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Thick-Film Multilayer Microwave Circuits for Wireless Applications

Charles Free

Advanced Technology Institute University of Surrey, UK

andZhengrong 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 dielectrics23

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Fine lines < 25 micron with 1 micron precisionHigh density, 4 micron thick conductorHigh conductivity - 95% of bulk

50m lines

96% Al

Photodefined conductors

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W

S

r1

r2W

S

MICROSTRIP RESONANT RING

TEST STRUCTURE

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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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• 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

Meander-line test structure

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Chamfering of the corners is a necessary precaution in microstrip to avoid reflections

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0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0 5 10 15 20 25 30 35 40 45

Frequency (GHz)

Lin

e L

oss

(d

B/m

m)

measured simulated

Comparison of measured and simulated loss in a 50 line fabricated on 99.6% alumina.

[substrate thickness = 254m and line width = 255m]

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0

0. 05

0. 1

0. 15

0. 2

0. 25

0. 3

4 8 12 16 20 24 28 32 36 40 44

Frequency (GHz)

Line

Los

s (d

B/wa

vele

ngth

)

Measured line loss: 50 thick-film microstrip line

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I

II

III

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

4 8 12 16 20 24 28 32 36 40 44

Frequency (GHz)

Lin

e L

oss (

dB

/mm

)

A

B

C

Typical microstrip line losses

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Skin effect: at RF and microwave frequencies currenttends 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

f

1

Significance: surface of conductors must be smoothand the edges well defined to minimise losses

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0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0 10 20 30 40 50

Frequency (GHz)

Line

loss

(dB

/mm

)

RGH=0.5

RGH=0.2

RGH=0.1

RGH=0

Effect of surface roughness on the lossin a microstrip line

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00.005

0.010.015

0.020.025

0.030.035

0.040.045

0.05

0 10 20 30 40

Frequency (GHz)

Tand=0.001 Tand=0.003Tand=0.005 Tand=0.0001

Effect of loss tangent on line loss

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0%

20%

40%

60%

80%

100%

LineLoss(%)

8 20 32 44

Frequency (GHz)

Bulk Conductor Loss Loss due to Surface Roughness Dielectric Loss

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0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

LineLoss(%)

Al LTCC

Different Material (evaluated at 2GHz)

Bulk Conductor Loss Loss due to Surface Roughness Dielectric Loss

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LTCC TECHNOLOGY

• LTCC technology is a well-established technology• Reliability established in the automotive market

Advantages for high frequency applications:

• 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 RECEIVERMICROWAVE RECEIVER

Feeder BPF1 BPF2LNA Mixer

Schematic of front-end of a microwave receiver

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RECEIVER NOISE PERFORMANCERECEIVER NOISE PERFORMANCE

System noise temperature (Tsys)

Feeder

BPF1 BPF2

LNA Mixer

........212

2

1

1 BPFpaBPFfeeder

m

BPFpafeeder

BPF

BPFfeeder

pa

feeder

BPFfeedersys GGGG

T

GGG

T

GG

T

G

TTT

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RECEIVER NOISE PERFORMANCERECEIVER NOISE PERFORMANCE

........212

2

1

1 BPFpaBPFfeeder

m

BPFpafeeder

BPF

BPFfeeder

pa

feeder

BPFfeedersys GGGG

T

GGG

T

GG

T

G

TTT

Significance of expression for Tsys:

• 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

Material r Tan x 10-3

99.5% AL9.98 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 CALCULATED RESULTS Noise figure variationNoise figure variation

Feeder

BPF1 BPF2

LNA Mixer

0

2

4

6

8

10

12

1 2 3 4 5

tand=0.005 tand=0.001 tand=0.0001

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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 10m

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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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3 Isolated port

Direct port 4

Ground plane

H

h1

εrW1

W2

εr1

1

2 Coupled port

l

S

Main substrate

Thick-film dielectric layer

Input port

Multilayer configuration

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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:

DC block

Directional coupler

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Single Layer Structure

Multilayer ConceptDirectional Coupler

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-35

-30

-25

-20

-15

-10

-5

0

0 1 2 3 4 5 6 7 8 9 10 11

Frequency (GHz)

2dB Directional Coupler - Measured Results

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-50

-40

-30

-20

-10

0

0 2 4 6 8 10 12 14 16 18 20

Frequency (GHz)

3dB Directional Coupler - Measured Results

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/4

Microstrip DC block

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300um

380um

Alumina

r = 3.9180um

Multilayer DC block

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1

1.2

1.4

1.6

1.8

2

1 2 3 4 5 6 7 8 9 10 11 12

Frequency (GHz)

VS

WR

Measured performance of multilayer DC block

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0.4

0.8

1.2

1.6

2

1 2 3 4 5 6 7 8 9 10 11 12

Frequency (GHz)

Inse

rtio

n L

oss

(d

B)

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