Semiconductor devices - Flexible and stretchable semiconductor devices - Part 3: Evaluation of thin film transistor characteristics on flexible substrates under bulging

IEC 62951-3:2018(E) specifies the method for evaluating thin film transistor characteristics on flexible substrates under bulging. The thin film transistor is fabricated on flexible substrates, including polyethylene terephthalate (PET), polyimide (PI), elastomer and others. The stress is applied by applying a uniformly-distributed pressure to the flexible substrate using the equipment.

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Status
Published
Publication Date
06-Nov-2018
Technical Committee
Current Stage
PPUB - Publication issued
Completion Date
07-Nov-2018
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IEC 62951-3:2018 - Semiconductor devices - Flexible and stretchable semiconductor devices - Part 3: Evaluation of thin film transistor characteristics on flexible substrates under bulging
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IEC 62951-3
Edition 1.0 2018-11
INTERNATIONAL
STANDARD
colour
inside
Semiconductor devices – Flexible and strechable semiconductor devices –

Part 3: Evaluation of thin film transistor characteristics on flexible substrates

under bulging
IEC 62951-3:2018-11(en)
---------------------- Page: 1 ----------------------
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IEC 62951-3
Edition 1.0 2018-11
INTERNATIONAL
STANDARD
colour
inside
Semiconductor devices – Flexible and strechable semiconductor devices –

Part 3: Evaluation of thin film transistor characteristics on flexible substrates

under bulging
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 31.080.99 ISBN 978-2-8322-6161-3

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

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC 62951-3:2018 © IEC 2018
CONTENTS

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

INTRODUCTION ..................................................................................................................... 5

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

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

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

4 Test piece ....................................................................................................................... 6

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

4.2 Size of a test piece ................................................................................................. 7

4.3 Measurement of dimensions ................................................................................... 7

4.4 Storage prior to testing ........................................................................................... 7

5 Test apparatus and procedure ......................................................................................... 7

5.1 General ................................................................................................................... 7

5.2 Test apparatus ........................................................................................................ 7

5.2.1 General ........................................................................................................... 7

5.2.2 Apparatus ........................................................................................................ 8

5.3 Test procedure and analysis ................................................................................. 12

5.3.1 Test procedure .............................................................................................. 12

5.3.2 Data analysis ................................................................................................. 14

6 Test report ..................................................................................................................... 17

Annex A (informative) Other types of electrical and mechanical test equipments ................. 18

A.1 Absorption type electrical and mechanical test equipment with heating

system .................................................................................................................. 18

A.2 Bulging-type electrical and mechanical test equipment with halogen lamp

heating system ...................................................................................................... 18

Annex B (informative) Failure pressure estimation ............................................................... 20

Bibliography .......................................................................................................................... 22

Figure 1 – Pressure chamber open window shapes ................................................................. 9

Figure 2 – Typical example of bulging-type mechanical and electrical measurement

test apparatus with heating system ....................................................................................... 11

Figure 3 – Exemplary schematics of pressure chamber, pressure chamber open

window ................................................................................................................................. 11

Figure 4 – Exemplary schematic of wire bonding................................................................... 12

Figure 5 – Exemplary DC characteristics for determining (a) μ (b) μ and (c) SS ............. 16

lin sat

Figure 6 – Representative bulge test data showing pressure-deflection relation for Ag-

Pd/SiN ................................................................................................................................ 17

Figure A.1 – Exemplary schematic of absorption-type electrical and mechanical test

equipment with heating system ............................................................................................. 18

Figure A.2 – Exemplary schematic of bulging-type electrical and mechanical test

equipment with halogen lamp heating system ....................................................................... 19

Figure B.1 – Schematic for failure pressure estimation for 100 μm-thick polyimide

assuming yield and tensile strength of 69 MPa and 231 MPa ................................................ 20

Table B.1 – Flexible substrate information (polyimide) .......................................................... 20

Table B.2 – Pressure calculation results ............................................................................... 21

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IEC 62951-3:2018 © IEC 2018 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SEMICONDUCTOR DEVICES –
FLEXIBLE AND STRECHABLE SEMICONDUCTOR DEVICES –
Part 3: Evaluation of thin film transistor characteristics
on flexible substrates under bulging
FOREWORD

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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is

indispensable for the correct application of this publication.

International Standard IEC 62951-3 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
47/2492/FDIS 47/2511/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.

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.

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– 4 – IEC 62951-3:2018 © IEC 2018

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

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

colour printer.
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IEC 62951-3:2018 © IEC 2018 – 5 –
INTRODUCTION

The International Electrotechnical Commission (IEC) draws attention to the fact that it is

claimed that compliance with this document may involve the use of a patent concerning the

method and apparatus for testing flexible elements.

IEC takes no position concerning the evidence, validity and scope of this patent right.

The holder of this patent right has assured the IEC that he/she is willing to negotiate licences

under reasonable and non-discriminatory terms and conditions with applicants throughout the

world. In this respect, the statement of the holder of this patent right is registered with IEC.

Information may be obtained from:
Industry Academy Cooperation Foundation of Sejong University,
Korea Institute of Machinery and Materials,
Electronics and Telecommunications Research Institute
1002 GwangGaeTo B/D
209, Neungdong-ro, Gwangjin-gu, Seoul, 05006,
KOREA

Attention is drawn to the possibility that some of the elements of this document may be the

subject of patent rights other than those identified above. IEC shall not be held responsible for

identifying any or all such patent rights.

ISO (www.iso.org/patents) and IEC (http://patents.iec.ch) maintain on-line data bases of

patents relevant to their standards. Users are encouraged to consult the data bases for the

most up to date information concerning patents.
---------------------- Page: 7 ----------------------
– 6 – IEC 62951-3:2018 © IEC 2018
SEMICONDUCTOR DEVICES –
FLEXIBLE AND STRECHABLE SEMICONDUCTOR DEVICES –
Part 3: Evaluation of thin film transistor characteristics
on flexible substrates under bulging
1 Scope

This part of IEC 62951 specifies the method for evaluating thin film transistor characteristics

on flexible substrates under bulging. The thin film transistor is fabricated on flexible

substrates, including polyethylene terephthalate (PET), polyimide (PI), elastomer and others.

The stress is applied by applying a uniformly-distributed pressure to the flexible substrate

using the equipment.
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.

IEC 62047-17, Semiconductor devices – Micro-electromechanical devices – Part 17: Bulge

test method for measuring mechanical properties of thin films

IEC 60747-8, Semiconductor devices – Discrete devices – Part 8: Field-effect transistors

3 Terms and definitions

For the purposes of this document, the terms and definitions given in IEC 62047-17, in

IEC 60747-8 and the following 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 substrate
substrate with flexibility onto which a thin film transistor is fabricated
4 Test piece
4.1 General

The test piece shall be prepared using the thin film transistor fabrication process on flexible

substrates. The mechanical and electrical properties of thin film transistors may depend on

the fabrication processes. Thin film transistors shall be prepared to prevent formation of

cracks or flaws and delamination from the substrate.
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IEC 62951-3:2018 © IEC 2018 – 7 –
4.2 Size of a test piece

As long as the size of a test piece is larger than that of the chamber open area, any test piece

will suffice. Since the change in electrical characteristics is related to strain or stress, it is

recommended that the thin film transistors be fabricated in a central region, where the strain

is uniform. To measure the electrical characteristics, attach lead wires to the source, drain

and gate pads of thin film transistors of the test piece.
4.3 Measurement of dimensions

The thickness and dimension of the thin film transistors and flexible substrate shall be

accurately measured respectively, because they are used to determine the mechanical and

electrical properties of thin film transistors. It is recommended that the thickness of thin film

transistors be smaller than that of the substrate in order to keep the deformation of the thin

film transistors uniform. The substrate material should be in-plane isotropic in order to keep

the stress and strain applied on the thin film transistor equibiaxial. There can be some

combinations of thin film transistor and substrate where it is difficult to fulfil the tolerance of

thickness measurement. In this case the average and the standard deviation of the thickness

measurement should be reported.
4.4 Storage prior to testing

In the case of thin film transistors, the storage environment may affect the electromechanical

properties of the thin film transistors. For example, oxidation on the test piece surface will

deteriorate the electrical and mechanical properties of the test piece. If there is an interval

between final preparation and testing, particular care should be taken in storing the test

pieces, and the specimens should be examined by appropriate means to ensure that the

surface has not deteriorated during the storage period. If any deterioration is observed that

was not present after the specimens were prepared, testing shall not be performed. However,

if the damage was introduced during the preparation processes, the test shall be performed.

5 Test apparatus and procedure
5.1 General

The test is performed by bulging a test piece at a specified temperature. To measure the

change in electrical characteristics along with the change in mechanical strain, carefully

select the measuring section. The section for measuring mechanical strain shall be coincident

with or scalable to that for measuring electrical characteristics. There are several types of

bulging equipment by which to measure the electromechanical property of thin film transistors.

It is not necessary that a certain type of bulging test method be preferred. As examples,

absorption-type electrical and mechanical test equipment with a heating system and

bulging-type electrical and mechanical test equipment with a halogen lamp heating system

are described in Annex A.
5.2 Test apparatus
5.2.1 General

By applying pressure to the specimen, the deformation response, i.e. the change in bulge

height as well as the electrical response of the thin film transistor on flexible substrate, shall

be measured. In general, test apparatus can be composed of pressuring device, pressure

chamber, pressure chamber open window, heating device (optional), bulging height

measurement unit and electrical measurement units as shown in Figure 2. Exemplary

schematics of pressure chamber, pressure chamber open window and wire bonding are given

in Figure 3 and Figure 4.
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– 8 – IEC 62951-3:2018 © IEC 2018
5.2.2 Apparatus
5.2.2.1 Pressuring device

The pressuring device should be equipped to apply a specified continuous pressure with a

controlled rate or a certain level of pressure to the pressure chamber open window to be

stressed. Pressure media can be oil, gas and distilled water. In general, the device can be

composed of a pressure sensor and pressure controller. The controller should show an

accuracy of 1 % in the full test scale.

NOTE At the pressures encountered in the tests, gas is over a million times more compressible than typical

liquids such as oil and distilled water.
5.2.2.2 Pressure chamber

The pressure chamber should be as compact as possible, to reduce the compliance of the

test system. The volume, which has to be pressurized and which potentially contributes to the

compliance, would be minimized.

In case liquid is used to pressurize the test system, the system shall contain as little air as

possible because even a small air bubble trapped inside the test system can dominate the

system’s compliance. It is recommended that the system including the chamber be designed

so that there are no places where air bubbles can hide and that the liquid can be refilled

easily. It is necessary that special care be taken not to introduce air bubbles when the test

piece is mounted and removed.

The material of the chamber should be chosen considering the pressure media for the test,

testing pressure range, measurement temperature range and interference with the electrical

measurements.

In case liquid is used to pressurize the test system, it is recommended that the testing

apparatus be made out of transparent plexiglass (polymethyl methacrylate) in order to see air

bubbles and then to minimize them trapped within the chamber.

The pressure chamber is connected to the pressuring device and thus allows a test piece to

be deformed with fine control. The test piece is mounted on the pressure chamber by

mechanical clamping or the epoxy gluing method, etc.

NOTE In the case of a capacitance measurement type, the pressure chamber has an electrode and a mechanical

spacer. The electrode, which measures the height change of a test specimen due to deformation, is made of Cu-

coated polychlorinated biphenyl (PCB). A mechanical spacer that is located between the specimen and the

electrode controls a sensitivity of capacitance change by adjusting the thickness of the spacer.

The pressure inside the chamber shall be monitored and measured through a suitable

pressure sensor, which can be installed directly in the chamber or connected though the tube

transporting the pressure without loss of the pressure to be measured.

It is recommended that exposition of the area of the pressure sensor to the pressure media be

minimized and that the area have no indentation or internal cavities trapping air.

It is recommended that the nonlinearity and hysteresis of the pressure sensor be less than

0,5 % and be calibrated according to the national standard.
5.2.2.3 Heating device (optional)

When a very large pressure is needed to deform a flexible substrate, an optional heating

device such as a hot plate or a halogen lamp can be added to increase the temperature of the

device under testing. A heating device can be used when a large elastic or plastic deformation

is needed through a reasonable pressuring device, since the elastic modulus of flexible

substrates such as PET or PI becomes smaller at a higher temperature. When a plastic

deformation is effected, the flexible substrate is deformed permanently and cannot be

recovered to its initial non-stressed state. When a heating device is used, the pressure

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IEC 62951-3:2018 © IEC 2018 – 9 –

chamber should be designed with materials that conduct heat easily from the heating device

to the flexible substrate under testing. The materials used for the pressure chamber should be

able to withstand the supplied heat.
5.2.2.4 Pressure chamber open window
5.2.2.4.1 Pressure chamber open window shapes

Pressure chamber open windows can be in the shape of a rectangle, square, circle or ellipse,

as shown in Figure 1. In case of a square or circle shape, the stress state of the sample is

biaxial, while in case of a rectangle or ellipse, the stress state may be uniaxial. In case of

measuring transistor characteristics, a square or circle shape is recommended, since the

alignment of the thin film transistor under test is not necessary with biaxial stress. The

pressure chamber open window is surrounded with a thick substrate frame or frame jig, which

is not deformed by pressure. It is recommended that the half width, a, of the rectangular,

square and elliptic chamber open window and the diameter, d, of the circular window be in the

range of 1 mm to 50 mm.
IEC
IEC IEC IEC
a) rectangular b) square c) ellipse d) circle
Figure 1 – Pressure chamber open window shapes
5.2.2.4.2 Measurement of pressure chamber open window dimension

To analyze the test results, the accurate measurement of the pressure chamber open window

dimension and pressure is required since the dimensions are used to extract mechanical

properties of the test piece. The dimension of the window (width and length or diameter)

should be measured with very high accuracy within less than ±1 %. Special care should be

taken to measure the window size with clear division of the window boundary.

Special care should also be taken to avoid damage to the test piece during the measurement.

5.2.2.5 Height measurement unit

The height measurement unit should be installed in a position suitable to measure the

deformation of the flexible substrate and perform the function of continuous measurement,

which is needed to determine the maximum deformation of the flexible substrate bulged with

applying pressure. The maximum deformation of the flexible substrate can be determined

from the measurement in full-field or on top of the bulged area using the laser interferometric

system or capacitance type measurement system, which is described in detail in Annex B of

IEC 62047-17:2015.

The resolution of the measurement device for the deflection measuring a bulged flexible

substrate by pressure should be in units of micrometers. A fine resolution of less than 0,1 %

in full scale is very important for an accurate measurement.
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– 10 – IEC 62951-3:2018 © IEC 2018
5.2.2.6 Electrical measurement unit

For DC characteristics of thin film transistors, testing is performed using an electronic device

test system with measurement sensitivity sufficient to give an accuracy of at least ± 0,1 %

(minimum sensitivity at or better than three orders of magnitude below expected signal level).

For example, the smallest current through a thin film transistor is often the gate leakage

current. If gate leakage is approximately 1 pA, the instrument shall have a resolution of 1 fA

or smaller. Additionally, due to the large (> 1 GΩ) impedances often encountered in thin film

transistors, the input impedance of all elements of the test system shall be at least three

orders of magnitude greater than the highest impedance in the device. Commercial

semiconductor systems with the capability to characterize thin film devices typically have

input impedance values of 10 Ω, which is a recommended minimum value.

This test method requires that the instrumentation be calibrated against a known and

appropriate set of standards (such as those of the National Institute of Standards and

Technology (NIST)). These calibrations may be performed by the equipment user or as a

service by the equipment vendor. The basic instrument operations (e.g., voltage, current and

resistance) are calibrated against some method traceable to a physical standard of the NIST

(or similar internationally recognized standards organization). Re-calibration is required

according to the instrument manufacturer’s recommendations or when the instrument is

moved or when the testing conditions change significantly (e.g., temperature change greater

than 10 °C, relative humidity change greater than 30 %).
5.2.2.7 Wire bonding

Wire bonding is needed to measure the in-situ electrical characteristics of thin film transistors

under bulging. Wire bonding is typically done between the contact pads outside the chamber

open area and the pads on flexible substrates. It is recommended that ohmic contact be

formed during wire bonding and that the wire bonding be kept intact during testing. The

electrical measurement unit is connected through the contact pads onto a device under test.

5.2.2.8 Test environment

It is recommended to perform a test under constant humidity after temperature is stabilized.

Temperature change can induce thermal drift during deflection measurement. Temperature

change during the test should be less than 2 ºC.
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IEC 62951-3:2018 © IEC 2018 – 11 –
Electrical
characteristic
measurement
Deflectio
...

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