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X-ray photoemission spectroscopy (XPS)
X-ray photoemission spectroscopy (XPS), a.k.a. x-ray photoelectron spectroscopy or electron spectroscopy for
chemical analysis (ESCA) is a surface composition and chemical structure analysis technique. It uses soft x-ray
photons (AlKa, in monochromatised mode) to irradiate sample resulting in the removal of electrons from core
levels within the atoms. The energies of these core levels (binding energies ) are characteristic of a particular
element or its chemical bonding to the others in the sample.
XPS can detect all elements except hydrogen and helium. The practical detection limits are about
0.02 - 0.2 at. % or 1 - 10% of a monolayer. The detection depth of XPS is within 100Å from sample surface for
best results. The best lateral resolution can be 10µm. XPS also has the imaging capability with a lateral
resolution typically 100µm. It is nondestructive but requires a vacuum-compatible sample.
XPS has a broad applications in thin film analysis. Particularly, it can measure the film composition
(except for H and He), film chemical structure, semiconductor electronic structure (such as Si, GaAs and InP),
and film thickness. Polymers structure and its reaction, catalysis and corrosion science are other common
field ideal for XPS. Back
Field emission scanning Aguer electron spectroscopy (FE-AES)
Field emission Auger electron spectroscopy is a surface composition and depth profile analysis technique.
It utilizes focused energetic electron beam to irradiate the sample surface resulting in the emission of Auger
electrons from the sample surface. These Auger electrons have discrete energies which are element dependent.
AES can quantitatively detect all elements except hydrogen and helium. The detection limits are about 0.05 to
0.1 at.% for most of elements. The most advantage aspect for FE-AES is its high lateral resolution in the
neighborhood of 10 to 20 nm. Its imaging capability is also readily available but usually slow in acquisition.
The depth profile is one of the strong capability AES retains. The technique is generally non-destructive but
requires a vacuum compatible sample.
FE-AES has a wide spectrum of applications in analyzing sub-µm features in electronic materials, semiconductor,
information storage devices, and thin film structures. Particularly it is widely used in small defect analyses. It
can also provide limited information for the chemical
structure of the surface materials. Back
Time-of-flight secondary ion mass spectrometry (TOF-SIMS)
TOF-SIMS is a surface composition and structural analytical technique that utilizes a pulsed low intensity ion
beam bombarding the sample surface to generate the secondary ions into a time-of-flight mass spectrometer.
The mass resolution is achieved by dispersing the secondary ions in time according to their velocities. The
spectrum is consisting of both elements and molecular/compound fragments in a range typically of 1 - 1200
atomic mass units at a mass resolution, m/Dm > 6000 and a extremely high sensitivity down to 10 ppm. All
elements in the periodic table can be detected with various sensitivity. Generally, the detection sensitivity
decreases with the increase of the atomic number of the element. TOF-SIMS has a strong imaging capability
that enables simultaneously imaging of more than 10 ions or fragments with a lateral resolution typically of 1 µm.
The technique is generally non-destructive but requires a vacuum compatible sample.
TOF-SIMS technique is frequently combined with XPS to provide the broad spectra of surface information
including, sample surface structure (any solid materials such as polymers, electronic materials, package materials,
minerals, thin film overcoats, organic and biological materials) and its reaction products. The imaging capability
also provides surface distribution of ions and molecular/compound fragment. TOF-SIMS also can provide
ultra-shallow depth profiles to analyze the near surface composition of
the sample. Back
Field
emission scanning electron microscopy (FE-SEM)
with
energy dispersive x-ray analysis (EDX)
Field emission scanning electron microscopy together with the energy dispersive x-ray analysis is a high
magnification surface imaging and composition analysis technique. It utilizes the secondary electrons emitted
from the sample surface induced by an incident focused electron beam. The primary electron beam is raster
scanned on the sample surface while the secondary electrons are used simultaneously to modulate the brightness
of a cathode ray tube screen forming the image. The image quality is usually depends on the surface conditions,
a rougher surface usually yields higher contrast. FE-SEM can provide a lateral resolution about 1 nm. The
element composition analyses are done with energy dispersive x-ray analysis which uses high energetic electron
irradiating the sample surface resulting the emission of the characteristic x-ray for each element. It can measure
elements from boron to uranium with limited quantification capability (~ 5% error).
FE-SEM/EDX is frequently used in fast analysis of the composition of small feature of a material. It usually
has a sampling depth of a few µm. It can provide visual information and a rough composition, which meet most
of the industrial analytical need. FE-SEM also has the imaging capability but it is usually slow in acquisition.
Except for some electron beam damage, it is usually considered as non-destructive. The sample should however
be vacuum compatible.
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Laser Raman spectroscopy measures surface vibrational structures of a materials. A single line of a continuous
gas (such as argon gas) laser excites the vibrational modes backscattered from the sample surface. The lateral
resolution of Raman spectroscopy depends on the microscopic system, usually is about 1µm. The laser sampling
depth is a bout a few µm.
Raman spectroscopy is widely used in chemical finger-printing of a material such as ceramics/glass, fibers and
gels. Thin film structure, stress and sometimes hardness are also applied. The sample can be in any forms as it
operates in air. Raman spectroscopy can also provide imaging capability of a material with a specific structure.
Fourier transform infrared spectroscopy (FTIR)
The Fourier transform infrared spectroscopy (FTIR) measures the sample constituent vibrational motions. The
incident infrared radiation interacts with the sample surface and the detector compares the radiation intensity
before and after it interacts with the sample surface as a function of light frequency. The spectra are usually the
finger-prints of the specific structure of the materials in the sample. It is a compensation to Raman spectroscopy
since some vibrational motions of the materials are infrared active but not Raman active and verse visa.
The FTIR is not element sensitive but used to identify the functional groups/materials in the sample. It probes
a depth of 10 nm to µm in range. There is no specific requirement for sample preparation and solid, liquid or gas
in all forms can be measured. It is frequently used for qualitative and quantitative determination of chemical species
for solid and thin films. However, for quantitative measurement, a standard sample is usually required. It can
also be
used for film stress and structural inhomogeneity measurement.
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The n&k Optical Analyzer basically is an UV-visible spectrophotometer which determine film optical properties
relying on the measurement of the light intensity, through either reflectance or transmittance. Based on measured
light signal, the n&k spectra of a film can be deduced with various models such as Cauchy, Kramers-Kronig,
Forouhi-Bloomer etc.. Whenever accurate measurement of film thickness, spectra of optical constant (n and k),
energy bandgap (Eg) are required, the n&k Optical Analyzer determines these quantities simultaneously.
The n&k spectra range 190 to 1000 nm (extrapolated range: 100 - 3000 nm) and can measure film thickness:
1 to 20000 nm. The analytical
size is about 1 mm with a requirement for sample size > 1 cm x 1 cm.
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Atomic force microscopy measures surface microscopic morphology and properties of a solid material. The
topographic image is obtained by the interaction of a fine tip and a solid surface. Depending on the sample
surface condition, the tip either can contact the analyzed surface at a given force or does not contact the
surface. In the contact mode, a very soft cantilever is used to sense the change of the surface morphology
while in the non-contact mode, a stiff cantilever oscillated near its resonant frequency is applied. AFM
provides a lateral resolution in nanometer scale and a depth resolution about 0.1 nm. AFM can also provide
limited information about the material structure such as phases and chemical properties.
AFM is generally used for the analysis of surface morphology with very high resolution. Particularly, it can
provide information for the surface modification process. Almost all materials can be measured without
specific sample preparations. But samples with a very rough surface
are usually not suitable candidate. Back
High power optical microscopy can capture contrast details from most surfaces. The current state-of-art
microscope can provide sufficient image brightness and detailed micro-features can be readily resolved at a
resolution down to micrometer scale. The microscope can also be interfaced to a high resolution color video
camera that allows television display and digitize the video images for further analysis.
The optical microscope is used for many initial investigations of surface- related problems prior to more
sophisticated analyses, and sometimes it can provide enough visual insight of the samples under investigation.
Transmission electron microscopy (TEM)
Transmission electron microscopy measures thin film atomic structure and morphology with high lateral
resolution. The monoenergetic highly focused electron beam bombard and propagate through the sample.
The interaction and scattering between the incident electron beam and the sample forms film image and the
diffracted electrons forms the pattern corresponding to the sample crystalline structure. TEM can have the
lateral resolution upto 0.2nm. It requires a special sample preparation process and the sample has to be vacuum
compatible.
TEM is frequently used for thin film crystalline structure measurement. The cross-section of the sample can
also provide the accurate film thickness
data with a 0.2nm resolution. Back
Rutherford backscattering spectrometry (RBS)
Rutherford backscattering measures thin film (multilayered thin films) structure, composition and constituent
distribution. Ion beams (protons, deuterons, alpha particles) typically with MeV energy are supplied by an
accelerator and interact with the sample and the backscattered ions are analyzed by the charged particles detectors.
By comparing the energy loss of the incident ion beams and the backscattered ion beams, element composition
and profile as well as film structure can then be determined. The depth resolution of RBS is typically about
1 to 2 nm and the detection limit for the element from 0.5 to 5 at%. RBS can detect elements from lithium to
uranium. It is generally considered a non-destructive analytical technique with limited radiation damage. The
sample must be vacuum compatible.
RBS is frequently used in multilayered film structure, crystal structure and impurity distribution analyses.
Applications includes electronic materials doping, diffusion, thin film composition and interface diffusion.
Radiation damage is also a routine application. Back
Nuclear reaction analysis (NRA)
Nuclear reaction analysis measures thin film composition and constituent distribution. It utilizes the high
energetic ion beams impinge onto the sample surface initializing the nuclear reaction. The particles yielded
from the reaction are then analyzed by a particle detector. Compared to the Rutherford backscattering
spectrometry, the NRA is more accurate particularly for the films composing of light elements on a substrate
composing of heavier elements. The detection limit of NRA is about 0.1 to 1 at.% and elements from lithium
to uranium can be detected. NRA also has depth resolution by calculation of the energy loss of the incident
ion beams.
NRA is generally considered a non-destructive analytical technique with limited radiation damage. The sample
must be vacuum compatible. NRA is frequently used for thin film or multilayered thin film composition
measurements. Back
Dynamic secondary ion mass spectroscopy (SIMS)
Dynamic secondary ion mass spectroscopy measures the depth distribution of the constituent elements or impurities
in a sample. A primary ion beams (such as oxygen) bombard the sample surface resulting in sputtering of the materials
from the sample. The materials from the sample in the ionic form (secondary ions) are accelerated into a mass
spectrometer and analyzed. SIMS can measure all elements in the periodic table. It has very high sensitivity in
the ppb to ppm range) but usually a standard sample with known concentration is required for calibration. It also has
the imaging capability with a lateral resolution from 10nm to 1µm. The depth resolution is about 1 to 2 nm. SIMS is
a destructive analytical technique and the sample must be vacuum compatible.
SIMS is frequently used for depth profile of trace elements, such as
dopant, and constituent elements. Back
X-ray diffraction measures the crystal structure and thin film structure. A collimated x-ray beam is incident onto the
sample and is diffracted by the crystalline phase of the sample. The intensity of the diffracted x-ray is measured against
the diffraction angle and the sample orientation. By measuring the position of the pattern, the crystalline structure
and the corresponding materials can be determined. XRD can detect all elements but the sensitivity for light element
usually is low. The detection limit is about 0.1 to 2 at% depending on the x-ray intensity. XRD is a non-destructive
technique. In most cases, there is no requirement for the sample except for the synchrotron applications where vacuum
compatible sample may be required.
XRD is frequently used to analyze single or polycrystalline thin films. It can determine the crystal orientation, strain,
defects, atomic arrangement and crystal size. It can also determine the atomic arrangement in an amorphous material
and measure film thickness.
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