Download improve accuracy of on-wafer tests via lrm. calibration

IMPROVE ACCURACY
OF ON-WAFER TESTS
VIA
LRM.
CALIBRATION
The line-reflect-match (LRM) calibration method
combines the advantages of other techniques.
T
WO techniques are commonly utilized to calibrate network
analyzers and wafer-probing systems. The short-open-load-through
(SOLT) method requires three impedance standards and a throughline. The through-reflect-line (TRL)
technique requires only a throughline and one or more offset-length
transmission lines as references.
A new calibration technique, the
line-reflect-match (LRM) method,
combines some of the advantages of
SOLT and TRL calibration without
their disadvantages. Each type of
calibration determines a twelveterm error model that quantifies
systematic errors such as signal
leakage, impedance mismatches,
and frequency response. However,
the calibration standards used. by
any method determine its relative
STEPHEN LAUTZENHISER. Product Marketing Engineer, Hewlett-Packard
Co., Santa Rosa, CA, (707) 577-3140;
and ANDREW DAVIDSON, Develop
ment Engineer, and KEITH JONES,
Product Development Manager.
Cascade Microtech, Inc., Beaverton,
OR, (503) 601-1000.
Reprinted with permission from MICROWAVES & RF - January 1990
Copyright 1990 Penton Publishing, Inc.
D
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stations.
A variety of impedance and transmission-line standards can be used
to calibrate on-wafer measurement
systems. These electrical references
are typically fabricated on a substrate employed specifically for calibration. Because the SOLT and
LRM techniques require only one
transmission-line standard, they can
be used with fixed probes. The TRL
method requires transmission-line
standards of different lengths and
cannot be used with fixed probes.
Calibration with the SOLT method requires four known references,
all of which must be accurately modeled. Open standards are usually
provided by open-circuited probes.
However, open coplanar probes exhibit capacitance that must be empirically determined for each probe
tip.’ On-wafer Short references
have an inductance that must also
be measured. As frequency increases, accurate modeling of Short
and Open standards becomes more
complex due to the increasing effects of parasitic capacitances and
inductances. Broadband loads and
transmission lines are usually less
difficult to model.
The TRL technique requires only
a through-line (Through) and a
transmission-line offset (Line) as
references. The TRL method also
employs highly reflecting impedances (Reflects) as standards, but
their exact electrical characteristics
are not required.’ However, Reflect
standards must be accurately repeated at each test port.
A limitation of the TRL technique
is the limited bandwidth of Line
standards. Most Line standards can
only be used over an 8:l frequency
range. For broadband measurements, several Line standards may
be required. At low frequencies,
Line standards can become inconveniently long.
The SOLT calibration technique is
often preferred when S-parameters
are measured with respect to an ideal characteristic impedance such as
S
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N
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able for obtaining S-parameters
with respect to the impedance of onwafer transmission lines. When accurate on-wafer Line standards are
available, the TRL method usually
offers better accuracy than the
SOLT technique.
The LRM calibration method is
similar to the TRL technique.3 However, a single transmission line is
used as the Through standard, and
broadband Loads provide impedance references. The LRM technique takes advantage of the highquality coplanar transmission lines
and loads that can be fabricated on
many microwave substrates.? By using a pair of coplanar Loads instead
of offset transmission lines, the
LRM method avoids the low-frequency limitations of the TRL technique. Additionally, the LRM method can be used with fixed probes,
and accurate Short and Open refer-
U
R
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ences are not required.
To compare the accuracy of the
SOLT, TRL, and LRM calibration
techniques, each method was used
to calibrate a Cascade Microtech
probe station and an HP 8510B network analyzer from Hewlett-Packard Co. The measurement system
was calibrated from 45 MHz to 40
GHz using a coplanar. impedance
standard substrate.
COMPARING ACCURACY
For the ‘SOLT calibration, ‘Load,
Short, and Through standards were
provided by an impedance standard
substrate. Open-circuited probe tips
served as Open. references. The
same transmission line used for the
Through standard in the SOLT calibration was employed as the Line
standard in the LRM calibration.
Additionally, the LRM calibration
utilized the same loads as those used for the SOLT
calibration.
The TRL calibration employed two Line standards
with delays of 8 and 40 ps to cover 1.5 to 40 GHz. For
measurements at lower frequencies, SOLT calibration
data was used. The Open standard in the SOLT calibration was modeled with a capacitance of 15 fF, while the
Short was assumed to have zero inductance. The SOLT
Load had 7-pH parasitic inductance and the Through
had l-ps delay.
Measurements of a 15-ps transmission line indicated
the relative accuracy of the calibration. techniques (Fig.
1). Results from-the LRM and TRL calibrations are similar, but data obtained using the SOLT method has relatively poor accuracy.’ The errors resulting from the
SOLT calibration are mainly due to inaccurate definitions of the Open and Short standards.
The same transmission line was measured as an open
as skin-effect and radiation losses increase. Both the
using the TRL calibration technique are low by about
0.1 dB at 2 GHz. This is probably the result of losses in
the Line standard. The TRL method assumes that the
characteristic impedance of Line standards are real values (resistive). However, signal loss produces a complex
value for the characteristic impedance of a transmission
line.
Dispersion also affects calibration accuracy. To illustrate this, a 50-R resistor was measured (Fig. 3). Although an on-wafer resistor is essentially a series resistor-inductor circuit at low frequencies,” the data obtained using TRL calibration does not follow such a
model. Dispersion and losses in the Line standards can
6. The overall quality of an LRM calibration Is illustrated by measuring an
open stub and displaying the results on a Smith chart.
n
to verify that the correct amount of overlap is used
standard. Excessive overlap on the Load standard durthe Open standard. Too little overlap has the opposite
effect.
Using probes with
pitch, overlap during calibration was varied from an ideal value of 25
Measurements of the Open standard indicate the effects of
varying overlap during calibration (Fig. 4). When a 40-ps
offset Open was measured, calibration errors were
more pronounced (Fig. 5). The overall quality of an LRM
calibration is illustrated by a broadband measurement
of a 15-ps open stub (Fig. 6). A smooth spiral indicates
an accurate calibration.
The accuracy of the LRM calibration method demonstrates its superiority over the SOLT and TRL techniques. Requiring fewer standards and offering wider
frequency coverage, LRM will likely become the preferred method of calibrating on-wafer microwave measurements.
Cascade Microtech, Inc., 206 NW 206th Avenue, Beaverton, Oregon 97006, USA
Tel: (503) 601-1000 Fax: (503) 601-1002 E-mail: [email protected]
Japan: (03) 5478-6100; Europe: +44 (0) 1295-812828
WWW site: www.cascademicrotech.com E-mail: [email protected]
IMPLEMENTING THE
LINE-REFLECT-MATCH (LRM)
CALIBRATION TECHNIQUE FOR
ON-WAFER MEASUREMENTS
This supplement details how to define each of the LRM standards
within the HP 8510B TRL feature set. It also offers some practical
guidelines for making the highest quality on-wafer calibrations.
DEFINING THE LRM CALIBRATION
STANDARDS
The LRM calibration is implemented by changing the definitions of the HP 85108 TRL standards. Before calibrating, the
calibration standards must be manufactured and then be
defined in a calibration kit. An HP 8510B and good onwafer through-lines, shorts, and fixed loads amount to all the
hardware required to make LRM possible.
The Standard Definitions Table and the Standard Class
Assignments Table below show how each LRM standard is
defined within and assigned to its corresponding TRL class.
The LRM tine, LRM Reflect, and LRM Match standards are
respectively assigned to the TRL Thru, TRL Reflect, and TRL
Line standard classes. The standard number of the LRM
Match must, for example, be assigned to the TRL tine class
of standards. The standard numbers selected are only one
example; other numbers may be used.
Cascade Microtech’s latest Impedance Standard Substrate
(P/N 101-190) and accompanying cal kit tape, probe 1.2 (P/N
019-010), are specifically arranged to help users perform LRM
calibrations quickly and accurately. Quick contact of the tine,
Reflect, and Match standards is possible since each set of
standards are equidistant. These ground-signal-ground coplanar standards are defined collectively as cal kit PROBE.LRM
and are specified below in Table 1 and Table 2.
Why a Load can be defined in the
TRL Line class
The zero-length definition of the LRM tine standard allows
the substitution of fixed loads as transmission lines in the
TRL Line class of standards. In measuring the Match as a
TRL tine standard, the HP 8510B measures all 4 S-parameters of probes terminated in loads. However, only the
Match’s reflection parameters are used in the error correction
algorithm. The Match’s transmission parameters are ignored
since all the required transmission information has already
been taken in measuring a zero length LRM line.
Impedance Reference
The measured impedance of the load at each probe tip
establishes the reference impedance. If the load’s impedance is not 50 ohms, another value may be specified when
defining it in the cal kit. If the Match’s defined impedance is
50 ohms in a 50 ohm environment, then the Match is as-
COMMENTS:
LRM LINE
The LRM tine must currently be defined to have zero
electrical length even though the Line must have some practical length for probe contact. The 1 picosecond coplanar line
on Cascade’s ISS is the recommended choice for the LRM
tine standard.
Defining the LRM tine standard to have zero offset length
sets the reference plane at the center of the tine after calibration. However, the reference plane is usually desired at the
probe tips. The reference plane can be set at the probe tips
by specifying, in the HP 8510B Port Extensions feature, a
negative test port delay equal to half the Line standard’s
delay.
Using Port Extensions in this manner subtracts only the
Line standard’s phase and not its loss; however, the loss of
coplanar lines as short as those recommended here 1 ps.)
is typically negligible.
LRM REFLECT
The LRM Reflect standard has exactly the same require
ments as the TRL Reflect. Like TRL, LRM only requires that
the reflection at each probe tip be the same. No modification
of the TRL Reflect standard definition needs to be performed
before it can be used as an LRM Reflect.
On-wafer opens and shorts serve as good Reflect standards since they are highly reflective. The simplest Reflect is
an open created by lifting each probe tip at least 10 mils
above the wafer. This reflect eliminates probe placement
errors since the probes do not contact the substrate. However, measuring the same on-wafer short at each probe tip
offers the most identical reflection provided each probe tip’s
overtravel is identical.
LRM MATCH
The LRM Match standard is a pair of on-wafer loads de
fined to have a fixed impedance.
imperfect reflection is measured as part of the calibration’s
raw directivity.
Pair of Loads vs Single Load
Either a pair of loads or a single load may be used as the
LRM Match. LRM assumed the reflection of the Match standard at each probe tip is identical. Since most on-wafer loads
are nearly identical, LRM calibrations using a pair of loads are
highly accurate. When using a pair of loads, simultaneous
probe contact to each load must be made before the HP 8510
measures each load. Probes fixed in position by probe cards
must use a pair of loads.
Probes mounted on independent arms can use a single,
high-quality load to attain an even more accurate calibration.
If the probe overlap is the same for both connections, a single
load will provide a more identical match at each probe tip.
However, using a single load in this manner is less convenient
since a single load requires manual probe switching during its
measurement.
To measure a single load at both probe tips,
(1) contact port 1 probe tip to the load, and make Averaging
the active function by pressing its softkey,
(2) press the softkey labeled “Line: LRM Match” at the TRL
2-port Cal Menu,
(3) The HP 8510 will begin measuring the load contacted to
the Port 1 probe as a normal TRL Line standard.
Averaging Factor to a high number (eg. 4000) to slow down
the sweep.
(5) lift port 1 probe up and contact the port 2 probe on the
(6) Reset the Averaging Factor to its original value, and let
SUMMARY
The tables and comments above provide enough information to implement an LRM calibration using the HP 8510B and
on-wafer calibration standards such as those on Cascade
Microtech’s latest ISS.