Semiconductor devices - Flexible and stretchable semiconductor devices - Part 6: Test method for sheet resistance of flexible conducting films

IEC 62951-6:2019 specifies terms, as well as the test method and report of sheet resistance of the flexible conducting film under bending and folding tests. The measurement methods include the 2-point probe, 4-point probe and Montgomery method, which can be applied to in-situ and ex-situ measurement and the measurements of anisotropic sheet resistance.

Dispositifs à semiconducteurs - Dispositifs à semiconducteurs souples et extensibles - Partie 6: Méthode d’essai pour la résistance de couche des couches conductrices souples

L’IEC 62951-6:2019 spécifie les termes, ainsi que la méthode et le rapport d’essai de la résistance de couche d’une couche conductrice souple soumise à des essais de courbure et de pliage. Les méthodes de mesurage comprennent la méthode de la sonde 2 points, la méthode de la sonde 4 points et la méthode de Montgomery, qui peuvent être appliquées à un mesurage sur site ou hors site et aux mesurages de résistance de couche anisotrope.

General Information

Status
Published
Publication Date
05-May-2019
Technical Committee
Current Stage
PPUB - Publication issued
Completion Date
06-May-2019
Ref Project

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IEC 62951-6
Edition 1.0 2019-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Semiconductor devices – Flexible and stretchable semiconductor devices –
Part 6: Test method for sheet resistance of flexible conducting films
Dispositifs à semiconducteurs – Dispositifs à semiconducteurs souples
et extensibles –
Partie 6: Méthode d’essai pour la résistance de couche des couches
conductrices souples
IEC 62951-6:2019-05(en-fr)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC 62951-6
Edition 1.0 2019-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Semiconductor devices – Flexible and stretchable semiconductor devices –
Part 6: Test method for sheet resistance of flexible conducting films
Dispositifs à semiconducteurs – Dispositifs à semiconducteurs souples
et extensibles –
Partie 6: Méthode d’essai pour la résistance de couche des couches
conductrices souples
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 31.080.99 ISBN 978-2-8322-6871-1

Warning! Make sure that you obtained this publication from an authorized distributor.

Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé.

® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale
---------------------- Page: 3 ----------------------
– 2 – IEC 62951-6:2019 © IEC 2019
CONTENTS

FOREWORD ........................................................................................................................... 4

1 Scope .............................................................................................................................. 6

2 Normative references ...................................................................................................... 6

3 Terms and definitions ...................................................................................................... 6

4 Atmospheric conditions for evaluation and conditioning ................................................... 7

5 In situ measurements using 2-point probe method ........................................................... 8

5.1 General ................................................................................................................... 8

5.2 Sample preparation ................................................................................................. 8

5.3 Test methods .......................................................................................................... 8

5.3.1 Test apparatus ................................................................................................ 8

5.3.2 Measurement and data analysis ...................................................................... 9

5.4 Report of results ..................................................................................................... 9

6 Uniformity measurement using 4-point probe method .................................................... 10

6.1 General ................................................................................................................. 10

6.2 Test methods ........................................................................................................ 10

6.2.1 Test apparatus .............................................................................................. 10

6.2.2 Measurement and data analysis .................................................................... 10

6.3 Report of results ................................................................................................... 11

7 Anisotropic measurement using the Montgomery method .............................................. 12

7.1 General ................................................................................................................. 12

7.2 Test methods ........................................................................................................ 12

7.2.1 Test apparatus .............................................................................................. 12

7.2.2 Measurement and data analysis .................................................................... 12

7.3 Report of results ................................................................................................... 13

Annex A (informative) Bending tests .................................................................................... 14

Annex B (informative) 4-point probe measurements ............................................................. 15

B.1 General ................................................................................................................. 15

B.2 Correction for finite sample size ............................................................................ 15

B.3 Correction factors accounting for finite size probe tips .......................................... 20

Annex C (informative) Montgomery method.......................................................................... 22

C.1 General ................................................................................................................. 22

C.2 Sample preparation ............................................................................................... 22

C.3 Measurement of sheet resistance of isotropic sample ........................................... 23

C.4 Measurement of anisotropic sheet resistance ........................................................ 24

Bibliography .......................................................................................................................... 25

Figure 1 – Possible electric connection of 2-point probe measurement ................................... 8

Figure 2 – Gauge section of bending test ................................................................................ 9

Figure 3 – Example of measuring positions ........................................................................... 11

Figure 4 – Direction of bending and collinear probes ............................................................. 11

Figure 5 – Resistance measurement with the Montgomery method ....................................... 13

Figure A.1 – Two common bending test methods for flexible substrates ................................ 14

Figure B.1 – Schematic diagram of 4-point probe .................................................................. 15

Figure B.2 – Correction factor of square sample depending on length/probe spacing [2] ....... 17

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IEC 62951-6:2019 © IEC 2019 – 3 –

Figure B.3 – Correction factor depending on measuring position when collinear probes

are directed vertically ............................................................................................................ 18

Figure B.4 – Correction factor depending on measuring position when collinear probes

are directed horizontally ........................................................................................................ 18

Figure B.5 – Correction factor, f depending on measuring positions and direction of

collinear probes .................................................................................................................... 19

Figure B.6 – Example of probe with a finite contact diameter (e.g. 2mm) comparable to

inter-distance between probes (e.g. 5 mm) ........................................................................... 20

Figure B.7 – Dimensional sketch of probe with a finite contact diameter ............................... 21

Figure C.1 – Possible contact placements of square or rectangular sample .......................... 22

Figure C.2 – Correction factors for finite contact size on resistivity measurement [4] ............ 23

Figure C.3 – Resistance measurement of Montgomery method ............................................. 24

---------------------- Page: 5 ----------------------
– 4 – IEC 62951-6:2019 © IEC 2019
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SEMICONDUCTOR DEVICES –
FLEXIBLE AND STRETCHABLE SEMICONDUCTOR DEVICES –
Part 6: Test method for sheet resistance of flexible conducting films
FOREWORD

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International Standard IEC 62951-6 has been prepared by IEC technical committee 47:

Semiconductor devices.
The text of this International Standard is based on the following documents:
FDIS Report on voting
47/2547/FDIS 47/2566/RVD

Full information on the voting for the approval of this International Standard can be found in

the report on voting indicated in the above table.

This document has been drafted in accordance with the ISO/IEC Directives, Part 2.

---------------------- Page: 6 ----------------------
IEC 62951-6:2019 © IEC 2019 – 5 –

A list of all parts in the IEC 62951 series, published under the general title Semiconductor

devices – Flexible and stretchable semiconductor devices, can be found on the IEC website.

The committee has decided that the contents of this document will remain unchanged until the

stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to

the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates

that it contains colours which are considered to be useful for the correct

understanding of its contents. Users should therefore print this document using a

colour printer.
---------------------- Page: 7 ----------------------
– 6 – IEC 62951-6:2019 © IEC 2019
SEMICONDUCTOR DEVICES –
FLEXIBLE AND STRETCHABLE SEMICONDUCTOR DEVICES –
Part 6: Test method for sheet resistance of flexible conducting films
1 Scope

This part of IEC 62951 specifies terms, as well as the test method and report of sheet

resistance of the flexible conducting film under bending and folding tests. The measurement

methods include the 2-point probe, 4-point probe and Montgomery method, which can be

applied to in-situ and ex-situ measurement and the measurements of anisotropic sheet

resistance.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their

content constitutes requirements of this document. For dated references, only the edition

cited applies. For undated references, the latest edition of the referenced document (including

any amendments) applies.
ISO 291:2008, Plastics – Standard atmospheres for conditioning and testing
3 Terms and definitions
For the purpose of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following

addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
resistivity
inverse of the conductivity when this inverse exists
[SOURCE: IEC 60050-121:1998, 121-12-04]
3.2
sheet resistance

resistance of thin films that are nominally uniform in thickness, which is the resistivity divided

by the thickness of conducting film
3.3
resistance

for a resistive two-terminal element or two-terminal circuit with terminals A and B, quotient of

the voltage (IEC 60050-131:2008, 131-11-56) u between the terminals by the electric

current i in the element or circuit
𝑢𝑢
𝐴𝐴𝐴𝐴
𝑅𝑅=
𝑖𝑖
---------------------- Page: 8 ----------------------
IEC 62951-6:2019 © IEC 2019 – 7 –

where the electric current is taken positive if its direction is from A to B and negative if its

direction is from B to A
Note 1 to entry: A resistance cannot be negative.

Note 2 to entry: The term "resistance" is also a short term for “resistance to alternating current”

(IEC 60050-131:2013, 131-12-45).

Note 3 to entry: In French, the term "résistance" also denotes a device, in English "resistor" (see

IEC 60050-151:2001, 151-13-19).
Note 4 to entry: The coherent SI unit of resistance is ohm, Ω.
[SOURCE: IEC 60050-131:2013, 131-12-04]
3.4
contact resistance

resistance between the surface of a material and the electric contact made to the surface

3.5
radius
distance from the centre of a circle to the circumference
Note 1 to entry: The radius of a sphere is the radius of a great circle.
[SOURCE: IEC 60050-113:2011, 113-01-25]
3.6
radius of curvature
at a point of a curve, radius of the osculating circle

Note 1 to entry: The osculating circle is the circle tangent to a curve at a point that approaches at best the curve

in the vicinity of the point.
[SOURCE: IEC 60050-113:2011, 113-01-30]
3.7
2-point probe method

method for measuring the resistivity of a material, using two electric contacts to the material

Note 1 to entry: The measured value is dependent on the probe resistance.
3.8
4-point probe method

method for measuring the resistivity of a material, using four electric contacts to the material

Note 1 to entry: This avoids many contact resistance problems.
3.9
Montgomery method

technique used to measure the resistivity of two-dimensional sample by placing the electrodes

on its perimeter
4 Atmospheric conditions for evaluation and conditioning

The standard atmosphere for evaluation (test and measurement) and storage of the specimen

shall be a temperature of 23°C ± 2°C and relative humidity of (50 ± 10) %, conforming to

standard atmosphere class 2 specified in ISO 291:2008. If a polymer substrate is used for a

test piece coated with a conductive layer, the standard atmosphere for evaluation shall be a

temperature of 23°C ± 1°C and relative humidity of (50 ± 5) %, conforming to standard

atmosphere class 1 specified in ISO 291:2008.
---------------------- Page: 9 ----------------------
– 8 – IEC 62951-6:2019 © IEC 2019

If conditioning is necessary, the same standard atmosphere as specified above shall apply.

5 In situ measurements using 2-point probe method
5.1 General

The 2-point probe method for measuring the sheet resistance of a conductive film uses two

electric contacts. It is well known that the measured value includes the error caused by the

probe resistance and the contact resistance. However, other methods (i.e. 4-point probe and

Montgomery method) are not convenient or impossible to use for in-situ measurement during

the bending or folding test. Consequently, the 2-point probe method is often necessary for in-

situ measurements.
5.2 Sample preparation

To minimize the error caused by the probe resistance and the contact resistance, the

following should be satisfied.

• The sample resistance should be 20 times larger than the probe resistance to guarantee

the error within 5 % (e.g. if the sheet resistance is about 50 ohms/square and the probe

resistance is about 5 Ω, the ratio of length to width, L/W can be larger than 2).

• The probe electric contact should be made securely using highly conductive adhesive,

such as silver paste.

• When the width W is comparable or larger than the length L, of the sample, the conducting

bar (using highly conductive adhesive, such as silver paste) should be securely attached

to the sample to minimize the spreading resistance in the width direction. (see Figure 1)

W W
W ≥ L W << L
IEC
Key
W width of the sample
L length of the sample
B conductive bar
Figure 1 – Possible electric connection of 2-point probe measurement
5.3 Test methods
5.3.1 Test apparatus

The appropriate evaluation for flexible electronics is bending the sample to a given radius. For

this, either the collapsing radius test (see IEC 62951-1) or the X–Y–θ test can be used

(refer to Annex A). It is noted that the gauge section (where the bending radius r is observed)

should be measured in the collapsing radius test.
---------------------- Page: 10 ----------------------
IEC 62951-6:2019 © IEC 2019 – 9 –

It is noted that the folding test is also similar to the bending test. The difference is the fact

that the permanent deformation occurs in folding due to the relatively small radius of

curvature.
5.3.2 Measurement and data analysis

Acquisition of temporal resistance data requires digital multimeter, whose reading rate should

be 10 times faster than the bending frequency to measure the resistance change during one

cyclic bending. It is noted that the applied current can cause heating of the material, which

can change its resistivity. To avoid this problem, make sure the measured resistance is

constant with time (the average resistance should not drift more than 10 % in a few minutes).

For the 2-point probe method, the sheet resistance, R can be calculated from the measured

resistance, R, as shown by Formula (1):
RR= (1)

In addition, the bending radius should be measured by fitting circles to optical images of

curvature (especially when collapsing radius test is used). When the whole area of sample

does not experience the same bending radius (Figure 2), the sheet resistance at the gauge

section can be obtained from the initial resistance, R as shown by Formula (2):
 LL−  W
bend
R R− R
(2)
si 
 
bend

After the bending test, it is recommended to measure the sheet resistance of the sample

using a 4-point probe. The comparison of measurements between the 2-point and 4-point

probe can ascertain the secure electrical connection of the 2-point probe after the bending

test and can further reduce the measurement error by the 2-point probe. For the same reason,

it is also recommended to measure the sheet resistance using a 4-point probe before the

bending test.
bend
bend
IEC
Figure 2 – Gauge section of bending test
5.4 Report of results
The report shall include the following items:
a) specimen identification;
b) date of test;
---------------------- Page: 11 ----------------------
– 10 – IEC 62951-6:2019 © IEC 2019
c) atmospheric conditions of test;
d) bending radius;
e) sample dimension and the actual bending area (gauge section);
f) frequency of bending;

g) temporal sheet resistance curve (or equivalent sheet resistance over bending area);

h) optical observation permanent deformation;
i) (optional) 4-point probe measurement before and after the bending test.
6 Uniformity measurement using 4-point probe method
6.1 General

The 4-point probe method is an electrical measuring technique that uses separate pairs of

current-carrying and voltage-sensing electrodes to make more accurate measurements than

the simpler 2-point probe sensing. Separation of current and voltage electrodes eliminates the

lead and contact resistance from the measurement. 4-point probes can accurately measure a

resistance below 100 Ω, and therefore it is a suitable technique to evaluate the sheet

resistance uniformity of thin films.
6.2 Test methods
6.2.1 Test apparatus

The sheet resistances are measured by pressing collinear 4-point probes against the surface

of the film. A current is applied between the outer two points, while the voltage is measured

across the inner two points.

For the soft conducting film on flexible substrate, the use of the special collinear probe with a

finite contact area equipped with internal springs is recommended. An example of this probe

pin is shown in Figure B.6.
6.2.2 Measurement and data analysis

From the induced current, I and the measured voltage, V, the sheet resistance is calculated

as:
πV V
R f,4 5324 f
(3)
ln2 II

Here, f is the correction factor considering the finite size of the sample and the finite size of

the probe contact area and is detailed in Annex B. See Figure B.1 and Figure B.7.

To evaluate the uniformity of the sheet resistance of the conductive film, many measurement

points are required. There is not a preferred map for measuring positions, but it is

recommended that the positions are located 10 % inside from the edge. An example of

measuring positions is shown in Figure 3.

In the case of a long roll of flexible conductive film, it is recommended to acquire the samples

from both ends, but not the extremities of the roll and to evaluate the uniformity.

---------------------- Page: 12 ----------------------
IEC 62951-6:2019 © IEC 2019 – 11 –
0,2W 0,15W 0,15W 0,15W 0,15W 0,2W
IEC
Figure 3 – Example of measuring positions

In the case of a sample after the bending test, anisotropic resistivity may appear. Therefore, it

is recommended that the sheet resistance is measured with the collinear probe placed in both

directions, parallel and perpendicular to the bending direction, as depicted in Figure 4.

IEC
Key
1 direction of collinear probes
2 bending direction
Figure 4 – Direction of bending and collinear probes
6.3 Report of results
The report shall include the following items:
a) specimen identification;
0,2W 0,15W 0,15W 0,15W 0,15W 0,2W
---------------------- Page: 13 ----------------------
– 12 – IEC 62951-6:2019 © IEC 2019
b) date of test;
c) atmospheric conditions of test;
d) sample history;
e) sample dimension and measuring positons;
f) direction of collinear probes;
g) sheet resistance data and its statistical uniformity information;
h) (optional) data of correction factor.
7 Anisotropic measurement using the Montgomery method
7.1 General

The Montgomery method is a technique used to measure the sheet resistance of a sample. Its

advantage lies in its ability to accurately measure the sheet resistance of a sample and to

measure an anisotropic resistivity as well. However, for this method, electrodes should be

placed
...

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