Capabilities of related techniques are summarized as follows

• X-ray diffraction: Provides bulk crystallographic information

• Optical microscopy: Faster, less expensive, and provides superior image quality on relatively flat samples at less than 300 to 400*

• Scanning transmission electron microscopy, Auger electron microscopy: Compared in Table 2.

• Transmission electron microscopy: Provides information from within the volume of material, such as dislocation images, small angle boundary distribution, and vacancy clusters. Superior resolution, but requires thin samples

Table 2 Comparison summary of scanning electron beam instruments equipped with secondary electron and x-ray detectors

Instrument

Features optimized

Minimum area

Comments

Surface pictures

Microchemical analysis

Scanning electron microscope

Surface pictures: above ^SOOx on polished and etched samples; at all magnifications on high depth-of-field surfaces; accuracy and sensitivity of microchemical analysis

4-5 nm (conventional scanning electron microscope) 2-3 nm (in-lens)

1-3 .'■m (EDS and/or WDS)

Can be equipped with a WDS x-ray detector that maximizes sensitivity and light-element analysis. WDS: elements with Z > 4, EDS: elements with Z>10

Scanning transmission

Small area microanalysis of thin films; small area diffraction

2-3 nm (SEM mode, in-lens)

5-30 nm (EDS)

Samples must be thinned; generally also functions as a transmission

electron microscope

electron microscope; allows chemical analysis of particles characterized by transmission electron microscope observation

Scanning Auger microscope

Chemical analysis of monolayers on surfaces made by in situ fracturing, and low-Z elements on surfaces cleaned by in situ ion etching

■■ 100 nm (Auger) 10 nm (SEM mode)

(Auger) 1-3 /'m (EDS)

Requires UHV and careful surface preparation; can also detect electron loss signal

Signals Generated by the Scanning Electron Beam. A picture of the scanned surface region can be taken using any signal generated by the electron beam. In the scanning Auger microscope, it is possible to use as many as five different signals to generate pictures. To understand the potential utility of these instruments, it is necessary to understand some elementary ideas on the nature of the signal generation when the electron beam interacts with sample surface.

When an electron beam strikes a solid surface, electrons and x-rays are emitted from the surface. The energy distribution of these signals is shown qualitatively in Fig. 2 (electromagnetic radiation of lower energies than x-rays is also emitted, which is termed cathodoluminescence). In addition to the secondary electron detector (which can also be used to detect backscattered electrons), most scanning electron microscopes are equipped with an x-ray detector, and specialized backscattered detectors are available at relatively low cost. The addition of an x-ray detector allows determination of the energy of the emitted characteristic x-ray shown in Fig. 2. Two types of x-ray detectors are used: wavelength-dispersive spectrometers and energy-dispersive spectrometers. These x-ray methods are discussed in the section "Electron Probe X-Ray Microanalysis" in this article.

Fig. 2 Energy distribution of signals generated by the electron beam

In general, the electrons generated by the electron beam can be partitioned into three types: secondary, Auger, and backscattered (Fig. 3). The intensity scale shown in Fig. 3 has been increased to reveal the details not apparent in Fig. 2. The backscattered electrons can be further partitioned into three types:

Type 1: Elastically scattered

Type 2: Plasmon and interband transition scattered

Type 3: Inelastically scattered

Fig. 3 The energy distribution of emitted electrons at (a) low beam energy (around 1 keV) and (b) a higher beam energy (around 5 keV)

Elastically scattered electrons emerge with essentially the same energy as the beam energy; inelastically scattered electrons generally undergo many scattering interactions and emerge with a spectrum of energies lower than the beam energy. The type 2 electrons are scattered by interactions that produce a plasmon oscillation of the electrons in the sample material or a transition of sample electrons between different energy bands.

Comparison of Fig. 3(a) and 3(b) illustrates that the energies of the secondary and Auger electrons are fixed, but the backscattered electrons shift their energy values as the primary beam energy is changed.

Summary. Tables 2 and 3 provide an overview of the three types of scanning electron beam instruments and summarize the source of the signals used. Table 2 compares the scanning transmission electron microscope and scanning Auger microscope with regard to top surface analysis. The scanning transmission electron microscope enables direct probing through the sample. With the scanning Auger microscope, surface films can be probed through by ion sputtering. In general, these instruments are multifunctional.

Table 3 Comparison summary of signals used in scanning electron beam instruments

Signal

Type

Energy

Source

X-ray

Characteristic (fluorescent)

Discrete values; different for each element: Cu Ktï = ~ 8000 eV; Si Ktt = —1800 eV

Interband transitions: L ■ >K

= KCt, M —>K = kA KO!: lose K electron, L ■ >K, photon ejects_

Chemical analysis from micro areas in SEM, STEM, and SAM

Continuous

Continuous

Decleration electron

None (background noise)

Electron Auger

Discrete values; different for each element range: 100-1500 eV; Si LMM = —100 eV; Cu LMM = ~900 eV

Interband transitions; LMM: lose L electron, M ■ >L, M electron ejects

Monolayer surface analysis in SAM

Backscattered (elastic)

Essentially same as beam energy

Beam electron scattered back after elastic collision

Atomic number contrast, channeling contrast, channeling patterns, and magnetic contrast in SEM_

Backscattered (inelastic)

Energies less than beam energy

Beam electron scattered back after inelastic collision

Atomic number contrast, channeling contrast, channeling patterns, and magnetic contrast in SEM_

Backscattered (plasmon and interband transition interactions)

1-1000 eV less than beam energy

Beam electron scattered back after collision producing plasmon oscillations or interband transition

Surface analysis in SAM; light-element analysis in STEM where scattering is in forward direction

Secondary

Loosely bound electrons scattered from surface

Main signal for formation in SEM

image

SEM, scanning electron microscopy; STEM, scanning transmission electron microscopy; SAM, scanning Auger microscopy

To interpret correctly the physical significance of the various signals used in the three scanning electron beam instruments, the volume below the surface from which the signal is originating must be known. In general, it is not possible to analyze quantitatively particles with diameters less than approximately 1 to 2 /' m using the electron probe microanalyzer, although these particles appear to be huge on the scanning electron microscope screen. It is important to realize that the x-ray sample volume and shape vary with the electron beam voltage and the sample atomic number. Table 4 lists some estimates of the sample volume for the remaining signals. The inelastic backscattered electrons emerge from a volume roughly one-half the depth of the scattered electron range. The Auger electrons are collected from sample depths of 0.5 to 3 nm below the surface, depending on their energy. The Auger electron energies are relatively low, and only those electrons near the sample surface escape without suffering additional energy loss.

Table 4 Estimation of the volume of the various signals produced in iron by a 20 keV electron beam

Signal

Approximate

volume dimensions

Diameter

Depth

X-ray

"" 1 !:m

"" 1 !:m

Backscattered electrons

Inelastic (Type 3)

""0.8 .'-m

""0.5 .'-m

Energy loss (Type 2)

»Mb

Elastic (Type 1)

""dB

Auger electrons

■■■:■ 1.1dB

■■■:■ 0.5-2 nm

Secondary electrons

"" 1.2dB

■-■■' 10 nm

0 0

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