Semiconductor devices - Flexible and stretchable semiconductor devices - Part 2: Evaluation method for electron mobility, sub-threshold swing and threshold voltage of flexible devices

IEC 62951-2:2019 specifies terms, definitions, symbols, configurations and evaluation methods that can be used to evaluate and determine the performance characteristics of flexible thin‑film transistor (TFT) devices. This document specifies test methods and characteristic parameters for accurately evaluating the performance and reliability in practical use of flexible TFT devices under the bending status.

Dispositifs à semiconducteurs - Dispositifs à semiconducteurs souples et extensibles - Partie 2 : Méthode d’évaluation pour la mobilité des électrons, la pente en régime de sous-seuil et la tension de seuil des dispositifs souples

L’IEC 62951-2:2019 spécifie les termes, définitions, symboles, configurations et méthodes d’évaluation pouvant être utilisés pour évaluer et déterminer les caractéristiques de performance des dispositifs à transistors en couche mince (TFT) souples. Le présent document spécifie les méthodes d’essai et les paramètres caractéristiques permettant d’évaluer précisément, dans le cadre d’une utilisation pratique, la performance et la fiabilité des dispositifs TFT souples soumis à une contrainte de courbure.

General Information

Status
Published
Publication Date
16-Apr-2019
Technical Committee
Current Stage
PPUB - Publication issued
Completion Date
17-Apr-2019
Ref Project

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IEC 62951-2
Edition 1.0 2019-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Semiconductor devices – Flexible and stretchable semiconductor devices –
Part 2: Evaluation method for electron mobility, sub-threshold swing, and
threshold voltage of flexible devices
Dispositifs à semiconducteurs – Dispositifs à semiconducteurs souples et
extensibles –

Partie 2: Méthode d’évaluation pour la mobilité des électrons, la pente en régime

de sous-seuil et la tension de seuil des dispositifs souples
IEC 62951-2:2019-04(en-fr)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC 62951-2
Edition 1.0 2019-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Semiconductor devices – Flexible and stretchable semiconductor devices –
Part 2: Evaluation method for electron mobility, sub-threshold swing, and
threshold voltage of flexible devices
Dispositifs à semiconducteurs – Dispositifs à semiconducteurs souples et
extensibles –

Partie 2: Méthode d’évaluation pour la mobilité des électrons, la pente en régime

de sous-seuil et la tension de seuil des dispositifs souples
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 31.080.99 ISBN 978-2-8322-6817-9

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 62951-2:2019 © IEC 2019
CONTENTS

FOREWORD ........................................................................................................................... 3

1 Scope .............................................................................................................................. 5

2 Normative references ...................................................................................................... 5

3 Terms and definitions ...................................................................................................... 5

4 Test method .................................................................................................................... 6

4.1 General ................................................................................................................... 6

4.2 Test of electrical characteristics before bending ...................................................... 7

4.3 Test of electrical characteristics under bending ....................................................... 8

4.4 Test report .............................................................................................................. 9

Bibliography .......................................................................................................................... 10

Figure 1 – Procedure for measurement of flexible thin-film transistor ...................................... 7

Figure 2 – Schematic circuit diagram of the test ...................................................................... 8

Figure 3 – Configuration for the TFT bending test ................................................................... 9

---------------------- Page: 4 ----------------------
IEC 62951-2:2019 © IEC 2019 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SEMICONDUCTOR DEVICES –
FLEXIBLE AND STRETCHABLE SEMICONDUCTOR DEVICES –
Part 2: Evaluation method for electron mobility, sub-threshold swing, and
threshold voltage of flexible devices
FOREWORD

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International Standard IEC 62951-2 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
47/2541/FDIS 47/2564/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: 5 ----------------------
– 4 – IEC 62951-2:2019 © IEC 2019

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: 6 ----------------------
IEC 62951-2:2019 © IEC 2019 – 5 –
SEMICONDUCTOR DEVICES –
FLEXIBLE AND STRETCHABLE SEMICONDUCTOR DEVICES –
Part 2: Evaluation method for electron mobility, sub-threshold swing, and
threshold voltage of flexible devices
1 Scope

This part of IEC 62951 specifies terms, definitions, symbols, configurations and evaluation

methods that can be used to evaluate and determine the performance characteristics of

flexible thin-film transistor (TFT) devices. This document specifies test methods and

characteristic parameters for accurately evaluating the performance and reliability in practical

use of flexible TFT devices under the bending status.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes 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
flexible thin-film transistor
flexible TFT

thin-film transistor fabricated on mechanically flexible substrates such as polymers and metal

foils
Note 1 to entry: This note applies to the French language only.
3.2
mobility

quantity equal to the quotient of the modulus of the mean velocity of a

charge carrier (electron) in the direction of an electric field by the modulus of the field strength

[SOURCE: IEC 60050-521:2002, 521-02-58, modified — "electron" has been added.]
3.3
sub-threshold swing

parameter for quantifying how sharply the transistor is turned off by the gate voltage, defined

by the following formula
s ln(10) (1+ )
---------------------- Page: 7 ----------------------
– 6 – IEC 62951-2:2019 © IEC 2019
where C and C represent depletion layer capacitance and gate-oxide capacitance,
d ox
respectively
3.4
threshold voltage

gate-source voltage at which the magnitude of the drain current reaches a specified low value

[SOURCE: IEC 60050-521:2002, 521-07-24]
3.5
gate voltage
voltage between gate and source
3.6
drain voltage
voltage between drain and source
3.7
drain current
current between drain and source
3.8
transconductance

ratio of the increment in the drain current to a corresponding incremental change of the

gate-source voltage with the drain-source voltage held constant
[SOURCE: IEC 60050-521:2002, 521-07-25]
4 Test method
4.1 General

To investigate the reliability of the flexible TFTs, bending tests are performed as follows

(see Figure 1):

a) prior to any bending, the electrical characteristics of the TFTs are measured;

b) under the mechanical bending state, the electrical characteristics of the TFTs are

re-measured as shown in Figure 3.
---------------------- Page: 8 ----------------------
IEC 62951-2:2019 © IEC 2019 – 7 –
Start
Gate voltage
Drain current
Drain voltage
Electrical Characterization
Before Mechanical Bending
Temperature
Gate current
Illumination
Gate voltage
Drain voltage
Drain current
Electrical Characterization
Temperature Under Mechanical Bending
Gate current
Illumination
Bending radius
End
IEC
Figure 1 – Procedure for measurement of flexible thin-film transistor
4.2 Test of electrical characteristics before bending

The stability test of a flexible TFT is carried out using four kinds of biased evaluation. The

negative-bias-stress (NBS) test is carried out with a V of −20 V at a fixed V of 10 V under

GS DS
o o
C and 60 C. The
dark and the substrate temperature is maintained at 20

negative-bias-illumination-stress (NBIS) test is carried out with a V of −20 V at a fixed V

GS DS

of 10 V under illumination with a white light-emitting diode of 300 cd/m brightness and the

o o

substrate temperature is maintained at 20 C and 60 C. The positive-bias-stress (PBS) test is

carried out with a V of +20 V at a fixed V of 0,1 V under dark and the substrate
GS DS
o o

temperature is maintained at 20 C and 60 C. The positive-bias-illumination-stress (PBIS)

test is carried out with a V of +20 V at a fixed V of 0,1 V under illumination with a white

GS DS

light-emitting diode of 300 cd/m brightness and the substrate temperature is maintained at

o o
20 C and 60 C.

In the test procedure, the drain current is measured at room temperature by sweeping the

gate voltage from −30 V to 30 V, at a fixed drain voltage of 0,1 V. Afterwards, the device

stability tests are performed for 3 h. The field-effect mobility shall be calculated using

Formula (1). The mobility is extracted from the linear-regime transconductance g at the low

drain voltage of 0,1 V as follows:
(1)
μ =
C ( )V
ox DS

where, L and W are the channel length and width, respectively. The sub-threshold swing shall

be obtained using the slope of transfer curves in the sub-threshold regime by application of

Formula (2).
---------------------- Page: 9 ----------------------
– 8 – IEC 62951-2:2019 © IEC 2019
∂logI
DS −1
S= () (2)
1/2

Threshold voltage shall also be determined by plotting (I ) versus V and extrapolating

DS GS

the curve to zero drain current. The on/off ratio is estimated from the transfer curves by

calculating the ratio between the maximal on-current (at V = 30 V) and the minimal

off-current. Finally the gate leakage current shall be obtained measuring the gate-to-source

current at 30 V of the gate voltage applied. A schematic circuit diagram is shown in Figure 2.

Gate [Constant bias]
Source [0 V] Drain [0 V or constant bias]
Scan
Load
IEC
Figure 2 – Schematic circuit diagram of the test
4.3 Test of electrical characteristics under bending

To characterize the effects of mechanical stress on flexible TFTs, strain is applied on the

TFTs by bending the device convexly (i.e., inducing a tensile strain on the active devices) or

concavely (i.e., inducing a compressive strain on the active devices) with varied radius of

curvature R as shown in Figure 3. The bending direction is parallel or perpendicular to the

drain-to-source current path. The TFTs are first bent to the maximum R for 1 min and then

released to the flat state and are measured. This test cycle is repeated at decreasing R down

to the minimum R. After each mechanical bending, the TFT on-current, the off-current and the

gate leakage current are monitored.
Data
---------------------- Page: 10 ----------------------
IEC 62951-2:2019 © IEC 2019 – 9 –
TFT film
IEC
Figure 3 – Configuration for the TFT bending test

In the bended state, all bending radii (R) are converted to percent strain (ε) using Formula (3)

dd+
T S
(3)
2× R

Where, d and d are the corresponding thicknesses of the TFT stack and the flexible

T S

substrate. Simple mechanical fatigue testing is performed by putting the flexible TFTs through

repeated bending (i.e., at constant R) and flattening up to 50 000 times. The substrate

o o

temperature is maintained at 20 C and 60 C. The TFTs are periodically measured while flat

without removal from the bending apparatus.

The four kinds of test shall proceed under the bending state. To obtain the results of negative-

bias-stre
...

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