibration standards. For example, Cascade Microtech uses
loop-under grounds on the calibration structures to minimize the unwanted ground
slot mode of propagation. (See figure 4)
Comparison of S12 on an open stub using two
different calibration standards, one with and one without
a loop-under ground. Note spikes occur in the data when
loop-under ground is absent.
FIG. 4
Hybrid Calibration for 4-Port: The Best of LRRM and SOLR
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 onwafer measurements and less accurate and repeatable for making high-frequency
measurements.
Cascade Microtech auxiliary chuck mounts for
ISSs are constructed of microwave absorber material to
suppress unwanted modes.
FIG. 5
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.
Cascade Microtech’s 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, Cascade Microtech has 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 Cascade Microtech WinCal XE calibration and accuracy enhancement software.
(See figure 6)
APP NOTE
::
Making Accurate and Reliable 4-port On-wafer Measurements
::
Cascade Microtech, Inc. 3
A Complete 4-Port Measurement System
A precision 4-port RF measurement system will offer the following advantages:
• Low signal-to-signal crosstalk
• Low sensitivity to probe placement on standards
• Minimized undesired modes
• Less dependence on electrical standard definitions
• Fixed probe positioning with automatic wafer moves
• Ability to properly account for non-ideal 4-port standard loop-back thrus
Beyond calibration, WinCal XE software provides many other powerful features for
4-port including guidance for 4-port setup, post-processing of S-Parameters for
single-ended to mixed-mode conversion, along with example reports. (See figure 7)
Conclusion
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. Cascade Microtech has engineered its dual probes, dual-impedance
standards, and calibration software to optimize 4-port performance.
WinCal XE provides the unique LRRM-SOLR
calibration for superior calibration of 4-port systems. It
also includes a broad set of tools that enhance accuracy
and productivity of RF measurements.
FIG. 6
WinCal XE allows you to measure raw or corrected
S-Parameters and process the data for viewing in your
preferred format.
FIG. 7
A high-performance 4-port
system from Cascade Microtech
includes wafer probe station
with multiple-view digital
microscope, Infinity dual-signal
probes, impedance standard
substrate, phase-stable RF cables
and WinCal XE calibration and
measurement software.
© Copyright 2007 Cascade Microtech, Inc.
All rights reserved. No part of this document may be
reproduced or transmitted in any form or by any means,
electronic or mechanical, including photocopy, recording,
or any information storage and retrieval system, without
permission in writing from Cascade Microtech, Inc.
Data subject to change without notice
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