Transcript Thin Film Metrology

Thin Film Metrology
Rardchawadee Silapunt
AIT
09/15/2010
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Commonly found metrology
• Scanning Electron Microscopy (SEM)
• Transmission Electron Microscopy (TEM)
• Auger Electron Microscopy (Auger)
• Raman spectroscopy (RAMAN)
• Fourier Transform Infrared Spectroscopy (FTIR)
• Atomic Force Microscopy (AFM)
• Secondary Ion Mass Spectrometry (SIMs)
• Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMs)
• X-ray Photoelectron Spectroscopy (XPS)
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Introduction and History of Electron
Microscopes
• Electron microscopes are scientific instruments that use a beam of
energetic electrons to examine objects on a very fine scale.
• Electron microscopes were developed due to the limitations of Light
Microscopes which are limited by the physics of light.
• In the early 1930's this theoretical limit had been reached and there was a
scientific desire to see the fine details of the interior structures of organic
cells (nucleus, mitochondria...etc.).
• This required 10,000x plus magnification which was not possible using
current optical microscopes.
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Characteristic of information
• Topography: The surface features of an object or "how it looks", its
texture; direct relation between these features and materials properties
• Morphology: The shape and size of the particles making up the object;
direct relation between these structures and materials properties
• Composition: The elements and compounds that the object is composed
of and the relative amounts of them; direct relationship between
composition and materials properties
• Crystallographic Information: How the atoms are arranged in the object;
direct relation between these arrangements and material properties
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Physical comparison of OEM, SEM and
TEM
Note: OEM = Optical microscope
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Scale and Microscopy Techniques
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Electron-Solid Interactions
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Electron-Solid Interactions
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Secondary Electrons
• These electrons arise due to
inelastic collisions between
primary electrons (the beam) and
loosely bound electrons of the
conduction band or tightly bound
valence electrons. The energy
transferred is sufficient to
overcome the work function
which binds them to the solid and
they are ejected.
• The interaction is Coulombic in
nature and the ejected electrons
typically have ≈ 5 -10eV. 50eVis
an arbitrary cut-off below which
they are said to be secondary
electrons.
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Backscattered Electrons
• Backscattered electrons (BSE)
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arise due to elastic collisions
between the incoming electron
and the nucleus of the target
atom (i.e. Rutherford scattering).
Elastic scattering results in little
(< 1eV) or no change in energy of
the scattered electrons, although
there is a change in momentum
(p). Since p =mv and the mass of
the electron doesn’t change, the
direction of the velocity vector
must change. The angle of
scattering can range from 0 to
180°.
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Specimen Interaction Volume
The volume inside the specimen in which interactions occur while
interacting with an electron beam. This volume depends on the following
factors:
•Atomic number of the material being examined; higher atomic number
materials absorb or stop more electrons , smaller interaction volume.
•Accelerating voltage: higher voltages penetrate farther into the sample
and generate a larger interaction volume
•Angle of incidence for the electron beam; the greater the angle (further
from normal) the smaller the interaction volume.
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SEM Mechanisms 1
• Condenser lens* is used to both
form the beam and limit the
amount of current in the beam. It
works in conjunction with the
condenser aperture to eliminate
the high-angle electrons from the
beam.
• A set of coils then "scan" or
"sweep" the beam in a grid
fashion (like a television),
dwelling on points for a period of
time determined by the scan
speed (usually in the
microsecond range).
* This mode is usually controlled by the "coarse and fine probe current knobs".
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SEM Mechanisms 2
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The objective lens focuses the
scanning beam onto the part of the
specimen desired.
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When the beam strikes the sample
(and dwells for a few microseconds)
interactions occur inside the sample
and are detected with various
instruments.
•
Before the beam moves to its next
dwell point these instruments count
the number of e-interactions and
display a pixel on a CRT whose
intensity is determined by this
number (the more reactions the
brighter the pixel).
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A look inside the column
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Electron Detectors and Sample Stage
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Breakdown of SEM
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Sample preparation
• Sample Coating is necessary to reduce charging that
causes:
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Deflection of SE’s
Increased emission of SE’s in cracks
Periodic SE bursts
Beam deflection
• Sputter coating with C, Cr, or Au-Pd
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Gold Sputter
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Go to TEM
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The analogy of TEM
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A TEM works much like a slide
projector. A projector shines a beam
of light through (transmits) the slide,
as the light passes through it is
affected by the structures and
objects on the slide.
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These effects result in only certain
parts of the light beam being
transmitted through certain parts of
the slide. This transmitted beam is
then projected onto the viewing
screen, forming an enlarged image of
the slide.
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TEM Basic Mechanisms
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TEMs work the same way
except that they shine a
beam of electrons (like the
light) through the ultra thin
specimen (like the slide).
Whatever part is
transmitted is projected
onto a phosphor screen for
the user to see.
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Transmission Electron Microscope
(TEM)
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TEMs are capable of imaging at a
significantly higher resolution than
light microscopes, owing to the small
wavelength of electrons. This enables
the instrument to be able to examine
fine detail—even as small as a single
column of atoms, which is tens of
thousands times smaller than the
smallest resolvable object in a light
microscope.
TEM forms a major analysis method
in a range of scientific fields, in both
physical and biological sciences. TEMs
find application in cancer research,
virology, materials science as well as
pollution and semiconductor
research.
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Spec comparison of SEM and TEM*
*Semiconductor measurements and instrumentation by W. R. Runyan, T. J. Shaffner
Most resent research TEMs allow information on features on the scale of 0.1
nm to be obtained (resolutions down to 0.05 nm have been achieved)
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Technique comparison with SEM
• Substantially superior image resolution and contrast
• Need no reference standard or require assumption
or prior knowledge about the sample to get accurate
results
• Do an excellent job on buried layers and small
features
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CMOS Image Comparison with SEM
SEM
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TEM
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Another example of TEM image
Si based memory device
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Sample preparation is suffocating!
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TEM specimens are required to be at most hundreds of nanometres thick,
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High quality samples will have a thickness that is comparable to the mean free
path of the electrons that travel through the samples, which may be only a few
tens of nanometres.
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Preparation of TEM specimens is specific to the material under analysis and the
desired information to obtain from the specimen. As such, many generic
techniques have been used for the preparation of the required thin sections.
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Materials that have dimensions small enough to be electron transparent, such as
powders or nanotubes, can be quickly prepared by the deposition of a dilute
sample containing the specimen onto support grids or films.
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The specimens tend to be naturally resistant to vacuum, but still must be prepared
as a thin foil, or etched so some portion of the specimen is thin enough for the
beam to penetrate. Ion etching is favorable.
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TEM Limitations
• Many materials require extensive sample preparation to produce a
sample thin enough to be electron transparent, which makes TEM analysis
a relatively time consuming process with a low throughput of samples.
• The structure of the sample may also be changed during the preparation
process.
• Also the field of view is relatively small, raising the possibility that the
region analyzed may not be characteristic of the whole sample.
• There is potential that the sample may be damaged by the electron beam,
particularly in the case of biological materials.
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Scanning TEM (STEM)
• A TEM can be modified into a scanning transmission
electron microscope (STEM) by the addition of a
system that rasters the beam across the sample to
form the image, combined with suitable detectors.
Go back
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Auger Spectron Electroscopy (AES)
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AES is an analytical technique used to determine the
elemental composition of top few atomic layers (~2-10 nm
depth) of features as small as ~ 25 nm.
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AES can detect most elements with detection limits of 0.1-1
atomic percent for most species.
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AES is normally used for analyzing semi- and conductive solids
but might be difficult for insulators.
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AES can perform small-area compositional depth profiling
when used in combination with an ion sputter source.
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Basic mechanisms of AES
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Ionization: A hole in an inner shell (here: K
shell) is generated by an incident high-energy
electron that loses the corresponding energy
E transferred to the ejected electron.
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Auger-electron emission: The hole in the K
shell is filled by an electron from an outer
shell (here: L2). The superfluous energy is
transferred to another electron which is
subsequently ejected (here: from the L3 level)
as Auger electron.
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Since Auger electrons have an energy in the
range of some 100 eV to a few KeV, they are
strongly absorbed by the specimen.
Consequently, only Auger electron from the
surface can be measured, making Auger
spectroscopy a surface method.
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Surface sensitivity
• AES is usually performed with
only 3-5 keV incident electron
beam (much lower than SEM
and TEM ‘s)
• The Auger electron carries 601600 eV.
• Auger electron originates
right from the point of the
impact.
• AES is most sensitive to the
low atomic number elements.
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AES image comparison with SEM
• A particle of interest is identified in the SEM micrograph. Auger survey
spectra from areas on and around the particle identify Cu and Ti in the
particle residing on an SiO2 surface. Elemental maps obtained from area
containing the particle identify distinct Cu and Ti phases in the particle
and the Si from the surrounding substrate.
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AES image comparison with EDX
• Link
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Sample preparation
• Samples must be solid and vacuum compatible.
• Insulating samples can be more difficult. Because
Auger is a surface sensitive technique, gold or carbon
coating must be avoided.
• Sample Size: Up to 2" diameter
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Atomic Force Microscopy (AFM)
• The AFM works in the same way as our fingers which touch and probe the
environment when we cannot see it.
• By using a finger to "visualize" an object, our brain is able to deduce its
topography while touching it. The resolution we can get by this method is
determined by the radius of the fingertip.
• To achieve atomic scale resolution, a sharp stylus (radius ~1-2 nm)
attached to a cantilever is used in the AFM to scan an object point by
point and contouring it while a constant small force is applied to the
stylus.
• With the AFM the role of the brain is taken over by a computer, while
scanning the stylus is accomplished by a piezoelectric tube.This simple
technique provides a peep into the microscopic world.
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Atomic Force Microscope
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AFM principles
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A fine stylus is mounted on a cantilever spring and scanned over the surface. At sufficiently
small forces the corrugations of the scanning lines represent the surface topography of the
sample.
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The vertical deflection of the cantilever is detected by reflecting a laser beam onto a 2segment photodiode. The photodiode signal is used to drive a servo system which controls
the movement of the piezo xyz-translator. In this manner the applied force between the
stylus and the sample can be kept constant within some tens of a pico newton.
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Atomic resolution of a mica surface recorded in aqueous solution. The distance between
adjacent protrusions is 5.4 Å.
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AFM probe tip
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Imaging modes
• Static (contact mode)
 the static tip deflection is used as a feedback signal.
Because the measurement of a static signal is prone to
noise and drift, low stiffness cantilevers are used to boost
the deflection signal.
 the force between the tip and the surface is kept constant
during scanning by maintaining a constant deflection.
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Imaging modes
• Dynamic mode
 The cantilever is externally oscillated at or close to its
fundamental resonance frequency or a harmonic.
 The oscillation amplitude, phase and resonance frequency
are modified by tip-sample interaction forces; these
changes in oscillation with respect to the external
reference oscillation provide information about the
sample's characteristics.
 Schemes for dynamic mode operation include frequency
modulation and the more common amplitude modulation.
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Technique comparison with SEM
• Unlike the electron microscope which provides a two-dimensional
projection or a two-dimensional image of a sample, the AFM provides a
true three-dimensional surface profile.
• Samples viewed by AFM do not require any special treatments (such as
metal/carbon coatings) that would irreversibly change or damage the
sample (can analyze any type of samples).
• Most AFM modes can work perfectly well in ambient air or even a liquid
environment.
• In principle, AFM can provide higher resolution than SEM. It has been
shown to give true atomic resolution in ultra-high vacuum (UHV) and,
more recently, in liquid environments. High resolution AFM is comparable
in resolution to TEM.
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Technique comparison with SEM
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A disadvantage of AFM compared with the (SEM) is much
smaller image size.
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An incorrect choice of tip for the required resolution can lead
to image artifacts.
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AFM requires several minutes for a typical scan, while a SEM
is capable of scanning at near real-time (although at relatively
low quality) after the chamber is evacuated. The relatively
slow rate of scanning during AFM imaging makes it less suited
for measuring accurate distances between artifacts on the
image.
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Time-of-Flight Secondary Ion Mass
Spectrometry (TOF-SIMs)
• TOF-SIMS is a surface analytical technique that
focuses a pulsed beam of primary ions onto a sample
surface, producing secondary ions in a sputtering
process.
• It is the mass spectrometry of ionized particles which
are emitted from the surface when energetic
primary particles bombard the surface.
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TOF-SIMs
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TOF-SIMs mechanisms
• The pulsed primary ions with the energy of 1-25 keV, typically liquid metal
ions such as Ga+, Cs+ and O-, are used to bombard the sample surface,
causing the secondary elemental or cluster ions to emit from the surface.
• The secondary ions are then electrostatically accelerated into a field-free
drift region with a kinetic energy.
• The ion with lower mass has higher flight velocity than one with higher
mass. Thus they will reach the secondary-ion detector earlier.
• The mass separation is obtained in the flight time from the sample to the
detector.
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TOF-SIMs mechanisms
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Attractive Capabilities of TOF-SIMs
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Identifying the elemental composition and the chemical status near the surface
(around 5 angstrom) with high sensitivity (~1ppm) and high mass resolution
(~9000).
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Distinguishing the different isotopes of the same element.
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Imaging the topography of surface using the secondary electrons.
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Line-scanning of chemical species.
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Mapping chemical species on the submicron scale.
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Ultra-thin depth profiling.
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Database of the compound spectra.
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Identifying automatically peaks with the database of fragments.
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Attractive Capabilities of TOF-SIMs
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The ToF-SIMS can be used for surface analysis of inorganic, organic materials and
biological cells, applied to conductors, insulates and semiconductors.
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Strength of TOF-SIMs Analysis
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Specific molecular information on thin (sub-monolayer)
organic films/contaminants
Surface analysis that allows more complete characterization
of a surface
Excellent detection limits (ppm) for most elements
Quantitative element analysis of Si and GaAs
Probe size ~0.2 µm for imaging
Insulator and conductor analysis
Non-destructive
Depth profiling is possible
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Limitations of TOF-SIMs Analysis
• Usually not quantitative without standards
• Samples must be vacuum compatible
• It can be too surface sensitive:
 Sample packaging and prior handling may impact quality
of results
 Analysis order is important, surface-damaging tests should
be done after TOF-SIMS
• Very surface specific—only examines top couple of
monolayers
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Differences of TOF-SIMS from other
Surface Analysis Techniques
• TOF-SIMS, AES, XPS, and FTIR
• AES can provide better spatial resolution images for
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elemental species with poorer sensitivity.
XPS provides quantitative concentrations and
chemical bonding information that is not normally
obtained directly by TOF-SIMs.
FTIR has a greater information depth.
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The END
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