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NIST Technical Note 1520

Dielectric and Conductor-Loss

Characterization and Measurements

on Electronic Packaging Materials

James Baker-Jarvis

Michael D. Janezic

Bill Riddle

Christopher L. Holloway

N.G. Paulter

J.E. Blendell

QC

100

.U5753

#1520

2001 tl

istitute of Standards and Technology

idministration, U.S. Department of Commerce NIST CENTENNIAL

NIST Technical Note 1520

Dielectric and Conductor-Loss

Characterization and Measurements

on Electronic Packaging Materials

James Baker-Jarvis

Michael D. Janezic

Bill Riddle

Christopher L. Holloway

Radio-Frequency Technology Division

Electronics and Electrical Engineering Laboratory

N.G. Paulter

Electricity Division

Electronics and Electrical Engineering Laboratory

J.E. Blendell

Ceramics Division

Materials Science and Engineering Laboratory

July 2001

^^ATBS O* ./

U.S. Department of Commerce

Donald L. Evans, Secretary

Technology Administration

Karen H. Brown, Acting Director

National Institute of Standards and Technology
National Institute of Standards and Technology Technical Note 1520

Natl. Inst. Stand. Technol. Tech. Note 1520, 156 pages (July 2001)

CODEN: NTNOEF
Contents

1 Introduction 2

2 Dielectric Materials 3

2.1 Overview of Packaging Technology and Materials 3

2.2 Ceramic Substrates 5

2.3 Organic Substrates 9

2.4 Other Substrates 15

3 Dielectric Measurement Methods for the

Permittivity and Permeability of

Unmetalized Thin Substrates 17

3.1 Overview of the Measurement Problem 17

3.2 Resonant Methods 27

3.2.1 Reentrant Cavity 28

3.2.2 Split-Cylinder Resonators 30

3.2.3 Cavity Resonators 30

3.2.4 TEoi Split-Post Dielectric and Magnetic Resonators 31

3.2.5 Courtney Technique 34

3.2.6 Whispering-Gallery Resonators 36

3.2.7 Fabry-Perot Resonators 37

3.3 Transmission-Line Methods 39

3.3.1 Coaxial Lines and Waveguides: Transmission/Reflection and

Short-Circuited Line 39

3.3.2 Open-Ended coaxial lines and waveguides 46

3.4 Capacitive Techniques 50

4 Dielectric Measurement and Metallic Loss

Characterization Using Patterned Fixtures 52

4.1 Introduction to Metalized Structures 52

ni
4.2 Conductors on Printed Wiring Boards 53

4.3 Transmission-Line Parameters for

Circuit-Board Applications 55

4.4 Conductor-Loss Measurement 58

4.5 Time-Domain PWB Measurement Methods 59

4.6 Full-Sheet Resonance Method for Thin Sheets 61

4.7 Techniques for Transmission Lines on Substrates 64

4.7.1 Microstrip Resonator 65

4.7.2 Striphne 70

4.7.3 Coplanar Strip (CPS) 78

4.7.4 Coplanar Waveguide (CPW) 80

4.7.5 Increased Power Loss Associated with Surface Roughness ... 81

4.8 T and Ring-Resonator Methods for Substrates 90

4.9 On-Wafer Transmission-line Methods for Substrates 90

5 Dielectric Measurement Software 92

6 Ambient Temperature Mezisurements 93

7 Elevated Temperature Measurements 118

7.1 Thermal Properties 118

7.2 Elevated Temperature Measurement Results 122

8 Conclusion 131

9 References 132

Appendix A: Mixture Equations 151

Appendix B: Broadband-Plastic Measurements 152

IV
Dielectric and Conductor-Loss

Characterization and Measurements on

Electronic Packaging Materials

James Baker-Jarvis* Michael D. Janezic * Bill Riddle*

Christopher L. Holloway * N. G. Paulter t J. E. Blendell *

National Institute of Standards and Technology

325 Broadway, Boulder CO 80305

This is an overview of dielectric measurement methods and metrology on substrate materials used in electronic packaging. These substrates include low

and high temperature co-fired ceramic substrates, printed wiring boards, and

other packaging materials. The most cormnonly used methods are presented.

We particularly emphasize methods that can be used nondestructively either

in the laboratory or on the assembly line. We begin with an overview of pertinent electrical properties of substrate materials and conductors. We then

describe frequency-dependent metal losses on conductors. We also overview

cavity and dielectric resonators, full-sheet resonance, coaxial probe, stripline,

and capacitive methods. The frequency ranges of applicabihty and typical

uncertainties associated with the methods are summarized. We also present

measurement results of many substrate materials.

Key words: dielectric; full-sheet resonance; loss factor; LTCC; metal loss; microwave; permeability; permittivity; printed wiring board; PWB; stripline;

substrate.

* Radio-Frequency Technology Division, MS 813, email: jjarvis@boulder.nist.gov

^Electricity Division, MS 811, National Institute of Standards and Technology, Gaithersburg,

MD 20899

^Ceramics Division, MS 852, National Institute of Standards and Technology, Gaithersburg, MD

20899
1. Introduction

This technical note is an introductory overview of dielectric and magnetic measure- ment metrology on packaging materials.

Dielectric materials have many important functions in the microelectronics industry. New packaging technology requires substrates with low permittivity, interconnections made of high-conductivity metals, high wiring density, and embedded

passive circuit elements. The use of fine-line signal conductors requires thinner,

laminated, printed wiring board (PWB), thin films, low-temperature co-fired ceramics (LTCC) , and other substrate materials. As electrical components are miniaturized, the need for well-characterized dielectric measurements on thin materials

increases [1-10]. Accurate measurement of complex permittivity is needed for circuit

design, minimization of crosstalk, and characterization of signal-propagation speed.

In Section 2 we review important fundamental concepts related to dielectric properties of microelectronic substrate materials. This includes the types of materials and

laminates commonly used and the definitions of important electrical concepts. We

also provide an review of the relevant parameters of transmission-line theory. In Section 3 we review various frequency-dependent permittivity measurement methods for

thin materials. We also study metal losses in various transmission-line structures.

In Section 6, we present measurements on substrate and circuit-board materials. In

Section 7, we overview thermal properties and present a technique for measurements

at elevated temperatures. Measurement results for a number of commonly used

materials are also given.
2. Dielectric Materials

2.1 Overview of Packaging Technology and Materials

Substrates are used in PWBs, central processing units, and for thin films. Important

properties of LTCC, PWB, and other substrate materials include low electrical loss,

high thermal conductance, low thermal expansion, and high interfacial adhesion to

metal surfaces or other films (see table 1). Low electrical loss decreases heating and

signal attenuation, high thermal conductivity rapidly removes heat from the circuit,

and low thermal expansion promotes circuit durability.

In high-speed or high-frequency circuits, the speed of signal propagation is important. The signal-propagation delay depends on both the dielectric constant and the

transmission-line structure. This dependence is manifest in the equation for propagation delay for transverse electromagnetic (TEM) propagation modes which, in a

lossless hne, is

t, = b/SK^ (1) c where c is speed of light in vacuum, / is line length, fi'^ is the real part of the relative

permeability given by // = /io[A*r ~"

i/^r']? 4 ^^^ ^r ^^^ ^^^ ^^^^ ^^^ imaginary parts

of the relative permittivity, e = [e^ — J€"]eo, and Cq and //q are the permittivity and

permeability of vacuum. 4 provides a measure of the relative phase change as a

sinusoidal signal propagates through a material, e^' is related to attenuation of the

signal and includes both dielectric and dc-conductivity losses. Generally, the loss in a material is expressed in terms of the loss tangent, tan 6 = ^. Dielectrics with low

loss provide reduced attenuation and heating in circuits. They also provide greater

signal integrity. The propagation delay per unit length in a lossless line can be

expressed in terms of the distributed capacitance, C, and inductance, L

td = y^'rLCyac^ (2)

where L and Cyac are the inductance and equivalent vacuum capacitance of the
Table 1. Summary of substrate properties and applications.

Property Advantages Desired range

Low permittivity Minimize cross talk

Decreased propagation delay

< « 1 - 4

High-medium permittivity Smaller circuits

Generate phase changes

< « 8 - 500

Low loss Less heat generated

Less signal attenuation.

tan^<5 X 10"^

Low thermal expansion Less circuit failure

Less impedance

temperature dependence

a < 10-5(°C-i)

High thermal conductance Cooler operation A;>30 (W/m«C)

line [11]. The propagation delay can be reduced by using substrates with a low value

of 45 by using different transmission-hne structures such as microstrip, stripline,

coplanar waveguide, coplanar strips, or by varying the geometries of signal line widths

and conductor spacings of the transmission-line structure. Crosstalk is also an important parameter in the design of high-speed or high-frequency circuits. Crosstalk can increase the noise and spurious content on signal lines, which wiU affect both

analog and digital circuits. A low value for e^ can decrease the signal crosstalk between conductors by decreasing the capacitive coupling. Materials with low dielectric

constants include Teflon^ (polytetrafluoroethylene (PTFE)), cross-linked polystyrene

"Specific materials that are commonly used are mentioned for informational purposes only.

This does not imply or constitute any endorsement by the National Institute of Standards and

Technology.
(CPS), and fused silica (Si02).

High-permittivity materials also have a niche in microelectronic applications. At

low frequencies, high-permittivity materials are used to keep dimensions of circuits

small. This can be understood by considering how the TEM mode wavelength in a

material, A, is related to the permittivity and frequency, /, by

Compact antenna arrays require high-permittivity substrates for maintaining phase

shifts between elements. In addition, large capacitances can be obtained with highpermittivity materials. High-permittivity materials include silicon carbide, polycrystalline alumina, and monocrystalline materials, such as sapphire, lanthanum aluminate, aluminum nitride, ferrimagnetic materials, titanates, and gallium arsenide [1].

2.2 Ceramic Substrates

Ceramic substrate designs include thick film, plated copper, photo-patterned thick

film, high-temperature co-fired ceramics (HTCC), and LTCC. The advantages of

ceramic materials over polymers for substrates are durability, low thermal expansion

coeflScient, and relatively high thermal conductivity. High permittivity manifests

itself in slower propagation speed and larger crosstalk in circuits. The propagation

speed varies roughly as the square root of e^. The permittivity of ceramics is strongly

influenced by the microstructure [12]. Ceramics with lower than theoretical density

have a lower permittivity and higher loss due to interfacial charges on the pore

surfaces. Grain size also influences permittivity [13,14]. LTCC are commonly used

in applications where ruggedness is important. LTCC are also being used in circuitry

for cable-free wireless interconnections between devices, (for example: Bluetooth

Technology).

The materials used for ceramic substrates [15] include aluminum oxide (alumina),

cordierite (2Mg02Al2035Si02), aluminum nitride, silicon carbide, silicon nitride.
beryllium oxide, and boron nitride. LTCC materials usually are formed from alumina, cordierite or MgO combined with Si02/glass, with additions of Pb205 and

Ba203. Beryllium oxide, aluminum nitride, alumina, silicon carbide, and silicon nitride have a permittivity higher than thoses of most organic materials [16,17]. Silicon

carbide is a semiconductor and is often combined with BeO to obtain a low-loss substrate. Beryllium oxide, silicon carbide, and aluminum nitride have high thermal

conductivity and are useful when rapid heat transfer is required. These materials have advantages when higher power levels are used. A potential problem with

beryllium oxide, however, is its toxicity in powder form. Aluminum nitride has the

disadvantage of being difficult to metalize and is lossy below 10 GHz.

Multilayer ceramic substrates can be classified by sintering temperatures. HTCC

are usually composed of glass, alumina, and aluminum nitride. HTCC green tapes

are sintered at approximately 1500 °C. LTCC green tape has a lower volumetric ratio

of ceramic to glass than HTCC, and the sintering temperature is approximately 900

°C. An advantage of low-temperature sintering is that a wider range of metals can

be incorporated into a circuit. In LTCC technology, it is possible to fire the ceramics with embedded passives and conductors. LTCC has the advantage of allowing

direct attachment to gallium arsenide and silicon circuits with high conductivity

conductors. The capability of integrated multilayer technology is shown in figure 1. LTCC and HTCC ceramic substrates are made by mixing powders of ceramics,

glass, organic binders, and solvents that are tape cast into large sheets of material.

These green (unfired) tapes can be cut, punched, patterned, and laminated to form

substrates of a desired geometry, as shown in figure 2. Some of the organic components are evaporated from the green tape during drying and the rest are decomposed

during firing. Residual carbon left after firing tends to increase the permittivity [18].

Firing in a more oxidizing atmosphere can decrease the amount of carbon, but the

choice of atmosphere may be limited due to the propeties of the metals used for

conductors.
Table 2. Resistivities of Metals [1,18].

Material Cu Ag Au Mo Pb Al Sn Pd Pt

Bulk resistivity (//fi.cm)

Thick-film resistivity (^O.cm)

Maximum temperature (°C)

1.7 1.6 2.3 5.2 20.6 2.65 11 10.8 10.6

4 5 5 12 - - - -

950 950 950 1500 327 660 230 1550 1768

There are many conductor materials used in ceramic-based circuits (see table 2).

In this technology the conductors are applied as a paste or ink on the green-state

substrates. The type of metal used as a conductor for a specific application is related

to its melting temperature, resistivity, migration resistance, cost, line resolution, and

solderability. Typical conductors used in LTCC are aluminum, copper, gold, silver,

and palladium- silver alloys. Gold produces a very high definition and reliable circuit,

but is expensive. Silver tends to migrate into substrates causing warping of the substrate. HTCC requires metals of higher melting temperature, such as molybdenum,

palladium, platinum, and tungsten. In the firing process, the conductor's properties

change and carbon is generated in the substrate from decomposition of the organic

binders, which increases both the effective permittivity and the loss. Ceramic multichip modules (MCM-C) can be fabricated using LTCC materials.

The green ceramic tapes can be patterned and machined, and then pressure laminated and fired. Integrated passive components such as inductors, resistors, and

capacitors are easily incorporated into LTCC design. Cavities for filters can also be

included. LTCC can have a large number of layers, for example 40 or more [19].

Interlevel connections are obtained through drilled or punched via holes and also

through heat transfer vias. The embedded passives include resistors made from lossy

metallic films yielding resistances from milliohms to megaohms, inductors made from

spiraled or serpentine conductors of width 125 fim to 150 fim, and capacitors made
from high-permittivity materials yielding capacitances from 1 pF to 1000 pF. The

change in resistivity in co-fired resistors as a function of temperature is reviewed in

reference [20].

The ability to include components into the modules reduces the number of interconnects which increases reliability and reduces the size and the cost. LTCC

technology also allows for a high density of signal lines throughout the module and

vias as small as 0.075 mm in diameter. Another advantage of MCM-C over PWBbased multichip modules is the lower thermal expansion coefficient. Heat transfer

is facilitated by via arrays mounted on high-thermal conductivity materials such as

AlSiC.

The permittivity of low-loss ceramics is relatively constant with frequency. In

most ceramics the loss tangent increases as the frequency increases. The loss tangent

for many ceramic materials and some polymers obey a quasi-linear dependence with

frequency /, of the form tan 6= af+ b, where a is usually a positive number. This

dependence is not applicable to all ceramics, for example, aluminum nitride.

Low-loss ceramic dielectric materials within a cavity can resonate if excited by

an electric field of the appropriate frequency. Useful high-Q resonators can be made

by enclosing a dielectric in a metallic structure. Dielectric resonators are used as

oscillators, filters for rejection of unwanted frequencies, and combiners. Examples of

dielectric resonators used in the wireless industry are shown in figure 3 and are made

of low-loss, temperature stable, high-permittivity materials. Materials commonly

used for high-Q resonators include alumina, sapphire, and titanate compounds. The

relevant properties and requirements for dielectric resonators are summarized in table

4. Polymers have been combined with barium titanate powder to obtain a substrate

of relatively high effective permittivity, that is less brittle than a pure ceramic board,

but with a higher loss [21].
Table 3. Ceramic and glass substrates 1 .

Material Operational Anisotropy

or composite temperature range ("C)

Alumina -200 to 300 no

Sapphire -200 to 300 yes

Fused silica -200 to 500 no

Rutile -200 to 500 yes

Beryllium oxide -100 to 300 no

Aluminum nitride -200 to 300 no

Borosilicate/Alumina -200 to 500 no

2.3 Organic Substrates

Organics (plastics or polymers) are commonly used in packaging materials. Substrate

materials for PWB are usually composite organic materials and may be anisotropic.

Laminations and woven-glass cloth are usually the cause of dielectric anisotropy.

Examples of laminates are fiberglass-epoxy composites (FR-4), high-temperature

fiberglass-epoxy composites (FR-5), bismalimide triazine-epoxy, cyanate ester, arimidepoxy, polyimide-glass, and polyimide-quartz [22]. The composites commonly consist

of a mixture of plastics, glass, and/or ceramics, together with reinforcing materials.

Plastics usually are reinforced with glass fibers or impregnated with glass or ceramics. Typical reinforcing materials used are paper fabric, woven glass cloth, random

fiberglass fibers, and aramid fiber cloth (see figure 4). The fabric and fiber weaving

have some variability due to manufacturing limitations, and this translates into a

variability in permittivity.
Conductors

Thermal via

Vias

V

JWC^N.

:;;i^

'Buried passive

Stacked Vias

Ceramic layers

Figure 1. Geometry of typical LTCC substrate or multilayered PWB.

Green material Press Fire

Figure 2. Fabrication steps of LTCC substrates.

There are two types of organic wiring boards, rigid PWB and flexible printed

wiring board (FWB). Rigid PWBs are fabricated from fiber-reinforced thermoset

polymers or ceramics. FWBs are formed from polymer films. Composite PWB

boards are laminates of plastic or ceramic materials. Multichip PWB modules

(MCM-L) are used to increase wiring density and are fabricated with laminated

PWB boards. Interlevel connections are obtained through via holes.

Metal foils are electrodeposited or are attached to PWB laminates by adhesives

composed of polymidies, epoxies, acrylics, and polyesters. A cross section of a single10
Ring Resonator Dielectric Resonator

Figure 3. Two types of dielectric resonator.

Table 4. Summary of important properties for dielectric resonators.

Property Requirements

High permittivity

Low loss

Low thermal expansion

High thermal conductance

Miniaturization

Large Q, close channel spacing

Frequency stability

Cooler operation

layer PWB is shown in figure 5 and that for a two-layer system in figure 6. The foils

are from 20 /im to 100 /im thick. These copper foils are patterned, typically by

etching, to form interconnect lines.

The properties of plastic materials are summarized in tables 5 and 6. Typical

bulk polymers are PTFE, CPS, fiberglass, polyimides, fluropolymers, and composites made from fiberglass, fused silica, ceramics, polyethylene, aerogels, and foams.

These polymers all have low e'^. Plastics, however, have large coefficients of thermal

expansion (CTE) and low mechanical strength. Moreover, the thermal expansion coefficients have a nonlinear temperature dependence. Very low e^ can be achieved by

introducing porosity, use of low-permittivity materials, or by forming hollow stripline

11
i I I I 1 1 1 I i I 1

I II I I I I I I I I I

Figure 4. Weaving structure in glass FR-4.

ceramic structures [23].

Polymer resins are classified into thermoplastics or thermosets. Thermoplastics,

for example polyethylene or polystyrene, soften when heated. Thermosets, such as

epoxies and phenolics, do not soften or melt when heated. Epoxies are polyethers

and have good chemical resistance and adhesive properties. Common resins for lowcost laminates are phenolics. Phenolics, such as Bakelite, are thermosetting plastics. When heat and pressure are applied in the moulding process, thermosetting

plastics react to form a cross-linked structure. This structure yields excellent dimensional and thermal stability and superior load-bearing capability at low to near

glass-transition temperatures. Recently, more complex low-permittivity polymers

such as polyquiolines and benzocyclobutenes have been developed. Phenolic resins

are products of the condensation reaction of phenol and formaldehyde. Other polymers used in PWB materials are polyimides, cyanate ester [24], bismalimide triazine

12
o

Figure 5. Geometry of PWB metal-clad substrates.

(BT), fluropolymers, and polysulfone. Polyimides, bismalimide triazine, and cyanate

ester can withstand higher temperatures than can other polymers. Fluropolymers

and polysulfone have low dielectric loss. Polymers may be crystalline, amorphous,

or a combination. Dielectric properties of these materials are functions of both frequency and temperature. Dielectric properties of many commonly used polymer

materials are summarized in [25].

Cost is crucial for the widespread acceptance of a specific material. In PWB

applications, epoxy-glass, such as FR-4, is the least expensive material and, as a

consequence, holds a large market share. There are many variations of FR-4 epoxyglass materials, with a variation in permittivity from 3.8 to 4.6. In increasing order

of cost are epoxy-glass, polyimide, polyimide-quartz, and PTFE.

The reinforcements in PWB include inorganic and organic glass fibers as shown

in figure 4. For example, aramid is a fiber made from Kevlar. Teflon and fused

silica fibers are also used. Laminates used in PWB include E-glass/epoxy (FR-4),

glass/polyimide, aramid/polyimide, aramid/epoxy, glass/PTFE. The polymer films

13
Table 5. Typical properties of plastic substrates [1]. Material < tan (5 Useful range of Anisotropy

or composite (average) temperatures (°C)

PTFE 2.06 0.0002 -20 to 150 no

PTFE/Fiberglass 2.25 0.0010 -20 to 150 yes

PTFE/Quartz 2.50 0.0005 -20 to 200

PTFE/Ceramic 10.0 0.0010 -20 to 200

CPS 2.54 0.0004 -20 to 100 no

CPS/Fiberglass 2.70 0.0010 -20 to 100 yes

CPS/Quartz 2.70 0.0005 -20 to 100 yes

Polyolefin 2.30 0.0005 -20 to 100 no

Polyimide 3.50 0.0010 -20 to 130 no

Table 6. General properties of substrate materials.

Class Permittivities Range of tan^ CTE Metal Adhesion

low-K polymers 2-5 0.0001 - 0.01 poor poor-good

Plastic/Ceramic composites 3-50 0.0001 - 0.01 good poor-good

Glasses 3- 12 0.00005 - 0.01 good good

Ceramics 10 - 500 0.00005 - 0.005 good good

Ceramic/Glass composites 4- 100 0.00005 - 0.005 good good

14
Copper /

Material 1

Material 2

Figure 6. Dual-material PWB structure,

in FWB are primarily PTFE, polyimides, and nylon.

2.4 Other Substrates

There are a number of other materials commonly used as substrates. In this section

we overview some of the properties of other substrates [26] . Semiconducting materials

such as silicon and gallium arsenide are used as substrates. Silicon is very lossy at

low frequencies, whereas gallium arsenide has low loss. A substrate becoming more

commonly used is gallium nitride. Anisotropic substrates are single-crystal sapphire,

rutile, silicon, and quartz. Sapphire has a high thermal conductivity and very low

dielectric loss. Sapphire and quartz are both brittle and difficult to drill and cut.

Glasses such as fused silica and borosilicate have low loss and medium values of

permittivities. Fused silica and most glasses have poor thermal conductivity. Rutile

15
has a very high permittivity and medium loss.

Tunable or frequency-agile ferroelectric and ferrite materials in bulk or thin-film

form are being used for device design and consequently there is a need for unbiased

and biased dielectric and magnetic measurements. In ferromagnetics or ferrimagnetics, the permeabihty can be a function of the applied magnetic field. Permittivity

tuning by applied voltage requires strong electric fields, so most ferroelectric materials are used as thin films. In ferroelectric materials the permittivity is a function of

the applied bias field. Microscopic domains consisting of electric dipoles are formed

from strong dipole-dipole interactions in ferroelectric materials. Ferroelectric material losses are affected strongly by temperature. The loss tangent of ferroelectrics

increases with the temperature approximately as T^. Applications for ferroelectric

materials include phase shifters for antenna arrays. The loss in these materials is

minimized by adding manganese to the solid. The most commonly used ferroelectric

material is barium-strontium titanate. High-temperature performance of dielectric

and ferromagnetic materials is also important for high-power systems.

16
3. Dielectric Measurement Methods for the

Permittivity and PermeabiHty of

Unmetalized Thin Substrates

In this section we review a number of dielectric measurement methods for unmetalized or unpatterned materials. In Section 3.1 we overview the important issues surrounding dielectric measurements and in Section 3.2 unmetalized resonant dielectric

measurement methods are overviewed. We then present a summary of waveguide

and coaxial line measurement methods. Metalized-substrate dielectric measurement

methods are overviewed in Section 4.

3.1 Overview of the Measurement Problem

Dielectric properties of a specimen depend on frequency, homogeneity, anisotropy,

temperature, surface roughness and, in the case of ferroelectrics, applied dc bias field

[2,27]. Dielectric materials may be isotropic or anisotropic, crystalline, amorphous,

clad or unclad, bulk or film. No single technique can accurately characterize all

materials over all frequencies and temperatures. Each frequency band and loss regime

usually requires a different method.

Anisotropy in PWB materials is usually a result of weaving fiber in the material.

In bianisotropic materials, there are two independent permittivity components. In

such cases we distinguish between in-plane and out-of- plane permittivity. In table

7, some of the various parameters pertaining to permittivity measurements are summarized. The measurement of thin materials presents a special challenge in that

uncertainties in thickness of the specimen translates into uncertainty in the permit17
Table 7. Measurement parameters.

Component of permittivity (in-plane or normal)

Frequency

Loss measurement

Specimen thickness

Temperature

tivity. In this report, we consider thin materials as those having a thickness less than

3 mm. Measurement methods on thin films that depend less on sample thickness

and more on transverse dimensions yield more Eiccurate results for e^. The reentrant cavity, stripline, microstrip, and full-sheet resonance techniques

are useful for measurement of the component of permittivity norm2J to the sample

face. However, if the sample is not metalized, measurements with this type of field

orientation can suffer from the effects of air gaps between the sample and electrode.

TMq^ cavities can yield a measurement with the electric field normal to the specimen face. Using higher modes, these cavities can measure over a frequency band

of 1 GHz to 10 GHz, The stripline technique has the advantage of easily accommodating PWB materials. It gives reasonable estimates for ej., but dubious estimates

for e^'. The split-cylinder and split-post resonators are useful for obtaining in-plane

permittivity and loss. Whispering-gallery modes can be used to obtain very accurate measurements on low-loss samples. Fabry-Perot resonators are useful at high

frequencies. The coaxial probe techniques are useful in obtaining permittivity of

isotropic materials with medium accuracy. Many of the important features of the

most important measurement techniques are summarized in tables 8 and 9. The full characterization of thin, anisotropic materials generally requires two

18
Table 8. Dielectric measurement categories compared with typical uncertainties with

each method.

Technique Field Advantages A4/< A tan Sr

Full-sheet reson. TEio < ±2%

coaxial Line TEM,TEio Broadband ±1 - 10 % ±0.005

or waveguide

Capacitor Normal E-field Low f ±1% ±10-4

Cavity- TEoi Very Accurate ±0.2 % ±5 X 10-5

Cavity TMom 4 ±0.5 % ±5 X 10-4

Dielectric reson. TEoi Very Accurate ±0.2 ±5 X 10-5

Coax probe TEM, TMoi Nondestructive ±2 - 10 % ±0.02

Fabry-Perot TEM High frequency ±2% ±0.0005

techniques, one for the component of permittivity normal to specimen face and one

for in-plane permittivity. The loss measurement is not usually affected as much as e^

by anisotropy, and a single measurement of loss often suffices. For example, resonant

transmission-line methods can be used for e'^. and the loss could be obtained by more

accurate in-plane techniques such a TEqi resonator. However, there are materials

where the loss is significantly anisotropic.

Measurement of magnetic substrates requires a strong magnetic field. Magnetic

materials can be measured by a split-post magnetic resonator, also by TMno [28] or

TEoii cavities, whispering-gallery modes, or TEqh cavities or dielectric resonators

(see table 9).

The ASTM (American Society for Testing and Materials) and the IPC (formerly

known as Institute for Interconnection and Packaging Electronic Circuits) standards

19
Table 9. Magnetic measurement methods compared with typical uncertainties.

Technique Field Advantages A^;//.; A tan Sr

Coaxial Line TEM,TEio Broadband ±2% ±0.01

or waveguide

Cavity TEoii Very accurate ±0.5% ±5 X

10-^

Cavity TMiio t^'r. ±0.5% ±5 X 10-^

Dielectric reson. TEou Very accurate ±0.5% ±5 X 10"^

Whispering-gallery Hybrid Very accurate ±1 ±5 X

10-6

Courtney TEoi Very accurate ±1 ±5 X 10"^

committees have developed a number of standard dielectric measurement techniques

[29]. The techniques applicable to thin materials are summarized in tables 10 and

11.

20
Table 10. ASTM standard techniques for dielectric measurements.

ASTM No. Applicability Method Frequency

D150 discs capacitor 1 Hz to 10 MHz

D1531 thin sheeting bridge network 1 kHz to 1 MHz

D1673-94 polymers capacitor 10 kHz to 100 MHz

D2149 discs capacitor 50 Hz to 10 MHz

D2520 small rods rectangular resonator 0.5 GHz to 50 GHz

D3380 clad substrates stripline 8 GHz to 12 GHz

D5568 cylindrical samples coaxial line 1 MHz to 20 GHz

Table 11. IPC standard techniques for dielectric measurements.

IPC No. Applicability Method Frequency

2.5.5A clad substrates capacitor 0.01 MHz to 1 MHz

2.5.5.2A clad substrates capacitor 0.01 MHz to 1 MHz

2.5.5.5 clad substrates stripline 10 GHz to 13 GHz

2.5.5.5.1 clad substrates stripline 500 MHz to 10 GHz

2.5.5.6 clad substrates full-sheet resonance 500 MHz to 10 GHz

2.5.5.1 clad substrates capacitor 0.1 MHz to 10 MHz

2.5.5.2 unclad substrates capacitor 0.1 MHz to 10 MHz

2.5.5.3 unclad substrates capacitor 0.1 MHz to 10 MHz

2.5.5 unclad substrates capacitor 0.1 MHz to 10 MHz

2.5.5.8 unclad films capacitance .01 MHz to 1 MHz

2.5.5.9 clad substrates transmission line 1 MHz to 1.5 GHz

21
Field orientation is important for measurements of anisotropic materials. The

characterization of anisotropic materials generally requires two techniques, one for

the component of permittivity perpendicular to the plane of the sample and one for

the in-plane permittivity.

Measurement fixtures in which the electromagnetic fields are tangential to the

air-material interfaces, such as in TEqi cavities and dielectric resonators, generally

yield more accurate results than fixtures where the fields are normal to the interface.

If the electric field is tangential to a gap between a material and a metallic wall of a

measurement fixture, then the measurement of e^ is minimally infiuenced by the gap

and no gap correction is normally required. Unfortunately, for many applications

it is not always possible or preferable to measure in-plane field orientations. For

example, circuit boards and PWBs operate with the electric field primarily normal

to the plane of the sheet and, therefore, this component of the permittivity is of

primary interest. However, measurements with the electric field perpendicular to the

sample face may suffer from consequences of air-gap depolarization. In such cases,

the air gap must be either accepted, mitigated by metalization of sample surfaces or

application of conductive pastes, or corrections made by numerical techniques [4,5].

The gap correction is due to the capacitance of air in the gap and effectively depends

on frequency only through the dependence of e^ in the gap-correction formula.

In the radio-frequency and microwave bands, e^ is a monotonically decreasing

function of increasing frequency (see figures 7 and 8). In the radio-frequency and

low microwave bands, any increase in ^ as the frequency increases is almost always

due to a fixture geometrical resonance or other fixture-dependent resonance. For

lossy materials e^ decreases faster as frequency increases (dispersion) than it does in

nonlossy materials. On the other hand, e^' does not necessarily decrease as frequency

22
High-loss material

Low-loss material

er

Frequency

Figure 7. Typical frequency dependence in low loss and Debye materials.

increases. It may increase as frequency increases or form a Debye loss peak. The

loss tangent (tan S = e^'/^) ^^ most ceramics increases nearly linearly as frequency

increases; in such cases, it is possible to extrapolate measurement data from one

frequency range to another. Materials of very low-loss have loss tangents in the microwave spectrum that are nearly frequency independent. In materials with a finite

dc conductivity, 4' increases at low frequencies as 1/u;, where u is radial frequency,

(for example, < 1 kHz). Conducting materials are difficult to measure at low frequencies (/ < 1 MHz). This is because the boundary layer between the sample and

electrode produces an electrode-polarization capacitance that must be removed from

the measurement result.

23
Frequency

Figure 8. Typical permittivity variation for polar materials.

The real and imaginary parts of the permittivity, excluding dc conductivity, a/u)

in ej.', are related by Kramers-Kronig dispersion relations [30]. This is a result of

causality and yields a relationship between the real and imaginary parts of the permittivity. A consequence of these relations is that frequency-dependent changes in

the real and imaginary parts of the permittivity are correlated (see, for example, the

correlation in figure 7).

There are many models used for extracting permittivity and permeability. Methods based on Maxwell's equations are single or full-mode models that relate the

resonant frequency or transmission and reflection coefficients to functions of the permittivity (see table 12). These types of models are accurate, but require complicated

solutions to Maxwell's equations and can be computationally intensive.

At lower frequencies, dielectric and magnetic material properties can be extracted

from a transmission-line model using the impedance Zc and propagation constant 7

24
Table 12. Permittivity models

Model Advantage Disadvantage

Lumped element simple limited to low frequency

Distributed element simple medium-low frequency

Single-mode based on Maxwell's eqs cannot handle discontinuities

Full-mode based on Maxwell's eqs can handle discontinuities

oI Port 1 O

Outer Conductor

h Air

II

Sample Air

=-|

Outer Conductor

O

m

Port 2

O

Figure 9. waveguide transmission line with sample.

(see figure 9). For cases where both dielectric and magnetic properties are required

one must use both the impedance and propagation constant simultaneously [5,27].

For dielectrics, either the impedance or propagation constant may be used in isolation for determination of the permittivity. In both cases, a model of the transmissionline structure is required that relates the material properties to the reflection and

transmission coefficients. The models may be based on lumped circuits or distributed

elements. For distributed-element models, usually an approximation to Maxwell's

equations in terms of current and voltage waves in the transmission line are used.

We now overview the basic approach for determining the permittivity or permeability for a filled transmission line (see, for example, in reference [27]). We will

25
go into more detail in a later section for rectangular waveguide and coaxial line

measurement methods. The voltage reflection coefficient is

where Zq is the impedance of the empty transmission line, typically 50 Q. This

equation can be rewritten as Zc = \^Zo. (5) 1- Pv

The permittivity (permeability) can be found by solving eq (5), given the permeability (or permittivity) of the transmission line and then using it in a model for the

transmission line that relates material properties to the characteristic impedance of

the transmission line.

A transmission line of length L has a transmission coefficient

TL = exp(-7L), (6)

where "f = a + jl3 is the propagation constant. It is possible to use two or more

lengths of identical transmission lines of lengths Lj to extract the permittivity and

permeability. Or one can use both the impedance and propagation constant equations

to obtain four independent equations for the four electrical parameters, as is done in

scattering- matrix approaches [5]. The attenuation constant is computed from

L2

a = - —In

Lii — 1/2 L-iLlJ

The permittivity can be determined from the phase, which is calculated from

(7)

(5 = -— arctan

/m(e"^^e-^^^)

AL [i?e(e"^^e-T^^)_

26

(8)
Due to the inverse tangent, this function is multivalued. Therefore, the correct

physical root must be determined. Frequently the transmission line has more than

one dielectric material. In these cases, an effective permittivity is measured and a

mixture equation may be used to separate the various component permittivities (see

Appendix).

Surface roughness of the conductor increases the capacitance, resistance, and inductance. The capacitance is increased because the effective thickness of the rough

material between two conductors is less than the measured thickness [31]. The roughness of the surface of the conductors can also affect the resistance due to the increased

surface area.

3.2 Resonant Methods

In this section, we overview the most commonly used resonant measurement techniques for unmetalized thin materials. We discuss and compare the strengths and

weaknesses of each technique. We will also compare the techniques in light of destructive versus nondestructive measurements. For each technique, we either overview the

theory or refer to published work. We will indicate which fixtures can be used for

obtaining permittivity in or out of the plane of the sample. We also give an uncertainty range on e^ and tan S for each fixture. For thin films, we refer the interested

reader to recent publications [32-34].

For resonant measurements, the permittivity or permeability is determined from

measurements of the resonance frequency and quality factor (Q) as shown in figure

10. The quality factor is calculated from Q = /o/A/, where /o is the resonance

frequency and A/ is the frequency difference between 3 dB points.

27
/^^ ^

C/5

3.864 3.865 3.866 3.867 3.868

Frequency (GHz)

Figure 10. Measuring resonant frequency and Q.

3.2.1 Reentrant Cavity

Resonant measurement methods are the most accurate ways of obtaining permittivity and loss tangent. Most resonant techniques yield permittivity in the plane

of the sample. The resonant reentrant cavity is an exception in that it yields the

permittivity component normal to the face of the material at frequencies from 100

MHz to 1 GHz.

The reentrant cavity depicted in figure 11 consists of a coaxial line or other transmission line with a gap in the inner electrode. The specimen is inserted into this gap.

The cavity is then resonated and the capacitance of the gap produces a frequency

shift. If the specimen gap region is at the very top or bottom of the cavity then

the system is called a singly reentrant cavity, whereas, if the sample gap is in the

middle of the cavity it is called a doubly reentrant cavity. The loss tangent is determined from quality-factor measurements with and without the specimen in place.

28
Sample

Figure 11. Schematic cross section of the reentrant cavity. The coaxial line inner and

outer diameters are 2ri and 2r2, and specimen diameter is 2rs, as indicated. The

cavity height is L.

Typical expanded uncertainties (U) (with coverage factor k = 2 (number of standard

deviations) [35], for a well-characterized system for low to medium permittivity are

Ue = 0.01, and for loss tangent U = ±0.001.

Two approaches have been used for modeling the reentrant cavity. The first is

a lumped-circuit approximation [36,37]. The other approach is a rigorous modematching technique [38-40]. The full- field model is given in reference [38]. The

software for this fixture is usually limited to samples greater than 1 mm in thickness.

29
3.2.2 Split-Cylinder Resonators

The split-cylinder resonator is used to nondestructively measure the complex permittivity of low-loss, unclad dielectric materials at microwave frequencies. This method

uses a cylindrical cavity which is separated into two halves as shown in Figure 12. A

sample is placed in the gap between the two shorted cylindrical waveguide sections.

A TEon resonance is excited with the coupling loops, and from measurements of the

resonant frequency and the quality factor, one determines the complex permittivity

of the sample. Generally, the resonator is operated such that the TEqi mode propagates within the sample and is evanescent outside the sample. Note that for this

mode the electric field is in the plane of the sample.

In terms of sample geometry, the only requirements are that the sample must

extend beyond the diameter of the two cavity sections and the sample must be flat.

This provides the accuracy of a resonator technique without having to machine the

sample. Typical uncertainties are for the real part of the permittivity U = ±0.005

and loss tangent U = ±5 x 10~^, k = 2. The original theory for this technique is summarized in references [41-43]. However, an improved theoretical model, based on the mode-matching method exists

that properly takes into account the fringing fields in the sample region outside of

the cylindrical waveguide sections [44].

3.2.3 Cavity Resonators

Cavities are also used to measure the permittivity of thin materials. The most

common cavity is the TEqi resonator [45]. In this case, the permittivity in the

plane of the specimen is measured. In order to obtain a stronger electric field, the

sample usually is elevated above the cavity floor. The TMo^ cavity is also commonly

30
Coupling ^ loop

Sample

Figure 12. TEqi split-cylinder resonator for thin dielectrics.

used. In this cavity, there is a strong Ez component of the electric field at the

center. If a specimen is placed near the center, then it is possible to measure the Eg

component of the permittivity [46]. Either rod or bulk specimens can be used. In

these measurements, a thin rod specimen is inserted into a hole at the center of the

cavity [28]. A rod specimen has an advantage of lower depolarization effects than

with a flat sample.

Rectangular resonators are also commonly used (see ASTM test method 2520).

In this method a rod sample is inserted into a waveguide and a perturbation model

is used to obtain the permittivity. For accurate measurement with this technique, a

correction for the hole in the waveguide is necessary.

3.2.4 TEoi Split-Post Dielectric and Magnetic Resonators

The split-post dielectric and magnetic resonator techniques are methods for measurement of thin materials (see figures 13 and 14) [47-49]. In the split-post method,

31
Dielectric resonator Post

Copper

^

Sample

Coupling loops

Figure 13. Schematic diagram of a split dielectric resonator fixture.

a thin material or film is inserted between two fixed dielectric resonators. The posts

that support the resonators have low permittivity and low loss. The quality factor

of the support post is influenced by the presence of the specimen. Split dielectric

resonators were developed by Nishikawa et al. [48], Maj [49], and DelaBalle [50].

There are many variations of this technique reported in the literature. Some fixtures have the specimen extending to the back wall of the cavity. The lowest-loss

technique is when the dielectric resonators are supported by low-permittivity posts.

This configuration has the advantage of reduced wall loss. Various techniques have

been employed for analyzing this method: finite difference, mode-matching, and

finite-element methods [48].

The geometry of a split dielectric resonator fixture used in our measurements is

shown in figure 13. Such a resonator operates with the TEqi mode, which has only

an azimuthal electric field component so that the electric field remains continuous

across the dielectric interfaces. This also minimizes systematic air-gap uncertainties.

32
Dielectric resonator Post

Copper

Coupling loop

Sample

Nmmmmm\\\\\\\\\\^^^^

Ground plane ^

Figure 14. Schematic diagram of a post magnetic resonator fixture.

The resonator measures the permittivity component in the plane of the specimen. A

useful feature of the split-post resonator is that it can be designed to operate in the

low gigahertz region. We have successfully used dielectric resonators operating from

1.5 GHz to 12 GHz. Loop coupling is used in these dielectric resonators.

Thin magnetic materials can be measured with the magnetic post resonator technique as shown in figure 14. This fixture consists of a dielectric post as shown in

figure 14 above a ground plane. The ground plane induces a region of strong magnetic field. The thin magnetic material is inserted between the posts and then the

resonance frequency and quality factors are measured. The same sample can also be

measured with the split-post resonator. From these two measurements it is possible

to solve simultaneously for both e and /i. The main source of uncertainty in ^ in the split-post, as with the split-cylinder

resonator measurement, is from the uncertainty in the thickness of the specimen. We

have found that the following equation is useful in estimating the relative uncertain33
ties C/e, fc = 1, in 4, AeJ. for a specimen of thickness L:

rr .AL

17 « 2-—. (9)

Typical uncertainties in the split-post resonator £ire Ue{r) = ±0.002. Here Ue{r),

/c = 2, is the expanded uncertainty [35]. The uncertainty in the dielectric loss

tangent depends on other factors such as the accuracy with which the quality factor

can be measured. The uncertainty in the dielectric loss tangent is usually absolute,

but is relative for 4. For a properly chosen specimen thickness, it is usually possible

to resolve dielectric loss tangents with uncertainty U = ±2 x 10~^ for quality-factor

measurements made with accuracies of ±2 %.

3.2.5 Courtney Technique

Sleeve

Cavity wall

Coupling loop

Figure 15. Courtney fixture with rod sample.

The Courtney fixture, as shown in figure 15, can be used to measure the dielectric

properties of materials at gigahertz frequencies [51-57]. In this technique, the TEqu

34
mode is used to determine the dielectric properties. For a dielectric measurement,

a rod sample is placed between plates, and the resonante frequency and Q are measured. Low-loss, high-permittivity sleeves can be used around the sample to lower

the resonante frequency, thereby increasing the frequency coverage. If the material

is magnetic, two measurements are required to get both the permittivity and permeability of a material. First, the material is measured in the resonator without an

applied magnetic field and then the Courtney fixture is placed between the poles of

a magnet so that both e^ and fir can be determined.

For a rod resonator (without a sleeve), of height L, in a cavity with walls at

position r = 6, the following transcendental equation can be solved for 4 [58, 59] in

terms of Bessel functions Ji and modified Bessel functions Ki and 7i:

Mksa){Ko{ksa) - j^Koiksa)) {K,{k,a) - Mh^I,{k,a))Jo{k,a)

where

+ -—^

-—'''";"> ————^ = 0, (10)

.,=j^

-'

E<i, is assumed to vanish at r = 6. The quality factor is related to losses in the sample

and conductors

'

^ +^, (13)

where

Q Qo,c Qo,d

2407r2e, 24U7r-e, f/L\^y 1 + RK

35
Coupling loops

Figure 16. Whispering gallery mode resonator. The specimen is in form of a disk.

and Rs is surface resistance of the end plates.

1 + R

Qo,d =

tan <^,

(15)

where tan 6s is the loss tangent of the rod and

R =

ksJo{ksa)

JSJ?{ksr)rdr

(16) Srka{Ko{kaa)-j^Io{kaa))_

In addition, there are metal losses on the cavity outer wall located at r = 6.

3.2.6 Whispering-Gallery Resonators

Low-loss dielectric and magnetic materials can be measured in the microwave frequency range by use of whispering-gallery mode resonators as shown in figure 16

[60-63]. Whispering-gallery modes have been used in measurements of both 4 ^ind

tan 6. Highly accurate measurements of dielectric loss can be made using this technique. In this dielectric-resonator technique, the specimen is usually supported from

36
the bottom with a dielectric rod. The support rod does not unduly influence the measurement because the modes do not penetrate appreciably into the rod [50,64-69].

Measurements using whispering-gallery modes on low-loss materials are advantageous since conductor loss can be made very small.

3.2.7 Fabry-Perot Resonators

Coupling Aperture

Mirror

Sample

Mirror

Figure 17. Fabry-Perot resonator with specimen on the post.

Open resonators have been used for measuring low-loss materials in the millimeter

range in the configuration shown in figure 17 [70-74]. Open Fabry-Perot resonators

consist of two separated mirrors with a coupling aperture on one of the mirrors. In

the confocal setup both mirrors are concave, whereas in the semi-confocal arrangement only one of the mirrors is concave and the other is flat (see figure 17). Since they

are open structures they suffer from leakage of radiation. The concave feature of the

mirror minimizes radiation leakage from the open sides of the resonator because it

focuses the beam onto a smaller area of the specimen under test, thereby minimizing

37
specimen edge diffraction effects. Fabry-Perot resonators have large quality factors,

typically 20,000 to 100,000, and are useful for measurements on thin, low-loss materials. The metal in the mirrors limits the accuracy of the loss measurement. The

tensor permittivity values can be obtained by measuring at different angles of incidence. Thin substrate materials can be measured with this technique by positioning

the specimen on the flat mirror. The permittivity is found by solving [75]

tan {nkt — $t) -f ntan {kd — $d) = 0, (17)

where n = y/e^, d = D—t, ^t = arctan {t/nzo), $d = arctan (d"/2o)—arctan (t/n^zo),

Zq = Jd"{Ro — d")^ d" = d + t/n^, D is distance between mirrors, t is sample thickness, Rq is radius of curvature of the mirror, w is the beam width, and k = 27r/A,

where A is the wavelength in free space.

A correction for the flat mirror in a Gaussian beam is given by

where

A =

if n2- 1 1A sin^' [nkt --*t)

/ n4 k'^wn2

2(iA-h d)

'

n^ cos^ [nkt — $t) -h sin^ [nkt — $t)

*

The loss tangent is found from

2nkt{d + tA)

tanS =

and the quality factor is

QeA{2nkt - sin {nkt - $t))

Qe Qd Ql

where Qa is quality factor with the sample present and

2{tA + d) ^' - ^'D(A + 1)'

where Qq is the quality factor without the sample present.

(18)

(19)

(20)

(21)

(22)

38
3.3 Transmission-Line Methods

3.3.1 Coaxial Lines and Waveguides: Transmission/Reflection and ShortCircuited Line

Coaxial lines and waveguides are commonly used to measure samples of medium

to high loss [5,76-78] (see figure 9). A coaxial line of 7 mm diameter can be used

to measure samples from 1 MHz to approximately 18 GHz. A coaxial line of 14

mm diameter can be used to measure samples from 100 kHz to 2 GHz. X-band

waveguides can be used to measure from 10 GHz to 13 GHz. Waveguide bands of

other frequencies can be used to measure samples; however samples become large

in the low gigahertz region. This technique is limited by air-gap effects to use with

low-to-medium dielectric constants unless the sample surfaces in contact with the

waveguide walls are metalized.

In two-port simultaneous measurements of permittivity and permeability, a sample is inserted into either a waveguide or a coaxial line, and the sample is subjected

to an incident electromagnetic field. The scattering equations are found from an

analysis of the electric field at the sample interfaces. In developing the scattering

equations usually only the fundamental waveguide mode is assumed to exist.

As a special case, consider a perfectly dimensioned sample in a perfectly dimensioned waveguide as indicated in figure 9. In this case no mode conversion occurs

because the eigenfunctions in the air and sample regions are orthogonal with respect to cross-sectional coordinates. Therefore the modes may be decoupled, and

the evanescent modes are not of concern. For a 2-port device the expressions for the

measured scattering parameters are obtained by solving the boundary value problem

subject to boundary conditions. The scattering parameters are defined in terms of

the reflection coefficient T and transmission coefficient Z by:

39
'S'li — Ri

S22 = R2

S21 = R1R2

rr(i -z')]

1- p2^2

J

'

rr(i -z')]

1- Y2Z2 J

'

'z{i-T^y

'U-_r2z2

(23)

(24)

(25)

where transmission coefficients are

Ri = exp(-7oLi), (26)

R2 = exp(-7oL2), (27)

are the respective reference plane transformations and

7o = Ji -)-(r)-

i

(28)

where c^ac and ciab are respectively the speed of light in vacuum and laboratory, and

Ac is the cutoff wavelength. Also the transmission coefficient Z is

Z = exp(-7L), (29)

where L is the sample length and

7 = 3

\ ^vac V Ac /

and a reflection coefficient

M _ M

•p _ 7 7D_

it A. no.' 7 70

40

(30)

(31)

t
where e* and ji* are the complex permittivity and permeabihty. For coaxial line, F

reduces to

F = ii^iVj^ . (32)

Clab V e*

For nonmagnetic materials, eqs (23), (24), (25) contain e^, 4', L, and the reference plane transformations i?i,i?2 as unknown quantities. For magnetic materials,

we have the complex permeability as an additional unknown. Additionally, the total

sample holder length Lair = L + Li + L2 is assumed known. We have measurements

at each frequency point of all S-parameters. Therefore we have an over-determined

system of equations. This abimdance of information can be exploited by using optimization data reduction algorithms.

For dielectric materials we generally solve the following equation

p2 _ ^2

SnS22 - S21S12 = exp{-27o(Lair - L)}——^ (33)

for the permittivity in an iterative fashion [27,76].

For magnetic materials, in order to obtain both the permittivity and the permeability from the scattering parameter relations, we need at least two independent

measurements [7, 79] . These independent measurements could involve two samples

of different lengths or they could use a full 2-port measurement on a single sample.

In the full S-parameter solution, we solve equations that are invariant to reference

planes for e and //. A set of equations for single-sample magnetic measurements is

eq (33) in combination with

(^21 + Sn)/2 = exp{-7o(L.,. - L)}^^^^. (34)

41
We can also obtain an explicit solution to eqs (33) and (34). Let x = {S21S12 —

5ii522)exp{27o(I/oir - L)} and y = {(^21 + Sn) /2} e-xp{'yo{Lair - L)}] then it can

be shown that the physical roots for the transmission coefficient for \Z\ < 1 are [79]

x + 1 2y \'x + lV

2y

1. (35)

The reflection coefficient is

The ambiguity in the plus-or-minus sign in eq (36) can be resolved by considering

the reflection coefficient calculated from Sn alone

ri(Z' - 1) ± Jv'Z'^ + 2Z2(2Su - v') + v' ^' =

2SnZ^

• ^^^>

where rj = exp(—270L1). The correct root for Fs is picked by requiring iFsl < 1. An estimate of Li is needed in eq (37). If r2 is compared with Fs, then the plusor-minus sign ambiguity in eq (36) can be resolved and therefore F2 is determined.

The permeability and permittivity are then

where n = 0, ±1, ±2... Also

a;^ Ac L'^

The correct value of n is picked using the group delay comparison. At low frequencies

the correct value is more easily identified since the roots are more widely spaced.

Equation (39) has an infinite number of roots for magnetic materials, since the

logarithm of a complex number is multi-valued. In order to pick out the correct root,

we have to compare the measured group delay to the calculated group delay. The

42
calculated group delay is related to the change of the wave number k with respect

to the angular frequency

Tcalc.group --t£ — '-' ir Kf^;P 1

C2 l e't^tP 1

(40)

(41)

The measured group delay is

' meas.group n jr i V^^J

where (j) is the phase of Zi. To determine the correct root, the calculated group

delays are found from eq (41) for various values of n in the logarithm term, where

In Z = In |Z| +jf(^ + 27rn), and n = 0,±l,±2,.... The calculated and measured group

delays are compared to yield the correct value of n. Where there is no loss in the

sample under test, eq (39) is divergent at integral multiples of one-half wavelength in

the sample. This occurs because for small |5ii|, the phase of 5ii cannot be accurately

measured. Also in this limit, both scattering equations reduce to the relation Z^ ^

1,

which is a relation for the phase velocity only and therefore solutions for e* and /z* are

not separable. This singular behavior can be minimized in cases where permeability

is known a priori.

For a short-circuited line the equation, for the reflection coefficient is _ tanh7L -|- /3tanh7oAL — (5{1 + /3 tanh 7L tanh 70AL) . . ~ tanh 7L -h /3 tanh 70AL + (5{l + (3 tanh 7L tanh 70AL)

^

where AL is the distance of the sample from the short circuit, and

/3 = ^^. (44)

43
A list of cutoff frequencies and operating frequencies for various waveguide sizes

is given in tables 13 and 14. Coaxial line has the distinct advantage of having

no cutoff frequency; however, coaxial line becomes multi-moded above a certain

frequency. These multi-modes are due to bead resonance, which couples in the TEn

and higher modes. The approximate upper frequency limit due to bead resonance

is given in table 15. At very low frequencies (< lOOkHz), skin-depth effects change

the impedance due to changes in the inductance.

Table 13. Cutoff frequencies for rectangular waveguides.

EIAWR Band Cutoff frequency(GHz)

650 L 0.908

430 W 1.372

284 S 2.078

187 c 3.152

90 X 6.557

42 K 14.047

22 Q 26.342

Table 14. Rectangular waveguide dimensions (a), (6) and operating frequencies in

air. EIAWR Band a (cm) b (cm) TEio Operating Frequency(GHz)

650 L 16.510 8.255 1.12- 1.7

430 W 10.922 5.461 1.7-2.6

284 S 7.710 3.403 2.6 - 3.95

187 c 4.754 2.214 3.95 - 5.85

90 X 2.286 1.016 8.2 - 12.4

42 K 1.067 0.432 18 - 26.5

22 Q 0.569 0.284 33-50

44
Table 15. Air-filled coaxial cable operating frequencies.

Coaxial cable dimensions (mm) Useful operating frequency (GHz)

3^5 0.001 - 34.5

7.0 0.001 - 18.2

14.0 0.001 - 08.6

45
3.3.2 Open-Ended coaxial lines and waveguides

• Open-Ended Coaxial Probes

Open-ended coaxial lines and waveguides [80, 81] have been used for years as

nondestructive testing tools. In the open-ended coaxial or waveguide measurement, the probe is pressed against a specimen and the reflection coefficient is

measured and used to determine the permittivity. Coaxial probes (see figure

18) have also been used for elevated temperature measurements of solids and

liquids [82] . The technique has been popular and studied extensively. Although

nondestructive, the method has definite limitations. At low frequencies there

is little field interaction with the material. Since the coaxial probe has electric

field components in both axial and radial directions, the measurement contains

components of both the axial and radial permittivities. The method is sensitive to air gaps since the probe has both E^ and Ep electric field components.

The open-ended waveguide technique has the advantage of a tangential electric

field at the probe end, thus minimizing the effect of an air-gap. In process

control, for example in thickness testing, a noncontacting probe may be required [82-89]. For this reason it is important to have a model of a coaxial

probe which includes lift-off [86], or inclusion of an air gap between sample and

probe, as shown in figure 18. The coaxial probe operates at a frequency where

only the fundamental TEM mode propagates in the coaxial line. Evanescent

TMon modes are also assumed to exist in the coaxial line near the probe end. In

the coaxial line and the material under test, the magnetic field is assumed to be

azimuthally symmetric. Therefore, only the H(f, component need be calculated.

Two coaxial probes can be used to form a two-port test device. In this type

46
(1)

ground plane

l^ra ^ra ^1^ gap (2)

^rsKs specimen (3)

termination(4)

.z =

.z = d

z^L + d

Figure 18. The open-ended coaxial probe over a specimen with an air gap between

specimen and probe.

of fixture, a specimen is inserted between the probes and both reflection and

transmission data is obtained and inverted for permittivity. The full-mode

model is given in reference [90]. The analytical solution follows the same line

of reasoning used in the full-mode single coaxial-probe problem. Experimental

results indicate that the coaxial probe is useful for broadband material measurements on thin materials as shown in figures 19 and 20. Uncertainties in

open-ended coaxial measurements include dimensional and scattering parameter measurement uncertainties.

In figures 19 and 20, coaxial probe measurements on FR-4 are compared to

results on the reentrant cavity and full-sheet resonance methods. We see that

the coaxial probe measurement has higher uncertainty, but covers a larger

frequency band.

The coaxial probe has the advantage of nondestructive testing capability, but

the uncertainties are larger than for resonant techniques.

• Short-circuited Open-ended Probes

Another method that has been used over the years is a variation of the coaxial

probe, where the sample is positioned at the end of the inner conductor as

47
5.0

4.5-

co

4.0-

• One-RDTt Coaxial Robe

Reentrant Cavity I Full-Sheet Resonance

Split-RDSt Resonator

' 1

3

5

1 '''' I "I "I ' 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1

'012345

Frequency (GHz)

Figure 19. 4 from measurements of FR-4 material using a coaxial probe compared

to measurements made by reentrant cavity, split-post resonator, and full-sheet reso- nance techniques.

shown (a) in figure 21 [91-93]. A full-mode model of configuration (b) in figure

21 has been published [94]. Since the sample rests on a metal surface, the

method has limited loss resolution.

• Shielded Open-Circuited Holders

A useful measurement method for liquids, semisolids, and solids over broad

frequencies is the shielded open-circuited holder as shown in figure 22. The

shield extends beyond the inner-conductor termination. The open-circuit posi48
0.5

cur 0.0-

-0.5

i »

>

4 >

> <

<

•

1

>

(

(

(

1 1 'i

'

(

'

• •

>

• One-FbrtCoaxii

Reentrant Cavity

Split-RDSt Resonator " I ll I III I I II I I 1 I I I II I I ll I I I I I III I III! I I I II I II I ill I I ll I I I I 12 3 4 5 6

Frequency (GHz)

Figure 20. e^' from measurements of a FR-4 material using a coaxial probe compared to measurements made by reentrant cavity, split-post resonator, and full-sheet resonance techniques.

tion is below the fill level of the liquid or powder. The frequency of operation

is assumed to be such that the modes in the shield region are evanescent. The

dominant TEM mode is assumed to be incident upon the open circuit where

a reflected TEM wave is formed together with evanescent TMon modes. No

energy is lost to the evanescent modes unless the lines are lossy or a lossy

material is in the coaxial line or shield.

49
Connector

Outer Conductor

nner Conductor

(a)

Sample

(b)

Figure 21. Open-ended sample method, (a) depicts the sample only under the inner

post, and (b) the sample extends across the entire diameter of the coaxial Hne.

3.4 Capacitive Techniques

Capacitance techniques are important at frequencies in the range of dc to 100 MHz

[95-99] . In these techniques, the electric fields are nearly normal to the sample plane.

The difficulties with these measurements pertain to air-gap depolarization, electrode

polarization, and minimizing the effects of the fringing field. An air gap at the

conductor/metal interface will yield a low value for the real part of the permittivity.

The fringing field is usually partially eliminated by measuring the capacitance with

and without a specimen and subtracting the results. Guards can reduce the effect of

fringing fields, as shown in figure 23, if the guard and unguarded electrodes extend

beyond the guard gap by a distance of at least twice the sample thickness (see ASTM

method D-150). In a guarded capacitor the permittivity is

, Cs-jGs/u;

Cair ~ jGair/'^

(45)

50
Liquid fill level

Open circuit

Connector

Figure 22. The shielded open-circuited holder.

where Cairi Gair and Cg, Gs are the capacitances and conductances with holder

filled with air and specimen, respectively. At very low frequencies, for conducting

materials, electrode polarization causes an anomalously high value for e^. This is due

to an extra capacitance that forms a double layer between the sample and electrode.

Two-fluid capacitor measurements are frequently used on PWB materials [100].

The technique has the advantage that the material thickness does not enter the

calculation. The capacitance is measured in air and then in a fluid, both with and

without the specimen. The equations are then solved for the permittivity. The

fluids used in the method, such as n-heptane, must have very low loss and be well

characterized.

51
Guard gap

\

Capacitor measurement

Guarded electrode Guard electrode

Sample

Unguarded Electrode

Figure 23. A sample in a capacitor with electrode guards.

4. Dielectric Measurement and Metallic Loss

Characterization Using Patterned Fixtures

4.1 Introduction to Metalized Structures

Many transmission lines used in microelectronics have conductors deposited directly

on a substrate. Examples of these include stripline, microstrip, coplanar waveguide,

parallel plate, and coplanar strip. In the process of depositing metal films on the

substrates, some metallic particles may penetrate into the substrate surface, causing

a boundary-layer effect. This will affect both ^^ and tan J. Also, since the metal

films are very thin, the fields can penetrate through the film and cause additional

loss as the wave travels through the dielectric. Therefore, in these types of structures

it is important to characterize the system as a whole and attempt to deconvolve the

52
metal from the dielectric losses. This becomes more important at higher frequencies

where the dielectric and metal losses increase as approximately ^fu. In this section

we introduce various measurement structures containing metalization and indicate

how to characterize both the permittivity and metal losses. The metal losses in a

resonant structure are related to the losses in the dielectric in terms of the quality

factor of the dielectric, Q^ and conductor, Q^ as 111 , ,

Calibration of patterned structures is complicated because the impedance is not

always known and in addition usually is complex. Reference-plane calibration for patterned structures can be accomplished by transmission-reflection-load (TRL) algorithms (see Marks [101,102]). In these methods, standard transmission lines of varying lengths, short circuits, and open circuits are patterned onto well-characterized

substrates.

4.2 Conductors on Printed Wiring Boards

The conductors on PWB can be separated into signal or ground/power conductors.

These in turn can be separated by behavior as either lumped (resistance, inductance, capacitance) or distributed (transmission-line) elements. The differentiation

between distributed and lumped is somewhat arbitrary and is based on the frequency

at which the conductor's behavior is expected to change. The wavelength A is used

to differentiate between lumped and distributed behavior as shown in figure 24. Each

conductor has an electrical length that can be given in terms of the wavelength(s)

of the signal(s) propagating in the conductor. The wavelength for a given frequency

depends on the effective permittivity of the conductor environment. If the conductor's length is much greater than the wavelength {Lc » nX) then the conductor's

53
n/A

1/ALumped Distributed

nf

Figure 24. Lumped versus distributed parameters in a transmission line. For distributed circuits Lc » nX.

behavior is that of a transmission Hne. If the conductor length is much less than a

wavelength {Lc « nX) then the conductor behavior is that of a lumped element.

Typically, the integer n is between 3 to 10.

Power and ground conductors are typically treated as lumped elements because

of their size and configuration. Signal conductors, however, are either lumped or distributed, as defined above. If the signal conductor behaves as a lumped element, the

relevant circuit parameters are inductance (L), capacitance (C), and resistance (R).

Conductance G is related to dielectric losses. Conductors behaving as transmission

lines are discussed in the next section.

54
4.3 Transmission-Line Parameters for

Circuit-Board Applications

Important parameters for transmission lines in PWB and LTCC applications are

characteristic impedance, Zc, and propagation constant 7. Knowledge of the characteristic impedance is important for transfer of power between circuits and circuit

elements, reducing aberrant triggering caused by reflections, and maintaining signal

integrity. The characteristic impedance is given by [11,103]

Zc =

R{u;) + ju;L{u})

\G{uj)+jujC{ujy

where G is the dielectric conductance, u = 27r/, and L and R are the distributed

inductance and resistance which may be frequency dependent. The capacitance C

for low-loss dielectrics depends very weakly on frequency. The reason L is a function

of frequency is that it can be expressed as L = Lint + Lext, where Lint is the internal

self inductance of the conductors and Lgxt is the external inductance between the

conductors. R and Lint depend on the skin depth. The skin depth, S, is related to

the conductivity a by

^ = 7=7=' (^)

where a is the dc conductivity. Equation (48) implies that the fields at lower fi:equencies penetrate further into materials than fields at higher frequencies. Very thin

conductors allow the fields to penetrate appreciably through the metal. This increases resistance because the field exists in both the metal and the dielectric. As

a consequence of the skin depth, the internal inductance decreases with increasing

frequency, whereas R increases with frequency. The resistance is given by

55
Table 16. Conductivities of metals.

Material Conductivity, a{S/m) Skin depth ^(m) at (10 GHz)

/{WS/m) /(IQ-^m)

6.60

8.26

6.42

7.85

17.0

12.70

19.0

Cu 5.80

Al 3.72

Ag 6.17

Au 4.10

In 0.87

70-30 brass 1.57

Typical solder 0.71

where Lg, Ag are the length, and effective area of where the current passes through the

conductor. Using eq (48) in eq (49), we see that resistance increases with frequency.

Electromagnetic losses due to metals increase as frequencies increase due to the

surface resistivity defined in ohms/square as

1 UfflofJ'r ^^-^- (50)

Total conductor loss is a function of conductivity of the metal and of the thickness,

width, surface roughness, and boundary layer between metal and substrate. Clad

materials can have appreciable surface roughness, in order to promote adhesion. The

copper foil that is applied to the PWB material has a roughened surface. Surface

roughness affects the measured capacitance, inductance, and resistance. The effect

of surface roughness on the resistance is small at low frequencies. However, above

1 GHz, the resistance of rough copper increases noticeably over that of smooth

copper [2,31]. Bulk electrical conductances of commonly used metals are given in

table 16 [103].

The conductance G of the transmission line depends on the dielectric substrate.

At low gigahertz frequencies or below, these losses are smaller than conductor losses

56
for PWB applications. However, the losses in the dielectric increase roughly in proportion to u, whereas conductor losses increase as y/u. Therefore, at high frequencies,

above approximately 10 GHz to 30 GHz, the dielectric loss can exceed metal losses.

In an ideal lossless transmission line for typical wireless frequencies, we can ap>-

proximate eq (47) with

'L

C'

Zc{ideal) ^ xIt^- (51)

Equation (51) shows that the characteristic impedance of the lossless transmission

line is real and depends only on L and C. Z^ can be complex [101, 104]. An approximation for the characteristic impedance at high frequencies can be obtained by a

Taylor-series expansion in R. Assuming G = 0,

R R'

2ujL

"^ ^u^L\ ^^^^

The propagation constant 7 is also important in transmission-line performance

because it describes the attenuation and phase response of the line

^ = a + jl3^ MR{uj)+juL{u)){G + juC)

2Zn

+ juoy/LC 1- R^

8uj^L^

, (53)

where C,G,L,R are measured distributed circuit parameters per unit length. The

last expression in eq (53) is a Taylor-series expansion of 7 with G = in powers of

R. a and (3 are attenuation and phase parameters, a includes loss due to conductor,

dielectric, and radiation: a = ac + ad + ar- Here ac can be approximated by [11]

R{uj)

2Zc[ideal)

(54)

and

O^d = -zGZciideal) = |y^4' = |\A^4^tan5, (55)

57
where e^ = eo(^r ~ j^") is the permittivity of the substrate, cq is the permittivity of

vacuum and tan^ = ejf/ej, is the loss tangent in the material. The loss tangent in

terms of distributed circuit parameters and frequency, /, is tan J = G/{27rfC).

The imaginary contribution to the propagation constant defines the phase of an

electromagnetic wave. The phase velocity in a transmission line is given by i; = uj//3.

For a uniform lossless line v = l/y/LC. For a lossy line

Since the propagation time varies as a function, in a LTCC or PWB applications the

signals with high harmonic content, such as pulsed waveforms, will exhibit dispersion

in propagating along the line. Furthermore, the pulse will experience frequencydependent losses that will increase the dispersion in the signal.

4.4 Conductor-Loss Measurement

The insertion loss (L/) is commonly used to measure line loss [105]. The definition

of insertion loss is

L, = 101og:^, (57)

where Pi is the power delivered to a load in the absence of the network and P2 is

the power delivered to the load with the network inserted in the system. Therefore,

the insertion loss depends on both the impedance match between the load and the

characteristics of the network. Dissipative losses can be separated from reflection

power losses as [106]

Ln = J

[-10 log (1 - \Sn\') - 101og-J%L

L [ 1 - |5ii|^_

The attenuation (a) is assumed to be the insertion loss (L/) for a reflectionless

system. Here Ln denotes distributed insertion loss (dB/m).

58

i

(58)
The main issue when measuring insertion loss is accounting for effects of reflections at connectors. Other methods that are commonly used include a measurement

of two line lengths, Li and L2,

I/l — 1/2

Here L/2 denotes distributed insertion loss (dB/m). Equation (59) is equivalent to eq

(58) except it used two line lengths. This method has the advantage of subtracting

out the effects of the calibration and sample holder mismatch, assuming it is the same

for each measurement. Another method is based on the transmission coefficient [107]

T = l + r = l + |i-^, (60)

where

Lj = -201og|T| (dB). (61)

A measurement of attenuation from the propagation constant [102] using coplanar

waveguide on gallium arsenide is given in figure 25. The measurements were calibrated with NIST-developed software called MULTICAL.

4.5 Time-Domain PWB IVIeasurement IVIethods

Broadband methods using a time-domain reflectometry (TDR) technique have been

developed to extract e'^ from PWB dielectrics [7,10,44,108]. Janezic compared timedomain versus frequency-domain measurement accuracy [44]. A technique that uses

frequency averaging is given in reference [10]. In this method the average permittivity over the frequencies contained in the pulse is determined. The specimen used

in this technique is a parall'el-plate transmission line (PPTL) and is placed in the

holder shown in figure 26. The permittivity of the dielectric can be found in two

59
600

E

500 —

_

CO ( "O

400

</>

(/> o

300

c

o^

200

CD

( </3 1 c

100

Frequency (GHz)

Figure 25. Insertion loss for coplanar waveguide on gallium arsenide using (58), (59),

attenuation coefficient, and uncalibrated scattering parameters. Conductor width is 50 /zm, conductor thickness is 3 /im.

ways. In the first way, several specimens are used each with a different line width.

The relationship between the reciprocal of the measured impedance of the line versus

line width is a straight line with a slope proportional to permittivity (this assimaes

a hnear material); the intercept of the line provides information on the effects of the

fringing field. In the second, the fringing field effects are approximated by adding a

parallel fine transmission line to the ideal PPTL model. However, this approximation

does not work for large ratios of thickness to width. The measurements using this

TDR method are in close agreement with those obtained from frequency-domain

resonant methods[80,10]. A detailed uncertainty analysis is available in the refer60
i^ Clad sample (PWB)

I.

Single line contact

SMA connector

Figure 26. Sample for time-domain method.

ences[80] . The advantage of this method is that relatively inexpensive TDR systems

and specimen holders can be used, and specimens fabricated with a router or shear.

No photolithography or wet processing is required.

4.6 Full-Sheet Resonance Method for Thin Sheets

The full-sheet resonance technique is used to determine the permittivity of clad

circuit and PWB materials (see figure 27) [67,109,110]. It is a nondestructive test

and measures the bulk property of a substrate. The frequency of measurement

is hmited by the specimen dimensions since the method uses one-half wavelength

61
Coaxial Couplers

Figure 27. Full-sheet resonance technique.

resonances. The specimen is usually open at the sides, however some researchers

measure with the sides closed by conductors. The open edges make the electrical

length of the specimen slightly longer than the actual physical dimensions. The

permittivity is measured by transmission and reflection coefficients of test signals

introduced by coaxial feeds. This method causes the cladded structure to resonate

at integral multiples of one-half wavelength of the wave in the specimen. Higher

modes can also be used to extend the frequency coverage. The main difficulty in this

technique is mode identification and degeneracy. The full-sheet resonance method

has limited ability in determination of the loss factor since the surface area of the

metal cladding is large relative to the volume of the specimen.

A set of measurements using the full-sheet resonance method (FSR) on a number

of modes is shown in tables 17 and 18 and is compared to measurements with the

reentrant cavity [109]. A circle fit procedure yields a more accurate quality factor

and is summarized in [109].

62
(62)

The relative permittivity is obtained from the resonant frequencies of a cavity.

The resonance condition for TEmn modes is

where c is the speed of Hght, a and 6 are plate dimensions, m and n are mode

numbers, and fmn are the resonant frequencies. Note that the equation does not

depend on specimen thickness. This lack of sensitivity to thickness has an advantage

over other techniques.

The measured resonant frequencies calculated from eq (62) differ from the theoretical frequencies because of the open edges, conductivity of the plates, and radiation losses. The very low quality factors in this technique produce frequency-pulling

(that is, lowering of the resonant frequency due to losses) that must be taken into £iccount. Mode interference can also be a problem. Generally, better mode separation

is possible if the specimen is rectangular rather than square.

CoUin [111] derived an expression that ax3COunts for frequency pulling due to finite

wall conductivity

where fi and f^i refer respectively to loaded and unloaded resonant frequencies.

Qr is the radiation quality factor, and Yr = Gr + jBr is the open-side aperture

admittance [109].

The quality factors Qc, Qr, and Qe of cavity, radiation, and coupUng ports, are

related to the loaded Ql by— - — -i- —- ^ + ^ Ql Qc Qr Qe Qo

where k is the cavity-coupling coefficient.

63
Table 17. FSR on 0.38 mm thick PWB panel using various modes. The reentrant

cavity result was <. = 2.45, 0.02 at 440 MHz.

TE Mode f (MHz) < Circle fit FSR Uncertainty (U 1^2)

01 310 2.476 2.488 ±0.007

10 439 2.466 2.478 ±0.009

11 539 2.459 2.449 ±0.017

02 621 2.474 2.478 ±0.011

Table 18. FSR on 1.41 mm thick panel of FR4. The reentrant cavity result was

4 = 4.22, ±0.01 at 568 MHz. Open-ended coaxial probe yielded e^ = 4.23 ±5% at

580 MHz.

TE Mode f (MHz) < Circle fit FSR Uncertainty(U , k=2)

01 234 4.059 4.184 ±0.060

10 333 4.092 4.158 ±0.080

11 405 4.001 4.185 ±0.128

02 473 4.084 4.118 ±0.096

12 578 4.019 4.140 ±0.080

Equation (62) can be corrected for the open sides of the specimen by use of the

capacitance of the plates, C, and fringing capacitance, Cf to give [109]

e: = C + Cf

C + Cf/e', 2fn

{-?

a + &. (65)

4.7 Techniques for Transmission Lines on Substrates

Transmission-line techniques are useful in PWB applications since the electric field

orientation is roughly the same as in real applications. Various transmission-line

techniques have been used for nondestructive determination of substrate properties [27,76-78,112-115]. These techniques can be broadly grouped under resonant

or nonresonant techniques. The permittivity of a substrate or substrate-thin film

composite can be obtained by forming either a stripline (see figure 32), microstrip

64
(see figure 28), or coplanar waveguides (see figure 40), and then either resonating

the system or looking at the combined transmission and reflection response.

In numerical modeling, an effective permittivity eg// usually is defined that includes the effects of the fields in the air region surrounding the transmission line. As

a result, the effective permittivity depends both on the fields in the specimen and the

fringing fields. Theoretical models are usually available that extract the material's

permittivity from the effective permittivity [116-118]. These models also estimate

the characteristic impedance. In many applications, transmission lines have open surfaces. Open structures can radiate and have a complicated field structure [3,116-130].

Conductors may be etched away from wiring board cladding to form transmission

lines, or alternatily, the conductor may be deposited on a substrate or thin film.

Green tape LTCC materials are not usually clad, so conductors must be deposited

on the material. PWB materials are thin and usually clad by copper, making measurements with closed waveguide or coaxial transmission lines difficult. Also, many

substrates and PWB materials are laminated, so the component of permittivity normal to the sample face is different from the in-plane permittivity.

4.7.1 Microstrip Resonator

A microstrip consists of a strip and a ground plane separated by a dielectric. A drawing of a microstrip is given in figure 28. The quasi-TEM field in a microstrip exists

in both the air region above the strip and in the dielectric substrate. Measurements

can be made with the microstrip as a transmission line or resonator.

In a microstrip resonator, the effective permittivity can be estimated in terms

of the length L, fringing-field correction AL, frequency /, and resonance number N

65
as [131]

€e// = (66)

Sanple

Copper

Figure 28. Cross section of a microstrip.

In a microstrip fixture with thin dielectrics or wide signal lines such that w/h > 1,

where w is the width, and h is thickness of the substrate, €e// can be written as [131]

ee// =

ei + 1 +ei-1

2 2./l + 12-^

V w

(67)

Solving for 4 in sq 66 yields

, ^(t)' + 7T-bl-i

1 +

\/l+12^

(68)

When w;//i < 1

et + 1 . ei-1 W,

+ 0.04(1-^)2

V1 + I2J ^ . (69)

Solving for ^ gives

. ^(t)'-i+[7rTT^ + o-o4(i-f)^^

er =

1 + + 0.04(1 -f)2

(70)

The attenuation constant in a microstrip has been calculated [132]. However, if

the ratio t/S (where S is the relevant skin depth and t is the thickness of the conductor) is small or close to unity [133,134], Wheeler's rule breaks down and gives

66
unreliable results. The failure of analytical methods for thin conductors can be a serious limitation for the design and modeling of planar structures. Recently, Holloway

presented loss expressions that are valid for arbitrary conductor thickness [133-136].

These expressions include skin effects and field penetration through conductors. In

figures 29 through 31 attenuation in microstrip as a function of various parameters

is displayed.

a;(dB/cm)

1.00

0.60

0.20 4-5 t(f'm)

10

Figure 29. Strip conductor loss as a function of strip thickness for a microstrip

line at 10 GHz, with geometries having W = 30/im, H = 200/im, ^ = 12.9, (t —

3.33 X lO^S/m.

67
0.8

<y(dB/cm) 0.5 --

0.2 10 20

Frequency (GHz)

Figure 30. Strip conductor loss as a function of frequency for a microstrip line with

geometries having W = SO/^m, H = 300/im, t = 6^m, < = 12.9, o" = 1.77 x lO^S/m.

1.6 --

cr(dB/cm)

0.8 +

0.0 20 40

Frequency (GHz)

Figure 31. Strip conductor loss as a function of frequency for a microstrip line with

geometries having W = 20//m, H = 100//m, t = 3/im, ^ = 12.9, cr = 4.1 x lO^'S/m.

68
Table 19. Values of the stopping distance for 90° and 45'' edges.

t2d

tA t26 i t IS

tA t26tA

90° 45° 90° 45° 90° 45° 90° 45°

0.03 9.18 6.57 0.87 138.42 115.75 1.87 266.73 306.15 5.0 174.33 327.01

0.04 9.18 6.57 0.94 169.39 144.36 2.00 244.95 288.35 6.0 185.89 336.93

0.05 9.19 6.57 1.0 200.50 173.95 2.18 221.57 271.18 7.0 193.43 357.86

0.06 9.19 6.58 1.07 235.98 209.06 2.29 210.25 265.33 8.0 195.96 383.06

0.10 9.25 6.62 1.12 258.21 232.17 2.51 200.43 264.15 9.0 196.58 410.96

0.14 9.45 6.77 1.15 270.22 245.19 2.76 189.28 274.11 9.49 196.94 426.04

0.25 11.76 8.43 1.22 299.73 279.65 3.00 178.57 288.74 10.49 198.36 461.40

0.50 33.97 26.72 1.32 324.12 314.30 3.55 170.73 317.89 12.25 203.04 543.07

0.64 61.90 49.41 1.50 329.88 342.89 4.00 168.50 327.08 13.0 205.58 587.90

0.71 81.32 65.51 1.63 312.74 339.00 4.53 171.59 327.42 14.0 209.24 659.63

0.79 108.83 89.23 1.73 293.30 326.77 4.74 172.81 326.99 16.0 217.25 862.14

For a microstrip line, the attenuation constant associated with the strip conductor

is expressed as

OCstrip — R^\n{fi-1)

(71)

where w is the strip width, t is the strip thickness, and A is the stopping point for

the integrations, as defined in [133] and given in table 19 for normal conductors.

Reference [135] gives values for A for superconductors. Rsm is the modified

Horton impedance boundary condition as discussed in [133] and given by

cot (kct) -f- CSC (/i;c^)\

Ram = ^fJ'ct Im

Krt (72)

where kc is the wavenumber in the conductor. Zq is the characteristic impedance

69
given by [137]

7. Vo

47r V e^ + 1 ina +

32/1

w

h

w

re +

fhr^

^Vw

+

V(e^ + 1)

'

16e,.

(73)

where h is the height of the substrate, rjo is the impedance of free space and

/ ,^,„ 0.5160 0.0788

re = 4/0.4052 + + . (74)

The total conductor loss has contributions from both the strip conductor and the

ground plane. For narrow microstrip, the ground plane loss is a small part of the total

loss, whereas for wide microstrip, the ground plane loss becomes more important,

see [133] and [136]. In fact, for w/h >> 1, the ground plane loss nearly equals the

strip loss. The ground plane loss can be obtained from [138]

Rs

Oigr — / \Jgr{x)\ dx , J —00

(75) 2ZoP J~oo

where Jgr is the current density on the ground plane given in reference [133], and Rs

is the Leontovich surface impedance [139] is given by eq (50). This integral can be

evaluated to yield

Rs

CXgf

wttZq

Thus, the total conductor loss is given by

H^°^-^"(' + (^)l)-

(76)

OiT{Tnicro) — OCstrip H~ O^gr- (77)

4.7.2 Stripline

In stripline, as shown in figure 32, the field is confined primarily to the region between

ground planes. Striplines have been used both in resonant and nonresonant modes

70
of operation. A commonly used technique is the stripline-card method [119]. In this

technique, a stripHne is etched into a large sheet of PWB material. The stripline is

then resonated and, thereby, an estimate of the permittivity is obtained. In another

approach, the specimen sheet and copper conductors are stacked and then pressure

is applied to form a stripline resonator [140].

Sanpk

Figure 32. Cross section of a stripline resonator [141].

In stripline, the TEM mode resonates at frequencies corresponding to integral

multiples of one-half wavelength. 4 for stripline is given by

i2

^r =

nc

(78)

2fr{L + AL)\ '

where n is the number of half wavelengths, c is the speed of light, L is the physical

length of transmission line, AL is an effective length extension due to fringing fields,

and fr is the resonance frequency. The loss tangent is calculated from

tan J =

1 J_

Q unl Qc

(79)

where Q „„, is the quality factor not including coupling port losses, but includes losses

due to the dielectric, and the metal, and Qc is the quality factor of the coupling

probes [127,142].

71
As with the microstrip, the line loss for a stripline consists of components from

both the strip and its ground planes. Holloway has calculated the attenuation constant for stripline [133,136], which is

^T{stTipline) ^^ ^strip i Otgroundi \.""j

where

Once again, A is the stopping distance and is given in table 19. Rsm is given by eq

(72) and Zq is the impedance _ fWKjk)

where K is the elliptical integral of the first kind, k = sech(7rw;/4/i), k' = y/1 — P, w

is the width of strip, and h is the distance between the inner and outer conductors.

The ground losses are

abound = ^F{^1 • (83)

where Rs = 1/(t6 is the surface impedance, a is the metal conductivity, and S is the

skin depth in conductor, and

ir(^) = -^ r [tan-i(e(^-'"'^/^'^)) - tan-i(e(^+--/4'^))l ' dy. (84)

A plot of F{w/h) for different values of w/h is given in reference [133]. Variation

of attenuation with frequency, metal thickness, and conductivity for a stripline are

shown in figures 33 through 37.

The dielectric losses vary as

a, = ;^»Vi;ta„i. (85)

72
1.0

a((iB/cm)

0.1 --.

0.01

10

Frequency (GHz)

Figure 33. Total attenuation of dielectric, ground plane, and strip as a function

of frequency for a stripline line with geometries having t = 10/im, w = 200/im,

h = 455/xm, tanS = 0.005, o" = 2.5 x lO^S/m [136].

73
100.00

80.00

60.00

PQ

40.00 -

20.00

0.00

-

/

/

/

/

/

/

r -\

—

t = 6.0^un

• •

/

/

/

1

1 1

1 ' 1 ' 10 20 30 Frequency [GHz]

40

Figure 34. Attenuation for two different thicknesses (t) plotted against frequency

for a stripline with geometries having h = 100/zm, w = 70//m, e^ = 12.9, a =

3.6 X lO^S/m.

74
tuu

350-

- 300-

r f=20GHz

- f=2 GBz

250 - ^ J ?

PQ 200-

s

I

150 —

|\

100 —

50-

l\

\\\\

\\

N

A

U 1

1

1

1 11 1

1

' 1 'Ml '

1

'

1 '1 12 3 4 5 6 7

t Qim]

9 10

Figure 35. Attenuation for two frequencies plotted against thickness for a stripline

with geometries having h = lOO/^m, w = 70//m, e^ = 12.9, a = 3.Q x lO^S/m.

75
80.00

60.00 -

m 40.00

20.00 -

0.00

/

G-= 3.6e7S/m - - -- =:6.0e7S/m

J

"1

'—\—'—

10 20 30

Frequency [CHz]

40 50

Figure 36. Dependence of attenuation for two frequencies as a function of metal

conductivities for a stripline with geometries having h = 100/xm, w = 70^m, ej, =

12.9, t = S/zm.

76
itu.uu —

( > ^^

^

120.00 - w— 20 |im

w = 70jim y^

I J y^

100.00 - X

80.00 - y/ ¥

X .^ ^ ^

- / -"''''

1—1 / ^-'-' a 60.00 - // y / y / y / y / y

40.00 - / y / y / y

/ y

/ y

y

y

y

20.00 -

f\ f\i\

y

/

u.uu —

' 1 ' 1 ' 1 ' 1 10 20 30

Frequency [Clfe]

40

Figure 37. Dependence of attenuation at two frequencies on frequency for various

conductor widths for a stripline with geometries having h = lOO^m, t — 3//m,

< = 12.9, t = 3//m, (T = 3.6 X lO^'S/m.

77
4.7.3 Coplanar Strip (CPS)

Figure 38. Coplanar strip cross section.

In CPS, as shown in figure 38, the characteristic impedance is given by [136]

Zo =

2/xo K{k)

eo{er + l)K{^n^n^)' (86)

The attenuation is [136]:

OiCPS

tiam.O

4.ZoK'^{k'){h^ - 0?)

1, ,2a + A6-a-A, 1, ,26-A6-a-A,

- In ( 1 1 t) + t In ( —I—i t)

a ^ A 6 + a-A^ h ^ A 6 + a + A^

(87)

Variation of attenuation with frequency for CPS is shown in figure 39.

78
1.0 --

a(dB/cm)

0.5 -1-

10 100

Frequency (GHz)

Figure 39. Total attenuation plotted against frequency for a CPS line with t = l.bfim, a = 10/im, b = 25/zm, e = 12.9, o" = 3 x lO^S/m [136].

79
4.7.4 Coplanar Waveguide (CPW)

•Copper

Sample

Figure 40. Coplanar waveguide fixture cross section.

The attenuation in CPW (see figures 40 through 43) was calculated by Holloway

[134, 135]

> 2

Zo 1 + Vi +

4F

Zoymo

where

and

F =

otcpw ^ ^e

{Z.^Zrr^}? ri (2a h-a\ 1 /2b b-a

lQK^k){l>^ - a^) [a^yAb + a)'^ b^'yAb + a^

Imo = j^y/m^S

'Cr + 1 2 ' Z.=

' /JQ K{k')

IGeoeeff K{k) '

e^ + l ee// =

^« = -ii

-^m = -J^

fJ'O

ec - ja/uj

cot{kct),

fJ'O csc{kct),

and <7 is the conductor's conductivity, Ec is the conductor's permittivity, and

k' = y/l^l^.

80

(88)

(89)

(90)

(91)

(92)

(93)

(94)

(95)
B

no

100 —

90 —

so —

70 —

60 —

50 —

40

30 —

20 —

10

t = 1 .61 \i3XL

t = 3.0 \i3Xi

- t = 6.0 \i3Xi

1 I I I I I I 1 r

10

Frequency [GHz]

100

Figure 41. Attenuation for three thicknesses (t) plotted against frequency for a CPW

Une with geometries having a = 35.6//m, b = 84.6/im, ^ = 12.9, (j = 3.6 x lO^'S/m.

4.7.5 Increased Power Loss Associated with Surface Roughness

If the surfaces of the conductors are rough, then the conductor's power loss increases.

The amount of increase will depend on the direction of the roughness with respect to

81
the current flow. A recent study discussing these effects is given in reference [143].

Figures 45 through 48 show the increase in power loss (P) of a rough conductor relative to that (Po) of a smooth conductor for two different polarizations and different

roughness profiles as a function of p/6, where S is skin depth and p is effective length

characterizing the roughness. These results are for the two-dimensional rectangular

profile shown in figure 44.

It was shown that as the roughness dimensions get large compared to the skin

depth, there are noticeable roughness effects. Typical surface roughness cannot be

represented accurately by two-dimensional periodic profiles, but are instead more

likely to have three-dimensional, random profiles. For very small ratios of the period

to skin-depth (p/S), there is no physical reason to expect that the more general

random profiles will exhibit drastically different P/Pq behavior from that of the twodimensional periodic surfaces. From our analysis of two-dimensional roughness, we

see that for large p/S, the additional power loss resulting from a rough interface can

be significant. While quantitative predictions of this loss cannot be made from our

analysis of the two-dimensional profiles for randomly rough surfaces, we can conclude

that at these relatively large values of p/6, the additional power loss associated with

a randomly rough surface is also expected to be significant.

82
400

350 -

300-

250 —

100 —

—^

f=20(2fe

f=2Clfe

'Ml

2 3

I

'

I

'

I 'I

' I 4 5 6 7 8

t \iim]

9 10

Figure 42. Attenuation as a function of thickness of conductor (t) for two frequencies

for a CPW line with geometries having a = 35.6/im, b = 84.6/im, 4 = 12.9, a =

3.6 X lO^S/m as a function of thickness (t).

83
1 iw —

/

100 — r > — 3 .oe / a/ m /

/

90- a = 6e7S/m

//

/ / 80- / / / / / / 70 - / / X / / / / / 60- X 1 y^ / ^r / ^^ /

50- ^^ / ^^ / ^— / ^^^^— •

^„,,,,»»"'^ •

40 — ""^ x

y •

^ *» ^ ^ 30- 40'

"

*"

onzu — \ 1 1 1 1 1 1 1 1 1 1 1 1 1 M

10 100 Frequency [Cife]

Figure 43. Attenuation as a function of frequency for two different conductivities for a CPW line with geometries having t = 3//m, a = 35.6//m, 6 = 84.6//m, e'^ = 12.9.

84
- c^I'V

p •y

W 1

«x

Figure 44. Two-dimensional rectangular profile.

85
3.00

2.80-

2.60-

2.40-

2.20- ^ 2.00 H

1.80-

1.60

1.40-

1.20-

1.00

h/p=1.00, w/p=0.50

h/p=0.50, w/p=0.05

h/p=0.25. w/p=0.50

T T T T

1.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8. p/800 I

Figure 45. Power ratio for grooves transverse to current flow with — = 0.5 £ind ^

equal to 0.25, 0.5, and 1.0.

86
Q.

i.uu —

^~ '

1.90 —

-

- - - h/p=0.50, w/p=0.75

h/p=0.50, w/p=0.50

1.80 — - IVp=0.50, w/p=0.26 ^ , -^

^^_ _^ ^ " ^^.^-"'''''^ ^ ^r

• j/^ 1.70 —

f /

1.60- / /

1.50 —

/ / /

1.40 — ' / ^

' / /

1.30 — ' / /

'' / / 1.20 —

/

/ y ,y^'' 1.10 — ' >^ ,y' '/ ,,

' y /.^»- — ' '"

i Ofx i.uu — *

1

' 1'

1 '1

'

1

' 1' 1' 1 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 p/8

Figure 46. Power ratio for grooves transverse to current flow with - = 0.5 and —

equal to 0.25, 0.5, and 0.75.

87
i.ou —

h/p=1.00, w/p=0.50

1.50- u/__rt c,r\ uu/n—n ^=[0 n/p—u.ou, w/p—u.ou

h/p=0.25. w/p=0.50

1.40-

1.30 —

1

1

^ ^

9:°

_ _

Q.

1.20 —

f

1 /

1 /

; / ,^''''

1 /

/ / /

1.10-

1 /

1 / , / /

/

1 ftA i.uu —

'

1

1 1

1

1

' 1' 1' 1

'

1

' 1 0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16, p/5

00

Figure 47. Power ratio for grooves parallel to current flow with — = 0.5 and - equal

to 0.25, 0.5, and 1.0.

88
1 I—" \

^

I r—1 ^

\ '\

^

\ r

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.( p/S

Figure 48. Power ratio for grooves parallel to current flow with - = 0.5 and — equal

to 0.25, 0.5, and 0.75.

89
4.8 T and Ring-Resonator Methods for Substrates

The T and ring-resonator methods have been developed in order to obtain relatively

broadband measurements using small, resonating devices (see figures 49 and 50).

Broadband permittivity measurements using the ring resonator can be obtained by

use of higher-order modes. However, accuracy of these measurement techniques

degrades for the high modes [144]. For the ring resonator, usually a very simpUstic

model of medium accuracy is used which neglects radiation [145].

where n is the number of half wavelengths, / is frequency, < d > is the mean

diameter, and c is the speed of light.

A T-resonator length correction is commonly performed (see Amey et al. [144]).

The open-ended transmission-line stub in the T-resonator resonates at integral multiples of one-quarter wavelength.

Measurement of substrate loss is limited to tan J > 0.001 with these methods.

Simple models usually suffice for characterizing the ring resonator; however, the

system can be modeled by a Green function method that yields an integral equation

for the permittivity. This equation must be solved numerically for the permittivity

[145-147].

4.9 On-Wafer Transmission-line IVIethods for Substrates

Using on-wafer transmission-line techniques, one can measure the broadband permittivity and loss tangent of substrates in the microwave region. Specifically, from

measurements of the propagation constant and characteristic impedance of a coplanar waveguide on a dielectric substrate, one can determine the permittivity of the

90
Input Transmission

T-pattem Resonator

Figure 49. T-resonator.

input ^m ^B '^ output

coupling

Ring Resonator

Figure 50. Ring resonator.

91
Figure 51. Interdigitated capacitor.

substrate. Reference [32] provides measurements results for several low-loss dielectric

substrates over the frequency range of 50 MHz to 40 GHz. Although these methods

are broadband, the measurement uncertainty is higher than those for single-frequency

resonator methods, especially for the loss tangent. This is due mainly to the fact that

the conductive losses in the transmission line are usually larger than the dielectric

losses in the substrate. Although one can separate the two losses, the measurement

sensitivity is decreased, leading to a larger uncertainty in the substrate's loss tangent.

At lower frequencies, substrate permittivity has been measured using planar interdigitated capacitors, as shown in figure 51. This structure uses the many fingers

to increase the capacitance and hence decrease the measurement uncertainty [148].

5. Dielectric Measurement Software

Software has been developed in the Electromagnetic Properties of Materials Project

at NIST, Boulder. Table 20 summarizes the available software at the time of publication.

92

I
Table 20. Software summary.

Transmission line (coaxial line and waveguide)

Coaxial probe

Shielded open-circuited holder

Reentrant cavity

Split-cylinder

Permeameter

Courtney

6. Ambient Temperature Measurements

Figures 52 and 53 show results using the split-cylinder and split-post resonators on

FR-4. The darker square at 9 GHz is a measurement from another spatial position

on the same sample.

In figures 54 and 55, the results of measurements on a sample of aluminum nitride

are shown using various measurement fixtures. Note that the loss tangent decreases

as frequency increases above 3 GHz.

Figures 56 and 57 plot the permittivity of high-resistivity gallium arsenide as

a function of frequency. These measurements were made by a mode-filtered TEqi

X-band cavity.

93
CD

•f.OU

- D Split-Cylinder Resonator

_ O Split-Post Resonator

4.48 "

4.46 - o D

D

4.44 -

A AO -I_L III .. ..I....I.. ..I....I.. ..1 ....1... .1....

1 2 3 4 5 6 7

Frequency (GHz)

8 9 10

Figure 52. Measurements of the real part of the permittivity of FR-4.

94
CO

c

CO

0.03

0.02 -

0.01 -

0.00

- D Split-Cylinder Resonator

o Split-Post Resonator

- o i

- o

-LJ iiIimiIimiImmIiiiiIii 1 ihlllllM ill III

10

Frequency (GHz)

Figure 53. Loss tangent of FR-4.

9.0

8.5 -,

cJ- 8.0 -

7.5 -

7.0

-

i

fii a i I

i

1 M 2 GHz Split Post

D 3 GHz Split Post

: A 5 GHz Split Post

o 10 GHz Split Cylinder 7x10 GHz Split Post

35 GHz Split Cylinder 0.244 mm thick

V 60 GHz Fabry- Perot

". ... 1 .... 1 .... 1 ... . • 1 .036 nnm thick

1 .... 1 .... 1 ... . 10 20 30 40 50 60 70

Frequency (GHz)

Figure 54. Measurements of the real part of aluminum nitride from 2 to 60 GHz.

95
0.020

0.015 -

to

C 0.010

0.005 - A

0.000

I a

o

•

0.244

1.036

rrni

rrrn

thick

thick n

A

O

X

o

V

2 GHz Split Post

3 GHz Split Post

5 GHz Split Post

10 GHz Split Cylinder

10 GHz Split Post

35 GHz Split Cylinder

60 GHz Fabry-Perot

: m^

-

•

:

1 J_

o

1

1

i T

10 20 30 40 50

Frequency (GHz)

60 70

Figure 55. Loss tangent of aluminum nitride from 2 to 60 GHz.

12.97

12.96

12.95

uT 12.94

12.93

12.92 h

Start Middle

Terminus

-AA—

12.91 ' '

I

I I I I I 8 10 11 12

Frequency (GHz)

Figure 56. e^ of gallium arsenide using an X-band cavity [149]. Start, middle, and

terminus refer to different specimens taken from the same boule.

96

(
0.80

9 10 11 12

Frequency (GHz)

Figure 57. Loss tangent of gallium arsenide using an X-band cavity [149].

97
Table 21. Split-post dielectric resonator measurement for low-loss materials [150].

The alumina are from different manufacturers.

Material f(MHz) Q h(mm) < tan 6 (x 10^) UW,),k = 2 U{UmS){xlO^)

Teflon 5587.07 9040 0.620 2.04 2.7 ±0.03 ±1.0

AI2O3 5559.31 9639 0.842 9.95 0.4 ±0.03 ±0.5

AI2O3 5321.06 9115 0.999 9.75 1.0 ±0.06 ±0.5

AI2O3 5406.28 8081 0.740 9.23 2.7 ±0.06 ±0.5

Measurement results for diff^erent materials using a split-post dielectric resonator

operating at a frequency of approximately 5.6 GHz with associated uncertainties

are shown in tables 21 through 25 [151]. In table 21, three almnina samples from

three manufacturers are given. Notice the variation in results. This is due to density

variations between the various alumina samples. Measurement results for stacked

or layered dielectrics using this method are nearly independent of the number of

layers (see table 21). This is a result of the field being in the plane of the sample for

the split dielectric resonator method and, therefore, it is insensitive to the presence

of air gaps between stax^ked films. Some dielectric substrates may exhibit uniaxial

anisotropy. Therefore, additional measurements in the direction perpendicular to the

substrate are necessary to characterize fully the dielectric properties. Measurement of

the component of permittivity normal to the sample plane can also be accomplished

using the reentrant cavity technique.

In table 25 measurements on PWB materials using the split-post resonator method

are displayed. Tables 26 to 32 report measurements of many materials often used

for substrates. The results are presented as additional data on substrate materials

reproduced from reference [151].

98

I
Table 22. Measurements with split-post dielectric resonators operating unloaded at

9.6 and 12.6 GHz [150].

Material f(MHz) Q li(mm) < tan J (xlO^) Wr), k=2 U(tan(5 xlO^)

AI2O3 (Vistal) 9570.2 9740 0.85 10.0 0.24 ±0.05 ±0.5

Rexolite 10214 7666 0.98 2.53 4.16 ±0.02 ±1

GaAs 9719.5 7134 0.51 12.9 2.75 ±0.02 ±1

Teflon 10304 8709 0.62 2.05 1.61 ±0.03 ±1

Teflon 12727 5416 0.62 2.03 1.94 ±0.05 ±1

LaSrAlOs 11319 5081 0.50 23.2 1.78 ±0.07 ±1

AI2O3 11955 5560 0.70 9.94 1.18 ±0.05 ±0.5

Table 23. Measurement on polymer films from split-post dielectric resonators operating unloaded at nominally 1.46 GHz and 2.05 GHz [150].

Material f(MHz) Q h(mm) < tanJ(xlO^) C/(e;) U{tan6xW)

film 2046.00 2020.7 0.96 3.34 324 ±0.02 ±50

film 1455.27 2487.6 0.96 3.34 323 ±0.02 ±50

Table 24. Split-post (in-plane) measurements on Rogers PWB materials

Material Prequency(GHz) 4 U(e;, k=2) tan 6 U(tanJ)

RO3003 3.36 2.96 ±0.015 9.6 X 10-4 5 X 10-^

RO3003 3.36 3.01 ±0.015 8.7 X 10-4 5 X 10-5

RO4350(Lot 1) 3.36 3.61 ±0.015 3.7 X 10-3 5 X 10-5

RO4350(Lot 2) 3.36 3.75 ±0.015 3.8 X 10-3 5 X 10-5

RO4003 3.36 3.55 ±0.015 2.3 X 10-3 5 X 10-5

RO3003 3.36 3.60 ±0.015 2.4 X 10-3 5 X 10-5

RO3010(Lot 1) 3.36 11.2 ±0.025 1.4 X 10-3 5 X 10-5

RO3010(Lot 2) 3.36 11.4 ±0.025 2.5 X 10-3 5 X 10-5

RO3010(Lot 3) 3.36 11.5 ±0.025 2.5 X 10-3 5 X 10-5

99
Table 25. Split dielectric resonator measurements on PWB materials [150].

Specimen f (MHz) Q h (mm) < tan J (xlO^) C/„ Ut ,fc=2(xl0^)

5608.71 8515 ±0.02 ±4

1 5584.71 2823 0.402 2.77 177 ±0.02 ±4

2 5585.05 2829 0.412 2.71 176 ±0.02 ±4

3 5584.66 2828 0.402 2.78 176 ±0.02 ±4

4 5584.66 2818 0.399 2.79 177 ±0.02 ±4

5 5585.21 2825 0.405 2.73 178 ±0.02 ±4

6 5582.34 5561 0.393 3.28 46.5 ±0.02 ±1

7 5579.97 5413 0.380 3.25 45.9 ±0.02 ±1

8 5579.41 5346 0.380 3.29 46.8 ±0.02 ±1

Table 26. Measurements on glass specimens. The expanded uncertainties , k=2

are as follows: parallel-plate dielectric resonator: Ue' = 0.5 %, UtanS = 2 x 10~^,

split-cylinder: Ue' = 1 %, C/tan<5 = 1 x lO""*, TEqi cavity resonator: U^> = 1 %,

UtanS = 1 X 10~^, Fabry-Perot resonator: Ue' = ±2 %, C/tan* = 2 x 10""*, reentrant

cavity: Ue' = ±1 %, C/tan5 = 2 x 10"!

Material Value Parallel plate Split cavity 60 mm TEoi Split post Reentrant

Corning

7980

34604

f(GHz)

tan J

5.07

3.84

6.85 X 10-^

8.85

3.827

11.9 X 10-5

9.64

3.84

1.40 X 10"^

2.03

3.84

4.40 X 10-5

Corning

7980

34605

f(GHz)

tan^

5.07

3.844

6.8 X 10-^

8.82

3.822

11.7 X 10-5

9.63

3.843

1.4 X 10-^

2.03

3.838

3.1 X 10-5

Corning

7980

34606

f(GHz)

tan (5

5.26

3.844

6.90 X 10-5

8.86

3.825

1.80 X 10-^

9.64

3.826

1.5 X 10-^

2.03

3.84

4.9 X 10-5

Corning

7940

44608

f(GHz)

tan J

4.89

3.847

6.83 X 10-5

8.86

3.825

11.6 X 10-5

9.64

3.826

1.4 X 10-^

2.03

3.839

3.6 X 10-5

Corning

H. Bussey

1723 (1959)

f(GHz)

tan 6

9.0

6.20

5.36 X 10-3

1.0

6.21

3.2 X 10-3

100
Table 27. Measurements on plastic specimens of polytetrafluoroethylene

(PTFE), cross-linked polystyrene (CPS), fluoroethylenepropylene (FEP), polymethylmethacrylate (PMMA), and nylon. The standard type A expanded imcertainties, k = 2, are as foUows: parallel-plate dielectric resonator: Ue' = 0.5 %,

UtanS = 2 X 10-^, TEoi cavity resonator: Ue' = 1 %, UtanS = 1 x 10-4, reentrant

cavity: U^' = 2 %, f/tan* = 2 x 10-4.

Material Value Parallel plate 60 ram TEoi Reentrant cavity

FEP f (GHz)

tan (5

6.59

2.064

7.04 X 10-4

9.837

2.025

3.12 X 10-4

-

PTFE f (GHz)

tan J

9.93

2.05

2.0 X 10-4

9.816

2.055

2.1 X 10-4

1.082

2.06

3. X 10-4

CPS f (GHz)

tan J

5.7

2.542

5.1 X 10-4

9.982

2.533

4.2 X 10-4

1.032

2.56

7 X 10-4

Nylon f (GHz)

tan J

4.75

3.08

8.4 X 10-2

9.742

3.01

7.1 X 10-2

PMMA f (GHz)

tanS

5.721

2.634

7.2 X 10-2

9.813

2.626

4.4 X 10-2

101
Table 28. Measurements on cubic single crystals. The standard type A expanded

uncertainties, k = 2, are as follows: parallel-plate dielectric resonator: Ue' = 0.5 %,

Uts^aS = 2 X 10"^, whispering gallery Mode resonator: Ue' = 0.1 %, UtanS = 2 x 10"^.

Anisotropic materials were measured either by two dominant modes (TEqh and

HEiii) using the parallel plate rod resonator or two families of whispering gallery

modes.

Material Value Parallel Plate

LaAlOa

(cubic)

freq. (GHz)

e' ta.n6

18.38

23.99

1.0 X 10-5

NdGaOa

(cubic)

f (GHz)

tan 6

18.49

21.82

1.07 X 10-^

Zirconia

(cubic)

f (GHz)

e' tan^

4.30

27.8

3.28 X 10"^

GGG

(cubic)

f (GHz)

e' tan J

15.7

12.43

2.64 X 10"^

SrTiOs

(cubic)

f (GHz)

tan J

7.20

310

4.01 X 10-^

MgO

(cubic)

f (GHz)

e' tanS

17.46

9.70

2.5 X 10-6

102
Table 29. Measurements on noncubic single crystals. The A expanded uncertainties,

A; = 2, are as follows: parallel-plate dielectric resonator: U^' = 0.5, C/tan<5 = 2 x 10~^,

whispering gallery Mode resonator: Ue' = 0.1 %, UtanS = 2 x 10~^. Anisotropic

materials were measured either by two dominant modes (TEom and HEm) using

the parallel plate rod resonator or two families of whispering gallery modes.

Material Value Parallel plate Whispering gallery

AI2O3 f (GHz) 10.0 12.4

1 c-axis €' 11.58 11.58

sapphire tan J 2 X 10"^ 5. X 10-6

± c-axis €' 9.40

tan J 2 X 10-^ 7. X 10-6

LiNbOa f (GHz) 10.19

c-axis €' 32.97

tan<5 8. X 10"^

LiNbOa f (GHz) 11.56

± c-axis ^ 42.34

tan (5 8.4 X 10-^

Quartz f (GHz) 7.75

-L c-axis e' 4.59

tan (5 1.5 X 10"^

Quartz f (GHz) 9.03

c-axis €' 4.443

tan 6 1.3 X 10"^

Ti02 f (GHz) 4.2

c-axis €' 163.7

tan J 1 X 10-^

_L c-axis €' 85.6

tanS 8 X 10-^

103
Table 30. Measurements on low-medium permittivity TransTech ceramic speci- mens. The A expanded uncertainties, A; = 2, are as follows: paraUel-plate dielectric resonator: Ue' = 0.5 %, UtanS = 2 x 10"^, TEqi cavity resonator: Ue' = 1 %,

UtauS = 1 X lO""*, whispering gallery mode resonator: Ue> = 0.1 %, Utaas = ±2 x 10"^.

Material Value Parallel Plate 60 mm Cavity Whispering gaUery

Ceramic

4

f (GHz)

e' tan^

3.675

4.704

6.8 X 10-^

9.76

4.686

5.80 X 10-^

Ceramic

6

f (GHz)

e' tanS

3.17

6.592

4.58 X 10"^

9.74

6.592

5.30 X 10-^

Ceramic

9

f (GHz)

tan^

2.713

9.808

2.55 X 10-^

9.74

9.73

2.50 X 10-^

Alumina

(1986)

f (GHz)

e' tan^

3.27

10.04

3.9 X 10-5

9.53

10.02

3.38 X 10-5

9.528

9.98

3.2 X 10-5

Alumina

(1996)

f (GHz)

e' tanS

9.00

9.992

1.30 X 10-3

Additional measurements on plastics are given in Appendix B. In table 32 and

figures 58, 59, and 60, the permittivities of 7940 and 1723 Corning glass are displayed

using various measurement fixtures.

Figures 62 through 67 and tables 33 through 35 tabulate measurements of a number of ceramics, polymers, and PWB laminate materials made by other researchers.

104
Table 31. Measurements on high-permittivity TransTech ceramic specimens. The A

expanded uncertainties, A; = 2, for the parallel-plate dielectric resonator: U^ = 0.5

%, Uta.n5 = 2 X 10~^, TEoi cavity resonator: Ue' = 1%, UtanS = 1 x 10"^, whispering

gallery mode resonator: Ue' = 0.1 %, C/tan<J = ±2 x 10~^

Material Value Parallel plate 60 mm cavity Whispering gallery

Ceramic

13

f (GHz)

tan^

1.987

13.51

1.07 X 10-^

9.72

13.55

1.10 X 10-^

9.577

13.54

1.01 X 10-4

Ceramic

16

f (GHz)

tanS

1.834

16.60

2.43 X 10"^

9.71

16.64

0.4 X 10-^

9.692

16.66

5.70 X 10-5

Ceramic

20

f (GHz)

tan^

1,617

20.77

8.4 X 10-5

9.70

20.80

2.00 X 10-^

Ceramic

31

f (GHz)

tan^

1.328

30.96

2.4 X 10-5

9.44

31.03

5.0 X 10-5

6.023

30.99

4.9 X 10-5

Ceramic

35

f (GHz)

e' tan 6

1.200

35.15

4.8 X 10-5

9.46

35.19

1.6 X 10-^

5.460

35.14

1.18 X 10-4

Ceramic

36

f (GHz)

tan^

1.116

36.55

3.3 X 10-5

9.46

36.56

1.2 X 10-^

4.812

36.55

8.8 X 10-5

Ceramic

38

f (GHz)

tan^

1.109

38.65

5.8 X 10-5

9.73

38.77

3.0 X 10-^

Ceramic

78

f (GHz)

tan^

2.83

78.29

3.1 X 10-^

Ceramic

79

f (GHz)

tan 6

0.855

79.27

1.63 X 10-^

9.44

79.29

7.8 X 10-4

4.448

79.11

4.40 X 10-4

105
1723 Corning Glass (Bussey- 1972 Round Robin)

CO

6.20

- D Split-Cylinder Resonator

O Split-Post Resonator

- O

6.18 -

- n

6.16 - n

- o

6.14 -

ft 1*?

-

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 L_L..I. .I.,,l. 1 L-1-L-I-Xj-I-L.I..L.I 1.1.1. 1.1 I.I 1

1 2 3 4 5 6 7 8 10

Frequency (GHz)

Figure 58. Measurements of the real part of the permittivity of Corning 1723 glass.

CO

tz

1723 Corning Glass (Bussey - 1972 Round Robin)

0.01

0.001 ' '

Split-Cylinder Resonator

O Split-Post Resonator

o

I I ' ' I I I I 8 9 10

Frequency (GHz)

Figure 59. Measurements of the loss tangent of Corning 1723 glass.

106
Table 32. The 1964 round-robin measurements on Corning 1723 and 7940 glasses

compared with recent measurements [151]. The round-robin results from 1964 are given below [152,153]. The A expanded uncertainties, k = 2, are as follows: parallelplate dielectric resonator: Ue' = 0.5 %, UtsaS = 2 x 10-"*, TEqi cavity resonator:

Ue' = 1 %, f/tan5 = 1 X 10-^

Material Value NBS(1964) Parallel Plate TEoi (50 mm) TEoi (60 mm)

1723 Glass

(1964 RR)

f (GHz)

tan^

9

6.20

5.3 X 10-3

10

6.26

4.8 X 10-3

1723 Glass

(1972 RR)

f (GHz)

tanS

9

6.155

4.7 X 10-3

9.58

6.16

4.9 X 10-3

10

6.146

4.7 X 10-3

7940 Glass

(1972 RR)

f (GHz)

tan (5

9

3.826

1.2 X 10-^

9

3.822

1.4 X 10-^

10

3.828

1.03 X 10-^

9.82

3.829

1.2 X 10-^

u

o.»

- A

X O V D

O ^

3.8

+

o

- A

60 GHz Fabry- Perot

35 GHz Split Cylinder

10 GHz Split Post

10 GHz Split Cylinder

+

3.7

-5 «

V

" o

5 GHz Split R)st

3 GHz Split Post

2 GHz Split Post

1 GHz Split Post

1 1 1 1 1 1 t 1 1 ^..TMi 1 4 5 6 7 8 910

5 6 7

Frequency (GHz)

Figure 60. Measurements on the real part of the permittivity of Corning 7940 glass.

107
6x10

CO

c

CVS

_

: + 60 GHz Fabry-Perot - )K 35 GHz Split Cylinder +

\ O 10 GHz Split Post

L A 10 GHz Split Cylinder

: D 5 GHz Split Post

: V 3 GHz Split Post

- O 2 GHz Split Post

: X 1 GHz Split Post ^

: A

o

- n

: X o ^

1 5 6 7 8 910

5 6 7

Frequency (GHz)

Figure 61. Measurements of the loss tangent of Corning 7940 glass.

108
Table 33. Typical dielectric measurements on materials (no uncertainties assigned).

Material Crystal f (GHz) < tan^

Aluminum nitride [154] No 8.5 8.5 0.0030

Barium borate Yes 0.001 4.6 0.0005

Beryllium oxide [154] Yes 8.5 6.86 0.0003

Boron nitride [154] No 5.0 4.30 0.0002

Calcium carbonate [154] Yes 0.0001 8.5 0.0004

Calcium fluoride [154] Yes 1 6.3 0.0006

Germanium [154] Yes 10 16.6

Lithium niobate [154] Yes 10 29

Neodymium gallate [154] Yes 10 20 0.0030

Nickel oxide [154] No 1 7.5 0.0001

Silicon nitride [154] No 8.5 5.5 0.0036

Potassium bromide [154] Yes 10 4.9

Silicon [154] Yes 14 12 0.0090

Silicon carbide [155] No 10 47 0.0010

Sodium chloride [154] Yes 0.001 5.7 0.0001

Strontium lanthanum aluminate [154] Yes 10 17 0.0008

Strontium lanthanum gallate [154] Yes 10 22 0.0001

Topaz [154] Yes 0.01 6.5 0.0001

Yttrium iron garnet [154] Yes 10 14.9 0.0001

Yttrium oxide [154] No 0.001 11.3 0.0006

Zinc oxide [154] No 0.01 8.3 0.0300

109
Table 34. Dielectric measurements on polymers (no uncertainties assigned).

Material f (GHz) < tan (5 PMMA [154] 3.0 2.61 0.0057

Nylon [154] 3.0 2.84 0.0117

Polyethylene [154] 3.0 2.26 0.0003

Styrofoam [154] 3.0 1.03 0.0002

Polyimide [148] 0.001 3.50 0.003

BPDA-PDA Polyimide [148] 0.001 3.8

Polysulfone [156] 1.0 3.02 0.0090

Polyolefin [156] 1.0 2.75 0.0200

Cyanate Ester [148] 0.001 3.48 0.01

110
Table 35. Dielectric measurements of PWB composites (no uncertainties assigned).

Material Range of e'^. FR-4 [140 4.2-5.0

FR-5 (high temp) [140 4.3-4.6

BT/epoxy [140 4.0-4.2

Cyanate ester [140 3.5-3.7

Arimid/epoxy [140 3.7-3.9

Polyimide/glass [140 3.9-4.2

Polyimide/quartz [140 3.4-3.5

111
CO

t.UJ

\

4.00 \

-A-298K

A --373K

; \ --473K

3.95 :\

-t-673K

3.90 ;V\^

3.85|

f ! A A

1 1

1 1 1 1 1

1 1 1

10^ 10^ 10^ 10^ 10^ 10^ 10^ 10^ 10^° 10^^

Frequency (Hz)

Figure 62. Measurements of e^ of Si02 as a function of temperature [154] (no uncertainties assigned).

112
CO

CO

10^ 10^ 10* 10^ 10^ 10^ 10^ 10^ 10^°

Frequency (Hz)

Figure 63. Measurements of the loss tangent part of Si02 as a function of temperature

[154] (no uncertainties assigned).

113
Frequency (Hz)

Figure 64. Measurements of 4 for rutile with electric field perpendicular to axis as a function of temperature [154] (no uncertainties assigned).

114
Frequency (Hz)

Figure 65. Measurements of 4' for rutile with electric field perpendicular to axis as a function of temperature [154] (no uncertainties assigned).

115
Frequency (Hz)

Figure 66. Measurements on ej. for silicon nitride as a function of temperature [154]

(no uncertainties assigned).

116
O)o 0.1 r

Frequency (Hz)

Figure 67. Measurements on 4' for silicon nitride as a function of temperature [154]

(no uncertainties assigned).

117
7. Elevated Temperature Measurements

7.1 Thermal Properties

When measuring permittivity as a function of temperature, it is important to use accurate thermal expansion data at each temperature. The two thermal properties

relevant to substrates for electronic packaging are the coefficient of thermal conductivity {k) and coefficient of thermal expansion {a cte)- The CTE of many polymers is

strongly temperature dependent and care must be taken to account for this variable

in a packaged circuit.

The coefficient of thermal expansion a cte is defined as

1 «CTE =

I

dL

(97) dT p LAT

where L is material thickness, P is pressure, and AL is change in thickness as temperature changes by AT. For temperature-dependent thermal expansion coefficients

we can integrate eq (97) to get

L = Lo exp (r a{T)dT\ . (98)

Tj^ical coefficients of thermal expansion are given for a number of materials in

tables 36 and 37 [1, 17,155,158]. If the temperature is not specified, then it is for 295

K. These results were taken from various sources and in some cases from company

specification sheets [159]. Polymers can have large temperature-dependent CTE and

low glass transition temperatures Tg (see table 38). The glass transition temperature

is where a polymer starts to change phase to a more Uquid-like state. The temperature coefficient of permittivity is defined as

1

^^ = T^r L

de'r ' ^<

(99) dT P < AT

Tg is usually expressed as parts in 10^ [52]. Materials with negative temperature

coefficients can be combined with materials with positive temperatmre coefficients to

118

I
Table 36. Typical coefficients of thermal expansion (CTE)(no uncertainties as- signed). Temperature 295AT unless otherwise specified.

Material a xlO« (°C-^)

ABS plastic 70

acetal 85

acrylic 68

alumina [1 6.7

aluminum [1 24

aluminum nitride [1 3.0

aluminum oxide [1 2.6

aluminum titanate 0.4

aramid/epoxy [140 6-9

beryllium oxide [1 6.5

bismalimide triazine/epoxy [140 15-17

borosilicate glass [155 3.1

calcium oxide 14.8

copper [1 17

cordierite [1 1.0

cyanate ester [140 13

diamond [1 1.02

epoxy [157 54

FR-4 [1] 17-18

FR-5 [1 17-18

fused quartz 0.5

fused Si02 -0.53(100 K), 0.5(300 K)

fused silica [1 0.5

gallium arsenide [6 5.7

gallium gadolinium garnet 8.0

germanium 6.1

kovar 5.0

119
Table 37. Typical coefficients of thermal expansion (continued) (CTE)(no uncertainties assigned) . Temperature 295K unless otherwise specified.

Material a xlO^ (^C-^

lanthanum aluminate

lithium niobate

magnesium oxide (MGO) [1] neodymium gallate

nylon

PMMA

polycarbonate

polyethylene (high density)

polyimide [155]

polyimide/glass [1] polyimide/quartz [140]

polypropylene [157]

quartz (crystalline) [1] Rexolite [6] rutile [1] sapphire [1] silicon [1] silicon carbide [1] spinel [1] strontium titanate

strontium lanthanum aluminate [6] strontium lanthanum gallate [6] Teflon [6] tridymite [1] zirconia oxide

Zr-Sn titanate [1]

9.0

15 ±, 4.1 II 8.0

9.0

50(193K), 93(300 K), 146(400 K)

11(40 K), 61(300 K), 172(450 K)

66

33(50 K)), 175(300 K), 940(400 K)

40

14-15

6-8

76(180 K), 94(250 K), 205(293 K)

11.2

26(30 K), 71(300 K)

7.0

5.3

3.0

3.7

6.6

9.0

7.0

10

30 (30 K), 124(330 K), 258(500 K)

17.5

7.5

-6.9

120
Table 38. Typical glass transition temperature [140].

Material Tg {°C)

Polyimide/quartz 250

Cyanate ester 250

Polyimide/glass 250

Teflon 200

BT/epoxy 190

FR-5 170

Rexolite 120

FR-4 130

produce a material with a small composite change with temperature; however the

mixture properties need to be well-characterized.

The thermal coefficients of expansion for composites can be calculated by mixture

formulas such as "eff=li^. (100)

where ki^ 0;^, and 6i are the bulk modulus, coefficient of expansion, and volume

fraction of the ith phase [160].

One method for quantifying the effects of temperature on resonant frequency is

the temperature coefficient of resonance frequency

1 di

dT -,% (101) /AT ^ ^

Many ceramics have t/ that are well-represented by a quadratic polynomial in temperature.

In addition to thermal expansion there is the problem of thermal warping of

substrates. Warping or camber is a result of differential thermal expansion. A

121
measure of camber can be estimated by

^= ^=^, - (102) L a where AVF is the height of the warp of the material from a plane surface, a^ is

coefficient of thermal bending, L is the length of material, and d is thickness [161].

7.2 Elevated Temperature Measurement Results

Many substrate and PWB permittivities vary strongly with temperature. In this

section, we describe a system for making measurements at elevated temperature. We

also present permittivity measurements as a function of temperature for a number

of substrate materials. For these measurements we used a TEqi^ dielectric resonator

as shown in figure 68. In order to minimize wall losses, the specimen is supported

by a low-loss dielectric rod. The temperature system, as shown in figure 69, is an environmental chamber that operates from -150 °Q to 150 "C. In this figure, a

coaxial line system is shown; however, for the measurements in this report we used

a resonant device. We can use cavities, transmission lines, and reentrant cavities

in the chamber. The chamber has bulkhead adapters on the sides for insertion of

coaxial feeds for the cavities or transmission lines. The chamber can also be purged

with nitrogen gas to reduce oxidation and water vapor.

Measurements of some common low-permittivity materials are given in figures 70

to 75.

122
Metal shield

Figure 68. TEqi^ resonator for measurements at elevated temperatures.

123
Plug

7 mm to 14 mm

adapter (2 ea.)

Stainless steel

air lines adapter

(2 ea.)

14 mm Air Line and

Adapter in Chamber

Inner dimensions:

30 cm wide x 75 cm deep x 75 cm high Outer dimensions:

49 cm wide x 75 cm deep x 75 cm high

Top and side port are 10 cm diameter

"^1

Thermocouple

Thermocouple wires

(*— 15 cm -*\

Plugs are to allow the use of

14 and 17 mm coaxial lines

Plug

/

Inert gas port

^H&

Length of

adapter is 20 cm

Figure 69. Environmental chamber for measurements at elevated temperatures.

124
100 150 200 250 300 350 400

Temperature (K)

Figure 70. Real part of the permittivity of Teflon as a function of temperature, 1^=2.

125
40

30-

20-

X

e/O

-2 10

0-

10

(I

(I

(I

T "

" T ii ..

T T T * •

:

*' ..

100 150 200 250 300 350

Temperature (K)

400

Figure 71. Loss tangent of Teflon as a function of temperature, expanded uncertainies, k=2.

126
100 150 200 250 300 350

Tenperature(K)

Figure 72. Real part of the permittivity of Rexolite as a function of temperature,

expanded uncertainies, k=2.

127
100 150 200 250 300 350 400

Temperature (C)

Figure 73. Loss tangent of Rexolite as a function of temperature, expanded uncertainies, k=2.

128
CO

20 40 60 80 100 120 140 160

Temperature, °C

Figure 74. Real part of the permittivity of alumina as a function of temperature,

expanded uncertainies, k=2.

129
O.H •

5.2 -

5.0 ' T

n

u, 4.8

; T .(

(1

. ^ 4.6 X

• •

"2 4.4 ^ 4.2

• •

(

(

1

1 >

4.0 4

3.8 , 1

'id

'

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

20 40 60 80 100 120 140 160

Temperature, °C

Figure 75. Loss tangent of alumina as a function of temperature, expanded uncertainies, k=2.

130
8. Conclusion

Dielectric measurements for LTCC and PWB materials are required from low megahertz to high gigahertz frequencies. Each technique we have reviewed has a niche

in the frequency band, field behavior, and loss characteristics of LTCC, PWB, and

substrate materials. One technique alone is insufficient to characterize materials

over all of the necessary parameters. Measurements of substrate materials present a

metrological challenge because of uncertainty in thickness and anisotropy. Typical

PWB materials have a thickness of between 0.2 mm to 3 mm. Large uncertainties

in 4 of thin materials are caused by large uncertainties in thickness. It is critical to

use accurate thermal expansion data for temperature dependent permittivity measurements. For methods using conductors, metal losses need to be accounted for and

removed. As frequencies increase metal losses and insertion losses increase.

131
We would like to acknowledge J. Krupka of Warsaw Technical University and Y.

Kantor of Rafael, Israel for measurement assistance; P. Kabos, and R. Geyer, and

R. Kaiser for help with measurements and drawings; also G. R. Traut from Rogers

Corporation for obtaining samples. We would also like to thank S. Preiman and

the NIST Ceramic Division for working with us on collaborations with the LTCC

community.

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150
Appendix A: Mixture Equations

It is often of interest to estimate the permittivity of a mixture of a number of distinct materials. The effective permittivity of a mixture Cg// of constituents with

permittivity e^ and volume fractions 9i can be approximated in various ways. For an

arbitrary number of constituents the following mixture equation apply [160]

3e; C^ + se;)"

The Bruggeman equation is useful for binary mixtures [162]

or the Maxwell-Garnett mixture equation can be used [162]

€',// + 24 Vi+26'2'

where e[ is e^ of the matrix and 4 is 4 of the filler [163]

(103)

^'^-- = ^1^^. (105)

151
Appendix B: Broadband-Plastic Measurements

^=4=*=*:

2 -

• Nylon -- RV^W

-A- FVC

W Rexlolite Teflon

_i I I i_ _i I I I

' I i_ 5 10 15

Frequency (GHz)

Figure 76. Measurements on ej. on polymers.

20

0.03

0.02

0.01

0.00

Nylon

FVC

Rexlolite Teflon

5 10 15

Frequency (GHz)

Figure 77. Measurements on tan S on polymers.

20

152

*U.S. GPO 2001-0-673-019/40198
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official source of information in the Federal Government regarding standards issued by NIST pursuant to the Federal Property and Administrative Services Act of 1949 as amended. Public Law 89-306 (79 Stat. 1127),

and as implemented by Executive Order 11717 (38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations).

NIST Interagency Reports (NISTIR)—A special series of interim or final reports on work performed by

NIST for outside sponsors (both government and non-government). In general, initial distribution is handled

by the sponsor; public distribution is by the National Technical Information Service, Springfield, VA 22161,

in paper copy or microfiche form.
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