IEC 62951-5:2019

Semiconductor devices - Flexible and stretchable semiconductor devices - Part 5: Test method for thermal characteristics of flexible materials

IEC 62951-5:2019 specifies the test method for thermal characteristics of flexible materials. This document includes terms, definitions, symbols, and test methods that can be used to evaluate and determine thermal characteristics of flexible materials for practical use. The measurement method relies on non-contact optical thermometry that is based on temperature dependent optical reflectance. This document is applicable to both substrate and thin-film flexible semiconductor materials that are subjected to bending and stretching.

Dispositifs à semiconducteurs - Dispositifs à semiconducteurs souples et extensibles - Partie 5 : Méthode d’essai pour les caractéristiques thermiques des matériaux souples

L’IEC 62951-5:2019 spécifie la méthode d’essai pour les caractéristiques thermiques des matériaux souples. Le présent document inclut les termes, définitions, symboles et méthodes d’essai pouvant être utilisés pour évaluer et déterminer les caractéristiques thermiques des matériaux souples, dans le contexte d’une utilisation pratique. La méthode de mesure s’appuie sur la thermométrie optique sans contact, qui est elle-même basée sur le facteur de réflexion optique, qui dépend de la température. Le présent document s’applique aux semiconducteurs souples en substrat et en couche mince, qui sont soumis à des contraintes de courbure et d’étirement.

General Information

• Status: Published
• Publication Date: 26-Feb-2019
• ICS: 31.080.99 - Other semiconductor devices
• ICS: 33.160.10 - Amplifiers
• ICS: 33.180.30 - Optic amplifiers
• Technical Committee: TC 47 - Semiconductor devices
• Drafting Committee: WG 6 - TC 47/WG 6
• Current Stage: PPUB - Publication issued
• Start Date: 27-Feb-2019
• Completion Date: 29-Mar-2019

Overview

IEC 62951-5:2019 is an international standard developed by the International Electrotechnical Commission (IEC) that specifies the test method for evaluating the thermal characteristics of flexible semiconductor materials. These materials, including both substrates and thin films, are subject to bending and stretching in practical applications. The standard introduces a non-contact optical thermometry technique based on temperature-dependent optical reflectance-known as thermoreflectance-to measure and analyze thermal properties with precision.

This standard covers terminology, symbols, and detailed test procedures to ensure consistent evaluation of flexible and stretchable semiconductor devices. It addresses the unique challenges posed by flexible materials, whose thermal characterization requires sensitive and accurate methods to account for their mechanical flexibility and optical properties.

Key Topics

• Thermoreflectance Principle
  The test method relies on thermoreflectance, which measures changes in optical reflectance of a material surface as a function of temperature without physical contact. This technique exploits temperature-dependent variations in refractive index to determine thermal behavior.

• Material Categorization

  • Substrate materials: Defined as materials thicker than approximately 12.7 µm (20 times the probing laser wavelength). Their optical reflectance typically changes linearly with temperature.
  • Thin-film materials: Defined as materials thinner than approximately 12.7 µm, exhibiting highly non-linear optical reflectance behavior with temperature, requiring more advanced measurement techniques.

• Test Apparatus and Measurement Techniques
  For substrates, single-wavelength optical probing suffices, while thin films require dual-wavelength probing to compensate for non-linearities and thickness-related variations. The apparatus includes laser sources, optical setups for reflectance measurement, and calibration procedures.

• Testing Procedures
  The standard specifies procedures for measuring thermoreflectance signals during bending or uniaxial stretching of flexible semiconductor materials. It defines how to calibrate signals, perform optical measurements, and document thermal parameters such as local and average temperatures, initial and final temperature states, and thermal time constants.

• Data Reporting
  Test results include reflectance values as functions of temperature, thermal response curves, and calculated thermal time constants. Documentation supports reproducibility and comparison across devices and materials.

Applications

IEC 62951-5:2019 applies to the following practical use cases:

• Flexible Electronic Devices
  Evaluation of thermal behavior in flexible semiconductors used in wearable electronics, foldable displays, and bendable sensors.

• Stretchable Semiconductor Devices
  Characterizing thermal performance in devices designed for uniaxial stretching or complex deformation during operation.

• Thin-Film Flexible Materials
  Optimizing and validating thin-film materials for use in advanced semiconductor applications including flexible photovoltaics and bio-integrated electronics.

• Thermal Management in Emerging Technologies
  Providing essential data to improve heat dissipation and reliability of flexible electronic systems under mechanical stress.

• Product Development and Quality Assurance
  Enabling manufacturers and researchers to standardize thermal testing, ensuring product safety and performance consistency.

Related Standards

• IEC 62951 Series
  Part of a broader series covering flexible and stretchable semiconductor devices, providing complementary guidelines on material properties and testing methods.

• IEC 60050-845
  Electrotechnical vocabulary standard related to definitions of reflectance and optical properties, referenced for terminology consistency.

• ISO/IEC Directives, Part 2
  Governs the drafting and formatting principles for IEC and ISO standards, ensuring clarity and harmonization across documentation.

• Other Optical and Thermal Measurement Standards
  Related guidelines on non-contact temperature measurement and semiconductor device characterization provide foundational methods supporting thermoreflectance techniques.

---

Keywords: IEC 62951-5, thermoreflectance, flexible semiconductor devices, stretchable semiconductors, thermal characteristics, optical thermometry, test method, non-contact temperature measurement, thin-film materials, substrate materials, thermal time constant, flexible electronics, standardization, semiconductor testing

Standards Content (Sample)

IEC 62951-5 ®
Edition 1.0 2019-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
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Semiconductor devices – Flexible and stretchable semiconductor devices –
Part 5: Test method for thermal characteristics of flexible materials

Dispositifs à semiconducteurs – Dispositifs à semiconducteurs souples et
extensibles –
Partie 5: Méthode d’essai pour les caractéristiques thermiques des matériaux
souples
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IEC 62951-5 ®
Edition 1.0 2019-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Semiconductor devices – Flexible and stretchable semiconductor devices –

Part 5: Test method for thermal characteristics of flexible materials

Dispositifs à semiconducteurs – Dispositifs à semiconducteurs souples et

extensibles –
Partie 5: Méthode d’essai pour les caractéristiques thermiques des matériaux

souples
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 31.080.99 ISBN 978-2-8322-6611-3

– 2 – IEC 62951-5:2019 © IEC 2019
CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Testing method . 6
4.1 General . 6
4.2 Test apparatus . 7
4.3 Test procedures . 13
4.3.1 General . 13
4.3.2 Substrate specimen . 13
4.3.3 Thin-film specimen . 14
4.4 Report of results . 14
Annex A (informative) Example of 3D design of thermoreflectance thermometry . 15
Bibliography . 16

Figure 1 – Thermoreflectance signals of substrate and thin-film materials as functions
of temperature . 7
Figure 2 – Reflectance vs. temperature of silicon thin-films (thicknesses of 1,62 µm,
1,64 µm, and 1,67 µm) for the wavelength of 633 nm . 7
Figure 3 – Schematic of thermoreflectance thermometry with one laser source that is
used for calibration . 8
Figure 4 – Schematic of thermoreflectance thermometry with one laser source that is
used for measurement . 9
Figure 5 – Schematic of thermoreflectance thermometry with two lasers of different
wavelengths used for calibration . 10
Figure 6 – Schematic of thermoreflectance thermometry with two lasers of different
wavelengths used for measurement . 11
Figure 7 – Optical reflectance of a silicon thin-film (1,526 µm) at 532 nm and 633 nm
as a function of temperature . 12
Figure 8 – Reflectance ratio of a silicon thin-film as a function of temperature
(λ = 633 nm and λ = 532 nm) . 12
1 2
Figure 9 – Suspended bending or uniaxial stretching of flexible or stretchable
semiconductor materials . 13
Figure A.1 – 3D design of dual wavelength thermoreflectance setup . 15

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SEMICONDUCTOR DEVICES –
FLEXIBLE AND STRETCHABLE SEMICONDUCTOR DEVICES –

Part 5: Test method for thermal characteristics of flexible materials

FOREWORD
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International Standard IEC 62951-5 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
47/2534/FDIS 47/2543/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.

– 4 – IEC 62951-5:2019 © IEC 2019
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
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• replaced by a revised edition, or
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SEMICONDUCTOR DEVICES –
FLEXIBLE AND STRETCHABLE SEMICONDUCTOR DEVICES –

Part 5: Test method for thermal characteristics of flexible materials

1 Scope
This part of IEC 62951 specifies the test method for thermal characteristics of flexible
materials. This document includes terms, definitions, symbols, and test methods that can be
used to evaluate and determine thermal characteristics of flexible materials for practical use.
The measurement method relies on non-contact optical thermometry that is based on
temperature dependent optical reflectance. This document is applicable to both substrate and
thin-film flexible semiconductor materials that are subjected to bending and stretching.
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
reflectance
ρ
ratio of the reflected optical power to the incident optical power at a given wavelength and
temperature for a given surface of materials
Note 1 to entry: Reflectance can be defined as the ratio between reflected and incident radiant or luminous flux.
[SOURCE: IEC 60050-845:1987, 845-04-58, modified – temperature dependence of optical
reflectance is added.]
3.2
thermoreflectance
temperature dependent optical reflectance of a given surface of materials
Note 1 to entry: Thermoreflectance has nothing to do with thermal reflectance.
3.3
local temperature
T
loc
temperature at a local position in a spatially distributed device or system

– 6 – IEC 62951-5:2019 © IEC 2019
3.4
average temperature
T
avg
temperature within a substrate averaged over an area of interest at a given time
(1)
where
n is the total number of measurement point
th
T (i) is the local temperature at the i measurement point
loc
3.5
initial temperature
T
i
local or average temperatures when the sample is about to be powered on (t = 0)
3.6
final temperature
T
f
local or average temperatures when the sample is about to be powered off (t = t )
f
3.7
thermal time constant
τ
time taken to reach 63,2 % of the difference between the initial and the final temperatures
3.8
substrate
materials that are more than 20 times thick of the wavelength of the probing laser
Note 1 to entry: For 633 nm, the thickness criterion to determine the substrate materials is larger than
approximately 12,7 µm.
3.9
thin-film
materials that are less than 20 times thick of the wavelength of the probing laser
Note 1 to entry: For 633 nm, the thickness criterion to determine the thin-film materials is less than approximately
12,7 µm.
4 Testing method
4.1 General
Thermoreflectance is one of non-contact optical thermal characterization techniques that
relies on the change of refractive index of materials as a function of temperature. Depending
on their thickness, flexible semiconductor materials can be categorized as either substrate or
thin-film. For substrate materials, their optical reflectance values change linearly with
temperature. However, optical reflectance values of thin-film materials show highly non-linear
behaviours as shown in Figure 1. For thin-films, non-linear optical reflectance is also strongly
dependent on the sample thickness as shown in Figure 2. Such a non-linearity requires
reflectance measurements at multiple wavelengths. Once optical reflectance values at one or
more wavelengths are calibrated at various temperatures, thermal characterization is enabled.

Key
1 Substrate
2 Thin-film
Figure 1 – Thermoreflectance signals of substrate and
thin-film materials as functions of temperature

Figure 2 – Reflectance vs. temperature of silicon thin-films (thicknesses of
1,62 µm, 1,64 µm, and 1,67 µm) for the wavelength of 633 nm
4.2 Test apparatus
In case of substrate materials, thermoreflectance signals at a given wavelength tend to
change linearly with increasing temperature. Thin-film materials, however, exhibit highly non-
linear thermoreflectance behaviours as temperature increases. Therefore, single wavelength
optical probing is necessary and sufficient for substrate flexible semiconductor materials and
at least dual wavelength probing is required for thin-film materials. Thermoreflectance ratio at
different wavelengths is still non-linear with temperature but can provide an acceptable match
with the theoretical estimation and more precise temperature measurements. Figure 3 and
Figure 4 show schematics of thermoreflectance thermometry with one laser source used for
calibration and measurement (for example, wavelength, λ = 633 nm herein but other
wavelength can be used), respectively. For calibration, heating or cooling blocks with fixed
temperature are used. The substrate is joule heated with DC or AC (simple periodic sine)
power supplies for actual measurements.

– 8 – IEC 62951-5:2019 © IEC 2019

Key
1 Goniometer 2 Motorized xy stages
3 Fixed stage 4 Motorized linear stage
5 Cold temperature block 6 Hot temperature block
7 Objective lens 8 Halogen lamp
9 Multimeter 10 CCD
11 Variable terminal 12 Silicon detector
13 Laser
NOTE The wavelength of the laser, λ, is 633 nm for this example but other wavelength can be used.
Figure 3 – Schematic of thermoreflectance thermometry with
one laser source that is used for calibration

Key
1 Goniometer 2 Motorized xy stages
3 Fixed stage 4 Motorized linear stage
5 DC or AC power supply 6 Objective lens
7 Halogen lamp 8 Multimeter
9 CCD 10 Variable terminal
11 Silicon detector 12 Laser
NOTE The wavelength of the laser, λ , is 633 nm for this example but other wavelength can be used.
Figure 4 – Schematic of thermoreflectance thermometry with one laser source
that is used for measurement
Figure 5 and Figure 6 show schematics of thermoreflectance thermometry with two lasers of
different wavelengths that are well suited for thin-film materials but can be generally
applicable to substrate materials. Three dimensional design of the dual wavelength
thermoreflectance setup is also shown in Annex A. For calibration, heating or cooling blocks
with fixed temperature are used. The substrate is joule heated with DC or AC (simple periodic
sine) power supplies for actual measurements.
For tight focusing (or smaller focal spots), lasers in setups shown in Figures 3, 4, 5 and 6 are
expanded by a beam expander and focused onto a sample under test using a microscope
objective lens attached to a turret. The focal spot ( d ) is determined by
2 fλ
d =
(2)
D
– 10 – IEC 62951-5:2019 © IEC 2019
where, is the focal length of the lens, is the beam diameter at the lens entrance, and
f D
λ is the wavelength. Typically, 5x expansion is sufficient. The reflected light travels
backwards and is guided towards a silicon photodetector. The photocurrent generated in the
silicon detector is converted into a voltage via a variable terminal or a transimpedance
amplifier and measured with a digital multimeter or a lock-in amplifier.

Key
1 Goniometer 2 Motorized xy stages
3 Fixed stage 4 Motorized linear stage
5 Cold temperature block 6 Hot temperature block
7 Objective lens 8 Halogen lamp
9 Multimeter 10 CCD
11 Variable terminal 12 Silicon detector
13 Low or high pass filter 14 Laser 1
15 Laser 2
NOTE The wavelengths of the laser 1 and laser 2, λ and λ , are 633 nm and 532 nm for this example but other
1 2
wavelengths can be used.
Figure 5 – Schematic of thermoreflectance thermometry with two lasers of
different wavelengths used for calibration

Key
1 Goniometer 2 Motorized xy stages
3 Fixed stage 4 Motorized linear stage
5 DC or AC power supply 6 Objective lens
7 Halogen lamp 8 Multimeter
9 CCD 10 Variable terminal
11 Silicon detector 12 Low or high pass filter
13 Laser 1 14 Laser 2
NOTE The wavelengths of the laser 1 and laser 2, λ and λ , are 633 nm and 532 nm for this example but other
1 2
wavelengths can be used.
Figure 6 – Schematic of thermoreflectance thermometry with two lasers of
different wavelengths used for measurement
A sample specimen is mounted between two blocks with constant temperature which are
located on a fixed jig and a motorized linear stage, respectively. In general, the temperature
blocks are running during calibration (reflectance vs. temperature at a given wavelength).
Figure 7 shows typical results with a silicon thin-film during calibration where wavelength
dependent non-linear behaviours can be seen. By taking the reflectance ratio as shown in
Figure 8, a calibration curve for a thin-film can be obtained. The thickness of each thin-film
sample is not necessarily measured.

– 12 – IEC 62951-5:2019 © IEC 2019

Figure 7 – Optical reflectance of a silicon thin-film (1,526 µm) at 532 nm and 633 nm
as a function of temperature
Figure 8 – Reflectance ratio of a silicon thin-film as a function of temperature
(λ = 633 nm and λ = 532 nm)
1 2
For actual measurements, the sample specimen is joule-heated by using either DC or AC
(simple periodic sine) powers in general. For highly resistive samples, heating methods other
than joule heating shall be used. As shown in Figure 9, the single-axis motorized stage
enables either bending or stretching for flexible or stretchable semiconductor materials, the
dual-axis (xy) motorized stages are used for sample scanning for obtaining an average
temperature, and the goniometer rotates the sample to maintain normal incidence of the
probing laser.
Figure 9 – Suspended bending or uniaxial stretching of flexible or
stretchable semiconductor materials
4.3 Test procedures
4.3.1 General
The test procedure is as follows:
Determine whether the specimen is substrate or thin-film by measuring reflectance as a
function of temperature varying within a range of user’s interest using the setup shown in
Figure 3. The specimen is mounted on two temperature blocks. To minimize the interface
thermal resistance, a thermal paste is gently and uniformly applied to the contact area. If the
result is linear, the specimen can be regarded as a substrate. If not, it shall be treated as a
thin-film.
4.3.2 Substrate specimen
The test procedure for substrate specimen is as follows.
a) Use the result obtained during the determination of substrate or thin-film specimen as a
calibration plot.
b) Configure the substrate specimen as shown in Figure 4. The thermal paste is gently and
uniformly applied to contact areas.
c) Deform the specimen to result in a finite bending or stretching. Then, power up the sample
specimen with a DC supply and wait a while for the specimen to reach a steady-state
temperature.
d) Obtain local temperatures by measuring optical reflectance at discrete points of interest
and using the calibration plot. Use the goniometer to maintain the normal incidence of the
probing laser.
e) Obtain an average temperature by measuring optical reflectance within an area of interest,
obtaining local temperatures with the aid of calibration plot, and calculating the mean
value. Use the goniometer to maintain the normal incidence of the probing laser.
f) Power up the sample specimen with an AC supply (simple periodic sine). Make sure that
the digital multimeter connected to the silicon detector through the variable terminal is
synchronized with the AC supply.

– 14 – IEC 62951-5:2019 © IEC 2019
g) Take reflectance measurements with a time interval (Δt) that is typically equal to or shorter
than 1/500 of the pulse duration. Draw a reflectance vs. time plot and convert it to a
temperature vs. time plot. To obtain the thermal time constant τ, fit the temperature vs.
time plot with
(3)
where T and T are initial and final temperatures, respectively.
i f
h) Unload the sample after any deformation is removed.
4.3.3 Thin-film specimen
The test procedure for thin-film specimen is as follows.
a) Measure reflectance at the midpoint of the suspended sample specimen as a function of
temperature varying within a range of user’s interest at two wavelengths (λ and λ )
1 2
using the setup shown in Figure 5. The local temperature at the midpoint is the average of
the hot and cold temperature blocks. Draw the calibration plot that is the reflectance ratio
( ) vs. temperature graph.
b) Configure the thin-film specimen as shown in Figure 6. The thermal paste is gently and
uniformly applied to contact areas.
c) Deform the specimen to result in a finite bending or stretching. Then, power up the sample
specimen with a DC supply and wait a while for the specimen to reach a steady-state
temperature.
d) Obtain local temperatures by measuring the reflectance ratio at discrete points of interest
and using the calibration plot. Use the goniometer to maintain the normal incidence of the
probing laser.
e) Obtain an average temperature by measuring the reflectance ratio within an area of
interest, obtaining local temperatures with the aid of calibration plot, and calculating the
mean value. Use the goniometer to maintain the normal incidence of the probing laser.
f) Power up the sample specimen with an AC supply (simple periodic sine). Make sure that
the digital multimeter connected to the silicon detector through the variable terminal is
synchronized with the AC supply.
g) Take reflectance measurements at two wavelengths with a time interval (Δt) that is
typically equal to or shorter than 1/500 of the pulse duration. Obtain the reflectance ratio
vs. time plot and convert it to the temperature vs. time plot. To obtain the thermal time
constant τ, fit the temperature vs. time plot with Formula (3).
h) Unload the sample after any deformation is removed.
4.4 Report of results
The report shall include the following items:
a) specimen identification;
b) date of test;
c) atmospheric conditions of test;
d) sample dimension;
e) bending radius or stretching length;
f) local temperature with coordinate;
g) average temperature with area;
h) time constant.
Annex A
(informative)
Example of 3D design of thermoreflectance thermometry
Figure A.1 shows a three-dimensional design of dual wavelength thermoreflectance
thermometry composed of two lasers with different wavelengths that is well suited for thin-film
samples.
IEC
a) Dual wavelength setup
IEC
b) Zoomed-in view
Figu
...