Energy Dispersive X-Ray Spectroscopy (2)

Elemental Mapping - Characteristic x-ray intensity is measured relative to lateral position on the sample. Variations in x-ray intensity at any characteristic energy value indicate the relative concentration for the applicable element across the surface. One or more maps are recorded simultaneously using image brightness intensity as a function of the local relative concentration of the element(s) present. About 1 µm lateral resolution is possible.

Line Profile Analysis - The SEM electron beam is scanned along a preselected line across the sample while x-rays are detected for discrete positions along the line. Analysis of the x-ray energy spectrum at each position provides plots of the relative elemental concentration for each element versus position along the line.

TYPICAL APPLICATIONS

• Foreign material analysis
• Corrosion evaluation
• Coating composition analysis
• Rapid material alloy identification
• Small component material analysis
• Phase identification and distribution

SAMPLE REQUIREMENTS

Samples up to 8 in. (200 mm) in diameter can be readily analyzed in the SEM. Larger samples, up to approximately 12 in. (300 mm) in diameter, can be loaded with limited stage movement. A maximum sample height of approximately 2 in. (50 mm) can be accommodated. Samples must also be compatible with a moderate vacuum atmosphere (pressures of 2 Torr or less).

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Energy Dispersive X-Ray Spectroscopy (1)

DESCRIPTION OF TECHNIQUE

Energy Dispersive X-Ray Spectroscopy (EDS or EDX) is a chemical microanalysis technique used in conjunction with scanning electron microscopy (SEM). (See Handbook section on SEM.)The EDS technique detects x-rays emitted from the sample during bombardment by an electron beam to characterize the elemental composition of the analyzed volume. Features or phases as small as 1 µm or less can be analyzed.

When the sample is bombarded by the SEM's electron beam, electrons are ejected from the atoms comprising the sample's surface. The resulting electron vacancies are filled by electrons from a higher state, and an x-ray is emitted to balance the energy difference between the two electrons' states. The x-ray energy is characteristic of the element from which it was emitted.

The EDS x-ray detector measures the relative abundance of emitted x-rays versus their energy. The detector is typically a lithium-drifted silicon, solid-state device. When an incident x-ray strikes the detector, it creates a charge pulse that is proportional to the energy of the x-ray. The charge pulse is converted to a voltage pulse (which remains proportional to the xray energy) by a charge-sensitive preamplifier. The signal is then sent to a multichannel analyzer where the pulses are sorted by voltage. The energy, as determined from the voltage measurement, for each incident x-ray is sent to a computer for display and further data evaluation. The spectrum of x-ray energy versus counts is evaluated to determine the elemental composition of the sampled volume.

ANALYTICAL INFORMATION

Qualitative Analysis - The sample x-ray energy values from the EDS spectrum are compared with known characteristic x-ray energy values to determine the presence of an element in the sample. Elements with atomic numbers ranging from that of beryllium to uranium can be detected. The minimum detection limits vary from approximately 0.1 to a few atom percent, depending on the element and the sample matrix.

Quantitative Analysis - Quantitative results can be obtained from the relative x-ray counts at the characteristic energy levels for the sample constituents. Semi-quantitative results are readily available without standards by using mathematical corrections based on the analysis parameters and the sample composition. The accuracy of standardless analysis depends on the sample composition. Greater accuracy is obtained using known standards with similar structure and composition to that of the unknown sample.

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Auger Electron Spectroscopy Part 2

ANALYTICAL INFORMATION

Survey Scan - The position of the peaks in the AES spectrum obtained in a survey scan identifies the elemental composition of the uppermost 20 Å of the analyzed surface.

Multiplex Scan - A higher resolution analysis of the Auger spectrum in the region around a characteristic peak is used for determination of the atomic concentration of the elements identified in the survey scans and, in some cases, chemical state information.

Quantitation - The AES analysis results can be quantified without standards by using the area under the peaks in the AES spectrum and corrections based on elemental sensitivity factors.

Mapping and Line Scans - These are imaging techniques that measure the lateral distribution of elements on the surface. The electron beam is scanned across the sample surface, either along a fixed line (line scan) or across a given area (mapping) while the AES signal is analyzed for specific energy channels. The AES signal intensity is a function of the relative concentration of the element(s) corresponding to the selected energy channel(s). Spatial resolution is approximately 0.3 µm.

Depth Profile - Material is removed from the surface by sputtering with an energetic ion beam concurrent with successive AES analyses. This process measures the elemental distribution as a function of depth into the sample. Depth resolution of < 100 Å is possible.

TYPICAL APPLICATIONS

Microscopic particle identification
Passive oxide film thickness
Contamination on integrated circuits
Quantitation of light element surface films
Mapping spatial distribution of surface constituents

SAMPLE REQUIREMENTS

Samples should be no larger than approximately 3/4 in. by 1/2 in. (18 mm by 12 mm). Height of samples should not exceed 1/2 in. (12mm). Samples must be conductive or area of interest must be properly grounded. Insulating samples, including thick insulating films (>300 Å), cannot be analyzed. Samples must also be compatible with a high vacuum environment (<1x10^-9 style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 318px; height: 198px;" src="http://3.bp.blogspot.com/_C_N3x2dSff0/SdwG6dQXqnI/AAAAAAAAAIc/Nt8Nqh3e2L0/s320/8.JPG" alt="" id="BLOGGER_PHOTO_ID_5322136461000551026" border="0">

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Auger Electron Spectroscopy Part 1

DESCRIPTION OF TECHNIQUE

Auger Electron Spectroscopy (AES) provides information about the chemical composition of the outermost material comprising a solid surface or interface. The principal advantages of AES over other surface analysis methods are excellent spatial resolution (< 1 µm), surface sensitivity (~20 Å), and detection of light elements. Detection limits for most elements range from about 0.01 to 0.1 at%.

AES uses a primary electron beam to excite the sample surface. When an inner-shell electron is ejected from a sample atom by the interaction with a primary electron, an electron from an outer shell fills the vacancy. To compensate for the energy change from this transition, an Auger electron or an xray is emitted. For light elements, the probability is greatest for the emission of an Auger electron, which accounts for the light-element sensitivity for this technique.

The energy of the emitted Auger electron is characteristic of the element from which it was emitted. Detection and energy analysis of the emitted Auger electrons produces a spectrum of Auger electron energy versus the relative abundance of electrons. Peaks in the spectrum identify the elemental composition of the sample surface. In some cases, the chemical state of the surface atoms can also be determined from energy shifts and peak shapes.

Auger electrons have relatively low kinetic energy, which limits their escape depth. Any Auger electrons emitted from an interaction below the surface will lose energy through additional scattering reactions along its path to the surface. Auger electrons emitted at a depth greater than about 2 - 3 nm will not have sufficient energy to escape the surface and reach the detector. Thus, the analysis volume for AES extends only to a depth of about 2 nm. Analysis depth is not affected by the energy of the primary electron energy.

The AES instrumentation can include a tungsten filament or field emission electron gun for the primary electron beam. The instruments are equipped for secondary electron imaging (SEM) to facilitate location of selected analysis areas, and micrographs of the sample surface can be obtained. The sample chamber is maintained at ultrahigh vacuum to minimize interception of the Auger electrons by gas molecules between the sample and the detector. Some instruments include special stages for fracturing samples to examine interfaces that have been freshly exposed within the vacuum chamber. A computer is used for acquisition, analysis, and display of the AES data.

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Atomic Force Microscopy Part 2

Image Analysis - Since the images are collected in digital format, a wide variety of image manipulations are available for AFM data. Quantitative topographical information, such as lateral spacing, step height, and surface roughness are readily obtained. Images can be presented as two-dimensional or three-dimensional representations in hard copy or as digital image files for electronic transfer and publication.

Nanoindentation - A specialized probe tip is forced into the sample surface to obtain a measure of the material’s mechanical properties in regions as small as a few nanometers. (See the Handbook section on Nanoindentation Hardness Testing.)

TYPICAL APPLICATIONS

• 3-dimensional topography of IC device
• Roughness measurements for chemical mechanical polishing
• Analysis of microscopic phase distribution in polymers
• Mechanical and physical property measurements for thin films
• Imaging magnetic domains on digital storage media
• Imaging of submicron phases in metals
• Defect imaging in IC failure analysis
• Microscopic imaging of fragile biological samples
• Metrology for compact disk stampers

SAMPLE REQUIREMENTS

No sample preparation is typically required. Samples can be imaged in air or liquid. Sample height is limited to about 1.5 inches. Areas up to 8 inches in diameter can be fully traversed without repositioning. Larger samples can be fixtured for imaging within a limited area. Total surface roughness in the image area should not exceed about 6 µm.

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