Materials Characterization
Page II: Scanning Probe Microscopy
Page III: Non-Destructive Testing
Materials Characterization represents many different disciplines depending upon the
background of the user. These concepts range from that of the scientist, who thinks of it
in atomic terms, to that of the process engineer, who thinks of it in terms of properties,
procedures, and quality assurance, to that of the mechanical engineer, who thinks of it in
terms of stress distributions and heat transfer. The definition selected for the ASMInternational Materials Characterization Handbook is as follows: "Characterization
describes those features of composition and structure (including defects) of a material
that are significant for a particular preparation, study of properties, or use, and suffice for
reproduction of the material." This definition limits the characterization methods included
in the Handbook [and this web page] to those that provide information about
composition, structure, and defects and excludes those methods that yield information
primarily related to materials properties, such as thermal, electrical, and mechanical
properties.
Taken from Metals Handbook Ninth Edition, Volume 10: Materials Characterization,
ASM Handbook Committee, ASM-International, Metals Park,
Ohio.
An important component of the materials engineering
methodology involves knowledge of the structure of materials.
Typical structure is necessarily viewed through a microscope....
an optical microscope, the electron microscope (imaging of
electrons passed through a thin specimen in the transmission
electron microscope TEM; or imaging by collecting electrons
emitted from the surface of the material of interest in the
scanning electron microscope SEM). Here are some structure
related Internet images:
Here is a typical optical microscope. The principles are the same
for the operation of a SEM or TEM; except the light source for
an electron microscope is an electron gun, and the lenses are electromagnetic (they are
not made from optical-grade glass). Electron microscopes require a vacuum column
because electrons readily interact with air molecules and would become rapidly adsorbed.
The reference source URL for the optical microscope image was the Lucent Technologies
Microscapes site, which is no longer available. Do visit the Bell Labs web pages to see
related images.
On the upper right we see a photomicrograph of pyramid-shaped etch pits on the surface
of a silicon wafer. What is the reason for formation of an 'etch-pit?' Well, there are
defects in crystalline materials called dislocations. Where dislocations intersect a surface,
a feature called an etch-pit can form. The pit will form if atomic
impurities in the crystal are segregated to the dislocation (by thermal
treatment) and if the specimen is subsequently etched (ie, the surface
chemically attacked with an appropriate chemical etchant). The
reference source URL is the Bell Labs web pages.
This is a scanning electron micrograph (left) of a dendrite. When
solid metal grows from the liquid state, a tree-like feature grows
which is called a dendrite. The dendrite is, essentially, a single crystal feature. The
growth of dendrites occurs when liquid metals solidify, and the phenomenon is analogous
to the process of formation of ice crystals from supersaturated water vapor, for example.
For a most interesting look at the dendrite growth process, visit this Cal Tech web site.
Here is a scanning electron micrograph of a device region (right) on an integrated circuit.
The white features are metallization lines. Here, we are looking normal to the surface of
the silicon wafer and see the top-level of connective lines. In the next image, we will see
a cross-section through metallization lines. The reference source URL is Philips
Research, part of Philips Electronics.
Next, let us consider a cross-section through an integrated
circuit (IC). View the series of images below.
First, to get a good idea as to the various layers that makeup a modern IC, go to the following LINK where a
schematic of a typical cross-section of a complementary
metal oxide semiconductor (CMOS) device is presented
(origin is Texas Instruments). For cross-sectional views and
an operational description of six different IC devices, visit
these International Rectifier web pages.
The SEM image below, left, is an actual cross-section of a modern IC device showing the
various metallization lines (yellow) embedded in passivation material (silicon nitride, for
example). Being able to deposit a number of planar layers has made possible the scalingdown of the size of device regions and made possible more punch per chip! The reference
source URL is IBM Corporation.
On the right is an SEM section through 4 MB DRAM (Dynamic Random Access
Memory) cells. The section was made by a new instrument called the Focused Ion Beam
Microscope. It is capable of performing corrective micro-surgery on IC chips. The
reference source for this cross-section is FIB International of Santa Clara, CA.
To see impressive detail within the FIB section, click here.
If you have further interest into the structure and operation of IC devices, I encourage you
to visit the Semiconductor Applet Service web pages.
This is a polycrystalline array of grains in an
aluminum metallization line on an integrated
circuit. It is a color photomicrograph. Each
colored grain is a single crystal of aluminum,
oriented differently with respect to its
neighbors. The reference source URL is the
Bell Labs web page.
Photomicroscopy is an important tool in the
characterization of engineering materials.
Another excellent Internet source of solidstate photomicrographs is the Manchester Materials Science Center (at the University of
Manchester; UK) site. Link to MATERIALS to view various micrographs over a range of
magnifications. Link to FEATURES to view special features which help the materials
engineer to determine the type of alloy and its process history, and thus to help predict
performance in service.
The scanning electron micrograph (left) shows a typical TiC
coating of 150 micrometers on Poco AXF-5Q graphite. These
TiC coatings feature both an equiaxed structure adjacent to
the coating-substrate interface, and columnar grains away
from the interface. Additionally, these coatings adhere well
to their graphite substrates even after fracture. Thicknesses
for these coatings can be controlled from a few micrometers
to 250 micrometers. The reference source URL is Lanxide
Coated Products
If you wish to know more about materials characterization, I invite you to turn to PAGE
II. PAGE II will introduce you to other examples of materials characterization, including
scanning probe microscopy, which offers the possibility to view, and manipulate,
individual atoms and molecules on surfaces. Then, there is PAGE III. Page III will
introduce you to Non-Destructive Testing, useful methods to find and characterize
internal flaws within a material, for example. Make sure to visit these two additional
pages.
The materials engineer is often involved in a function called failure analysis. Materials
characterization is an important component of any failure analysis. For further
information about this most interesting aspect of materials engineering, please visit the
Failure Analysis web pages at this site.