HP-SN4291-8_Materials Characterization With a New Dielectric Spectrometer

8/14/2019 HP-SN4291-8_Materials Characterization With a New Dielectric Spectrometer

1/8

Introduction

Dielectric spectroscopy is a

valuable tool for the

characterization of materials

properties. The complex dielectricfunction reflects the molecular

relaxation and transport processes

of the material. Since = 'i"

depends on many different

physical quantities, dielectric

measurements often are done as a

function of frequency,

temperature, time and DC bias(superimposed static electric

field). The Novocontrol BDS 6000

system based on the HP 4291A RF

Impedance Analyzer automatically

performs these measurements.

The HP 4291A RF Impedance

Analyzer was designed especially

for impedance measurements and

uses a new technique developed

by Hewlett-Packard. This

technique offers more accurate

characterization of devices and

materials. Compared to systemsbased on spectrum or network

analyzers, it has much higher

resolution and a broader

impedance range, which are key

considerations.

Materials Characterization

with a New Dielectric Spectrometer

Solution Note 4291-8

Dr. Gerhard Schaumburg

Novocontrol GmbH

Novocontrol BDS 6000

System based on the

HP4291A RF Impedance

Analyzer

New technology offers improved material analysis over a wide range of temperature and

frequency.


2/8

Dielectric Behavior and

Materials Properties

Temperature analysis is

particularly important for thecharacterization of materials, as

the electric response measured by

the dielectric function strongly

depends on temperature.

From temperature dependence, a

lot of information on the physical,

chemical and electrical material

properties can be obtained. First,

it can separate isolators,

polymers, plastics,

semiconductors, metals and

superconductors. Second, solid,glass, liquid, gas state and other

structural phase transitions of the

material can be distinguished and

characterized. For each material

class and state, the temperature

dependence contains special

information.

For isolators, the magnitude andtemperature dependence of theresidual conductance givesinformation about the amount andnature of impurities.

For polymers and plastics, thedielectric spectra are shifted bytemperature. From temperaturedependence, information onmolecular dynamics likeactivation energy and glasstemperature can be obtained.

For semiconductors, thetemperature dependence of thedielectric function allowsdetermination of importantelectric parameters like gap

energy, charge carrier density, andmobility.

In metals, the temperaturedependence of the dielectricfunction (which here istraditionally expressed in terms ofthe complex conductivity) givesinformation on the electron

phonon interaction and theresidual resistance.

At low temperatures, properties ofsuperconductors can also bestudied. This is of particularinterest for high temperature

superconductors with transitiontemperatures up of -140C andabove. In this case, thetemperature dependence of thedielectric function givesinformation on the mechanism ofhigh temperaturesuperconductivity, which is stillnot resolved.

The above examples mention only

the most familiar topics related to

dielectric temperature analysis. In

addition there is an unlimited

number of special problems whichcan be evaluated by temperature

analysis of the complex dielectric

function. Therefore, adding

temperature control to a dielectric

measurement system multiplies its

performance.

System Performance

In the BDS 6000 system, the

HP4291A RF Impedance Analyzer

is enhanced with a high

performance cryogenictemperature control system which

was especially designed for

dielectric applications. The

complete system is controlled by a

PC with the MS-Windows software

package WinDETA. System

description:

1 MHz1.8GHz

-160C to +400C

-40 V to +40 V

2 s107s.

Moreover any combination of

these parameters is supported.

These features make the BDS 6000

superior for world-class scientific

and engineering applications.

Applications of Dielectric

Spectroscopy

Key scientific applications

include:Relaxation processes on the

molecular dynamics of liquid

crystals, polymers and liquids

Charge transport in

semiconductors, organic crystals,

ceramics, etc.

Timing development of chemical

reactions, polymerization and

curing processes

Structural material properties like

phase compositions, phase

transitions and crystallization

processesNon-linear electrical and opticaleffects

Industrial applications include:

Quality control and

characterization of isolating and

semiconductor materials

Materials that change theirdielectric function if exposed togases or liquids which are used insensor applications

Another interesting application is

the characterization of printedcircuit boards. This is necessary,

as the functional behavior of

boards, operated with high speed

electronics up to the GHz range, is

largely influenced by the dielectric

properties of the board materials.

2


3/8

System Components

The BDS 6000 dielectric

spectrometer consists of the

following components shown in

Figure 1:

A special RF sample cell with the

sample capacitor,

A precision microwave extension

line connecting the sample cell to

the impedance analyzer,

HP 4291A RF Impedance Analyzer,

Temperature control system

allowing exposure of the sample

temperatures from -160C to

+400C with 0.01C stability and

very short settling times,

PC with the MS-Windows softwarepackage WinDETA and WinFITthat controls the measurementflow, operates all devices and theevaluates the measured data.

Principle of Operation

The material under test generally

is placed between two electrodes

creating a capacitor as shown inFigure 2.

Electrodes

Sample material

Figure 2. Sample material placed into ameasurement capacitor.

The capacitor plates are round

with a diameter from 1 mm6mm.The spacing between the plates

may be selected between 2mm

and 10 mm. For special

applications other sample

geometries may be used. The

sample impedance Zs is related to

the dielectric function by

(1) = i =j

2nfZ1C0

where fdenotes frequency and C0is the vacuum capacity of the

empty sample capacitor. The

sample material is placed between

two electrodes in a special high

frequency sample cell shown in

Figure 6. The sample cell is

mounted in a cryostat and

exposed to a heated gas stream

evaporated from a liquid nitrogen

dewar (Figure 1). The test head of

the HP 4291A RF Impedance

Analyzer is connected to the

sample cell by a precision

microwave extension line.

3

Figure 1. Novocontrol Dielectric Spectrometer BDS 6000 based on HP4291A RFImpedance Analyzer.

APC7 ConnectorAPC7 Connector


4/8

Measurement Example

The dielectric function of glycerol

for temperatures between 193 K

and 333 K is shown in Figures 3and 4. Figure 3 shows the real part', Figure 4 shows the imaginary

part '' of the dielectric function.

The curves represent the main

dielectric relaxation

(alpha-relaxation) due to

orientation polarization of

permanent dipoles in the applied

electric field. ' is correlated to thepolarization strength, '' measures

the electric losses of the

relaxation and the dc-conductivity.

For a fixed temperature and low

frequency, the electric dipoles

follow the external field without

delay. The polarization is at a

constant high level. As frequency

increases, the dipoles more slowly

follow the field and are only partly

oriented. As can be seen from

Figure 3., the polarization

decreases. The electric losses

show a maximum at the crossover

frequency from high to low

polarization (Figure4).

At higher temperatures, the

mobility of the molecular dipoles

increases and the relaxationfrequency is shifted to higher

values. In addition, the

dc-conductivity caused by thermal

activated free charge carriers

increases. This can be seen in

Figure 4 in the increase of'' on

the left from the dielectric

relaxation in the 1 MHz range.

WinFIT does an analytical

evaluation of the measured data.

The solid lines in Figure 3 and 4

are calculated according to the

relaxation function of Havriliak-Negami (including the

Cole-Davidson and Debye

functions) in combination with an

ac-conductivity term by the

evaluation, software WinFIT

which is a standard part of the

BDS 6000 system. From this

evaluation information about the

molecular dynamics of the

material like

Activation energy

Glass transition temperature

AC- and DC-conductivity

Time domain behavior and others

can be obtained.

4

#

$

%

&

'

F r e q u e n c y [ H z ]

!

"

#

E

p

s

'

[

]

G l y z e r o l

T e m p . [ K ] = 1 9 3 . 1 5 T e m p . [ K ] = 2 0 3 . 1 5 T e m p . [ K ] = 2 1 3 . 1 5





Figure 3.Real part ' ofthe dielectric

function ofGlycerol for

severaltemperatures.

#

$

%

&

'

F r e q u e n c y [ H z ]

#

#

E

p

s

'

'

[

]

G l y z e r o l






Figure 4.

Imaginary part" loss index ofthe dielectricfunction of

Glycerol forseveral

temperatures.


5/8

Impedance Measurement

In contrast to the low frequency

techniques, above 10 MHz coaxial

connecting cables alwaysintroduce errors to impedance

measurements. Above

approximately 30 MHz additional

standing waves arise at the line

and a direct measurement of the

sample impedance completely

fails. This can be avoided by

application of microwave

techniques taking the

measurement line as the main part

of the measured impedance into

account. Therefore, the

connection from the analyzer test

head to the sample cell is made by

a coaxial precision line with

defined and nearly temperature-

independent propagation

constants.

The sample capacitor is used as

the termination of the extension

line. The complex reflection factor

r(l) at the analyzer end of the line

is dependent on sample

impedance and is measured with

the HP 4291A RF Impedance

Analyzer. The incoming and

reflected waves are separated

with two directional couplers and

are phase sensitive measured.ris

defined as the ratio of the voltages

(or electrical fields) of the

reflected wave to the incoming

wave on the line. It depends on

the location of the measurement

on the line (see Figure 5).

(2)r x

U x

U x

Ref

In

( )( )

( )=

For an ideal line,r(l) which is

measured by the analyzer can be

transformed to the reflection

factorr(0) at the sample end of

the line by

(3)r r l el j

( ) ( )( )

02= +

where alpha is the damping

constant and the = 2l/ ( : wavelength)propagation constant of the line.

From (5), the sample impedance iscalculated by

(4)Z Z

r

rS =

+

0

1 0

1 0

( )

( )

whereZ0 is the wave resistance of

the line (50 ).

In practice, lines are not ideal and

sophisticated calibration

procedures must be applied. Theline parameters , must be

homogenous over the entire line

and also independent of

temperature, as the calibration

generally only can be carried out

at room temperature.

Figure 5. Reflection factors and

impedance on a line terminated by thesample.

The same criteria mentioned for

the line apply to the sample cell

shown in Figure 6. Therefore, an

additional calibration which

eliminates the influence ofinternal impedances in the sample

cell is applied. In order to keep the

wavelength as short as possible,

the test head of the HP 4291A RF

Impedance Analyzer is directly

mounted at the top of the cryostat

with the sample cell.

The frequency range is 1 MHz to

1.8 GHz with a resolution of

tan()< 10-3 and overlaps with

other Novocontrol Dielectric

Analysis Systems in the lowerfrequency range. This is

important, as the accuracy of

measurements at the limits of the

range rapidly decreases.

5

Figure 6.

RF sample cell withcoaxial short and

sample capacitor.


6/8

Quatro Cryosystem

The Quatro Cryosystem allows the

user to automatically set any

sample temperature required. Thesystem is modular and may be

combined with any Novocontrol

Dielectric Analysis System. The

main parts of the system shown in

Figure 1 are the cryostat, the gas

jet, and the microprocessor

controlled Quatro system. The

Quatro consists of 4 independent

loops controlling the sample

temperature, the gas temperature

(at the gas heater), the

temperature of the liquid nitrogen

in the dewar, and the pressure in

the dewar.

The liquid nitrogen evaporator

heats the liquid nitrogen until a

specified pressure in the dewar is

reached. The actual pressure is

controlled by channels 1 and 2 of

the Quatro controller. The

pressure is automatically adjusted

for optimal performance

depending on the setpoint of the

sample temperature (high

pressure at low temperatures, low

pressure at high temperatures).

Due to the pressure, the cold gas

flows out of the dewar through the

gas jet to the cryostat. The

transfer line from the dewar to the

gas jet is vacuum isolated. The gas

jet is controlled by channel 3 and 4

of the Quatro controller in order to

match the actual temperature at

the sample to the desired

temperature setpoint. The gas jet

is mounted directly onto the

vacuum isolated cryostat in whichthe sample cell is located.

Due to the high stability of the gas

pressure and the two circuit

arrangement of the gas heating,

the temperature stability is better

than 0.01 C. Moreover, as the gas

flows directly along the sample

cell, this design allows a very fast

and save operation which is

required for automatic operating

systems. Even if the sensors may

be damaged or the liquid nitrogendewar becomes empty, there will

be no damage to the system or the

sample.

A very important requirement to

the cryosystem for dielectric

measurements is a fast

temperature settling time, as

settling times constitute most of

the measurement time and slow

down the system performance. On

the other hand, the dielectric

response may be vary temperaturesensitive (e.g. in the region of

phase transitions). Therefore, a

cryosystem that is both fast and

very accurate is required. Another

key point is, that after a setpoint

step, the sample temperature

should approach the setpoint, but

should not exceed it. This is

important if samples near a phase

transition are measured, but the

transition temperature must not

be exceeded as the sample will

change its structure or evenbecome damaged as in the case

when it melts.

The Quatro Cryosystem meets

these three requirements very

well. A typical performance

diagram of a temperature

frequency measurement from

-160C to +400C with 22 steps is

shown in Figure 7. Typical

stabilization times (for 0.1C

stability) are about 8 minutes per

temperature step (20 minutes for0.01C). This means, that the

demonstration measurement

(Figure 3, 4) with 15 temperature

steps is performed in less than

2hours. Moreover, Figure 7 shows

that the sample temperature

follows the setpoint with almost

no oscillations and without

overshooting.

6

0 100 200 300 400 500-200

-150

-100

-50

0

50

100

150

200

250

300

350

400

450

Process ValueSet Point

Figure 7.Temperaturesetpoint and

sampletemperature in

dependence oftime for the

QuatroCryosystem.


7/8


8/8

For more information on Hewlett-Packard

Test and Measurement products,applications, or services please call your

local Hewlett-Packard sales office. Acurrent listing is available via the Web

through AccessHP at http://www.hp.com. Ifyou do not have access to the internet,

please contact one of the HP centers listedbelow and they will direct you to your

nearest HP representative.

United States:Hewlett-Packard Company

Test and Measurement Organization5301 Stevens Creek Blvd.

Bldg. 51L-SCSanta Clara, CA 95052-8059

1 800 452 4844

Canada:Hewlett-Packard Canada Ltd.

5150 Spectrum Way

Mississauga, OntarioL4W 5G1(905) 206 4725

Europe:

Hewlett-PackardEuropean Marketing Centre

P.O. Box 9991180 AZ Amstelveen

The Netherlands

Japan:

Hewlett-Packard Japan Ltd.

Measurement Assistance Center9-1, Takakura-cho, Hachioji-shi,

Tokyo 192, Japan(81) 426 48 3860

Latin America:

Hewlett-PackardLatin American Region Headquarters

5200 Blue Lagoon Drive9th Floor

Miami, Florida 33126U.S.A.

(305) 267 4245/4220

Australia/New Zealand:Hewlett-Packard Australia Ltd.

31-41 Joseph StreetBlackburn, Victoria 3130

Australia131 347 ext. 2902

Asia Pacific:Hewlett-Packard Asia Pacific Ltd17-21/F Shell Tower, Times Square,

1 Matheson Street, Causeway Bay,Hong Kong

(852) 2599 7070

8

Copyright 1996Hewlett-Packard Company

Data subject to changePrinted in U.S.A. 01/965964-6522E

HP-SN4291-8_Materials Characterization With a New Dielectric Spectrometer

Documents

Transcript of HP-SN4291-8_Materials Characterization With a New Dielectric Spectrometer