Scanning beam-specimen

In a confocal microscope, invented in the mid-1950s, a focused spot of light scans the specimen. The fluorescence emitted by the specimen is separated from the incident beam by a dichroic mirror and is focused by the objective lens through a pinhole aperture to a photomultiplier. Fluorescence from out-of-focus planes above and below the specimen strikes the wall of the aperture and cannot pass through the pinhole (Figure 11.3). 

The instmment shown in Figure 2.4 is a double-axis instrument. The first axis is the adjustment of the beam conditioner, the second is the scan of the specimen through the Bragg angle. It is irrelevant to this definition that a practical diffractometer may contain a dozen or more controlled axes , for example, to tune and to align the beam conditioner, to locate the specimen in the beam, to align and to scan the specimen and to control shts. It is the differential movement of the two main axes that make the measnrement and determine the precision and accuracy of the instrument. This is the basic high resolntion diffractometer, which is now widely nsed for measniements of crystal perfection, epilayer composition and thickness. 

After calibration, a block-shear specimen was positioned in the profilometer. Two stepper motors, controlled by timed relays, were used to maneuver the specimen under the laser. Specimen position was measured by two linear variable differential transformers (LVDTs), one placed on each axis. A data acquisition system was configured to capture sensor outputs at the rate of 30 Hz. Initially the specimen was scanned across the non-bonded portion of the adherend (Fig. 5). About 1000 sensor readings were acquired for each profile (or about one reading per 25 pm). The specimen was then advanced 1 mm lengthwise and ain scanned across its width. This process was repeated up to 25 times to define a precise beam grid (Fig. 5) for scanning all specimens. 

Iron microscope can be used with thicker specimens and forms a perspective im e, although the resolution and m nification are lower. In this type of instrument a beam of primary electrons scans the specimen and those that are reflected, together with any secondary electrons emitted, are collected. This current Is used to modulate a separate electron beam in a TV monitor, which scans the screen at the same frequency, consequently building up a picture of the specimen. The resolution is limited to about 10-20 nm. See also field-emission microscope EIELD-IONIZATION MICROSCOPE. 

Figure 7.19 C-Scan images of thermosetting polymer beam specimens embedded with 9.9% by volume shape memory polymer fibers having pre-tensions of 0%, 50%, and 100% (left) pre-crack before recovery and (right) crack closure after the first healing. Source [21] Reproduced with permisson from...

Ignoring electron diffusion effects within the specimen, SEM resolution is determined by the diameter of the electron beam which scans the specimen. The minimum useable beam diameter, and therefore the optimum resolution, is determined by a number of instrument parameters, which include the brightness of the electron gun, the SE collection efficiency, and the aberrations of the final focusing lens in the electron optical column. This very critical lens is normally referred to as the objective lens. While aberrations are associated with the condenser lenses, their magnitude is much smaller than those of the objective lens, and they can be ignored. 

Figure 2 The primary electron beam-specimen interaction in a scanning electron microscope.

The electrons emitted by the electron gun are focused on the specimen by the Wehnelt cylinder and two to three electromagnetic lenses. As the narrowly focused electron beam strikes the specimen surface, it has a focal diameter of 2-10 nm. A sweep generator is controlled in such a way that the electron beam scans the specimen surface line-by-line and point-by-point. The electron beam also sweeps across a cathode-ray tube. These two motions of the electron beam are synchronized so that each point on the specimen surface is depicted on the screen. The image on a second screen is recorded by a camera. 

Type 3, scanning beam A focused beam (laser tight or electron beam) scans across the specimen, resulting in a reflected beam from the surface (as in a confocal laser scanning microscope) or in secondary or backscattered electrons (in scanning electron microscopes) thick and thin specimens can be studied. 

Type 4, a focused scanning beam passes through the thin specimen (scanning transmission electron microscopes). 

They appear as though the specimen is viewed from the source of the scanning beam and illuminated by a light at the detector position. 

So far, it has been assumed that the result of microanalysis is the elemental composition of a small region of the specimen. This is obtained from the x-ray spectrum produced when the electron beam is stationary. It is often more useful to show the concentration of a specific element as a function of position on the specimen. This is elemental mapping. The map is formed by using the intensity of x-ray emission in a specific energy range to modulate the intensity on a display as the beam scans the specimen. The energy region, or window, is set to include... 

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