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Thick-Film Multilayer Microwave
Circuits for Wireless Applications
Charles Free
Advanced Technology Institute
University of Surrey, UK
and
Zhengrong Tian
Formely with Middlesex University
Now with NPL
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University of Surrey
 Located in Guildford  30km south of London
 Approx. 5000 students
 Single campus - lot of student accommodation on-site
 Technological university
 Research-led university
 Top of UK research ratings in Electronic Engineering
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School of Electronics: Research Groups
 Surrey Space Centre
Small satellites: design + construction + control
 Advanced Technology Institute
Semiconductors + ion beam applications + microwave systems
 Centre for Communication Systems Research
Mobile + satellite communications
 Centre for Vision, Speech and Signal Processing
Medical + Multimedia + Robotics
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Advanced Technology Institute
Microwave Systems:
- MMIC design
- RF and Microwave MCMs
- Microwave circuits and antennas
- thick-film (including photoimageable) processing
- access to clean rooms (class 1000 and class 100)
- measurement capability to 220GHz
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Thick-Film Multilayer
Microwave Circuits
for Wireless Applications
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CONTENTS
 Introduction
 Thick-film technology
 Significance of line losses
 Single layer microwave circuits
 Multilayer microwave circuits
 Summary
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INTRODUCTON
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Typical frequencies for wireless applications:
Current mobile: 0.9GHz - 2GHz
3G systems: 2.5GHz
Bluetooth: 2.5GHz
GPS: 12.6GHz
LMDS: 24GHz and 40GHz
Automotive: 77GHz
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Driving forces created by the wireless market:
 lower cost
 higher performance
 greater functionality
 increased packing density
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Microstrip: basic microwave interconnection
structure
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Summary of key material requirements at RF:
Conductors:- low bulk resistivity
- good surface finish (low surface roughness)
- high line/space resolution
- good temperature stability
Dielectrics: - low loss tangent (<10-2)
- good surface finish
- precisely defined r (stable with frequency)
- isotropic r
- consistent substrate thickness
- low Tf (< 50 ppm/oC)
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RF Transceiver Architecture
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Features of an RF MCM
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THICK-FILM TECHNOLOGY
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Thick-Film Technology
Advantages:
Low Cost
Feasibility for mass production
Adequate quality at microwave frequencies
Potential for multi-layer circuit structures
Difficulty:
Fabrication of fine line and gaps: limited
quality by direct screen printing
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Standard range of materials is used:
CONDUCTORS: - gold
- silver
- copper
DIELECTRICS: - ceramic (alumina)
- green tape (LTCC)
- thick-film pastes
- laminates
Plus photoimageable conductors and dielectrics
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Fine lines < 25 micron with 1 micron
precision
High density, 4 micron thick conductor
High conductivity - 95% of bulk
96% Al
50m lines
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Photodefined conductors
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MICROSTRIP RESONANT RING
TEST STRUCTURE
r2
W
W
r1
S
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Microstrip Resonant Ring
• can be used to measure total line loss and vp
(measure Q  loss, measure fo  vp )
• does not separate conductor and dielectric loss
• ring is loaded by input and output ports - source
of measurement error
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Meander-line test structure
• can be used to measure total line loss and vp
(measure Q  loss, measure fo  vp )
• does not separate conductor and dielectric loss
• ring is loaded by input and output ports - source
of measurement error
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Chamfering of the corners is a necessary
precaution in microstrip to avoid reflections
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0.04
0.035
Line Loss (dB/mm)
0.03
0.025
0.02
0.015
0.01
0.005
0
0
5
10
15
20
25
30
35
40
45
Frequency (GHz)
measured
simulated
Comparison of measured and simulated loss in a 50 line
fabricated on 99.6% alumina.
[substrate thickness = 254m and line width = 255m]
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0.3
Line Loss (dB/wavelength)
0.25
0.2
0.15
0.1
0.05
0
4
8
12
16
20
24
28
32
36
40
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Frequency (GHz)
Measured line loss: 50 thick-film microstrip line
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0.04
III
0.035
Line Loss (dB/mm)
0.03
0.025
C
II
B
I
0.02
0.015
A
0.01
0.005
0
4
8
12
16
20
24
28
32
36
40
44
Frequency (GHz)
Typical microstrip line losses
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Skin effect: at RF and microwave frequencies current
tends to flow only in the surface of a conductor
Skin depth (): depth of penetration at which the magnitude
of the current has decreased to 1/e of the surface value
 
1
 f 
Significance: surface of conductors must be smooth
and the edges well defined to minimise losses
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0.045
RGH=0.5
0.04
Line loss (dB/mm)
0.035
0.03
RGH=0.2
0.025
RGH=0.1
RGH=0
0.02
0.015
0.01
0.005
0
0
10
20
30
40
50
Frequency (GHz)
Effect of surface roughness on the loss
in a microstrip line
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Effect of loss tangent on line loss
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100%
80%
Line
Loss
(%)
60%
40%
20%
0%
8
20
32
44
Frequency (GHz)
Bulk Conductor Loss
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Loss due to Surface Roughness
Dielectric Loss
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100%
90%
80%
70%
L in e
L o ss
(% )
60%
50%
40%
30%
20%
10%
0%
Al
LTC C
D iffe re n t M a te ria l (e va lu a te d a t 2 G H z)
B u lk C o n d u cto r L o ss
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L o ss d u e to S u rfa ce R o u g h n e ss
D ie le ctric L o ss
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LTCC TECHNOLOGY
•
•
LTCC technology is a well-established technology
Reliability established in the automotive market
Advantages for high frequency applications:
•
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parallel processing (→ high yield, fast turnaround,
reduced cost)
• precisely defined parameters
• high performance conductors
• potential for multi-layer structures
• high interconnect density
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LTCC TECHNOLOGY
Microwave applications:
 LTCC can meet the physical and electrical
performance demanded at frequencies above
1GHz
 Increases in material and circuit production
are reflected in lower costs: LTCC is now
comparable to FR4
 Significant space savings when compared to
other technologies, such as FR4
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SIGNIFICANCE OF LINE LOSSES
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MICROWAVE RECEIVER
Feeder
BPF1
LNA
BPF2
Mixer
Schematic of front-end of a microwave receiver
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RECEIVER NOISE PERFORMANCE
Feeder
LNA
BPF1
BPF2
Mixer

Tm
System noise temperature (Tsys)
T sys  T feeder 
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T BPF 1
G feeder

T pa
G feeder G BPF 1

T BPF 2
G feeder G pa G BPF 2
G feeder G BPF 1G pa G BPF 2
 ........
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RECEIVER NOISE PERFORMANCE
T sys  T feeder 
T BPF 1
G feeder
T pa

G
feeder

G BPF 1
T BPF
G feeder G pa G BPF
Tm

2
2
G feeder G BPF 1G pa G BPF
Significance of expression for Tsys:
 ........
2
• noise performance dominated by first stage
• a lossy first stage introduces noise:
Tfeeder = (L -1) 290
• a lossy first stage magnified noise from
succeeding stages: Gfeeder < 1
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Dielectric Properties @ 9GHz
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Material
r
Tan  x 10-3
99.5% AL
9.98
0.1
LTCC1
7.33
3.0
LTCC2
6.27
0.4
LTCC3
7.2
0.6
LTCC4
7.44
1.2
LTCC5
6.84
1.3
LTCC6
8.89
1.4
Published material data
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CALCULATED RESULTS
Feeder
LNA
BPF1
Noise figure variation
BPF2
Mixer
12
10
8
6
4
2
ta n d = 0 .0 0 5
ta n d = 0 .0 0 1
ta n d = 0 .0 0 0 1
0
1
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3
4
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SINGLE-LAYER MICROWAVE
CIRCUITS
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Single-layer microstrip circuits:
 all conductors in a single layer
 coupling between conductors achieved through
edge or end proximity (across narrow gaps)
Problem:
 difficult to fabricate (cheaply in production) fine
gaps, possibly  10m
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End-coupled
filter
Directional
coupler
Examples of single-layer microstrip circuits
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DC break
Edge-coupled
filter
Examples of single-layer microstrip circuits
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MULTI-LAYER MICROWAVE
CIRCUITS
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Multilayer microwave circuits:
 conductors stacked on different layers
 conductors separated by dielectric layers
 allows for (strong) broadside coupling
 eliminated need for fine gaps
 registration between layers not as difficult to
achieve as narrow gaps
 technique well-suited to thick-film print technology
 also suitable for LTCC technology
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Direct port 4
3 Isolated port
l
Thick-film dielectric layer
Multilayer
configuration
W2
Main substrate
εr1
h1
εr
H
S
W1
Ground plane
Input port
1
2 Coupled port
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Thick-film technology is particularly suitable for the
implementation of multilayer circuits:
 higher packing density
 integration of antenna
 close coupling between conductors
Circuit examples:
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
DC block

Directional coupler
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Directional Coupler
Multilayer Concept
Single Layer
Structure
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2dB Directional Coupler - Measured Results
0
-5
-1 0
-1 5
-2 0
-2 5
-3 0
-3 5
0
1
2
3
4
5
6
7
8
F re q u e n c y (G H z )
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3dB Directional Coupler - Measured Results
0
-1 0
-2 0
-3 0
-4 0
-5 0
0
2
4
6
8
10
12
F re q u e n c y (G H z )
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/4
Microstrip DC block
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Multilayer DC block
380um
r = 3.9
Alumina
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180um
300um
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VSW R
1 .8
1 .6
1 .4
1 .2
1
1
2
3
4
5
6
7
8
9
10
11
12
F re q u e n cy (G H z)
Measured performance of multilayer DC block
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In s e rtio n L o s s (d B )
2
1 .6
1 .2
0 .8
0 .4
0
1
2
3
4
5
6
7
8
9
10
11
12
F re q u e n cy (G H z)
Measured performance of multilayer DC block
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SUMMARY
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SUMMARY:

Thick-film technology provides a viable fabrication
process for wireless circuits at microwave frequencies

Multilayer microwave circuits can offer enhanced
performance for coupled-line circuits

Photoimageable thick-film materials extend the usable
frequency range to mm-wavelengths
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C.Free@surrey.ac.uk
www.ee.surrey.ac.uk
www.ee.surrey.ac.uk/ati
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