GB2285169A - Scanning electron microscope grain imaging - Google Patents

Abstract

An apparatus for studying grain structure includes means (16) for mounting a specimen (25), means for scanning a surface of the specimen with a beam (17) from an electron gun (18), a plurality of Forward Mounted Back Scattering Detectors (FMBSDs) (13) positioned to collect electrons back scattered from the surface, and means such as a computer for combining images formed on the FMBSDs to provide a combined image. There will usually be three FMBSDs, each connected to an individual colour from a choice of red, green and blue. The combined image might be viewed directly, photographed or searched by the computer to locate grain boundaries. <IMAGE>

Description

SCANNING ELECTRON MICROSCOPE GRAIN IMAGING The present invention relates to scanning electron microscopes and in particular to such microscopes using Forward Mounted Back-Scatter Detectors (FMBSDs).

Scanning electron microscopes (SEMs) are frequently used for studying the surface of specimens. One SEM based technique, used to examine the crystalline nature of materials is Electron Back-Scatter Diffraction (EBSD), in which a beam of electrons is directed at an angle to the surface and electrons scattered from the surface are collected by a phosphor screen to farm an image which can be analysed.

As an addition to the use of the phosphor screen a technique has been developed for studying grain orientation. This involves imaging grain structure directly on detectors mounted forward of the surface and analysing the effect of back scattered electrons falling thereon.

These are known as FMBSDs. The surface of the specimen must be mechanically flat and polished. The resultant image is known as an Orientation Contrast Image (OCI), and shows the different grains as different shades of grey. It has been found, however, that a single FMBSD can be insensitive to certain grains of certain orientations.

According to the present invention an apparatus for studying grain orientation includes means for mounting a specimen, means for scanning a surface of the specimen with a beam from an electron gun of a scanning electron microscope, a plurality of FMBSDs positioned to collect electrons back scattered from the surface, and combination means for combining images formed on the FMBSDs to provide a combined image.

In this specification the term "combined image" is used to cover the situation where signals from the FMBSDs are fed to and processed simultaneously by a computer as well as the situation where a visible image is produced.

The combination means might include colour facilities, for example by using red, blue and green rasters. In this case one preferred form of the invention uses three FMBSDs each connected to one of the three prime colours of the combination means.

The combined image might be viewed directly or photographed, or, especially when there are more than three FMBSDs, searched by a computer system, for example to locate grain boundaries.

The apparatus preferably includes adjustment means for adjusting the relative positions and angles of the specimen, the beam, the FMBSDs and apertures over the FMBSDs. The adjustment means might include a conventional phosphor screen, as used in Scanning Electron Microscope work, and sighting wires to assist in precise specimen alignment.

The individual images may be background corrected, to compensate for an images uneven background by subtracting from the image a heavily blurred version of itself which contains almost no detail, and contrast expanded, making features in the image more visible by increasing the contrast over a selected intensity range, before being combined.

The applicant has found that for specimens of many materials the best image is obtained when the FMBSDs are positioned below the imaging phosphor; for some materials even below glancing incidence.

The invention also provides a method of analysing specimens using a combination of at least three FMBSDs.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, some diagrammatic, of which: Figure 1 is a perspective view of a scanning electron microscope including a three FMBSD scanning arrangement, Figure 2a is an end elevation of the scanning electron microscope shown in Figure 1, Figure 2b is a side elevation of the microscope, Figure 3 is a schematic view of a an apparatus according to the invention including analysis equipment, Figures 4a, b, c, d and e, and Figures 5a, b, c, d and e show images from each of three FMBSDs, a combined image in shades of grey and a combined image in colour.

A scanning electron microscope 30 (Figure 3) has an irradiating and scanning zone, shown generally at 10 in Figure 1, including structure 11 (Figures 1, 2, 3) on which are positioned a phosphor screen 12 and three FMBSDs 13. Also mounted on the structure 11 are four support rods 14 carrying sighting wires 15. A specimen holder 16 is positioned at about 700 to the horizontal in the path of a vertical electron beam 17 from an electron gun 18 (Figure 3). A SIT camera 19 is positioned behind the structure 11.

The SIT camera 19 is connected to a camera control unit 20 (Figure 3) which is associated with a Frame store 21 and a computer unit 22 which has a monitor 23 and a display 24. The monitor 23 is capable of displaying OCIs from a single FMBSD in monochrome or combinations in colour. Multiple OCIs can be simply added together to form a grey level image, or, by assigning one of red, green or blue to each of the images, a colour image can be produced.

In use, a specimen 25 is prepared with a flat polished surface and is carefully positioned on the specimen plate 16. The electron beam 17 is directed at the specimen 25 and the relative positions and angles of the various items, particularly the FMBSDs 13 and specimen plate 16, are adjusted by trial and error or by calculation to give optimum grain contrast. Adjustment is carried out using the display 24 and sighting wires 15.

The electron beam 17 is then raster scanned over the surface of the specimen 25, reflections 26 (Figures 1 and 2) being imaged on the phosphor screen 12 and reflections 27 imaged on the FMBSDs 13.

Signals from the FMBSDs are amplified by, for example, a Four Quadrant Back-Scatter Detector (4QBSD) which is normally a built in component of a SEM, and can then be viewed separately, or in combination as either monochrome or colour images on monitor 23 which can be slaved to move in synchronism with the electron beam 17. As the beam 17 is raster scanned many Electron Back-Scattering Patterns (EBSPs) are formed, and when the scan encounters a new grain the EBSP will change, so altering the contrast detected by the FMBSDs. The resultant images can be analysed to, for example, locate the grain boundaries, size the grains and measure grain orientation. Figure 4 shows, at Figures 4a, b and c the individual images from the three FMBSDs, at Figure 4d the combined image in shades of grey, and at Figure 4e the combined image in colour.By repeated sectioning of the specimen and by polishing it is possible to recreate the actual grain volumes.

The image also allows microstructural information to be directly compared with orientation measurements (Texture) and grains of differing character to be identified and analysed- for example recrystallised grains can be located and their texture measured separately.

The red, blue, and green images are preferably background corrected and contrast expanded before being combined, so considerably increasing the composite image quality. The image can be blurred to generate a background shading reference which is subtracted from the original image. Images formed in this way are illustrated in Figures 5, which correspond in designation with Figures 4. By comparing images 4d and 5e it can be seen that the colour combined image shows grains that are invisible in the other images.

With more than three detectors it is necessary to use a computer to "look" at the images and select the combination of images that produces the clearest colour images, or to search for grain boundaries.

For samples of the same material the relative dispositions of the various items will usually be the same and may therefore be pre-set. It has been found that, for some materials, for example steel, optimum results are obtained when the FMBSDs are positioned to collect electrons scattered at below glancing incidence.

The detectors used for the FMBSDs might typically be squares of silicon mounted on a substrate. For use with a typical scanning electron microscope the detectors might be, for example, ten millimetres square.

It will be realised that many variations of the above described apparatus and method are possible within the scope of the invention.

For example the size of the image falling on each FMBSD may be varied using an adjustable aperture, as shown at 40 in figure 2.

Claims (19)

1. What is claimed is: 1. An apparatus for studying grain structure including means for mounting a specimen, means for scanning a surface of the specimen with a beam from an electron gun of a scanning electron microscope, a plurality of Forward Mounted Back Scattering Detectors (FMBSDs) positioned to collect electrons back scattered from the surface, and combination means for combining images formed on the FMBSDs to provide a combined image.
2. 2. An apparatus as claimed in Claim 1 wherein the combination means might include colour facilities.
3. 3. An apparatus as claimed in Claim 2 wherein the colour facilities are provided by using red, blue and green rasters.
4. 4. An apparatus as claimed in Claim 3 having three FMBSDs each connected to one of the three prime colours of the combination means.
5. 5. An apparatus as claimed in any one of Claims 1 to 4 wherein the combined image is viewed directly or is photographed.
6. 6. An apparatus as claimed in any one of Claims 1 to 5 wherein the combined image is searched by a computer system.
7. 7. An apparatus as claimed in any one of Claims 1 to 6 including adjustment means for adjusting the relative positions and angles of the specimen, the beam and the FMBSDs.
8. 8. An apparatus as claimed in Claim 7 wherein the adjustment means includes a conventional phosphor screen.
9. 9. An apparatus as claimed in Claim 8 wherein the adjustment means includes sighting wires.
10. 10. An apparatus as claimed in any one of Claims 1 to 9 wherein the FMBSDs are positioned at below glancing incidence.
11. 11. A method for studying grain structure including the steps of mounting a specimen, scanning a surface of the specimen with a beam from a scanning electron microscope, positioning a plurality of FMBSDs to collect electrons back scattered from the surface, and combining images formed on the FMBSDs to provide a combined image.
12. 12. A method as claimed in Claim 11 in which the images formed on the FMBSDs are combined on combination means including colour facilities.
13. 13. A method as claimed in Claim 12 wherein the combination means use red, blue and green rasters.
14. 14. A method as claimed in Claim 13 wherein there are three FMBSDs each connected to one of the three prime colours of the combination means.
15. 15. A method as claimed in Claim 11 wherein the combined image is processed directly by a computer.
16. 16. A method as claimed in any one of Claims 11 to 15 wherein the FMBSDs are positioned at below glancing incidence.
17. 17. A method as claimed in any one of Claims 11 to 16 wherein the images are background corrected and contrast expanded before being combined.
18. 18. Apparatus for studying grain structure substantially as herein described with reference to Figures 1 to 5 of the accompanying drawings.
19. 19. A method for studying grain structure substantially as herein described with reference to Figures 1 to 5 of the accompanying drawings.