NIST Next-Generation Nanometrology research.
Nanometrology is a subfield of metrology, concerned with the science of measurement at the nanoscale
level. Nanometrology has a crucial role in order to produce
nanomaterials and devices with a high degree of accuracy and reliability
in nanomanufacturing.
A challenge in this field is to develop or create new measurement
techniques and standards to meet the needs of next-generation advanced
manufacturing, which will rely on nanometer scale materials and
technologies. The needs for measurement and characterization of new
sample structures and characteristics far exceed the capabilities of
current measurement science. Anticipated advances in emerging U.S.
nanotechnology industries will require revolutionary metrology with
higher resolution and accuracy than has previously been envisioned.
Introduction
Control
of the critical dimensions are the most important factors in
nanotechnology. Nanometrology today, is to a large extent based on the
development in semiconductor technology. Nanometrology is the science of measurement
at the nanoscale level. Nanometer or nm is equivalent to 10^-9 m. In
Nanotechnology accurate control of dimensions of objects is important.
Typical dimensions of nanosystems vary from 10 nm to a few hundred nm
and while fabricating such systems measurement up to 0.1 nm is required.
At nanoscale due to the small dimensions various new physical
phenomena can be observed. For example, when the crystal size is smaller
than the electron mean free path the conductivity
of the crystal changes. Another example is the discretization of
stresses in the system. It becomes important to measure the physical
parameters so as to apply these phenomena into engineering of
nanosystems and manufacturing them. The measurement of length or size,
force, mass, electrical and other properties is included in
Nanometrology.
The problem is how to measure these with reliability and accuracy. The
measurement techniques used for macro systems cannot be directly used
for measurement of parameters in nanosystems. Various techniques based
on physical phenomena have been developed which can be used for measure
or determine the parameters for nanostructures and nanomaterials. Some
of the popular ones are X-Ray diffraction, transmission electron microscopy, High Resolution Transmission Electron Microscopy, atomic force microscopy, scanning electron microscopy, field emission scanning electron microscopy and Brunauer, Emmett, Teller method to determine specific surface.
Nanotechnology is an important field because of the large number
of applications it has and it has become necessary to develop more
precise techniques of measurement and globally accepted standards. Hence
progress is required in the field of Nanometrology.
Development needs
Nanotechnology can be divided into two branches. The first being molecular nanotechnology which involves bottom up manufacturing and the second is engineering nanotechnology
which involve the development and processing of materials and systems
at nanoscale. The measurement and manufacturing tools and techniques
required for the two branches are slightly different.
Furthermore, Nanometrology requirements are different for the
industry and research institutions. Nanometrology of research has
progressed faster than that for industry mainly because implementing
nanometrology for industry is difficult. In research oriented
nanometrology resolution is important whereas in industrial
nanometrology accuracy is given precedence over resolution.
Further due to economic reasons it is important to have low time costs
in industrial nanometrology it is not important for research
nanometrology. The various measurement techniques available today
require a controlled environment like in vacuum, vibration
and noise free environment. Also, in industrial nanometrology requires
that the measurements be more quantitative with minimum number of
parameters.
Standards
International standards
Metrology standards
are objects or ideas that are designated as being authoritative for
some accepted reason. Whatever value they possess is useful for
comparison to unknowns for the purpose of establishing or confirming an
assigned value based on the standard. The execution of measurement
comparisons for the purpose of establishing the relationship between a
standard and some other measuring device is calibration. The ideal
standard is independently reproducible without uncertainty. The
worldwide market for products with nanotechnology applications is
projected to be at least a couple of hundred billion dollars in the near
future.[citation needed] Until recently, there almost no established internationally accepted standards for nanotechnology related field. The International Organisation for Standardization TC-229 Technical Committee on Nanotechnology recently published few standards for terminology, characterization of nanomaterials and nanoparticles using measurement tools like AFM, SEM, Interferometers,
optoacoustic tools, gas adsorption methods etc. Certain standards for
standardization of measurements for electrical properties have been
published by the International Electrotechnical Commission.
Some important standards which are yet to be established are standards
for measuring thickness of thin films or layers, characterization of
surface features, standards for force measurement at nanoscale,
standards for characterization of critical dimensions of nanoparticles
and nanostructures and also Standards for measurement for physical
properties like conductivity, elasticity etc.
National standards
Because
of the importance of nanotechnology in the future, countries around the
world have programmes to establish national standards for nanometrology
and nanotechnology. These programmes are run by the national standard
agencies of the respective countries. In the United States, National Institute of Standards and Technology
has been working on developing new techniques for measurement at
nanoscale and has also established some national standards for
nanotechnology. These standards are for nanoparticle characterization, Roughness Characterization, magnification standard, calibration standards etc.
Calibration
It is difficult to provide samples using which precision instruments can be calibrated at nanoscale. Reference or calibration
standards are important for repeatability to be ensured. But there are
no international standards for calibration and the calibration artefacts
provided by the company along with their equipment is only good for
calibrating that particular equipment. Hence it is difficult to select a
universal calibration artefact using which we can achieve repeatability
at nanoscale. At nanoscale while calibrating care needs to be taken for
influence of external factors like vibration, noise, motions caused by thermal drift and creep and internal factors like the interaction between the artefact and the equipment which can cause significant deviations.
Measurement techniques
In
the last 70 years various techniques for measuring at nanoscale have
been developed. Most of them based on some physical phenomena observed
on particle interactions or forces at nanoscale. Some of the most
commonly used techniques are Atomic Force Microscopy, X-Ray Diffraction,
Scanning Electron Microscopy, Transmission Electron Microscopy, High
Resolution Transmission Electron Microscopy, and Field Emission Scanning
Electron Microscopy.
Block Diagram of atomic force microscope.
Atomic force microscopy
(AFM) is one of the most common measurement techniques. It can be used
to measure Topology, grain size, frictional characteristics and
different forces. It consists of a silicon cantilever with a sharp tip
with a radius of curvature of a few nanometers. The tip is used as a
probe on the specimen to be measured. The forces acting at the atomic
level between the tip and the surface of the specimen cause the tip to
deflect and this deflection is detected using a laser spot which is
reflected to an array of photodiodes.
Diagram of Scanning tunneling microscope.
Scanning tunneling microscopy (STM) is another instrument commonly
used. It is used to measure 3-D topology of the specimen. The STM is
based on the concept of quantum tunneling. When a conducting tip is
brought very near to the surface to be examined, a bias (voltage
difference) applied between the two can allow electrons to tunnel
through the vacuum between them. Measurements are made by monitoring the
current as the tip's position scans across the surface, which can then
be used to display an image.
Another commonly used instrument is the scanning electron
microscopy (SEM) which apart from measuring the shape and size of the
particles and topography of the surface can be used to determine the
composition of elements and compounds the sample is composed of. In SEM
the specimen surface is scanned with a high energy electron beam. The
electrons in the beam interact with atoms in the specimen and
interactions are detected using detectors. The interactions produced are
back scattering of electrons, transmission of electrons, secondary
electrons etc. To remove high angle electrons magnetics lenses are used.
The instruments mentioned above produce realistic pictures of the
surface are excellent measuring tools for research. Industrial
applications of nanotechnology require the measurements to be produced
need to be more quantitative. The requirement in industrial
nanometrology is for higher accuracy than resolution as compared to
research nanometrology.
Nano coordinate measuring machine
A coordinate measuring machine
(CMM) that works at the nanoscale would have a smaller frame than the
CMM used for macroscale objects. This is so because it may provide the
necessary stiffness and stability to achieve nanoscale uncertainties in
x,y and z directions. The probes for such a machine need to be small to
enable a 3-D measurement of nanometre features from the sides and from
inside like nanoholes. Also for accuracy laser interferometers need to
be used. NIST has developed a surface measuring instrument, called the
Molecular Measuring Machine. This instrument is basically an STM. The
x- and y-axes are read out by laser interferometers. The molecules on
the surface area can be identified individually and at the same time the
distance between any two molecules can be determined. For measuring
with molecular resolution, the measuring times become very large for
even a very small surface area. Ilmenau Machine is another nanomeasuring
machine developed by researchers at the Ilmenau University of
Technology.
Dimensional metrology using CMM.
The components of a nano CMM include nanoprobes, control hardware, 3D-nanopositioning platform, and instruments with high resolution and accuracy for linear and angular measurement.
List of some of the measurement techniques
Traceability
In
metrology at macro scale achieving traceability is quite easy and
artefacts like scales, laser interferometers, step gauges, and straight
edges are used. At nanoscale a crystalline highly oriented pyrolytic graphite (HOPG), mica or silicon
surface is considered suitable used as calibration artefact for
achieving traceability. But it is not always possible to ensure
traceability. Like what is a straight edge at nanoscale and even if take
the same standard as that for macroscale there is no way to calibrate
it accurately at nanoscale. This so because the requisite
internationally or nationally accepted reference standards are not
always there. Also the measurement equipment required to ensure
traceability has not been developed. The generally used for traceability
are miniaturisation of traditional metrology
standards hence there is a need for establishing nanoscale standards.
Also there is a need to establish some kind of uncertainty estimation
model. Traceability is one of the fundamental requirements for
manufacturing and assembly of products when multiple producers are
there.
Tolerance
Integrated circuit made using monolithic integration technique.
Tolerance
is the permissible limit or limits of variation in dimensions,
properties, or conditions without significantly affecting functioning of
equipment or a process. Tolerances are specified to allow reasonable
leeway for imperfections and inherent variability without compromising
performance. In nanotechnology the systems have dimensions in the range
of nanometers. Defining tolerances at nanoscale with suitable
calibration standards for traceability is difficult for different nanomanufacturing methods. There are various integration techniques developed in the semiconductor industry that are used in nanomanufacturing.
Integration techniques
- In hetero integration direct fabrication of nanosystems from compound substrates is done. Geometric tolerances are required to achieve the functionality of the assembly.
- In hybrid integration nanocomponents are placed or assembled on a substrate fabricating functioning nanosystems. In this technique, the most important control parameter is the positional accuracy of the components on the substrate.
- In monolithic integration all the fabrication process steps are integrated on a single substrate and hence no mating of components or assembly is required. The advantage of this technique is that the geometric measurements are no longer of primary importance for achieving functionality of nanosystem or control of the fabrication process.
Classification of nanostructures
There
are a variety of nanostructures like nanocomposites, nanowires,
nanopowders, nanotubes, fullerenes nanofibers, nanocages,
nanocrystallites, nanoneedles,
nanofoams, nanomeshes, nanoparticles, nanopillars, thin films,
nanorods, nanofabrics, quantumdots etc. The most common way to classify
nano structures is by their dimensions.
SEM of nanowire.
Dimensional classification
| Dimensions | Criteria | Examples |
|---|---|---|
| Zero-dimensional (0-D) | The nanostructure has all dimensions in the nanometer range. | Nanoparticles, quantum dots, nanodots |
| One-dimensional (1-D) | One dimension of the nanostructure is outside the nanometer range. | Nanowires, nanorods, nanotubes |
| Two-dimensional (2-D) | Two dimensions of the nanostructure are outside the nanometer range. | Coatings, thin-film-multilayers |
| Three-dimensional (3-D) | Three dimensions of the nanostructure are outside the nanometer range. | Bulk |
Classification of grain structure
Nanostructures
can be classified on the basis of the grain structure and size there
are made up of. This is applicable in the cas of 2-dimensional and
3-Dimensional Nanostructurs.
Surface area measurement
For nanopowder to determine the specific surface area the B.E.T. method is commonly used. The drop of pressure of nitrogen in a closed container due to adsorption of the nitrogen
molecules to the surface of the material inserted in the container is
measured. Also, the shape of the nanopowder particles is assumed to be
spherical.
- D = 6/(ρ*A)
Where "D" is the effective diameter, "ρ" is the density and "A" is the surface area found from the B.E.T. method.
