Laboratory Techniques
Auger Electron Spectroscopy (AES)

Auger Electron Spectroscopy (AES), also referred to as Scanning Auger Microscopy (SAM), uses a focused electron beam to create Auger electrons near the surface of a solid sample. These electrons have energies characteristic of the elements and, in some cases, chemical bonding of the atoms in the sample. AES can provide elemental maps, and when combined with sputtering, gives elemental and chemical information as a function of depth (depth profiling). Due to the nature of the Auger electron emission process, AES can detect all elements except H and He. It is usually nondestructive, except when depth profiling. Concentrations as low as 0.1 atom% can be detected. The sampling depth is 15-30 monolayers (50-100Å) and it can distinguish features as small as 0.1 µm, AES dos elemental analysis of unknown conductive materials well.

Scanning Auger Microprobes
Our Physical Electronics Mode 610 Scanning Auger Microprobe provides scanning Auger microscopy (SAM), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS). The system uses AES to perform compositional analysis and compositional depth profiling of selected elements by recording the Auger intensities as a function of energy or sputtering time, respectively. SAM uses AES in a raster scanning mode to provide an image of the lateral distribution of elements on the sample surface. SIMS detects secondary ions sputtered from the surface to identify elements or molecular species by their mass-to-charge rations. SIMS analyses can be performed in either static or dynamic mode.

The Model 610 is often used in cases where fine x-y spatial resolution (300Å) is needed. The Model 610 can operate at lower current levels than other electron beam instruments. This is advantageous when analyzing nonconductive samples. Additionally, the system has EBIC (electron beam induced current), EBIV (electron beam induced voltage) and AVC (Auger voltage contrast) functions for performing microelectrical analysis.

Auger Electron Spectra
Auger spectra identify surface constituents or determine the composition of specific surface features. Spectra can be generated in two ways: the point analysis mode, where a stationary beam probes a selected point, or the area-averaged mode where the beam is rastered over the surface. With either method, spectral data are collected and stored in the N(E) format, i.e., the electron energy analyzer is stepped through a selected energy range and the electron signal level at each step is measured and stored. It can be scanned repetitively to obtain the desired signal level. Mathematical routines such as differentiation, smoothing, expansion and background subtraction can then be applied to the stored data. A multiplex format, where selected energy regions containing Auger peaks of interest are scanned, is also available. It is used for automatic data acquisition and quantification, e.g., sputter depth profiling, using computer routines and elemental sensitivity factors derived from standards. Auger analysis performed simultaneously with ion beam sputter etching provides information on elemental composition as a function of depth into the material and is termed "compositional depth profiling."

Multiple-Point Analysis
Multiple-point analysis is appropriate when several features within the image area are of interest. Using an image stored on the storage monitor as a guide and the scanning system controls, the coordinates for up to twenty separate points within the imaged area are selected by the operator and stored in memory. The operator then specifies acquisition parameters and the system automatically sequences through the points, performs the analysis, and stores the data for processing and printout. 

Auger Line Scans
An Auger line scan shows the relative concentration of a specific element along a line across the specimen. The electron beam is stepped point-by-point along a selected line. The Auger peak height and background level are measured and the difference stored. The system can scan up to five vertical or horizontal lines and will monitor up to ten elements per line in a single sequence. Stored data can be printed or superimposed on a photomicrograph. Line scans can also be corrected for topography using a special computer algorithm.

Auger Compositional Maps
An Auger image, or elemental map, shows the surface distribution of an element. An elemental map is obtained by setting the analyzer to a specific Auger peak energy and scanning the electron beam over the selected sample area point-by-point. The peak intensity above the adjacent background at each point is measured and stored for later output. The matrix size a s specified by the operator can be up to 250 lines with up to 250 points per line. The stored image can be photographed with a 4-to 256-level gray-scale format or a pseudo-color format.

Auger Line-Shape Analysis
Auger line-shape analysis involves the resolution of observed changes in KVV, LVV, MVV, NVV Auger transitions due to different chemical environments in a material. AES data is recorded in N(E) mode and corrected for secondary electron background as well as inelastic backscatter. Curve fitting is then accomplished by using a weighted-least-squares fit of a Gaussian function to the data values by means of stepwise Gauss-Newton iterations. A Gaussian fit to the actual data is used since the data are taken in a pulse count mode and the pulse-height distribution is Gaussian. In most cases, theoretical Auger line shapes are used as models for the curve fit parameters.

Auger Voltage Contrast
Auger voltage contrast (AVC) involves the measurement of energy shifts (about 0.5 eV) of Auger transitions associated with an internal potential at a p-n junctions. A narrow energy window is set on the slow-energy slope of an Auger peak while the electron beam is scanned across the sample in the Auger compositional mapping mode. The small energy differences are measured and displayed as a surface potential map of the p-n junction. This technique requires an angle lapped surface to be useful.

Back-Scattered Electron Images
Back-scattered electron (BSE) images are useful for locating phase differences and finding specific features on a sample. When a high energy electron interacts with an atom, it undergoes either inelastic scattering with the nucleus. BSEs result from elastic scattering and leave the sample with energies comparable to the primary beam. The likelihood of backscattering increases with the atomic number (Z) of the material. High-Z materials give a stronger signal (brightness) than low-Z materials, thereby giving image contrast from elemental differences.

Applications
The Auger techniques described above may be applied to a wide range of problems including the study of:

  • surface chemistry
  • depth profiling
  • corrosion
  • contamination
  • diffusion studies
  • grain boundary segregation
  • interfacial contamination
  • oxide thickness
  • layers in digital storage media
  • biomaterials
  • semiconductors
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X-ray Photoelectron Spectroscopy (XPS)

In XPS, also known as Electron Spectroscopy for Chemical Analysis (ESCA), X-rays bombard a sample creating ionized atoms and ejecting free electrons. The energies of these free electrons are related to their binding energies in the original atom. By measuring these characteristic energies, XPS identifies the chemical elements present in the sample. XPS provides both elemental and, to a certain extent, chemical information in the top 3-30 atomic layers (10-100Å) in solid samples. The sensitivity varies between 0.01-1 atom% dependent upon the element. It can do nondestructive depth profiling to 100 Å and detect all elements except H and He. Ion sputtering combined with XPS is used to accomplish deeper profiling. XPS is especially good for obtaining elemental surface composition of unknown materials, including conductors and insulators.

Critical problem solving with surface analysis is enhanced by reducing the probe area when using XPS. Small-spot XPS instruments probe for composition, chemistry, and contamination in 0.01 mm2 areas. It also makes XPS sputter depth profiles a reality.

One of the primary reasons for using XPS to analyze samples is its inherent high surface sensitivity. This results from the fact that nearly all of the electrons which are used for analysis escape from only the outermost four to five atomic layers of the material. This high surface sensitivity permits the easy detection of most surface concentrated elements that would be undetectable by bulk or quasi-bulk techniques, e.g. XRD, XRF, EDS or Electron Microprobe. Remember, chemistry begins at the surface.

Imagine that a sample surface is contaminated by 20% coverage of Si from a silicone lubricant. Using XPS, the Si atoms represent ~6% of the atoms present in the 4-atom deep sampling volume. However, by using one of the bulk or quasi-bulk techniques, the Si atoms now represent ~0.03% or less of the >1 µm deep sampling volume. Given that surface Si concentrations as low as 0.10.% can be detected, the advantage of XPS over bulk techniques is readily apparent.

One very important reason for using XPS is that it is nondestructive. XPS uses very soft (low energy) x-rays that produce minimum energy input to the sample during analysis. Electron beam analysis techniques concentrate a high amount of energy in a small region and can be very destructive toward organic materials or other thermally sensitive compounds. Bulk analysis techniques often require that the sample be powdered and placed in a matrix material introducing a high probability of altering or entirely losing some surface species.

In addition to providing a detailed elemental surface composition, XPS provides even more information about the detected elements. Changes in the chemical environment or oxidation state of an atom can cause corresponding changes in the energies of the electrons that are ejected and analyzed. These energy shifts or "chemical shifts" have been well studied and tabulated for many different compounds. By measuring these shifts, it is possible in most cases to accurately assign the chemical environment of a given element.

Another important advantage of XPS over electron beam techniques, i.e. AES, Electron Microprobe, etc., is its ability to analyze insulating specimens with relative ease. Since the analysis beam (x-rays) does not consist of charged particles, the insulating specimen is not required to conduct away any charge buildup due to incidence of the analysis beam itself. The specimen is only required to conduct away enough charge to compensate for the small number of electrons which were ejected from the sample. This small positive charge buildup is easily compensated for by use of a "flood gun", which directs low energy electrons to the sample surface.

In addition to the inherent advantages of using XPS generally, the small-spot instrument that Rocky Mountain Laboratories' employs has a number of special features that give an enormous edge over other instruments. The sample transfer and sample chamber configuration allows the analysis of samples a s large as 3.75" diameter x 0.375" high. Or, many specimens may be mounted and measured by software automation, if they are of uniform size and shape. The minimum size is limited only by the size of the smallest x-ray beam (50 µm), which has been used to analyze a single
10 µm organic fiber.

The largest x-ray spot (image of the x-ray beam on the sample) is 1-2 mm and is used primarily for rapid data acquisition during survey scans. The smallest x-ray spot is most often used for analysis of small heterogeneous features on a larger sample or simply for analysis of a very small sample. Because the x-ray spot is smaller than in other XPS instruments, remarkably rapid and precise depth profiles are now routine, since both raster size and beam voltage of the ion etching gun can be greatly reduced.

Rocky Mountain Laboratories' XPS provides:

  • high surface sensitivity
  • nondestructive analysis
  • chemical bonding information
  • insulating specimen capability
  • small x-ray spot
  • rapid, precise depth profiles
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Secondary Ion Mass Spectrometry (SIMS)

In Secondary Ion Mass Spectrometry (SIMS), a solid sample is bombarded with a narrow beam of primary ions that are energetic enough to cause ejection (sputtering) of secondary ions, neutral atoms, electrons, and photons. A mass spectrometer separates the secondary ions, neutral atoms, electrons, and photons. A mass spectrometer separates the secondary ions according to their mass-to-charge ratio (referred to as m/e and expressed in amu) and counts them. The m/e suggests the element or compound based on atomic or molecular weight, and the counts give information about the concentration. Since the sputtering process inherently erodes the sample, dynamic SIMS (DSIMS) provides useful depth profiling data during the analysis. SIMS can detect every element in the periodic table with detection limits well below the ppm range. It is a destructive technique. In static mode (SSIMS) the analysis depth is one to three monolayers or less than 30 angstroms.

There are several types of mass spectrometers used in SIMS. The two most common types in commercially available instruments are the quadrupole mass filter and the time-of-flight (TOF) mass analyzer. Both can be used for SSIMS or DSIMS.

The quadrupole operates by applying RF and DC potentials to a set of four rods, which causes the ions to be separated by their mass as they travel through the quadrupole. The voltages can be changed quickly which allows relatively rapid scanning of the mass range. Although typically limited to about 1,000 amu, instruments with much higher mass ranges have been built. Quadrupoles have only moderate mass resolution, which generally allows them to separate nominal mass numbers.

The TOF analyzers require the primary ion beam to be pulsed prior to striking the sample. The extracted secondary ions travel through a drift tube to the detector. Mass separation is accomplished by noting that ions having different masses take different times to reach the detector, e.g., lighter ions take less time to traverse the drift tube than heavier ions. With this method of mass separation, the entire spectrum can be collected in a few microseconds. TOF's main advantage is that it can measure masses up to thousands of amu.

The primary difference between SSIMS and DSIMS is the rate of sputtering utilized. In SSIMS the objective is to analyze only the top few atomic monolayers of material and to minimize sample damage. This is accomplished with a lower primary beam current and energy The chemical integrity of the surface is maintained during analysis such that whole molecular or characteristic fragment ions are removed from the surface and measured in the mass spectrometer. This provides a chemical rather than elemental characterization of the true surface. SSIMS is often used in conjunction with X-Ray Photoelectron Spectroscopy (XPS or ESCA), which provides chemical bonding information. The two techniques combined can yield a complete picture of the molecular composition of the sample surface.

In DSIMS, a focused energetic ion beam is used to sputter material from a specific location on the surface. The most common application of DSIMS is depth profiling of elemental dopants and contaminants in materials at trace levels. DSIMS provides little or no chemical or molecular information because of the violent sputtering process. It provides a measurement of the elemental impurity as a function of depth with very low detection limits. 
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Scanning Electron Microscopy (SEM)

In the SEM, an electron beam is focused into a fine probe and rastered over a small rectangular area. As the electrons interact with the sample, various signals, including those from secondary electrons, are created and detected. These highly localized signals are used to modulate the brightness of a CRT display rastered synchronously with the electron beam. The image formed on the CRT is a highly magnified image of the sample. The technique is usually nondestructive, although instrumental sample requirements often mandate alteration of the sample before analysis. Magnification is from 10X to 300,000X and lateral resolution is 10-50nm, allowing features as small as 100 Å to be seen. The analysis depth is 20-50 Å. SEM provides excellent topographical data, and in backscatter mode atomic number information (compositional) also. Images can be e-mailed or delivered on various storage media for client manipulation. 
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Energy Dispersive Spectroscopy (EDS)

EDS, also called Energy Dispersive X-ray Spectroscopy (EDX), is a technique based on the collection and energy dispersion of X-rays created when high energy electrons bombard a sample. The EDS is attached to the Scanning Electron Microscope (SEM) and the two techniques are often used together. The X-rays have energies that are characteristic of the elements in the sample. The instrument's electronics process the signals to give histograms of energy vs. signal strength, the latter being related to relative concentration. EDS can also provide elemental maps of the sample that can be compared to the electron micrographs. The method detects all elements from boron to uranium and is usually nondestructive. Detection limits range from 0.05-2% for high to low Z elements, respectively. Features as small as 1 µm. EDS is particularly good for relating elemental composition to topographical features. 
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Stylus Profilometry

In this technique, roughness and the sizes of surface features are measured by the mechanical movement of a diamond stylus over the sample. The trace of the surface is digitized and stored in a computer for display on a CRT and output to a printer. The stylus force can be adjusted to protect fragile surfaces from damage. The Stylus Profilometer can measure laterally to 1 µm and vertically to 5 Å with effective magnification of 20-100,000X. It provides good two dimensional topographical data and is nondestructive. 
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Optical Microscopy

This technique uses direct visual observation of a sample with white light through a series of lenses. Resolution of about 0.2 µm can be achieved. Morphology, color, opacity, and optical properties are often sufficient to characterize a material. It is nondestructive. 
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Scanning Probe Microscopy (SPM), Atomic Force Microscopy (AFM)

Description

Scanning Probe Microscopy (SPM) refers to a group of techniques used to measure mechanical, magnetic, electrical, and electrochemical surface topography on a nanometer scale. It can measure features from as small as interatomic spaces to a tenth of a millimeter. The three dimensional resolution of SPM is unparalleled by any other technique.

The ability to measure surfaces in three dimensions provides information that cannot be obtained with optical or typical electron microscopes. Unlike many stylus profilometers that only scan in a straight line, SPMs raster over a square area and give better and more precise topographical data.

SPMs can be operated in air and liquids. This allows for the examination of a very extensive range of materials and environments, generally with no sample preparation. Examinations of biological materials can be carried out in appropriate solutions to preserve their vital characteristics.

Applications

SPM is applied to problems in many industries from space materials testing to semiconductor fabrication to compact disk manufacturing. A few examples include:

  • silicon wafer roughness
  • compact disk texture
  • polymer membrane imaging
  • biological structure imaging
  • cancer research
  • biomedical pathology
  • immunoassay process development
  • thin film and coating
  • integrated optics
  • particle/surface forces
  • nucleation
  • contact lenses
  • catalysis
Types of data/output:
  • high resolution color images
  • surface roughness
  • grain size
  • cross sections
  • fractal roughness
  • bearing ratios
  • microsurface area
  • particle counting
Data can be supplied as hard copy or in digital files for electronic transfer and further processing by the client.

Sample Requirements

SPM can analyze solids, including biomaterials. Generally , no costly sample preparation or coating is required, since both conductive and nonconductive samples can be imaged. Usually, smooth samples are best and those that have macroscopic roughness with greater that 5 µm maximum relief cannot be analyzed. Sample sizes up to 8 inches in diameter may be analyzed.

To obtain the most accurate results, surface features should not be mobile, because the stylus could potentially move them giving erroneous position information.

Analysis times are in the order of thirty to sixty minutes dependent upon sample characteristics. Since SPM is a nondestructive technique, Rocky Mountain Laboratories can combine it with other analytical methods for subsequent analysis on the same sample to provide complementary information. 
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Fourier Transform Infrared (FTIR) Spectroscopy

Infrared spectroscopy is the study of interactions between matter and electromagnetic radiation. Atoms in molecules and crystals continuously vibrate with natural frequencies in the range of 1013 to 1014 cycles per second, which is the frequency of infrared radiation. Vibrations which are accompanied by a change in dipole moment cause absorption of infrared radiation. Several vibration modes may occur for a particular atomic group, each at a particular frequency which is normally independent of the other modes. If the amount of radiation absorbed by a substance is plotted against the incident wavelength, the resulting graph reflects the presence of specific chemical bonds and can therefore be used for structural identification. Whereas spectra associated with atoms are caused by electrons moving from one electronic energy level to another, inter-atomic spectra are usually characterized by either bond stretching or bending vibration modes. In addition, to these fundamental absorption bands, there are multiples of the fundamental frequencies (overtones) and frequencies which are the sum and difference of two or more fundamental frequencies, called combination lines. Wavelength is conventionally expressed in microns (µ) and frequency in wavenumber - the number of cycles per centimeter, with units of cm-1.

Infrared spectroscopy is probably the most powerful single technique available to qualitatively identify organic materials and to determine molecular structure. Mass spectrometry gives the molecular weight and formula, and nuclear magnetic resonance the number and type of protons, but only infrared indicates in a direct manner the presence of key functional groups. A match between the infrared spectra of an unknown sample and a reference sample is a simple, and almost positive, method of identification. Currently, over 130,000 reference spectra are available, most of which are available in indexed form. Spectroscopic theory will be explained in terms of very simple molecules, and then extensions will be briefly indicated to use with construction composites.

from: Composite Construction Materials Handbook, Nicholls, R. Prentice-Hall, Inc., Englewood Cliffs, NJ, 1976, pp. 528

Currently we match with a library of ~ 250,000 spectra.  
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NanoScope™ and TappingMode™ are registered trademarks of Digital Instruments, Inc.

Rocky Mountain Laboratories is currently upgrading all of our laboratory equipment. Thus, not all the techniques listed above may be available in-house, at any given time.