Scanning Probe Microscopy (SPM)

Exploring Materials Engineering



Materials Characterization: PAGE II

Scanning Probe Microscopy

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 ASM-International 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.

One way to study the structure of materials is to use the Transmission Electron Microscope (TEM), discussed on PAGE I . It is called a transmission electron microscope because the electron beam used to create the image is transmitted through the specimen of interest. Shown below is a HREM (high-resolution electron micrograph) of ordered icosahedra in an annealed TiMn quasicrystal. The image results from 10-fold scattering centers orthorhombically arrayed in an image 64 Å wide. A single atom is only about 1.2 Å in diameter! Until recently, icosahedra were thought to be an "impossible" building block of solid, crystalline materials. Check out the other images and information on the reference URL, University of Missouri-St. Louis, St. Louis, Missouri.



Another way to image individual atoms (or at least an image caused by the electrical field that surrounds individual atoms in a crystal) is by use of the Field-Ion Microscope (FIM). This very simple microscope generates projected images of the fine tip of a single crystal, shaped much like the scanning probe microsope (SPM) tips described below. Here is an image of the tip of a tungsten single crystal. What is observed is a bunch of bright spots. These correspond to positions on the tip where the electric field is particularly high - where the local radius of curvature is particularly small. This happens near atomic steps and adatoms (ie, atoms sitting on the surface of the tip). So the projected image allows us to see 'atoms', sort of as a 'ball model' of the tip of the tungsten. A source of FIM images is in an article titled Modeling of Atomic Imaging and Evaporation in the Field Ion Microscope. in the Journal of Sensors.


Another new and powerful characterization tool to help the materials engineer better meet materials requirements through structural interpretation and manipulation, is the Scanning Probe Microscope (SPM). Actually, there are a variety of modes by which one can perform SP microscopy, each giving a "different" look at the material. One mode, atomic force microscopy (AFM) using the constant-height mode, allows atomic resolution, and this mode will first be introduced. Another mode, magnetic force microscopy (MFM), will then be demonstrated.

Before we look at images created by SPM's, let us consider a schematic of the basic instrument. This is illustrated below. A basic tutorial explaining the operation of the Atomic Force (or Scanning Probe) microscope is available at University of Illinois at Urbana-Champaign web pages.

An image of the tip used to generate images using the scanning probe microscope (SPM) is shown below. The reference URL is no longer available; but you may refer to the Applied Physics and Soft Matter Group web pages in Germany. Consult the Nanosensors web page for more views of specific SPM probes! For those of you who recall seeing a phonograph needle, the probe is very similar. Yes? Maybe just a tad smaller. Look at the scale marker on this image! The tip is somewhat less than 2 micro-meters long. About how thick is it? What is your best guess at the tip-radius? Now are you beginning to understand why SPM's are capable of resolving atomic structure?

One more point. How would you manufacture such a small needle? This topic too is the realm of the materials engineer. Maybe you will find some helpful "tips" in the Nanosensors .pdf file.



Just like the TEM, SPM's are capable of resolving individual atoms on the surface of a crystalline solid. This is illustrated (left) by an image that shows a 60 Å by 60 Å area (recall an atom is only about 1 Å to 3 Å in diameter!) of KCl. AFM imaging with atomic resolution opens up the field to the investigation of insulators as well as conductors and semiconductors. For a collection of AFM/SPM images, visit the NT-MDT gallery.

It is often the case that a more macro-structural (or further back) view will be helpful to predict or alter the properties or performance of a material. Some examples of macroscopic characterization using the SPM are illustrated below.

For magnetic imaging, the probe has a magnetic tip. The specimen is moved in a raster pattern by the piezoelectric scanner, and the specimen's stray fields cause shifts in the resonance of the cantilevered probe that are measured by laser detection. Typical SPM, using the magnetic imaging mode are offered, below. For more about magnetic-force microscopy, download this Nanosensors .pdf file.

The Magnetic Force Microscopy (MFM) is an ideal way to characterize magnetic media in data storage devices (like the magnetic recording layer on a hard-disk of the disk drive on a computer). For example, by scanning a tiny ferromagnetic probe over a specimen, MFM maps the stray magnetic fields close to the specimen surface. In the MFM image below, overwritten tracks on a textured hard disk are shown. The topography (left) was imaged using what is termed the TappingMode; the magnetic force image of the same area (right) was captured with LiftMode (lift height 35 nm) by mapping shifts in cantilever resonant frequency. The LiftMode is obviously sensitive to the magnetic properties of the "track" regions: the TappingMode is not. The reference URL for this image pair was Veeco.com; but the image-source is no longer available. Visit the Park Systems Image Gallery

for a collection of AFM images of various subjects.

Track width and skew, transition irregularities, and the difference between erased and virgin areas are visible in the above image. Note that the area scanned is only 25µm by 25µm or about one-fifth the area of a hair on your head!

You may not be aware of how digital data are 'stored' on recording media. For magnetic films deposited on thin substrates, where the film thickness is less than 500 Angstroms (about 250 atoms thick!), data-bits can form as triangular features. The 'properties' of these individual bits depends on how close they are to each other. The accompanying figure illustrates 'uniformity' of the magnetic bits when the spacing is 'reasonable' (left); and interactions that interfere with proper data recording when the triangular domains are 'written' too close to each other. These are MFM images of magnetic domains. Read more about this by visiting this British link.

As you might imagine, the SPM is an excellent tool to "profile" the surface topography of a specimen. A great example of this is the surface topography of a SRAM (Static Random Access Memory) area of an IC (integrated circuit) on a Si (silicon) wafer. What you see below is an AFM representation of the TEOS (tetraethylorthosilicate) surface at the center of the SRAM after a bit of CMP (chemical-mechanical polishing). The TEOS film, which is essentially glass, acts as an insulator of the embedded metallization lines within and has been deposited by CVD (chemical vapor deposition). To see a cross-section of typical embedded lines in an IC device, select this link .

By the way, you had better get used to acronyms if you plan to follow an engineering or science career path!

It should be noted that, as with most AFM images, the z-scale is expanded relative to the x- and y- scales to take advantage of the high vertical resolution of the measurement. For more information about AFM, SPM, and CMP, please go to the Park Systems web pages and be sure and visit the Park Systems image gallery.

If you wish to return to PAGE I of Materials Characterization, click here.

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.

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Please send any comments to Patrick P. Pizzo, Professor Emeritus of Materials Engineering
Created by Dr. Pizzo on October 18, 1997.
Last Revision, January 03, 2015