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A Practical Guide for Accurate Broadband On-Wafer Calibration in
RF Silicon ApplicationsAndrej Rumiantsev
1st International MOS-AK Meeting, Dec.13 2008, San Francisco, CA
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Outline
Motivation
Standard Technique vs. On-Wafer Calibration
Suitable Calibration Methods
Implementation
Practical Results
Summary
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Motivation
The operation frequency of modern RF CMOS devices approaches 110GHz and beyond
Accurate device characterization at operation frequencies is a must
Limited accuracy of the standard technique beyond 40 GHz (with the pad parasitic de-embedding)
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Motivation
Historical view: Microwave community: customized on-chip calibration is the standard technique Digital community (Si): off-chip calibration + de-embedding
Progress in development of customized calibration on silicon over the last years
Work from NIST (US), PTB (Germany)
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Outline
Motivation
Standard Technique vs. On-Wafer Calibration
Suitable Calibration Methods
Implementation
Practical Results
Summary
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Standard Technique
Line Reflect Load (Match) Reference plane at probe-tips after step 1
(off-wafer calibration)
Step 2: Reference plane after de-embeddingStep 1: calibration to the probe tip end
with commercial calibration substrateStep 2: de-embedding of DUT contact pads and interconnects
Step1:
off-wafer calibration
Standard technique: a two-step approach
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Standard Technique
Why de-embedding failsBased on the impedance equivalent model of pads and interconnectsAssumes that a dummy pad represents (a part of) the parasitic impedanceSubtracts impedances
Frequency ↑ accuracy ↓
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
On-Wafer Calibration
One step approachExample: LRM+ calibration with wafer embedded standards
Thru Load Reflect (Short)
⇒+ +
Measurement reference plane after the on-wafer calibration
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
On-Wafer Calibration
Why calibration worksApplies the measurement theory: the concept of systematic measurement errorsUses (partly) known standards to define error termsDetermines DUT parameters over the error correction procedure
No frequency limitation
P1
DUT
P2 P1
DUT
P2Forward Direction Reverse Direction
Error Box Error Box Error Box Error Box
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Outline
Motivation
Standard Technique vs. On-Wafer Calibration
Suitable Calibration Methods
Implementation
Practical Results
Summary
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Suitable Calibration Methods
Multiline TRL:Developed at NIST in early 90s [1]Original application: semi-insulating wafers (GaAs) [2]End of 90s: application techniques for Si [3]
LRM+:First application: SiGe:C [4] Comparison vs. multiline TRL: GaAs [5], bulk Si [6]
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
On-Wafer TRL/ NIST Multiline TRL
AdvantagesFundamental calibrationDoes not require ideal Open or ShortMeasures line propagation constantSelf-consistent
ChallengesMany lines are required for broadband measurementsReference impedance is the line Z0
Measurement of the line Z0 on lossy substrates
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
On-Wafer TRL: Residual Calibration Errors
0 20 40 60 80 100 120Frequency (GHz)
Calibration Accuracy Verification
0
0.05
0.1
0.15
0.2
0.25
Upper
bound |
Sij'-
Sij|/
|Sij|
and |
Sii'
-Sii|
TRL
System Drift
0 20 40 60 80 100Frequency (GHz)
Re(
Zo),
Ohm
45
47.5
50
52.5
55CPW Line Impedance (ISS)
Alumina CPW TRL (4 Lines) with respect to the 50 Ohm calibration [7]
Methods to characterize the line accuracy improvement(NIST approaches), e.g. [3]
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
On-Wafer LRM+
AdvantagesNot sensitive to Load asymmetry Arbitrary impedance elements can be used as LoadsDoes not require known Open or ShortSelf-consistent
ChallengesRequires known Loads and Thru
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Outline
Motivation
Standard Technique vs. On-Wafer Calibration
Suitable Calibration Methods
Implementation
Practical Results
Summary
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Characterization of Customized Standards
Measurement• Lines (multilineTRL)• Lumped standards• “Chicken and egg…”
Model• EM-simulation• Network• Fabrication tolerances?
Mixed• Meas.-based model
(e.g. LRM+)
Mixed approach addresses fabrication tolerances
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Implementation Challenges
High loss of silicon substrate:Difficulties in implementing TRL calibration
Many metal layers:Difficulties in 3D EM simulation of on-wafer standards
High fabrication tolerances:Inaccurate lumped (open, short and load) and distributed (thru and line) standards
High price of wafer space
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Design of Standards
GSG pad set
Ground and signal pads are at the high-level metal
Lines: high-level metal
Load, short, and open are at the bottom of the via stacks, at the plane “D”
Special methods to improve electrical properties of the line
DUT Plane
“D”Plane
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Characterization of Standards: Simulation
EM simulation technique:Not practical for the complete structures
Simplified representation: A line with a solid, reference conductor, and a single dielectric (addressing physical dimensions)
Simulated model of the load standard
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Characterization of Standards: Measurements
Measurement technique:γ of the line from the NIST Multiline TRLZ0 of the line: from the load method
Load impedance: From the reference NIST multiline TRLFrom the ‘Mixed Technique’
Z modeledR* measuredZ* reconstructed [4]
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Outline
Motivation
Standard Technique vs. On-Wafer Calibration
Suitable Calibration Methods
Implementation
Practical Results
Summary
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Characterization of Standards
Line parameters, extracted from multiline TRL [6]
0 20 40 60 80 100 120FREQUENCY (GHz)
40
42
44
46
48
50
Rea
l(Zo)
(Ohm
)
-4
-3
-2
-1
0
1
Im(Z
o)
(Ohm
)
Real(Zo) (L)
Imag(Zo) (R)
0 20 40 60 80 100 120FREQUENCY (GHz)
0
2
4
6
8
Loss
(dB/c
m)
1
1.5
2
2.5
3
Rel
ativ
e Ph
ase
Const
ant
Loss (L)
Phase Constant (R)
Propagation constant Characteristic Impedance
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Characterization of Standards
Thru: measurement vs. simplified simulation [6]
0 20 40 60 80 100 120FREQUENCY (GHz)
-0.05
0
0.05
Mag
nitude
(dB)
-7.5
-5
-2.5
0
2.5
Phas
e (D
egre
e)
DB(|S[2,1]|) (L)Measured
DB(|S[2,1]|) (L)Simulated
Ang(S[2,1]) (R)Measured
Ang(S[2,1]) (R)Simulated
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Characterization of Standards
Load standard: fabrication tolerances within one test chip (mixed model vs. measurement) [6]
Mixed model agrees with measurement results
Mag
nitude
(dB)
0 20 40 60 80 100 120FREQUENCY (GHz)
-60
-50
-40
-30
-20
-10
0DB(|S[2,2]|)Load
DB(|S[2,2]|)Open-Resistor
DB(|S[2,2]|)Short-Resistor
DB(|S[2,2]|)Load Model
Return Loss of the Load
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Calibration Accuracy Comparison: NIST Method
Calibration comparison technique for:Contact repeatability (on Si)Instrument drift* LRM+ accuracy
Reference calibration:NIST Multiline TRL
* Experimental time: 8 hours
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Calibration Accuracy Comparison: NIST Method
0 20 40 60 80 100 120FREQUENCY (GHz)
0
0.05
0.1
0.15
0.2
0.25
0.3U
pper
bound for
|Sij'-
Sij|/
|Sij|
and |
Sii'
-Sii|
Sum of Contact and Instrument Drifts
Instrument Drift
LRMplus
LRM+ is in agreement with the reference multiline TRL
LRM+ vs. Multiline TRL [6]
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Calibration Accuracy Comparison: DUT
Verification for FET: advanced 0.13 μm RF-CMOS NFET, [6]
20 40 60 80 100 12020
25
30
35
40
45
50
55
60
65
70Cgs
(fF)
FREQUENCY (GHz)
NIST Multiline TRL
Off-wafer SOLT+ De-embedding
SUSS LRM+
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Calibration Accuracy Comparison: DUT
Verification for FET : advanced 0.13 μm RF-CMOS NFET, [6]
20 40 60 80 100 12010
12
14
16
18
20Cgd (
fF)
FREQUENCY (GHz)
NIST Multiline TRL
Off-wafer SOLT+ De-embedding
SUSS LRM+
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Outline
Motivation
Standard Technique vs. On-Wafer Calibration
Suitable Calibration Methods
Implementation
Practical Results
Summary
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Summary
Improved design of standards and a consistent reference plane increased on-wafer calibration accuracy
Simplified 3D EM simulation of standards demonstrated good results
Wafer-embedded NIST Multiline TRL and LRM+ significantly improve measurement accuracy at mm-wave frequencies
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Conclusion
LRM+ and NIST Multiline (with 3 lines) are comparable for measurement accuracyLRM+ is more practical for implementation:
Requires fewer standards: 3 vs. 5 Saves test chip size by a factor of ~14xTwice as fast: 10 sweeps vs. 20 sweeps
LRM+ does not require motorized positioners (or operator):
Cheaper and faster
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
Acknowledgement
Susan L. Sweeney and Phillip L. Corsonfrom IBM Microelectronics
for their tremendous contribution
A Practical Guide for Accurate Broadband On-WCalibration in RF Silicon Applications, A.Rumiantsev, 1st International MOS-AK MeetingDec. 13, 2008 © All Rights Reserved
References
[1] R. B. Marks, "A multiline method of network analyzer calibration," Microwave Theory and Techniques, IEEE Transactions on, vol. 39, pp. 1205-1215, 1991.
[2] D. Williams, R. Marks, K. Phillips, and T. Miers, "Progress toward MMIC on-wafer standards," in ARFTG Microwave Measurements Conference-Fall, 36th, 1990, pp. 73-83.
[3] D. F. Williams, U. Arz, and H. Grabinski, "Accurate Characteristic Impedance Measurement on Silicon," in ARFTG Conference Digest-Spring, 51st, 1998, pp. 155-158.
[4] R. F. Scholz, F. Korndorfer, B. Senapati, and A. Rumiantsev, "Advanced technique for broadband on-wafer RF device characterization," in ARFTG Microwave Measurements Conference-Spring, 63rd, 2004, pp. 83-90.
[5] R. Doerner and A. Rumiantsev, "Verification of the wafer-level LRM+ calibration technique for GaAs applications up to 110 GHz," in ARFTG Microwave Measurements Conference-Spring, 65th, 2005, pp. 15-19.
[6] A. Rumiantsev, S. L. Sweeney, and P. L. Corson, "Comparison of on-wafer multiline TRL and LRM+ calibrations for RF CMOS applications," ARFTG Microwave Measurements Conference-Fall, 72nd, pp. 132-135, 2008.
[7] A. Rumiantsev, "A review of VNA calibration methods," in European Microwave Conference, 38th. Workshop WMO-3, Determining and Dealing with Errors in Microwave Measurements, 2008.
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