On Wafer S-Parameters& Uncertainties
Dr. Andrej Rumiantsev
Director RF Technologies
MPI Corporation

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visit: www.mtt-archives.org/~mtt11

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A.Rumiantsev ARFTG86th-NIST SC-20152

Outline• Introduction• Criteria for the “right method”• Calibration residual errors• Conclusion

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The Last Inch to the DUT…leads to a different world
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Coaxial Calibration Standards

Picture: Spinner Group• Same media• Well-defined electrical characteristics at thecalibration plane
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Wafer-Level Standards• Coplanar design:– Dispersion• Not shielded:– Coupling and radiation• Manufacturing
inaccuracy

Picture: FBH, Berlin

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Wafer-Level Standards (cont.)
Two discontinuities:• Coaxial media to planarprobe tips
• Probe tips to coplanar
devices (standards)

Picture: FBH, Berlin

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Outline• Introduction• Criteria for the “right method”• Calibration residual errors• Conclusion

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Calibration and Self-Calibration• Calibration methods: known standards– SOL, SOLT, QSOLT• Self-calibration standards:– Reflect, reciprocal thru, line

• Solution criteria:– Frequency range– Self-calibration methods with optimal set ofstandards

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Example: SOLTStandardRequirements

Typical Definition

Error Terms

ShortFully known

R=0; L=9pH2Open

Fully known

R=inf; C=0.3fF

2Load

Fully known

R=50; L=10.6pH

2Thru

Fully known

Z0=50Ω
α=0, τ=1ps

4

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Open/Short are not Ideal
Open and Short (multiline TRL)

1Mag(S11) (dB)

Short
Open

0.5

0-0.5

-10Company Confidential

20406080
Frequency (GHz)A.Rumiantsev ARFTG86th-NIST SC-2015

10012011

Open/Short are not Ideal
190

Open and Short (multiline TRL)

20Open (R)
180

10170

0160
0-10120

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Frequency (GHz)A.Rumiantsev ARFTG86th-NIST SC-2015

100

Phase(S11) (degree)

Phase(S11) (degree)

Short (L)12

Test DUT: SOLT

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Calibration Outcome
Removes systematic errors:– Shifts the reference plane– Defines reference impedance ZREF
Error Model

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DUT

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Reference Impedance: Lumped Methods• ZREF is defied by the load ZLOAD
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Calibration Plane: Middle of the Thru

• Thru must be fully known – crucial!
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Calibration Challenges at mm-Wave Frequencies• Probe tip Load:– Less space on the calibration substrate– More elements on the same size

• Acceptable compromize for lowfrequency measurements• Load impedance at mm-wave range?
• Calibration accuracy above 110 GHz?
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Calibration Challenges at mm-Wave Frequencies• Accurate definition of the equivalent impedance requires
uniformed structure of the E/M waves at the reference point• Where is the point of the uniformed waves?

321?

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Calibration Challenges at mm-Wave Frequencies•

Away from the probe tips:––––

Transition to CPW complete
Uniformed fields
Well-established traveling waves
Well-defied reference
impedance!

Company Confidential•

At the probe tip:––––

Transition point from tip to standard
Discontinuity
Not-uniformed fields
Load impedance??? (undefined!)

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Solution for Calibration at mm-Wave Frequencies• Move load away from the probe-tips.
• However:– Load with long offset impractical– How to define its equivalent impedance in wide frequency
range?

• Solution: NIST multiline TRL– Load not required!
– Requires just sections of transmission lines– Calibration standards easy to make on-wafer
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Distributed Standard-Based Methods:
TRL and mTRL
• Calibration reference impedance is the characteristic
impedance of the line
• Lines are dispersive

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Dispersion Phenomenon• S-parameters:– ZREF=ZLINE• Pseudo S-parameters:– ZREF=50 ΩUseful reading: D. Williams, “Traveling waves and power waves”, Microwave Magazine, Nov-Dec 2013.

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Distributed Standard-Based Methods:
TRL and mTRL
• Transformation to 50 Ω reference impedance required• Measurement of line characteristic impedanceCompany Confidential

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Probe Tip vs. on-Wafer Calibration
Wafer Probe
Probe Tip

On-Wafer
DUT (HBT)

A. Rumiantsev, “On-Wafer calibration techniques enabling accurate characterization of high-performance silicon devices at the mm-wave range and beyond,” Fakultät für Maschinenbau,Elektrotechnik und Wirtschaftsingenieurwesen, BTU Cottbus, Cottbus, 2014.

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Probe Tip vs. on-Wafer Calibration
TMR

TRL

Off-wafer standards

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On-wafer: CMOS

A.Rumiantsev ARFTG86th-NIST SC-2015

On-wafer: BiCMOS

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On-Wafer Cal: Distributed Standards

Top metal

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On-Wafer Cal: Lumped Standards

Metal1 (2)Company Confidential

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Outline• Introduction• Criteria for the “right method”• Calibration residual errors• Conclusion

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Calibration
VNAError Box[A]Company Confidential

Planar Standards

A.Rumiantsev ARFTG86th-NIST SC-2015

Error Box

[B]-1
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Real Situation: Residual Errors
VNAError Box

Residual

ResidualError Box

[A]

[A]*

[B]*-1-1[B]

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30Example: MAG 2x25μm Cascode Cell

Probe type I

Probe type A

Shinghal, P.; Sloan, R.; Duff, C.I.; Cochran, S., "Observations on the sensitivity of on-wafer cascode cell S-parameter measurements due to probing uncertainties," ARFTG Microwave
Measurement Conference (ARFTG), 2014 83rd , vol., no., pp.1,3, 6-6 June 2014

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Sources of Calibration Residual Errors• Inaccuracies of calibration standards– Design, fabrication and probe positioning• Higher-order mode propagation– Calibration standards and DUT• Coupling with nearby structures– Between elements– RF probes and elements• Limitations of calibration methods

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(Co)Planar Calibration Standards• Dispersion• Manufacturing inaccuracy• Probe positioning

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Dispersion: Example for AC-2

MPI Probe Selection Guide. MPI Corporation, Hsinchu, Taiwan, 2014

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Operating with Multiple Lines• Multiline TRL definition:– Offset of each line vs. Thru.
– Just physical length, not electrical parameters needed• Wafer-level multiline TRL system requirements:– Motorized positioner(s)– Typical positioning error +/- 5 micrometers– Painful investment

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Minimize Positioning Error• Digital gauge for X-axes
T• Positioning
resolution 1µmhr• Set/adjust/measureΔl foru
multilne TRL

ThruΔlLinePicture: MPI

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Membrane CPW Designfor Improved Line Characteristics
Conventional CPWε0εr

Al2O3Membrane CPWU. Arz, M. Rohland, and S. Büttgenbach, Improving the Performance of 110 GHz Membrane-Based Interconnects on Silicon: Modeling, Measurements, and Uncertainty Analysis, in EEETrans. on Components, Packaging and Manufacturing Tech., vol. 3, No. 11, Nov. 2013

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Comparison for Relative Phase Constant

U. Arz, M. Rohland, and S. Büttgenbach, Improving the Performance of 110 GHz Membrane-Based Interconnects on Silicon: Modeling, Measurements, and Uncertainty Analysis, in EEETrans. on Components, Packaging and Manufacturing Tech., vol. 3, No. 11, Nov. 2013

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CPW Boundary Conditions• Multi-mode propagation at mm-wave frequencies• Critical for every calibration methodCeramic

Metal

Courtesy: FBH, Berlin

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Optimized Chuck Material
0Imag(Z0) (Ohm)On Metal-0.2

On Ceramic

On Absorber

-0.4

-0.6

-0.8
020

40
60
80
Frequency (GHz)100120Picture: MPI

A. Rumiantsev, R. Doerner, and E. M. Godshalk, "The influence of calibration substrate boundary conditions on CPW characteristics and calibration accuracy at mm-wave frequencies," ARFTG
Microwave Measurements Conference-Fall, 72nd, pp. 168-173, 2008.

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Coupling with Nearby Structures• Ex-field in the CPW• CPW mode in theneighborhood

V. Krozer, R. Doerner, A. Rumaintsev, N. Weinmann, F. Schmückle, and W. Heinrich, On-Wafer Small- and Large- Signal Measurement Systems at sub-THz Frequencies, BCTM-2014, pp. 1-8, 2014.

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Higher-Order Modes• Coupling between probe ground and on-waferground:– Parasitic propagating signal• Parallel-plate mode in substrate:– CPW ground and the backside ground• Other modes guiding:– Between coaxial probe ground and wafer backside
metallization

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Coupling and Higher Modes

G.N. Phung, F.J. Schmückle, W. Heinrich, “Parasitic Effects and Measurement Uncertainties in Multi-Layer Thin-Film Structures,” Proc. 43th European Microwave Conf. (EuMC 2013),
Nuremberg, Germany, Oct. 7-10, pp. 318-321

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Coupling and Higher Modes:
Microstip Elements

PPL tube mode
CPW needle mode
MS mode
PPL mode
G.N. Phung, F.J. Schmückle, W. Heinrich, “Parasitic Effects and Measurement Uncertainties in Multi-Layer Thin-Film Structures,” Proc. 43th European Microwave Conf. (EuMC 2013), Nuremberg,Germany, Oct. 7-10, pp. 318-321

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110 GHz RF Probe:
Similar Concept, Different Tip Design

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A.Rumiantsev ARFTG86th-NIST SC-201545RF Probe Design:
Coax-to-CPW Transmission Variety
Picoprobe

ACP

A. Rumiantsev and R. Doerner, "RF probe technology," Microwave Magazine, IEEE, vol. 14, pp. 46-58, 2013.

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Infinity

TITAN (Allstron)

2.5'A''P'

2.0'I''AL'

1.5
1.0
0.5
0.0

0

102030Frequency (GHz)40

Relative Phase Constant (unitless)Attenuation Constant (dB/cm)

Impact of the Probe Tip Design
2.300'A''P'

2.275'I''AL'

2.250
2.225
2.200

0102030

40

Frequency (GHz)A. Rumiantsev, R. Doerner, Method for Estimating Probe-Dependent Residual Errors of Wafer-Level TRL Calibration, ARFTG-83rd, 2014.

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Worst-Case Estimate:Error for Four Probe Technologies
0.5'I''A''P''AL'
ΔSMAX, ΔSMIN

Mag(S21) (dB)

0.0

-0.5

-1.0

-1.5
020

40

60

80

100120Frequency (GHz)A. Rumiantsev, R. Doerner, Method for Estimating Probe-Dependent Residual Errors of Wafer-Level TRL Calibration, ARFTG-83rd, 2014.

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Outline• Introduction• Criteria for the “right method”• Calibration residual errors• Conclusion

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Conclusion• Calibration residual errors have complex nature• Contributing factors:– Design of standards– Probe type• Solutions for reference CPW line available• Design optimization of the calibration chip:– Reduce support of higher-order modes– Minimize coupling effects
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THANK YOU
FOR YOUR ATTENTION
For more information, please visit:www.mpi-corporation.com

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