Scanning electron microscope (SEM)

Novel approach for surface scanning

1. Scanning Electron Microscope by : MAHESH BIRADAR Dept. of genetics and plant breeding UAS DHARWAD

2. History The first scanning electron microscope with higher magnification were developed by - Manfred von Ardenne 1937. First SEM developed for bulk samples - Zworykin et al . in 1942 First commercial SEM developed - Cambridge Scientific Instrument Company as “ Stereoscan ” in 1965 . Manfred von Ardenne

3. Scanning Electron Microscope (SEM) Produces images of a sample by scanning it with a focused beam of electrons in a raster scan pattern . Electrons interact with atoms and produces various signals that contain information about the sample's surface topography and composition. Resolution Depends on the size of the electron spot, which in turn depends on the magnetic electron-optical system which produces the scanning beam. Is not high enough to image individual atoms , as is possible in the TEM … so that, it is 1-20 nm

4. M. von Ardenne's first SEM Schematic representation of SEM

5. PRINCIPLE Basic principle : A beam of eˉ is generated by a suitable source ( tungsten filament or a field emission gun). Electron beam is then accelerated through a high voltage (e.g. 20 kV). Passed through a system of apertures and electromagnetic lenses to produce a thin beam of eˉ.

6. Then the beam scans the surface of the specimen. Electrons are emitted from the specimen by the action of scanning beam. Collected by a suitable positioned detector.

7. + ve - ve

8. C O M PONE N TS Tungsten filament 2400 o C

9. Electron Gun W hairpin LaB6 crystal FEG

10. 2. LENSES  Condenser lens – determines the number of electrons in the beam which hit the sample by reducing the diameter of the electron beam.  Objective lenses - changes the position of the point at which the electron are focused on the sample.

11. 3. SCANNING COILS Are used to raster/scan the e-beam across the sample surface  The e-beam can be scanned in a rectangular raster across the surface of the sample by means of a series of “scan coils” situated above the objective lens . 4. SAMPLE CHAMBER A place where the sample was mounted on.

12. X-ray ph o t on

13. SCATTERED ELECTRONS

14. 5. DETECTORS detect the secondary and backscattered electrons. Have + charges

15. E l e ct ron D e t e c t o r s O b j e c t i v e lens Sample stage

16. 6. VACUUM CHAMBER Used to protect the electronic beam from interference with air. control the number of electrons which reach the sample. control the final convergence angle of the electron beam onto the sample 7. Aperture

17. HOW THE IMAGE WILL CR E ATED?

18. Beam spot image at different stage of heating

19. Topographical Contrast Topographic contrast arises because SE generation depend on the angle of incidence between the beam and sample. B r i gh t D ark

20. SAMPLE PREPARATION Appropriate sized samples will be prepared to fit in to SEM M o u nt e d r i gidl y on a spe c imen ho l der - specimen stub For imaging in the SEM, specimens must be 1. Electrically conductive 2. Electrically grounded Cleaning the surface of the specimen Stabilizing the specimen Rinsing the specimen Dehydrating the specimen Drying the specimen Mounting the specimen Coating the specimen

21. 1. Cleaning the surface of the specimen Su r f a c e co n tain s m a ny unwante d d e posit s , such as dust, mud, soil etc 2. Stabilizing the specimen Hard, dry materials such as wood, bone, feathers, dried insects, or shells can be examined with little further treatment, but living cells and tissues and whole, soft-bodied organisms usually require chemical fixation to preserve and stabilize their structure. Stabilization is typically done with fixatives. 3. Rinsing the specimen Sample must be rinsed -- remove excessive fixatives. Fixation -- performed by incubation in a solution of a buffered chemical fixative, such as glutaraldehyde, sometimes in combination with formaldehyde and other fixatives. Fixatives that can be used are:- Aldehydes, Osmium tetroxide, Tanic acid, Thiocarbohydrazides

22. 4. Dehydrating the specimen Water must be removed Air-drying causes collapse and shrinkage, this is commonly achieved by replacement of water in the cells with organic solvents such as ethanol or acetone. D e hydr a t i o n - - pe r fo r m e d wit h a gra d ed series of ethanol or acetone. 5. Drying the specime Specimen should be completely dry Otherwise the sample will be destroyed 6. Mounting the specimen Specimen has to be mounted on the holder Mounted rigidly on a specimen holder called a specimen stub Dry specimen -- mounted on a specimen stub using an adhesive such as epoxy resin or electrically conductive double-sided adhesive tape.

23. Cha r g e - up

24. Sam p le co a t i n g is intended to prevent charge-up phenomenon by allowing the charge on the specimen surface go to ground through the coated conductive film. This charge up phenomenon can be prevented by the coating the non - con d uc t o r sample w i th metal (conductor).

25. Coating the specimen To increase the conductivity of specimen and to prevent the high voltage charge on the specimen C o ated with thin layer i . e . , 20n m- 30nm of conductive metal. All metals are conductive and require no preparation before being used. Coating the specimen Non-metals need to be made conductive , which is done by usin g a d evic e call e d a "sputter coater.” Conductive material s Gold , Gold-palladium Alloy , Platinum , Osmium , Iridium , Tungsten , Chromium and Graphite

26. A spider coated with gold

27. AD V AN T AGES Wide array of applications. 3D & topographical imaging. Versatile information gathered from different detectors. Works faster ( completing SEI, BSE & EDS). Most SEM samples require minimal preparation action. DISADVANTAGES Expensive, large & must be housed in an area free of electric, magnetic and vibration interference. Maintenance involves keeping a steady voltage, currents to electromagnetic coils and circulation of cool water. Special training is required to operate an SEM as well as prepare samples. SEMs are limited to solid, inorganic samples small enough to fit inside the vacuum chamber that can handle moderate vacuum pressure.

28. SE Images - Topographic Contrast 1  m Defect in a semiconductor device The debris shown here is an oxide fiber got stuck at a semiconductor device detected by SEM Molybdenum trioxide crystals

29. SCANNING ELECTRON MICROSCOPIC IMAGE OF THE TONGUE

30. Scanning electron micrographs of the early human embryo

31. Pollen and Stamens Wool fibers

32. THANK YOU…