Introduction to On-Wafer Characterization at Microwave Frequencies

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Introduction to
On-Wafer Characterization
at Microwave Frequencies
Chinh Doan
Graduate Student
University of California, Berkeley
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Introduction to
On-Wafer Characterization
at Microwave Frequencies
Dr. Tariq Alam
Senior Applications Engineer
Cascade Microtech, Inc.
E-Mail: tariqa@cmicro.com
presented by: Chinh Doan,
University of California, Berkeley
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Presentation Outline
• Microwave Probing Technology
– Air CoplanarTM Probes
• On-Wafer Calibration Methods
– SOLT, TRL/LRM, SOLR, LRRM
• On-Wafer Verification Methods
• Layout Rules
• Calibration and Measurement Software
– WinCal TM
What Are My Measurement Objectives ?
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• Now
– Determine S-parameters of on-wafer
active devices between 500 MHz to 5 or
50 GHz or beyond
• Future?
– ↑ Wafer size (6 to 8 to 12 inch)
– ↑ Frequency range of interest
– ↑ Need for thermal measurements
– ↑ Test automation for throughput
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What Equipment Do I Need ?
•
•
•
•
•
•
•
•
Vector Network Analyzer
Cables
Probes
Probe positioners
Probe station
Controller
Contact Substrate
Impedance Standard Substrate (ISS)
Only Cascade Microtech provides the Total Measurement Solution
from the Test Ports of the VNA down to the wafer level
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Microwave Probing
• On-wafer fixturing needs:
• Electrically
– wide BW transmission
lines
– low contact R
• Mechanically
– consistent probe shape
– placement
– durability
Optimize for loss, impedance match, power and current handling
capability, contact force, tip visibility...
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Microwave Probe Transmission Line Contacts
for better
measurements
faster
Poor
Variable loop inductance
prevents calibration
(Requires repeatable transition)
Better
Long path to single ground
contact limits bandwidth
Best
Precise line impedance right
to the ground-signal-ground
contacts
ACP Probe Technology
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K-Connector
Block
Hard
Absorber
Absorber
Air Coplanar
Waveguide Tip
Soft
Absorber
Low-Loss
Cable
Low-loss, low density teflon dielectric coax
Microwave absorber
- consistent attenuation
- termination of coaxial shield energy
- provides rigidity
15 W CW at 10 GHz
5 A DC current
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Low-loss ACP
• Low-loss and
standard ACP
Probes
• Application
– Noise
measurements
– Load Pull
Air-Coplanar Tips
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• Precision tip fabrication
for tight impedance control
l
BeCu tips for Au or Cu pads
Tungsten tips for Al pads
High tip visibility for
consistent placement
on pads
Clear view of
contact point
Preferred 23°
contact angle
Wide contact
area
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Installing and Using ACP Probes
for better
measurements
faster
φ
Use Cable strain relief on
positioners
Use Contact Substrate to
planarize probes
*definition: planarization the ability to insure all
contacts are at the same
height
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Using ISS Alignment Marks
for better
measurements
faster
Internal Apex
Initial contact
Full skate and overtravel
• Used to set ‘skate’ and probe separation
Maintaining ACP Probes
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• Keep tips clean of dirt and debris
• Inspect and clean connectors
• Electrical verification -- Probe Test
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Calibration
ao
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measurements
faster
°°
bo
Port 1
Forward
•
vv v
°
°
•
Perfect
Reflectometer
Switch
Error
Adapter
DUT
[S]
Reverse
Port 2
a3
°°
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How Do I Calibrate My VNA ?
b3
Microwave Errors (Forward)
Directivity
Port-1 Match
Reflection Tracking
Port-2 Match
Transmission Tracking
Crosstalk
Calibration Standards
Open
Short
Load
Thru
Line
Etc.
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One-port Network Analyzer Model
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faster
ER
aO
ED
ES
ΓDUT
bO
1
ER = frequency response of measurement channel
ED = Directivity of coupler
ES = Port match
Calibrate with three known reflection coefficients
Short - open - load (SOL)
Short - offset short 1 - offset short2
l
l
Two-port Network Analyzer Model
(Forward Model)
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EX
S21
ER
aO
b3
DUT
ED
bO
ET
ES
1
S22
S11
S12
ET = Models imperfections in transmission response
EL = Models signal reflected back into DUT from P2
Calibrate with short-open-load on each port
plus a Thru (SOLT) (uses 10 knowns)
EL
VNA Calibration
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bO
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measurements
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a3
Ideal
Network
Analyzer
aO
b3
DUT
e00
e01
e10
S21
e11
S11
S12
S22
e22
e23
N1
One port or
two port
calibration standards
N2
N3
• Switch modelled and measured separately
– signals simultaneously measured
– two two-port error boxes
e32
e33
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Calibrating the Probe Tips with Coplanar
Waveguide Impedance Standards
ISS
GROUND
GROUND
GROUND
GROUND
GROUND
GROUND
SIGNAL
SIGNAL
SIGNAL
SIGNAL
SIGNAL
SIGNAL
GROUND
GROUND
GROUND
GROUND
GROUND
GROUND
Thru
Short
Loads
• Electrical behavior of standards
– standard dimensions
– probe pitch
– probe placement (use alignment
marks)
– cal coefficients supplied with probe
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SOLT Calibration
Short
Open
(probes in air)
Lshort
Copen
Load
Thru
Lterm
DelayThru
• All standards must be
perfectly known
– available on virtually every
vector network analyzer (CalKit
required)
– open has capacitance (often
negative)
– short and load have
inductance
– sensitive to probe placement
– mathematically
overdetermined
– unpredictable behavior
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TRL/LRM Calibration
Thru
Reflect
Line(s)
OR
Match
• Thru-Reflect-Line
–
–
–
–
–
–
requires least info about standards
S-parameters referenced to line Zo
reference plane at centre of Thru
requires multiple probe spacings
Design rules
Zo is inherently complex at low
frequencies
– not suitable for fixed spacing probes
(e.g., probe card)
• Line-Reflect-Match
– referenced to Zmatch
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Multi-line TRL Benchmark Cal
Method
Thru
Reflect
Line(s)
• Modified TRL algorithm developed by
the U.S. National Institute of
Standards and Technology
• Benchmark on-wafer calibration
method
• Takes an optimal weighted average
of all the line measurements
• On-wafer standards (with DUT)
preferred
• Renormalizes the S-parameter
impedance to 50 Ohms
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SOLR Calibration
Short
Load
Open
(probes in air)
Reciprocal
OR
• Short-Open-LoadReciprocal Thru
– reciprocal Thru requires only S12 =
S21
– tolerant to lossy or highly reactive
insertion standard
– convenient for use with fixed probe
spacing in probe cards
– Does not require a custom
Thru
– convenient for use when DUT
terminals are orientated at 90°
– available in WinCal
SOLR for Right Angle Measurements
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• Carefully constructed right
angle ‘Thru’ standard
• Thru is non-ideal, large dip
at 20 GHz
• Errors in standard cal’s
• SOLR immune to Thru
errors
1.0
0.5
Orthogonal SOLT
|S21|
Orthogonal LRRM
[dB]
0.0
Orthogonal SOLR
-0.5
Straight LRRM
-1.0
0
5
10
15
20
25
30
[GHz]
35
40
45
50
LRRM
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Line
• Cascade Microtech Calibration Research
– Line-Reflect-Reflect-Match Calibration
– like TRL, only Match acts as infinitely high loss line
Reflect
Reflect
(probes in air)
Match
– one transmission line standard only allows fixed probe
spacing calibration
– Thru (line) delay, Match resistance must be known
– measurements referenced to laser trimmed resistor
– required measurement of only one load standard
– load inductance compensation
– uses off-wafer standards (ISS)
– same standards as SOLT only - no need for cal kit
– available in WinCal
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LRRM Calibration
l
l
System drift baseline
LRRM compares with
system drift limit
–
l
best fixed probe
position calibration
SOLT /LRM
–
–
–
growing error w/freq
possible CalKit error
possible ref plane
error
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SOLT
TRL
Popular Calibration Methods
for Wafer Probing
Z0
Reference
Trimmed
Resistor
Inherently
Consistent
No
Probe Card
Support
Absolute
Accuracy
Fair
Fair
Best (if
Corrected)
Transmission
Lines
Yes
Poor
LRM/
LRRM
Trimmed
Resistor
Yes
Fair
Good
SOLR
Trimmed
Resistor
Yes
Best
Good
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Calibration Verification
How Do I Know If My Calibration is Successful ?
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GROUND
GROUND
GROUND
SIGNAL
SIGNAL
SIGNAL
GROUND
GROUND
GROUND
Pad
Open stub
GROUND
GROUND
SIGNAL
SIGNAL
GROUND
GROUND
Line (delay)
Calibration Verification Standards
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Thru
0.1 GHz
0.5
1.0
40.0 GHz
0.10
0.05
[dB]
•Unity Gain
0.00
•1 pS Line
-0.05
•Phase Lag
-0.10
0
5
10
15
20
[GHz]
25
30
35
40
for better
measurements
faster
Reflect
0.5
(probes in air)
0.4
0.3
0.2
0.1
40.0
10
GHz
[dB]
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Calibration Verification Standards
0.0
-0.1
-0.2
20
50
0.1
-0.3
GHz
-0.4
10 2050
-0.5
-50
-20
0
5
10
15
20
[GHz]
-10
•Unity Gain
•Negative Capacitance
25
30
35
40
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Calibration Verification Standards
Match
S21 of Standard Loads
0
-10
-20
-30
-40
[dB]
-50
-60
HPC40 on
HPC ISS
-70
-80
0
5
10
15
20
[GHz]
25
30
35
40
• Measured S21 when probes placed on 50ohm loads
for better
measurements
faster
1
0.8
1.5
0.6
2
G
0.4
Open
stub
S
G
3
4
0.2
5
10
20
0.2
0.4
0.6
0.8
1
1.5
2
3
4 5
10 2050
50
0.1
-50
-20
-10
0.0
-0.2
-5
-4
-0.5
40.0 GHz-3
-0.4
[dB]
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Independent Calibration Verification
Standards
-2
-1.0
-0.6
-1.5
-0.8
-1
-1.5
0
5
10
15
20
[GHz]
25
30
35
40
• Linear Phase Lag
GHz
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Common Calibration Errors
• Poor Calibration
Verification/
Repeatability
•
•
•
•
Inaccurate Probe Placement
Inaccurate Probe Definitions
Poor Probe Contact
LRM Sensitivity to Differences
in Load Standards
• Incorrect Models for Lines
• Calibration Drift
• Poor Phase Stability of Cable
• Narrow Band ‘Suck out’ • Dirty Connectors
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Beware of an Unconnected Substrate
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measurements
faster
GSG pads shield like CPW
Fields terminate on
backside of wafer
on one side
GS pads fringe to the ground plane or chuck
• Parasitic couplings to the conductors near DUT
• GSG shields magnetic and electric fields better than GS
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Layout Guidelines
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Device Characterization with Coplanar
Waveguide Microwave Probes
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measurements
faster
GROUND
SOURCE
DRAIN
GATE
SIGNAL
SIGNAL
GROUND
GROUND
SOURCE
GROUND
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Pad Size (Passivation Window)
• Recommended minimum pad size is 80um x 80um for
ACP Probes when performing automated
measurements
• Smaller pad dimensions can be used for manual
probing
• HPC Probe allows 40um x 70um manual probing
• Passivation height must be considered
• Pad height variation must not exceed 25um
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Process Control Monitor Device
Layout
G
G
S
S
G
G
GSG Test Device
Open Pads & Metal
(Remove Ypad)
Shorted Metal
(Remove Zmetal)
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PPR Corrected H21 Measurement
0.25 µm CMOS Transistor
H21
60
40
Corrected for pad parasitics
FT = 33 GHz
As measured
FT = 25 GHz
20
0
-20
.1
1
10
FREQUENCY (GHz)
100
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WinCal
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• VNA Calibration and Measurement Tool
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Calibration Features
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measurements
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• Calibration
– SOL, SOLT, SOLR,
LRM, LRRM, TRL
– Stability tests
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Setting Up a CalKit
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measurements
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CalKit can be easily defined and downloaded to VNA
– Removes one of the most common sources of error
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Measurement Features
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measurements
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• Tools
– Read/Save SParameters
– Pad Parasitic
Removal
– Probe Test
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Monitoring the Calibration
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measurements
faster
• Stability check checks
system drift
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Pad Parasitic Removal
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measurements
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• Measures intrinsic devices
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Calibration Repeatability
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measurements
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LRRM automatic calibration is
very repeatable
Operator dependent probe
placement errors
– manual cal’s are not
as repeatable
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Summary
• Microwave measurements require careful
calibration, verifications and attention to detail
• Many new applications to accommodate
the varied needs of growing wireless and
high-speed digital needs
• Let Cascade Microtech help you keep up with
future innovations
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E-Mail: tariqa@cmicro.com