IEC TS 62607-6-5:2022

Nanomanufacturing - Key control characteristics - Part 6-5: Graphene-based materials - Contact and sheet resistance: transmission line measurement

IEC TS 62607-6-5:2022(E) establishes a standardized method to determine the key control characteristics
contact resistance, and
sheet resistance  for graphene-based materials and other two-dimensional materials by a
transmission line measurement.  The method uses test structures applied to the 2D material by photolithographic methods consisting of several metal electrodes with increasing spacing between the electrodes. By a measurement of the voltage drop between different pairs of electrodes, sheet resistance and contact resistance can be calculated.
The method can be applied to any other two-dimensional materials which are subject to electrical metal contact on top of the materials.
The method provides accurate and reproducible results, if the electrical contact formed between the two-dimensional material and the metal electrodes provides ohmic contact property.

General Information

• Status: Published
• Publication Date: 13-Dec-2022
• ICS: 07.120 - Nanotechnologies
• ICS: 17.140.20 - Noise emitted by machines and equipment
• ICS: 97.040.50 - Small kitchen appliances
• Technical Committee: TC 113 - Nanotechnology for electrotechnical products and systems
• Drafting Committee: PT 62607-6-5 - TC 113/PT 62607-6-5
• Current Stage: PPUB - Publication issued
• Start Date: 13-Jan-2023
• Completion Date: 14-Dec-2022

Standards Content (Sample)

IEC TS 62607-6-5
®

Edition 1.0 2022-12
TECHNICAL
SPECIFICATION

colour
inside


Nanomanufacturing – Key control characteristics –
Part 6-5: Graphene-based materials – Contact resistance and sheet resistance:
transmission line measurement
IEC TS 62607-6-5:2022-12(en)

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IEC TS 62607-6-5

®


Edition 1.0 2022-12




TECHNICAL



SPECIFICATION








colour

inside










Nanomanufacturing – Key control characteristics –

Part 6-5: Graphene-based materials – Contact resistance and sheet resistance:

transmission line measurement

























INTERNATIONAL

ELECTROTECHNICAL


COMMISSION





ICS 07.120 ISBN 978-2-8322-6224-5




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


® Registered trademark of the International Electrotechnical Commission

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– 2 – IEC TS 62607-6-5:2022 © IEC 2022
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
3.1 General terms . 7
3.2 Key control characteristics . 9
4 General . 10
4.1 Measurement principle . 10
4.2 Recommended sample preparation method. 11
4.3 Recommended measurement equipment and apparatus . 12
5 Measurement of sheet resistance and contact resistance . 14
5.1 Recommended measurement procedure . 14
5.2 Suggested I-V measurement to ensure ohmic contact . 15
5.3 Constraint in using TLM for Schottky contact devices . 16
6 Results to be reported (case studies) . 16
6.1 Measured results of the contact resistance and sheet resistance of graphene . 16
6.2 Measured results of the contact resistance of MoS . 18
2
6.3 TLM patterns for the bilayer graphene and the results obtained by the four-
point probe method . 18
Annex A (informative) Measurement results and the simulation results from various
setups . 20
Bibliography . 23

Figure 1 – TLM pattern structure . 10
Figure 2 – Determination of contact resistance and sheet resistance . 11
Figure 3 – Optical microscopy pictures of rectangular TLM channels and differently
spaced TLM electrodes defined by electron beam lithography . 12
Figure 4 – TLM structure and its equivalent circuit of two-point probe (2PP) TLM for
contact resistance and sheet resistance. 12
Figure 5 – Schematic view of four-point probe (4PP) TLM pattern for measuring
contact resistance and sheet resistance. 13
Figure 6 – Experimental setup for contact resistance and sheet resistance
measurements . 14
Figure 7 – Output curves drawn for total current (I ) as a function of voltage applied
ds
(V ) for different spacings between electrodes from a bilayer graphene TLM pattern . 15
ds
Figure 8 – Output curves drawn for total current (I ) as a function of voltage applied
ds
(V ) for different spacings between TLM electrodes from a MoS TLM pattern . 15
ds 2
Figure 9 – Resistances of graphene measured by TLM . 16
Figure 10 – Schematic views of the graphene TLM pattern where L > L > L > L >
5 4 3 2
L and optical microscopy images of the TLM pattern . 17
1
Figure 11 – Contact resistance of MoS device fabricated by forming benzyl viologen
2
(BV) polymeric interlayer. 18
Figure 12 – TLM patterns for the bilayer graphene and the results obtained by the four-
point probe method . 19

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IEC TS 62607-6-5:2022 © IEC 2022 – 3 –
Figure A.1 – Comparison of TLM two-point probe and four-point probe setups . 21

Table 1 – Contact resistance and sheet resistance of graphene, obtained from
different plasma etching conditions . 17
Table A.1 – Contact resistance measurement by TLM and conventional four-point

probe methods based upon the case study of Figure 12 . 20
Table A.2 – Sheet resistance measurement by TLM and conventional four-point probe
methods based upon the case study of Figure 12 . 21
Table A.3 – Simulation of contact resistance and sheet resistance results from TLM
two-point probe and four-point probe methods . 22

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INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________

NANOMANUFACTURING – KEY CONTROL CHARACTERISTICS –

Part 6-5: Graphene-based materials – Contact resistance and sheet
resistance: transmission line measurement

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
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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.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
IEC TS 62607-6-5 has been prepared by IEC technical committee 113: Nanotechnology for
electrotechnical products and systems. It is a Technical Specification.
The text of this Technical Specification is based on the following documents:
Draft Report on voting
113/677/DTS 113/709/RVDTS

Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Technical Specification is English.

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IEC TS 62607-6-5:2022 © IEC 2022 – 5 –
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement,
available at www.iec.ch/members_experts/refdocs. The main document types developed by
IEC are described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC TS 62607 series, published under the general title
Nanomanufacturing – Key control characteristics, 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 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 document 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.

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– 6 – IEC TS 62607-6-5:2022 © IEC 2022
INTRODUCTION
Technical Specifications for contact resistance and sheet resistance of two-dimensional
materials provide a proper definition of contact resistance and sheet resistance measurement
and an electrical characterization of two-dimensional materials. This document includes
recommended conditions for a sample preparation and recommended method to measure
contact resistance and sheet resistance of two-dimensional materials under test in the
referenced background research results. Here, the transmission line measurement (TLM) is
used which had been used to measure both contact resistance and sheet resistance for
conventional bulk semiconductor devices including silicon devices. TLM devices are formed
with various spacings between contacts from which contact resistance and sheet resistance
are determined from voltage measured. Thickness of the atomic thin 2D materials cannot be
defined clearly when the layers are ultrathin near monolayer, and therefore it is difficult to
express the thickness-dependent electronic resistivities of the devices fabricated by using 2D
materials. TLM is used conveniently to determine contact resistance and sheet resistance of
2D materials since it does not require thickness of tested materials to be included in the
calculation procedure.
The objectives of this document are to
a) define the contact resistance and sheet resistance of two-dimensional materials;
b) specify the methodology for contact resistance and sheet resistance measurements of
two-dimensional materials using transmission line measurement (TLM);
c) provide a contact formation method for two-dimensional materials with ohmic contact
property which is an essential prerequisite;
d) establish units for the quantitative characteristics of contact resistance and sheet
resistance for two-dimensional materials;
e) provide relevant case studies;
f) provide relevant references.
This document is meant to be a general document that can be applied to two-dimensional
materials and their applications. It is the intent of this document to be compatible with and
work in conjunction with the performance standards defined in the IEC TS 62607 series.

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IEC TS 62607-6-5:2022 © IEC 2022 – 7 –
NANOMANUFACTURING – KEY CONTROL CHARACTERISTICS –

Part 6-5: Graphene-based materials – Contact resistance and sheet
resistance: transmission line measurement



1 Scope
This part of IEC TS 62607 establishes a standardized method to determine the key control
characteristics
• contact resistance, and
• sheet resistance
for graphene-based materials and other two-dimensional materials by a
• transmission line measurement.
The method uses test structures applied to the 2D material by photolithographic methods
consisting of several metal electrodes with increasing spacing between the electrodes. By a
measurement of the voltage drop between different pairs of electrodes, sheet resistance and
contact resistance can be calculated.
• The method can be applied to any other two-dimensional materials which are subject to
electrical metal contact on top of the materials.
• The method provides accurate and reproducible results, if the electrical contact formed
between the two-dimensional material and the metal electrodes provides ohmic contact
property.
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 General terms
3.1.1
detail specification
DS
specification based on a blank detail specification with assigned values and attributes
Note 1 to entry: The properties listed in the detail specification are usually a subset of the key control
characteristics listed in the relevant blank detail specification. The industrial partners define only those properties
which are required for the intended application.
Note 2 to entry: Detail specifications are defined by the industrial partners. Standards development organizations
will be involved only if there is a general need for a detail specification in an industrial sector.

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– 8 – IEC TS 62607-6-5:2022 © IEC 2022
Note 3 to entry: The industrial partners can define additional key control characteristics if they are not listed in the
blank detail specification.
3.1.2
graphene
graphene layer
single-layer graphene
monolayer graphene
single layer of carbon atoms with each atom bound to three neighbours in a honeycomb
structure
Note 1 to entry: It is an important building block of many carbon nano-objects.
Note 2 to entry: As graphene is a single layer, it is also sometimes called monolayer graphene or single-layer
graphene and abbreviated as 1LG to distinguish it from bilayer graphene (2LG) and few-layer graphene (FLG).
Note 3 to entry: Graphene has edges and can have defects and grain boundaries where the bonding is disrupted.
[SOURCE: ISO/TS 80004-13:2017, 3.1.2.1]
3.1.3
bilayer graphene
2LG
two-dimensional material consisting of two well-defined stacked graphene layers
Note 1 to entry: If the stacking registry is known, it can be specified separately, for example, as "Bernal stacked
bilayer graphene".
[SOURCE: ISO/TS 80004-13:2017, 3.1.2.6]
3.1.4
few-layer graphene
FLG
two-dimensional material consisting of three to ten well-defined stacked graphene layers
[SOURCE: ISO/TS 80004-13:2017, 3.1.2.10]
3.1.5
two-dimensional material
2D material
material, consisting of one or several layers with the atoms in each layer strongly bonded to
neighbouring atoms in the same layer, which has one dimension, its thickness, in the
nanoscale or smaller and the other two dimensions generally at larger scales.
Note 1 to entry: The number of layers when a two-dimensional material becomes a bulk material varies depending
on both the material being measured and its properties. In the case of graphene layers, it is a two-dimensional
material up to 10 layers thick for electrical measurements, beyond which the electrical properties of the material
are not distinct from those for the bulk (also known as graphite).
Note 2 to entry: Interlayer bonding is distinct from and weaker than intralayer bonding.
Note 3 to entry: Each layer can contain more than one element.
Note 4 to entry: A two-dimensional material can be a nanoplate.
[SOURCE: ISO/TS 80004-13:2017, 3.1.1.1]

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IEC TS 62607-6-5:2022 © IEC 2022 – 9 –
3.2 Key control characteristics
3.2.1
key control characteristic
KCC
material property or intermediate product characteristic which can affect safety or compliance
with regulations, fit, function, performance, quality, reliability or subsequent processing of the
final product
Note 1 to entry: The measurement of a key control characteristic is described in a standardized measurement
procedure with known accuracy and precision.
Note 2 to entry: It is possible to define more than one measurement method for a key control characteristic if the
correlation of the results is well-defined and known.
3.2.2
sheet resistance
measure of the resistance of a thin film that is nominally uniform in thickness
Note 1 to entry: Sheet resistance can be measured together with contact resistance by TLM for 2D materials, as
shown in Figure 2.
Note 2 to entry: Sheet resistance is one of a material’s properties. The SI unit of measure of sheet resistance is
the ohm per square (Ω/sq.).
3.2.3
contact resistance
measure of the contribution of the contacting interfaces to the total resistance of thin films
that are nominally uniform in thickness
Note 1 to entry: Contact resistance can be measured together with sheet resistance by TLM for 2D materials, as
shown in Figure 2.
Note 2 to entry: Contact resistance is a property that exists between a metal and a semiconducting (or
conducting) material. Contacts need to supply necessary electrical current.
[SOURCE: Schroder [1], Pages 127, 131]
3.3 Terms related to the measurement method
3.3.1
transfer length
L
T
measuring distance over which most of the current flows from a semiconductor (or conducting
material) into a metal or from a metal into a semiconductor (or conducting material)
[SOURCE: Schroder [1], Page 140]
3.3.2
transmission line measurement
TLM
measuring method to determine sheet resistance of a layer and contact resistance between a
layer and an applied electrode by the formation of a set of electrodes and a measurement of
the voltage drop between the electrodes
Note 1 to entry: In some cases, TLM is used as abbreviation for transfer length measurement, but this represents
the same technique as the transmission line measurement.
[SOURCE: Schroder [1], Pages 139-141]

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4 General
4.1 Measurement principle
TLM involves making a series of contacts separated by various distances. See Figure 1 for
TLM pattern structure. Probes are applied to pairs of contacts, and the resistance between
them is measured by applying a voltage across the contacts and measuring the resulting
current. The current flows from the first probe, into the metal contact, across the metal–
semiconductor junction, through the sheet of semiconductor, across the metal–semiconductor
junction again into the second contact, and from there into the second probe and into the
external circuit to be measured by an ammeter. The resistance measured is a linear
combination (sum) of the contact resistance of the first contact, the contact resistance of the
second contact, and the sheet resistance of the semiconductor between the contacts.

Figure 1 – TLM pattern structure
If several such measurements are made between pairs of contacts that are placed at different
distances, a plot of resistance against contact separation can be obtained. If the contact
separation is expressed in terms of the ratio L/W – where L and W are the length and width of
the area between the contacts – such a plot should be linear, with the slope of the line being
the sheet resistance. See Figure 2 for determination of contact resistance (R ) and sheet
C
resistance (R ).
sh

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IEC TS 62607-6-5:2022 © IEC 2022 – 11 –

Figure 2 – Determination of contact resistance and sheet resistance
NOTE 1 Transfer length (L ) is defined as the distance over which voltage drops to "1/e" compared to the voltage
T
at the edge of the metal contact. That is, electron flow between metal and channel preferentially occurs at the edge
of the metal contact and is reduced exponentially inside the metal in a way to minimize total resistance (R ). [2],
total
1
[3] Particularly, L can be very small for 2D-material-based devices due to its ultra-thinness. L is approximately
T T
characterized by
R LW
C
L = (1)
T
R
ch
where R is the contact resistance and R is the channel resistance.
C ch
NOTE 2 Channel lengths, L , L , …, can be determined by availability of device dimension and capability of
1 2
lithography technology.
NOTE 3 The edge roughness and inhomogeneity of the 2D material can give rise to error of the data.
4.2 Recommended sample preparation method
The TLM pattern shall be designed and implemented by lithography for device fabrication. For
example, a graphene channel width (W) of 30 µm and a channel length (L) in the range from
10 µm to 50 µm, in increments of 10 µm, can be fabricated. For comparison, other patterns
with L = 5 µm, 10 µm, 15 µm, 20 µm and 25 µm can be fabricated at the same time. These
dimensions need to be determined by trial and error to ensure reliability of data, depending on
the parasitic resistances of a tested device and the thickness uniformity of the 2D material
sample. For example, if L is too small, the error range is large. In contrast, if L is too large,
the non-uniformity of the sample can cause deviations from linearity.
For the fabrication of 2D material TLM pattern, 2D materials formed by mechanical exfoliation
or chemical vapour deposition (CVD) are used. To fabricate TLM patterns, the use of
degenerately doped Si wafer covered with thermally grown SiO layer which serves as the
2
global back gate electrode, the gate insulator, and the substrate is recommended. The wafer
is ultrasonically cleaned in acetone and rinsed in isopropyl alcohol to remove chemical
residues before using. The TLM pattern was defined by the general photolithography process
as in the field effect transistor (FET) device fabrication. However, in the case where 2D
material samples are too small for patterning by the general photolithography, electron-beam
lithography (EBL) is commonly conducted using spin-coating of an electron-beam resist layer
onto the 2D material sample. The TLM patterns are formed as rectangular shape by
lithography to define the electronic transport channels in the pattern. This is often referred to
as the channel definition patterning process. Figure 3 a) and Figure 3 b) show optical
microscopy images of a graphene and a MoS TLM pattern, respectively. The metal
2
electrodes are deposited via electron beam evaporation to form TLM structures.
___________
1
 Numbers in square brackets refer to the Bibliography.

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It is recommended to check if the 2D material has been degraded before measurements. One
possible way of checking the degradation of 2D material of the TLM pattern is to find out the
significantly deviating measured points in Figure 2 where the slope line consists of measured
points resulting a linear behaviour. When there is a significantly deviating measured point
from the linear slope line, this indicates that in some parts the 2D material underneath the
metal electrode has been degraded.

a) TLM patterns on graphene b) TLM patterns on MoS
2
Figure 3 – Optical microscopy pictures of rectangular TLM channels
and differently spaced TLM electrodes defined by electron beam lithography
4.3 Recommended measurement equipment and apparatus

Key
V voltmeter
I ammeter
Figure 4 – TLM structure and its equivalent circuit of two-point
probe (2PP) TLM for contact resistance and sheet resistance
Figure 4 shows TLM pattern and its equivalent circuit of two-point probe (2PP) measurement
for contact resistance and sheet resistance. The electrical measurements setup typically
includes a probe station combined with a temperature controller, as shown in Figure 4. The
device under test shall be placed and probed inside the chamber. Since 2D materials are air
and moisture sensitive, it is recommended to measure KCC in a vacuum chamber. For more
general measurement of contact resistance and sheet resistance, a temperature-dependent
test can be performed. In t
...