Journal of Computer and Communications, 2013, 1, 25-29
Published Online November 2013 (http://www.scirp.org/journal/jcc)
http://dx.doi.org/10.4236/jcc.2013.16005
Open Access JCC
25
Limitations of On-Wafer Calibration and De-Embedding
Methods in the Sub-THz Range
Manuel Poterea u 1, Christian Raya2, Magali De Matos1, Sébastien Fregonese1, Arnaud Curutchet1,
Min Zhang2, Bertrand Ardouin2, Thomas Zimmer1,2
1IMS Laboratory, University of Bordeaux 1, Talence, France; 2XMOD Technologies, Bordeaux, France.
Received August 2013
ABSTRACT
This paper investigates frequency limitations of calibration and de-embedding techniques for S parameter measure-
ments. First, the TRL calibration methods are analysed and the error due to the probe movement when measuring the
different line lengths is quantified, next the coupling between the probe-heads and the wafer surface is investigated and
finally an upper frequency validity limit for the standard Open-Short de-embedding method is given. The measured
results have been confirmed thanks to the use of an electro-magnetic simulator.
Keywords: TRL; Open-Short; De-Embedding; Calibration
1. Introduction
The frequency range in electronic applications is con-
tinuously increasing during the past few years to reach
the range of hundreds of Gigahertz. The associated bipo-
lar and CMOS transistors used as elementary compo-
nents for these high frequency applications have maxi-
mum oscillation frequency reaching the half Terahertz
[1]. Hence, characterisation of advanced SiGeC HBT and
CMOS devices and their associated passive elements are
more and more challenging. First, different calibration
techniques of the network analyser such as TRL and
SOLT at frequency higher than 50 GHz need to be veri-
fied. Moreover, associated device modelling requests
accurate characterisation of the intrinsic device, e.g. free
of parasitic elements such as pads, vias and interconnects.
Hence, conventional de-embedding techniques such as
Open-Short need to be refined for higher frequencies. It
has already been shown that calibration and de-embed-
ding techniques are less accurate when the frequency
increases [2]. Some parasitic effects, formerly negligible,
are now strong enough to modify significantly the S-
parameters measurement [3,4]. This paper investigates
three different errors that can commonly occur in Sub-
THz measurements and which are mostly not taken into
account: First, a practical li mitation of the TRL technique
is presented due to the measurements of different line
lengths. Then, a second limitation of the calibra tion tech-
nique due to the difference between the calibration sub-
strate and the device under test structure is explored. Fi-
nally, a limitation of the conventional de-embedding
Open-Short method is highlighted on active and passive
elements.
2. Influence of the Probe Movement during
Measurement
The TRL calibration method is based on the measure-
ment of three different standards:
• a reflect, which can be an open or a short and need to
be as symmetrical as possible
• a short line, (called through in the rest of the paper)
• a long line, (called line in the rest of the paper)
The TRL is based on the measurement of two lines
with different lengths [5]. In between the measurements
of these lines, one probe is moved as described on the top
of the Figure 1. When this probe is moved, the meas-
urement environment is altered at high frequency. For
example, crosstalk between probes is reduced, the probe
contact resistance can be changed and the positio n of the
cable is slightly changed when moving the probe head.
Due to these small modifications of the measurement
environment the calibration is less accurate at high fre-
quencies. Hence, we expose a method to quantify the
error introduced by moving the probe he ad . A special test
structure has been designed using two open structures
with different distances between the probes, see Figure
1. First, the impedance of the Open structure “A” is
measured. Then, the left probe is moved away along the

Limitations of On-Wafer Calibration and De-Embedding Methods in the Sub-THz Range
Open Access JCC
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Figure 1. Sketch of the probes movement front (up) and top
view (dow n) .
x-axis as shown by the white arrow on Figure 1. The
Open structure “B” is measured and then the two meas-
urements are compared.
On the Figure 2, the admittance (y-parameter) and
capacitance for the r ight probe are shown. This probe is
not moved between the two measurements.
The admittance is exactly the same for each measure-
ment and the capacitance has the same behaviour, only
an offset is visible.
Figure 3 shows the admittance and capacitance for th e
left port before (red curve) and after the probe movement
(blue curve).
On this figure, the admittance’s ma ximu m i s sh ift ed to
lower frequencies by 15 GHz and raised by 0.5 mS
(~50%) between the two measurements. The graph also
shows a modification in the behaviour of the test struc-
ture’s electrical characteristic above 50 GHz. There is a
drop in the capacitance value. This result is not-physical
because the test structure is still the same!
As a conclusion, above 50 GHz, a probe movement
can introduce measurement errors. This can be critical
because the line measurement is one of the key-steps
during TRL-calibration.
3. Coupling between Probes and Test
Structures under the Probes
Due to the cost of the silicon area, test structures are
placed as close as possible to each other.
The RF probes are large compared to the pad size, and
a part of the probes is right above the neighbouring
structures. Th e structures under the probe s make a strong
coupling with the RF probes [3]. To quantify this cou-
pling, we carried out the following measurements shown
on Figure 4. The red and blue triangles are the RF
probes in GSG configuration. The cell called “A” does
not have any coupling under its left p robe. The cell called
“C” does not have any couplin g under its right probe
while the structure “B” is totally symmetric and has a
coupling on both sides. The three measured test-struc-
tures are identical and symmetrical (open). Consequently,
the measured characteristics should be symmetrical (S11
Figure 2. Port 2 admittance (up) and capacitance (down) before and after left probe move.
frenquency (GHz) [ E+9]
Y 1 cel l A Y1 cell B [E-3]
020 4060 80100
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
frenquency (GHz) [ E+9]
C1 c el l A C1 c el l B [E-15]
020 4060 80100
35.0
37.5
40.0
42.5
45.0

Limitations of On-Wafer Calibration and De-Embedding Methods in the Sub-THz Range
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Figure 3. Port 1 admittance (up) and capacitance (down) before and after left probe move.
Figure 4. Placement of the measured test structures.
= S22). The difference that we can see is due to the cou-
pling between probe and the neighbour test structures
under it.
The middle graph of the Figure 5 shows the coupling
between probes and test structures under it. Thanks to the
symmetry of the structure, the behaviour of the two ports
of the cell “B” is completely symmetrical (S11 = S22).
On the top and bottom graph, we can see that the
measurement is no longer symmetrical; this is due to the
absence of test structures under the left probe for the first
graph and under the right pr obe for the third one.
For on-wafer measurement, the coupling between
Figure 5. Measurement results for the cell A (up), B (mid-
dle), C (down).
frenquenc y (GHz ) [ E+9]
Y 2 cel l A Y2 c el l B [E-3]
020 4060 80100
-1.0
-0.5
0.0
0.5
1.0
1.5
frenquenc y (GHz ) [ E+9]
C2 c el l A C2 cel l B [E-15]
020 4060 80100
35.0
37.5
40.0
42.5
45.0
f requency (GHz) [E+9] S11 mag m ag(S .m .22) [DB ]
020 40 60 80100
-0.4
-0.2
0.0
0.2
0.4
f requency (GHz) [E+9]
S11 mag S22 m ag [D B ]
020 40 60 80100
-0.6
-0.4
-0.2
0.0
0.2
0.4
f requency (GHz) [E+9]
S11 mag S22 m ag [D B ]
020 40 60 80100
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4

Limitations of On-Wafer Calibration and De-Embedding Methods in the Sub-THz Range
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probes and test-structures can be controlled by the inser-
tion of dummies on the first and the last column of the
chip. E.g. Single port calibration standards can be in-
serted in those columns.
Another question arises here. Usu ally the calibration is
done with an ISS-standard substrate. The surface of this
calibration substrate is completely different compared to
the wafer surface and consequently the coupling during
calibration is different compared to the measurement.
The only solution to correct for the coupling is to use on-
wafer calibration structures and no longer use the ISS-
standard.
4. Open-Short De-Embedding Limitation
During the high frequency measurement of a large multi-
finger transistor (HBT), the S11 (Figure 6) shows a non-
physical behaviour after the de-embedding with the
Open-Short methodology (OS).
The impedance (represented by the magnitude of S11)
is higher after the de-embedding (red line) because the
losses (especially through the pads) are compensated.
The Figure 6 also shows that the de-embedded magni-
tude is increasing with frequency after 60 GHz which is
not physical for the measured input impedance of the
HBT. This non-physical behaviour is due to over com-
pensation of the pad capacitance during de-embedding.
In order to validate this hypothesis, EM simulation has
been carried out. The methodology is explained on Fig-
ure 7.
First, an inductance is simulated with HFSS without
the ba ck-end. From a technical point of view, when using
an electromagnetic simulator, it is not possible to define
a port near a discontinuity. A work-around was to add a
short line, make the simulation of the whole structure and
then to remove the short line using the standard de-em-
bedding procedure. This is represented on the first line of
the Figure 7.
In a second step, the same inductance is simulated
with the back-end (see second line of Figure 7). A Shor t
and an Open are also simulated and the OS de-embed-
ding is performed. The simulation results are shown on
Figure 8, upper pa rt: magni t ude, lowe r pa rt: phase.
The simulation results of the inductance without back-
end are given by the red curve. The simulation results of
the inductance after OS de-embedding are given by the
blue curve. Theoretically, a perfect superposition of the
red and blue curves is expected, but a large difference
appears above 40 GHz, especially for the magnitude. A
similar increase of the magnitude in the high frequency
range is visible as already before seen during measure-
ments (Figure 6, red line). We can conclude that above
60 GHz, the OS de-embedding introduces a non negligi-
ble error.
For accurate measurements, a new de-embedding me-
thod is needed. The 6 dummies method developed in [2]
and [6] is highly recommended in this frequency range.
5. Conclusions
High frequency calibration and de-embedding techniques
have been analyzed and their validity range has been
checked: 1) the TRL calibration need the measurements
of two different line lengths introducing the movement of
the probe-heads for on wafer-measurements. This move-
ment can introduce errors and make the calibration less
precise above 40 GHz. 2) the coupling between the
probe-heads and the underlying wafer-surface introduces
an error when the structures on the wafer that are under
the probe-heads are not identical. A work around is to
pay particular attention when designing the test-struc-
tures to take this effect into account.
The most widely used de-embedding technique (Open-
Short) introduces major errors in measurement above 60
GHz. The 6 dummies method developed in [6] is highly
recommended above this freque nc y range.
Figure 6. Magnitude (up) of a transistor S11 without OS
(blue line ) and after OS (red line ).
Figure 7. The five structures simulated with HFSS and the two methods of extraction.

Limitations of On-Wafer Calibration and De-Embedding Methods in the Sub-THz Range
Open Access JCC
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Figure 8. Simulation magnitude (up) and phase (down) of
an inductance; intrinsic (red) and extracted with OS de-
embedding (blue).
6. Acknowledgements
This work is part of the DotSeven project supported by
the European Commission through the Seventh Frame-
work Program for Research and Technological Devel-
opment. The authors would like to thank STMicroelec-
tronics for wafer supply.
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